MEMS Actuator Design and Simulation
MEMS have transitioned from being a research curiosity to widespread adoption in consumer electronic products and have forever changed the user experience. MEMS actuators, miniaturized devices that transduce energy to mechanical motion, compose greater than 50% of this rapidly growing market and are used in a variety of RF, optical and industrial applications. Examples include micro-relays for VLSI, phase-shifters, camera micro-shutters, next generation displays, micro-grippers for robotic surgery, confocal microscopy, and micro-valves.
Figure 1 – MEMS Actuator market segmented by actuation mechanism
Design Challenges
The key challenge for designers of MEMS actuators is to create a device that exceeds the performance criteria for force, displacement, frequency, resolution, hysteresis, temperature stability, and size. To succeed, designers must properly account for 3D electrostatic fringing, coupled electromechanical contact, buckling, parametric-bias voltage for amplification, gas damping, anchor loss, and package effects. On top of this, designers face increasing pressures to reduce both time-to- market and costs. These pressures render traditional design flows based on build-and-test obsolete and the development of in-house code, impractical.
Coventor’s platform provides significant advantages in speed, accuracy, and capacity to address these challenges. In addition, the platform enables greater team productivity by enabling rapid exploration at the device level to occur simultaneously with optimizing the control circuitry, evaluating package effects, and/or investigating the effects of process and/or material changes.
Figure 2 – Coventor’s integrated platform for a pre-stressed comb drive actuator
- Select and optimize device architectureDesigners first evaluate and optimize the MEMS actuator across a large number of geometric changes, materials and process tolerances that impact device performance and manufacturability. Here, simulation speed and capacity is a key advantage. The ability to rapidly explore and determine the most optimal combination from such a large number of variables will help designers avoid committing to non-optimal and/or overly-costly architectures early in the design cycle.
Coventor’s MEMS+ provides an excellent solution. A parametric design of the device is created using an extensive library of MEMS specific components. These components are higher-order finite elements, specially crafted to run fully-coupled multi-physics simulations extremely fast in Matlab, Simulink, and/or Cadence Spectre. It is not uncommon for users to experience 100x -1,000x speed advantage vs. traditional FEA approaches, making it possible to explore hundreds of highly sensitive design changes within a single day.
Consider the simulation of pull-in and lift-off hysteresis. Traditional approaches use Newton’s method, where the solver employs a converged solution from the previous voltage step. This can result in excessively-long simulation times and convergence failures near the instabilities. Coventor uses an Arc-Length Corrector approach which adaptively implements predictor and corrector numerical schemes at different parts of the pull-in curve, making it robust for MEMS design Coventor’s technique delivers fast, reliable results, enabling designers to explore hundreds of designs in a fraction of the time it takes using traditional methods.
- Device level design and verificationNext, a more detailed analysis that includes the effects of anchors, packaging stress, and the impact from various damping mechanisms are evaluated. At this stage, accuracy is critical and hence Coventor’s customized FEA Tools, (Designer & Analyzer) are used to fine tune and validate the design. The specialized algorithms ensure contact electro-mechanics and relevant 3D fringing fields are considered, the solution is meshed optimally, and charge based and/or voltage-based coupled-electromechanics are available to ensure convergence on the most difficult of problems.
Coventor achieves this using our proprietary Hybrid FEM & BEM solver. As stated above, this solver adaptively refines the finite element mesh used for electrostatics, providing an accurate result without burdening the user with unnecessary mesh refinement. In addition, unlike traditional FEA approaches, the air volume in Coventor’s hybrid method is not meshed. This is most significant when solving for contact hysteresis and evaluating other fine details associated with the motion and reliability of these devices. Using our approach, the simulation is more accurate, more robust, and significantly faster.
Figure 4 – Coventor’s mesh for electrostatics is optimized for MEMS design
The non-linear behavior of piezoelectric and electro-thermal actuators, as seen below, can also be simulated. Designers can determine contact forces, insertion loss, transient switching time and perform shock analysis (drop-tests). Additionally, tools to simulate squeeze-film, slide film and Stokes gas-damping, anchor losses and thermo-elastic damping enable users to predict energy losses rather relying on simple analytical formulae or back-fitting to experimental results.
Figure 5 Thermal Actuator
- System level integration and verificationMEMS actuators do not operate in isolation; they perform their tasks in the context of a system that includes an electronic circuit. Hence, prior to fabrication, the complete system needs to be simulated and tuned. To accomplish this, designers have traditionally built handcrafted, reduced-order models based on analytical expressions or extracted from FEA simulation. The problem with this approach is two-fold. First, building the reduced order models can be very time-consuming. Second, and more important, the analytical equations used are over-simplified and the reduced order models created do not include non-linear effects. Taken together, this traditional approach significantly impacts the IC and system designers and can result in very costly redesigns at the very end of the design cycle.
Coventor’s MEMS+ solution is tailored to address this need. A parametric model which includes the non-linear behavior of the mechanical MEMS actuator is created and run in the Mathworks Simulink or Cadence Spectre. System designers can evaluate the full behavior of the device including the IC circuit. They may also simulate an array of MEMS devices using this approach. Device and system variables can be parametrically varied to control linearity and perform sensitivity analysis. Designers can also simulate actuator force and displacement along with drive current/voltages and also view motion in 3D.
Figure 6 Cadence array of 4-terminal NEM relays
A Complete Platform
Coventor’s platform for MEMS Actuator design and analysis is fast, accurate, and comprehensive. The unique solvers offer users with the best numerical methods tuned for MEMS design. Using this platform, designers can solve individual MEMS actuators, simulate MEMS arrays, co-simulate these devices in the surrounding package, and include the control circuitry. This integrated-flow and these customized solvers make it practical for today’s actuator designers to optimize the device performance and meet their increasingly difficult time to market goals.
References
MEMS actuators and sensors: observations on their performance and selection for purpose – D J Bell, T J Lu, N A Fleck and S M Spearing –6/2005
A novel large displacement electrostatic actuator: pre-stress comb-drive actuator, Journal of Micromechanics and Microengineering, J C Chiou and Y J Lin
Electrostatic Zipping Actuators and Their Application to MEMS – Jian Li 1/2004
Stable zipping RF MEMS varactors, Suan Hui Pu, Andrew S Holmes, Eric M Yeatman, Christos Papavassiliou and Stepan Lucyszyn, Imperial College London, UK – 3/2010
Repulsive-force out-of-plane large stroke translation micro electrostatic actuator – S He1, R Ben Mrad2 and J Chong2 – 6/2011
System Integration of High Voltage Electrostatic MEMS Actuators – Jean-François Saheb, Jean-François Richard, R. Meingan, M. Sawan, and Y.Savaria – DALSA Semiconductor Inc., 5/2005
Low-voltage small-size double-arm MEMS actuator – N. Biyikli, Y. Damgaci and B.A. Cetiner – 3/2009
Pull-In Analysis of Torsional Scanners Actuated by Electrostatic Vertical Combdrives – Daesung Lee and Olav Solgaard – Journal of MicroElectroMechanicalSystems, 10/2008
A fully wafer-level packaged RF MEMS switch with low actuation voltage using a piezoelectric actuator – Jae-Hyoung Part et. al – 9/2006
