Aluminum grain interaction with metal etch induced by TiN film thickness

In Texas Instruments\´ DMOS5 wafer fabrication, yield loss occurred in a consistent spatial signature on 0.35 μm product wafers. The defect was termed the "Beard" due to its crescent shape on the wafers. It was found that the Beard could be reduced, but not eliminated, by an increase in metal overetch. The metal stack consisted of a sandwich of a top TiN anti-reflective coating, an aluminum/0.5% copper interconnect, a bottom TiN layer and a Ti sticking layer.. Ultimately, an interaction was found between the thickness of the bottom TiN layer with the Beard. Specifically, a thinner TiN film had more fails. Texture analysis shows that the aluminum film decreases in [111] texture with a thinner underlying TiN film. This change in aluminum grain texture is the likely cause of the observed localized regions of unetched aluminum and TiN. By thickening the TiN by 1.5X and thereby achieving a greater percentage of columnar [111] grains, residual TiN shorts do not occur as confirmed by electrical test of the product wafers.