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This study investigates the general mechanism of Direct Laser Interference Patterning (DLIP) involved in the structuring process of polymer materials. An empirical model is developed taking into account experimental observations of DLIP-treated pigmented and transparent polycarbonate substrates with UV (263 nm) and IR (1053 nm) laser radiation. Depending on the used laser processing conditions, the type of material as well as the spatial period of the interference pattern, four different structuring mechanisms can be identified. The treated surfaces are investigated using confocal microscopy, scanning electron microscopy and focus ion beam and as a result from the experimental data analysis, the developed model predicts the material surface topography after the patterning process, by means of a set of material-dependent coefficients.
© 2017 Optical Society of America
1. Introduction
It is well known that micro and sub-micrometer periodical structures play a significant role on the properties of a surface [1–7]. Ranging from friction reduction to the bacterial adhesion control, the modification of the material surface is the key for improving the performance of a device or even creating a completely new function [8–13]. Among different laser processing techniques, Direct Laser Interference Patterning (DLIP) is one of the most efficient routes for texturing materials up to the nanometer scale with high throughput [14–16]. The DLIP process relies on the local surface modification process which is obtained when two or more coherent laser beams interfere on the surface of a material, producing periodic surface patterns with controllable pitch and geometry [17–20]. While the spatial period can be controlled by changing the angle of the incident beams, the shape can be defined by the number of used laser beams as well as the polarization or the intensity, which brings a high degree of freedom in producing more complex patterns [21].
In the last years it has been proved not only the feasibility of the DLIP method to treat different materials [22–24], but also the possibility to realize advanced structures for a large number of applications, such as enhancing the growth of nanorods [25, 26] for optoelectronics, increasing the optical absorbance [27], fabricating micro-lenses [28, 29], patterning of graphene oxide [30] for humidity sensing and scaffold fabrication for tissue engineering [31, 32].
Although the geometry and characteristic pitch of an interference pattern can be perfectly controlled as mentioned before, identical experimental conditions applied to different polymers can result on totally different topologies [33, 34].
The aim of this study is to develop a general model to describe the structuring mechanisms of DLIP on polymers. Due to previous investigations of polymer materials treated through DLIP showing unusual structuring behavior, pigmented and transparent polycarbonate (PC) substrates were chosen [35]. The treated surfaces are investigated using confocal microscopy, scanning electron microscopy and focus ion beam (the last for cross sectional analyses). Finally, a universal model to calculate the material surface topography after DLIP is presented.
2. Two-beams direct laser interference patterning
2.1 Materials and methods
For all the DLIP-structuring experiments, commercial LEXAN polycarbonate materials (SABIC, The Netherlands) have been employed. The transparent sheets (LEXAN SLX 11010BA) have a nominal thickness of 0.250 mm while the black sheets (LEXAN FR25A) contain a black dopant (0.22% of carbon black pigment) and are also 0.250 mm thick. The samples surface was cleaned with ethanol prior the laser processing.
A two-beam interference setup was utilized to produce line-like surface patterns (Fig. 1(a)). An example of an interference pattern obtained by overlapping two Gaussian beams (TEM00 mode) is shown in Fig. 1(b). Two laser systems (UV and IR) have been employed for the structuring of the polymers. For the UV (263 nm) processing, a Q-switched diode pumped solid state laser (Laser Export, TECH-263 Advanced) was used, with a pulse duration of 3 ns, a repetition rate of 1 kHz and a maximal pulse energy of 50 µJ. The IR (1053 nm) system was also a Q-switched diode pumped solid state laser (Laser Export, TECH-1053 Advanced) with a pulse duration of 6 ns, a repetition rate of 1kHz and a maximal pulse energy of 1000 µJ. For each experiment, single laser pulses have been used.
Fig. 1 (a) Two-beams interference patterning principle; (b) calculated intensity distribution obtained by overlapping two Gaussian (TEM00) laser beams with a spatial period of 6 µm; (c) DLIP-µFAB compact system for direct laser interference patterning equipped with an IR-DLIP optical head.
The experiments were conducted on a compact self-developed DLIP system (DLIP-µFAB, Fraunhofer IWS, Fig. 1(c)) producing pixels containing the line-like interference pattern with diameters of 20 µm and 120 µm for the UV and IR wavelengths, respectively (see Fig. 1(b)). To cover larger areas, the substrate was translated in x and y directions and a specific distance between the pixels is defined. The optical head is designed to split the main beam into two beams using a diffractive optical element, which are later recombined using a prism and a lens. By varying the distance between the prism and the lens, the spatial period can be adjusted automatically (see [36] for further information). For each experiment the laser fluence has been retrieved measuring the laser power distribution and estimating the area in which the two laser beams interfere.
The morphology of structured samples was characterized using confocal microscopy (Sensofar S Neox) employing a 150x magnification objective with a nominal lateral resolution of 140 nm and vertical resolution of 2 nm. Topographical inspections have been carried out also by means of Scanning Electron Microscopy (SEM), previously coating the processed samples with a 30 nm thick gold/palladium layer. Focus Ion Beam (FIB) cross-sectional analyses were performed on the black-doped PC material (JEOL JIB 4610F). Prior to the FIB treatment, the sample was partially protected with a carbon layer.
2.2 Structuring of transparent polycarbonate
In a first set of experiments, transparent polycarbonate substrates were irradiated with UV laser radiation with spatial periods Λ ranging from 0.57 to 2.13 µm at different laser fluences (energy densities) up to 0.64 J/cm2 (Fig. 2(a) and 2(b)). The results revealed that well-defined line-like patterns, which perfectly match the periodic intensity distribution (Fig. 1(b)), are obtained only for a small range of laser fluences (see in Fig. 2(a) the structure obtained at 0.1 J/cm2). For higher laser fluences, the upper part of the structured features appear erased, leading to a partially structured volume (see for example the structures produced at 0.38 and 0.56 J/cm2 in Fig. 2(a)). The same effect was observed keeping the laser fluence constant but reducing the spatial period (Fig. 2(b)). For example, in the case of the lowest period (Λ = 0.57 µm) and a fluence of 0.25 J/cm2, approximately 77% of the structured volume is missing.
Fig. 2 Confocal microscope profiles showing periodic line-like structures produced on the transparent Lexan SLX treated with 3 ns single pulses at 263nm laser wavelength; in Fig. 2a the influence of the laser fluence on the pattern morphology is shown, while in 2b the spatial period is varied at constant laser fluence.
A possible explanation of this structuring mechanism can be related to the plasma formation and expansion during the ablation process. Since the plasma is ignited in all the volume of the irradiated material, the positions corresponding to the interference maxima close to the surface experience a higher concentration of expanding plasma and the structures are more likely to be unselectively ablated. This effect is greater for higher laser fluences and for shorter distances between the interference maxima (smaller period). As an example, a SEM image of a DLIP-treated transparent PC foil, with high laser fluence (3.5 J/cm2) and small structure period (0.56 µm) is shown in Fig. 3. Clearly, a Gaussian non-periodic ablated part can be observed in the central zone (diameter of ~10 µm) of the DLIP-pixel (diameter of ~25 µm).
Fig. 3 SEM image of a DLIP-treated transparent Lexan SLX polycarbonate showing the Gaussian non-periodically ablated zone in the center of the DLIP pixel (wavelength: 263 nm, pulse duration: 3 ns, fluence: 3.5 J/cm2, spatial period: 0.56 µm).
2.3 Structuring of pigmented polycarbonate
Adding a dopant to the polymer matrix to change its optical properties can result in a completely different structuring process. For example, in a previous study, a black PC substrate was structured using DLIP with a nanosecond pulsed IR laser [35]. Cross sectional investigations revealed the presence of pores beneath the material surface at the interference maxima positions which induced the local swelling of the polymer. Thus, in a second set of experiments, PC substrates doped with a black dye were used to quantitatively investigate the swelling process under interference patterns for IR radiation at 1053 nm (the transparent PC material could not be structured using the same IR wavelength).
In this case, the laser fluence was varied between 0.55 J/cm2 and 1.29 J/cm2 and the structure period from 2.31 µm to 7.13 µm. Surface profiles of the obtained topographies on this polymer are depicted in Fig. 4(a)-4(b). Similarly to the ablation case, it was found that only for low laser fluences and especially for large spatial periods (see for example in Fig. 4(a) the pattern obtained at 0.86 J/cm2) the polymer was only swelled at the interference maxima positions.
Fig. 4 Confocal microscope profiles showing periodic line-like structures produced on the black Lexan FR25A treated with 6 ns single pulses at 1053nm laser wavelength; in Fig. 3(a) the influence of the laser fluence on the pattern morphology is shown, while in 3b the spatial period is varied at constant laser fluence.
In order to better understand the structuring mechanism of the black doped PC, the absorption spectra of the treated materials were analyzed in the region going from UV to near-IR. As it can be seen in Fig. 5, the transparent PC exhibits negligible absorption for wavelength larger than 400 nm, while the black-doped PC has a nearly constant absorption (~90%) all over the analyzed spectral range. Thus, the reason for the swelling process can be attributed to the black dye which absorbs the IR photons and dissociates into gaseous byproducts producing pores and thus increasing the local volume [37].
Fig. 5 Absorption spectra of the transparent PC (Lexan SLX, red line) and the black-doped PC (Lexan FR25A, black line) for wavelengths ranging from 200 to 1500 nm.
To demonstrate this assumption, SEM analyses have been carried out on DLIP-treated black-doped PC substrates. Figure 6(a) and 6(b) show the effect of the change of laser fluence (0.86 J/cm2 to 1.3 J/cm2) for the structuring of the black-doped PC, carried out employing the 1053 nm laser radiation and with a spatial period of 7.13 µm. It can be seen how it is possible to switch from simple line-like swelled ridges to double-scale swelled structures by just increasing the laser fluence. Moreover, focused ion-beam (FIB) cuts performed onto the black-doped polycarbonate sample show the pores that are formed beneath the surface after the DLIP treatment (Fig. 6(c)). In this case, the sample was treated with a laser fluence of 1.3 J/cm2 and spatial period 7.13 µm. Besides, the dynamics of creation and expansion of the pores can be considered independent on the interference period, for a fixed laser fluence and wavelength. Therefore when the dimension of the expanding pores is lower than the interference spatial period, the pores quench forming non-periodic structures (e.g. see Fig. 4(b) for Λ = 2.31 µm).
Fig. 6 SEM images of the DLIP-treated black Lexan FR25A using a wavelength of 1053 nm, showing (a) a swelled single-scale line-like ridges (fluence: 0.86 J/cm2, spatial period: 7.13 µm), (b) a swelled double-scale structure (fluence: 1.3 J/cm2, spatial period: 7.13 µm) and (c) a FIB-cross section of a swelled double-scale pixel (fluence: 1.3 J/cm2, spatial period: 7.13 µm).
Based on these results, two different contributions to the general geometry of the produced features could be defined: (i) upper swelled features showing a periodic topographical variation and (ii) the non-periodic swelling modulation (Gauss-like swelling). Similarly to this structuring behavior, the creation of double-scale swelled features was already observed in doped polymers employing a two step-technique with nanoimprinting and laser swelling [38].
In a third set of experiments, the black doped PC substrates were also irradiated with UV interference patterns (263 nm). At this wavelength, both the matrix and the dye can absorb the laser radiation and therefore a combined ablation-swelling behavior is expected. The experiment has been performed irradiating the material with fluences ranging between 0.01 J/cm2 and 0.66 J/cm2 and spatial periods between 0.57 µm and 2.13 µm. An overview of the structuring is presented in Fig. 7. The threshold fluence for the structuring process was 0.20 J/cm2. At this fluence, only the swelling mechanism is visible. However, the height of the swelled features was only a few tens of nanometers, being much less visible than with the IR structuring. At higher laser fluences (e.g. 0.30 J/cm2), the black doped PC substrate was locally ablated at the interference maxima positions obtaining similar results as the transparent material treated with the UV radiation. In consequence, at these laser fluences the ablation process dominates over the swelling mechanism.
Fig. 7 Confocal microscope profiles showing periodic line-like structures produced on the black Lexan FR25A, treated with 3 ns single pulses at 263 nm laser wavelength with 2.13 µm structure period, showing the influence of the laser fluence on the pattern morphology.
3. Modeling of direct laser interference patterning
3.1 The structuring mechanisms
After determining the different structuring mechanisms on polymers materials using DLIP, a model for predicting the three-dimensional geometry as function of the laser wavelength, spatial period and laser fluence was developed.
The most common way to quantitatively characterize the structuring process (ablation) in polymer substrates is retrieving an ablation curve, i.e. reporting the variation of the ablation depth as a function of the laser fluence. For polymers exhibiting mainly a photochemical ablation mechanism, this curve is expected to follow a logarithmic dependence and the structure depth can be fitted using the Lambert-Beer law expressed in Eq. (1) [39, 40]:
where d(x) is the structure depth as function of the coordinate x, α(λ) [cm−1] is the absorption coefficient, F(x) is the laser fluence at the position x, and Fth [J/cm2] is the ablation threshold. Typically, these analyses are performed considering the maximal depth at the center of the Gauss distribution (for TEM00) of the laser beam.So far, under specific experimental conditions, the occurrence of four competing structuring phenomena was observed: (i) interference structured ablation, (ii) upper non-structured ablation, (iii) interference structured swelling and (iv) lower non-structured swelling. All these mechanism cannot be simultaneously calculated using the Lambert-Beer law since they cannot be explained by a pure ablation process. For this reason, the Lambert-Beer ablation law was modified by considering four terms related to each specific structuring process. This procedure is described in Eq. (2):
where dI is the depth of the periodic modulation corresponding to the interference ablation, dG is the depth of the upper non-periodic ablated region (Gauss-like), dS is the height of the swelled features due to the interference pattern and dGS is the height of the lower non-periodic swelled region (Gauss-like).For the calculation of each term of Eq. (2), also a logarithmic correlation with the laser fluence and the threshold fluence for each individual process was considered, as in the Lambert-Beer ablation law:
where Fth is the ablation threshold, FthS is the swelling threshold and k [µm] are the rate coefficients for each of the four structuring mechanism, calculated as function of the spatial period [µm]. The ablation coefficients kI and kG are related to the interference structuring ablation and the non-periodic Gauss-like ablation, respectively, while the swelling coefficients kS and kGS referrers to the upper periodic swelled modulation and the non-periodic Gauss-like lower swelled region. These coefficients represent the speed rate of the process and can be calculated from a fitting procedure based on structuring experiments as a function of the spatial period as follows.In the case of a pure ablation process (i.e. transparent PC treated with UV-DLIP) the structuring mechanism is described by dI(x) and dG(x). Measuring how these two depths vary with the laser fluence, two ablation curves can be retrieved as shown in Fig. 8(a). This leads to a double Lambert-Beer-like plot, whose slopes provide the information about the efficiency of each the process (i.e. the coefficients kI and kG). Similar graphs can be obtained by repeating the ablation experiments over different structure periods and considering the slopes of these ablation curves (not shown here). The result of this procedure is presented in Fig. 9(a), where both ablation coefficients kI and kG are reported as function of the spatial period. Furthermore, the plot shows a clear trend: while kI increases with the spatial period, decreases, which means that the periodic structure is predominant at large spatial periods while the non-periodic modulation dominates for short periodic distances (the inverse of ablation coefficients k can be related to the depth of the structure produced for each regime).
Fig. 8 (a) Ablation curves reporting the periodic (dI ) and non-periodic (dG) contributions for the transparent Lexan SLX treated with 266 nm laser radiation for line-like structuring with a period of 2.13 µm; (b) swelling curves reporting the periodic (dS ) and non-periodic (dGS) contributions for the black Lexan FR25A treated at 1053 nm laser radiation with a 7.31 µm periodic line-like interference pattern.
Fig. 9 Dependence of the kI, kG and kS coefficients on the spatial period for (a) the transparent Lexan SLX treated at 263 nm, (b) the black Lexan FR25A treated at 1053 nm and (c) the black Lexan FR25A treated at 263 nm laser wavelength. kI, kG, kS and kGS denote the periodic ablation, non-periodic ablation, periodic swelling and non-periodic swelling contributions, respectively.
To retrieve the material-only related parameters as function of the spatial period (k(Λ)), the polynomial function reported in Eq. (4) has been employed for fitting the obtained coefficient trends.
As a result, four constants (a, b, c, d) can be used to describe each single k coefficient and thus the behavior of the polymer as function of spatial period. Important to mention is that this equation does not describe any physical mechanism and has been only used to explain the experimental results. Furthermore, in the case of the ablative process, to prevent negative values in the fitting of the kG-coefficient, the Λ2-dependence has not been considered (d = 0).
In the case of the swelling behavior produced by IR irradiation, the same procedure was employed to calculate the process-dependent coefficients. This is showed for example in Fig. 8(b), where the height of the swelled areas (Lambert-Beer plots) was measured as function of the pulse energy for the black Lexan FR25A treated at 1053 nm laser radiation with a 7.31 µm period. The two quantities taken into account are the height of the periodic modulated structures dS and of the lower non-periodic swelled part dGS (both taken as negative depths). The slopes of these two curves are the coefficients kS and kGS and their dependence with the spatial period is reported in Fig. 9(b).
The variation of the kS and kGS swelling coefficients with the spatial period denote that, while for short spatial periods the non-periodic Gauss-like swelling dominates, for large periods the periodic swelled patterns are predominant. Furthermore, these results show that for this material the rate of the periodic modulated swelling is always higher than the non-periodic part.
As for the ablation case, these trends can be fitted with the same polynomial law (Eq. (4)) leading to two other sets of material-related constants. Also in this case, it must be remarked that the Λ2-dependence has not been taken into account for the fitting of the kGS coefficient, in order to prevent unrealistic fitting behaviors. Lastly, the same procedure was applied for the doped black PC substrates irradiated with UV radiation. In this case, three different structuring behaviors were observed: periodic ablation, non-periodic ablation and periodic swelling, represented by the depths dI, dG and dS and their coefficients kI, kG and kS, respectively. These three behaviors act together during the structuring with an extent defined by the laser fluence and with a proportion regulated by the period of the interference pattern. These results are represented in Fig. 9(c). Also in this case, the coefficients kI, kG and kS were fitted using the polynomial expression of Eq. (4). It is worth to mention that the a-constant has not been considered for the fitting of the kI coefficient, in order to prevent negative values in the fitting procedure, which do not correlate with the experimental observations.
3.2 Calculation of surface topographies of DLIP irradiated polymers
A summary of the obtained results has been compactly reported into Table 1. The table shows the constants a, b, c and d from Eq. (4) to calculate ablation and swelling coefficients k for both the transparent and doped PC materials irradiated at 263 nm and 1053 nm wavelengths. The reported values not only permit to understand the structuring behavior of these two materials under IR and UV radiation, but also to predict the geometry of periodic patterns produced using DLIP. This can be realized taking into account the periodic intensity distribution of the incoming laser beam irradiating the material.
Table 1. Constants a, b, c and d (from Eq. (4) to calculate both ablation ( and ) and swelling ( and ) coefficients
In our experimental setup, two individual Gaussian beams are overlapped at the surface of the material (Fig. 1(b)). The interference between two plane waves with a Gaussian intensity distribution has been previously studied [41] and can be described by the following equation:
with and where [J] is the pulse energy, [µm] is the diameter of the laser illuminating area, [cm−1] is the wave-vector and is the angle between the two interfering beams. This intensity distribution can be inserted into Eq. (3), retrieving the overall depth/height of the produced patterns. It can be shown that the terms related to the periodic structuring (dI and dS) depend on the interference angle , while for describing the terms that unselectively ablate or swell the material (dGS and dGS) Eq. (5) is calculated using = 0.In order to directly compare the experimental results with the calculated patterns, four different structuring conditions have been considered. Figure 10(a) shows experimental measurement of the topography for a complete periodic-modulated ablation, a predominant Gaussian-modulated ablation mechanism, a simultaneous ablation and swelling periodic modulated pattern and combined periodic and Gaussian-modulated swelling. The corresponding simulated patterns are depicted in Fig. 10(b), showing in general a good agreement with the experimental results.
Fig. 10 Comparison between (a) experimental and (b) calculated profiles for different structuring conditions: complete periodic-modulated ablation (Lexan SLX, λ = 263 nm, Λ = 2.13 µm, Ep = 2 µJ), predominant Gaussian-modulated ablation mechanism (Lexan SLX, λ = 263 nm, Λ = 0.56 µm, Ep = 0.37 µJ), simultaneous ablation-swelling periodic modulated pattern (Lexan FR25A, λ = 263 nm, Λ = 2.13 µm, Ep = 1.1 µJ), and combined periodic and Gaussian-modulated swelling (Lexan SLX, λ = 1053 nm, Λ = 7.31 µm, Ep = 0.24 mJ). Also a comparison between (c) experimental and (d) calculated double-scale swelled surface is shown, created on Lexan FR25A with λ = 1053 nm, Λ = 7.31 µm, Ep = 0.19 mJ and 100 µm pixel distance.
Finally, the same model was used for calculating the 3D topography of a black doped PC substrate treated at 1.14 J/cm2 with a spatial period of 7.1 µm as shown in Fig. 10(c) and 10(d). Figure 10(c) presents the structured polymer, while Fig. 10(d) its calculated surface structuring using the same processing parameters. The distance between the individual pixels was 100 µm and a hexagonal arrangement between the pixels was used. The results show also in this case a good agreement between the experiment and the calculation. For example, the measured periodic and non-periodic swelling heights were 1.13 µm and 1.94 µm, respectively (Fig. 10(c)). For the calculation (Fig. 10(d)), the heights were 1.31 µm and 2.14 µm, showing deviation smaller than 16% and 10%, respectively.
4. Conclusions
In conclusion, this work reported on the different structuring mechanism on doped and non-doped PC materials using the Direct Laser Interference Patterning method through UV and IR wavelengths. It was found that for the transparent polycarbonate, two main ablation mechanisms can describe the structuring process. For large spatial periods and low laser fluences, very well defined patterns are produced where the material is selectively ablated at the maxima positions. However, for short spatial periods as well as at high fluences, the upper part of the periodic modulation is erased, leading to a structure geometry combining a periodic distribution with a Gaussian-like envelop. For multi-component materials such in colored polycarbonate materials, a very different behavior was observed. If the structuring process is performed with an IR wavelength, swelled structures were produced, also with a periodic and a non-periodic (Gaussian-like) contribution, especially for short spatial periods. The swelling mechanism has been attributed to the creation of pores beneath the surface, due to the dissociation of the polymer dopant. If the same polymer is treated with an UV laser radiation, where the PC matrix is mainly absorbing the laser wavelength, not only both ablation behaviors where observed but also a periodic-swelling contribution at low laser fluences.
Using different sets of equations, it was also possible to develop a simple model, capable to predict the contribution of the four different observed structuring mechanisms (periodic modulated ablation and swelling and non-periodic/Gaussian-like ablation and swelling). The comparison between the experiments and the simulations were in good agreement. Furthermore, this model can be applied in the future to describe the DLIP structuring process of any polymer substrate and to design more complex structures, even employing the combination of ablated and swelled structures.
Funding
Horizon 2020 Framework Programme (H2020) under the Marie Skłodowska-Curie grant agreement (No 675063); German Research Foundation (DFG).
Acknowledgments
The authors acknowledge F. Caccavale (SABIC, The Netherlands) for the useful discussions, and J. Bretschneider (Fraunhofer IWS) for the FIB-cross sectional investigations of the processed polymers. A. I. Aguilar Morales (Fraunhofer IWS) is also acknowledged for his support in the SEM analysis.
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