Laser-induced Photodetachment Research Articles

On the possibility of negative ion concentration growth between pulses of discharge current in oxygen

We present a critical analysis of data available in literature on the degradation of low-pressure oxygen plasma. Special attention is paid to two effects repeatedly observed in the initial stage of plasma degradation: a sharp growth in the flux of negative ions incident on the wall and an increase in the probing current during the laser-induced photodetachment. Both these phenomena are attributed by various authors to an absolute growth in the negative ion concentration. Our analysis shows that the whole body of experimental data cannot be explained based only on the influence of bulk plasmachemical reactions without taking into account special features of the transport processes in electronegative gases. An alternative mechanism is proposed which involves a two-stage degradation of the electronegative gas plasma accompanied by a change in the diffusion regime.

Photodetachment Study of Capacitively-Coupled RF C4F8 Plasma

Behaviors of electrons and negative ions in a capacitively coupled RF (13.56 MHz) plasma of c-C4F8 (perfluorocycrobutane) were studied using a laser-induced photodetachment technique in combination with a microwave cavity resonance technique. The results indicate that the negative ion formation process strongly depends on the RF power because of the dissociation of c-C4F8 even in the low power range (<100 W) studied in this work. The negative ion density estimated on the assumption that all negative ion species have the same photodetachment efficiency as that for F- is larger by a factor of ∼50 than the electron density in the typical condition in this work (100 mTorr C4F8, 10 W RF power). However, the electron attachment rate and ion recombination rate suggest that the actual negative ion density should be significantly higher than that derived on the above assumption; this discrepancy could be attributed to the low photodetachment efficiency for large molecular negative ions which may exist abundantly in the low RF power region.

Regular and irregular vibrational states: Localized anharmonic modes and transition-state spectroscopy of Na3

We have calculated the lowest 900 vibrational energy levels and wave functions for the quartet (14A2′) state of Na3. The equilibrium geometry of the trimer is triangular, but the calculations include many states that lie above the barrier to linearity. Most of the high-lying states are irregular, but there are a few relatively localized states embedded in the irregular bath. The localized modes observed include a “horseshoe” mode and a symmetric stretch centered on the linear transition state. The density of states and couplings are such that in most cases the “horseshoe” character is spread over several bath states, while the symmetric stretch states exist in a purer form. The localized states could be observed in laser-induced fluorescence, stimulated-emission pumping or ion photodetachment spectroscopy from a state with a linear equilibrium geometry.

The effect of laser-induced photodetachment in rf discharges

Using the relaxation continuum (RCT) model, numerical analysis of oxygen rf glow discharges between parallel plates with laser irradiation parallel to the electrode was performed. The oxygen discharge sustained by a sinusoidal wave voltage at a frequency of 13.56 MHz, amplitude of 200 - 230 V and pressure of 0.5 - 1.0 Torr is irradiated by the laser with a power of 300 W, width of 2 mm and wavelength of . Under these conditions, laser-induced photodetachment occurs with a large enough rate to affect the discharge structure significantly. When the plasma is irradiated continuously, the number density of ions in the incident region decreases by about 30%. The electron number density and the flux in the discharge when the pulsed laser is turned on and off every ten rf cycles, show prominent temporal variation with the variation of the electric field. Electron cooling is enhanced during the laser irradiation.

Dust formation and charging in an Ar/SiH4 radio-frequency discharge

The formation and charging of submicrometer dust particles in a low pressure argon/silane radio-frequency (rf) discharge was studied using laser-induced photodetachment in combination with a microwave resonance technique. This method allows a measurement of the spatially averaged electron density, the spatially resolved negative ion density, and/or the charge on small clusters in the plasma as a function of time during particle formation. The loss frequency of photodetached electrons yields information about the recharging of small clusters. During the first second after plasma ignition dust particles are formed. Simultaneously, the electron density decreases from about 2×1015 m−3 to about 4×1014 m−3. In the first 10 ms after discharge ignition, charged particles are not present in the plasma and the photodetachment experiment gives a negative ion density of 4×1015 m−3. During the first 50 ms after plasma ignition, nanocrystallites are formed, which is reflected by a strong increase of the loss frequency of photodetached electrons. After 50 ms the particles start to coalesce and acquire a negative charge, which results in a strong increase of the photodetachment signal. After 1 s of plasma operation, the charge density on particles is about 8×1016 m−3. The photodetachment signal decreases with the gas flow rate, indicating that the clusters are expelled from the plasma by the gas flow.

Negative ions and particle formation in low-pressure halocarbon discharges

The growth of particles in a radiofrequency (RF) (13.56 MHz) plasma at pressures from 25 to 200 mTorr in mixtures of CF4, CF2Cl2 and argon has been studied experimentally. A planar configuration was used, with a silicon wafer on the powered electrode. The electron density has been measured with microwave resonance spectroscopy using a cylindrical cavity surrounding the plasma. The same geometry has been used to measure the density of various species of negative ions by detecting the extra electrons created by laser-induced photo detachment. It appears that the negative ion density is much larger in the case of CF2Cl2 than in the case of CF4. There seems to be hardly any dependence of the negative ion concentration on the CF2Cl2/Ar partial pressure ratio in the range where powder growth occurs. However, the attachment rate to chlorine is found to be much higher than to fluorine. Furthermore the gas phase discharge chemistry has been studied using infrared absorption spectroscopy. Both a tunable diode laser system, and a Fourier transform spectrometer have been applied. The CF2 concentration appears to decrease strongly when powder growth occurs. The SiF4 concentration then has a maximum. The results indicate that the presence of chlorine in the plasma feed gas is essential. In CF4 no particle formation is detected. The wafer surface is blackened during powder formation. SEM inspection indicates that this is caused by micromasking. Considering all the information, we arrive at the conclusion that particle growth is initiated by micromasking at the Si surface combined with a highly directional etching process. Due to residual isotropic etching the particles are released from the surface and enter the plasma, where they start coalescing and growing under the influence of CF2 polymerization.

Plasma diagnostics in negative ion sources

The use of thin probes and laser photodetachment for measuring the negative ion density are described. In the photodetachment method, the work done by detecting the free electrons produced by laser-induced photodetachment is described. Several detection means for the change in electron density produced by laser-induced photodetachment are reviewed: microwave cavity, electrostatic probes and optogalvanic detection. The problems of the choice of the laser and of identification of the negative ions are also discussed. The two-laser photodetachment technique, applied for the measurement of the negative hydrogen and deuterium ion temperature, is described, as are time-resolved measurements of negative ion density.

Detection of Cl and chlorine-containing negative ions in rf plasmas by two-photon laser-induced fluorescence

Chlorine atoms have been detected in rf etching plasmas of CClF3 and CCl2F2 with three-dimensional spatial resolution using a two-photon laser-induced fluorescence technique. The spin-forbidden 4p(4S0)–3p(2P0) transition is pumped by absorption of two 233.3-nm laser photons. Decay of the excited state results in 725–775 nm emission. Spatially resolved plasma concentration measurements under certain etching conditions indicate an anomalously large signal spike at the plasma/sheath boundary, an effect attributed to an aggregation of chlorine-containing negative ions which are also detected by this technique. The negative ions are detected by laser-induced photodetachment followed by two-photon excitation of atomic Cl.

Dye laser-induced photodetachment of electrons from SH − studied by ion cyclotron resonance spectroscopy

The photodetachment of electrons from gaseous SH − ions has been studied in an ion cyclotron resonance mass spectrometer using a flashlamp-pumped organic dye laser as a light source. The onset of the photodetachment process at 538.7 ± 0.3 nm (2.301 6 ± 0.001 3 eV, 18563 ± 10 cm −1) agrees well with that obtained in an earlier study. Coarse structure in the photodetachment curve with a spacing of 11.8 nm (0.052 eV, 422 cm −1) has been identified with spin-orbit coupling in the SH . radical. Finer structure, with a spacing of ca. 2.5 nm (0.011 eV, 89 cm −1), has also been observed in the curve, but remains unexplained.