Understanding Smart Sensors - Nomads.usp

Micromachining 41being used in micromachined devices. The following four examples demonstratethe variety of approaches being pursued.2.6.1 Diamond as an Alternative Sensor MaterialSelectively deposited diamond film has been used as the thermal element in aflow sensor [34]. The bulk micromachined structure with diamond film isshown in Figure 2.14. Processing for the boron-doped heater resistors is performedat temperatures above 1,000°C. After surface preparation, the diamondfilm is grown in the desired regions by microwave plasma chemical vapor depositionin a mixture of hydrogen and methane gas and at a substrate temperatureof 900°C. The aluminum-pad contact to the resistors is the next step. Last, theback of the wafer is anisotropically etched to form the bridge structure for theflow sensor. Besides higher operating temperatures, diamond’s resistance tocorrosive and abrasive environments makes it attractive as a flow sensor.2.6.2 Metal Oxides and Piezoelectric SensingIn addition to semiconductor materials, various metal oxides can be depositedon micromachined structures. A pressure sensor with a zinc oxide (ZnO) piezoelectricsensing element has been made in an IC-compatible process [35].Surface micromachining is used to create the cavity. As shown in Figure 2.15, athermal oxide layer (Tox) is grown to isolate the sensor from the silicon substrate.The lower polysilicon electrode is encapsulated in Si 3 N 4 . The spaceroxide layer is sacrificially etched. Polysilicon is used to form an electrically conductivestructural support and cavity for the sensor. The 0.95-mm-thick activeZnO layer is deposited by RF sputtering. In addition to forming the active piezoelectricfilm, the sputtering also seals the sidewalls of the structure with anO 2 -Ar mixture at 10 mtorr inside the cavity. The sensor exhibits approximately0.36 mV/mbar sensitivity at 1.4 kHz and 3.4 dB variation over the rangeDiamond filmAluminumBoron resistorSiliconSilicon nitrideFigure 2.14 Diamond film flow sensor. (After: [34].)