Wetting and dissolution studies of fluoropolymers used in 157 nm photolithography applications

Photolithography using the $F2$ excimer laser at 157 nm, a technology to bridge traditional optical lithography and next generation lithographies, promises to enable ultralarge scale integrated devices with sub-70 nm design rules. Chemically amplified resists based on fluoropolymers have previously been shown to be good candidates for 157 nm microlithography. In our research, hexafluoroisopropyl alcohol (HFIPA) groups have been incorporated into polymers to improve the base solubility and to increase the transparency needed for new photoresists at 157 nm. These new polymers have absorbance values at 157 nm ranging from 1.7 to $3.9 μm−1.$ The introduction of fluorine groups increases their hydrophobicity and makes these polymers more difficult to wet at the surface. We have studied the effect of fluorine content on hydrophobicity of fluorinated polymers by measuring contact angle data over short time intervals. The ability to combine fluoropolymer synthesis with extensive contact angle studies has proven to be valuable in understanding solubility characteristics of these resins. These studies indicate that many of the fluorinated polymers have a very hydrophobic surface. Lowering contact angles can help to achieve the desired base solubility. Wetting studies also indicate that addition of certain surfactants to 0.26 N aqueous tetramethylammonium hydroxide (TMAH) help to reduce the contact angle. In addition to contact angle studies, we have used a quartz crystal microbalance to determine the dissolution rate of polymers in an aqueous base. Preliminary dissolution studies indicate a correlation to contact angle findings. Aqueous base solutions of 0.26 N TMAH containing surfactants were found to significantly lower the contact angle of an HFIPA-containing polymer, and increase the dissolution rate by an order of magnitude.