Vacuum Ultraviolet Emission Research Articles

Optical and scintillation properties of Nd differently doped YLiF4 from VUV to NIR wavelengths

Nd 0.1, 0.5, 1, and 3mol% doped YLiF4 crystalline scintillators were prepared by Tokuyama Corp. and their optical and scintillation properties from vacuum ultraviolet (VUV) to near infrared (NIR) wavelengths were investigated. 4f–5d absorption bands appeared in VUV and several absorption bands due to 4f–4f transition were observed in visible-NIR wavelengths. Nd3+ 5d–4f emission was observed around 180nm under 160nm excitation and intense line due to 4F3/2→4I11/2 at 1064nm appeared under 500nm excitation. In X-ray induced radioluminescence, both VUV and NIR emissions were observed. Scintillation decay times of VUV emission was 40–80ns while that of 4f–4f ones at visible wavelength were around 12–15μs. When 252Cf neutron was irradiated, VUV scintillation light yield resulted around 90 ph/n.

Temporally resolved electron density of a repetitive, nanosecond pulsed microdischarge

Using high speed spectroscopic diagnostics, temporally resolved optical emission spectroscopy is performed on a nanosecond, repetitively pulsed microdischarge. The microdischarge operates in an argon–hydrogen gas mixture (99%/1%) to provide a Lyman-α vacuum ultraviolet emission. Based on the Stark broadening of the 486.1 nm, Balmer-β hydrogen transition, the temporally resolved electron density was determined. Experimental electron density data are compared with the results of a 0D rate equation model. Peak electron density is estimated to be 5.6 · 1015 cm−3, corresponding to a ~0.25% degree of ionization. Using the approximate experimental ionization rate, the electron temperature is estimated to be ~3.5 eV.

System for time-discretized vacuum ultraviolet spectroscopy of spark breakdown in air.

A system for time-discretized spectroscopic measurements of the vacuum ultraviolet (VUV) emission from spark discharges in the 60-160 nm range has been developed for the study of early plasma-forming phenomena. The system induces a spark discharge in an environment close to atmospheric conditions created using a high speed puff value, but is otherwise kept at high vacuum to allow for the propagation of VUV light. Using a vertical slit placed 1.5 mm from the discharge the emission from a small cross section of the discharge is allowed to pass into the selection chamber consisting of a spherical grating, with 1200 grooves/mm, and an exit slit set to 100 μm. Following the exit slit is a photomultiplier tube with a sodium salicylate scintillator that is used for the time discretized measurement of the VUV signal with a temporal resolution limit of 10 ns. Results from discharges studied in dry air, Nitrogen, SF6, and Argon indicate the emission of light with wavelengths shorter than 120 nm where the photon energy begins to approach the regime of direct photoionization.

Pattern Roughness Mitigation of 22 nm Lines and Spaces: The Impact of a H2Plasma Treatment

As the semiconductor industry pursues Moore's law, the demand to obtain smaller features continues. Extreme ultraviolet (EUV) lithography remains one of the primary options for sub-20 nm patterns. However, the technology struggles to meet the line width roughness (LWR) specifications. In this article, we present the significance of plasma treatment as a roughness smoothing technique. Two EUV photoresists with 22 nm lines are exposed to various plasma processes. We highlight the advantages of a hydrogen plasma treatment and its vacuum ultraviolet (VUV) emission as an optimal smoothing process and discuss the effect of photoresist thickness, initial LWR and the VUV plasma emission. Even though a H2 plasma treatment results in a successful reduction of LWR/LER, the target value of below 2nm is not yet achieved.

(Invited) Ballistic Hot Electron Effects in Nanosilicon Dots and Their Photonic Applications

The efficient generation of ballistic hot electrons in nanosilicon dots under an electric field is based on the intrinsic carrier transport therein. Due to suppressed phonon scattering losses followed by enhanced tunneling rate, electrons become hot at the early stage of injection [1]. Generated ballistic hot electrons can be utilized either inside or outside the nanosilicon dot layer. The topics on this effect are discussed here toward possible photonic applications. The nanosilicon device is a kind of MIS diode consisting of a thin film surface electrode, a nanosilicon layer (~1 μm thick), a crystalline silicon wafer substrate, and a back contact. The nanosilicon layer includes parallel chains of quantum-sized nanosilicon dots (~3 nm in mean diameter) interconnected with tunnel oxides. When a positive voltage is applied to the surface electrode, electrons injected from the substrate into the nanosilicon layer are drifted toward the outer surface, and then some of them reach the surface electrode as ballistic hot electrons.One possible application of this effect is the avalanche photoconduction by sequential impact ionization in nanosilicon dots. Actually, the photo-carriers multiplication effect was observed in thin oxidized nanosilicon diodes under the reverse bias condition [2]. For a monochromatic near-uv light incidence at low temperatures (e.g., 77 K), the photoconduction quantum efficiency was increased up to 2500% at an external electric field of 9×105 V/cm. The experimental data on the temperature, electric field, and sample thickness dependencies of the photocurrent suggest that the field-induced avalanche multiplication of photo-excited carriers occurs inside the nanosilicon dot layer. From theoretical analyses of carrier dynamics [3], it has been clarified that the impact ionization rate is significantly enhanced in nanosilicon dots compared with in bulk silicon owing to the efficient transition between quantized discrete energy states and the enhanced Coulomb interaction in nanosilicon dots. The threshold electron energy of impact ionization is decreased in realistic nanosilicon dots with disordered surface structures [4]. These facts provide useful information for the development of efficient photo-sensors and photovoltaic cells.Ballistic hot electrons generated in the nanosilicon layer uniformly ejects to outside by tunneling through the thin surface electrode [5]. The most characteristic feature of this emission phenomenon is the high kinetic energy of electrons and its controllability. At applied voltages of 15-20 V, for instance, the mean energy of output electrons varies from 5 to 7 eV. No conventional cold cathodes can emit such energetic electrons. Being a supplier of electrons with an extremely high physical and electrochemical activity, this emitter is available for the operation in every media: vacuum, atmospheric gases, and solutions.In vacuum, this device is applicable to the exposure source for parallel EB lithography [6]. By integrating the emitter array with active-matrix CMOS driver circuit and combining with focusing electron optics system, alternative advanced nanofabrication technology would be developed. Another promising application in vacuum is a probing source for an extremely high-sensitivity image pick-up.The usefulness of the nanosilicon ballistic emitter in atmospheric gases has been demonstrated in air and in Xe gases as a negative ion source and a vacuum-ultraviolet (VUV) light generator, respectively. The former relates to the dissociative hot electron attachment on O2 molecules. In the latter, the output electron energy match well with the internal excitation of Xe gas molecules followed by VUV emission without any impact ionizations.This emitter acts even in solutions as an active electrode which supplies highly reducing electrons. Based on the experimental confirmation of the usefulness in aqueous solutions for H2 generation and pH control, it has been demonstrated that the operation of the emitter in metal-salt solutions is available for depositing thin metal (Cu, Ni, Co, Zn, and so on) films without using any counter electrodes.In contrast to the exchange of thermalized electrons, the ballistic electro-reduction scheme is effective in SiCl4 and GeCl4 solutions. When driven in these solutions, thin Si and Ge films are deposited uniformly on the emitting surface [7]. As in the case in metal-salt solutions, injected energetic electrons can directly reduce Si4+ and Ge4+ ions at the interface leading to the nuclei formation and the subsequent film growth with neither contaminations nor byproducts. Because of a large electrochemical window, it is difficult to get uniform thin Si and Ge films by electroplating. The deposition rate in this unilateral electro-reduction process is comparable to that in the conventional dry process. The structural analyses indicate that the deposited thin Si and Ge films are of amorphous. Visible photoluminescence was observed in ultra-thin Si films. In addition, it is possible to deposit thin SiGe films by the operation in mixture (SiCl4+GeCl4) solutions. Since the ballistic electro-reduction is an alternative clean approach, this is attractive for the low-temperature fabrication of thin-film and multilayered photoelectronic devices.This work was partially supported by the JSPS through a Grant-in-Aid for Scientific Research and the FIRST Program and by the NEDO through the R&D Project.

Vacuum ultra-violet emission of plasma discharges with high Xe partial pressure using a cathode protective layer with high secondary electron emission

In this work, the mechanism of the vacuum ultra-violet (VUV) emission of plasma discharges, with high Xe partial pressure and high ion-induced secondary electrons emission protective layer, is studied by measuring the VUV light emission directly and comparing it with two-dimensional simulations. From the panel measurement, we find that the high intensity of excimer VUV mainly contributes to the high luminous efficacy of SrCaO-plasma display panels (PDP) at a low sustain voltage. The unchanged Xe excitation efficiency indicates that the electron temperature is not decreased by the high secondary electrons emission protective layer, even though the sustain voltage is much lower. From the two-dimensional simulations, we can find that the ratio of excimer VUV to resonant VUV, which is determined by the collision rate in the discharge, is only significantly affected by the Xe partial pressure, while it is independent of the sustain voltage and the secondary-electrons-emission capability of protective layer. The unchanged average electron energy at the moment when the electric field becomes maximum confirms that the improvement of the VUV production efficiency mainly is attributed to the increase in electron heating efficiency of a PDP with high ion-induced secondary electrons emission protective layer. Combining the experimental and the simulation results, we conclude about the mechanism by which the VUV production is improved for the plasma display panel with a high Xe partial pressure and a cold cathode with high ion-induced secondary electrons emission.

Comparison of surface vacuum ultraviolet emissions with resonance level number densities. I. Argon plasmas

Vacuum ultraviolet (VUV) photons emitted from excited atomic states are ubiquitous in material processing plasmas. The highly energetic photons can induce surface damage by driving surface reactions, disordering surface regions, and affecting bonds in the bulk material. In argon plasmas, the VUV emissions are due to the decay of the 1s4 and 1s2 principal resonance levels with emission wavelengths of 104.8 and 106.7 nm, respectively. The authors have measured the number densities of atoms in the two resonance levels using both white light optical absorption spectroscopy and radiation-trapping induced changes in the 3p54p→3p54s branching fractions measured via visible/near-infrared optical emission spectroscopy in an argon inductively coupled plasma as a function of both pressure and power. An emission model that takes into account radiation trapping was used to calculate the VUV emission rate. The model results were compared to experimental measurements made with a National Institute of Standards and Technology-calibrated VUV photodiode. The photodiode and model results are in generally good accord and reveal a strong dependence on the neutral gas temperature.

Vacuum ultraviolet field emission lamp consisting of neodymium ion doped lutetium fluoride thin film as phosphor.

A vacuum ultraviolet (VUV) field emission lamp was developed by using a neodymium ion doped lutetium fluoride (Nd3+ : LuF3) thin film as solid-state phosphor and carbon nanofiber field electron emitters. The thin film was synthesized by pulsed laser deposition and incorporated into the lamp. The cathodoluminescence spectra of the lamp showed multiple emission peaks at 180, 225, and 255 nm. These emission spectra were in good agreement with the spectra reported for the Nd3+ : LuF3 crystal. Moreover, application of an acceleration voltage effectively increased the emission intensity. These results contribute to the performance enhancement of the lamp operating in the VUV region.

VACUUM ULTRAVIOLET EMISSION SPECTRUM MEASUREMENT OF A MICROWAVE-DISCHARGE HYDROGEN-FLOW LAMP IN SEVERAL CONFIGURATIONS: APPLICATION TO PHOTODESORPTION OF CO ICE

We report measurements of the vacuum ultraviolet (VUV) emission spectra of a microwave-discharge hydrogen-flow lamp (MDHL), a common tool in astrochemistry laboratories working on ice VUV photoprocessing. The MDHL provides hydrogen Ly-α (121.6 nm) and H2 molecular emission in the 110-180 nm range. We show that the spectral characteristics of the VUV light emitted in this range, in particular the relative proportion of Ly-α to molecular emission bands, strongly depend on the pressure of H2 inside the lamp, the lamp geometry (F type versus T type), the gas used (pure H2 versus H2 seeded in He), and the optical properties of the window used (MgF2 versus CaF2). These different configurations are used to study the VUV irradiation of CO ice at 14 K. In contrast to the majority of studies dedicated to the VUV irradiation of astrophysical ice analogs, which have not taken into consideration the emission spectrum of the MDHL, our results show that the processes induced by photons in CO ice from a broad energy range are different and more complex than the sum of individual processes induced by monochromatic sources spanning the same energy range, as a result of the existence of multistate electronic transitions and discrepancy in absorption cross sections between parent molecules and products in the Ly-α and H2 molecular emission ranges.

Low-k films modification under EUV and VUV radiation

Modification of ultra-low-k films by extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission with 13.5, 58.4, 106, 147 and 193 nm wavelengths and fluences up to 6 × 1018 photons cm−2 is studied experimentally and theoretically to reveal the damage mechanism and the most ‘damaging’ spectral region. Organosilicate glass (OSG) and organic low-k films with k-values of 1.8–2.5 and porosity of 24–51% are used in these experiments. The Si–CH3 bonds depletion is used as a criterion of VUV damage of OSG low-k films. It is shown that the low-k damage is described by two fundamental parameters: photoabsorption (PA) cross-section σPA and effective quantum yield φ of Si–CH3 photodissociation. The obtained σPA and φ values demonstrate that the effect of wavelength is defined by light absorption spectra, which in OSG materials is similar to fused silica. This is the reason why VUV light in the range of ∼58–106 nm having the highest PA cross-sections causes strong Si–CH3 depletion only in the top part of the films (∼50–100 nm). The deepest damage is observed after exposure to 147 nm VUV light since this emission is located at the edge of Si–O absorption, has the smallest PA cross-section and provides extensive Si–CH3 depletion over the whole film thickness. The effective quantum yield slowly increases with the increasing porosity but starts to grow quickly when the porosity exceeds the critical threshold located close to a porosity of ∼50%. The high degree of pore interconnectivity of these films allows easy movement of the detached methyl radicals. The obtained results have a fundamental character and can be used for prediction of ULK material damage under VUV light with different wavelengths.

An atmospheric pressure inductively coupled microplasma source of vacuum ultraviolet light

An inductively coupled microplasma source operating under atmospheric pressure was fabricated as a vacuum ultraviolet (VUV) light source. The microplasma source consisted of a quartz capillary tube (OD: Ø1.5mm, ID: Ø1mm) surrounded by a coil antenna. A floating wire electrode was set inside the tube to induce a high electrical potential. Under He gas flow (0.5slm), 100MHz VHF power was supplied to the coil antenna. The microplasma source easily achieved plasma ignition under ambient conditions. To extract the VUV emissions from the microplasma, an optical system was designed. VUV emissions of N, O, and H were obtained from the microplasma source operated at 10W of VHF power at 100MHz. The inductively coupled microplasma source operating at atmospheric pressure was further miniaturized using MEMS technology with the goal of developing an easily transportable device. The size of the fabricated microplasma chip was 20mm×3mm. The chip generated a point-like microplasma of Ø200μm at a VHF power of 24W. The point-like microplasma emitted clear spectral lines.

Features of VUV emission from plasmas generated by nanosecond laser pulses in air

Abstract Temporally-integrated characteristics of vacuum ultraviolet (VUV) plasma emission induced onto a steel target by 1064 nm laser radiation are investigated by spatially-resolved emission spectroscopy in air atmosphere. Emissions from the C (III) spectral line at 97.7 nm are recorded under the influence of several experimental parameters and conditions. These include laser wavelength, laser pulse energy, spatial distribution of emitting species as well as entrance slit-width of the spectrometric system. The overall objective of the work is to improve the detection capability of the laser-induced breakdown spectroscopy (LIBS) technique in the VUV regime. We also outline the role of LIBS in the field of elemental composition analysis of solid materials, and briefly describe its fundamental principles. The results obtained demonstrate a set of optimum conditions for maximum spectral line intensities and signal-to-background ratios of laser-generated plasmas in the VUV region. Moreover, in general, the emission characteristics of the vacuum ultraviolet-based spectra were similar to those previously investigated in the UV-visible spectral range.

The effect of preionization on the shock wave evolution in a fast cylindrical discharge

The present work is devoted to the study of the properties of the shock wave, appearing in a fast cylindrical discharge (dI/dt about 1012 A/s). The phenomenon under study is the influence of the preionization current (about 500 A) upon the evolution of the shock wave. The discharge was produced in an alumina tube with inner diameter 6 mm, filled with argon to pressures ranging from 80 to 320 Pa. The shock wave was detected from the vacuum ultra violet emission of the hot plasma behind the front, using a pin-hole camera and a microchannel plate detector with a gating time of 10 ns. The measured velocities of the shock wave front are in the range (1–4) × 106 cm/s. The main effect of the preionization is the earlier start of the shock wave from the inner surface of the alumina discharge tube and about 1.5 times increase of the velocity of the front. The time delay of the shock wave cumulation in the absence of the preionization results in the appearance of the eroded wall material in the plasma.

Inter-pulse delay optimization in dual-pulse laser induced breakdown vacuum ultraviolet spectroscopy of a steel sample in ambient gases at low pressure

Time-integrated spatially-resolved Laser Induced Breakdown Spectroscopy (LIBS) has been used to investigate spectral emissions from laser-induced plasmas generated on steel targets. Instead of detecting spectral lines in the visible/near ultraviolet (UV), as investigated in conventional LIBS, this work explored the use of spectral lines emitted by ions in the shorter wavelength vacuum ultraviolet (VUV) spectral region. Single-pulse (SP) and dual-pulse LIBS (DP-LIBS) experiments were performed on standardized steel samples. In the case of the double-pulse scheme, two synchronized lasers were used, an ablation laser (200mJ/15ns), and a reheating laser (665mJ/6ns) in a collinear beam geometry. Spatially resolved and temporally integrated laser induced plasma VUV emission in the DP scheme and its dependence on inter-pulse delay time were studied. The VUV spectral line intensities were found to be enhanced in the DP mode and were significantly affected by the inter-pulse delay time. Additionally, the influence of ambient conditions was investigated by employing low pressure nitrogen, argon or helium as buffer gases in the ablation chamber. The results clearly demonstrate the existence of a sharp ubiquitous emission intensity peak at 100ns and a wider peak, in the multi-microsecond range of inter-pulse time delay, dependent on the ambient gas conditions.

Vacuum ultraviolet excited luminescence properties of Ca3Gd7(SiO4)5(PO4)O2:Re3+ (Re3+=Tb3+, Dy3+) phosphors

A series of Ca3Gd7(SiO4)5(PO4)O2:Re3+ (Re3+=Tb3+, Dy3+) phosphors were synthesized by a solid-state reaction, and their vacuum ultraviolet excitation and emission characteristics were measured. All the excitation spectra show a broad band in the region of 140–210nm, which can be mainly assigned to the host absorption. For Tb3+-doped sample, the absorption bands at 230nm and 281nm are respectively ascribed to the f–d spin-allowed and spin-forbidden transitions of Tb3+. In Dy3+-doped sample, the f–d spin-allowed transitions of Dy3+ and O2−→Dy3+ charge transfer band have not been clearly distinguished probably because of the overlapping with the strong host absorption band. The weak bands at 267 and 288nm are attributed to be the f–d spin-forbidden transitions of Dy3+. The concentration dependence of the emission intensity upon the excitation at 172nm indicates that the optimal doping concentrations of Tb3+ and Dy3+ are 11mol% and 5mol%, respectively. The optimal Tb3+ and Dy3+-activated samples respectively exhibit yellowish green and white emitting colors due to their characteristic emissions.

Parallel transport studies of high-Z impurities in the core of Alcator C-Mod plasmas

Measurements of poloidal variation, ñz/⟨nz⟩, in high-Z impurity density have been made using photodiode arrays sensitive to vacuum ultraviolet and soft x-ray emission in Alcator C-Mod plasmas. In/out asymmetries in the range of −0.2<nz,cos/⟨nz⟩<0.3 are observed for r/a<0.8, and accumulation on both the high-field side, nz,cos<0, and low-field side, nz,cos>0, of a flux surface is found to be well described by a combination of centrifugal, poloidal electric field, and ion-impurity friction effects. Up/down asymmetries, −0.05<nz,sin/⟨nz⟩<0.10, are observed over 0.5<r/a<0.9 with nz,sin>0 corresponding to accumulation opposite the ion ∇B drift direction. Measurements of the up/down asymmetry of molybdenum are found to disagree with predictions from recent neoclassical theory in the trace limit, nzZ2/ni≪1. Non-trace levels of impurities are expected to modify the main-ion poloidal flow and thus change friction-driven impurity density asymmetries and impurity poloidal rotation, vθ,z. Artificially modifying main-ion flow in parallel transport simulations is shown to impact both ñz/⟨nz⟩ and vθ,z, but simultaneous agreement between measured and predicted up/down and in/out asymmetry as well as impurity poloidal rotation is not possible for these C-Mod data. This link between poloidal flow and poloidal impurity density variation outlines a more stringent test for parallel neoclassical transport theory than has previously been performed. Measurement and computational techniques specific to the study of poloidal impurity asymmetry physics are discussed as well.

Fine structure of the sources of quasi-periodic pulsations in “single-loop” solar flares

Analysis of the observational data obtained with a high angular resolution in the ranges of vacuum ultraviolet (1″, TRACE) and hard X-ray (4″, RHESSI) emissions in some solar flares previously considered “single-loop” ones shows that they are not such flares. The thick single loops with a diameter of 13″–21″ observed in these flares in the microwave range with an angular resolution of 5″–10″ (NoRH) are actually arcades of thinner loops with a diameter of less than 3″. In this case, the observed quasi-periodic pulsations of microwave emission are not a consequence of the oscillations of an isolated thick loop, as is usually assumed, but a result of the successive involvement of many relatively thinner loops in the process of flare energy release. The established facts impose significant constraints on the generation models of pulsations in flares.

Vacuum ultraviolet emission from microwave Ar-H2 plasmas

Vacuum ultraviolet emission from Ar-H2 wave driven microwave (2.45 GHz) plasmas operating at low pressures (0.1–1 mbar) has been investigated. The emitted spectra show the presence of the Ar resonance lines at 104.8 and 106.7 nm and of the Lyman-α,β atomic lines at 121.6 nm and 102.6 nm, respectively. The increase of the hydrogen amount in the mixture results in an abrupt increase of the Werner and Lyman molecular bands intensity. The Lyman-β intensity shows little changes in the range of 5%–30% of hydrogen in the mixture while the Lyman-α intensity tends to decrease as the percentage of hydrogen increases.

Modification of organosilicate glasses low-k films under extreme and vacuum ultraviolet radiation

Degradation of chemical composition of porous low-k films under extreme and various vacuum ultraviolet emissions is studied using specially developed sources. It is shown that the most significant damage is induced by Xe line emission (147 nm) in comparison with Ar (106 nm), He (58 nm), and Sn (13.5 nm) emissions. No direct damage was detected for 193 nm emission. Photoabsorption cross-sections and photodissociation quantum yields were derived for four films under study. 147 nm photons penetrate deeply into low-k films due to smaller photoabsorption cross-section and still have sufficient energy to excite Si-O-Si matrix and break Si-CH3 bonds.

A combined matrix isolation spectroscopy and cryosolid positron moderation apparatus

We describe the design, construction, and operation of a novel apparatus for investigating efficiency improvements in thin-film cryogenic solid positron moderators. We report results from solid neon, argon, krypton, and xenon positron moderators which illustrate the capabilities and limitations of our apparatus. We integrate a matrix isolation spectroscopy diagnostic within a reflection-geometry positron moderation system. We report the optical thickness, impurity content, and impurity trapping site structures within our moderators determined from infrared absorption spectra. We use a retarding potential analyzer to modulate the flow of slow positrons, and report positron currents vs. retarding potential for the different moderators. We identify vacuum ultraviolet emissions from irradiated Ne moderators as the source of spurious signals in our channel electron multiplier slow positron detection channel. Our design is also unusual in that it employs a sealed radioactive Na-22 positron source which can be translated relative to, and isolated from, the cryogenic moderator deposition substrate. This allows us to separate the influences on moderator efficiency of surface contamination by residual gases from those of accumulated radiation damage.