Characteristics of etching residues on the upper sidewall after anisotropic plasma etching of silicon

Highlights

• Characteristics of Si nano-trench sidewall etching residue were investigated.

• SiO₂ and suboxide etching residue was formed on Si nano-trench sidewalls.

• Sidewall surface state after cleaning process was evaluated via tilted XPS.

Abstract

Anisotropic Si nano-trench structures were fabricated using inductively coupled HBr + Cl₂ plasmas for sidewall etching residue analysis. The sidewall etching residues formed inside the Si nano-trench patterns were analyzed via tilted X-ray photoelectron spectroscopy. The changes in the chemical composition and binding state of the etching residues formed on the Si nano-trench sidewalls were investigated with various gas mixing ratios in HBr + Cl₂ plasma etching processes. The sidewall chemistry of the plasma-etched Si nano-trench patterns was examined at various take-off angles. SiO₂ and suboxide groups (Si_xO_y, x ≤ 2, y ≤ 3) were formed on the Si nano-trench sidewalls after the plasma etching. An increase in the chlorine content of the gas plasma resulted in the increased formation of SiO₂ and suboxide residual groups on the Si nano-trench sidewalls. Additionally, the changes in the chemical states of the Si nano-trench sidewalls after a wet-cleaning process were examined using our designed experimental technique.

Introduction

With the continuous shrinking of the feature size to the nanoscale in the semiconductor industry, various plasma etching techniques have been developed for fabricating highly integrated nanostructures on wafers. In particular, in Si etching processes that require anisotropic patterning, such as shallow-trench isolation and transistor-gate formation, the shapes of the etched features, as well as the critical dimension deviations, are significantly affected by the non-saturated residual layers, which passivate the pattern sidewalls [1], [2], [3], [4], [5]. If these residual layers are not sufficiently removed via a wet- or dry-cleaning process, they can cause yield loss of semiconductor devices [6], [7], [8], [9]. Accordingly, detailed information on the sidewall etching residue formed by the plasma etching process and the surface state of pattern sidewalls after the etching or cleaning process is essential for obtaining the desired feature profiles and device yields. Thus, the importance of sidewall analysis for fine patterns is increasing.

In numerous studies on Si etching in HBr- and Cl₂-based plasmas, non-saturated silicon halides (SiBr_x or SiCl_x) and/or silicon oxyhalides (SiO_xBr_y or SiO_xCl_y) were formed on a nonpatterned (blanket) Si surface via the deposition of etch byproducts and neutral species from the bulk plasma [10], [11], [12], [13], [14]. For patterned samples, researchers have investigated the chemical composition of the sidewall passivation layers formed in the plasma via X-ray photoelectron spectroscopy (XPS) [4], [15], [16]. The electrostatic charging effects have been used to distinguish different portions of the Si trench structures (e.g., mask surface and trench sidewalls). For differentiating the signal between the mask and the trench sidewalls, the pattern should consist of a dielectric mask and conducting sidewalls. Apart from this, little research has been performed on experimental techniques for sidewall analysis.

Most previous studies for pattern sidewalls involved patterns that were larger than microscale and isolated. Moreover, few studies have examined the chemical characteristics of the sidewall etching residue formed on nanosized patterns after the plasma etching process, because of the limited lateral resolution of existing surface-analysis techniques [4], [17]. Hence, to analyze the pattern sidewall, cumbersome pre-treatment steps are necessary (e.g., sample cleavage along the center of patterns and coating of specific materials on pattern surfaces) [18], [19]. However, this is difficult to implement for nanosized patterns. Such a situation does not allow accurate information on the sidewall etching residue caused by chemical (coating of other materials) and/or physical deformation (sample cleavage) to be obtained. Therefore, an experimental technique for examining the chemical characteristics of the nanosized pattern sidewalls should be developed. It is believed that detailed information on the sidewall surface states of nanopatterns before and after the cleaning process can make an important contribution for confirming the effects of wet- and dry-cleaning processes performed after the etching process.

Accordingly, in this study, we investigated the chemical characteristics of the etching residue formed on Si nano-trench sidewalls after HBr + Cl₂ plasma etching using our designed experimental technique. The changes in the atomic composition and binding state of etching residues formed on both the nonpatterned Si surfaces and Si nano-trench sidewalls were investigated with various gas mixing ratios in HBr + Cl₂ plasma via XPS. To obtain information on the etching residues originating only from the sidewalls, the remaining SiO₂ mask layer after plasma etching was completely removed via in situ Ar-ion sputtering in the XPS analysis chamber. This approach for the sidewall analysis of nanopatterns has not been previously reported. The sidewall chemistry of Si nano-trench patterns depending on the profile depth was examined with different take-off angles. Additionally, the change in the sidewall surface state of the Si nano-trench patterns after the wet-cleaning process was evaluated via the tilted XPS approach.