Excimer Laser Wavelength Research Articles

Mammalian Cell Interaction with Periodic Surface Nanostructures.

Here, we report on the nanopatterning of different aromatic polymer substrates achieved by KrF excimer laser treatment. The conditions for the construction of the laser-induced periodic surface structures, the so-called LIPSS pattern, were established by optimized laser fluence and a number of pulses. The polymer substrates were polyethylene naphthalate (PEN), polyethersulfone (PES), and polystyrene (PS), which were chosen since they are thermally, chemically, and mechanically resistant polymers with high absorption coefficients at the excimer laser wavelength. The surface morphology of the treated substrates was investigated by atomic force microscopy and scanning electron microscopy, and the roughness and effective surface area on the modified samples were determined. Elemental concentration was characterized by energy-dispersive (EDX) analysis, surface chemistry was determined with X-ray photoelectron spectroscopy (XPS). The samples with the formation of LIPSS induced by 10 mJ·cm−2 with 1000, 3000, and 6000 pulses were used for subsequent in vitro cytocompatibility tests using human cells from osteosarcoma (U-2 OS). The LIPSS pattern and its ability of significant cell guidance were confirmed for some of the studied samples. Cell morphology, adhesion, and proliferation were evaluated. The results strongly contribute to the development of novel applications using nanopatterned polymers, e.g., in tissue engineering, cell analysis or in combination with metallization for sensor construction.

First Steps Towards a Joint Measurement System for Light-Addressable Potentiometric Sensors and Light-Addressable Electrodes

Introduction A rapid diagnostic of the patients’ disease is essential for the success of the therapy. To support the doctor, lab-on-a-chip devices could be used. These devices contain different components, which one would primarily find in an analytical laboratory. Typically, sensors, liquid-handling devices or actuators are integrated in a microfluidic channel to allow a medical analysis on a single chip. Through the miniaturization of the single components, only a small volume e.g., of the patients’ blood, is needed and a first diagnosis can be done on-site.In the past research, mostly rigid sensor and actuator structures such as electrodes are integrated in lab-on-a-chip devices. For their implementation, photolithographic fabrication processes are necessary. On the contrary, devices based on semiconductor structures can be spatially and temporally resolved by light, whereby the sophisticated structuring process is no longer required. For sensing, light-addressable potentiometric sensors (LAPS) can detect different target species in the analyte [1,2], while with light-addressable electrodes (LAE), photoelectrochemical reactions can be triggered [3,4].In this work, the joint implementation of these two technologies integrated in a microfluidic channel will be presented for the first time, as depicted in figure 1. Methods and Materials LAPS are based on a semiconductor-insulator-transducer sandwich structure. An applied d.c. bias voltage will induce a space-charge region at the semiconductor-insulator interface. By illumination with an intensity-modulated light source from the rear-side, electron-hole pairs are generated leading to a detectable a.c. photocurrent. Local variations of the analyte concentration at the transducer surface will influence the width of the space-charge region and therewith the photocurrent. Therefore, moving the light source in a scanning like-manner, a spatially resolved mapping of the photocurrent, which is proportional to the analyte concentration, can be recorded.To fabricate the Al/p-Si/SiO2/Ta2O5 field-effect structure of the LAPS chip, SiO2 was grown by a thermal dry oxidation step on a p-doped silicon wafer. Subsequently, Ta was deposited by electron-beam evaporation followed by an oxidation step to achieve the Ta2O5 layer. In a final step, an Al ohmic rear-side contact was prepared and an illumination window was etched inside the Al layer for rear-side illumination.The LAE can be used for the spatially resolved manipulation inside the microfluidic channel. Semiconductors can be applied as electrode materials where charge carriers are generated upon illumination. In contrast to LAPS, LAEs have no insulating layer, which enables a direct charge transfer with the analyte and triggering of surface reactions. By spatially resolved illumination, a localized photocurrent will occur at the illuminated spot, while the remaining electrode is still isolated.A titanium dioxide layer was fabricated by means of pulsed laser deposition on a SnO2:F (FTO) glass substrate. The TiO2 target was vaporized by a KrF excimer laser (wavelength = 248 nm) with a repetition rate of 10 Hz at a pressure of 2x102 hPa O2. During deposition, the FTO glass substrate has been heated up to 400 °C. Results and Conclusions A possible strategy for the implementation of both technologies into a microfluidic channel was introduced. Furthermore, as a proof-of-concept, by local illumination, the pH value inside a microchannel was changed photoelectrocatalytically by the LAE. Simultaneously, a chemical image of the localized pH shift has been recorded by the LAPS. Since both systems can be addressed by light, dynamic manipulation and sensing is possible, which enables an easy adjustment of both technologies at run time. A detailed description of the experiment and the obtained results will be presented and discussed. Acknowledgements This work was supported by the German Federal Ministry of Education and Research (BMBF) within “NanoMatFutur“ (13N12585).

Импульсное лазерное облучение светоизлучающих структур со слоем (Ga,Mn)As

InGaAs/GaAs heteronanostructures with a (Ga,Mn)As layer on the surface were fabricated by MOCVD epitaxy and pulsed laser deposition, and the effect of a pulsed excimer laser (wavelength 248 nm, pulse duration ~ 30 ns) on their radiative, structural and galvanomagnetic properties was studied. The radiation energy density was varied in the range from 200 to 360 mJ/cm2. In the studies, photoluminescence spectroscopy was used, which makes it possible to analyze the polarization characteristics of the radiation of the structures. The crystalline perfection of the initial and laser-irradiated samples was studied using Raman spectroscopy. The elemental composition of the structures and its depth distribution were studied by secondary ion mass spectrometry. The effect of pulsed laser annealing on the ferromagnetic properties of heteronanostructures was characterized by the behavior of the magnetic field dependences of the Hall resistance and magnetoresistance at temperatures of 10–300 K in the range of magnetic fields ± 3600 Oe. At room temperature, the study was carried out in magnetic fields reaching values of ± 28000 Oe. To obtain the calculated temperature distributions along the sample thickness and in time using a model of the laser annealing process based on solving the problem of heat propagation in a one-dimensional GaAs system taking into account the (Ga,Mn)As layer on the surface, we used the original data on the thermal conductivity coefficient of structures with a layer (Ga,Mn)As obtained using a modified frequency separation technique (3ω-method).

First Steps Towards a Joint Measurement System for Light-Addressable Potentiometric Sensors and Light-Addressable Electrodes

Introduction A rapid diagnostic of the patients’ disease is essential for the success of the therapy. To support the doctor, lab-on-a-chip devices could be used. These devices contain different components, which one would primarily find in an analytical laboratory. Typically, sensors, liquid-handling devices or actuators are integrated in a microfluidic channel to allow a medical analysis on a single chip. Through the miniaturization of the single components, only a small volume e.g., of the patients’ blood, is needed and a first diagnosis can be done on-site.In the past research, mostly rigid sensor and actuator structures such as electrodes are integrated in lab-on-a-chip devices. For their implementation, photolithographic fabrication processes are necessary. On the contrary, devices based on semiconductor structures can be spatially and temporally resolved by light, whereby the sophisticated structuring process is no longer required. For sensing, light-addressable potentiometric sensors (LAPS) can detect different target species in the analyte [1,2], while with light-addressable electrodes (LAE), photoelectrochemical reactions can be triggered [3,4].In this work, the joint implementation of these two technologies integrated in a microfluidic channel will be presented for the first time, as depicted in figure 1. Methods and Materials LAPS are based on a semiconductor-insulator-transducer sandwich structure. An applied d.c. bias voltage will induce a space-charge region at the semiconductor-insulator interface. By illumination with an intensity-modulated light source from the rear-side, electron-hole pairs are generated leading to a detectable a.c. photocurrent. Local variations of the analyte concentration at the transducer surface will influence the width of the space-charge region and therewith the photocurrent. Therefore, moving the light source in a scanning like-manner, a spatially resolved mapping of the photocurrent, which is proportional to the analyte concentration, can be recorded.To fabricate the Al/p-Si/SiO2/Ta2O5 field-effect structure of the LAPS chip, SiO2 was grown by a thermal dry oxidation step on a p-doped silicon wafer. Subsequently, Ta was deposited by electron-beam evaporation followed by an oxidation step to achieve the Ta2O5 layer. In a final step, an Al ohmic rear-side contact was prepared and an illumination window was etched inside the Al layer for rear-side illumination.The LAE can be used for the spatially resolved manipulation inside the microfluidic channel. Semiconductors can be applied as electrode materials where charge carriers are generated upon illumination. In contrast to LAPS, LAEs have no insulating layer, which enables a direct charge transfer with the analyte and triggering of surface reactions. By spatially resolved illumination, a localized photocurrent will occur at the illuminated spot, while the remaining electrode is still isolated.A titanium dioxide layer was fabricated by means of pulsed laser deposition on a SnO2:F (FTO) glass substrate. The TiO2 target was vaporized by a KrF excimer laser (wavelength = 248 nm) with a repetition rate of 10 Hz at a pressure of 2x102 hPa O2. During deposition, the FTO glass substrate has been heated up to 400 °C. Results and Conclusions A possible strategy for the implementation of both technologies into a microfluidic channel was introduced. Furthermore, as a proof-of-concept, by local illumination, the pH value inside a microchannel was changed photoelectrocatalytically by the LAE. Simultaneously, a chemical image of the localized pH shift has been recorded by the LAPS. Since both systems can be addressed by light, dynamic manipulation and sensing is possible, which enables an easy adjustment of both technologies at run time. A detailed description of the experiment and the obtained results will be presented and discussed.Figure 1: Schematic illustration of the combination of a light-addressable electrode and a light-addressable potentiometric sensor. The pH gradient, generated by the LAE can be measured by the LAPS. Acknowledgements This work was supported by the German Federal Ministry of Education and Research (BMBF) within “NanoMatFutur“ (13N12585).

Optical Property Change of Silver Nanowire Thin Films in Laser Patterning Process

Silver nanowire thin films are attractive materials for flexible electrodes as an alternative for conventional tin-doped indium oxide films. An appropriate patterning process is required to apply nanowire-based transparent electrodes into the optoelectronic devices. In this paper, we report the optical property change in silver nanowire transparent electrodes during laser ablation process. We discovered that, when silver nanowires were patterned by nanosecond pulsed lasers at infrared wavelength, their optical properties were changed. The change of optical properties was caused by nanoresidues remaining on the surface. It was analyzed that the color and transparency of thin films were changed because the residual nanoparticles absorbed the visible light wavelengths where silver nanoparticles possess their peak values in the absorption spectra. The nanoparticles were derived from insufficient vaporization of silver nanowires during infrared laser ablation. In the case of using an excimer laser for patterning, the nanoparticles were not left more on the surface due to complete removal of silver nanowires, since silver has higher absorption coefficient at the wavelength of excimer laser. As a result, high-quality patterning of silver nanowire thin films could be obtained without any deterioration of optical properties in transparent electrodes.

Electron beams produced by innovative photocathodes based on nanodiamond layers

The investigation of two different photocathodes (PCs) based on nanodiamond (ND) layers, irradiated by a KrF nanosecond excimer laser (wavelength, $\ensuremath{\lambda}=248\text{ }\text{ }\mathrm{nm}$; photon energy, ${E}_{\mathrm{Ph}}=5\text{ }\text{ }\mathrm{eV}$) is reported. The ND layers were deposited by means of a pulsed spray technique. Specifically, the active layer of each PC consisted of untreated (as-received) and hydrogenated ND particles, 250 nm in size, sprayed on a $p$-doped silicon substrate. The ND-based photocathodes were tested in a vacuum chamber at ${10}^{\ensuremath{-}6}\text{ }\text{ }\mathrm{mbar}$ and compared to a Cu-based one, used as reference. All the photocathodes were irradiated at normal incidence. The quantum efficiency (QE) of the photocathodes was assessed. QE values of the ND-based photocathodes were higher than that of the reference one. In particular, the hydrogenated ND-based PC exhibited the highest QE due to the negative electron affinity that results from the surface terminated by hydrogen. Additionally, the photocathode surface/local temperature and the multiphoton process contribution to the electron emission were studied.

Corneal Endothelial Cell Changes after Photorefractive Keratectomy for Low to Moderate Myopia With and Without Mitomycin-C

AbstractBackground: Mitomycin-C was suggested for application during surface ablation procedures to reduce post-operative haze formation. It can effectively reduce haze formation and hence, improve the predictability of visual outcomes following refractive surgery. Despite these advantages, MMC can po-tentially damage all three main corneal cell types including epithelial (differentiated epithelium and limbal cells), stromal (keratocytes), and endothelial cells.Aim of Study: To investigate the croneal endothelial cell changes after Photorefractive Keratectomy (PRK) with and without Mitomycin-C 0.02% for low to moderate myopia.Patient and Methods: This prospective, randomized, comparative study included 60 eyes of 30 patients who were chosen from Ophthalmology Clinic at Tanta University Hos-pital and Shorouk Laser Vision Correction Centre for 6 month from December 2017 to May 2018 and scheduled for Photo-refractive Keratectomy (PRK) using193nm wavelength exci-mer laser system Allegretto EX500, (Wave Light Technologie., USA). These patients were randomly divided into two groups. Group (A) were scheduled for (PRK) with adjunctive mito-mycin-C 0.02% and patients of Group (B) were scheduled for (PRK) without mitomycin-C. Informed consents were obtained from all study participants.Results: In Group A the mean Endothelial Cell Density (ECD) was 2891.61±272.41 and at three months after surgery was 28 13.04±276.55cells/mm2 (p=0.289), pre-operative mean of Coefficient of Variation (CV) was 34.87±4.37 and post-operatively was 34.70±4.06 (p=0.877), the pre-operative mean of Central Corneal Thickness (CCT) was 523.00±26.14 m and 467.36±40.61 m post-operatively (p=0.001), the mean of hexagonality pre-operatively was 64.26±8.64% and 59.96± 8.15% post-operatively (p=0.059) which was statistically insignificant. In Group B the mean Endothelial Cell Density was (ECD) 2874.50±314.48 and at three months after surgery was 2777.23±302.44cells/mm2 (p=0.227), pre-operative mean of Coefficient of Variation (CV) was 33.49±4.98 and post-operatively 33.55±4.10 (p=0.962), pre-operative mean of central corneal thickness was 538.03±26.03 and 484.50±20.55 post-operatively (p=0.001), the mean of hexagonality pre-operatively was 63.00±7.74 and 61.43±7.29% post-operatively (p=0.423).Conclusions: There's no significant effect on endothelium in patients underwent to PRK in low to moderate myopia with or without mitomycin-C. We can't judge the same results for patients with high myopia because the ablation depth will be more and theoretically the diffusion of mitomycin-C to en-dothelium would affect its function or count.

Laser-assisted stress reduction in molybdenum microstructures for CMOS compatible MEMS integration

The fusion of micro-cantilevers over complementary-metal-oxide-semiconductor (CMOS) wafers (with driving electronics) is essential integration requirement for the development of portable sensing devices. In general, the stress gradient in metal cantilevers (after release) lead to structure warping and limits the performance and stability of the final fabricated devices. This work is focused to solve the requirement of (i) having a stress-free metal based micro-electro-mechanical structures (MEMS) without using high-temperature annealing technique and (ii) process compatibility with vapor HF release. We demonstrate a laser-assisted stress controlling scheme to achieve free-hanging metal cantilever without affecting the underneath (essential requirement for CMOS-MEMS integration) deposited layers. The molybdenum (Mo) metal is deposited by physical vapor deposition (PVD) technique at the temperature of 200 °C and then patterned to obtain the microcantilever beams of different length. High-pulse-energy excimer laser (wavelength =248 nm with pulse duration =24 ns) is used to anneal these metal cantilevers and their residual stress is measured by analyzing the final displacement of the released microbeams. Different laser energy density ranging from 200 × 10−3 J/cm2 to 450 × 10−3 J/cm2 is used to study the optimized parameters in the annealing process. Proposed pulsed laser-annealing (PuLA) technique reduces not only the stress gradient but also aid stress-tuning ability that has numerous advantages especially in electrostatic and piezoelectric MEMS devices. Furthermore, we also present the impact of laser action on the grain microstructures and electrical conductivity. Results show that molybdenum microstructures with localized laser heat treatment achieve a reduced surface roughness (from 7.5 nm to 3.8 nm) and improved electrical conductivity. Results demonstrate that the proposed PuLA scheme is a promising technique for (i) eliminating stress in metal interconnects in CMOS-MEMS integrations, (ii) building a stress-free and smooth surface micromechanical platform for N/MEM (resonators, sensors and actuators) devices and (iii) establishing molybdenum as an anti-diffusion barrier in Cu interconnect technology owing to its excellent bulk property.

Low energy Ar+ ion irradiation effects in monoclinic zirconia

Argon ion beam induced phase transformation (monoclinic to tetragonal) and bubble formation in monoclinic zirconia (ZrO2) has been studied using grazing incidence X-ray diffraction, electron microscopy, Raman scattering and photoluminescence spectroscopy. Monoclinic zirconia samples were irradiated with low energy (120 keV) Ar+ ions at temperatures 300 K and 143 K. The as-sintered and Ar+ ion irradiated zirconia samples were then irradiated using an excimer laser of wavelength 248 nm (energy 5 eV) to study the defect rich material (after ion irradiation) behavior at extreme condition (where ionization process is dominant). Zirconia was found to undergo a partial phase transformation from monoclinic to tetragonal (≈20%; ion fluence 2 × 1017 ions/cm2) upon Ar+ ion irradiation at 143 K. Further, Ar bubbles were formed and the size of the bubble is found to increase with the ion fluence, irrespective of sample temperature during ion irradiation. The oxygen vacancies (produced during ion irradiation) lead to strain field in the sample and this strain field is the prime cause for phase transformation. The rate of phase transformation was found to be faster in case of ion irradiation at 143 K. Further it was observed that Ar+ ion irradiation lead to creation of defects and laser irradiation on as-sintered zirconia leads to annihilation of defects. However, when laser irradiation was carried out on ion irradiated zirconia, there is a competition between production and annihilation of defects where the production of defects by the Ar+ ion irradiation is a dominant one. Further, the present work confirms the role of O vacancy in the monoclinic to tetragonal phase transformation and the results are consistent with the earlier report (Simeone et al., 2000).

Growth of two-dimensional WS2 thin films by pulsed laser deposition technique

Tungsten disulfide (WS2) is a promising material for potential applications in next-generation 2D-electronics. However, for such applications large-area films will be required. Fabrication techniques involving exfoliation give films of high crystal quality but suffer from the problem of extremely small size of the films which makes it very challenging to realize real devices for industrial applications. In this study, we are reporting the growth of large area two-dimensional (2D) films of WS2 using pulsed laser deposition technique. A polycrystalline WS2 target was ablated using a pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns). The ablated material was deposited over a c-axis aligned single crystal sapphire substrate maintained at 500°C in an ambient vacuum of 10−5 Torr. Thickness of the resulting films was controlled by controlling the number of incident laser pulses. Detailed characterization of the films was carried out using several state-of-the-art techniques including atomic force microscopy, Raman spectroscopy, and ultraviolet–visible spectroscopy. Relationship between the number of monolayers in the WS2 films and the spacing between the A1g(Γ) and E2g1 (Γ) Raman peaks (Δf) was determined. Effect of post annealing on the optical properties of the films was also investigated.

Effect of laser annealing on ZnO nanorods grown by chemical bath deposition on flexible substrate

Integration of nanostructures of semiconducting materials such as zinc oxide with flexible substrates represent a great opportunity to obtain unique sensing platform especially for the biomedical market. However, the attractive optical and electrical properties of ZnO are usually obtained by growing or annealing nanostructured films at relative high temperatures (>400 °C), which are not suitable to substrates made of polymeric materials.We demonstrate in this work that high quality ZnO nanostructures can be processed at relative low temperature (<100 °C) by combining low temperature deposition process and XeCl excimer laser (wavelength 308 nm), used to locally anneal and recrystallize ZnO without affecting the underlying polymeric substrate. We obtain unique post-annealed ZnO nanostructures, grown and processed both on rigid and flexible substrates, that show comparable optical and structural properties of ZnO films that are processed at high temperatures. We discuss the effect of the laser energy density on the nanostructure by analyzing the recrystallized ZnO films through photoluminescence and scanning electron microscopy.

Exploring femtosecond laser ablation in single-particle aerosol mass spectrometry

Abstract. Size, composition, and mixing state of individual aerosol particles can be analysed in real time using single-particle mass spectrometry (SPMS). In SPMS, laser ablation is the most widely used method for desorption and ionization of particle components, often realizing both in one single step. Excimer lasers are well suited for this task due to their relatively high power density (107–1010 W cm−2) in nanosecond (ns) pulses at ultraviolet (UV) wavelengths and short triggering times. However, varying particle optical properties and matrix effects make a quantitative interpretation of this analytical approach challenging. In atmospheric SPMS applications, this influences both the mass fraction of an individual particle that is ablated, as well as the resulting mass spectral fragmentation pattern of the ablated material. The present study explores the use of shorter (femtosecond, fs) laser pulses for atmospheric SPMS. Its objective is to assess whether the higher laser power density of the fs laser leads to a more complete ionization of the entire particle and higher ion signal and thus improvement in the quantitative abilities of SPMS. We systematically investigate the influence of power density and pulse duration on airborne particle (polystyrene latex, SiO2, NH4NO3, NaCl, and custom-made core-shell particles) ablation and reproducibility of mass spectral signatures. We used a laser ablation aerosol time-of-flight single-particle mass spectrometer (LAAPTOF, AeroMegt GmbH), originally equipped with an excimer laser (wavelength 193 nm, pulse width 8 ns, pulse energy 4 mJ), and coupled it to an fs laser (Spectra Physics Solstice-100F ultrafast laser) with similar pulse energy but longer wavelengths (266 nm with 100 fs and 0.2 mJ, 800 nm with 100 fs and 3.2 mJ). We successfully coupled the free-firing fs laser with the single-particle mass spectrometer employing the fs laser light scattered by the particle to trigger mass spectra acquisition. Generally, mass spectra exhibit an increase in ion intensities (factor 1 to 5) with increasing laser power density (∼ 109 to ∼ 1013 W cm−2) from ns to fs laser. At the same time, fs-laser ablation produces spectra with larger ion fragments and ion clusters as well as clusters with oxygen, which does not render spectra interpretation more simple compared to ns-laser ablation. The idea that the higher power density of the fs laser leads to a more complete particle ablation and ionization could not be substantiated in this study. Quantification of ablated material remains difficult due to incomplete ionization of the particle. Furthermore, the fs-laser application still suffers from limitations in triggering it in a useful time frame. Further studies are needed to test potential advantages of fs- over ns-laser ablation in SPMS.

TE modes of UV-laser generated waveguides in a planar polymer chip of parabolic refractive index profile

The UV-laser lithographic method is used for the preparation of Polymeric integrated-optical waveguides in a planar polymer chip. The waveguide samples are irradiated by an excimer laser of wavelength 248 nm with different doses and with the same fluencies. The refractive index depth profile for the waveguides, in the first zone is found to have a parabolic shape and Gaussian shape in the second one that can be determined by Mach–Zehnder interferometer. Both the mode field distribution and the effective mode indices for the first zone only are determined by making use of the theoretical mode and the experimental data. It is found that the model field distribution is strongly dependent on the refractive indices for each zone.

Surface, structural and mechanical properties of zirconium ablated by KrF excimer laser radiation

We study the effect of the ambient – dry (air) and wet (propanol) – environment on the surface, structural and mechanical properties of zirconium samples after irradiation with a KrF excimer laser (wavelength of , pulse duration of and repetition rate of ). The samples are exposed to an increasing number of laser pulses varying from 500 to 2000 in both media. Various features of the treated targets such as surface morphology, chemical composition, crystalline structure and hardness are analysed by complementary characterisation techniques, such as scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy and Vickers hardness tester, respectively. Surface morphology and hardness are explained on the basis of modifications in crystalinity, residual pressures and chemical properties of the sample surface after its irradiation.

Regular pattern formation on surface of aromatic polymers and its cytocompatibility

In this work, we describe ripple and dot nanopatterning of three different aromatic polymer substrates by KrF excimer laser treatment. The conditions for regular structures were established by laser fluence and number of pulses. Subsequently, the influence of the angle of incidence of a laser beam was investigated. We have chosen polyethersulfone (PES), polyetherimide (PEI) and polyetheretherketone (PEEK) as substrates for modification since they are thermally, chemically and mechanically resistant aromatic polymers with high absorption coefficients at excimer laser wavelength. As a tool of wettability investigation, we used contact angle measurement and for determination of the absorption edge, UV–vis spectroscopy was used. Material surface chemistry was analyzed using FTIR and the changes caused by modification were gained as differential spectra by subtraction of the spectra of non-modified material. Surface morphology was investigated by atomic force microscopy, also the roughness and surface area of modified samples were studied. The scans showed the formation of regular periodic structures, ripples and dots, after treatment by 8 and 16mJcm−2 and 6000 pulses. Further, initial in vitro cytocompatibility tests were performed using U-2 OS cell line growing on PES samples subjected to scanning electron microscopy analysis. The structure formation mapping contributes strongly to development of new applications using nanostructured polymers, e.g. in tissue engineering or in combination with metallization in selected electronics and metamaterials construction.

Enabling laser applications in microelectronics manufacturing

In this experimental study, we report on high-pulse-energy excimer laser drilling into high-performance build-up films which are pivotal in microelectronics manufacturing. Build-up materials ABF-GX13 from Ajinomoto as well as ZS-100 from Zeon Corporation are evaluated with respect to their viability for economic excimer laser-based micro-via formation. Excimer laser mask imaging projection at laser wavelengths of 193, 248 and 308 nm is employed to generate matrices of smaller micro-vias with different diameters and via pitches. High drilling quality is achievable for all excimer laser wavelengths with the fastest ablation rates measured in the case of 248 and 308 nm wavelengths. The presence of glass fillers in build-up films as in the ABF-GX13 material poses some limitations to the minimum achievable via diameter. However, surprisingly good drilling results are obtainable as long as the filler dimensions are well below the diameter of the micro-vias. Sidewall angles of vias are controllable by adjusting the laser energy density and pulse number. In this work, the structuring capabilities of excimer lasers in build-up films as to taper angle variations, attainable via diameters, edge-stop behavior and ablation rates will be elucidated.

Optimization of substrate-target distance for pulsed laser deposition of tungsten oxide thin films using Langmuir probe

The paper investigates the spatial and temporal variation of laser produced plasma of tungsten oxide using a Langmuir probe. The plasma was produced by laser ablation of tungsten oxide target using an Excimer laser of wavelength 248 nm. Our experimental studies confirmed that oxygen partial pressure (P) of 2× 10−2 mbar is sufficient enough to get stoichiometric tungsten oxide thin films and the plume dynamics was diagnosed for their spatial and temporal behaviour at the above optimised oxygen pressure. Spatial distribution was recorded with the target to substrate distance (D) ranging from the target position to a distance of 75 mm away from the target, whereas the temporal variation was taken in the range of 0–50 μ S with an interval of 0.5 μ S. The average electron densities were found to be maximum at 30 mm from the target position. However, ion density was constant beyond the probe distance of 45 mm from the target. The plasma current was found to be maximum at 28 μ S. The target to substrate distance was optimized for homogenous adherent good quality thin films using plasma parameters such as ion density and average electron density obtained at different oxygen pressure. The target distance and background gas pressure were correlated as PD scaling law and fitted as PD3 in the model.

Optimization of substrate-target distance for pulsed laser deposition of tungsten oxide thin films using Langmuir probe

The paper investigates the spatial and temporal variation of laser produced plasma of tungsten oxide using a Langmuir probe. The plasma was produced by laser ablation of tungsten oxide target using an Excimer laser of wavelength 248 nm. Our experimental studies confirmed that oxygen partial pressure (P) of 2× 10−2 mbar is sufficient enough to get stoichiometric tungsten oxide thin films and the plume dynamics was diagnosed for their spatial and temporal behaviour at the above optimised oxygen pressure. Spatial distribution was recorded with the target to substrate distance (D) ranging from the target position to a distance of 75 mm away from the target, whereas the temporal variation was taken in the range of 0–50 μ S with an interval of 0.5 μ S. The average electron densities were found to be maximum at 30 mm from the target position. However, ion density was constant beyond the probe distance of 45 mm from the target. The plasma current was found to be maximum at 28 μ S. The target to substrate distance was optimized for homogenous adherent good quality thin films using plasma parameters such as ion density and average electron density obtained at different oxygen pressure. The target distance and background gas pressure were correlated as PD scaling law and fitted as PD3 in the model.

Ultra-Low Temperature (~180°C) Solid-Phase Crystallization of GeSn on Insulator Triggered by Laser-Anneal Seeding

Solid-phase crystallization (SPC) of amorphous-GeSn (a-GeSn) films on insulating substrates has been developed combining with laser-annealed seeding, to realize next generation thin-film devices. By this technique, we have realized crystallization of GeSn (>10%) at low temperatures (~180oC), which is applicable for flexible thin-film devices on low cost plastic substrates with low softening temperatures (~200oC). In addition, the starting point of crystallization can be controlled by seeding, which is big advantage in device designing. Introduction In order to realize next generation flexible devices, novel functional materials have to be crystallized at controlled positions on insulating substrates below the softening temperature of low cost plastic substrates (~200oC). GeSn is attracting material for this purpose, since GeSn has higher carrier mobility than Si or Ge, and moreover, GeSn shows direct band structure by introducing substitutional Sn concentration more than 8%. Under such background, we have investigated solid-phase crystallization (SPC) of GeSn [1] and realized growth of GeSn at 180oC. However, to initiate the SPC at controlled positions, the samples have to be annealed above the melting point of Sn (~232oC) to form seeding regions, which is not applicable on flexible plastic substrates. In line with this, we propose an idea to use laser-annealing (LA) to form crystal seeds. By forming seeds by LA, lateral SPC is expected at process temperature below 200°C. Moreover, by decreasing the process temperature, we can expect that GeSn crystals with high substitutional Sn concentration should be obtained. Experimental Procedure In the experiment, amorphous Ge0.8Sn0.2 layers (100 nm thickness) were deposited on quartz substrates by a molecular beam technique. The samples were irradiated with a KrF excimer laser (wavelength: 248 nm, energy: 160 mJ/cm2) with 100 pulses in 1 sec to form seed regions, followed by thermal annealing (180 - 220 oC) for lateral SPC as shown in Fig. 1. Results and Discussion Nomarski images of samples as LA and after annealing at 180oC for 48h are shown in Figs. 2(a) and 2(b), respectively. The laser annealed seed region is formed as shown in Fig. 2(a). From the seed, a bright region spreads after 180oC annealing. The Raman spectra obtained at points A, B, and C in Figs. 2(a) and 2(b) are shown in Fig. 2(c). From these spectra, we can clearly say that lateral SPC is initiated from only the seed and no spontaneous nucleation occurs. From the length of lateral SPC region, growth rates of GeSn at various annealing temperatures (180, 200, 220 oC) are evaluated. It is found that the growth rate is about 1 μm/h at 180 oC, which is nearly 106 time as large as that of Ge [2]. The activation energy of the growth rate of Ge0.8Sn0.2 was estimated to be ~1.5 eV, which was smaller than that of Ge (~2.0 eV). This is attributed to the weakening atomic bonds by Sn atoms. This is a very useful technique to realize next generation flexible thin-film devices. Substitutional Sn concentration in grown layer obtained after annealing at 180oC for 48h is evaluated by Raman measurement, which is shown in Fig. 3. Substitutional Sn concentration in SPC region shows higher value (>10%) than that of LA region (~7.5%), which is attributed to difference of non-equilibrium conditions between SPC and LA. Interestingly, substitutional Sn concentration shows very high values exceeding 15% near the growth edge. The Sn concentration decreases with increasing distance from the edge. Decrease in the Sn concentration from >15% to ~10% is attributed to the post annealing effect of grown regions. Detailed physics will be discussed at the presentation.

Nanodrilling of fused silica using nanosecond laser radiation

The fast laser drilling of dielectric surfaces with hole diameters in the sub-μm range and a high aspect ratio is a challenge for laser methods. In this study, a novel laser structuring method for the production of randomly and periodically distributed holes in a fused silica surface will be presented using a self-assembling process. A fused silica surface was covered with a 10nm thick magnetron-sputtered molybdenum film. The metal film was irradiated by a KrF excimer laser (wavelength λ=248nm, pulse duration Δtp=25ns) with low laser fluences (Φ<1J/cm2) and the laser-induced heating resulting in a melting of the metal film and finally in a self-assembled formation of randomly distributed metal droplets due to the surface tension of the metal liquid phase using a top hat beam profile. Furthermore, the usage of a periodically modulated laser beam profile allows the fabrication of periodically distributed droplet pattern. The multi-pulse irradiation of the laser-generated metal droplets with higher laser fluences results in a stepwise evaporation of the metal and in a partial evaporation of the fused silica near the metal droplets. Finally, the laser-induced stepwise evaporation process results in a formation of cone-like holes in the fused silica surface where the resultant holes are dependent on the size of the generated metal droplets and on the laser parameters. The “drilling” process allows the fabrication of holes with a depth up to 1μm at a hole radius in the sub-μm range. Furthermore, the hole density can be adjusted by the laser fluence where a hole density up to ∼1.3μm−2 can be achieved. Furthermore, the process was simulated by finite element method using a simple thermodynamic model and the theoretical results were compared with the experimental findings.