Novel Materials for MEMS
Monday, April 10, 2000 10:30 AM in Room Atlas Foyer
H2-1 Nanocrystalline Diamond Thin Films for MEMS and Moving Mechanical Assembly (MMA) Devices
A.R. Krauss, O. Auciello, D.M. Gruen, A. Jayatissa (Argonne National Laboratory); E.M. Meyer (University of Bremen, Germany); H.G. Busmann (Fraunhofer Institute for Applied Materials Science, (IFAM), Germany); J. Tucek, A. Sumant (Argonne National Laboratory); M.Q. Ding (Beijing Vacuum Electronics Research Institute, People’s Republic of China); N. Moldovan, D.C. Mancini (Argonne National Laboratory); M.N. Gardos (Raytheon Systems Company)
MEMS devices are currently fabricated primarily in silicon because of the available surface machining technology. However, Si has very poor mechanical and tribological properties, and current MEMS devices are largely limited to applications involving only bending and flexural motion, such as cantilever accelerometers and vibration sensors. However, because of the poor flexural strength and fracture toughness of Si, and the tendency of Si to adhere to hydrophilic surfaces, even these simple devices have limited dynamic range. Future MEMS applications that involve significant rolling and sliding contact will require the use of new materials with significantly improved mechanical and tribological properties, and the ability to perform well in harsh environments. Diamond is a superhard material of high mechanical strength, exceptional chemical inertness, and outstanding thermal stability. The brittle fracture strength is 23 times better than that of Si, and the projected wear life of diamond MEMS moving mechanical assemblies (MMAs) is 10,000 times greater than that of Si MMAs. However, as the hardest-known material, diamond is notoriously difficult to fabricate. Conventional CVD thin film deposition methods offer an approach to the fabrication of ultra-small diamond structures, but the films have large grain size, high internal stress, poor intergranular adhesion, and very rough surfaces, and are consequently unsuited for MEMS-MMA applications. A thin film deposition process has been developed that produces phase-pure nanocrystalline diamond (NCD) with morphological and mechanical properties that are ideally suited for MEMS applications in general, and MMA use in particular. We have developed lithographic techniques for the fabrication of NCD microstructures including cantilevers and multi-level devices, acting as precursors to micro-bearings gears and motors, making NCD a promising material for the development of high performance MEMS devices. Work supported by the DOE Office of Basic Energy Sciences under contract W-31-109-ENG-38.
H2-3 Amorphous Diamond Mems*
T.A. Friedmann, J.P. Sullivan, R.J. Hohlfelder, M.P. de Boer, B.D. Jensen, D.A. LaVan, C.I.H. Ashby, M.A. Mitchell, M.M. Bridges (Sandia National Laboratories)
Single-level micromechanical structures of amorphous hydrogen-free diamond-like carbon (aD) have been fabricated. These structures consist entirely of aD and are not simply coated parts. aD based microelectromechanical systems (MEMS) should be inherently more reliable than the current standard systems that use poly-Si. aD is chemically inert, extremely hard, stiff, wear resistant, low friction, and low stiction. aD has not been used to this point in MEMS because of the high biaxial (in-plane) residual compressive stresses (6-10 GPa) and out-of-plane stress gradients typically found in as-grown films. These high average stresses exceed by 3-4 orders of magnitude the maximum stress permitted for successful MEMS device manufacture (<2 MPa), since moderate residual stress causes released devices to distort rendering them useless. In addition, through-thickness stress gradients can cause released structures to warp out-of-plane. A process for routinely fabricating 1-2 µm thick aD films with low residual stress (<2 MPa) and low out-of-plane stress gradients has been developed. These thick films have been processed into simple single-mask level MEMS structures using standard MEMS manufacturing techniques. The structures manufactured include single- and double-clamped cantilever beams, tensile test rings, comb-drive actuators, and resonant fatigue structures. @paragraph@An environmentally-controlled interferometric microprobing station was used to measure deflections of electrostatically-acuated singleand double-clamped beams. This data was combined with finite element modeling to extract information on stiction, residual stress, strain gradients, and the elastic modulus of the aD film. Microtensile testing has also provided tensile strength, Weibull distribution in strength and Young's modulus. Early evidence strongly suggests that problems with stiction in these devices are greatly reduced over poly-Si based MEMS. @paragraph@In addition, simple actuation of resonant structures with electrical biasing has been demonstrated. Importantly, stress-free aD appears to be entirely compatible with current MEMS production techniques. Fabrication of devices with three aD levels (one ground plane and two structural levels) is in progress. @paragraph@*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.
H2-4 Design and Experimental Characterization of the In-Plane Tip Force and Deflection Achieved with Asymmetrical Polysilicon Electrothermal Microactuators
E.S. Kolesar, J.T. Howard, P.B. Allen, J.M. Wilken, N.C. Boydston, S.Y. Ko (Texas Christian University)
Several microactuator technologies have recently been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion are the most common modes of microactuator operation. This research focuses on the design and experimental evaluation of two types of asymmetrical MEMS electro-thermal microactuators. The motivation is to present a unified description of the behavior of the electro-thermal microactuator so that it can be adapted to a variety of MEMS applications. Both MEMS polysilicon electro-thermal microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. In the first design variant, when current is passed through the microactuator, the larger current density in the “hot” arm causes it to heat and expand more than the “cold” arm. Since both arms are joined at their free (released) ends, the actuator tip is forced to move in an arc-like pattern. Removing the current from the device allows it to return to its equilibrium state. In the second variant, the current is passed through two identical and parallel “hot” arms. The resulting thermal expansion causes the tip-connected “cold” arm to move in a similar arc-like pattern. To be a useful MEMS device, an electro-thermal microactuator will need to produce in-plane tip deflections that span 0-10 microns while generating force magnitudes on the order of 10 micro-Newtons. The electro-thermal microactuator designs were accomplished with the L-Edit CAD program, and they were fabricated using the Multi-User Microelectromechanical Systems (MEMS) Process (MUMPs) foundry at the Microelectronics Center of North Carolina (MCNC). The dimensions of the electro-thermal microactuator design variants were systematically varied so that an optimum design of each type could be established. The performance of the elements in the arrays of the electro-thermal microactuator design variants was experimentally characterized. For example, when the applied power was 30 mW, the 250-micron long single “hot” arm design variant produced the largest tip deflection observed (20 microns), and a tip force of 3.3 micro-Newtons. When the applied power was 80 mW, the 200-micron long dual “hot” arm design variant produced a 12.8-micron tip deflection and a tip force of 5 micro-Newtons.
H2-5 Chemical, Optical and Tribological Characterization of Perfluoropolymer Films as an Anti-stiction Layer in Micromirror Arrays
K.K. Lee, N.G. Cha, J.G. Park (University, South Korea); H.J. Shin (Company, South Korea)
Fluorocarbon (FC) thin films were deposited on aluminum (Al) and tetraethylorthosilicate (TEOS) oxide substrates by thermal vapor-phase (VP) deposition to prevent stiction of micromirrors (50 µm x 50 µm, 100 x 100 arrays) during their touchdown operation between Al mirrors and TEOS pads. The FC chemicals (from the 3M company) and PFDA (perfluorodecanoic acid) were used as precursors for film deposition. Static contact angles (SCA) of the FC films on Al and TEOS were about 115@super o@ and 110@super o@, respectively. Dynamic contact angle (DCA) analysis of the films showed poor surface coverage and poor homogeneity of the films. However, very low surface energies, less than 13 and 11 dynes/cm, were calculated from the contact angle measurements on Al and TEOS, respectively, after film deposition. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis showed the presence of CF, CF@sub 2@, CF@sub 3@, and C-C stretching bands on deposited FC films. Optical characterization of films, using variable angle spectroscopic ellipsometry (VASE), was carried out to determine optical and dielectric properties of the films. Thickness and the high frequency dielectric constant of FC films were calculated by applying the Lorentz model. Atomic force microscopy (AFM) analysis showed changes of surface morphology and roughness after film deposition. Friction and wear tests of films were performed as a function of time for different temperature, atmosphere, and humidity conditions. The final reliability of films was evaluated by the actual operation of micromirror arrays at 3 kHz as a function of time in a dry N@sub 2@, atmosphere. The number of mirrors that became stuck was counted during operation. We found that micromirrors were movable without stiction occurring, even after 2x10@super 8@ touchdown cycles in the presence of FC films on surfaces.