F2 Excimer Laser Research Articles

Electronic and optical properties of zinc based hybrid organic-inorganic compounds

There is significant interest in hybrid organic-inorganic (HOI) compounds since these materials offer multiple functionalities and properties that can be tailored at the mesoscopic and nanoscale levels. HOIs investigated for photovoltaic applications typically contain lead or mercury. There is considerably less work done on Zn-based HOIs. These could potentially be considered in biomedical applications due to presence of organic components and the biocompatibility of Zn cations. Using a systematic materials selection approach, we have carried out a detailed search of Zn-HOI compounds in two comprehensive experimental crystallographic repositories: Inorganic Crystal Structure Database and American Mineralogist Crystal Structure Database. Thirteen Zn-HOI compounds are discovered: CuZnO2(CO3), Zn(C2O4), ((CH3)2NH2)Zn3(PO4)(HPO4)2, (CH3NH3)Zn4(PO4)3, Zn(N(CH2PO3H)3)(H2O)3, (CH3NH3)Zn(HCO2)3, Zn4(CO3)2, Zn8(HPO4)16(C2H8N)8, Zn5(CO3)2, (Mg2Zn)8(CO3)2(OH), Zn7(CO3)2(OH)10, Ca3Zn2(PO4)CO3(OH).2H2O, and Zn(CO3). We have then performed first principles calculations via density functional theory with hybrid functional treatment to determine the electronic band gap and optical response of these materials. Our computations show that eleven of the thirteen compounds have insulating properties with band gaps ranging from 2.8 eV to 6.9 eV. Ten of these are found to have a high absorbance in the far ultra-violet (FUV) region of 200–112 nm wavelength. For example, the absorption coefficient of (CH3NH3)Zn(HCO2)3 is ∼0.75 × 105 cm−1 for F2 excimer laser energy (wavelength ∼157 nm) which is more than three orders higher than the average tissue absorbance (∼101.5 cm−1) and the refractive index of 1.85 is larger than typical biological matter which is in the range 1.36–1.49. These results suggest that Zn-HOIs could potentially find applications in photothermolysis and UV protection.

Generation of coherent soft X-ray radiation in short FEL with harmonic multiplication cascades and two-frequency undulator

The generation of powerful nanometer x-ray radiation in cascaded high-gain free-electron lasers (FELs) with two-frequency undulator is studied. An advanced phenomenological model of single pass FEL is developed, which describes both linear and nonlinear harmonic generation from seed and initial shot noise; it includes calibrated corrections for all loss factors, specific for each FEL harmonic in each cascade. In this way, it differs from other phenomenological models, which account for the energy spread or divergence equally for all FEL harmonics. The new model, calibrated with a FEL experiment and numerical 3D simulations, is employed for the analysis of the FEL power dynamics to obtain maximum high-harmonic power in the X-ray band at the shortest possible FEL length with a low seed frequency and electron energy. The advantages of the two-frequency undulators in FEL prebunchers are demonstrated; their use in the cascaded FELs is proposed. Several such FELs with two-frequency undulators are simulated. The nanometer X-ray radiation production is studied in high-gain cascaded FELs with the seed wavelengths, matching peak reflectivities of MoRu/Be and Mo/Si. Dozens of megawatts of X-ray radiation are modeled at ≈40 m in a multi-cascade FEL, seeded from commercially available F2 excimer laser.

Luminescence of phosphorus doped silica glass

A fiber preform with P-doped silica core is studied by luminescence methods. P-doped silica was synthesized via the SPCVD method on a substrate tube made of pure silica glass F300. Two luminescence bands were detected under excitation of the F2 excimer laser (157nm). One band is in UV range at 4.6eV (265 nm) with two time constants ~30ns and 5μs and the other at 3.1eV (400 nm) with time constant ~5.5ms. Fast decay of the blue band with time constant ~20ns was also observed. The main excitation band of the UV luminescence is at 7.1eV (~170 nm) and that for blue band is at 6.3eV (~195 nm). These bands belong to two different luminescence centers, however, both are associated with the presence of phosphorus. The UV band is similar to the one observed in many different oxide materials containing phosphorus and is ascribed to PO43- complex ion. The blue band is ascribed to a twofold coordinated phosphorus. Both the blue and the UV luminescences participate in the recombination process due to electron trapping. These luminescences appear due to thermal stimulation upon recombination of liberated self-trapped holes. Other than the detected phosphorus-related oxygen-hole-centers, there is no other recombination luminescence.

Ultraviolet luminescence of ScPO4, AlPO4 and GaPO4 crystals

The luminescence of self-trapped excitons (STEs) was previously observed and described for the case of tetragonal-symmetry ScPO4 single crystals. The subject band in this material is situated in the UV spectral range of ∼210 nm or ∼5.8 eV. In the present work, we are both expanding this earlier luminescence study and seeking to identify similar luminescence phenomena in other orthophosphate crystals, i.e., AlPO4 and GaPO4. These efforts have proven to be successful—in spite of the structural differences between these materials and ScPO4. Specifically we have found that for AlPO4 and GaPO4, in addition to an α-quartz-like STE, there is a UV luminescence band that is similar in position and decay properties to that of ScPO4 crystals. Potentially this represents an STE in AlPO4 and GaPO4 crystals that is analogous to the STE of ScPO4 and other orthophosphates. The decay kinetics of the UV luminescence of ScPO4 was studied over a wide temperature range from 8 to 300 K, and they exhibited some unusual decay characteristics when subjected to pulses from an F2 excimer laser (157 nm). These features could be ascribed to a triplet state of the STE that is split in a zero magnetic field. A fast decay of the STE was detected as well, and therefore, we conclude that, in addition to the slow luminescence corresponding to a transition from the triplet state, there are singlet–singlet transitions of the STE. Time-resolved spectra of the slow and fast decay exhibit a small shift (∼0.15 eV) indicating that the singlet–triplet splitting is small and the corresponding wavefunction of the STE is widely distributed over the atoms of the ScPO4 crystal where the STE is created.

Luminescence of unfused 95%SiO2–5%GeO2 amorphous films with fluorine additive: No evidence for presence of GeODC(I) defects found

Photoluminescence (PL) of unfused amorphous germanosilicate films with fluorine additive is studied in 2–8.5eV spectral range. Experiments are based on films deposited on silica substrates by means of the surface-plasma chemical vapor deposition (SPCVD). Films of about 100μm in thickness with “high F” (~4.2wt.%) and “low F” (~0.5wt.%) fluorine content have been fabricated for the experiments. KrF (248nm), ArF (193nm) and F2 (157nm) excimer lasers are used to pump PL. It is found that absorption and luminescence associated with germanium oxygen deficient centers (GeODCs) in “high F” and “low F” films differ. In the “high F” unfused film absorption coefficient of the band at 5eV as well as intensity of the blue PL band at 3.1eV are significantly greater. This film proves features of the so called GeODCs(II), which symbolize twofold coordinated germanium defects in silica network. In the “low F” unfused film absorption band at 5eV is feebly marked. Poorly resolved PL intrinsic to GeODCs(II) can be detected in this film under the KrF laser pump. The most significant PL features are revealed under deeper UV pump by ArF and F2 lasers. Spectral positions of PL bands excited by these lasers correspond to GeODCs(II). However PL decay kinetics dramatically differs from that one intrinsic to GeODCs(II). Noticeable growth of PL intensity caused by permanent (half an hour and more) exposure to ArF and/or F2 lasers takes place, indicating GeODC(II) formation. It is found that considerable body of fluorine additive has the same effect as profusion for “low F” and/or fluorine free germanosilicate amorphous material synthesized by SPCVD. In the “high F” film yield of GeODC(II) PL pumped by the F2 laser remains high. This speaks for the suppression of the competitive 7.6eV absorption band associated with SiODCs(I) by fluorine additive indicating a decrease in the content of this type of defects in the material. High yield of GeODC(II) luminescence pumped by deep UV photons as well as hypothetical similarity of SiODC(I) and GeODC(I) permit one to conclude that GeODCs(II) are the only defects dominating in the materials under study.

Reaction dynamics of Cl + butanol isomers by crossed-beam sliced ion imaging

Butanol is now prominent among the prototype renewable biofuels. We have studied oxidation of a variety of butanol isomers under single collision conditions using chlorine atom as the oxidizing agent to gain detailed insight into the energetics and dynamics of these reactions. The interaction of chlorine atom radicals with butanol isomers: n-butanol, iso-butanol, sec-butanol, and tert-butanol have been studied by crossed-beam dc slice ion imaging techniques. The hydroxybutyl radicals generated from the H-abstraction processes were probed by single photon ionization using an F2 excimer laser. After background subtraction and density-to-flux correction of the raw images, translational energy distribution and product angular distributions were generated. At low collision energy, the hydroxyalkyl products are backscattered with respect to the alcohol beam and the scattering shifts to the forward direction as the collision energy is increased. The translational energy distributions are reminiscent to that of Cl + pentane reactions we studied earlier, i.e. a sharp forward peak -80% of the collision energy appears at the high collision energy. Isomer-specific details of the reactions will be discussed.

Depth profiling of anodic tantalum oxide films with gold cluster ions

AbstractIn order to investigate the merits of cluster bombardment for inorganic sputter depth profiling, thin (50‐nm) anodic Ta2O5 films were analyzed by ToF‐SNMS using 25 keV Aun+ projectile ions. The same focused primary ion beam was employed both for sputter erosion and (static) data acquisition. Sputtered neutral particles were postionized by means of single photon ionization using a 157 nm F2 excimer laser. The results show that, for the amorphous films investigated here, the transition from mono‐ to polyatomic projectiles does not lead to a significant improvement of the apparent depth resolution. Copyright © 2010 John Wiley & Sons, Ltd.

Effect of preionization, fluorine concentration, and current density on the discharge uniformity in F2 excimer laser gas mixtures

The discharge homogeneity in F2-based excimer laser gas mixtures and its dependence on various key parameters, such as the degree of preionization, preionization delay time, F2 concentration and current density, is investigated in a small x-ray preionized discharge chamber. The spatial and temporal evolution of the discharges is monitored by taking photographs of the discharge fluorescence with a fast intensified CCD camera. It is found that a preionization electron density of about 107 cm−3 bar−1 is sufficient to initiate a streamer-free homogeneous discharge in gas mixtures of helium and fluorine with multiatmospheric gas pressure. The accompanying optimum time delay between the application of the x-ray pulse and voltage across the discharge electrodes is determined to be about 20 ns. It is shown that in spite of these optimum initial conditions, a homogeneous glow discharge eventually transforms into an inhomogeneous discharge containing numerous filaments. Our experiments show that the higher the initial F2 concentration, the initial current density or the pump power density, the shorter the time interval over which the discharge stays homogeneous. By a quantitative characterization and defining a detailed measure of the observed discharge inhomogeneity we find that halogen depletion, as suggested from the theory, is responsible for the temporal instability of discharges in such laser gas mixtures, as the experimental results are in good agreement with the theory on the halogen depletion instability mechanism.

F2 excimer laser (157 nm) ablation of polymers: relation of neutral and ionic fragment detection and absorption

The ablation products of various polymers (triazene polymers, polyimides and glycidyl azide polymer) with 157 nm F2 laser irradiation were studied with time-of-flight mass spectroscopy, ion probe and white-light interferometry. In contrast to the ablation with longer UV wavelengths, 157 nm irradiation results in non-preferential bond-breaking and a much more pronounced fragmentation into fragments with masses <50 amu. This result suggests a photochemical ablation process that occurs at any bond in the molecule. In addition, ions have been detected at very low fluence similar to the threshold of neutral detection, which is below 30 mJcm−2 for the triazene polymers and Kapton. The observation of ions from the onset of ablation suggests a 2-photon ablation mechanism and possibly involves an excited neutral as an intermediate step. The low thresholds were verified by ion probe measurements of the ablation plume and white-light interferometry of the ablated target surface.

Fabrication of intrinsic fibre Fabry–Perot cavities in silica optical fibres via F2-laser ablation

This paper reports on a technique for fabricating intrinsic fibre Fabry–Perot cavities in fused silica optical fibres. An F2-excimer laser was used to micromachine parallel walled cavities through the diameter of optical fibres. A custom-designed beam delivery and target alignment system was used to enable the production of high aspect-ratio cavities in fibres. The optical response of the micro-cavities ablated in SMF 28 and SM 800 fibres at suitable energy densities showed Fabry–Perot fringes produced as a result of interference of Fresnel reflections at the cavity wall surfaces. The intrinsic fibre Fabry–Perot cavities were characterized using optical and scanning electron microscopy. Preliminary tests have shown that these intrinsic cavities can be used as strain sensors.

Deep etching of epitaxial gallium nitride film by multiwavelength excitation process using F2 and KrF excimer lasers

The deep etching of GaN(0001) thin films epitaxially grown on Al2O3(0001) has been investigated by laser ablation using F2 and KrF excimer lasers. The simultaneous irradiation with F2 and KrF excimer lasers markedly improves etching quality compared with single-KrF excimer laser ablation and provides almost the same quality as that in the case of single-F2 laser ablation with a high etching rate. Additionally, the present method achieves the deep etching of a GaN film of more than 5 μm in depth with steep side walls at an angle of 87° by changing the laser incidence angle.

Deep-ultraviolet-induced damage of charge coupled device sensors

In order to facilitate the inspection of deep-submicron features, a generation of semiconductor inspection systems are being pushed to image using deep-ultraviolet (UV) sensitive cameras at increasingly shorter wavelengths. In this article, we present a deep-UV sensitive thinned front-illuminated linear charge coupled device (CCD) image sensor structure and investigate its behavior after exposure to F2 (λ=157 nm) excimer laser. Two key manifestations of radiation damage are observed: (1) Extrinsic quantum efficiency drifts with increasing 157 nm exposure, and (2) dark current increases almost exponentially with 157 nm exposure. These fluctuations in CCD parameters can be caused by several factors including UV-induced color center formation in the oxide, charge generation in the oxide, interface modification, and structural rearrangement. These UV-induced effects alter the optical and electrical properties of the oxide and Si–SiO2 interface, resulting in both temporal and permanent shifts in device performance. The experimental results suggest that careful control of the oxide thickness and the Si–SiO2 interface quality are critical for realizing CCD sensors with high responsivity and stability for deep-UV imaging.

Optical-gradient antireflective coatings for 157-nm optical lithography applications.

We demonstrate an optical-gradient bottom antireflective coating (BARC) film, which can be easily prepared by treating a silicon nitride film with oxygen plasma. The oxygen composition is gradually decreased inside the silicon nitride film. The optical constants of the silicon nitride film are also changed gradually. A reflectance of less than 1% for various highly reflective substrates with high thickness-controlled tolerance has been achieved. The optical-gradient film is also shown to have high thermal stability during the postexposure bake procedure. Results indicate that the optical-gradient-type BARC is suitable in both ArF and F2 excimer lasers for sub-70-nm lithography applications.

Intense narrow-bandwidth extreme ultraviolet laser system tunable up to 20 eV

Narrow-bandwidth, broadly tunable vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) radiation has been generated up to 20 eV by resonance-enhanced four-wave mixing in argon using the five two-photon resonances of argon between 105 000 and 109 000 cm−1 corresponding to the optically accessible fine-structure components of the 3p54p electron configuration. These two-photon resonances were reached by using the output of an F2 excimer laser (157 nm) and the tripled output of a dye laser. The highest VUV/XUV intensities were obtained using the 3p54p′[1/2]0←3p6(1S0) two-photon resonance at 108 722.62 cm−1 in combination with the main excimer line. The conversion efficiency reaches an optimum for photon energies around 16 eV and slowly decreases when the photon energy rises to 20 eV. The use of the argon resonances also facilitates the generation of intense VUV laser radiation around 90 000 cm−1, a region that is not easily accessible by four-wave mixing with the commonly used two-photon resonances of krypton (202.3 and 212.5 nm) and xenon (222.6 and 249.6 nm). The bandwidth of the VUV/XUV laser radiation was measured to be less than 0.2 cm−1 over the entire range between 11 and 20 eV. The VUV/XUV laser system was used to measure the isotopic shift of the 2p54s[3/2]1←2p6(1S0) resonance in neon at 158 796 cm−1, and fully rotationally resolved photoelectron spectra of C2H2 around 92 000 cm−1 and CO2 between 140 000 and 146 000 cm−1. Improved values of the first adiabatic ionization energy of C2H2 (91 953.5±0.5 cm−1) and CO2 (111 112.3±0.8 cm−1) were determined.

Wetting and dissolution studies of fluoropolymers used in 157 nm photolithography applications

Photolithography using the F2 excimer laser at 157 nm, a technology to bridge traditional optical lithography and next generation lithographies, promises to enable ultralarge scale integrated devices with sub-70 nm design rules. Chemically amplified resists based on fluoropolymers have previously been shown to be good candidates for 157 nm microlithography. In our research, hexafluoroisopropyl alcohol (HFIPA) groups have been incorporated into polymers to improve the base solubility and to increase the transparency needed for new photoresists at 157 nm. These new polymers have absorbance values at 157 nm ranging from 1.7 to 3.9 μm−1. The introduction of fluorine groups increases their hydrophobicity and makes these polymers more difficult to wet at the surface. We have studied the effect of fluorine content on hydrophobicity of fluorinated polymers by measuring contact angle data over short time intervals. The ability to combine fluoropolymer synthesis with extensive contact angle studies has proven to be valuable in understanding solubility characteristics of these resins. These studies indicate that many of the fluorinated polymers have a very hydrophobic surface. Lowering contact angles can help to achieve the desired base solubility. Wetting studies also indicate that addition of certain surfactants to 0.26 N aqueous tetramethylammonium hydroxide (TMAH) help to reduce the contact angle. In addition to contact angle studies, we have used a quartz crystal microbalance to determine the dissolution rate of polymers in an aqueous base. Preliminary dissolution studies indicate a correlation to contact angle findings. Aqueous base solutions of 0.26 N TMAH containing surfactants were found to significantly lower the contact angle of an HFIPA-containing polymer, and increase the dissolution rate by an order of magnitude.

Fabry–Pérot structures for attenuated phase-shifting mask application in ArF and F2 lithography

We demonstrate a structure for an attenuated phase-shifting mask (APSM) which is based on a three-layer Fabry–Pérot structure for ArF (193 nm) and F2 (157 nm) excimer laser based lithographies. The APSM structure is composed of a metal/dielectric/metal stack. The optical characteristics of the Fabry–Pérot structure meet the following requirements: 180° phase shift, transmittance in the range of 4%–15%, reflectance of less than 10% at the exposure wavelength, and high inspection contrast at the inspection wavelength. Common dielectric and metal materials, such as chromium, tungsten, amorphous silicon, silicon dioxide, aluminum oxide, and aluminum nitride films, can be used to construct the Fabry–Pérot structure. The top metal layer can also prevent charge accumulation during electron-beam writing.

Spectroscopic properties of Pr3+ luminescence in complex fluoride crystals

Spectroscopic properties of several complex fluoride crystals K2YF5, KYF4, LiKYF5, CsY2F7 and CsGd2F7 doped with Pr3+ have been studied under synchrotron radiation and F2 excimer laser excitation. The Pr3+ doped K2YF5 and LiKYF5 crystals with only a single crystallographic site for Pr3+ ions show typical properties as in the case of spectroscopic systems where the low edge of the 4f5d Pr3+ configuration lies slightly below the 1S0 level of the 4f2 Pr3+ configuration. The Pr3+:KYF4 crystal has the same properties caused with only the 5d–4f luminescence of Pr3+ in spite of the fact that in this crystal there are six sites suitable for Pr3+ ions. The luminescence spectrum of the Pr3+:CsY2F7 crystal is determined by both the 5d–4f luminescence and the 4f–4f emission originating from the 1S0 Pr3+ level, which is due to two groups of optical Pr3+ centers with the energy levels of the 1S0 state just between the lowest 5d levels of Pr3+ for these two kinds of centers.

Approach for High-resolution of Chemically Amplified Resists Using Rate Editor Software

In discussions of resist resolution and process margins, knowledge of the resist development characteristics is more important than of primary importance. However, at present, there is insufficient understanding of the extent of the impact of various development characteristics on different process margins. The authors therefore analyzed photoresist development rate data obtained for various exposure doses, and developed rate analysis software that allows the development, and hence characteristics, of the resists to be modified freely. The design rule of VLSI devices is expected to reach the 70 to 100 nm scale by 2005 through lithography using F2 excimer lasers (157 nm). The software developed in this study was used to determine which development characteristics need be changed and to what extent, to obtain high resolution and a broad defocus margin for processes involving chemically amplified positive resist for use with ArF excimer lasers. The resist was improved based on the findings of this study, and the validity of the results was confirmed.

Observation of Species Ablated from Fused Silica by F2-KrF Multiwavelength Excitation Process.

Dynamics of ablation of fused silica by multiwavelength excitation process using F2 and KrF excimer laser has been investigated by energy analyzed mass spectrometry of ablated species. The number of generated Si+ ion by multiwavelength excitation process corresponds to that by single-F2 laser ablation and to approximately 2.1 times higher than that by single-KrF excimer laser ablation. In addition, kinetic energy distribution of Si+ ablated by multiwavelength excitation process shows almost same as that by single-F2 laser ablation. We regard that absorption of KrF excimer laser by excited state generated by F2 laser (excited-state absorption: ESA) causes effective photoionization, resulting in enhancement of Si+ with higher kinetic energy and then in high-quality ablation.

Development of High-Reflection Mirrors of Fluoride Multilayers for F2 Excimer Laser by Ion Beam Sputtering Method

High-reflection mirrors of fluoride multilayers were developed for a F2 excimer laser, using the ion beam sputtering method. Fluoride multilayer mirrors deposited by ion beam sputtering method are known to have good morphology and low scattering loss, but it was rather difficult to reduce their absorption loss. In this work, the reflectance of the mirror composed of AlF3/LaF3 layers was improved to 93.6% at a wavelength of 157 nm with an incidence angle of 45° by the optimization of deposition conditions. This value of reflectance is almost equivalent to that of mirrors deposited by the conventional deposition method.