This paper describes a new surface cleaning approach for photomask cleaning. This non-conventional cleaning method uses momentum transfer between aerosolized, frozen, CO<sub>2</sub> particles and the contaminants for effective removal from surface. The purity of CO<sub>2</sub> used is an important component in the determination of cleaning efficiency. The authors will present two methods developed to analyze hydrocarbon contamination in CO<sub>2</sub>. These analytical methods were used to compare different grades of CO<sub>2</sub> including Ultra High Pure (UHP) grade developed for sub-micron particle removal from photomasks. Using the UHP grade CO<sub>2</sub>, it was shown that greater than 99% particle removal efficiency is possible from silicon wafer surfaces, with higher removal efficiency of sub-micron particles compared to larger size range. This particular characteristic of particle removal by cryogenic aerosol method is theoretically derived in an earlier paper. In this paper results of CO<sub>2</sub> cryogenic aerosol cleaning with respect to electrostatic discharges on two different binary masks are presented. The paper also shows the removal of 99.9% of the progressive defects such as haze of 0.5 to 1.0 μm size. Cleaning characterization of attenuating phase shift masks with MoSiON films indicate 0.04% change in transmission and 0.37% change in phase angle after 16 cleaning cycles, suggesting that cryogenic cleaning has minimal effect on transmission and phase of att-PSM.
This paper describes the mechanism and cleaning results of a dry cleaning technology using CO<sub>2</sub> cryogenic aerosols. The cleaning mechanism relies on momentum transfer from the aerosol particles to overcome the force of adhesion of the contaminant particles on the surface. Particle removal is possible without degradation or etching of underlying film or the need for drying with IPA as in wet cleaning. A theoretical model of particle removal based on momentum transfer is described, predicting higher removal efficiency for sub-micron particles compared to larger particles. Experimental results with Si<sub>3</sub>N<sub>4</sub> particles on silicon wafers show that removal of sub-micron particles is 10% higher than larger particles up to 30 μm, as predicted by the model. The paper also shows experimental results of various types of contaminant particle removal in photomask cleaning. Results of post mechanical repair cleaning of photomasks show effective removal of the quartz particles without damage to the adjacent chrome lines. Inorganic contaminants such as ammonium sulphate, commonly known as "haze", is removed by cryogenic aerosol cleaning with 99% efficiency as seen using optical inspection tool. The effect of cleaning on the phase and transmission of the mask is measured with multiple cleaning. The results show that over 16 cleaning cycles, the change in transmission is 0.04% an the change in phase is 0.37°. Thus a non-invasive cleaning for sub-micron particles from photomasks is possible with CO<sub>2</sub> cryogenic aerosols.