Introduction: 

Many applications in Optics require very flat and accurately displaceable micromirrors. This can be difficult to achieve due to the residual stress induced in the device during fabrication. Additional mechanical stress can be introduced to the device due to thermal and packaging effects.

The Piston-Tip-Tilt actuator and mirror device uses three symmetrically arranged C-Shaped suspension beams. Vertical actuation is achieved by using asymmetric electrostatic comb actuators. The height of the mobile comb fingers (attached to the mirror) is less than the static comb fingers. Thus in the initial state, the mobile and static comb fingers are vertically offset. A potential applied to the static combs creates a vertical force due to the asymmetric electric field between static and mobile combs, so lifting up the mobile combs and the mirror. Pure piston motion is achieved by applying the same potential on all three static comb electrodes, whereas a small tip-tilt can be generated by applying different potentials to the static electrodes. Figure 1 shows the 3D model of the device.

Figure 1: 3D model of Piston-Tip-Tilt actuator and mirror (Click here for enlarged image) 

In this example, Architect is employed to build a fully parametric 3D model of the device using rigid plates, beams and comb finger elements from the Parameterized Electromechanical Parts Library. Figure 2 shows the Architect schematic of the Piston-Tip-Tilt actuator and mirror.

Figure 2: Schematic of Piston-Tip-Tilt actuator and mirror (Click here for enlarged image) 

The triangular mirror is modeled using the component labeled 'Rigid Plate'. This component also allows you to assign mobile comb fingers to the mirror. The three static comb drives are modeled by 'Straight comb' components. 'Freefrom Beam' components are used to define the C shaped suspension beams. A polynomial is used to define the shape of the beam. A dc source is used for actuating the comb drive.

Figure 3 shows the result of a DC Transfer analysis in Architect. The figure plots voltage applied to the static combs on X-axis and electrostatic force in z direction and displacement of mirror center on the Y-axis. You can see that as the voltage is increased, the electrostatic force also increases causing the mirror to lift up. This fully coupled electro-mechanical simulation takes just 25 seconds to run on a 2GHz laptop.

Figure 3: Result of DC transfer analysis in Architect (Click here for enlarged image) 

Figure 4 shows the animation of DC transfer analysis.This animation has been created using Scene3D. For clarity, the displacement in Z is scaled by a factor of 1,000.

Figure 4: Animation for DC transfer analysis (Click here for enlarged image) 

Customers who have an active CoventorWare license can access this example using their download account. Register for a download account!

Click here to access the files for piston tip-tilt actuator and mirror example. Save the zipped file and extract the components in your Design files directory. This will create a new folder named 'Example_TipTiltMirror'. When you start CoventorWare, this name will appear in Project Browser window. Open the project and go to the Architect tab. Select ' Example_TipTiltMirror.proc' and ' Example_TipTiltMirror.ai_sch' as the process and schematic respectively. Open the schematic and simulate it. You can use ' Example_TipTiltMirror.ai_frm' as the form for DC transfer analysis.

You will need CoventorWare 2008 to run this example.

Reference:

Proc. of SPIE Vol. 6467: Compact large-stroke piston-tip-tilt actuator and mirror W. Noell (a), A. Hugi (a), T. Overstolz (b), R. Stanley (b), S. Waldis (a), and N. F. de Rooij (a) a SAMLAB, Institute of Microtechnology (IMT), University of Neuchâtel; b Swiss Center for Electronics and Microtechnology (CSEM)