AVS2001 Session MM-ThM: Characterization of MEMS Materials

Thursday, November 1, 2001 8:20 AM in Room 130

Thursday Morning

Start	Invited?	Item
8:20 AM	Invited	MM-ThM-1 Mechanical Properties of MEMS Materials W. Sharpe (Johns Hopkins University) The "mechanical" part of "microelectromechanical systems" (MEMS) requires knowledge of mechanical properties to predict relations between forces and displacements. Young’s modulus and Poisson’s ratio are needed for elastic response, and the strength of the material is needed to determine the allowable forces or displacements. Tensile testing is the preferred approach for structural materials because its uniform stress and strain fields enable direct determination of mechanical properties according to their definitions. Tensile testing of small thin-film specimens presents three challenges - preparation and handling of the specimen, measurement of small forces, and measurement of strain in the specimen. The author and colleagues at Hopkins have developed techniques and procedures for tensile testing of polysilicon, silicon nitride and silicon carbide. It is easier to measure mechanical properties of MEMS materials indirectly by modeling microdevices and extracting properties. One can fabricate a comb-driven resonant structure and use the measured resonant frequency to determine the modulus. Thin membranes of different shapes can be pressurized, and the measured displacements used to determine both Young’s modulus and Poisson’s ratio. Cantilever or fixed-end beams can be deflected electrostatically to measure modulus. However, none of these indirect approaches permit measurement of all the three properties (modulus, ratio, strength) simultaneously as does the tensile test. This presentation summarizes the current state-of-the-art in terms of test methods and the values of the polysilicon and other materials used in MEMS.
9:00 AM		MM-ThM-3 The Beam vs. Plate Distinction for Si Strips Mechanically Loaded in Bending S.K. Kaldor (Columbia University); I.C. Noyan (IBM T.J. Watson Research Division) Silicon structures used in microelectromechanical systems (MEMS) are generally anisotropic and possess dimensions that make it difficult to determine whether a beam or plate solution is more appropriate. Since a plate has an increased stiffness over that of a beam, errors of up to 10% in predicted displacements and stresses can occur if the proper bending solution is not employed. For single crystal Si samples loaded in four-point bending, we report both finite element modeling results and x-ray curvature measurements that illustrate the effects of boundary conditions (bending jig rollers used to apply displacements), specimen anisotropy, and specimen dimensions. We find that the transverse, or anticlastic, bending effects, which are ignored by 2-D solutions, should be considered as they can result in non-uniform loading across the sample width, and they are important in deciding whether a beam or plate solution should be used. While the sample's width-to-thickness ratio is typically the only criterion used to differentiate between beam and plate structures, we show that it is necessary to consider not only the sample's width and thickness but also the amount of applied bending; this was first considered by Searle¹ in 1908. We show that the Searle parameter, width²/(thickness * bending radius), can be used to accurately differentiate between beam and plate structures. Furthermore, the difference in stiffness between a beam and a plate depends on the Poisson's ratio of the bent material. Since Poisson's ratio in Si can vary from 0.06 to 0.36 with crystallographic orientation, controlling the bending direction of a single crystal is a possible method for tailoring the specimen's flexural rigidity. ¹ G.F.C. Searle, "Experimental Elasticity," 2nd ed. Cambridge UP, 1920.
9:20 AM		MM-ThM-4 Amorphous Diamond MEMS J.P. Sullivan, T.A. Friedmann, M.P. de Boer, M.T. Dugger, M. Mitchell, R.G. Dunn, R. Ellis (Sandia National Laboratories); D.A. LaVan (Massachusetts Institute of Technology) Microelectromechanical systems (MEMS), including electrostatic comb drives, simply-supported beams, and tensile test specimens, have been fabricated from amorphous diamond (aD), a pure carbon material with mechanical properties similar to crystalline diamond. Measurements using aD MEMS revealed that the material has high strength (8 GPa), fracture toughness (8 MPa.m^1/2), and elastic modulus (800 GPa). These properties, combined with good inherent wear resistance, makes the material useful for achieving long lifetime MEMS that have rubbing surfaces or experience impact loading. Hydrophobicity and bio-compatibility of aD were also evaluated. The water contact angle was found to range from 84° for the as-prepared MEMS material up to 94° after annealing to 850°C. The increase in contact angle with annealing is similar to that found for crystalline diamond surfaces, which is due to O desorption that leaves an H-terminated surface. The hydrophobic nature of aD greatly reduces stiction in MEMS, thus permitting release without the use of applied hydrophobic coatings or supercritical drying. Bio-compatibilty was tested through the use of cultured cell growth, using bovine capillary endothelial cells, on bare and fibronectin-coated aD surfaces. Limited cell growth and adhesion was found for the uncoated aD surface, while good growth and adhesion was found for the fibronectin-coated aD. This is desirable for the creation of bioMEMS. Finally, the very high elastic modulus of this material is desirable for achieving mechanical structures with high resonant frequency. A key requirement for mechanical oscillators used for electrical signal processing is the need for high quality factor, Q. The Q for aD MEMS oscillators operating in vacuum will be reported and compared to that found for silicon oscillators. Sandia is a multiprogram lab operated by Sandia Corp., a Lockheed Martin Co., for the U.S. D.O.E. under contract DE-AC04-94AL85000.
9:40 AM		MM-ThM-5 Fabrication Techniques and Integration Processes for a New Ultrananocrystalline Diamond (UNCD) -Based MEMS Technology and Characterization of UNCD Mechanical Properties O. Auciello, A.V. Sumant, D.M. Gruen, J.A. Carlisle, J. Birrell, N.A. Moldovan, D.C. Mancini, M. Angadi (Argonne National Laboratory); H.D. Espinosa, B.C. Prorok (Northwestern University) State-of-the-art Si-based MEMS components exhibit serious performance limitations due to the relatively poor mechanical and tribological properties of Si. Diamond and diamond-like materials are investigated for MEMS applications, but they also have microstructural /properties, and/or processing limitations. A novel diamond coating technology developed at ANL yields phase-pure UNCD coatings with 2-5 nm grains and smooth surfaces,in addition to hardness of 97 GPa and friction coefficient of ~ 0.01, both similar to pure diamond. The unique growth process (involving C60 or CH4 /Ar microwave plasmas), based on C2 dimer insertion into the growing film, results in low activation energy for growth of UNCD on various substrates down to a record low temperature of ~350 °C. We demonstrated the fabrication of high-resolution UNCD-based 2-D and 3-D MEMS components, such as micro-gears, pinwheels,cantilevers, strain-gauges, and a microturbine, via growth of UNCD on Si and sacrificial SiO₂ layers, and selective etching. UNCD coatings can be grown conformally on high aspect ratio Si structures. UNCD coatings exhibit excellent mechanical and tribological properties, in addition to extremely low threshold voltage for electron field emission, which allows to produce MEMS sensors using the uniquely combined mechanical/electron emission properties of UNCD. We will discuss fabrication issues and UNCD properties applicable to MEMS. Work supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38.
10:00 AM		MM-ThM-6 Silicon Carbide Films by Low Temperature CVD for MEMS Applications D. Gao, C.R. Stoldt, W.R. Ashurst, C. Carraro, R. Maboudian (University of California, Berkeley) The single source CVD precursor, 1,3-disilabutane, is used to grow polycrystalline cubic silicon carbide (SiC) films for MEMS applications at temperatures below 1000 C. Using this process, SiC films are integrated into surface and bulk micromachining technologies to obtain SiC-based micromechanical structures. SiC cantilever beam arrays and strain gauges are fabricated and used to characterize film stress and stress gradients. Also, released polysilicon microstructures are coated with thin SiC films, and exhibit superior physicochemical characteristics. For instance, SiC-coated lateral resonators are functional after HF and hot KOH treatments and display increased resonant frequencies.
10:20 AM		MM-ThM-7 Thermal Characteristics of Microswitch Contacts X. Yan, N.E. McGruer, G.G. Adams (Northeastern University); S. Majumder (Analog Devices, Inc.) Electrostatically actuated microswitches and relays developed at Northeastern University are approximately 100 x 100 µm in size and have been tested beyond 10⁹ cycles with a current of 2 mA per contact. For gold-gold contacts, the microswitches fail in a permanently closed mode in less than 10 cycles for currents exceeding 300 mA. At currents of approximately 1 A, the drain electrode melts, resulting in a permanently open switch. A number of authors have reported on various aspects of heat conduction through larger contacts. Hyman and Mehregany have discussed the contact physics of microcontacts, and modeled their thermal behavior.¹ However, they do not consider the effect of the thin film traces leading up to the contacts in most MEMS switch designs. Finite element modeling and experiments have been used to study the thermal characteristics of microswitches. Because of the asymmetry in the contact geometry, the highest temperature is located in the thin film contact trace rather than at the contact interface. Contributions from convection and radiation are negligible, and conduction through the gas is marginally important. The hottest spot moves away from the contact as the contact radius increases, from 0.3 µm for a 100 nm contact radius to 2.7 µm for a 500 nm contact radius. Measurements show a sharp decrease in the contact resistance at a switch voltage of about 0.08 V and a current of 0.15 A, which may be due to softening of the contact surfaces and/or removal of surface contaminants. The result is in rough agreement with the onset of softening predicted by the model. The contact trace melts at a switch current of 1 A. The melted region is between 3.5 and 6 µm away from the center of the contact, slightly further than is predicted by the model. ¹Daniel Hyman and Mehran Mehregany, Proc. of 44th IEEE Holm Conf. on Electrical Contacts (Arlington, VA, October 26-28, 1998, pp.133-140).
10:40 AM		MM-ThM-8 MBE-grown Single-crystal Ferromagnetic Shape Memory Ni₂MnGa Thin Films J.W. Dong, J. Lu, J.Q. Xie, Q. Pan, J. Cui, S. McKernan, R.D. James, C.J. Palmstrom (University of Minnesota) Ni₂MnGa is a ferromagnetic shape memory alloy which goes through a thermodynamically reversible martensitic phase transformation and demonstrates ferromagnetic properties. In bulk, Ni₂MnGa with the stoichiometric composition has a Curie temperature ~376 K and the martensitic phase transformation occurs ~202 K. Above 202 K, Ni₂MnGa adopts a cubic L2₁ crystal structure with weak magnetic anisotropy. Below 202 K, it will transform to a tetragonal structure with greatly enhanced magnetic anisotropy. In this low-symmetry tetragonal phase, a twinning structure will be formed by three types of martensitic variants with different magnetic easy axes. External magnetic and/or stress fields can be employed to adjust the volume fraction of the twinned martensitic variants by the motion of twin boundaries. This will result in large reversible strain and this novel mechanism is thus called ferromagnetic shape memory effect. In bulk single crystals of Ni₂MnGa, strain as large as 6.1% has been demonstrated. This makes it a promising candidate for magnetic field driven actuator material. For micro-electro-mechanical-system (MEMS) actuators, conceptual designs based on single-crystal Ni₂MnGa films have been proposed. The first single-crystal growth of Ni₂MnGa thin film has been reported by the authors.¹ The 300 Å-thick film grows pseudomorphically on a (001) GaAs substrate with a unique tetragonal structure (a = b = 5.65 Å, c = 6.12 Å). The Curie temperature was measured to be ~320 K. Moreover, martensitic phase transformation is observed in a partially released 450 Å-thick epitaxial film. In this presentation, we will report the growth, characterization, and patterning of 900 Å-thick single-crystal Ni₂MnGa films to elucidate the concept of using it in MEMS actuators. ¹ J.W. Dong, L.C. Chen, C.J. Palmstrom, R.D. James, and S. McKernan, Appl. Phys. Lett., 75, pp. 1443-1445 (1999).
11:00 AM		MM-ThM-9 Stability of Alkylsilane Monolayer Films in Humid Environments T.M. Mayer, H.I. Kim, M.G. Hankins, M.P. de Boer (Sandia National Laboratories) Alkylsilane monolayer films on SiO₂ are used to prevent adhesion in micromechanical (MEMS) devices. We have studied the stability of these films in humid environments, where degradation may lead to loss of hydrophobic character, water adsorption, and adhesion of MEMS components by capillary condensation. In this work we study silane monolayer films with both fluorocarbon and hydrocarbon side chains, deposited by both solution and chemical vapor deposition methods. In-situ ellipsometry and interfacial force microscopy measurements examine water vapor adsorption and its effect on adhesion and friction. Ex-situ atomic force microscopy and x-ray reflectivity measurements examine the morphology and density of the films before and after exposure. We find that chemical binding of the film to the surface is critical for its stability. Silanol films are not strongly bound to the surface and exhibit substantial water adsorption. This is accompanied by an irreversible increase in friction when probed with a similarly functionalized tip. In the presence of high humidity at room temperature, the silanol film restructures to form small droplets on the surface, leading to increased adhesion in cantilever beam MEMS test structures. In contrast, silanol films that have been annealed to react with surface hydroxyls are strongly bound to the surface and display negligible water adsorption, no effect on adhesion or friction, and no surface restructuring after exposure to high humidity (>80% RH) for short periods (10 hr) at room temperature. Stability of these films after more severe exposure (longer times at higher temperature), mechanisms of degradation, and long-term effects on the performance and reliability of MEMS devices will be addressed. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the U. S. Dept. of Energy under contract DE-AC04-94AL85000.