MEMS Simulation and Design Optimization with FEA-Based Tools

By Robert Calvet
Optomechanical Engineer
SiWave, Inc.
Arcadia, CA

The design of any product can benefit from computer-based simulation and optimization tools prior to prototype testing. The growing MEMS (Micro Electro Mechanical Systems) industry is no exception. In fact, optimizing products as small as a MEMS device offers a considerable incentive for using computer simulation prior to prototype testing. While MEMS are inexpensive to produce in high quantities, set-up costs significantly drive up the price of producing prototypes, making multiple prototypes prohibitively expensive.

Sidebar: What are MEMS? MEMS are micromachines the size of a grain of salt or the eye of a needle that integrate mechanical elements, sensors, actuators and electronics on a common silicon substrate. In addition to optical switches within telecommunication and networking systems, MEMS applications include accelerometers in automotive airbags, inkjets in desktop printers and sensors in medical testing equipment. MEMS are manufactured using photolithographic-based semiconductor processes that selectively etch away parts of the silicon wafer or add new layers to form the devices. This batch process makes MEMS very economical to produce in large quantities. The photo on the left shows a MEMS optical switch for fiber optic networks. The image on the right shows the switch on a penny to demonstrate how small MEMS really are.

While many MEMS-specific software packages exist, SiWave chose a general-purpose FEA package from ALGOR, Inc. to analyze telecommunication optical switching components because it has the analysis and modeling capabilities that are needed and can be used to optimize packaging as well as MEMS devices.

Flexible Modeling

I begin my designs in SolidWorks. CAD software, such as SolidWorks, is ideal for creating the intricate features of a MEMS device. Then, I use ALGOR’s InCAD technology to seamlessly capture the geometry for analysis. This technology offers the user a choice of integration with CAD packages, because it works with Autodesk Inventor, CADKEY, Mechanical Desktop, Pro/ENGINEER and Solid Edge as well as SolidWorks. The software automatically generates a high-quality FEA mesh. I often use Superdraw, a precision finite element modeling program that comes with every ALGOR software package. Superdraw provides the capability to create and change the initial mesh, add or remove features for design optimization and extrude a model from 2-D profiles. I frequently use this last capability because MEMS geometry tends to be almost 2-D.

Calvet uses Superdraw, a precision finite element modeling program that comes with every ALGOR software package, to create and change the mesh, extrude a model from 2-D profiles and add or remove features from CAD geometry for design optimization.

A Wide Variety of Analysis Capabilities 

Once geometry has been acquired, the next challenge of designing and optimizing MEMS devices is to consider multiple, interdependent physical phenomena to which MEMS are sensitive. Dynamics, electrostatics, heat transfer, stress and even fluid effects are important in the design and optimization of the optical network components I design. 

ALGOR’s Mechanical Event Simulation (MES) is used for accurately predicting dynamic effects. MES combines large-scale motion and stress analysis of a finite element model in a single process and simulates real-world deformation and failure behavior. For example, I recently used MES to simulate the effects of a shock load on an optical telecommunications switch. The shock test is a requirement of the Telcordia standard for all telecommunication components. The results of the MES showed displacements and stresses over time, so that I could verify that the part would hold up under a 500 g shock load and that the displacement would not result in any interference with its surrounding components.

ALGOR’s Mechanical Event Simulation software was used to calculate stress and displacement results on this MEMS optical switch over time for an applied shock load.

MES even eliminates the need for kinematics software because it can simulate large-scale motion, such as the actuation of a MEMS device. It also can include linear and nonlinear materials. A wide range of material models is important since silicon, a common material in MEMS, is orthotropic while other materials that may be used can require nonlinear material models. 

In addition to MES, ALGOR’s electrostatic capabilities are useful for determining electrostatic forces, which are often used to actuate MEMS devices. These forces can be applied to a static stress analysis or MES to determine the displacement and resulting stresses that result from electrostatic forces. In the case of the optical switch, an electrostatic analysis was used to verify that the proposed electrostatic potential would actuate the working design as anticipated.

ALGOR’s heat transfer capabilities are also frequently used in MEMS design. Although I work with optical components, which don’t experience temperatures as high as some MEMS, there is some heat generated from light absorption, which may result in warpage because mirrors are not perfect reflectors. Beyond the actual MEMS device, it must attach to other parts of the telecommunication system, frequently through thermal bonding. When such a technique is likely to be used, heat transfer analysis is conducted to ensure that the MEMS device will not be adversely affected during manufacturing.

Finally, static stress analysis is used for the assembly to verify the structural integrity of the design and consider the effects of phenomena such as gravity sag.

All these analysis capabilities are based on the trusted finite element method. In contrast, many MEMS-specific software packages may use a combination of analysis methods such as boundary elements, p-elements and finite difference instead of or in addition to finite elements. Having used FEA for about 20 years, I prefer finite elements and trust the accuracy of this analysis method. 

All of ALGOR’s CAD support, modeling and analysis capabilities work within the same point-and-click, Windows-style interface. Having structural capabilities such as MES and static stress integrated with multiphysics enables me to simulate how electrostatic forces actuate a device or how heat might cause parts to deform from thermal stresses. 

The interface is consistently easy to use and was a deciding factor in our choice of a MEMS design solution. Some MEMS-specific software packages require the user to create user-defined code routines in order to use advanced analysis capabilities. In ALGOR, all tools and analysis capabilities, even advanced controls, are accessible through the user interface.

All of ALGOR’s CAD support, modeling and analysis capabilities work within the same point-and-click, Windows-style interface.

More Than Just MEMS

Some think that once you’ve designed the actual MEMS device, the job is done. At the end of the day, however, the MEMS device needs to live in the real world. It must be a product that you can put in an envelope and send to a customer. In MEMS design, everything that is not the actual MEMS device is grouped under the catch-all “packaging,” including components that interface the MEMS device with the macro-scale world as well as the components that house and protect the device. 

Designing and optimizing packaging for a MEMS device is an important part of the design process – one that can benefit from analysis and simulation as much as the design and optimization of the actual MEMS device. One advantage to selecting general-purpose FEA software such as ALGOR over MEMS-specific software is that the general-purpose software can easily be used for packaging optimization because of its capability to model and analyze virtually any type of design.

While there are a few other tools on the market for MEMS design and optimization, few are as flexible, easy to learn, simple to use and have the full range of FEA-based analysis capabilities for Mechanical Event Simulation and multiphysics as ALGOR.

Robert Calvet has been using ALGOR FEA for 20 years. SiWave, Inc., is a supplier of optical switching components and subsystems. SiWave, Inc., is funded by Draper Fisher Jurvetson (www.dfj.com), the leading, early-stage information technology venture capital firm.

Click here for more on how SiWave, Inc. used ALGOR software.