My Ph.D. project was presented at the Technical University of Denmark the 16th of April 2010.

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?

4. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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

8. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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

9. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Smaller
Lower fabrication costs
Higher integration
Comparable performance

10. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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)

11. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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

12. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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)

14. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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

15. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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)

16. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Proposal for Triaxial Design
FEM model
y z
y
x
x

17. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Fabrication
How can we integrate PZT thick film and MEMS?

18. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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)

19. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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

20. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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)

22. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Top electrode
Lift-off process
20 μm thick photo resist
No sonication

23. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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)

24. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Packaging
LTCC on front side
Pyrex wafer on backside

25. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Characterization
How is the accelerometer performing?

26. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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

27. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Impedance Sweep (dielectric, mechanical, piezoelectric)

28. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Impedance Sweep (dielectric, mechanical, piezoelectric)

29. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Coupling Coefficient (dielectric, mechanical, piezoelectric)

30. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Shaker Setup
Charge Amplifier
LabView
Accelerometer
Shaker Wave Generator

31. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Frequency Sweep (mechanical)
Quality factor: 150 - 250

32. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Resonance Frequency (mechanical)

33. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Charge Sensitivity (mechanical, piezoelectric)

34. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
Range and Linearity (dielectric, mechanical, piezoelectric)

35. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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

36. Piezoelectric Accelerometer Motivation Design Fabrication Characterization Conclusion
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, ...