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Optical emission spectroscopy study of low frequency inductively coupled plasmas : application to plasma processing
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Type
Thesis
Author
A. B. M. Shafiul Azam
Supervisor
Xu, Shuyan
Abstract
This thesis presents optical emission characteristics of low frequency, planar coil inductively coupled plasma and real-time in situ, ex situ analysis for the nitriding process.
The intensities of plasma emission are found to have shoulders near the periphery in radial profile and gradually reduce with the distance away from the discharge coil. In pure argon discharge, no excited species from impurities is noticed while those are noticed in pure nitrogen, methane or mixture plasmas. It is seen that the emission is dominated by nitrogen species in the mixture of N2+Ar, or N2+H2, or N2+Ar+H2 and almost independent of their composition ratios, provided that the other discharge conditions remain unchanged. The variation of input power and operating gas pressure leads to hysteresis in the optical emission intensity (OEI), which is associated with the transitions between the electrostatic (E-) and electromagnetic (H-) discharge operating regimes. The characteristics of the hysteresis loops and character of mode transitions appear to be different in pure gases and gas mixture. It is found that the E- to H- transition is always discontinuous, while the H- to E- transition appears smooth in pure nitrogen and N2 -dominated discharges. The OEI of nitrogen species can efficiently be controlled by small Ar or H2 admixtures. Addition of argon enhances the optical emission of different nitrogen species, whereas the effect of hydrogen admixture is found to be opposite. Relatively at higher power, the electron impact reaction plays an important role, which is seen in methane plasma.
For nitriding stainless steel, it is revealed that DC (negative) bias is essential. Without any bias applied to the substrate, seemingly it is impossible to get any significant amount of hardness and thickness of nitrided layer from nitrided steel samples using this low frequency device. It is seen that plasma composition does not change at all at various bias voltages. Though hydrogen addition lowers the intensities of excited species of nitrogen, above 50 sccm of hydrogen addition and -100 V bias on substrate are found to produce maximum hardness and thickness of the nitrided samples.
Gas desorption from both the chamber inner wall and the substrate holder is noticed during processing of material in this low frequency device. Within the chamber the desorbed gas increases the pressure much at the early period of the processing. This changed plasma parameter degrades the impedance matching between the RF (radio frequency) generator and the plasma load. Thereby readjustment of impedance matching is required in order to maintain the input RF power unchanged throughout the processing. To minimize the effects of gas desorption for processing of samples, chamber cleaning by argon prior to process materials is found to be important. This chamber cleaning procedure can avert discharge to some extent to be unstable at the beginning of processing, and finally improve the quality of the nitrided layer.
The intensities of plasma emission are found to have shoulders near the periphery in radial profile and gradually reduce with the distance away from the discharge coil. In pure argon discharge, no excited species from impurities is noticed while those are noticed in pure nitrogen, methane or mixture plasmas. It is seen that the emission is dominated by nitrogen species in the mixture of N2+Ar, or N2+H2, or N2+Ar+H2 and almost independent of their composition ratios, provided that the other discharge conditions remain unchanged. The variation of input power and operating gas pressure leads to hysteresis in the optical emission intensity (OEI), which is associated with the transitions between the electrostatic (E-) and electromagnetic (H-) discharge operating regimes. The characteristics of the hysteresis loops and character of mode transitions appear to be different in pure gases and gas mixture. It is found that the E- to H- transition is always discontinuous, while the H- to E- transition appears smooth in pure nitrogen and N2 -dominated discharges. The OEI of nitrogen species can efficiently be controlled by small Ar or H2 admixtures. Addition of argon enhances the optical emission of different nitrogen species, whereas the effect of hydrogen admixture is found to be opposite. Relatively at higher power, the electron impact reaction plays an important role, which is seen in methane plasma.
For nitriding stainless steel, it is revealed that DC (negative) bias is essential. Without any bias applied to the substrate, seemingly it is impossible to get any significant amount of hardness and thickness of nitrided layer from nitrided steel samples using this low frequency device. It is seen that plasma composition does not change at all at various bias voltages. Though hydrogen addition lowers the intensities of excited species of nitrogen, above 50 sccm of hydrogen addition and -100 V bias on substrate are found to produce maximum hardness and thickness of the nitrided samples.
Gas desorption from both the chamber inner wall and the substrate holder is noticed during processing of material in this low frequency device. Within the chamber the desorbed gas increases the pressure much at the early period of the processing. This changed plasma parameter degrades the impedance matching between the RF (radio frequency) generator and the plasma load. Thereby readjustment of impedance matching is required in order to maintain the input RF power unchanged throughout the processing. To minimize the effects of gas desorption for processing of samples, chamber cleaning by argon prior to process materials is found to be important. This chamber cleaning procedure can avert discharge to some extent to be unstable at the beginning of processing, and finally improve the quality of the nitrided layer.
Date Issued
2000
Call Number
QD96.I47 Aza
Date Submitted
2000