Pressure Sensor Lecture introduction and working
- 1. Pressure Sensor Theory Two Main Types of Pressure Sensors Capacitive Sensors • Work based on measurement of capacitance from two parallel plates. • C = εA/d , A = area of plates d = distance between. • This implies that the response of a capacitive sensor is inherently non-linear. Worsened by diaphragm deflection. • Must use external processor to compensate for non-linearity
- 2. Pressure Sensor Theory Piezoresistive Sensors Work based on the piezoresistive properties of silicon and other materials. Piezoresistivity is a response to stress. Some piezoresistive materials are Si, Ge, metals. In semiconductors, piezoresistivity is caused by 2 factors: geometry deformation and resistivity changes.
- 3. Pressure Sensor Theory Our Sensor is a Piezoresistive Sensor based on a Wheatstone Bridge Configuration. Resistors are made with Boron Diffusion.
- 4. Pressure Sensor Process Overview Initial Wafer is 525 µm thick, n-type, <100> double-side polished (DSP).
- 5. Pressure Sensor Process Overview – Step 2 Thermal Oxidation Wet Oxidation Followed by Dry Oxidation Si + O2 → SiO2 (Dry Oxidation) Si + 2H2O2 → SiO2 (Wet Oxidation)
- 6. Pressure Sensor Process Overview – Step 3 Photolithography for Piezoresistive Elements Contact Lithography Si + O2 → SiO2 (Dry Oxidation) Si + 2H2O2 → SiO2 (Wet Oxidation)
- 7. Pressure Sensor Process Overview – Step 3 Cont. DNQ Method using Mercury Lamp Diazonap. Changes to carboxylic acid via Wolf re-arrangement Carboxylic Acid is more soluble in a base than Novolak. So exposed areas dissolve. Use TMAH (a base) mixture to develop
- 8. Pressure Sensor Process Overview – Step 4 - Diffusion Creates Resistors in Substrate Three Methods 1. Solid Evap. (Tetramethyl Borate, Boron Nitride) - Rare 2. Gaseous – Diborane (B2H6) – Dangerous!! 160 ppm for 15 min life threatening 3. Liquid – Borosilicate polymer in ethanol. Creates borosilicate glass, boron oxide, and unused boron. 5 Squares 1 Square 3 Squares Ref: Jaeger, Richard. “Introduction to Microelectronic Fabrication”
- 9. Pressure Sensor Process Overview – Wafer Testing 5 Squares 1 Square 3 Squares R=lw/xj Rs=/xj R=(squares)/xj Contacts Equivalent: Rs=0.65 squares Resistive Element: 6.3 squares