Microelectromechanical Systems (MEMS)Reliability

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Microelectromechanical Systems
(MEMS)Reliability

• Richard L. Doyle, PE
• 5677 Soledad Rd.
• La Jolla, CA 92037
• Email r.doyle_at_ieee.org
• Web http//www.laacn.org/firms/doyle

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Microelectromechanical Systems Reliability

• Richard L. Doyle
• Dallas, TX
• 200 PM
• Jan. 22, 2009
• IEEE Reliability Society Symposium
• UTD - Session - 2B

• This reliability presentation applies to a broad
  scope of all small Mechanical Devices and applies
  to a MEMS Reliability Analysis

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Purpose - Provide

• An overview of all aspects of Microelectromechanic
  al Systems (MEMS) reliability engineering.
• A comparison of various mechanical reliability
  predictions and sources of data along with
  helpful methodology in using them.
• An understanding of important relationships with
  other reliability/design disciplines.

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Trustworthy - MEMS Airbag Accelerometers

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Thoratec HeartMate II LVAS

• By Thoratec and the Texas Heart Institute(THI)
• FDA approval of left-ventricular assist device
  (in process)

Non-Diseased
Diseased
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Introduction

• Reliability is established by the design and
  manufacturing process.
• You can demonstrate micro -mechanical reliability
  by test (high failure rate) but you need to use
  design principles to improve reliability.
• Prior to production, only a limited time and test
  samples are available.

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Reliability Approach

• High reliability is attained through good
  controls and analytical verification.
• First, define operational requirements or MTBF.
• MTBF can be translated into Safety Factor
  guidelines.
• Stress levels determine mechanical failure rates.
• Math model and mission profile predict
  availability.

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Customer Requirements

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Basic Reliability Equations

• li The ith Part Failure Rate, FITs
• lp The System Failure Rate

lp Series Equation (Non Redundant) MTBF Mean
Time Between Failure, Hours MTBF 1/ lp
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Basic Reliability Equations

• For Constant Failure Rate
• P(s) Probability of Success
• t Time without failure
• P(s) exp (- lp t)
• P(s) P(1) P(2) ...
• Q(s) Probability of Failure
• Q(s) 1 - P(s)

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Basic Reliability

• Use standard exponential reliability formulas and
  assume that the failure rates are constant.
• Parts which wearout are replaced prior to any
  appreciable increase in failure rate.
• Primary failure mode is wearout

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Environment and its affect on reliability

• The Environment contributes to part failures
• High Temperature causes failure
• High Humidity causes failure (corrosion)
• High Altitude causes failure (heat)
• Vibration causes failure
• Mechanical Shock causes failure

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High Failure Parts

• Highly stressed and high wear parts are major
  problems in the design. These parts must be
  replaced many times during the service life of
  the system.

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Customer Requirements

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Prediction - Present Techniques

• Electronic parts
• Millions of similar parts
• Billions of hours of operation
• Failure rate data with defined environmental and
  electrical stress conditions.
• Microelectromechanical System (MEMS) part is
  designed for a specific configuration and use.
• Subjected to large variations in stress levels,
  environment and temperature.

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Failure rate and Reliability data bases

• Microelectromechanical Systems Hardware
• JPL Publication 99-1 MEMS Reliability Assurance
  Guidelines For Space Applications

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Failure rate and reliability reference

• Chapters 18, 19 and 20 (By Doyle, Richard L),
  Handbook of Reliability Engineering and
  Management, Published by McGraw-Hill, Inc.
  January 1996.

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Example No. 1

• PART QTY CYCLIC TOTAL DESCRIPTION FAILURE
  RATE
• Micro-Switch 6 N/A 14.4 86.4
• Micro Relay 4 N/A 16.8 67.2
• Micro Motor 1 40/hr 15.2 15.2
• TOTAL Sys Failure Rate 168.8 (f/106 Hr)
• MTBF 1/0.0001688 5924 HOURS

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Bearings

• Jewel bearings
• Sleeve bearings
• Bearing materials
• Friction coefficients
• Wearout, L10 life

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L10 Bearing Life

• Number of hours/cycles at given load that 90
  will survive
• Equations MTTF, L10 lambda B1
• MTBF B1 L10

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Micro-Stress Analysis

• Perform analysis during development
• Identify and control high stresses
• Maximize life and confirm MS
• Identify life limiting stresses
• Compare strength versus max loads

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Stress

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Microelectromechanical Systems

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Microelectromechanical Systems

Digital Micromirror Device (DMD)
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Microelectromechanical Device Manufacturing

• MEMS Lithography
• MEMS Metal deposition
• MEMS Metal deposition
• MEMS Plasma etching
• MEMS Deep Reactive Ion Etching
• MEMS Wet etching
• MEMS Electroplating
• Bonding
• Dicing
• MEMS Characterization
• Not achievable due to cost and size

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Mechanical Failure Modes

• Tensile yield strength failure
• Ultimate tensile strength failure
• Compressive failure
• Failure due to shear loading
• Bearing failure
• Fatigue failure

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Mechanical Failure Modes (2)

• Metallurgical failures
• Brittle fracture
• Bending failure
• Failure due to stress concentration
• Failure due to flaws in material
• Instability failure

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Probabilistic FR Analysis

• Tool for random loading and various tolerances
• Design so product never fails
• Not achievable due to cost and size
• Design for low probability of failure
• Design values, Material properties and Applied
  loads have mean values and variations

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Normal Curve (Gaussian)

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Taylors Series

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Example

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Mechanical Reliability programs

• MECHREL (Mechanical Reliability Prediction
  Program)
• MRP (Mechanical Reliability Prediction Program
• RAM Commander for Windows
• Relex Mechanical

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Computing Reliability

• Use FR models developed in previous MEMS designs
• Probabilistic stress and strength analysis for
  each part
• Probabilistic stress and strength analysis for
  each critical part, with generic data for all
  others

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Key Benefits

• Predict Failure Rates Based on Scientific
  Calculations
• Identify Weak Structural Sections
• Identify High Areas of Wear
• Use Information to Improve Design
• Meet or Exceed Customer Requirements

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Next Steps

• Apply the Structural Analysis Equations to Your
  Design
• Apply the Probabilistic Approach to Your Next
  Project. It will improve your design and provide
  a high level of confidence in the design.