Threshold Ionization Mass Spectrometry Research Articles

On the origins of the m/z = 19 signals in residual gas analysis

Previous investigations of mass spectra obtained in very high vacuum systems have disagreed as to the origin and identity of the mass peak frequently observed at m/z = 19 amu. Recent work with quadrupole mass spectrometry instruments that can also be used for threshold ionization mass spectrometry has shown how the apparent conflict between different sets of published data may be resolved.

Characterizing Nuclear Fusion

SummaryConventional mass spectrometry techniques can be used to monitor vacuum quality and contaminant species ahead of the fusion process. However, their use in characterizing species used in fusion reactions is limited by the mass interferences that occur with the isotopes of light gases, deuterium and helium for example appear at mass 4. This article describes the use of Threshold Ionization Mass Spectrometry (TIMS) and Ultra‐high resolution Mass Spectrometry, both are effective in the characterization of typical species found in nuclear fusion. The TIMS technique uses the control of ionization potential to selectively ionize species at the same nominal mass, deuterium, D2 a typical fuel used in nuclear fusion and the fusion by‐product 4He, both appear at m/z 4 for example. The development of ultra‐high resolution mass spectrometry, using the second stability zone, “Zone H”, further improves these capabilities, with the detection of interference species by mass. Ultrahigh resolution allows improvements in sensitivity of a factor of 100 over the TIMS method.

Deuterium and helium outgassing following plasma discharges in WEST: Delayed D outgassing during D-to-He changeover experiments studied with threshold ionization mass spectrometry

• New calibration method for threshold ionization mass spectrometry (TIMS) in tokamak. • Uncertainty evaluation of TIMS in plasma discharge and post-discharge. • He and D gas balance in D-to-He plasma changeover in WEST tokamak. • Difference in outgassing time constants for He and D 2 in a full-W tokamak. • Delayed outgassing for D 2 depends on the plasma termination: disrupted or not? Threshold ionization mass spectrometry (TIMS) is one of two methods envisioned in ITER to quantify the helium (He) fusion product in the exhaust pumping lines during plasma discharges. We present the first demonstration of another potential application of TIMS in a tokamak environment, namely, the analysis of deuterium (D) and He outgassing following a plasma discharge i.e. during the post-discharge. This method has been tested with sub-second temporal resolution in WEST during its first He plasma discharges in the so-called He changeover experimental campaign. The calibration method of TIMS using a D plasma discharge is presented while the uncertainties related to TIMS during rapid pressure variations, i.e. upon plasma breakdown and plasma termination, are discussed. The first results obtained with TIMS during consecutive D and He plasma discharges in the full tungsten (W) tokamak WEST are reported. It is found that the time evolutions for He and D outgassing in the post-discharge are markedly different. On one hand, He outgassing is instantaneous and decays within 60 s until the He signal gets below detection level. On the other hand, D outgassing can reach a maximum up to several tens of seconds after the termination of the plasma and this outgassing can last for about 10 min. These striking differences should be related to different retention and outgassing from WEST plasma facing components, presently constituted of actively-cooled ITER-like W units and inertially cooled W-coated graphite. Potential mechanisms at the origin of the different outgassing behavior for D and He in W plasma facing components are discussed in light of a systematic analysis of the He and D gas balance and a macroscopic rate equation modeling of the D outgassing from the divertor strike points.

Improved Process Stability on an Extremely Thin Amorphous/Crystalline Silicon Interface Passivation Layer by Using Predeposition on the Chamber Wall

In this study, in situ plasma diagnostic systems of optical emission spectrometry (OES) and quadrupole mass spectrometry (QMS) were used to monitor an extremely thin (5–10 nm) intrinsic amorphous/crystalline Si interface passivation film as deposited using plasma-enhanced chemical vapor deposition (PECVD). We observed a dramatic improvement in the quality of the passivation layer with a chamber background environment, even with a background pressure of 1E-6 Torr. When the chamber walls were coated (at a predeposition time of approximately 150 min) to a specified thickness of a few hundred micrometers, the minority carrier lifetime increased by more than 26 times, as compared with an insufficiently coated counterpart with a predeposition of approximately 30 min (from approximately 30 μs to 800 μs). In this predeposition process, the tendency of species to concentrate could be systematically obtained using the in situ OES system, with the chamber environment monitored using residual gas analysis and threshold ionization mass spectrometry of QMS. We found that the optimal predeposition time was 150 min, which enabled the OES intensity of Si*/SiH*, Hβ/Hα, and Hα/SiH* to become stable. OES and QMS were incorporated in this study and were validated using the Fourier-transform infrared spectroscopy absorbance spectra on the films. The plasma condition was stabilized and the minority carrier lifetime was improved to 800 μs. The proposed predeposition process stabilized the chamber environment and gas discharge. Therefore, the optimized value of the minority carrier lifetime could be consistently obtained. A transmission electron microscopy photograph showed a compact a-Si:H layer (of approximately 10 nm) on the c-Si substrate interface passivation layer with a void-free and crystallites-free interface after a predeposition time of 150 min.

Determination of the accurate ionization energy and cationic structure of cyanopyrazine by one-photon mass-analyzed threshold ionization spectroscopy

The ionization energy and cationic structure of cyanopyrazine have been determined for the first time by pulsed-field threshold ionization mass spectrometry. The ionization energy was accurately measured to be 80,305±6cm−1 (9.9566±0.0007eV) from the 0-0 band position in the mass-analyzed threshold ionization (MATI) spectrum obtained for the direct S0→D0 transition using a vacuum ultraviolet laser pulse generated by four-wave mixing in Kr. The highest occupied molecular orbital (HOMO) and singly occupied molecular orbital (SOMO) calculations indicated the extent of conjugation of nonbonding orbitals on nitrogen atoms with the π orbital in cyanopyrazine, and clarified the effect of substitution of H with CN in pyrazine on the ionization energy of cyanopyrazine. The observed MATI spectrum was interpreted using the Franck–Condon factors and vibrational frequencies calculated through adjustment of the geometrical parameters of the cyanopyrazine cation in the ground electronic state at the B3LYP/cc-pVTZ level. From rigorous analyses, it has been concluded that upon ionization, the cyanopyrazine cation in the D0 state should retain a planar structure with Cs symmetry, similar to the ground state molecule.

Investigation of Electron Cyclotron Resonance Chemical Vapor Deposition Process for a-Si:H Deposition, Film Characterization and In Situ Plasma Diagnostics

We present the application of quadrupole mass spectrometry (QMS) with threshold ionization mass spectrometry (TIMS), optical emission spectroscopy (OES) with an actinometry method, and Langmuir probe (LP) as an integrated technique for in situ plasma characterization of the electron cyclotron resonance chemical vapor deposition (ECR-CVD) of hydrogenated amorphous silicon (a-Si:H). The main aim of this study was to determine the relationship among the process parameters, plasma characteristics, and thin film properties, including the microstructure parameter (R*), hydrogen content (CH), and a-Si:H film growth rate. The physical properties of plasma, such as the electron density (Ne), electron energy (Te), and sheath potential (Vs), were studied using an LP and OES. In addition to the general plasma properties, the ion density (Ni) and plasma electronegativity were examined. The results indicate the ECR plasma source has high potential for producing a high-quality a-Si:H film because of the availability of few high silanes (SinH2n+2, n > 2); furthermore, the results indicate an obvious difference in the QMS analysis results between plasma and gas. Therefore, the consumption of parent molecules must be considered. The TIMS method was applied to estimate the relative concentration of ground-state silane radicals (SiHx, x < 4) and consumption of SiH4. The results indicate that, in addition to the degree of ionization, the consumption of SiH4 and Vs are key factors affecting the thin film growth rate. When microwave power density increases, the values of Ni, Ne, and Vs increase considerably, but the plasma electronegativity remains unchanged. Furthermore, we compared our results with the TIMS analysis reported by other authors and found that the ratio of SiH2 to SiH3 obtained using QMS and TIMS can be considered an indicator of film quality for R* in the ECR-CVD process.

CVD Characteristics and Mechanism of a-Si:H thin Films in Electron Cyclotron Resonance H2-Ar-SiH4 Plasma

This study investigated the use of quadrupole mass spectrometer (QMS), optical emission spectrometer (OES) and Langmuir probe to diagnose the H2-Ar-SiH4 plasma characteristics in cyclotron resonance chemical vapor deposition (ECR-CVD) process. The plasma physical characteristics such as electron density (Ne), electron temperature (Te), plasma and floating potential were obtained by Langmuir probe. The QMS and OES were used to identify the radicals in the plasma. Furthermore, the relative concentration and consumption of SiH4 in ECR-CVD process could be estimated by the TIMS (threshold ionization mass spectrometry) method based on the QMS results. The relationship between the film and plasma characteristics with varying process parameters was also discussed. As results shown, besides the degree of ionization, the consumption of SiH4 and the sheath potential (Vs) were key factors to affect thin film deposition rate (DR). When microwave power density rose from 1 w/cm2 to 3w/cm2, the DR has increased 194%, and it was nearly equal to the product of 104% in SiH4 consumption and 187% in Vs increase. The high hydrogen dilution ratio (H2/SiH4) would lead to higher electron temperature, density and Vs, but decrease the SiH4 consumption and DR. Moreover, the declining extent of DR was significantly greater than SiH4 consumption and Vs increase. It seems that the effect of hydrogen etching plays an important role due to the increase of high hydrogen dilution ratio (H2/SiH4), This interpretation can be received from supporting evidence in OES measurements.

Integrating the Quadrupole Mass Spectrometer, Optical Emission Spectroscopy and Langmuir Probe as a Plasma Characterization Tools for ECR-CVD with a-Si:H

We present the application of quadrupole mass spectrometer (QMS), optical emission spectroscopy (OES), and Langmuir probe (LP) as an integrated technique for plasma characterization of the process during electron cyclotron resonance chemical vapor deposition (ECR-CVD) of hydrogenated amorphous silicon (a-Si:H). The main aim of this study was to investigate the use of QMS to determine the relationship between plasma and thin film characteristics, in particular microstructure parameter (R*) and hydrogen content (CH). The OES was used to assist the QMS in measuring the light and simple radicals, such as the H, Si and SiH, and the associated physical plasma properties which include electron density (Ne), electron energy (Te) and sheath potential (Vs) would be studied with the Langmuir probe. The effects of the two ECR-CVD parameters, namely the power density (1 ~ 4 w/cm2) and hydrogen dilution ratio (H2/SiH4, 0 ~ 25) were discussed. The results indicated that there is a very obvious difference in QMS analysis between plasma and gas, and it is due to the fact that the QMS analysis of plasma is accompanied with the vapor and surface reactions. Therefore, the consumption of parent molecule (SiH4) needed to be considered. The TIMS (Threshold ionization mass spectrometry) method which is usually applied to estimate the relative density trends of the ground-state silane radicals (SiHx, x < 4) was used in this study. It was found that increasing power density and hydrogen dilution ratio would lead to higher electron density and more SiH2 generated in the plasma. Furthermore, when comparing with the TIMS analysis reported by other authors, we found the ratio of SiH2 to SiH3 from QMS could be considered as a better indicator of film quality for R*.

Plasma Composition by Mass Spectrometry in a Ar-SiH4-H2 LEPECVD Process During nc-Si Deposition

Mass spectrometry has been used to assess plasma composition during a low-energy plasma-enhanced chemical vapor deposition (LEPECVD) process using argon-silane-hydrogen (Ar-SiH4-H2) gas mixtures with input flows of 50 sccm Ar, 2–20 sccm SiH4 and 0–50 sccm H2 at total pressures of 1–4 Pa. Energy-integrated ion densities, residual gas analysis and threshold ionization mass spectrometry have been used to characterize the transition from amorphous (a-Si) to nano-crystalline silicon (nc-Si) deposition at constant LEPECVD operating parameters. While relative ion densities have a marked decrease with H2 input, the densities of SiHx (x < 4) radicals show evolution trends depending on the SiH4 and H2 input. For conditions leading to nc-Si growth a turning point is reached above which SiH is the main radical. Observed SiHx density trends with H2 input are explained based on kinetic reaction rates calculated from previously obtained Langmuir probe data.

Surface radicals in silane/hydrogen discharges

Using threshold ionization mass spectrometry, radical densities have been measured at the substrate surface of a radio frequency discharge in silane plus hydrogen vapor. The conditions are 100–300 Pa pressure and a ratio (R) of hydrogen flow/silane flow typical of discharges used to produce large area amorphous (R=20) and microcrystalline (R=40) silicon. For comparison, we include measurements in pure-silane vapor. The neutral radicals observed in the mixed gases are H, SiH3, Si2H2, and Si2H5, with decreasing flux in that order. (Si2H4 is also seen in pure silane and SiH2 for R=20.) The H flux is sufficient for major film etching and restructuring, particularly for R=40. The ion-bombardment species were also measured, establishing that SiyHn+ (y=1–3) ion flux is much larger than the Hn+ (n=1–3) flux. The ion flux provides 15±4% of the total Si flux to the film for R=20 and 37±10% for R=40. This is larger than in pure-silane discharges, and it may be very important to film properties due to the impact energy.

Mass spectrometric detection of short-living radicals produced by a plasma needle

A plasma needle is a radio-frequency (rf) micro-discharge operated in a mixture of helium and air at atmospheric pressure. This source is designed for medical treatment of living tissues. Therapeutic effects of plasma treatment depend on generation of short-living active radicals: reactive oxygen and nitrogen species (ROS/RNS). In this work we determine the concentration of several ROS and RNS (atomic oxygen, nitrogen and hydroxyl radical) by means of threshold ionization mass spectrometry. It is shown that molecular oxygen and nitrogen are substantially dissociated in the plasma. Atomic nitrogen and oxygen are the most abundant radicals: the densities are on average few times 10−4. Hydroxyl radicals are less abundant (10−5 fraction of the total gas density). As expected, the densities of active species increase with increasing plasma power. Spatial (axial) distributions have been determined; the radical density reaches a maximum at 2.5 mm away from the rf powered electrode, and it decreases at distances larger than 3.5 mm. The amount of active radical species is reasonably high, which explains the effectiveness of plasma in bacterial inactivation and tissue treatment.

Observations of long-lived H−2 and D−2 ions from non-thermal plasmas

Strong mass signals of H−2 and D−2 ions have been observed from low-pressure dielectric barrier discharge hydrogen and deuterium plasmas via molecular beam mass spectrometry. The observed H−2/H− and D−2/D− ratios (∼0.35–0.4) are over five orders of magnitude higher than those observed by other techniques. The kinetic energy of H−2 and D−2 ions sampled from the plasmas was determined to be widely distributed, from a few eV to >100 eV, giving lifetimes greater than ∼40 µs for H−2 and ∼55 µs for D−2. The highest vib-rotational excitation of neutral H2 species in the plasma was determined to be about J = 0, v = 5 or J = 19, v = 0 via threshold ionization mass spectrometry. The possible pumping mechanisms for generating H−2 with further high J, required by the current high-rotation model, have been proposed. Similar to the lifetime of D−2 determined recently by another group, the H−2 lifetime observed in this work is about two orders of magnitude longer than that predicted by the current theoretical model. To explain these experimental observations regarding the meta-stability of long-lived H−2 and D−2 ions, the improved current high-rotation model or other new models, including the possible existence of some long-lived electronically excited states of H−2/D−2, need to be developed.

N, NH, and NH2 radical densities in a remote Ar–NH3–SiH4 plasma and their role in silicon nitride deposition

The densities of N, NH, and NH2 radicals in a remote Ar–NH3–SiH4 plasma used for high-rate silicon nitride deposition were investigated for different gas mixtures and plasma settings using cavity ringdown absorption spectroscopy and threshold ionization mass spectrometry. For typical deposition conditions, the N, NH, and NH2 radical densities are on the order of 1012cm−3 and the trends with NH3 flow, SiH4 flow, and plasma source current are reported. We present a feasible reaction pathway for the production and loss of the NHx radicals that is consistent with the experimental results. Furthermore, mass spectrometry revealed that the consumption of NH3 was typically 40%, while it was over 80% for SiH4. On the basis of the measured N densities we deduced the recombination and sticking coefficient for N radicals on a silicon nitride film. Using this sticking coefficient and reported surface reaction probabilities of NH and NH2 radicals, we conclude that N and NH2 radicals are mainly responsible for the N incorporation in the silicon nitride film, while Si atoms are most likely brought to the surface in the form of SiHx radicals.

Downstream ion and radical densities in an Ar–NH3 plasma generated by the expanding thermal plasma technique

A full characterization of the Ar–NH3 expanding thermal plasma source applied in industrial processes for high-rate silicon nitride deposition is presented in terms of absolute densities of reactive species such as ions and radicals. The ion composition of the plasma was identified by mass spectrometry, while absolute ion density information was obtained by Langmuir probe measurements. N radicals were detected by threshold ionization mass spectrometry, whereas NH and NH2 radicals were measured by cavity ringdown spectroscopy. It was found that the ion density decreases from 1013 cm−3 in a pure Ar plasma to 1010–1011 cm−3 when NH3 is injected, with ArH+, , , and being the most abundant ions. The densities of N and NH both saturate at a level of 3 × 1012 cm−3 at NH3 flows above 3 sccs while the density of NH2 increases linearly with the NH3 flow and reaches a level of 4 × 1012 cm−3. When the plasma source current is increased, the densities of N and NH increase linearly, while the NH2 density remains approximately constant. Furthermore, it is revealed that most of the consumed NH3 is converted into N2 and H2 in the plasma.

On the hexamethyldisiloxane dissociation paths in a remote Ar-fed expanding thermal plasma

In this paper we present primary hexamethyldisiloxane (HMDSO) molecule dissociation paths in a remote argon-fed expanding thermal plasma, where the HMDSO dissociation is initiated by charge exchange reactions with argon ions followed by dissociative recombination reactions with electrons. Investigation of the argon/HMDSO plasma chemistry by means of cavity ring down spectroscopy and threshold ionization mass spectrometry has allowed the detection and identification of radical species, such as CHx, SiCxHy and SiCHxO, and oligomerization products. The charge exchange reaction rate constant between argon ions and HMDSO molecules has been found to be equal to (4 ± 2) × 10−16 m3 s−1. This reaction has a dissociative character, i.e. the dissociation occurring at the Si–C and Si–O bonds accompanied by the abstraction of radicals, e.g. CH3 and OSi(CH3)3. Under the experimental conditions investigated, the ions produced via charge exchange reactions lead to negligible ion-induced oligomerization routes. Instead, they undergo recombination reactions with electrons, leading to Si–O bond dissociation and further abstraction of methyl radicals.

Detailed TIMS Study of Ar/C2H2 Expanding Thermal Plasma: Identification of a-C:H Film Growth Precursors

Acetylene chemistry is studied by means of threshold ionization mass spectrometry (TIMS) in remote Ar/C(2)H(2) expanding thermal plasma to identify the growth precursors of hydrogenated amorphous carbon (a-C:H) films. More than 20 hydrocarbon species are measured, enabling a comprehensive study of acetylene chemistry in the plasma environment. It is shown that the plasma composition is controlled by the initial ratio between the acetylene flow into the reactor and argon ion and electron fluence emanating from the remote plasma source. Complete decomposition of acetylene to C, CH, CH(2), C(2), and C(2)H radicals is achieved in subsequent charge transfer and dissociative recombination reactions under low acetylene flow conditions. The formation of soft polymer-like a-C:H films can be attributed to C, C(2), and also partially to CH and C(2)H deposition. At acetylene flows higher than argon ion and electron fluence, reactions of C, CH, C(2), and C(2)H radicals with acetylene lead to the formation of various hydrocarbon species, whose behavior is dependent on whether the number of carbon atoms is even or odd. The detected resonantly stabilized C(3), C(3)H, and probably also C(5) and C(5)H radicals are unreactive with acetylene in the gas phase and are, therefore, abundantly present close to the substrate. The C(3) radical has among them the highest density, and it is identified as the significant growth precursor of Ar/C(2)H(2) expanding thermal plasma deposited hard a-C:H films.

Threshold ionization mass spectrometry of reactive species in remote Ar∕C2H2 expanding thermal plasma

Triple stage threshold ionization mass spectrometry is successfully implemented in an Ar∕C2H2 expanding thermal plasma setup. More than 20 hydrocarbon radicals and molecules formed in the plasma, including for example the C2H, C3, C3H, and C3H2 radicals or C3H4 or C4H2 molecules, are measured and their absolute densities are determined. Thanks to a careful design, a high sensitivity of the instrument is achieved and species with densities as low as 1×1016m−3 can be detected. Issues related to the absolute density calibration procedure are considered. The proper determination of the background signal by means of a beam chopper and the influence of the chopper on the measurement are discussed and the possible composition distortion of the sampled beam due to the collisions in the sampling orifice is checked. Furthermore, reported values of electron impact ionization cross sections of hydrocarbon species are compared and, based on their similarity in the near threshold energy region, the C2H2 electron impact ionization cross section is proposed as a reasonable approximation for other hydrocarbon radicals. The results for C, CH, and C2 radicals are compared with previous cavity ringdown absorption spectroscopy measurements and a good agreement is obtained.

Analysis and Kinetics of Transient Species in Electrode Near Plasma and Plasma Boundary Sheath of RF Plasmas in Molecular Gases

AbstractModern diagnostic methods are applied for investigation of spatial and temporal behaviour of selected reactive species in oxygen and fluorocarbon rf plasmas. Comprehensive investigations of the spatial distribution of atomic oxygen were done by means of two‐photon absorption laser induced fluorescence spectroscopy. Exemplarily, axial and radial O‐concentrations are shown for an asymmetric capacitively coupled rf plasma in pure oxygen. The time resolved threshold ionization mass spectrometry was applied for investigation of small fluorocarbon radicals. In pulsed CF4/H2 rf plasma the concentrations of the extracted radicals CF2, CF3, C2F5 as well as the intermediate product C2F4 in the plasma on and off phase are presented. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Threshold ionization mass spectrometry study of singlet molecular oxygen in the deposition of SnO2 by PACVD

Threshold ionization mass spectrometry (TIMS) has been used to measure the excited molecular oxygen states O2 (1Δg) and during plasma-assisted chemical vapour deposition of tin oxide (SnO2) thin films. The latter, composed of nanosized features, was deposited by feeding in a mixture of Ar, O2 and tetramethyltin (Sn(CH3)4 or TMT) in a capacitively coupled RF discharge reactor. Langmuir probe measurements were performed along with TIMS to measure the electron temperature and density. The correlations between these two diagnostic methods have been investigated. The observed densities of O2 (1Δg) and in the γ mode of the discharge are maximum at a low electron temperature and high density. Furthermore, these results have been shown to be correlated to the trend of the electronic conductivity of the deposited SnO2 thin films.

Determination of atomic hydrogen in non‐thermal hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry

Atomic hydrogen plays important roles in chemical vapor deposition of functional materials, plasma etching and new approaches to chemical synthesis of hydrogen-containing compounds. The present work reports experimental determinations of atomic hydrogen near the grounded electrode in medium-pressure dielectric barrier discharge hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry (MB-TIMS). At certain discharge conditions (a.c. frequency of 24 kHz, 28 kV of peak-to-peak voltage), the measured hydrogen dissociation fraction is decreased from approximately 0.83% to approximately 0.14% as the hydrogen pressure increases from 2.0 to 14.0 Torr. A simulation method for extraction of the approximate electron beam energy distribution function in the mass spectrometer ionizer and a semi-quantitative approach to calibrate the mass discrimination effect caused by the supersonic beam formation and the mass spectrometer measurement are reported.