Optical MEMS Design and Simulation

Optical MEMS or MOEMS have had the greatest success to date in digital projection.  The original DLP (digital light projection) technology was developed by Texas Instruments. DLP technology is now used in projectors, television sets, digital cinema and in additive manufacturing.   MOEMS are being increasingly adopted for other applications, including passive-light display technologies, optical networking, focusing and zoom mechanisms for miniature cameras, and scanning mirrors used in LIDAR devices. To meet complex MOEM design requirements, engineers are adopting a hybrid design methodology that offers significant advantages over custom model development while reducing the need for time consuming and costly build-and-test cycles.

Examples of optical MEMS devices

Figure 1: Examples of optical MEMS devices

Design Challenges

The main design challenge for a typical MOEMS device is to optimize switching speed, which involves understanding mode shapes and frequencies of oscillation. Other critical design parameters include contact dynamics, frequency hysteresis, temperature drift, shock dynamics, package deformation, gas damping, and power. To maximize process yield, process and material tolerances also need to be taken into account in the design phase. Furthermore, successful MOEMS design requires the combined expertise of MEMS device and ASIC Engineers, particularly for MEMS arrays where RC delays induce cross talk, reducing screen performance.

Coventor’s hybrid design flow helps MEMS teams to meet their performance criteria while simultaneously reducing cost and time to market. The methodology leverages Coventor’s MEMS+ to enable fast device design and co-simulation with the drive electronics. By simulating the MEMS device together with the control system early in the design flow, costly redesign is avoided late in the design cycle. Layout information from the MEMS+  software can also be transferred to Coventor’s Analyzer (in CoventorWare), so that detailed device designs can be completed using MEMS-specific FEM solvers in CoventorWare.

Coventor's novel hybrid methodology for MOEMS design (applied to a scanning mirror in this example)

Figure 2: Coventor’s novel hybrid methodology for MOEMS design (applied to a scanning mirror in this example)

New Hybrid Methodology

1. Rapid Design Exploration with Higher Order Finite Elements

Designers first create a geometric MEMS model in MEMS+ , using components from the extensive library of higher-order electromechanical finite elements. In the mirror example shown in Figure 3 (below), mechanical elements are assembled to model the suspension and gimbal.   Comb fingers with a circular backbone are built to model the non-linear electrostatic actuation of the mirror and gimbal. The elements use conformal mapping to simulate fringing field capacitance and comb finger out-of-plane fringing force actuation. Using higher order parametric elements, users can assemble and test hundreds of design solutions in the time it takes to execute one traditional finite element analysis (FEA) simulation.

<em>MEMS</em>+ high-order components used to assemble a model of a scanning mirror

Figure 3: MEMS+ high-order components used to assemble a model of a scanning mirror

The MEMS+ models have been optimized to run in MATLAB Simulink® or Cadence®. For an electrostatically driven micro mirror, frequency response, drive angle with drive voltage and transient response are commonly evaluated after the geometric MEMS model is built in MEMS+. Users can rapidly evaluate differences in device performance, by carrying out Monte-Carlo based yield analysis using process and material information provided by their foundry.

2. Detailed Analysis and Verification with Field Solvers

After the MEMS+ model is developed, the next step is to utilize field solvers to perform detailed analysis of the MEMS device. In our micro mirror example, Coventor’s Analyzer is employed to extract the damping coefficients for MEMS+ and investigate the stresses in the suspension beams. To provide additional confidence in the simulation prior to fabrication, MEMS+ and Analyzer solutions can be validated by comparing resonant frequencies.

Position-dependent gas damping coefficients can be simulated in CoventorWare. These values can be inserted in the <em>MEMS</em>+ model and interpolated for transient simulations of the device.

Figure 4: Position-dependent gas damping coefficients can be simulated in CoventorWare. These values can be inserted in the MEMS+ model and interpolated for transient simulations of the device.


CoventorWare simulation of suspension stress distribution and maximum stress with rotation angle

Figure 5: CoventorWare simulation of suspension stress distribution and maximum stress with rotation angle

3. MEMS+ IC System Simulation

Next, co-simulation of the device and its affiliated electronics can be completed in Cadence circuit design tools or the MathWorks Simulink environment. The MOEMS model built in CoventorMP can also be constructed into an array for a full system analysis of the entire display.

An example schematic is shown in Figure 6 (below). The MEMS+ model array is connected to a driving SRAM actuation circuit and an RC network that models the line resistance. Because these models are parametric, the designer can quickly and easily adjust the circuit and/or change the MEMS device to explore which strategies yield the best performance.

Schematic of a mirror array in Cadence and transient simulation results shown in a 2D graph and 3D animation

Figure 6: Schematic of a mirror array in Cadence software, with transient simulation results shown in a 2D graph and 3D animation

A Complete Platform For MEMS Design

Coventor’s MEMS+ and Analyzer provide a powerful platform for MOEMS design.  They address the critical design challenges which cannot be solved using general FEM tools. Using this platform, designers may simulate the complex multi-physics of the actuation and sensing mechanism, along with the electronics and MEMS packaging. Furthermore, Monte-Carlo analyses can be used to study and improve manufacturing yield.

This methodology has numerous advantages. First, engineers do not need to hand craft reduced-order or equivalent models from FEA or analytical expressions; all essential physics of the MOEMS are included in the MEMS+ model. This saves valuable time and resources. Second, rapid exploration of process and design variations is possible, because the MEMS+ models are fully parametric and simulate extremely fast compared to conventional FEA models. Third, by utilizing a platform that integrates with best-in-class simulators like MATLAB®, Simulink, and Cadence Spectre®, engineers have access to the best combination of accuracy and capacity. As Coventor, Mathworks and Cadence all make speed and capacity improvements to their tools, the benefits multiply. Going forward, simulation performance will exceed the user’s growing demands.

Reference:

Design Optimization of MEMS 2D Scanning Mirrors with High Resonant Frequencies, Ma,W., Chan,H.Y., Wong,C.C., Chan,Y.C., Tsai,C.J., Lee,F.C.S., MEMS 2010

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