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.
Laser-induced backside wet etching (LIBWE) allows the etching of transparent materials with pulsed UV-lasers. The laser-etched structures are characterized by a high fidelity and a low surface roughness. For the etching of periodically sub-micron structures on a solid surface interfering laser beams were used produced by projection of diffraction masks. In conjunction with LIBWE surface relief gratings were realized on planar and curved fused silica substrates in one-step direct fabrication process. The fundamentals of topography and roughness evolution of surfaces etched by LIBWE are investigated in detail. The etching of sub-μm 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 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 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. The knowledge on the effects of submicron resolved laser irradiation by LIBWE approach is from great importance for applying laser backside etching to nanometer grating fabrication. To demonstrate the capabilities of the processing approach a rectangular binary grating with a period was subsequently patterned with sub-micron relief gratings.
Presently, there is a growing demand from the industry for microprocessing of materials. For applications in microsystems technology and biotechnology it is particularly necessary to produce structures with dimensions down to the micrometer scale. This refers especially to materials that can not or not in a sufficiently quality be processed by conventional methods of silicon technologies.
In order to fulfil the industrial demands we have investigated the structuring of anodic bondable PYREX glass and of polymers by means of laser microprocessing using the excimer laser mask projection technique (193 nm wavelength, 10 ns pulse duration, 8 mJ pulse energy, 500 Hz repetition rate). In our paper we will show the dependence of the obtained quality especially the roughness of the generated surfaces on the processing parameters. The possibilities of our technology including the creation of holes, channels and three dimensional microstructures will be presented too. Single structures e.g. bridges and grooves are attainable with widths of 10 micrometers and below. Some of the drilled holes (diameter about 50 µm) have been successfully filled inside with aluminium by a laser assisted CVD process.