Abstract
To be able to create small integrated wireless radio front ends, variable
capacitors that can be monolithically integrated on the same chip as the
electronic circuits are needed. Today’s radio front ends are based on solidstate
diode varactors which suffer from poor tuning range and phase noise
and do not meet the requirements for a fully integrated wide band radio
front end. In this thesis we will look at possibilities for implementation of
a monolithic integrated MEMS varactor.
A post-CMOS-MEMS process is used to manufacture the structures. The
post-process is a combination of anisotropic RIE dry etch steps and an
isotropic release step, which makes monlithic integration simple and low
cost. The actuation mechanism is based on an out of the plane curled
cantilever beam. Initial curling of the structure as a result of residual
stress in the CMOS layers of the chip is utilized to initially levitate
the actuator over the chip plane. To actuate the varactor combs, the
actuator is heated with a joule-heating element placed under the structure.
When the the actuator is heated to the in-plane alignment temperature,
maximum capacitance is obtatined. Simulations and analytic models are
used to predict the performance of the actuator and varactor. The analytic
models presented and developed make development of out of plane curled
varactors more efficient and simpler than with FEM simulations only. The
complete varactor design was included in a test circuit sent to production
in November 2008.
Simulations and models yield good results for tuning range and Qualityfactor.
The tuning range is predicted to be between 240 % and 325 % and
the Q-factor is calculated to be 30@1.5GHz. The tuning voltage required
for operation of the varactor is within 8 V and the varactor does not suffer
from pull-in, which is the case for electrostatic based varactors.