Analysis of stress/strain in Electroless Copper Films

Abstract

Polymer substrates were chemically coated with copper using various electroless copper baths and the internal strain/stress, as well as the adhesion quality, in the resulting copper films were studied during and after deposition as a function of the deposit thickness and the operation parameters of the electroless bath. The appearance of internal compressive stress in the copper film correlates to the probability of buckle driven delamination failure (blistering). Based on a simple theoretical concept we derived limits of allowed compressive stress in the copper film without inducing this failure mode. Furthermore depth-resolved X-ray diffraction (XRD) measurements in up to 1 μm thick electroless films indicate an approximately linear internal stress profile from about +200 MPa tensile stress at the substrate/adsorbate interface to −100 MPa compressive stress at the surface of the deposit. This will be explained in terms of a possible composition gradient of nickel in the copper film.