A thermal actuator (TA) is the thermal compliment of electrostatic actuators. A TA has several advantages over other microactuation methods. They can provide relatively large forces (^N) and large displacements (>10 jam) at CMOS compatible voltages and currents. The main disadvantage of a TA is it large power consumption. A TA can be used in basic building-block MEM devices such as stepper motors, optical-component positioners, and grippers. The TA shown below in Figure 36 converts electrical energy into mechanical via ohmic heating and deflects due to asymmetric heating. The disparity in the widths of the Cold (wide) beam and the Hot (thin) beam causes an uneven current density to flow through the TA when an electrical bias is placed across the ends of the two beams. The higher current density in the Hot beam causes it to expand, due to thermal expansion, more than the Cold beam. This results in the sweeping of an arc in the plane of the wafer by the free end of the TA. Figure 37 shows a TA in its electrically biased state. Figure 36: Thermal actuator in the steady-state position. Figure 37: Thermal actuator in biased state. I propose to design, develop a fabrication process, fabricate, and test microsystems that integrate a polycrystalline silicon TA with a photodetector. The microsystem uses a photodetector as a position sensor to indicate the TA position in real-time. The process flow simulation will be accomplished using Silvaco Athena Process Simulator as a part of the design process. Various polycrystalline silicon TA will be fabricated in order to verify the effects of the design parameters. The design parameters are the length and width of the Hot Arm. The chip design will incorporate test structures to aid in the analysis of the structures and photodiodes and will use the Mentor Graphics IC layout editor. The design will not incorporate on-chip signal amplification. The fabrication process used will incorporate standard integrated circuit technologies, processes and standard surface micromachining processes. Optical and Scanning Electron Microscope photographs of the devices will be taken after important processing steps. Tests will include actuator position determination via photodetector current, maximum deflection, photodiode current-voltage characteristics, and TA cyclic fatigue. Video of the electrical testing will be taken. An empirical model of the relationship between the deflection and the current will be developed.
Library of Congress Subject Headings
Detectors; Actuators; Microelectromechanical systems
Department, Program, or Center
Microelectronic Engineering (KGCOE)
Munger, Kevin, "Polycrystalline silicon thermal actuators integrated with photodiode" (2002). Thesis. Rochester Institute of Technology. Accessed from
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