AVS2004 Session MN-MoM: Processing and Characterization for MEMS and NEMS

Monday, November 15, 2004 8:20 AM in Room 213C

Monday Morning

Start	Invited?	Item
8:20 AM	Invited	MN-MoM-1 Porous Thin Films for MEMS and Nano Applications C.J. Kim (University of California, Los Angeles) The talk will summarize various porous thin-films developed at the UCLA Micro and Nano Manufacturing Laboratory over the past several years. (1) Micromachining of aerogel-like thin film has been developed, including photolithographic steps and surface micromachining procedures for silica and alumina. Mechanical properties have been measured by direct bending tests with such fabricated free-standing aerogel microbeams. (2) Polysilicon thin film on silicon dioxide, which represents a typical surface micromachining process, has been converted permeable by post-deposition electrochemical etching, allowing on-chip vacuum encapsulation of micro and nano structures finally practical. (3) Silicon wafer with high-aspect-ratio pores serves as a mold in developing three-dimensional nanobatteries. Although most projects start from development of pore formation processing steps, the main goals for all are to explore specific new applications that take advantage of the unique property of the materials or the processing procedures.
9:00 AM		MN-MoM-3 Nanotribological Characterization of Perfluoropolymer Thin Films for BioMEMS Applications K. Lee, B. Bhushan, D. Hansford (The Ohio State University) The undesired adhesion of micro-organisms and biomolecules to surfaces and biofilm development called biofouling may cause detrimental effects to the performance of most biomedical microelectromechanical system (BioMEMS) devices. A vapor phase deposition technique to modify surfaces with perfluoropolymer and silane thin films was developed to reduce or prevent protein or cell interactions, critical for their use. The surface properties of these devices and therefore the surface modifications become increasingly important for BioMEMS applications as the channel dimensions decrease within these systems. Compared to dip coating or spin coating, the vapor phase deposition is more effective for smaller channels, especially at the nanoscale. Since nanotribological behaviors such as surface topography, adhesive and frictional properties and mechanical stability of these films play a very important role in forming uniform, conformal and ultra thin films on the surface and reducing protein or cell interactions, coating effects of these films were characterized extensively using an atomic force microscopy in this study.
9:20 AM		MN-MoM-4 Multi-scale Friction Experiments Using Atomic Force Microscopy and Surface Micromachined Interfaces E.E. Flater, M.D. Street (University of Wisconsin-Madison); A.D. Corwin, M.P. de Boer (Sandia National Laboratories); R.W. Carpick (University of Wisconsin-Madison) Friction and wear are major limiting factors for the development and commercial implementation of devices fabricated by surface micromachining techniques. We use atomic force microscopy (AFM) to determine the constitutive relation for friction for a single asperity nanoscale contact on micromachined surfaces. Friction is measured using AFM SiO₂- and alkyl-monolayer terminated tips sliding on alkyl-terminated single crystal silicon. The alkyl monolayer coatings include octadecyltrichrolosilane (OTS), octadecene, and fluorinated monolayers (FOTAS). Frictional information at the nanoscale is then used to predict tribological properties of a polycrystalline silicon nanotractor device interface. This microscale friction and wear test device provides abundant, quantitative information about friction and wear at an actual microelectromechanical system (MEMS) interface. This in-situ approach to measuring tribological properties of MEMS, combined with high-resolution atomic force microscope images of device wear, provides insight into the effects of wear and prescriptions for avoiding it.
9:40 AM		MN-MoM-5 Deposition and Characterization of Nitrogen-Doped Polycrystalline SiC Films for MEMS Applications J. Trevino, X.-A. Fu, S. Rajgopal, M. Mehregany, C.A. Zorman (Case Western Reserve University) This presentation reports on the development of processes to deposit undoped and nitrogen-doped, polycrystalline silicon carbide (poly-SiC) films on large-area substrates in a high-throughput, low pressure chemical vapor deposition (LPCVD) reactor using SiH2Cl2, C2H2 and NH3 precursor gases. The films were deposited in a customized deposition system constructed around a resistively-heated, horizontal furnace similar in design to a conventional polysilicon furnace and capable of holding up to 100, 150 mm-diameter substrates. To the best of our knowledge, this is the largest furnace designed specifically for the production of poly-SiC films for MEMS. Depositions were performed on 100 mm-diameter Si and SiO2-coated Si wafers using a SiH2Cl2 flow rate of 35 sccm, a C2H2 (5% in H2) flow rate of 180 sccm and NH3 (5% in H2) flow rates ranging from 10 to 90 sccm. The furnace temperature was held at 900C while the deposition pressures ranged from 2.5 to 4 Torr. Stoichiometric poly-SiC films were deposited over this entire range. The films exhibit a strong (111) 3C-SiC texture regardless of pressure. Films having a thickness of up to 2 microns are uniform, with less than a 5% variation across both the wafers and the boat. Four-point probe measurements indicate that the highest conductivities are achieved at a NH3 flow rate of 90 sccm. Wafer-scale residual stresses were measured using an optical curvature measurement technique. The residual stresses in the heavily-doped films are tensile with values decreasing to around 100 MPa in films deposited at 4 Torr. Single-layer, surface mechanical properties test structures, such as cantilever beams, stress pointers and lateral resonators were fabricated, successfully released and used to characterize the films. Likewise, bulk micromachined membranes were fabricated and tested using a load-deflection technique. Stress measurements from these micromachined structures confirm the wafer-scale residual stress measurements.
10:00 AM		MN-MoM-6 Characterization of Nanotribological Properties and Surface Chemistry of Advanced Nanostructured Carbon Materials for MEMS and NEMS Applications A.V. Sumant, D.S. Grierson (University of Wisconsin-Madison); J.E. Gerbi, J.P. Birrell, J.A. Carlisle, O.H. Auciello (Argonne National Laboratory); T. Friedmann, J.P. Sullivan (Sandia National Laboratories); R.W. Carpick (University of Wisconsin-Madison) Despite rapid advances in micro- and nanofabrication technologies, the implementation of reliable, high endurance devices that involve sliding contacts remains elusive. At small length scales, device properties are dominated by surface chemistry rather than bulk properties, and therefore materials with superior tribological properties and optimized surface chemistry are needed. Ultrananocrystalline diamond (UNCD) and tetrahedra amorphous carbon (taC) thin films have exceptional physical, chemical and tribological properties at the macroscale (nearly equivalent to those of single crystal diamond) and are being considered promising materials for the fabrication of high performance MEMS devices. However, little is known about the surface chemistry of these materials, and how it affects their nano- and micro-scale tribological performance. We have developed detailed methodologies to characterize nanotribological properties and surface chemistry of UNCD and taC at the tribologically relevant interface by using a combination of near-edge X-ray absorption fine structure spectroscopy (NEXAFS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). We show that the surface chemistry, sp2-sp3 ratio, and nanoscale friction and adhesion can be different on the etched underside of the film (the side which becomes exposed and makes tribological contact after a MEMS release process) as compared with the top side of the film. We also discuss the effect of hydrogen plasma treatment, which in the case of UNCD renders the surface extremely inert and chemically pure, and reduces nano-scale friction and adhesion dramatically. Adhesion, as measured with tungsten carbide AFM probes, is reduced to the van der Waals limit indicating full saturation of dangling surface bonds and elimination of surface contaminants.
10:40 AM		MN-MoM-8 Critical Issues in Epitaxial Growth of Pulse Laser Deposited AlN Films for MEMS and NEMS based RF Resonators S. Hullavarad, R. Vispute, T. Venkatesan (University of Maryland); A. Wickenden, L. Currano, M. Dubey, T. Takacs, J. Pulskamp (U.S. Army Research Laboratory) AlN exhibits strong piezo-electric properties suitable for RF resonator applications. In this work we report the growth of highly oriented AlN films for MEMS and NEMS resonator devices. A multiple flexural structure of Pt/SiO2/Si is used as a substrate and films are grown by Pulse Laser Deposition (PLD) technique at a pulse energy of ~2J/cme2 with a repetition rate of 10 Hz. The process is optimized for the growth of AlN on different thicknesses of underlying SiO2. The films are characterized by XRD, RBS and techniques for crystalline quality and stoichiometry respectively. The interface analysis of underlying structures is analyzed in detail by RBS and oxygen content in the film is monitored by Resonant Oxygen Scattering technique. The morphology of AlN films is studied by scanning electron and atomic force microscopies. We have obtained highest Q factors for PLD grown AlN MEMS resonator beams of Q = 8,000 at fo = 2.5 MHz and Q = 17,400 at fo = 0.44 MHz We also address in this work critical issues related to (1) thickness of SiO2 (2) method of growth of SiO2 in fabricating MEMS and NEMS devices. These factors are very essential for the growth of high quality AlN films. However, SiO2 provides a amorphous underlayer for the growth of AlN leading to non in plane aligned AlN with respect to substrate. A lattice matching, epitaxial oxide layer like Y2O3 in place of SiO2 is going to be a unique solution for eventual epitaxial growth of AlN. We address the epitaxial issues of AlN and underlying oxide for improving the resonator properties of AlN based MEMS and NEMS devices.
11:00 AM		MN-MoM-9 Vapor Phase Uptake of Mobile Organophosphates for MEMS Lubrication Purposes D.A. Hook, W. Neeyakorn, C. Jaye, J. Krim (North Carolina State University) MEMS devices are highly susceptible to surface forces that can cause suspended members to deflect towards the substrate, collapse and/or adhere permanently to the substrate. A number of surface treatments have met with varying degrees of success for alleviation of MEMS-related stiction/adhesion problems, but friction and wear remain problematic. We report here the results of a quartz crystal microbalance (QCM) study of the nanodynamics and uptake characteristics of organophosphate (tricresylphosphate and t-butyl phenyl phosphate) layers adsorbed from the vapor phase onto silicon and silane treated silicon surfaces. Silanes applied from the liquid phase as self-assembled monolayers are in common use as anti-stiction treatments for silicon MEMS devices, but degrade at elevated temperatures. Organophosphates are highly stable at temperatures in excess of 600ï,°C, act as antioxidants, and have well-documented tribological performance for certain materials combinations. We observe that organophosphates adsorb readily onto selected silanes. The silane +organophosphate combinations moreover exhibit interfacial slippage and/or viscoelasticity in response to the oscillatory motion of the QCM. Such effects have previously been linked to beneficial tribological performance.[1] Work is in progress to assess the tribological performance of these materials on actual MEMS devices. Work supported by AFOSR and NSF. ¹M. Abdelmaksoud, J.Bender and J. Krim, Phys. Rev. Lett. 92, 176101 (2004).
11:20 AM		MN-MoM-10 Mechanical and Electrochemical Characterization of Gold Membranes on a Drug Delivery MEMS Device Y. Li, M.J. Cima (Massachusetts Institute of Technology) Our drug delivery MEMS device was designed to release multiple substances with complex profiles in order to maximize the efficacy of drug therapies. The device consists of arrays of microreservoirs etched into a silicon substrate to contain different types and doses of drug. The release of drug is achieved through the electrochemical dissolution of the gold membranes that seal individual reservoirs. The mechanical and electrochemical properties of the gold membranes are important parameters in evaluating the reliability of device performance. A bulge test apparatus was constructed to measure the mechanical properties of the gold membranes. The apparatus is pressurized, and the resutling deflection of the membranes is measured using interferometry. The biaxial modulus of elasticity and residual stress in the membranes extracted from the bulge test were 126-168 GPa and ~100 MPa (tensile) respectively for membranes with in-plane sizes ranging from 20 to 200 mm. An in situ experimental set up was constructed to observe the electrochemical disintegration process of the gold membranes when voltage was applied. The bulge test was used to evaluate the mechanical integrity of gold membranes corroded for different duration of time. The decrease in the membrane burst pressure with longer corrosion time under the bulge test confirmed a gradual loss of mechanical integrity of the gold membranes due to corrosion. Observation of the membrane morphology with an optical profiler indicated an abrupt transition in the membrane stress state from slightly tensile to highly compressive after five seconds of corrosion. This suggests that the gold membrane disintegration occurs by a combination of thinning through active dissolution and accumulation of compressive stress.
11:40 AM		MN-MoM-11 Microfabrication and Nanomechanical Characterization of Polymer MEMS for Biological Applications G. Wei, B. Bhushan, N. Ferrell, D. Hansford (The Ohio State University) Polymer Microelectromechanical System (MEMS) devices are promising for biological applications such as development of biosensors and biomechanical devices. The relatively low stiffness and improved biological interface between cells and polymeric materials make polymer cantilever and beam structures attractive as highly sensitive force sensors for measuring cellular and biomolecular nanomechanics. In order to develop polymer Bio-MEMS, novel polymer microfabrication techniques are required, and the nanomechanics studies including measurement of the mechanical properties of the polymer materials in the nano scale must be carried out. This paper presents the development of soft lithography based polymer Bio-MEMS microfabrication techniques and systematic studies on the nanomechanical characterization of the polymer materials, polymer beams and polymer cantilevers. Poly (methyl methacrylate) (PMMA) and poly (propyl methacrylate) (PPMA) are used to make the polymer beams and cantilevers, which are 5 µm wide, 10-30 µm long and 200 nm-5 µm thick, for MEMS integration. The hardness, creep behavior and scratch resistance of the PMMA and PPMA microstructures were measured using nanoindentation/nanoscratch technique with a Nano Indenter II system, and the nanomechanical properties are compared with the bulk values. The elastic modulus of the polymer beam was obtained from the bending tests performed by nanoindentation, and the nano scale fatigue of the polymer cantilever was measured using the nanoindentation Continuous Stiffness Measurement (CSM) technique. To simulate the working environment of the polymer Bio-MEMS, PMMA and PPMA beams and cantilevers were also placed in an aqueous solution (saline, DI water, etc.), and nanoindentation experiments were performed on such samples. The results are discussed along with the dry condition values.