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© Meggitt Sensing Systems. 120.10 NON-TECHNICAL DATA DEFINED UNDER THE ITAR Distribution Statement: Public Release.

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 18th, 2012

The information contained in this document is considered non-technical data defined under section 120.10 of the International Traffic in Arms Regulations (ITAR) and is approved for public distribution.


Meggitt organization and capabilities


Company history and achievements in MEMS

; Endevco Laboratories founded in Los Altos, CA

First MEMS sculptured silicon diaphragm (1976)

First optimally-damped MEMS crash-test accelerometer (2007) Smallest single-chip triaxial PR accelerometer (2008)

Most survivable accelerometer (2008)

First wafer-level packaged, surface-mountable accelerometer (2004)

First and only 200,000g MEMS accelerometer (1982) First MEMS VC accelerometer with internal electronics (1984) First monolithic MEMS accelerometer for crash testing (1987)


Meggitt’s markets


Meggitt’s MEMS fab

SAN FRANCISCO

Sunnyvale, CA MEMS facility

Silicon Valley


Meggitt MEMS facility

!  20,000 square feet !  3,500 square feet of Class 10,000 or better clean room

!  Wafer Fabrication !  Wafer Bonding and Dicing !  Assembly & Test

Assembly & Test

Diffusion

Etch

Photo

Thin films


Features differentiating Meggitt’s MEMS designs

!   Small die size (1-3 mm2 footprint, 0.2-1 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


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)


Example 1: Pacemaker accelerometer


Requirements for implantable accelerometers

Critical: !   Reliability !   Low power consumption !   Small size Important: !   Sensitivity !   Transverse sensitivity !   Dynamic range !   Linearity !   Noise !   Frequency response


Endevco® model 29950 accelerometer

!   Used for over 15 years in implantable heart devices without field returns !   Variable Capacitance Accelerometer !   2.1 x 2.9 x 0.8 mm3


Endevco® model 29950 accelerometer in surface-mount package


Endevco® model 40366 wafer-level packaged, surface-mount accelerometer

Covered by US Patents 4,999,735 and

7,696,083


Over-range and anti-stiction features in Endevco® model 40366

Meggitt Sensing Systems



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)


Shock test (2300g)




Temperature cycling test (-55 to 125ºC)




Hermeticity test




Ball shear test


Endevco® model 40366 performance and physical specifications

Parameter Value

Nominal range +/-2 g

Shock limit 10,000 g

Dimensions [mm] 2.1 (W) x 2.9 (L) x 0.8 (H)

Nominal sensitivity 0.15-0.35 pF/g

Transverse sensitivity 0.5%

Frequency response -20 dB @ 1 kHz

Hermeticity <5E-8 cc/s (MIL-STD-883)

Non-destructive shear 2,000 gram

Electrical isolation >10 GΩ

Storage temperature -55 to 125ºC (minimum)


Example 2: Bunker-buster accelerometer


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


Endevco® bunker-buster accelerometer

US Patented


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


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


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


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


Survivability test data


Zero-shift-after-shock results

!   20,000g (1x) sensitive axis < 30g !   80,000g (4x) sensitive axis < 40g !   80,000g (4x) cross axis < 40g


Base strain test results


Very-High G shock machine


Cannon and sled tests

http://www.youtube.com/watch?v=2_JTT3OsDJQ


Flight test


Example 3: Aircraft tire pressure sensor


Requirements for pressure sensors on aircraft tires

Critical: !   Reliability !   Output stability !   High-temperature survivability Important: !   Pressure over-range !   Acceleration sensitivity !   Breakdown voltage


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


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


Design features of Endevco® model 30225 high-temperature pressure sensor

!   Silicon-On-Insulator starting material !   High-temperature capable metal stack

Meggitt Sensing Systems Meggitt Sensing Systems


Thermal cycling profile (-55 to 260ºC)

Time

Time


Summary

!   Parameters measured to evaluate device reliability include zero-measurand 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


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, 2006.

[3] T. Kwa, G. Pender, J. Letterneau, K. Easler, R. Martin, “A new generation of high-shock accelerometers with extreme survivability performance”, 53rd NDIA Fuze Conference, Orlando, FL, May 19-21, 2009.

[4] T. Kwa, G. Pender, J. Letterneau, “Triaxial accelerometer for placement in the ear canal”, Nanotech 2010, Anaheim, CA, June 21-24, 2010.

[5] “Model 2925 AACS Comparison Shock Calibrator (POP)”, Datasheet No. 2925, Meggitt (San Juan Capistrano), Inc.


Contact info

Tom Kwa, PhD, Design and Development Manager Meggitt Sensing Systems 355 N. Pastoria Ave Sunnyvale, CA 94085 Tel: +1 (408) 739-3533 [email protected]

© Meggitt Sensing Systems. 120.10 NON-TECHNICAL DATA DEFINED UNDER THE ITAR Distribution Statement: Public Release. 44

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