This document summarizes the design, fabrication, and characterization of a piezoelectric MEMS accelerometer. It discusses the motivation for using MEMS and piezoelectric materials for accelerometers. It then describes the design of a square and circular accelerometer with analytical models. The fabrication process integrates PZT thick film and MEMS techniques. Characterization shows the accelerometer has a resonance frequency of 25.8 kHz, voltage sensitivity of 0.28 mV/g, and charge sensitivity of 0.46 pC/g, meeting design goals.

1. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Piezoelectric MEMS Accelerometer - PiMEMS
Christian Gammeltoft Hindrichsen
Supervisor: Professor, Erik V. Thomsen

2. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Piezoelectric Accelerometer
Motivation
Design
Fabrication
Characterization
Conclusion

3. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Piezoelectric Accelerometer
What is piezoelectricity and how does an accelerometer work?

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Piezoelectric effect
http://electronicdesign.com/

5. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Accelerometer operating in bending mode MEMS springs:
Cantilever
Piezoelectric
V
Plate
acceleration
F = m·a
Mass Bridge
Spring
Fixed frame
Membrane

6. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Motivation
Why use MEMS and what is state-of-the-art?

7. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Applications
1 mm 100 μm 10 μm 1 μm 100 nm
Bulk PZT Thick Film PZT Thin Film Nanotech

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Accelerometer market
Measurement and
control
Piezoelectric
MEMS Capacitive
Performance
(bulk micromachined)
MEMS
Piezoelectric
Movement monitoring
and event detection
MEMS Capacitive
(surface micromachined) Special applications:
Piezoresistive
1 10 100 1000 Optical
Price [$] Servo

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Smaller
Lower fabrication costs
Higher integration
Comparable performance

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State-of-the-art
Kunz et al. (2001) Yu et al. (2003)
Resonance Voltage Charge Chip
frequency sensitivity Sensitivity size
[kHz] [mV/g] [pC/g] [mm]
Yu et al. 22 0.47 0.19 4x4
Kunz et al. 0.5 - 22 7x7
Beeby et al. 7.6 - 16 12 x 12
Beeby et al. (2000)

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Goal
The Toolbox The MEMS Accelerometer
Integrate PZT thick film and MEMS Design, model, fabricate, characterize
an accelerometer:
Design process flow
• Resonance frequency > 20 kHz
Optimize cleanroom processes
• Chip size in the mm-range
• Sensitivity as high as possible

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Design
What is special about this accelerometer?

13. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Square Design
Resonance Voltage Charge Chip
frequency sensitivity Sensitivity size
[kHz] [mV/g] [pC/g] [mm]
12.5 0.36 0.04 10 x 10
Hindrichsen et al., Analytical Model of a PZT Thick Film Triaxial Accelerometer for Optimum Design, IEEE Sensors Journal (2009)
Hindrichsen et al., Triaxial MEMS Accelerometer with PZT Thick Film, Journal of Electroceramics (2010)

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Circular Design
Electrodes
Dimensions:
PZT
Membrane radius: 1.8 mm
Membrane Membrane
Seismic mass: 6 mg
Mass Silicon membrane: 20 μm
z
PZT thickness: 9 – 27 μm
x
x
y
x

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Sensitivity-Resonance Balance
Resonance frequency > 20 kHz
Chip size in the mm-range
Sensitivity as high as possible
>1 mm
10 m m
Membrane > 30 μm: thick film Membrane < 10 μm: thin film
Hindrichsen et al., Advantages of PZT Thick Film for MEMS Sensors , Sensors and Actuators A (in review)

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Proposal for Triaxial Design
FEM model
y z
y
x
x

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Fabrication
How can we integrate PZT thick film and MEMS?

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Oxidation Screen printing
UV lithografy
Plasma etch
Metal evaporation
Lou-Møller et al., Screen-printed piezoceramic thick films for miniturised devices, Journal of Electroceramics (2007)

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Process Flow
Silicon substrate
PZT thick film
Bottom electrode
Silicon oxide
Top electrode
Fragile substrate
High temperature sintering
Diffusion barrier layer
Adhesion
Thin film electrodes
Wafer scale production
Reproducibility

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Diffusion barrier layer (DBL)
31-mode
PZT
33-mode
DBL
Si

21. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Stylus Profiler Atomic Force Microscopy
PZT
Silicon
Hindrichsen et al., Investigation of Top/Bottom Electrode and Diffusion Barrier Layer for PZT Thick Film MEMS Sensors , Ferroelectrics (2009)

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Top electrode
Lift-off process
20 μm thick photo resist
No sonication

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58 accelerometers
4 mask fabrication process
Price estimation:
70 DKK per accelerometer
Hindrichsen et al., Circular Piezoelectric Accelerometer for High Band Width Application, IEEE Sensor Conference (2009)

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Packaging
LTCC on front side
Pyrex wafer on backside

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Characterization
How is the accelerometer performing?

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Accelerometer Specifications
Capacitance [pF]
Couplings coefficient [%] Impedance analyzer
Resonance frequency [kHz]
Sensitivity [pC/g]
Shaker setup
Quality factor
Range [g]
PZT thickness: 9 μm – 27 μm
Dielectric, mechanical & piezoelectric properties

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Impedance Sweep (dielectric, mechanical, piezoelectric)

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Impedance Sweep (dielectric, mechanical, piezoelectric)

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Coupling Coefficient (dielectric, mechanical, piezoelectric)

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Shaker Setup
Charge Amplifier
LabView
Accelerometer
Shaker Wave Generator

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Frequency Sweep (mechanical)
Quality factor: 150 - 250

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Resonance Frequency (mechanical)

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Charge Sensitivity (mechanical, piezoelectric)

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Range and Linearity (dielectric, mechanical, piezoelectric)

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Screen printed PZT on membranes with thicknesses down to 10 μm
Designed process flow
Diffusion barrier layer & thin film top electrode
Detailed analytical and FEM model
Square and circular design for triaxial sensing presented
Accelerometers are fabricated with high yield
Performance:
Resonance Voltage Charge Capacitance
frequency sensitivity Sensitivity [nF]
[kHz] [mV/g] [pC/g]
Yu et al. 22 0.47 0.19 2
Measured 25.8 0.28 0.46 1.6
Model 32.5 0.57 0.94 1.6

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Thank you for your attention
Acknowledgement
Erik Thomsen, Thomas Pedersen, Christian Hansen, Jesper Olsen, Tobias, Jack Larsen, Johan
Nagstrup, Ninia, Simon, Louise, Gustav, MEMS Applied Sensor Group, Rasmus Lou-Moller,
Karsten Hansen, Ferroperm, Ole Hansen, Lars Kofoed , Danchip people, Kristian Hvass, Anne, ...