Ultrashort Pulse Laser Ablation
The energy of a focused laser beam can be used to remove material by so called ablation mechanisms. The laser light excites electrons in a material which rapidly relax to lower energy states by transferring the energy to the lattice of the material. Thereby the material can melt and evaporate which can be used to remove material in a relatively controlled way.
With ultrashort laser pulses the energy is deposited in a time that is shorter than the relaxation time between the electrons and the lattice. This creates very high energy densities and allows ablation to take place before heat conduction to the bulk material becomes important. As a result ultrashort laser pulses can be used to process materials without thermally affecting the surrounding bulk material. This can be used to machine materials more precisely with much less heat-related negative effects.
Typical pulse durations used in ultrashort pulse laser processing range from tens of femtoseconds to several tens of picoseconds. Lasers with a pulse length of about 10 picoseconds have become a standard in industrial micromachining with ultrashort pulses and have proven to be suitable for 24/7 production. These pulse durations are about a thousand times shorter compared to the pulses in conventional short pulse laser machining systems and thereby avoid several detrimental effects like a heat effected zone, burrs, and material cracks.
A second advantage of ultrashort pulse processing is that practically any material can be processed. This is caused by the extremely high energy-densities that allow for multi-photon absorption. Materials that are transparent to the laser wavelength – like glass for 1064 nm laser radiation – can still be processed. Multi-photon absorption can also be used for local modification inside transparent materials.
Modern picosecond pulse lasers offer considerable flexibility such as very high repetition rates and burst mode processing. Using these options processes can be made more thermal again, due to heat accumulation during irradiation with multiple pulses. This offers the possibility of very precisely generating heat and melting effects in materials, which can be used for micro-welding, polishing effects, or just to increase the material removal rate.
A single ultrashort laser pulse influences an area of approximately the size of the laser spot with a typical diameter between 10 and 40 micrometer. Depending on the material and pulse energy a single pulse normally removes between a few nanometers up to a micrometer of depth. To process larger volumes and specific structures laser scanning techniques are used to rapidly move the focused laser beam over the workpiece. The pattern in which the laser pulses are applied to the material determines the resulting structure. This is calleddirect-write processing – the focused laser beam writes structures into the material. Unlike with mask based processes this approach results in a very flexible tool.
The area over which the laser beam can be scanned is relatively small. To machine larger parts the surface is divided into tiles. A multi-axis motion system is used to position the laser scanner over a tile which then is machined using the scanner. After that the machine steps to the next tile. This step-and-scan method is fast, because the use of the scanner allows fast manipulation of the laser spot; and it is accurate, because the motion system only makes point-to-point movements and does not move during processing.
Lightmotif operates high precision 3-axis and 5-axis motion systems, allowing complex 3D products to be machined. The machines are controlled using custom software that Lightmotif specifically developed for laser micromachining. More details on our products can be found here.