Indirect laser etching of fused silica: Towards high etching rate processing

The indirect laser processing approach (LIBWE) laser-induced backside wet etching allows defined microstructuring of transparent materials at low laser fluences with high quality. The optical and the thermal properties of the solid/liquid interface determine the temperatures and therefore the etching mechanism in conjunction with the dynamic processes at the interface due to the fast heating/cooling rates. The exploration of organic liquid solvents and solutions such as 0.5 M pyrene/toluene results in low etch rates (∼20 nm/pulse). By means of liquid metals as absorber here, demonstrated for gallium (Ga), etch rates up to 600 nm/pulse can be achieved. Regardless of the high etch rates a still smooth surface similar to etching with organic liquid solutions can be observed. A comparative study of the two kinds of absorbing liquids, organic and metallic, investigates the etch rates regarding the fluence and pulse quantity. Thereby, the effect of incubation processes as result of surface modification on the etching is discussed. In contrast to pyrene/toluene solution the metallic absorber cannot decompose and consequently no decomposition products can alter the solid/liquid interface to enhance the absorption for the laser radiation. Hence, incubation can be neglected in the case of the silica/gallium interface so that this system is a suitable model to investigate the primary processes of LIBWE. To prove the proposed thermal etch mechanism an analytical temperature model based on a solution of the heat equation is derived for laser absorption at the silica/gallium interface.     

Backside laser etching of fused silica using liquid gallium

Laser-induced backside wet etching (LIBWE) that is regularly performed with hydrocarbon solutions is demonstrated with the liquid metal gallium as a new class of absorbers for the first time. Well-contoured square etch pits in fused silica with smooth bottoms and well-defined edges were achieved already with the first pulse from a 248 nm excimer laser. The etching is characterized by a threshold fluence of 1.3 J/cm² and a straight proportional etch rate growth with the fluence up to 8.2 J/cm². In addition, the etch depth increases linearly for onward pulsed laser irradiation and gives evidence for an only marginal incubation effect. The high fluences necessary for etching originate from the high reflection losses as well as the high thermal conductivity of the metallic absorber. The suggested etch mechanism comprises the heating of the fused silica up to or beyond the fused silica melting point by the laser heated gallium and the removing of the softened or molten fraction of the material by mechanical forces from shock waves, bubbles, high pressures, or stress fields. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

Laser processing of micro-optical components in quartz

Laser induced backside wet etching combined with the diffractive gray tone phase mask has been used for the fabrication of a micro-lens array with a single lens diameter of 1 mm and a micro-prism in quartz. The micro-lens array was tested as beam homogenizer for high power XeCl excimer laser yielding a clear improvement in the quality of the laser beam.The optimum fluence range for fabrication of micro-lenses by laser induced backside wet etching using 1.4 M pyrene in THF solution and 308 nm irradiation wavelength is 1-1.6 J/cm². The etching mechanisms of LIBWE are based on a combination of pressure and temperature jumps at quartz-liquid interface.     

Laser ablation of toluene liquid for surface micro-structuring of silica glass

Microstructures with well-defined micropatterns were fabricated on the surfaces of silica glass using a laser-induced backside wet etching (LIBWE) method by diode-pumped solid state (DPSS) UV laser at the repetition rate of 10 kHz. For a demonstration of flexible rapid prototyping as mask-less exposure system, the focused laser beam was directed to the sample by galvanometer-based point scanning system. Additionally, a diagnostics study of plume propagation in the ablated products of toluene solid film was carried out with an intensified CCD (ICCD) camera.     

Resist-assisted atom lithography with group III elements

Resist-assisted atom lithography with group III elements, specifically with gallium and indium, is demonstrated. Self-assembled monolayers (SAM) of nonanethiols prepared on thin sputtered gold films were exposed to a beam of neutral gallium and indium atoms through a physical mask. The interaction of the Ga and In atoms with the nonanethiol layer, followed by a wet etching process, creates well defined structures on the gold film, with features below 100 nm. The threshold of the lithographic process was estimated by optical methods and found to be around 3 gallium atoms and 12 indium atoms per thiol molecule. Our experiments suggest that resist-assisted atom lithography can be realized with group III elements and possibly extended to new neutral atomic species. PACS 07.77.Gx; 42.82.Cr; 81.16.Ta     

Comparing study of subpicosecond and nanosecond wet etching of fused silica

The effectiveness of the laser induced backside wet etching (LIBWE) of fused silica produced by subpicosecond (600 fs) and nanosecond (30 ns) KrF excimer laser pulses (248 nm) was studied. Fused silica plates were the transparent targets, and naphthalene-methyl-methacrylate (c = 0.85, 1.71 M) and pyrene-acetone (c = 0.4 M) solutions were used as liquid absorbents. We did not observe etching using 600 fs laser pulses, in contrast with the experiments at 30 ns, where etched holes were found. The threshold fluences of the LIBWE at nanosecond pulses were found to be in the range of 360-450 mJ cm⁻² depending on the liquid absorbers and their concentrations. On the basis of the earlier results the LIBWE procedure can be explain by the thermal heating of the quartz target and the high-pressure bubble formation in the liquid. According to the theories, these bubbles hit and damage the fused silica surface. The pressure on the irradiated quartz can be derived from the snapshots of the originating and expanding bubbles recorded by fast photographic setup. We found that the bubble pressure at 460 mJ cm⁻² fluence value was independent of the pulse duration (600 fs and 30 ns) using pyrene-acetone solution, while using naphthalene-methyl-methacrylate solutions this pressure was 4, 5 times higher at 30 ns pulses than it was at 600 fs pulses. According to the earlier studies, this result refers to that the pressure should be sufficiently high to remove a thin layer from the quartz surface using pyrene-acetone solution. These facts show that the thermal and chemical phenomena in addition to the mechanical effects also play important role in the LIBWE procedure.     

Laser-induced backside wet etching of fluoride and sapphire using picosecond laser pulses

Laser-induced backside wet etching (LIBWE) is a promising process for microstructuring of rigid chemical resistant and inert transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated in a number of studies. LIBWE in a time scale of femtosecond and picosecond pulse durations has been investigated only in a few studies and just on fused silica. In the present study LIBWE of fluorides (CaF₂, MgF₂) and sapphire with a mode-locked picosecond (t _p=10 ps) laser at a UV wavelength of λ=355 nm using toluene as absorbing liquid has been demonstrated. The influence of the laser fluence and the pulse number on the etching rate and the achieved surface morphology was investigated. The etching rate grows linearly with the laser fluence in the low and high-fluence ranges with different slopes. The achieved etching rates for CaF₂ and for sapphire were in the same range. Contrary to CaF₂ and sapphire the etching rates of MgF₂ were one magnitude less. For backside etching on sapphire at high fluences smooth surfaces and at low fluences ripples pattern were found, whereas fluoride surfaces showed a trend towards crack formation.     

Influence on the laser induced backside dry etching of thickness and material of the absorber, laser spot size and multipulse irradiation

Laser induced backside dry etching method (LIBDE) was developed - analogously to the well-known laser induced backside wet etching (LIBWE) technique - for the micromachining of transparent materials. In this procedure, the absorbing liquid applied during LIBWE was replaced with solid metal layers. Fused silica plates were used as transparent targets. These were coated with 15-120 nm thick layers of different metals (silver, aluminium and copper). The absorbing films were irradiated by a nanosecond KrF excimer laser beam through the quartz plate. The applied fluence was varied in the 150-2000 mJ/cm² range, while the irradiated area was between 0.35 and 3.6 mm². At fluences above the threshold values, it was found that the metal layers were removed from the irradiated spots and the fused silica was etched at the same time. In our experiments, we investigated the dependence of the main parameters (etch rate and threshold) of LIBDE on the absorption of the different metal layers (silver, copper, aluminium), on the size of the irradiated area, on the film thickness and on the number of processing laser pulses.     

Micromachining of quartz crystal with excimer lasers by laser-induced backside wet etching

We fabricated a well-defined pattern of lines and spaces on the surface of a quartz crystal plate (c-SiO₂) with micron-sized features, using laser-induced backside wet etching (LIBWE). The line patterns obtained using LIBWE showed a high aspect ratio of about 3. The etch rates of fused silica (a-SiO₂) ranged from 5 to 25 nm/pulse with KrF laser irradiation from 0.4-1.3 J/cm². Threshold fluences for a-SiO₂ and c-SiO₂ were 0.23 and 0.34 J/cm², respectively. The single-pulse etch depth was not affected by the repetition rates of laser pulses from 1-50 Hz.     

A MOVPE method for improving InGaN growth quality by pre-introducing TMIn

We propose a metal organic vapor phase epitaxy(MOVPE) method of pre-introducing TMIn during the growth of uGa N to improve the subsequent growth of In Ga N and discuss the impact of this method in detail. Monitoring the MOVPE by the interference curve generated by the laser incident on the film surface, we found that this method avoided the problem of the excessive In Ga N growth rate. Further x-ray diffraction(XRD), photoluminescence(PL), and atomic force microscope(AFM) tests showed that the quality of In Ga N is improved. It is inferred that by introducing TMIn in advance, the indium atom can replace the gallium atom in the reactor walls, delivery pipes, and other corners. Hence the auto-incorporation of gallium can be reduced when In Ga N is grown, so as to improve the material quality.     

Mechanism of backside etching of transparent materials with nanosecond UV-lasers

In consequence of high interest in micro- and nanomachining of transparent materials by laser irradiation, studies on the mechanism of laser-induced backside wet etching (LIBWE) are presented. To reveal the role of the surface modification due to LIBWE the backside ablation (BSA) of LIBWE-modified fused silica (mFS) surfaces at 248 nm was investigated. The threshold fluence and the etch rate of BSA are similar to that of LIBWE and amount ∼250 mJ/cm² and 30 nm for 1 J/cm², respectively. The sample transmission after backside ablation of mFS increases and proves the decreasing thickness of the absorbing layer. Time-resolved reflection studies at LIBWE and BSA of mFS show similar patterns in the backside reflection that can be assigned an ablation process as the comparison to thin polymer films demonstrates. By fitting the BSA data to an exponential decay absorption model a modification depth and a surface absorption of about 38 nm and α ^S ∼1.3×10⁷ m⁻¹ were calculated, respectively. In conclusion of the results a new model for LIBWE is proposed.     

Competitive segregation of gallium and indium at heterophase Cu-MnO interfaces studied with transmission electron microscopy

This paper concentrates on the possible segregation of indium and gallium and competitive segregation of gallium and indium at atomically flat parallel {111}-oriented Cu-MnO interfaces. The segregation of gallium at Cu-MnO interfaces after introduction of gallium in the copper matrix of internally oxidized Cu-1 at.% Mn could be hardly detected with energy-dispersive spectrometry in a field emission gun transmission electron microscope. After a heat treatment to dissolve indium in the copper matrix, gallium has a weak tendency to segregate, that is 2.5 at.% Ga per monolayer at the interface compared with 2 at.% in the copper matrix. The striking result is that this gallium segregation is observable because it does not occur at the metal side of the interface but in the first two monolayers at the oxide side. Using the same heat treatment as for introducing indium in the sample, but without indium present, gallium segregates strongly at the oxide side of the Cu-MnO interface with a concentration of about 14.3 at.% in each monolayer of the two. In contrast, the presence of gallium has no influence on the segregation of indium towards Cu-MnO interfaces, because the outermost monolayer at the metal side of the interface contains 17.6 at.% In, that is similar to previously found results. This leads to the intriguing conclusions, firstly, that, in contrast with antimony and indium, gallium segregates at the oxide side of the interface and, secondly, that the presence of indium strongly hampers gallium segregation. The results from analytical transmission electron microscopy on gallium segregation are supported by high-resolution transmission electron microscopy observations.     

Zur Supraleitung einiger Mischsysteme von Verbindungen mit A15-Struktur auf Vanadiumbasis

Ternary intermetallic compounds on the base of V₃Ga and V₃Si have been prepared by arc melting, substituting gallium and silicon partly by aluminium, gallium, indium, silicon, and germanium. In similar way part of the niobium in the compounds Nb₃Al and Nb₃Ga has been replaced by vanadium. All these ternary compounds have been investigated for superconductivity. Only by substituting gallium in V₃Ga partly by aluminium or silicon, the transition temperature could be raised.     

Gallium- and iodine-co-doped titanium dioxide for photocatalytic degradation of 2-chlorophenol in aqueous solution: Role of gallium

Visible-light-driven TiO₂-based catalysts for the degradation of pollutants have become the focus of attention. In the present work, iodine-doped titania photocatalysts (I-TiO₂) were improved by doping with gallium (Ga,I-TiO₂) and the resulting physicochemical properties and photocatalytic activity were investigated. The structural properties of the catalysts were determined by X-ray diffraction, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis and transmission electron microscopy. We found that Ga probably enters the TiO₂ framework for doping levels <0.5 mol%. A further increase in Ga content probably leads to dispersal of excess Ga on the TiO₂ surface. The photocatalytic activity of Ga,I-TiO₂ catalysts was evaluated using 2-chlorophenol (2-CP) as a model compound under visible and UV-vis light irradiation. The results indicate that 0.5 mol% Ga loading and calcination at 400 °C represent optimal conditions in the calcining temperature range 400-600 °C and with doping levels from 0.1% to 1 mol%. The effective enhancement of 2-CP degradation might be attributed to the formation of oxygen vacancies by Ga doping, which could decrease the recombination of electron-hole pairs.     

In situ reflectivity investigations of solid/liquid interface during laser backside etching

In situ reflectivity measurements of the solid/liquid interface with a pump-probe setup were performed during laser-induced backside wet etching (LIBWE) of fused silica with KrF excimer laser using toluene as absorbing liquid. The intensity, the temporal shape, and the duration of the reflected light measured in dependence on the laser fluence are discussed referring to the surface modification and the bubble formation.The vaporisation of the superheated liquid at the solid interface causes a considerable increase of the reflectivity and gives information about the bubble lifetime. The alterations of the reflectivity after bubbles collapse can be explained with the changed optical properties due to surface modifications of the solid surface. Comparative studies of the reflectivity at different times and the etch rate behaviour in dependence on the laser fluence show that the in situ measured surface modification begins just at the etch threshold fluence and correlates further with etch rate behaviour and the etched surface appearance. The already observed surface modification at LIBWE due to a carbon deposition and structural changes of the near surface region are approved by the changes of the interface reflectivity and emphasizes the importance of the modified surface region in the laser-induced backside wet etching process.     

Effect of synthesis conditions on microstructures and photoluminescence properties of Ga doped ZnO nanorod arrays

Ga doped ZnO nanorod arrays were prepared on silicon substrates in a mixture solution of zinc nitrate hexahydrate, methenamine, and gallium nitrate hydrate. Effect of synthesis conditions on crystal structures, morphologies, surface compositions, and optical properties was analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence techniques (PL). Experimental results reveal that Ga doping amount can reach 1.67 at% with the increase of gallium nitrate concentration. Ga doping greatly affects the morphologies of ZnO nanorod arrays. The photoluminescence spectra show a sharp UV emission and a broad visible emission. With Ga doping, UV emission has an apparent broadening effect and its peak shifts from 3.27 eV to 3.31 eV. The intensity ratio of UV emission to visible emission demonstrates that appropriate Ga doping amount is beneficial for the improvement of ZnO crystalline quality.     

Imprinting by hot embossing in polymer substrates using a template of silica glass surface-structured by the ablation of LIBWE method

Laser-induced backside wet etching (LIBWE) of silica glass plates was performed to fabricate an imprinting template for hot embossing in polymer substrates such as polystyrene and silicone resin. Well-defined inverse surface-micropatterns of gratings and grid arrays on the substrates were produced by the hot embossing using a surface-structured silica glass as the template. These results indicate that the LIBWE method allows us to generate robust glass molding tools that exhibit the inverse shapes of the intended microstructures. PACS 52.38.Mf; 68.47.Mn; 81.05.Kf; 81.05.Lg; 83.50.Uv     

生长条件对Ga掺杂ZnO薄膜微观结构及光致发光性能的影响

利用热氧化法在不同参数条件下生长了Ga掺杂范围较宽的ZnO薄膜, 研究了ZnO薄膜的表面微观结构和光致发光性能. 研究表明: Ga以Ga³⁺存在并掺入ZnO晶格取代Zn²⁺, Ga的掺入改变了ZnO薄膜中的缺陷类型及浓度、化学计量比、薄膜表面结晶质量, 进而影响了薄膜的光致发光性能. 随着热氧化温度升高, Ga掺杂量增大, ZnO薄膜的晶粒尺寸增大, 尺寸更均一, 紫外光与可见光强度比增大. 随着热氧化时间延长, Ga掺杂量降低, ZnO薄膜的晶粒尺寸均一性变差, 紫外光与可见光强度比减小.     

The influence of laser-induced surface modifications on the backside etching process

Spectroscopic measurements in the UV/VIS region show reduced transmission through laser-induced backside wet etching (LIBWE) of fused silica. Absorption coefficients of up to 10⁵ cm⁻¹ were calculated from the transmission measurements for a solid surface layer of about 50 nm. The temperatures near the interface caused by laser pulse absorption, which were analytically calculated using a new thermal model considering interface and liquid volume absorption, can reach 10⁴ K at typical laser fluences. The high absorption coefficients and the extreme temperatures give evidence for an ablation-like process that is involved in the LIBWE process causing the etching of the modified near-surface region. The confinement of the ablation/etching process to the modified near-surface material region can account for the low etch rates observed in comparison to front-side ablation.