LASERSTRUCTURING OF POLY- AND MONOCRYSTALLINE DIAMOND TOOLS

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Figure1: Prototype of a grooving tool, length: 200μm, aspect ratio 1:10

Its outstanding thermal conductivity and mechanical hardness make diamond the preferred material for tools for ultra-precision manufacturing of metals, metal alloys, plastics and semi‑conductors. However, in turn these properties make it hard to manufacture or to refurbish diamond tools. On the one hand, the spectrum of possible tool geometries is technologically limited. On the other, generating the tool tips with commonly used abrasive techniques like grinding, lapping and polishing is a time-consuming and material-intensive process. Laser contouring and manufacturing by ultra-short laser pulses is a promising alternative technology to overcome some of these limitations [1]. This technique allows a fast processing of the diamond independent of its crystal orientation to generate almost every arbitrary shape. Furthermore, the application of energy within femtoseconds is confined both locally and temporary, reducing damage to the diamond to a minimum [2]. This technique is therefore suitable for ultra-precision processing. The laser beam is focussed directly onto the diamond surface by a microscope objective without any prior beam shaping or preparation. The extreme high intensity leads to ultra-fast material removal within some picoseconds in the laser focus area [3]. Within this period of time, diffusion of heat and mechanical stress is negligible.

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Figure 2: Prototype of a saw-tooth-like structure for a diamond micro mill, Dimensions: 100μm x 30μm

The applied energy in the nanojoule domain minimizes the amount of energy inside the diamond and therefore limits the ablation volume. Hence one single laser shot only removes some cubic microns of diamond material. Moving the diamond in up to three dimensions (x,y,z) while processing, almost every arbitrary shape can be carved into the diamond surface as well as into the bulk material. Structures with micrometer-sized, filigree contours and shapes are representable. So far, some prototypes of possible tool geometries have been produced, e.g. needle-like shapes in single crystal diamond as a prototype for a diamond grooving tool (Fig. 1) for UP-turning or -milling as well as saw‑tooth‑like shapes in polycrystalline diamond as a prototype of a micro mill (Fig. 2). The structures can be sized down to the mircometer domain with a surface roughness as small as 30 nm (rms). Cutting edges show a sharpness (edge radius) as small as 400 nm. The easily accessible surface of the diamond can be manufactured by the laser with sub-micrometer precision. Furthermore, nanoscopic surface structures can be generated by choosing appropriate process parameters like laser pulse energy and writing speed. On the chip face of a diamond tool, the right combination of microscopic structures superimposed by a nanoscopic pattern (Fig. 3) can guide chips while UP-turning and adhere or guide cooling lubricant as well. The investigations show that the structuring of diamond into almost any shape is possible with the ultra-short pulse laser technique. At the present time, this technique is capable of generating highprecision diamond micro mills. The laser processing is also suitable for manufacturing diamond turning tools by precision-cutting the tool shape by the laser, if an additional polishing step for finishing is applied.

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Figure 3: Microscopic chip guiding structure (period 5μm) with superimposed nanoscopic tern (period 400nm) on a diamond surface

This work is funded as part of the research and development project "nanoreplica" by the German Federal Ministry of Education and Research (BMBF), fund number FKZ 03IP609, and is managed by the Project Management Agency Projektträger Jülich (PtJ).

[1] A. Joswig, S. Risse, R. Eberhardt, A. Tünnermann: "Laser generated and structures prototypes of dimond tool tips for microoptics fabrication" ASPE (2010).

[2] B. Rethfeld, K. Sokolowski-Tinten, D. von der Linde, S.I. Anisimov: "Timescales in response of materials to femtosecond laser excitation" Appl. Phys. A 78, 767-769 (2004).

[3] C. Momma, S. Nolte, B.N. Chichkov, F. v. Alvensleben, A. Tünnermann: "Precise laser ablation with ultrashort pulses" Appl. Surf. Sci., 109/110, 15-19 (1997).