Improvement of Etching Anisotropy in Fused Silica by Double-Pulse Fabrication

Abstract

Femtosecond laser-induced selective etching (FLISE) is a promising technology for fabrication of a wide range of optical, mechanical and microfluidic devices. Various etching conditions, together with significant process optimisations, have already been demonstrated. However, the FLISE technology still faces severe limitations for a wide range of applications due to limited processing speed and polarization-dependent etching. In this article, we report our novel results on the double-pulse processing approach on the improvement of chemical etching anisotropy and >30% faster processing speed in fused silica. The effects of pulse delay and pulse duration were investigated for further understanding of the relations between nanograting formation and etching. The internal sub-surface modifications were recorded with double cross-polarised pulses of a femtosecond laser, and a new nanograting morphology (grid-like) was demonstrated by precisely adjusting the processing parameters in a narrow processing window. It was suggested that this grid-like morphology impacts the etching anisotropy, which could be improved by varying the delay between two orthogonally polarized laser pulses.

Keywords: double pulses; femtosecond; fused silica; selective chemical etching.

Figure 1

The double-pulse experimental setup. P—Brewster angle polarizer, PBS—polarising beam splitter, BS—beam splitter, D—dump, OB—microscope objective.

Figure 2

Spatial and temporal beam alignment: (a) two-beam angle adjustment principle; (b) intensity profiles of two beams with different temporal delay and polarisation orientation; (c) interference patterns of two beams with various misalignment angles at the 0 fs delay.

Figure 3

Schematic illustration of experiments: (a) recording and polishing of the lines written with double pulses for nanogratings observation by scanning electron microscope (SEM); (b) the line writing procedures for the investigation of the double-pulse influence on the chemical etching rate; (c) recording of the vertical bow-like structures for the investigation of directional etching dependence.

Figure 4

The etching rate dependence on the delay between two pulses for modifications recorded in fused silica with the 200–400 nJ pulse energy and orthogonal polarisations: (a) 600 fs pulse duration with a negative chirp; (b) 290 fs pulse duration; (c) 600 fs pulse duration with a positive chirp; (d) The microscope picture of etched channels for the 0 fs pulse delay at 290 fs pulse duration. Red curve: the etching rate dependence on the pulse energy for a single pulse. Samples were etched 30 min in 10% diluted hydrofluoric acid (HF) acid. The inset shows the SEM pictures from the side (b,c) and top (d).

Figure 5

SEM pictures of the nanograting morphology evolution depending on the delay between two pulses. The inset shows the enlarged area of the grid-like nanograting structure. The nanostructures recorded with a total 400 nJ pulse energy (200 nJ + 200 nJ).

Figure 6

The pictorial explanation of the nanograting shape only for temporally adjusted pulses (0 fs delay) when a slight spatial beam misalignment is induced in the Y direction.

Figure 7

The microscope measurement of the etching dependence on the double-pulse delay for the bow-like vertical structures from the sample side (ZX plane): (a) the structures recorded with 200 nJ and 400 nJ pulse energy and (b) the structures inscribed with 600 nJ and 800 nJ pulse energy. Two channels with 500 and 1000 ppµ density were recorded at the constant pulse energy. Etching performed for 30 min. in 10% HF. The inset shows the bow-like trajectory view on the XY plane with a predicted nanogratings orientations.

Figure 8

Etching of bow-like structures fabricated with the single-pulse regime at 600 nJ and 800 nJ pulse energy (a) and dependence of the etching isotropy factor K on the pulse delay for etching from the top sample surface of the vertical surface. (b). The insets show the microscope pictures with etched vertical surfaces.