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.
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