CF A 2Σ+–X 2Π and B 2Δ–X 2Π study by broadband absorption spectroscopy in a plasma etch reactor: Determination of transition probabilities, CF X 2Π concentrations, and gas temperatures

Abstract

Broadband absorption spectroscopy was applied to study the CF A 2Σ+–X 2Π and B 2Δ–X 2Π transitions in a plasma etch reactor. We report a previously unobserved band, which is assigned as CF A 2Σ+–X 2Π (3,0). This band is significantly broadened by predissociation, and we estimate the average collision-free lifetime of the CF A 2Σ+ v′=3 level to be 0.30±0.08 ps. Experimental relative oscillator strength measurements, together with ab initio calculations, Rydberg–Klein–Rees-based wave functions and experimental lifetimes were used to calculate a full set of transition probabilities for the CF A 2Σ+–X 2Π and B 2Δ–X 2Π bands. The maximum observed number densities of CF X 2Π were ∼2×1013 cm−3 with sensitivity to measure to 1010 cm−3. The excited state and ground state temperatures were determined by comparing the spectra to simulations. The ground state rotational temperature was 450±30 K and the vibrational temperature was 850±80 K near the substrate surface. The CF B 2Δ excited state rotational temperatures are higher than those of the ground state. We show that this absorption technique is practical for determining gas temperatures and absolute concentrations in plasma etch reactors.

Use of the ultraviolet absorption spectrum of CF2 to determine the spatially resolved absolute CF2 density, rotational temperature, and vibrational distribution in a plasma etching reactor

“…17,18 The vibrational temperature for CF X 2 ⌸ was higher than the rotational temperature and about 850 K. The moderate number of vibrational levels populated for a relatively light diatomic molecule precludes the determination of a bimodal distribution, as has been shown for CF 2 in the present work. The results from both absorption studies confirm the rotational distributions as a good estimate of the gas kinetic energy, while the vibrational distributions tend to give information on excitation mechanisms like electron impact vibrational excitation and exothermicity of the chemical reaction producing the reactive species.…”

“…9,17 For example, the CF A -X(1,0) band can be seen in Fig. 8͑a͒ at 224 nm and the weaker CF A -X(0,0) band can be seen at 233 nm.…”

“…Most importantly, the CF 2 species detected in the outer volume have a lower gas temperature than CF 2 molecules in the plasma, which simplifies the analysis of the spectra. In addition, the relative contribution to the spectrum from the CF radical 17,18 is reduced by this arrangement, since CF is more reactive than CF 2 and therefore has much lower densities outside the plasma volume. Finally, this arrangement avoids the collection of plasma optical emission by the detection optics, simplifying the measurement of the absorption spectrum.…”

Time-resolved Optical Emission Spectroscopy in Water Electrical Discharges

“…The (0, 0) band of the OH radical (Q 1 branch forms the head at 307.8 nm) was detected to be the strongest line among other OH radical band heads when the W electrodes were used in the process, while for the Ta electrodes the (1, 1) band of the OH radicals (R 2 branch forms the head at 312.6 nm) was the strongest. The shift in the maximum intensity of the OH [18,19]. The line width is given in this experiment by the instrumental line width (2.4 nm), and the rotational structure of the molecular band could not be observed.…”

Radical kinetics in an inductively-coupled plasma in CF4

“…As the temperature is increased, more population is transferred from the probed lower level into higher rotational states, lowering the overall detection sensitivity; therefore the correction factors C increase with temperature. Finally, the corrected profiles were put on an absolute scale by broad-band UV absorption measurements 9,10 . This technique gives absolute line-integrated radical densities.…”