Femtosecond laser micromachining is a rapidly advancing area of deployment for ultrashort pulse lasers. It has been shown that optical pulses of femtosecond duration offer many advantages in the selective ablation and modification of materials. In linearly absorbing materials, collateral damage can be largely avoided. Moreover, femtosecond optical pulses in the linear transmission band of a material can be used to modify materials in sub-surface regions. In this case, multiphoton processes can be employed at the laser focus to deposit energy in a selective and spatially well-defined manner.

Femtosecond Laser Ablation and Micromachining Setup

In our research we use the following laser systems:

A schematic diagram and a photograph of our laser micromachining setup are shown below. (Click to enlarge the images.)

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Our Research

We study the advantages and feasibility of using femtosecond pulses for direct writing, cutting, drilling and modification of a wide variety of materials, such as semiconductors, metals, dielectrics and thin films. In our studies we concentrate on detailed post mortem analysis of the material after femtosecond irradiation. The final state of the material has been characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) along with focussed ion beam (FIB) techniques, atomic force microscopy (AFM), and optical techniques.

The study of morphology, crystal structure and chemical composition can give insight into physical processes taking place during the ablation. Figures below show examples of single pulse ablation craters. (Click to enlarge the images.)

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Figure: 1. Light microscopy image of single pulse ablated Si surface. 2. Light microscopy image of single pulse ablated Si surface at a lower fluence.

Irradiation of samples with multiple pulses leads to interesting surface structuring effects, such as the formation of laser induced periodic surface structures (LIPSS). Figures below show formation of such structures on various semiconductor surfaces. (Click to enlarge the images.)

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Figure: 1. SEM images of copper drilling at different pulse durations. 2. Light microscopy image of multi-shot laser irradiated silicon sample, showing ripples of two different spatial frequencies and fine bumps. 3. SEM image of multi-shot laser irradiated InP sample, showing ripples of two different spatial frequencies. 4+. SEM and cross-sectional TEM images of multi-shot laser irradiated GaP sample, showing ripples with high spatial frequency. The cross-sectional specimen was prepared using the FIB technique.

Many practical laser applications involve cutting, drilling and scribing. We investigate the ablation rates, morphology, geometry as well as collateral damage caused by laser micromachining. (Click to enlarge the images.)

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Figure: 1. McMaster Engineering Logo micromachined on Si surface. The MPEG clip shows diffraction effects after illumination of the pattern with white light at various angles. 2*. SEM images of the side and top surfaces of grooves micromachined in diamond. The laser beam was focused into the bulk and scanned across the polished surface out of a rough side edge. 3. Microstructuring of Si surface (black area) on prepatterned SiO2-on-Si sample. Deposition of ejected nanoparticles form the colour rings. 4. MPEG clip of laser writing. 5. SEM image of pointed structures on diamond. Machining conditions were similar to that of Figure 2. 6. Microstructured Si surface.

⁺This image appeared in APL 92, 221112 (2008) and may be found here. Copyright (2008) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

^*This image appeared in APA 103, 185 (2011) and may be found springerlink.com.