CMOS/BICMOS Self-assembling and Electrothermal Microactuators for Tunable Capacitors

 
 
  Altug Oz


Advisor:
Gary Fedder
   
  Project:

Advanced RF systems on chip will benefit from microelectromechanical (MEMS) tunable capacitors integrated on a CMOS or BICMOS chip with high quality factor (Q) and large tuning range. RF circuits with on-chip CMOS/BICMOS MEMS tunable capacitors will have small footprints and will not have the reduction in tuning range coming from fixed capacitance between off-chip circuit parts. CMOS-MEMS micro-movers that use the principles of self-assembly and electrothermal actuation are successfully designed, modeled, fabricated and characterized for use in RF tunable capacitors, latch mechanisms and nanometer-scale gap-closing structures.

The micro-movers exploit the lateral stress gradient setup by embedding metal layers into CMOS-MEMS beams that are offset from the centerline of the beam. Built-in residual stress in the aluminum and silicon dioxide layers creates a gradient driving self-assembly upon microstructural release. Electrothermal actuation generates a stress gradient from the different temperature coefficients of expansion of the offset materials. This actuation has relatively low driving voltage of around 4 V maximum, which is compatible with IC technology and silicon substrates. Various micro-mover designs in four different CMOS/BICMOS processes are characterized. The largest lateral displacement from self assembly is 11 Ám in a 200 Ám by 40 Ám footprint (Figure-1). The largest lateral displacement from electrothermal actuation is 25.5 Ám in an actuator with the same footprint. Frequency response of the micro-movers is limited by the thermal time constant with the fastest measured 3dB bandwidth ! of 178 Hz. The largest tuning range achieved among tunable capacitor designs is 352.4% with a Q of 52 at 1.5 GHz (Figure-2). For zero-power stand-by operation of RF MEMS capacitors, mechanical latch structures are developed by sequencing micro-movers. Such mechanisms are also applied to assembly of lateral nanometer-scale sidewall gaps for large capacitance and large electrostatic force per area. Mechanical latch and nanometer-scale gap-closing mechanisms are successfully fabricated and tested.

 

   
     


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