Two lateral RF MEMS varactors and an experimental switch made using a CMOS-MEMS process

Abstract

An expanding wireless industry creates a demand for smaller and smarter devices, while maintaining the high performance requirements. Todays solutions often use off-chip components to meet these demands, which lead to additional area, cost and parasitic components. RF MEMS can beneficially replace a great number of these components, and in integration with CMOS offer small devices at low cost with on-chip signal processing.

One important component in RF systems is the variable capacitor, or varactor. This thesis presents two varactor designs made using a CMOS-MEMS process that enable monolithic integration of MEMS and CMOS. The varactors are electrothermal actuated and based on interdigitated combs moving lateral, parallel to the substrate. A latch mechanism is used so that the varactors only require power when switching. Since the fabricated circuit did not arrive in time for measurements only calculated and simulated results are presented. Bimetal actuator theory is used for theoretical understanding of the actuator and the various design parameters are thoroughly analyzed and discussed. The capacitances of the varactors is in the range of 133-897fF, with tuning ranges of 440% and 489% and Q-factors of ~30 at 2GHz.

In addition an experimental lateral DC series switch has been made. The purpose of this switch is to investigate how well defined the sidewalls of this CMOS-MEMS process are to determine if it is possible to obtain a good metal-to-metal connection. This work will be important for future work on CMOS-MEMS switches.

All simulations have been done using Coventorware, while the layout of the chip has been made in Cadence. The chip has been fabricated in a 0.25um CMOS process from STMicroelectronics through the broker service Circuit Multi Projects (CMP), France. The post-process has been done at Carnegie Mellon University (CMU), USA and Sintef, Norway.