Papers

AVS2007 Session MN-MoM: Materials Processing, Characterization and Fabrication Aspects

Monday, October 15, 2007 8:00 AM in Room 615

Monday Morning

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8:00 AM		MN-MoM-1 Materials for the Realization of High Performance Radio Frequency MEMS Devices S.P. Pacheco (Freescale Semiconductor, Inc.); G. Piazza (University of Pennsylvania) RF MEMS technology has rapidly evolved and matured over the last decade. More than 60 companies are currently involved in RF MEMS development with around 25% shipping commercial products or samples to customers. According to industry projections, by 2009, the RF MEMS market will break the $1 billion barrier with about 40% of the total market dominated by Bulk Acoustic Wave (BAW) devices.¹ Major opportunities for other RF MEMS devices will continue to expand as the rest of the market hits its stride in terms of both high-volume and high-end applications as issues with reliability, packaging, and CMOS integration are solved. The remaining market will be split between micro-mechanical resonators and oscillators for consumer and IT applications and RF MEMS switches for military applications and RF Test and Automated Test Equipment (ATE). This paper will describe proven material systems that are being presently commercialized as well as examine innovative materials that are starting to gain popularity for RF MEMS micro-resonators and switches. Benefits and challenges associated with each of these material systems will be presented. Topics such as CMOS/MEMS monolithic integration as well as the use of high acoustic velocity materials such as silicon carbide and diamond-like films for the realization of high performance, compact frequency references will be discussed. Additionally, the introduction to CMOS-compatible, low-loss GHz-range bandpass filters based on piezoelectric aluminum nitride contour-mode MEMS resonators will be covered. Piezoelectricity is also being investigated as an actuation mechanism for RF MEMS switches that would allow handset front-end compatible bias voltages in the 2-4 V range. Lastly, packaging breakthroughs using wafer-level techniques, including 3-D integration and surface micromachining, have the potential to enable low-cost, high-reliability, high-performance RF MEMS devices. ¹ J. Bouchaud, B. Knoblich, RF MEMS Market and Industry Overview, WTC - Wicht Technologie Consulting, Munich, http://www.wtc-consult.de.
8:40 AM		MN-MoM-3 Nano-Scale, Directional Contact Metal Transfer in Hot-Switched MEMS Actuators Z. Yang, D. Lichtenwalner, A. Kingon (North Carolina State University) We have investigated failure mechanisms of metal-contact Micro-electro-mechanical Systems (MEMS) switches using a new accelerated lifetime test facility. The facility utilizes a double-contact upper cantilever from commercial MEMS contact switches, and tests these against bottom gold contact pads within a modified AFM. After a number of switching cycles, both the upper contacts and bottom electrode were characterized by atomic force microscopy (AFM). In this paper, a new phenomenon we term "nano-scale, directional contact metal transfer" during hot switching is reported. The material (gold) in the contact area transfers between the bottom contacts and upper contacts during hot-switching, following the electric field direction. The field-directional transfer was confirmed by DC signal (open circuit voltage, Voc = 6 V) test and reversed DC signal (Voc = -6 V) test. It has been found that the material transfer process is accelerated with the increase of cycling number. Volume analysis of the damaged contact area shows that using an AC signal (with the amplitude of Voc = 6 V) yields an order of magnitude less material transfer damage than DC current, and the material transfer under AC does not have any directional characteristics. This indicates that under the condition of "hot-switching", signal type (AC/ DC; biased or not) may have a significant effect on MEMS switches' failure mechanism. Different material transfer related theories and models are reviewed and examined. A new micro-contact degradation mechanism is proposed.
9:00 AM		MN-MoM-4 Growth of AlN by Pulsed Laser Deposition and Reactive Sputtering Techniques and Fabrication of RF MEMS Resonators S. Hullavarad (University of Alaska Fairbanks) AlN is a desirable piezoelectric material for MEMS and NEMS resonators and micro-switches for high frequency filtering applications. The theoretical maximum frequency of AlN is >180 GHz, as opposed to <60 MHz for PZT. The Young’s modulus of bulk ceramic AlN at 25°C is 345 GPa, and the density is 3260 kg m-3. This compares to a Young’s modulus of 56 GPa for thin film PZT, and a density of 7600 kg/m3 measured for bulk PZT. Additionally, AlN is better suited for the integration of MEMS devices into silicon-based electronics due to its complete compatibility with conventional silicon technologies.¹ In this work, the growth of AlN by Pulsed Laser Deposition method and comparison of properties of AlN thin films with reactive sputter deposition method will be presented. The fabrication and performance of MEMS resonators on AlN thin films deposited by both Pulsed laser deposition and sputtering techniques on substrates consisting of Pt/SiO2/Si structures will be discussed. Special emphasis will be given to the quality (thermal stability, stress) of SiO2 (thermal or PECVD) used as a support layer in fabrication of MEMS resonators. The results obtained by Rutherford Back Scattering Spectroscopy and X-Ray Diffraction techniques to understand the structural stability, composition, crystalline quality of SiO2, Pt and AlN films will be discussed. The quality factor of the resonators in air and vacuum will be compared. It will be shown that the crystalline quality of the films affects the actutation properties of the resonator beams. ¹ S.S.Hullavarad, B. Nagaraj, V.N.Kulkarni, S.Dhar, R.D.Vispute, T. Venkatesan, K.A. Jones, M. Derenge, T. Zheleva, M. Ervin, A. Lelis, C.J. Scozzie, D. Habersat, A.E. Wickenden, L. Currano, M. Dubey, J. Electronic Materials 35, 777 (2006) - A short review.
9:20 AM		MN-MoM-5 Science and Technology of Piezoelectric/Diamond Hybrid Heterostructures for High Performance MEMS/NEMS Devices O. Auciello, A.V. Sumant, J. Hiller, B. Kabius (Argonne National Laboratory) A new generation of low power microelectromechanical and nanoelectromechanical system (MEMS/NEMS) devices will require new materials and the integration of dissimilar materials, and new micro and nanofabrication processing techniques to achieve high device performance. Most MEMS devices are currently based on silicon because of the available surface micromachining technology. However, the poor mechanical and tribological properties of Si are not suitable for many high-performance MEMS/NEMS devices, such as resonators and switches. A novel ultra-nano crystalline diamond (UNCD) material developed in thin film form at Argonne exhibits exceptional mechanical and tribological properties that make UNCD a suitable material for a new generation of high-performance MEMS/NEMS devices. Piezoelectric-based MEMS attracts much attention due to their high sensitivity and low electrical noise in sensing applications and high-force output in actuation applications. Piezoelectric Pb(ZrxTi1-x)O3 (PZT) thin films have been intensively investigated over the past decade due to its potential applications in a wide variety of devices, such as non-volatile ferroelectric memories and piezoelectrically actuated MEMS/NEMS devices, which can be actuated at comparatively lower voltages (5-10 V) to those actuated by electrostatic action that required higher voltages. Therefore, the integration of functional PZT thin films with the UNCD-based MEMS/NEMS structures opens up the tantalizing possibility of advanced MEMS/NEMS devices. However, the integration of PZT and UNCD is challenging, mainly due to the PZT/UNCD interface and the need to grow PZT at high temperature in oxygen in the presence of a carbon-based material such as diamond. We will review in this paper the fundamental and applied materials science performed in our laboratory to achieve integration of PZT as a piezoelectric actuation material and UNCD as a mechanically superior platform for MEMS/NEMS, and the development of fabrication processes to produce high-performance hybrid PZT/UNCD MEMS.NEMS devices. We will also present data from test of hybrid PZT/UNCD piezo-actuated resonator structures. This work was supported by the US Department of Energy, BES-Materials Sciences, under Contract DE-AC02-06CH11357.
9:40 AM		MN-MoM-6 NEMS Resonators of Carbon Nanotube Network and Metal-Carbon Nanotube Composites J.H. Bak, Y.D. Kim, B.Y. Lee, S.S. Hong, Y.D. Park (Seoul National University, Korea) We present nanomechanical torsional resonator and doubly-clamped beam resonator structures fabricated from aluminum-carbon nanotube (CNT) and palladium-CNT composites. In addition, we realize nanoelectromechanical systems (NEMS) structures suspended by self-assembled carbon nanotube network on GaAs surface by adopting highly selective wet-etching and reactive ion etching techniques. Carbon nanotubes have been spotlighted for its great potential as a promising material as well as a future candidate material for nanoelectronics, with CNT’s unique electrical and mechanical properties. NEMS structure combined with CNT can be applied to elucidate the nanotube’s physical properties as well as further applications. Furthermore, metallic based NEMS resonator structures are of interest due to higher optical reflectivity, ductility, and conductivity compared to insulator- and semiconductor- based NEMS structures. The resonators are electrostatically driven and are detected at room temperatures under moderate vacuum conditions using optical modulation techniques. From identifying fundamental flexural and rotational modes as well as applying continuum mechanics equations, we observe a significant enhancement of the Young’s modulus in metallic resonators structures with added CNTs. We will also discuss the characterization of mechanical properties of the structures by AFM force deflection spectroscopy and compare the two characterization techniques.
10:20 AM		MN-MoM-8 Addition and Removal of Stress to Drastically Tune Frequency and Quality Factor of Nanomechanical Resonators S.S. Verbridge, D. Finkelstein Shapiro, H.G. Craighead, J.M. Parpia (Cornell University) We have used optical drive and detection to study the mechanics of flexural nanostring resonators. Beam stress in devices made of both silicon and silicon nitride is tuned by macroscopically bending the resonator chip, resulting in a drastic tuning of the frequency of the correctly oriented doubly clamped beams. Frequency tuning by as much as several hundred percent is achieved with this technique. Over this wide range of frequency tuning, quality factor is also observed to be tuned by as much as several hundred percent. Highly stressed devices display the highest quality factors, and we therefore conclude that stress can be used as a parameter to increase device performance by increasing both resonant frequency as well as quality factor. Frequency can be drastically tuned and quality factor positively impacted by the addition of both tensile, as well as compressive stress. We discuss the sources of dissipation for these devices, and demonstrate a high tensile stress doubly-clamped beam resonator with sub-micron cross-sections, and a quality factor of 390,000 at 3.7 MHz, in vacuum, and at room temperature. The high frequency and quality factor exhibited by the high stress devices, as well as the significant tuning attained with the chip-bending technique, should prove useful for applications of nanomechanical resonant devices.
10:40 AM		MN-MoM-9 Noise Temperature and Thermodynamic Temperature of Ultrasensitive Cantilevers Below 1 K A.C. Bleszynski, W.E. Shanks (Yale University); B. Ilic (Cornell University); J.G.E. Harris (Yale University) Micromechanical systems can be fabricated with the sensitivity necessary for detecting ultra-small forces arising from quantum mechanical effects. We use cantilevers as torsional magnetometers to study the magnetic properties of systems mounted directly on a cantilever. Our goal is to study persistent currents in normal metal rings. The properties of these currents remain an outstanding controversy in mesoscopic physics. As with all sample-on-cantilever arrangements, there are two distinct temperatures that determine the performance of the experiment: the cantilever’s Brownian motion temperature (T_n) and the temperature of the sample mounted on the cantilever (T_s). T_n is associated with a single macroscopic degree of freedom extended over the length of the cantilever. T_s on the other hand is associated with the very large number of microscopic degrees of freedom in the sample. For a high-Q cantilever, T_n, which sets the cantilever’s ultimate force sensitivity, is in weak contact with the thermal bath at temperature T_b. T_s is in contact with the bath via phonon conduction through the cantilever. This contact can also be weak for a small, electrically insulating cantilever at low temperatures. It is thus a priori uclear whether in a practical experiment T_s and T_n will equilibrate with each other or even with T_b. It is also unclear how they will respond to a localized heat source, e.g. a laser used to monitor the cantilever’s motion. We have used our sample-on-cantilever system to realize two primary thermometers to measure both T_n and T_s. We infer T_n by monitoring the cantilever’s Brownian motion and T_s from the critical magnetic field of a superconducting sample mounted on the cantilever. We find that for modest laser powers incident on the sample, these two temperatures stay equilibrated to each other and to T_b down to 300mK. For higher laser powers T_s and T_n remain equal to each other but are hotter than T_b. The temperature difference is well-described by a simple model of phonon transport along the cantilever beam. We have also fabricated single crystal silicon cantilevers with integrated micron-scale metal rings. We have demonstrated attonewton force sensitivity with these devices and will present measurements of the rings’ susceptibility in the normal and superconducting states.
11:00 AM		MN-MoM-10 Process Development and Material Characterization of Polycrystalline BiTe and PbTe Thin Film Alloys on Si for MEMS Thermoelectric Generators I. Boniche (University of Florida); B.C. Morgan, P.J. Taylor (U.S. Army Research Laboratory); C.D. Meyer, D.P. Arnold (University of Florida) Numerous opportunities exist in commercial and military applications for thermoelectric (TE) energy scavengers to act as integrated power sources. Bulk TE materials and modules are commercially available but are often tailored for heating/cooling applications, rather than power generation. Additionally, these bulk technologies limit the miniaturization of TE modules. This work seeks to develop and characterize vapor-deposited polycrystalline TE thin films on Si substrates for integration with MEMS devices, specifically investigating Bi₂Te₃ and PbTe alloys for both room and high-temperature applications. P-type polycrystalline Bi₂Te₃ and PbTe films from 0.4 µm to 9 µm thick have been successfully deposited on bare and etched Si, thermally oxidized Si, and Si/SiO₂ substrates with patterned metal traces. The films were vapor-deposited in UHV using congruent sublimation of the solid-source parent compounds. Fundamental microfabrication techniques for Bi₂Te₃ films, such as patterning and metallization, have recently been developed to augment previous work on PbTe alloys.¹ Dry etch rates of 0.4 µm/min and 0.7 µm/min were obtained for Bi₂Te₃ and PbTe, respectively. Wet etch rates of ~3 µm/min were achieved using bromine-based chemistries, but at the expense of mask undercut. Films have been characterized electrically using van der Pauw and transfer length method test structures. As-deposited resistivity was 23 mΩ-cm for Bi₂Te₃, and 126 mΩ-cm for PbTe films. Contact resistivities of 2x10^-4 Ω-cm² were achieved for Cr/Pt/Au on Bi₂Te₃, and 4x10^-4 Ω-cm² for Cr/Au on PbTe. The Seebeck coefficient was measured to be 94 μV/K for Bi₂Te₃ and ~100 μV/K for PbTe alloys. Analytical modeling of in-plane MEMS TE generators showed that film resistivity is a limiting factor for power generation. Various post-deposition annealing treatments were explored to reduce film resistivity, and thus enable higher power delivery. The results show that successive rapid thermal annealing in nitrogen at 400°C can reduce the resistivity of PbTe. The integration of these materials into prototype generator structures will also be discussed, particularly towards developing fabrication compatible TE, heat exchanger, and mechanical MEMS structures. ¹I. Boniche, et al, PowerMEMS Conf., Nov. 2006.
11:20 AM		MN-MoM-11 Fabrication of Metal-based High Aspect Ratio Microscale Structures by Compression Molding J. Jiang, F.H. Mei, W.J. Meng (Louisiana State University) Metal-based high aspect ratio microscale structures (HARMS) are basic building blocks for metallic microdevices such as micro heat exchangers^1,2 and micro electromagnetic relays.^3,4 Metallic microdevices may function better when subjected to high stresses, high temperatures, and other harsh conditions. Metal-based HARMS can be fabricated by combining X-ray/UV lithography and electrodeposition, following the Lithographie/Galvanoformung (LiG) protocol.⁵ Such primary HARMS made by LiG are expensive. In comparison, production of secondary HARMS by molding replication from HARMS inserts is fast and simple.^5,6 We have demonstrated successful molding replication of HARMS in Pb⁷ Al⁸, and Cu.⁹ Molding replication of metal-based HARMS entails extensive plastic deformation within the molded metal. Understanding the mechanics of microscale compression molding is important for accurately assessing the capabilities and limitations of this technique. The present paper summarizes our results on instrumented compression molding of Pb, Al, and Cu as a function of the molding temperature. Measured molding responses are rationalized with companion elevated-temperature tensile testing of metals using a simple mechanics-based model of the micromolding process. The present results suggest that stresses on the insert during micromolding are determined primarily by the yield stress of the molded metal at the molding temperature and the frictional tractions on the insert sidewalls. Additional factors of complication during high temperature micromolding will be discussed. ¹D. B. Tuckerman, R. F. W. Pease, IEEE Elect. Dev. Lett. 1981, 2(5), 126 ²F. Arias, S. R. J. Oliver, B. Xu, R. E. Homlin, G. M. Whitesides, J. MEMS 2001, 10, 107 ³I. J. Busch-Vishniac, Sensors and Actuators 1992, A33, 207-220 ⁴J. D. Williams, W. Wang, Microsystem Technologies 2004, 10, 699 ⁵E. W. Becker, W. Ehrfeld, D. Munchmeyer, H. Betz, A. Heuberger, S. Pongratz, W. Glashauser, H. J. Michel, and V. R. Siemens, Naturwissenschaften 1982, 69, 520 ⁶M. Madou, Fundamentals of Microfabrication, CRC Press, Boca Raton, Florida, 2000 ⁷ D. M. Cao, W. J. Meng, K. W. Kelly, Microsystem Technologies 2004, 10, 323 ⁸ D. M. Cao, W. J. Meng, Microsystem Technologies 2004, 10, 662 ⁹D. M. Cao, J. Jiang, W. J. Meng, J. C. Jiang, W. Wang, Microsystem Technologies 2007, 13, 503.
11:40 AM		MN-MoM-12 Process Characterization of Vapour Phase Sacrificial Etching A. O'Hara, G. Pringle, M. Leavy (MEMSSTAR, UK) The manufacture of MEMS devices has primarily used processes and techniques developed for the semiconductor industry. The process characterization is well established for these methods and are then adapted to the MEMS structure. One process unique to MEMS manufacture is isotropic etching of a sacrificial layer. Historically these processes have been developed using wet etch methods, stagnant gas techniques or gas flow processes with limited process capability. Wet processing and stagnant gas processes employ a one process fits all approach. However, it is seen that different MEMS structures require significantly different process optimization and control. Using memsstar systems for etching, based on controlled continuous flow technology CCFT the process is optimised to the structure being etched. In this example for XeF₂ etching, a carrier gas is employed to transport a precise flow of XeF₂ to the process chamber. The flow of the carrier gas determines the flow of the XeF₂. When etching a structure with a large open access to the sacrificial material the etch is seen to be transport limited. The etch rate is dependent on the flow of XeF₂ into the chamber, the higher the flow the higher the etch rate. When the open access to the sacrificial material is very limited the etch is seen to be reaction limited. In this case the etch rate is dependent on the partial pressure of the XeF₂, the higher the partial pressure the higher the etch rate. Using controlled continuous flow of the process gases combined with fine chamber pressure control the sacrificial etch process can be tuned to the MEMS structure being manufactured. Experimentation with different structures is discussed to show that the etch process performance and process window varies depending upon the mechanical materials and dimensions.