Chapter 1 Introduction

Transcription

1 Chapter 1 Introduction MEMS switches and relays have many properties that make them promising candidates to replace conventional relays or solid-state switches in a number of low-power applications. They are smaller and can switch faster than the smallest existing mechanical relays (reed relays). They also have lower on-resistance and lower parasitic capacitances than transistor switches. Also, electrostatically actuated MEMS switches consume practically zero steady-state power, unlike mechanical relays and PIN diode switches. Potential applications of MEMS switches include RF applications such as in cell phones, phase shifters and smart antennas [Rebeiz 2001], as well as relay-replacements in Automatic Test Equipment (ATE) and industrial and medical instrumentation. Most MEMS switches reported in the literature use one of two actuation mechanisms electrostatic and electromagnetic. Electrostatic switches generally consume less power than electromagnetic switches; electromagnetic switches usually draw steady-state power in the on-state (an exception is the device reported by Ruan, et al [Ruan 2001], which uses a permanent magnet in the steady-state). Also, electrostatic switches are generally smaller and faster, and easier to build. On the other hand, electromagnetic switches have higher contact forces, and therefore have lower and more reliable contact resistance, particularly when operated in air. Therefore, the two categories of switches are suited for different sets of applications, although there may be some overlap. 1

2 A fundamental constraint in electrostatic microswitches is the low available actuation force. In most devices, the electrostatic actuator can be treated approximately as a parallel-plate capacitor, with a movable plate attached to the substrate by a spring, and a fixed electrode (Figure 1.1). Applying a voltage V between the two plates results in an electrostatic force which pulls the movable plate towards the fixed plate, and closes a path between two fixed contacts. When the voltage is removed, the restoring force in the spring returns the movable plate to its original position, and opens the circuit. The electrostatic force is given by F 2 εav = 2, 2x( V ) (1.1) where A is the overlap area between the two plates, x(v) is the gap between the two plates for a given V, and ε is the permittivity of the gap medium. Typically, the area A is mm 2, the gap x(v) in a closed switch is µm, and the actuation voltage is 100 V x 0 x(v) V (a) (b) Figure 1.1 Spring-capacitor model of an electrostatically actuated microswitch. 2

3 or less. These values are constrained by the drive voltage available in an application, and the fabrication-related difficulty of maintaining a small gap over a large area. With an air gap, the actuation force is therefore typically no more than a few mn, and often several hundred µn. Part of the available actuation force deflects the beam to close the switch, and the rest of the actuation force provides the contact force. Therefore, both the contact force and the spring force are usually limited to several hundred µn. The corresponding forces are generally 2-3 orders of magnitude higher in reed relays, and 1-2 orders of magnitude higher in MEMS electromagnetic relays. There are two implications. Because of the low contact force, it is difficult to obtain a low and stable contact resistance at the least, an inert, hermetic environment is required. At the same time, because of the low spring force, adhesion between the contact surfaces (due to van der Waal forces, or metallic bonds) becomes an important consideration; in the extreme case, the adhesive force between the contacts exceeds the restoring spring force, holding the beam in its on position even when the drive voltage is removed, and causing a stuck-closed failure. The two problems (low contact force and low restoring force) are inter-related. Within the constraint of a certain available actuation force, the designer may try to obtain a higher contact force - thereby reducing the restoring force, and also creating a more intimate contact, which in turn tends to make the switch more susceptible to the adhesion force. Also, a low available contact force tends to encourage the choice of gold as a contact material, because it is soft (resulting in a larger contact area and more intimate contact for a given contact force), and inert (resulting in a clean, truly metallic contact surface). However, as will be discussed later, a large contact area and a clean-metal contact increase the adhesion force, and increase the possibility of contact stiction. 3

4 The objective of this thesis is to develop a basic understanding of adhesion in microswitch contacts, and how it affects microswitch performance. To support this work, I have used measurements of a microswitch with gold-on-gold contacts, developed at Northeastern University [Zavracky 1997]. Our experience with stiction in gold contacts has shown that pure gold is probably not the right material for microswitch contacts (we have since had more success with other contact materials). However, for the same reason, a gold micro-contact is a good vehicle for the study of contact adhesion. Also, it is relatively easy to obtain a clean gold surface, avoiding the complications introduced by surface contaminants or films. In the rest of this chapter, I will provide a brief introduction to the Northeastern University microswitch. I will also discuss briefly published work in the field of microswitch contacts. 1.1 The device Figure 1.2 is a schematic representation of a microswitch, and Figure 1.3 shows an SEM micrograph of a fabricated device. The microswitch is based on a double cantilever beam, and has 3 electrical terminals, labeled source, gate and drain in Figures 1.2 and 1.4. Applying a voltage between the gate and source terminals creates an electrostatic force between the gate electrode and the cantilever, and pulls the beam down to create electrical contact between the drain and source terminals. Contact between the beam and the drain electrode occurs through a pair of bumps on the lower surface of the beam, near 4

5 its free end. Figure 1.3 shows a detailed view of a contact bump on the bottom of a beam that was flipped over with a probe. Typical dimensions are marked in the schematic of Figure 1.5. Devices measured for this work had a slightly modified version of this geometry, with an extra pair of source and drain terminals to permit Kelvin measurements of the contact resistance. A schematic of this geometry is shown in Chapter 2. The device shown in Figure 1.5 just closes at a gate-to-source voltage of about V. This is referred to as the threshold voltage. The device is generally operated at a gate-tosource voltage of V, corresponding to a contact force range (at each contact bump) of µn. When the gate voltage is removed, the spring force, or restoring force acting on the cantilever beam to restore it to its original position is about 140 µn, or 70 µn per contact bump. The switch is actuated in a nitrogen ambient, at atmospheric pressure, with current being applied to the contacts during each cycle only when the switch is in a closed position. The current through the switch is between 4 ma and 20 ma. Switches with gold-on-gold contacts typically have an initial contact resistance between Ω. Cycling the switches results in a reduction in the contact resistance over the first switching cycles, to Ω. The lifetime of the switches is typically of the order of 10 6 cycles, and is limited by permanent stuck-closed failures. Microswitches with 2 contact bumps usually have a shortened lifetime if the current though the microswitch exceeds 20mA. 5

6 Contact Detail Gate Drain Beam Source 100 µm Figure 1.2 SEM micrographs of the Northeastern University microswitch. A simplified process flow for the fabrication of the microswitch is shown in Figure 1.6. Thin layers of chromium (200 A) and gold ( µm) are first deposited, and the source, gate, and drain are defined by wet etch (Figure 1.6a). Next, a sacrificial layer (0.6 µm copper) is deposited, to define the gap between the beam and the bottom electrodes (Figure 1.6b). The sacrificial layer is then etched to define the beam anchor, and then partially etched to define the contact bump (Figure 1.6c). A thin layer of gold (0.1 µm) is deposited and patterned in the shape of the beam, to serve as a seed layer for electroplating. The beam itself is then formed by electroplating (Figure 1.6d). Finally the sacrificial layer is wet-etched to free the beam (Fig. 1.6e). 6

7 Figure 1.3 SEM micrograph showing close-up of a contact bump on a microswitch that was flipped over. 1.2 Related work There is a large body of work on microscopic contacts, and also specifically on metallic contacts. Much of this work is based on true single point contacts (such as those obtained in Atomic Force Microscopes), and most of it is concerned solely with mechanical contact. This work will be referred to in Chapter 2 and 3, in relation to the model developed in this work. There is relatively little literature that deals specifically with microswitch or microrelay contacts. Work in this area is briefly discussed below. 7

8 Preliminary versions of the adhesionless model presented in Chapter 2 have been reported in [Majumder 1997a, 1997b, 1998, 2001]. A preliminary version of the model including contact adhesion was presented in [Majumder 1999]. High current measurements and thermal modeling of microswitch contacts have been reported by Hyman and Mehregany [Hyman 1998], and by Yan, et al [Yan 2001]. Hyman and Mehregany studied contact between gold plated tungsten tips (of radii about µm), and a thin (unspecified thickness) layer of plated gold on a silicon substrate. They showed experimental evidence of material transfer at high currents (up to 200 ma through a 100 mω contact). They also calculated the temperature distribution at the contact based on a finite-difference model, assuming a single contact spot and heat generation only at the contact interface. Yan, et al studied the contact of the Northeastern microswitch, with a similar contact geometry as that studied in this work. They found G S DRAIN GATE SOURCE D DRAIN GATE SOURCE Figure 1.4 Schematic of the microswitch. 8

9 experimentally and through finite element modeling that the thermal constriction could occur in the drain trace rather than at the contact interface. They also calculated temperature distributions at various currents. Other authors have used simpler contact models, generally from the viewpoint of device design. Kruglick, et al reported a device with lateral actuation and gold contacts, and applied simple thermal and contact resistance models to their contact resistance measurements [Kruglick 1999]. Lafontan, et al reported a microswitch with gold contacts, and extracted the contact area from a simple contact resistance model, assuming no adhesion [Lafontan 2001]. In both cases, the reported contact resistance was high (1 Ω or greater), and a clean-metal model may not have been applicable. Φ= DRAIN GATE SOURCE Figure 1.5 Dimensions of microswitch, in microns. 9

10 There has also been some work in looking at contact resistance in the force range applicable to microswitches, using an experimental set-up instead of real devices. This allows direct measurement of the contact force, and also allows the force to be varied over a larger range. Beale and Pease carried out such a study in 1992, using a tungsten AFM tip on a gold surface. An experimental study of contact resistance in the 100 µn - 10 mn force range was reported by Schimkat [Schimkat 1998], for three different contact metals gold, AuNi5, and Rh. Contact resistance was reported as a function of contact force. Adherence force was also reported, although the contact force from which the unloading took place was not clearly specified. An experimental study of gold-gold contact resistance as a function of contact force over the force range 200 µn 10 mn was reported by Bromley and Nelson [Bromley 2001]. In both the above papers, the effect of contact cycling on resistance and adherence was not reported. Since the first MEMS switch reported by Petersen in 1979 [Petersen 1979], many other electrostatic and electromagnetic devices have been reported, most of which have relatively little emphasis on characterization and modeling of the contacts. Electrostatically actuated devices reported since 1995 include [Yao 1995, Randall 1996, Wright 1996, Schlaak 1996, Taylor 1996, Hiltmann 1999, Schiele 2001, Komura 2001]. Electromagnetically actuated devices include [Taylor 1996, Schiele 2001, Komura 2001]. 1.3 Organization of this thesis The rest of the thesis is organized in three chapters. In Chapter 2, I will develop a contact resistance model that ignores adhesion, and discuss the discrepancies with measurements 10

11 resulting from this assumption. I will then develop a model incorporating adhesion in Chapter 3, and compare the model with measurements. In Chapter 4, I will summarize and discuss future work, including effects not considered by the current model. References [Rebeiz 2001] G. M. Rebeiz and J. B. Muldavin, RF MEMS Switches and Switch Circuits, IEEE Microwave Magazine, Dec. 2001, pp [Ruan 2001] M. Ruan, J. Shen, C. B. Wheeler, Latching micromagnetic relays, Journal of Microelectromechanical Systems, Vol. 10, pp , [Zavracky 1997] P.M. Zavracky, N.E. McGruer, and S. Majumder, Micromechanical Switches Fabricated using Nickel Surface Micromachining, Journal of Microelectromechanical Systems, Vol. 6, pp. 3-9, [Majumder 1997a] S. Majumder, P.M. Zavracky, N.E. McGruer, Electrostatically Actuated Micromechanical Switches, Journal of Vacuum Science and Technology B, v 15, pp. 1246, [Majumder 1997b] S. Majumder, N. E. McGruer, P.M. Zavracky, G. G. Adams, R. H. Morrison, J. Krim, Measurement And Modeling Of Surface Micromachined, Electrostatically Actuated Microswitches, Transducers 97, Chicago, IL (1997). [Majumder 1998] S. Majumder, N.E. McGruer, P.M. Zavracky, G.G. Adams, R.H. Morrison, and J. Krim, Contact Resistance in Electrostatically Actuated Micromechanical Switches, Proceedings of the 44th IEEE Holm Conference on Electrical Contacts, pp , 1998, Washington, DC. [Majumder 2001] S. Majumder, N.E. McGruer, G.G. Adams, A.P. Zavracky, P.M. Zavracky, R.H. Morrison and J. Krim, Study of Contacts in an Electrostatically Actuated Microswitch, Sensors and Actuators A, Vol. 93, pp , [Majumder 1999] S. Majumder, N.E. McGruer, G.G. Adams, P.M. Zavracky, R.H. Morrison, and J. Krim, Adhesion Properties of Gold-on-Gold Microswitch Contacts, American Vacuum Society, 46 th Annual Symposium Abstracts, pp. 141,

12 [Schimkat 1998] J. Schimkat, Contact materials for microrelays, Proceedings of the 1998 IEEE 11 th Annual Workshop on Micro Electro Mechanical Systems, January 25-29, 1998, Heidelberg, Germany, pp [Hyman 1998] D. Hyman and M. Mehregany, Contact physics of gold microcontacts for MEMS switches, Proceedings of the th Holm Conference on Electrical Contacts, October 26-28, 1998, Arlington, VA, pp [Yan 2001] X. Yan, N. E. McGruer, G. G. Adams, S. Majumder, "Thermal Characteristics of Microswitch Contacts," Proceedings of the National Association of Relay Manufacturer s (NARM) 49th Annual International Relay Conference, April 23-25, [Kruglick 1999] E. J. J. Kruglick and K. S. J. Pister, Lateral MEMS microcontact considerations, Journal of Microelectromechanical Systems, Vol. 8, 1999, pp [Lafontan 2001] X. Lafontan, C. Dufaza, M. Robert, F. Pressecq, G. Perez, Concepts, characterization and modeling of MEMS micro-switches with gold contacts in MUMPS, Proceedings of SPIE The International Society for Optical Engineering, v 4408, 2001, Design, Test Integration and Packaging of MEMS/MOEMS 2001, April 25-27, 2001, Cannes, France, pp [Beale 1992] J. P. Beale and R.F.W. Pease, "Apparatus for Studying Ultrasmall Contacts", Proceedings of the 38 th IEEE Holm Conference on Electrical Contacts, Philadelphia, PA (1992). [Petersen 1979] K. E. Petersen, Micromechanical membrane switches on silicon, IBM Journal of Research and Development, Vol. 23, pp , [Yao 1995] J. J. Yao and M. F. Chang, A surface micromachined miniature switch for telecommunications applications with signal frequencies from DC up to 4 GHz, The 8 th International Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, June 25-29, [Randall 1996] J. N. Randall, C. Goldsmith, D. Denniston, T-H. Lin, Fabrication of micromechanical switches for routing radio frequency signals, Journal of Vacuum Science and Technology B, v 14, n 6,

13 [Wright 1996] J. A. Wright, Y-C. Tai, S-C. Chang, Large-force, fully integrated MEMS magnetic actuator, Proceedings of the 1997 International Conference on Solid-State Sensors and Actuators, Part 2, Jun 16-19, 1997, Chicago, IL, USA, pp [Schlaak 1996] H. F. Schlaak, F. Arndt, J. Schmikat, M. Hanke, Silicon-microrelay with electrostatic moving wedge actuator new functions and miniaturisation by micromechanics, Proceedings of Fifth International Conference on Microelectrooptomechanical Systems and Components, Potsdam, pp , [Hiltmann 1999] K. Hiltmann, W. Keller, W. Lang, Micromachined switches for low electric loads, Sensors and Actuators A, Vol. 74, pp , [Taylor 1996] W. P. Taylor, M. G. Allen, C. R. Dauwalter, A fully integrated magnetically actuated micromachined relay, Solid-state Sensor and Actuator Workshop, Hilton Head, South Carolina, June 2-6, 1996, pp [Schiele 2001] I. Schiele, J. Huber, B. Hillerich, F. Kozlowski, Surface-micromachined electrostatic microrelay, Sensors and Actuators A, Vol. 66, pp , [Komura 2001] Y. Komura, M. Sakata, T. Seki, K. Sano, S. Horiike, K. Ozawa, Micro machined relay for high frequency applications, Proceedings of the National Association of Relay Manufacturer s (NARM) 49th Annual International Relay Conference, April 23-25,

14 Source Gate Drain (a) (b) (c) (d) (e) Figure 1.6 Simplified fabrication process flow of microswitch. 14