Debonding of photoresist caused by Case II diffusion

A glassy polymeric photoresist bonded to a thin copper substrate was immersed in an organic penetrant environment. Debonding of the polymer layer from the substrate was observed by monitoring the deflection of the composite strip. The diffusion of the environment into the polymer layer was followed using Rutherford backscattering spectrometry. For all environments investigated the diffusion showed the characteristics of Case II diffusion, i.e. a uniformly swollen layer formed behind a sharp front and propagated into the polymer at a constant velocity. Even though the front velocity could be varied over three orders of magnitude by varying the environment or the temperature, debonding always occurred when the front had penetrated only about one-fifth of the total layer thickness. It is concluded that debonding is driven by release of the elastic strain energy stored in the composite strip rather than a specific attack of the interface by the environment. Additions of a smaller organic molecule to a predominately large organic molecule environment were found to produce a marked increase in the kinetics of debonding and a corresponding increase in the Case II front velocity. To discover the mechanism of this effect, experiments were carried out with mixtures of iodomethane and 1,1,1-trichloroethane (TCE). Rutherford backscattering spectra showed that the smaller iodomethane diffused ahead of the main Case II diffusion front of the TCE. It is proposed that the increase in Case II front velocity in the mixture results from the fact that the faster diffusing iodomethane preplasticizes the polymer ahead of the front.