Process for low temperature deposition of strain gauge materials based on Chromium nitride thin films

As a first step AlN thin films were made but no significant strain sensing effect could be produced. Next CrN thin films of varying resistivity were deposited on oxidized silicon wafers by DC reactive magnetron sputtering, using a 99.95% Cr sputter target, a wafer at 200 °C in various nitrogen gas concentrations. The processed wafers were diced into long beams for testing. After the devices were wire-bonded, the cantilever detection method was employed to measure the sheet resistance change due to the strain. The resistances of over stochiometric CrN thin films (nitrogen rich) show a thermal sensitivity resembling semiconductors. The B-parameter equation curve fitting was adopted to describe the resistor's temperature dependency. B parameters from 570 to 690 were found in several samples. The largest longitudinal gauge factor achieved in these films is 5.2. Because of the isotropic piezoresistive effect in x-y plane, the best results are obtained when an elongated rectangular CrN resistor is placed parallel to the object elongation direction. Best strain gauges had a resistivity of 690 Ohm / square, layer thickness 260 nm, with a stress of +245 MPa.