System.T. Dobrev, … S.S. Dimov, in 4M 2006 – Second International Conference on Multi-Material Micro Manufacture, 2006
2 Process overview
Laser cleaning is a form of laser ablation performed on the laser milling system where instead of having the target material in the focal point of the laser beam, an offset is introduced to reduce the laser fluence . As a result of this the substrate is only heat to reach a temperature close to the melting point of the particular material and thus to “flatten” any debris and other recast contaminants on the surface .
Figure 1 presents the set-up for a normal laser ablation operation. Where the laser beam is focuse on the top surface of the substrate. In this way a maximum power density could achieve on the surface. That is sufficient to raise the material temperature to its boiling point. Any deviation from the accurate focal distance normally results in a decrease of the volume of material. We remove it from the target and hence this has a detrimental effect on the process performance. Thus, in normal conditions this is an undesirable effect that should be avoided. Therefore, an optical sensor system is integrat in the machine to measure the distance to the target surface and if require to modify the laser power output so that a constant material removal rate is maintain.
However, to carry out laser cleaning the system should operate in this defocus state (see Figure 1). As a result of the offset, the laser beam does not reach a power density sufficient to perform material removal. While it is enough to soften the top-most surface layer and any debris and we deposit it during the machining.