Rise of chemical amplification resists from laboratory curiosity to paradigm enabling Moore's law

Abstract

The concept of chemical amplification resists was proposed from IBM Research about 25 years ago. Although initially considered laboratory curiosity even within IBM, the tBOC resist based on acid-catalyzed deprotection was employed in a negative mode in mass production of 1 Mbit DRAMs by deep UV lithography in IBM in the mid 80's. Development of positive 248 nm resists faced a devastating postexposure delay problem, which threatened the future of chemical amplification resists. Tracing the cause to contamination of the resist film surface with airborne basic substances resulted in development of environmentally stable resist systems, which cemented the industry-wide acceptance of chemical amplification resists, enabling the semiconductor industry to follow the Moore's law. The migration from 248 to 193 nm necessitated abandonment of the etch-resistant but absorbing phenolic structure and introduction of alicyclic structures for transparency and etch resistance. Several platforms were developed, including polymethacrylates, all-norbornene systems, cycloolefin-maleic anhydride co- and terpolymers (COMA), and vinyl ether-maleic anhydride (VEMA) systems. Replacement of phenol with carboxylic acid for transparency and aqueous base development resulted in swelling. Lactones were incorporated to increase the polarity of hydrophobic alicyclic polymers. Hexafluoroalcohol was introduced to replace carboxylic acid and became employed ubiquitously in 157 nm resists, for transparency and base development. Although 157 nm lithography has been abandoned, the fluoroalcohol group has been heavily utilized in dry and wet 193 nm resists and immersion topcoats, and as additives for surface segregation. The chemical amplification resists initially developed for 1 μm patterning can now print <30 nm features. The question is how far chemical amplification resists can go in terms of resolution, maintaining sensitivity (or even increasing the sensitivity) while improving line edge roughness (LER).