Investigation of Acid diffusion during laser Spike Annealing with Systematically designed Photoacid generators

The post-exposure bake in the lithographic process has 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. We utilize a laser (CO2) based scanned heating system to achieve sub-millisecond 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 submillisecond heating. Summary of Research: The requirement for smaller feature dimensions continues to drive lithography research forward. In order to meet the requirements of the ITRS roadmap, it is necessary to investigate not only resist materials, but every step of the lithography process as well. By optimizing processing steps, it may be possible to push materials past their current limit on state of the art lithography systems. The post exposure bake (PEB) step in the lithography process has the most effect on the shape and size of the patterns produced [1]. The heat catalyzes the deprotection or crosslinking of chemically amplified resists by mobilizing the PAG in the resist matrix. Some diffusion of acid is necessary to start the reaction, however, uncontrollable acid diffusion has been held accountable for limiting resolution and increasing line edge roughness (LER) of patterned resists [2,3]. Controlling acid diffusion by altering resist chemistry has been studied extensively with the creation of polymer-bound PAGs, where the PAG is chemically bonded to the resist chain, thereby limiting its mobility [4,5]. These resists have shown reduced LER, but at the expense of sensitivity [5]. Figure 1: Structures of PAG molecules. Our research is associated with changing the method of the PEB to improve resist performance. Resists baked on a hotplate are heated to a modest temperature, in the range of 100°C to 150°C for seconds to minutes. By heating the wafer with the laser, the film is subject to a high rate of heating and cooling, in the submillisecond timeframe. We have shown that laser-post exposure bake (l-PEB) has resulted in improved LER, along with greater sensitivity, demonstrating the excellent performance capability of the laser [6]. We have synthesized a set of PAG molecules with differing anion sizes, shown in Figure 1. The anion structure has been systematically varied to investigate the effects of anion size on diffusion behavior. The acid diffusion length was measured using a bilayer technique that has been previously reported [7, 8]. The resist on the PDMS stamp was exposed using an ABM contact aligner with dose steps from 0-20 mJ/cm2, and tested at a variety of bake temperatures with both the hotplate and laser. Figure 2 shows the acid diffusion length of the various photoacids as a function of bake temperature. The acid diffusion length is shortened with l-PEB treatment, even though those wafers were brought to a higher temperature than those tested on the hotplate. Figure 2: Acid diffusion behavior of the PAGs with hotplate and l-PEB.