Structural and physical properties of highly piezoresistive nickel containing hydrogenated carbon thin films

Nickel containing amorphous hydrogenated carbon (Ni:a-C:H) thin films prepared by reactive sputtering have a high potential for use as piezoresistive sensors. Investigations by means of X-ray diffraction (XRD), transmission electron microscopy, energy-dispersive X-ray spectroscopy, and magnetic characterizations indicate that sputtering parameters and heat treatment influence the film composition, the microscopic structure, and some relevant macroscopic physical properties. The films are heterogeneous in nature and consist of either nanometer sized hcp nickel, nickel carbide (these phases being indistinguishable by XRD), or fcc nickel clusters encapsulated by graphite-like carbon shells. The nature of the metal clusters in the thin films has a strong effect on its magnetic properties. For approximately 55at.% Ni the electrical resistivity of the film is nearly temperature independent over a broad temperature range from 100K to 400K. The strain sensitivity, with a gauge factor of 20, is up to ten times higher than conventional temperature independent strain sensitive films. Compared to industry standard NiCr functional layers used for pressure sensors, Ni:a-C:H films provide a ten fold higher output signal. ► Piezoresistive Ni:a-C:H thin films exhibit gauge factors >20. ► Temperature independent resistance can be achieved by a proper Ni/C ratio. ► High gauge factors and temperature independent resistance simultaneously demonstrated. ► TEM analysis reveals Ni-clusters are embedded in a carbon matrix. ► Homogeneously distributed H (~12at.%) over the film thickness was found.