Micromachining of Alumina Using a High-Power Ultrashort-Pulsed Laser

Abstract

We report on a comprehensive study of laser ablation and micromachining of alumina using a high-power 1030 nm ultrashort-pulsed laser. By varying laser power up to 150 W, pulse duration between 900 fs and 10 ps, repetition rates between 200 kHz and 800 kHz), spatial pulse overlap between 70% and 80% and a layer-wise rotation of the scan direction, the ablation efficiency, ablation rate and surface roughness are determined and discussed with respect to an efficient and optimized process strategy. As a result, the combination of a high pulse repetition rate of 800 kHz and the longest evaluated pulse duration of 10 ps leads to the highest ablation efficiency of 0.76 mm3/(W*min). However, the highest ablation rate of up to 57 mm3/min is achieved at a smaller repetition rate of 200 kHz and the shortest evaluated pulse duration of 900 fs. The surface roughness is predominantly affected by the applied laser fluence. The application of a high repetition rate leads to a small surface roughness Ra below 2 μm even for the usage of 150 W laser power. By an interlayer rotation of the scan path, optimization of the ablation characteristics can be achieved, while an interlayer rotation of 90° leads to increasing the ablation rate, the application of a rotation angle of 11° minimizes the surface roughness. The evaluation by scanning electron microscopy shows the formation of thin melt films on the surface but also reveals a minimized heat affected zone for the in-depth modification. Overall, the results of this study pave the way for high-power ultrashort-pulsed lasers to efficient, high-quality micromachining of ceramics.

Keywords: Alumina; ceramics; high rate ablation; processing strategy; ultrashort pulse laser.

Figure 1

Schematic of (a) the laser setup for high-power micro machining and (b) pulse deposition.

Figure 2

Accumulated intensity by equidistant placement of nine adjacent pulses with an overlap of (a) 70% and (b) 80%.

Figure 3

Ablation efficiency for 200, 400 and 800 kHz in dependence of the applied laser pulse length and laser fluence.

Figure 4

Depth rate for 200, 400 and 800 kHz in dependence of the applied laser pulse length and laser fluence.

Figure 5

Surface roughness Ra for 200, 400 and 800 kHz in dependency of the applied laser pulse length and laser fluence.

Figure 6

Ablation rate of Alumina as a function of the applied laser power (a) and resulting roughness values (b) for selected parameter combinations leading to a high rate ablation.

Figure 7

SEM analysis of unprocessed Al₂O₃ surface (a,e) and laser processed surfaces using an overlap of 80% and a high average power of 150 W in combination with (b,f) 200 kHz and 0.9 ps, (c,g) 800 kHz/0.9 ps and (d,h) 800 kHz/10 ps.

Figure 8

Cross section of an ablated cavity showing the surface melt effect of Al₂O₃.

Figure 9

Influence of the interlayer rotation for 0${}^{°}$, 11${}^{°}$ and 90${}^{°}$ on the achieved ablation depth and surface roughness. (b–d) Height map of the resulting surfaces.