Removal of Titanium Oxide Grown on Titanium Nitride and Reduction of Via Contact Resistance Using a Modern Plasma Asher

In making multi-level interconnects, a via layer is generally made of a dielectric layer grown on a TiN/AlSiCu/TiN metal stack. The via contact hole to the lower level metal is first etched after metal patterning and intermetal oxide deposition, then the resist mask for via etch is stripped in an oxygen plasma at high temperature before filling the via with tungsten or aluminum. However, during the resist stripping process, the titanium nitride (TiN) is exposed to oxygen radicals and becomes oxidized. This oxidized surface leads to high via resistance and may cause yield loss and poor adhesion of the subsequently deposited metal. Thus, the oxide layer must be removed completely before filling the via hole.Utilizing a modern asher and combining microwave downstream plasma and low damage RIE we have successfully developed an integral process to strip the resist and to remove any oxidized TiN film. This process also gives clean vias that are free of polymer residues from the etching process.Samples were characterized using Auger electron spectroscopy (AES), scanning electron microscopy (SEM), ellipsometry and via resistance measurements. The AES depth profile measurements and ellipsometry show clearly that a titanium oxide has grown onto TiN during the oxygen downstream ashing and is completely removed when adding the RIE step. The electrical measurements show that the resistance of vias cleaned with this two step process in a single tool is comparable to that of vias cleaned with Ar sputtering using a separate tool after the oxygen ashing process.