Oxygen plasma and high pressure H<Subscript>2</Subscript>O vapor heat treatments used to fabricate polycrystalline silicon thin film transistors

Oxygen plasma and high pressure H₂O vapor heat treatment were applied to fabrication of n-channel polycrystalline silicon thin film transistors (poly-Si TFTs). 13.56 MHz-oxygen-plasma treatment at 250 °C, 100 W for 5 min effectively reduced defect states of 25-nm-thick silicon films crystallized by 30 ns-pulsed XeCl excimer laser irradiation. 1.3×10⁶-Pa-H₂O vapor heat treatment at 260 °C for 3 h was carried out in order to improve electrical properties of SiO_x gate insulators and SiO_x/Si interfaces. A carrier mobility of 470 cm²/V s and a low threshold voltage of 1.8 V were achieved for TFTs fabricated with crystallization at 285 mJ/cm². Received: 18 November 2002 / Accepted: 25 November 2002 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +81-42/388-7109, E-mail: tsamesim@cc.tuat.ac.jp     

Boron isotope enrichment in nanosecond pulsed laser-ablation plume

Boron isotopic enrichment is observed in the laser ablation of B₄C target using nanosecond (ns) wide 532 nm laser beam of a Nd-YAG laser. B¹⁰/B¹¹ ratio of 0.9 against the natural abundance of 0.25 is obtained at a laser power density of 8×10⁸ W/cm² (fluence of 6.4 J/cm²). The enrichment as a function of laser power density is demonstrated using a quadrupole mass spectrometer. Apart from higher enrichment factor, only singly charged ions are found in the laser plume from the B₄C target, in contrast to the multiply charged ions from the BN target reported in a recent report using femtosecond (fs) laser pulses. This study indicates the possibility of using less expensive, widely used ns lasers, which can also yield a higher throughput per pulse than a fs laser for isotope enrichment. Received: 28 September 2001 / Accepted: 4 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +91-4114/480-065, E-mail: mj@igcar.ernet.in     

Production of biologically inert Teflon thin layers on the surface of allergenic metal objects by pulsed laser deposition technology

Allergic-type diseases are current nowadays, and they are frequently caused by certain metals. We demonstrated that the metal objects can be covered by Teflon protective thin layers using a pulsed laser deposition procedure. An ArF excimer laser beam was focused onto the surface of pressed PTFE powder pellets; the applied fluences were 7.5–7.7 J/cm². Teflon films were deposited on fourteen-carat gold, silver and titanium plates. The number of ablating pulses was 10000. Post-annealing of the films was carried out in atmospheric air at oven temperatures between 320 and 500 °C. The thickness of the thin layers was around 5 μm. The prepared films were granular without heat treatment or after annealing at a temperature below 340 °C. At 360 °C a crystalline, contiguous, smooth, very compact and pinhole-free thin layer was produced; a melted and re-solidified morphology was observed above 420 °C. The adhesion strength between the Teflon films and the metal substrates was determined. This could exceed 1–4 MPa depending on the treatment temperature. It was proved that the prepared Teflon layers can be suitable for prevention of contact between the human body and allergen metals and so for avoidance of metal allergy. Received: 12 June 2002 / Accepted: 13 June 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. E-mail: bhopp@physx.u-szeged.hu     

Wet bone ablation with mechanically Q-switched high-repetition-rate CO2 laser

2 laser using a miniature water spray is demonstrated. An ablation threshold of 1.4 J/cm², an optimal energy density of 9–10 J/cm², and a corresponding specific ablation energy of 25–30 J/mm³ are found for pig thighbone compacta at λ=9.57 μm and a beam waist diameter of 0.5 mm. The water spray alleviates tissue carbonization even at high laser pulse repetition rates and increases ablation efficiency. Received: 9 March 1998/Revised version: 6 July 1998     

Influence of storage time on laser cleaning of SiO<Subscript>2</Subscript> on Si

The influence of the ‘storage time’ τ_s on the threshold fluence φ_cl and the efficiency in dry laser cleaning is investigated. τ_s denotes the time between the deposition of particles and the cleaning. As a model system we employed silica spheres with diameters of 500 nm and 1500 nm on commercial silicon wafers and single-pulse KrF excimer laser radiation (τ_FWHM=28 ns). For the 1500-nm silica spheres, φ_cl was found to increase from about 65 mJ/cm² to 125 mJ/cm² for storage times of 4 h and 362 h, respectively. For 500-nm silica spheres the increase in the threshold fluence was less than 20% for storage times up to 386 h. Received: 12 July 2002 / Accepted: 12 July 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. E-mail:dieter.baeurle@jku.at     

Tailored ablation processing of advanced biomedical hydroxyapatite by femtosecond laser pulses

The micromachining of hydroxyapatite (HAp) is highly important for orthopedics and dentistry, since human bone and teeth consist mainly of HAp. We demonstrate ultrashort Ti:sapphire laser ablation of HAp, using pulse-widths of 50 fs, 500 fs, and 2 ps at a wavelength of 820 nm and at 1 kpps. The crucial medical issue is to preserve the chemical properties of the machined (ablated) surface. If the chemical properties of HAp change, the human bone or tooth cannot re-grow after laser processing. Using X-ray photoelectron spectroscopy, we observe chemical properties of HAp ablated in air. The HAp is ablated at laser fluences of 3.2 J/cm² (6.4×10¹³ W/cm² at 50 fs), 3.3 J/cm² (6.6×10¹² W/cm² at 500 fs), and 9.6 J/cm² (4.8×10¹² W/cm² at 2 ps), respectively. As a result it is found that the ablated surface is unchanged after laser ablation over the pulse-width range used in this experiment. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-45/566-1533, E-mail: obara@obara.elec.keio.ac.jp     

VUV F<Subscript>2</Subscript> laser ablation of sodium chloride

We report an investigation of the ablation of NaCl crystals at the 157-nm wavelength of the F₂ laser where there is very strong excitonic absorption. Probe-beam deflection and etch-rate measurements show that the interaction is characterised by a low ablation threshold (∼80 mJ cm^-2) and a capability for controllable material removal at the nanometer level. Scanning electron microscopy of the exposed surfaces show this to be microscopically smooth but with fine cracks present. It is demonstrated that micron-scale features can be formed in NaCl using 157-nm laser ablation, a result attributed to the strongly localised optical and thermal nature of the interaction. The results are discussed within the framework of a thermal vaporisation model. Received: 29 May 2002 / Accepted: 17 July 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1482/465606, E-mail: p.e.dyer@hull.ac.uk     

Characteristics of SrBi2Ta2O9 ferroelectric films on GaAs with a TiO2 buffer layer

Mainly [115]-oriented SrBi₂Ta₂O₉ (SBT) films were prepared on GaAs(100) substrates with TiO₂ buffer layers. Both the SBT films and the TiO₂ buffer layers were deposited by pulsed laser deposition (PLD) using a KrF excimer laser. The depth profile of the constituent elements observed by Auger electron spectrometry (AES) shows no remarkable diffusion at both the interfaces between SBT and TiO₂ and between TiO₂ and the GaAs substrate. The electrical characteristics of the Pt/SBT/TiO₂/GaAs(100) structures show a ferroelectric hysteresis loop with a small remanent polarization (∼0.5 μC/cm²). Received: 1 March 2002 / Accepted: 3 March 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/3595535, E-mail: xhliu81@hotmail.com RID="**" ID="**"Present address: Data Storage Institute, DSI Building, 5, Engineering Drive 1 (off Kent Ridge Crescent, NUS) 117608 Singapore     

Fundamentals and applications of polymers designed for laser ablation

The ablation characteristics of various polymers were studied at low and high fluences for an irradiation wavelength of 308 nm. The polymers can be divided into three groups, i.e. polymers containing triazene groups, designed ester groups, and reference polymers, such as polyimide. The polymers containing the photochemically most active group (triazene) exhibit the lowest thresholds of ablation (as low as 25 mJ cm^-2) and the highest etch rates (e.g. 250 nm/pulse at 100 mJ cm^-2), followed by the designed polyesters and then polyimide. Neither the linear nor the effective absorption coefficients have a clear influence on the ablation characteristics. The different behavior of polyimide might be explained by a pronounced thermal part in the ablation mechanism. The laser-induced decomposition of the designed polymers was studied by nanosecond interferometry and shadowgraphy. The etching of the triazene polymer starts and ends with the laser pulse, indicating photochemical ablation. Shadowgraphy reveals mainly gaseous products and a pronounced shockwave in air. The designed polymers were tested for an application as the polymer fuel in laser plasma thrusters. Received: 21 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/3104-412, E-mail: thomas.lippert@psi.ch     

Ion synthesis and laser annealing of Cu nanoparticles in Al2O3

Al₂O₃ samples with Cu nanoparticles, synthesised by ion implantation at 40 keV with a dose of 1×10¹⁷ ion/cm² and a current density from 2.5 to 12.5 μA/cm², were annealed using ten pulses from a KrF excimer laser with a single pulse fluence of 0.3 J/cm². The copper depth distribution, formation and modification of metal nanoparticles under the ion implantation and laser treatment were studied by Rutherford backscattering (RBS), energy dispersive X-ray (EDX) analysis, atomic force microscopy (AFM) and optical spectroscopy. It was found that laser annealing leads to a reduction in the nanoparticle size without diffusion of metal atoms into the bulk. The change in particle size and the possibility for oxidation of the copper particles are examined in the framework of Mie theory. Calculations presented show that under excimer laser treatment, Cu nanoparticles are more likely to be reduced in size than to undergo oxidation. Received: 19 April 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Ablation with a single micropatterned KrF laser pulse: quantitative evidence of transient liquid microflow driven by the plume pressure gradient at the surface of polyesters

A microscopic flow of a transient liquid film produced by KrF laser ablation is evidenced on targets of PET and PEN. Experiments were done by using single pulses of the excimer laser beam micropatterned with the aid of submicron projection optics and grating masks. The samples of various crystalline states, ablated with a grating-forming beam (period Λ=3.7 μm), were precisely measured by atomic force microscopy, in order to evidence any deviation from the ablation behavior predicted by the current theory (combination of ablation curve and beam profile). This was confirmed by comparing various behaviors dependent on the polymer nature (PC, PET and PEN). PC is a normally ablating polymer in the sense that the ablated profile can be predicted with previous theory neglecting liquid-flow effects. This case is called ‘dry’ ablation and PC is used as a reference material. But, for some particular samples like crystalline PET, it is revealed that during ablation a film of transient liquid, composed of various components, which are discussed, can flow under the transient action of the gradient of the pressure of the ablation plume and resolidify at the border of the spot after the end of the pulse. This mechanism is further supported by a hydrodynamics theoretical model in which a laser-induced viscosity drop and the gradient of the plume pressure play an important role. The volume of displaced liquid increases with fluence (0.5 to 2 J/cm²) and satisfactory quantitative agreement is obtained with the present model. The same experiment done on the same PET polymer but prepared in the amorphous state does not show microflow, and such an amorphous sample behaves like the reference PC (‘dry’ ablation). The reasons for this surprising result are discussed. Received: 31 October 2002 / Accepted: 4 November 2002 / Published online: 22 January 2003 RID="*" ID="*"Present address: ST Microelectronics, Crolles, France RID="**" ID="**"Corresponding author. Fax: +33-556/84-6645, E-mail: s.lazare@lpcm.u-bordeaux1.fr     

PbS-doped phosphate glasses saturable absorbers for 1.3-μm neodymium lasers

Passive mode locking and saturable absorber Q-switching of neodymium lasers at 1.3 μm with PbS-doped phosphate glasses are demonstrated. Q-switched pulses of 120 ns (0.1 μJ) in duration (energy) and the average output power of 3 mW from a quasi-cw diode-pumped Nd³⁺:KGW laser and ultrashort pulses of a maximum of 250 μJ in energy and 150 ps in duration from a Nd³⁺:YAP laser were obtained. The bleaching decay rate of the samples was found to increase with the Quantum Dot’s size decreasing due to the enhancement of quantum confinement effects for smaller dots and stronger overlapping of the electron and trap state wave functions. Received: 23 January 2002 / Revised version: 2 April 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +375-17/232-6286, E-mail: savitski@eudoramail.com     

Properties of a Co/Cu/Co spin-valve system prepared by an optimized 193 nm pulsed laser deposition process

Layered CoO/Co(7 nm)/Cu(6 nm)/Co(7 nm) spin-valve systems capped by a 5 nm boron top layer, which exhibit giant magnetoresistance (GMR), were prepared by pulsed laser deposition (PLD) with a 193 nm ArF excimer laser. To reduce atomic intermixing at the various layer interfaces, the areal energy density of the laser was lowered to 4.2 J/cm² for the first 2 nm of the intermediate Cu and the Co top layers, while applying 11 J/cm² for the rest. By this procedure, a clear improvement of the GMR could be accomplished as compared to an identical reference system prepared by exclusively applying the high value of 11 J/cm², pointing to the importance of minimizing interface mixing.Additionally, the effect of the laser intensity on the areal density and shape of m-sized droplets co-deposited onto the substrates by PLD was studied. It turned out that for the two limiting values applied in the present case, no negative influence of the droplets on the magnetic hysteresis of pure Co layers or spin-valve systems could be observed. PACS 81.15.Fg; 75.47.Np; 75.70.Cn     

Field emission from single-walled carbon nanotubes aligned on a gold plate using a self-assembly monolayer

Field emission from single-walled carbon nanotubes (SWNTs) aligned on a patterned gold surface is reported. The SWNT emitters were prepared at room temperature by a self-assembly monolayer technique. SWNTs were cut into sub-micron lengths by sonication in an acidic solution. Cut SWNTs were attached to the gold surface by the reaction between the thiol groups and the gold surface. The field-emission measurements showed that the turn-on field was 4.8 V/μm at an emission current density of 10 μA/cm². The current density was 0.5 mA/cm² at 6.6 V/μm. This approach provides a novel route for fabricating CNT-based field-emission displays. Received: 3 May 2002 / Accepted: 6 May 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +82-54/279-8298, E-mail: ce20047@postech.ac.kr     

Interaction of laser radiation with TNT with respect to the laser neutralisation of AP mines

According to UN estimations there are between 80 and 115 million activated landmines worldwide. These mines, or other unexploded ordnance (UXO), can be triggered accidentally and kill or injure more than 2000 civilians per month. The most common explosive in these mines is trinitrotoluene (TNT). In this paper, the potential of some of the most promising lasers for mine neutralisation is investigated, namely an ArF laser, a KrF excimer laser and a Nd:YAG solid-state laser. We have studied the interaction between laser beams emitting at λ=193 nm, 248 nm and 1060 nm and a bare solid sample of TNT of approximately 15 mg. Using pulsed excimer radiation at λ=193 nm, with an energy density up to 1 J/mm², ablation of the TNT without any deflagration has been achieved. At λ=248 nm, using the KrF excimer laser with a pulse duration of 30 ns and a repetition rate of 5 Hz, the TNT sample started melting and burning after an irradiation of 10 s. Preliminary results with the Nd:YAG solid-state laser operating in cw emission have shown that the irradiated sample exhibits the desired burning behaviour even after the exposure is stopped. Received: 14 December 2000 / Accepted: 18 December 2000 / Published online: 20 June 2001     

Surface modulation of silicon surface by excimer laser at laser fluence below ablation threshold

Controlled single step fabrication of silicon conical surface modulations on [311] silicon surface is reported utilizing KrF excimer laser [λ=248 nm] at laser fluence below ablation threshold laser fluence. When laser fluence was increased gradually from 0 to 0.2 J/cm² for fixed 200 numbers of shots; first nanopores are observed to form at 0.1 J/cm², then very shallow nanocones evolve as a function of laser fluence. At 0.2 J/cm², nanoparticles are observed to form. Up to 0.15 J/cm² the very shallow nanocone volume is smaller but increases at a fast rate with laser fluence thereafter. It is observed that the net material volume before and after the laser irradiation remains the same, a sign of the melting and resolidification without any ablation.     

InAsSb/InAsSbP current-tunable laser with narrow spectral line width

The article describes the manufacture and testing of a new type of semiconductor laser working at low temperatures (12–100 K) in the wavelength range 3200–3300 cm^-1. This kind of laser can be tuned in the modal range up to 6 cm^-1 and is characterized by a narrow spectral line width (about 7 MHz). Received: 12 September 2002 / Final version: 29 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +420-286/591-766, E-mail: civis@jh-inst.cas.cz     

Dual-beam ablation of fused silica by multiwavelength excitation process using KrF excimer and F2 lasers

A new technique of dual-beam laser ablation of fused silica by multiwavelength excitation process using a 248-nm KrF excimer laser (ablation beam) coupled with a 157-nm F₂ laser (excitation beam) in dry nitrogen atmosphere is reported. The dual-beam laser ablation greatly reduced debris deposition and, thus, significantly improved the ablation quality compared with single-beam ablation of the KrF laser. High-quality ablation can be achieved at the delay times of KrF excimer laser irradiation shorter than 10 ns due to a large excited-state absorption. The ablation rate can reach up to 80 nm/pulse at the fluence of 4.0 J/cm² for the 248-nm laser and 60 mJ/cm² for the F₂ laser. The ablation threshold and effective absorption coefficient of KrF excimer laser are estimated to be 1.4 J/cm² and 1.2᎒⁵ cm^-1, respectively.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.     

Micro-lens arrays generated by UV laser irradiation of doped PMMA

Micro-lenses with well-defined optical parameters are generated on polymethylmethacrylate (PMMA) substrates doped with diphenyltriazene (DPT) by controlled use of a swelling effect generated under conditions of subablative excimer laser illumination. The surface profiles depend on the laser spot size and energy density. A sensitively balanced combination of matrix softening, substrate volume expansion due to photochemical nitrogen release, and surface tension is responsible for the final shape of the lenses. Complete arrays of identical lenses with 15 μm diameters and a focal length of 30 μm are produced by irradiation of (0.25 wt. %) DPT-PMMA with a single laser pulse at a wavelength of 308 nm and a fluence of 3 J/cm². It is shown experimentally and theoretically that appropriate volume expansion is possible without introducing internal light scattering due to the formation of small bubbles. Received: 7 April 1999 / Accepted: 8 April 1999 / Published online: 5 May 1999