SiNx Deposition at Low Temperature Using UV-Irradiated NH3
Authors: Shiba Yoshinobu , Teramoto Akinobu , Suwa Tomoyuki , Ishii Katsutoshi , Shimizu Akira , Umezawa Kota , Kuroda Rihito , Sugawa Shigetoshi 
Published: February 2020

Introduction
Technology for depositing silicon nitride (SiNx) films at low temperature has been extensively studied. Plasma-enhanced chemical vapor deposition and photo-chemical vapor deposition can deposit SiNx films at low temperature (1-3). However, a substrate exposed to plasma or UV light is damaged (4). Deposition equipment with low plasma damage has also been developed (1-2); however, such damage is difficult to avoid. In this paper, we suggest a low-temperature SiNx film deposition technique that does not cause plasma or UV damage. Further, we deposited SiNx film to evaluate its characteristics.
Experiment
Fig. 1 shows a schematic diagram of the SiNx film deposition equipment. The equipment was constructed with a UV irradiation chamber and a tube reactor for SiNx film deposition. Ammonia (NH3) gas was excited by UV light and then supplied to the tube reactor. Si2H6 gas was directly supplied to the tube reactor without the UV irradiation. The UV light was emitted perpendicular to the NH3 flow in the UV irradiation chamber separate from the tube reactor and Si2H6. Because the UV light did not reach the substrate, the deposited films are not damaged and the film is not deposited the glass window for the irradiation chamber. The wavelength range of UV light of 160 – 210 nm included the absorption wavelength of NH3 (5). The deposition temperatures were 30˚C and 450˚C. We evaluated the effect of the UV-irradiated NH3 for the formation of high quality SiNx film. The nitrogen content of the films was analyzed by X-ray photoelectron spectroscopy (XPS).
Result and discussions
The effect of UV-irradiated NH3 is shown in Fig. 2. The vertical axes show the thickness and refractive index evaluated by ellipsometry. Horizontal axes indicates measurement positions shown in Fig. 1. NH3, N2 or H2 were supplied to the tube reactor with and without UV irradiation. A film having a refractive index near 2.0 was clearly deposited only when UV-irradiated NH3 was supplied. When N2 and H2 was supplied, regardless of the UV irradiation, almost the same films having thickness and refractive index were obtained. These results indicate that UV irradiation of N2 and H2 gas did not affect the reaction of not only N2 and H2, but also Si2H6. Fig. 3 shows composition ratio measured by XPS of the film deposited on the 300 nm thick SiO2. Figs. 3(a)-3(b) show that the film containing nitrogen was formed at 30°C and 450°C. The N/Si ratio of the film deposited at 450˚C with UV-irradiated was near 1.33, and the film’s depth profile had a region where no oxide was detected. These results show that the stoichiometric composition of the deposited film was close to Si3N4, and the film cannot be oxidized by the atmosphere of the clean room at R.T.
Conclusion
We suggested a low-temperature SiNx film deposition technique using UV-irradiated NH3 and demonstrated the deposition of SiNx films at low temperature. When UV-irradiated NH3 was used, nitrogen-containing film with an N/Si ratio close to 1.33 was clearly deposited at 450°C. In addition, the UV light introduced into the irradiation chamber excited only NH3 molecules, and it did not reach the substrate in the tube reactor. These results show that UV irradiation with NH3 is able to deposit SiNx films without UV damage at low temperature.

Figure 1