Paper PS+EM-WeM2
Mechanism of Highly Selective SiO₂ Etching over Si₃N₄ using a Cyclic Process with BCl₃ and Fluorocarbon Gas Chemistries

Wednesday, October 24, 2018, 8:20 am, Room 104A

Multiple patterning techniques require extremely high selectivity to various materials and controllability of cross-sectional pattern profiles with atomic scale precision. In these fine patterning techniques, SiO₂ etching over Si₃N₄ requires an advanced process to form a thinner protection layer on Si₃N₄. For example, SiO₂ atomic layer etching with the fluorocarbon (FC) passivation of C₄F₈ plasma followed by Ar⁺ bombardment has been investigated [1]. To achieve a high selectivity to Si₃N₄ using conventional FC plasma, the thickness and composition of the FC film should be controlled to protect only the Si₃N₄ surface from reaction with the FC film [2]. In our previous study, we proposed a cyclic SiO₂ etching process over Si₃N₄ by using BCl₃ and FC gas chemistries [3]. BCl₃ plasma was applied because it was expected to form a thin protection layer, which was suitable for selective etching at fine patterns when a low wafer bias was used. The thin protection layer formed by BCl₃ plasma was also expected to inhibit the spontaneous etching of Si₃N₄ by F radicals.

In this study, we investigated a cyclic process using BCl₃ and fluorocarbon gas chemistries for a fine pattern structure with a space width of 20 nm. The relationships between etching parameters and cross-sectional pattern profiles were also analyzed to control the pattern profiles. This process alternately performs two steps: an adsorption step using BCl₃ mixed-gas plasma and an etching step using BCl₃/CF₄/Ar plasma with applying a wafer bias. The mechanism of the cyclic process was investigated by analyzing the surface chemistry at each step. At the adsorption step, a thicker BCl_x layer was formed on the Si₃N₄ surface than on the SiO₂ surface. Then, CCl_x films were formed on both surfaces at the etching step. We found that the thicker BCl_x layer formed on Si₃N₄ at the adsorption step protected Si₃N₄ from etching by reaction of BCl_x with CF_x and F radicals at the etching step. The B atoms in the BCl_x layers desorbed from the surfaces by forming BF_x, BCl_xF, and CCl_x. In contrast, the BCl_x layer became thinner on SiO₂ than that on Si₃N₄ to promote ion-assisted etching of SiO₂. This is because the BCl_x component has high reactivity with SiO₂, and the CF_x component was also consumed by the etching reaction with SiO₂. We also found that ion-flux should be controlled to etch without shoulder-loss, and ion-energy should be controlled to etch without footing shape at the bottoms of the pattern.

[1] D. Metzler et al., J. Vac. Sci. Technol. A 32, 020603 (2014).

[2] M. Matsui et al., J. Vac. Sci. Technol. A 19, 2089 (2001).

[3] M. Matsui et al., Jpn. J. Appl. Phys. 57,(2018) to be published.