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1 High-Reliability Accelerometer and Pressure Sensor Design and Test (invited paper) Presentation at the 4th Annual Conference on MEMS Testing and Reliability by Tom Kwa, Meggitt Sensing Systems October 18 th, 2012 The Meggitt information Sensing contained Systems. in this document is NON-TECHNICAL considered non-technical DATA DEFINED data defined UNDER under THE section ITAR of the International Traffic in Arms Distribution Regulations Statement: (ITAR) Public and is Release. approved for public distribution.

2 Meggitt organization and capabilities

3 Company history and achievements in MEMS ; Endevco Laboratories founded in Los Altos, CA First MEMS sculptured silicon diaphragm (1976) First and only 200,000g MEMS accelerometer (1982) First MEMS VC accelerometer with internal electronics (1984) First monolithic MEMS accelerometer for crash testing (1987) First wafer-level packaged, surface-mountable accelerometer (2004) First optimally-damped MEMS crash-test accelerometer (2007) Smallest single-chip triaxial PR accelerometer (2008) Most survivable accelerometer (2008)

4 Meggitt s markets

5 Sunnyvale, CA MEMS facility Meggitt s MEMS fab SAN FRANCISCO Silicon Valley Meggitt Sensing Systems NON-TECHNICAL DATA DEFINED UNDER THE ITAR

6 Meggitt MEMS facility! 20,000 square feet! 3,500 square feet of Class 10,000 or better clean room Photo! Wafer Fabrication! Wafer Bonding and Dicing! Assembly & Test Diffusion Etch Assembly & Test Thin films

7 Features differentiating Meggitt s MEMS designs! Small die size (1-3 mm 2 footprint, mm height)! Robust and reliable designs (used in mission-critical applications, e.g., fuzes and pacemakers)! High performance (high sensitivity, resonance, over-range) Meggitt Sensing Systems US Patented

8 Origins of device unreliability! Mechanical Fracture (over-load, stress concentration) Stiction (humidity, particles) Stress (base strain, conditioning)! Electrical Shorts (ESD, metal diffusion) Opens (ESD, metal corrosion) Drift (mobile ions, contact surface, stress relief)

9 Example 1: Pacemaker accelerometer

10 Requirements for implantable accelerometers Critical:! Reliability! Low power consumption! Small size Important:! Sensitivity! Transverse sensitivity! Dynamic range! Linearity! Noise! Frequency response

11 Endevco model accelerometer! Used for over 15 years in implantable heart devices without field returns! Variable Capacitance Accelerometer! 2.1 x 2.9 x 0.8 mm 3

12 Endevco model accelerometer in surfacemount package

13 Endevco model wafer-level packaged, surface-mount accelerometer Covered by US Patents 4,999,735 and 7,696,083

14

15 Over-range and anti-stiction features in Endevco model Meggitt Sensing Systems Meggitt Sensing Systems

16 Acceleration test 1400g (x, y and z) Mechanical Fracture (over-load, stress concentration) Stiction (humidity, particles) Stress (base strain, conditioning) Electrical Shorts (ESD, metal diffusion) Opens (ESD, metal corrosion) Drift (mobile ions, contact surface, stress relief) Meggitt Sensing Systems Meggitt Sensing Systems

17 Shock test (2300g) Mechanical Fracture (over-load, stress concentration) Stiction (humidity, particles) Stress (base strain, conditioning) Electrical Shorts (ESD, metal diffusion) Opens (ESD, metal corrosion) Drift (mobile ions, contact surface, stress relief)

18 Temperature cycling test (-55 to 125ºC) Mechanical Fracture (over-load, stress concentration) Stiction (humidity, particles) Stress (base strain, conditioning) Electrical Shorts (ESD, metal diffusion) Opens (ESD, metal corrosion) Drift (mobile ions, contact surface, stress relief)

19 Hermeticity test Mechanical Fracture (over-load, stress concentration) Stiction (humidity, particles) Stress (base strain, conditioning) Electrical Shorts (ESD, metal diffusion) Opens (ESD, metal corrosion) Drift (mobile ions, contact surface, stress relief)

20 Ball shear test

21 Endevco model performance and physical specifications Parameter Nominal range Shock limit Dimensions [mm] Nominal sensitivity Value +/-2 g 10,000 g 2.1 (W) x 2.9 (L) x 0.8 (H) pf/g Transverse sensitivity 0.5% Frequency response Hermeticity Non-destructive shear Electrical isolation Storage temperature khz <5E-8 cc/s (MIL-STD-883) 2,000 gram >10 GΩ -55 to 125ºC (minimum)

22 Example 2: Bunker-buster accelerometer

23 Requirements for accelerometers in high-shock applications Critical:! Reliability! Survivability! Minimal zero-shift after shock Important:! Small size! Sensitivity! Transverse sensitivity! Dynamic range! Resonance modes! Frequency response

24 Endevco bunker-buster accelerometer US Patented

25 Survivability enhancements in Endevco model 72 accelerometer! Mechanical stops prevent damage to die from high-g over-range inputs Base and lid serve as stops (z-axis) walls for x, y Approximately 3 times full-scale range! Light damping attenuates resonance to prevent damage due to ringing Mechanism is squeeze-film gas damping 5% nominal (can be adjusted) Additional benefit is preventing saturation of signal conditioning circuitry

26 Endevco model 2925 Comparison Shock Calibrator for low-g shock testing

27 Shock survivability test set-up! Testing on Hopkinson Bar! Tested over temperature (hot and cold)! 20,000g range unit survived to 240,000g in sensitive and cross axes

28 Endevco model 2973A Hopkinson bar for high-g shock testing

29 Survivability test data

30 Zero-shift-after-shock results! 20,000g (1x) sensitive axis < 30g! 80,000g (4x) sensitive axis < 40g! 80,000g (4x) cross axis < 40g

31 Base strain test results

32 Very-High G shock machine

33 Cannon and sled tests

34 Flight test

35 Example 3: Aircraft tire pressure sensor

36 Requirements for pressure sensors on aircraft tires Critical:! Reliability! Output stability! High-temperature survivability Important:! Pressure over-range! Acceleration sensitivity! Breakdown voltage

37 General factors affecting die stability! Materials E.g., Impurities, metal stack! Design Layout Geometry! Design Process E.g., Bonding! Facility Equipment E.g., Uniformity in Photo, Diffusion, Etch! Facility Environment Particle count, contaminants

38 General factors affecting pressure transducer stability! Materials E.g., Substrate, adhesives! Design E.g., Package build-up, mounting technique! Post-assembly treatment E.g., Burn-in scheme

39 Design features of Endevco model hightemperature pressure sensor! Silicon-On-Insulator starting material! High-temperature capable metal stack Meggitt Sensing Systems Meggitt Sensing Systems

40 Thermal cycling profile (-55 to 260ºC) Time Time

41 Summary! Parameters measured to evaluate device reliability include zeromeasurand output (zero offset), sensitivity, and resistance, before, during and after subjecting the devices to their full-scale operating measurement range while also subjecting the devices to elevated temperatures, humidity and vibration levels to screen for or induce early mechanical or electrical failure! Design features required to ensure high reliability are driven by the application! Despite careful design and various qualifications, extensive in-production testing is unavoidable to guarantee high reliability which comes at a cost

42 Further reading [1] Practical Understanding of Key Accelerometer Specifications, Technical Paper No. 328, Meggitt (San Juan Capistrano), Inc. [2] T. Kwa, Wafer-level packaged accelerometer with solderable SMT terminals, IEEE Sensors 2006, Daegu, Korea, Oct 22-25, [3] T. Kwa, G. Pender, J. Letterneau, K. Easler, R. Martin, A new generation of high-shock accelerometers with extreme survivability performance, 53 rd NDIA Fuze Conference, Orlando, FL, May 19-21, [4] T. Kwa, G. Pender, J. Letterneau, Triaxial accelerometer for placement in the ear canal, Nanotech 2010, Anaheim, CA, June 21-24, [5] Model 2925 AACS Comparison Shock Calibrator (POP), Datasheet No. 2925, Meggitt (San Juan Capistrano), Inc.

43 Contact info Tom Kwa, PhD, Design and Development Manager Meggitt Sensing Systems 355 N. Pastoria Ave Sunnyvale, CA Tel: +1 (408)

44 Thank you 44