Fundamental investigation of femtosecond pulsed laser micromachining mechanisms for single-crystal diamond

Single-crystal diamond (SCD) has excellent mechanical, thermal and physical properties that rank among the highest of all substances. It has widespread applications in diverse fields such as cutting tools, optical windows, spintronics and power electronics. However, SCD is difficult to machine via mechanical processes due to its extremely high hardness. In recent years, laser processing has attracted attention since it enables non-contact efficient machining regardless of material hardness. In this study, femtosecond pulsed laser, which is considered to be capable of high-precision machining with low thermal effect, was used for micromachining of SCD, and the effects of laser irradiation parameters on the surface texture, roughness and profile geometry, as well as subsurface material structural changes, were investigated. It was found that even under femtosecond pulses thermal effect existed and that using a higher laser scanning speed was helpful to eliminate thermal cracks formation. The laser machined surfaces were covered with nanoscale periodical surface ripples with a surface roughness in the level of 0.10 μm Ra. Raman analysis of the SCD surface after laser machining showed that amorphous carbon and nanocrystalline graphite existed as a composite material. The graphitization depended on the laser fluence and hatching width and was particularly pronounced at low fluence and small hatching width. It was also demonstrated that acid cleaning could remove most of the graphite layer and reduce surface roughness. As samples, a few three-dimensional micro cavities with flat bottoms and sharp corners were created. This study demonstrates that femtosecond pulsed laser is a suitable method for machining micro three-dimensional shapes in SCD while it is important to optimize the laser machining conditions to avoid thermal damage and improve the surface quality. • Femtosecond pulsed laser micromachining characteristics of single-crystal diamond were investigated. • By increasing laser scanning speed, crack generation due to thermal effect was suppressed. • Flat surfaces with surface roughness of 0.10 μm Ra were successfully obtained under suitable conditions. • A laser-machined surface was covered with a layer of amorphous carbon and nanocrystalline graphite. • The graphite layer thickness and surface roughness depended on the overlap of the laser beam wings.