speed, accuracy and automation in mems simulation...
TRANSCRIPT
Speed, Accuracy and Automation in MEMS Simulation and Development C. J. Welham, Coventor, Paris
MEMS Design & Simulation Challenges
• Overview
• Simulation Challenges and Approaches
• Validation
• Case Studies
• Conclusion
• MEMS have evolved from DLP, inkjet and
airbag applications to high volume Consumer
Electronic devices >20% CAGR
• Democratization of MEMS causing a shift
in business growth from traditional MEMS
companies to semi-foundries and fabless
design houses.
• Consumer Electronic applications require
shorter product development cycles.
• Pressure to reduce size of MEMS
components whilst increasing performance
and integration.
Market data from Yole Development
Demand for MEMS is Changing
MEMS-based products are becoming smaller and more integrated
Decreasing size and greater integration pressures require more sophisticated models and simulations
• More sensitive spurious coupling between components
• More important to consider non-linearities
• More sensitive to parasitic effects
• More chance of design errors in interconnect
1-axis accel. 2-axis accel. 3-axis accel. IMU: 3-axis accel. + 3-axis gyro
Simulations must include more coupled
physics, non-linearity, parasitic effects
and the connected circuit/system
Simulation Challenges
Two Approaches
3D Design Entry
in Graphical UI
High Order FEA
- Designed for MEMS
High-order, coupled physics elements
Scripting in
MATLAB
or Python or
Meshed Model
Small 10 to 1000 DOF
Conventional Low Order FEA
- General Elements
Library of
generic, low-order
finite elements
Brick, tet, shell, and beam elements
Mesh
Generator
Meshed Model
Large 10k to 10M DOF
3D Geometry
High Order Benefits
Compatibility
Speed and Accuracy
Parameterization
3D Design Entry
in Graphical UI
High-order, coupled physics elements
Scripting in
MATLAB
or Python or
Meshed Model
Small 10 to 1000 DOF
High Order FEA
- Designed for MEMS
Compatibility
Internal Simulator
MATLAB
Simulink
Virtuoso
Simulate and Analyze Enter Design in 3D Visualize Results in 3D
Models with a small number of DOF
can run directly in MATLAB, Simulink and Cadence
High Order Finite Element Library:
MEMS coupled-physics specific, 3D, high-order, parametric
VerilogA
Models that run directly in other simulators allow MEMS and IC
Designers to collaborate easily
Design Capture
High Order FEA
Algorithmic Level
(Simulink)
Behavioral Level
(Virtuoso)
Transistor Level
(Virtuoso)
Physical Level
(Virtuoso)
High Order Elements allow
the automatic transfer of
models for use by IC or
System Designers
IC Designers MEMS Designers
Compatibility
Model Export
• High Order Models can be inherently parametric
– Explore design envelope
– Manufacturing/Sensitivity Analysis
Sidewall
Angle
CD loss
Mass
thickness
Parameterization
Mode Y
Variation of Frequency Y - 1000 Samples
Mode Z
Variation of Frequency Z - 1000 Samples
• Results from Monte Carlo Analysis
– Random input variables defined from mean and std. deviation
• Modal Analysis, 1000 samples
• 7s per sample
Parameterization
Variation of Frequency X - 1000 Samples
Mode X
*Self-aligned Vertical Electrostatic Comb drives for Micromirror Actuation”, Krishnamoorthy, U., Lee, D., Solgaard, O., JMEMS2003
• Electrostatic MEMS element 100s of times faster
Speed and Accuracy
Comb Finger - Element Level Validation
High Order Model
Low Order BEM Model
*Experimental Validation of Aluminum Nitride Energy Harvester Model with Power Transfer Circuit
S. Matova, D. Hohlfeld, R. van Schaijk, C. J. Welham, S. Rouvillois
Eurosensors XXIII conference 2009
• Compare both approaches and measurement
Validation – Device Level
Energy Harvester - Coupled-Physics, Multi-Domain
• Virtual fabrication MEMS device
– Voxel modelling for robust model build
• Silicon accurate 3D geometry
• Test design is correct before tape-out
• Communication with Foundry
– Process development
Validation - Process
Voxel Model
Release
Oxide
Bulk Silicon Thermal
Oxide
Polysilicon
Electrode
Epitaxial
Polysilicon
Hard Mask
Release Oxide
Deposition Via to Electrode Epi-poly growth and
Planarization DRIE Release Etch
Based on Chipworks teardown
Case Study - Gyroscope Benefits of MEMS specific element approach
• “The power of high order element models allows very rapid design studies
and optimize the design”
• “A 3D parametric study in low order conventional FEA can take significant
time, even though point simulations (modal) only take minutes. With high
order element models simulations take seconds: total simulation time is
therefore much lower”
• “Geometry variation is more difficult in conventional FEA ”.
Key challenges solved by Model Export
• “Writing a (VerilogA) model takes weeks and requires significant expertise.
Enhancements (e.g. spurious modes, parasitic Cs) and design variations
takes time, which is not always available.”
• “Simple hand written analytical models miss non-linearities and other
second order effects.”
• Upgrading and maintaining hand written models for ASIC design partners
is painful
Case Study - Microphone
Perform noise analysis
• Model supports noise analysis in Cadence Spectre
and accurately predict thermo-mechanical noise
• Model supports all relevant noise sources in your MEMS+IC system,
enabling you to evaluate noise cancelling strategies
Model includes mechanics,
electrostatics and fluidic
effects
Cadence Virtuoso schematic Noise analysis
in Cadence Spectre
Microphone
Case Study - Switch Switch Dynamic Response
Time (s)
Tip
position
(m)
High order elements are
very fast and can easily
combine mechanical,
electrical and fluidic effects
in a transient simulation
RF switch tip position, closing and opening
Simulated gas pressure on
electrodes as RF switch closes
Other Examples
DLP mirror, 11 DoF
RF Switch
119 DoF
Gyroscope, 96 DoF Accelerometer, 67 DoF
The High Order approach is general: it has been used to create
compact, accurate models of many real-world designs
Ring Gyro, 345 DoF
Ring Resonator, 727 DoF
Conventional FEA
Is there a place for conventional FEA models
– YES!
– Some problems are better modelled with conventional FEA
• Stress, packaging, multi-conductor, parasitic and damping problems
– TED, Gas, Anchor
– Some geometries are not suited to high order elements
Simulate stress Simulate package
Simulate FBAR
In Conclusion • Use High Order FEA for Speed, Accuracy and Automation
– MEMS-specific geometries and coupled physics
• Benefits over traditional FEA
– Much faster simulations due to reduced degrees of freedom
– Parameterization enables rapid design exploration and optimization
– Compatible with MATLAB, Simulink and Cadence simulators
– Simulate the full dynamic response of sensors and actuators
– Simulate closed-loop operation of sensors
– Deliver models of MEMS devices to system and IC designers
– Perform noise analysis of sensors
• Conventional FEA still useful for certain design tasks
• Leverage Process Modeling
– Validate design with process prior to tape out