MEMS Actuator Design and Simulation

MEMS actuators, devices that convert electrical energy to mechanical motion, comprise more than 50% of the rapidly growing MEMS market. They are used in a variety of optical, RF, and industrial applications. Examples include variable capacitors (varactors), micro-relays for low-power VLSI, optical phase-shifters, next generation displays, micro-grippers for robotic surgery, and focusing mechanisms for cameras in mobile devices.

MEMS actuators have a wide variety of actuation mechanisms

Figure 1: MEMS actuators have a wide variety of actuation mechanisms

Design Challenges

Designers face increasing pressure to reduce both time-to-market and cost. These pressures are forcing changes in the traditional reliance on in-house tool development and repetitive build and test cycles. The key challenge for designers of MEMS actuators is to create a device that meets the performance criteria for force, displacement, switching time, power, size, temperature stability, and reliability. To succeed, designers must properly account for 3D electrostatic fringing, along with coupled electromechanics with contact, buckling, and gas damping effects.

Coventor’s MEMS design platform provides significant advantages in speed, accuracy, and capacity to address these challenges. The platform promotes greater team productivity, by allowing multiple design areas to be investigated concurrently.   Rapid and simultaneous exploration at the device level can occur, while control circuitry is simultaneously optimized and the effects of process and/or material changes are investigated.

Coventor’s integrated platform applied to a pre-stressed electrostatic comb drive actuator

Figure 2: Coventor’s integrated platform applied to a pre-stressed electrostatic comb drive actuator

1. Select and optimize device architecture

Designers can first evaluate and optimize the MEMS actuator across a wide variety of geometric changes,  material specifications and process tolerances. Each of these changes might impact device performance and manufacturability separately, and the complex interaction between them needs to be examined in real-time.   Simulation speed and capacity is a key advantage in this design process. The ability to rapidly explore and choose an optimal design combination, when using such a large number of variables,  can help designers avoid sub-optimal and overly-costly approaches early in the design process.

Coventor MEMS+ provides an excellent solution to this problem.  Using MEMS+, a fully-parametric design of the device can be created using an extensive library of MEMS specific components. These MEMS specific components are higher-order finite elements, specifically built to quickly execute fully-coupled multi-physics simulations within MATLAB®, Simulink®, and/or Cadence Spectre®. It is not uncommon for users to experience 100-to-1,000X speed advantages in MEMS+ compared to conventional FEA approaches.  This makes it possible to explore hundreds of process and design variations within a single day. Consider the simulation of the pull-in and lift-off instabilities of an electrostatic actuator. Simulations in conventional FEA tools may take 8 hours or more, versus a few minutes with a MEMS+ model. Transient simulations that include gas damping effects are not feasible with conventional FEA, but quite tractable with MEMS+ models.

2. Device level design and verification

Next, 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.

Coventor’s customized FEA Tools (Designer & Analyzer) are next used to fine tune and validate the MEMS+ design.  Specialized algorithms in the FEA tools ensure that:

  • 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.

CoventorWare achieves this using our proprietary Hybrid FEM & BEM solver. 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.

Coventor’s mesh for electrostatics is optimized for MEMS design

Figure 3: Coventor’s mesh for electrostatics is optimized for MEMS design

The non-linear behavior of piezoelectric and electro-thermal actuators, as seen in Figure 4 (below), can also be simulated. Designers can determine contact forces, insertion loss, transient switching time and can also perform shock analysis (drop-tests). Additionally, tools to simulate squeeze-film, slide-film and Stokes gas damping enable users to calibrate or verify MEMS+ models rather than relying on simple analytical formulae or experimental measurements.

Thermal Actuator

Figure 4: Thermal Actuator

3. System-level integration and verification

MEMS 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 may increase the chances of costly redesigns late in the development 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 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 even view actuator motion in 3D.

Array of 4-terminal NEM relays composed by inserting <em>MEMS</em>+ models in a Cadence schematic

Figure 5: Array of 4-terminal NEM relays composed by inserting MEMS+ models in a Cadence schematic

A Complete Platform

Coventor’s platform for MEMS actuator design and simulation is fast, accurate, and comprehensive. Using this platform, designers can simulate individual MEMS actuators and arrays,  including their control circuitry. This new, integrated methodology based on MEMS-specific tools enables today’s actuator designers to optimize device performance and meet their increasingly challenging 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
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