School of Electrical and Computer Engineering Microsensors and Microactuators Group WIRELESS...

School of Electrical and Computer EngineeringMicrosensors and Microactuators Group

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


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.

CompressorCombustor

Sensor placed here

Pratt and Whitney


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.


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.


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

evacuatedcavity

CB

ACapacitorelectrodes

Inductor coil

evacuatedcavity

C(P)L


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.

Bottom view of a typical ceramic pressure sensor.

ElectroplatedCu Coil

ElectroplatedCu electrode

ConductivePaste

3.8mmDC sputteredCu electrode

161µm

Cross-sectional diagram

3.8mm


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.

AntennaCoil

Feedthroughs

To Impedance Analyzer

Pressure vessel or Vacuum oven

d(P)


Experimental ResultsWireless Ceramic Pressure Sensor

.

363534333231302984

85

86

87

88

89

90

91

92

Frequency (MHz)

Phase versus frequency for zero and full-scale applied pressure (0-1 bar)

electrode radius = 5mmmembrane thickness = 96µmgap spacing = 161µm

Sensitivity = 2.6 MHz/bar

Full Scale Pressure Zero PressureP

has

e (D

egre

es)



.

1.21.00.80.60.40.20.031.0

31.5

32.0

32.5

33.0

33.5

34.0

T=25 deg C

T=200 deg C

Pressure (Bar)

Frequency versus pressure for 25°C and 200°C (0-1 bar)

electrode radius = 5mmmembrane thickness = 96µmgap spacing = 161µm

Fre

qu

ency

(M

Hz)



.

10080604020026.00

26.05

26.10

26.15

26.20

26.25

26.30

26.35

26.40

Pressure (Bar)

Frequency versus pressure for high pressure (0-100 bar)

electrode radius = 3.8mmmembrane thickness = 96µmgap spacing = 161µm

Sensitivity = 6.4 kHz/bar

Fre

qu

ency

(M

Hz)


Comparison of Theoretical and Experimental Results

.

1.21.00.80.60.40.20.0

31.00

31.25

31.50

31.75

32.00

32.25

32.50

32.75

33.00

33.25

33.50

33.75

34.00

Measured

Theoretical

Pressure (Bar)

• 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.

Fre

qu

ency

(M

Hz)


Experimental ResultsPressure Sensor Array

.

4035302520151050

60

70

80

90

100

Frequency (MHz)

• 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.

Sensor 1Sensor 3

Sensor 2

Ph

ase

(Deg

rees

)


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.


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.

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