|M.Sc Student||David Eyal|
|Subject||Development of Variable MEMS Capacitor|
|Department||Department of Electrical Engineering||Supervisor||Professor Emeritus Yael Nemirovsky|
|Full Thesis text - in Hebrew|
Variable capacitor (varactor) is a basic component in microwave circuits, and its characteristics have a large impact on transceiver performance. We investigate Micro-Electro-Mechanic-Systems (MEMS) varactors because of their potential characteristics including: high quality factor, high operating frequencies, high linearity, high tuning rang and low power consumption. Yet it is challenging to meet all these requirements simultaneously with MEMS technology.
In this work we present a new MEMS variable capacitor with a comb-like structure that utilizes the wide tuning range of the comb varactor yet avoids unwanted coupling through the mechanical spring. A conventional comb varactor consists of two interdigit comb-like structures, one anchored to the substrate, and the other held by mechanical springs which allow it to move toward the anchored comb. The varying capacitance is between these two combs and the electrical connection between them is through the mechanical spring. In our design there are three comb-like structures. Two combs are anchored to the substrate, and the third (movable comb) is held by mechanical springs. The capacitance is from one anchored comb to the movable comb and backward to the second anchored comb, so the contact to the movable comb is capacitive and there is a total separation between the mechanical spring and the RF capacitor. This new design allows independence design of the spring and the RF capacitor and this freedom can yield better performance.
Our new varactor design was micromachined by a bulk micromachining process on a SOI (Silicon On Insulator) wafer with a high resistivity silicon substrate. The varactor was implemented in the device layer (10mm of conductive silicon) coated with a 1mm gold layer. We achieved a full displacement of the actuator with a control voltage of 19V - 24V, capacitance ratio of 200% (100fF - 200fF), resonance frequency of 35GHz and quality factor of 11@1GHz - 20GHz. A failure during the silicon etches caused a narrowing of the capacitor teeth and reduced capacitance compared to predictions. The reduced quality factor is attributed to significant energy absorption at 1.5GHz, due to acoustic resonance. In order to increase the quality factor of the device, the handle area should be reduced or replaced by lossless material. In this work we demonstrate a new design of comb varactor that can achieve high capacitance ratio while keeping its high resonance frequency. Its losses are too high, but we suggest a way of reducing them.