Metastable Argon Research Articles

ガス分析用大気圧ペニングイオン源の開発

A direct gas introduction type atmospheric pressure Penning ionization source for on-site analysis of environmental organic pollutants was developed. The long-lived metastable argon atoms Ar* (3P2 and 3P0) were utilized for Penning ionization. As the excitation energies of Ar* are 11.5 eV (3P2) and 11.8 eV (3P0), the major components of air such as N2, O2, CO2, CO, and H2O are not ionized with Ar* but most of the organic compounds can be ionized. Atmospheric pressure Penning ionization mass spectra of all samples studied in our experiment showed abundant molecular-related ions (M+, [M+H]+, and [M-H]+) as base peaks. In the mass spectra of sample gases (M) mixed with methanol, intense ion [M+CH3]+ was observed in addition to [M+H]+ and [2M+H]+. The appearance of [M+CH3]+ is due to the occurrence of the methyl cation transfer reaction, CH3OH2++M→CH3+M+H2O.

Optimal Pulse Shaping for Plasma Processing

Thin-film etching and deposition using low-pressure plasma reactors is an integral part of the fabrication of very-large-scale integrated (VLSI) circuits. Standard operation of plasma reactors uses an RF power source with constant average power to excite a plasma in the vacuum chamber. Recently, several researchers have shown empirically that operation of plasma reactors with a periodic power input has the promise to increase the flexibility of plasma processing, in the sense that a greater range of operating conditions is achievable. This paper presents a numerical analysis of a global model for an argon plasma with the aim of answering the following questions: First, can a periodic input achieve effective operating conditions that cannot be achieved using steady-state inputs? Second, if the answer to the first question is yes, what is the shape of the periodic input required to achieve a particular operating point? This technique was utilized to give answers to these questions in one particular case. It was shown that periodic operation of an argon plasma can create variations in the ratio of metastable argon density to ionized argon density beyond that achievable using constant power. In addition, strong evidence is given that arbitrarily shaped modulating waveforms have advantages over simple pulse width modulation.

Influence of metastable atoms in radio-frequency argon discharges

A planar one-dimensional (1-D) particle-in-cell (PIC) simulation with Monte Carlo collisions (MCC) has been used to study the effect of including metastable argon on the characteristics of a 13.56-MHz argon discharge. Reactions such as metastable creation, ionization from the metastable state, metastable quenching to resonant level, and metastable pooling were taken into account. At low pressure (0.05 torr), metastables were found to have little influence on the discharge. However, at a pressure of 1 torr, ionization from the metastable state and collisions between two metastables represent a significant percentage of the total ionization rate, strongly influencing the electron heating and the discharge potential. The effect of pressure and applied voltage on the discharge parameters is studied and a comparison between discharges with and without metastables is presented. A PIC-MCC simulation was compared to the fluid model of Lymberopoulos and Economu (1993) using their published parameters, showing that using PIC simulation the main process of metastable loss is metastable pooling.

Saturated Absorption Spectra of Metastable Argon Atoms in a Thin Discharge Cell

Saturated absorption spectra of the 1s 3 –2p 2 (Paschen notation) line of metastable argon (Ar * ) atoms in a 1 mm thick glow discharge cell have been observed at room temperature. We examined the pressure dependence of the spectra from 0.025 to 0.5 torr. The profiles of the spectra were compared with the velocity-changing collision kernels of the Ar * (1s 3 )–Ar collisions. The results suggest that the saturated absorption spectroscopy with the thin cell is useful to detect collision kernels of metastable atoms.

Dynamics of Metastable Argon Atoms in a Thin Discharge Cell: Single Beam Absorption Spectroscopy and a Monte Carlo Study of the Velocity Distribution

Velocity distributions of the 1s 3 (Paschen notation) metastable argon atoms (Ar * ) in a 1 mm thick glow discharge cell have been examined by a Monte Carlo simulation model including Ar * (1s 3 )–Ar elastic collisions and collisional quenching of Ar * atoms at the cell surface. In order to describe the elastic collisions, the differential cross sections, the collision kernels and the collision rate coefficients were calculated from Ar * (1s 3 )–Ar potentials. The calculated velocity distributions with the model were in good agreement with the absorption spectra of the Ar * (1s 3 ) atoms measured by single beam absorption spectroscopy with the thin cell.

Modeling the effect of dust on the plasma parameters in a dusty argon discharge under microgravity

A dusty radio-frequency argon discharge is simulated with the use of a two-dimensional fluid model. In the model, discharge quantities, such as the fluxes, densities, and electric field are calculated self-consistently. The charge and density of the dust are calculated with an iterative method. During the transport of the dust, its charge is kept constant in time. The dust influences the electric potential distribution through its charge and the density of the plasma through recombination of positive ions and electrons on its surface. Results are presented for situations in which the dust significantly changes the discharge characteristics, both by a strong reduction of the electron density and by altering the electric potential by its charge. Simulations for dust particles having a radius of 7.5 microm show that a double space charge layer is created around the sharp boundary of the dust crystal. A central dust-free region (void) is created by the ion drag force. Inside this void a strong increase of the production of argon metastables is found. This phenomenon is in agreement with experimental observations, where an enhanced light emission is seen inside the void.

Self-Assembled Monolayers Exposed to Metastable Argon Beams Undergo Thiol Exchange Reactions

Self-assembled monolayers (SAMs) formed from alkanethiols on thin films of gold were exposed to a beam of metastable argon atoms through a stencil mask. The changes in the organizational structure of the alkyl chains in the SAM that resulted from exposure were characterized using reflection-absorption infrared spectroscopy. All spectroscopic evidence suggested that the SAMs become disordered after exposure to metastable argon atoms and that no apparent oxidation of the alkane chain occurred. The alkanethiolates in the regions of a SAM of dodecanethiolate damaged by the atom beam exchanged readily upon immersion in a solution of 16-mercaptohexadecanoic acid. The exchange reaction was selective for the regions of the SAM exposed to metastable argon atoms with patterns containing critical dimensions of <50 nm. A thin (<5 nm) polymeric multilayer was covalently linked to the carboxylic acid groups in the exposed regions of the SAM. The polymeric layer served as an improved resist against a commercial KI/I 2 -based etchant used to transfer the pattern into the thin film of gold.

Stern Gerlach interferometry with metastable argon atoms: an immaterial mask modulating the profile of a supersonic beam

A new Stern Gerlach interferometer operating with a nozzle beam of metastable argon atoms Ar* (3p5 4s, 3 P 2) is described. The selection of incoming (polarisation) and outgoing (analysis) Zeeman sublevels is achieved by use of laser induced transitions at two wavelengths, 811.5 nm (closed J = 2 → J = 3 transition) and 801.5 nm (open J = 2 → J = 2 transition). Linear superpositions of Zeeman sublevels, just beyond the polariser and just before the analyser, are prepared by means of two zones where Majorana transitions take place. In between, a controlled magnetic field configuration (the phase object) is produced within a triple μ-metal shielding. Standard interference patterns are obtained by scanning the field and detecting the atoms by secondary electron emission from a Faraday cup. When a static radial magnetic gradient is used, the beam profile is modulated by interference. The transverse pattern, which can be translated at will by adding a homogeneous field, is observed for the first time using a multi-channel electron multiplier followed by a phosphor screen and a CCD camera. The results satisfactorily agree with all theoretical predictions.

Probing excitation/ionization processes in millisecond-pulsed glow discharges in argon through the addition of nitrogen

The addition of N 2 to a millisecond-pulsed glow discharge (PGD) allows diagnostic measurements of the PGD but is found to drastically influence the transient signals arising from the argon and sputtered analyte atoms. Penning excitation between metastable argon atoms and ground state nitrogen molecules and charge transfer between argon ions and the added nitrogen reduce the degree of ionization of sputtered atoms during the power-on, plateau, period by a factor of ∼10 (at 1% N 2 by vol.). The added nitrogen affects sputtered atom emission signals less at this time because electron excitation dominates the excitation of these species. Upon power termination, afterpeak, the added nitrogen prevents plasma recombination in two major mechanisms: (i) the nitrogen reduces the number of argon ions available for recombination in the afterpeak; and (ii) vibrationally excited states of nitrogen slow the thermalization of electrons thereby decreasing recombination efficiency. The argon ion population contributes significantly to the afterpeak increase in the number of metastable argon atoms. These atoms are essential for the afterpeak ionization of sputtered atoms. Judicious selection of the nitrogen partial pressure can tune the delay time of afterpeak ionization/recombination by up to 200 μs. This could be particularly beneficial for time-resolved optical or mass spectrometric analyses.

Simultaneous trapping of rubidium and metastable argon in a magneto-optical trap

We have simultaneously confined rubidium and metastable argon in a dual-species magneto-optical trap (MOT). Here a binary mixture of atomic species from different groups of the periodic table have been optically confined at ultracold temperatures. We describe the apparatus and characterize the individual, single species MOTs and the dual-species MOT. Both fluorescence and ion production are monitored. With both species trapped, we observe $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${}^{85}\mathrm{Rb}$ atoms and $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{6}$ ${}^{40}{\mathrm{Ar}}^{*}$ atoms. Realization of the dual-species trap opens the way for detailed studies of Penning and associative ionization, photoassociative spectroscopy, and eventually for the production of bound, ultracold RbAr molecules.

Numerical simulation of silane decomposition in an RF plasma

A model of silane decomposition in a radio-frequency argon plasma is constructed. The concentrations of SiH4 decomposition products, as well as products of synthesis (higher silanes), are calculated. The role of metastable argon atoms in the formation of SiH3 radicals and the higher silanes is analyzed.

Modulation-free laser frequency stabilization and detuning

We have developed a modulation-free technique for stabilizing a laser to a Doppler-free saturated absorption spectrometer without the need for phase sensitive detection. By using two acousto-optic modulators, we can lock, with variable offset, to atomic resonances. The new technique works equally well with both vapor and discharge reference cells. We demonstrate the technique using rubidium and metastable argon as references.

A rf discharge cell for saturated absorption spectroscopy of metastable argon

We have produced a rf discharge in Ar40 and used saturated absorption spectroscopy to offset lock a Ti:Sapphire laser to the absorption peak of the 43P2→43D3 cooling transition at 811 nm. We describe the procedure for fabrication of the cell and production of the discharge.

A spatially resolved spectroscopic study of ionization in the planar magnetron discharge

Spatially resolved optical emission spectroscopy has been used to investigate the sputtering and ionization of titanium in a DC planar magnetron using argon as a sputtering gas. Maps of argon and titanium atomic and ionic emission have been obtained. Characteristic emission patterns have been identified and explained by the sputtering of atoms from the magnetron target and their subsequent ionization in the dense plasma at the magnetron target. The inverse Abel transformation was used to reconstruct plasma emission profiles, revealing that the non-uniform erosion profile at the magnetron target leads to a non-uniformity in plasma properties some distance into the discharge. An enhanced metal ionic content has been found in the plasma on the axis of symmetry of the discharge, and this has been explained by an enhanced lateral diffusion of metal ions arising from the change in momentum of sputtered metal during collisional ionization by metastable argon atoms.

Collision Energy Dependence Measurement of the Total De-excitation and the CF3* Chemiluminescence Cross Sections in the Ar(3P) Reaction with CF3H

Abstract The collision energy dependences of the total de-excitation and the CF3* chemiluminescence cross sections were measured in the reaction of Ar(3P) with CF3H using conventional and cross-correlation TOF methods. In the latter method, which is favorable for detecting weak signals, we employed a pseudorandom pulse discharge to generate a metastable argon beam. The collision energy dependence for the CF3* chemiluminescence channel gives a positive slope as a function of the collision energy with a threshold of 0.10 eV. The collision energy dependence of the total de-excitation in the entrance channel, on the other hand, shows a steep decrease as a function of collision energy without any threshold. These results provide insight into the detailed dynamics at crossing points of the potential energy surfaces.

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s5 (3P2) and 1s3 (3P0) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the 3P2 (11.55 eV) and the 3P0 (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.

Metastable rare gas atoms scattered by nano- and micro-slit transmission gratings

The transmission of metastable argon atoms through nano-slit or micro-slit gratings is studied by use of time of flight and angular analysis. This transmission departs from the simple geometric one essentially by two ways: (i) the elastic or diagonal part of the van der Waals (vW) interaction with the solid causes an angular narrowing of the emerging beam; (ii) the off-diagonal vW interaction induces the exothermal fine structure transition 3 P 0↦3 P 2 (ΔE = 175 meV) leading to large scattering angles; the resulting angular distribution is very sensitive to the roughness of the surface in the direction of the depth. An extension of these experiments to transversally coherent beams is proposed. It should be considered as a first step towards a new type of interferometer in which the inelastic diffraction makes the gratings work as beam splitters or mirrors.

Effect of Ar addition to an O2 plasma in an inductively coupled, traveling wave driven, large area plasma source: O2/Ar mixture plasma modeling and photoresist etching

We report on the effect of Ar addition to an O2 plasma on photoresist etching in an inductively coupled, traveling wave driven, large area plasma source (LAPS). We also develop a simplified spatially varying O2/Ar mixture discharge model corresponding to the LAPS in a two-dimensional geometry in order to account for the effect of Ar addition. A photoresist etch kinetics model and spatially varying O2/Ar mixture discharge model are used to explain the experimental data. We find that the addition of 50% Ar increases the plasma density and etch rate approximately by a factor of 2. From the simulation we find that argon metastables (Ar*) play an important role in the mixture plasma. The simulation predicts an enhancement in O-atom density due to Ar addition, even in the presence of dilution of the feed gas. The experimental data and predicted etch rates from the simulation are generally in good agreement, indicating that the increase in the etch rate with Ar addition is due to both the increase in the plasma density and the enhancement in O-atom density attributable to the dissociation of O2 by Ar*.

Pressure dependent mode transition in an electron cyclotron resonance plasma discharge

Despite the wide range of applications of the electron cyclotron resonance (ECR) plasma sources, the ECR plasma processing control is tricky at certain operation parameters. There are several reports of regimes where abrupt changes and instabilities in plasma parameters occur. In the present work we report extensive probe measurements of plasma potential, electron temperature, ion beam energy, and velocity as well as plasma density over a mode change appearing when the neutral argon gas pressure is changed. The parameters were measured over the entire pressure range from 0.15 to 7 mTorr. We found a large drop in electron temperature and plasma potential when the pressure increased from 0.15 to 0.4 mTorr. At 0.4 mTorr the temperature reached a minimum and the density a local maximum, while at 1 mTorr the density reached a minimum. When increasing the pressure above 1.2 mTorr the temperature decreased and the density increased rapidly. While the plasma appearance at low and high pressure can be explained by a global conservation model, the behavior in the intermediate pressure between 0.4 and 1.2 mTorr needs a deeper investigation. We discuss the possibility that the mode change is connected to the competition between stepwise and direct ionization of neutral argon and excitations to metastable argon atoms.

Elimination of the concentration dependence in mass isotopomer abundance mass spectrometry of methyl palmitate using metastable atom bombardment

An important problem in mass isotopomer abundance mass spectrometry (MIAMS) is the dependence of measured mass isotopomer abundances on sample concentration. We have evaluated the role of ionization energy on mass isotopomer abundance ratios of methyl palmitate as a function of sample concentration. Ionization energy was varied using electron impact ionization (EI) and metastable atom bombardment (MAB). The latter generates a beam of metastable species capable of ionizing analyte molecules by Penning ionization. We observed that ionization of methyl palmitate by EI (70 eV) showed the greatest molecular ion fragmentation and also showed the greatest dependence of relative isotopomer abundance ratios on sample concentration. Ionization using the 3 P 2 and 3 P 0 states of metastable krypton (9.92 and 10.56 eV, respectively) resulted in almost no molecular ion fragmentation, and the isotopomer abundances quantified were essentially independent of sample concentration. Ionization using the 3 P 2 and 3 P 0 states of metastable argon (11.55 and 11.72 eV, respectively) showed molecular ion fragmentation intermediate between that of EI and MAB(Kr) and showed an isotopomer concentration dependence which was less severe than that observed with EI but more severe than that observed with MAB(Kr). The observed decrease in the dependence of isotopomer abundance on sample concentration with a decrease in molecular ion fragmentation is consistent with the hypothesis that proton transfer from a fragment cation to a neutral molecule is the gas phase reaction mechanism responsible for the concentration dependence. Alternative explanations, e.g., hydrogen abstraction from a neutral molecule to a molecular cation, is not supported by these results. Moreover, the MAB ionization technique shows potential for eliminating one source of error in MIAMS measurements of methyl palmitate, in particular, and of fatty acids methyl esters, in general.