Suppression of Microwelding in RF MEMS Direct Contact Switches

the resistive loss of the signal line which includes theresistance of the signal line and the contact resistance. Athigher frequencies, the insertion loss can be attributed to boththe resistive loss and the skin depth effect [3-4].micromachining technique with a total of six basic maskinglevels. Gold sputtered using RF magnetron sputtering systemis used as the structural material and AZ1512 photo resist isused as the sacrificial material. Figure 5 is an abbreviatedcross sectional schematic illustration of the process sequence.Fig. 3 Insertion loss, return loss and isolationsimulated by HFSS v8.0.Section AA’Section BB’Port 1(a) CPW line pattering with dimples (W or Mo)and dielectric layer patterning (SiN)Port 2 Port 3Fig. 4 Switch Pattern for S-Parameter Simulation.We made short circuit between port 1 and port 2,while port 1and port 3 are open circuit with gold dimple switch (Fig. 4).Insertion loss will be increased slightly when W or Mo dimplewas used. However, this value can be ignored because of lowloss characteristic of the switch.(b) Sacrificial layer spin coating and anchor patterning(c) Thermal treatment for sacrificial layerand Au cantilever patterning4 FABRICATION4.1 Overall ProcessRF MEMS switches were fabricated based on the conventionalsemiconductor manufacturing process. A coplanar waveguideis a planar transmission line made up of a center conductorseparated by gaps the ground planes on either side. Thecharacteristic impedance of the coplanar waveguide is set bythe width of the center conductor, the width of the gap, and therelative permittivity of the substrate. Upon the application of asufficiently large voltage between the actuation cantilevers andelectrodes, the cantilevers collapses downward until itconforms to dielectric layers, which prevents it from shortingto the ground plane.MEMS switch is manufactured using a surface(d) Connecting bar patterning (stress controlled SiN)Fig. 5 Fabrication process for RF MEMS Switch.4.2 Stress control of connecting barThe performance of MEMS devices is still significantlyrestricted by residual stress of thin films (Fig. 6). To reducethe effect of the total stress in released structures, manyapproaches were proposed and successfully demonstrated[6–8]. They used additional stress compensation layers and

suitable mechanical designs.Fig. 6 Deformed shape after inducing residual stress.In this study, we controlled residual stress of silicon nitride(SiN) layer formed by PECVD (Plasma Enhanced ChemicalVapor Deposition). It has been demonstrated that the residualstress of SiN is affected by the frequency used in PECVDprocess. In case of the low frequency (i.e. 50~100 kHz), morethe ions in the plasma are able to oscillate according to thealternating electric field, and hence, transfer energy to thegrowing SiN film, causing a densification. For this reason, theSiN film is compressively stressed against the substrate.Figure 7 shows curved SiN film used for connecting bar withcompressive residual stress, which is undesirable for metalcontact.In case of high frequencies (i.e. 13.56 MHz), not all the ionscan follow the alternating field, and thus, the SiN filmbecomes less dense and tensely stressed against the substrate[2]. Figure 8 shows SiN film used for connecting bar withtensile residual stress. The SiN film was formed by using onlyhigh frequency in PECVD Process.5 EXPERIMENTAL RESULTSTo examine the dynamic behavior of MEMS switch test. Anamplified function generator signal for device actuation andmulti-meter to monitor contact resistance are required. Sinewave with peak amplitude of 20 ~ 40 Volts is applied. Theelectrostatic force on the MEMS switch is proportional to V 2 ,and therefore a bipolar voltage will always result in a constantattractive force on the cantilever. And bipolar actuationresulted in a vast improvement in the reliability of the LincolnLaboratories and the University of Michigan switches. Andreliability can be improved by tailoring the actuation voltagewaveform to reduce the impact energy and the resulting pittingand hardening of the contact area. This was demonstrated byRockwell Science Center and University of Michigan [9]. Inthis case, sine wave (Fig. 10) with peak amplitude of 20~40Volts are applied which can include bipolar and tailoring theactuation voltage in Figure 9.VoltageVoltageTimeTime(a)(b)Fig. 9 (a) bipolar, and (b) tailored actuation voltages.VoltageTimeFig. 10 Sine waveform actuation voltage.Fig. 7 Compressively stressed silicon nitride film (200 MPa).Contact resistance was measured by using probe station andfunction generator at atmospheric pressure. The results showedthat the resistance of gold-on-gold contacts was between 0.2and 1.9 Ω and gold-on-tungsten contacts was 2 times as highas that of gold-on-gold contacts. It means if tungsten is usedfor switch dimple the insertion loss will be -0.4 ~ -0.2 dB [1].6 FUTURE WORKFollowing tests with three types of switches will be done bythe end of April, which have different contact materials: Au,W, Mo.Fig. 8 Tensely stressed silicon nitride film (50MPa).6.1 RF power handling testThe maximum power-handling capability of three kinds ofswitches with different contact material will be evaluated atseveral GHz frequency range.

6.2 Insertion loss and isolation testInsertion loss and isolation will be evaluated for three kinds ofswitches.6.3 Lifetime test.The lifetime of the switches will be evaluated at several GHzfrequency range.7 CONCLUSIONWe have made an attempt to suppress the in-use stiction in RFMEMS direct contact switches due to microwelding. Twokinds of refractory metals, W and Mo, were coated onto thecontact point of the switches and the effect of refractory metalscoating was investigated. The results showed that refractorymaterials have higher contact resistance than gold. Eventhough higher contact resistance of refractory metals isdetrimental to insertion loss, the use of refractory metals isexpected to do good on power handling capability and lifetimeextension.The insertion loss, isolation and lifetime for three types ofswitches will be evaluated by the end of April.REFERENCES[1] G. Rebeiz, J. Muldavin, “RF MEMS Switches and SwitchCircuits”, IEEE Microwave Magazine, pp.59-71, 2001[2] A. Tarraf, J. Daleiden, “Stress investigation of PECVDdielectric layers for advanced optical MEMS”, J. ofMicromechanics & Microengineering, 14, pp317-323, 2004[3] J. Jason Yao, M. Frank Chang, “ A surface micromachinedminiature switch for telecommunications applications withsignal frequencies from DC up to 4GHz”, The 8 th InternationalConferences on Solid-State Sensors and Actuators, 1995[4] J. Jason Yao, “RF MEMS from a device perspective”, J. ofMicromechanics & Microengineering, 10, pp9-38, 2000[5] Tarn, H. William, “CRC Handbook of metal etchants”,CRC Press, 1991[6] F. R. Gass, D. J. Dagel, D. P. Adams, G. D. Grossetete, O.B. Spahn, S. A. Kemme, S. S. Mani and K. J. Malloy, “Stressand curvature in MEMS mirrors”, Proc. SPIE, 4983,pp87–93, 2003[7] M. T. Hou, K. Liao, H. Yeh, B. Cheng, P. Hong and R.Chen, “Fabrication of micromachined focusing mirrors withseamless reflective surface”, Proc. SPIE, 4983, pp59–66, 2003[8] J. J. Taghander, “Shape control and heat transfer in opticalMEMS”, IEEE LEOS Newsletter, 16, pp3–8, 2002[9] G.Rebeiz, “RF MEMS Theory, Design, and Technology”,Wiley Interscience, 2003