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ENGINEERING: MEMS


electrostatics. Finally, there is an issue with materials. ‘In the MEMS world you essentially make your own materials, and the foundry must have data about them.’ Coventor’s flagship product, CoventorWare,


Spider mite with legs on a mirror drive assembly. (Image courtesy Sandia National Laboratories)





Various levels of integration In looking at commercial MEMS software, note that all packages are based on a multiphysics engine and then add other features to various degrees. With its strong emphasis on multiphysics, Comsol offers an add-on called the MEMS Module for solving problems that couple structural mechanics, microfluidics and electromagnetics. It provides a user interface to account for both slide-film and squeezed-film damping and also provides predefined couplings for microfluidics modelling. For example, the ‘flow with species transport’ interface can be used to model the convection and diffusion of species within a fluid, and the ‘electro-osmosis’ interface can model electrically conductive fluids. Ionic solutions subjected to electric fields, diffusion, convection and migration to drive transport can also be modelled using the ‘electrokinetic flow’ interface. As for dealing with the electronics and process aspects, it provides lumped-parameter extraction and connection to external electrical circuits through SPICE netlists. Other packages have a stronger focus on bridging the gap between MEMS simulations and EDA (electronic design automation) environments for the co-design, optimisation and verification of MEMS and their accompanying ICs. Two of the most popular suppliers of such software are Coventor and Intellisense, both MIT spinoffs with similarly sophisticated product offerings. Interestingly, as its multiphysics engine Coventor uses Ansys, while Intellisense chose Abaqus. Covergent president Mike Jamiolkowski


notes that his company has added extensive MEMS-specific knowledge to his simulator. Damping is very important at the micro level, and ‘we’re experts at damping algorithms’. Then there are also contact issues through


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is a suite composed of three sub-products. First, Architect3D provides a library of MEMS building blocks that allow the quick assembly and simulation of a MEMS design in a multiphysics simulator. Second, Designer includes an all-angle layout tool with MEMS-specific features including design rule checks and makes it easy to go from a 2D layout to a 3D solid model. Third, Analyzer provides finite-element solvers, but being based on behavioural modelling it also requires no meshing. Models derived from the MEMS behavioural modelling library have the advantages of being fully parametric with respect to manufacturing and design variables, and accurately modelling nonlinear effects that are crucial to the function of some MEMS (such as angular rate sensors) and undesirable in others.


‘If gravity plays a far smaller role in the micro domain, virtually all other physical, electromechanical and thermal effects take on major significance’


Links to EDA packages For system-level simulation there’s also an interface into Matlab’s Simulink. And more recently, the company announced SEMulator3D, which emulates MEMS and semiconductor fabrication steps to perform virtual fabrication of a MEMS device before running actual silicon. In addition, MEMS+ for Cadence Virtuoso implements a 3D user interface for MEMS design entry, automatic creation of MEMS symbols, netlists, models and PCells and is compatible with the IC design flow within Cadence. The second product in this group of companies is the IntelliSuite from Intellisense, which likewise consists of a number of modules. Synple handles schematic capture, component-based design exploration, meshing, masking and 3D synthesis; Blueprint is for physical design, parametric studies, layout/design-rule checking/and tapeout;


SCIENTIFIC COMPUTING WORLD OCTOBER/NOVEMBER 2010


CleanRoom is dedicated to process flow design, debug and visualisation; FastField covers the multiphysics solvers; while EDA Linker integrates MEMS and ASIC (application specific IC) design flows. It provides interfaces to tools from Cadence, Mentor, Synopsis, Matlab and Tanner Research. Users can convert electromechanical devices such as gyroscopes, resonators or switches into an equivalent hardware-description language such as VHDL, Verilog, HSPICE, PSPICE and Simulink HEX flows. In this regard, associate professor Sazzadur Chowdhury of the Electrical and Computer Science Department at the University of Windsor, who heads up its MEMS effort, says that these two packages continually leapfrog each other so they have much the same features. However, with Intellisense he is particular impressed with RECIPE, a RIE/ ICP (reactive ion etch/inductive coupled plasma) etch process simulation tool for use in designing microstructures. With it, you can layout a microstructure and automatically simulate popular types of etching processes, and it supports the simulation of both ion- assisted etching and deposition processes and is claimed to be the only tool on the market for simulating the deep etching of silicon and other substrates.


Coming from the EDA domain This last product emphasises the importance of process capabilities, and a third software supplier, SoftMEMS, comes from this industry, being a spinoff of EDA-tool vendor Tanner Research. MEMS Pro is for Windows, while MEMS Xplorer is the Unix version. Functionalities include mixed MEMS/IC schematic capture and simulation, full custom mask layout capability and verification, 3D model generation and visualisation, behavioural model creation and links to 3D analysis packages. Interesting here is the MEMS Modeler, which generates behavioural models ready for system simulation with electronics and packaging from 3D data from finite-element analysis. Complex, finite- element models involving a large number of degrees of freedom are reduced to behavioural models with a few master degrees of freedom. The company’s products support SAT, IGES and APDL file formats, and for multiphysics studies they interface with Ansys, Comsol and Oofelie Multiphysics.


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