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Micro force sensor with piezoresistive amorphous carbon strain gauge

: Peiner, E.; Tibrewala, A.; Bandorf, R.; Biehl, S.; Lüthje, H.; Doering, L.


French, P.J.:
Transducers '05, 13th International Conference on Solid-State Sensors Actuators and Microsystem. Selected papers : Seoul, Korea, 5 - 9 June 2005
Amsterdam: Elsevier, 2006 (Sensors and actuators. A, Physical 130/131.2006)
International Conference on Solid-State Sensors, Actuators and Microsystems <13, 2005, Seoul>
Conference Paper, Journal Article
Fraunhofer IST ()
amorphous carbon; hopping conduction; piezoresistive gauge factor; bulk silicon micromachining; micro force sensor

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.