Hiroki Ohno

Insights into Watermark Formation and Control

Introduction Watermarks have long been known to occur when hydrophobic silicon is exposed after HF wet processing, particularly where patterns include hydrophilic and hydrophobic regions [1-3]. Historically these defects have been associated with an inadequate drying process, and were a driving factor in the semiconductor industry’s transition from spin to IPA-based drying in FEOL batch systems. Silicon is oxidized by oxygen dissolved in DI water during rinsing, and this oxide is dissolved into the water. Upon thinning of a water layer or droplet on the wafer, dissolved silica and/or silicic acid precipitate on the wafer. Avoiding oxygen in the water and evaporation during drying have been identified as key factors in preventing watermark formation. We systematically studied mechanisms of formation of such defects after dilute HF processing in a single wafer wet cleaning chamber. As a result, different types of defects were identified that are distinct from previously reported watermarks. Subsequently the conditions under which these defects can be formed were found, so that they may be prevented.