The cracking resistance of nanoscale layers and films

Thin film cracking has been studied for a nanometer brittle/ductile layered system consisting of Si and Cu. Single Si films and Cu/Si/Cu trilayers were fabricated by physical vapor deposition. The films were deposited onto a ductile substrate consisting of stainless steel with a thin polyimide interlayer. Straining of the substrate induced cracking of the Si. Cracking strains ≥1% were observed, particularly in layers ≤100 nm thick. The critical cracking strain was found to depend upon the Si layer thickness, as well as the thickness and elastic/plastic properties of the adjacent ductile layers. Si cracking was suppressed by the presence of adjacent Cu layers. The measured strains were compared with lower-bound critical strains for tunnel and channel cracking. Comparisons indicated that these mechanisms control the critical strain found in trilayers, because trilayer fabrication introduces edge flaws larger than the Si layer thickness. Conversely, for Si films, the measured critical strains exceed the channel cracking strain, because the flaws in these films are relatively small.