Mechanical stability of PECVD silicon nitride protective films over bondwires, bonds and bondpads during thermal stress

Abstract

PECVD silicon nitride thin films have potential application as protective diffusion barriers against water and aggressive ions which might corrode aluminum bondpads, bonds, and bondwires in packaged microelectronic assemblies. Test articles representing mounted and bonded devices were coated with silicon nitride films of thicknesses from 50 to 20,000 Å, then subjected to temperature and chemical stresses. These thin films exhibited less tendency to crack under temperature stress than would be predicted based on the physical properties of bulk silicon nitride. Films of less than one micron thickness over Al structures did not crack under standard industrial temperature cycling and showed good coverage near the bond interfaces and around the bondwires. The spread of Al metallization corrosion under these films proceeded at a slower rate beneath the thinner films due to their more favorable mechanical properties. Although the intrinsic stresses could not be measured, the decreased tendency for the thinner films to crack is primarily a result of higher ultimate strain and not initial compressive stresses.