Paper PS-MoM3
Effect of the Amount of Hydrogen During SiN etching on Etching Properties

Monday, November 7, 2016, 9:00 am, Room 104B

Silicon nitride (SiN) is an essential film in complementary metal oxide semiconductor devices. The amount of hydrogen contained in SiN films depends on the process conditions used in chemical vapor deposition (CVD), and strongly affects the etching properties of etching rate (ER), C-F polymer thickness (T_C-F), and damage. Therefore, revealing the mechanism of how hydrogen influences etching is very important to predict and control damage distribution considering the etching profile, and to develop etching processes with high selectivity for SiN over SiO₂.

To model SiN etching under the effect of hydrogen, we performed experiments using a dual-frequency capacitively coupled plasma system. We prepared three kinds of SiN_x:H_y films with y = 2.6%, 16.8%, and 21.9% (denoted as LP-SiN, Low-H SiN, and High-H SiN, respectively) on Si substrates using different CVD processes. The films were treated with CH₂F₂/O₂/Ar plasma under a gas pressure of 20 mTorr and Vpp of 350 V, for which we measured ER and T_C-F values. We also measured plasma and surface conditions using various monitoring techniques. We analyzed the results through a first-principles calculation with VASP [1].

We found that for the low CH₂F₂/(CH₂F₂+O₂) ratio where few C-F polymer layers existed, the ER values of High-H SiN were 20%–40% smaller than those of LP-SiN and Low-H SiN. In contract, inverse behavior was observed in the case of a high CH₂F₂/(CH₂F₂+O₂) ratio. Considering variation of OES data and reactivity estimated by the VASP calculation, under the assumption that the etching front consisted of two layers (C-F polymer layer and reactive layer) [2], not only reaction between the outflux of H from the reactive layer and F from plasma but also termination of Si dangling bonds by H seems to cause variation of the ER value when the CH₂F₂/(CH₂F₂+O₂) ratio is low. Because a C-F polymer layer of moderate thickness existed over the reactive layer at high CH₂F₂/(CH₂F₂+O₂) ratio, some H was consumed by reaction with C in the polymer layer, which weakened the effects of H such as deactivation of F and termination of Si dangling bonds. This seems to lead to the inverse behavior observed at high CH₂F₂/(CH₂F₂+O₂) ratio.

We formulate the above effect of H and include it in our SiN surface reaction model using the 3D voxel-slab method [3], reproducing ER and T_C-F values. We also demonstrate SiN side-wall etching of fin-type field-effect transistors and discuss how to control etching profile and damage distribution.

[1] G. Kresse and J. Furthmüller, Phys. Rev. B 54, 11169 (1996).

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

[3] N. Kuboi et al., J. Vac. Sci. Technol. A 33, 061308 (2015).