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Absolute Pressure SensorsAbsolute Pressure Sensors

Z. Celik-Butler, D. Butler and M. Chitteboyina

Nanotechnology Research and Teaching Facility University of Texas at Arlington

http://www.uta.edu/engineering/nano/

Absolute Pressure Sensor Model in CoventorWare

Cavity

Flexible superstrate

Flexible substrate

Sealed Pressure SensorWheatstone Bridge

Configuration

Bond Pads

Stress, Strain and Displacement simulations for the pressure sensor

is currently on-going

0.14cm x 0.14cm

Clean silicon wafer

Fabrication Process Flow for the Absolute Pressure Sensor

40 µm flexible polyimide substrate

Fabrication Process Flow for the Absolute Pressure Sensor

Insulation layer 0.5 µm

Fabrication Process Flow for the Absolute Pressure Sensor

Thick polyimide sacrificial layer 2.5 µm

Fabrication Process Flow for the Absolute Pressure Sensor

Thin polyimide sacrificial layer 0.5 µm

Fabrication Process Flow for the Absolute Pressure Sensor

Membrane layer 1µm

Etch holes

Fabrication Process Flow for the Absolute Pressure Sensor

Piezoresistors 0.5 µm

Fabrication Process Flow for the Absolute Pressure Sensor

Aluminum metallization 0.5 µm

Fabrication Process Flow for the Absolute Pressure Sensor

Encapsulant layer 0.5 µm

Bond pads

Etch holes

Fabrication Process Flow for the Absolute Pressure Sensor

Release both the sacrificial layers

Release both the sacrificial layers

Cavity

cross-section

Fabrication Process Flow for the Absolute Pressure Sensor

Deposit final sealing layer 0.6 µm

Bond padscross-section

Cavity

Fabrication Process Flow for the Absolute Pressure Sensor

Bond pads

Spin coat 40 µm flexible polyimide superstrate

Fabrication Process Flow for the Absolute Pressure Sensor

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Absolute Pressure Sensor ResultsSensor A (0.0 – 2.0 psi)

DISPLACEMENT PLOTSTRAIN YY PLOT STRAIN XX PLOTMISES STRESS PLOT

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STRAIN YY PLOT STRAIN XX PLOT

Absolute Pressure Sensor DesignSensor H (14.0 – 16.0 psi)

DISPLACEMENT PLOTMISES STRESS PLOT

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Absolute Pressure Sensor Fabrication Steps

STEP 1 On a clean wafer spin-coat ~ 40 µm flexible polyimide as the substrate layer followed by 600 nm passivation layer

STEP 2 Spin coat thick polyimide and cure

STEP 3

STEP 4

NEXT STEPS Deposit ~ 500 nm thick aluminum as the metallization layer Ash the sacrificial layer using oxygen plasma to suspend the membrane Deposit 0.5 µm encapsulation layer followed by 0.5 µm as the final sealing layer Etch the silicon wafer from the back side to get access to the bond pads and characterize the absolute pressure sensors

Piezoresistors ~ 25 nm

Passivation layer ~ 600 nm

membrane layer ~ 0.1 µm

Thick Sacrificial layer ~ 7 µm

Spin coat thin polyimide and cure

Deposit 0.1 µm membrane layer STEP 5 Deposit ~ 25 nm piezoresistor layer

Thin Sacrificial layer ~ 0.5 µm

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Absolute Pressure Sensor Fabrication Steps

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Absolute Pressure Sensor Characterization

1 2 3 4Vout Vin Vout Vin

R1

R2 R3

R4

Vin Vout

R1

R3R2 R4

The electrical circuit is complete with two active piezoresistors (R1 and R3) and two passive piezoresistors (R2 and R4) in a Wheatstone bridge configuration

The current-voltage characteristics is plotted and the true resistances are found Pressure is applied on top of the membrane and the change in output voltage vs. input voltage is plotted