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.
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.
The analysis of entrapped debris provides a useful complementary method of investigating the laser ablation mechanism in laser processing of polycrystalline metal samples using a femtosecond laser (Clark MXR, CPA2001). Morphological investigations of the laser- processed areas, for a range of laser fluences and pulse number, were recorded using optical and scanning electron microscopies (SEM) and white light interferometry. Data obtained on ablation rates, ejected particle sizes, and crater morphologies prove that ablation changes from a smooth to an explosive process at high fluences, as identified with changes in the material removal mechanisms. The build-up of laser-induced mechanical stresses, due to the heating and cooling of the samples between successive laser shots, plays an important role in the material modification process, leading to the observed dependence of ablation threshold on shot number. The strength of the dependence is governed by the incubation coefficient, S, which has been measured for all materials studied. In this paper, additional insight is derived from the analysis of the debris generated for metal samples, which can be attributed to laser ablation mechanisms based on vaporization, spallation, phase explosion, and fragmentation.
The aim of the current work is two-fold: First, the aim is to investigate the transition, for a number of metals, from a smooth ablation process to an explosive one. Secondly we aim to study the dependence of the ablation threshold in metals on the applied laser shot number. Ablation of polycrystalline metal samples was performed with multiple pulses from a femtosecond laser (Clark MXR, CPA2001). Morphological investigations of the laser processed areas were recorded using optical and scanning electron microscopies (SEM) and white light interferometry. The investigations have been carried out on sample matrices which were processed for a range of laser fluences and applied laser shots for four metals. Data obtained on ablation rates, ejected particle sizes and crater morphologies prove that ablation changes from a smooth to an explosive process at high fluences, as identified with changes in the material removal mechanisms. Threshold fluences were measured for both the smooth and explosive ablation processes. The ablation threshold fluence depends on the number of pulses applied to the same spot. It was found that the build up of laser induced mechanical stresses, due to the heating and cooling cycles of the samples between consecutive laser shots, plays an important role in the material modification process. It leads to the observed dependence of ablation threshold on shot number, which is described by a power law based on a mechanical fatigue model. The strength of the dependence is governed by the incubation coefficient, S, which has been measured for all materials studied. It is expected that the build up of laser energy or incubation leads to the accumulation of material defects and residual stresses which has the effect of lowering the energy required to cause ablation using a large number of incident laser shots.
Femtosecond laser micromachining of silicon offers the potential to realize precision components with minimal thermal damage. In this work, an assessment of the damage observed in bulk silicon during femtosecond laser micromachining is presented. The different analysis methods used to determine the structural and chemical changes to wafer grade silicon is first described. The analysis is at or above the ablation threshold - defined as the point where laser induced crystalline- damage is first observed for 1 kHz laser pulses, of 150 fs duration, at a wavelength of 775nm. Structural analysis is based upon electron and optical microscopies, with different sample preparation techniques being used to reveal the micro-machined structure. A key feature of the work presented here is the high-resolution Scanning Transmission Electron Microscope (STEM) images of the laser-machined structures. Below the ablation threshold, electrical experiments were performed with silicon under femtosecond laser excitation to provide a direct method for determining the accumulation of damage to the silicon lattice.
Based on this analysis, it will be shown that laser machining of silicon with femtosecond pulses can produce features with minimal thermal damage, although lattice damage created by mechanical stresses and the deposition of ablated material both limit the extent to which this can be achieved, particularly at high aspect ratios.
In the current work ablation of metal targets in air with femtosecond laser pulses is studied. The laser pulses used for the study were 775 nm in wavelength, 150 fs in pulse duration and the repetition rate was 100 Hz. Ablation thresholds have been measured for a number of metals including stainless steel niobium, titanium and copper. The ablation depth per pulse was measured for laser pulse fluences ranging from the ablation threshold (of most metals) ~ 0.1 J/cm2 up to 10 J/cm2. It has been shown previously that there are two different ablation regimes. In both cases the ablation depth per pulse depends logarithmically on the laser fluence. While operating in the first ablation regime the ablation rate is low and is dependant on the optical penetration depth, α-1. While in the second ablation regime the ablation rate is greater and is characterized by the 'electron heat diffusion length' or the 'effective heat penetration depth'. In the present study good qualitative agreement in the ablation curve trends was observed with the data of other authors, e.g. Nolte et al (1997).
This article describes the development and application of a femtosecond laser micro-machining workstation geared towards the machining of damage free micro-geometries. Much attention has been paid to ultrafast laser micro-machining in recent years given the reported possibilities for machining materials in the absence of thermal damage, and the minimum dimensions that can be machined. The laser systems themselves have evolved from table top lasers to fully packaged commercial systems. The work described in this article details the development of a workstation around a femtosecond laser source to enable controllable micro-machining. A femtosecond laser source with a 1 kHz repetition rate, 800mJ pulse energy, and a pulse width of the order of 150fs was used. A prototype workstation was built around the laser source to incorporate laser monitoring and control, control of laser parameters, high resolution motion, and vacuum technology. Using the system, percussion drilling and surface structuring was performed on stainless steel, aluminium and silicon substrates, and these results are reported.
In this paper the interaction of ultra-short pulses (150fs) of laser radiation (wavelength 775nm) over a range of fluences with wafer grade Silicon material in air was analysed using optical and electron microscopy. Optical microscopy was performed by the use of a white light interferometer and a high power optical microscope (magnification 100X). The resolution of both these methods was only sufficient to resolve large dimensions relative to the wavelength of light. For smaller geometries and greater detail, electron microscopy (resolution 1.5nm, 1KV) was used to obtain more information due to its greater resolution and depth of focus. When used in conjunction with surface, cross sectional and transmission imaging, this technique provided the greatest level of detail on the physical processes involved. Using these analysis techniques it was possible to provide a qualitative understanding of the ablation process as a function of laser fluence and to quantitatively describe the depth per pulse over a range of laser fluences, from which a value for the ablation threshold for Silicon (0.17Jcm-2) could be derived.