Efficient Ozone, Sulfate and Ammonium Free Resist Stripping Process
Conventional resist strip processes employ a variety of steps geared towards highly effective removal of organic and inorganic contamination from the mask surface. Figure 1 summarizes the process steps in mask cleaning and the mostly used chemistries employed in 193i mask cleaning. For proper wetting of the photomask surface with cleaning chemicals, higher surface energy is desired. A photomask surface is needed to be in hydrophilic state prior to the application of wet chemistries. Hydrophilic surfaces promote better liquid distribution and uniform chemical effects across the surface; as part of the POR cleaning process flow a 172nm excimer VUV step was used to achieve a low water contact-angle on the surface. The UV radiations under the oxygen atmosphere create oxygen radicals leading to surface organic bond cleavage as well as direct surface activation for better wetting [8,9] . However, the high energy radiations exposure of the photomask surface may also cause interface stress or material diffusion, which eventually transforms into unpredictable mask registration shifts. Ozone water (DIO 3 ) or conventional SPM (H 2 SO 4 + H 2 O 2 ) was used for resist stripping and organic removal, however haze formation [10,11] or oxidative degradation of photomask materials [12,13] has been observed as a result of the very high oxidation potential of Ozone and Sulfate ions getting trapped on the surface and reacted with ammonium ions used in the following process steps. Such material damage can affect optical properties of the materials and can also cause CD shift.