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3.9 Summary
V-beam electrothermal actuators have been fabricated in SCS by using DRIE. These high-aspect-ratio actuators were found to be more robust than their polysilicon equivalents, avoiding problems such as stiction and out-of-plane bowing. A model was developed to predict actuator deflection and force for a given current. Because of temperature-dependent properties of silicon, a revised, numerical solution of temperature distribution is required for acceptable accuracy. Once the thermal expansion is calculated, the flexibility method of virtual work is used to predict deflection and force. Simplified equations were developed to easily calculate the effects of changing design parameters.
Deflections exceeding 25 mm and forces exceeding 2.5 mN were observed for a single V-beam actuator. Significantly higher deflections than expected were realized due to the decrease in thermal conductivity and increase in the thermal expansion coefficient of silicon at elevated temperatures. Transient behavior was also examined. Because of their large thermal mass, the high-aspect-ratio SCS actuators respond more slowly than surface micromachined polysilicon thermal actuators. Finally, a design strategy was presented that optimizes actuator dimensions to meet the high-force requirements of 3DMEMS devices.