A New Thermal Characterization Technique For Thermal Actuators Using A Frequency Response Analyzer

1. Abstract : We have, fabricated microrelays with a thermally driven polycrystalline silicon microbridge actuator and characterized their thermal parameters. An electrical equivalent circuit for microactuator has been modeled in functions of experimental parameters to describe the thermal characteristics. Then thermal conductance and thermal capacitance of microactuator have been derived at once from the model and frequency response analysis results. A radiation source is not needed in this method because signal voltage applied by frequency response analyzer heats microactuator. The method also enables a elimination of error from ambient temperature variation due to independence on ambient temperature. 2. Introduction : Extraction and modeling of thermal parameters is useful to design the speed and power consumption of thermal actuators and thermal sensors. According to reported works [1][2], the measurement of thermal conductance is achieved by dc method and thermal capacitance is measured by ac method using the radiation source. Therefore these are necessary to use both dc method and ac method. Also ac method needs the radiation source. In this work, we have proposed a new thermal characterization technique for thermal actuators using a frequency response analyzer. 3. Results and Discussion : The structure of microrelay is shown in Fig. 1. Microbridges have lengths of 300, 500 and 7 0 0 ~ and widths of 40 and 60m and thickness of 3m. For heavily doped polycrystalline silicon, the temperature characteristics of resistivity obey the linear relationship [3]. Therefore the resistance of polycrystalline silicon microbridge actuator can be assume the equation as Where R, is the resistance of actuator at temperature To, a is the temperature coefficient of resistance, r is the average temperature of actuator, To is the ambient temperature. Fig. 2 shows the simple lumped model for a thermal equivalent circuit of thermal microactuator. The average temperature variation of microactuator can be calculated from the relationship between Jole’s heating generated by signal voltage and the thermal impedance of the thermal equivalent circuit. Since the electrical impedance of microactuator is a function of average temperature variation, the thermal conductance (G,) and thermal capacitance (C,) can be expressed as R ( T ) = R,, {1+ a(T T u ) } (eq. 1)