ANALYSIS: MICROMACHINING


High-throughput, high-quality micromachining of semiconductors with a 1kW sub-picosecond laser


Daniel Holder, of the IFSW at the University of Stuttgart, shares how new USP laser technologies could facilitate the rapid micromachining of silicon wafers


Co-authored by: Christoph Röcker, Rudolf Weber, Marwan Abdou Ahmed and Thomas Graf, of the IFSW at the University of Stuttgart; David Bruneel of Lasea; Gerhard Kunz of Bosch; and Martin Delaigue of Amplitude.


machining volumes. Examples include post-processing of additively manufactured metal components to improve their shape accuracy and surface quality1


, or the removal of


damaged areas in the repair of carbon fibre-reinforced plastic (CFRP) components2


. An emerging topic with


For the high-power micromachining of silicon wafers, a novel ultrashort pulse (USP) laser beam source with an average power exceeding 1kW, and a process for machining high-quality surfaces at high throughput, were developed in the EU project Hiperdias, which concluded in 2019. The project not only achieved


a record-breaking material removal rate of 3.8mm³/s at low roughness Sa < 0.6µm, but also saw high-power micromachining with over 1kW average laser power at sub-picosecond pulse duration demonstrated for the first time. Micromachining with USP


lasers enables the creation of technical, high-quality surfaces with low roughness and minimised melt formation, due to the low and local heat input. A fast and local removal of the material is necessary for the application of laser micromachining in processes with larger


18 LASER SYSTEMS EUROPE AUTUMN 2021


applications in medical imaging, security and communication is the manufacture of optics for terahertz (THz) radiation. The optics can be manufactured from silicon wafers by micromachining with USP lasers. In the past, laser micromachining has been limited in many potential applications due to the low available average laser power and resulting low throughputs. With the recent development of beam sources in the kW range, new challenges are now emerging in converting the high laser power into high quality at high throughputs.


Equipment setup To address these challenges, an application laboratory for high-power USP lasers in the kW range was established at the IFSW at the University of Stuttgart3


. The beam source


developed in Hiperdias was built at the IFSW4 and emitted laser pulses with duration <600fs at a wavelength of 1,030nm and a beam quality of M² < 1.5. The linearly polarised laser beam


Figure 1: Processing station from Lasea for micromachining with a 1kW sub-picosecond laser from the IFSW


was guided into a Lasea LS 5-1 processing station (see figure 1) by means of deflection mirrors. After the focusing optics with


a focal length of 580mm and a galvanometer scanner for fast beam deflection, a maximum available average power of 1,010W could be measured on the workpiece. The laser was operated at a repetition rate of 500kHz, which corresponds to a maximum available single pulse energy of 2.02mJ. The single pulse energy could be evenly distributed over five sub-pulses in a burst train, each with a time interval of 23ns. The minimum beam diameter on the workpiece surface with the focus position on the workpiece surface was about 90µm. The process development


was carried out in an ambient atmosphere on the polished side of 100mm diameter, 1mm-thick silicon wafers. Squares of 5 x 5mm² were scanned along parallel offset lines to create cavities. Figure 2 shows an example of the resulting surfaces after laser micromachining with different processing parameters. To increase the depth of the cavities, several scans were performed.


Machining strategy Machining with the focal position on the sample surface resulted in a laser fluence (energy density) per sub-pulse of 12J/cm² on the workpiece at 950W average laser power, which is a factor of 120 above


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Lasea


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