An investigation into the microgrooving process for germanium substrates using a hybrid laser-waterjet technology.

An extensive literature review of the traditional and advanced laser machining processes has been conducted to appreciate the fundamentals of laser physics, laser material removal mechanisms, the formation and effect of plasma, as well as the development of advanced laser ablation processes for minimizing thermal damages. It is shown that laser ablation is a viable method for material processing, whereas the laser induced thermal damage is a highly undesirable phenomenon that affects the cut quality. Water assistance has been introduced into laser machining in several ways to reduce thermal damages, among which the hybrid laser-waterjet technology appears to be a promising method. In this technology, laser is used to locally heat and soften the material and the softened material is then expelled by a high pressure waterjet. The waterjet also introduces a cooling effect. It has been reported that this technology can remove material in its soft solid status and thermal damage can be reduced to a negligible level.An experimental investigation of the cutting performance in hybrid laser-waterjet microgrooving of germanium wafers is performed, with a view to assess the machining process. Relevant process parameters are considered, such as water pressure, laser pulse overlap, pulse energy and focal plane position. It is found that the hybrid technology can machine microgrooves on germanium wafers at commercially viable cutting rates with negligible thermal damages. A Raman spectroscopy study did not reveal any crystalline change of the material on the machined surfaces. The effects of process parameters on the heat-affected zone and groove characteristics are amply discussed. It is shown that good grooves of within 100 μm in top width and up to 300 μm in depth can be machined with high material removal rates, and the heat-affected zone size can be controlled to within 20 μm on each side of the grooves. Recommendations are also made on the appropriate process parameters that may be used in the practice. A thermal model for the hybrid laser-waterjet microgrooving process has been developed, in which laser induced plasma (or optical breakdown) in the water layer and its associated shielding effect on laser beam are considered. Other relevant physical phenomena are also incorporated in the model which include laser absorption and heat transfer in germanium substrates, waterjet cooling and impinging effects, as well as the gradual material removal process. Model assessment is carried out by comparing the quantities whose data are either available in the literature or can be measured experimentally with the calculated data from the model under the corresponding conditions. Reasonable agreements are found between the measured and model-calculated results, so that it may be stated that the model is able to adequately evaluate the heat evolution process associated with the hybrid laser-waterjet micromachining technology. Using the developed thermal model, a simulation study of the hybrid micromachining process has been performed to study the relevant physical phenomena involved in the hybrid process. The simulation results show that the shielding effect of the laser induced plasma increases with an increase in laser pulse energy, and the laser energy transmission rate in the beam centre can be reduced to less than 40% when the pulse energy is increased to 0.5 mJ. In addition, the irradiated material can be removed by a waterjet at its soft-solid status, and heat accumulation in the workpiece is effectively removed by the continuous waterjet cooling effect, so that laser heating induced thermal damage is minimised. The effects of processing parameters on material removal are also numerically analysed. Specifically, an increase in laser pulse energy increases the groove depth, while at the same time a more intensive laser beam yields a stronger shielding effect on the incident laser beam by the laser-induced plasma. Further, an increase in the water pressure decreases the threshold workpiece temperature for material removal; however, it is noted that the increased cooling effect by a higher pressure waterjet requires more laser energy input to heat the material. The effect of laser wavelength and pulse duration on the laser induced breakdown threshold have also been studied, where the breakdown threshold is found to decrease with an increase in laser wavelength or pulse duration, while the simulated groove depth shows an increasing trend with the laser wavelength