Thermally-driven microactuators have simpler structures and fabrication process than other actuators using electrostatic force, electromagnetic force, piezoelectric force and etc., in addition they are expected to have advantages such as relatively large force and displacement. Modeling of thermal characteristics and extracting of thermal parameters, mechanical and electrical properties are prerequisite for investigating operation speed and power consumption of thermal actuators and thermal sensors.
In this study, thermally-driven polycrystalline silicon microbridges and microswitches are fabricated to investigate the thermal, mechanical and electrical properties.
A new characterization method have been introduced to extract the thermal conductance and the thermal capacitance of thermal microactuators using a FRA (Frequency Response Analyzer). A radiation source and a chopper, which are needed to oscillate the temperature of thermal sensors in other measurement method, are not necessary in this method because signal voltage supplied by a FRA heats the thermal microactuators. The method can derive several thermal parameters at the same time and also eliminates the error from the ambient temperature variation because it considers the change of dc resistance affected by the ambient temperature fluctuation.
A electrically small signal equivalent circuit for the thermal microactuator has been modeled in functions of experimental parameters to measure the thermal parameters from frequency response analysis results. The electrically small signal equivalent circuit of polycrystalline silicon microbridge with a negative TCR(Temperature Coefficient of Resistivity) has inductance component, but in the case of a positive TCR it have capacitance component. According to induced equivalent circuit, it is shown that the diameter and the angular frequency at the top point of impedance semicircle are related to the thermal conductance and the thermal capacitance respectively. The impedance value is equal to the dc resistance of a microbridge at high frequency.
The thermal conductance and the thermal capacitance are measured from polycrystalline silicon microbridges that have a bridge width of 5㎛, a thickness of 3㎛ and lengths of 200, 300, 400, 500㎛. As average temperature of microbridge increased, they were increased by 1~2%/℃ because the temperature of a part of pad connected to microbridge was increased to be included into that of microbridge. It is needed to modify the thermally equivalent circuit after contact between microbridge and substrate since semicircles were distorted by other thermal conductances and thermal capacitances whose mean heat paths via contact area between the center of microbridge and substrate.
Especially, the proper design of thermally-driven microactuators requires the understanding of the process dependence of basic material properties. From this experiments, the residual stress of polycrystalline silicon microbridge measured 13MPa, resistivity $6×10^{-4} Ωm, TCR $2.6×10^{-3}/℃, thermal expansion coefficient was extracted $2.9×10^{-6}/℃ using microgauge, elastic modulus was 143GPa using α-step.
A Microswitch with a thermally-driven polycrystalline silicon microbridge actuator was fabricated to characterized their thermal parameters. For thermal microactuator having a bridge length of 500㎛, a width of 40 ㎛ and a thickness of 3㎛, the thermal conductance and the thermal capacitance were $3.3×10^{-4}W/℃ and $6.9×10^{-7}J/℃ at 20mTorr respectively. And operation power and contact resistance measured.