Symposium Organizers
Frank W. DelRio National Institute of Standards and Technology
Maarten P. de Boer Carnegie Mellon University
Christoph Eberl Karlsruhe Institute of Technology (KIT)
Evgeni P. Gusev Qualcomm MEMS Technologies
S1: Fabrication Methods
Session Chairs
Monday PM, November 29, 2010
Room 207 (Hynes)
9:30 AM - **S1.1
Transfer Printing With Advanced Stamps For Applications in MEMS and Other Areas.
John Rogers 1
1 , University of Illinois, Urbana, Illinois, United States
Show AbstractFabrication in MEMS often demands formation of complex, three dimensional layouts and heterogeneous collections of materials. This talk summarizes some work on the use of soft, elastomeric stamps for transfer printing solid objects of silicon and other materials relevant to MEMS. Advanced stamp designs that incorporate strategically located microtips enable large, passive switching of adhesion with a contrast ratios approaching 1000 times, thereby providing unmatched capabilities in transfer. We present theoretical and experimental studies of the mechanics of these stamps, and demonstrate their use in forming two and three dimensional structures of silicon, metal and oxides in representative devices.
10:00 AM - S1.2
Fabrication and Replication of Hierarchically Textured Polymer Microstructures Using Carbon Nanotube Master Molds.
Davor Copic 1 , Sameh Tawfick 1 , Sei Jin Park 1 , Michael De Volder 1 2 , A. John Hart 1
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 , IMEC, Leuven Belgium
Show AbstractFabrication of polymer structures and surfaces having controlled shape and texture at a hierarchy of length scales is essential to understanding and controlling the behavior of soft matter, such as liquid wetting, dry adhesion, and cell-substrate interactions. The advent of soft lithography using PDMS transformed our capability for cost-effective fabrication of structures for these applications; however, master mold features and the resulting replica structures typically have straight cross-sections, and it is particularly difficult to create vertical and multi-directional nanoscale textures on microstructures. To meet this need, we introduce use of carbon nanotube (CNT) composite master molds to fabricate textured polymer microstructures and polymer surfaces. These novel master molds are made by infiltration of vertically aligned CNT microstructures with SU-8, followed by replication using standard soft lithography methods. The master molds are made by (1) CNT growth by thermal CVD from a lithographically patterned catalyst; (2) capillary densification of the CNTs by condensation and evaporation of a solvent onto the substrate; and (3) infiltration of the densified CNTs with SU-8. The densification step critically improves the robustness of the CNT microstructures while maintaining the aligned nanoscale texture of the sidewalls of the microstructures, which is precisely copied into the final polymer replicas as quantified by atomic force microscopy. Uniform centimeter-scale arrays of structures having critical dimensions ranging from 5-1000 micrometers have been made in both SU-8 and PDMS. These include high-aspect-ratio needles, micro-foams with hexagonal cells, and thin sheets with radially anisotropic surface texture. The hierarchical roughness of the master and replica surfaces is controlled by the CNT diameter and packing density, and is demonstrated to enable uniform and tunable SERS enhancement on Au-coated and Ag-coated replicas. This study indicates promise for batch fabrication and replication of polymer features that capture complex nanoscale shapes and textures, while retaining compatibility with existing microfabrication methods.
10:15 AM - S1.3
Carbon Nanotube Based NEMS Actuators.
Michael Forney 1 , Jordan Poler 2
1 Nanoscale Science, UNC Charlotte, Charlotte, North Carolina, United States, 2 Chemistry, UNC Charlotte, Charlotte, North Carolina, United States
Show AbstractCarbon nanotubes (CNTs) have been widely studied because of their superior mechanical properties as well as their ballistic one-dimensional electronic behavior. We have grown vertically aligned CNTs (VA-CNTs) grown onto Octosensis microcantilever arrays, which provide an architecture for novel actuators. SEM and Raman spectroscopy indicate that the CNTs are small multi-walled carbon nanotubes. Electrical characterization shows reasonable electrical resistance (~ 70 kΩ) from the chip body to the microcantilever tips. By applying a voltage, we load charge onto the array of VA-CNTs. The electrostatic repulsion among the charged CNTs provides surface stress that induces microcantilever deflection. COMSOL Multiphysics modeling results for the actuator design show that only a few electrons per CNT are needed to produce measureable deflections, and experimental actuators are currently being characterized using SEM, Raman spectroscopy, an I-V probe station, and an AFM optical lever system. Other microcantilever actuators in the literature have been successful at inducing deflections of tens or hundreds of nanometers, and we expect larger deflections, coupled with faster response times. Actuators based on this architecture could be used for nano-manipulation, release of drugs from a capsule, or nano-valves that are controlled by electrical inputs.
10:30 AM - S1.4
Environmental Effects on the Elasticity of Cross-linked Poly(methyl methacrylate) Nano-wires Produced by Two-photon Lithography.
Satoru Shoji 1 , Tomoki Hamano 1 , Shota Kuwahara 1 , Thomas Rodgers 1 , Satoshi Kawata 1
1 Department of Applied Physics, Osaka University, Suita, Osaka Japan
Show AbstractRecent novel technologies allow us to study the intrinsic properties of polymers in the micro/nano-scale, which can be quite different from those of macro-scale polymers. Previously, we presented experimental evidence by our laser lithography and laser manipulation techniques [1-3] that the elastic modulus of polymers starts to show size-dependence when the size of the polymers is less than a micrometer. The size-dependent mechanical properties of poly(methyl methacrylate) (PMMA) nano-wires fabricated by two-photon lithography were observed as an enhancement of the elasticity and a concurrent decrease of the phase transition temperature. Although the mechanism of these size-dependent features is not fully understood yet, it is clear that the size-dependence is seen only when the polymer is thinner than a certain critical size. One of the possible factors is the effect of molecules surrounding and/or penetrating into the surface of the polymer. In this presentation, we show a comparison of the elasticity of PMMA nano-wires in different circumstances. We prepared crosslinked-PMMA polymer nano-wires in the shape of coil springs with different radii from 100 to 500 nm. We performed stress tests for each nano-wire by means of laser trapping technique and/or atomic force microscopy for two cases where the nano-wires are dried in air, and are wet in ethanol. We found that the elastic modulus of the polymer nano-wires significantly drops by three orders of magnitude when they are immersed in ethanol. This significant atmosphere-dependent change of the elasticity was seen when the size of the polymer wires is smaller than 500 nm. It is known that, in general, ethanol does not have a good affinity to PMMA. We speculate that, although it is very weak, the interaction of ethanol with PMMA macromolecules in a thin layer of the polymer surface could induce such significant modulation of elasticity. In the presentation we also introduce our methods for the preparation and the tensile test of the polymer nano-wires with high precision. [1]S. Shoji, S. Nakanishi, T. Hamano, and S. Kawata, MRS Proc. 1224, 1224-FF06-05-DD06-05 (2009). [2]S. Nakanishi, S. Shoji, H. Yoshikawa, Z. Sekkat, and S. Kawata, J. Phys. Chem. B 112, 3586 (2008). [3]S. Nakanishi, S. Shoji, S. Kawata, and H.-B. Sun, Appl. Phys. Lett. 91, 063112(2007).
10:45 AM - S1.5
Xenon Difluoride Etching of Germanium for Free-standing III-V Heterostructures.
Garrett Cole 1 , Yu Bai 2 , Markus Aspelmeyer 1 , Eugene Fitzgerald 2
1 Physics, University of Vienna, Vienna, Vienna, Austria, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe have developed a novel fabrication technique for the realization of free-standing monocrystalline AlGaAs heterostructures of arbitrary aluminum content. This process is enabled by recent advances in high quality III-V/Ge epitaxy and utilizes the noble gas halide, xenon difluoride (XeF2), in order to rapidly and selectively remove a sacrificial germanium (Ge) underlayer in a room temperature gas-phase etching procedure. Combining polar and non-polar semiconductor multilayers allows for the first time the extension of dry selective undercutting, as has been pioneered in silicon-based microstructures, to high performance monocrystalline compound semiconductor devices. We demonstrate two possibilities for exploiting this unique procedure: 1) bulk micromachining of an optomechanical resonator consisting of an epitaxial GaAs/AlAs distributed Bragg reflector (DBR) grown on a Ge substrate, and 2) epitaxial lift-off (ELO) of submicron-thickness GaAs films via removal of an embedded Ge sacrificial layer.All samples are prepared using a low-pressure metal organic CVD system. The optomechanical resonators are fabricated from an epitaxial DBR consisting of 40.5 periods of alternating GaAs (high index) and AlAs (low index), grown on a 2” diameter, epi-ready (100) Ge substrate, offcut 6 degrees toward the [011] direction. The use of the offcut substrate is necessary to obtain high quality GaAs epitaxy on Ge by inhibiting anti-phase boundaries. Resonator fabrication entails a single-mask bulk micromachining process utilizing a pulsed XeF2 etching system used for selective underetching of the Ge substrate. In order to explore the limits of this process, we further demonstrate ELO of sub-micron thickness, single-crystal GaAs films. Two distinct materials structures are used in this experiment: both structures are grown on an epi-ready (100) GaAs substrate offcut 6 degrees toward the [011] direction, followed by 190 nm of Ge (1 μm in the second sample), capped with 180 nm (or 320 nm) of GaAs. We record respective lateral etch rates of 30 and 50 μm/min in these samples; a factor of 5 larger than that typically found in silicon.Post etch characterization yields selectivities between 0.26×10^6 and 1.8×10^6 for Ge versus AlGaAs etching. Compared with typical results for hydrofluoric-acid-based ELO, our demonstration XeF2 process shows similar selectivity, is an order of magnitude faster, and additionally enables the release of epitaxial films with arbitrary aluminum content. Along with a rapid etch rate and high selectivity to all AlGaAs alloy compositions, further advantages of this etchant include the elimination of surface tension forces in the release of suspended structures, and the alleviation of ion-induced damage associated with plasma exposure.
11:30 AM - **S1.6
Production and Characterization of Molded Micro-parts Made of Metals and Ceramics.
Oliver Kraft 1
1 izbs, Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractIn the past decades, miniaturization of micro electro-mechanical systems (MEMS) based on silicon technology has led to ever smaller, cheaper and better devices demonstrating strong economic success in a broad range of technological applications. Meanwhile, functional systems to be developed for biological or medical applications, for micro-chemical engineering or in many other relevant areas of science and technology call for a much wider selection of materials and for parts with true three-dimensional shapes. For this, the development of production routes other than typical silicon processing is required. In particular, for the fabrication of micro-components, made of metals and ceramics, manufacturing processes known from the macro world are to be scaled down. Therefore, it has been the major goal of a Collaborative Research Center at the Karlsruhe Institute of Technology to investigate the limits of this approach and to develop the full process chain for micro-molding including powder injection-molding as well as micro-casting. This talk will give an overview of the major achievements with a particular emphasis on the processing-microstructure- property relationship of micro-parts made from cast alloys as well as high strength ceramics. For mechanical characterization at the micro-scale, a number of micro-testing methods have been developed to study strength, toughness and fatigue endurance of the produced materials. Overall, it turns out that at a scale between microns and a few hundred microns scaling effects play an important role for the mechanical behavior as samples and components have a large surface to volume ratio. However, an equally important aspect relates to the fact that the microstructure and the resulting properties of the produced materials are quite sensitive to details of the production route. Nevertheless, it is shown by the successful manufacturing of complex parts and demonstrators that micro-molding allows for processing a large variety of materials at small scale.
12:00 PM - S1.7
All-Oxide MEMS Devices Based on Free-standing Structures of Epitaxial Transition Metal Oxides.
Luca Pellegrino 1 , Michele Biasotti 1 2 , Renato Buzio 1 , Emilio Bellingeri 1 , Nicola Manca 2 , Cristina Bernini 1 , Antonio Sergio Siri 2 1 , Daniele Marre 2 1 , Teruo Kanki 3 , Hidekazu Tanaka 3
1 , CNR SPIN , Genova Italy, 2 Physics Department, University of Genova, Genova Italy, 3 Institute of Scientific and Industrial Research, Osaka University, Osaka Japan
Show AbstractMost of the applications envisaged for Transition Metal Oxides (TMO) concern electronic or optoelectronic devices such as memories, transistors, LED. An additional route toward the realization of devices employing the rich properties of TMOs is the fabrication of free-standing elements of epitaxial TMO thin films for applications in smart Microelectromechanical Systems (MEMS) devices. Our fabrication process starts from the deposition of epitaxial oxide films or multilayers that are micromachined by conventional microlithography. A key factor of the process is the use of crystalline oxide sacrificial layers that are selectively removed by acids thus leaving free-standing elements. As an example, the combined use of HF and HCl allowed the fabrication of free-standing structures made of crystalline SrTiO3 (001) films using sacrificial layers of (La,Sr)MnO3 [1,2]. In some cases, combination of dry etching and acids is employed like in the fabrication of epitaxial rutile TiO2 (110) microcantilevers. After a brief description of the fabrication protocol used to obtain microcantilevers and suspended bridges based on different TMOs, we will show possible applications of suspended structures based on epitaxial (La,Sr)MnO3, (La,Sr)CoO3 and VO2 films. Strain generator devices, in which reversible modulation of the electrical resistance of TMO thin films is induced by crystalline SrTiO3(001) microcantilevers, will be illustrated. By bending downward the microcantilever through an AFM tip or gate electric fields, tensile strain is produced at the upper surface of the cantilever and the overgrown film is thus strained through epitaxial lock. MEMS-based strain devices are expected to influence properties of TMOs inducing shifts of Metal-Insulator Transitions - thus affecting properties of applicative interest such as the temperature coefficient of resistance (TCR) - or even modifying magnetic ordering. Further perspectives such as the realization of micro-hotplates and micro bolometers based on functional epitaxial oxides will be introduced as well as the possibility of measuring interacting forces between correlated oxides and external fields using oxide micromechanical oscillators. The combination of strain, mechanical vibration and high thermal insulation together with the enormous possibilities offered by TMO epitaxial heterostructures is a fascinating field that could open interesting perspectives for applications of such materials. [1] L. Pellegrino, M. Biasotti, E. Bellingeri, C. Bernini, A. S. Siri, D. Marré Adv. Mater. 21, 2377 (2009)[2] M. Biasotti, L. Pellegrino, E. Bellingeri, C. Bernini, A. S. Siri, D. Marré Procedia Chemistry 1 839–842 (2009)
12:15 PM - S1.8
A New Route To Fabricating PDMS-Based Microfluidic Components.
Roger Diebold 1 2 , David Clarke 2
1 Materials Department, UC Santa Barbara, Santa Barbara, California, United States, 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Show AbstractIn the efforts to build standardized microfluidic components for ‘Lab-on-a-chip’ purposes, Polydimethylsiloxane (PDMS) is a structural material which has gained much attention. However, the low surface energy of cured PDMS limits the use of optical lithography since many of the traditional Novolak photoresists used in optical lithography dewet. Several different materials (metal, parylene, etc) have been applied as adhesive layers on cured PDMS for the purpose of lithographic patterning but in doing so necessitate undesirable alterations to the PDMS surface via oxygen plasma treatment. In addition, soft lithographic methods used in fabricating microfluidic devices also require oxygen plasma treatment of the PDMS surface for bonding purposes, which has been previously shown to have irreproducible bond strength and inherently imprecise feature alignment. In this paper, the authors will present an alternative route for microfluidic device fabrication which is completely solution processable, does not require plasma treatment, and fully compatible with basic optical lithography equipment.Polydimethylglutarimide (PMGI) is a commercially available deep UV and electron beam photoresist that provides sufficient adhesion to cured PDMS surfaces permitting traditional optical lithography techniques to be applied, allowing the fabrication of multilayer elastomeric structures useful in microfluidic applications. As an adhesive underlayer supporting a positive Novolak-based photoresist, PMGI is completely solution processable and easily removed using N-methyl-2-pyrrolidone-based photoresist strippers. To demonstrate the utility of PMGI in fabricating microfluidic devices, the authors have fabricated a PDMS-based, electrostatically actuated peristaltic pump without alteration of the PDMS surface. We anticipate that the technique presented will be applicable in fabricating other microfluidic device components.
12:30 PM - S1.9
Integrating Nanomaterials with Micromachined Structures Using Electron-beam Lithography.
Kaushik Das 1 , Pascal Hubert 1 , Srikar Vengallatore 1
1 Mechanical Engineering, McGill University, Montreal, Quebec, Canada
Show AbstractIntegrating nanomaterials (in the form of quantum dots, nanotubes, nanowires, nanocrystalline thin films, and nanocomposite films) with micromachined structures and devices can enable the design of microelectromechanical systems with multiple functionalities, improved performance, and higher reliability. However, achieving this integration is a daunting challenge because the processing techniques used for synthesizing the nanostructures are usually different from, and often incompatible with, the standard methods used for micromachining. Nanomaterials can be grown, or self-assembled, on micromachined structures, but it is difficult to control their dispersion, alignment, pattern density, and location. These parameters can be controlled if the nanostructures are patterned on micromachined devices using high-resolution lithography or direct-write techniques. Here, we report an approach for integrating polymeric and metallic nanowires directly on microcantilevers platforms that are commonly employed in MEMS-based sensors. Single-crystal silicon microcantilevers were micromachined using photolithography, anisotropic wet etching, and deep reactive-ion etching. These beams were then sputter-coated with thin aluminum films, spray coated with poly methyl methacrylate (PMMA), and then patterned using electron-beam lithography. The nanostructures comprise of an array of oriented trenches that are 200 nm wide and 450 nm deep, and the spacing between adjacent trenches ranges from 1 μm to 5 μm. A metal lift-off process was used to fabricate an array of oriented chromium nanowires (10 nm thick and 130 nm wide) with controlled spacing of 2 μm and 5 μm between adjacent lines. By using deposition after patterning, aluminum/PMMA/aluminum nanocomposites were also fabricated on the microcantilever. The use of these nanostructures for applications in resonant sensing, and for fundamental studies of idealized nanocomposite materials, will be discussed.
12:45 PM - S1.10
C-MEMS Structures as Three Dimensional Current Collectors for Micro-supercapacitors.
Majid Beidaghi 1 , Wei Chen 1 , Chunlei Wang 1
1 Mechanical & Materials Engineering, Florida International University, Miami, Florida, United States
Show AbstractDevelopment of miniaturized electronic systems has stimulated the demand for miniaturized power sources that can be integrated into such systems. Micro-supercapacitors with high power density can be coupled with energy harvesting devices to store the generated energy. Moreover, they can also be paired with micro-batteries to provide the peak power and improve the cycle lifetime. Electrically conducting polymers, such as polyaniline (PANI), polypyrrole (Ppy) and their derivatives, and transition metal oxides, such as RuO2 and MnO2 are promising electro-active materials for supercapacitors. In this work, we are aiming to develop on-chip supercapacitors based on interdigitated C-MEMS electrode microarrays, which are employed as three dimensional (3D) current collectors of pseudo-capacitive materials. Fabrication of C-MEMS structures involves a two-step photolithography on silicon oxide wafer followed by a pyrolysis step. Ppy and MnO2 were electrochemically deposited on the 3D interdigitated C-MEMS electrodes. Effects of different experimental parameters on the performance of micro-supercapacitor cells are investigated by Cyclic Voltammetry (CV), Galvanostatic Charge-discharge and Electrochemical Impedance Spectroscopy (EIS). Detailed results will be presented at the conference.
S2: Materials Development and Optimization
Session Chairs
Monday PM, November 29, 2010
Room 207 (Hynes)
2:30 PM - **S2.1
Towards the Development of Ni-base Superalloys as High Temperature MEMS Materials.
Devin Burns 1 , Michael Teutsch 2 , S. Suresha 1 3 , Klaus Bade 4 , Jarir Aktaa 2 , Sara Johnson 5 , Tresa Pollock 6 , Kevin Hemker 1
1 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Institute for Material Research II, Karlsruhe Institute of Technology, Karlsruhe Germany, 3 National Center For Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Institue for Microstructure Technology, Karlsruhe Institute of Technology, Karlsruhe Germany, 5 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 6 Materials, University of California Santa Barbara, Santa Barbara, California, United States
Show AbstractElectrodeposited LIGA Ni micro-structures offer an attractive balance of toughness, stiffness, and room temperature strength, but the elevated temperature strength of traditional LIGA Ni components and molds are far from optimal. The technological motivation for the work to be presented is derived from a desire to expand the temperature capabilities of current MEMS materials. Cast and wrought Ni-base superalloys with highly developed two-phase (γ−γ’) microstructures are ubiquitous in land-based and aero turbines. The processing routes required to shape MEMS and NEMS components with sub-micron precision do not, however, lend themselves to traditional casting and forging processes. Two alternative processing routes will be presented and contrasted. The first involves control and optimization of electrodeposition parameters to uniformly entrain Al nano-particles in ED Ni micro-components, the development of heat treat schedules required to transform the as-deposited green composites into γ−γ’ superalloy microstructures, and characterization of the resultant microstructures and attendant mechanical properties. The second employs vapor-phase aluminization of existing LIGA Ni micro-structures to achieve desired alloy compositions, as well as subsequent heat treating and characterization of the resultant microstructures and mechanical properties. This talk will review the properties of traditional LIGA Ni micro-structures, outline recent attempts to develop solid solution and oxide dispersion strengthened LIGA Ni alloys, and then focus on the development of processing routes for the fabrication of Ni-base superalloys for use in MEMS and NEMS applications.
3:00 PM - S2.2
Mechanical and Piezoelectric Behavior of Thin Film PZT Composites for MEMS.
Ioannis Chasiotis 1 , Sivakumar Yagnamurthy 1
1 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractThe mechanical and piezoelectric properties of freestanding PZT composite films, comprised of SiO2, Pt and PZT layers, were measured from microscale tension specimens. Since PZT thin films for MEMS are not fabricated in freestanding form, thin film stacks were fabricated in combination with SiO2 and Pt. The specimens tested were stacks of SiO2-TiPt-PZT-Pt, SiO2-TiPt-PZT, and individual SiO2 and Pt thin films with gauge lengths of 1,000 μm and widths of 50-100 μm and thicknesses of 400-1,200 nm. Full-field strain measurements were conducted with the aid of a fine speckle pattern (1 μm particle size) generated on the samples and analyzed by digital image correlation. The PZT films demonstrated very high tensile strengths and non-linear responses at strains higher than 0.35%, which was due to domain switching. Furthermore, the d31 coefficient was calculated from the out-of-plane deflection of biased PZT specimens in conjunction with analytical solutions for the bending response of multilayer piezoelectric beams. The field induced in-plane stress hysteresis loops were asymmetric at small in-plane stresses becoming of similar magnitude as the stress increased beyond 300 MPa. Similarly, the intersection of hysteresis loops shifted from negative to positive electric field at stresses larger than 150 MPa. Finally, the applied stress resulted in reduction of the hysteresis magnitude due to mechanical constraints imposed on 90° domain switching. The aforementioned measurements on the hysteresis and stress controlled piezoelectric response of PZT films are critical for the design and reliability of active MEMS devices.
3:15 PM - S2.3
Nanomechanical Characterization of Thin, Compliant Coatings.
Michelle Oyen 1
1 Engineering Dept. , Cambridge University, Cambridge, 0, United Kingdom
Show AbstractExpansion of the functionality of advanced MEMS and NEMS devices requires the inclusion of a diverse materials set. In modern MEMS applications, particularly but not exclusively in the domain of biomedical applications, there is a need for coatings of polymeric materials and hydrogels with significant mechanical integrity. This is particularly challenging because the materials themselves are relatively compliant (and thus also typically exhibit time-dependent mechanical behavior) such that traditional mechanical techniques for material properties characterization are not available. In this study, polymer and hydrogel coatings are characterized using nanoindentation techniques. Material constitutive laws are considered as viscoelastic for polymers and poroelastic for hydrogels. Both analytical and finite element analyses are used to ascertain the effects of coating thickness on observed mechanical response and to deconvolute true material properties of the coatings. These techniques show great promise for MEMS development in the context of robust polymer-based functional layers.
3:30 PM - S2.4
Pt/TiO2 Growth Templates for PZT Films and Quality MEMS Devices.
Daniel Potrepka 1 , Glen Fox 2 , Luz Sanchez 1 3 , Ronald Polcawich 1
1 RDRL-SER-L, U.S. Army Research Laboratory, Adelphi, Maryland, United States, 2 , Fox Materials Consulting, LLC, Colorado Springs, Colorado, United States, 3 Materials Science, University of Maryland, College Park, Maryland, United States
Show AbstractThe crystallographic texture of PZT thin films strongly influences the piezoelectric properties used in MEMS applications. When PZT films are poled to saturation, the piezoelectric response increases sequentially on transforming from random orientation to {111} texture to {001} texture. Textured growth can be achieved by relying on crystal growth habit, but it can also be initiated by the use of a seed layer that provides a heteroepitaxial template. The choice of template and the process used to form it determines the structural quality and ultimately influences performance and reliability of MEMS PZT devices such as switches, filters, and actuators. This study focuses on how {111} textured PZT is generated by a combination of crystal habit and templating mechanisms that occur in the PZT/bottom electrode stack . The sequence begins with {002} oriented Ti deposited on PECVD or thermally grown SiO2 on a Si wafer. The Ti is then converted to {200} TiO2 with the rutile structure through thermal oxidation and then {111} Pt can be grown to act as a template for {111} PZT. The Ti and Pt are deposited by DC magnetron sputtering. Optimization of the TiO2 and Pt film textures and structure were studied by variation of sputtering deposition times, temperatures and power levels, and anneal conditions. The relationship between Ti, TiO2, and Pt texture and their impact on PZT growth will be presented.
3:45 PM - S2.5
Contact Resistivity of Laser Annealed SiGe for MEMS Structural Layers Deposited at 210°C.
Joumana El Rifai 1 2 3 , Sherif Sedky 2 4 , Ahmed Abdel Aziz 2 , Robert Puers 1 3 , Chris Van Hoof 1 3 , Ann Witvrouw 1
1 , IMEC, Leuven Belgium, 2 Youssef Jameel Science and Technology Research Center, The American University in Cairo, Cairo Egypt, 3 , Katholieke Universiteit Leuven, Leuven Belgium, 4 Physics Department, The American University in Cairo, Cairo Egypt
Show AbstractThis work examines, for the first time, the contact resistivity between a laser annealed SiGe MEMS structural layer and a bottom TiN electrode. By using laser induced crystallization a TiN-SiGe contact resistivity as low as 2.14×10-3 Ωcm2 was obtained for SiGe deposited at 210°C.Lowering the SiGe deposition temperature will enable a transition from rigid Si substrates to flexible polymer or other temperature sensitive substrates, thus increasing the integration flexibility of SiGe-based MEMS [1]. However, depositing SiGe at 210°C by plasma enhanced chemical vapor deposition will yield amorphous films with high stress, stress gradient and resistivity. A post-deposition laser annealing treatment has already been used to produce SiGe films with improved stress gradient and low resistivity suitable for MEMS applications [2,3]. In this paper we investigate an additional property of the laser annealed SiGe, namely its contact resistivity to 100 nm thick TiN layers. As previously investigated, increasing the laser energy leads to a decrease in electrical resistivity and an increase in crystallization depth. The same is transferable to contact resistivity, as shown in this work. The laser annealing with a 248 nm KrF excimer laser of the SiGe layer was carried out both in air and vacuum. Laser energy densities up to 100 and 240 mJ/cm2 in air and vacuum, respectively, were applied. Using higher laser energy densities leads to damage in the Si-oxide dielectric layer situated in between the SiGe and TiN layers and eventually the SiGe layer itself.It was found that an energy density of 100 mJ/cm2 applied to a 1.0 µm thick SiGe layer is sufficient to produce a contact resistivity of 3.02×10-1 Ωcm2 for a 263 µm2 contact area if the laser annealing is conducted in an air ambient. Changing the annealing environment to a vacuum ambient will produce layers with a reduced surface roughness and allows the usage of higher laser energies [4]. Whereas the previous sample can only withstand a maximum of 100 mJ/cm2 in air before damage, it can be subjected to a 240 mJ/cm2 laser pulse in vacuum and produce a contact resistivity of 1.88×10-1 Ωcm2. Using a soft sputter etch and a Ti-TiN layer between SiGe and the TiN interface leads to a further reduction in the contact resistivity. SiGe layers with a 0.7 µm thickness and a Ti-TiN interlayer had a contact resistivity as low as 2.14×10-3 Ωcm2 for a 3 µm2 contact area after laser annealing in vacuum with a single pulse of 200 mJ/cm2. This beneficial effect of the Ti/TiN interlayer on contact resistivity has also been seen for poly-SiGe layers deposited at 450°C [5].References[1]A. Witvrouw, Mater. Res. Soc. Symp. Proc., vol. 1075, 2008.[2]S. Sedky et al., JMEMS, vol. 16, no. 3, pp. 581-588, 2007.[3]J. El-Rifai et al., Mater. Res. Soc. Symp. Proc., vol. 1153, 2009.[4]A. T. Voustas et al., J. Electrochem. Soc., vol. 146, no. 9, pp. 3500-3505.[5]G. Claes et al., Mater. Res. Soc. Symp. Proc., vol. 1222, 2010.
S3: Strength Characterization
Session Chairs
Monday PM, November 29, 2010
Room 207 (Hynes)
4:30 PM - **S3.1
Making Silicon Stronger.
Brad Boyce 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractSilicon microfabrication has seen many decades of development, yet the structural reliability of microelectromechanical systems (MEMS) is far from optimized. The fracture strength of Si MEMS is limited by a combination of poor toughness and nanoscale etch-induced defects. A MEMS-based microtensile technique has been used to characterize the fracture strength distributions of both standard and custom microfabrication processes. Recent improvements permit 1000’s of test replicates, revealing subtle but important deviations from the commonly assumed 2-parameter Weibull statistical model. Subsequent failure analysis through a combination of microscopy and numerical simulation reveals salient aspects of nanoscale flaw control. Grain boundaries, for example, suffer from preferential attack during etch-release thereby forming failure-critical grain-boundary grooves. We will discuss ongoing efforts to quantify the various factors that affect the strength of polycrystalline silicon, and how weakest-link theory can be used to make worst-case estimates for design. 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.
5:00 PM - S3.2
Mechanical and Electromechanical On-chip Testing of Mono- and Poly-crystalline Silicon Nanobeams.
Jean-Pierre Raskin 2 3 , Vikram Passi 2 , Umesh Bhaskar 2 , Azeem Zulfiqar 1 2 , Thomas Pardoen 1
2 ICTEAM, université catholique de Louvain, Louvain-la-Neuve Belgium, 3 CERMIN, Université catholique de Louvain, Louvain-la-Neuve Belgium, 1 IMMC, Université catholique de Louvain, Louvain-la-Neuve Belgium
Show AbstractThe application of well defined levels of mechanical stress in Si and PolySi is essential to characterize and make use of electromechanical couplings such as piezo-resistivity effects.One of the main limitations for stress driven applications of Si and PolySi, such as for any MEMS, is the inherent defect sensitivity leading to the statistical brittle failure behavior. A versatile on-chip suite of nanomechanical testing units has been developed in order to combine mechanical and electromechanical testing on the same specimen. The concept of actuation is based on the use of internal stress present in one material (tensile LPCVD silicon nitride, named the actuator) to load another specimen, in the present case Si or PolySi beams. Several thousands of tests structures with various lengths and widths, involving electrical connections, are produced on a single wafer, delivering statistically representative fracture data.Here we report tensile tests results on Si nanobeams of various lengths with thickness ranging between 40 to 200 nm and width ranging between 50 and 500 nm patterned by e-beam lithography. The smallest specimens exhibit elastic strains larger than 5%, corresponding to fracture stress on the order of 9 GPa, i.e. about 40% of the theoretical fracture stress of perfect Si. Simple and reliable statistical failure analysis has been demonstrated on both Si and PolySi films. In addition, the surface roughness of released mono-crystalline Si nanobeams has been scanned with AFM for various applied stress values, changing the length of the actuator. The experimental results show a decrease by a factor of 1.5 of the Si surface roughness moving from an unstrained Si nanobeam (0.2 nm) towards a Si nanobeam stressed at around 1 GPa (0.13 nm).Giant piezoresistance effects have been demonstrated in Si nanowires recently under limited applied stress (100 MPa) using a 4-point bending setup. Adding electrical contacts to the on-chip suite of nanostructures used for nanomechanical analyses, the current-voltage characteristics as a function of induced stress (different actuator lengths) can be measured up to values of several GPa. Transient electrical effects due to surface charges and saturation of piezoresistance coefficients in Si nanowires at high level of stress are demonstrated.
5:15 PM - S3.3
Electron Back Scatter Diffraction and Confocal Raman Microscopy in situ Studies of Stress Applied to Polysilicon Grains.
Siddharth Hazra 1 , Ryan Koseski 2 , Frank DelRio 2 , Jack Beuth 1 , Mark Vaudin 2 , Robert Cook 2 , Maarten de Boer 1
1 Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 Ceramics Division, National Institute of Standards and Technology, Gaithersburgh, Maryland, United States
Show AbstractWe have recently developed an in situ, on-chip high throughput microelectromechanical tensile tester for measuring fracture strength of polycrystalline silicon (polysilicon) specimens. This device uses a thermal actuator to mechanically grip and generate tens of milliNewtons of force to fracture polysilicon tensile bars of 70 µm and 2.25×2 µm2 nominal length and cross-sectional area respectively. Stresses resulting from sample-actuator misalignment are negligible and fracture occurs under uniaxial tension. These features, along with its small size, render the device suitable for in situ, small working distance, high-resolution microscopy techniques. Because the measured strength is averaged over a flaw population, the data does not provide direct insight into the mechanics of brittle fracture. It has been reported that fracture in polycrystalline silicon initiates at the side-wall grain boundary flaws. However obtaining quantitative assessments of the stresses acting within such small regions has proven elusive. In this work, we explore confocal Raman microscopy (CRM) and electron back scatter diffraction (EBSD) for in situ local stress mapping on this specimen. CRM provides lower spatial resolution (~100-200 nm), but has the potential for three-dimensional stress characterization. Preliminary Raman results indicate clear stress contours as the fillet necks down to the gage section. EBSD provides greater spatial resolution (~10-20 nm) but is more surface sensitive, and hence detail two-dimensional data can be collected. Preliminary results indicate that individual grains of less than 500 nm diameter are well resolved. Therefore, this tensile tester is amenable to in situ stress studies using these techniques. In this talk we will discuss the feasibility of these techniques in mapping stress within single grains of polysilicon as a function of in situ applied stress.
5:30 PM - S3.4
Characterizing the Effect of Uniaxial Strain on the Surface Roughness of Si Nanowire MEMS-based Microstructures.
Enrique Escobedo-Cousin 1 , Sarah Olsen 1 , Thomas Pardoen 2 , Umesh Bhaskar 2 , Jean-Pierre Raskin 2
1 , Newcastle University, Newcastle upon Tyne United Kingdom, 2 , Universite catholique de Louvain, Louvain-la-Neuve Belgium
Show AbstractThis work uses an original MEMS concept to strain released silicon beams in order to analyze the relationship between on-chip applied strain and nanoscale surface roughness in Si. Roughness affects carrier mobility through surface roughness scattering at high electric fields. The rms surface roughness and correlation length are key parameters to model carrier mobility in MOSFET inversion layers. Simulations indicate that only a reduction in rms roughness compared with bulk Si values can explain the high values of electron mobility observed experimentally in tensile strained silicon devices. However due to the limited characterization techniques available to measure roughness accurately on a nano and sub-nanoscale, to date such assertions have remained largely unconfirmed. An initial observation of strain-induced reduction in surface roughness was recently reported for biaxially strained Si compared with unstrained Si but only one level of strain was investigated [O. Bonno, S. Barraud, D. Mariolle and F. Andrieu, J. Appl. Phys. 103, 63715 (2008)]. Furthermore uniaxial strain was not studied despite offering a wider range of device options and performance benefits compared with biaxial strain. For device modellers investigating transport in uniaxial strain either bulk Si roughness parameters are assumed or limited biaxial data is used. This work addresses the paucity of roughness measurements by reporting on roughness parameters in uniaxial strained Si beams relevant for state of the art MOSFETs, nanowire and MEMS devices, with varying degrees of strain. Roughness is characterized using ultra-high resolution AFM techniques while strain is characterized by Raman spectroscopy. Microstructures comprising a silicon nitride actuator are used to induce a wide range of stress levels in Si beams. The microstructures also allow the comparison of surface evolution in the strain direction (along the Si beam) compared with the unstrained direction (across the Si beam). A gradual reduction in rms roughness amplitude and increase in roughness correlation length in the direction of the applied stress are found for increasing values of strain. In contrast, surface roughness in the direction perpendicular to the applied stress remained largely unchanged from the unstrained initial state. This is the first time that roughness in uniaxially strained structures has been studied and provides unequivocal confirmation that a reduction in rms roughness accompanies increasing tensile strain. Moreover the results suggest that the reduction in rms roughness often assumed by device modellers to generate agreement between measured and modelled data may be overestimated; a proportion of the enhanced carrier mobility observed is likely to instead originate from the increased correlation length. This correlation length parameter has previously been assumed to remain unchanged relative to bulk Si values.
5:45 PM - S3.5
Effects of Plasma-etched Silicon Surfaces on the Strength of Theta-like Micromechanical Test Structures.
Michael Gaither 1 , Frank DelRio 1 , Richard Gates 1 , Robert Cook 1
1 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractIn order for the microelectromechanical systems (MEMS) industry to continue to grow and advance, it is critical that methods are developed to determine the mechanical reliability of MEMS devices. Such methods are a crucial element in the “feedback” cycle that optimizes materials and processing selections for device reliability. This is particularly so for advanced devices with contacting, moving components, for which component strength is a key factor in determining reliability. The etching processes used to produce MEMS devices leave residual surface features that typically limit device strength and, consequently, device lifetime and reliability. In order to optimize MEMS device reliability, it is therefore necessary to understand and characterize the effects these etching processes have on MEMS-scale device strengths. At the micro and nano scales, however, conventional strength testing methods cannot be used, and a standardized test method for MEMS-scale strength measurement has yet to be established. The micro-scale NIST theta specimen, shaped like the Greek-letter theta, acts as a tensile test specimen when loaded in compression by generating a uniform tensile stress in the central web of the specimen. Utilizing the theta specimen for strength measurements allows for simple micro-scale strength testing and assessment of etching effects, while removing the difficulties associated with gripping and loading specimens as well as minimizing potential misalignment effects. Three sets of MEMS-scale single crystal silicon theta specimens are fabricated using two deep reactive ion etching (DRIE) recipes and a temperature-controlled cryogenic plasma etching recipe, each set resulting in a different specimen surface quality. DRIE is a silicon etch technique that results in high-aspect-ratio sidewalls and nearly uniform etch steps, or scallops, along these sidewalls. The cryogenic plasma etching technique results in smooth, high-aspect-ratio sidewalls. Each sample is tested by instrumented indentation and finite element analysis (FEA) is used to determine sample strength. Equations developed via FEA translate load-displacement response at the load-point into stress-strain behavior across the theta web region. Strength values for each set of specimens are examined via Weibull statistics. The resulting surface roughness for each etching recipe is determined by atomic force microscopy and sample fragments are examined via field-emission scanning electron microscopy. Surface roughness topography and fracture origins located during fractographic analysis of tested samples are compared with strength-limiting flaw size calculations.
S4: Poster Session: Devices and Fabrication
Session Chairs
Tuesday AM, November 30, 2010