Papers

ICMCTF2002 Session H3-1: Materials and Processes for MEMS

Wednesday, April 24, 2002 8:30 AM in Room Royal Palm 4-6

Wednesday Morning

Start	Invited?	Item
8:30 AM		H3-1-1 Tribological Coatings for LIGA MEMS S.V. Prasad, T.R. Christenson (Sandia National Laboratories) Deep X-ray lithography (DXRL) based techniques such as LIGA (German acronym representing lithography, galvanoformung and abformung) can be used to fabricate net shape components for microelectromechnical systems (MEMS). Unlike other microfabrication techniques, LIGA lends itself to a broad materials base including metals, alloys, polymers, as well as ceramics and composites. Currently, Ni and Ni alloys are the materials of choice for LIGA Microsystems. While Ni alloys may meet the structural requirements for MEMS, their tribological (friction and wear) behavior remains somewhat undefined. For instance, the friction coefficient of pure Ni (m = 0.6 to 1.2) does not meet the design criteria. Additionally, generation of wear debris and the stick-slip behavior could interfere with the performance and reliability. In a number of microsystems applications such as gear trains, comb drives and transmission linkages, tribological considerations, particularly sliding contacts amongst sidewalls, is of paramount importance. The miniature nature of LIGA MEMS elements--several hundred microns to millimeters in size--poses a tough challenge to the coating technology. This paper gives an overview of the issues and challenges involved in coating the sidewalls of intricate LIGA MEMS parts to mitigate friction and wear problems. A novel methodology that was successfully applied to coat LIGA Ni MEMS parts with diamond like nanocomposite coatings by a commercial plasma enhanced CVD technique will be presented. Tribological measurements of DLN films on electrodeposited Ni test coupons showed much improved tribologcal behavior with friction coefficient of 0.04 and practically no signs of debris generation or stick-slip behavior. .
9:10 AM		H3-1-3 Carbon Films for Microelectromechanical Systems (MEMS): a Nanotribological Study I.S. Forbes, J.I.B. Wilson (Heriot-Watt University, United Kingdom) Tetrahedral amorphous carbon (ta-C) films have been deposited by microwave plasma-enhanced chemical vapour deposition of Ar/CH₄ gas mixtures in a novel coaxially-bladed reactor. The films are compared with nanocrystalline and microcrystalline diamond films grown in this system and in a more conventional microwave plasma system. Comparisons are also made with hard carbon films deposited elsewhere by previously reported methods. Our materials have been examined by ultra-violet and visible Raman spectroscopy, and subjected to nanotribological investigation by scanning force microscopy (SFM). The choice of film type for microelectromechanical systems (MEMS) is discussed, based on their relative merits.
9:30 AM		H3-1-4 Ultrananocrystalline Diamond MEMS: Properties and Applications D.M. Gruen, O. Auciello, J.A. Carlisle (Argonne National Laboratory) The remarkable properties of diamond make it potentially useful as a new MEMS material. However, the Feynman criterion, that feature resolution in polycrystalline MEMS is limited by grain size, has restricted the use of MEMS made by conventional CVD diamo nd methods. These techniques characteristically employ low renucleation rates that typically result in 1-10 µm crystallites. Our development in recent years of CVD diamond high-renucleation rate regimes allows one to fabricate MEMS structures compo s ed of ultrananocrystalline diamond of 3-5 nm randomly oriented grains. Submicron feature resolution can now be routinely achieved by well-established photolithographic and etching techniques. Fascinating 0-, 1-, 2-, and 3-dimensional diamond MEMS have b ee n produced by selecting etchants that take exquisite advantage of the dramatic differences in the chemistries of carbon and silicon. The basic science of the new CVD diamond methodology -- growth species, growth and nucleation mechanisms, plasma diagnostics -- will be discussed. The various characterization techniques used to ensure the phase-pure nature of the ultrananocrystalline diamond (UNCD) films will be briefly reviewed. For MEMS application, the mechanical and electrical properties of UNCD are extremely important. Work on fracture toughness and stiction will be reported. The ability to n-type dope UNCD has motivated recent development efforts aimed at integrated MESFET-MEMS structures. Specific applications in fields as diverse as photonic and RF switching, microfluidics, electrochemical sensors, neural prostheses, and field emission devices will be enumerated. This work is supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38.
10:30 AM		H3-1-7 Tribology of Carbon Films and Carbon-based Mems Devices A. Erdemir, O. Auciello, D.M. Gruen, J.A. Carlisle (Argonne National Laboratory); A. Sumant (LigthMatrix Techologies); N. Moldovan, D. Mancini (Argonne National Laboratory) Carbon is rather unique and offers the kind of flexibility needed in the design and fabrication of one-, two-, and three-dimensional structures ranging in sizes from nano to meso scales for the fabrication of nanoelectromechanical systems (NEMS) and microelectromechanical systems (MEMS). Recently, carbon was used in our laboratory to produce ultrananocrystalline diamond (UNCD) and nearly frictionless carbon (NFC) films that can provide friction coefficients as low as 0.001 and wear rates of 10-11-10-10 mm3/N.m even under dry sliding conditions and at very high contact pressures. Using advanced micro fabrication and chemical vapor deposition methods, our research team has pioneered the development of UNCD-based MEMS structures that can offer exceptional physical, chemical, mechanical, electrical, and tribological properties. Combination of such exceptional properties in one material is rather unusual but urgently needed by industry to meet the increasingly multifunctional needs of advanced micro systems and devices. This talk will provide an overview of recent progress in the study and understanding of tribological phenomena on carbon-based coatings that can be used for MEMS applications. Surface engineering aspects of these devices and the principles of superlubricity in amorphous and crystalline forms of carbon films will also be presented. Examples of current and future applications for two- and three-dimensional carbon-based MEMS devices will be elaborated.
11:10 AM		H3-1-9 Lubrication and Wear Control of Microelectromechanical Systems (MEMS) Using Fluorocarbon Monolayers and Diamond-Like Carbon J.S. Zabinski, S.T. Patton (Air Force Research Laboratory); S. Sastry (Surmet Corporation); K.C. Eapen (Air Force Research Laboratory) Integration of sensors, actuators, and signal processing may be accomplished by utilizing microelectromechanical systems (MEMS). MEMS devices are increasingly being integrated into products, but most marketable designs cannot include contacting surfaces in constant relative sliding motion. The reason is that stiction, friction, and wear significantly reduce the reliability of these types of systems. This is especially true for systems that require operation in vacuum, such as satellites and spacecraft. In this report, the wear mechanisms of a MEMS electrostatic output motor are determined in high vacuum and compared to those in dry and moist air. Wear in vacuum is the most severe and proceeds by a fundamentally different mechanism. In vacuum, wear proceeds by adhesion and grain pullout, whereas in air wear occurs by an oxidative mechanism and creates nanometer sized SiO₂ particles. To control wear, two lubrication schemes have been studied: (1) bound and mobile monolayers have been applied to MEMS motors and model Si surfaces to provide lubricant replenishment through the mobile phase, and (2) diamond-like-carbon was deposited on released motors, using a non line-of-sight process to ensure uniform coatings. Both strategies increased motor lifetimes by over two orders of magnitude. Lubrication and wear mechanisms in different environments will be discussed in terms of tribo-chemistry and surface forces.
11:30 AM		H3-1-10 MEMS Devices Made from Stress-free Tetrahedral Amorphous-carbon* T.A. Friedmann, J.P. Sullivan, M.P. de Boer, T.E. Buchheit, M.T. Dugger, R.V. Ellis, W.K. Schubert, M.A. Mitchell (Sandia National Laboratories) We are using low stress (<10 MPa) amorphous hydrogen-free tetrahedral amorphous-carbon(ta-C) as a material for manufacturing microelectromechanical systems (MEMS). These structures consist entirely of ta-C and are not simply coated poly-Si parts. The motivation for using ta-C as a MEMS structural material is based on its excellent mechanical properties - ta-C is chemically inert, extremely hard, stiff, wear resistant, low friction, and low stiction - and should outperform poly-Si MEMS in applications that require rubbing surfaces with low stiction. Simple single- and double-level devices (one ground plane and a structural level) have been fabricated using standard photolithographic lift-off and etching techniques. The focus of this investigation is to not only evaluate the suitability of ta-C for MEMS applications, but also to use the fabricated devices to measure materials properties on length scales appropriate to MEMS applications. The structures manufactured include single- and double-clamped cantilever beams, tensile test rings, comb-drive actuators, micro-xylophone and flexural plate-wave resonators, and friction test structures run by comb drives. A description of the fabrication process as well as results of device testing will be presented. *This work was supported by the U.S. DOE under contract DE-AC04-94AL85000 through the Laboratory Directed Research and Development Program, Sandia National Laboratories.