MEMS ModelingDr. Junwei LuSchool of Microelectronic Engineering

Contents of MEMS Modelingz 1.Computational Methods for MEMSDesignz 2. Simulation and Optimization of MEMSz 3. Applications of RF MEMS

1. Computational Methods forMEMS Designz 1.1Microelectromechanical Machines andComputationz 1.2 Efficient MEMS Simulation: Threemain computational challenges

1.1 MicroelectromechanicalMachines and ComputationzWhat is MEMS?– Using the fabrication techniques and materials ofmicroelectronics as a basis, MEMS processes constructboth mechanical and electrical components.– MEMS is a fabrication approach that conveys theadvantages of miniaturization, multiple components,and microelectronics to the design and construction ofintegrated electromechanical systems.– All MEMS fabrication approaches share these three keycharacteristics.

MiniaturizationzzzMiniaturization brings many advantages to theperformance of electromechanical devices andsystems.Structures that are relatively small and light leadto devices that have relatively high resonantfrequencies. These high resonant frequencies inturn mean higher operating frequencies andbandwidths for sensors and actuators.Thermal time constants – the rates at whichstructures absorb and release heat – are shorter forsmaller, less massive structures.

zzBut miniaturization is not the principle drivingforce for MEMS.Because MEMS devices are by definitioninteracting with some aspect of the physical worldsuch as– Pressure,– Inertia,– Fluid flow,– Light,

MEMS devise

Multiplicityz Multiplicity,or the batch fabricationinherent in photolithographic-based MEMSprocessing, is as important asminiaturization.z It provides two important advantages toelectromechanical devices and systems.z Multiplicity makes it possible to fabricateten thousand or a million MEMScomponents as easily and quickly as one.

zzzThe second, equally important advantage ofmultiplicity is the additional flexibility in thedesign of massively parallel, interconnectedelectromechanical systems.The multiplicity characteristic of MEMS hasalready been exploited in the development andrecent demonstration of a digital micromirrordisplay.In an array about the size of two standard postagestamps, over a million mirrors-each the size of ared blood cell-collectively generate a complete,high-resolution video image.

MicroelectronicszzzFinally, neither the miniaturization nor themultiplicity characteristics of MEMS could befully exploited without microelectronics.The microelectronics integrated into MEMSdevices provides the latter with intelligence andallows closed-loop feedback systems, localizedsignal actuator arrays.The considerable investment that has been put intomicroelectronics materials and processing, and theexpertise built up in this field, is helping thedevelopment of MEMS devices.

MicroelectromechanicalMachines and Computationz MEMSis an exciting applications areaz MEMS devices use fabrication technologysimilar to traditional semiconductor chipsbut operate as mechanical as well aselectrical devicesz The design of MEMS devices is still verymuch an art based on practicalexperimentation

z MEMSdesign inherits all the difficulties ofmodeling electrical semiconductors;additionally, it requires thermal andstructural modeling to simulate themechanical properties of MEMS devices.z Surface tension from humidity can alsobecome a major physical force at themicroscopic level.

1.2 Efficient MEMS Simulation: Threemain computational challengesz Theaction of MEMS devices such assilicon micro-accelerometer involvesseveral physical effects:– mechanical motion,– air damping,– electrostatic action,– and capacitive position detection.

z Detailedknowledge of all of these effects isa prerequisite for effective and efficientdesign.z The first step in reducing design time andallowing for aggressive design strategies isto develop simulation tools that will letdesigners try “what if” experiments in hoursinstead of months.

Three computational challengeszzFirst, faster algorithms are being developed forcomputing surface forces due to fields or fluidsexterior to geometrically complex, flexible threedimensional structures.Second, since the performance of mostmicromachined structures is due to a complicatedinteraction between structural stress, electrostaticor magnetic forces, and fluid tractions orpressures, approaches for coupling efficientdomain-specific solvers are under investigation.

zzzFinally, when it is embedded in a completed system,designers need accurate dynamical models that permitrapid simulation of system performance under a widevariety of inputs and scenarios, such as being inserted intoa feedback loop.And since direct simulation of 3D structures involvesthousands of degrees of freedom, all coupled together, fullnonlinear direct dynamic simulation can becomputationally infeasible in a typical workstationenvironment.Furthermore, designers tend to think in terms of modelswith only a few degrees of freedom that are well correlatedto modifiable parameters like dimensions or materialproperties.

MEMS design inherits all the difficulties of modeling electrical semiconductors; additionally, it requires thermal and structural modeling to simulate the mechanical properties of MEMS devices. z. Surface tension from humidity can also become a major physical force at the microscopic level.