Atomic Oxygen and Its Application to Plasma Monitoring

The line profile of the S2−P1 transition of atomic oxygen was measured by diode laser absorption spectroscopy. As a result, it was found that the absorption line profile had a wing component in the wavelength range detuned from the line center and was not fitted with a Gaussian function. The wing component was considered to be originated from dissociative excitation of molecular oxygen. We fitted the absorption line profile with the superposition of two Gaussian functions corresponding to high and low translational temperatures. We propose that the ratio of the high-temperature to low-temperature components is useful for monitoring the relative degree of dissociation of molecular oxygen in oxygen-containing plasmas. The ratio of the hightemperature to low-temperature components was compared with the survival ratio of molecular oxygen, which was evaluated from the lifetime of O(S2) in the afterglow of pulsed discharges.


Introduction
Low-pressure plasmas with electron densities below 10 13 cm −3 are widely used for various material processing such as dry etching and plasma-enhanced chemical vapor deposition. The spectral line profiles of atoms and molecules in low-pressure, low-density plasmas are governed by Doppler broadening, which represents the velocity distribution function of atoms and molecules in plasmas. Since collisions among neutral species in plasmas used for material processing are frequent, it is expected that the velocity distribution functions of atoms and molecules are approximated by Maxwellian functions with widths corresponding to the species temperatures. However, there are several processes which deviate the velocity distribution functions of neutral species from Maxwellian functions.
Spectral profiles of hydrogen Balmer lines have been investigated intensively by optical emission spectroscopy, and many authors have reported the existence of large Doppler broadening in their spectral line profiles [1][2][3][4][5][6][7][8][9][10]. The existence of large Doppler broadening means that the velocity distribution function of emitting species contains a high-energy component. A possible mechanism for the production of the high-energy component is collision between molecular hydrogen and ions. Another process for explaining the existence of the high-energy component is dissociative excitation of molecular hydrogen. This is because electron impact dissociation of a diatomic molecule is divided into two steps. The first step is electron impact excitation to an electronic state having a repulsive potential curve without changing the distance between nuclei. The second step is automatic separation of nuclei along the repulsive potential curve. Since the energy of the dissociated state is lower than that of the repulsive potential curve immediately after electron impact excitation, atoms produced after dissociation have kinetic energies corresponding the energy difference.
A reason why the investigations of the spectral line profiles are concentrated in atomic hydrogen may be the smallest mass number. Since the spectral resolution of optical emission spectroscopy is not high, atomic hydrogen, which has the widest Doppler broadening width (the smallest mass number), is suitable for investigating the spectral line profile by optical emission spectroscopy. In this work, we examined the spectral line profile of the 5 S o 2 − 5 P 1 transition of atomic oxygen by diode laser absorption spectroscopy. Since diode laser absorption spectroscopy has a much finer resolution than optical emission spectroscopy, we can examine the 2 International Journal of Spectroscopy detailed structure of the spectral line profile. In addition, we propose a method for monitoring the relative degree of dissociation of molecular oxygen in oxygen-containing discharges from the spectral line profile of the 5 S o 2 − 5 P 1 transition. The plasma processing industry requires a simple, economical technique which is applicable to the monitoring of plasma processing tools. The proposed method has a potential as a plasma monitoring tool because of the compactness, simplicity, and the economical price of a diode laser.

Experiment
The plasma source and the system for diode laser absorption spectroscopy are the same as those used in a previous work [11], where we evaluated the gas temperatures in hydrogen plasmas from the absorption line profile of the Balmer-α line. We used pure oxygen at pressures from 30 to 100 mTorr for discharge in this experiment. Helicon-wave plasmas were produced by applying rf power at 13.56 MHz to a helical antenna wound around a glass discharge tube of 1.6 cm inner diameter. The plasma column with the same diameter as the glass tube was confined radially by the uniform magnetic field along the cylindrical axis of a vacuum chamber. The strength of the magnetic field was adjusted to be 70 G to avoid the distortion of the spectral line profile due to the Zeeman effect. The plasmas were produced in a pulsed mode with a discharge duration of 40 milliseconds and a repetition frequency of 2 Hz to avoid the overheating of the plasma source.
A commercial diode laser (TOPTICA, DL100) beam was injected into the plasma from the radial direction of the cylindrical vacuum chamber. The wavelength of the diode laser beam was tuned around the line center of the 5 S o 2 − 5 P 1 transition (777.539 nm). The tuning of the laser wavelength was triggered at 15 milliseconds after the initiation of the pulsed discharge, when the plasma reached the steady-state condition. Sweeping the laser wavelength for ±20 pm needed 20 milliseconds. The wavelength tuning of the diode laser beam was monitored using a spectrum analyzer. The power of the diode laser beam was attenuated below 10 μW to avoid saturation. The laser beam transmitted through the plasma was detected using a photomultiplier tube via a monochromator.

Absorption Line Profile and the Interpretation
For the sake of comparison, we produced an argon plasma and measured the absorption line profile of the 4s [3/2]

low-energy component in the velocity distribution function of O( 5 S o
2 ) is produced by the three-step process represented by

Application to the Monitoring of Degree of Dissociation
According to the aforementioned production processes, the densities of the slow (low-energy) and fast (high-energy) components in the velocity distribution function are given by respectively, where τ is the lifetime of O( 5 S o 2 ), n e is the electron density, and [X] stands for the density of species X. The rate coefficients for (3) and (4) are represented by k ex and k diss , respectively. Therefore, the density ratio of molecular oxygen to atomic oxygen is evaluated by The

Comparison with Degree of Dissociation Evaluated from Lifetime of O( 5 S o 2 )
The major loss processes of O( 5 S o 2 ) are diffusion, spontaneous emission, and collisional quenching with O 2 , and the lifetime τ in (5) and (6) are given by where k q is the rate coefficient for collisional quenching, τ d is the diffusion time constant, and τ r is the radiative lifetime. The values of k q and 1/τ r are given in literature [12,13] as k q = 2.2×10 −10 cm 3 /s and 1/τ r = 5.6×10 3 s −1 . The diffusion time constant of O atoms in O 2 plasmas produced in the plasma source used in this experiment has been examined in a previous work and was estimated as 1/τ D < 2 × 10 3 s −1 at pressures higher than 30 mTorr [14,15]. Therefore, the dominant term in the right-hand side of (8) Figure 5 shows the temporal variation of the absorbance at the line center (− ln(I t /I 0 ) with I 0 and I t being the incident and transmitted laser intensities, resp.) in the afterglow phase of a pulsed discharge at an rf power of 1.5 kW and a pressure of 50 mTorr. The slight increase in the absorbance immediately after the termination of the rf power may be due to the production of O( 5 S o 2 ) by recombination reactions. After that, the absorbance decreased exponentially as shown in Figure 5,  The survival ratio of O 2 thus evaluated is plotted in Figure 6 as functions of the rf power and the pressure. It is reasonable that the survival ratio decreases with the rf power as shown in Figure 6 Figure 4(b) is roughly constant. A possible explanation for this discrepancy is the increase in k ex /k diss in (7) with the pressure. This is because k ex /k diss decreases with the electron temperature since the threshold electron energy for (4) is higher than that for (3). Since the electron temperature is usually a decreasing function of the pressure, it is expected that the decrease in the survival ratio of O 2 is compensated by the increase in k ex /k diss , resulting in the roughly constant [O( 5 S o 2 , fast)]/[O( 5 S o 2 , slow)] with the pressure. Further investigation is necessary to evaluate the discharge conditions where this method is applicable.

Conclusions
In this work, we investigated the absorption line profile of the 5  We propose a method for monitoring the relative degree of dissociation of O 2 by the ratio of the high-temperature to low-temperature components in the absorption line profile. The relative degree of dissociation estimated by the proposed method was compared with the survival ratio of O 2 evaluated from the lifetime of the 5 S o 2 state. As a result, a reasonable agreement was obtained in the rf power dependence of the relative degree of dissociation, but the agreement was insufficient in the pressure dependence.