| New method for structuring brittle materials |
| Hard and brittle materials, like certain types of glass and sapphire, are difficult to process—even for lasers. Poor absorption at most wavelengths combined with poor heat-transfer properties make it difficult or impossible to realize structures in these materials.
However, the In-Volume Structuring team at Fraunhofer ILT (Institute for Laser Technology), Aachen, Germany, has successfully developed and demonstrated a technique for the high-resolution structuring of these hard, brittle materials. The two-step process—called In-volume Selective Laser Etching, or ISLE—involves applying the ultrashort pulses (10-15 second) of a femtosecond laser.
The direct-laser-writing technique can be used for surface and in-volume structuring, and has potential applications in the manufacture of microfluidic devices, sensing applications and, perhaps, gas-flow dynamics.
The technique involves application of focused laser pulses that change the chemical structure of a material such that subsequent immersion in a wet etching agent promotes the selective etching of only the exposed area, leaving the unexposed area untreated.
The provided energy is absorbed only in the focal volume, where the intensities are high enough for nonlinear absorption processes. This has the effect of changing the structural properties of the lattice, resulting in a density and refraction-index change of the exposed area.
The fs-laser pulse is used in conjunction with a microscope objective in order to attain the required high intensities on target. The resulting spot size is about 1µm. A high-repetition-rate laser must be used with a fast scanner in order to irradiate the volume at speeds up to v >0.3m/s. However, the repetition rate must not be so high as to trigger heat-accumulation effects.
After exposure, the sample is immersed in a wet etching agent that selectively removes the exposed material. Etch ratios are material-specific. For instance, the etching ratio between sapphire and the resulting amorphous sapphire is about 1:10,000.
The ISLE technique can produce features with a surface-area roughness of Sa = 64nm, which is approximately the roughness value of highly polished mechanical devices like bearings. Furthermore, there is very little material loss. Parts can be totally excised from the bulk with less than 2 percent total material loss.
Fraunhofer's ISLE technique has been successfully demonstrated in a range of applications. Below are several examples:
In-volume marking. With ISLE, micromarking in transparent materials is achieved below the polished surface. Figure 1 shows a field of squares, in fused silica, at a depth of approximately 200µm. Each square was structured in less than 3 seconds using intersecting line segments, with a varied 3µm-to-10µm pitch between lines. The modified lines exhibit a refractive-index change with no observable cracks. Due to the diffraction from the micro grid and interference in the observers eye, colors are observed when varying the viewing angle.
2-D cuts. Figure 2 shows a cross section of a slit in sapphire. The dimensions are 1µm wide (X) × 1cm (Y) × 125µm deep (Z). The slit was made by using 50 overlapping tracks while adjusting the Z position between tracks to achieve the desired depth. This strategy also can be used to produce 3-D parts, including hollow, free-form structures and channels for microfluidic devices. The dimensions of the structures are controlled by the number of adjacent tracks.
Current practice is to etch channels on two plates, then join them together. This technique creates numerous problems and involves many steps. With the ISLE technique, large, hollow structures are created in bulk material in a two-step process. Cubes measuring 50µm × 10µm × 10µm have been produced. A condition of the technique is that channels must be etched from the surface to allow the wet etching agent to flow into the designed cavity.
3-D microparts, microholes. Figure 3 shows a cylinder removed from a bulk substrate. This was accomplished by having the laser exposure and etch penetrate the sample's entire depth. Both the diameter and part thickness are 500µm. After etching, the cylinder easily fell out of the bulk material.
Figure 4 shows a section of a 1.4µm cut made when producing a cube, before the cube was extracted from the material. The cube dimensions are about 450µm × 500µm × 500µm. The volume of the kerf is 450µm × 2,000µm × 1.4µm. The volume of the kerf compared to the volume of the cube shows a material loss of less than 2 percent.
Figure 5 shows gears extracted from bulk, 1mm-thick fused silica. ISLE can laser-irradiate 1mm-dia. × 1mm-high structures in 20 to 40 seconds.
The ISLE technique works with existing CAD/CAM software, enabling the rapid manufacturing of microstructured parts and in-volume micromarkings by exploiting the potential of today's high-repetition-rate fs lasers.
reference: http://micromanufacturing.com/
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