CUTTING OF CVD DIAMOND BY OPTICAL FIBRE GUIDED Nd:YAG LASER

摘要

The objective of this work is to study the performance of the Nd:YAG laser to cut thick free standing CVD diamond films. The high flexibility of this type of laser, given by the possibility to guide the laser beam by means of an optical fibre represents a great advantage in order to automate the cutting process in the production lines. Laser cutting technology is imperative to shape non-conductive CVD diamond parts for electronic, optical and mechanical applications, namely heat sinks, infrared transparent windows and small tips to braze onto machining tool holders. Concerning diamond tool production, the rapid cutting method by laser here proposed provides an advantage to CVD diamond when compared to conventional polycrystalline diamond (PCD) blanks that are cut by the Electro-Discharge Machining (EDM), a time-consuming and very expensive technique due to the electrodes cost. Free-standing diamond films were grown on silicon wafers by microwave plasma chemical vapour deposition (MPCVD) method in a ASTeX PDS 18 unit. The diamond growth parameters were: microwave power 4.35kW; total pressure 110Torr; H2 flow 400s.c.c.m.; CH4 flow 30s.c.c.m; deposition time 70h. The final film thickness was approximately 500←m. The diamond plates were then released by chemical dissolution of the Si substrate in a nitric/hydrofluoric acids mixture. The equipment employed to perform the cutting experiments consisted of a pulsed Nd:YAG laser delivering a maximum average power of 500W at a wavelength of 1064 nm. The laser beam was coupled to an optical fibre of 400←m diameter and 10 m long. The pulsed beam coming out of the fibre was focused onto the surface of the free standing diamond coating by means of a commercial cutting head, having a lens of 80 mm focal length, in which the laser beam was coaxial to the Ar gas jet used as assist gas. In this paper we discuss the results of the work carried out to analyse the influence on cuts quality of laser processing parameters such as: cutting speed 5-20mm/s; pulse width 0.3-1ms; pulse energy 0.8-4.6J; power 80-470W. Scanning electron microscopy was used to look into film cracking events and to appraise the cut width and linearity. Micro-Raman spectroscopy allowed the evaluation of the graphitisation level on the cut surface.