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fabricatiON OF robust electrothermal MEMS with customizable thermal response time and power consumption requirements 

Motivation/Objectives:

• Devices actuated by electrothermal bimorphs provide large displacement, e.g. micromirrors that can scan large angles are used for biomedical imaging
• SiO₂ is often used as thermal isolation to minimize power consumption. The brittle nature of SiO₂ increases susceptibility to impact failure.
• For metal-SiO₂ bimorphs, the low thermal diffusivity of SiO₂ results in slow thermal response
• A fabrication process that allows engineers to custom design thermal isolation at one or both ends of the actuators can be used to trade-off power requirements for speed and vice versa.

Approach:

• Material pairs with widely different thermal expansion coefficients and high thermal diffusivity are shortlisted. Three material pairs stand out — Al-DLC, Al-Invar, Al-W. Al-W was chosen to form the bimorphs.
• W also acts as a resistive heater.
  
• High temperature polyimide was chosen for thermal isolation as it has low thermal conductivity.

Results:

• Micromirrors were fabricated on SOI wafers.

Fig. 1 (a) One-dimensional (b) Two-dimensional micromirrors actuated by Al-W bimorphs

• Robustness of the new designs were compared with older designs that are actuated by Al-SiO₂ bimorphs. The two-ball setup shown in Fig. 3 was used to generate single impact events.
  

• The new devices could withstand accelerations in excess of 4000g. The  new mirrors >4 times more robust than Al-SiO₂ based designs.

Publications and Patents:

• S. Pal, H. Xie, “Fabrication of robust electrothermal MEMS with fast response”, UF.945P (Pending)
  
• S. Pal, H. Xie, “A PROCESS FOR FABRICATING ROBUST ELECTROTHERMAL MEMS WITH CUSTOMIZABLE THERMAL RESPONSE TIME AND POWER CONSUMPTION REQUIREMENTS”, submitted to Transducers 2011