Strain in Electroless Copper Films: Analysis by in situ X-Ray Diffraction and Curvature Methods

Abstract

Strain in chemically deposited copper films on polymer substrates was determined by means of in situ X-ray diffraction (XRD), deposit stress analyzer (DSA) and spiral contractometer (SC). The strain evolution of the films was studied as a function of copper film thickness and electroless copper bath parameters, during and after deposition. The results are not indicative of a preferred crystallite orientation or texturing in the deposit. The copper film stress is controllable over a wide range of some 100 MPa from compressive to tensile stress by appropriate variation of bath parameters (e.g. temperature, concentration of bath components such as nickel, stabilizer and formaldehyde). A higher tendency of blister generation for relaxed or compressively stressed films is apparent, which implies that a sufficient level of tensile stress throughout the deposition promotes film adhesion. An observable change from tensile to compressive film stress during the cooling of the sample from bath operation to rinse water temperature is discussed in terms of substrate-induced thermal stress to the copper film. In this context, the difference in the substrate materials required for XRD (polymer), DSA (copper) and SC (stainless steel) may be a significant factor contributing to the diverging measured stress behaviors of the methods. Moreover, it is questionable whether SC stress data can be compared with XRD and DSA stress data, due to the low resolution of the SC method (~60 MPa).