IEEE-MEMS-2005-F

A Lateral, Self-cleaning, Direct Contact MEMS SwitchYong Shi1 and Sang-Gook Kim21Stevens Institute of Technology, Hoboken, New Jersey, USA 2Massachusetts Institute of Technology, Cambridge, Massachusetts, USAABSTRACTA novel direct contact MEMS switch has been developed with compliant lateral metal contacts to address the needs of low contact resistance and long life cycles. The device is unique in self-alignment of the contact surfaces, self-cleaning of particles generated during operations and mechanical anchoring method of the contact metal in the Su-8 switch structures. It has been demonstrated experimentally that a low contact resistance of about 0.1 Ω is maintained for up to 10 billions of cycles of operation. The switch design is compatible with our thin film PZT actuating technology and can be readily integrated into a fully functional MEMS switch.1. INSTRODUCTIONRecently RF MEMS switches have drawn great deal of research interests from both academic and industry, which was driven by their wide applications, hybrid technology and also the huge potential market. Since the first MEMS switch reported by Petersen in 1979 [1],different kinds of MEMS switch have been developed by a number of companies and universities. The switches can be classified as either vertical contact [2] or lateral contact [3], direct metal-metal contact [3] or capacitive type [4]. The actuators used for these switches are typically electrostatic [2], thermal-electric [5], electromagnetic [6] and piezoelectric [7]. The typical life cycle requirement for a RF switch for radar systems and other instrumentation systems is over 40 to 100 billion cycles [8]. At the present time, there is no commercial MEMS switch to meet all these requirements. The objective of this research is to understand the functional requirements and the failure modes of MEMS switch, and develop a cost effective, compact and highly reliable direct contact MEMS switch.2. DESIGN AND MODELINGFunctional requirements of an RF switch have been investigated systematically. There are several parameters which have been used to define the performance of an RF switch, such as cut-off frequency, isolation, insertion loss, power carrying capacity, switching speed, contact resistance and reliability etc. However, not all of these parameters are independent. Contact resistance is one of the dominant factors. Another independent parameter is the isolation that can be determined by the separation of the two switching members. The functional requirements for a RF MEMS switch can be summarized as how to provide and maintain the low contact resistance and high isolation over a long cycles of operation.Functional requirements• Provide low resistivity at contact• Prevent particles between contact surfaces• Provide low off-state capacitanceEach of the functional requirements is to be satisfied by an appropriate design parameter or solution. The new design parameters are generated from our knowledge base to meet individual functional requirement as well as not to couple other functional requirements.Design parameters• Maximize true contact area by high force piezoelectric actuation• Micro-grooved contact surface with limited sliding motion (self-cleaning)• Piezoelectric actuation with amplified strokesThe switch design is shown in Figure 1. It’s a lateral contact series switch that consists of fixed switching members, movable switching members and the position stoppers to prevent excessive contact forces. Each switching member consists of two parallel beams with associated angled contact surfaces at the tips that induce small scale sliding between fixed and movable contacts. Gold or other noble metals are to be electroplated on the sidewall of the angled contact surfaces as well as the transmission line along the beams and pads. When the two switch members meet under a linear controllable motion, the physical contacts between the two pairs of angled surfaces will create a short circuit in the transmission line from one of the fixed members to the other. As is shown in Figure 1, the arrowed path indicates the signal line, while the “In” and “Out” arrows shows the direction of the switched signal.The angled contact orientation to the actuation makes the undulated surface slide over the static surface, which pushes entrapped particles and/or generated micro-weldments into the micro-grooves. By cleaning the surface at every cycle of switching, the tribological behavior of the micro-undulated surface ensures a low contact resistance over a long life cycle. Undulation of the low friction surface was first developed by Suh [9].The sliding distance is limited by a mechanical stop, which limits the maximum contact force to prevent a premature damage of the contact. The large stroke piezoelectric actuator ensures a large off-state gap for the high isolation and a high contact force to lower the contact resistance.Figure 1:Self- cleaning lateral contact switchSwitch isolation and insertion lossThe modeling of the switch consists of the switch performances modeling, contact resistance modeling and switch-actuator coupled analysis [10]. The equivalent model of the lateral contact switch is a resistor at on state, and a capacitor at off state. Isolation can be found from the S parameter, S 21, as following [8],20222214Z C S s ω= or as01021220)(10Z C Log S Log s ω= (2- 1) where S 21 is the S parameter of transmission coefficient from port 1 to port 2; ω is the signal frequency, C s is the off-state capacity and Z 0 is the characteristic impedance. Insertion loss can be described as [8],)21(200Z R Log oss InsertionL C−−= (2- 2) where R c is the contact resistance. From the models above, it’s predicted that off-state gaps of 3~5 µ are required to ensure –47 dB isolation for up to 40 GHz signals. For a contact resistance of 0.1 Ω, the insertion loss can be as low as –0.01 dB assuming the characteristic impedance to be 50 Ω for both cases.Switch contact resistanceThe modeling of the contact resistance consists of a few more steps. First the contact surface is characterized to determine the distribution of asperity diameter and height, then a single asperity contact analysis is made to find the relation between applied pressure and the radius of the contact area. For an elastic contact, the relation follows just the Hertz’s Law. For a plastic contact, Chang’s model is used [11]. With the radius of contact, we can find the constriction resistance [12]. Finally, the total contact resistance is found by integrating all the contact asperities over the whole area.For an elastic and plastic contact, the total conductance is expressed in equations (2-3) and (2-4), respectively [11].dz z d z NRG dc )()(221211φρ∫∞−=(2- 3)dz z d z NRG cd c C )(])(2[221211φαρα∫∞+−−=(2- 4)where Gc is the total conductance; N is the number of asperities; R 1 is the radius of the asperity; ρ is the resistivity; d is the distance between two contacts; z is the Z coordinates and ϕ (z) is the probability density function of the asperities.Analytical model for the contact force-resistance relation predicts necessary contact force of 50 µN for the resistance of 0.1 Ω as shown in Figure 6 assuming the asperities on the contact surfaces have radius from 50 nm to 200 nm with normal distribution.Switch-actuator coupled analysisSwitch-actuator coupled analysis is conducted to determine the dimension of the switch, so that it exerts enough contact force as well as maintains the off-state gap of 3~5 µm as required from the isolation modeling. The strain amplified PZT actuator developed by one of the authors [13] has large stroke of 9 µm, and lateral stiffness of 47 N/m, as well as low driving voltage of less than 10.The switch-actuator pair is a coupled system and can be simplified as an actuator working against a structural spring. Assuming the switch actuation is quasi-static, the actuator and the switch can be described by (2-5) and (2-6) respectively [14].⎭⎬⎫⎩⎨⎧=⎭⎬⎫⎩⎨⎧ΘΘ−111111Q f V x C K SPZT E PZT(2- 5) s s s x K f = (2- 6)where EPZT K is the stiffness of PZT actuator under constant electric field; Θ1 is the electro-mechanical coupling term; sPZT C is the actuator capacitance under constant stress. V 1 and x 1 are the voltage applied and displacement of the actuator. f 1 and Q 1 are the external force and charge, while f s , K s and x s are the force, stiffness and displacement of the switch. The stiffness of the switch can be found by1111x K x V K PZTs −Θ=(2- 7)which can be further simplified asPZT s K x x V d K 11131−=(2- 8) where d 31 is the piezoelectric coefficient in 3-1direction.The dimensions of the switch can now be completely determined by the models above.3. FABRICATIONTwo methods to deposit Au on the sidewall of the structure have been evaluated: e-beam evaporation or electroplating. The e-beam evaporation process is simple but the deposited film is less strong and there is line of sight limit to deposit film on the sidewall of a narrow gap. Electroplating is used to form the lateral metal contacts into predefined trenches. The mold trenches should have near vertical sidewall to guarantee the plated metal walls can make a parallel ohmic contact. SU-8 has the reputation of creating near vertical sidewalls, but it is not easy to be removed due to its cross-linked epoxy structure after curing. Positive photo resist AZ9260 is easy to remove but is hard to form vertical sidewall. Su-8 is eventually chosen as both the mold and structural materials. The process steps of the MEMS switch are shown in Figure 2. From steps a) to c) in Figure 2, PZT actuator is deposited and patterned. In step d), Su8 mold is coated and patterned. In step e), electroplating of the contact metal is conducted. In step f), the electroplating mold is removed and in step g), the switch structural Su8 is coated and patterned. The device is finally released using plasma etch to remove SiO2 first and then XeF2 to etch the Silicon substrate underneath.Among several plating processes such as DC electroplating, pulsed plating and pulse reverse plating, pulsed electroplating is chosen in this paper for the acceptable deposit quality (with 1000 Hz pulse frequency and 10% duty cycle). Temperature control and the current control are the two key parameters to prevent mold de-bonding and under-plating. Bottom electrode needs to be cleaned thoroughly for plating metal into the sharp bottom corners of the 10µm thick Su-8 beam structure.Figure 2: Switch fabrication process stepsFigure 3 shows the electroplated contacts after removed from the mold. It clearly shows that the well grooved contact surfaces and the mechanical anchors to reinforce the connection between the metal and Su-8 structure. The fabricated switch contacts before release are shown in Figure 4, while the released device and the contacts are shown in Figure 5. 4. TESTING RESULTSContact resistance is measured using 4-probe method, which measures the voltage drop directly at the contacts and also the current in the circuit. Lateral movement is provided by a linear piezoelectric stack actuator. The contact force and contact resistance relationship is characterized to determine the driving force level for the cycling tests. The contact force is derived from the switch displacement, which is converted from the driving voltage to the actuator. The force-contact resistance curves shown in Figure 6 correlate very well with the theoretical prediction. The long lifecycle test shown in Figure 7 demonstrated that the contact resistance has been maintained below 0.1 Ω over1010 cycles of switching and is expected to maintain the low resistance far beyond 1012 cycles, the test of which is underway. The contact surfaces created by molded electroplating are also denser and smoother than that by e-beam evaporation as shown in Figure 8. Our switch shows that the self-cleaning works well and opens the possibility of direct contact MEMS switch for high power and low cost RF applications.Figure 3: SEM of the electroplated gold contacts and themechanical anchorsFigure 4: Fabricated device before release (a)Amplified view of the contacts (b)Figure5: Fabricated device after release (a)Amplified view of the contacts (b)5. CONCLUDING REMARKSA lateral self-cleaning direct contact MEMS switch and the in-plane piezoelectric micro-actuator for its actuation have been demonstrated. The grooved contact surfaces have shown the self-cleaning effect and a low contact resistance of 0.1 Ω has been maintained over 10 billion cycles. Major failure modes of MEMS switch of surface damages and micro-welding are largely eliminated. Further integration of the switch with the thin film PZT actuator will provide a full functional MEMS switch for wireless communication applications. Thorough modeling of contact resistance, friction and wear rate will be critical to further understand the contact physics and improve theswitch performances.Figure 6: Relationships between Contact resistances andcontact forcesFigure 7: Switch contact resistance measurements overnumbers of operation cyclesFigure 8: SEM of the contact surfaces by molded electroplating (a) and e-beam evaporation (b)AcknowledgementAuthors gratefully acknowledge KIMM for the support of this research.6. REFERENCES[1] K. E. Petersen, “Micromechancal membrane switches on silicon,“ IBM J. Res, Dev.23, 376-85, 1979[2] S. Majumder et al., “Measurement and modeling of surface micromachined, electrostatically actuated micro-switches,” Proc. IEEE Transducers’97, pp1145-1148[3] Y. Wang, et al, “Low-voltage lateral-contact microrelays for RF applications,” IEEE, 2002[4] C. 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