Optical emission plasma diagnostics

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

Knowledge on the plasma species can be obtained by the use of plasma diagnostics techniques, such as optical emission spectroscopy (OES) and mass spectroscopy (MS). Both techniques are able to probe atomic and molecular, neutral or ionized species present in plasmas. OES is based on measuring the light emission spectrum that arises from the relaxation of plasma species in excited energy states. MS, on the other hand, is generally based on the measurement of mass spectra of ground state species. 

Some of the most powerful tools for in situ discharge diagnostics are optical (62). Plasma-induced emission spectroscopy, laser-induced fluorescence, laser absorption, and laser optogalvanic spectroscopy have all been... 

Optical spectroscopies. These techniques are the least intrusive in situ plasma diagnostic methods. The most commonly used techniques are emission spectroscopy, absorption spectroscopy, laser-induced fluorescence. 

Important plasma diagnostics include Langmuir probes, optical emission spectroscopy, laser induced fluorescence, absorption spectroscopy, mass spectrometry, ion flux and energy analysis, and plasma impedance analysis. A plasma reactor equipped with several of these diagnostics is shown in Fig. 51 [35, 160]. A capacitively coupled plasma is sustained between the parallel plates of the upper (etching) chamber. The lower (analysis) chamber is differentially pumped and communicates with the etching chamber through a pinhole on the lower electrode. 

Fig. 51. A RE capacitively-coupled diode with a variety of plasma diagnostics, including Langmuir probe, optical emission spectroscopy, molecular beam mass spectrometry, and ion flux/energy analysis systems. After [35).

The most direct need for plasma diagnostic techniques results from the determination of the etch end point for a given process. In addition, plasma diagnostic techniques are used for process monitoring and provide information on the types of species present in a plasma etching, the concentration, and the energy content. Laser interferometry (or reflectance) and optical emission spectroscopy (OES) are two commonly used techniques for EPD and require only an appropriate optical window attached to the chamber. They are easily implemented to obtain information about etching plasmas [1]. 

Pastol, A. Catherine, Y. (1990). Optical emission spectroscopy for diagnostic and monitoring of CH4 plasmas used for a-C H deposition. /. Phys. D Appl. Phys., Vol. 23, pp. 799-805... 

Progresses in Experimental Study of N2 Plasma Diagnostics by Optical Emission Spectroscopy... 

Robinson D. and Lenn P. D. (1967) Plasma diagnostics by spectroscopic methods, Appl. Opt. 6 983-1000. Kalnicki D. j., Kniseley R. N. and Fassel V. A. (1975) Inductively coupled plasma optical emission spectroscopy. Excitation temperatures experienced by analyte species, Spectrochim. Acta, Part B 30 511-525. Corliss C. H. and Bozman W. R. (1%2) Experimental transition probabilities for spectral lines of 70 elements derived from the NBS tables of spectral line intensities. The... 

In laser diagnostic methods developed to study the vaporization behavior of ZrC (7), a vapor phase was produced by laser ablation of a ZrC target. The temperatures of the plasmas are estimated to be between 9000 and 12,000 K. Thermodynamic calculations for 9000 K predict that C3 has the highest partial pressure, followed by C2 and C5. Zirconium has the lowest calculated partial pressure. The dominant neutral gas species of an expanding plasma plume are predicted to be Zr and C followed by, in decreasing order of importance, C2, C3, C4, and Cs. The optical emission spectra of the ablated ZrC from 200 to 500 nm at delay times from 10 p.s to 1 ms (Fig. 5) contain lines only for excited Zr. Emission peaks from C, C2, and C3 were absent from the spectra, apparently because of the inherently low emission intensities of these species compared with that of Zr, which has a very strong spectrum in the ultraviolet frequency range. 

Optical diagnostic techniques are convenient and popular for alkali metal plasmas, because the strong and well-understood emission lines and continua can be easily observed in the visible and near-visible region. 

Fundamental quantities, such as wavelengths and transition probabilities, determined using spectroscopy, for atoms and molecules are of direct importance in several disciplines such as astro-physics, plasma and laser physics. Here, as in many fields of applied spectroscopy, the spectroscopic information can be used in various kinds of analysis. For instance, optical atomic absorption or emission spectroscopy is used for both qualitative and quantitative chemical analysis. Other types of spectroscopy, e.g. electron spectroscopy methods or nuclear magnetic resonance, also provide information on the chemical environment in which a studied atom is situated. Tunable lasers have had a major impact on both fundamental and applied spectroscopy. New fields of applied laser spectroscopy include remote sensing of the environment, medical applications, combustion diagnostics, laser-induced chemistry and isotope separation. 

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