Femtosecond laser micromachining of silicon with an external electric field

Femtosecond (fs) lasers have been shown to be superior to long-pulse lasers in micromachining a wide range of materials. The ultrashort pulses and their extremely high peak power lead to minimum thermal energy diffusion into the material and ensure fast heating and vaporizing of materials with reduced melting. Therefore, fs laser micromachining results in a much reduced heat-affected zone of the machined surfaces. However, plasma generation is normally associated with fs laser processing due to the substrate material breakdown. The plasma cloud may fall back onto the substrate as debris on the surface. In the case of fs laser micromachining of silicon (Si), a large number of micro/nano-sized particles are observed on the Si surface. Some of the particles are loose particles that can be easily removed. The remaining particles are however bonded to the substrate that cannot be cleaned off. For microelectronics applications, such surface contaminants affect the reliability of the device and have to be avoided. In our study, we employed an external electric field during the fs laser micromachining. It was found that the external electric field strength had a significant effect on the cleanliness of the machined Si surface.