P6.3 - Novel Carbon-Based Materials for Pressure and Force Sensors

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
P6 - Technology, Materials
H. Schmid-Engel, D. Göttel, G. Schultes - University of Applied Sciences, Saarbücken, R. Koppert - Siegert TFT GmbH, Hermsdorf, U. Werner - INM, Leibniz Institute for New Materials, Saarbrücken (Germany)
800 - 805


Amorphous carbon–based materials are well known for their high mechanical hardness and chemical inertness making them ideal candidates for wear-resistant coatings. We however focus on the electrical and morphological properties of metal containing hydrogenated amorphous carbon (Me:a-C:H) thin films. Depending on the metal content, the conductivity changes over 12 orders of magnitude. Moreover the material acts as a piezoresistor, reacting on mechanical strain as well as on hydrostatic pressure. One key aspect for the application as sensor material is the combination of a high strain sensitivity (gauge factor) and a temperature coefficient of electrical resistance (TCR) close to zero. Especially for nickel containing hydrogenated amorphous carbon (Ni:a-C:H) thin films an enhancement of the strain sensitivity by a factor of 10 compared to commercial metallic strain gauges has been achieved with a bandwidth of TCR of plus/minus 50 ppm/K. Hydrostatic pressure applied directly on the thin film yields a resistance change of approx. 1 per mill / 100 bar. Ni:a-C:H thin films have been prepared by means of a reactive r.f. sputtering process using a carbon containing gas as precursor. TEM analysis revealed crystalline nickel or nickel carbide clusters with diameters between 10 and 20 nm completely encapsulated by layers of highly disordered graphite-like carbon. Annealing at temperatures above approx. 500 centigrade destroys the carbon encapsulation which leads to larger graphitic areas next to larger metallic areas. Cross sectional TEM analysis shows that the vertical film composition starts from globular Ni particles which merge into an area of elongated Ni particles. Some of the Ni columns densify towards the surface of the films. It is assumed that the high strain sensitivity originates from a conductivity mechanism of the graphite-like carbon separating the metallic areas.

To demonstrate the high application potential of these novel carbon-based materials, Ni:a-C:H strain gauges have been deposited on insulated stainless steel membranes of pressure sensors. The output signal of these sensors (characteristic value of approx. 20 mV/V) is ten times higher compared to state of the art pressure sensors based on metallic strain gauges, e.g. CrNi. Furthermore the errors of linearity and hysteresis (less than 0.175 percent FS and less than 0.06 percent FS respectively) are comparable to CrNi sensors of the same geometry. Thus, the errors mentioned above are caused basically by the non-linear deformation of the membrane. This new kind of functional layer opens up new possibilities for pressure and force sensor applications. One example could be the low pressure range of steel based pressure sensors below 10 bar. Another application might focus on a type of membraneless pressure sensor utilizing the hydrostatic sensitivity for very high pressure ranges.