ANSYS Microsystem Analysis Example Application

Thermoelectric Actuator

Device Description:

The device consists of a surface micromachined thermal actuator (electro-thermal actuated beam). The actuator consists of a blade connected to electrical contact pads by two thin beams. A potential difference is applied to the electrical contact pads and current flows through the thin beam & blade. The device is constructed from Polysilicon which has a finite , temperature dependent resistivity. The current flow produces Joule heating that in turn imparts a large thermal stress on the device, concentrated in the long thin beam. The thermal expansion of the thin beam causes the device to bend at the short thin beam. The blade rotates in the plane of the substrate. The tip of the blade is typically connected to pushrods that are used move gears and ratchet mechanisms (e.g for mirror positioning).

Analysis Capabilities:

Determine the nature of the mechanical motion of the device.
Determine the tip deflection for range of voltages.
Determine maximum temperature for a range of voltages.
Incorporate temperature dependent material properties.
Incorporate orthotropic material properties.
Incorporate heat transfer to surrounding fluid
Compute fluid-structural damping
Build reduced order macro model of device.

Analysis Methods:

The device can be analyzed in either 2-D or 3-D. A two dimensional, static, sequential coupled analysis, where the device is built using the ANSYS solid modeler. This 2-D model can easily be extruded into 3-D with the ANSYS solid modeler, or a 3-D solid can be imported from CAD.

3-D solid model of thermal actuator:

A two dimensional, static, sequential coupled analysis, was performed where the device is built using the ANSYS solid modeler.(This 2-D model can easily be extruded into 3-D with the ANSYS solid modeler). A thermal-electric simulation is performed first followed by a thermal-structural. Only the polysilicon of the device is modeled, and is meshed with PLANE67 elements, in plane with thickness. The DC drive voltage is applied and the joule heating calculated. The resulting temperatures are then applied as loads to the thermal-structural part of the simulation. The following animation of the deforming part with temperatures was generated.

Results:

Animation of 2-D results (temperature contours):

Animation of 3-D model results - voltage contours (Note lower electrode pad (blue) is at ground potential):

Conclusion:

This 2-D sequential coupled technique is a small problem size that runs efficiently and illustrates one technique for solving this coupled physics problem. In future we will also show a matrix coupled, 3-D analysis where the device geometry is imported from a MEMS layout front end design tool. The device is meshed with SOLID98 elements that support structural, thermal and electric degrees of freedom. A DO .. LOOP can be employed in the input file to run the solution for a range of voltages. A table of results is thus constructed. The following results are from a 3-D analysis:

40.0

Voltage / V	Current / pA	Maximum Displacement / um	Maximum Temperature / K
10.0	3.712E+6	0.12	288.5
20.0	7.424E+6	0.47	329.0
30.0	1.111E+7	1.05	394.9
40.0	1.485E+7	1.87	485.0
50.0	1.856E+7	2.92	600.5

Acknowledgment:

ANSYS, Inc. would like to thank Prof. Victor Bright at UC Colorado for permission to use the SEM images & material properties reported in his recent papers.

References & Further Reading:

Thermal Actuator References on the Web:

MUMPS Thermal Actuator- Simon Fraser University

Thermal Vertical Bimorph Actuators - Southampton University Microelectronics Center

Compliant Mechanisms at Brigham Young University