by M. David Levenson, Editor-in-Chief, Microlithography World
First-generation resist systems for immersion lithography employed a topcoat material to protect the resist and prevent leaching of resist components that might damage the optics. By making the topcoat surface hydrophobic and controllable, topcoat materials suppressed defects and facilitated rapid wafer scan. However, a hydrophobic surface could interfere with resist development, and so the topcoat material also had to be removed efficiently by developer or some dedicated process step.
These topcoat systems were successful in advancing immersion lithography and suppressing characteristic (round) immersion defects, but particles, evidently originating at the wafer edge, affected the total defect level. The mechanism for producing those particles became clear at the 2007 SPIE Advanced Lithography Symposium: topcoats do not stick well to silicon, SiO2, TEOS, SiN, or other common substrate materials. If the topcoat film extends beyond the BARC and resist layers at the wafer edge, it will flake off, contributing particles, according to Junichi Kitano of TEL. The challenge is to control the wafer edge bead area, while maintaining a 2mm edge exclusion.
However, another option has appeared: topcoat-less resists. A new generation of these materials has appeared that develops a hydrophobic surface in situ due to the self-segregation of a resist component during the coating process. Soichi Owa of Nikon reported that such materials gave the lowest defect density of any materials in production with the NSR-609B immersion tool: down to 0.009 defects/cm2 or 5/wafer. As lens contamination recedes as a concern, simpler topcoat-less resist processes should become dominant.
Of course, a hydrophobic surface is just what you don’t want during development. Chemists at IBM’s Almaden Research Center presented formulations in which the contact angle of the surface switches with pH. The key trick is to create a self-segregating molecular structure with fluoroalcohol groups that present a fluorine-rich hydrophobic surface to pure water but ionize to create a hydrophylic surface in the high pH of TMAH developer. Thus there is more to water resist interactions than simple “polarity.”
Particles will remain and continue to be moved around by the fluid unless removed. Using clean wafers helped, according to ASML and Nikon, as did bevel edge drains that sucked around the edge of wafers. However, both toolmakers reported that flushing out the immersion system with “cleaning fluid” in situ periodically reduced defects in production. – M.D.L.