Use of high temperature and high humidity to test the adhesion of sputtered copper to a polyimide surface modified by an AC nitrogen glow discharge

We have used an extreme environmental stress test to study the adhesion of a thin sputtered copper film (0.5 μm) to flexible polyimide (PI) substrates between 25 and 125 μm thick. The polyimide types include Kapton (PMDA-ODA) and Upilex (BPDA-PDA). When there was no surface modification on the PI, the adhesion of the film to Upilex type S was better than the adhesion to Upilex type R or Kapton type HN. When the polymer surface was treated with a simple AC nitrogen glow discharge (NGD), there was an improvement in the adhesion of the film to each of these polyimides. This improvement in adhesion became apparent after the film/substrate combination was subjected to either boiling water or steam for 30 min or more; the difference became quite clear after 2 h. A simple tape test was used to quickly estimate a relative adhesion strength. In order to compare the effect of our AC NGD treatment with other substrate surface modification methods, we used it to improve the coupling of a thick (> 10 μm) layer of copper (via a thin intermediate chromium layer) to a rigid PI substrate, formed from spin coating its precursor onto a silicon wafer. Peel test results were within a factor of 2-3 of the corresponding results obtained with a radio frequency (RF) plasma and ion beam treatments.