Vacuum Ultraviolet Absorption Spectroscopy Research Articles

Spatial distributions of the absolute CF and CF2 radical densities in high-density plasma employing low global warming potential fluorocarbon gases and precursors for film formation

Behaviors of gas species in electron cyclotron resonance (ECR) plasmas employing low global warming potential fluorocarbon gases of hexafluorobutadiene (C4F6) and hexafluoropropene (C3F6) with unsaturated carbon bonds together with the conventional octafluorocyclobutane (c-C4F8) are investigated. The spatial distributions of the absolute CF and CF2 radical densities are measured by combining the single-path infrared diode laser absorption spectroscopy and laser-induced fluorescence techniques. CF2 radicals have hollow-type distributions at all conditions in the ECR plasma reactor. However, the spatial distribution of the CF radical density differs greatly from that of the CF2 radical density. Behaviors of carbon atoms measured by vacuum ultraviolet absorption spectroscopy disagree with those of CF and CF2 radical densities. The behaviors of ion and fluorine species and the gas pressure in the plasma have been also evaluated. Fluorocarbon films with low dielectric constant of about 3.0 are deposited at a h...

Behavior of hydrogen atoms in ultrahigh-frequency silane plasma

We have investigated the behavior of the absolute density of hydrogen (H) atoms in ultrahigh-frequency (UHF), (500 MHz) silane (SiH4) plasma by using a vacuum ultraviolet absorption spectroscopy technique with a microdischarge hollow cathode lamp. In the UHF plasma using SiH4 highly diluted with hydrogen molecule (H2) at a pressure of 20 Pa, an UHF power of 1000 W, and a total flow rate of 200 sccm, the absolute density of H atoms slightly increased from 7.4×1011 to 7.9×1011 cm−3 with increasing the SiH4 flow rate ratios from 0% to 2.5% and then the H atom density decreased at the ratio of 5%. The decrease of the density is due to the increase of the reaction between the H atom and the SiH4 molecule. The behavior of the absolute density of H atoms was compared with that of the Balmer α(Hα) emission intensity. It was found that the behaviors of the absolute H atom density and the Hα emission intensity were quite different. Moreover, the kinetics of H atom density in SiH4 plasmas have been clarified on the basis of measured results.

Development of vacuum ultraviolet absorption spectroscopy technique employing nitrogen molecule microdischarge hollow cathode lamp for absolute density measurements of nitrogen atoms in process plasmas

We have developed a vacuum ultraviolet absorption spectroscopy (VUVAS) technique employing a high-pressure nitrogen molecule (N2) microdischarge hollow cathode lamp (N2 MHCL) as a light source of the atomic nitrogen (N) resonance lines for measuring absolute N densities in process plasmas. The estimations of self-absorption and the emission line profiles of the N2 MHCL, which are necessary for absolute N density determination, were carried out. The measurement of absolute N densities have been demonstrated for an inductively coupled N2 plasma using the VUVAS system employing the N2 MHCL.

ラジカル，プラズマの形成と計測 マイクロプラズマを光源に用いた真空紫外吸収分光法による原子密度計測

プロセスプラズマ中の水素原子絶対密度計測のため, マイクロプラズマを光源に用いた真空紫外吸収分光システムの開発を行った.光源の自己吸収は, 水素分圧7Pa以下にすることで, 計測に影響のないレベルまで低減できることが判明した.また, 光源の発光プロファイルは, 水素原子温度400Kのドップラー拡がりとドップラー拡がりの2倍のローレンツ拡がりからなるフォークト型であると見積もられた.このシステムを用い, 誘導結合型水素プラズマ中の水素原子絶対密度計測を行った.水素原子密度は, パワー100W, 流量100sccmにおいて, 圧力1.33Paから66.5Paの増加に伴い, 1.4×1011cm-3から1.2×1012cm-3へと増加した.

VUV absorption spectroscopy measurements of the role of fast neutral atoms in a high-power gap breakdown

The maximum power achieved in a wide variety of high-power devices, including electron and ion diodes, z pinches, and microwave generators, is presently limited by anode-cathode gap breakdown. A frequently discussed hypothesis for this effect is ionization of fast neutral atoms injected throughout the anode-cathode gap during the power pulse. We describe a newly developed diagnostic tool that provides a direct test of this hypothesis. Time-resolved vacuum-ultraviolet absorption spectroscopy is used to directly probe fast neutral atoms with 1-mm spatial resolution in the 10-mm anode-cathode gap of the SABRE 5 MV, 1 TW applied-B ion diode. Absorption spectra collected during Ar RF glow discharges and with CO2 gas fills confirm the reliability of the diagnostic technique. Throughout the 50-100 ns ion diode pulses no measurable neutral absorption was seen, setting upper limits of (0.12-1.5)x10(14) cm(-3) for ground-state fast neutral atom densities of H, C, N, O, and F. The absence of molecular absorption bands also sets upper limits of (0.16-1.2)x10(15) cm(-3) for common simple molecules. These limits are low enough to rule out ionization of fast neutral atoms as a breakdown mechanism. Breakdown due to ionization of molecules is also found to be unlikely. This technique can now be applied to quantify the role of neutral atoms in other high-power devices.

Loss kinetics of carbon atoms in low-pressure high density plasmas

Vacuum ultraviolet absorption spectroscopy (VUVAS) with a carbon hollow cathode lamp was applied to the measurement of decay rate of C atom density in the afterglow of CO and CO/H2 inductively coupled plasmas. The transition line used for the measurement was 2p3s 3P2–2p2 3P2 at 165.7 nm. The influence of background absorption by the species in plasma other than C atoms on the transition line of C atoms was found to be negligible. This was clarified by measuring the absorption intensities around the center wavelength of C atoms in plasmas with VUVAS employing a xenon microhollow cathode lamp. Moreover, the dependence of the decay rate of C atom density on pressure revealed that C atoms were dominantly lost at the surface rather than in the gas phase in both CO and CO/H2 plasmas. However, in the case of CO/H2 plasma at higher pressures over about 5.0 Pa, C atoms were lost in the gas phase as well as at the surface. The diffusion constants of C atoms in both CO and CO/H2 plasmas were also determined to be 3.1×104 and 3.7×104 cm2 Pa s−1, respectively.

Measurement and control of absolute nitrogen atom density in an electron-beam-excited plasma using vacuum ultraviolet absorption spectroscopy

The absolute nitrogen (N) atom density in an electron-beam-excited plasma (EBEP) operating at an ultralow pressure has been investigated by vacuum ultraviolet absorption spectroscopy, employing a microdischarge hollow-cathode lamp. The measured N atom density was estimated to be around 6×1011 cm−3, and the dissociation fraction was 4.9% at a N2 pressure of 0.05 Pa, an electron-beam current of 10 A, and an electron-beam acceleration voltage of 120 V. The EBEP potentially enables us to control the electron density and electron energy independently with the electron-beam current and electron-beam acceleration voltages, respectively. It was found that N atom densities increased with increasing electron-beam current and electron acceleration voltage under low-pressure conditions. The EBEP shows great promise as a N atom source operating at an ultralow pressure.

Kinetics and role of C, O, and OH in low-pressure nanocrystalline diamond growth

A simple low-pressure condition at 80 mTorr has been employed to study the kinetics and role of C, O, and OH in diamond growth by using inductively coupled CO/CH4/H2 and O2/CH4/H2 plasmas. Vacuum ultraviolet absorption spectroscopy (VUVAS) and actinometric optical emission spectroscopy (OES) were used to examine the densities of ground-state C atoms and emissive species such as OH, C2, and O, respectively. Diamond films consisting of nanocrystallites with sizes as small as 20 nm were obtained on positively biased Si substrates only when CH4 was fed. Both diamond and nondiamond growth were enhanced with increasing CO for a fixed CH4 concentration of 5%, while diamond growth was suppressed with increasing O2. Comprehensive discussion along with the VUVAS and OES results suggested that the C atoms resulting mainly from CO by electron impact dissociation had a close relation with the formation of C2 or still larger species as the precursors to nondiamond phase, while the OH radicals resulting predominantly by loss reactions of the byproduct O atoms with H2 and CH4 were highly responsible for the enhanced diamond growth. A large amount of O atoms from O2 was shown to affect the initial nucleation stage seriously. The results support the growth chemistry of diamond from H-hybridized carbon radicals fragmented from CH4 rather than from H-stripped carbon radicals.

Vacuum ultraviolet absorption spectroscopy employing a microdiacharge hollow-cathode lamp for absolute density measurements of hydrogen atoms in reactive plasmas

We have developed a measurement technique for absolute H-atom densities in process plasmas using vacuum ultraviolet absorption spectroscopy employing a high-pressure microdischarge hollow-cathode lamp (MHCL) as a Lyman α (Lα, 121.6 nm) emission light source. Characterization of the Lα emission-line profile could be simplified by using a high-pressure discharge at about 1 atm. The effect of self-absorption in the MHCL was reduced to an insignificant level by decreasing the H2 partial pressure. The contribution of the collisional broadening to the Lα emission profile was estimated from the saturation characteristics of the absorption intensity when the optical thickness of the plasma was varied. The technique was applied to the measurement of the absolute H-atom density in an inductively coupled H2 plasma.

Diamond Deposition and Behavior of Atomic Carbon Species in a Low-Pressure Inductively Coupled Plasma

Diamond was successfully synthesized by using H2-rich CH4/CO/H2 and H2-rich CH4/H2 inductively coupled plasmas at a pressure of 11 Pa. The ratio of particle size to deposition time, as a criterion of the diamond growth rate, in H2-rich CH4/CO/H2 mixture gas plasmas was higher than that in H2-rich CH4/H2 mixture gas plasmas. The deposits in H2-rich CH4/H2 and H2-rich CH4/CO/H2 mixture gas plasmas were found to contain nondiamond phases, as confirmed by Raman spectroscopy. In order to investigate the mechanism involved in diamond formation, C-atom densities in the plasmas were measured by vacuum ultraviolet absorption spectroscopy with a carbon hollow cathode lamp. In addition, CH, OH and H-atom emission intensities were measured by optical emission spectroscopy. As a result, it was found that the C-atom densities increased considerably with increasing mixture ratio of CO to CH4. On the basis of the correlation between the quality of deposits and the C-atom densities, C-atoms were determined to probably contribute to the formation of nondiamond phases in the deposits.

Vacuum-ultraviolet laser absorption spectroscopy for absolute measurement of fluorine atom density in fluorocarbon plasmas

Absolute density measurement of fluorine atoms has been performed by a vacuum-ultraviolet (VUV) laser absorption technique in fluorocarbon plasmas. A VUV laser tunable around 95 nm covering the resonance lines of F atoms has been produced in Xe gas by a two-photon resonance four-wave-mixing technique. In this method, the background absorption by the parent gases and species produced in the plasma can be eliminated by scanning the wavelength, and the absolute density of F atoms can be derived accurately from the integrated absorption line profile. The measured values of the density varied from 1×1011 to 4×1012 cm−3, depending on the source gas species and the operating conditions of an inductively coupled radio-frequency (400 kHz) discharge.

Determination of fluorine atom density in reactive plasmas by vacuum ultraviolet absorption spectroscopy at 95.85 nm

Vacuum ultraviolet absorption spectroscopy was developed for the measurement of absolute fluorine (F) atom density in reactive plasmas. In order to minimize the influence of radiation trapping (self-absorption) in the light source, fluorescence at a wavelength of 95.85 nm from the F atoms in an electron–cyclotron resonance (ECR) CF4 plasma, which was operated with a low microwave power (0.1 kW) and a low gas pressure (1 mTorr), was employed as the probe emission. A windowless transmission system for the probe emission was constructed by connecting the ECR light source with the target plasma and the detection system using vacuum tubes having small slits. The connection tubes were differentially evacuated with turbomolecular pumps to prevent neutral particles from passing through between the ECR and target plasmas. The present method was applied to high-density CF4 and C4F8 plasmas produced by helicon-wave discharges. The accuracy of the measurement was examined carefully by evaluating various sources of error. In the present article, we have emphasized the evaluation of the radiation trapping effect in the light source plasma.

Surface Kinetics of CFx Radicals and Fluorine Atoms in the Afterglow of High-Density C4F8 Plasmas

Temporal variations of absolute densities of CF, CF2 and atomic fluorine (F) were measured in the afterglow of high-density C4F8 plasmas generated by helicon-wave discharges. Laser-induced fluorescence (LIF) spectroscopy was adopted for CF and CF2 radicals, while vacuum ultraviolet (VUV) absorption spectroscopy was employed for the F atom. CF and F densities gradually decreased for 20–80 ms after the extinction of the rf power, while CF2 density steadily increased during the same period. This slow increase in CF2 density can be explained by surface kinetics of the radicals. In the afterglow of discharges with a high degree of dissociation, the increase in CF2 density is approximately equal to CF density at the beginning of the afterglow. The mechanism for the surface production of CF2 in the afterglow is discussed based on the close relationships between the temporal variations of CF and CF2 densities.

A study of O2(a 1Δg) with photoelectron spectroscopy using synchrotron radiation

The atmospherically important species O2(a 1Δg) has been studied by photoelectron spectroscopy using vacuum ultraviolet radiation from a synchrotron as the photon source. Constant-ionic-state (CIS) spectra, recorded for vibrational levels of O2+(X 2Πg) v+=0,1,2,3 accessed from O2(a 1Δg) v″=0, exhibit intense signals in the photon energy region 14.0–15.5 eV which are shown to arise from autoionization from a Rydberg state with an O2+(C 2Φu) core. On the basis of the results obtained and earlier evidence derived from vacuum ultraviolet absorption spectroscopy, this state is assigned as a (C 2Φu,3sσg) 1Φu Rydberg state. Photoelectron spectra recorded for O2(a 1Δg) at positions of strong resonances have allowed extended vibrational structure to be obtained in the first photoelectron band. The relative vibrational component intensities in the resonant photoelectron spectra are in good agreement with computed relative intensities obtained via Franck–Condon calculations, confirming the vibrational numbering of the resonances in the 1Φu state. Competition between autoionization and predissociation in the 1Φu Rydberg state is discussed on the basis of the results obtained. Weaker structure is observed in CIS spectra recorded in the photon energy regions 12.5–13.5 and 15.0–20.0 eV. Suggestions are made for the nature of the highly excited states of O2 associated with this structure, based on available ionization energies and spectroscopic constants of known ionic states accessible from O2(a 1Δg). For example, two broad bands centered at ≈16.4 and ≈17.75 eV are assigned to excitation to Rydberg states arising from the configurations (D 2Δg,3pπu) and (D 2Δg,4pπu), respectively.

Absolute density and reaction kinetics of fluorine atoms in high-density c-C4F8 plasmas

Absolute density and reaction kinetics of fluorine (F) atoms in high-density octafluorocyclobutane (c-C4F8) plasmas were examined using vacuum ultraviolet absorption spectroscopy. The F atom densities, corresponding to electron densities ranging from 1×1011 to 5×1012 cm−3, were 1×1012–5×1013 cm−3 for gas pressures of 2–7 mTorr and rf powers of 0.2–1.5 kW. The F atom density was linearly dependent on the electron density for ne<1.5×1012 cm−3. According to lifetime measurements in the afterglow, two decay processes were found in the F atom density: exponential (first-order kinetics) and linear (zero-order kinetics) decay components. The linear-decay component became significant at high gas pressures. The time constant of the exponential-decay component ranged from 5 to 100 ms, which corresponds to surface loss probabilities of 10−1–10−3. The surface loss probability varied inversely with the F atom density.

Kinetics of fluorine atoms in high-density carbon–tetrafluoride plasmas

Reaction processes of fluorine (F) atoms in high-density carbon–tetrafluoride (CF4) plasmas were investigated using vacuum ultraviolet absorption spectroscopy. A scaling law nF∝(nenCF4)0.5–0.7 was found experimentally, where nF is the F atom density and ne and nCF4 stand for the electron and parent gas (CF4) densities, respectively. The lifetime measurement in the afterglow showed that the decay curve of the F atom density was composed of two components: a rapid decay in the initial afterglow and an exponential decrease in the late afterglow. The decay time constant in the initial afterglow τ1 satisfied the scaling law τ1∝(nenCF4)−(0.3–0.4), which is a consistent relationship with the scaling law for the F atom density. The two scaling laws and the lifetimes of CFx radicals suggest that the major loss process of F atoms in the initial afterglow is the reaction with CFx radicals (probably, x=3) on the wall surface. The loss process in the late afterglow was simple diffusion to the wall surface. The surface loss probability of F atoms on the chamber wall was evaluated from the decay time constant in the late afterglow, and was on the order of 10−3.

Vacuum ultraviolet absorption spectroscopy for absolute density measurements of fluorine atoms in fluorocarbon plasmas

Absolute densities of fluorine (F) atoms at the ground state (2p5 2P°) were measured in helicon-wave excited high-density CF4 plasmas by vacuum ultraviolet (VUV) absorption spectroscopy. By employing an electron cyclotron resonance CF4 plasma as a light source in the VUV wavelength range, an absorption spectroscopy system with no vacuum windows was constructed. As a result, the density of F atoms was approximately 1×1013 cm−3 for an rf power of 1 kW and a CF4 gas pressure of 2.5 mTorr, which was one-order higher than the density of CFx radicals and was one-order lower than the density of the parent gas.

Vacuum ultraviolet absorption spectroscopy in the spectral interval of Lyman-α of atomic hydrogen and deuterium in an ion source plasma

Atomic hydrogen and deuterium in the plasma of a magnetic multipole source are investigated with respect to the fraction of the atomic component and the energy distribution of the atoms. Information is obtained by analysis of the wings of the optically thick Lyman-α transitions, and by admixture of small amounts of the other isotope as a thermometer gas in order to warrant transparency in the center of the line. The atom to molecule density ratio is found to be around 15%; the analysis of the energy distributions yields a dominant cold component (Tcold∼400 K), a hot component (Thot≳2000 K) which comprises about 20% of the atoms, and a small but significant amount of fast atoms, described by Tfast with energies up to several eV; the relative amount of these atoms is below 1%.

Absolute density measurements of ammonia synthetized in N2–H2 mixture discharges

Measurements of absolute density of ammonia in nitrogen–hydrogen mixture dc glow discharges were performed by employing photofragment translational spectroscopy associated with laser induced fluorescence (LIF) of the H ground state atoms. The dissociation energy of ammonia using the photolysis at 205 nm was determined to be (4.34±0.07) eV. The H atom LIF measurements were calibrated by vacuum ultraviolet absorption spectroscopy on the Lyman-α (121 nm) line. The ammonia synthesized in the discharge has a maximum density of about 8×1012 cm−3 for a 75% H2–25% N2 mixture, a discharge current of 50 mA and a pressure of 2 Torr.

ChemInform Abstract: Predissociation Supported High‐Resolution Vacuum Ultraviolet Absorption Spectroscopy of Excited Electronic States of NH3.

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