Krypton Fluoride Excimer Laser Research Articles

Laser-perforated polymer films for possible use in tissue engineering

Ablation experiments were carried out to modify surfaces of poly(3-hydroxybutyrate) (PHB), poly(lactic acid) (PLA), poly(methyl methacrylate) (PMMA), and polyurethane (PU)/poly(dimethylsiloxane) (PDMS) films using a krypton fluoride excimer laser, which was employed to produce cavities and orderly perforated holes on the films for tissue engineering. Surface patterning was assisted by prefabricated stainless steel masks with different mesh sizes. Film surfaces modified in this manner were studied and analyzed for different laser parameters (such as pulse energy and pulse repetition rate), and characterized using optical microscopy and surface profilometry. The present study demonstrated that laser surface modification of PHB, PLA, PMMA, and PU/PDMS films can be achieved with a high degree of success and precision. The effect of the surface changes on the tendency of cells to adhere to the polymers, as well as its effect on cell growth, were subsequently investigated. Cell–polymer film interfacial interactions were examined based on the proliferation of human fibroblast cells cultured on the laser-modified membranes in comparison to unexposed membranes and tissue culture dishes as controls. Surface topographical features created by excimer laser modification seem to improve the attachment and growth of human fibroblast cells on some of the films.

Fabrication and characterization of Bragg gratings in perfluorinated polymer optical fibers and their embedding in composites

Fiber Bragg gratings (FBG) have attracted interest especially in the field of structural health monitoring (SHM) and online process monitoring. The main objectives of this study are an ultraviolet laser based generation of Bragg gratings in a perfluorinated polymer optical fiber (POF) and their optical, thermo-mechanical and mechanical characterization. This kind of polymer fiber has a higher optical transparency than typical polymer fibers based on polymers with carbon hydrogen bonds like polymethylmethacrylate, which is the most used material for polymer optical fibers to date. Until recently only gratings inscribed by the phase mask technique in thin slabs of the amorphous fluoropolymer CYTOP (cyclic transparent optical polymer) made from polymer fibers were successfully detected. Here infrared spectra of Bragg gratings in the core of perfluorinated polymer optical fibers are presented and the embedding of the bare perfluorinated polymer fibers without the over-cladding (reinforcement) layer are described for the first time. The inscription of the gratings in the core of the polymer optical fiber was done using a krypton fluoride excimer laser (248nm, 5eV) and the well-known phase mask method. An advantageous mechanical characteristic of the perfluorinated polymer optical fibers compared to glass optical fibers is the failure strain. In structural health monitoring applications, where strains higher than 10% have to be measured, polymer optical fibers are feasible in contrast to glass optical fibers. The results are also promising regarding stability and reliability. This could open a new field of POF sensing of parts, structures and devices.

Ultraviolet laser quantum well intermixing based prototyping of bandgap tuned heterostructures for the fabrication of superluminescent diodes

The ultraviolet laser induced quantum well intermixing process has been investigated for prototyping of multiple bandgap quantum well (QW) wafers designed for the fabrication of superluminescent diodes (SLDs). The process takes advantage of a krypton fluoride excimer laser (λ=248nm) that by irradiating an InP layer capping GaInAs/GaInAsP QW heterostructure leads to the modification of its surface chemical composition and formation of point defects. A subsequent rapid thermal annealing step results in the selective area intermixing of the investigated heterostructures achieving a high quality bandgap tuned material for the fabrication of broad spectrum SLDs. The devices made from a 3-bandgap material are characterized by ~100nm wide emission spectra with relatively flat profiles and emission exceeding 1mW.

Tunable stoichiometry of SiO x - BaTiO y - BO z fabricated by multitarget pulsed laser deposition

Oxide materials of desired stoichiometry are challenging to make in small quantities. Nanostructured thin films of multiple oxide materials were obtained by using pulsed laser deposition and multiple independent targets consisting of Si, BaTiO3, and B. Programmable stoichiometry of nanostructured thin films was achieved by synchronizing a 248-nm krypton fluoride excimer laser at an energy of 300 mJ/pulse, a galvanometer mirror system, and the three independent target materials with a background pressure of oxygen. Island growth occurred on a per pulse basis; some 500 pulses are required to deposit 1 nm of material. The number of pulses on each target was programmed with a high degree of precision. Trends in material properties were systematically identified by varying the stoichiometry of multiple nanostructured thin films and comparing the resulting properties measured using in situ spectroscopic ellipsometry, capacitance measurements including relative permittivity and loss, and energy dispersive spectroscopy (EDS). Films were deposited ∼150 to 907 nm thickness, and in situ ellipsometry data were modeled to calculate thickness n and k. A representative atomic force microscopy measurement was also collected. EDS, ellipsometry, and capacitance measurements were all performed on each of the samples, with one sample having a calculated permittivity greater than 20,000 at 1 kHz.

Suitability of polycarbonate safety glasses for UV laser eye protection

Commonly used impact resistant polycarbonate (PC) safety glasses were evaluated for suitability of use in a high power, ultra-violet (UV) laser lab in place of laser eyewear. Product bulletins for the glasses tested all specified 99.9% or greater UV absorbance for their PC materials. Safety glasses from various manufacturers were exposed to 1,501 pulses of UV light (248 nm) from a 0.6 J krypton fluoride (KrF) excimer laser over a 30 s period. Radiant energy incident on the eye wear was reduced to a nominal 200 mJ through attenuating filters. Surface damage to lens coatings was rapid. Calculations for this laser system indicated that the safety glasses tested had a minimum optical density (OD) of 2.6 (2.58–3.40). At this wavelength the safety glasses would protect from an intra-beam (direct) exposure up to pulse energies of 800 μJ at the lowest OD rating. They would not be protective for an intra-beam exposure to the system as employed for these tests (minimum OD of 5 required). The damage threshold for surface coating destruction was 100 mJ/cm2. Damage only occurred at radiant energies above which the lenses could provide sufficient UV attenuation. That is to say ocular damage could result before damage to the coating became evident. As damage was allowed to continue the PC material began to blister, char and distort. At this point the damage resulted in small increases in optical densities. In general, PC safety glasses would not be suitable for high energy laser applications (all Class 4 lasers and many 3B lasers) whenever intra-beam viewing was possible. Safety glasses may be suitable for diffuse viewing (indirect) situations when it can be determined that the OD provided is sufficient to attenuate the scattered energy.

Simultaneous Visualization of Water and Hydrogen Peroxide Vapor Using Two-Photon Laser-Induced Fluorescence and Photofragmentation Laser-Induced Fluorescence

A concept based on a combination of photofragmentation laser-induced fluorescence (PF-LIF) and two-photon laser-induced fluorescence (LIF) is for the first time demonstrated for simultaneous detection of hydrogen peroxide (H2O2) and water (H2O) vapor. Water detection is based on two-photon excitation by an injection-locked krypton fluoride (KrF) excimer laser (248.28 nm), which induces broadband fluorescence (400-500 nm) from water. The same laser simultaneously photodissociates H2O2, whereupon the generated OH fragments are probed by LIF after a time delay of typically 50 ns, by a frequency-doubled dye laser (281.91 nm). Experiments in six different H2O2/H2O mixtures of known compositions show that both signals are linearly dependent on respective species concentration. For the H2O2 detection there is a minor interfering signal contribution from OH fragments created by two-photon photodissociation of H2O. Since the PF-LIF signal yield from H2O2 is found to be at least ∼24,000 times higher than the PF-LIF signal yield from H2O at room temperature, this interference is negligible for most H2O/H2O2 mixtures of practical interest. Simultaneous single-shot imaging of both species was demonstrated in a slightly turbulent flow. For single-shot imaging the minimum detectable H2O2 and H2O concentration is 10 ppm and 0.5%, respectively. The proposed measurement concept could be a valuable asset in several areas, for example, in atmospheric and combustion science and research on vapor-phase H2O2 sterilization in the pharmaceutical and aseptic food-packaging industries.

Characterization of Fogging and Develop-Loading Effects in Electron-Beam Direct-Writing Technology

We investigated long-range critical dimension (CD) error factors, such as fogging and develop loading, to improve CD uniformity in electron-beam direct-writing (EBDW) technology. It was found that the impact of both effects reached 20 mm and the CD of the monitor pattern decreased by no less than 10%. Fogging and develop loading were separated by comparing the newly designed test patterns that were exposed using both EB and a krypton-fluoride excimer laser. We confirmed that the impact of fogging and develop loading by arranging dummy patterns with a density of 40% was estimated to be +8.9 and -18.9% in the CD, respectively. Based on success in separating each effect, fogging and develop loading were decreased by applying an antistatic agent and multipuddle development, respectively.

A high performance size exclusion chromatographic study on the depth-dependent gradient in the molecular weight of aged triterpenoid varnish films

The depth-profiles of aged triterpenoid resin films, dammar and mastic, were uncovered by optimized krypton fluoride excimer laser ablation (248nm, 25ns) and the etched varnishes were examined by high performance size exclusion chromatography (HP-SEC). The use of two detectors monitored the molecular weight distribution across depth as a function of absorbance at ultraviolet (240nm) and visible (400nm) light. The absorption of the films at both wavelengths is separated in a small fraction of sesquiterpenoids (200Da), triterpenoid molecules (400/500Da), dimerized and oxidized triterpenoid molecules (900–1000Da) and condensed entities (10–80kDa) that are formed upon the radical polymerization of polycadinene and cis-1,4-poly-β-myrcene, which are the polymers of dammar and mastic, respectively. The results establish the generation of depth-dependent gradients in radical polymerization, condensation and cross-linking across the depth-profiles of the triterpenoid films as a consequence of aging.

Fabrication of an integrated optical Mach–Zehnder interferometer based on refractive index modification of polymethylmethacrylate by krypton fluoride excimer laser radiation

It is known that deep ultraviolet (UV) radiation induces a refractive index increase in the surface layer of polymethylmethacrylate (PMMA) samples. This effect can be used for the fabrication of integrated optical waveguides. PMMA is of considerable interest for bio and chemical sensing applications because it is biocompatible and can be micromachined by several methods, e.g. structuring by photolithography, ablation and hot embossing. In the presented work direct UV irradiation of a common PMMA substrate by a krypton fluoride excimer laser beam through a contact mask has been used to write integrated optical Mach–Zehnder interferometers (MZI). MZI are used as sensitive bio and chemical sensors. The aim was to determine contact mask design and laser irradiation parameters for fabricating single-mode MZI for the infrared region from 1.30 μm to 1.62 μm. Straight and curved waveguides have been generated and characterized to determine the optical losses. The generation of channel waveguide structures has been optimized by a two step irradiation process to minimize the lithographic writing time and optical loss. By flood exposure to UV laser radiation in the first step the optical absorption of PMMA can be increased in the irradiated region. The required refractive index profile is then achieved with a second lithographic irradiation. The spectral behaviour of an unbalanced, integrated optical MZI fabricated by this excimer laser based contact mask method is shown for the first time. Further the optical intensity at the output port of a MZI has been measured while the optical path length difference was tuned by creating a temperature difference between the two arms of the MZI.

Depth-profile investigations of triterpenoid varnishes by KrF excimer laser ablation and laser-induced breakdown spectroscopy

The ablation properties of aged triterpenoid dammar and mastic films were investigated using a Krypton Fluoride excimer laser (248 nm, 25 ns). Ablation rate variations between surface and bulk layers indicated changes of the ablation mechanisms across the depth profiles of the films. In particular, after removal of the uppermost surface varnish layers there was a reduction of the ablation step in the bulk that was in line with a significant reduction of carbon dimer emission beneath the surface layers as detected by laser-induced breakdown spectroscopy. The results are explicable by the generation of condensation, cross-linking and oxidative gradients across the depth profile of triterpenoid varnish films during the aging degradation process, which were recently quantified and established on the molecular level.

Pattern evaluation for in-plane displacement measurement of thin films

Ultra-lightweight spacecraft incorporating “gossamer” structures are extremely compliant, which complicates control, design, and ground testing in full scale. One approach to model the behavior of a full-scale gossamer structure is to construct a small-scale model test article that can be used to verify a corresponding small-scale computer model. Once the predictions of the computer model have been verified by measurement of the physical test article, it can be scaled up to allow computation of the full-scale structure behavior. As model verification requires accurate deflection measurements at multiple points along the surface of the structure, a sensing system that provides full-field data without changing the dynamic response of the structure must be developed. Hence, an optical approach is taken. Since the thin films used in gossamer space structures are typically smooth and featureless, targets must be incorporated into the film surface to enable tracking of both in-plane and out-of-plane displacements. A krypton fluoride excimer laser system was used to etch 35 μm wide linear features approximately 0.1 μm into the surface metallization of both 50.8 μm polyester and 127 μm polyimide films. These optically diffuse surface features, designed mainly to investigate the precision of the laser etching method, were used as targets for ultra-close-range photogrammetry, the method chosen for displacement tracking. A force applied to the surface of the etched mirror (test article) produced in-plane and out-of-plane deformations that were resolved via ultra-close-range photogrammetry. To measure the in-plane tracking resolution, 1.5 and 3.0 mm circular dots were added (using ink) to the surface of the thin film, and some of these targets were tracked as the test article was translated on a precision linear stage. In-plane tracking resolution using ultra-close-range photogrammetry was related to the ground sample distance of the camera, which in this case was 51.25 μm pixel−1 (equal to the ratio of sample dimension to number of pixels in the field of view). Using a manual technique to identify features of the etched pattern for tracking, the mean tracking error was about 13 μm (σ=43 μm). Using an automated, subpixel marking technique to identify the 1.5 mm circular targets, the mean tracking error was 22 μm (σ=13 μm). Neither of these methods achieved the desired 10 μm tracking resolution.

Pattern Evaluation for In-plane Displacement Measurement of Thin Films

Ultra-lightweight spacecraft incorporating “gossamer” structures are extremely compliant, which complicates control, design, and ground testing in full scale. One approach to model the behavior of a full-scale gossamer structure is to construct a small-scale model test article that can be used to verify a corresponding small-scale computer model. Once the predictions of the computer model have been verified by measurement of the physical test article, it can be scaled up to allow computation of the full-scale structure behavior. As model verification requires accurate deflection measurements at multiple points along the surface of the structure, a sensing system that provides full-field data without changing the dynamic response of the structure must be developed. Hence, an optical approach is taken. Since the thin films used in gossamer space structures are typically smooth and featureless, targets must be incorporated into the film surface to enable tracking of both in-plane and out-of-plane displacements. A krypton fluoride excimer laser system was used to etch 35 μm wide linear features approximately 0.1 μm into the surface metallization of both 50.8 μm polyester and 127 μm polyimide films. These optically diffuse surface features, designed mainly to investigate the precision of the laser etching method, were used as targets for ultra-close-range photogrammetry, the method chosen for displacement tracking. A force applied to the surface of the etched mirror (test article) produced in-plane and out-of-plane deformations that were resolved via ultra-close-range photogrammetry. To measure the in-plane tracking resolution, 1.5 and 3.0 mm circular dots were added (using ink) to the surface of the thin film, and some of these targets were tracked as the test article was translated on a precision linear stage. In-plane tracking resolution using ultra-close-range photogrammetry was related to the ground sample distance of the camera, which in this case was 51.25 μm pixel−1 (equal to the ratio of sample dimension to number of pixels in the field of view). Using a manual technique to identify features of the etched pattern for tracking, the mean tracking error was about 13 μm (σ=43 μm). Using an automated, subpixel marking technique to identify the 1.5 mm circular targets, the mean tracking error was 22 μm (σ=13 μm). Neither of these methods achieved the desired 10 μm tracking resolution.

Selective electroless copper plating micro-coil assisted by 248 nm excimer laser

In the study, template of micro-coil patterns created by the excimer laser and activated by reactants for electroless copper plating is described. The generated micro-coil patterns are transformed into copper patterns on the substrate and formed copper microstructures. This method simplifies the manufacturing process of making circuits on boards comparing with the conventional lithography process of forming copper patterns on the substrate. Micro-patterns generated by the excimer laser result in the change of surface electric properties and activation selectively. A chemical reaction through the selective activated area may deposit metal, such as copper. The krypton–fluoride excimer laser (KrF, 248 nm in wavelength) not only provides simple and fast machining patterns, but also uses its high energy density to drill holes and circuits directly. Palladium ions are added as mediators in the electroless plating solution to enable a continuous electroless copper deposition. According to the experiment of excimer laser assisted electroless copper plating, the procedures of pretreatment and post cleaning are key factors resulted in excellent selective plating. The samples were pretreated by sodium dodecyl sulfate (SDS) and post cleaned by acetone and diluted nitric acid resulting in distinct micro-patterns. The deposition area is confined in the excimer laser ablated portion resulting in well selective plating.

Development of a Silicon-Based Electron Beam Transmission Window for Use in a KrF Excimer Laser System

Princeton Plasma Physics Laboratory, in collaboration with the Naval Research Laboratory, is currently investigating various novel materials (single-crystal silicon, 〈100〉, 〈110〉, and 〈111〉) for use as electron beam transmission windows in a krypton fluoride (KrF) excimer laser system. The primary function of the window is to isolate the active medium (excimer gas) from the excitation mechanism (field-emission diodes). The chosen window geometry must accommodate electron energy transfer >80% (750 keV) while maintaining the structural integrity during the mechanical load (1.3- to 2.0-atm base pressure differential, ~0.5-atm cyclic pressure amplitude, 5-Hz repetition rate) and the thermal load across the entire hibachi area (~0.9 W⋅cm−2). In addition, the window must be chemically resistant to attack by fluorine free radicals (hydrofluoric acid, secondary). In accordance with these structural, functional, and operational parameters, a 22.4-mm square silicon prototype window, coated with 500-nm thin-film silicon nitride (Si3N4), has been fabricated. The window consists of 81 square panes 0.019 ± 0.001 mm thick. The stiffened (orthogonal) sections are 0.065 mm wide and 0.500 mm thick (approximate). Assessment of silicon (and silicon nitride) material properties and computer-aided design modeling/analysis of the window design suggest that silicon may be a viable solution to inherent parameters and constraints.

Analysis of Laser Irradiated Organo-Platinum Films

A study of the surface electronic configuration and surface morphology of excimer laser irradiated organo-platinum films [cis-dichlorobis(triphenylphosphine)platinum(II)] is presented. The films were deposited by thermal evaporation to a thickness of approximately 1 µm. The organo-platinum films were irradiated in air with 248 nm UV light in order to produce metallic deposits. The irradiation source used was a krypton fluoride excimer laser with a pulse length of approximately 20 ns. After irradiation, the surface morphology of the films was studied by atomic force microscopy, which yielded information concerning the effect of exposure dose on the surface roughness. The composition of the deposits was investigated using X-ray photoelectron and Fourier-transform infrared spectroscopy. The effect of annealing the deposits has also been studied. XPS analysis of the annealed samples has provided evidence for the formation of platinum silicide (PtSi).

Accurate Measurement of Spherical and Astigmatic Aberrations by a Phase Shift Grating Reticle

A new grating structure is proposed for measuring aberrations in lithography lenses. The grating structure on a reticle comprising opaque lines, naked lines and π/2-phase-shifted grooves with their width ratio equal to 2/1/1 has a property such that either of the two first-order diffracted rays disappears entirely. The other first-order ray interferes with the zeroth-order ray to form tilted standing waves. Since a resist pattern of the grating is formed on top lines or valley lines of the standing waves, it moves with perfect linearity upon defocus. In this paper the quantitative metrology of even aberrations in a lithography lens is described along with a demonstration of an application of the grating reticle for a krypton fluoride excimer laser scanner with a numerical aperture (NA) of 0.73. Variation of the grating period along with a small-opening stopper inserted into the illuminator enlarge the measurable range of the pupil radius. The measurement technique described in this paper achieves high accuracy which is sufficient for small aberration lenses in state-of-the-art scanners.

A poly-methylmethacrylate electrophoresis microchip with samplepreconcentrator

A microstructure on poly-methylmethacrylate (PMMA) for sample concentrationand electrophoresis was fabricated. This microfabricated structure was able toincrease the detection signal and lower the amount of sample used inelectrophoretic analysis. The thin-film electrode located at theT-intersection of the sample injection and separation channels provides thecurrent path for the injection channel, but restrains the DNA molecules frompassing through. This can accumulate DNA molecules and increase theconcentration before performing the electrophoretic analysis. This microstructurewas fabricated using krypton fluoride (KrF) excimer laser photo-ablation andfusion bonding techniques. The excimer laser photo-ablation performs rapidprototyping with great flexibility in design changes. The PMMA material ismuch cheaper than other materials, for example glass and silicon, used in capillaryelectrophoresis and concentration. The applied electrical field was300 V cm-1for the DNA concentration in this microstructure. Experiments show that theDNA concentration was saturated within 200 s after the DNA moleculesfirst reached the injection tee. The DNA fragments can be concentrated up tofive times greater than samples without a concentrator at the injection tee.The separation results also demonstrated that the detected signal intensitiesof the separated DNA fragments in the tee-type chip with a samplepreconcentrator were five times greater than that obtained in a conventionalcross-type capillary electrophoresis chip with an identical initial sample concentration.

Aberration Monitoring toward Wavefront Matching with Device Patterns

Wavefront matching is a new technique for compensating uncontrollable aberration effects on a certain reticle design which involves moving controllable aberrations. Such a compensation concept is crucial for producing system-on-chip devices using photolithography. A high-precision wavefront monitor is an essential item for the wavefront matching. The resist-based aberration measurement technique based on the three-beam interference theory is a candidate for monitoring. In the present work, the wavefront in the lithography lens was driven by the lens controller, and then the slight change from the initial state was measured using the technique. For the experiment, a krypton fluoride excimer laser scanner with a numerical aperture (NA) of 0.68 was used. The measurement results verified that the technique has sufficient sensitivity to monitor the wavefront matching.

Highly Accurate and Precise Measurement Technique for Effective Exposure Dose

A highly accurate and precise measurement technique for effective exposure dose in optical microlithography has been developed. The effective exposure dose can be obtained by a dose monitor mark in a photomask, called the effective dose meter, which consists of multiple segments including grating patterns with a pitch below the resolution limit and different duty ratios gradually. Since the residual thickness of the photoresist after development changes according to the duty ratio, the effective exposure dose can be obtained by monitoring the film thickness for each segment of effective dose meter. It was found that an extremely high dose resolution of less than 0.01% was achieved in this technique. A dynamic range expansion technique has been also proposed, which has wide measurement range and improved repeatability. Across-wafer variation of the effective exposure dose in the current krypton fluoride excimer laser lithography process was measured as a demonstration of these technologies.

Investigation of High-Precision Lithography Lens Aberration Measurement Based on Three-Beam Interference Theory: Sensitivity versus Coherent Factor and Variations with Dose and Focus

The three-beam interference technique with high coherent illumination is considered promising for application to the measurement of lens aberrations due to its ease of use and convertibility of the results to Zernike coefficients. The coefficients widely describing the characteristics of aberrations are crucially important for the prediction of aberration impacts on a specific pattern, and are expected to minimize the impacts. Furthermore, intensive study of the three-beam interference technique is necessary for highly accurate prediction. This paper discusses the expected characteristics of three-beam interference based on theoretical considerations and experimental results. In one experiment, a krypton fluoride excimer laser scanner with a numerical aperture of 0.68 was used. Having practicable highly coherent illumination affects sensitivity in detecting aberrations, and makes results independent of exposure dose and focus. Such characteristics improve repeatability under realistic conditions that the exposure dose is not uniform and the wafer surface is not flat.