HBr/O2 plasma treatment followed by a bake for photoresist linewidth roughness smoothing

With the decrease of CMOS device dimensions, the linewidth roughness (LWR) of transistor gates is being recognized as a major concern. In integrated circuit manufacturing, an HBr plasma treatment has been used so far to reduce the LWR of photoresist (PR) lines printed by 193 nm lithography before transfer into the gate stack by plasma etching. In this article, a more efficient treatment based on HBr/O2 plasma exposure followed by a bake is developed, and the plasma conditions are optimized. We show that by controlling the dose of vacuum ultra violet (VUV) light emitted by the HBr/O2 plasma, the PR bulk can be either softened (photolysis) or strengthened (crosslinking). Moreover, the PR surface properties can be controlled by the O2 content of the plasma. The surface and bulk properties of the PR line after plasma treatment determine the LWR behavior during the subsequent thermal cure treatment. A soft resist bulk combined with a thin surface carbon layer is the ideal case to get the best LWR smoothing effect during the subsequent bake without reflowing. By optimizing both the plasma oxygen content and the VUV dose, we obtained a 59% LWR decrease (from 7.3 nm to 3.0 nm) using an HBr/O2 plasma treatment followed by a bake. The detailed study on the impact of the VUV dose during the plasma treatment permitted to determine the sidewall smoothing mechanisms involved. During both the plasma and the thermal treatments, the outgassing of the products of photolysis or deprotection reactions followed by the re-densification of the polymer material is shown to be the main sidewall smoothing enabler.