Bhushan, B. “Micro/Nanotribology and Micro/Nanomechanics of MEMS .” Handbook of Micro/Nanotribology. Ed. Bharat Bhushan Boca Raton: CRC Press LLC, 1999 © 1999 by CRC Press LLC 16 Micro/Nanotribology and Micro/Nanomechanics of MEMS Devices Bharat Bhushan 16.1 Introduction Background • Tribological Issues 16.2 Experimental Techniques Description of Apparatus and Test Procedures • Test Samples 16.3 Results and Discussion Micro/Nanotribological Studies of Virgin, Coated, and Treated Silicon Samples • Micro/Nanotribological Studies of Doped and Undoped Polysilicon Films, SiC Films, and Their Comparison to Single-Crystal Silicon • Macroscale Tribological Studies of Virgin, Coated, and Treated Samples • Boundary Lubrication Studies • Component Level Studies 16.4 Closure References 16.1 Introduction 16.1.1 Background The advances in silicon photolithographic process technology since 1960s have led to the development of microcomponents or microdevices, known as microelectromechanical systems (MEMS). More recently, lithographic processes have been developed to process nonsilicon materials. These lithographic processes are being complemented with nonlithographic micromachining processes for fabrication of milliscale components or devices. Using these fabrication processes, researchers have fabricated a wide variety of miniaturized devices, such as acceleration, pressure and chemical sensors, linear and rotary actuators, electric motors, gear trains, gas turbines, nozzles, pumps, fluid valves, switches, grippers, tweezers, and optoelectronic devices with dimensions in the range of a couple to a few thousand microns (for an early review, see Peterson, 1982; for recent reviews, see Muller et al., 1990; Madou, 1997; Trimmer, 1997; and Bhushan, 1998a). MEMS technology is still in its infancy and the emphasis to date has been on the fabrication and laboratory demonstration of individual components. MEMS devices have begun © 1999 by CRC Press LLC to be commercially used, particularly in the automotive industry. Silicon-based high- g acceleration sensors are used in airbag deployment (Bryzek et al., 1994). Acceleration sensor technology is slightly less than a billion-dollar-a-year industry dominated by Lucas NovaSensor and Analog Devices. Texas Instruments uses deformable mirror arrays on microflexures as part of airline-ticket laser printers and high-resolution projection devices. Potential applications of MEMS devices include silicon-based acceleration sensors for anti-skid braking systems and four-wheel drives, silicon-based pressure sensors for monitoring pressure of cylinders in automotive engines and of automotive tires, and various sensors, actuators, motors, pumps, and switches in medical instrumentation, cockpit instrumentation, and many hydraulic, pneumatic, and other con- sumer products (Fujimasa, 1996). MEMS devices are also being pursued in magnetic storage systems (Bhushan, 1996a), where they are being developed for supercompact and ultrahigh-recording-density magnetic disk drives. Horizontal thin-film heads with a single-crystal silicon substrate, referred to as silicon planar head (SPH) sliders are mass-produced using integrated-circuit technology (Lazarri and Deroux-Dauphin, 1989; Bhushan et al., 1992). Several integrated head/suspension microdevices have been fabricated for contact recording applications (Hamilton, 1991; Ohwe et al., 1993). High-bandwidth servo-controlled microactuators have been fabricated for ultrahigh-track-density applications which serve as the fine-position control element of a two-stage, coarse/fine servo system, coupled with a conventional actuator (Miu and Tai, 1995; Fan et al., 1995b). Millimeter-sized wobble motors and actu- ators for tip-based recording schemes have also been fabricated (Fan and Woodman, 1995a). In some cases, MEMS devices are used primarily for their miniature size, while in others, as in the case of the air bags, because of their high reliability and low-cost manufacturing techniques. This latter fact has been possible since semiconductor-processing costs have reduced drastically over the last decade, allowing the use of MEMS in many previously impractical fields. The fabrication techniques for MEMS devices employ photolithography and fall into three basic categories: bulk micromachining, surface micromachining, and LIGA a German acronym (Lithographie Galvanoformung Abformung) for lithography, electroforming, and plastic molding. The first two approaches, bulk and surface micromachining, use planar photolithographic fabrication processes devel- oped for semiconductor devices in producing two-dimensional (2D) structures (Jaeger, 1988; Madou, 1997; Bhushan, 1998a). Bulk micromachining employs anisotropic etching to remove sections through the thickness of a single-crystal silicon wafer, typically 250 to 500 µm thick. Bulk micromachining is a proven high-volume production process and is routinely used to fabricate microstructures such as acceleration and pressure sensors and magnetic head sliders. Surface micromachining is based on depos- iting and etching structural and sacrificial films to produce a free-standing structure. These films are typically made of low-pressure chemical vapor deposition (LPCVD) polysilicon film with 2 to 20 µm thickness. Surface micromachining is used to produce surprisingly complex micromechanical devices such as motors, gears, and grippers. LIGA is used to produce high-aspect ratio (HAR) MEMS devices that are up to 1 mm in height and only a few microns in width or length (Becker et al., 1986). The LIGA process yields very sturdy 3D structures due to their increased thickness. The LIGA process is based on the combined use of X-ray photolithography, electroforming, and molding processes. One of the limi- tations of silicon microfabrication processes originally used for fabrication of MEMS devices is lack of suitable materials which can be processed. With LIGA, a variety of nonsilicon materials such as metals, ceramics and polymers can be processed. Nonlithographic micromachining processes, primarily in Europe and Japan, are also being used for fabrication of millimeter-scale devices using direct material microcutting or micromechanical machining (such as micromilling, microdrilling, microturning) or removal by energy beams (such as microspark erosion, focused ion beam, laser ablation, and machining, and laser polymerization) (Friedrich and Warrington, 1998; Madou, 1998). Hybrid technologies including LIGA and high-precision micromachining techniques have been used to produce miniaturized motors, gears, actuators, and connectors (Lehr et al., 1996, 1997; Michel and Ehrfeld, 1998). These millimeter- scale devices may find more immediate applications. © 1999 by CRC Press LLC Silicon-based MEMS devices lack high-temperature capabilities with respect to both mechanical and electrical properties. Recently, researchers have been pursuing SiC as a material for high-temperature microsensor and microactuator applications (Tong et al., 1992; Shor et al., 1993). SiC is a likely candidate for such applications since it has long been used in high-temperature electronics, high-frequency and high-power devices, such as SiC metal–semiconductor field effect transistors (MESFETS) (Spencer et al., 1994) and inversion-mode metal-oxide-semiconductor field effect transistors (MOSFETS). Many other SiC devices have also been fabricated including ultraviolet detectors, SiC memories, and SiC/Si solar cells. SiC has also been used in microstructures such as speaker diaphragms and X-ray masks. For a summary of SiC devices and applications, see Harris (1995). Table 16.1 compares selected bulk properties of SiC and Si(100). Because of the large band gap of SiC, almost all devices fabricated from SiC have good high- temperature properties. This high-temperature capability of SiC combined with its excellent mechanical properties, thermal dissipative characteristics, chemical inertness, and optical transparency makes SiC an ideal choice for complementing polysilicon (polysilicon melts at 1400°C) in MEMS devices. Since MEMS devices need to be of low cost to be viable in most applications, researchers have found low-cost techniques of producing single-crystal 3C-SiC (cubic or β -SiC) films via epitaxial growth on large area silicon substrates (Zorman et al., 1995). This technique allows high-volume batch processing and has the advantage of having silicon as the substrate, an inexpensive material for which microfabrication and micromachining technologies are well established. It is believed that these films will be well suited for MEMS devices. 16.1.2 Tribological Issues In MEMS devices, various forces associated with the device scale down with the size. When the length of the machine decreases from 1 mm to 1 µm, the area decreases by a factor of a million and the volume decreases by a factor of a billion. The resistive forces such as friction, viscous drag, and surface tension that are proportional to the area, increase a thousand times more than the forces proportional to the volume, such as inertial and electromagnetic forces. The increase in resistive forces leads to tribological concerns, which become critical because friction/stiction (static friction), wear and surface contamination affect device performance and in some cases, can even prevent devices from working. Examples of two micromotors using polysilicon as the structural material in surface micro- machining — a variable capacitance side drive and a wobble (harmonic) side drive — are shown in Figures 16.1 and 16.2, which can rotate up to 100,000 rpm. Microfabricated variable-capacitance side- drive micromotor with 12 stators and a 4-pole rotor shown in Figure 16.1 is produced using a three- layer polysilicon process and the rotor diameter is 120 µm and the air gap between the rotor and stator is 2 µm (Tai et al., 1989). It is driven electrostatically to continuous rotation (by electrostatic attraction between positively and negatively charged surfaces). The intermittent contact at the rotor–stator interface and physical contact at the rotor–hub flange interface result in wear issues, and high stiction between the contacting surfaces limits the repeatability of operation or may even prevent the operation altogether. Figure 16.2 shows the SEM micrograph of a microfabricated harmonic side-drive (wobble) micromotor TABLE 16.1 Selected Bulk Properties a of 3C ( β - or cubic) SiC and Si(100) Sample Density (kg/m 3 ) Hardness (GPa) Elastic Modulus (GPa) Fracture Toughness (MPa m 1/2 ) Thermal Conductivity b (W/m K) Coeff. of Thermal Expansion b ( × 10 –6 /°C) Melting Point (°C) Band-Gap (eV) β –SiC 3210 23.5–26.5 440 4.6 85–260 4.5–6 2830 2.3 Si(100) 2330 9–10 130 0.95 155 2–4.5 1410 1.1 a Unless stated otherwise, data shown were obtained from Bhushan and Gupta (1997). b Obtained from Shackelford et al. (1994). © 1999 by CRC Press LLC (Mehregany et al., 1988). In this motor, the rotor wobbles around the center bearing post rather than the outer stator. Again friction/stiction and wear of rotor-center bearing interface are of concern. There is a need for development of bearing/bushing materials that are both compatible with MEMS fabrication processes and which provide superior friction and wear performance. Monolayer lubricant films are also of interest. Figure 16.3 shows the SEM micrograph of an air turbine with gear or blade rotors, 125 to 240 µm in diameter, fabricated using polysilicon as the structural material in surface micromachining. The two flow channels on the top are connected to the two independent input ports and the two flow channels at the bottom are connected to the output port. Wear at the contact of gear teeth is a concern. In microvalves used for flow control, the mating valve surfaces should be smooth enough to seal while FIGURE 16.1 (a) SEM micrograph, and (b) schematic cross-section of a variable capacitance side-drive micromotor fabricated of polysilicon film. (From Tai et al., 1989, Sensors Actuators A21–23, 180–83. With permission.) FIGURE 16.2 SEM micrograph of a harmonic side-drive (wobble) micromotor. (From Mehregany, M. et al., 1990, in Proc. IEEE Micro Electromechanical Systems, pp. 1–8, IEEE, New York. With permission.) © 1999 by CRC Press LLC maintaining a minimum roughness to ensure low friction/stiction (Bhushan, 1996a, 1998b). Studies have been conducted to measure the friction/stiction in micromotors (Tai and Muller, 1990), gear systems (Gabriel et al., 1990) and polysilicon microstructures (Lim et al., 1990) to understand friction mechanisms. Several studies have been conducted to develop solid and liquid lubricant and hard films to minimize friction and wear (Bhushan et al., 1995b; Deng et al., 1995; Beerschwinger et al., 1995; Koinkar and Bhushan, 1996a,b; Bhushan, 1996b; Henck, 1997). In a silicon planar head slider for magnetic disk drives shown in Figure 16.4, wear and friction/stiction are an issue because of the close proximity between the slider and disk surfaces during steady operation and continuous contacts during start and stops (Lazzari and Deroux-Dauphin, 1989; Bhushan et al., 1992). Hard diamondlike carbon (DLC) coatings are used as an overcoat for protection against corrosion and wear. Two electrostatically driven rotary and linear microactuaters (surface-micromachined, poly- silicon microstructure) for a magnetic disk drive shown in Figure 16.5, consist of a movable plate connected only by springs to a substrate, on which there are two sets of mating interdigitated electrodes which activate motion of the plate in opposing directions. Any unintended contacts may result in wear and stiction. Figure 16.6 shows an SEM micrograph of a micromechanical switch (Peterson, 1979). As the voltage is applied between the deflection electrode and the p + ground plane, the cantilever beam is deflected and the switch closes, connecting the contact electrode and the fixed electrode; wear during contact is of concern. Figure 16.7 shows an SEM micrograph of a pair of tongs (Mehregany et al., 1988). The jaws open when the linearly sliding handle is pushed forward, demonstrating the linear slide and the linear- to-rotary motion conversion; for this pair of tongs, the jaws open up to 400 µm in width. Wear at the teeth is of concern. As an example of nonsilicon components, Figure 16.8a shows a DC brushless permanent magnet millimotor (diameter = 1.9 mm, length = 5.5 mm) with an integrated milligear box which is produced with parts obtained by hybrid fabrication processes including the LIGA process, micromechanical machining, and microspark erosion techniques (Lehr et al., 1996, 1997; Michel and Ehrfeld, 1998). The motor can rotate up to 100,000 rpm and deliver a maximum torque of 7.5 µNm. The rotor, supported on two ruby bearings, consists of a tiny steel shaft and a diametrically magnetized rare earth magnet. The rotational speed of the motor can be converted by the use of a milligear box to increase the torque for a specific application. Gears are made of metal (e.g., electroplated Ni–Fe) or injected polymer materials (e.g., POM) using the LIGA process, Figures 16.8b and c. Optimum materials and liquid and solid lubrication approaches for bearings and gears are needed. FIGURE 16.3 SEM micrograph of a gear train with three meshed gears, in an air turbine. (From Mehregany, M. et al., 1988, IEEE Trans. Electron Devices 35, 719–723. With permission.) © 1999 by CRC Press LLC There are tribological issues in the fabrication processes as well. For example, in surface microma- chining, the suspended structures can sometimes collapse and permanently adhere to the underlying substrate, Figure 16.9 (Guckel and Burns, 1989). The mechanism of such adhesion phenomena needs to be understood (Mastrangelo, 1997). Friction/stiction and wear clearly limit the lifetimes and compromise the performance and reliability of microdevices. Since microdevices are designed to small tolerances, environmental factors, surface contamination, and environmental debris affect their reliability. There is a need for development of a fundamental understanding of friction/stiction, wear, and the role of surface contamination and envi- ronment in microdevices (Bhushan, 1998a). A few studies have been conducted on the tribology of bulk silicon and polysilicon films used in microdevices (Bhushan and Venkatesan, 1993a,b; Gupta et al., 1993; Venkatesan and Bhushan, 1993, 1994; Gupta and Bhushan, 1994; Bhushan and Koinkar, 1994; Bhushan, 1996b). Mechanical properties of polysilicon films are not well characterized (Mehregany et al., 1987; Ericson and Schweitz, 1990; Schweitz, 1991; Guckel et al., 1992; Bhushan, 1995; Fang and Wickert, 1995). The advent of atomic force/friction force microscopy (AFM/FFM) (Bhushan, 1995, 1997; Bhushan et al., 1995a) has allowed the study of surface topography, adhesion, friction, wear, lubrication, and measure- ment of mechanical properties, all on a micro- to nanometer scale. Recently, microtribological studies FIGURE 16.4 Schematic (a) of a silicon planar head slider and (b) of cross section of the slider for magnetic disk drive applications. (From Bhushan, B. et al., 1992, IEEE Trans Magn. 28, 2874–2876. With permission.) © 1999 by CRC Press LLC have been conducted using the AFM/FFM on undoped and doped silicon and polysilicon films and SiC films that are used in MEMS devices (Bhushan, 1996b, 1997, 1998; Bhushan et al., 1994, 1997a,b, 1998; Li and Bhushan, 1998; Sundararajan and Bhushan, 1998). This chapter presents a review of macro- and micro/nanotribological studies of single-crystal silicon and polysilicon, oxidized and implanted silicon, doped and undoped polysilicon films and SiC films. A summary of limited component-level tests is also presented. FIGURE 16.5 Schematics of (a) a microactuator in place with magnetic head slider, and (b) top view of two electrostatic, rotary and linear microactuators (electrode tree structure). (From Fan, L.S. et al., 1995; IEEE Trans. Ind. Electron. 42, 222–233. With permission.) © 1999 by CRC Press LLC 16.2 Experimental Techniques 16.2.1 Description of Apparatus and Test Procedures 16.2.1.1 Micro/Nanoscale Tests A modified AFM/FFM (Nanoscope III, Digital Instruments, Santa Barbara, CA), was used for the micro/nanotribological studies. Surface roughness and microscale friction measurements were simulta- neously made over a scan size of 10 × 10 µm with an Si 3 N 4 tip (tip radius ~ 50 nm, cantilever stiffness ~ 0.6 N/m) sliding over the sample surface orthogonal to the long axis of the cantilever at 25 µm/s. A coefficient of friction and conversion factors for converting the friction signal voltage to force units (nN) were obtained through the methods developed previously by Bhushan and co-workers (Bhushan, 1995). The normal loads used in the friction measurements varied between 50 to 300 nN. The reported values are each an average of six separate measurements. FIGURE 16.6 SEM micrograph of single-contact and double-con- tact (with two orientations of the fixed electrodes) designs of micro- mechanical switches (Peterson, 1979). (From Peterson, K.E., 1979, IBM J. Res. Dev. 23, 376. With permission.) FIGURE 16.7 SEM micrograph of a partially released pair of tongs. (From Mehregany, M. et al., 1988, IEEE Trans. Electron. Devices 35, 719–723. With permission.) © 1999 by CRC Press LLC FIGURE 16.8 Schematics of (a) permanent magnet millimotor with integrated milligear box, (b) of wolfrom-type system made of Ni–Fe metal (Lehr et al., 1996), and (c) of multistage planetary gear system made with microinjected POM plastic showing a single gear and the gear system. (From Thurigen, C. et al., 1998, in Tribology Issues and Opportunities in MEMS, B. Bhushan, ed., Kluwer Academic, Dordrecht. With permission.) [...]... (1996b), “Nanotribology and Nanomechanics of MEMS Devices,” in Proc Ninth Annual Workshop on Micro Electro Mechanical Systems, pp 91–98, IEEE, New York Bhushan, B (1997) Micro/ Nanotribology and Its Applications, E330, Kluwer Academic, Dordrecht, Netherlands Bhushan, B (1998a), Tribology Issues and Opportunities in MEMS, Kluwer Academic, Dordrecht, Netherlands Bhushan, B (1998b), “Contact Mechanics of Rough... Martelet, C., and Belin, M (1995b), “Microtribological Characterization of Self-Assembled and Langmuir–Blodgett Monolayers by Atomic Force and Friction Force Microscopy,” Langmuir 11, 3189–3198 Bhushan, B and Gupta, B.K (1997), Handbook of Tribology: Materials, Coatings and Surface Treatments, Reprint edition, Krieger, Malabar, FL Bhushan, B and Koinkar, V.N (1997a), “Microtribological Studies of Doped... Mathieson, D., Rueben, R.L., Yang, S.J., and Taghizadeh, M (1995), “Wear at Microscopic Scales and Light Loads for MEMS Applications,” Wear 181–183, 426–435 Bhushan, B (1992), Mechanics and Reliability of Flexible Magnetic Media, Springer-Verlag, New York Bhushan, B (1995), Handbook of Micro/ Nanotribology, CRC, Boca Raton, FL Bhushan, B (1996a), Tribology and Mechanics of Magnetic Storage Devices, 2nd ed.,... speed of 0.8 mm/s c Measured using an AFM with a diamond tip of radius of 100 nm a b TABLE 16.3 Surface Roughness and Coefficients of Micro- and Macroscale Friction of Selected Samples Material Si(111) C+-implanted Si(111) rms Roughness (nm) Coefficient of Microscale Frictiona Coefficient of Macroscale Frictionb 0.11 0.33 0.03 0.02 0.18 0.18 a Versus Si N tip, tip radius of 50 nm in the load range of 10–150... Schematics of microstructures during fabrication using surface micromachining before and after removal of sacrificial/spacer layer For the scratch and wear tests, specially fabricated diamond microtips were used (Bhushan et al., 1997a; Sundararajan and Bhushan, 1998) These microtips consisted of single-crystal natural diamond, ground to the shape of a three-sided pyramid, with an apex angle of 60° and tip... Koinkar, V.N and Bhushan, B (1996b), “Microtribological Studies of Unlubricated and Lubricated Surfaces Using Atomic Force/Friction Force Microscopy,” J Vac Sci Technol A14, 2378–2391 Koinkar, V.N and Bhushan, B (1997), “Scanning and Transmission Electron Microscopies of Single-Crystal Silicon Microworn/Micromachined Using Atomic Force Microscopy,” J Mater Res,12, 3219–3224 Lazarri, J.P and Deroux-Dauphin,... and Polysilicon Films for MEMS Devices,” Sensors Actuators A 57, 91–102 Bhushan, B and Li, X (1997b), “Micromechanical and Tribological Characterization of Doped SingleCrystal Silicon and Polysilicon Films for Microelectromechanical Systems,” J Mater Res 12, 1–10 Bhushan, B., Sundararajan, S., Li, X., Zorman, C.A., and Mehregany, M (1998), Micro/ Nanotribological Studies of Single-Crystal Silicon and. .. “Diagnostic Microstructures for the Measurement of Intrinsic Strain in Thin Films,” J Micromech Microeng 2, 86–95 Gupta, B.K and Bhushan, B (1994), “Nanoindentation Studies of Ion Implanted Silicon,” Surf Coat Technol 68/69, 564–570 Gupta, B.K., Chevallier, J., and Bhushan, B (1993), Tribology of Ion Bombarded Silicon for Micromechanical Applications,” ASME J Tribol 115, 392–399 Gupta, B.K., Bhushan, B., and. .. Experimental and Theoretical Investigation of Ploughing, Cutting and Wedge Forming during Abrasive Wear,” Tribol Int 21, 51–57 Hutchings, I.M (1992), Tribology: Friction and Wear of Engineering Materials, CRC Press, Boca Raton, FL Jaeger, R.C (1988), Introduction to Microelectronic Fabrication, Vol 5, Addison-Wesley, Reading, MA Koinkar, V.N and Bhushan, B (1996a), Micro/ Nanoscale Studies of Boundary Layers of. .. (Koinkar and Bhushan, 1996a,b) and Langmuir–Blodgett and chemically grafted self-assembled monolayer films (Bhushan et al., 1995b) 16.3 Results and Discussion Reviews of five studies are presented in this section The first study compares micro/ nanotribological properties of various forms of virgin, coated, and treated silicon samples The second study is composed of similar studies conducted on SiC film and . Bhushan, B. Micro/ Nanotribology and Micro/ Nanomechanics of MEMS .” Handbook of Micro/ Nanotribology. Ed. Bharat Bhushan Boca Raton:. Raton: CRC Press LLC, 1999 © 1999 by CRC Press LLC 16 Micro/ Nanotribology and Micro/ Nanomechanics of MEMS Devices Bharat Bhushan 16.1 Introduction Background