Integrated equipment-feature modeling investigation of fluorocarbon plasma etching of SiO2 and photoresist

Abstract

As the microelectronics industry continues to shrink feature size and increase feature density in the back-end of integrated circuits, the traditional empirical approach to plasma etch process development is becoming prohibitively expensive and time consuming. Fundamental physics based models can prove useful in driving down process development time and cost. In this article, an integrated equipment-feature scale modeling infrastructure for SiO2 and photoresist (PR) etching in fluorocarbon based plasma discharges is described. The model correlates process conditions with plasma properties, surface interactions, and etch results. A validated plasma chemistry for Ar/c–C4F8/CF4 and detailed plasma–surface reaction mechanisms for SiO2/PR etching have been incorporated in the model. Major surface reactions for SiO2 etching include neutral surface passivation, fluorocarbon radical polymerization, and ion assisted etching of volatile products. The mechanism for PR erosion includes energy/angle dependent ion sputtering, ion activation, F atom etching with ion assistance, and fluorocarbon radical deposition. Computed SiO2 and PR etch profiles and rates have been validated by comparing with experimental results in a commercial inductively coupled plasma (ICP) etch tool. The validated model is used for a detailed investigation of SiO2/PR etching in a representative 300 mm wafer ICP tool. It is found that SiO2 etch rate is a nonlinear function of Ar/c–C4F8 ratio, where the highest etch rate is obtained when sufficient neutral passivation takes place while polymer deposition is still small. Deviating from this condition reduces SiO2 etch rate by either excessive polymerization or insufficient passivation. PR etch rate and facet size, however, increase monotonically with Ar/c–C4F8 ratio due to reduced polymer deposition. The effect of CF4 ratio in the Ar/c–C4F8/CF4 source gas on SiO2 etching depends on the Ar fraction. When Ar fraction is large, replacing c-C4F8 with CF4 reduces surface passivation and thereby decreases SiO2 etch rate. However, at small Ar fractions, CF4 addition reduces polymer formation and increases the SiO2 etch rate. For the range of conditions explored, SiO2 etch characteristics are insensitive to bias frequency as the ion energies are well above the threshold energy for etching. The plasma zone height (PZH) impacts the fluxes of etchants to the wafer and consequently the SiO2/PR etch rates. PZH, however, does not influence etch uniformity noticeably as diffusion is dominant at low gas pressures.