Strain sensitivity of nickel-containing amorphous hydrogenated carbon (Ni:a-C:H) thin films prepared by r.f. sputtering using substrate bias conditions

Thin films of nickel-containing amorphous hydrogenated carbon Ni:a-C:H were deposited by means of a reactive r.f. powered sputter process with different argon/ethene gas mixtures. The amount of incorporated nickel, measured by energy dispersive X-ray analysis (EDX) and the electrical conductivity of the films were obtained as a function of the ethene concentration in the processes, showing that the conductivity is strongly dependent on the nickel concentration in the films and may be varied over 8 orders of magnitude.The temperature coefficient of resistivity (TCR) and the strain sensitivity in terms of the gauge factor were measured for films under various process conditions. It turned out that low TCR values (300 ppm/K) and higher gauge factors of approx. 8 could be produced simultaneously, yet only with the help of an additional substrate r.f. bias power. In addition the long term stability of the produced resistors was improved by means of substrate bias sputtering. Therefore these films proved to be very promising candidates as strain gauge devices for sensors able to measure mechanical quantities such as pressure, force, weight and torque.