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Multi physics FEM simulation of MEMS thermal actuator using EMS

Applications


Introduction

Micro-electro-mechanical systems (MEMS) make it possible to reduce the size of complex engineering devices to a micrometer scale, for various applications: micro positioning, micro fixturing, micromanipulation etc. Micro-thermal actuators convert thermal energy into force and motion. Here, we study a low voltage micro-thermal actuator that develops relatively large thermally-induced deflections.

Electron microscope image of thermal actuator [1]

Figure 1 -  Electron microscope image of thermal actuator [1]

Problem description

The U-shaped actuator analysed in this study consists of two parallel arms: cold and hot arm. They are connected to the pads substrate at one side and to each other at the other side, like in Figure 2.

EMS simulation coupled to thermo-structural analysis is used to compute the mechanical displacement of the micro-device caused by temperature expansion effect.

The basic design of the U-shaped thermal actuator
Figure 2 - The basic design of the U-shaped thermal actuator
 

Table 1 - Model dimensions [2]

Part Dimension (µm)
Hot arm length 495
Hot arm width 2
Cold arm length 470
Cold arm width 30
Arms separation 10
Connecting bar width 10
Pad length 40
Pad width 30

Simulation Setup

In order to compute and visualize the mechanical displacement of the studied thermal actuator device, the Magnetostatic module is used along with the steady state thermal and structural analysis.
The simulation setup consists of the following steps:
  1. Select an appropriate material.
  2. Define the necessary electromagnetic inputs.
  3. Define the necessary thermal inputs.
  4. Apply the structural boundary conditions.
  5. Mesh the entire model and run the solver.

In our case study, the following properties of material are used (Table 2):

 Table 2 - Polysilicon material properties
 
Property Electrical conductivity
(S/m)
Thermal conductivity
(W/m. K)
Thermal expansion coefficient
(/K)
Elastic Modulus
(GPa)
Poissons ratio
Polysilicon 43.5 E+03 150 2.9 E-06 169 0.22

The micro actuator is defined as a solid coil, carrying a voltage of 5V, with the hot-arm connected pad serving as the entry port. The initial temperature applied to both anchored pads is 0 °C. The thermal convection is defined on the ambient air body, by setting the initial (ambient) temperature to 273.15 K, and the convection coefficient to 10 W/m²K . A “Fixed” constraint structural boundary condition is applied to both sides of the anchored pads, as shown in the figure 3:

Applied mechanical boundary conditions

Figure 3 - Applied mechanical boundary conditions
 

The whole model uses a fine mesh control for a better results accuracy (Figure 4).

Meshed model

Figure 4 - Meshed model

Results

The final simulation results for the thermal distribution are shown in figure 5. The voltage difference across the pads causes a temperature difference between the two arms, therefore achieves a maximum value of 864°C at the most critical zone of the thin arm.

Temperature distribution across the actuator
Figure 5 -
 Temperature distribution across the actuator

 

The mechanical displacement caused by the thermal expansion effect reaches 16.99 µm. The actuator is moving mostly in the area connecting the two arms.

Resultant displacement plot 
Figure 6 - Resultant displacement plot
 

The comparison between EMS and the reference [2] results for the actuator deflection can be found in Table 3.

Table 3 - Comparative table between EMS and the reference [2] results.
 
  EMS Reference [2]
Deflection (µm) 16.99 17

Conclusion

EMS tool enables the validation and prediction of the micro-machined thermal actuators performance. It successfully evaluated and validated its capacity to produce large deflection under low supply voltages.

Reference

[1]. http://www.sfu.ca/adm/heatuator.html
[2]. Hristov, Marin Hristov, et al. "Design and Investigation of a Thermal Actuator." PROCEEDINGS OF THE XVII INTERNATIONAL SCIENTIFIC AND APPLIED SCIENCE CONFERENCE–ELECTRONICS ET 2008. 2008.