UV-Femtosecond-Laser Structuring of Cyclic Olefin Copolymer

Abstract

We report on the laser ablation of cyclic olefin copolymer using an amplified ultrashort pulsed laser in the ultraviolet spectral range. In addition to a high ablation depth per laser-structured layer up to 74 μm at a fluence of 22 J cm-2, an excellent mean roughness Ra of laser-patterned surfaces down to 0.5 μm is demonstrated. Furthermore, with increasing fluence, increasing ablation efficiencies up to 2.5 mm³ W^-1 min^-1 are determined. Regarding the quality of the ablation, we observed steep ablation flanks and low debris formation, though for fluences above 10.5 J cm-2 the formation of troughs was observed, being attributed to multiple reflections on the ablation flanks. For comparison, laser ablation was performed under identical conditions with an infrared laser wavelength. The results highlight that UV ablation exhibits significant advantages in terms of ablation efficiency, surface roughness and quality. Moreover, our results show that a larger UV focus spot accelerates the ablation process with comparable quality, paving the way for high-power UV ultrashort pulsed lasers towards an efficient and qualitative tool for the laser machining of cyclic olefin copolymer. The production of complex microfluidics further underlines the suitability of this type of laser.

Keywords: cyclic olefin copolymer; femtosecond pulse laser; laser ablation; microfluidics; ultraviolet laser.

Figure 1

Schematic illustration of the laser-machining setup with the wavelengths 343 $\mathrm{n}$$\mathrm{m}$ and 1030 $\mathrm{n}$$\mathrm{m}$.

Figure 2

Focal points of the wavelength in (a) UV Z = 0 $\mathrm{m}$$\mathrm{m}$ (b) UV defocused (Z = −1 $\mathrm{m}$$\mathrm{m}$) (c) IR with BET (Z = 0 $\mathrm{m}$$\mathrm{m}$).

Figure 3

(a) Schematic representation of laser ablation with pulse spacing $P_d$, hatch spacing $H_d$, scan velocity $v_L$, jump velocity $v_{Jump}$. (b) Representation of the hatch rotation and dynamic refocusing.

Figure 4

Simulated 2D cumulative fluence in pulse or hatch directions with overlap of (a) 35% (b) 50% (c) 65% for a single pulse fluence of 10 J cm⁻² following [35].

Figure 5

SEM images of an ablation cavity on COC with a fluence of $10.5$ J cm⁻² in different magnifications: (a) overview; (b) magnified section of (a) (dashed line); (c) tilted image; (d) magnified section of (b); (e) magnified section of (a) (solid line) with trough; (f) line scans in X and Y direction of (c).

Figure 6

Ablation values of COC by the UV laser: (a) ablation per pass and roughness; (b) ablation efficiency.

Figure 7

SEM image of the topography evolution of COC by the focused UV laser at the fluence of (a) 4.6 (b) 10.5 (c) 16.8 (d) 22 J cm⁻².

Figure 8

A 45${}^{\circ }$-tilted SEM image of ablation cavities on COC with different wavelengths: (a) UV; (b) IR under same conditions at a fluence of $10.5$ J cm⁻² (circles show reattached particles).

Figure 9

Comparison of the ablation values of COC of the different wavelengths UV and IR under same conditions: (a) ablation (b) roughness (c) ablation efficiency.

Figure 10

Comparison of the removal values of COC at UV with different focus positions: (a) material removal, (b) roughness, (c) removal efficiency, (d) removal rate plotted on fluence.

Figure 11

Exemplification of an UV USP laser ablated microfluidic Tesla mixer leaning on [48]. (a) Top left: design of one Tesla sturcture geometry with l = 790 $\mathsf{\mu }$m, w = 200 $\mathsf{\mu }$m and h = 600 $\mathsf{\mu }$m. Bottom left: optical micrograph of one of eight mixers. Right: 45${}^{\circ }$-tilted SEM image. (b) LSM image.