Strength and fracture resistance of amorphous diamond-like carbon films for MEMS

The mechanical strength and mixed mode I/II fracture toughness of hydrogen-free tetrahedral amorphous diamond-like carbon (ta-C) films, grown by pulsed laser deposition, are discussed in connection to material flaws and its microstructure. The failure properties of ta-C were obtained from films with thicknesses 0.5-3 m and specimen widths 10-20 m. The smallest test samples with 10 m gage section averaged a strength of 7.3 ± 1.2GPa, while the strength of 20- m specimens with thicknesses 0.5-3 m varied between 2.2-5.7GPa. The scaling of the mechanical strength with specimen thickness and dimensions was owed to deposition-induced surface flaws, and, only in the smallest specimens, RIE patterning generated specimen sidewall flaws. The mode I fracture toughness of ta-C films is K Ic =4.4±0.4MPa m, while the results from mixed mode I/II fracture experiments with cracks arbitrarily oriented in the plane of the film compared very well with theoretical predictions.