Paper PS-FrM8
A Study on the Oxidation Properties of W Surface by O₂ Plasma and Reduction of WO_x Layer by H₂ Plasma in Sub 50nm Patterning Process

Friday, October 19, 2007, 10:20 am, Room 606

As the feature size gradually shrinks down to nano scale of sub 50nm, various metals like as W, Ti, and Co, have been introduced as a low resistance material for word line. Recently, W has become to the most powerful candidate and widely used for several applications. For the formation of W pattern, the most challenging problem is the oxidation of W surface. Generally, we treat wafer with O₂ plasma to remove the organic mask layer and polymeric residues after W etching process. W surface is oxidized to WO_x layer by reactive oxygen radicals during plasma ashing. The oxidation of W surface brings increase of pattern resistance and variation of critical dimension. In this study, we controlled the reactivity between W surface and oxygen radical varying process parameters such as radical flux, RF power and especially electrode temperature. Also, we characterized the W surface using XRR, SFX and checked the change of line resistance to compare the oxidation amount. Oxygen radicals activated moderately, could not react with W surface, while organic polymer could be sufficiently removed. Using properly activated oxygen radical, we can find the reasonable process window which is nearly free from surface oxidation and polymeric residues. Even though we find the optimum process condition, there are still thin WO_x layer after ashing. In order to remove this layer, we also investigated post treatment process using H₂ containing plasma. We found that hydrogen radical allowed a quick recovery of WO_x layer to their initial state of W. Finally, we proposed a multi-step plasma treatment process to keep W surface from oxidation during plasma ashing.