Biaxial elastic modulus of very thin diamond-like carbon (DLC) films

The biaxial elastic modulus of very thin diamond-like carbon (DLC) films wa s measured by the recently suggested free overhang method. The DLC films of thickness ranging from 33 to 1100 nm were deposited on Si wafers by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) or by the filtered vacuum arc (FVA) process. Because the substrate was partially remo ved to obtain sinusoidal free overhang of the DLC film, this method has an advantage over other methods in that the measured value is not affected by the mechanical properties of the substrate. This advantage is more signific ant for a very thin film deposited on a substrate with a large difference i n mechanical properties. The measured biaxial elastic moduli were reasonabl e values as can be judged from the plane strain modulus of thick films meas ured by nanoindentation. The biaxial elastic modulus of the film deposited by r.f.-PACVD was 90 +/- 3 GPa and that of the film deposited by FVA proces s was 600 +/- 50 GPa. While the biaxial elastic modulus of the film deposit ed by FVA is independent of the film thickness, the film deposited by r.L-P ACVD exhibited decreased elastic modulus with decreasing film thickness whe n the film is thinner than 500 nm. Although the reason for the different be havior could not be clarified at the present state, differences in structur al evolution during the initial stage of film growth seem to be the reason. (C) 2001 Elsevier Science B.V. All rights reserved.