Laser etching of transparent materials at a backside surface adsorbed layer

The laser etching using a surface adsorbed layer (LESAL) is a new method for precise etching of transparent materials with pulsed UV-laser beams. The influence of the processing parameters to the etch rate and the surface roughness for etching of fused silica, quartz, sapphire, and magnesium fluoride (MgF₂) is investigated. Low etch rates of 1 nm/pulse and low roughness of about 1 nm rms were found for fused silica and quartz. This is an indication that different structural modifications of the material do not affect the etching significantly as long as the physical properties are not changed. MgF₂ and sapphire feature a principal different etch behavior with a higher etch rate and a higher roughness. Both incubation effects as well as the temperature dependence of the etch rate can be interpreted by the formation of a modified near surface region due to the laser irradiation. At repetition rates up to 100 Hz, no changes of the etch rate have been observed at moderate laser fluences.     

Excimer laser-induced etching of sub-micron surface relief gratings in fused silica using phase grating projection

Phase grating projection using a Schwarzschild objective is presented in conjunction with a process that allows laser-induced etching of transparent materials at the interface of liquids, in order to generate periodical surface relief structures with sub-micron resolution in fused silica. The achieved sinusoidal gratings exhibit a period of 780 nm and a depth of up to 180 nm, having a roughness lower than 5 nm r.m.s. The depth and roughness of the gratings are related to the applied laser fluence and pulse number. In addition to the grating formation, an overall removal of material at higher laser fluences was found. Received: 3 December 2001 / Accepted: 5 December 2001 / Published online: 11 February 2002     

Laser etching of fused silica using an adsorbed toluene layer

A new method for laser etching of transparent materials with a low etch rate and a very good surface quality is demonstrated. It is based on the pulsed UV-laser backside irradiation of a transparent material that is covered with an adsorbed toluene layer. This layer absorbs the laser radiation causing the etching of the solid. The threshold fluence for etching of fused silica amounts to 0.7 J/cm². The constant etch rate of about 1.3 nm/pulse that has been observed in a fluence interval from 2 to 5 J/cm² is evidence of a saturated process. The limited thickness of the adsorbed layer causes the low etch rates and the rate saturation. The etched surface structures have well defined edges and low surface roughness values of down to 0.4 nm rms. PACS 81.65.Cf; 81.05.Kf; 79.20.Ds; 61.80.Ba; 42.55.Lt     

High-speed etching of hexagonal GaN by laser ablation and successive chemical treatment

Photostimulated direct etching of GaN has been demonstrated with extremely high etching rate up to 135 nm/pulse. The process consists of laser irradiation and ex-situhydrochloric acid treatment. Not only deep etching but also a highly planarized surface are obtained by an increase in laser fluence and the number of pulses. Seven-pulse irradiation at 1 J/cm² decreases surface average roughness (R_a) to ~2 nm from ~10 nm of the untreated sample. No deep-level emission (450-600 nm) is detected in photoluminescence measurement on the samples irradiated with laser fluences as high as 3 J/cm².     

Using IR laser radiation for backside etching of fused silica

Laser-induced backside etching of fused silica with gallium as highly absorbing liquid is demonstrated using pulsed infrared laser radiation. The influences of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography were studied and the results are compared with these of excimer laser etching. The high reflectivity of the fused silica-gallium interface at IR wavelengths results in the measured high threshold fluences for etching of about 3 J/cm² and 7 J/cm² for 18 ns and 73 ns pulses, respectively. For both pulse lengths the etch rate rises almost linearly with laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. The etching process is almost free from incubation processes because etching with the first laser pulse and a constant etch rate were observed. The etched surfaces are well-defined with clear edges and a Gaussian-curved, smooth bottom. A roughness of about 1.5 nm rms was measured by AFM at an etch depth of 0.95 μm. The normalization of the etch rates with respect to the reflectivity and the pulse length results in similar etch rates and threshold fluence for the different pulse widths and wavelengths. It is concluded that etching is a thermal process including the laser heating, the materials melting, and the materials etching by mechanical forces. The backside etching of fused silica with IR-Nd:YAG laser can be a promising approach for the industrial usage of the backside etching of a wide range of materials. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

Surface characteristics of SiO2-TiO2 strip fabricated by laser direct writing

SiO2-TiO2 sol-gel films axe deposited on SiO2/Si by dip-coating technique.The SiO2-TiO2 strips are fabricated by laser direct writing using an ytterbium fiber laser and followed by chemical etching.Surface structures,morphologies and roughness of the films and strips are characterized.The experimental results demonstrate that the SiO2-TiO2 sol-gel film is loose in structure and a shrinkage concave groove forms if the film is irradiated by laser beam.The surface roughness of both non-irradiated and laser irradiated areas increases with the chemical etching time.But the roughness of laser irradiated area increases more than thalt of non-irradiated area under the same etching time.After being etched for 28 s,the surface roughness value of the laser irradiated area increases from 0.3 nm to 3.1 nm.     

Coupling of focused laser pulse to surfaces of transparent materials studied by time-resolved imaging technique

Short and intense laser pulse can process the surface and the inside of transparent materials by focusing the pulse at the desired position. Here we report the interaction of fundamental radiation (1064 nm) of the Q-switched Nd:YAG laser to the surface of PMMA as observed by an imaging system with nanosecond time resolution. The system used fundamental radiation of a Q-switched Nd:YAG laser as a processing laser and second harmonic radiation (532 nm) of another Nd:YAG laser as illuminating light. We observed shock waves which propagate into the material and into the atmosphere by shadowgraph and photoelastic method. Surface roughness of a sample is expected to affect the coupling of light and transparent materials for both normal and focused laser light. Our results have revealed the effects visually. For roughness larger than 0.6 m, all energy is absorbed at the surface, while the larger part of the energy is absorbed inside the material as the surface becomes smoother. PACS 52.38.MF; 79.20.DS; 87.63.Lk     

Dry‐etching‐assisted femtosecond laser machining

Femtosecond laser machining has been widely used for fabricating arbitrary 2.5 dimensional (2.5D) structures. However, it suffers from the problems of low fabrication efficiency and high surface roughness when processing hard materials. To solve these problems, we propose a dry‐etching‐assisted femtosecond laser machining (DE‐FsLM) approach in this paper. The fabrication efficiency could be significantly improved for the formation of complicated 2.5D structures, as the power required for the laser modification of materials is lower than that required for laser ablation. Furthermore, the surface roughness defined by the root‐mean‐square improved by an order of magnitude because of the flat interfaces of laser‐modified regions and untreated areas as well as accurate control during the dry‐etching process. As the dry‐etching system is compatible with the IC fabrication process, the DE‐FsLM technology shows great potential for application in the device integration processing industry.

     

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.     

Self-organized near-field etching of the sidewalls of glass corrugations

Using soda-lime glass with a nano-stripe pattern as a test specimen, we demonstrated self-organized near-field etching with a continuum-wave laser (λ=532 nm) light source. Atomic force microscopy confirmed that near-field etching decreases the flank roughness of the corrugations as well as the roughness of the flat surface.     

Crackless linear through-wafer etching of Pyrex glass using <Emphasis Type="Italic">liquid-assisted</Emphasis> CO<Subscript>2</Subscript> laser processing

Pyrex glass etching is an important technology for the microfluid application to lab-on-a-chip devices, but suffers from very low etching rate and mask-requiring process in conventional HF/BOE wet or plasma dry etching as well as thermal induced crack surface by CO₂ laser processing. In this paper, we applied the liquid-assisted laser processing (LALP) method for linear through-wafer deep etching of Pyrex glass without mask materials to obtain a crackless surface at very fast etching rates up to 25 μm/s for a 20 mm long trench. The effect of laser scanning rate and water depth on the etching of the 500 μm thick Pyrex glass immersed in liquid water was investigated. The smooth surface without cracks can be achieved together with the much reduced height of bulge via an appropriate parameter control. A mechanism of thermal stress reduction in water and shear-force-enhanced debris removal is discussed. The quality improvement of glass etching using LALP is due to the cooling effect of the water to reduce the temperature gradient for a crackless surface and natural convection during etching to carry away the debris for diminishing bulge formation. An erratum to this article can be found at     

Processing of transparent materials using visible nanosecond laser pulses

Laser micromachining of transparent materials is an intensively studied research area from the point of view of microoptical element fabrication. One of the most promising indirect processing methods is the laser-induced back-side dry etching (LIBDE). During this method, transparent targets are contacted with solid thin layers, which absorb and transform the pulse energy resulting in etching. The applicability of LIBDE technology for processing of fused silica using a visible nanosecond dye laser (λ=500 nm, FWHM=11 ns) and a 100-nm-thick aluminium absorbing layer was investigated. The applied fluence was varied in the range of 0–3050 mJ/cm²; the illuminated area was 0.1 mm². The threshold fluence of the LIBDE etching of fused silica was found to be approximately 540 mJ/cm². The chemical composition of the surface layers on and around the etched holes was investigated by field-emission scanning electron microscopy and energy-dispersive X-ray spectrometry. It was found that on average 0.4±0.3 at. % aluminium is built into the upper ∼1-μm-thick volume of the illuminated fused silica, while the aluminium content fell below the detection limit in the case of the original surface. Our experiments proved that the LIBDE procedure is suitable for microprocessing of transparent materials using visible nanosecond laser light. PACS 42.62.-b; 61.80.Ba; 81.16.Rf; 81.65.Cf     

Effect of composition gradient on CuIn3Te5 single-crystal properties and micro-Raman and infrared spectroscopies

The topography and roughness evolution of surfaces etched by laser-induced backside wet etching (LIBWE) is investigated in detail. The etching of sub-m gratings with a period of 760 nm into flat surfaces by means of interfering laser beams shows a saturation of the grating depth within 20 pulses. The over-etching of already microstructured surfaces results in the change of the cross section and in reduction of the microstructure height with increasing pulse number. The decrease in height of sub-micron gratings from 125 to less than 10 nm within 15 laser pulses causes a substantial roughness reduction. The depth limitations in etching of the gratings as well as the height reduction of microstructures are the result of the influence of the surface topography to the heat flow. The more efficient heating of surface peaks in contrast to the valleys results in higher etch rates and probably causes the smooth surfaces observed in LIBWE processing. The thermal diffusion length determines the structure dimension influenced by this smoothing effect. PACS 81.65.Cf; 81.05.Je; 42.70.Ce; 42.55.Lt     

Laser backside etching of fused silica with ultra-short pulses

Laser backside etching of transparent materials like fused silica at the interface to liquids with sub-picosecond UV laser pulses using a pyrene/toluene solution is achieved. For the experimental conditions applied, the etching effect is rather weak with measured rates of the order of 0.1 nm/pulse. A linear dependence of the etched volume upon the laser pulse energy or the pulse number was extracted from the experimental data obtained. At low pulse numbers the etched surface exhibits a feature-free, smooth morphology, while quasi-periodic ripple formation is observed for prolonged laser exposure. In addition, the etching process is accompanied by enhanced carbon deposition at and in the vicinity of the etched surface. The etching mechanism proposed comprises the primary interaction of the laser radiation with the liquid, a surface-modification phase, and the etching of the modified fused-silica surface. PACS 81.65.Cf; 81.05.Kf; 79.20.Ds; 61.80.Ba; 42.55.Lt; 68.45.Da     

Real-time monitoring of SiO2/Si(1 1 1) interlayer etching by Brewster-angle reflectometry

A transitory etching regime after SiO₂ dissolution and before bulk Si(1 1 1) etching in neutral NH₄F solutions was monitored by in situ Brewster-angle reflectometry (BAR). An observed intermediate increase of the BAR reflectance signal is attributed to a fast dissolution of a stressed/strained interlayer beneath the SiO₂/Si(1 1 1) interface. Similar effects were observed on thin thermal oxides (18.2 nm), grown on float zone silicon, as well as on ultra-thin native oxides (1.2 nm) on Czochralsky silicon. Native oxide covered samples showed an increased surface roughness in the course of interlayer dissolution while the surface is progressively covered with compounds of fluorinated silicon. The etch rate, determined by atomic force microscopy (AFM) and compared to the etch rate of bulk silicon, is increased by a factor of four. In the limit of extended etching, the known low etch rates for silicon in 40% NH₄F are observed. Structural and chemical properties of the interfacial layer were analyzed by synchrotron radiation photoelectron spectroscopy (SRPES) which confirmed the presence of Si^3+/4+ valence states throughout the interlayer and by near open-circuit potential (N-OCP) dark current measurements. As a result, oxide etch rates in NH₄F in the pH-range 7–8 as well as the silicon interlayer depth can be assessed by in situ BAR.     

脉冲激光与电化学复合的应力刻蚀加工质量研究

脉冲激光电化学复合加工可以有效去除激光辐照区域内的电解产物, 提高加工效率, 改善加工质量. 针对高性能金属材料的微细加工要求, 采用脉冲激光电化学复合的应力刻蚀加工方法对铝合金的刻蚀特性进行理论和试验研究. 通过比较激光直接刻蚀加工和激光电化学复合加工的特点, 应用扫描电子显微镜、光学轮廓仪等检测技术分析了刻蚀区域的形貌特征. 根据力学电化学原理, 探讨了金属材料微结构加工的应力去除机理. 通过加工试验, 研究了工艺参数和加工方式对加工质量的影响, 采用优化的工艺参数, 加工出了质量较好的微结构. 试验结果表明, 激光电化学复合的连续扫描加工稳定性好, 可以有效地降低表面粗糙度, 提高加工质量. 关键词： 激光电化学 应力刻蚀 加工质量 工艺参数     

Backside etching of fused silica with Nd:YAG laser

The laser-induced backside etching of fused silica with gallium as highly absorbing backside absorber using pulsed infrared Nd:YAG laser radiation is demonstrated for the first time. The influence of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography was studied. The comparable high threshold fluences of about 3 and 7 J/cm² for 18 and 73 ns pulses, respectively, are caused by the high reflectivity of the fused silica-gallium interface and the high thermal conductivity of gallium. For the 18 and 73 ns long pulses the etch rate rises almost linearly with the laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. Incubation processes are almost absent because etching is already observed with the first laser pulse at all etch conditions and the etch rate is constant up to 30 pulses.The etched grooves are Gaussian-curved and show well-defined edges and a smooth bottom. The roughness measured by interference microscopy was 1.5 nm rms at an etch depth of 0.6 μm. The laser-induced backside etching with gallium is a promising approach for the industrial application of the backside etching technique with IR Nd:YAG laser.     

聚合物阵列波导光栅的制作技术

研究了聚合物阵列波导光栅AWG制作的几个关键技术.首先,为了克服反应离子刻蚀过程中单独使用光刻胶作掩膜而导致的光波导形状和尺寸偏离设计的缺点,采用了光刻胶与金属掩膜相结合的双掩膜技术进行器件制作.详细介绍了双掩膜技术制备聚合物AWG的过程,并得出铝膜作为掩膜的最佳厚度为100nm左右.测试给出了使用和没有使用双掩膜的对比结果,该结果表明使用双掩膜技术制作的波导质量明显好于单独使用光刻胶作掩膜制作的结果.其次,采用蒸气回溶技术来减小反应离子刻蚀产生的波导表面和侧壁的起伏,从而降低了波导的散射损耗.结果表明,蒸气回溶技术使所制作的波导表面的均方根粗糙度从41.307nm降低到24.564nm.     

Imprinted optical pattern of low-softening phosphate glass

Thermal imprinting of transparent tin phosphate glass was performed at 250 degrees C using a fine-patterned silica mold. The glass sample was prepared by a conventional melt-quenching method and polished with a roughness of < or =10 nm for imprinting experiments. The imprinting temperature is optimized based on experimental viscosity data. Scanning electron microscope and atomic force microscope observations revealed that a square grid pattern has a surface roughness of < or =10 nm and 5 microm x 5 microm squares with ~1 microm intervals and 90-100 nm depth. Diffraction spots due to the micropattern are demonstrated by illuminating He-Ne laser light.     

Microstructuring of fused silica by laser-induced backside wet etching using picosecond laser pulses

The laser-induced backside wet etching (LIBWE) is an advanced laser processing method used for structuring transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated for various materials, e.g. oxides (fused silica, sapphire) or fluorides (CaF₂, MgF₂), and applied for the fabrication of microstructures. In the present study, LIBWE of fused silica with mode-locked picosecond (t_p = 10 ps) lasers at UV wavelengths (λ₁ = 355 nm and λ₂ = 266 nm) using a (pyrene) toluene solution was demonstrated for the first time. The influence of the experimental parameters, such as laser fluence, pulse number, and absorbing liquid, on the etch rate and the resulting surface morphology were investigated. The etch rate grew linearly with the laser fluence in the low and in the high fluence range with different slopes. Incubation at low pulse numbers as well as a nearly constant etch rate after a specific pulse number for example were observed. Additionally, the etch rate depended on the absorbing liquid used; whereas the higher absorption of the admixture of pyrene in the used toluene enhances the etch rate and decreases the threshold fluence. With a λ₁ = 266 nm laser set-up, an exceptionally smooth surface in the etch pits was achieved. For both wavelengths (λ₁ = 266 nm and λ₂ = 355 nm), LIPSS (laser-induced periodic surface structures) formation was observed, especially at laser fluences near the thresholds of 170 and 120 mJ/cm², respectively.