1. WIRELESS MICROMACHINED CERAMIC PRESSURE SENSORS
Jennifer M. English and Mark G. Allen
School of Electrical and Computer Engineering
Georgia Institute of Technology
Atlanta, Georgia

2. Background and Motivation MURI project - Intelligent turbine engines.
Goal: extend the operational range of turbine engines using sensing and active feedback control techniques
Push operating curve of engine by active measures to eliminate surge and stall
Monitoring of compressor output pressure (static and dynamic) required to provide input data for active control scheme.

3. Pressure Sensor Requirements
Environment inside the turbine compressor:
Temperature range °C.
Pressure range (1-50 atm).
Pressure fluctuations 2 kHz.
Pressure sensor issues:
Materials with high temperature stability.
Pressure sensitivity at both low and high pressures.
Temperature sensitivity.
Data retrieval compatible with high temperature and hostile environments.
MEMS technologies offer: potential for multiple sensors, spatial resolution, reduction or elimination of wiring harnesses of conventional sensors.

4. Ceramic Pressure Sensor
Our approach - Utilize key design and fabrication techniques from the silicon sensor and microelectronics packaging infrastructures to develop a ceramic pressure sensor.
Silicon sensor infrastructure:
Flexible membrane, capacitive sensing.
Microelectronics packaging infrastructure:
Ceramic tape and complex package processing techniques.
Benefits - Batch fabrication capabilities, self-packaged devices, possible high temperature stability.

5. Ceramic Pressure Sensor - Design
Three layer design using Dupont 951-AT LTCC ceramic tape.
Layer A: 1 sheet, Layer B: > 1 sheet with a punched hole, Layer C: > 1 sheet.
Integrate metal capacitor electrodes and planar, spiral inductor (DC sputtering, E-beam evaporation, screen printing).

6. Ceramic Pressure Sensor - Fabrication
Four sheets are aligned and laminated in a hot press at 3000 psi and 70°C for 10 min under ambient vacuum.
Inductor and bottom capacitor electrode are electroplated with copper.
Top electrode is DC sputtered copper.
High temperature conductive paste connects inductor to top electrode.
3.8mm
Cross-sectional diagram
Bottom view of a typical ceramic pressure sensor.

7. Wireless Ceramic Pressure Sensor Operation
No physical connections to the sensor are necessary. Sensor can be placed on moving parts.
Impedance analyzer records the phase of the antenna coil over a frequency range that includes the sensor center frequency while the ambient pressure and temperature are varied.
Phase of the antenna is +90°except at the fo of the sensor. At fo, the sensor couples to the antenna and causes a dip in the phase.
As the ambient pressure increases, the ceramic membrane deflects. The capacitance increases and the fo decreases.

8. Experimental Results Wireless Ceramic Pressure Sensor
Phase versus frequency for zero and full-scale applied pressure (0-1 bar) . 84 85 86 87 88 89 90 91 92
electrode radius = 5mm
membrane thickness = 96µm
gap spacing = 161µm
Full Scale Pressure
Zero Pressure
Sensitivity = 2.6 MHz/bar
Phase (Degrees) 3 6 5 4 2 1 9 F r e q u e n c y ( M H z )

9. Experimental Results Wireless Ceramic Pressure Sensor
Frequency versus pressure for 25°C and 200°C (0-1 bar) . 3 1 . 5 2 4
electrode radius = 5mm
membrane thickness = 96µm
gap spacing = 161µm T = 2 5 d e g C
Frequency (MHz) 1 . 2 8 6 4 P r e s s u r e ( B a r )

10. Experimental Results Wireless Ceramic Pressure Sensor
Frequency versus pressure for high pressure (0-100 bar) .
electrode radius = 3.8mm
membrane thickness = 96µm
gap spacing = 161µm 2 6 . 5 1 3 4
Sensitivity = 6.4 kHz/bar
Frequency (MHz)
100 80 60 40 20 P r e s s u r e ( B a r )

11. Comparison of Theoretical and Experimental Results. 3 1 . 2 5 7 4
Exp. sensitivity = 2.6MHz.
Theor. sensitivity = 2.2MHz.
Theoretical model allows for only one membrane (electrode) to deflect.
Actual sensor allows deflection of the top membrane and some deflection of the bottom membrane. M e a s u r d T h o t i c l
Frequency (MHz) 1 . 2 8 6 4 P r e s s u r e ( B a r )

12. Experimental Results Pressure Sensor Array
Three pressure sensors designed with distinct resonant frequencies monitored by the same antenna simultaneously.
Magnitude of the dip depends on the proximity of the sensor to the antenna coil.
The number of sensors monitored by a single antenna is limited only by bandwidth. 50 60 70 80 90
100 .
Phase (Degrees) 40 35 30 25 20 15 10 F r e q u e n c y ( M H z )

13. Conclusions
Design, modeling, fabrication and testing of a passive wireless ceramic pressure sensor has been performed.
Sensor is fabricated from ceramic tape layers to create a sealed cavity structure with a flexible ceramic membrane.
The ceramic structure is integrated with a fixed L/ varying C resonant circuit.
A passive, wireless scheme is used to retrieve the pressure data.
Pressure and temperature tests were performed and shows the concept is valid. Theoretical modeling compares well with the experimental results.
Pressure sensor array concept was demonstrated.

14. Acknowledgments
Work is supported by Army Research Office Intelligent Turbine Engines MURI Program (contract DAAH ), under the direction of Dr. David Mann.
Microfabrication carried out in the Georgia Tech Microelectronics Research Center
Professors D. Hertling and R. Feeney of Georgia Tech.