Ablation effects and mechanism of sintered silicon carbide ceramics by an ArF excimer laser

The ablation of sintered silicon carbide ceramics by an ArF excimer laser was studied. Three zones are generated: the ablation zone that presented molten morphology and was composed by the Si and C phase; the condensation zone formed by vaporized SiC; and the oxidation zone that showed the characteristics of thermal oxidation. The ablation depth and oxidation range increase linearly with fluence and pulses within 0.5-4 J/cm~2,but the normalized ablation efficiency is constant(3.60 ± 0.60 μm·mm~2/J). The theoretical photochemical ablation depth supplies 25% of the total depth at 1 J/cm~2 but decreases to 16% at 4 J/cm~2. The ablation is dominated by the photothermal effect and conforms to the thermal evaporation mechanism.     

Low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation

The low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation has been studied. Two fabrication methods are presented. One is photoenhanced direct nitridation of a silicon surface with NH₃ for very thin gate insulators, and the other is photo-enhanced deposition of silicon nitride films with Si₂H₆ and NH₃ gases for stable passivation films. The ArF excimer laser irradiation dissociates the NH3 gas producing NH and NH₂ radicals which proved effective in instigating the nitridation reaction. The quality of both films has been much improved and the growth temperature has been lowered by using laser irradiation. These photo-enhanced processes seem to be promising ULSI techniques because they do not depend on high temperatures and are free from possible reactor contamination.     

Laser-induced periodic surface structures formed on the sidewalls of microholes trepanned by a femtosecond laser

Laser-induced periodic surface structures (LIPSSs) were observed on the sidewalls of 300-μm-diameter holes trepanned on cemented tungsten carbide using femtosecond laser pulses at a wavelength of 800 nm. For a circularly polarized beam, LIPSSs were formed at a period of 300 nm and oriented perpendicularly to the plane of incidence on the sidewalls. For a linearly polarized beam, LIPSS formation was dependent on the relative angle α between the polarization direction and the plane of incidence. For relative angles α from 0° to 70° and from 110° to 180°, LIPSS spacing was 300 nm. However, there were two types of LIPSSs coexisting from 70° to 110°. One had a spacing of 120 nm and the other had a spacing that varied from 500 to 760 nm. It was found that the orientation angle of LIPSSs measured between the LIPSS orientation and the plane of incidence had a nonlinear dependence on α. To understand this dependence, a model was proposed in which LIPSSs are assumed to align perpendicularly to the direction of the absorbed electric field lying in the tangent plane of the sidewall of a drilled hole. The calculated results from this model showed good agreement with the experimental results.     

Photochemical welding of silica microspheres to silicone rubber by ArF excimer laser

Transparent fused silica (SiO₂) microspheres 2.5 μm in diameter were photochemically welded to transparent, flexible silicone rubber ([SiO(CH₃)₂]_n) substrate by 193 nm ArF excimer laser induced photochemical modification of silicone into silicon oxide. Single layer of silica microspheres was easily formed on an adhesive silicone rubber before laser irradiation after dropping of silica microspheres dispersed in ethanol and subsequent tape peeling. The welding rate, the percentage of welded microspheres tested by ultrasonic cleaning with ethanol, was examined by varying the single pulse fluence and irradiation time of ArF excimer laser. The welding layer underneath microsphere, silicon oxide, was also found to emit white light of strong intensity under UV light illumination.     

Simultaneous detection of atomic and molecular hydrogen using a tunable ArF excimer laser

We report preliminary experiments which demonstrate the simultaneous detection of both atomic and molecular hydrogen using a single tunable ArF laser. The tunable ArF laser was modified to lase simultaneously at two wavelengths by the addition of a second grating to the oscillator stage as in earlier work by Ketterle et al. for KrF operation. To our knowledge, this is the first demonstration of dual-wavelength capability with a tunable ArF excimer. The H atom diagnostic utilized tunable ArF excimer laser output at 193.29 nm which was Raman shifted in D₂ (first Stokes) to generate 205.14 nm radiation which then excited the H atom to its (3s, 3d) states via a two-photon transition. Fluorescence was detected at 656 nm on the (3s, 3d) 2p transition. The second wavelength from the tunable excimer was used to excite several rovibronic LIF transitions of hydrogen molecule via two-photon absorption on the E ¹ _g ⁺ X ¹ _g ⁺ (2,0) band. Fluorescence occurs at 750 nm and 830 nm on the (E ¹ _g ⁺ B ¹ _g ⁺ ) transitions.     

放电泵浦ArF准分子激光动力学模拟与参数分析

利用一维流体模型研究了放电泵浦ArF准分子激光动力学过程,得到气体放电过程中放电电路的电流电压波形,得到了电子密度、光子密度、电场的时空分布特性,分析了放电参数对激光输出的影响。结果表明,电路参数、工作气压、氟气比例均对激光输出有显著影响,放电电路中的电感值对输出影响较小,参数范围较广,而电路中较小的峰化电容值有利于获得长脉冲输出,气压太大或太小都会使输出能量降低,同样,氟气比例太大或太小也会使输出能量降低。     

Preparation of palladium film by coating photolysis process using KrF or ArF excimer laser

The preparation of palladium (Pd) films has been investigated using KrF or ArF laser irradiation on a Pd acetate (PdAc) coated substrate. A crystalline Pd film could be obtained by KrF laser irradiation (fluence = 15-40 mJ/cm²) but PdAc was found to remain in the film. An increase in the substrate temperature to 423 K decreased the inclusion of the unreacted precursor and produced a better crystallinity. An amorphous and uniform Pd film composed of very fine particles was found to be formed by this process under reduced pressure, which is probably due to the preferential ablation of the crystalline nuclei. ArF laser irradiation is more effective for decomposing the PdAc and for producing a Pd film with a better crystallinity and no (or smaller) organic inclusion.     

Characterization of calcium phosphate coatings doped with Mg, deposited by pulsed laser deposition technique using ArF excimer laser

Calcium phosphate layers were deposited on Ti6Al4V substrates with TiN buffer layers by use of pulsed laser deposition method. With this technique three pressed pellets consisted of tricalcium phosphate (TCP, Ca(3)(PO(4))(2)), hydroxyapatite (HA, Ca(10)(PO(4))(6)(OH)(2)) and hydroxyapatite-doped with magnesium (HA with 4% of Mg and trace amount of (Ca,Mg)(3)(PO(4))(2)) were ablated using ArF excimer laser (lambda=193 nm). The using of different targets enabled to determine the influence of target composition on the nature of deposited layers. The obtained deposits were characterized by means of Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction method (XRD). The obtained Fourier spectras revealed differences in terms of intensity of spectral bands of different layers. The analysis from XRD showed that Mg-doped HA layer has crystalline structure and TCP and HA layers composition is characterized by amorphous nature.     

Laser-induced damage in SiO2 and CaF2 with picosecond and femtosecond laser pulses

Single- and multiple-shot damage thresholds and plasma-emission thresholds for fused silica and CaF₂ are reported for 790 nm photons as a function of laser pulse width (190 fs – 4.5 ps). The results are compared with single-shot plasma-emission measurements [1] and with multiple-shot damage measurements [2]. Both the damage threshold and the plasma-emission threshold are shown to decrease with decreasing pulse width over the entire pulse-width range investigated.     

Melting threshold of crystalline and amorphized Si irradiated with a pulsed ArF (193 nm) excimer laser

Modifications induced by a pulsed ArF excimer laser at surface of implanted silicon were investigated by a new and simple optical method which consists to follow the evolution of solid reflectivity, at 633 nm wavelength, resulting from the amorphouspolycrystalline (or monocrystalline) transition during the laser melting process. These results, which have been compared to those obtained using time resolved reflectivity experiments have demonstrated the capability of this simple technique to determine the melting threshold of implanted silicon.     

Two-dimensional ‘hat-scratch’ periodic structures induced by a single femtosecond laser pulse on silica glass

Two-dimensional ‘hat-scratch’ structures are fabricated on silica glass by the interference of three non-coplanar beams originating from a single femtosecond laser pulse. The scanning electron microscope (SEM) characterizations show that the as-formed structures are composed of hat holes and scratch marks. The experimental results indicate that the structures are dependent on the intensity of laser beam. The formation of the two-dimensional ‘hat-scratch’ structures is mainly due to the combined laser ablation effects including ionization, shock wave, plasma expansion, and phase explosion.     

Tuning ranges of KrF and ArF excimer laser amplifiers and of associated vacuum ultraviolet anti-Stokes Raman lines

Measurements of tuning ranges for narrow-bandwidth ( ) radiation amplified in KrF and ArF laser amplifiers are reported. Generation of high-order anti-Stokes Raman lines in H₂ down to 116 nm is described as well as measurements of the corresponding tuning ranges. Generation of rotational side-bands opens the possibility to tune most of the vacuum-ultraviolet (vuv) lines over intervals in excess of 1400 cm^–1. Absolute values of the power measured for the various vuv lines are communicated.     

Formation of periodic surface structures by ultrashort laser pulses

The formation of periodic surface structures by ultrashort laser pulses was observed experimentally and explained theoretically. The experiments were performed on graphite with picosecond laser pulses. The spatial period of the structures is of the order of the wavelength of the incident radiation, and the orientation of the structures is correlated with the direction of polarization of the light. The key point of the theoretical model proposed is resonance excitation of surface electromagnetic waves, which under conditions such that the temperature of the electronic subsystem is decoupled from the temperature of the crystal lattice causes a “temperature grating” to be written on the flat solid surface of the sample while the laser pulse is being applied on account of the temperature dependence of the surface impedance. The formation of a periodic surface profile from the temperature grating occurs by the volume expansion of a melted layer near the surface of the material. For typical values of the surface tension and viscosity for metals, there is not enough time for the periodic profile to be resorbed before the liquid layer solidifies. The formation of periodic surface structures is delayed in time relative to the laser pulse. Zh. éksp. Teor. Fiz. 115, 675–688 (February 1999)     

共掺Na~ 的Yb~(3 )∶CaF_2晶体的荧光分析与低阈值激光运转

基于975nm激发的室温下荧光光谱测量,系统地研究了在新型钠、镱共掺的氟化钙晶体中钠离子的作用机理,分析了掺钠离子浓度与激光阈值的关系,获得了具有最低激光阈值的掺杂浓度优化配比.激光实验表明对于2%Yb3 离子浓度的氟化钙,掺入3%的钠离子能够获得最低阈值的激光运转,这与理论和荧光分析完全一致.     

The magnetic structures of NdCu2 determined by single crystal neutron diffraction

We investigated the magnetic structure of NdCu₂ by means of neutron diffraction as a function of temperature between 1.5 K and 8 K in zero external field. The diffraction data were obtained on two single crystals with different orientations using the triple-axis-spectrometer TAS6 at the DR-3 reactor at Risø. Two magnetic phases were observed between 1.5K andT _N =6.5K. From 1.5 K to 4.1 K the magnetic reflections can be described by the commensurate wave vector =(3/5 0 0) and its higher harmonics 3 and 5. Below 2.5K the structure is completely squared-up. For 4.1 KT6.5 K the magnetic structure is incommensurate with the chemical lattice and can be described by the wave vector=(3/5 0 0) and its higher harmonies 3 and 5M. Below 2.5 K the structure is completely squared-up. For 4.1 K T 6.5 K the magnetic structure is incommensurate with the chemical lattice and can be described by the wave vector ^*=(0.62 0.044 0). In both phases the Nd-moments are oriented along the easyb-direction.     

Cascaded second-harmonic and sum-frequency generation of a CO2 laser by use of a single quasi-phase-matched GaAs crystal

Cascaded second-harmonic and sum-frequency generation were used to obtain the third harmonic of a CO(2) laser in a single quasi-phase-matched GaAs crystal. Both continuous-wave and pulsed regimes with a single-pass configuration were studied. The continuous case confirmed the good capability of the GaAs stack to bear high average power density. In the pulsed regime a 0.66% peak power conversion efficiency was achieved for the third harmonic when the laser was pumping at 8.2 MW/cm(2) , in fair agreement with theoretical predictions.     

准分子激光辐照K9玻璃的热力效应分析

基于传热学理论, 利用有限元法对KrF准分子激光辐照K9玻璃样品中的热力效应进行了数值分析, 并比较了脉冲数目和重频对损伤效果的影响。研究表明, 较低的准分子激光能量就能够使K9玻璃在表面和体内产生热应力损伤, 热应力损伤在光斑区域内主要由压缩热应力控制, 在光斑边缘和材料内部则主要由拉伸热应力控制。在激光脉冲结束时刻, 产生的温度和热应力最大, 且热应力以热冲击波的形式在材料内传播, 随时间变化而来回振荡, 逐渐减弱。这种热应力的反复冲击会对材料产生持续的损伤增长效应, 增加了材料的损伤时间, 并使材料更容易断裂。脉冲数目和重复频率对损伤效果有着较大影响, 在高重复频率下, 损伤累积效应明显。     

Measurement of optical and electrical properties of silicon microstructuring induced by ArF excimer laser at SF6 atmosphere

The irradiation of ArF excimer laser (193 nm) on Si wafer (〈1 1 1〉, n-type, arsenic-doped, 0.01 Ω cm) in SF₆ atmosphere, from vacuum to 1000 mbar, creates a regular self-assembled microstructure owning to a great number of microconical spikes covered with SiF₂ (fluorosilyl) layer containing sulfur impurities. The geometry of microstructure as well as the layer thickness varies with the gas pressure and the laser parameters, particularly duration, pulse energy and the dose. In this work, the electrical properties of the layer on the microstructured silicon have been investigated based on electrical impedance spectroscopy (EIS). The measured impedance significantly changes regarding to the unirradiated samples. It was shown that the corresponding electrical conductance and the dielectric constants of the layer are strongly dependent on the gas pressure and UV dose. The layer thickness was also determined in terms of SF₆ pressures.