High-sensitivity white-light absorption spectroscopy

High-Sensitivity White-Light Absorption Spectroscopy 

Either a photodiode array or a charge-coupled device (CCD) array, placed at the output plane of the spectrometer, is used as the detector. This discussion concentrates on the use of the diode array, with some comments on the use of a 

CCD array at the end of the section. Typical photodiode arrays from Princeton Instruments and other manufacturers have individual diodes with a center-to-center spacing width of 25 tm and a height of 2 mm. This height matches the height of the 1-to-l image of the arc in a Xe lamp. The center-to-center spacing of the photodiodes and the dispersion of the spectrometer determine the spectral resolution of the system. 

The quantity T differs from T because the response of the diode array is not a perfectly linear function of the number of photons incident on the diodes, so that clie, and % are not the same constants of proportionality. It can be shown that 

From this equation, it is easily seen that if a, a , and were all identical, then T would be equal to T. The absorbance of the glow discharge is given by 

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USES OF HIGH-SENSITIVITY WHITE-LIGHT ABSORPTION SPECTROSCOPY IN CHEMICAL VAPOR DEPOSITION AND PLASMA PROCESSING... 

II. High-Sensitivity White-Light Absorption Spectroscopy. 296... 

HIGH-SENSITIVITY WHITE-LIGHT ABSORPTION SPECTROSCOPY 297... 

Fig. 1. Schematic diagram of the hollow-cathode glow-discharge CVD and high-sensitivity white-light absorption spectroscopy apparatus. (Reprinted with permission from Menningen et ai, 1995a, Contrih. Plasma Phys. 35, 359, 1995 Wiley-VCH, Inc.)...

As stated in the introduction, high-sensitivity white-light absorption spectroscopy has been used more extensively in studying the gas-phase composition during the CVD of diamond films than in other applications of CVD. In this section we review the use of high-sensitivity white-light absorption for the measiuement of n for the various free radicals and molecules present in the CVD of diamond under different conditions. 

High-sensitivity white-light absorption spectroscopy was used in the detection of CH3 and C2H2 in the CVD of diamond by Childs et al. (1992) in a hot-filament... 

High-sensitivity white-light absorption spectroscopy has been used to measure the CH densities in various CVD reactors. These measurements show remarkable sensitivity. The CH and CH3 densities have been used to obtain the dissociation ratio [H]/[H2]. 

In this section, we have reviewed the use of high-sensitivity white-light absorption spectroscopy to study the C2H2 density during diamond growth in hot-filament CVD. Many interesting effects due to filament poisoning have been observed. 

The high-sensitivity white-light absorption spectroscopy of hydrogen-deficient microwave CVD used for nanocrystalline diamond growth done by Goyette et al. (1998a) used the (0,0) vibrational band of the cPYl — a II electronic transition of C2, known as the Swan system. Figure 20 shows a typical absorption spectrum from this band. The bandhead for the (0,0) Swan band is located at 516.5 nm. The... 

High-sensitivity white-light absorption spectroscopy has also been used in the determination of gas kinetic temperature in diamond CVD environments. The gas kinetic temperature is an important parameter in these reactive environments, since the rates and endpoints of chemical reactions are temperature-dependent, fri addition, knowledge of the gas kinetic temperature is necessary in order to calculate column or absolute densities of species from their measured absorbances. Absorption cross sections are temperature-dependent, and the relative populations of the various levels represented in the integrated equivalent width are also temperature-dependent. Determination of gas kinetic temperatures in reactive discharges is therefore important. 

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