Multi-cycle Chamber Conditioning for Plasma Etching of SiO2: From Optimization to Stability in Lot Processing

Abstract

Hydrofluorocarbon gas chemistries have long been favored for SiO₂ etching. However, the fluorocarbon polymer generated during the process not only assists in obtaining a high selectivity, but also leads to chamber wall contamination. The adhesion efficiency of the polymer depends on the chamber wall temperature, which needs to be well-controlled to ensure controllable polymer deposition rate and etch characteristics. Similarly, the increasing gas temperature during the process is also expected to increase the production rate of polymer precursors. Hence, it is important to properly condition the chamber so that a sufficiently high and stable chamber temperature is reached before starting the actual process. This work utilizes an Inductively Coupled Plasma Reactive Ion Etcher to optimize a multi-cycle chamber conditioning process for two C₄F₈/H₂-based chemistries. We use the integrated optical emission spectroscopy (OES) tool to show that the dependence of etch characteristics on conditioning time is much stronger for the highly polymerizing chemistry. For a low conditioning time (< 15 min), the instability of plasma species indicate that the chamber temperature has not yet plateaued, resulting in a ⁓60% decrease of recess in the underlying silicon layer during the lot processing time. By conducting systematic etch tests, we analyze the behavior of key OES peaks to identify the optimal conditioning time (≥ 30 min) for this recipe, which results in only a 13% decrease in silicon recess depth during the processing time. Subsequently, a method to assess the stability of plasma species during the conditioning process is devised, assisting in advance to identify the optimal moment to initiate the lot process. By comparing the experimental results of the two etch recipes, we also highlight the important correlation between conditioning time and polymerizing degree of the chemistry.