WIRELESS MICROMACHINED CERAMIC
PRESSURE SENSORS
Jennifer M. English and Mark G. Allen
School of Electrical and Computer Engineering
Georgia Institute of Technology
Atlanta, Georgia 30332-0250
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

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.
Pratt and Whitney
Compressor
Combustor
Sensor placed here
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

Pressure Sensor Requirements
•
Environment inside the turbine compressor:
– Temperature range 400 - 500°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.
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

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.
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

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).
External pressure
A
B
C evacuated cavity
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

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.
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

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 f o of the sensor. At f o
, 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 f o decreases.
To Impedance Analyzer
Feedthroughs d(P)
Antenna
Coil
Pressure vessel or Vacuum oven
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

Experimental Results
Wireless Ceramic Pressure Sensor
Phase versus frequency for zero and full-scale applied pressure (0-1 bar)
92
Full Scale Pressure Zero Pressure electrode radius = 5mm membrane thickness = 96µm gap spacing = 161µm
91
90
Sensitivity = 2.6 MHz/bar
89
88
87
86
85
84
29 30 31 32 33
Frequency (MHz)
34 35 36
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

Experimental Results
Wireless Ceramic Pressure Sensor
Frequency versus pressure for 25°C and 200°C (0-1 bar)
34.0
33.5
T=200 deg C
T=25 deg C electrode radius = 5mm membrane thickness = 96µm gap spacing = 161µm
33.0
32.5
32.0
31.5
31.0
0.0
0.2
0.4
0.6
Pressure (Bar)
0.8
1.0
1.2
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

Experimental Results
Wireless Ceramic Pressure Sensor electrode radius = 3.8mm
membrane thickness = 96µm gap spacing = 161µm
Frequency versus pressure for high pressure (0-100 bar)
26.40
26.35
Sensitivity = 6.4 kHz/bar
26.30
26.25
26.20
26.15
26.10
26.05
26.00
0 20 40 60
Pressure (Bar)
80 100
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

Comparison of Theoretical and Experimental
Results
•
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.
34.00
33.75
33.50
33.25
33.00
32.75
32.50
32.25
32.00
31.75
31.50
31.25
31.00
0.0
Measured
Theoretical
0.2
0.4
0.6
0.8
Pressure (Bar)
1.0
School of Electrical and Computer Engineering
Microsensors and Microactuators Group
1.2

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.
100
90
80
70
60
50
10 15
Sensor 3
Sensor 1
Sensor 2
20 25
Frequency (MHz)
30 35 40
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

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.
School of Electrical and Computer Engineering
Microsensors and Microactuators Group

Acknowledgments
•
Work is supported by Army Research Office Intelligent
Turbine Engines MURI Program (contract
DAAH049610008), 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.
School of Electrical and Computer Engineering
Microsensors and Microactuators Group