Determination of elastic modulus and Poisson's ratio of diamond-like carbon films

A simple technique to measure the elastic modulus and Poisson's ratio of diamond-like carbon (DLC) films deposited on Si substrate was suggested. This technique involved etching a side of Si substrate using the DLC film as an etching mask. The edge of the DLC overhang, which is free from constraint of the Si substrate, exhibits periodic sinusoidal shape. By measuring the amplitude and the wavelength of the sinusoidal edge, we can determine the strain of the film required to adhere to the substrate. Combined with an independent stress measurement by laser reflection method this technique allows calculation of the biaxial elastic modulus, E/(1 - ν) where E is the elastic modulus and ν Poisson's ratio of the DLC films. By comparing the biaxial elastic modulus with plane-strain modulus E/(1 - ν²) measured by nanoindentation, we could further determine the elastic modulus and Poisson's ratio, independently. The mechanical properties of DLC films deposited by r.f. PACVD were characterized using this technique. The films were prepared by using C₆H₆ r.f. glow discharge at a self bias voltage of 400 V and a deposition pressure of 1.33 Pa. The elastic modulus and Poisson's ratio were 87 ± 18 GPa and 0.22 ± 0.33, respectively. The effects of the etching depth and the film thickness were also discussed.