of 0.05 ϫ 10 5 Hz ⅐ cm 2 /ng, which is ϳ5 orders of magnitude higher than the sentitivity (0.057 Hz ⅐ cm 2 /ng) of the conventional 5-MHz QCMs. The ultra-mass-sensitive device will be very useful for future chemical-sensor or biosensor applications. ACKNOWLEDGMENTThis work was partially supported by the Radio Education and Research Center (RERC) of Information and Communications University. [1][2][3]. Traditionally, these plates are limited to planar geometries and lie parallel to the substrate. These devices are constructed from layers that are typically thinner than 5 m. They are normally actuated electrostatically, or thermally. Most of these devices have been fabricated using silicon-based thin-film processes, such as MUMPs, which are lossy at high frequencies. An alternate configuration is a lateral-comb structure [4,5]. In this geometry, adjusting the overlap of the capacitor fingers changes the capacitance. The direction of actuation is perpendicular to the actuation in parallel-plate-type capacitors. Device layers as thick as 80 m have been reported [5]. These layers were constructed using a highly refined deep silicon etch. HIGH-Q LIGA-MEMS VERTICAL CANTILEVER VARIABLE CAPACITORS FOR UPPER MICROWAVE FREQUENCIESOf the many variations of MEMS variable capacitors in existence, most are designed to operate at the lower end of the microwave-frequency range. Very few are able to achieve Qfactors greater than 100 at frequencies above 4 GHz. The reported variable capacitors which are capable of high-Q operation at high frequencies make use of a shunt-mounted design [6,7]. In this configuration, coplanar-waveguide transmission lines are used. A bridge is created over the center conductor and the two air gaps. A change in air gap between the bridge and the center conductor changes the capacitance. In [6], a shunt-mounted capacitor with thermal actuation was designed. A minimum Q-factor of 197 for 0.272 pF at 10 GHz was obtained. In [7], a shunt-mounted capacitor that is electrostatically actuated was developed. A Q-factor of 95-100 for 80 fF at 34 GHz was obtained. The LIGA process for deep sub-micron structuring effectively removes the restriction of planar geometries for MEMS fabrication. The LIGA process makes it possible to fabricate very tall metal structures (hundreds of microns) with lateral feature sizes that are smaller than a micron. These unique properties have led to an interest in LIGA for the development of high-performance microwave devices. Existing work using LIGA for microwave devices has concentrated mainly on statically operating structures such as transmission lines, filters, couplers, and antennas [8 -10].This paper describes simulation results for four variable capacitors suitable for fabrication using the LIGA process. These are parallel-plate capacitors, but the plates are oriented vertically rather than horizontally and are perpendicular to the substrate. The gap is varied by electrostatic actuation. The capacitors were simulated using 345-m-thick nickel and copper device layers on an a...

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