Volume 92, Issue 12, 15 December 2002
Index of content:
- PLASMAS AND ELECTRICAL DISCHARGES (PACS 51-52)
Calculated characteristics of radio-frequency plasma display panel cells including the influence of xenon metastables92(2002); http://dx.doi.org/10.1063/1.1521258View Description Hide Description
Although alternating-current plasma display panels (ac PDPs) are now produced by several companies, improvements are still necessary. In particular, the overall efficiency of the discharge in the standard configuration is low, on the order of 1 lm/W i.e., about 0.5% of the power dissipated in the discharge is transformed into useful visible photons. One way to substantially improve the efficiency of PDPs is to use radio-frequency (rf) excitation because, when compared to ac PDPs, less of the electrical energy input is dissipated by ions in the sheath and relatively more power is deposited in excitation of the xenon, which produces the ultraviolet photons used to excite the phosphors. In this article, we show calculated dischargecharacteristics for typical rf PDP conditions and pay particular attention to the role of the xenon metastable atoms in the ionization balance. Our discussion is limited to the sustaining regime, the “on-state,” of a PDP cell.
92(2002); http://dx.doi.org/10.1063/1.1519950View Description Hide Description
The chemistry of high-density plasma discharges is not well characterized. In this article, a combination of computational modeling and experimental diagnostics has been utilized to understand charged species dynamics in an inductively coupled plasma discharge. The model is based on the two-dimensional HybridPlasma Equipment Model with a detailed plasma chemical mechanism for In the experiments, absolute electron density and total negative ion density have been measured using microwave interferometry and laser photodetachment, respectively. In addition, we have also utilized prior measurements of mass and energy resolved ion fluxes by Goyette et al. [J. Vac. Sci. Technol. A 19, 1294 (2001)]. Computational results show that all ions are present in the plasma discharge. Important negative ions include and Electron and positive ion densities increase with coil power due to enhanced ionization. However, negative ion densities decrease with coil power as the main negative ion precursor, is lost through neutral dissociation. An increase in concentration in the gas mixture decreases electron density due to enhanced electron loss through (dissociative) attachment, which enhances negative ion densities. RF bias power does not have an appreciable impact on the ion and electron densities for the parameter range of interest. Experiments show that electron density decreases with gas pressure while the total negative ion density increases up to 25 mTorr. This is due to a decrease in electron temperature, which enhances electron loss through (dissociative) attachment. Although the model is able to capture most of the experimentally observed trends, there are discrepancies regarding the impact of gas pressure on electron density and relative flux of large positive ions.
Comparison of excessive Balmer α line broadening of glow discharge and microwave hydrogen plasmas with certain catalysts92(2002); http://dx.doi.org/10.1063/1.1522483View Description Hide Description
From the width of the 656.3 nm Balmer line emitted from microwave and glow dischargeplasmas, it was found that a strontium–hydrogen microwaveplasma showed a broadening similar to that observed in the glow discharge cell of 27–33 eV; whereas, in both sources, no broadening was observed for magnesium–hydrogen. Microwave helium–hydrogen and argon–hydrogen plasmas showed extraordinary broadening corresponding to an average hydrogen atom temperature of 180–210 eV and 110–130 eV, respectively. The corresponding results from the glow dischargeplasmas were 33–38 eV and 30–35 eV respectively, compared to for plasmas of pure hydrogen, neon–hydrogen, krypton–hydrogen, and xenon–hydrogen maintained in either source. Similarly, the average electron temperature for helium–hydrogen and argon–hydrogen microwaveplasmas were high, and respectively; compared to and for helium and argon alone, respectively. External Stark broadening or acceleration of charged species due to high fields can not explain the microwave results since no high field was present, and the electron density was orders of magnitude too low for the corresponding Stark effect. Rather, a resonant energy transfer mechanism is proposed.
92(2002); http://dx.doi.org/10.1063/1.1521518View Description Hide Description
Methanedissociation, followed by the formation of hydrocarbons, in a pulsed microwave discharge in methane was investigated by mass spectrometry and optical emission spectroscopy(OES). Long microwave pulses are characterized by a pronounced dehydrogenation, but have a disadvantage in the saturation of the methane conversion at relatively low values, due to methane depletion toward the end of the pulse. For shorter pulses, the conversion degree increases approximately linearly as a function of energy input, and a maximum conversion of 90% with 80% selectivity toward acetylene was obtained for 60 μs pulses at 1 kHz repetition frequency. A further decrease of the pulse duration (20 μs) at higher frequency, in order to ensure a similar energy input, resulted in a decrease in conversion and dehydrogenation. The explanation of the effect of the pulse duration is based on information provided by optical emission spectroscopy of active species generated in the discharge. Atomic hydrogen, formed by methanedissociation, was found to play an essential role in methane plasma chemistry. A qualitative estimation of the variation of H atom concentration with operating conditions was done by actinometry, since time-resolvedOES provides evidence that atomic hydrogen is mainly formed in the ground state and dissociative excitation can be neglected. In addition to the concentration of atomic hydrogen, the second key parameter is the gas temperature. It was determined from the relative intensity distribution in the rotational structure of the Swan band and of the Fulcher-α band. Gas temperatures between 1500 and 2500 K were determined for the present discharge conditions. The hydrogen abstraction by hydrogen atoms, favored at high temperature, is responsible for the high methane conversion and low energy requirement achieved (9–10 eV/molecule) and for the distribution of the reaction products.
Analytical model for ion angular distribution functions at rf biased surfaces with collisionless plasma sheaths92(2002); http://dx.doi.org/10.1063/1.1524020View Description Hide Description
The article presents an analytical model for evaluation of ion angular distribution functions (IADFs) at a radio frequency (rf)-biased surface in a high-density plasma reactor. The model couples a unified rf sheathmodel to an assumed ion velocity distribution function-based formulation for determining the IADF under any general rf-bias condition. Under direct-current (dc) bias conditions the IADF profile shape shows a strong dependence on the bias voltage and the ion temperature is relatively independent of the plasma electron temperature, ion density, and the ion mass. The model establishes the importance of rf-bias frequency in determining the IADF. For conditions where the sheath current wave form is sinusoidal, low bias frequencies result in a large-angle tail contribution to the IADF which can potentially lead to poor anisotropicplasma etching behavior. The large-angle tail is absent at higher bias frequencies. An increase in bias power leads to a general narrowing of the IADF, but the large-angle tail for the IADF at low frequencies persists despite increasing bias powers. Therefore, plasmaetchanisotropy can be improved by increasing bias powers only if the bias frequency is sufficiently high. Tangential ion drift velocities introduce azimuthal angle dependence on the IADF and a shift in the peak IADF to off-normal polar angles. While the location of the peak IADF in the azimuthal direction is dictated purely by the direction of the drift velocity, the shift in peak IADF in the polar angle depends on both the drift velocity as well as the bias frequency.