Joerg Bagdahn Fraunhofer Center for Silicon Photovoltaics
Norman Sheppard MicroCHIPS, Inc.
Srikar Vengallatore McGill University
Kevin Turner University of Wisconsin-Madison
DD1: Resonators and High Frequency Devices
Monday PM, November 30, 2009
2:30 PM - **DD1.1
Silicon micromechanical resonators for precision sensing
Ashwin Seshia 1 Show Abstract
1 Engineering, University of Cambridge, Cambridge United Kingdom
Micromachined silicon resonators can be adapted as highly sensitive monitors of their physical and chemical environment. Non-dissipative physical processes resulting in structural and material perturbations in the resonator can be monitored by measuring induced resonant frequency shifts. Dissipative processes coupling to resonator dynamics can be monitored through changes in the quality factor. Accurate models for resonator dynamics, their interaction with the environment and integration into more complex sensor and signal processing systems are essential to realize applications in precision sensing. A number of underlying research threads are discussed here. First, a description of quality factor enhancement in micromachined silicon resonators through mode shape selection and resonator design is presented. Micromachined silicon resonators with quality factors of over 1 million at room temperature under moderate vacuum levels and associated frequency-quality factor products in excess of 10^13, close to the limits defined by intrinsic thermal loss, have been demonstrated. The quality factor of these resonators are increased by limiting vibrational energy loss to the substrate and substantially reducing thermoelastic dissipation through mode shape selection. Second, the role of electromechanical transducers and electrical interfaces in resonator design will be presented. Electrical transduction methods are described that utilize non-linear electrostatic actuators and readout mechanisms that leverage the piezoresistive properties of silicon to allow enhanced all-electrical probing of micromechanical vibratory response. Additionally, by incorporating actuators internal to the micromechanical structure, further enhancement in coupling the mechanical and electrical domains is attained, particularly at high frequencies. Thirdly, by constructing coupled resonator arrays, parametric sensitivities orders of magnitude higher than that for a single degree of freedom resonator are obtainable. This increase in parametric sensitivity can be accompanied by an intrinsic rejection of common mode environmental drift by leveraging the principle of vibration localization. The underlying research will be highlighted by application case studies. Oscillators incorporating CMOS-compatible high Q silicon square-extensional and wine-glass mode micromachined resonators exhibit short-term frequency stabilities of better than 1 part per billion. These oscillators can be viewed as building blocks for sensors and related systems. Two particular resonant sensor case studies are discussed viz. (1) thin film monitors demonstrating sensitivities of 136 Hz/nm and a mass flux resolved noise floor of under 17 pg/cm^2 and (2) tuning fork strain sensors with a resolution of close to 10 pε in a 1 Hz bandwidth. These sensors form essential components of systems currently under development for environmental and infrastructure monitoring.
3:00 PM - DD1.2
MEMS Microresonators for High Temperature Sensor Applications.
Dharanipal Doppalapudi 1 , Jeremie LeClair 1 , Richard Mlcak 1 , Harry Tuller 1 2 Show Abstract
1 , Boston MicroSystems Inc., Woburn, Massachusetts, United States, 2 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Microelectromechanical Systems (MEMS) are being extensively investigated as a means of miniaturizing piezoelectric sensors thereby offering higher sensitivity, reduced power consumption, and ability to form compact multi-sensor arrays. Such devices typically employ one or more silicon micromechanical elements (e.g. membranes, cantilever beams and tethered proof masses) driven electromechanically by a polycrystalline piezoelectric film. The use of polycrystalline materials results in inherently less stable and irreproducible device characteristics. For elevated operating temperatures, more robust and refractory materials are required. In this paper, we describe a MEMS microresonator array capable of operating to temperatures exceeding 600°C enabled by the integration of epitaxially grown piezoelectric AlN films onto virtually stress-free tethered single crystal SiC membranes. The operation of the microresonators as sensors is illustrated by examining their response to temperature and chemical analytes. Detection limits for the chemical analytes and response time of the sensor at high temperatures will be presented.
3:15 PM - DD1.3
A Study of Dissipation in Nanocrystalline Diamond MHz Frequency MEMS and NEMS.
Matthias Imboden 1 , Pritiraj Mohanty 1 Show Abstract
1 Physics, Boston University, Boston, Massachusetts, United States
Enabled by recent technological advances, it is now possible to create a variety of nanomechanical oscillators with resonances at microwave frequencies. Such nano- and micro-electromechanical systems (NEMS and MEMS) have become established devices as both commercial products as well as tools for basic research. Diamond, well known for its extraordinary mechanical properties, such as being the hardest known natural material, is an ideal material for NEMS and MEMS applications. Using CVD growth processes, diamond thinfilms can be grown controllably to engineer novel NEMS devices. Here, MEMS and NEMS performance is discussed in doubly clamped structures with emphasis on dissipation at both room temperature and sub Kelvin temperatures.We present frequency and dissipation scaling laws for doubly clamped diamond resonators . The device lengths range from 10 to 19 microns corresponding to frequency and quality-factor ranges of 17 to 66 MHz and 600–2400, respectively. The authors find that the resonance frequency scales as 1/L2 confirming the validity of the thin-beam approximation. The dominant dissipation comes from two sources: for the shorter beams, clamping loss is the dominant dissipation mechanism, while for the longer beams, surface losses provide the most significant source of dissipation.We also present Kelvin to millikelvin-temperature measurements of dissipation and frequency shift in megahertz-range resonators fabricated from ultra-nanocrystalline diamond . Frequency shift δf/f0 and dissipation Q-1 demonstrate temperature dependence in the millikelvin range similar to that predicted by the glass model of tunneling two level systems. The logarithmic temperature dependence δf/f0 is in good agreement with such models, which include phonon relaxation and phonon resonant absorption. Dissipation shows a weak power law, Q-1 ~ T1/3, followed by saturation at low temperature. A comparison of both the scaled frequency shift and dissipation in equivalent micromechanical structures made of single-crystal silicon and gallium arsenide indicates universality in the dynamical response. M. Imboden, A, Gaidarzhy, P. Mohanty, J. Rankin, and B.W. Sheldon, Scaling of dissipation in megahertz-range micromechanical diamond oscillators, Appl. Phys. Lett., 90, 173502 (2007) M. Imboden, P. Mohanty, Evidence of universality in the dynamical response of micromechanical diamond resonators at millikelvin temperatures, Phys. Rev. B 79, 125424 (2009)
3:30 PM - DD1.4
Thermo-mechanical Stability of Ultrananocrystalline Diamond Resonators.
Vivekananda Adiga 1 , Suresh Sampath 2 , Arindom Datta 2 , John Carlisle 3 , Robert Carpick 4 1 Show Abstract
1 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 , Innovative Micro Technology, Santa Barbara, California, United States, 3 , Advanced Diamond Technologies, Romeoville, Illinois, United States, 4 Mechanical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Tetragonal sp3-bonded diamond has the highest known atomic density. The nature of the bond and its high density enable diamond to have superior physical properties such as the highest Young’s modulus and acoustic velocity of all materials, and excellent tribological properties. These make diamond promising for high frequency micro/nanomechanical devices. Recently, conformal thin diamond films have been grown at CMOS-compatible temperatures in the form known as ultrananocrystalline diamond (UNCD). We have measured the Young’s modulus (E), Poisson’s ratio and the quality factors (Q) for microfabricated overhanging ledges and fixed-free beams composed of UNCD films grown at lower temperatures.1 The overhanging ledges exhibited periodic undulations due to residual stress. This was used to determine a biaxial modulus of 838 ± 2 GPa. Resonant excitation and ring down measurements of the cantilevers were conducted under ultra high vacuum (UHV) conditions on a customized atomic force microscope to determine E and Q. At room temperature we found E = 790 ± 30 GPa, which is ~20 % lower than the theoretically predicted value of polycrystalline diamond, an effect attributable to the high density of grain boundaries in UNCD. From these measurements, Poisson’s ratio for UNCD is estimated for the first time to be 0.057 ± 0.038. We discuss how adjusting for overhang is critical for these measurements. We also measured the temperature dependence of E and Q in these cantilever beams from 30 K to 300 K. Mechanical stiffness of these cantilevers followed Watchman’s empirical relation whereby E increased linearly with the reduction in temperature until 150 K where it then saturates. This is the first such measurement for UNCD and strongly suggests that the nanostructure plays a significant role in modifying the thermo-mechanical response of the material. The Q varied from 5000 to 16000 and showed a moderate increase as the cantilevers were cooled below room temperature. The results suggest that defects in the grain boundaries significantly contribute to the observed dissipation in UNCD resonators as opposed to thermoelastic dissipation and dissipation due to coupling between thermal and acoustic phonons. Influence of the overhanging ledges at the cantilever bases on temperature dependent resonant frequency shifts and quality factors will also be discussed.1. “Mechanical stiffness and dissipation in ultrananocrystalline diamond resonators” Adiga V.P., A. V. Sumant, S. Suresh, C. Gudeman, O. Auciello, J. A. Carlisle, R.W. Carpick, Vol. 79, pp 245403.1-8, Physical Review B, 2009
4:15 PM - **DD1.5
ZnO Thin Film Surface Acoustic Wave based Lab-on-a-Chip.
Jack Luo 1 , Yongqing Fu 2 , Xiaoye Du 3 , Sunglyul Maeng 4 , Daesik Lee 4 , Andrew Flewitt 3 , William Milne 3 Show Abstract
1 Centre for Material Research & Innovation, University of Bolton, Bolton United Kingdom, 2 Department of Mechanical Engineering, School of Engineering and Physical Sciences,, Heriot Watt University,, Edinburgh United Kingdom, 3 Department of Engineering, University of Cambridge, Cambridge United Kingdom, 4 , Electronics and Telecommunications Research institute, Daejeon Korea (the Republic of)
Lab-on-a-chip (LOC) is one of the most important microsystems with promising applications in microanalysis, drug development, diagnosis of illness and diseases etc. LOC typically consists of two main components: microfluidics and sensors. Integration of microfluidics and sensors on a single chip can greatly enhance the efficiency of biochemical reactions and the sensitivity of detection, increase the reaction/detection speed, and reduce the potential cross-contamination, fabrication time and cost etc. However, generally the mechanisms used for microfluidics and sensors are different, making the integration of the two main components complicated and increasing the cost of the systems. We have developed a new type of micropump and mixer using surface acoustic wave (SAW) as an actuation mechanism using low-cost ZnO piezoelectric films on Si-substrates, with a great potential for microfluidic applications. The SAW-based micropump and micromixer are simple in structure and fabrication, are low cost and can act as active pumping and mixing devices without moving-parts, hence are reliable and effective. SAW devices were fabricated on nanocrystalline ZnO thin films deposited on Si substrates using sputtering. When an AC signal at the intrinsic resonant frequency is applied to the interdigited transducer, acoustic waves are generated through the piezoelectric effect and travel on the surface. Coupling of acoustic waves into a liquid induces acoustic streaming and motion of a droplet if it is on a hydrophobic surface. A streaming velocity up to ~5cm/s was obtained. When the surface energy of the SAW device is reduced by using a self-assembly monolayer, the acoustic wave can be effectively used to pump droplets up to a speed of ~1cm/s. It was also found that a higher order mode wave, the Sezawa wave is more effective in streaming and transportation of microdroplets. A novel SAW device on a ZnO island was also fabricated which can be used to pump and mix liquids remotely, thus avoiding direct contact of the very-reactive ZnO film with biochemical solutions. The SAW devices have also been utilized for biosensing. The well defined resonant frequency of the SAW devices decreases once additional mass is attached to the surface, and the sensitivity of the acoustic devices is proportional to the operating frequency. The high frequency Sezawa wave of the ZnO SAW devices has been utilized to detect reactions between pro-state antigens and its antibodies. The shift of the resonant frequency of the SAW device was found to increase as the concentration of the pro-state antigens increases, verifying its potential for early stage pro-state cancer detection. These results clearly demonstrated the feasibility of using a single actuation mechanism for both microfluidics and sensing, and hence show its suitability for the LOC. This greatly simplifies the fabrication and operation of the microsystems, and enhances the sensitivity and performance of the microsystems.
4:45 PM - DD1.6
A New Approach for Accurate Measurement of Internal Friction in Thin Films: Applications to Microresonators used in MEMS.
Sairam Prabhakar 1 , Guruprasad Sosale 1 , Srikar Vengallatore 1 Show Abstract
1 Mechanical Engineering, McGill University, Montreal, Quebec, Canada
Accurate measurement of internal friction is essential for fundamental studies of the mechanical behavior of thin films, and for the design of high-Q microresonators employed in MEMS for applications in sensing and communications. Typically, these measurements are performed by depositing the thin film on a relatively thick substrate, and then measuring the structural damping of the composite film/substrate system in flexural or torsional vibration under reduced pressure. However, the measured damping is due to losses from several sources, including anchor losses at the supports, thermoelastic damping in the composite beam, and internal friction in the thin film. Identifying the magnitude of internal friction from the measured structural damping has been a long-standing problem in this field. Here, we propose a solution to this problem by taking advantage of recent developments in our understanding of thermoelastic damping (TED). This mechanism provides an absolute, thermodynamically-mandated, lower-bound on structural damping in flexural mode resonators. Furthermore, microscale resonators fabricated using single-crystal silicon can be designed to operate at the thermoelastic limit, which ensures that all other losses (including viscous air damping and clamping losses) are demonstrably negligible in these monolithic devices. Now, if these resonators are coated with a thin film, then the measured damping contains only two components: internal friction in the thin film, and thermoelastic damping in the film/substrate composite. The latter is calculated using an exact theory that does not require any free parameters , and then subtracted from the measured damping to yield an accurate estimate of the internal friction in the film.As an illustration of this approach, we present data for the internal friction in Al films with thickness ranging from 75 to 500 nm. First, a set of single-crystal silicon beams with thickness ranging from 100 to 150 μm, and fundamental natural frequency ranging from 0.1 to 1.0 kHz, were fabricated. Measurements at room temperature and low pressure showed that these devices operate at the thermoelastic limit with 10-6 < Q-1 < 5x10-5. Thin films of Al were then sputter deposited on these silicon beams, and the internal friction extracted from the measured structural damping. The effects of thickness and frequency on internal friction in these films, and implications for the selection of metallic coatings in the design of layered microresonators, will be discussed. S. Prabhakar and S. Vengallatore, “Thermoelastic damping in bilayered micromechanical beam resonators,” J. Micromechanics Microengineering, vol. 17, pp. 532-538 (2007)
5:00 PM - DD1.7
Micromachined Active Piezoelectric Structures for Applications above 600 °C.
Jan Sauerwald 1 , Denny Richter 1 , Erik Ansorge 2 , Bertram Schmidt 2 , Holger Fritze 1 Show Abstract
1 Department of Physics, Metallurgy and Materials Science, Technische Universitaet Clausthal, Goslar Germany, 2 Institute of Micro and Sensor Systems, Otto-von-Guericke-Universitaet Magdeburg, Magdeburg Germany
The operation of active devices in harsh environments requires appropriate materials and adapted operation principles. For example, high-temperature transducers based on single crystalline piezoelectric langasite (La3Ga5SiO14) meet both requirements. Bulk acoustic wave (BAW) langasite resonators coated with gas sensitive films could be operated up to at least 1000 °C and are demonstrated to be selective in-situ gas sensors. The materials properties and the application limits of langasite are well known. Further, large size and high quality wafers are available. Those capabilities motivate the creation of micromachined active langasite components. The specific objective of this work is to prepare thin membranes for BAW gas sensors and field emission diodes.The fundamental concept includes monolithic structures in order to minimize thermal stress. Therefore, conductive structures are prepared by doping. Heavy Sr doping increases the conductivity of langasite by about four orders of magnitude. Locally doped areas serve as monolithic electrodes as demonstrated by the excitation of resonators up to about 800 °C. In order to control the preparation process the chemical diffusion coefficients of potential dopants like Pr, Sr and Nb are determined.Initially, the thermal stability of miniaturized structures is tested since short diffusion distances and large surface to volume ratios might result in additional degradation processes in comparison to bulky structures. Test structures like tips and edges are prepared and annealed for 48 h at temperatures up to 1300 °C. SEM photographs do not indicate any rounding or other types of degradation. Therefore, langasite is expected to show dimensional long-term stability at typical operation temperatures up to 1000 °C. BAW langasite resonators are prepared by wet chemical etching in form of thin membranes. Resonance frequencies of up to 60 MHz and, thereby, increased mass sensitivities are achieved. Nearly isotropic etching results in slightly concave surfaces leading to moderate resonator quality factors (Q factor). Convex surfaces prepared by multi-step processes improve the energy trapping of the resonators. Biconvex membranes show a clear improvement of the Q factor in comparison to nominally planar resonators. At 700 °C, 16 MHz biconvex and planar resonators show Q factors of 820 and 400, respectively. In addition, arrays of membranes are machined. Further, field emission diodes are prepared and demonstrated to be operational. The radius of the tips is estimated to be as low as 40 nm. Those diodes and other active electronic elements are intended to process e.g. sensor signals already close to the sensor element.Finally, application examples demonstrate the capabilities of langasite membranes. For example, simultaneous determination of mechanical and electrical properties of thin sensor films by arrays of resonant sensors enables detection of CO in hydrogen containing atmospheres.
5:15 PM - DD1.8
Fatigue Properties of Silicon Thin Films in Harsh Environments.
Michael Budnitzki 1 , Pierre-Olivier Theillet 1 , Eva Baumert 1 , Olivier Pierron 1 Show Abstract
1 Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
MEMS devices have promising applications in harsh environments. For devices that cannot be hermetically packaged, the reliability of the structural materials, such as silicon (Si) thin films, must be assessed in these harsh environments. This study investigated the fatigue degradation properties of mono- and poly-crystalline silicon thin films as a function of temperature (from 30 to 80 °C), relative humidity (from 30 to 90% RH), testing frequency (static test, 4 vs. 40 kHz), and nanometer-scale surface coating. 2-micron-thick polycrystalline Si notched cantilever beam structures subjected to fully reversed sinusoidal loading at resonance (stress amplitudes between ~1.45 and 1.6GPa, ~40kHz) were used to investigate fatigue degradation in a high-temperature (80°C), high-humidity (90%RH) environment. Comparison to a reference set of S-N data obtained at moderate conditions (30°C and 50%RH) reveals a strong tendency for faster degradation in harsh environment, with damage rates exceeding the reference by two orders of magnitude. Transmission electron microscopy (TEM) on vertical through-thickness slices shows highly localized oxide thickening (up to 50nm) after cycling. Protection from the environment by means of ~20nm alumina, deposited on the specimen’s surface using atomic layer deposition (ALD) technique, results in a drastically different frequency evolution behavior at 30°C and 50%RH. No oxide thickening was observed in TEM for coated run-out specimens. A model is proposed to explain the different degradation behavior of the ALD-alumina coated samples. Thickened oxides after cycling appear consistent with the reaction-layer fatigue mechanism, even though the critical processes such as room-temperature, stress-assisted oxidation remain elusive.The low-cycle and high-cycle fatigue properties of 10-micron-thick, single-crystal Si films were also investigated using kHz-frequency resonating structures. The influence of resonant frequency (4 vs. 40 kHz) and environment (from 30°C/50%RH to 80°C/90%RH) on the resulting S-N curves and fatigue rates was monitored. Fatigue rates were assessed based on the resonant frequency decrease rate (0.01Hz resolution in resonant frequency measurement). The fatigue rates are most sensitive to stress, with 4 orders of magnitude decrease from 3.2 to 1.5-2 GPa. The fatigue rates are also much more sensitive to relative humidity than temperature or partial pressure of water, indicating the effective environmental parameter is the adsorbed water layer. Last but not least, unambiguous frequency effects are reported. The underlying mechanisms for the fatigue behavior of Si films are discussed in light of these experimental data.
DD2: Poster Session: Material Characterization and Devices
Monday PM, November 30, 2009
Exhibit Hall D (Hynes)
9:00 PM - DD2.1
Piezoelectric Nanogenerator by Using p-type ZnO Nanowire Arrays.
Jinhui Song 1 , Ming-Pei Lu 2 , Ming-Yen Lu 1 3 , Min-Teng Chen 3 , Yifan Gao 1 , Lih-Juann Chen 3 , Zhong Lin Wang 1 Show Abstract
1 Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , National Nano Device Laboratories, Hsinchu Taiwan, 3 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan
Harvesting energy from environment is a potential approach for building self-powered nanodevices/nanosystems. Piezoelectric nanogenerators using nanowires (NWs) are a technology for converting mechanical energy into electricity. Utilizing the semiconductor and piezoelectric properties possessed by ZnO NWs, an energy generator has been demonstrated for converting mechanical energy into electricity [1,2]. Under the straining created by an external force, a piezoelectric potential is created in the NW as a result of elastic deformation, which drives the flow of charge carriers through an external load. A Schottky barrier formed between the electrode and the NW serves as a “gate” that controls the flowing direction of the charge carriers. This is the principle of the nanogenerator. All of the existing literatures on nanogenerator are established using n-type ZnO NWs, while little is known about the characteristics of p-type ZnO NWs for energy harvesting. In this paper, we demonstrate the nanogenerators which could convert mechanical energy to electricity by using the p-type ZnO NWs . The p-type ZnO NWs produce positive output voltage pulses when scanned by a conductive AFM in contact mode. The output voltage pulse is generated when the tip contacts the stretched side (positive piezoelectric potential side) of the NW. In contrast, the n-type ZnO NW produces negative output voltage when scanned by the AFM tip; and the output voltage pulse is generated when the tip contacts the compressed side (negative potential side) of the NW. In reference to theoretical simulation, these experimentally observed phenomena have been systematically explained based on the mechanism proposed for nanogenerator. Wang, Z. L.; Song, J. H. Science 2006, 312, 242 Wang, X. D.; Song, J. H.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. Lu, M.P.; Song, J.H.; Lu, M.Y.; Chen, M.T.; Gao, Y.F.; Chen, L.J.; Wang, Z.L. Nano Letters., 9 2009 3, 1223. Research supported by DARPA, DOE, NIH.
9:00 PM - DD2.10
The Effect of Ion Delivery on Polypyrrole Strain and Strain Rate under Elevated Temperature.
Yenmei Keng 1 , Priam Pillai 1 , Ian Hunter 1 Show Abstract
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Conducting polymers can act as actuators when an electrochemical stimulus causes the materials to undergo volumetric changes. Ion flux into the polymer causes volumetric expansion and ion outflow causes contraction. Polypyrrole is an attractive actuator material due to its ability to generate up to 30 MPa active stress and 10-26% maximum strain with voltage supply lower than 2 V. The polymer’s mechanical performance depends upon the solvent used and the dominating ion species. In this study, we used 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) to characterize the effect of temperature increase on ion flow and how it contributes to strain and maximum strain rate of polypyrrole. In this solvent, the cation BMIM+ diffuses in and out of the polymer under applied voltage to cause strain changes. For approximately each increment of 10oC from 27-83oC, isotonic tests were done with +/-0.8 V square pauses, using a custom built device that is capable of performing temperature controlled dynamic mechanical analyses and electrochemistry simultaneously. Results showed that, independent of voltage polarity, from 27-83oC the charge per polymer volume increased by 146% and the strain increased in a similar way from 1.2% to 5.4%. Therefore, the increase in strain resulted from the charge delivered to the polymer in higher quantities at higher temperature. Moreover, both the maximum charge and strain rates were higher at positive voltage than at negative voltage throughout the same temperature range. Positive voltage caused the maximum strain rate to increase exponentially from 0.28%/s to 1.8%/s, while negative voltage caused it to increase more linearly from 0.23%/s to 0.88%/s. The results suggest that BMIM+ ions get expelled faster than being attracted in to the polymer, perhaps due to the ions preferentially remaining in the bulk solution. As the temperature increased, the ionic mobility increased and as a result, BMIM+ ions get populated back into the solvent even faster.
9:00 PM - DD2.11
Effect of Humidity on Dielectric Charging Process in Capacitive RF MEMS Switches Based on Kelvin Force Microscopy Surface Potential Measurements.
U. Zaghloul 1 2 , G. Papaioannou 3 1 , F. Coccetti 1 , P. Pons 1 , R. Plana 1 2 Show Abstract
1 MINC, LAAS/CNRS, Toulouse France, 2 UPS, University of Toulouse, Toulouse France, 3 Solid State Physics Section, University of Athens , Athens Greece
Reliability issues especially the dielectric charging have prevented the commercialization of RF-MEMS Capacitive switches. In spite of the extensive study performed on this topic, a comprehensive understanding of the dielectric charging process is still missing. Moreover, a very few work has been performed in order to study the effect of the humidity on the dielectric charging process. On the other side, Kelvin Force Microscopy (KFM) has recently proven to be a very efficient method for assessing the dielectric charging. Yet, all of the previous KFM measurements had been performed in ambient air and hence the effect of humidity has not been considered.In this work we present for the first time the effect of humidity and hence the surface charge on the charging process in PECVD silicon nitride films based on KFM. The measurements have been performed under different relative humidity levels; in ambient air (30% - 40% RH) and under a N2 flow (RH 10%). The effect of the dielectric film thickness and the substrate nature on the charging process have been investigated through depositing SiN films with different thicknesses ranges from 100nm to 500nm over bare silicon substrates and over evaporated Au layers.For both measurements performed in ambient air and under the N2 flow, the surface potential distribution, hence the injected charge is found to decay exponentially with time following the stretched exponential law. The decay time constant is found to be much higher in case of N2 flow (1.70E+07 sec) than in the case of ambient air measurements (558sec). Besides, it is found that the surface potential distribution becomes more confined as the relative humidity decreases. For example, the Full Width at Half Maximum (FWHM) at t=0 is measured to be 1.1µm in case of 40% RH measurements comparing to 0.72µm in case of 10% RH. Charge injection time has been controlled to range from 0.1 ms to 60 sec. In air, the FWHM is found to be always larger than the FWHM measured in N2 flow for different injection times. Moreover, the FWHM increase linearly with increasing the charge injection time for measurements performed under different relative humidity levels. However, the rate of increase in the FWHM is higher for ambient air measurements. Charge lateral diffusion has been observed in case of ambient air where there it has not been observed for the N2 flow measurements. Obviously, the more hydrophobic SiN material in case of N2 flow prevents water condensation at the surface and thereby inhibits lateral charge migration due to the electrical conductivity of a possible water film. The surface potential distribution is found to be more confined in thinner than in thicker films and the relaxation time is found to be larger in the thinner dielectric films, independently of the substrate nature. Finally, the decay time constant is found to be smaller in case of dielectric films deposited over Au layers comparing to films deposited over bare silicon substrates.
9:00 PM - DD2.12
Improved Characterization of ECD and PVD MEMS Processes by X-ray Methods.
Emmanuel Nolot 1 , Agathe Andre 1 , Assunta Vigliante 2 , Nikolai Vyshatko 2 , John Pointet 1 Show Abstract
1 , CEA-LETI-MINATEC, Grenoble France, 2 , BRUKER AXS GmbH, Karlrsruhe Germany
Optimizing and monitoring ECD and PVD MEMS processes first requires accurate thickness measurement in the 10nm to 30µm range, for low-Z (e.g Al) to high-Z (e.g Au) elements. Moreover, it implies precise determination of the composition for alloys (e.g NiFe magnetic alloys) and ternary systems (e.g SnAgCu lead-free solders) in multilayer complex stacks on both blanket and product wafers. Thus, introducing improved in-line X-ray characterization in the MEMS field is a key to accelerate the adoption of new materials that can expand the horizon for new products.We evaluate the performances of Bruker D8-Fabline automated equipment which combines high-resolution X-ray reflectometry (XRR) and micro-spot energy-dispersive X-ray fluorescence (µ-XRF). Based on round robin test, we demonstrate the accuracy the XRR module to monitor the thickness and the density of PVD metal films as thick as 350nm. Based on optical and mechanical profilometry measurements as well as particle-induced X-ray emission (PIXE) experiments, we quantify the thickness and composition accuracy of this equipment on blanket and product wafers with ECD-pad size below (75µm)2.
9:00 PM - DD2.13
Functionalized Nanocrystalline Diamond for Resonator Gas Sensors.
Frank Mendoza 1 2 , Nelson Sepulveda 1 3 , Brad Weiner 1 4 , Gerardo Morell 1 2 Show Abstract
1 Institute for Functional Nanomaterials, University of Puerto Rico, San Juan, Puerto Rico, United States, 2 Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 3 Electrical Engineering , University of Puerto Rico, Mayaguez, Puerto Rico, United States, 4 Chemistry, University of Puerto Rico, San Juan, Puerto Rico, United States
The detection of chemical agents is critically important for homeland security and industrial settings. Miniaturized electro-mechanical systems that employ nanocrystalline diamond (NCD) can be used to develop fast and specific gas sensors. NCD films were grown by hot filament chemical deposition vapor deposition on silicon substrates and made into a resonator structure by lithographic technique. We functionalized the NCD using different strategies, such as: hydrogenation, nitrogenation and sulfuration. These films were subjected to different concentrations of chlorine (Cl2), ammonium (CH3) and carbon dioxide (CO2) gases to study their adsorption. The surface adsorbates were studied by FTIR and XPS, and the surface wettability was studied using contact angle measurements. The suitability of the functionalized NCD surface for gas detection and the actual integration of NCD into electro-mechanical systems are discussed.
9:00 PM - DD2.14
Mechanical Characterization of Atomic Layer Deposited (ALD) Alumina for Applications in Corrosive Environments.
Kuang-Shun Ou 1 , I-Kuan Lin 2 , Ping-Hsin Wu 1 , Kuo-Shen Chen 1 Show Abstract
1 Mechanical Engineering, National Cheng-Kung University, Tainan Taiwan, 2 Mechanical Engineering, Boston University, Boston, Massachusetts, United States
Reliability and longevity of microelectronics and MEMS devices for corrosive environment applications, such as devices for oceanic signal transductions and those for oil well sensing, are traditionally major concerns of device performance evaluation. The metallic circuits are, in general, vulnerable under the attack of corrosive agents such as salty water or toxic gases. As a result, a barrier layer is required for enhancing the anti-corrosion capability of the devices. Recently, due to their nature of low temperature deposition, atomic layer deposited (ALD) dielectric films have become potential barrier materials for the above applications. Thin ALD alumina was fabricated for characterizing its capability as a barrier material for corrosive environments in this study. The specimens were fabricated (via Savannah S100, Cambridge NanoTech Inc.) by using trimethyl aluminum (TMA) and water vapor under a pressure of 0.93 Torr at 100, 200, and 300°C. The substrates were 4” silicon wafer (525μm) and Aluminum foils (380μm). The deposition rate was approximately 0.9 nm per cycle and a final thickness of 200nm was achieved after 2280 cycles. In our preliminary study, under a 90°C salty water environment, the measured resistance of aluminum wire without ALD alumina protection increased from 0.7Ω to 18MΩ in just two hours. On the other hand, under the protection of 200 nm ALD alumina, the resistance was 0.8Ω and remained essentially unchanged. However, it was also observed that after a four-day soaking, certain portions of the ALD layer were peeled off. It was believed that the salty water diffused from the micro defects existed in the ALD alumina and oxidized the aluminum wires underneath. The volume expansion and stress generated during this corrosion process caused fracture and delamination of the ALD alumina films. As a result, environmental effects on the fracture behavior of ALD alumina must be characterized. Vickers micro indentations tests were conducted on ALD alumina coated on both silicon and aluminum substrates with a load of 490~1000 mN. For fresh ALD alumina, it was interesting to find that cracks generated on the alumina/silicon samples. On the other hand, the alumina/aluminum samples exhibited no visible cracks. However, for the samples under the corrosive environment, the situation was quite different. A program is currently underway to perform detail fracture toughness variations with respect to the environmental variables. Meanwhile, the fatigue behavior of ALD films is also being characterized since the cyclic thermal stress generated by the temperature produced underneath the device during operation could possibly induce subcritical crack propagation of the protective ALD layers, which could be a serious concern for the long term reliability of related electronics and MEMS.
9:00 PM - DD2.17
Laser Induced Spatial Non-Uniformities in the Shape Memory Response of NiTi Thin Films.
Andrew Birnbaum 1 , Gen Satoh 2 , Y. Lawrence Yao 2 Show Abstract
1 Materials Science and Technology, Naval Research Lab, Washington, District of Columbia, United States, 2 Mechanical Engineering Dept., Columbia University, New York, New York, United States
A new process and mechanism are presented for controlling the shape memory response spatially within monolithic NiTi thin film structures. This technique is shown to effectively control the martensitic phase transformation temperature and exhibits control over aspects of the mechanical and shape memory responses as well. Specifically, the martensitic phase transformation temperature decreases with incident laser energy density. Concomitant modifications are observed in both the mechanical and shape memory responses in laser processed films. Analysis and characterization is performed via temperature controlled optical microscopy, x-ray diffraction, atomic force microscopy and nanoindentation.
9:00 PM - DD2.18
Thin Film Bulk Acoustic Wave Resonators for Continuous Monitoring in the Physical, Chemical and Biological Realms.
Greg Ashley 1 , Paul Kirby 2 Show Abstract
1 CMRI, Bolton Univeristy, Bolton United Kingdom, 2 , Cranfield University, Cranfield United Kingdom
Thin Film Bulk Acoustic Wave Resonator (FBAR) is a miniaturized version of Quartz Crystal Microbalance (QCM) with dimensions a thousand times smaller, and is operated in the UHF/Microwave frequency range (300MHz-<10GHz).FBAR devices were fabricated on silicon substrate which was back etched to form an unsupported metal-piezoelectric-metal stack. The typical dimension of the FBAR is 100μm x 100μm, and the thickness of the ZnO or AlN piezoelectric films deposited by sputtering ranged 0.5μm to 3μm depending on the desired frequency of operation. A 50Ω impedance matched coplanar waveguide Ground-Signal-Ground configuration was used for the FBAR.FBAR characteristic resonance responses tend to be noisy with a low Q factor, typically 30 to 300. In order to extract the exact resonant frequency and to monitor its change with time, the FBAR resonance curves were processed in real-time with a second order polynomial fitting, making the output signal smooth and stable enough for sensing. FBARs have been used for various sensing applications with main results summarized.The FBAR is extremely sensitive changes in environmental temperature. Temperature fluctuations were measured with FBAR and even relatively low frequency devices (300MHz) gave responses ~50 KHz per 1oC with resolution of 10Hz. The responses were almost instant and were in proportional agreement with that measured by thermocouples.Also, FBAR was found to be responsive to incident light. Light fluctuations were measured with FBAR using an external light detector to correlate the incident brightness. From dark (0% saturation) to normal room florescent lighting (40% saturation) Sub GHz FBAR was shown to decrease output frequency by 2KHz.FBAR was adapted to act as a partially selective chemical sensor by the addition of the hygroscopic polymer Poly vinyl pyrrolidone (PVP). Typically the measurement ranges were <%20RH to %90RH, FBAR responded within <1 second, and with almost perfect proportional correlation to independent humidity meters measuring the same gas flow. PVP coated Sub GHz FBAR using second harmonic @~700MHz gave 4MHz change from %RH20 to %RH60.FBAR was used to detect antibody-antigen reaction. Direct comparison of FBAR performance against Quartz (QCM) for immunosensor applications was performed. 10MHz QCM units gave 1HZ frequency change per 1ng of additional antibody-antigen reaction, whereas a typical sub GHz FBAR gave 1KHz/ng, showing three orders of magnitudes greater response.The experimental results have shown it is responsive to changes in the physical realms and to addition of various coatings, demonstrated the potential as a universal sensing platform with unprecedented sensitivity.
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