Effect of Oxygen Impurity on Nitrogen Radicals in Post-Discharge Flows
Authors: Shiba Yoshinobu , Teramoto Akinobu , Suwa Tomoyuki , Watanabe Kensuke , Nishimura Shinichi , Shirai Yasuyuki , Sugawa Shigetoshi 
Published: February 2020

Introduction

Silicon nitride films (SiNx) formed at low temperature are strongly required in very shrunk LSI process. The plasma silicon nitride formation technology with low damage at low temperature has been reported [1]. We have reported that the amount of active nitridation species in the process is important to form to high quality SiNx, especially on the side wall [2]. When the oxygen impurity is remained in the process gases, the oxygen is excited during the plasma processing. Then the excited oxygen is degraded the SiNx film quality [3-6]. However, it has not been reported the effect of inactive oxygen molecules in the nitridation process. In this paper, we report the effect of the impurity oxygen in post charge flows on the process of nitride formation.

Experiment

Fig.1 shows the schematic diagram of the experimental setup used for the reaction products measurement in the gases. It has a mixing point placed at downstream of a Nitrogen Radical Generator [7]. The produced gases reacted from the nitrogen radicals and O2 gas are measured by Fourier Transform Infrared spectroscopy (FTIR). The impurity concentration of oxygen in N2 gas was less than 1 ppb, this indicates the oxygen impurity of supply gas did not affect the experimental results. Nitrogen radicals concentration was generated by the Nitrogen Radical Generator several ten ppm in the N2 gas [7]. The pressure in Nitrogen Radical Generator was varied from 30 to 120 kPa in the case of the N2 flow rate at 5, 7, and 10slm. The O2flow rate were varied from 8 to 190 sccm.

Results and discussions

Fig. 2 show the absorbance of (a)O3, (b)N2O produced from the nitrogen radicals and O2gas.
Horizontal axis of Fig. 2(a) denotes the product of the oxygen partial pressure and the oxygen molecules concentration. Here, this is equivalent to the collision probability of the oxygen molecules and the oxygen molecules. The produced O3concentration increases with the increase of the collision probability.
Then, horizontal axis of Fig. 2(b) denotes the product of the oxygen partial pressure and the elements' density exception of the oxygen molecules, that is, they’re nitrogen molecules and nitrogen radicals. Here, this is equivalent to the collision probability of the oxygen molecules and the nitrogen molecules including the nitrogen radicals. The produced N2O concentration decreases with the increase of the collision probability.
The oxygen molecules become oxygen radicals by the colliding with the nitrogen radicals because only O3 and N2O are generated. It’s considered that the numbers of nitrogen radicals decrease at the collision with oxygen molecules. Then, Fig. 2(a) and (b) show the oxygen radicals are more easier to react to the oxygen molecules or the oxygen radicals than the nitrogen molecules or the remained nitrogen radicals.
The nitrogen radicals easily transfer the energy to the oxygen, and the nitrogen become inactive. These results indicates that the oxygen impurity has strong impact on the radical nitridation process. The oxygen components, even O2molecules, must be reduced in the process using nitrogen radicals.

Acknowledgement

This research has been carried out at fluctuation free facility of New Industry Creation Hatchery Center, Tohoku University.

References

[1] Y. Nakao, et al., International Conference on Solid State Devices and Materials, Nagoya, 2011, pp905-906
[2] Y. Nakao, et al., ECS Trans. 45 (3) 421-428 (2012)
[3] X. Guo, et al., IEEE Electron Device Lett., Vol.19, No.6, pp.207 (1999).
[4] D.M.Brown, et al., J. Electrochem. Soc., Vol.115, No.3, pp.311 (1968).
[5] L.He, et al., Jpn. J. Appl. Phys., vol.35, pt.1, No.2B, pp.1503 (1996).
[6] V. A. Gritsenko, et al., Thin Solid Films, Vol.51, pp.353 (1978).
[7]Gaku Oinuma，et al., J. Phys. D: Appl. Phys. 41 (2008) 155204

Figure 1