Investigation of acid diffusion during laser spike annealing with systematically designed photoacid generators

Abstract

Chemically amplified resists (CARs) are the current workhorse for photolithography, where higher resolution and smaller feature size represent a continual driving force for the semiconductor industry. As the feature size decreases to sub-30 nm, LWR and gate critical dimension (CD) control become serious concerns. In order to reach the goals in the ITRS, an unprecedented level of control of photoacid diffusion while maintaining the high resist sensitivity and resolution during image formation is required. CARs require a post exposure bake (PEB), typically performed on a hot plate at 90-150°C for 30-120 seconds, to complete the resist deprotection after photoacid generation. This bake step is a primary influence on resist performance as the time/temperature profile controls both the diffusion of photogenerated acids and the deprotection of the resist backbone. Sufficient time must be provided to achieve the level of deprotection required for the solubility switching in a developer, but the seconds timeframe of conventional hot plate PEB leads to an undesirable amount of acid diffusion. As long as the activation energy of diffusion is less than that for deprotection, higher temperatures for optimized time durations will result in reduced diffusion. However, traditional hot plate PEB cannot access times shorter than a few seconds. We utilize a laser (CO₂) based scanned heating system to achieve sub-millisecond to milliseconds in heating durations with temperatures up to the thermal decomposition limit of the resist. This research is aimed at using synthetic techniques to vary the structure of the photoacid generator (PAG) in order to learn about the role of PAG size and structure on acid diffusion during sub-millisecond heating. A variety of PAGs with different anion sizes have been synthesized and tested on the CO₂ laser system, and their lithographic performance and effect on acid diffusion has been studied.