David La Van Yale University
Mark Spearing University of Southampton
Srikar Vengallatore McGill University
Mark da Silva Exponent Inc.
DD1: Micromechanics I
Monday PM, November 26, 2007
Room 313 (Hynes)
9:30 AM - **DD1.1
A Review of Tension Test Methods for Thin Films.
William Sharpe 1 Show Abstract
1 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States
The tensile test is the accepted standard for determining mechanical properties of materials used in large structures and components. It, along with the compression test, generates uniform states of stress and strain thereby permitting their direct measurement. However, tension test methods applicable to freestanding thin films were virtually non-existent only 15 years ago. While several researchers using different techniques and procedures now generate comparable results, there is as yet no accepted standard for tension testing of thin films. Neugebauer conducted tensile tests of gold films ranging in thickness from 50 nanometers to 1.5 microns in 1960! It is remarkable that no similar tests followed until the new test method of Read and Dally in 1992 – a gap of 30 years. Neugebauer evaporated gold films with a tensile specimen pattern onto rock salt crystals, glued the crystal ends to a unique test machine, and then dissolved the salt away from the tensile region. Read and Dally’s method is quite similar except the substrate is silicon crystal and removed by selective etching. An easier test method in which one end of the patterned tensile specimen remains attached to the silicon substrate was introduced by Tsuchiya in 1997. The free end of the specimen can be gripped by electrostatic attraction to a probe or by various gluing methods. Unique test machines are developed by each researcher.Strain can be measured by overall elongation of the specimen, which in many cases is sufficient since the specimen can be relatively long compared to its end conditions. Direct strain measurement is of course preferred, and various techniques are used. Sharpe introduced an interferometric technique in 1997. Long used optical digital image correlation (DIC) for gold films in 2001, and Chasiotis has used AFM images in the same manner. Currently, various forms of DIC are being used at this size scale, and it certainly appears to be the preferred technique. These DIC methods have advanced rapidly in the past few years and promise to be even more powerful with computer improvements.Finally, MEMS structures can be used to produce tiny test machines incorporating thin tensile specimens in the processing. Saif’s work of 2004 is an excellent example.This paper is a review of the development of thin-film test methods over the past 15 years. It will describe and discuss the merits of each and include a complete bibliography.
10:00 AM - DD1.2
Reliability of MEMS Materials: Mechanical Characterization of Thin-Films using the Wafer Scale Bulge Test and Improved Microtensile Techniques.
Joao Gaspar 1 , Marek Schmidt 1 , Jochen Held 1 , Oliver Paul 1 Show Abstract
1 Department of Microsystems Engineering (IMTEK), University of Freiburg, Freiburg, BW, Germany
The mechanical characterization of thin films is of great importance for their use as structural layers in MEMS or ICs since device functionality and reliability depend strongly on mechanical parameters. Towards a consistent extraction of mechanical properties, this paper reports on key improvements of both bulge and microtensile techniques applied to an extensive set of materials, brittle and ductile, typically used in MEMS applications.Membranes and microtensile specimens made of silicon nitride, silicon oxide, polycrystalline silicon and aluminum have been fabricated and characterized. Deposition methods such as LPCVD, PECVD, thermal oxidation and PVD have been used. Properties extracted from these materials included pre-stress σ0, elastic modulus E or plane-strain modulus Eps, Poisson’s ratio ν, Weibull parameters (strength μ and modulus m) and strain hardening coefficients n. Their dependence on deposition conditions will be emphasized.The bulge test uses a setup where up to 80 membranes on a silicon wafer can be characterized in a single series of sequential measurements. This enables the efficient acquisition of large data sets necessary, e.g., for the determination of reliable fracture data. The membrane profiles resulting from pressure loads are measured with an auto-focus sensor integrated with automated position tables for scanning purposes. A peculiarity of these bulge experiments is the use of diaphragms with large side length aspect ratios. This makes it possible to formulate and apply an accurate, analytical model of the diaphragm mechanics including multilayers with individual prestress and mechanical constants, and the support stiffness.The microtensile test exploits a wafer-detachable test structure design where the specimens are produced on a c-Si frame with coplanar fixed and mobile components defined by DRIE. Delicate handling of the tensile samples is thus avoided. The specimens bridge the gap between the frames experiencing truly uniaxial loads by geometrical constraint when displacements are imposed to the moving component. Such test structures allow the characterization of several material samples in a single experiment and thus contribute to the efficient acquisition of statistically relevant data. Force data are read from a load-cell while local strains are obtained from 2D-interferometric measurements, stroboscopic image analysis and calibrated actuators movement. Current work is developing a wafer scale tensile setup complementing the capability of the bulge test setup.More than 500 membranes of some films have been measured: from the brittle materials analyzed, LPCVD nitride films have the highest fracture strength and are highly predictable (m up to 60). In contrast, oxides are weak and have the lowest Weibull moduli (∼5). Elastic, fracture and ductile properties obtained from both techniques agree with each other and literature, however with narrower uncertainties due to the high throughputs of data.
10:15 AM - DD1.3
Multipurpose on-chip Nanomechanical Laboratory for Testing Thin Films.
Thomas Pardoen 1 2 , Raskin Jean-Pierre 1 3 , Coulombier Michael 1 2 , Sebastien Gravier 2 , Nicolas Andre 1 3 , Asmahan Safi 2 , Thomas Gets 2 , Vincent Delongueville 3 , Stanislas Sobieski 3 , Samer Houri 3 Show Abstract
1 Research Center in Micro and Nanoscopic Materials and Electronic, Université catholique de Louvain, Louvain-la-Neuve Belgium, 2 Department of Materials Science and Processes, Université catholique de Louvain, Louvain-la-Neuve Belgium, 3 Department of Electricity, Université catholique de Louvain, Louvain-la-Neuve Belgium
The measurement of the mechanical properties of submicron sized specimens is extremely challenging due to difficulties for manipulating samples, for applying small load and for extracting accurate stresses and strains. We present a novel, versatile concept of micromachines to measure the mechanical response, up to large strains and fracture, of thin films allowing multiple loading configurations and geometries. This new concept relies on MEMS microfabrication techniques. The first idea is to use internal stresses to actuate and impose the load to the tested sample which is directly deposited on a patterned substrate. The second idea is to multiply elementary machines rather than to build on single complex multi-purpose micromachine. The applied stress and strain are directly inferred from a local displacement measurement. The versatility of the concept allows other loading configurations than simple tension such as shear, biaxial, compression, three-point bending and double cantilever opening. About 5000 elementary micromachines have been processed on top of a single silicon wafer, delivering statistically representative information. The mechanical response of pure Al, Al/Si alloys, Ti and silicon nitride films, with thicknesses ranging between 80 and 500 nm, has been addressed. For instance, stress strain curves have been measured for the Al films, with the strength increasing linearly with 1/thickness to reach values larger than 1 GPa for the thinnest samples while keeping more than 8% of uniform elongation. The strength of Al/Si alloys is two times larger than for Al films. The strength of the Ti films reaches more than 2 GPa. The fracture stress of silicon nitride films attains about 4 to 5 GPa, approaching the theoretical cleavage stress. This method based on MEMS fabrication techniques constitute a combinatorial experimental platform for addressing several critical MEMS reliability issues.
10:30 AM - DD1.4
Fatigue of Nanostructured Ni MEMS Thin Films.
Yong Yang 1 , Guojiang Fan 2 , Benjamin Imasogie 3 , Brad Boyce 4 , Peter Liaw 2 , Winston Soboyejo 1 Show Abstract
1 Princeton Institute for the Science and Technology of Materials (PRISM) and The Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 2 Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee, United States, 3 Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife Nigeria, 4 Materials Science and Engineering Center, Sandia National Laboratories, Albuquerque, New Mexico, United States
This paper presents the results of an experimental study of fatigue in nanostructured Ni thin films produced using electro-deposition techniques. The underlying mechanisms of fatigue are elucidated in stress-life and short/long crack fatigue crack growth experiments. The studies reveal the potential role of pre-existing material defects in crack nucleation. They also show new experimental evidence of the role of microvoid formation and grain growth during short and long fatigue crack growth of nanostructured Ni MEMS thin films. The implications of the observed phenomena are discussed for potential applications of Ni MEMS structures with nanostructured grain sizes.
10:45 AM - DD1.5
Fatigue of Al Thin Films at ultra-high Frequencies: Discrete Dislocations and Damage Behavior.
Diana Courty 1 , Chris Eberl 1 2 , Werner Ruile 3 , Oliver Kraft 1 2 Show Abstract
1 izbs, University of Karlsruhe, Karlsruhe Germany, 2 Institute for Materials Research II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 3 , EPCOS AG, München Germany
Micro devices are widely used in applications for radio frequency communications. In many of these applications, they are exposed to cyclic mechanical loading at typical operating frequencies in the GHz regime. In our work, we have used patterned Al thin films on a piezoelectric substrate to study ultra-high-cycle fatigue. The working principal is based on frequency filtering by the to and fro transduction of an electrical signal into an acoustic wave. An alternating applied electric field generates acoustic deformations on the surface of the piezoelectric substrate by the transduction and can thus be used to stress metallizations at extremely high frequencies. At the resonance frequency, large deflection amplitudes and, hence, high stresses may occur. The devices are tested in the GHz-regime, which allows studying cycle numbers up to 10^14. Fatigue effects, such as the formation of voids and extrusions that result in a frequency shift or even in short circuits are observed.In our work, we have concentrated on the question how cyclic stresses at extremely high frequencies activate defect mechanisms, in particular if they are able to drive dislocations in thin metal films leading to the observed irreversible damage.For this, we have conducted discrete dislocation dynamics simulations showing that dislocations are still able to move in the GHz-regime. It is observed in these simulations that even small asymmetries in the loading conditions lead very quickly to an accumulated plastic strain due to the extremely high frequencies. As a result, the role of the microstructure of the metal films such as grain size or texture is discussed.In order to explore further possible failure mechanisms, tested Al thin film devices were analyzed by Focused Ion Beam microscopy (FIB) and Scanning Electron Microscopy (SEM) evaluating quantitatively the observed extrusions and voids. Furthermore FIB micrographs were used to determine the mean grain size, to investigate whether or not there is a significant influence of the grain size on the extrusion and void formation. Our attempt to correlate details of the dislocation behavior with the damage formation at ultra high cycle fatigue in thin metal films is of general importance for the reliability of micro devices operated at radio frequencies. It becomes obvious that even very moderate loading conditions may become detrimental due to the high frequency connected to an extremely fast accumulation of strain.
11:30 AM - **DD1.6
Strategies for Testing of Material and Geometrical Properties of MEMS.
Joerg Bagdahn 1 , Matthias Ebert 1 , Ronny Gerbach 1 , Falk Naumann 1 , Ralf Schaefer 1 , Jan Schischka 1 Show Abstract
1 Microsystems, Fraunhofer Inst for Mechanics of Materials, Halle/Saale Germany
Testing of materials properties of micro electromechanical systems (MEMS) is required during the development stage of devices, during fabrication and for analysis of failed components. The testing approaches can be classified in techniques for testing on wafer-level, on die-level and on lift-out samples. Testing on wafer-level is a key element in in-line quality management during fabrication. A fast (few seconds) and non destructive test procedure is required. The testing on wafer-level or die-level is used to detect faulty devices or divide devices on a wafer into different quality classes. The possibility of the detection of bad / or good devices on the wafer-level is an essential cost factor, since unnecessary packing and assembly cost can be avoided and the yield during inte-gration of several dies, e.g. in stacking applications, is dramatically increased. Furthermore ma-terials and also geometric parameters can be monitored during fabrication. Due to the complex-ity of MEMS components a simple electrical measurement is not sufficient. Rather mechanical, chemical, optical, electrical excitations are required and either optical or electrical measurement methods are needed. In the papers the focus is given on techniques for mechanical stimulation and evaluation. Techniques for excitation of MEMS structures on the wafer level, methods for detection of static deformation as well as dynamic vibrations and numerical methods for data evaluation will be presented. Tests on die-level are used to fabricate dedicated test structures, which are tested off-line. The off-line test allows also the destructive-testing of devices. An example of such an test struc-ture are micro-chevron-samples, which can be used for strength characterization of wafer-bonded interfaces. In the paper the current status of this test approach will be shown. Lift-off techniques are required if materials properties of parts of devices has to be measured, e.g. in failed components. An new approach for testing of parts of MEMS structures was devel-oped. The test includes the preparation of test samples by focused ion beam technique milling, handling of test samples and subsequent testing with dedicated test equipment. The approach allows the measurement of mechanical properties like tensile or bend strength or fatigue prop-erties of already fabricated devices. Results of strength investigations will be presented.
12:00 PM - **DD1.7
Adhesion of Microstructures in Dry and Humid Ambients.
Maarten deBoer 1 , Frank DelRio 2 , Martin Dunn 3 Show Abstract
1 MEMS Core Technologies, Sandia National Labs, Albuquerque, New Mexico, United States, 2 Chemical Engineering, UC Berkeley, Berkeley, California, United States, 3 Mechanical Engineering, Univ. of Colorado at Boulder, Boulder, Colorado, United States
The miniaturization of functional structures to the micron and nanometer scales is of widespread technological and scientific interest. Examples include nanotubes, nanowires, mechanical switches, robotic tools, and non-volatile memory elements. As dimensions decrease, surface-to-volume ratios increase, and the effects of properties such as adhesion, friction and damping on system performance become increasingly important. Here, we address the interaction between adhesion and surface topography in dry and humid ambients. To measure adhesion, we employ microcantilever structures using a fracture-mechanics based methodology. In dry ambients, adhesion is as low as several microjoules per square meter. Analysis of the surface topography shows that the highest surface asperities control the average surface separation, but that the main contribution to adhesion is from non-contacting areas. In humid ambients, we observe a strong interaction between relative humidity and surface roughness with millijoule per square meter adhesion values. To analyze the results, a constitutive model for an asperity bridged by a liquid condensate to a flat surface is needed. Under equilibrium conditions, it can be shown that the work of adhesion is less than the surface energy created. This apparent paradox is resolved by a thermodynamic control-volume analysis, and is due to heat-induced evaporation of the liquid during the pull-off process. A more general Griffith criterion results. Experimentally, topographic correlations between the upper and lower surfaces, asperity plasticity and disjoining pressure of the ambient vapor are also important factors. When these effects are all taken into account, good agreement between the experimental and calculated results are found. This work will help enable more reliable micro and nanostructures.
12:30 PM - DD1.8
Vapor Phase Lubrication of Silicon MEMS.
Michael Dugger 1 , David Asay 2 , James Ohlhausen 1 , Seong Kim 2 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Pennsylvania State University, University Park, Pennsylvania, United States
MicroElectroMechanical Systems (MEMS) that have contacting surfaces necessitate surface modification to mitigate adhesion, friction and wear. Most MEMS applications avoid rubbing surfaces because of the challenges associated with reliability in such devices. Surface treatment for non-contacting MEMS is focused mainly on obtaining structures that survive handling associated with production and assembly. A lubrication approach to create reliable sliding contacts would enable a significant expansion of the design space.Chemisorbed monolayers have been used to modify the surface energy of oxidized silicon after etching the sacrificial oxide layers, in order to resist water adsorption and capillary adhesion. A variety of other surface treatments for silicon microsystems have also been investigated, including carbides and oxides to improve wear resistance. Recent emphasis in MEMS lubrication has been on systems that allow for damaged areas to be healed by lubricant diffusion over time. This approach offers some promise, although surface diffusion may limit the effectiveness of this approach for applications involving long duration operation.Interfaces that are deeply buried in complex structures present particular challenges for lubrication. Line-of-sight surface treatment approaches are not applicable to these contacts, and chemisorbed monolayers have exhibited limited mechanical durability in repetitive contact. To address these problems, vapor phase lubrication has been developed for silicon MEMS to enable extremely long duration operation with little or no wear. Molecules in the vapor phase adsorb on silicon surfaces and react to form a friction and wear reducing film. Therefore, as long as the molecule is available in the environment, the lubricant film can be replenished. Micromachined silicon tribometers have been used to demonstrate the vapor phase lubrication concept on MEMS devices, and to compare friction and adhesion behavior with and without the wear-reducing film. Operational lifetime is increased from the order of 1E4 cycles with a chemisorbed monolayer alone, to in excess of 1E8 cycles with vapor phase lubrication. Macro-scale pin-on-disk measurements have been used to produce wear areas large enough for surface analysis by ToF-SIMS. These measurements indicate the formation of a high molecular weight product in the wear scar that is hypothesized to be responsible for the dramatic improvement in tribological behavior. Probable reaction pathways leading to the reactant formation will be discussed.
12:45 PM - DD1.9
MEMS Lubrication: An Atomistic Perspective of a Bound + Mobile Lubricant.
Douglas Irving 1 , Donald Brenner 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Chemically bound octadecyltrichlorosilane (ODTS) self-assembled monolayers have been successful in protecting microelectricalmechanical systems (MEMS) from stiction related failure in both the processing stage and the initial stage of use (i.e. “single shot” applications). This lubricant, however, is not successful for MEMS devices expected to cycle repetitively for extended time due, primarily, to the inability to replenish the ODTS in situ. Successful lubrication by ODTS is also limited to a narrow range of temperatures and pressures. A new “Bound + Mobile” lubrication scheme, which combines the bound ODTS with a replenishable mobile tricresyl phosphate (TCP) phase, has been proposed as a solution to these deficiencies. This talk will focus on recent atomistic simulations that characterize both the self-diffusion and the incorporation into defects of the mobile phase as well as the adhesive properties and interfacial structure of the newly formed ODTS/TCP/ODTS interface. This work has been supported by the Air Force Office of Scientific Research, MURI grant # FA9550-04-1-0381.
DD2: Micromechanics II
Monday PM, November 26, 2007
Room 313 (Hynes)
2:30 PM - **DD2.1
Fracture of Brittle Polycrystalline and Amorphous Materials for MEMS.
Ioannis Chasiotis 1 , Krishna Jonnalagadda 1 Show Abstract
1 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Fracture of brittle materials fabricated for surface micromachined devices hinges upon understanding the stochastic nature of material failure in conjunction with its microstructure and the material volume under stress. Experiments and numerical analyses were conducted to understand the deterministic and probabilistic parameters entering the problem of fracture in two material systems: Polycrystalline silicon and tetrahedral amorphous diamond-like carbon. Fracture investigations were carried out to determine the effect of material inhomogeneity and stress gradients in brittle films with prefabricated mode-I and mixed-mode critical cracks. It was shown that the location of the crack tip determines the macroscopic failure stress and, thus, the effective critical stress intensity factor for both mode-I and mixed-mode cracks. As a result, the stochastic nature of polysilicon strength is strongly affected by variations in the local critical stress intensity factor. This finding limits extrapolations of statistical probability of failure functions to small material domains that are comparable to grain size, or to specimen sizes that do not guarantee statistical homogeneity. Furthermore, it was shown that through-the-thickness stress gradients are critical in the determination of the material fracture toughness, especially for films that are thicker than one micron. These data were corroborated with an investigation of the accuracy of a single two-parameter Weibull cumulative probability density function (material scale parameter and Weibull modulus) used to describe the failure strength of brittle MEMS components with non-uniform geometries and stress distributions. It was concluded that failure of self-similar MEMS geometries is not always described by the same Weibull constants due to competing flaw populations whose severity is controlled by the stress distribution and the material volume subjected to large stresses.
3:00 PM - **DD2.2
On-chip Microscale Mechanical Properties Measurements.
H Kahn 1 , R. Ballarini 2 , A. Heuer 1 , S. Eppell 3 Show Abstract
1 Materials Science and Engineering, Case Western Reserve University, Cleveland, Ohio, United States, 2 Civil Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 3 Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio, United States
Because of the actuation schemes and processing techniques developed over the past years and their inherent micrometer size scales, MEMS are excellent platforms for performing materials properties investigations. For example, chemical vapor deposition can produce films with wide ranges of microstructures and residual stresses, and electrostatic actuation can generate well controlled forces for monotonic or resonant loading. Submicron displacement accuracy adds to the capabilities for highly precise mechanical measurements. This talk will demonstrate this capacity by describing results for on-chip tests, including: the stresses generated during oxidation of silicon, the effects of microstructure on fracture toughness and fracture strength of polysilicon, fatigue of polysilicon, and stress corrosion of silicon dioxide. In addition, MEMS devices can be used to test non-MEMS materials. The example of tensile testing of collagen fibrils will be presented.
3:30 PM - DD2.3
Effect of Surface Oxide Layer on Mechanical Properties of Single Crystalline Silicon.
Kenji Miyamoto 1 , Koji Sugano 1 , Toshiyuki Tsuchiya 1 , Osamu Tabata 1 Show Abstract
1 Microengineering, Kyoto University, Kyoto-shi, Kyoto, Japan
Single crystal silicon (SCS) is widely used for MEMS devices as structural material, because it has excellent mechanical properties and is suitable for microfabrication processes. Since silicon is a brittle material, the mechanical reliability, especially the fatigue fractures under long term cyclic loading, is concerning and the fatigue properties and mechanisms are being investigated widely. The previous works have revealed that the fatigue fracture of single crystalline silicon is sensitive to the humidity of the test environment and the native oxide layer on the surface affects the fatigue properties (1), (2). In this research, we focus on the effect of surface oxidation on the mechanical properties, such as the Young’s modulus, fracture strength and fatigue. The tensile testing of the surface oxidized SCS specimens was performed. The SCS specimens were fabricated from SOI (silicon-on-insulator) wafer using standard SOI surface micromachining, which contains deep reactive ion etchings (DRIEs) for specimen patterning and substrate removal and sacrificial etching. The thickness, width, and length of the specimens were 4 μm, 3 μm, and 120 or 600 μm, respectively. The specimen surface orientation was (100), and tensile axis was aligned to the <110> direction. Then about 200-nm-thick oxide layer was grown by dry thermal oxidization at 1100 °C for 90 min. The tensile testing was performed using the tensile tester for thin films that can control the humidity of test environment by covering the specimen with a small humidity-controlled chamber. The electrostatic chuck was used for specimen fixing(1). The tests were performed in two conditions, in lab air (humidity: 50 %RH) and in the humidity chamber (humidity: 90 %RH) both at room temperature (26 °C). The quasi static tensile tests showed that the averaged fracture strengths of surface oxidized SCS specimens were 2.3 GPa in the lab air and 1.8 GPa in the high humidity, respectively, which is lower than that without oxide layer, which was 2.8 GPa. The fracture surface of the surface oxidized SCS specimen indicates that the fracture was initiated from inside silicon near the corner, not from surface oxide layer. The oxidization of silicon caused the tensile strain in silicon near the oxide layer interface due to the large compressive strain in oxide layer, which may relate to the fracture behavior. Now the thickness effect on the tensile and fatigue strength of the silicon is being investigated and will be reported.(1)Y. Yamaji, et al. Proc MEMS 2007, pp.267-270.(2)C.L. Muhlstein, et al., J. Microelectromechanical Syst. 10(4), 2001, 593-600.
3:45 PM - DD2.4
A Comparison of the Strength of Poly- and Single-Crystalline Silicon Subject to Galvanic Corrosion in Hydrofluoric Acid.
David Miller 1 , Collin Becker 1 , Brad Boyce 2 , Conrad Stoldt 1 Show Abstract
1 Mechanical Engineering, University of Colorado, Boulder, Colorado, United States, 2 Microsystem Materials, Sandia National Laboratories, Albuquerque, New Mexico, United States
Immersion of silicon-based microelectromechanical systems (MEMS) in hydrofluoric acid (HF) solutions is a standard post-fabrication procedure used to render mechanically moveable structures. Recent studies have demonstrated that HF etching can result in autonomous corrosion of silicon, when an added metallic layer is present. We utilized microfabricated tensile specimens to examine the reduction in mechanical strength when silicon is galvanically coupled to a gold metallic layer. The resulting corrosion damage is compared between polycrystalline silicon (polySi) and single crystal silicon (SCS) tensile specimens, manufactured using the standard commercial Multi-User MEMS Process (MUMPs) technologies. The etching solutions used include undiluted 48% HF (UDHF), as well as HF mixed with either water (an oxidizing agent) or Triton-X-100 (a surfactant). Micro-tensile testing was performed using a custom electromechanical probe station where the applied load could be measured to 0.01 mN. The specimens’ fractured surface was examined in a field emission scanning electron microscope (FESEM), with representative specimens being further characterized in a transmission electron microscope (TEM). For polySi, strength decreased precipitously with etch time, either due to preferential grain-boundary attack (leading to intergranular fracture) to generalized material removal (leading to transgranular fracture), depending on the etch solution. Alternately for SCS, strength could increase or decrease with etch time, depending on the effect of etch chemistry in blunting or deepening sidewall crevices. For both types of silicon, the severity of damage and corresponding change in strength depended on etch duration and etch chemistry. Our results uniquely indicate that galvanic corrosion is dominated by the surface wetting characteristics of the HF-based chemistry utilized. For example, etching in UDHF resulted in cracks and voided regions. Alternately, etching in the presence of a surfactant resulted in a very different rounded etch-front. In all cases, the corroded surface layer primarily composed of oxidized silicon as observed using energy dispersive X-ray spectroscopy (EDX). We will discuss the results of mechanical and morphological examination, which compare the different types of silicon and etchant chemistries. Additional nanoindentation, mathematical modeling, electrical resistance, and electrochemical based studies support the mechanical test data and the related conclusions. Our work provides an understanding of the drastic degradation in mechanical properties for the commonly used silicon and gold material couple, with implications concerning the design, performance, and reliability of MEMS structures.
4:30 PM - DD2.5
MEMS Stress Camera for High-Throughput Materials Characterization.
Noble Woo 1 , Jonathan Petrie 2 , R. van Dover 2 Show Abstract
1 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States, 2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
4:45 PM - DD2.6
Indentation Characterization of Fracture Toughness and Interfacial Strength of PECVD Nitrides after Rapid Thermal Annealing.
Hong-Yi Yan 1 , Kuang-Shun Ou 1 , Kuo-Shen Chen 1 Show Abstract
1 Mechanical Engineering, National Cheng-Kung University, Tainan Taiwan
Plasma enhanced chemical vapour deposited (PECVD) silicon nitrides have been widely used in microelectromechanical systems (MEMS) and integrated circuits (IC) devices as mask or structural materials and their mechanical properties are traditionally important information for the structural design to enhance the device integrity. In particular, the mechanical properties of PECVD nitride significantly depend on the associate thermal processing history. Recently, rapid thermal annealing (RTA) has been widely applied in IC and MEMS processes for reducing thermal budget during fabrication. However, the influence of RTA on the mechanical properties of PECVD nitride, especially on the fracture toughness and interfacial strength, is still not clear. Since the information is vital for device longevity concern, a detail investigation is therefore required.This work presents the result of mechanical characterization for the fracture toughness and interfacial strength of PECVD silicon nitride films deposited on silicon subjected to rapid thermal annealing (RTA) processing between 200 and 800 oC. Both micro- and nano-indentation techniques are employed to perform the experiments. In conjunction with the model proposed by Marshall and Lawn for data reduction, the fracture toughness of un-heat treated nitride is obtained as 1.33 MPa.sqrt(m) based on a series of Vickers micro-indentation tests and this value is essentially unchanged if the RTA temperature is below 400 oC. Further increase in RTA temperature would significantly enhance the fracture toughness. On the other hand, using nanoindentation testing in conjunction with the model proposed by Marshall and Evans, the interfacial strength between the nitride and silicon is determined as 17.2 J/m2 for un-heat treatment nitrides and it could be significantly enhanced by RTA processes with temperature exceeding 400 oC. These results should be useful for related MEMS or IC structure fabrication for the concerns of maintaining the structural integrity and improve fabrication performance in related applications.
5:00 PM - DD2.7
Incomplete Martensitic Transformations of NiTi Shape Memory Alloy Thin Films.
Xu Huang 1 , Hoo-Jeong Lee 2 , Ainissa Ramirez 1 3 Show Abstract
1 Mechanical Engineering, Yale University, New Haven, Connecticut, United States, 2 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Kyongki-Do Korea (the Republic of), 3 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Nickel titanium thin films were deposited onto silicon wafers and subjected to thermal cycles to create incomplete martensitic transformations. With wafer curvature methods, it was found that the film stress plateaus briefly when forward (austenite-to-martensite) transformations follow incomplete transformations. No stress plateau was found in reverse transformations. X-ray diffraction measurements show the phases remained constant over this plateau when austenite converts to martensite, but changed immediately in the opposite direction. These observations are consistent with the thermodynamic model that suggests the elastic strain energy is stored in forward transformations and impedes them, yet assists them when released in reverse ones.
5:15 PM - DD2.8
A Novel Micro Tensile Testing Instrument with Replaceable Testing Specimen by Parylene Passivation Technique.
Yung-Dong Lau 1 , Tso-Chi Chang 1 , Hong Hocheng 1 , Rongshun Chen 1 2 , Weileun Fang 1 2 Show Abstract
1 Power Mechanical Engineering, National Tsing Hua University, Hsinchu Taiwan, 2 Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Hsinchu Taiwan
5:30 PM - DD2.9
Atomistic+Continuum Multi-scale Modeling of Single Asperity Gold-Gold Contact in an RF MEMS Device.
Wes Crill 1 , D. Irving 1 , C. Padgett 3 , O. Rezvanian 2 , M. Zikry 2 , D. Brenner 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Department of Chemistry and Physics, Armstrong-Atlantic State University, Savannah, Georgia, United States, 2 Mechanical and Aerospace Engineering , North Carolina State University, Raleigh, North Carolina, United States
The gold contacts of failed RF MEMS devices often show what appear to be melted and re-solidified asperity tips. The cause of these features is not yet understood, but it is thought that the formation of nanowires during contact pull-out could potentially be a precursor to this behavior. To understand the details of nanowire formation in gold-gold RF MEMS contacts, we have carried out simulations of the pull-apart of a single asperity contact using large-scale molecular dynamics simulations coupled to a continuum treatment of Joule heating and heat transfer. The initial asperity geometry, which was derived from a finite element fractal model of a contact blunted by plasticity, contained a 550 nm2 asperity on a 7500 nm2 substrate that is brought into contact with another flat substrate of the same size. As the contact is pulled apart, the simulations show dislocation emission in the substrate surrounding the asperity contact as well as a disordering of the region between the surfaces. Without Joule heating, the severed contact retains a non-cylindrical geometry that reflects that of the initial contact shape. With an applied current, Joule heating that starts at the edges of this contact drives the structure towards a more cylindrical geometry. The influence of this local heating on the formation of long nanowires will be discussed and compared to experimental structures observed for hot switching of gold MEMS.This work was supported by the Office of Naval Research and the Extreme Friction Multi-University Research Initiative sponsored by the Air Force Office of Scientific Research.
5:45 PM - DD2.10
Microscopic Motion and Deflection Quantifying Chip for Lab-on-chip Application.
Xiaoyu Zheng 1 , Xin Zhang 1 Show Abstract
1 , Boston University, Brookline, Massachusetts, United States
This paper presents a double layered polymer structure for quantifying local distortions on polymer micro/nano structures. Recently, researchers have been engineering polymer based periodic substratum for cell migration and cellular force measurement. The substratum ranges from thin elastomer films to micro beads or micro vertical pillar arrays which quantifies the force evolution on cells upon their substratum. However, the determination of beads or pillar motion was only based on measurement of direct observation on each individual measuring unit via optical microscopy, which has to rely on algorithms to determine the centroid for each unit and track their motions individually. Such method has several drawbacks. Firstly, it has to rely on the algorithm on determining the motion of unit individually without knowing their initial state, which, on the other hand, could make the measurement problematic especially after fabrication, elastomer materials such as poly dimethylsiloxane(PDMS), is distorted by stress, due to the contraction on curing of the elastomers and contact with a substrate. Such distortion could cause each individual sensors distribute non-uniformly which violate the assumption that the sensor were distributed evenly on the substratum hence affect the accuracy of such measurement. Secondly, conventional tracking of each individual unit requires visibility of not only each individual unit but also their distortions. As a result, this method confines the sensors within micro meter domains due to the capacity determined by the microscope and camera, therefore requires the sensors must have very low stiffness to make the distortion distinguishable, which in turn, violates the linear load-displacement assumption. In this paper, we fabricated double layered chip by soft lithography technique. The top layer is the substratum consisting of each individual unit sensor such as pillars or micro beads. The bottom layer consists of a pattern layer made of the same material but with a much higher rigidity which has slightly different scales than the top one and tilted by a small angle. A He-Ne 633nm laser was used as an imaging source that transmitted vertically through the plane of the substratum. A 10X to 50 X objective and CCD camera was followed by the placement of the substratum. A circular aperture 3mm diameter was mounted to increase the depth of focus. Upon saturation of laser, diffracted moiré pattern from the double layer was formed and visualized on monitor. The angle of the double layer was chosen such that the flexible fringes of moiré pattern and the spatial period satisfy measurement needs which map the distortion of pillars or motion of beads in two planar directions. Therefore, the information of initial state of periodic substratum can be visualized and the motion of every unit can be directly mapped in real time via diffracted moiré patterns.
DD3: Poster Session I
Tuesday AM, November 27, 2007
Exhibition Hall D (Hynes)
9:00 PM - DD3.1
Mechanical Stress Sensors for Copper Damascene Interconnects.
Romain Delamare 1 , Christian Rivero 2 , Sylvain Blayac 1 , Moustafa Kasbari 1 , Karim Inal 1 Show Abstract
1 PS2, Centre de Microelectronique de Provence, Gardanne France, 2 , STMicroelectronics, Rousset France
We propose embedded microsensors to investigate the mechanical stress in copper damascene lines. Stress induced voids and cracks in interconnect structures is known to be responsible in reduced yield at all levels of device processing: the local mechanical stress measurement during process steps is of major interest. In this study, we describe sensors based on silicon piezoresistive effect where strain in the active silicon is induced by orientated copper lines. The challenge is to correlate the electrical sensors signal directly to stress variation in lines. Experimental characterizations have been realized versus copper stress variation induced by both temperature and uniaxial external stress application. The relation between the carrier mobility and the silicon strain variation was extracted via four points bending measurements. In addition MEMS were integrated in interconnects levels to estimate the stress. A good agreement between Finite Element Analysis and measurements permit to propose an electro-mechanical model of our structures. Moreover, these microsensors are process compatible, thus it permits to show the relevance in the monitoring of stress induced by interconnects during the fabrication step process.
9:00 PM - DD3.10
Modification of Conductivity and Stiffness of Electroactive Polymer (EAP) Thin Films by Metal Ion Implantation.
Muhamed Niklaus 1 , Samuel Rosset 1 , Philippe Dubois 1 , Massoud Dadras 2 , Herbert Shea 1 Show Abstract
1 LMTS - Microsystems for Space Technologies Laboratory, EPFL - Ecole Polytechnic Federal de Lausanne, Neuchatel Switzerland, 2 Institute of Microtechnology, University of Neuchâtel, Neuchatel Switzerland
Compliant electrodes for micron-scale dielectric electroactive polymer actuators can be fabricated by implantation into polydimethylsiloxane (PDMS) of metal ions with energies from 2 to 35 keV , creating a thin conductive layer a few nm below the surface without forming a continuous metallic film. Electrical conductivity of a few kΩ/sq can be reached with less than 1 MPa increase in Young’s modulus for doses of order 1016 ions/cm2, allowing efficient downscaling of artificial muscle actuators.We present the impact of implantation method, ion species, energy, and dose on the mechanical and electrical properties of PDMS membranes. Dose (D) and energy are chosen to be in the range that produces useful electrodes for EAP actuators. Two implantation techniques were used: Filtered Cathodic Vacuum Arc (FCVA) and Low Energy broad beam Implanter (LEI). Ti and Au were implanted at 5 keV by FCVA, and Ti at 10 keV and 35 keV by LEI. Doses varied from 1016 to 6x1016 ions/cm2.Stress, Young’s modulus (E) and surface resistivity (R) were measured. Initially E was 0.85 MPa and R was >2000 MΩ/sq. All data sets show an increase in E and decrease of R with increasing dose. Different conditions however give up to a factor of 100 difference in E for a given dose.Best performance for actuators was obtained for FCVA implantations of Gold at 5 keV. For D=1.5x1016 ions/cm2, E increased by 100% (to 2 MPa) while R decreased to less than 1kΩ/sq. R vs. time shows two regimes: for R<1kΩ, R decreases slightly then stabilizes, while for R>1kΩ, R rapidly increases. We explain these two behaviors by Au diffusion which leads to annealing for thicker films, but can bring barely continuous films to below the percolation limit.For Ti implanted by LEI at 10 keV and 35 keV, to get comparable R a dose 50% larger than for FCVA Au 5 keV is required, consistent with simulations. However E is 2 orders of magnitude larger, primarily due to the nature of the implantation process: FCVA is pulsed (600 µs pulses at 1 Hz, 4x1013 ions/cm2/pulse) and generates large heating at every pulse. LEI rasters a 3 µA beam over the surface for several hours. Charging plays a large role in the final microstructure and the two techniques have orders of magnitude difference in beam current.Implantation changes surface roughness (SR) of polymers by sputtering, stress modification, charging and heating. The PDMS films had initial SR of 3 nm rms. SR increases with implant dose, up to 100 nm for FCVA, and over 1 µm for LEI. This is an indication of the dramatic difference in microstructure obtained at comparable doses with different implanters. TEM samples have been prepared.This study enabled the definition the implant conditions where stable low resistivity is reached with only limited stiffening of the elastomer membrane. To create compliant electrodes, FCVA type implanters are much better suited than conventional implanters.  P. Dubois et al, Sensors and Actuators A: Physical, 130-131, p.147 (2006)
9:00 PM - DD3.11
C-Axis Oriented ZnO Film by RF Sputtering and its Integration with MEMS Processing.
Sudhir Chandra 1 , Ravindra Singh 1 Show Abstract
1 Centre for Applied Research in Electronics, Indian Institute of Technology Delhi, New Delhi, Delhi, India
During recent years, extensive research has been carried out on zinc oxide (ZnO) thin films because of their applications in surface acoustic wave (SAW) devices, bulk acoustic wave (BAW) resonators, optical waveguides, transparent conducting coatings, light emitting diodes (LED), photodetectors and electroluminescence devices. These films can also be used in micro-electro-mechanical-systems (MEMS) as a sensing material because of its good piezoelectric properties, and as a sacrificial layer in surface micromachining. More recently, piezoelectric based MEMS have shown great potential for bio-sensor applications. For applications of ZnO films based on its piezoelectric properties, it is a requirement that the films should be c-axis oriented and have high resistivity. Furthermore, ZnO films deposition at comparatively low temperature is of great interest for realization of MEMS in post-CMOS integration process. Among various deposition techniques, sputtering is the preferred method as oriented and uniform films of ZnO can be obtained even at relatively low substrate temperatures. In the present work, we report the preparation of high resistivity c-axis oriented ZnO thin films on different substrates by RF magnetron sputtering. The self-heating of the substrate during sputtering process has been advantageously exploited to prepare c-axis oriented ZnO thin films.The ZnO films are very sensitive to most of the chemicals used in MEMS fabrication process such as H2SO4, hydrofluoric acid (HF), buffered HF (BHF), silicon anisotropic etchants (e.g. KOH, EPW), metal (e.g. chromium, gold, aluminum) etchants etc. In another words, the subsequent fabrication processes after ZnO deposition could degrade or damage ZnO film if it is not protected properly. Therefore, the integration of ZnO films with MEMS processing is a very challenging task. The present work also focuses on how to protect the ZnO film against degradation in subsequent micromachining processes. A modified fabrication process has been demonstrated to integrate the ZnO films with bulk-micromachined diaphragms. A Si3N4 layer was incorporated in the diaphragm structures to protect the ZnO film from the etchants of chromium and gold used for patterning the top electrodes. The square diaphragms of different dimensions were fabricated using standard IC fabrication techniques and bulk micromachining process. A mechanical jig has been used for protecting the front side (ZnO film side) of the wafer from ethylenediamine pyrocatechol water (EPW) during the anisotropic etching of silicon. The proposed technique can be advantageously applied for the fabrication of various MEMS devices such as micro-sensors, micro-actuators, accelerometers, SAW devices, bulk acoustic resonators, RF switches and bio-sensors etc.
9:00 PM - DD3.12
Optimization of the Geometry of the MEMS Electrothermal Actuator to Maximize In-Plane Tip Deflection.
Edward Kolesar 2 , Jeffrey Tippey 2 , Brandon Least 2 , Thiri Htun 2 Show Abstract
2 Department of Engineering, Texas Christian University, Fort Worth, Texas, United States
Several microactuator technologies have been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion represent the most common modes of microactuator operation. This investigation optimized the geometry of the asymmetrical electrothermal actuator to maximize its in-plane deflection characteristics. The motivation was to present a unified description of the behavior of the electrothermal actuator so that it can be adapted to a variety of microsensor and microactuator applications. The MEMS polysilicon surface micromachined electrothermal actuator uses resistive (Joule) heating to generate differential thermal expansion and movement. In the traditional asymmetrical electrothermal actuator design, the single-hot arm is narrower than the cold arm, and thus, the electrical resistance of the hot arm is greater. When an electrical current passes through the device (both the hot and cold arms), the hot arm is heated to a higher temperature than the cold arm. This temperature differential causes the hot arm to expand along its length, thus forcing the tip of the device to rotate about its flexure. In this investigation, a 3D model of the electrothermal actuator was been designed, and its geometry was optimized using the finite-element analysis (FEA) capabilities of the ANSYS computer program. The electrothermal actuator's geometry was systematically varied to establish optimum values of several critical geometrical ratios that maximize tip deflection. The value of the ratio of the length of the flexure component relative to the length of the hot arm was discovered to be the most sensitive geometrical parameter ratio that maximizes tip deflection.
9:00 PM - DD3.13
Polymer Gel-gate Field Effect Transistor for Quantitative Detection of Sugar Molecules.
Akira Matsumoto 1 , Naoko Sato 1 , Toshiya Sakata 1 , Chiho Kataoka 2 , Kazunori Kataoka 1 , Yuji Miyahara 1 2 Show Abstract
1 Center for NanoBio Integration, The University of Tokyo, Tokyo Japan, 2 Biomaterials Center, National Institute for Materials Science , Tsukuba Japan
Development of a Field Effect Transistor (FET) device capable of detecting cell membrane dynamics in a noninvasive manner is being attempted. Phenylborate derivative is used as a functional molecule exhibiting specific bindings with carbohydrate chains, particularly targeting to sialic acid, which generally exists with the largest population of all other sugar moieties on the cell membrane. Modification of the transistor gate with phenylborate moiety may therefore lead to a promoted cell adhesion onto the gate surface. Further, due to negative charges on the sialic acid molecules, construction of “cell transistor” may be possible through which cell membrane dynamics can be continuously and quantitatively analyzed. The introduction of phenylborate moiety onto the gate surface was accomplished by forming a copolymer gel: poly(N-isopropylacrylamide-co-3-acrylamidophenylboronic acid) (9:1 in molar ratio = NB10 gel) covalently attached onto the gate. The gel can undergo a reversible volume changes in response to the change in sugar concentrations. For the check of sugar recognizing ability of the transistor, it was tested for its glucose sensitivity. Change in threshold voltage (VT) of the gel-modified transistor was monitored with varying glucose concentrations using an FET realtime analyzer. It was clearly seen that the VT value increases with increase of the glucose concentration in linear or non-linear manners depending on the temperatures. Reversibility was also confirmed. Based on systematic evaluation of the glucose-sensitivity and comparison to previously reported phase diagrams of the NB10 gel, it was suggested that the glucose-dependent volume change causes a change in permittivity at the vicinity of the gate surface resulting in a change in VT. In ordinary types of FET devices, change in charge density is detectable only within the Debye length, which can reach in length no greater than 10 nm. Noteworthy is that, in the present gel-based device, very drastic and overall volume change is induced and, consequently, despite the large gel thickness, distinctive signal with high S/N ratio can be achieved. Under the physiological pH (7.4), binding constant between phenylborate group and sialic acid is about 10 times higher than that between phenyborate and glucose. Thus detection of sialic acid can be expected with an even better S/N ratio using the herein-proposed FET device. Further, some derivatives of sialic acid moieties are known, whose composition and density on the cell membrane vary depending on type of cells and between those of normal and diseased. With ability to distinguish such differences of cell membranes on molecular level, and to do so continuously in a noninvasive manner, the device may represent a candidate for a new type of cytology.
9:00 PM - DD3.14
Electroosmotic Micro-pump for Local Control of Droplets.
Amit Gupta 1 , Heather Denver 2 , Amir Hirsa 2 1 , Julie Stenken 3 , Diana-Andra Borca-Tasciuc 2 Show Abstract
1 Chemical Engineering Department, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Chemistry and Chemical Biology Department, Rensselaer Polytechnic Institute, Troy, New York, United States
This work demonstrates the principle of localized, low-voltage DC electroosmotic pumping across thin membranes- a novel approach to transport liquid droplets onto and off a surface. Localization of the pumping effect is obtained by employing ring electrodes, microfabricated on each side of a nanoporous alumina membrane and coating the membrane surface with a non-wetting agent, except for the designated pumping site. Low voltage actuation is achieved by having the separation of the two electrodes in the microscale range- the thickness of the membrane. The actuation voltage is more than an order of magnitude lower than typically used in electroosmotic pumping applications. The measured electroosmotic velocity increases linearly from 40 µm/s at 5V to 200 µm/s at 20V. Experimental results compare well with predictions of an electroosmotic flow model considering the nonuniform distribution of the electric field between the electrodes. These results establish the potential for microfabricated devices employing local, cross-membrane electroosmotic pumping. Possible applications include addressable liquid microlens arrays, reconfigurable, two-dimensional photonic crystals and highly localized sampling of extracellular fluid for brain studies.
9:00 PM - DD3.15
Aligned Low Temperature Wafer Bonding for MEMS Manufacturing: Challenges and Promises.
Viorel Dragoi 1 , Thorsten Matthias 1 , Gerald Mittendorfer 1 , Paul Lindner 1 Show Abstract
1 Technology Development, EV Group, Sankt Florian Austria
Wafer bonding became during last decade a very attractive technology for applications in various fields. Due to the specific 3D architecture of Micro-Electro-Mechanical Systems (MEMS), wafer bonding was relatively easy adopted as an important component of MEMS manufacturing tool box.The “early MEMS” applications (e.g. pressure sensors) were using mainly the advantage offered by this technique to join two substrates (typically silicon-silicon or silicon-glass), eventually with possibility to encapsulate certain vacuum levels. The complexity of current MEMS devices impose new requirements for wafer bonding. Among these can be mentioned low process temperature (<400°C), precise optical alignment of substrates, ability to bond a large variety of substrates and the possibility to bond with defined intermediate layers.Important efforts were focused during last years on developing various wafer bonding processes to fulfill the above requirements. The most used wafer bonding processes are: anodic bonding (known also as electrostatic assisted bonding) of silicon and glass, direct (fusion) bonding (based on molecular bonds between the two surfaces in contact), adhesive bonding using polymer intermediate layers, eutectic bonding (based on formation of an eutectic alloy as bonding layer), metal diffusion bonding (based on formation of metal bonds) or glass frit bonding (using low melting temperature glass as bonding layer).An important aspect in aligned wafer bonding is that alignment accuracy needs to be correlated to the type of bond process. Especially in the case of processes using intermediate layers the post-bond alignment accuracy will be given by the behavior of the bonding layers.If for mainstream IC industry standardization solves numerous technical problems, in MEMS and particularly in wafer bonding the lack of standards imposes additional challenges to MEMS manufacturing from the design stage and going through the entire process flow.This paper aims to present results illustrating the main criteria to be considered in defining aligned wafer bonding processes. Particularly bonding of substrates containing electronics (e.g. CMOS wafers) is currently of high technological interest.Results on fusion bonding, adhesive bonding, anodic bonding and metal bonding will be reviewed. Examples of applications and corresponding aligned wafer bonding requirements, as well as various wafer-to-wafer alignment processes will be presented.
9:00 PM - DD3.16
A Microfluidic Device for Interrogating Pathogenic Immune Response in Single Host Cells.
Matthew Moorman 1 , Bryan Carson 1 , Conrad James 1 , Ronald Manginell 1 , Anthony Martino 1 , Jamie McClain 1 , Jens Poschet 1 , Michael Sinclair 1 , Anup Singh 2 Show Abstract
1 , Sandia National Labs, Albuquerque, New Mexico, United States, 2 , Sandia National Labs, Livermore, California, United States
Real-time interrogation of single cells offers a promising method of studying cellular functions such as interactions between bacterial and viral pathogens and host cells. Single cell analysis eliminates the population averaging effects that are characteristic of bulk measurements, and high resolution confocal imaging enables dynamic analysis of the spatial and temporal characteristics of proteins involved in intracellular signaling cascades. We have developed a microfluidic platform that integrates cell sorting and sample preparation with single-cell capture, fluorescence imaging, and protein analysis including cytokine secretion. Here, we present the development of one component of the platform, a single-cell array (SCA) device used to capture, isolate, and expose multiple single cells to pathogenic insults while providing an optimized interface for high resolution confocal imaging and subsequent multi-spectral analysis. The SCA is fabricated in silicon using deep reactive ion etching to create microfluidic channels that contain in-line cell trapping structures. Chips are sealed with anodically-bonded coverslips, providing a confocal microscopy interface for a high NA oil immersion lens. The cell trapping structures are designed to fluidically isolate single cells from the secretions of neighboring cells, a feature that enables the interrogation of the primary immune response (host cells directly exposed to pathogens) and the secondary immune response (host cells exposed to the secretions of pathogen-challenged host cells). Fluorescent reporter constructs using green fluorescent protein (GFP) and labeled antibodies are used to monitor events in signal transduction in innate immunity pathways of macrophages and other immune cells. We have generated RAW264.7 macrophages with a GFP construct of RelA, an NFκB family member that regulates the transcription of numerous genes involved in the innate immune response. Here, we monitor the time- and dose-dependent translocation of RelA from the cytoplasm to the nucleus of macrophages in response to stimulation with lipopolysaccharide (LPS). Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
9:00 PM - DD3.18
Micromolding by Polymerization in Capillaries.
Chris Hughes 1 , Brian Augustine 2 , Jacob Forstater 1 Show Abstract
1 Physics, James Madison University, Harrisonburg, Virginia, United States, 2 Chemistry, James Madison University, Harrisonburg, Virginia, United States
We have developed a technique for the fabrication of microfluidic structures using the photopolymerization of PMMA on top of a silicon master, a technique we refer to as micromolding by photopolymerization in capillaries (μ-PIC). Conventional photolithography and cystallographic etching are used to create a silicon master of the high aspect ratio pattern to be fabricated.. A sheet of optical grade PMMA is then placed on top of the silicon master with 200 micron PMMA beads between to provide a thin gap between the PMMA blank and the silicon master. A solution of MMA monomer with the photoinitiator benzoin methyl ether is then drawn by capillary force into this gap. The monomer solution is polymerized by exposure to ultraviolet light from a mercury vapor lamp transferring an exact negative replica of the pattern from the silicon into the PMMA. Atomic force microscopy and scanning electron microscopy has been used to show that the topology of the mu-PIC layers matches nearly exactly the qualities of the silicon master. Furthermore, the thickness of the μ-PIC layer can be controlled by the appropriate choice of PMMA beads used to space the master from the PMMA blank. We have demonstrated that this provides a rapid, near room temperature method of fabricating microfluidic structures in PMMA and for creating more complicated structures such as buried reflecting layers and layers composed of novel co-polymers.
9:00 PM - DD3.19
Piezoelectric Microtransducer for Aural Assistance.
Aurelio Horacio Heredia Jimenez 1 , Berenice Suarez R 2 , Manuel Gonzalez Perez 3 , Laura Castro y Fernandez 4 Show Abstract
1 Mechatronic, UPAEP, Puebla, Puebla, Mexico, 2 Chemical, UPAEP, Puebla, Puebla, Mexico, 3 Ing. Biomedical, UPAEP, Puebla, Puebla, Mexico, 4 Laboratory, UPAEP, Puebla, Puebla, Mexico
Cochlear implants have supposed a revolution to them in the treatment of severe and deep deafness. During the last decades, especially in the last 10 years; cochlear implant has evolved considerably to be an effective solution for numerous cases of deafness. The cochlear implant that we design and construct is often referred as a bionic ear, it is a hearing aids, but does not amplify sound, it works by directly stimulating any functioning auditory nerves inside of the cochlea with electrical impulses. We aim at the research, manufacturing and testing a piezoelectric type micro transducer using a micro cavity based on the MEMS technology; for the electrical stimulation of the auditory nerve. The configuration chosen to generate electric charge in response to applied mechanical stress, the piezoelectric material was positioned on a floating silicon nitride membrane that becomes deformed under small pressures on range of mili Pascales. According to the parameters of the membrane and the piezoelectric material we obtain deformations 236 nm that produces voltages on range of mili volts. The devices having lateral dimensions from 700x700 μm2 and ~0.2μm of thick. The figure of merit is measured with a dedicated structure to room temperature, the pressure resolution is 20 mPa.
9:00 PM - DD3.2
Elastic and Viscoelastic Characterization of Polydimethylsiloxane (PDMS) for Cell-Mechanics Applications.
I-Kuan Lin 1 , Yen-Ming Liao 2 , Kuo-Shen Chen 2 , Xin Zhang 1 Show Abstract
1 Manufacturing Engineering, Boston University, Brookline, Massachusetts, United States, 2 Mechanical Engineering, National Cheng Kung University, Tainan Taiwan
The mechanical interaction of cells with their neighboring extracellular matrix is believed to be of fundamental importance in various physiological processes such as division, growth, migration, and force transmission. The mechanical force generated by living cells is typically on the order of pN to μN and can be measured by soft material probes such as polydimethylsiloxane (PDMS), due to their mechanical compliance and biocompatibility. Researches have constructed PDMS pillar arrays as extracellular matrices and the interactions have been characterized by converting the measured deflection of PDMS pillars using elastic mechanics of materials, where the elastic modulus of PDMS was assumed to be a constant. However, PDMS is a viscoelastic material and its elastic modulus changes with loading frequencies and elapsed time durations. As a result, by neglecting the time- and frequency-dependent nature of PDMS, it could result in significant errors in data interpretation and even on the mechanisms in cell responses. Therefore, it is important to perform a detail material characterization on PDMS materials for cell-mechanics applications. This paper presents a new approach to analyze the bending behavior for determining the cell force applied at PDMS micro pillar arrays. Both material characterization and finite element analysis (FEA) are performed, and the material constitutive law of PDMS is obtained by using the testing data obtained from a self-developed dynamic mechanical analysis (DMA) system and a five-parameter generalized Maxwell fluid model. On the other hand, a series of nanoindentation stress relaxation testing are conducted by using a nanoindention system (Hysitron TriboIndenter). Meanwhile, by incorporating the test data obtained from the DMA system, a finite element model is constructed to simulate the behavior of nanoindentation and the results agree to each other essentially. Finally, the bending behavior of PDMS pillars are analyzed and discussed. It can be found that with the viscoelastic effect, the deflection and the corresponding cell force is time dependent and the classical relationship could result in considerable errors in data reduction if the time scale of the cell-matrix experiments is close to the relaxation time constant of PDMS.
9:00 PM - DD3.20
Synthesis and Applications of Silicone Elastomer and Nanoporous Gold Composite.
Erkin Seker 1 , Matthew Begley 1 2 3 , Hilary Bart-Smith 2 , Robert Kelly 3 , Giovanni Zangari 3 , Michael Reed 1 Show Abstract
1 Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia, United States, 2 Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, Virginia, United States, 3 Materials Science and Engineering, University of Virginia, Charlottesville, Virginia, United States
Nanoporous gold (np-Au), produced by selectively removing silver from a gold-silver alloy, is a promising material for microdevices. Some desired properties of this material are large surface area-to-volume ratio, corrosion resistance, high conductivity, and relatively low elastic modulus. Polydimethylsiloxane (PDMS) is a silicone elastomer, which is widely used in microfluidic systems, due to its superior elastic properties and simple synthesis. Recently, we developed an inexpensive fabrication process to combine np-Au and PDMS to produce a composite material that exhibits advantages of both constituents. Np-Au was produced by dealloying a commercially available 12-karat “white gold leaf” (~10-by-10 cm square) by floating it on nitric acid, rinsing it with deionized water, and subsequently placing it on a photoresist-coated silicon wafer. A mixture of uncured PDMS was then prepared and the np-Au leaf (~200 nm-thick) on the silicon carrier was spin-coated with the PDMS (~20 µm-thick). The sample was subsequently transferred to a vacuum chamber to remove the air trapped in the pores, and fill them with PDMS by capillary action. After the “fusing” of np-Au and spun-PDMS, a stock PDMS film (~250 µm-thick) was placed on the uncured composite to facilitate handling of the final composite material. Finally, the sample was cured at 60°C for 10 hours, and the composite structure was released by dissolving photoresist in a methanol bath. Freestanding membranes of various dimensions were easily produced by punching holes in the stock PDMS before placing it onto the uncured composite. For more precise and complex freestanding features, a custom-molded PDMS can be placed on the composite. The composite membranes were placed on a metal plate, and electrostatically displaced to more than 250 µm peak deflections without losing electrical conductivity. Composite ribbons were prepared and elongated with a tensile testing apparatus while measuring their resistance change. The ribbons remained conductive up to more than 12% strain. This paper presents details of the fabrication process, and preliminary results from strain-resistance and electrostatic actuation experiments. Scanning electron micrographs combined with energy dispersive spectroscopy and white-light-interferometry observations complement the results, and provide insight into mechanical and metallurgical properties of the composite material. We expect this technology to be beneficial in developing flexible electrodes, strain sensors with a wide operation range, and conductive membranes for microfluidic applications.
9:00 PM - DD3.21
Variation in Dislocation Pattern Observed in SCS Films Fractured by Tensile Test: Effects of Film Thickness and Testing Temperature.
Shigeki Nakao 1 , Taeko Ando 1 , Shigeo Arai 2 , Noriyuki Saito 2 , Kazuo Sato 1 Show Abstract
1 Department of Micro-Nano Systems Engineering, Nagoya University, Nagoya Japan, 2 High Voltage Electron Microscope Laboratory, Nagoya University, Nagoya Japan
This paper reports a transition of fracture behavior of micron-sized single-crystal silicon (SCS) in MEMS structure with the change in dislocation patterns. We evaluated the fracture toughness of two types of SCS films, which has different thickness, at elevated temperatures by tensile test. We confirmed the effects of thickness and temperature on the dislocation activity.Because of the development of MEMS devices in many fields, researches have a great concern about the effects of environmental factor, particularly temperature, on the mechanical properties of thin film for its reliability. Many researchers have studied the fracture toughness of bulk SCS for determine the temperature of sharp transition from brittle to ductile behavior (about 600οC). In the case of micron-sized SCS materials, there are some experimental results of tensile and bending test at elevated temperatures. However, there are no experimental data of relations between fracture toughness and temperature for determine the BDT temperature. We succeeded the determination of the transition temperature for micron-thick SCS film by fracture toughness measurement using tensile test. The SCS film specimens have (100) surface orientation and <110> loading direction. The size of specimens was 50 or 100 μm length, 45 μm wide, and thickness was 1 or 4 μm. We fabricated the notch on one side of the film specimen to measure the fracture toughness by FIB.We tested the 4-μm thick specimens at temperatures ranging from RT to 500οC. The fracture toughness at RT was 1.28 MPam1/2, which remained at a similar value up to 60οC. However, it increased drastically at 70οC and stood at 2.60 GPam1/2 at 150οC, then saturated at higher temperatures. The fracture behavior of SCS film clearly changed at 70οC, although they didn’t show the complete ductile behavior. To confirm the difference of dislocation activity, we observed the dislocations on the SCS specimen fractured at RT and 300οC using TEM. We prepared the samples for TEM observation by FIB. We could see some small dislocations of about 200 nm in length were induced from the fracture surface at RT. On the other hand, they spread to 2–3 μm in length at 300οC. We think that this difference of dislocation activity certainly caused the transition of fracture behavior of 4-μm SCS films.We also evaluated the fracture toughness of 1-μm thick SCS films at temperatures ranging from RT to 300οC. The specimens showed high toughness value of 2.7 MPam1/2 even at RT. They didn’t showed drastic increase in fracture toughness at higher temperatures. To observe the dislocations on these specimens at RT without FIB fabrication, we used the High Voltage Electron Microscopy (HVEM). As a result, we amazingly observed the many dense dislocations were induced with length about 1-2 μm at RT. We could confirm the change in dislocation activity of micron-sized SCS films with the temperature and thickness, and it caused the transition of fracture behavior.
9:00 PM - DD3.22
Stability of Diamond-Like Carbon Coating on Nanoimprint Templates.
L. Tao 1 , S. Ramachandran 1 , L. Overzet 1 , M. Goeckner 1 , M. Kim 1 , G. Lee 1 , W. Hu 1 Show Abstract
1 Department of Electrical Engineering, University of Texas at Dallas, Richardson, Texas, United States
The major challenges for nanoimprint lithography (NIL) include defect control, template damage, and reproducibility issues, which are strongly related to the separation process of the template from either thermoplastic or UV curable resists. To facilitate the separation process, low surface energy fluorinated self-assembled monolayers (F-SAMs) like heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane (FDTS) and Fluorosyl FSD 4500, have been widely used as anti-adhesion template coatings for both thermal and UV nanoimprint. This coating layer is effective for template releasing from resists without leaving macroscopic defects on the wafer. However, F-SAMs gradually lose F atoms to air and were recently found to chemically react with UV resists forming hard-to-remove compounds. This causes defects on the printed wafer and also damage to the template, in addition to the presence of particles and contaminants from the environment. These issues have motivated us to explore new materials, such as diamond-like carbon (DLC) as a durable and stable coating for nanoimprint templates.Our previous studies have demonstrated the fabrication of nanostructured DLC films as scratch-proof, atomically smooth and anti-stick nanoimprint templates for both thermal and UV nanoimprint. In this paper, the physical and chemical stability of DLC coated templates is studied in comparison to F-SAMs coated templates. The 20-300 nm DLC films were deposited on Si and quartz substrates and then patterned using lithography and reactive ion etching to form nanostructured templates for thermal and UV nanoimprint respectively. A typical thermoplastic polymer, PMMA (~300 nm thick), was spin-coated on a clean silicon wafer for the thermal imprint at 180 oC and 5 MPa pressure, whereas the UV imprint was done with the same thickness of UV curable SU-8 at 80 oC and 5 MPa. The surface properties of the DLC films before and after the imprinting are characterized using X-ray photoelectron spectroscopy (XPS), contact angle goniometry, atomic force microscopy (AFM), and an n&k analyzer. The elemental composition of the DLC films is stable during multiple imprint cycles, while F-SAMS lost about 25% of its F-atoms. In addition, after imprints, the surface roughness (RMS) of DLC increased from 0.16 nm to 0.36 nm, while the RMS of F-SAMs changed from 0.29 nm to 0.99 nm. The surface variation of the F-SAMs coating is almost the same of its height, indicating the microscopic damage happened. A negligible change of the contact angles was observed for both films before and after imprints. AFM images of the imprinted resist structures demonstrate good pattern transfer fidelity. These results indicate that the DLC films offer better chemical inertness after contact with resists for both thermal and UV nanoimprint processes than F-SAMs coating.In the future work, the chemical stability of the DLC and F-SAM films will be presented after cleaning with various chemicals.
9:00 PM - DD3.23
A Comparison of Material Characteristics of a Bulk-micromachined Single-crystal Silicon-on-insulator Process and a Surface-micromachined Polycrystalline Silicon Process.
Brad Boyce 1 , David Miller 2 1 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado, United States
Design of silicon-based microelectromechanical systems (MEMS) requires an understanding of the material characteristics that are governed by processing. In this study, several design-critical characteristics of a single-crystal silicon-on-insulator (SOI) process (SOIMUMPs) have been examined and compared to a polycrystalline silicon process (polyMUMPs), both produced by MEMSCAP, Inc. Specifically, matching diagnostic structures were utilized in both processes to evaluate electrical resistivity, surface roughness, in-plane residual stress, through-thickness residual stress gradient, and tensile fracture strength. In general, most of the measured characteristics were similar for SOIMUMPs and polyMUMPs, in spite of their very different fundamental process constraints. For example, the characteristic fracture strength for SOIMUMPs and polyMUMPs (layer poly1) was 1.97 and 1.43 GPa, with associated Weibull moduli of 8.9 and 14.0, respectively. These characteristic strength values are surprisingly similar given the very different sources of silicon: mechanically thinned single crystal silicon wafers versus low pressure chemical vapor deposited polycrystalline films. Fractography suggests that fracture strength for the both technologies is controlled by surface topography features: individual sidewall etching defects drive SOIMUMPs failures whereas top-surface grain boundary crevices drive polyMUMPs failures. For this reason, the etching conditions have been shown to dramatically affect the resulting fracture strengths of both processes. Discussion focuses on the ramifications of the process-controlled material characteristics in SOIMUMPS and polyMUMPs on the design of MEMS components.
9:00 PM - DD3.24
The Plastic Biochip for Biomedical Diagnostics.
Zhengshan Zhao 1 , Gerardo Diaz-Quijada 2 , Regis Peytavi 1 , Ann Huletsky 1 , Eric LeBlanc 1 , Johanne Frenette 1 , Guy Boivin 1 , Jim Zoval 3 , Marc Madou 3 , Michel Dumoulin 2 , Teodor Veres 2 , Michel Bergeron 1 Show Abstract
1 Centre de recherche en infectiologie, Université Laval, Sainte Foy, Quebec, Canada, 2 Industrial Materials Institute, National Research Council, Boucherville, Quebec, Canada, 3 Department of Mechanical and Aerospace Engineering, University of California, Irvine, California, United States
Microfluidics is a rapidly expanding field due to its potential applications in analytical separations, drug discovery, detection of biothreats, biomedical diagnostics and chemical catalysis, etc. Microfludic devices possess several advantages such as short analysis time, reduced cost, portability and lower consumption of precious biological analytes. Since fabrication of microfluidic devices using traditional materials such as glass is rather expensive, there is a high interest in employing polymeric materials as a low cost alternative that is suitable for mass production.We present proof-of-concept plastic microfluidic device for the rapid detection of common respiratory viruses with a high degree of sensitivity and specificity. The method involves extracting the genetic material from the pathogens and performing a highly sensitive multiplexed Room Temperature Polymerase Chain Reaction (RT-PCR). The resultant amplicons are hybridized, without further purification, with an array of specific oligonucleotide probes (20mers) that had been covalently bound to a plastic substrate. The hybridization process is achieved within 5 minutes using a centrifugal microfluidic platform, setting the stage for a rapid medical diagnostic device for point-of-care use.
9:00 PM - DD3.26
Electrical Measurement of Mechanical Properties of Gold Films.
Nicholas Barbosa 1 , Robert Keller 1 Show Abstract
1 Materials Reliability Division, NIST, Boulder, Colorado, United States
We present a novel method for measuring strength and fatigue properties of gold metallizations for microelectromechanical systems. Mechanical testing of small or buried structures, passivated films, or metallizations for micromachined geometries continues to present significant challenges. We have developed an alternative approach to methods based on tensile testing, wafer curvature, or indentation. Application of controlled alternating current under conditions of high density and low frequency produces cyclic joule heating of films on substrates. Differences in thermal expansion properties between film and substrate result in cyclic thermal straining, which can be used to assess strength and fatigue properties of structures without the need for special test geometries. In this work, we demonstrate the application of this test method to gold films of thickness 0.5 μm and width 5 μm. We tested in the as-deposited condition and in the post-annealed (550 °C – 2 h) condition, in order to demonstrate the ability of this electrical test to distinguish properties associated with different microstructures. The microstructures corresponding to these two conditions differ largely in terms of grain size and texture. We will present measurements of ultimate strength and fatigue lifetimes. The electrical results are consistent with those obtained by the more conventional methods of microtensile testing and nanoindentation. Analysis of the evolution of microstructural damage by scanning electron microscopy and automated electron backscatter diffraction revealed that the gold films deformed by relatively low temperature mechanisms, consistent with the fact that the peak temperature reached during electrical stressing did not exceed approximately 250 °C, and that the mean temperature was only approximately 135 °C during the course of the tests.
9:00 PM - DD3.27
Three-dimensional Micro-Topography Enhances Directional Myogenic Differentiation of Skeletal Precursor Cells.
Yi Zhao 1 Show Abstract
1 Department of Biomedical Engineering, The Ohio State University, Columbus, Ohio, United States
Skeletal muscle precursor cells have recently attracted intensive attention due to their unique potential of differentiation into skeletal muscle cells. This is of critical importance in muscular tissue engineering where the differentiated cell assembly can be transplanted to the patients and benefit the damaged native tissues in various aspects. For example, clinical practices in different animal models have demonstrated that the contractility of the damaged myocardium recovers, to some extents, after the skeletal myoblasts have been injected using cell therapies. Although the mechanism of such cell therapies remains obscure, it is generally believed that the local environment of the native myocardium directs the injected skeletal myoblasts into certain phenotypes that synergistically coordinate with the neighboring cells. In order to provide a deep understanding of the complex interaction between the local environment and the injected skeletal myoblasts, we decouple different environmental parameters and report herein the influence of the micro-topography on myogenic differentiation.The micro-topography was generated as below: a glass substrate was spin-coated with photoresist AZ9260 and patterned with closely spaced line arrays (2 microns in line width and 2 microns in spacing). The premature PDMS prepolymer was poured on the substrate and thermally cured. The microscale line arrays formed after thermal curing and peeling off. In this work, the thicknesses of AZ4260 were systematically varied for microstructures with different aspect ratios.The cell culture was then conducted. After being suspended in the non-differentiation culture medium (DMEM supplemented with 10% FBS, 1% penicillin), C2C12 myoblasts were seeded on the PDMS substrate and incubated for three days. Meanwhile, the cell morphology was monitored. The results showed that the cells prefer proliferating along the longitudinal axis of the microstructures. Such directional proliferation strongly depends on the aspect ratio of the PDMS microstructures, i.e., higher structures result in more directional proliferation. Afterwards, the differentiation culture medium was used (supplement with 2%HBS, 1% penicillin and insulin) to trigger the myogenic differentiation. Highly oriented myogenic differentiation was observed on the high aspect ratio PDMS microstructures, in sharp contrast to the control group with plain PDMS substrates. This was further confirmed by directional organization of F-actin filaments and the cascading nuclei arrays along the longitudinal axis. This work thus demonstrates a unique method for realizing and controlling directional myogenic differentiation using extracellular micro-topography, which may shed some light in assembling functional skeletal muscle tissues in vitro.
9:00 PM - DD3.28
Modeling of RF MEMS Behavior: Surface Roughness and Asperity Contact.
Omid Rezvanian 1 , Mohammed Zikry 1 , J. Crill 2 , D. Irving 2 , D. Brenner 2 Show Abstract
1 Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Modeling predictions were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. Three-dimensional surface roughness profiles were generated, based on a Weierstrass-Mandelbrot fractal representation, to match the measured roughness characteristics of contact bumps of manufactured RF MEMS switches. Contact asperity deformations due to normal contact pressures, were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating, and contact forces. This finite-element modeling framework was used to understand how contact resistance evolves due to changes in real contact area, the number of asperities in contact, and the temperature and resistivity profiles at the contact points. The numerical predictions were qualitatively consistent with experimental measurements and observations of how contact resistance evolves as a function of deformation time history. Molecular dynamic simulations were also used to understand how an asperity behaves at the molecular level. This integrated modeling framework can be used for the design of reliable RF MEMS devices with extended life cycles.
9:00 PM - DD3.29
Comparison of Hermetic Packaging Techniques using Wafer Level Bonding for MEMS Devices.
Sumant Sood 1 , Sharon Farrens 1 Show Abstract
1 Wafer Bonder Applications, Suss Microtec, Waterbury Ctr, Vermont, United States
Wafer-wafer bonding has become a reliable technology for MEMS and MOEMS packaging and variety of wafer level bonding techniques are now available. MEMS components need to be encapsulated in micro-cavities to protect them from contamination during the subsequent packaging steps as well as from mechanical or environmental damage during their actual use. Hermetic sealing is required to avoid stiction and ensure correct functioning of MEMS devices such as in accelerometers, gyroscopes and sensors. Some of the interface materials used for packaging include silicon, oxides, glasses and various types of polymers. As the MEMS devices are getting smaller, a range of metals and eutectic compositions have also emerged into mainstream MEMS packaging. In addition to enabling smaller packages, metal bonds also allow for better hermetic seals as compared to glass-frit and polymer bonding. Figure 1 compares the seal widths required for hermetic sealing. Since the permeation rate of gases through materials depends upon the density and materials properties of the seal materials as well as the size of the gas atoms, a variety of choices are possible depending upon specific device goals.In this paper, a variety of wafer level bonding schemes will be compared head-to head in terms of reliability and ease of packaging. The effect of metal thickness on hermeticity will also be discussed. For metal bonding schemes, various eutectic compositions (Au-Si, Au-SiGe, Au-Sn) as well as thermo compression (Cu, Au) bonding will be reviewed and compared. In the last few years, the major focus has been to develop low temperature metal wafer bonding techniques to meet the demands of nanotechnology and complex integrated microsystems. Cavity sealing techniques using low melting point (<250oC) metals and eutectics (such as Indium and In-Au eutectic) and the effect of low temperature cavity sealing on hermeticity will be discussed. As advanced MEMS products move toward commercialization, an increasing number of devices are being fabricated with high vacuum cavity seals. New advances in bonding technology and equipment are enabling these new MEMS packaging schemes.
9:00 PM - DD3.3
Curvature Control of Microcantilever Based Infrared Detectors Using Thermal Loading Method.
I-Kuan Lin 1 , Katherine Yanhang Zhang 2 , Xin Zhang 1 Show Abstract
1 Manufacturing Engineering, Boston University, Brookline, Massachusetts, United States, 2 Aerospace and Mechanical Engineering, Boston University, Brookline, Massachusetts, United States
Infrared radiation (IR) detection and imaging are of great importance to a variety of military and civilian applications. Microelectromechanical (MEMS)-based IR detectors have recently gained a lot of interest because they potentially have extremely low noise equivalent temperature difference (NETD) while maintaining low cost to make them affordable to more applications. However, the curvature induced by residual strain mismatch severely decreases the device performance. Also, the present plate theory is not capable to predict the thermal responses of cantilever IR detectors. Therefore, the purpose of this research is twofold, i.e., to provide an engineering approach to flatten cantilever detector and to modify the present plate theory for the specific application of IR detectors.A cantilever IR detector is typically composed of SiNx/Al bimaterial actuation cantilevers and sensing plate. The IR detector was fabricated using surface micromachining with a sacrificial layer of polyimide. Previously, we demonstrated that the curvature of an IR detector can be modified using rapid thermal annealing (RTA) treatment. In this work, a microheating stage rather than the RTA unit is used so that the curvature change during the thermal cycling can be in-situ monitored with an interferometric microscope. The cantilever IR detector was heated and subsequently cooled for five cycles between room temperature to 320°C, with the maximum temperature in each successive cycle increasing by 20°C. Within each cycle, curvature was measured every 10°C at thermal equilibrium. The general behavior of the IR cantilever detector can be characterized by linear thermoelastic regimes and inelastic regimes. After the thermal cycling with a maximum temperature of 295°C, upon return to room temperature the IR detectors were flattened and the curvature decreases about 97%. Both finite element method and analytical solutions were used to model the linear thermoelastic regime. Modulus ratio of the bimaterial plate is obtained by fitting the finite element simulations to the curvature responses from the thermal cycling experiment. The present simple plate theory leads to the results significantly different from experimental data. This may be due to the complex geometry and boundary conditions of the IR detectors. Therefore, in this work, an equivalent modulus ratio is defined so that the analytical solutions from simple plate theory can be used to describe the linear thermoelastic response of the IR detector.
9:00 PM - DD3.32
The Effect of Hydrophobic Patterning on Micromolding of Aqueous-Derived Silk Structures.
Konstantinos Tsioris 1 , R. White 1 , D. Kaplan 2 , Peter Wong 1 Show Abstract
1 Mechanical Engineering, Tufts University, Medford, Massachusetts, United States, 2 Biomedical Engineering, Tufts University, Medford, Massachusetts, United States
Biopolymers are polymers synthesized by organisms with many advantages including excellent biocompatibility and adjustable biodegradability. Silk, for example, offers a wide spectrum of outstanding material properties such as a high elongation, high fracture toughness and excellent optical properties. Fabricating with biopolymers is also environmentally sound, since they can be processed in aqueous solutions. Due to these advantages biopolymers are often used as substrates in cell and tissue culture. However they are rarely used in MEMS applications.There is enormous potential for biopolymers in MEMS applications. In MEMS devices biopolymers could function as membranes or optical components. Devices which demand outstanding biocompatibility, such as implantable sensors, could be packed in or fully manufactured from biopolymers materials. The challenge today exists in understanding critical processing parameters in manufacturing structures with micron and submicron level features from biopolymers. In this research, the development of a micromolding technology, to produce microstructures from aqueous derived silk solutions was studied. In particular, well-defined cellular and tissue culture substrate (scaffold) fabrication was used as a model to study manufacturing methods. The manufacturing challenges consist in counteracting shrinkage caused by solvent evaporation, producing well defined porous structures and demolding of delicate structures.Methods developed for soft lithography were used to produce molding masters from polydimethylsiloxane (PDMS). Subsequently the PDMS surface was modified in the channels and on the apical master surface to compensate for shrinkage of the biopolymer and to produce porous structures. The mold test pattern comprised a segmented band pattern of microchannels 100 µm wide, 250 µm deep and with 200 µm interchannel spacing over an area of 10 mm x 10 mm. An approximately 7% silk fibroin aqueous solution was prepared from B. mori silk. To counteract the shrinkage and to provide a porous structure, the microchannels were manufactured to be less hydrophobic than the apical master surface. The hydrophobicity of the apical master surface repels the biopolymer solution, and as a result the microchannels are provided constantly with excess silk solution as the channel structure dries. The biopolymer fully filled the microchannels and left porous patterns. Investigation with SEM of the edges of the silk structures indicated the ability to replicate structural features at a sub micron level. The ability to produce well controlled structures from aqueous derived silk with features at the micron and sub-micron level allows this biopolymer to be integrated in the field of micro scale systems. By adjusting these novel manufacturing techniques other biopolymers could also be used in the future for MEMS.
9:00 PM - DD3.5
Fatigue Crack Growth of Vibrating LIGA Micro-beams.
Andrea Cambruzzi 1 , Jurg Dual 1 Show Abstract
1 Zfm -IMES, ETH Zurich, Zurich Switzerland
For the further development, commercialization and miniaturization of microelectromechanical system (MEMS) the mechanical properties of materials employed in MEMS devices must be understood. New test methods and new testing techniques suitable for micrometer-scale specimens are necessary to evaluate the mechanical properties of thin films and microstructures because these properties differ from those of bulk material. This paper presents an experimental technique to study fatigue crack growth and its results for UV-LIGA Ni micro-beam resonators produced by electroplating from a sulfamate bath. Free standing micro-samples are excited and maintained in resonance by a Phase Locked Loop (PLL) feedback control at a load suitable for nucleation and propagation of a fatigue crack from a notch root. The drop in the resonance frequency can be attributed to the local increase of compliance and through a mechanical model related to the stable growth of the fatigue crack and to the stress intensity factor. High stability in frequency and precise laser interferometer measurements are the key features that enable this technique to achieve high resolution (tens of nm) in crack length measurement. Using the model da/dN vs. ΔK curves are obtained. In particular near threshold behavior has been investigated using a decreasing stress intensity gradient method. The micro-beams have also been characterized by a micro-tensile test to determine the model parameters and critical stress values.
9:00 PM - DD3.6
A Direct Method of Determining Complex Depth Profiles of Residual Stresses in Thin Films on a Nanoscale.
Stefan Massl 1 , Jozef Keckes 2 , Reinhard Pippan 1 Show Abstract
1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria, 2 Department Materials Physics, University of Leoben, Leoben Austria
A cantilever method of determining complex depth profiles of residual stresses is presented. The technique developed is based on the fabrication of a micro-cantilever and the subsequent gradual reduction of the film thickness using a focused ion beam workstation. The deflection as a function of film thickness is measured directly from SEM images, and the stress distribution in the thin film is determined by means of a straightforward calculation procedure. The method can be applied to crystalline as well as to amorphous materials and permits the determination of 3D stress profiles. The depth profiles of residual stresses in a 1 micron thick PVD-deposited TiN film and a 2.5 microns thick thermally grown amorphous SiO2 film as well as the 3D stress field produced by a defined scratch in a 840nm thick PVD-deposited Ni film are presented.
9:00 PM - DD3.7
Normalized Rolling Resistance of an Elastic Cylinder in Adhesive Contact for Micro-manipulation.
Shigeki Saito 1 , Fumikazu Yoshizawa 1 , Toshihiro Ochiai 1 Show Abstract
1 Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Tokyo, Japan
Analyzing the rolling behavior of micro-object is greatly required so that we can establish a reliable micro-manipulation technique by mechanical-method. Thus in this paper, we obtain the rolling resistance of an isotropic and elastic micro-cylinder in adhesive contact to a rigid surface. In order to evaluate the rolling resistance, we assume this adhesive contact to be the plane strain problem, and calculate the total energy of this system as the numerical function of the contact area using finite element method (FEM). The total energy of this system is defined as the sum of the next three terms: the elastic energy of the cylindrical micro-object, the interface energy within the contact area, and the mechanical potential energy that depends on the external moment applied to the cylindrical micro-object. From careful consideration of the energy balance of the system, we obtain a normalized form of a rolling resistance, which surprisingly clarifies that the rolling resistance per unit cylindrical length is expressed by a work of adhesion times a cylindrical radius, independent of Young’s modulus and Poisson’s ratio.
9:00 PM - DD3.8
Optimized Voltage Sequence for Repeatable Manipulation of a Conductive Micro-particle by a Single Probe.
Masaki Sonoda 1 , Kenji Kurihara 1 , Shigeki Saito 1 Show Abstract
1 , Tokyo Institute of Technology, Tokyo Japan
With the miniaturization of MEMS (Micro Electro Mechanical Systems) technology, the demand for assembly of micro objects has risen in recent years. During micromanipulation, the influence of gravitational force becomes extremely small. The adhesional force is more significant for smaller objects. It is known that an adhered object can be detached by using electrostatic force. Thus we have investigated repeatable electrostatic micromanipulation methods by a single probe. In our previous studies, the electrostatic force generated by an applied voltage has been theoretically analyzed using a boundary element method (BEM). The system consists of a manipulation probe, a spherical micro-particle, and a substrate plate. These objects are all conductive. Based on this numerical solution, our group proposed the technique of non-impact electrostatic method by simplest voltage sequence with high switching frequency for repeatable micromanipulation. The success rate of this technique, however, was not yet high enough for the industrial application to date because of the delay caused by the difference between the voltage desirably calculated and the voltage actually applied with a certain slew-rate. Therefore, in this study we find out the optimized voltage sequence by BEM calculation with consideration of such delay; furthermore, we experimentally demonstrate the effectiveness.
9:00 PM - DD3.9
Scanning Multiprobe Microscopy for Electrical Measurements of Nanodevices.
Hesham Taha 1 , Andrey Ignatov 1 , Abraham Israel 1 , Oleg Zinovev 1 , Anatoly Komisar 1 , Alexander Krol 1 , David Lewis 1 Show Abstract
1 , Nanonics Imaging Ltd., Jerusalem Israel
Nanocomponents such as carbon nanotubes, metal nanoparticles and nanowires perform interesting electrical properties at the nanometer scale. These components are considered as the best candidates for nanoelectronics, molecular electronics and nanodevices. However, electrical probing of these small-scale devices is still challenging due to the difficulty of performing localized nanometric measurement, which meets the dimensions of these features. Here, we describe a methodology of using scanning probe microscopy systems based on multiple probes for both topographic inspection and for performing electrical measurements. It will be demonstrated that protocols have been developed to rapidly and effectively bring multiple probes in contact with one another, both in terms of instrumental aspects and in terms of probe geometries. Unique glass insulated platinum nanowires have been developed for both the topographical mapping of nanometric features and the electrical probing of the nanstructures with independent atomic force microscopic feedback of the multiple probes.The technological advances offer exceptional capabilities not only in molecular electronics, but also as semiconductor processing reaches the 45nm node, the semiconductor roadmap. In essence, this technology allows for padless electrical testing with up to four independently controlled atomic force microscopy electrical probes. Furthermore, the technology also opens new horizons to investigate molecular structure in terms of their electrical properties on non-conductive substrates.