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INSTITUTE OF PHYSICS PUBLISHINGJOURNAL OF MICROMECHANICS AND MICROENGINEERINGJ. Micromech. Microeng. 15 (2005) 1854–1863doi:10.1088/0960-1317/15/10/010A mechanically reliable digital-type singlecrystalline silicon (SCS) RF MEMSvariable capacitorJong-Man Kim1, Sanghyo Lee1, Jung-Mu Kim1,Chang-Wook Baek2,3, Youngwoo Kwon1and Yong-Kweon Kim11School of Electrical Engineering and Computer Science, Seoul National University,301-1118(007), Kwanak PO Box 34, Seoul 151-600, Korea2School of Electrical and Electronics Engineering, Chung-Ang University,211 Heuk-Seok Dong, Dong-Jak Gu, Seoul 156-756, KoreaE-mail: kjmjjw5@snu.ac.kr and cwbaek@cau.ac.krReceived 25 April 2005, in final form 1 July 2005Published 9 August 2005Online at stacks.iop.org/JMM/15/1854AbstractThis paper reports the first demonstration of a single crystallinesilicon(SCS)-based vertical gap-tuning MEMS variable capacitor with highmechanical reliability using the SiOG (silicon-on-glass) process. Theproposed variable capacitor was fabricated by silicon-based micromachiningtechnology, and its mechanical and electrical performances were tested. Byadopting SCS, which has nearly zero stresses and superior thermalcharacteristics as a structural material, we can improve themanufacturability, reproducibility and mechanical reliability of thefabricated devices compared with a conventional metallic variable capacitor.The variable capacitor described in this paper is fabricated on a coplanarwaveguide (CPW) transmission line and the top electrode in the driving partis incorporated with a SCS actuating membrane, resulting in a highly rigidstructure without structural deformations. The designed SCS variablecapacitor is actuated by an electrostatic force, and the total chip size is1.05 mm × 0.72 mm. The measured pull-in voltage was 37 V, and the RFcharacteristics of the fabricated SCS variable capacitor from dc to 40 GHzwere measured. With and without an applied dc bias voltage, the measuredS11and S21were changed from −15.6 to −5.1 dB, and from −0.49 to−2.3 dB at 30 GHz, respectively. The measured capacitance at the up andthe down states was 30 and 140 fF, respectively, which corresponds to thecapacitance ratio of 4.67. The mechanical lifetime of the fabricatedvariable capacitor was also measured. RF performance degradation andstiction problems were not observed, even after 108cycles of repeatedactuations.(Some figures in this article are in colour only in the electronic version)1. IntroductionThecommunication systems have motivated many researchers tofocus on RF MEMS technology. MEMS technology enablesgrowingdemandsforhighperformanceRF3Author to whom any correspondence should be addressed.the realization of RF passive components with low loss, smallsize, low power consumption and high linearity comparedwith conventional semiconductor-based passives [1–3].particular, MEMS technology has led to the development oflow loss and high-Q tunable RF devices by using a mechanicaltuning scheme, which is helpful in eliminating the higher lossproblem in semiconductor devices at high frequencies. AsIn0960-1317/05/101854+10$30.00© 2005 IOP Publishing LtdPrinted in the UK1854