ICMCTF2001 Session H2-1: Materials and Processes for MEMS

Tuesday, May 1, 2001 9:10 AM in Room Sunrise

Tuesday Morning

Start	Invited?	Item
9:10 AM	Invited	H2-1-3 Amorphous Diamond Mems* T.A. Friedmann, J.P. Sullivan, R.J. Hohlfelder, D.A. LaVan, M.P. de Boer, M.T. Dugger, C.I.H. Ashby, M.A. Mitchell (Sandia National Laboratories) We are using low stress (<10 MPa) amorphous hydrogen-free diamond-like carbon (aD) as a material for manufacturing microelectromechanical systems (MEMS). These structures consist entirely of aD and are not simply coated poly-Si parts. The motivation for using aD as a structural material is based on its excellent mechanical properties - aD is chemically inert, extremely hard, stiff, wear resistant, low friction, and low stiction - and should outperform poly-Si MEMS in applications t hat require rubbi ng surfaces with low stiction. Simple single level- and multilevel-devices (one ground plane and two structural levels) have been fabricated using standard photolithographic lift-off and etching techniques. The focus of this investigation is to not only evaluate the suitability of aD 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, friction test structures run by comb drives, and resonant fatigue structures. A description of the fabrication process as well as results of device testing will be presented with an evaluation of the suitability of a-D for MEMS applications. 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.
9:50 AM	Invited	H2-1-5 Mechanical and Tribological Properties of Ultrananocrystalline Diamond (UNCD) Thin Films for Multifunctional Devices A. Sumant, O. Auciello, D.M. Gruen, A.R. Krauss, J. Tucek, D.C. Mancini, N. Moldovan, A. Erdemir (Argonne National Laboratory); D. Ersoy (University of Illinois-Chicago); M.N. Gardos (Raytheon Electronic Systems) MEMS devices are currently fabricated primarily in silicon because of the available surface machining technology. However, Si has poor mechanical and tribological properties, and practical MEMS devices are currently limited primarily 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 hydrophyllic surfaces, even these simple devices have limited dynamic range. Future MEMS applications that involve significant rolling or 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 that of Si, and the projected wear life of diamond MEMS moving mechanical assemblies (MEMS-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 ill-suited for MEMS-MMA applications. A thin film deposition process has been developed that produces phase-pure ultrananocrystalline diamond (UNCD) 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 diamond microstructures including cantilevers and multi-level devices, acting as precursors to micro-bearings and gears, making UNCD a promising material for the development of high performance MEMS devices.
10:30 AM		H2-1-7 Amorphous Hydrocarbon Based Thin Films for High-Aspect-Ratio MEMS Applications D.M. Cao, T. Wang, B. Feng, W.J. Meng, K.W. Kelly (Louisiana State University) Amorphous hydrocarbon (a-C:H) and metal-containing amorphous hydrocarbon (Me-C:H) thin films possess interesting combinations of mechanical and tribological properties, such as moderately high hardness, low coefficient of friction, and low wear rate[1]. Mechanical properties of Ti-C:H thin films have recently been shown to vary systematically with Ti composition[2]. Deposition of Ti-C:H thin films occurs in a hybrid CVD/PVD mode[3], making conformal converage of high-aspect-ratio micro-scale structures by Ti-C:H coatings likely. Using a low-pressure, high-density plasma assisted hybrid CVD/PVD reactor, we demonstrate conformal Ti-C:H coating of high-aspect-ratio micro-scale Ni structures fabricated by deep X-ray lithography/electrodeposition (LIGA) as a function of structure geometry and aspect ratio. Coating characterization by SEM, TEM, AFM, and nanoindentation was carried out. Such conformal coating of high-aspect-ratio micro-scale metal structures opens the possibilities of tribological improvement of MEMS devices with parts in relative motion and of using advanced ceramic thin films as structural materials for MEMS. As an example, free standing Ti-C:H micro-scale tube structures will be demonstrated. ¹ C. Donnet and A. Grill, Surf. Coat. Technol. 94/95, 456 (1997). ² W. J. Meng and G. A. Gillispie, J. Appl. Phys. 84, 4314 (1998). ³ W. J. Meng, E. I. Meletis, L. E. Rehn, and P. M. Baldo, J. Appl. Phys. 87, 2840 (2000).
10:50 AM		H2-1-8 Patterning of Diamond and Amorphous Carbon Films Using Focused Ion Beam A.V. Stanishevsky (University of Maryland); L.Yu. Kriachtchev (University of Helsinki, Finland) For applications of diamond and amorphous carbon films in optics, microelectronics and micromechanics, it is necessary in some cases to form submicron size patterns. In this work, a finely focused beam of 50 keV Ga⁺ ions was used to mill a number of patterns in CVD diamond, amorphous carbon with various sp³/sp² ratios, and C:N films. The trenches sizes of 20 nm width and depth-to-width ratios up to 25 have been achieved. Submicron size tips with radii in the range of 30-50 nm and high height-to-width ratios have been fabricated in both single CVD diamond microcrystallites and amorphous carbon films. The influence of the focused ion beam parameters and ambient gas on the milling yield, surface morphology, and structure modification of the material was studied using scanning electron microscopy, atomic force microscopy (AFM), measurements of electric resistivity, and Raman spectroscopy. Modification of the ion damaged surface layer and its swelling during the irradiation can influence the shape and physical properties of the milled structures. For carbon material with a high sp³/sp² ratio, an ion dose of ~0.01 nC/µm² (6.24x10¹⁵ ion/cm²) results in an amorphous material with the sp³/sp² ratios of 0.5-1. Higher doses (>0.1 nC/µm²) reduce sp³-fraction to a negligible value. The effect of thermal annealing of the focused ion beam (FIB) irradiated films was also studied. Particularly, annealing at <600 °C in oxygen leads to growth of sp²-bonded clusters within the damaged layer and its faster removal rate by oxidation.
11:10 AM		H2-1-9 Three-Dimensional Structures Assembled from Polysilicon Surface Micromachined Continuous Hinges and Microrivets E.S. Kolesar, M.D. Ruff, J.T. Howard, P.B. Allen, J.M. Wilken, N.C. Boydston, R.J. Wilks, S.Y. Ko, J.E. Bosch (Texas Christian University) A new polysilicon surface micromachining technique for realizing three-dimensional structures has been developed. Single-layer polysilicon elements and laminated polysilicon panels with trapped-glass reinforcement ribs have been successfully attached to a silicon substrate with robust and continuous hinges that can be rotated out-of-plane and assembled. One of the elevatable panels are terminated with an array of open windows, and a matching set of microrivets with flexible barbs protrudes from the other rotatable element. Because the microrivet barb tip-to-barb tip separation is larger than the opening in the matching window, the barbs flex as they pass through the open window and result in a permanent latch condition and a three-dimensional structure. Three novel microrivet designs have been micromachined to facilitate the latching process, including a simple arrowhead design, a high-aspect ratio shank-to-barb length variant, and a rivet-like structure with a split shank and a hemispherical shaped cap. To minimize panel breakage after the sacrificial glass release etch process and facilitate alignment during assembly, a network of sacrificial electrothermally-actuated mechanical links ('fuses') have been integrated into the design to promote robust and stable three-dimensional MEMS structures.