M. V. V. S. Rao

Neutral gas temperature estimates in an inductively coupled CF4 plasma by fitting diatomic emission spectra

In this work we examine the accuracy of plasma neutral temperature estimates by fitting the rotational band envelope of different diatomic species in emission. Experiments are performed in an inductively coupled CF4 plasma generated in a Gaseous Electronics Conference reference cell. Visible and ultraviolet emission spectra are collected at a power of 300 W (∼0.7 W/cm3) and pressure of 30 mTorr. The emission bands of several molecules (CF, CN, C2, CO, and SiF) are fit simultaneously for rotational and vibrational temperatures and compared. Four different rotational temperatures are obtained: 1250 K for CF and CN, 1600 K for CO, 1800 K for C2, and 2300 K for SiF. The vibrational temperatures obtained vary from 1750 to 5950 K, with the higher vibrational temperatures generally corresponding to the lower rotational temperatures. These results suggest that the different species have achieved different degrees of equilibration between rotational and vibrational modes and may not be equilibrated with the transl...

Mass spectrometric and Langmuir probe measurements in inductively coupled plasmas in Ar, CHF3/Ar and CHF3/Ar/O2 mixtures

Absolute fluxes and energy distributions of ions in inductively coupled plasmas of Ar, CHF 3 /Ar, and CHF 3 /Ar/O 2 have been measured. These plasmas were generated in a gaseous electronics conference cell modified for inductive coupling at pressures of 10-50 mTorr and 100-300 W of 13.56 MHz radio frequency (RF) power in various feed gas mixtures. In pure Ar plasmas, the Ar + flux increases linearly with pressure as well as RF power. In mixtures, the Ar + flux decreases with increase in pressure and CHF 3 concentration in the mixture. The loss mechanism for Ar + is attributed to resonance charge exchange (Ar + + CHF 3 → products). Total ion flux in CHF 3 mixtures decreases with increase in pressure and also CHF 3 concentration. Relative ion fluxes observed in the present studies are analysed with the help of available cross sections for electron impact ionization and charge-exchange ion-molecule reactions. Measurements of plasma potential, electron and ion number densities, electron energy distribution function, and mean electron energy have also been made in the centre of the plasma with an RF-compensated Langmuir probe. Plasma potential values are compared with the mean ion energies determined from the measured ion energy distributions and are consistent. Electron temperature, plasma potential, and mean ion energy vary inversely with pressure, but increase with CHF 3 content in the mixture.

Langmuir probe and mass spectrometric measurements in inductively coupled CF4 plasmas

Abstract Electron and ion energy distribution functions and other plasma parameters such as plasma potential (V(sub p)) , electron temperature (T(sub e)), and electron and ion number densities (n (sub e) and n(sub i)) in low pressure CF4 plasmas have been measured. The experiments were conducted in a GEC cell using an inductively coupled plasma (ICP) device powered by a 13.56 MHz radio-frequency (rf) power source. The measurements were made at 300 W of input rf power at 10, 30 and 50 mTorr gas pressures. Langmuir probe measurements suggest that n(sub e), n(sub i) and V(sub p) remain constant over 60% of the central electrode area, beyond which they decrease. Within the limits of experimental error (+/- 0.25 eV), T(sub e) remains nearly constant over the electrode area. T(sub e) and V(sub p) increase with a decrease in pressure. n(sub e) and n(sub i) are not affected as significantly as T(sub e) or V(sub p) by variation in the gas pressure. The electron energy distribution function (EEDF) measurements indicate a highly non-Maxwellian plasma. CF3+ is the most dominant ion product of the plasma, followed by CF2+ and CF+. The concentrations of CF2+ and CF+ are much larger than that is possible from direct electron impact ionization of the parent gas. The cross-section data suggest that the direct electron impact ionization of fragment neutrals and negative ion production by electron attachment may be responsible for increase of the minor ions.

Detection of chamber conditioning by CF4 plasmas in an inductively coupled plasma reactor

During oxide etch processes, buildup of fluorocarbon residues on reactor sidewalls can cause run-to-run drift and will necessitate time for conditioning and cleaning of the reactor. Various measurements in CF4 and Ar plasmas are made in an attempt to identify a metric useable to indicate the chamber condition. Mass spectrometry and Langmuir probe data show that the buildup of fluorocarbon films on the reactor surface causes a decrease in plasma floating potential, plasma potential, and ion energy in argon plasmas. This change in floating potential is also observed in CF4 plasma operation, and occurs primarily during the first hour and a half of plasma operation. A slight rise in electron density is also observed in the argon plasmas. Because the change is seen in an argon plasma, it is indicative of altered physical, not chemical, plasma-surface interactions. Specifically, the insulating films deposited on metal surfaces alter the electromagnetic fields seen by the plasma, affecting various parameters inc...

Fourier-transform infrared and optical emission spectroscopy of CF4/O2/Ar mixtures in an inductively coupled plasma

Characterization of CF4/O2/Ar inductively coupled plasmas by Fourier-transform infrared (FTIR) spectroscopy and optical emission spectroscopy (OES) in a Gaseous Electronics Conference reference cell has been carried out. Characterization was performed at three mixture compositions (80/10/10, 60/20/20, and 40/30/30), powers of 200 and 300 W and pressures between 10 and 50 mTorr. Quantitative estimates of CF4, CO, COF2, and SiF4 etch products are made via FTIR, while the OES combined with actinometry allows for qualitative characterization of conversion of CF4 to atomic C, CF, and C2, and molecular oxygen to atomic oxygen. Qualitative measurements of F density and etch products Si and SiF are also made by OES. Results are explained by a combination of electron-impact dissociation and recombination processes. In the absence of significant capacitive coupling, etch species are believed to be formed primarily by window interactions with atomic F. Rotational temperatures are estimated by both FTIR and analysis ...

Neutral gas temperature estimate in CF4/O2/Ar inductively coupled plasmas

Neutral temperatures in inductively coupled CF4/O2/Ar mixture plasmas have been characterized by fitting the rotational envelope of emission spectra in a gaseous electronics conference reference cell. CF and CO are found to present different rotational temperatures with opposite trends with respect to pressure. It is proposed that CF is created as a rotationally and translationally hot molecule due to CF4 dissociation and the disparity in temperatures and temperature trend is indicative of incomplete equilibration of the CF with other neutral species in the plasma. This may have important consequences from the standpoint of understanding neutral reaction chemistries.

Fourier transform infrared spectroscopy of CF4 plasmas in the GEC reference cell

Fourier transform infrared spectroscopy (FTIR) has been used to characterize inductively coupled CF4 plasmas in a GEC reference cell in situ. In examining these FTIR spectra, several assumptions and approximations of FTIR analysis are addressed. This includes the density dependence of cross-sections, non-linear effects in the addition of overlapping bands and the effect of spatial variations in density and temperature. This analysis demonstrates that temperatures extracted from FTIR spectra may provide a poor estimate of the true neutral plasma temperature. The FTIR spectra are dominated by unreacted CF4, accounting for 40-60% of the gas products. The amount of CF4 consumption is found to have a marked dependence on power, and is nearly independent of pressure in the range of 10-50 mTorr. Small amounts of C2F6 are observed at low power. Also observed are etching products from the quartz window - SiF4, COF2 and CO - which occur in approximately equal ratios and together account for 17-19% of the gas at 300 W and 6-9% of the gas at 100 W. The concentrations of these species are nearly independent of pressure. CFx radicals are below the detection limit of this apparatus (~1013 cm-3).

Analysis of emission data from O2 plasmas used for microbe sterilization

In order to study the sterilization capabilities of radio frequency driven low pressure oxygen plasmas, the radiative emission was recorded at various pressures and input powers. A distinct transition from the bright mode (primarily inductively coupled) to a dim mode (primarily capacitively coupled) was observed as the pressure was increased and/or the power decreased. The data was further analyzed to estimate the electron temperature, rotational and vibrational temperatures, and various species concentrations. Based on the diffusion and rovibrational relaxation times, it is concluded that the rotational temperatures can be assumed to be in equilibrium with the translational temperature. The ions are produced “hot” and have little time to get equilibrated with the translational temperature. It is further determined that in the bright mode, which is more effective in microbe sterilization, the translational/rotational temperatures are in the 650–850 K range, the electron temperatures are low (3.5–4.5 eV), ...

Langmuir Probe Measurements in Inductively Coupled CF 4 ­ Ar Plasmas

Technological advancement in the microelectronics industry requires an understanding of the physical and chemical processes occurring in plasmas of fluorocarbon gases used as etchants to optimize various operating parameters. This paper reports data on electron number density and temperature, electron energy distribution function (EEDF), and plasma potential measured using Langmuir probe in inductively coupled plasmas of CF 4 -Ar mixtures of various compositions. The probe data were recorded at several radial positions providing radial profiles of these plasma parameters at 10-50 mTorr and 200 and 300 W of radio frequency (rf) power. The measurements indicate that the electron and ion number densities increase with power; the plasma potential and electron temperature decrease with an increase in pressure, and they depend weakly on rf power. The radial profiles show that the electron and ion number densities and the plasma potential peak at the center of the plasma and drop toward the wall. Within the experimental error, the electron temperature is nearly constant in the electrode region and decreases toward the wall. As the CF 4 content increases in the mixture, the electron temperature increases hut the electron density decreases. At low CF 4 concentration, the electron and ion densities increase with pressure, but the densities are nearly independent of pressures at high CF 4 concentrations. The EEDFs have a characteristic drop near the low energy end at all pressures and powers and their shapes represent a non-Maxwellian plasma.

Inductively-Coupled RF Powered O2 Plasma as a Sterilization Source

Low-temperature or cold plasmas have been shown to be effective for the sterilization of sensitive medical devices and electronic equipment. Low-temperature plasma sterilization procedures possess certain advantages over other protocols such as ethylene oxide, gamma radiation, and heat due to the use of inexpensive reagents, the insignificant environmental impacts and the low energy requirements. In addition, plasmas may also be more efficacious in the removal of robust microorganisms due to their higher chemical reactivity. Together, these attributes render cold plasma sterilization as ideal for the surface decontamination requirements for NASA Planetary Protection. Hence, the work described in this study involves the construction, characterization, and application of an inductively-coupled, RF powered oxygen (O2) plasma.