1 Absolute Pressure Sensors Z. Celik-Butler, D. Butler and M. Chitteboyina Nanotechnology Research and Teaching Facility University of Texas at Arlington

Absolute Pressure Sensor Model in CoventorWare Cavity Flexible superstrate Flexible substrate Sealed Pressure Sensor Wheatstone 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 Cavity cross-section Fabrication Process Flow for the Absolute Pressure Sensor

Deposit final sealing layer 0.6 µm Bond pads cross-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

15 Absolute Pressure Sensor Results Sensor A (0.0 – 2.0 psi) DISPLACEMENT PLOT STRAIN YY PLOT STRAIN XX PLOT MISES STRESS PLOT

16 STRAIN YY PLOT STRAIN XX PLOT Absolute Pressure Sensor Design Sensor H (14.0 – 16.0 psi) DISPLACEMENT PLOT MISES STRESS PLOT

17 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

18 Absolute Pressure Sensor Fabrication Steps

19 Absolute Pressure Sensor Characterization Vout Vin Vout Vin R1R1 R3R3 R2R2 R4R4  The electrical circuit is complete with two active piezoresistors (R 1 and R 3 ) and two passive piezoresistors (R 2 and R 4 ) 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