J.J. Sniegowski

Bio: J.J. Sniegowski is an academic researcher from University of Wisconsin-Madison. The author has contributed to research in topics: Surface roughness & Beam (structure). The author has an hindex of 7, co-authored 7 publications receiving 674 citations.

Characteristics of polysilicon resonant microbeams

Abstract: Polysilicon resonant microbeams can be used as strain-sensitive elements to replace conventional silicon piezoresistors in precision sensor applications, such as pressure sensors and accelerometers. These elements are combined with conventional silicon diaphragms or flexures with a proof mass to convert pressure or acceleration directly into a frequency output. Vacuum-enclosed resonant microbeam elements 200 or 400 μm long, 45 μm wide and 1.8 μm thick have been fabricated using LPCVD mechanical-grade polysilicon at the University of Wisconsin. Q-values determined using gain/phase analysis are typically over 25 000. Lower Q-values are primarily the result of residual gas in the cavity. Closed-loop operation from −60 to 180°C using piezoresistive sensor and electrostatic drive has been achieved with automatic gain control (AGC) to prevent overdrive. The characteristic resonance frequencies of the beams have been measured, with 550 kHz, 1.2, 2.2 and 5.2 MHz being typical of the frequencies of the one-dimensional bending modes for the 200 μm length. These measurements of the multiple resonance frequencies of a single beam provide a means of testing mathematical models of the dynamic behavior as well as determining the residual beam stress. The one-dimensional (1D) differential equation of motion of a doubly clamped single-span beam with an axial load can be solved analytically for lateral natural frequencies and mode shapes. These 1D solutions have been verified by 3D finite-element methods. In addition, the finite-element models are used to identify both lateral and torsional modes. The closed-form solutions agree closely with the numerical results and the experimental data.

The application of fine-grained, tensile polysilicon to mechanicaly resonant transducers

Abstract: Resonant force sensors are devices which convert axially applied forces to changes in resonant frequency. These structures are fundamentally wires or beams or more complicated structures which are in a vacuum envelope. They become interesting and useful if they can be miniaturized, can be fabricated from a single material in a cost effective manner and can be excited and read via simple techniques. The devices which are reported here satisfy most of the above criteria. The construction material involves a silicon substrate, tensile strain polysilicon films and strain-compensated silicon nitride deposits. Clamped-clamped beams of polysilicon, typically 400,μm long, 40,μm wide and 2μm thick are fabricated with an isoplanar process over an oxide filled tub. Low-pressure chemical-vapor-deposited (LPCVD) nitride is used as a second sacrificial layer which also serves to support a second polysilicon layer which is part of the vacuum envelope. Internal surface adhesion problems are avoided by freeze-sublimation procedures which remove surface tension-induced beam deflections. Passivation and sealing is accomplished via LPCVD nitride and reactive sealing. Excitation and sensing is accomplished via ion implanted resistors. Experimental results always produce quality factors, Q , above 35 000. Resonant frequencies to 750 kHz have been achieved. It is estimated that these devices can measure axially applied forces below 0.1 dyne with standard electronic interfaces.

Advances in processing techniques for silicon micromechanical devices with smooth surfaces

Abstract: The use of fine-grained polysilicon in the development of micromechanical devices (e.g. bearings, with smooth surfaces) is discussed. Fine-grained polysilicon can be produced with surface roughness near 8 AA r.m.s. (root mean square). The ability to anneal films of this type into tension eliminates size restrictions which are caused by compressive buckling. The use of these films in micromechanical devices has been restricted because hydrogen-fluoride-etched structures are covered by an etch residue which leads to contact welding. Contact between opposing surfaces is induced mainly by surface tension effects. This problem can be avoided by removing the deflection mechanism. Thus, freezing of a water-methanol rinse after sacrificial etching all but eliminates surface tension. Removal of the ice mixture via sublimation at 0.15 mbar occurs readily. Free-standing structures with smooth surfaces and small gaps are then passivated by silicon nitride deposition or other techniques. >

Performance characteristics of second generation polysilicon resonating beam force transducers

Abstract: Polysilicon resonating beam force transducers and their performance characteristics are studied. Doubly clamped beams in vacuum display a shift in resonant frequency with applied axial load. Functionality, miniaturization, and batch fabrication were accomplished using surface micromachining techniques with tensile, fine-grained polysilicon as the construction material. The device could be a very accurate, cost-effective alternative to presently available force transducers if it displays sharply defined and stable resonant frequencies. The devices are driven into resonance by means of electrothermomechanical or capacitive forces. Analytical expressions are derived to illustrate the detrimental effect the drive mechanisms have on the resonant frequency. Material and process sensitivities are calculated from a closed-form expression for the fundamental resonant frequency. Experimental beams have typical dimensions of 200- mu m length, 45- mu m width, and 2- mu m thickness. The fundamental unloaded resonant frequency is near 650 kHz and can be adjusted by processing. It will shift to nearly 900 kHz with an applied strain level of +0.1%. A temperature coefficient of the frequency of -75 p.p.m./ degrees C for the finished batch-fabricated device was demonstrated. Theory and experiment are in agreement, showing that the resonant frequency stability and not the ability to measure frequency limits the force resolution. >

The MEMS Handbook

Abstract: Part I: Background and Fundamentals Introduction, Mohamed Gad-el-Hak, University of Notre Dame Scaling of Micromechanical Devices, William Trimmer, Standard MEMS, Inc., and Robert H. Stroud, Aerospace Corporation Mechanical Properties of MEMS Materials, William N. Sharpe, Jr., Johns Hopkins University Flow Physics, Mohamed Gad-el-Hak, University of Notre Dame Integrated Simulation for MEMS: Coupling Flow-Structure-Thermal-Electrical Domains, Robert M. Kirby and George Em Karniadakis, Brown University, and Oleg Mikulchenko and Kartikeya Mayaram, Oregon State University Liquid Flows in Microchannels, Kendra V. Sharp and Ronald J. Adrian, University of Illinois at Urbana-Champaign, Juan G. Santiago and Joshua I. Molho, Stanford University Burnett Simulations of Flows in Microdevices, Ramesh K. Agarwal and Keon-Young Yun, Wichita State University Molecular-Based Microfluidic Simulation Models, Ali Beskok, Texas A&M University Lubrication in MEMS, Kenneth S. Breuer, Brown University Physics of Thin Liquid Films, Alexander Oron, Technion, Israel Bubble/Drop Transport in Microchannels, Hsueh-Chia Chang, University of Notre Dame Fundamentals of Control Theory, Bill Goodwine, University of Notre Dame Model-Based Flow Control for Distributed Architectures, Thomas R. Bewley, University of California, San Diego Soft Computing in Control, Mihir Sen and Bill Goodwine, University of Notre Dame Part II: Design and Fabrication Materials for Microelectromechanical Systems Christian A. Zorman and Mehran Mehregany, Case Western Reserve University MEMS Fabrication, Marc J. Madou, Nanogen, Inc. LIGA and Other Replication Techniques, Marc J. Madou, Nanogen, Inc. X-Ray-Based Fabrication, Todd Christenson, Sandia National Laboratories Electrochemical Fabrication (EFAB), Adam L. Cohen, MEMGen Corporation Fabrication and Characterization of Single-Crystal Silicon Carbide MEMS, Robert S. Okojie, NASA Glenn Research Center Deep Reactive Ion Etching for Bulk Micromachining of Silicon Carbide, Glenn M. Beheim, NASA Glenn Research Center Microfabricated Chemical Sensors for Aerospace Applications, Gary W. Hunter, NASA Glenn Research Center, Chung-Chiun Liu, Case Western Reserve University, and Darby B. Makel, Makel Engineering, Inc. Packaging of Harsh-Environment MEMS Devices, Liang-Yu Chen and Jih-Fen Lei, NASA Glenn Research Center Part III: Applications of MEMS Inertial Sensors, Paul L. Bergstrom, Michigan Technological University, and Gary G. Li, OMM, Inc. Micromachined Pressure Sensors, Jae-Sung Park, Chester Wilson, and Yogesh B. Gianchandani, University of Wisconsin-Madison Sensors and Actuators for Turbulent Flows. Lennart Loefdahl, Chalmers University of Technology, and Mohamed Gad-el-Hak, University of Notre Dame Surface-Micromachined Mechanisms, Andrew D. Oliver and David W. Plummer, Sandia National Laboratories Microrobotics Thorbjoern Ebefors and Goeran Stemme, Royal Institute of Technology, Sweden Microscale Vacuum Pumps, E. Phillip Muntz, University of Southern California, and Stephen E. Vargo, SiWave, Inc. Microdroplet Generators. Fan-Gang Tseng, National Tsing Hua University, Taiwan Micro Heat Pipes and Micro Heat Spreaders, G. P. "Bud" Peterson, Rensselaer Polytechnic Institute Microchannel Heat Sinks, Yitshak Zohar, Hong Kong University of Science and Technology Flow Control, Mohamed Gad-el-Hak, University of Notre Dame) Part IV: The Future Reactive Control for Skin-Friction Reduction, Haecheon Choi, Seoul National University Towards MEMS Autonomous Control of Free-Shear Flows, Ahmed Naguib, Michigan State University Fabrication Technologies for Nanoelectromechanical Systems, Gary H. Bernstein, Holly V. Goodson, and Gregory L. Snider, University of Notre Dame Index

Surface micromachining for microelectromechanical systems

Abstract: Surface micromachining is characterized by the fabrication of micromechanical structures from deposited thin films. Originally employed for integrated circuits, films composed of materials such as low-pressure chemical-vapor-deposition polycrystalline silicon, silicon nitride, and silicon dioxides can be sequentially deposited and selectively removed to build or "machine" three-dimensional structures whose functionality typically requires that they be freed from the planar substrate. Although the process to accomplish this fabrication dates from the 1960's, its rapid extension over the past few years and its application to batch fabrication of micromechanisms and of monolithic microelectromechanical systems (MEMS) make a thorough review of surface micromachining appropriate at this time. Four central issues of consequence to the MEMS technologist are: (i) the understanding and control of the material properties of microstructural films, such as polycrystalline silicon, (ii) the release of the microstructure, for example, by wet etching silicon dioxide sacrificial films, followed by its drying and surface passivation, (iii) the constraints defined by the combination of micromachining and integrated-circuit technologies when fabricating monolithic sensor devices, and (iv) the methods, materials, and practices used when packaging the completed device. Last, recent developments of hinged structures for postrelease assembly, high-aspect-ratio fabrication of molded parts from deposited thin films, and the advent of deep anisotropic silicon etching hold promise to extend markedly the capabilities of surface-micromachining technologies.

Squeeze film air damping in MEMS

Abstract: The paper presents an overview and reports the recent progress of research on squeeze film air damping in MEMS. The review starts with the governing equations of squeeze film air damping: the nonlinear isothermal Reynolds equation and various reduced forms of the equation for different conditions. After the basic effects of squeeze film damping on the dynamic performances of micro-structures are discussed based on the analytical solutions to parallel plate problems, recent research on various aspects of squeeze film air damping are reviewed, including the squeeze film air damping of perforated and slotted plate, the squeeze film air damping in rarefied air and the squeeze film air damping of torsion mirrors. Finally, the simulation of squeeze film air damping is reviewed. For quick reference, important equations and curves are included.

Stiction in surface micromachining

Abstract: Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces.

Micromachined pressure sensors: review and recent developments

Abstract: Since the discovery of piezoresistivity in silicon in the mid 1950s, silicon-based pressure sensors have been widely produced Micromachining technology has greatly benefited from the success of the integrated circuit industry, borrowing materials, processes, and toolsets Because of this, microelectromechanical systems (MEMS) are now poised to capture large segments of existing sensor markets and to catalyse the development of new markets Given the emerging importance of MEMS, it is instructive to review the history of micromachined pressure sensors, and to examine new developments in the field Pressure sensors will be the focus of this paper, starting from metal diaphragm sensors with bonded silicon strain gauges, and moving to present developments of surface-micromachined, optical, resonant, and smart pressure sensors Considerations for diaphragm design will be discussed in detail, as well as additional considerations for capacitive and piezoresistive devices Results from surface-micromachined pressure sensors developed by the authors will be presented Finally, advantages of micromachined sensors will be discussed