High Selectivity in Dry Etching of Silicon Nitride over Si Using a Novel Hydrofluorocarbon Etch Gas in a Microwave Excited Plasma for FinFET
Authors: Nakao Yukihisa , Matsuo Takatoshi , Teramoto Akinobu , Utsumi Hidetoshi , Hashimoto Keiichi , Kuroda Rihito , Shirai Yasuyuki , Sugawa Shigetoshi , Ohmi Tadahiro 
Published: February 2020

The SiNx gate spacer formation is one of the key processes in the miniaturized MISFETs, because the thickness of the gate spacer and the Si recess after the gate spacer formation etching have large impacts on the electrical properties of MISFETs [1,2]. High selectivity of SiNx over Si is strongly required in MISFETs such as extremely thin silicon on insulator (ETSOI) device and 3 dimensional (3D) MOSFET for the reduction of the series resistance [1,2]. Various etching gases were evaluated in order to obtain high selectivity of SiNx over Si[3-6]. It is considered that a plasma source and etching gases are important in order to obtain high selectivity dry etching of SiNxover Si. We consider that the plasma source with low electron temperature is advantageous in the improvement of selectivity, because it prevents excessive dissociation of the etching gas. The difference of the chemical structures of etching gas with the plasma source can make a difference of the selectivity. In this work, we utilized the microwave excited plasma source that has the characteristic of low electron temperature and a novel gas (SSY525 made by ZEON) [7].
Figure 1 shows the selectivity of SiNx to poly Si as a function of O2 flow rate. The flow rate of SSY525 and CH3F are 8 sccm and 10 sccm, respectively. The microwave power, the RF power of bottom electrode and the Ar gas flow rate were 1000 W, 60 W and 200 sccm, respectively. SiNx films were deposited by PECVD and LPCVD. Si/N ratio and density of SiNx by PECVD was lower than those by LPCVD [8]. In the case of SSY525, a higher selectivity of SiNx by PECVD and LPCVD to poly Si was obtained with O2 flow rate from 0 to 50 sccm to CH3F. Low etch rate of poly Si less than 0.3 nm/min and high selectivity of SiNx to Si over 50 were obtained with O2 flow rate of 0 sccm and more than 30 sccm. Figure 2 shows the analysis of poly Si surface by XPS after dry etching using SSY525. A high intensity of C 1s was obtained with O2 flow rate of 0 sccm. It is considered that high selectivity of SiNx to poly Si was obtained due to carbon deposition on poly Si surface. The intensity of C 1s decreases and the intensity of O 1s increases as the O2 flow rate increases. It is considered that High selectivity of SiNx to poly Si is obtained with O2 flow rate more than 30 sccm due to an oxidation of poly Si surface. Figure 3 (a) shows the schematic of 3D-MISFET structure. Figure 3 (b)-(d) show the cross-section SEM images from A to B in Fig. 3 (a) with various SSY525 flow rate. A 40 nm-thick SiNx was overetched at (b) 8 sccm, (c) 12 sccm and (d) 15 sccm. Si near the SiNx sidewall was etched with SSY525 of 8 sccm. It was not etched with SSY525 more than 12 sccm. About 5 nm-thick film containing carbon was deposited on the Si surface with 15 sccm. In the cases of (c) and (d), high selectivity of SiNx over Si was obtained. It is confirmed that SSY525 is effective in order to form the SiNxgate spacer in the miniaturized MISFET.
References
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