Moisture-induced interfacial oxidation of chromium on polyimide

Abstract

Interfacial strength of Cr/polyimide thin film structures and its durability to thermal exposure as a function of process variables was determined by 90° peel testing. Results were correlated with surface and interfacial characterization of thin film structures and peel failure surfaces using Auger, x-ray photoelectron spectroscopy, and secondary ion mass spectroscopy techniques. Initial adhesion in as-deposited Cr/polyimide structures exhibits a mild dependence on radio-frequency Ar plasma surface modifications and deposition process conditions. However, the durability of these structures under thermal process cycles depends strongly on these process parameters, greater doses of energetic ion bombardment were seen to yield greater durability. Characterization of peel failure surfaces revealed that fracture in the as-deposited and peeled samples occurs cohesively in the near surface region within the polyimide. Cohesive fracture was also seen in some samples after thermal process exposures, with peel strengths that were acceptable but somewhat less than the corresponding unannealed samples. Failure at an oxidized Cr to polyimide interface was observed in other thermally annealed samples whose peel strengths were very low. It is proposed that both types of degradation are caused by the water absorbed in the polyimide. The slight adhesion degradation appears to be due to water reaction and hydrothermal weakening of the modified near surface polymer region during thermal processing. The more catastrophic degradation is due to the oxidation of the Cr by the absorbed water during the anneal cycles. Results of critical experiments that verify these hypotheses are presented. © 1992, American Vacuum Society. All rights reserved.