Pulsed Radiofrequency Glow Discharge Research Articles

Combined Pulsed RF GD-OES and HAXPES for Quantified Depth Profiling through Coatings

Chemical characterization at buried interfaces is a real challenge, as the physico-chemical processes operating at the interface govern the properties of many systems and devices. We have developed a methodology based on the combined use of pulsed RF GD-OES (pulsed Radio Frequency Glow Discharge Optical Emission Spectrometry) and XPS (X-ray Photoelectron Spectroscopy) to facilitate the access to deeply buried locations (taking advantage of the high profiling rate of the GD-OES) and perform an accurate chemical diagnosis using XPS directly inside the GD crater. The reliability of the chemical information is, however, influenced by a perturbed layer present at the surface of the crater, hindering traditional XPS examination due to a relatively short sampling depth. Sampling below the perturbed layer may, however, can be achieved using a higher energy excitation source with an increased sampling depth, and is enabled here by a new laboratory-based HAXPES (Hard X-ray PhotoElectron Spectroscopy) (Ga-Kα, 9.25 keV). This new approach combining HAXPES with pulsed RF GD-OES requires benchmarking and is here demonstrated and evaluated on InP. The perturbed depth is estimated and the consistency of the chemical information measured is demonstrated, offering a new route for advanced chemical depth profiling through coatings and heterostructures.

Aluminum coating of twinning-induced plasticity plates via plasma-enhanced magnetron sputtering as a potential industrial technique

Challenges are encountered in the use of the conventional hot-dip galvanization process to deposit a protective layer on the surface of twinning-induced plasticity (TWIP) steel due to the low wettability, which is caused by manganese oxidation on the surface of the steel. In a vacuum environment and after conducting etching on the surface to remove surface oxides, corrosion-resistant aluminum (Al) coatings were prepared on the surface of TWIP steel via the hot-wire plasma-enhanced magnetron sputtering (PEMS) technique under various target and wire discharge currents (Ihot-wires/Itarget). The substrate ion current density at −50 V was as high as 0.88 mA/cm2 under a high hot wire discharge current of 20 A and a low target current of 3 A (20 A/3 A), which was 4 times the ion current under Ihot-wires/Itarget of 5 A/3 A, respectively. According to an analysis of the coatings via pulsed radio frequency glow discharge optical emission spectroscopy, Al, Fe and Mn elements diffused at the interface between the coating and steel substrate. The diffusion layers on both sides of the interface resulted in the Al coating remaining well adhered to the surface of the TWIP steel after Rockwell indentation, scratch testing and Erichsen cupping tests. As the sacrificial and passivated anode, the self-corrosion potential of the dense Al coating increased from −0.770 V of the TWIP steel to −0.471 V, and the corrosion current decreased from 3.91 × 10−4 A/cm2 to 6.14 × 10−5 A/cm2. The dense coating remained undamaged after electrochemical corrosion. Additionally, the structure of the TWIP steel was not affected by the PEMS coating processes.

Pulsed radiofrequency glow discharge time-of-flight mass spectrometry: Depth profile analysis of multilayers on conductive and non-conductive substrates

Combination of a pulsed glow discharge (PGD) source with time-of-flight mass spectrometry (TOFMS) enables for fast multielemental transient signals detection of solid samples. Such instrument is ideal for depth profiling of layered materials with depth resolution down to the low nm range. In this work, depth profile capabilities of radiofrequency (rf) PGD-TOFMS system have been investigated when dealing with thin layers (GexNi100-x/Ni/NdyNi100-y with total thicknesses between 15 nm and 75 nm) deposited on two substrates with different electrical properties (non-conductive glasses and silicon wafers). Attention has been paid towards the selection of the different PGD operational parameters aiming at achieving best instrument performance for each substrate in terms of depth resolution of the thin coating layers. Particularly, the applied pulse width has proven to have a direct influence in the temporal regions (prepeak, plateau and afterglow) responsible of the analytical signals detected by TOFMS. Experimental results showed that shorter pulses (0.16 ms was finally selected) and periods (1.32 ms) allowed for better depth profile resolution of the layers deposited onto the glass substrate. However, such selection for the nonconductive substrate hindered obtaining analytical information from the prepeak and plateau regions. On the other hand, the effect of pulse width on depth resolution was not so critical when coatings were deposited onto the silicon wafer.

Rapid evaluation of different perovskite absorber layers through the application of depth profile analysis using glow discharge – Time of flight mass spectrometry

Depth profile analysis of perovskite absorber layers deposited onto glass substrates is investigated by radiofrequency pulsed glow discharge - time of flight mass spectrometry (rf-PGD-ToFMS). Elemental depth profiles obtained for perovskite films fabricated using a double-step deposition route with different precursors (methylammonium iodide and PbI2, PbCl2 or PbBr2) show varying distribution of the principle components depending on the precursors employed. Furthermore, the results show that rf-PGD-ToFMS allows to identify traces of residue solvent used in the initial film preparation (dimethyl sulphoxide or dimethylformamide) and to identify differences produced by film thickness and oxygen uptake caused by exposure to ambient conditions. The approach also enables inspection of the differences in elemental diffusion and the degradation processes. By using rf-PGD-ToFMS, no ultra-high-vacuum is needed for processing and rapid analysis of absorber films can be obtained in less than 40 s. The demonstration of such powerful analytical technique for obtaining depth profile information could enable groups in the field to better optimize processing conditions and enhance stability.

A novel gas sampling introduction interface for fast analysis of volatile organic compounds using radiofrequency pulsed glow discharge time of flight mass spectrometry

An improved gas sample introduction interface is developed and characterized for gas chromatography coupling and for direct injection of volatile organic compounds (VOCs), in a pulsed glow discharge (pulsed-GD) ion source coupled to a time of flight mass spectrometer (TOFMS) that is typically used for direct solid analysis. The novel interface allows the introduction of the analytes in the flowing afterglow region of the GD (a few mm away from the negative glow region) to reduce plasma quenching effects. Analyte ion signals are acquired in the temporal afterglow region, where low fragmentation of the molecular species is produced, providing useful qualitative and quantitative molecular information (e.g. molecular ion). Analytical capabilities of the pulsed-GD ion source with the novel gas sampling interface provides improved performance compared to previous designs. In particular, limits of detection for the analysis of VOCs in air were below (better) that legally established limits according to Directive 2008/50/EC of the European Parliament.

Comparison of coherent forward scattering and atomic absorption profiles of O I 844.6-nm and Ar I 842.5-nm lines in the pulsed and continuous radio-frequency glow-discharge plasmas

Comparison of the coherent forward scattering (CFS) and atomic absorption (AA) of the O I 844.6- nm and Ar I 842.5 -nm lines was performed in pulsed and continuous radio-frequency glow discharge (GD) plasma. Zeeman splitting of the conventional AA spectra and the self-absorption profile of CFS radiation could be observed for different magnetic fields, above ~50 and ~10 mT, respectively. The calculation of the axial variation of the magnetic field of the GD tube and the magnetic field determined by the measured Zeeman splitting allowed us to determine the locations of the highest and the second highest relative maximum of the number density of excited argon atoms in the lower state of the Ar I 842.5-nm transition.

Transient response of the emission signal controlled by pulsated bias current in pulsed radio-frequency glow discharge optical emission spectrometry

The objective of this paper is to suggest a new method for controlling temporal response of the emission signal from a pulsed radio-frequency (RF) glow discharge plasma for optical emission spectrometry. It is a noticeable feature that the emission profile accompanies a transient stage in the pulsed operation, appearing as a pre-peak which sometimes becomes 10–20 times as large as the plateau-stage intensity. Therefore, it is worth studying how the spike-like transient signal can be modified to a smooth response following the square-wave timing signal by using any experimental procedure. For this purpose, a phenomenon regarding self-bias voltage in RF plasma was focused on. The self-bias voltage is induced near the RF-loaded electrode, and then, a bias current can be introduced through an electric circuit including the plasma body by connecting an external electric device with the glow discharge plasma source. The bias current could be also pulsated, while it was synchronized with the pulsed RF plasma. Larger pulses of the bias current could change the characteristics of the plasma for atomic emission spectrometry, extending the size of the RF plasma as well as enhancing the emission intensity, which enabled the plateau stage to become more prominent in the pulsed emission profile. Several experimental parameters, such as an amplitude of the bias current, duty ratios of both the RF power and the bias current, and a phase shift between them, were investigated to obtain a plateau-like transient response of the emission signal. It was found that the phase-shift angle should be regulated between timing pulses of the RF voltage and the bias current.

Technical note: Characterization of gold coated ceramics by radiofrequency pulsed glow discharge – time of flight mass spectrometry

Depth profile characterization of 6 mm thick industrial ceramics with a golden decorative layer is achieved by PP-TOFMS.

Emission characteristics of 6.78-MHz radio-frequency glow discharge plasma in a pulsed mode

This paper investigated Boltzmann plots for both atomic and ionic emission lines of iron in an argon glow discharge plasma driven by 6.78-MHz radio-frequency (RF) voltage in a pulsed operation, in order to discuss how the excitation/ionization process was affected by the pulsation. For this purpose, a pulse frequency as well as a duty ratio of the pulsed RF voltage was selected as the experimenter parameters. A Grimm-style radiation source was employed at a forward RF power of 70W and at an argon pressures of 670Pa. The Boltzmann plot for low-lying excited levels of iron atom was on a linear relationship, which was probably attributed to thermal collisions with ultimate electrons in the negative glow region; in this case, the excitation temperature was obtained in a narrow range of 3300–3400K, which was hardly affected by the duty ratio as well as the pulse frequency of the pulsed RF glow discharge plasma. This observation suggested that the RF plasma could be supported by a self-stabilized negative glow region, where the kinetic energy distribution of the electrons would be changed to a lesser extent. Additional non-thermal excitation processes, such as a Penning-type collision and a charge-transfer collision, led to deviations (overpopulation) of particular energy levels of iron atom or iron ion from the normal Boltzmann distribution. However, their contributions to the overall excitation/ionization were not altered so greatly, when the pulse frequency or the duty ratio was varied in the pulsed RF glow discharge plasma.

Characterization of thin film tandem solar cells by radiofrequency pulsed glow discharge – Time of flight mass spectrometry

Beside low production costs and the use of nontoxic and abundant raw materials, silicon based thin-film solar cells have the advantage to be built up as multi junction devices like tandem or triple junction solar cells. Silicon thin film modules made of tandem cells with hydrogenated amorphous silicon (a-Si:H) top cell and microcrystalline (μc) Si:H bottom cell are available on the market. In this work, the analytical potential of state-of-the art radiofrequency (rf) pulsed glow discharge (PGD) time of flight mass spectrometry (TOFMS) commercial instrumentation is investigated for depth profiling analysis of tandem-junctions solar cells on 2mm thick glass substrate with 1µm thick ZnO:Al. Depth profile characterization of two thin film tandem photovoltaic devices was compared using millisecond and sub-millisecond rf-PGD regimes, as well as the so-called “low mass mode” available in the commercial instrument used. Two procedures for sample preparation, namely using flat or rough cell substrates, were compared and the distribution of dopant elements (phosphorous, boron and germanium) was investigated in both cases. Experimental results obtained by rf-PGD-TOFMS as well as electrical measurements of the samples showed that a worse depth resolution of dopant elements in the silicon layers (e.g. distribution of boron in a thicker region that suggests a diffusion of this dopant in the coating of the sample) found using a rough sample substrate was related to a higher power conversion efficiency.

Analytical potential of rf-PGD-TOFMS for depth profiling of an oxidized thin film composite

The capabilities of radiofrequency pulsed glow discharge time of flight mass spectrometry (rf-pulsed-GD-TOFMS) for the analysis of thin film composite membranes have been investigated in this work.

Pulsed radiofrequency glow discharge time of flight mass spectrometry for coated glass analysis

The potential of rf-PGD-ToFMS for quantitative depth profiling analysis of thin layers on glasses has been investigated.

Depth profile analysis of amorphous silicon thin film solar cells by pulsed radiofrequency glow discharge time of flight mass spectrometry.

Among the different solar cell technologies, amorphous silicon (a-Si:H) thin film solar cells (TFSCs) are today very promising and, so, TFSCs analytical characterization for quality control issues is increasingly demanding. In this line, depth profile analysis of a-Si:H TFSCs on steel substrate has been investigated by using pulsed radiofrequency glow discharge-time of flight mass spectrometry (rf-PGD-TOFMS). First, to discriminate potential polyatomic interferences for several analytes (e.g., (28)Si(+), (31)P(+), and (16)O(+)) appropriate time positions along the GD pulse profile were selected. A multi-matrix calibration approach, using homogeneous certified reference materials without hydrogen as well as coated laboratory-made standards containing hydrogen, was employed for the methodological calibration. Different calibration strategies (in terms of time interval selection on the pulse profile within the afterglow region) have been compared, searching for optimal calibration graphs correlation. Results showed that reliable and fast quantitative depth profile analysis of a-Si:H TFSCs by rf-PGD-TOFMS can be achieved.

Improving pulsed radiofrequency glow discharge for time-of-flight mass spectrometry simultaneous elemental and molecular analysis

The performance of radiofrequency (rf) millisecond pulsed glow discharge (PGD) coupled to a fast orthogonal time-of-flight mass spectrometer (TOFMS) for chemical characterization and quantification of organic volatile compounds was investigated by using two different GD chamber designs. The designs investigated had substantial differences in the way that the volatile organic compound is introduced into the GD and the distance between the cathode and the sampling cone of the mass spectrometer. Bromochloromethane was selected as the model analyte because of the practical interest of determining trihalomethanes at low concentrations, and also because of both its low boiling point (to avoid problems associated with condensations in the interface) and the fact that it has two different heteroatoms, making the fragmentation patterns easier to follow. Pulse shapes of element, fragment, and molecular parent ions obtained by using the two GD chambers under investigation were critically compared. Results revealed the critical effect of the GD chamber geometry in obtaining the three types of chemical information, temporally discriminated. The spectra of the gaseous samples and of a polymer containing TBBPA (solid sample) were also compared. Detection limits for bromochloromethane in the order of low ng L(-1), and the required high tolerance of the plasmas to the introduction of organic vapours, were achieved using one of the proposed GD designs. The capability of the designed system for the analysis of other volatile compounds, for example dimethyl disulfide and dimethyl selenide, was also successfully evaluated, making use of the analytical potential of the information obtained from the different pulse time regions.

Radiofrequency pulsed glow discharge-ToFMS depth profiling of a CdTe solar cell: A comparative study versus time of flight secondary ion mass spectrometry

In this manuscript, the authors compare and contrast depth profile data generated on the same commercially available CdTe cell using two analytical techniques, Time of flight secondary ion mass spectroscopy (ToF-SIMS), which is a well-established technique, and radiofrequency pulsed glow discharge (rf-PGD)-ToFMS, which is an emerging technique. The authors demonstrate that pulsed-rf-GD-ToFMS allows for a rapid analysis over a large (4 mm diameter) area of the sample at moderate vacuum conditions, while ToF-SIMS analyzes smaller regions (typically about 200 × 200 μm2) of the sample at ultrahigh vacuum conditions; the authors note that ToF-SIMS enables three dimensional analysis at micron or better lateral resolution. Both techniques show, in general, good agreement; however, some discrepancies are observed and discussed herein.

Isotope dilution mass spectrometry for quantitative elemental analysis of powdered samples by radiofrequency pulsed glow discharge time of flight mass spectrometry

In recent years particular effort is being devoted to the development of pulsed glow discharges (PGDs) for mass spectrometry because this powering operation mode could offer important ionization analytical advantages. However, the capabilities of radiofrequency (RF) PGD coupled to a time of flight mass spectrometry (ToFMS) for accurate isotope ratio measurements have not been demonstrated yet. This work is focused on investigating different time positions along the pulse profile for the accurate measurement of isotope ratios. As a result, a method has been developed for the direct and simultaneous multielement determination of trace elements in powdered geological samples by RF-PGD-ToFMS in combination with isotope dilution mass spectrometry (IDMS) as an absolute measurement method directly traceable to the International System of Units. Optimized operating conditions were 70W of applied radiofrequency power, 250Pa of pressure, 2ms of pulse width and 4ms of pulse period, being argon the plasma gas used. To homogeneously distribute the added isotopically-enriched standards, lithium borate fusion of powdered solid samples was used as sample preparation approach. In this way, Cu, Zn, Ba and Pb were successfully determined by RF-PGD-ToF(IDMS) in two NIST Standard Reference Materials (SRM 2586 and SRM 2780) representing two different matrices of geological interest (soil and rock samples). Cu, Zn, Ba and Pb concentrations determined by RF-PGD-ToF(IDMS) were well in agreement with the certified values at 95% confidence interval and precisions below 12% relative standard deviation were observed for three independent analyses. Elemental concentrations investigated were in the range of 81–5770mg/kg, demonstrating the potential of RF-PGD-ToF(IDMS) for a sensitive, accurate and robust analysis of powdered samples.

Sputter depth profiling of Mo/B4C/Si and Mo/Si multilayer nanostructures: A round-robin characterization by different techniques

A round-robin characterization is reported on the sputter depth profiling of [60×(3.0nm Mo/0.3nm B4C/3.7nm Si)] and [60×(3.5nm Mo/3.5nm Si)] stacks deposited on Si(111). Two different commercial secondary ion mass spectrometers with time-of-flight and magnetic-sector analyzers, a pulsed radio frequency glow discharge optical emission spectrometer, and a home-built time-of-flight low-energy ion scattering and quadrupole-based secondary ion mass spectrometer were used. The influence of the experimental conditions, especially the type and energy of sputter ions, on the depth profiles of Mo/Si nanostructures with and without B4C barrier layers is discussed in terms of depth resolution, modulation factor and rapidity of analysis. The pros and cons of each instrumental approach are summarized.

A path towards a better characterisation of silicon thin‐film solar cells: depth profile analysis by pulsed radiofrequency glow discharge optical emission spectrometry

ABSTRACTThe analytical potential of radiofrequency pulsed glow discharge optical emission spectrometry (rf‐PGD‐OES) is investigated for quantitative depth profiling analysis of thin‐film solar cells (TFSC) based on hydrogenated amorphous silicon (a‐Si:H). This method does not require sampling at ultra‐high‐vacuum conditions, and so it facilitates higher sample throughput than do reference techniques. In this paper, the determination of compositional depth profiles of a‐Si:H TFSC was performed by resorting to a multi‐matrix calibration procedure. For this purpose, certified reference materials, as well as laboratory standards based on individual layers of doped a‐Si:H, were simultaneously employed to build the analytical calibration curves. Results show that rf‐PGD‐OES allows us to discriminate the different layers of photovoltaic devices: the front contact composed by ZnO:Al2O3 (AZO), the a‐Si:H layer (the B‐doped, intrinsic a‐Si:H and P‐doped films), the AZO/Al back contact and substrate. A good agreement with the nominal values for element concentrations (e.g. 0.4% of H, 1.5% of B and 3.7% of P) and layer thicknesses (in the range of 950 nm for the front contact and 13 nm for the P‐doped a‐Si:H layer) was obtained, demonstrating the ability of rf‐PGD‐OES for a direct, sensitive and high‐depth‐resolution analysis of photovoltaic devices. Moreover, diffusion processes between the coating layers, which could have an important influence on the final efficiency of TFSC, can be identified as well. Hence, the findings support the use of rf‐PGD‐OES as an analysis method in the development of photovoltaic thin films. Copyright © 2013 John Wiley & Sons, Ltd.

Further insights into prepeak emission in pulsed radiofrequency glow discharge

Side-on optical emission measurements, with spatial and temporal resolution, have been carried out on a modified Grimm type pulsed rf glow discharge, using a copper sample and argon as discharge gas. In particular, the early ignition phase of the pulsed discharge, prepeak, and the beginning of the plateau have been electrically characterized and spectroscopically evaluated along the plasma plume for several argon and copper emission lines. Different pulse repetition frequencies have been used while keeping a constant duty cycle of 25%, covering a range from the μs to the ms pulsed GD. In all the cases a spatially extended discharge has been observed during the first 10–20μs of the pulse, when the dc bias voltage has not been fully established yet. During this early phase of the pulse, the species present in the chamber are excited following the amplitude of the voltage symmetrical oscillations, and argon and copper emission can be detected at all the distances evaluated along the plasma axis at delays as short as 4μs. After the first 20μs approximately, the discharge has contracted close to the sample surface, the dc bias voltage has been almost established and the sputtering process is taking place. Emission registered through side-on observation, at positions relatively far from the sample, decreases and then progressively increases to reach a plateau if the voltage pulse lasts long enough. Moreover, the further the detection position is from the sample surface, the later the increase of the copper and argon emission is observed. Therefore transport phenomena involving both argon and copper species are now evidently leading to an estimated average speed of 100m/s.Additionally, end-on optical emission measurements have been carried out to evaluate the prepeak emission from the negative glow region close to the sample surface, and then to compare this emission with that occurring at farther positions (side-on detection experiments). The results have shown that the basis of both prepeaks might be different as the prepeak signal measured using end-on detection peaks around 10μs later than that observed using side-on detection.

Critical evaluation of the potential of radiofrequency pulsed glow discharge–time-of-flight mass spectrometry for depth-profile analysis of innovative materials

The combination of radiofrequency pulsed glow discharge (RF-PGD) analytical plasmas with time-of-flight mass spectrometry (TOFMS) has promoted the applicability of this ion source to direct analysis of innovative materials. In this sense, this emerging technique enables multi-elemental depth profiling with high depth resolution and sensitivity, and simultaneous production of elemental, structural, and molecular information. The analytical potential and trends of this technique are critically presented, including comparison with other complementary and well-established techniques (e.g. SIMS, GD-OES, etc.). An overview of recent applications of RF-PGD-TOFMS is given, including analysis of nano-structured materials, coated-glasses, photovoltaic materials, and polymer coatings.