Deep Ultraviolet Region Research Articles

Interferometric characterization of ultrashort deep ultraviolet pulses using a multiphoton ionization mass spectrometer

The temporal characterization of a femtosecond laser pulse in the deep ultraviolet region using an interferometric autocorrelation scheme is demonstrated. Two-photon ionization of a molecule in a time-of-flight mass spectrometer was used as a nonlinear detector to obtain an autocorrelation trace. This setup proved useful in not only providing a temporal characterization of a pulse but also investigating the ultrafast dynamics of photochemical processes.

Growth and Luminescence Properties of Subsequently Grown AlInN Layers on AlN Homoepitaxial Layers by Ammonia Gas Source Molecular Beam Epitaxy

We performed the epitaxial growth of Al-rich Al1-xInxN (x<0.02) ternary alloys that have subsequently been grown on AlN epilayers by ammonia gas source molecular beam epitaxy (GS-MBE). The In composition x of Al1-xInxN ternary alloys was controlled by changing In/(In+Al) flux ratio without phase separation. The optical properties of these alloys were investigated using cathodoluminescence measurements as a function of the In composition x of Al1-xInxN thin films. Near-band-edge emissions in a deep ultraviolet region (5.87–6.04 eV) were clearly observed in high-quality Al1-xInxN epilayers at 77 K.

Investigation on elemental and isotopic fractionation during 196 nm femtosecond laser ablation multiple collector inductively coupled plasma mass spectrometry

Despite the large number of successful applications of laser ablation, elemental and isotopic fractionation coupled to inductively coupled plasma mass spectrometry (ICP-MS) remain as the main limitations for many applications of this technique in the fields of analytical chemistry and Earth Sciences. A substantial effort has been made to control such fractionations, which are well-established features of nanosecond laser ablation systems. Technological advancements made over the past decade now allow the ablation of solids by femtosecond laser pulses in the deep ultraviolet (UV) region at wavelengths less than 200 nm. Here the use of femtosecond laser ablation and its effects on elemental and isotopic fractionation is investigated. The Pb/U system is used to illustrate elemental fractionation and stable Fe isotopes are used to illustrate isotopic fractionation. No elemental fractionation is observed beyond the precision of the multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) measurements. Without a matrix match between standard and sample, elemental fractionation is absent even when using different laser ablation protocols for standardization and samples (spot versus raster). Furthermore, we found that laser ablation-induced isotope ratio drifts, commonly observed during nanosecond laser ablation, are undetectable during ultraviolet femtosecond laser ablation. So far the precision obtained for Fe isotope ratio determinations is 0.1‰ (2 standard deviation) for the 56Fe/ 54Fe ratio. This is close to that obtainable by solution multiple-collector inductively coupled plasma mass spectrometry. The accuracy of the results appears to be independent of the matrix used for standardization. The resulting smaller particle sizes reduce fractionation processes. Femtosecond laser ablation carries the potential to solve some of the difficulties encountered during the two prior decades since the introduction of laser ablation.

Improvement of Vacuum-Ultraviolet Transparency of Silica Glass by Modification of Point Defects(Review)

Silica glass is an important optical material in deep-ultraviolet (DUV, λ≤300 nm) and vacuum-ultraviolet (VUV, λ≤200 nm) spectral regions. However, DUV-VUV transparency of silica glass is significantly influenced by the presence of structural imperfections (point defects). This article describes point defects in silica glasses of low metallic impurity contents (“synthetic” silica glasses), and optical and chemical properties of these point defects. It is shown that controlled doping with specific point defects can improve the transparency and radiation hardness of silica glass in DUV-VUV spectral region. Such doped species include network modifiers that relax the glass structure and mobile interstitial hydrogen molecules. Utilization of these techniques in developing photomask substrates for F2 laser photolithography and DUV optical fibers for ArF laser transmission is presented.

Novel absorber stack for minimizing shadow effect in extreme ultraviolet mask

Finding an optimized absorber stack is becoming a more critical issue in the fabrication of extreme ultraviolet (EUV) mask since it is directly related to the performance of lithography such as pattern fidelity and productivity. Optical simulation, deposition, and measurement have been conducted to establish an optimized absorber stack including antireflection coating (ARC), absorber layer, and capping (or buffer) layer, which satisfies major requirements for EUV mask applications. TaN and the other absorber candidates do not show acceptable reflectivity value (lower than 5%) in deep ultraviolet (DUV) wavelength region (199 or 257nm) for pattern inspection. DUV reflectivity can be lowered by applying C and Al2O3 layers as top ARCs for 199 and 257nm wavelengths, respectively, while keeping the EUV reflectivity at 13.5nm less than 1%. ARC-covered TaN absorber stacks result in a reduction of printed CD variation owing to the mitigation of the shadow effect. However, long-term stability and fabricability of these stacks should be examined further.

Extremely weak surface emission from (0001) c-plane AlGaN multiple quantum well structure in deep-ultraviolet spectral region

We studied the anisotropic optical properties of c-plane AlxGa1−xN multiple quantum wells and m-plane AlxGa1−xN single layer grown on a SiC substrate. Very weak surface emission was detected from c-plane samples with x=0.66–0.76 (emission wavelength of 228–240nm), although strong surface emission was detected from c-plane samples with x=0–0.41 (280–365nm) and m-plane samples with x=0–0.76 (240–365nm). These results indicate that crystal-field splitoff hole valence band plays an important role in (E‖c) polarization and isotropic emissions from AlxGa1−xN with x≧0.5.

Surface assessment of CaF_2 deep-ultraviolet and vacuum-ultraviolet optical components by the quasi-Brewster angle technique

The requirements for optical components have drastically increased for the deep-ultraviolet and vacuum-ultraviolet spectral regions. Low optical loss, high laser damage threshold, and long lifetime fluoride optics are required for microlithographic applications. A nondestructive quasi-Brewster angle technique (qBAT) has been developed for evaluating the quality of optical surfaces including both top surface and subsurface information. By using effective medium approximation, the negative quasi-Brewster angle shift at wavelengths longer than 200 nm has been used to model the distribution of subsurface damage, whereas the positive quasi-Brewster angle shift for wavelengths shorter than 200 nm has been explained by subsurface contamination. The top surface roughness depicted by the qBAT is consistent with atomic force microscopy measurements. The depth and the microporous structure of the subsurface damage measured by the qBAT has been confirmed by magnetorheological finishing. The technique has been extended to evaluate both polished and antireflection-coated CaF(2) components.

Density Measurement of Molten CaF2 by an Electrostatic Levitator

High-quality single crystalline calcium fluoride (CaF2) has been of considerable practical concern as an optical lens to an excimer laser stepper in the deep-ultraviolet region due to its excellent transparency. Highly accurate thermophysical properties of molten CaF2 are essential as input data for a numerical simulation of the crystal growth process. The density of molten CaF2 has been successfully determined in the stable and undercooled liquid states (1600–1820 K) with an electrostatic levitator. The temperature dependence of the density of molten CaF2 is given by $$\rho \hbox{/kg}\cdot \hbox{m}^{-3} = 3580-0.63T(\pm 1.6\% ).$$ The coefficient of cubical expansion of molten CaF2 has been determined to be $$\beta \hbox{/K}^{-\hbox{1}}=\frac{1}{5683-T}.$$

Achieving superior band gap, refractive index and morphology in composite oxide thin film systems violating the Moss rule

The interrelation between energy gap and high frequency refractive index in semiconductors and dielectrics is manifested by an inverse law which is popularly known as the Moss rule. This semi-empirical relationship is based on the fundamental principle that in a dielectric medium all energy levels are scaled down by a factor of the square of the dielectric constant. Such a rule is obeyed by most pure semiconductors and dielectrics with a few rare violations in composite materials which display several interesting parametric and microstructural evolutions. The present results are based on some specific oxide composite thin films involving Gd2O3/SiO2 and ZrO2/SiO2 codeposited systems that have displayed a superior refractive index and energy gaps violating the semi-empirical Moss rule. Also, morphological supremacy is also distinctly noticed in these composites. The novel microstructural and polarizability properties of such composite systems were probed through multi-mode atomic force microscopy and phase modulated spectroscopic ellipsometry using refractive index modelling, autocorrelation and height–height correlation functional analyses. These binary composite thin films have shown their potential as well as the possibility of meeting expectations in satisfying the challenging optical coating requirements of the deep ultraviolet spectral region.

Bridgman growth and characterization of calcium fluoride crystals

Calcium fluoride (CaF 2) single crystal is an important lens material for photolithographic processing of silicon integrated circuits (IC) at wavelengths in the deep ultraviolet (DUV) region. In this paper, CaF 2 single crystals have been grown by the Bridgman method using a graphite crucible. Commercial CaF 2 raw materials were treated in an electric resistance furnace to remove moisture as well as residual oxides. The scattering particles and absorption near 300 nm were attributed to the existence of residual moisture in the raw materials. The stress distributions were observed and it tended to accumulate at grain boundaries. By optimizing growth conditions, 〈1 1 1〉-oriented CaF 2 crystals up to 185 mm in diameter were grown and the transmittance was more than 93% in the range of 380–3500 nm.

Investigation of magnesium fluoride crystals for imaging acousto-optic tunable filter applications

Results of an investigation of acousto-optic (AO) cells using single crystals of magnesium fluoride (MgF2) are presented. Two acousto-optic tunable filter (AOTF) cells for imaging application have been designed and tested in the visible and ultraviolet (UV) regions of the spectrum from 190 to 490 nm. The two imaging filters were developed by using the wide-angle AO interaction geometry in the (010) and (11 0) planes of the crystal. These filters were used to obtain spectral images at the shortest wavelengths achieved so far. Advantages and drawbacks of this crystal are discussed and photoelastic, acoustic, and AO properties of MgF2 are examined. The investigation confirmed that MgF2-based AOTF cells can be used in the deep UV region up to 110 nm.

RETRACTED: Fractals and superstructures in gadolinia thin film morphology: Influence of process variables on their characteristic parameters

Characteristic topographic parameters like fractals and superstructures contribute substantially to the thin film morphology, which directly or indirectly influences the physical, optical and mechanical properties. Fractal geometry and scaling concepts can concisely as well as more effectively describe the complex rough surface morphology. In the present study, such microstructural analysis has been carried out for gadolinia thin films, a most promising candidate to develop multilayer optical coatings in the deep ultraviolet spectral region. Extended power spectral density (PSD) spectra for these films acquired through atomic force measurements have been modeled and analyzed using several thin film morphological functions appropriately modified to suit the present requirements. Analysis results of the PSD functions have distinctly indicated the presence of Brownian fractals and uniformly distributed superstructures in the most experimental films deposited through reactive electron beam deposition technique. Process variables such as the rate of deposition and the oxygen partial pressure have affected the microstructural and related morphological evolutions very differently. The rate parameter strongly influenced the intrinsic microroughness, whereas the oxygen pressure influenced the grain sizes of the gadolinia films. One can advantageously use such information in tailoring a desired surface morphology in thin films by choosing appropriate combinations of process variables.

Diffraction-limited performance of flat-substrate reflective imaging gratings patterned by DUV photolithography

We report on the first demonstration of flat substrate imaging gratings fabricated by deep ultraviolet (DUV) photoreduction lithography, which uniquely offers sub-100-nm resolution and spatial coherence over centimeter scales. Reflective focusing gratings, designed according to holographic principle, were fabricated on 300-mm silicon wafers by immersion DUV lithography. Spatial coherence of the fabrication process is evident in measured diffraction-limited imaging function. Flat-substrate gratings, with lines of arbitrary spacing and curvature, offer both dispersion and general spatial wavefront transformation combining the function of multiple optical elements. Fabrication at the sub-100-nm resolution level allows high-line-count, low-order efficient gratings even into the deep ultraviolet region. Nanoreplication of gratings at the wafer level provides a pathway to devices of ultimate low cost.

Ultraviolet Fluorescence Imaging of Fingerprints

We studied fluorescence imaging of fingerprints on a high-grade white paper in the deep ultraviolet (UV) region with a nanosecond-pulsed Nd-YAG laser system that consists of a tunable laser and a cooled CCD camera.Clear fluorescence images were obtained by time-resolved imaging with a 255- to 425-nm band-pass filter, which cuts off strong fluorescence of papers. Although fluorescence can be imaged with any excitation wavelength between 220 and 290 nm, 230 and 280 nm are the best in terms of image quality. However, the damage due to laser illumination was smaller for 266-nm excitation than 230- or 280-nm excitation.Absorption images of latent fingerprints on a high-grade white paper are also obtained with our imaging system using 215- to 280-nm laser light. Shorter wavelengths produce better images and the best image was obtained with 215 nm. Absorption images are also degraded slightly by laser illumination, but their damage is smaller than that of fluorescence images.

A new single grating spectrograph for ultraviolet Raman scattering studies

AbstractA state‐of‐the‐art single grating spectrograph for Raman scattering studies within the deep ultraviolet (DUV) region of the electromagnetic spectrum is discussed. It is based on a high throughput DUV version of a single‐stage monochromator originally designed for use in the visible spectral region. Its key components are two identical, newly designed calcium fluoride camera lenses each consisting of five different individual optical elements. The first of these lenses collimates the Raman scattered DUV radiation entering the spectrometer through its entrance slit. The second lens focuses the collimated beam of dispersed Raman scattered DUV radiation emerging from a high‐resolution reflection grating onto a charge coupled device (CCD) detector with enhanced DUV sensitivity. A novel high transmission edge filter is used as a blocking device for a sufficient rejection of the Rayleigh line generating a relatively sharp transmittance cutoff at a Stokes Raman wavenumber shift of about ∼450 cm−1 employing 257 nm DUV excitation. Overall, this new spectrograph enables rapid collection of Stokes DUV Raman scattered photons at f/2 wide apertures with sufficiently large signal‐to‐noise ratios (SNRs) in relatively short acquisition times and with an effective spectral resolution of approximately ∼6 cm−1. Backscattered Raman spectra of the following chemicals are presented as typical results illustrating the excellent performance characteristics of this new DUV spectrograph for a variety of experimental conditions within different scattering scenarios and for a relatively wide range of commonly used sample preparation techniques: neat cyclohexane, laboratory air, polycrystalline D‐glucose, single crystal L‐alanine and a dilute aqueous solution of 2′‐deoxyadenosine. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Measurement system to determine the total and angle-resolved light scattering of optical components in the deep-ultraviolet and vacuum-ultraviolet spectral regions

An instrumentation for total and angle-resolved scattering (ARS) at 193 and 157 nm has been developed at the Fraunhofer Institute in Jena to meet the severe requirements for scattering analysis of deep- and vacuum-ultraviolet optical components. Extremely low backscattering levels of 10(-6) for the total scattering measurements and more than 9 orders of magnitude dynamic range for ARS have been accomplished. Examples of application extend from the control of at-wavelength scattering losses of superpolished substrates with rms roughness as small as 0.1 nm to the detection of volume material scattering and the study into the scattering of multilayer coatings. In addition, software programs were developed to model the roughness-induced light scattering of substrates and thin-film coatings.

Growth, structure and carrier transport properties of Ga 2O 3 epitaxial film examined for transparent field-effect transistor

Growth conditions of heteroepitaxial thin films of tin-doped Ga 2O 3 were surveyed from the viewpoint of visible application to field-effect transistors (FETs). Films were deposited by pulsed laser deposition, and post-annealing was examined to improve film structures. Atomically flat surfaces were obtained for films grown on yttria-stabilized zirconia (111) plane and post-annealed at 1400 °C, but they were insulating. Conductive heteroepitaxial films applicable to FETs were obtained on α-Al 2O 3 (0001) at specific deposition conditions, i.e. substrate temperatures from 500 to 550 °C and oxygen pressures from 5 × 10 − 4 to 1 × 10 − 3 Pa. It was found that the resulting epitaxial films have a crystal structure different from that of β-Ga 2O 3. The crystal lattice for the films is determined to be orthorhombic with a large possibility of a higher-symmetry hexagonal or rhombohedral system. The films exhibited high transparency in the near infrared–deep ultraviolet region and had bandgap of ∼ 4.9 eV. The operation of top-gate MISFETs using the Ga 2O 3 film for the n channel was demonstrated.

Characterization of microroughness parameters in gadolinium oxide thin films: A study based on extended power spectral density analyses

Spectral microroughness is a performance-limiting factor for optical thin films like Gd 2O 3, which have dedicative applications in ultraviolet or deep ultraviolet region of the electromagnetic spectrum. Such a morphological parameter of a thin film surface can be very well characterized by power spectral density (PSD) functions. The PSD provides a more reliable description to the topography than the RMS roughness and imparts several useful information of the surface including fractal and superstructure contributions. Through the present study it has been noticed that deposition parameters like evaporation rate and oxygen pressure can play very definite, dominant and predictable roles in the evolution of fractal and superstructures in thin film topographies recorded through atomic force microscopy (AFM). In this work, the PSD functions derived from morphologies of various gadolinia thin films have been fitted with a novel multi peak-shifting Gaussian model along with fractal and k-correlation functions, to extract characteristic parameters of the precision surfaces. Using such information, roughness contributions of the fractal components (substrate dominated), pure film and the aggregates have been successfully extracted. Higher spectral fractal strengths have depicted lower refractive index values. The microroughness and grain sizes of the pure film have been influenced very differently with deposition rate and oxygen pressure. The oxygen pressure strongly influenced the grain sizes where as the deposition rate influenced the microroughness of the gadolinia films.

Ultraviolet Fluorescence Spectra of Fingerprints

We have studied inherent fluorescence spectra and imaging of fingerprints in the deep ultraviolet (UV) region with a nanosecond-pulsed Nd-YAG laser system that consists of a tunable laser, a cooled CCD camera, and a grating spectrometer. In this paper, we have studied UV fluorescence spectra of fingerprints under 266-nm illumination. Fluorescence spectra of fingerprints have two main peaks, around 330 nm (peak A) and 440 nm (peak B). At first, when a fingerprint has just been pressed, peak A is dominant. However, its intensity reduces as the total illumination time increases. On the other hand, peak B is weak at first. It appears after enough 266-nm illumination and its intensity increases as time elapses. After 3 h of illumination, peak A almost diminishes and peak B becomes dominant. By leaving the fingerprint under a fluorescent lamp in a room without laser illumination, peak A can be restored partly, while the intensity of peak B still increases.Time-resolved fluorescence spectra were also measured for these two peaks. The lifetime of each peak is 2.0 nsec (peak A) and 6.2 nsec (peak B) on average. Both peaks seem to consist of several components with different lifetimes. In the case of peak A, the 330-nm peak decays fast and a new component at 360 nm becomes dominant when the delay time exceeds 20 nsec. In the case of peak B, unlike peak A, no clear peak separation is observed, but the peak position seems to move from 440 to 460 nm when the delay time becomes larger.

Fluorine doped silica glass fiber for deep ultraviolet light

Optical fibers for the deep ultraviolet region were developed using fluorine doped silica glasses called ‘modified fused silica’. Optimizing the fiber drawing conditions improved the transmission of the fiber in the deep UV region. The transmittance of the fiber at 193nm reached more than 65% per 1m long (65% m−1) without reflection loss. Significant absorption bands from defects were not observed throughout the wavelengths of the deep UV–visible–infrared region. Hydrogen-impregnation into the fibers suppressed the degradation of the transmission induced by irradiating with an ArF excimer laser. Transmission in the DUV region and resistance to laser irradiation were drastically improved compared to high OH silica fibers. Dipping the fibers into a hydrofluoric acid solution automatically prepared fibers with a sharp-pointed edge that were suitable for optical microprobes in scanning near-field optical microscopes.