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.     

Interaction of 157-nm excimer laser radiation with fluorocarbon polymers

Two important fluoropolymers, polytetrafluoroethylene [PTFE—(C₂F₄)_N] and polyvinylidene fluoride [PVDF—(C₂H₂F₂)_N], respond to 157-nm laser radiation in dramatically different ways. At fluences sufficient to produce rapid etching, the volatile emissions from PTFE are dominated by (CF₂)_N fragments. The velocities of the fastest (CF₂)_N molecules at each mass are consistent with kinetic energies on the order of an electron volt—and change little with fluence. This fluence independence suggests that the velocities are not affected by collisions after emission. To account for the high kinetic energies and the unusual, half-monomer mass distribution, we propose that these fragments are produced by photochemical scission of the polymer backbone, and that a fraction of the excitation energy is delivered to each fragment as kinetic energy. In contrast, the principle neutral species from PVDF is HF. HF is produced by the scission of C-F bonds, followed by chemical reactions with nearby hydrogen. This process is accompanied by the conjugation of backbone C-C bonds. The photochemical cleavage of C-C bonds in PTFE and C-F bonds in PVDF is consistent with the lower C-C bond energy of PTFE.     

157-nm laser micromachining of N-BK7 glass and replication for microcontact printing

We report etch-rate and probe-beam-deflection studies of N-BK7 glass ablated using a 157-nm F₂ laser. It is found that controllable material removal at the nanometre level is possible above an ablation threshold of 250 mJ cm^-2. Contact mask printing shows well-defined features can be micromachined in this glass with sub-micron resolution capability. Micro-features produced in this way have been replicated by polydimethylsiloxane (PDMS) moulding and the stamps used to print arrays of fluorescent molecules with sub-micron fidelity. PACS 45.55.Lt; 52.38.Mf; 87.80.Mj; 42.70.Ce     

Tritium recovery from co-deposited layers using 193-nm laser

Recovery of tritium from co-deposited layers formed in deuterium–tritium plasma operations of the TFTR (Tokamak Fusion Test Reactor) was investigated by the use of an ArF excimer laser operating at the wavelength of 193 nm. At the laser energy density of 0.1 J/cm², a transient spike of the tritium-release rate was observed at initial irradiation. Hydrogen isotopes were released in the form of hydrogen-isotope molecules during the laser irradiation in vacuum, suggesting that tritium can be recovered readily from the released gases. In a second experiment, hydrogen (tritium) recovery from the co-deposited layers on JT-60 tiles that had experienced hydrogen-plasma operations was investigated by laser ablation with a focused beam of the excimer laser. The removal rate of the co-deposited layers was quite low when the laser energy density was smaller than the ablation threshold (1.0 J/cm²), but reached 1.1 μm/pulse at the laser energy density of 7.6 J/cm². The effective absorption coefficient in the co-deposited layers at the laser wavelength was determined to be 1.9 μm^-1. The temperature of the surface during the irradiation at the laser energy density of 0.5 J/cm² was measured on the basis of Planck’s law of radiation, and the maximum temperature during the irradiation decreased from 3570 K at the initial irradiation to 2550 K at the 1000th pulse of the irradiation. Received: 5 August 2002 / Accepted: 7 August 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +81-29/2825917, E-mail: shu@tpl.tokai.jaeri.go.jp     

Surface delamination of polyimide using 355-nm nanosecond pulse laser

In this paper, we investigate the phenomena of the surface delamination of polyimide (PI) using a 355-nm nanosecond laser. Under the threshold fluence of the ablation, the surface starts to delaminate in a layer-by-layer with nanoscale thickness. As the number of laser pulses with low fluence increases, much more surface delamination occurs. Regarding the chemical characteristics from XPS and XRD measurements, such delamination is due to the layer-stacking structure, a unique molecular bonding structure of the PI wherein the boundaries of the layers become separated and delaminated by the laser.     

157-nm coherent light source as an inspection tool for F(2) laser lithography

We have developed a 157-nm coherent light source by two-photon resonant four-wave mixing in Xe, with two tunable single-mode 1-kHz Ti:sapphire laser systems at 768 and 681 nm. This light source has been developed to determine the instrumental function of a vacuum ultraviolet spectrometer and to evaluate optical designs for ultra-line-narrowed F(2) laser lithography. The spectral linewidth of the source was less than 0.008 pm (FWHM), with an average power of 0.6 mW.     

Resonance transition 795-nm rubidium laser using He buffer gas

We report a demonstration of a 795-nm rubidium optical resonance transition laser using a buffer gas consisting of pure ³He. This follows our recent demonstration of a hydrocarbon-free 795-nm rubidium resonance laser which used naturally-occurring He as the buffer gas. Using He gas that is isotopically enriched with ³He yields enhanced mixing of the Rb fine-structure levels. This enables efficient lasing at reduced He buffer gas pressure, improved thermal management in high average power Rb lasers and enhanced power scaling potential of such systems.     

Parameters optimization for biological molecules patterning using 248-nm ultrafast lasers

Laser-induced forward transfer has been used for the deposition of photoactive biotin in micron-scale patterns. The process uses a 500 fs pulsed KrF laser beam to transfer small amounts of a liquid solution target as micron-size droplets to a substrate placed parallel and in close proximity to it. The biomolecules remain active after the transfer; this is demonstrated through fluorescence assays. In addition to the laser parameters, those regarding the target composition and the receiving surface for the miniaturization of the transferred patterns have been studied and optimized. Droplets as small as 5 μm have been obtained by reducing the target thickness and transfer energy; by increasing the percentage of glycerol added in the biomolecules solution and by using hydrophobic surfaces as receiving substrates. The influence of the glycerol addition and the hydrophobicity of the receiving surfaces on the activity of the transferred biomolecules have also been studied.     

Beam cleanup of a 532-nm pulsed solid-state laser using a bimorph mirror

A successful beam cleanup of a 5-mJ/200-μs pulsed solid-state laser system operating at 532-nm wavelength is demonstrated. In this beam cleanup system, a wave-front sensor-less adaptive optics (AO) system is set up with a 20-element bimorph mirror (BM), a high-voltage amplifier, a charge-coupled device camera, and a control software implementing the stochastic parallel gradient descent (SPGD) algorithm. The brightness of the laser focal spot is improved because the wave-front distortions have been compensated. The performance of this system is presented and the experimental results are analyzed.     

Tapered Bragg reflection waveguide edge emitting lasers with near circular twin-beam emission简

Experiments on developing a frequency-stabilized 555.8-nm laser are presented. The 555.8-nm laser＇ is obtained by frequency doubling of a l lll.6-mn diode laser through single-passing a periodically poled lithium niobate （PPLN） waveguide. Tile 555.8-nm laser is then locked to a stable high-finesse Fabry Perot （FP） cavity by tile Pound Drever--Hall （PDH） technique. Tile finesse of the cavity is measured by tile heterodyne cavity ring-down spectroscopy technique. The linewidth of the 555.8-nm laser is investigated. Alter the laser is locked, the laser line width is reduced to about 3 kHz. This frequency-stabilized 555.8-nm laser is used in experiments on the laser cooling and trapping of ytterbium atoms to develop an ytterbium optical clock.     

Hyperfine structure in Iodine at the 612-nm and 640-nm helium-neon laser wavelengths

The hyperfine structure of iodine-127 at 612 nm and 640 nm is observed by saturated absorption in a gas cell placed within a He-Ne laser cavity modified to operate at these wavelengths. At 612 nm, all the 21 components of the strong R 47 9-2 line lie within the laser gain profile, and the resulting saturated absorption peaks have a contrast of 10% at a gas pressure of 3 Pa.     

Applications of the 308-nm excimer laser in dermatology

Excimer lasers contain a mixture of a noble inert gas and a halogen, which form excited dimers only in the activated state. High-energy current is used to produce these dimers, which have a very short lifetime, and after their fast dissociation they release the excitation energy through ultraviolet photons. The application of these lasers proved to be successful in medicine, including the field of ophthalmology, cardiology, angiology, dentistry, orthopaedics, and, in recent years, dermatology. For medical purposes, the 193-nm argon fluoride, the 248-nm krypton fluoride, the 351-nm xenon fluoride, and the 308-nm xenon chloride lasers are used. Recently, the 308-nm xenon chloride laser has gained much attention as a very effective treatment modality in dermatological disorders. It was successfully utilized in psoriasis; later, it proved to be useful in handling other lightsensitive skin disorders and even in the treatment of allergic rhinitis. This review summarizes the possible applications of this promising tool in dermatology.     

Achievement of laser fusion of biological cells using UV pulsed dye laser beams

This paper reports a successful achievement of laser-induced biological cell fusion using an excimer laser excited dye laser. To our knowledge, we examined for the first time the dependence of fusion rate on laser pulse energy, number of laser pulses, and laser wavelength. Maximum fusion rate of approximately 50% was obtained by this laser fusion technology.     

Characteristics of blood fluorescence spectra using low-level, 457.9-nm excitation from Ar+ laser

We measured the fluorescence spectra of the whole blood, the red blood cell (RBC) and the hemoglobin using 457.9-nm Ar+ laser excitation. It was found that the fluorescence spectra of the whole blood and the RBC have much similarities in the intensity, the emission peaks and the emitting region, and abundant peaks can be found. But for the hemoglobin, fluorescence could only be found in the wavelength range 580-650 nm. It was concluded that in the wavelength range of 650-850 nm, the fluorescence spectra were emitted by the new fluorophores generated by the breakdown of some weak bonds on the RBC membrane,such as the C-C bond and the C-N bond. In the wavelength range of 590 - 650 nm, the fluorescence spectra are mainly emitted by the hemoglobin, but the hemoglobin solution of cracked RBC has a strong quencher effect on the fluorescence spectrum. The experimental result and the theoretical analysis are meaningful for the medical diagnostics and the therapy.     

Frequency stabilization of a 214.5-nm ultraviolet laser

An improved method for stabilizing a frequency-quadrupled 214.5-nm tunable diode laser system is reported. Improvements to the method include a homemade logic circuit and the use of a Fabry-Perot optical spectrum analyzer as a transfer cavity. Lasers locked with this method exhibit megahertz-level frequency stability measured with an optical frequency comb referenced to a cesium atomic standard. The laser can be locked for hours to days, depending on experiment requirements. Being relatively inexpensive, stable, and robust, the control method can be applied to stabilizing essentially all lasers of deep ultraviolet wavelengths.     

Saturated high-repetition-rate 18.9-nm tabletop laser in nickellike molybdenum

We report saturated operation of an 18.9-nm laser at 5-Hz repetition rate. An amplification with a gain-length product GL of 15.5 is obtained in the 4d 1S0-4p 1P1 laser line of Ni-like Mo in plasmas heated at grazing incidence with approximately 1-J pulses of 8.1-ps duration from a tabletop laser system. Lasing is obtained over a broad range of time delays and pumping conditions. We also measure a GL of 13.5 in the 22.6-nm transition of the same ion. The results are of interest for numerous applications requiring high-repetition-rate lasers at wavelengths below 20 nm.     

High-speed machining of glass materials by laser-induced plasma-assisted ablation using a 532-nm laser

+ :YAG laser (532 nm). The plasma generated from a silver (Ag) target by the laser irradiation effectively assists in ablation of the fused quartz substrate by the same laser beam, although the laser beam is transparent to the substrate. A grating with a cross-sectional shape like a square-wave (period ≈ 20 μm) is achieved using the mask projection technique. The ablation rate reaches several tens nm/pulse. In addition, LIPAA is applied to high-speed hole drilling (700 μm in diameter) of fused-quartz (0.5 mm thick) and Pyrex glass (0.5 mm thick). Received: 25 May 1998/Accepted: 19 June 1998     

Fabrication of strong long-period gratings in hydrogen-free fibers with 157-nm F2-laser radiation

Long-period gratings were fabricated in standard telecommunication fiber (Corning SMF-28) by use of what is believed to be record short-wavelength light from a 157-nm F(2) laser. Strong loss peaks were formed without the need for enhancement techniques such as hydrogen loading. The magnitude of the attenuation peak was sensitive to the single-pulse laser fluence, decreasing with increasing pulse fluence as a result of nonuniform 157-nm laser interaction with both the fiber cladding and core. The long-period fiber gratings have good wavelength stability (Dlambda~7 nm) under thermal annealing at 150 degrees C.     

Excitation of sodium atoms by 330-nm laser pulses

1/2 ) atoms in a dense sodium vapour irradiated by nanosecond laser pulses tuned near the 3S→4P transition was investigated. It was observed that the population of Na(4P) atoms remained high only within the laser pulse, in spite of the relatively long lifetime of the 4P level (110.ns)The 3P_1/2 level, which is populated as a result of cascade transitions from the higher levels, reached the highest population several nanoseconds after the laser shot. The fast population changes are explained by cascade-stimulated transitions between the excited atomic levels. Received: 16 July 1997/Revised version: 27 October 1997