(Invited) The Mechanism of Surface Modifications of Photoresist Material in Plasma Etching Nano Processes
Authors: Hori Masaru , Zhang Yan , Ishikawa Kenji 
Published: February 2020

Etching plasmas employed for the fabrication of ultra large scale integrated circuits (ULSIs) have been causing serious damages to photoresist materials such as surface roughness formations and structural changes. So far, there have been many reports on suppressing these damages by some process techniques such as the UV, HBr plasma cure and so on. However, effects of individual species and photons in the plasma on the damages of photoresist materials have not been clarified enough. Especially, the effect of vacuum ultraviolet (VUV) photons on damages have never been quantitatively understood. Furthermore, beside the VUV photons, the synergetic effects of individual species of ion and radicals enhance the damages. As a result, the chemical structure alteration and the surface roughness formation of polymer materials are considered to be brought about. In the fabrication of the sub-45 nm devices and integrated circuits, it is extremely important to clarify the reaction mechanism of surface modification of photoresist materials induced by the plasma etching process.

In this article, firstly, we have focused on the effects of VUV photons on damages on the 193 ArF photoresist materials by investigating the absolute density of VUV photon fluxes measured in the range of 115nm and 200nm in HBr plasma etching processes. The etching and measurement of VUV photons in the plasma were performed in the plasma beam system for understanding the fundamental reaction on the photoresists. This plasma beam system consists of an upper chamber (plasma chamber) and a lower chamber (process chamber). This equipment provided the beam source with the HBr plasma. The plasma beam system was pumped down to a base pressure of 10-5 Pa prior to the experiment. During the whole experiment, inductively coupled plasma (ICP) was generated by applying a radio-frequency (13.56 MHz) power of 400 W to the low-impedance antenna. Pure HBr, H2 or Ar gas was introduced into the plasma chamber at a flow rate of 1 sccm. The pressure of the plasma chamber was maintained at approximately 5 Pa. The chemical compositions of the surface of photoresists before and after the plasma beam irradiation were analyzed by in-situ XPS.

On the basis of measured results, effects by VUV photons emitted from HBr on the surface modifications of photoresist materials were clarified. The VUV photons cleaved the MAA groups below the photon dosage of 16 × 1016 photons/cm2. The crosslinking of the photoresist and release of volatile products simultaneously generated the surface roughness. The reaction became saturated with high dosages [1].

Secondary, the sequential irradiation by VUV/radicals from HBr or H2 plasma and Ar ions on a photoresist have been performed. The irradiation by VUV and radicals before the ion irradiation (pre-treatment) suppressed the roughness increase by the ion bombardment but the VUV and radical irradiation after the ion irradiation (post-treatment) increased the roughness. The surface morphologies were also analyzed in detail after pre- or post-treatment on the photoresist by using the power spectral density. Such pre-treatment processes generated a dense and hard crosslinked layers which improved the etch resistance of the photoresist and improved the roughness [2]. The enhancement of roughness by post-treatment was correlated with the removal of the MAA groups and the nonuniformity of crosslinking reactions on the roughened photoresist by the ion irradiation.

On the basis of these results, the mechanism of photoresist modifications by individual effects of VUV, ions and radicals and their synergetic effects by sequential irradiation experiments were systematically discussed and then perspectives for the damage less processes to be established in the etching nano processes are introduced.

[1] Y. Zhang et al., Journal of Vacuum Science & Technology A, 35, 060606 (2017).

[2] Y. Zhang et al., Japanese Journal of Applied Physics, 56, 126503 (2017).