Contamination and Debris
Figure 7.1: A slot machined in INVAR with nanosecond pulses.
Figure 7.2: Invar sheet machined with an ultrafast laser.
This sheet of INVAR was micromachined with a Clark-MXR ultrafast workstation. It should be compared with the previous picture, where the same material was machined with a long-pulse (10 nsec) laser. The femtosecond-processed sample was cleaned with a mild jet of dry nitrogen (just like the nanosecond-processed sample).
In conventional, (i.e. long-pulse), laser machining, large amounts of debris are created during the machining process. The debris, whose form depends on the material being machined, can be very difficult to remove.
In conventional machining, a large heat affected zone completely surrounds the work area. A heavy recast layer is present immediately along the edges of the slot. Outside the recast layer, an extended zone of debris (droplets of molten metal) is visible. This debris was still extremely hot when it landed on the surface. Removing this material will require substantial post processing efforts, if it can be done at all without damaging the surface. The situation is quite different when working with femtosecond lasers, as shown below.
With the ultrafast pulses very little debris was generated during the micromachining process, and what remains is not in the form of hot droplets that attached to the surface. Rather, the femtosecond process creates a fine dust that does not carry much heat, and therefore does not bind to the surface.
It should be noted that techniques have been developed to reduce the amount of debris created when machining with long-pulse lasers. Using powerful gas jets one can considerably reduce the amount of debris created, or one can push the debris far away from the work zone before re-deposition. A significant amount of work has gone in this debris mitigating effort. Users have developed jets with various geometries. Various gases have been used, including pure oxygen for rapid burn of the debris, etc.
These approaches can, in some cases, almost totally eliminate contamination of the sample by debris. (This of course does not alleviate the other problems associated with heat diffusion as encountered in long-pulse machining.)