Introduction of a High Selectivity Etching Process with Advanced SiNx Etch Gas in the Fabrication of FinFET Structures
Authors: Kojiri Takashi , Suwa Tomoyuki , Hashimoto Keiichi , Teramoto Akinobu , Kuroda Rihito , Sugawa Shigetoshi 
Published: February 2020

The FinFETs [1-3] is one of the candidates of further high performance device compared to the conventional planar MOSFETs. It is important to form the channel as high and thin structure for the performance of FinFETs. Such a high aspect channel is superior to the gate bias controllability to the channel and current capability per unit area. High selectivity etch technologies are required in the fabrication of high aspect fin channel. Those technologies can also realize a fabrication of fin as a fine structure, which leads to the possibility of a newly technology such as a applying of different orientation in Si fin surface [4]. In this work, we fabricated FinFET structure with high selectivity etching using newly developed SiNx etch gas SSY525 (a prototype developed by Zeon Corporation) and we found that SSY525 is one of the promising material for the fabrication of FinFETs.
We evaluated the etching characteristics of SSY525. The evaluations were carried out with a microwave-excited plasma etching equipment [5]. The microwave power, the RF power of bottom electrode, the chamber pressure and the flow rate of SSY525 were 1000 W, 90W, 5.3 Pa, and 15 sccm, respectively. Figure 1 (a) shows the etch rates of SiNx and the selectivity to Si and SiO2 as a function of the O2 flow rate. The etching characteristics of CH3F, which is conventionally used in SiNx etching is also plotted in Fig. 1 (b) as a comparison. The flow rate of CH3F was 15 sccm. The higher etching selectivity of SiNx to Si and SiO2 was obtained with SSY525 compared with CH3F. In the case of SSY525, the highest selectivity was obtained with O2 flow rate of 0 sccm, and then the selectivities were 62 to the Si and over 100 to the SiO2, respectively. Contrary to the SSY525, in the CH3F, the highest selectivity was obtained with O2 flow rate of 8 sccm, and then the selectivities were 8 to the Si and 9 to the SiO2, respectively. In the case of CH3F, the etching of poly-Si and SiO2 are suppressed by introducing a small amount of O2. Meanwhile in the case of SSY525, those etching are suppressed by depositing a film on Poly-Si and SiO2, therefore SSY525 realized a high selectivity etching [6]. Furthermore, in these conditions, the etch rates in the both gases were almost the same; the etching rate of SSY525 was 57 nm/min and of CH3F was 55 nm/min, respectively.
Next, we carried out the shapes evaluation after the etching with SSY525 and CH3F. We used a SOI wafers, which was patterned line and space (L/S=30 nm/100nm, height=75 nm) with Si represent a fin structure and deposited 40 nm SiNx film over the L/S pattern for evaluation. The SiNx film on the Si fin was etched under the 40% over-etching condition with SSY525 and CH3F, respectively. The just etching means here, the time to etch the Si fin height and SiNx film thickness. The flow rates of etchant gases were 15 sccm, and the O2 flow rates were set at the etch selectivity condition maximum, that is, 0 sccm in SSY525 and 8 sccm in CH3F, respectively. For both etchant conditions, the microwave power, the RF power of bottom electrode, and the chamber pressure were 1000 W, 90W, and 5.3 Pa, respectively. Then, these Si fin after etching were evaluated the shapes. The results were shown in Fig. 2. The SiNx film between the Si fins was completely removed with both gases. Though, with the CH3F, there were losses of the top of Si fin and bottom oxide during the over-etching, with SSY525, there were almost no less of those parts. Since SSY525 has higher selectivity than CH3F, therefore the top of Si fin and bottom oxide almost maintained the pristine structures.
Finally, we fabricated FinFET structure introducing SSY525 in the spacer etch process, and these indicate that the fabrication of the fine shape FinFET is possible by using SSY525 (Fig. 3).
Acknowledgment
This research has been carried out at fluctuation free facility of New Industry Creation Hatchery Center, Tohoku University.
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Figure 1