Femtosecond Laser Ablation of Si, GaAs, and InP

Abstract

This thesis presents the study of x-ray emission from femtosecond laser micromachining and laser ablation of semiconductors. Prior to femtosecond machining experiments we investigated the nature of radiation emitted during the irradiation of solid targets with 120 femtosecond pulses with energies between 500 nJ and 0.3 mJ at a 1 kHz repetition rate. We have shown that the majority of the radiation was emitted below 10 keV with the high energy edge extending up to 25 keV. Under our experimental conditions K line emission was observed from materials with Z<32. We have also measured the x-ray dose rates during laser machining of various targets on the order of 10 mSv/h at a distance of 13 cm from the target. The implications for work pace safety, micromachining control, and potential for pulsed x-ray line sources for spectroscopic and imaging applications are discussed. In our studies of single shot femtosecond ablation of selected semiconductors: Si, GaAs, and InP, we have concentrated on the studies of microstructure and composition of the material after irradiation with 120 femtosecond pulses with energies between 2 nJ and 2 µJ. The resulting surface morphology, structure and composition of the micron scale ablation features on the semiconductors were studied by electron microscopy and atomic force microscopy. We found that no sharp threshold in the surface morphology was observed with increasing pulse power; however three ablation stages were identified based on the characteristic features of the ablation craters. TEM analysis revealed essentially no crystal damage beneath and in the vicinity of the ablation craters. In case of the binary semiconductors 5-30 nm polycrystalline grains were found over the ablated surfaces. The results were discussed in terms of the existing state of knowledge of ablation dynamics. The implications for practical micromachining applications are also discussed.