A technique combining the use of microhardness indentations and an aluminum etch decoration process has been developed to measure the relative ability of polymer coatings to protect an integrated circuit (I.C.) device surface from mechanical damage. A threshold for inducing a crack in a passivated silicon surface was measured by applying a grid of indentation loads and measuring the load at which the aluminum etchant revealed damage in the passivation. A polyimidesiloxane and two polyimide materials, with widely different elastic moduli, were tested.
The ability of a coating to protect the chip surface was found to increase with increasing compliance of the film. The materials with the higher threshold loads had lower elastic moduli and allowed greater indenter penetration depth for a given load. Our finite element mechanical model supports the hypothesis that the stress generated by the indenter tip is absorbed and distributed by the lower modulus materials. The stress is transferred to the passivated silicon in the case of the higher modulus film, leading to damage at lower loads.