ANSYS Microsystem Analysis Example Application

RF Switch

Device description:

RF switches have applications in the wireless communication industry (wireless handsets, wireless LANs, broadband wireless access and wireless data link) and global positioning systems.

The RF switch analyzed here consists of a fixed-fixed thin metallic film suspended over a dielectric film deposited on top of the bottom electrode. When an electrostatic voltage is applied between the membrane and the bottom electrode, an electrostatic force is created to pull the membrane down.

In this application example, a static coupled electrostatic-structural analysis is performed using ESSOLV macro in ANSYS Multiphysics. However, it should be noted that ANSYS Multiphysics can also be used to perform a thermal-electrostatic-structural analysis to account for the switch response at various temperatures, and couple in a HF full wave analysis to allow a user for example to examine the effects of temperature on S parameters.

The following image shows an array of three MEMS RF switches:

Analysis Capabilities:

Compute electrostatic fields and forces at various applied voltages
Compute pull-in voltage of the switch
Compute structural deflection and mechanical stress and strain
Include effect of initial stress, which is of wide existence in MEMS devices
Include temperature effects
Extract capacitance between metallic film and bottom electrode using CMATRIX macro

Analysis Methods:

In this application example, a static coupled electrostatic-structural analysis is performed using ESSOLV macro in ANSYS Multiphysics.

The switch, bottom electrode and surrounding air are included and meshed in the model. The switch is modeled as shell element and the air is modeled with solid 3-D elements. Initial and deformed mesh are shown in the following image:

Since the model is symmetric along x and y axes, only a quarter of the model needs to be analyzed, (note the results can be expanded later on using the /EXPAND command). The following image shows the quarter symmetry mode, the purple region is the mesh of the air surrounding the switch, the green region is the switch itself. Note that the bottom electrode is not shown.

With the structural and field domain meshed, the ESSOLV macro was then used to automate the coupled electrostatic-structural solution in sequence.

Results:

The following image shows the voltage contour plot for the switch in its initial position, i.e., the results form the first ESSOLV step.

ESSOLV makes the results for both the electrostatic and structural domains available for display or listing in postprocessing. The image below is the displacement contour in Z-direction of the switch. Here the /EXPAND function has been used to restore the geometry based on the symmetry boundary conditions.

The image below shows the stress contour in the switch in the deformed position.

Conclusion:

Pull-in voltage of approximately 8 Volts is obtained from this electrostatic structural coupled analysis. The ESSOLV macro allows a user to list deflection at any node in the model versus load. The following graph shows maximum displacement (center of switch beam) versus applied voltage:

Acknowledgment:

ANSYS, Inc. would like to thank Prof. Horacio D. Espinosa and Yong Zhu of the Micro/Nano Mechanics Lab at Northwestern University for submitting this excellent material for publication on our website.

References & Further Reading:

Useful references on the Web:

Micro/Nano Mechanics Lab at Northwestern University

Alternative MEMS RF Switch Configurations from UC Berkeley

RF MEMS at University of Michigan