R. Böhme

Backside etching of fused silica with UV laser pulses using mercury

Laser-induced backside wet etching (LIBWE) is an indirect etching technique that was first demonstrated for fused silica applying a pyrene doped hydrocarbon solution and was recently demonstrated for gallium as a highly absorbing liquid. In this study the etching of fused silica with mercury as liquid absorber is demonstrated using a nanosecond UV laser (excimer laser ? = 248?nm, 25?ns pulses) radiation. The high absorption coefficient of mercury enforces the absorption of the UV laser radiation within a 100?nm region near the solid interface so that, despite the high reflectivity, the threshold for etching was measured to be 0.76?J?cm?2. With rising laser fluence the etch rate increases nearly linearly in the whole laser fluence range and reaches a value of 650?nm/pulse at a laser fluence of about 10?J?cm?2. The etched surfaces were investigated by scanning electron microscopy, atomic force microscopy and interference microscopy and show well defined etch groove edges and a flat, smooth etched bottom that features an rms roughness of about 2.2?nm. The laser-induced backside etching of fused silica with mercury combines a low etching threshold, high etch rates and a still smooth etching of well-defined patterns into material surfaces.

Sub-micron periodic structuring of sapphire by laser induced backside wet etching technique.

The periodic structuring of sapphire, by using laser induced backside wet etching technique (LIBWE) and 266 nm, 150 ps Nd:YAG laser radiation, is demonstrated here for first time. Sub-micron period Bragg reflectors are successfully patterned in sapphire wafers using modest energy densities and number of pulses. The gratings are characterized using diffraction efficiency measurements, AFM, and SEM. Issues related to the ablation process and to the phase mask holography are presented and discussed. The experimental results presented depict that the applied method is capable to produce relief structures of depth as deep as 100 nm, while maintaining high resolutions, close to the thermal diffusion length of the material corresponding to the ultrashort pulse duration.

Ultra-short laser processing of transparent material at the interface to liquid

Similarly to laser-induced backside wet etching (LIBWE) with nanosecond ultraviolet (ns UV) laser pulses, the irradiation of the solid/liquid interface of fused silica with sub-picosecond (sub-ps) UV and femtosecond near infrared (fs NIR) laser pulses results in etching of the fused silica surface and deposition of decomposition products from liquid. Furthermore, the etch threshold is reduced compared with both direct ablation with an fs laser in air and backside etching with UV ns pulses. Using 0.5 M pyrene/toluene as absorbing liquid, the thresholds were determined to be 70 mJ cm−2 (sub-ps UV) and 330 mJ cm−2 (fs NIR). Furthermore, an almost linear increase in the etch rate with increasing laser fluence was found. The roughness of surfaces backside etched with ultra-short pulses is higher in comparison with ns pulses but lower than that obtained using direct fs laser ablation. Hence a combination of processes involved in fs laser ablation and ns backside etching can be expected. The processes at the ultra-short pulse laser irradiated solid/liquid interface are discussed, considering the effects of ultra-fast heating, multi-photon absorption processes, as well as defect generation in the materials.

Simulation of laser-induced backside wet etching of fused silica with hydrocarbon liquids

The mechanism of laser-induced backside wet etching (LIBWE) is important for the optimization of application processes but is still ambiguous. Extremely high surface absorption coefficients of more than 40×104 cm−1 at λ=248 nm that decay exponentially within less than 25 nm were measured for LIBWE-etched fused silica surface. Therefore, the resulting laser-induced temperatures quickly exceed the boiling point and result in surface erosion of the modified material. Numerical calculations of the temperature considering the measured absorption and phase transitions have been used to evaluate the etching depth of fused silica with a pyrene/toluene solution that agrees well with the measured rates well. A model of LIBWE is proposed that bases on the laser ablation of the high-absorbing modified fused silica as the dominating erosion process.

Backside etching of fused silica with ultra-short laser pulses at the interface to absorbing liquid

The etching of fused silica substrates by employing the process of laser-induced backside wet etching (LIBWE) using the laser radiation of a 248nm, 500fs excimer laser and a 775nm, 130fs Ti:sapphire is presented here for the first time. Etched volume results are presented in combination with topological investigations of the etched areas performed by SEM scans, revealing new aspects of the nature and products of the process.

Direct laser etching of transparent materials: high quality surface patterning and figuring for micro-optical applications

The etching of transparent materials with high precision and high quality is still a challenge for laser processing. Laser backside etching allows the processing of transparent materials with pulsed UV-lasers. The laser-etched structures in fused silica are characterized by a high fidelity and a low surface roughness. Different machining techniques were applied for laser etching of binary and three-dimensional microstructures with micron and sub-micron sizes. Applying contour mask technique micro sized cylindrical lens and prism array were fabricated. Using small spot laser written gratings with uniform or variable depth was machined with nanometer depth resolution and the etching of free-form surfaces with a size of 1 mm2 and a P-V-value of less than 1 micron by means of laser scanning is demonstrated. Additionally, graded multilevel elements and submicron gratings were engraved with nanometer depth accuracy applying mask projection techniques.

Excimer laser-induced material modification to create nanometer high smooth patterns in glass using mask projection

Laser swelling of borosilicate and soda-lime glass is shown for wavelengths of 193 and 248 nm. Very smooth patterns up to 45 nm high were generated by KrF laser (248 nm) irradiation of borosilicate glass at a fluence of 1.5 J/cm2. At 193 nm laser wavelength, lower heights (up to 13 nm) and lower swelling threshold fluences (0.1 J/cm2) were observed due to higher material absorption. For the less absorbing soda-lime glass higher fluences than for the borosilicate glass are needed to establish elevated structures. Gratings in borosilicate glass with sub-micron periodicity demonstrate the high resolution of the method. The results can be explained by a thermo-physical model based on the change of the glass transition temperature due to fast cooling after the pulsed laser irradiation.

Influence of the kind of absorbing liquid on fundamentals and application at laser-induced backside wet etching

Wang et al. [1999] has introduced a method for laser etching of transparent materials using absorbing liquids called laser-induced backside wet etching (LIBWE). In this approach the etching process of the material is caused by the intense energy deposition near the interface to the solid due to the strong absorption of the high-energy laser light beam by the liquid. The kind and the properties of absorbing liquid are one of the important parameters of the etching process. The used liquid components influence the laser-induced decomposition chemistry directly and cause the formation of a surface layer containing the specific decomposition products. This laser-induced surface modification affects physical processes involved in laser etching and in consequence the etch ratesing and the applications of the etch process essentially basically.

Laser fabrication of micro sized diffractive and refractive optical devices in fused silica and glass

Laser ablation has proved to be a flexible method for direct micro-structuring of micro-mechanical, micro-fluid or micro-optical components. In this work we will present recent results on micro-machining of fused silica and different glasses. An excimer laser running at /spl lambda/=248 nm (KrF) or /spl lambda/=351 nm (XeF) (/spl tau//sub p/=20 ns) was used for the experiments. Various solutions of acetone, toluene and tetrachloroethylene containing pyrene in different concentrations (0.1-0.5 M [mol/1]) have been applied for studying the etching process. The influence of the laser and process parameters (e.g. laser fluency, scan velocity, etc.) on the etch rate and the surface roughness of the materials are investigated quantitatively with the aim of process optimization. Using these parameters the fabrication of three-dimensional micro-topographies in fused silica, pyrex and other glass by direct laser writing with the scanning contour mask technique will be shown. The etched cylindrical structures and micro-prisms show sharp edges and smooth surfaces with micro-roughness below 10 nm RMS at a etch depth of more than 30 /spl mu/m. Also the machining of linear or crossed phase gratings with a period less than 20 /spl mu/m is mentioned.