CC BY
21LP-I-17
New super-resolution method of direct laser writing on Zr films
V. Korolkov1, S. Mikerin2, R. Kuts1, A. Malyshev1
institute of Automation andElectrometry SB RAS, Diffractive optics laboratory, Novosibirsk, Russian Federation
2Institute of Automation and Electrometry SB RAS, Laboratory of laser physics, Novosibirsk, Russian Federation
The thermochemical effect of laser radiation on metal films has been studied for a long time. Laser-induced surface oxidation of thin chromium films with subsequent selective liquid etching is used for the manufacture of diffractive elements with a minimal feature of about 0.5-0.7 |im [1]. However, the modern trend in optical design requires further increase the resolution to the level of 100-200 nm. To jump over the gap a regime of through oxidation can be used. The regime is easier to get when using pyrophoric metals (for example Ti, Hf, Zr) having small thermal diffusivity, quite high hardness, the high melting temperature of the metal and the oxide [2]. We have found out that Zr film sputtered on fused silica substrates is optimal to make nanostructured regions on surfaces of optical components by through oxidation.
Here we demonstrate periodic gratings wrote on Zr films with 80 nm thickness by using circular laser writing system CLWS-300IAE. The system scans focused beam (700 nm diameter) of green CW DPSS laser at different scanning speeds and laser powers (in range of 10-30 mW).
S
1 um
Fig. 1. SEM image of the grating with 300 nm period: (a) the edge of the grating where the beam was switched off; (b) the edge of the grating when the beam was switched on. The yellow line is parallel to the scanning direction.
Scanning speed - 276 mm/s, beam power - 13 mW.
Fig. 1 shows an SEM image of the grating recorded with a period of 300 nm. From fig. 1b, it is obvious that the black track is formed along the contour of the thermal distribution since when the beam is turned on, it gradually shifts from the spot center to the periphery following the expansion of the heated area. In our opinion, the black tracks are cracks or deformations formed at the boundary between the metal film and the oxide track due to the high stress caused by the sharp thickening of the film after oxidation and different thermal expansion. Contour mechanism is confirmed at writing with laser pulses having a duration of 440 ns. However, in this case, the black contour is formed across the direction of beam scanning.
The revealed writing mechanism can be used to fabricate nanostructured optical components, since the phase of reflected light changes significantly.
References
[1] V P. Korolkov, AG. Sedukhin, D.A. Belousov, R.V. Shimansky, V.N. Khomutov, S.L. Mikerin, E.V. Spesivtsev, R.I. Kutz, "Increasing the spatial resolution of direct laser writing of diffractive structures on thin films of titanium group metals," Proc. SPIE 11030, 110300A (23 April 2019).
