A solid-state UV laser was used to make arrays of reproducible percussion-drilled micron-sized holes in polyimide. An optical switch was employed as a pulse picker to select specific patterns of pulses from the high repetition rate laser beam. The ability to control and vary the number of pulses per burst and the time between bursts enhanced the drilling rate while minimizing thermal damage around the holes. The optimum pulse patterns were determined experimentally. A photodiode acted as a breakthrough sensor to end the drilling and optimize the exit hole size and quality. Results were compared with computer simulations of the drilling process based on modeling of the laser/material interaction.
Polymers play an important role in many applications such as microelectronics and medical devices. Micro-channels and shaped holes can be produced by 2.5D micro-structuring with excimer laser sources using mask projection. The industrial cost associated with these processes can be greatly reduced by the use of solid-state lasers due to their lower cost and maintenance. For this purpose, we investigate the interaction of polyimide (Kapton) with solid-state lasers emitting in the UV (266 & 355 nm) spectral range. The study presents a comparison of the ablation profiles obtained for different laser sources and these are discussed in term of roughness and efficiency. Limitations on the actual motion system (scan-head) are evident and the need to control the material removed by a small Gaussian beam in terms of overlapping for the direct writing process will be highlighted.
A time-resolving Langmuir probe has been used to study the plasma plumes produced by ablation of silver with 200 femtosecond laser pulses at fluences of 1-12 J cm-2 at a central wavelength of 775 nm. Initial results have shown that surface contamination, and subsequent recontamination, can significantly influence the time of flight (TOF) signals obtained using the Langmuir probes. Surface conditioning techniques have been developed to overcome these influences. The TOF signals have been used to establish that the threshold fluence for the laser produced plasma in silver, under the present operating conditions, occurs at 1.04 J cm-2. The angular dependence of the magnitude of the ion yields and energies, at the time when the ion flux is maximized, agree with the predictions of Anisimov’s self-similar isentropic model of the plasma expansion.
Percussion drilling of blind holes and vias in Kapton film was investigated using Q-switched solid state lasers operating at UV (355nm) and VUV (266nm) wavelengths. Holes were analyzed using different methods such as scanning electron microscopy (SEM) and surface profilometry. Ablation rates for the two wavelengths are compared. No abrupt thresholds were found and there was no evidence of an incubation effect within the first few pulses. Introducing pauses during the drilling increased the number of shots required for perforation of the film. The effects of fluence on diameter, depth and taper of the holes are presented. Smaller and neater holes are achieved more accurately with a lower fluence. An observed skin effect brought about by long exposure to low fluence VUV laser light is also discussed.
The problem created by the re-deposition of ablated material when laser machining structures in silicon wafers is investigated. The study focuses on the specific case of machining wafer grade silicon with femtosecond pulses centered at a wavelength of 775 nm. Based on the evidence that a highly ionised plasma state exists immediately after laser ablation, this work explores the potential of using electric fields to channel the debris out of the laser machined feature before it becomes deposited. To this extent the work discusses the step-by-step development of different experimental arrangements, by first evaluating its effects, then identifying its limitations and finally by proposing and investigating potential solutions. It is found that a reduction in the amount of re-deposited debris is observed when a carrier-depleted region is generated in silicon materials.
First results and analysis of second harmonic generation with a Nd:YAG laser working in the long pulse, free running mode are presented in this paper. Second harmonic generation conversion efficiencies of up to 17.5% and pulse powers of 162W have been generated with a free running Nd:YAG laser and a KTP non-linear crystal. The conversion efficiency is limited by a saturation effect and optical damage occurring at ~50 times lower peak intensities than in the Q-switch mode. The saturation and damage mechanism involves creation of temporary 'color centers' induced by the second harmonic radiation and subsequent increased fundeamental wave absorption.
Frequency doubled Nd:YAG lasers represent an attractive alternative to other laser tools for many material processing applications, but frequency doubling with pulsed Nd:YAG lasers has been performed until now only with pulses of tens of nanoseconds. In material processing with longer pulses (10-1000 microsecond(s) ), such as encountered in typical 1.06 micrometers industrial Nd:YAG applications, the laser-material interaction is different and, in particular, higher material ablation rates are performed. Furthermore, the green light material processing permits a better focusability and a higher absorption in most materials. However, frequency doubling with long pulse lasers is much more difficult and less efficient up to now. The main problems are the generation of a fundamental 1.064 micrometers beam of high quality necessary for the non-linear process, and the low damage threshold of the non linear materials in the long pulse regime. Therefore, a zigzag slab laser, which has a high beam quality and an inherently linear polarization of the beam, is an ideal candidate for non-linear processes. The optical damage threshold in the non-linear materials is the main limiting parameter. The 140 W instantaneous power obtained for a 200 microsecond(s) pulse duration in extra-cavity configuration allows us to finely process sheets up to 200 micrometers thick.
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