F2 Excimer Laser Research Articles

Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation.

Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to approximately 1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nm(rms)) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.

Fluoride Glasses for Laser Materials. Transmittance and Refractive Index in VUV Region.

Optical transmission and refractive indexes in vacuum ultraviolet (VUV) region are very important and provide basic data for lens systems in micro lithography equipments and substrate materials using F2 excimer laser as a lightsource. High purity multi-component fluoroberyllate glasses are promising candidates for the deep UV transmitting optical materials due to their exceptionally large optical band gaps compared to those of the other fluoride glass systems. The optimum glass composition for VUV glass material were introduced in terms of glass forming ability and VUV absorption of impurities in the glasses. We also measured refractive indexes of the glasses in VUV region. In order to predict the refractive index, we presented compositional parameter for fluoride glass based on our measured data.

F2 Laser Resist with Fluorinated Polymers.

Fluorinated organic compounds are applied as F2 laser resists due to their high transparency at 157nm. The relationship between numbers/positions of fluorine atoms in a repeating unit of the fluorinated polymers and their physical/chemical properties (solubility parameter and hydrophilicity) for F2 laser resist is discussed. Fluorine at suitable position improves hydrophilicity without lowering the solubility parameter.We propose a novel polymer suitable for F2 excimer laser resists consisting of the polymer with fluorine atoms connecting to proper positions.

Influence of laser fluence and irradiation timing of F 2 laser on ablation properties of fused silica in F 2 -KrF excimer laser multi-wavelength excitation process

A collinear irradiation system of F2 and KrF excimer lasers for high-quality and high-efficiency ablation of hard materials by the F2 and KrF excimer lasers’ multi-wavelength excitation process has been developed. This system achieves well-defined micropatterning of fused silica with little thermal influence and little debris deposition. In addition, the dependence of ablation rate on various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number is examined to investigate the role of the F2 laser in this process. The multi-wavelength excitation effect is strongly affected by the irradiation timing, and an extremely high ablation rate of over 30 nm/pulse is obtained between -10 ns and 10 ns of the delay time of F2 laser irradiation. The KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing F2 laser fluence. Moreover, the ablation rate shows a linear increase with the logarithm of KrF excimer laser fluence when the F2 laser is simultaneously irradiated, while single KrF excimer laser ablation shows a nonlinear increase. The ablation mechanism is discussed based on these results.

Interaction of F2 excimer laser pulses with hydroxy groups in SiO2 glass: Hydrogen bond formation and bleaching of vacuum ultraviolet absorption edge

Excitation of vacuum ultraviolet absorption band of SiOH groups by F2 excimer laser photons (7.9 eV) was performed for synthetic SiO2 glasses. Irradiation induced two concurrent optical changes; red-shift of SiOH infrared absorption band and bleaching of vacuum ultraviolet absorption edge which is primarily controlled by SiOH absorption. These optical changes have the same origin, that is, the structural alternation of SiOH from isolated to metastable hydrogen-bonded states. The F2-induced hydrogen-bonded state was thermally stable to ∼300 °C and the kinetic barrier for the thermal relaxation was ∼1.7 eV. The formation of hydrogen-bonded state proceeded via one-photon absorption processes for F2 laser irradiation, but did not occur for ArF laser (6.4 eV) irradiation. This F2-induced hydrogen-bonded state is distinguished from laser-induced compacted state because the latter occurs for ArF or F2 laser irradiations via two-photon absorption processes.

Formation and decay of nonbridging oxygen hole centers in SiO2 glasses induced by F2 laser irradiation: In situ observation using a pump and probe technique

Formation and decay of nonbridging oxygen hole centers (NBOHC, an oxygen dangling bond) in SiO2 glasses by F2 excimer laser (7.9 eV) irradiation were in situ analyzed by monitoring 1.9 eV photoluminescence of NBOHC using a pump and probe technique. In wet SiO2, the SiO–H bond was efficiently photolyzed by F2 laser photons to form NBOHC with a quantum yield of ∼0.2. However, the recombination with dissociated hydrogenous species suppressed the buildup of NBOHC. In dry SiO2, in contrast, NBOHC formation by dissociation of strained Si–O–Si bonds was inefficient but NBOHC accumulated with the number of F2 pulses due to a negligibly slow reverse recombination reaction.

Modification of sol–gel-derived amorphous Al2O3 thin films by F2 excimer laser irradiation at ambient temperature

F2 excimer laser (157 nm) irradiation was applied to modify sol–gel-derived amorphous Al2O3 thin films at ambient temperature. The surface morphology and density of the film were significantly altered by the laser irradiation (power: 2 mW/cm2/pulse). The surface properties of the film were also changed from hydrophilic to hydrophobic. These alterations were not observed when using ArF excimer laser (193 nm) irradiation at the same laser power as that of the F2 laser. It was found that the changes induced by F2 laser irradiation mainly arose from the direct photoexcitation of C = O groups in ethylacetoacetate, which was added as a chelating agent of aluminum-alkoxides. Consequently, photochemical reactions of the Norrish-type occur, resulting in the formation of hydrocarbon or olefin and the elimination of carbon monoxide (CO) or decomposition products. The elimination of CO is considered to cause the marked change in structure and surface properties of the film. Patterning of the gel films was successfully performed by using these findings.

Delivery of F2-excimer laser light by aluminum hollow fibers

A hollow fiber composed of a glass-tube substrate and an aluminum thin film coated upon the inside of the tube delivers F2-excimer laser light. A smooth, aluminum thin film was deposited by using metal-organic chemical vapor deposition using dimethylethylamine:alane (DMEAA) as the precursor. It was shown that the transmission loss of the fiber with a 1.0-mm inner diameter was as low as 0.5 dB/m for the fiber with 1.0-mm diameter when the bore of the fiber is pressurized with an inert gas to remove the absorption of air. When the fiber is bent at the radius of 30 cm, the additional loss was 1.6 dB.

Interaction of F2 excimer laser with SiO2 glasses: Towards the third generation of synthetic SiO2 glasses

Changes in optical absorption spectra and defect formation of different types of synthetic SiO2 glasses were examined by irradiation with F2 excimer laser pulses (157 nm). Fluorine-doped, OH-free SiO2 glasses exhibit high optical transmittance (∼80%) at 157 nm in an as-delivered state and the intensity of F2-laser-induced absorption is much less than that in wet or dry F-free samples. The effect of F-doping on the blue shift of the absorption edge and suppression of color center formation was conspicuous up to 1 mol% but was slight upon further doping. It is suggested that elimination of strained Si–O–Si bonds upon F-doping is the primary reason of the improvement of resistance of SiO2 glasses to F2-laser light. Novel optical phenomena by F2-laser irradiation, bleaching of the vaccum UV (VUV) absorption edge and changes in the SiOH infrared absorption, were found in H2-impregnated, or wet SiO2 glasses. These results lead to the conclusion that F-doping to 1 mol% is an effective and practical method to obtain synthetic SiO2 glasses for F2 excimer laser optics as a photomask in optical lithography.

Development of Photochemical Analysis System for F2-Excimer Laser Lithography Processes

A system for photochemical analysis of F2-excimer laser lithography processes has been developed. The system, VUVES-4500, consists of 3 units: (1) an exposure and bake unit that uses the F2-excimer laser to carry out a flood exposure and then post-exposure bake (PEB), (2) a unit for measurement of the development rate of photoresists, and (3) a simulation unit that utilizes PROLITH of profile simulation software to calculate the resist profiles and process latitude using the measured development rate data. With this system, preliminary evaluation of the performance of F2-Excimer laser lithography can be performed without the use of a lithography tool capable of imaging and alignment. Profiles for 150 nm lines are simulated for the PAR-101 resist (manufactured by Sumitomo Chemical) and the SAL-601 resist (manufactured by Shipley), a chemically amplified resist that has sensitivity at the F2-excimer laser wavelength. The simulation successfully predicts the resist behavior. Thus it is confirmed that the system enables efficient evaluation of the performance of F2-excimer laser lithography processes.

Poly (.ALPHA.-methyl-p-hydroxystyrene-co-methacrylonitrile) Based Single-Layer Resists for VUV Lithography: (2) F2 Excimer Laser Exposure Characteristics.

We have developed a novel single-layer resist for VUV lithography using a terpolymer of methacrylonitrile, p-tert-butoxycarbonyloxy-α-methylstyrene and p-hydroxy-α-methylstyrene. The transmittance at 157nm of this resist was pretty high (-35% per 1000 A-thickness) due to methacrylonitrile units with low absorption. This resist indicated the positive-resist behavior without crosslinking. The 220nm line at 1500A-thick resist and 300nm line at 2000A-thick resist were obtained by F2 excimer laser contact exposure.

New aspects of micromachining and microlithography using 157-nm excimer laser radiation

The use of F2 excimer laser sources, emitting at 157 nm, constitutes a new promising tool for scientific, industrial and lithography applications. The 157-nm laser emission enables high-resolution processes and the high photon energy offers the unique possiblity of photoionizing molecules in a single step. Therefore a lower fragmentation or thermal loading takes place. The 157-nm radiation will enable fundamental research and development for deep UV (DUV) high-resolution optical microlithography in the manufacturing of integrated circuits. This is the next step from the technology of ArF lasers at 193 nm. Furthermore, benefits are expected for key technologies requiring high-resolution processing and the micromachining of tough materials like Teflon or fused silica for micro-optics fabrication. Such applications require F2 excimer laser sources with high performance, reliability and efficiency. The world of nanotechnology is just beginning to reveal its potential.

Fluorine-doped SiO_2 glasses for F_2 excimer laser optics:?fluorine content and color-center formation

Color-center formation in F-doped, OH-free synthetic SiO(2) glasses by irradiation with F(2) excimer lasers (157 nm) was examined as a function of the F content. The concentration of photoinduced E(') centers was reduced to approximately 1/20 by 1 mol.% F(2) doping and remained almost constant on further doping to 7.3 mol. %. The absorption edge was considerably shifted to a lower wavelength (157.4 nm -->153 nm for a 5-mm-thick sample) by 1-mol. % doping and decreased only slightly on further doping. The intensities of the Raman bands that are due to three- and four-membered ring structures were significantly reduced by 1-mol. % F doping. These results strongly suggest that elimination of strained Si-O-Si bonds by F doping plays a central role in the improvement of radiation resistance of SiO(2) glasses to F(2) laser light.

F-doped and H2-impregnated synthetic SiO2 glasses for 157 nm optics

Optical transmission change and point defect formation by irradiation with F2 excimer laser light (∼10 mJ/cm2/pulse×3.6×105 pulses, λ=157 nm) were examined for four types of synthetic SiO2 glasses: wet (OH content: 1.7×1019 cm−3), dry (OH content: <1×1016 cm−3), F-doped (F content: 2.2×1020–4.2×1020 cm−3, OH content: <1×1016 cm−3), and H2-impregnated (OH content; 1.9×1018 cm−3, H2 content; 1×1018 cm−3) SiO2 glasses. Intense optical absorption bands peaking at 4.8 and 5.8 eV, which are attributed to nonbridging oxygen hole center and E′ center, respectively, and a shoulder at ∼7 eV were induced in the wet and dry specimens after the irradiation. On the other hand, the intensities of absorption bands induced in the F-doped and H2-impregnated specimens were smaller by an order of magnitude than those in other specimens. The internal (reflection-corrected) transmittance at the wavelength of 157 nm for 5 mm thick samples after the irradiation was 87% for the F-doped, 82% for the H2-impregnated, 14% for the wet, and 2.4% for the dry silica glasses. The present results show that F doping or H2 impregnation is an effective way to improve resistance of SiO2 glasses to F2 laser damage.

Rotatable source crossed molecular beams apparatus with pulsed ultraviolet/vacuum ultraviolet photoionization detection

A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus employs two rotatable molecular beam sources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionization of reaction products. Electronically cold neutral supersonic transition metal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spectroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoionization of the ZrC2H2 products from the crossed beam reaction Zr+C2H4→ZrC2H2+H2 is carried out using the pulsed 157 nm radiation from a F2 excimer laser in the UHV region of the mass spectrometer. Compared to conventional electron impact ionization, 157 nm photoionization improves signal-to-noise ratios by more than two orders of magnitude for experiments using pulsed beam sources where cross correlation methods cannot be used. We also demonstrate the use of 1+1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atoms from decay of long-lived collision complexes. Due to the small reaction cross sections for Y+C2H6→YC2H4+H2 and Y+CD3CDO→DYCD3+CO, these reactions could not be studied using electron impact ionization. However, photoionization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoionization technique facilitates studies of transition metal systems not previously amenable to the crossed beams method.

CMOS device fabrication and the evolution of optical lithographic exposure tools

Advances in the performance of integrated circuits have long been fuelled by continuing progress in microlithography. For at least thirty years, the minimum feature size of leading edge semiconductor devices have been reduced to 70% of the previous generation every 3 years. In recent years, this extraordinary shrink rate in feature size has accelerated, so that production of 250nm generation microprocessors and DRAMs started in 1997, a year before the 1994 National Technology Roadmap for Semiconductors forecast, and initial production of 180nm generation devices is now forecast for 1999, a full two years before the 1994 roadmap predicted. In this paper, aspects of optical lithography being developed to address the requirements of advanced CMOS device production will be outlined. The extraordinary synergy between the requirements of IC fabrication and the evolution of optical lithographic projection systems will be explored. The 180nm and 150nm device generations will use 248nm lithography on high NA, KrF excimer laser steppers and scanners and DUV chemically amplified resists. It is likely that the 130 and 100nm generations will be supported by 193nm lithography using ArF scanning tools and VDUV CA resists. A further extension of optical lithography, to 157nm, using the F2 excimer laser as a source, is possible to address production at 100 and 70nm. Beyond the 70nm generation node, conventional optical lithography will become difficult, if not impossible, to use to fabricate the most critical masking layers and other, post-optical, lithography technologies may start to find their niches in IC manufacturing.

Vacuum-Ultraviolet Light Sources and Their Applications for Processings. VUV Lithography.

The semiconductor industry, whose long period of sustained growth is in no small measure due to the optical lithography process, is now on the verge of a dilemma. Optical lithography has arrived at a crossroads, and after many years of steady improvement in device performance, device integration, and cost reduction, the industry is facing a major crisis. In Japan, Europe and the U.S., consortiums comprising the entries from government, business, and the academic world have been formed in an effort to ward off the coming crisis. Their work seeks to extend the useful life of optical lithography as well as to foster the development of post-optical lithographic processes. A particular problem they are confirming is to ascertain how the development of sub-0.1μm lithographic technology will affect the economical manufacturing of semiconductors. This paper discusses the limits of current optical lithography such as VUV lithography, especially F2 excimer laser and worldwide trends in developing post-optical lithographic processes. Future miniaturization trends in semiconductor production are also discussed.

Optical Materials for Vacuum Ultraviolet Lithography: Interaction of Materials with Excimer Laser Pulses.

Recent advances in optical materials for vacuum ultraviolet (VUV) lithography are reviewed from a view of interaction of excimer laser light with materials. Synthetic silica glasses applicable for F2 excimer laser totally differs from those for KrF and ArF lasers. A novel phenomenon was found in H2-impregnated SiO2 glasses byirradiation with F2 laser light pulses, i. e., bleaching of VUV absorption edge. Degradation of optical transmissionat 193 nm of calcium fluoride single crystals is primarily determined by the concentration of specific impuritycations such as Y3+ and Tb2+. Time resolved fluorescence was an effective technique to make screening of the fluoride crystals with high tightness to ArF laser pulses.

Stability of clusters of fullerenes

We have studied the formation and stability of pure and mixed clusters of C60 and C60/C70 produced in a gas aggregation source. The source yields very smooth abundance distributions indicating an efficient cooling. The stability of the clusters was probed by varying the degree of heating with a XeCl excimer laser (hvI.P.) followed by single photon ionisation with a F2 excimer laser. The stability patterns show features related to geometrical packing structures but much stronger than previously seen in cluster spectra. The fragmentation patterns for the mixed clusters indicate that C60 and C70 have similar binding energies in the clusters in contrast to what would be expected from vapour pressure data of macroscopic samples.

Photoluminescence and Its Excimer Laser Irradiation Effects in SiO2 Film Prepared by Photo-Induced Chemical Vapor Deposition

Defects and other imperfections in photo-induced chemical vapor deposition (photo-CVD) SiO2 films have been characterized by photoluminescence excited by ArF and F2 excimer lasers. Photoluminescence peaks have been observed at around 2.7, 3.5 and 4.4 eV and are characteristic of both the deposition temperatures of photo-CVD and excitation energy. The intensity of the photoluminescence peak at around 2.7 eV initially increases with ArF excimer laser irradiation but decreases after about 2000 shots. On the other hand, it decreases monotonously with F2 excimer laser irradiation. The origins of these peaks have been discussed in terms of various dependences of photoluminescence and absorption spectra.