Experimental Studies On Femtosecond Laser Ablation Of Semiconductor And Metals

Because of the many advantages of femtosecond laser interaction with materials, such as shortinteraction time, small heat-affected zone and high machining accuracy, ultrashort laser pulsemicro-fabrication technology has been extensively studied in many scientific experiments. Thetechnology has wide potential applications in industrial fabrications and many other fields. In thisthesis, the double pulse and shaped laser pulse are used in the ablation of semiconductor and metalin different experiment conditions. The mechanisms of ablation are proposed, based on the analysisof the photoluminescence intensity, transmitted light intensity and ablation morphology, etc. Thecontents of this thesis are divided into the following parts:(1) Study of femtosecond double pulse laser ablation of silicon. The ablation mechanism hasbeen studied experimentally by means of photoluminescence and morphology analysis underdifferent experimental conditions like double pulse delay time, fluence and ambient pressure.Firstly, laser pulses with different fluences (40.6J/cm2-947J/cm2) are used to ablate Si target invacuum (2Pa) and atmospheric pressure condition, photoluminescence intensities are scanned byvarying the delay time between the double-pulse. The spectrum intensity at all wavelengths in thespectrum range studied (200nm–600nm) follows the same trend as a function of double pulsedelay. So in this work, the Si+emission line at505.6nm (3s24p(2P)3s24d(2measured. The experimental results show that, at lower laser fluences, photoluminescence intensitywill increases with increasing delay time. With the fluence further increasing, the emission intensitywill decreases initially, reach a minimum at certain delay time, then start to increase. This thesiscompared the results of different laser fluences and explained the reason of the above-mentionedphenomena. In addition, the size and the shape of the splashing material are analysed at differentdelay time. After the ablation by high fluence laser pulse, some splashing material is produced atthe sample surface. With the delay time increasing, the width of splashing material decrease, whilethe height increases. The shape of the droplet is more like a thin slab at shorter double pulse delay,and becomes more congregated at longer delay. At longer delay time, droplets are followed byliquid tails.In this thesis, the effect of ambient pressure on ablation is also studied at two laser fluences(40.6J/cm2and676J/cm2). The experimental results show that the emission intensity and crater depth increase as a function of pressure. The maxima reach at103Pa, and104Pa, respectively,followed by decline at even higher pressure. The trend of crater width is reversed compared to thatof the crater depth, the minimum is around104Pa. Base on the changes of crater depth and width,the crater volume fluctuates around0.2μm3and6.5μm3over the pressure range, with a slightincreasing trend as the pressure increases. By measuring and analysis of the morphology, the sizeand the shape of the splashing material are studied at different ambient pressure. At low fluence(40.6J/cm2), there is no splashing material around the crater, while a resolidified structure appearsat the bottom of the crater. The size of this structure increases with increasing ambient pressure, themaximum size appears at the order of104Pa. At high fluence (676J/cm2), the width of splashingmaterial decreases with increasing ambient pressure, while the height increases.(2) In the study of drilling of aluminum and copper by femtosecond laser pulse with differentrepetition frequency, the influences of relative position between laser focus and target, laser fluenceand ambient pressure on femtosecond laser drilling are studied by means of detecting thetransmitted light intensity and ablation morphology analysis. These studies are used to explore themechanism of femtosecond laser drilling of metal. These studies help to obtain the optimummethod to improve the quality of hole. Firstly, the effect of relative position between laser focusand target is studied. The minimum number of pulse required to penetrate and the maximum stableintensity of transmitted light are obtained when the laser focus is positioned at the target surface.And under this condition, the diameters of entrance and exit holes are the smallest and thedifference between them is the smallest too. When the laser focus moves away from the targetsurface, the number of pulse required to penetrate and hole diameters increase, while the stableintensity of transmitted light decreases.It is shown that the number of pulse required to penetrate decreases with increasing laserfluence, while the stable intensity of transmitted light, the hole diameter and the quantity ofsplashing material increase. When the laser fluence is in the range of100J/cm2-200J/cm2, thedifference between the diameters of the entrance and exit holes is small.When the ambient pressure is changed, the number of pulse required to penetrate decreaseswith increasing ambient pressure, while the stable intensity of transmitted light and the quantity ofsplashing material increase initially, reach a maximum at certain ambient pressure, then start todecrease. The diameter of the entrance hole decreases and reaches minimum around102Pa, then increases in even higher pressure region, reaches maximum around104Pa. Under the differentambient pressure condition, no significant variation of the exit hole diameter is observed.(3) The shaped femtosecond laser pulses are used to drill aluminum. The phenomenon isexplained by the two-temperature model (TTM). With different laser fluence, the ablationefficiency of shaped laser pulse is different. For the higher laser fluence, the ablation efficiency ofshaped laser pulse is higher than that of transform-limited pulse. The optimized ablation efficiencycan be obtained by increasing the time separation of each subpulse. For the lower laser fluence, theablation efficiency is the highest by using transform-limited pulse. Compared with original pulse,the diameter of ablation hole will be reduced by the shaped femtosecond laser. The ablationaccuracy is improved.