Quantitative depth profile analysis of metallic coatings by pulsed radiofrequency glow discharge optical emission spectrometry

In recent years particular effort is being devoted towards the development of pulsed GDs because this powering operation mode could offer important analytical advantages. However, the capabilities of radiofrequency (rf) powered glow discharge (GD) in pulsed mode coupled to optical emission spectrometry (OES) for real depth profile quantification has not been demonstrated yet. Therefore, the first part of this work is focussed on assessing the expected advantages of the pulsed GD mode, in comparison with its continuous mode counterpart, in terms of analytical emission intensities and emission yield parameters. Then, the capability of pulsed rf-GD-OES for determination of thickness and compositional depth profiles is demonstrated by resorting to a simple multi-matrix calibration procedure. A rf forward power of 50 W, a pressure of 600 Pa, 1000 Hz pulse frequency and 50% duty cycle were selected. The quantification procedure used was validated by analysing conductive layers of thicknesses ranging from a few tens of nanometer up to about 20 μm and varied compositions (hot-dipped zinc, galvanneal, back contact of thin film photovoltaic solar cells and tinplates).     

Endogenous and exogenous hydrogen influence on amorphous silicon thin films analysis by pulsed radiofrequency glow discharge optical emission spectrometry

During the last decade the photovoltaic industry has been growing rapidly. One major strategy to reduce the production costs is the use of thin film solar cells based on hydrogenated amorphous silicon (a-Si:H). The potential of pulsed radiofrequency glow discharge coupled to optical emission spectrometry (rf-PGD-OES) for the analysis of such type of materials has been investigated in this work. It is known that when hydrogen is present in the argon discharge, even in small quantities, significant changes can occur in the emission intensities and sputtering rates measured. Therefore, a critical comparison has been carried out by rf-PGD-OES, in terms of emission intensities, penetration rates and depth resolution for two modes of hydrogen introduction in the discharge, manually external hydrogen in gaseous form (0.2% H₂–Ar) or internal hydrogen, sputtered as a sample constituent. First, a comparative optimisation study (at 600 Pa and 50 W) was performed on conducting materials and on a silicon wafer varying the pulse parameters: pulse frequency (500 Hz–20 kHz) and duty cycle (12.5–50%). Finally, 600 Pa, 50 W, 10 kHz and 25% duty cycle were selected as the optimum conditions to analyse three types of hydrogenated samples: an intrinsic, a B-doped and a P-doped layer based on a-Si:H. Enhanced emission intensities have been measured for most elements in the presence of hydrogen (especially for silicon) despite the observed reduced sputtering rate. The influence of externally added hydrogen and that of hydrogen sputtered as sample constituent from the analysed samples has been evaluated.     

Time- and wavelength-resolved emission line profiles for pulsed Cu and Ag hollow cathode lamps

Time-resolved, line emission wavelength profiles were obtained for a copper and a silver pulsed hollow cathode lamp with the use of a piezoelectrically driven interferometer. A computer was used to control the timing of the sweep of the interferometer, the pulsing of the source, and the acquisition, averaging, sorting and display of data. Minor manipulation of the data stored in core facilitated the presentation of intensity vs wavelength emission profiles in a time-resolved form. Ten sequential line profiles spaced at 21 μsec time intervals were obtained.The copper hollow cathode lamp was driven at 100 Hz with pulse currents of up to 400 mA. Pulses lasted for up to 300 μsec. Line profiles changed dramatically during a pulse and showed extreme self-reversal of the resonance emission lines after 100 μsec. Variations of line profile with dc background current level and spatial position within the discharge were also investigated. Uncorrected line widths of each of the doublets of the Cu(I) 324.7 nm resonance line measured during the first 21 μsec of the discharge, where they were narrowest, ranged from 0.0012 to 0.0022 nm, including an instrumental broadening contribution estimated to be less than 0.0004 nm. The silver lamp showed extreme self-reversal even during the first 21 μsec.     

The vertical spatial characteristics of analyte emission in the inductively coupled plasma

Utilizing a photodiode array based spatial profiling spectrometer vertical spatial profiles of analyte emission have been measured for a large number of neutral atom and ion lines in the inductively coupled plasma. The lines can be subdivided into two basic categories. One group (called “ soft ” lines after Boumans) have spatial emission behaviour that is very dependent on power, aerosol flow and analyte excitation and ionization characteristics. The second group (called “ hard ” lines after Boumans) have spatial emission behaviour that is relatively insensitive to all of the above parameters. In all cases ion lines have hard line behaviour as do the more energetic atom lines. Under fixed ICP conditions all hard lines have their peak emission at essentially the same position in the discharge which is always higher in the discharge than that for soft lines. It is also shown that the spatial behaviour of soft lines can be directly correlated with normal temperature.     

Early stage emission spectroscopy study of metallic titanium plasma induced in air by femtosecond- and nanosecond-laser pulses

In this work the laser induced plasma obtained in air at atmospheric pressure by the interaction of a fs (femtosecond) or a ns (nanosecond) laser pulse with a metallic titanium target has been investigated by optical emission spectroscopy. The temporal evolution of plasma parameters such as electron number density and excitation temperature has been determined in order to highlight the processes involved when the emission spectra are acquired at short time delays from the ablating laser pulse. A survey of elementary processes implicated during plasma formation and expansion of ns- and fs-Laser Induced Plasma has been performed. Departures from equilibrium conditions are even discussed. The dynamic aspects corresponding to ns- and fs-LIP have been investigated by optical time of flight (TOF) and by fast emission imaging. The overall results have been used for clarifying the basic mechanisms occurring during plasma expansion due to either ns or fs laser source when experimental conditions usually used for laser-induced breakdown spectroscopy (LIBS) applications are employed.     

Spectral,spatial and temporal characterization of a millisecond pulsed glow discharge: copper analyte emission and ionization

Two-dimensional maps of the spatial distributions of excited and ionized sputtered copper atoms are presented for a millisecond pulsed argon glow discharge. These maps demonstrate the temporal as well as spatial dependence of different excitation and ionization processes over the pulse cycle. Transitions from the low energy electronic states for the atom, characterized by emission such as that at 324.75 nm (3.82→0.00 eV), dominate the plateau time regime at a distance of 2.5 mm from the cathode surface. These processes originate from the electron excitation of ground state copper atoms. Transitions from high-energy electronic states, such as that characterized by emission at 368.74 nm (7.16→3.82 eV), predominate during the afterpeak time regime at a distance of 5.0–6.0 mm from the cathode surface. This observation is consistent with the relaxation of highly excited copper atoms produced by electron recombination with copper ions during the afterpeak time regime. Analyses of afterpeak and plateau intensities for a series of copper emission lines indicate an electron excitation temperature equivalent to 5.78 eV at 0.8 torr and 1.5 W. Temporal profiles exhibit copper ion emission only during the plateau time regime.     

Hydrogen effects on copper,zinc and nickel atomic emission lines in argon radiofrequency glow discharge optical emission spectrometry

The effect of hydrogen (0.5%, 1% and 10% v/v) added to the argon plasma gas on the emission spectra of selected atomic lines for copper, zinc and nickel has been studied by radiofrequency glow discharge optical emission spectrometry (rf-GD-OES). Conductive homogeneous samples containing different concentrations of the elements under study in different matrices have been investigated. Results show different trends of the emission intensity lines with increasing hydrogen concentration in the rf-GD, depending on the line characteristics. In most cases, the emission yields of the lines under study did not change or increased when hydrogen was added to the discharge (no decreases were observed). The emission yields of certain lines showed much higher increases than other lines of the same element (for example, lines 213.86 nm of Zn and 231.10 nm of Ni). Our experiments indicate that such notorious increases could be related with the possible decrease of the self-absorption when hydrogen is added to the discharge. Overall, the results obtained for the emission yield changes of certain lines of a given element in different matrices (with different analyte content) showed that while for resonance emission lines very notorious increases are observed, the values for non-resonance lines do not change significantly (specially if the matrices employed are similar).     

The impact of molecular emission in compositional depth profiling using Glow Discharge-Optical Emission Spectroscopy

The scope of this paper is to investigate and discuss how molecular emission can affect elemental analysis in glow discharge optical emission (GD-OES), particularly in compositional depth profiling (CDP) applications. Older work on molecular emission in glow discharges is briefly reviewed, and the nature of molecular emission spectra described. Work on the influence of hydrogen in the plasma, in particular elevated background due to a continuum spectrum, is discussed. More recent work from sputtering of polymers and other materials with a large content of light elements in a Grimm type source is reviewed, where substantial emission has been observed from several light diatomic molecules (CO, CH, OH, NH, C₂). It is discussed how the elevated backgrounds from such molecular emission can lead to significant analytical errors in the form of “false” depth profile signals of several atomic analytical lines. Results from a recent investigation of molecular emission spectra from mixed gases in a Grimm type glow discharge are presented. An important observation is that dissociation and subsequent recombination processes occur, leading to formation of molecular species not present in the original plasma gas. Experimental work on depth profiling of a polymer coating and a thin silicate film, using a spectrometer equipped with channels for molecular emission lines, is presented. The results confirm that molecular emission gives rise to apparent depth profiles of elements not present in the sample. The possibilities to make adequate corrections for such molecular emission in CDP of organic coatings and very thin films are discussed.     

A pulsed arc-glow hollow cathode lamp

A copper pulsed arc-glow discharge has been designed, and time-resolved Fabry-Perot emission line profiles have been obtained for the Cu(I) 324.7 nm doublet. Results when compared to a standard copper hollow cathode lamp operated under the same pulsed conditions showed that the arc-glow lamp gave 17 per cent greater time- and wavelength-integrated intensity, and a line profile that was not as severely self-reversed. Total radiant power emitted by the arc-glow lamp was less because of the reduced size of the emission region. Line widths during the first 21 μsec of a pulse were in the range 0.0012–0.0025 nm including an instrumental broadening contribution estimated to be less than 0.0004 nm. Argon pressures for the discharge ranged from 15 to 26 Torr. Pulse durations of 300 μsec with a repetition rate of 100 Hz and pulse currents up to 300 mA were used.     

Parametric studies of emission from a plasma gun source for atomic spectroscopy

A detailed parametric study of the effects of electrical discharge parameters on the spatially and temporally resolved emission from the expelled plume of a plasma gun atomic emission source was performed. Expulsion properties were found to depend on the dI/dt of the discharge current. Low-inductance discharges produced plasmas with propagation velocities (about 1.2 km s⁻¹) which were independent of discharge energy and probably controlled by the properties of the support gas in the discharge chamber. Added-inductance discharges produced plumes with significantly lower propagation velocities and less efficient atomization and expulsion of analyte. Continuum background emission was attributable to two processes, depending upon the spatial location in the plasma: decaying background from the rapidly cooling expelled plasma (lower regions) and collision of the initially-expelled plasma with some of the continuously-expelled plasma gun vapor (upper regions). The line-to-background ratio is optimized when using the highest-energy, low-inductance discharges coupled with integration of emission only from the upper regions of the plume (>10 mm) after the first emission pulse (> 75 μs).     

Microsecond-pulse glow discharge atomic emission

A microsecond pulsed glow discharge was produced with high pulse magnitude and small duty cycle. Time resolved emission and absorption spectroscopy was applied to study the processes of atomization, excitation and ionization in the glow discharge. Experimental results show that, without overheating the sample, the emission peak intensity is several orders greater than that obtained in the conventional dc mode. This implies that a much more intense plasma is generated during pulsed "on" region.     

Depth profiles of ceramic tiles by using orthogonal double‐pulse laser induced breakdown spectrometry

The potential of a double pulse (DP) excitation scheme for in‐depth characterization of ceramic samples using laser induced breakdown spectrometry (LIBS) has been demonstrated. For this purpose, two Q‐switched Nd:YAG lasers in orthogonal configuration were employed, the first one to ablate the sample (1064 nm) and the second one (532 nm) to excite the ablated material. Light emission was collected by a spectrograph and detected by an intensified charge‐coupled device (CCD) detector. Optimal conditions such as relative laser beam positions, laser pulse energies, inter‐pulse separation and CCD delay time were studied. Depth profiles were evaluated on the basis of various elemental compositions in both layers of ceramic samples. The depth resolution with DP configuration was improved by almost twofold as compared to the single‐pulse approach. The reproducibility of the depth profiles is also twice better with double pulse LIBS. Copyright © 2009 John Wiley & Sons, Ltd.     

Feasibility of depth profiling of Zn-based coatings by laser ablation inductively coupled plasma optical emission and mass spectrometry using infrared Nd:YAG and ArF* lasers

The feasibility of depth profiling of zinc-coated iron sheets by laser ablation (LA) was studied using an Nd:YAG laser (1064 nm) with inductively coupled plasma optical emission spectrometry (ICP-OES), and an excimer ArF* laser (193 nm) with a beam homogenizer. The latter was coupled to an ICP with mass spectrometry (ICP-MS). Fixed-spot ablation was applied. Both LA systems were capable of providing depth profiles that approach the profiles obtained by glow discharge optical emission spectroscopy (GD-OES) and electron probe X-ray microanalysis (EPXMA). For Nd:YAG laser an artefact consisting of zinc depth profile signal tailing appeared, enlarging thus erroneously diffusional coating–substrate interface profile. However, the ArF* system partially reduced but not suppressed that phenomenon. For both LA systems the Fe signal from the substrate increased with depth as expected and reached a plateau. The depth resolution (depth range corresponding to 84%–16% change in the full signal) achieved was several micrometers. Ablation rate was found to depend on ablation spot area at constant irradiance. Consequently, ablated volume per shot dependence on pulse energy exhibits deviation from linear course.     

Surface elemental mapping using glow discharge—optical emission spectrometry

A glow discharge optical-emission spectrometry source was evaluated for use in imaging elemental surface distributions. A 1.8 cm² area of a nickel–chromium alloy was sampled and copper emission was observed directly above the surface of a copper inclusion. By pulsing the glow discharge, the resolution was improved greatly over measurements using direct-current powering. Ranges of gas flows, pulse frequencies, pulse potentials, pulse widths, and pressures were explored to determine their effects on spatial resolution and were related to atom transport in the glow discharge cell. Pressure, pulse width, and pulse frequency were all found to have a significant effect on resolution.     

Optical emission spectroscopy of nitrogen species and plasma plume induced by laser ablation combined with pulse modulated radio-frequency discharge

Combined laser ablation and pulse modulated radio-frequency (RF) discharge for deposition of CNalpha films was studied by the use of optical emission spectroscopy. Chemically active nitrogen was produced by RF discharge, concentrated between two small electrodes. Influence of RF power, nitrogen pressure, modulation frequency and pulse rate on nitrogen species production was researched. For the same system plasma expansion rate, kinetic energy and concentration of carbon ions emitted by laser from graphite target were determined by Langmuir probes measurement.     

CN emission spectroscopy study of carbon plasma in nitrogen environment

Spectroscopic emission diagnostics of a carbon plasma created by an excimer KrF laser pulse at three laser fluences (12, 25 and 32 J/cm²) is performed under nitrogen ambient at pressures of 0.5 and 1 mbar. By following the time evolution of the radical CN spectral emission profiles, we notice, at a certain distance from the target surface, the existence of twin peaks for the time of flight distribution. This double structure depends on laser fluence and gas pressure parameters. The first peak moves forward in relation with the plasma expansion whereas the second peak moves backward and it is attributed to CN species undergoing oscillations or reflected shocks.     

Self-absorption and Doppler temperatures of emission lines excited in a glow discharge lamp

The shapes of the emission lines of calcium and chromium emanating from a Grimm type glow discharge lamp have been determined by use of a pressure-scanning Fabry—Perot interferometer using various excitation conditions for different concentrations of these metals in standard matrices. Assuming a certain model for the light source, theoretical line profiles were calculated.The Doppler temperature as a function of current at constant voltage was determined from a comparison of the experimental and theoretical profiles. Also determined was the degree of absorption in emitting and non-emitting layers of the model.It was found that considerable self-absorption and reversal occur in the lamp at higher currents and concentrations.     

Instrument-induced effects in the analysis of polycyclic aromatic compounds by capillary gas chromatography with atomic emission detection (GC-AED)

Peak splitting of high molecular weight polycyclic aromatic compounds originating from the microwave plasma of an atomic emission detector (AED) coupled to a GC has been described and evaluated. The influence of the solute structure, solute concentration, and physical conditions in the AED (such as detector temperature, make-up gas flow, concentration of reagent gases and distance of column end from the plasma) have been studied. An explanation is presented for peak splitting, which is based on an insufficient solute decomposition and solute mass flow in the discharge tube. Modification of the instrument by introduction of additional make-up gas applied through the transfer line has been shown to improve peak shape and solute response.     

Effects of matrix on spatial profiles of emission from an inductively coupled plasma

Effects of easily ionizable elements and other concomitants on spatial profiles of analyte emission were studied in an inductively coupled plasma. The measuring system consists of a self-scanning linear photodiode array mounted vertically in the focal plane of a monochromator and a microcomputer for data acquisition and processing. Easily ionizable elements had complex effects on spatial profiles of analyte emission depending on the excitation characteristics of the spectral lines and the operating parameters of the plasma. On the other hand, sulfuric acid simply depressed the emission over the whole observation height independent of the operating parameters. These data are useful for a better understanding of the complex interference effects found in optimisation experiments and may add to an understanding of the excitation mechanisms responsible for them.     

Microwave plasma in argon produced by a surface wave: study of the effect of pressure on the optical emission and the potentials for analysis of gaseous samples

The possibility of chemical analysis of gaseous samples by optical emission spectroscopy has been evaluated using a microwave induced plasma created by a surface wave at 210 MHz. Methane has been introduced at low concentration (1–20 ppm) in argon gas. The emission of excited CH, CN, C₂ at atmospheric pressure, was observed along the discharge and studied as a function of the methane concentration. The influence of the pressure on CH emission is presented from 10 Torr to atmospheric pressure. Contrary to usual predictions, the emission of CH bands is maximum at about 100 Torr and not at atmospheric pressure.