Micro force sensor with piezoresistive amorphous carbon strain gauge

In this contribution we report for the first time on the successful integration of amorphous carbon (a-C) as a piezoresistive strain gauge into a silicon micro cantilever force sensor. Sputter-deposited a-C layers showing excellent tribological properties contain a percentage of nearly 20% of tetrahedral sp3 carbon bonds as observed by optical absorption and Raman spectroscopy. Temperature-dependent transport measurements revealed hopping conduction between conducting sp2 carbon sites embedded in the insulating skeletal matrix of sp3 bonds. Changing their distance by strain a change of resistivity could be expected, which was investigated with layers sputter-deposited on a silicon membrane and structured by the lift-off technique using photo resist. Cantilevers comprising a-C strain gauges were etched out of this membrane using tetra methyl ammonium hydroxide (TMAH) and potassium hydroxide (KOH) solutions in a bulk silicon micromachining process. Realised prototypes were tested by applying a variable load to the cantilever free end. We found linear characteristics of the strain gauge resistance versus the applied force in the range of 0 to ±600 µN revealing piezoresistive gauge factors of a-C within 36-46.