Study of the airborne SO_2 and NH_3 contamination on Cr, MoSi and quartz surfaces of photomasks

摘要

The study of the airborne SO_2 and NH_3 contamination on Cr, MoSi and Quartz photomask surfaces was addressed bythe intentional exposure to controlled contaminated air. Experimentally, mask-like layers (representative Cr, MoSi andSiO_2 layers) deposited on wafers were used and then characterized by LPE-IC.Results showed that Cr surfaces clearly present a higher ability to be contaminated than MoSi and even more thanQuartz, both for SO_2 and NH_3 (Cr » MoSi > Quartz). For each mask surface, the NH_3 contamination occurred morerapidly than the SO_2 deposition.The contamination of Cr and MoSi surfaces respectively for the 2 contaminants and for SO_2, are governed byLangmuir-type adsorption models allowing then the forecasting of the deposited contaminant amounts depending on thetime and the airborne concentration. A very slow and low adsorption process of SO_2 on Quartz is observed whereas theNH_3 deposition on MoSi and quartz appear very rapid at ambient concentrations such as a way saturation levels arelikely reached in few minutes. Furthermore, a significant enhancement of the SO_2 and NH_3 deposition is characterizedin clean room humidity versus drier conditions, this effect being extremely drastic for SO_2 on Cr.The knowledge of the contamination behavior of the SO_2 and NH3 on photomask surfaces is very relevant for thecontrol of the crystal growth /haze occurrence on reticles. Their application led to recommend SO_2 and NH_3 thresholdlevels lower than 10 pptv in the mask environment to avoid haze beyond one year.