# High Density RF-DC Plasma Nitriding under Optimized Conditions by Plasma-Diagnosis
## 3 Experimental Results and Discussion
### Effect of Hydrogen Gas Content on the NH Radicals  
In the plasma nitriding with the use of N\({}_{2}\) + H\({}_{2}\) mixture gas, both N\({}_{2}^{+}\) ions and NH-radical work to supply the nitrogen atom flux onto the substrate surface for nitriding. Considering that only N\({}_{2}^{+}\) works as a mother ion to drive the nitrogen flux when using the N\({}_{2}\) gas, the effect of hydrogen gas content on nitriding is parametrically described by the ratio (R\({}_{\rm NH}\)) of the NH-radical peak intensity at \(\Lambda\) = 336.1 nm to the N\({}_{2}^{+}\)-ion peak intensity at \(\Lambda\) = 394 nm in the plasma diagnosis experiment. As shown in Figure 7, this  
Figure 5: Variation in N\({}_{2}^{+}\) peak intensity with the increase in RF-voltage in the plasma diagnosis, with and without the use of the hollow cathode device in cases where \(p\) = 50 Pa and V\({}_{\rm DC}\) = \(-\)500 V.  
Figure 6: Detection of NH-radical by the N\({}_{2}\) + H\({}_{2}\) plasma diagnosis with the use of OES through wide and narrow scanning steps.  
ratio increases with the increase in hydrogen gas content and maximizes at [H\({}_{2}\)] = 20 to 25%. For [H\({}_{2}\)] > 25%, this ratio monotonously decreases with [H\({}_{2}\)]. In the regime for 0 < [H\({}_{2}\)] < 25%, the N\({}_{2}\) + H\({}_{2}\) plasma is enriched with N\({}_{2}^{+}\)-ion and NH-radicals enough to drive a sufficient nitrogen atom flux onto the substrate. On the other hand, it is poor in the population of NH-radical for [H\({}_{2}\)] > 25%. At the same time, the depletion of N\({}_{2}\) yields less N\({}_{2}^{+}\) ions since more H\({}_{2}\) molecules participate in the N\({}_{2}\)-H\({}_{2}\) gas plasma mixture.  
This poor population of NH-radicals in higher hydrogen gas content is explained by the hydrogen quenching effect [19]. In the presence of hydrogen molecules with high content, two activated nitrogen molecules of N\({}_{2}\)\(\left(\text{A}^{3}\sum_{\text{u}}^{+}\right)\) and N\({}_{2}\)\(\left(\text{a}^{\prime}\sum_{\text{u}}^{-}\right)\) react to a more stable state of N\({}_{2}\)\(\left(\text{X}^{1}\sum_{\text{g}}^{+}\right)\) by the following reactions:  
\[\text{N}_{2}\left(\text{A}^{3}\sum_{\text{u}}^{+}\right)+\text{H}_{2}\to \text{N}_{2}\left(\text{X}^{1}\sum_{\text{g}}^{+}\right)+\text{H}+\text{H} \tag{4}\]  
\[\text{N}_{2}\left(\text{a}^{\prime}\sum_{\text{u}}^{-}\right)+\text{H}_{2}\to \text{N}_{2}\left(\text{X}^{1}\sum_{\text{g}}^{+}\right)+\text{H}+\text{H} \tag{5}\]  
Since two hydrogen atoms recombine to H\({}_{2}\), these reactions advance by themselves to quench the activation of nitrogen molecules and to reduce a yield of nitrogen atoms.  
In the regime with less hydrogen molecule content, the above reactions become inferior to the following reaction between the enriched N\({}_{2}^{+}\) and H\({}_{2}\) and between H\({}_{2}^{+}\) and N\({}_{2}\);  
\[\text{N}_{2}^{+}+\text{H}_{2} \to \text{N}_{2}\text{H}^{+}+\text{H} \tag{6}\]  
\[\text{H}_{2}^{+}+\text{N}_{2} \to \text{N}_{2}\text{H}^{+}+\text{H} \tag{7}\]  
This transient ion of N\({}_{2}\)H\({}^{+}\) further reacts by its collision with electrons in the following,  
\[\text{N}_{2}\text{H}^{+}+\text{e}\to\text{NH}+\text{H}\,\text{N}_{2}\text{H}^ {+}+\text{e}\to\text{N}_{2}+\text{H} \tag{8}\]  
As suggested by [20; 21], the reaction cross-section for N + N \(\to\) N\({}_{2}\) is thought to be less than that for N + H \(\to\) NH. These hydrogen atoms have the potential to react with nitrogen atoms from activated nitrogen molecules to yield NH-radicals.