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.

Main Impact Factors on Internal Stress of Electroless Deposited Copper Films

“…For electroless copper a moderate tensile stress is essential for the film substrate adherence. The experimentally observed appearance of blisters is explainable in a semi-quantitative concept of critical compressive stress [3]:…”

Effects of Chemical Compositions on Plating Characteristics of Alkaline Non-Cyanide Electrogalvanized Coatings

“…Moreover, these factors could also affect the development of internal stresses, which could subsequently result in the failure of deposited films, by mechanisms such as blistering or delamination. The effects of process variables on residual stress and how it, in turn, affects the properties of the deposits have been examined, primarily focusing on copper and nickel films [ 12 , 13 , 14 , 15 ]. However, the study on electrodeposited zinc is very limited and has been largely focused on the acid bath and the effects of organic additives [ 16 , 17 , 18 , 19 , 20 ].…”

Novel formaldehyde-free electroless copper for plating on next-generation substrates

“…Unfortunately, the smooth substrate topography leads typically to a limited adhesion of the electroless copper layer, which increases the risk of delamination and blister formation. To prevent this, the intrinsic physical properties of the metal film (and therefore the chemical properties of the copper bath) are critical [15]. A key factor is that the metal deposition occurs under the generation of internal tensile stress so that the blister tendency is minimized.…”