On-Chip Soil Moisture And Temperature Sensor ...

Csensor Csensor = nϵ

lt d

f=

Iavg (6Cload + 2Csensor )V

ρ=

1 µne e ne

µ

ρ

Si

Start with oxidation on ntype silicon wafer

SiO2

Photoresist

Metal (Al)

n+ region

Photo lithography and oxide etching using BOE etch

Phosphorus diffusion doping and sputtering of aluminum

Lift off of photoresist using acetone in ultrasound bath, removing excess aluminum

Spin coat very thick layer of photoresist

Pattern photoresist

Sputter metal for capacitor capacitor aluminum

Liftoff of photoresist, leaving only capacitor behind

Spincoat and pattern polyimide, similar to photoresist

polyimide

Mesurement Technology

Moisture

Temperature Temperature Range (°C)

Paxnas: Decagon Devices: HCX-TR 5TM (Our Design) Polymer Soil Capacitance Capacitance Thermistor Thermistor -40-100 °C -40 to 50 °C

Hygrometrix: HMX2000

Spectrum Technologies: Waterscout

Irrometer: Watermark 200SS

Piezoresistive

Soil Capacitance

Soil Resistance

Soil Capacitance

Thermistor -40 to 85 °C

Thermistor .5 to 80 °C

N/A N/A

0-100% 1% 1% 1.25 V .500 mA $75 11.31 10.34 5.06

0-100% 3% 1 °C 3V 10 mA $189 70 30 19.05

0-100% 3% (est) N/A 5V 20 mA $70 83 22 22

N/A N/A 0-100% (Requires special calibration) 4% N/A 5V 3 mA $95 89 18 7

RH Range Moisture Temperature Volts Amps

Accuracy Max Power Requirements Cost Dimensions

Length (mm) Width (mm) Thickness (mm)

0-100% 2-3% (est) .1 °C (est) 2.5 V 4 mA $0.63 (Fabrication) 10 10 5

0-100% 2-3% 0.1 °C 14 V 10 mA $150 100 32 7

Vernier: SMS-BTA