Lactones in 193 nm resists: What do they do?

Abstract

Lactones are almost ubiquitously employed in 193 nm resists to increase the polarity of hydrophobic alicyclic polymers. What else do lactones do in 193 nm resists? We studied the behavior of methacrylate (MA) resists consisting of different protecting groups, hexafluoroalcohols, and norbornane lactone methacrylate (NLM, 2-oxo-3-oxatricyclo[4.2.1.04,8]nonan-5-yl methacrylate). When the protecting group is large [ethylcyclooctyl (ECO) and methyladamantyl (MAd)], thinning of the resist film that occurs in highly exposed areas upon postexposure bake (PEB) is significantly smaller than what is expected from the polymer composition. When the concentration of isopropylhexafluoroalcohol methacrylate (iPrHFAMA) is increased in the ECOMA-NLM polymer, the thinning increases and reaches 100% of theory and the ECOMA-norbornenehexafluoroalcohol methacrylate (NBHFAMA) resist loses quantitative thickness in highly-exposed areas upon PEB at 90°C. This indicates that small lactones which are more basic than esters can trap deprotection fragments especially when the protecting group is large. Such entrapment was detected by IR spectroscopy and also observed at temperatures as high as 200°C in thermogravimetric analysis (TGA). Incorporation of lactone appears to decrease the bake temperature sensitivity and the sensitivity of the resist perhaps due to trapping of photochemically generated acids by basic lactone. The lactone ring can be hydrolyzed during aqueous base development but does not seem to affect the dissolution rate, indicating that hydrolysis occurs in aqueous base solution after dissolution. Poly(methacrylic acid-NLM) dissolves as fast as poly(methacrylic acid) in 0.26 N tetramethylammonium hydroxide (TMAH) aqueous solution. While exposed P(ECOMA47-NLM53) resist dissolves in 0.26 N developer at about the same rate as authentically prepared poly(methacrylic acid47-NLM53), the dissolution rate of highly-exposed P(MAdMA44-NLM56) resist is much slower, indicating that the deprotection fragment from the former does not interfere with the development but that from the latter does. When the NLM concentration is increased to 75 %, highly exposed P(ECOMA-NLM) resist dissolves slowly at ca. 600 A/sec and swells significantly, indicating that NLM can be a dissolution inhibitor and swelling enhancer when its concentration is high. Low activation energy protecting groups such as ethylcyclooctyl allows imaging at temperatures as low as 60°C. However, the temperature dependence of the dose to dear is very large and the chemical contrast is quite small in the low temperature range. Thus, for PEB temperature stability and contrast enhancement, baking 20-30°C above the lowest practical temperature is recommended.