Non-destructive in situ analysis of interface processes and thin film growth

Optical second harmonic generation (SHG) was applied to monitor the growth of films of polyamic acid on polycrystalline gold substrates. Different diamines such as 4,4'-oxydianiline (ODA), 4,4'-diaminodiphenyl disulfide (DAPS) and 4,4'-diaminobiphenyl (benzidine) were codeposited from the gas phase with pyromellitic dianhydride (PMDA). Depending on the diamine, the second harmonic signal varies differently during formation of the polymer/metal interface and the subsequent film growth. A three-layer model which takes into account the optical properties of thin films reveals that the shape of the thickness-dependent SH signal is related to the orientation of the molecules in the film. A fit of the experimental data based on the model indicates a structural transition that occurs several tens of nanometers above the substrate for PMDA/ODA and PMDA/DAPS. The experiments demonstrate that SHG can be applied to monitor the growth of thin films and to extract structural information.