Laser cleaning of optical elements in 157-nm lithography

In-situ laser cleaning is shown to be an effective tool for removal of organic contaminants on CaF2 windows. To study laser cleaning in a controlled fashion, CaF2 substrates were pre-contaminated with 5 to 10 nn of poly(methyl methracylate), poly(4-hydroxy styrene), poly(norbornene), and poly(beta-pinene) thin films. Irradiation of all the polymer films showed similar trends. Initially, a high rate of material removal occurs, which depends on the chemistry of the polymer. During this period, the material also undergoes significant bond rearrangement, forming a more tightly bound highly conjugated network. Removal of this residual "graphitized" film is significantly more difficult, but can be accelerated by the presence of modest levels of oxygen. For oxygen concentrations between 10 - 1000 ppm, the measured removal rate is approximately 3 nn / (kJ/cm(2)) / ppm oxygen. No effect on removal rate was observed as pulse energy or purge gas flow rate was varied over ranges expected to be used in practical systems.