1. Introduction

Recently, great attempts have been made on various laser-induced microstructuring of polymers using excimer lasers. Surface modification of polymers is considered as an attractive field in laser-material processing due to their unique characteristics such as light weight, low cost, flexibility, tunable properties and widespread applications. Since early 1980s, serious efforts have been made in order to perform surface modifications on several polymers using pulsed lasers in wide UV spectral range [1–20]. CR39 is known as an optical material because of its excellent transparency within the visible region and its high absorbance in UV range is promising for various applications. Those consist of UV protection, industrial replacement for glasses, sunglasses, lenses and substrate for microfluidics lab-on-a-chip devices.

Kokreja et al. [21] reported the successful irregular surface modification on CR39 using CO₂ laser. The regular changes of CR39 morphology appeared via high dose UV lasers irradiation [22–24]. Zakaria et al. have observed sharp tip conical microstructures using molecular Fluorine laser [22]. Bashir et al. have shown the laser induced periodic surface structure (LIPSS) using Ti: sapphire laser [23]. Kam et al. reported craters by means of KrF laser covering a random oriented periodic microstructure [24].

In fact, this structure is a reproducible surface rippling. The exact mechanism is not understood yet, however this microstructure is originated from the interference of the incident UV beam and the scattered light. Furthermore, it is lucidly enhanced by UV ablation. LIPSS has been previously reported on the other polymers such as polyethylene terephthalate, polyether sulfone, polyimide and polyether ether ketone [25–27]. However, this effect was reported on CR39 by Parvin et al. in 2014 [28]. The generally accepted view on the formation of LIPSS is suggested by Emmony et al. [29]. It is fundamentally based on the interference of incident beam with the wave scattered off from the sample surface. The scattering source is assumed to be originally existing or additional laser-induced granules generated on the irradiated surface. The interference causes a modulated laser intensity distribution, mostly imprinted onto the material with wavelength-scale microstructure. These structures may be created along with the incident electric field or its perpendicular direction [26–34]. The laser-induced microstructures are initially reported by Niino et al. on poly ether sulfone using XeCl laser irradiation at 308 nm having the spatial period larger than laser wavelength [35]. Dorronsoro et al. have recently found periodic microstructures on Filofocon A copolymer, irradiated with ArF laser pulses as well [31].

The regular self-aligning on CR39 was created by non-polarized normal-incidence ArF laser shots at different fluences and doses [28, 36]. The self-microstructuring was established in the form of contours after 10 shots (300 mJ/cm²). Subsequently, the random contours produced at lower doses can notably enlarge their effective radius to create the parallel line patterns at higher doses. In addition, at higher UV doses, the self-aligning microstructures with a spatial period of ~350 nm (at 300 mJ/cm²) linearly scale up as wide as 800 nm (at 700 mJ/cm²). The spatial period remains nearly invariant at higher fluences up to 1300 mJ/cm². However no systematic angular investigation has been so far carried out. Here, the effect of incident angles on the self-aligning is extensively investigated.

The created microstructures are highly suitable for use in MEMS, microlithography, nano technology, microbiology as well as micro-optics particularly the fabrication of micro diffraction gratings.

2. Experimental

Figure 1(a) depicts schematics of irradiation set up. At first, CR39 sheets was cut in to 1.5mm × 10mm × 10mm, placed on an angular adjustable mount (GMB1, Thorlabs) with 10 mrad precision angles. ArF laser (LPX 220i, Lambda Phyzik) was employed with 120 mJ/pulse, 15 ns duration and 1 Hz repetition rate to be spatially distributed over ~0.9cm × 2.5cm beam cross-section with 1 and 3 mrad divergences in perpendicular directions, respectively. The profile is uniform in X direction and hat-top in Y direction. The laser beam traverses an aperture and is focused on samples through a cylindrical MgF₂ lens (f = 20 cm). The CR39 samples have been irradiated with various fluences, F, ranging 0-60° incident angles, using multiple shot numbers N. The morphological changes are inspected using SEM (XL30-philips) having 10 nm spatial resolution magnifications. Two dimensional-fast Fourier transform (2D-FFT) analysis, as an accessory of ImageJ software, is employed to determine the surface quality. Moreover, scanning tunneling microscopy (STM) Natsyco Nama-SS3 is exploited to verify the height H and spatial period Λ of the regular microstructures.

 

Fig. 1 a) Schematic illustration of UV irradiation on CR39 where n̂ is the normal to the surface (the normal incidence takes place at φ = 0°). The incident plane includes $\widehat{n}$ and the incident beam. b) Typical SEM images and corresponding FFTs. Insets show 2D-FFTs: the contours are described by circles whereas a couple of focused spots are attributed to the self-aligning structure. The solid line and spots are artificially added for better clarification. The dot line is a hypothetical line that connects the spots to exhibit that it is normal to the self-aligning direction. The SEM images of laser induced self-microstructuring in terms of incident angle for c) φ = 0°, d) φ = 10°, e) φ = 30°, f) φ = 60°. g) ${\overrightarrow{V}}_R$ indicates self-aligning direction that is vector summation of two components of orthogonal P-polarization of incident beams. h) Random contour with no self-created direction. Note that un-polarized ArF laser shot is used here. Top insets are the magnified SEM images (2 μm scale).

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The self-aligning microstructures significantly improve when the incidence angles varies from normal up to 60° as shown in Fig. 1(a). Double arrows depict the direction of parallel lines that are subsequently formed on the surface.

3. Result and discussion

Figure 1(b) depicts typical SEM images and corresponding FFTs. The latter can obviously discriminate the contours from the regular self-aligning microstructures. Figures 1(c)-1(f) depict the SEM images of the CR39 samples (F = 700 mJ/cm² and N = 500 shots) irradiated by the non-polarized UV laser at typical incidence angles: c) φ = 0°, d) φ = 10°, e) φ = 30° and f) φ = 60° respectively. Despite the contours emerges at normal incidence and small irradiation angles φ, however the self-aligning patterns are dominantly formed at larger angles (30°-60°). The transition from contours, Fig. 1(c), to self-aligning structures, Fig. 1(e-f) is identified by FFT analyzer coupled with SEM micro graphs. The corresponding 2D-FFT analyses of each SEM images are shown in the insets.

In fact, the self-aligning area at normal irradiation exhibits to cover ~4000 μm² (corresponding to 3000 shots) [28]. This would expand over the whole treated zone to achieve ~12 mm² simply after 100 shots under typical φ = 60°. The UV irradiation at optimal incident angle creates large treated zone using relatively low UV doses respect to the normal exposure.

Furthermore, the self-aligning microstructures are aligned parallel to the interface of the incidence plane and the target surface i.e., the P-polarization component of the incident beam as shown by double arrows in Fig. 1(a).

A couple of experiments were carried out to exhibit that the self-aligning direction aligns with the vector summation of the P-polarization component of incident beams. At first, a typical CR39 sample is irradiated by the first UV shot and then the treated sample is rotated 90° around its normal axis before the next exposure. Afterwards, the sample is revolved back to its initial status to receive another shot. This process alternatively goes on up to 1000 multiple shots (500 shots impart to each status). The self-aligning appears along with the ${\overrightarrow{V}}_R$ direction that is oriented 45° respect to each primary orthogonal axis. In fact, ${\overrightarrow{V}}_R$ is the resultant of two equivalent orthogonal vectors as shown in Fig. 1(g).

Moreover, a rotating mount is used with different angular frequencies (600, 900 and 1200 rpm) to revolve another sample during UV irradiation. Here, the random contour microstructure is established and no regular self-aligning is created at the treated zone. There is no selective direction available, therefore the resultant P-polarization component is null as shown in Fig. 1(h). Consequently, at normal incidence, slight non-zero incidence angle and local slope of the samples surface due to the laser induced roughness as well as the convergence angle of the focused laser beam contribute together to provide locally P-polarization component to align the periodic structure along a certain axis.

Figure 2(a) illustrates the dependence of the microstructure’s spatial period on incidence angle for F = 700 mJ/cm² and N = 500. The spatial period linearly decreases versus the angle of incidence. Accordingly, the line density of those structures, as shown in the inset, varies from 1200 to 1700 mm⁻¹. Similar behaviors are seen at various F and N accordingly.

 

Fig. 2 a) The spatial period at different incidence angles. b) Self-aligning spatial period versus incident fluence at φ = 0° (blue rectangles) and φ = 30° (red triangles) incident angles. Insets delineate the variation of the correspondent line density in terms of incident angle and fluence respectively.

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Figure 2(b) depicts the variations of microstructure’s spatial period versus incident fluence at typical incidence angles i.e. φ = 0° and φ = 30°. Both graphs are similar but, demonstrate a sensible difference of ~50 nm in spatial period. The use of cylindrical lens narrows the range of spatial period in opposite of what happens by focusing spherical lens [28]. Narrowing refers to the shortening range of spatial period. The latter is larger when spherical lens is used (350-800 nm), whereas by making use of cylindrical lens the range of spatial period is narrowed within the interval of 600-800 nm.

Figure 3(a) displays the spatial period of the self-aligning patterns versus shot numbers at different incidence angles of φ = 0° and φ = 30° with 700 mJ/cm² incidence fluence. It indicates that the spatial periods linearly increase in terms of shot numbers exhibiting a relatively greater gradient at larger incident angles. The corresponding SEM images for 10 and 1000 laser shots at typical 30° angle of incidence are depicted in Fig. 3(a) insets. The microstructure initiates directly as parallel segments covering the treated area. The pattern evolves from discrete segments to a nearly perfect self-aligning. Self-correction refers to the transition from parallel segments recorded on the CR39 surface (at low UV doses) to the perfect regular self-aligning pattern (at high UV doses). In fact, the successive UV laser shots induce a structural order on the polymer surface.

 

Fig. 3 a) Variation of self-aligning spatial period versus shot numbers at φ = 30° incidence angle (red triangles) and normal-incidence (blue rectangles), with 700 mJ/cm² fluence. Inset delineates the variation of the corresponding SEM images. b) Threshold fluence for regular self-aligning on CR39 in terms of the incidence angles.

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Moreover, it was found that the threshold fluence is strongly dependent on the incidence angle. This linearly reduces from ~225 mJ/cm² at normal-incidence to ~160 mJ/cm² at 60°. Figure 3(b) depicts the threshold fluence versus the irradiation angles. In fact, it linearly decreases in terms of the incident angle.

The ripple height and period are measured with STM micro-analyzer. Figure 4 depicts the height of self-aligning pattern as a function of UV dose. It reveals a rise from 90 nm at 30 J/cm² to 170 nm at ~52 J/cm² and then it decreases to 120 nm for ~87 J/cm². The corresponding STM images are shown as peripheral insets while the central inset depicts the typical STM graph, due to periodic self-aligning patterns exposed by 193 nm ArF laser (F = 520 mJ/cm², N = 100, φ = 30°).

 

Fig. 4 The height variation of self-aligning in terms of UV dose; the corresponding STM images are shown as the peripheral insets with the area of 8μm × 8μm. The central inset depicts the typical STM graph of patterns with 52 J/cm² UV exposure.

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In general, more than four hundred CR39 samples were irradiated and examined successfully in order to characterize the reproducible microstructures in terms of laser properties particularly the incident angle, fluence and shot numbers. We conclude that UV irradiation on CR39 creates the desired self-aligning at certain fluence, dose and oblique angle.

UV laser induces skin absorption which in turn relates wavelength dependent absorption coefficient (the imaginary component of polymer refractive index) to the microstructuring. Despite the properties are mainly dependent on the material parameters such as absorbance and refractive index; however the way of irradiation may enhance the features of microstructures too. The latter is formed on the CR39 due to high UV absorption at 193 nm that exhibits to be drastically different from the structure created on PMMA (poly methyl methacrylate). We have also examined several CR39, PMMA and PC (poly carbonate) films using similar UV doses. The microstructure is related to UV absorbance on CR39 surface (5 × 10⁵ cm⁻¹). The smaller absorbance for PMMA at 193 nm (2 × 10³ cm⁻¹) features no microstructure on the polymer surface. In the case of PC, the absorbance is high at 193 nm (5.5 × 10⁵ cm⁻¹), however conical microstructure [10–12] takes place instead of self-aligning pattern. This indicates that the material properties are dominant to form the desired microstructure under proper UV doses.

The self-aligning pattern on CR39 is suitable for micro-grating fabrication because of its high durability and mechanical strength. This polymer can tolerate high laser doses. It accounts as an advantage for CR39 respect to the current holographic gratings that are fabricated based on the photosensitive polymers. Therefore, we introduce CR39 as an alternative optical material that exhibits high potential in micro-optics applications, particularly in the micro-grating fabrication.

4. Conclusion

We have previously reported that ArF laser induces self-aligning periodic structures on CR-39 at high dose normal exposure [28]. Here, it is shown that this effect is significantly enhanced at oblique irradiation using relatively lower doses. Subsequently, the self-aligning microstructures on the treated CR-39 samples are characterized in terms of incidence angle, fluence and UV dose. Although slightly sensitive to UV dose, the self-aligning pattern is predominantly an angular-dependent phenomenon. The self-aligning area drastically enlarges from 4000 μm² at normal incidence to ~12 mm² at proper oblique exposure. Furthermore, the fluence threshold linearly reduces with incident angle from 225 mJ/cm² at normal incidence to 160 mJ/cm² at 60°. The spatial period is measured to range 0.6-0.8 μm corresponding to 1200-1700 lines/mm.