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Fast and efficient manufacturing method of one- and two-dimensional polyethylene terephthalate transmission diffraction gratings by direct laser interference patterning.


Diffraction gratings are important elements for various areas of science and technology, in the form of spectrophotometers (1), astronomic telescopes (2), integrated optic systems, and data storage (3) among others. Simple and versatile diffractive optic elements are commonly fabricated by multistep processes (4,5) and in a large variety of materials and geometries that define their final application (6,7).

Diffraction gratings made of polymeric materials have gained popularity in advanced optical systems (8) because they offer good performance at lower production cost when compared with their glass-made counterparts (9). In particular, polyethylene terephthalate (PET) is well suited for optical applications because it has high birefringence, good optical transmittance, excellent mechanical and chemical properties, and thermal stability (10).

Diffraction gratings made of different polymers can be fabricated by different methods including micromolding (11,12), hot embossing (13), replication molding (14), soft lithography, and holographic techniques (15,16). However, these technologies require fabrication of molds or masks or require several processing steps, which increase the fabrication cost, at least in the first development phase. As an alternative to these methods, direct laser interference patterning (DLIP) is proposed as a new flexible and maskless microfabrication technique of diffraction gratings on PET.

DLIP is a relatively new-developed technique (17,18) that has been used extensively for micromachining of polymers, ceramics, and metal substrates. This technique allows producing well-defined micropatterns up to the sub-micrometer scale in a fast one-step procedure. An interference pattern is obtained when two or more coherent and collimated beams are superimposed at a certain angle. If a high-power laser system is used, the energy density at the interference maxima can surpass a certain threshold value allowing the local modification of the material at such positions. In polymers, this process produces local ablation of the polymer surface at the interference maxima positions based on a photothermal, photochemical, or photophysical interaction. In this method, periodic micropatterns can be produced over large areas over the desired polymer substrate. In a two beam interference patterning arrangement (see Fig. la), a linear periodic intensity distribution of the laser radiation is obtained and its repetitive distance between the interference maxima or minima (spatial period, A) can be calculated as follows:

A = [lambda] / 2sin(0/2) (1)

where it is the laser wavelength and 0 is the included angle between the laser beams. Therefore, the spatial period can be theoretically adjusted up to [lambda]/2, corresponding to an angle of 180 [degrees]. Due to the high-energetic laser pulses that are used, polymers can be directly processed, saving several steps such as photoresist spincoating, wet developing, and etching of the substrate (19)

In this study, DLIP is used to fabricate transparent PET diffraction gratings. By varying the laser-processing parameters (such as energy density and pulse number) as well as the optical setup arrangement, different periodic gratings with spatial periods of 1, 3, and 5 [micro]m were produced. The efficiency of the diffraction gratings prepared using this method was studied in terms of the topological structure of the produced microarrays.


PET substrates (from Goodfellow Huntingdon, England) with 175 [micro]m of thickness were used as received after rinsing with ethanol. DLIP was performed using a two-beam arrangement to create microstructured surfaces on PET film specimens, as shown in Fig. la. In this set up, a circular diaphragm and a lens were used to obtain the desired-beam diameter (approximately 6 mm). After the lens, the beam was split into two sub-beams using a beam splitter (50%. reflection). By positioning mirrors after the beam splitter, the laser beams were guided to the PET surface, where they interfered under a specific angle producing a line-like interference pattern. PET is transparent from ~1200 to 320 nm. Thus, the third harmonic (266 nm) of a high-power pulsed Nd-YAG laser (Quanta-Ray PRO 290, Spectra Physics) was used for the laser interference experiments.

The PET films were irradiated using laser energy densities ranging from 100 to 300 mJ/[cm.sup.2] and 1-15 laser pulses. These parameters were investigated using three different spatial periods ([GAMMA]): 1, 3, and 5 pm.

The efficiency of the first diffraction orders (+1 and --1) of the gratings was measured using the arrangement shown in Fig. 1b. In this simple set up, a 633-nm nonpolarized He: Ne laser (Uniphase model 1216-2) in continuous wave mode with 10 mW of power was used to irradiate normally the surface of the sample. Then, a diode power meter (Coherent, Molectron model PS 10Q) was used to measure the intensity of each diffracted beam. Losses due to reflection of the back face of the diffraction grating and absorption losses due to multiple scattering effects inside de substrate were corrected to calculate the relative diffraction efficiency.

The structured PET films were analyzed using high resolution Scanning Electron Microscopy (SEM) at an operating voltage of 5 kV (Philips XL30 ESEM-FEG). All the samples were coated with ca. 2 nm gold layer to improve the electrical conductivity prior to imaging. In addition, an atomic force microscope (AFM) in noncontact mode (JEOL JSPM 5200) was used to determine the groove depth as well as the quality of the obtained structures of the gratings.


In the direct fabrication of the PET diffraction gratings, two main laser processing parameters were varied: energy density and number of pulses. Scanning electron micros-copy and AFM images of the resulting parameters are shown in Figs. 2 and 3, respectively. Figure 2 shows a selection of gratings with 1 (Fig. 2a and b), 3 (Fig. 2c and d), and 5 [micro]m (Fig. 2e and f) spatial periods and processed under different conditions. The overall uniformity of the topography was monitored using an optical microscope with magnification factor of 200, 500, and 1000. Micro patterns fabricated using only one pulse and low fluences are very uniform and present few defects (Fig. 2a for [GAMMA] = 1 [micro]m, 1 laser pulse, 150 mJ/[cm.sup.2]; Fig. 2c for [GAMMA] = 3 [micro]m, 1 laser pulse, 100 mJ/[cm.sup.2]). The imperfections that can be observed in these images can be attributed to redeposit ion of ablated material on the surface after laser irradiation. By increasing the laser fluency (Fig. 2b for [GAMMA] = 1 [micro]m, 1 laser pulse, 300 mJ/[cm.sup.2]) some of the structures collapsed building bridges between two grooves. This can be attributed to the photothermal ablation of the PET material at the used laser wavelength ([lambda] = 266 nm) where the material was partially molten. On the other hand, for bigger spatial periods ([GAMMA] = 3 and 5 [micro]m) no bridges between two grooves were observed due to the large separation between the structures.

Another processing parameter that had a strong influence in the morphology of the periodic structures is the number of pulses used to irradiate the substrates. As shown in Fig. 2d and f, if the sample surface was irradiated with several laser pulses (5 and 15 for Fig. 2f and d, respectively) then the number of irregularities in the microstructure increased. Compared with the patterns produced using a single-laser pulse, when several pulses are used, the material is heated and molten several times at the interference maxima positions. This can thus explain the origin of the irregularities observed in the micropatterns, indicating that surface debris become a problem for multiple pulse irradiation conditions.

From the two-and three-dimensional topography profiles shown in Fig. 3, it can be seen that both laser energy density and the number of pulses have a significant effect on the depth of the microstructures (see for example Fig. 3a and b for 1 laser pulse, 1 [micro]m spatial period, and 100 and 150 mJ/[cm.sup.2], respectively). PET gratings obtained by single pulse irradiation at 100 mJ/[cm.sup.2] are very similar, with groove depths ranging from 0.64 to 0.75 [micro]m. These values are comparable to the absorption depth of PET at a wavelength of 266 nm (~0.3 pm) suggesting that material absorption controls the depth of the ablated material when irradiating with a single pulse (20). On the other hand, increasing the number of laser pulses also increased the depth of the grooves. For example, with a laser energy density of 100 mJ/[cm.sup.2] and a spatial period of 3 [micro]m, groove depths of 0.75, 1.20, and 1.30 [micro]m were observed for 1, 5, and 15 pulses, respectively (see Fig. 3c and d for 1 and 15 pulses, respectively).

The relative transmission efficiency for the first diffraction orders (+1 and -1) of the fabricated gratings was measured using the set up shown in Fig. 1 b. The results of these measurements are summarized in Fig. 4. In this article, the authors can see that the best efficiencies (first diffraction orders) for all investigated periods (1, 3, and 5 [micro]m) correspond to patterns produced using one laser pulse and low laser intensities (100-150 mJ/[cm.sup.2]). As previously discussed, using these irradiation conditions renders the most uniform structures. However, these patterns do not present the deeper structures. In contrast, PET gratings made, at moderate and high energy densities (from ~200 mJ/[cm.sup.2]) and/or several laser pulses showed lower efficiencies (Fig. 4) but the deeper grooves.

There are several possible explanations for the observation that transmission efficiency is generally inversely proportional to laser energy density. First, it is evident that higher laser fluences result on deeper grooves. In the case of 1 pm period gratings, a groove depth of 0.64 and 0.78 [micro]m was observed for 100 and 150 mJ/[cm.sup.2], respectively. For this spatial period, these laser fluences permitted to fabricate almost defect-free periodic arrays and the best efficiency was measured for PET substrates irradiated with 150 ral/[cm.sup.2]. This grating presents also the deeper grooves. Using higher laser intensities, a significant deterioration of the quality of the microstructures occurs due to defect formation. This effect leads to a reduction of the diffraction efficiency, especially for gratings where bridges between two grooves appear.

For PET substrates irradiated with three or more laser pulses, significant losses of the diffraction efficiency were observed. As reported earlier (21), the production of surface contaminants from residual ablation products by multipulse UV laser ablation is a factor that in the general decreases optical transmission of the material. In addition to this, samples irradiated with more than 5 pulses presented a yellowish and slightly hazy appearance, which results from marked redeposition of molten material and polymer degradation during the multipulse ablation process.

In reference to the influence of spatial period on efficiency in the first diffraction order, several aspects had to be noted. As shown in Fig. 5, by irradiating a simple 1 D diffraction grating with a laser beam (633 nm He:Ne) a Fourier diffraction pattern is produced. The diffraction patterns produced with grating with spatial periods of 1, 3, and 5 pm have a distinctive feature: the wider the periodicity of the micropattern, the more the number of significant diffraction orders that can be measured. For example, for a grating with A = 1 [micro]m, three beams, which correspond to the 0 order (or transmitted beam) and the first diffraction orders (+1 and -1), are visible (Fig. 5a). For the 3-pm gratings, seven spots are visible corresponding to the transmitted beam (0 order) and the other ones corresponding to the first, second, a third diffraction orders (-3, -2, -1, +1, +2, +3; Fig. 5b). Finally, for the diffraction gratings fabricated with a spatial period of 5 [micro]m, 11 spots are observed, corresponding to the transmitted beam (0 order) and up to the fifth diffraction order (-5, -4, -3, -2, -1, +1, +2, +3, +4, +5; Fig. 5c). Because the intensity of the original laser beam is divided into three, seven, or 11 parts depending on the spatial period of the periodic pattern, then the smaller the spatial period the lower the number of diffraction orders. In consequence, the highest efficiency for the first diffraction orders (-1, +1) is observed for periodic gratings with smaller spatial periods (1 [micro]m) because the beam is split in fewer orders. Additionally, for 1 [micro]m gratings produced with 150 mJ/[cm.sup.2] and 1 laser pulse, up to 95% of the total transmitted laser radiation was measured in the first diffraction orders (42.5% on order +1, and 42.5% on order -1, see Fig. 4a). In contrast, the lower efficiencies were measured for diffraction gratings with large periods (3 and 5 [micro]m) as shown in Fig. 4b and c (22,23). Comparing the efficiencies of the 3 and 5 pm gratings (Fig. 4b and c), better efficiencies are observed for the 5[micro]m arrays (Fig. 3e). As reported earlier (24), for a given grating period, there is an optimum structure depth to achieve a maximum theoretical diffraction efficiency. For gratings with 3 and 5 pm periods and a sinusoidal structure, the optimum depth is about 0.63 [micro]m (25) (calculated for a refraction index of 1.55, which is very close to the refraction index of PET (~1.60) at the used wavelength). This value is very close to the measured structure depth of the gratings produced for this period (~0.70 [micro]m, see Fig. 3). For gratings with a 3-[micro]m period, the measured structure depth was 0.75 [micro]m. This value is not as close to the optimum depth, as in the 5-[micro]m case, and a lower efficiency was measured. Apart from this optimum value, even a small variation of the structure depth can reduce significantly the relative transmission efficiency in the first diffraction order (24), what also explains the observed behavior.

In addition to line-like structures, two-dimensional patterns were also fabricated using DLIP. This is possible by irradiating the sample using a two-beam interference setup, then rotating the patterned substrate at certain angle and irradiating the substrate again. For example, Fig. 5d shows a diffraction pattern of a lattice-like array (Fig. 5e), where the second irradiation step was performed with 90[degrees]. Besides this, by irradiating the PET substrates 20 times with rotation angles of 9[degrees], other complex diffraction gratings can be made (Fig. 5f) an therefore obtaining two-dimensional diffraction patterns (Fig. 5g).


DLIP is presented as a method to the fabricate transmission diffraction gratings on PET substrates. Depending on the laser processing conditions used in the fabrication process, variations of the diffraction efficiency were observed. The best diffraction efficiencies were measured in gratings fabricated with low energy densities (100-150 mJ/[cm.sup.2]) and one laser pulse. Using these parameters rendered well defined and almost defect-free periodic structures. In contrast, higher laser fluences or more laser pulses deteriorated significantly the quality of the microstructures, reducing their efficiency. Thus, the performance of the PET gratings can be directly related to the uniformity of the micropatterns. On the other hand, higher laser intensities and several laser pulses increased the depth of the microstructures. In conclusion, a balance between high uniformity (few defects) and deep grooves provides the most efficient diffraction gratings. Finally, it was shown that DLIP is not only suitable to fabricate one-dimensional diffraction gratings but also can be used to produce complex two-dimensional arrays. Due to the excellent optical transmission as well as thermal stability of PET, diffraction gratings made by DLIP could be used for several optical systems operating in the infrared region of the electromagnetic spectra.


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Heidi Perez-Hernandez, (1), (2) Andres F. Lasagni (1)

Correspondence to: Dr. Andres Lasagni; e-mail: andres-labian.lasagni@ Hi s .ff aunhofer .de

Contract grant sponsor: FhG Internal Programs; contract grant number: Attract 692174.

Contract grant sponsor: Deutscher Akademischer Austausch Dienst (DAAD).

(1) Fraunhofer Institut fCir Werkstoff -und Strahltechnik (IWS), Winterbergstrabe 28, 01277 Dresden, Germany

(2) Institute for Laser and Surface Technology, Technical University of Dresden, 01062 Dresden, Germany

DOI 10.1002/pen.23149
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Author:Perez-Hernandez, Heidi; Lasagni, Andres F.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EUGE
Date:Sep 1, 2012
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