MEMS Variable Capacitors

Figure 1

In recent years, MEMS variable capacitors have begun to replace varactor diodes in RF and microwave electronics. These MEMS capacitors use parallel plate design architecture to enable traditional MEMS processing of the devices. These devices operate by using a dc bias voltage to move one conducting electrode relative to a reference plate, and by either changing the area of overlap of the conducting plates or the distance between the plates, a capacitive tuning range is established based on the dimensions of the device. Conventional parallel plate variable capacitors use gaseous dielectric materials with low relative permittivities. The main disadvantage of this design is that the tuning range of the capacitor is greatly limited by the permittivity of the dielectric medium. Additionally, traditional parallel plate variable capacitors require moving parts to enable changing the electrode area or gap distance. By requiring moving parts, the tunability of the devices is also limited by the effects of stiction; beyond the pull-in voltage, the electrostatic forces in the device can cause the moving plate to snap to the reference plate rendering the device useless.

My research seeks to overcome the limitations of traditional MEMS variable capacitor designs by using an electric field to move multiple liquid dielectric materials through a microfluidic channel between the electrodes of a parallel plate capacitor. By moving the liquid dielectrics via an electric field, the volume ratio of these materials between the parallel plates of the capacitor can be altered, thereby varying the capacitance of the device by changing the relative permittivity seen between the plates. The use of liquid dielectrics with large permittivity differences will allow for a greater range of capacitance values for a given total electrode area and higher overall capacitance than previously possible with conventional MEMS variable capacitors. Additionally, because the conducting electrodes are not moving, problems associated with electrode stiction should be non-existent.

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