Laser intensity-based geometry control of periodic submicron polymer structures fabricated by laser interference lithography

1. Introduction

Over the past decade, submicron periodic structures gained the significant attention of the researchers all over the world due to the potential applications in various fields. These structures are under intensive investigations for the development of the resonant grating waveguides [1,2], microlens arrays [3], photonic crystals [4,5], nanoimprint stamps [6,7], anti-reflective surfaces [8–11], diffraction gratings [12] and grating based optical elements [13]. Periodic micro- and nanostructures are also essential for the efficiency enhancement of solar cells [14], holographic data storage [15], tissue engineering [16–18], inducement of directional cell growth [19], modulation of tribological properties [20] and fabrication of hydrophilic and hydrophobic surfaces [11,21].

There are a variety of both additive and subtractive methods to fabricated such structures: laser ablation [1], direct laser writing [22], photolithography [23], electron beam lithography [24,25], ion beam lithography [26], nanoimprint lithography [7,11,23,27], or self-organization [28]. Laser interference lithography (LIL) is a rapid and flexible approach to fabricate periodic micro- and nanostructures over the large area by a single laser exposure of photosensitive layer. Compared to the other techniques, LIL is advantageous, because it does not require expensive equipment, a special environment like a vacuum, particular gasses or clean room, as well as no mask is needed for this approach. Also, the geometry of fabricated structures via LIL is controlled by the manipulation of the interference intensity distribution, which strongly depends on the number of interfering beams, their wavelength, the angle of incidence [29], polarization [30] and phase [31].

After a photoresist is exposed to light, a series of chemical reactions, such as generation of primary radicals, polymer chain propagation, inhibition and termination are induced. Therefore, photo-polymerization is a highly complicated process, which kinetic behaviour is strongly dependent on how the exposure energy is delivered [32]. Thus, the final distribution of polymer molecules and, consequently, the geometry of fabricated polymer structures depends on the laser exposure parameters.

Zhao and Mouroulis [33] have demonstrated that the ratio of the monomer diffusion coefficient and the irradiation intensity directly influence the spatial distribution profile of the polymer concentration. Further research by Colvin et al. [34] and Liu et al. [32] developed detailed theoretical models of polymerization processes during the laser exposure by the periodic beam intensity distribution. It was showed theoretically that the photo-polymerization using high enough irradiation intensities resulted in the polymer concentration profiles deviated from the cosinusoidal exposure intensity pattern and even had a dimple where the exposure intensity was maximum. That is a fairly counterintuitive result, and it was explained by the presence of a monomer diffusion, which took place due to the spatially modulated light exposure of the photoresist and was triggered by a concentration gradient of the monomer molecules. The monomer diffusion takes place from the low-intensity zones to the irradiation maxima zones, where monomer concentration has been depleted by the photo-polymerization. As the laser pulse peak intensity is increased, the photo-polymerization reaction proceeds more rapidly making the monomer diffusion distance shorter due to the significant monomer consumption by the photoreaction before it reaches the most monomer-depleted region, which is the central part of the structure. Consequently, monomers supplied from the dark regions are polymerised in the peripheral regions of the structure, which, eventually, overgrows the central area of the structure. The resulting polymer structure profile, therefore, has a dimple in the center.

In this work, we demonstrate experimentally the geometry control of submicron periodic polymer structures by altering the polymerization kinetics with variations of laser pulse duration and peak intensity, simultaneously, which leads to the fabrication of the dimple-like polymer structures predicted theoretically by Liu.

2. Materials and methods

Submicron structures were fabricated from the hybrid, organic-inorganic, negative photoresist SZ2080 (chemical formula C₄H₁₂SiZrO₂) [35]. This material exhibits low shrinkage, high mechanical stability and high optical resistance to laser beam radiation, comparable with the damage thresholds of the conventional optical elements [36]. The photosensitivity of SZ2080 was increased by enriching it with the photo-initiator Thioxanthen-9-one, also known as THIO (chemical formula C₁₃H₈OS). The concentration of THIO by weight was equal to 1%. In order to form a thin layer of SZ2080 + 1% THIO mixture, the viscosity was reduced by mixing it with the isopropanol by the volume ratio of 1:10 (volume fraction of SZ2080 + 1% THIO in isopropanol was 10%). Following mixture was coated on the glass substrate by the spin-coating method by using the angular velocity of 12000 RPM for 90 seconds, which resulted in the formation of uniform, ~200 nm-thick photopolymer layer.

The fabrication process of submicron polymer array is depicted in Fig. 1. At first, the deposited layer of SZ2080 + 1% THIO mixture, consisting of monomer and photo-initiator molecules as seen in Fig. 1(a), was irradiated by the four-beam laser interference intensity distribution shown in Fig. 1(b). Interference was formed by splitting the laser beam into four symmetrically arranged beams by the diffractive optical element (Holo-Or, Ltd.) and transferring them on the sample using the 4F imaging system. We used the second harmonics (515 nm) wavelength of the Yb:KGW femtosecond laser (Pharos, Light Conversion, Ltd.). The period of the interference intensity distribution, which depends on the laser wavelength and the angle of the beam incidence, was set to 600 nm. In the exposed areas, photo-initiator molecules were converted into free radicals, which induced the monomer conversion to polymer molecules. Subsequently, the concentration of photo-initiator and monomer molecules was locally decreased, and their diffusion to the exposed areas took place as demonstrated in Fig. 1(c). Figure 1(d) shows the polymer molecules, which were formed after the laser exposure. Compared to monomers, polymers have different solubility characteristics, therefore by rinsing the sample in the 4-methyl-2-pentanone solution, only the unexposed areas, which correspond to the interference intensity minima, were dissolved as seen in Fig. 1(e). After 5 minutes of rinsing, the sample was dried at the room temperature and the submicron polymer structures with the period of 600 nm were formed as depicted in Fig. 1(f). Fabricated structures were characterized by the atomic force microscope (AFM) Veeco Dimension Edge.

 

Fig. 1 Fabrication process of the submicron polymer structure array: a) microscopic view of SZ2080 + 1% THIO mixture; b) four laser beams overlapping to form the interference intensity distribution on the deposited photoresist layer; c) laser exposure by 600 nm-spaced interference intensity maxima and the directions of monomer and photo-initiator molecule diffusion; d) polymer chains formed in the exposed areas; e) dissolving of unexposed areas in 2-methyl-4-pentatone solution; f) fabricated submicron periodic polymer structures.

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3. Results and discussion

Six arrays of submicron structures were fabricated by the four-beam interference exposure of spin-coated, ~200 nm-thick, photopolymer SZ2080 + 1% THIO. For every fabrication, we used the same pulse energy (10 μJ), pulse repetition rate (10 kHz) and exposure duration (45 s). Therefore, the energy dose was maintained constant. The pulse peak intensities were varied from 0.57 GW/cm² to 18.1 GW/cm² (calculated by averaging over the area defined by the beam radius at 1/e² of the maximum intensity value) by changing the pulse duration from 8 ps to 250 fs.

The average area of fabricated arrays is ~0.02 mm², and each array consisted of thousands of 600 nm-pitched polymer structures. We characterized only 5x5 μm² square area in the center of each fabricated array by the atomic force microscope. Measured AFM images are shown in Fig. 2. Fabrication by the lowest laser pulse peak intensity, seen in Fig. 2(a), results in the fairly small diameter of structures with large spaces in between them. As the pulse peak intensity was increased, polymer structures were broadened and, eventually, came into contact with each other as shown in Fig. 2(b-d). By the further increase of the pulse peak intensity, a formation of a small dimple in the center of each structure was observed as seen in Fig. 2(e-f).

 

Fig. 2 2D AFM amplitude images of the fabricated polymer submicron structures by using different pulse peak intensities: a) 0.57 GW/cm²; b) 0.91 GW/cm²; c) 1.51 GW/cm²; d) 4.53 GW/cm²; e) 7.55 GW/cm²; f) 18.11 GW/cm². The scale bars correspond to 1 μm.

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Three-dimensional AFM amplitude images of arrays fabricated by 0.57 GW/cm²; 0.91 GW/cm²; 7.55 GW/cm² and 18.11 GW/cm² pulse peak intensities are depicted in Fig. 3(a-d), respectively. The change of periodic polymer structure geometry was further analysed by plotting the height profiles of separate structures from each array as shown in Fig. 3(e-h). Photo-polymerization by 0.57 GW/cm² pulse peak intensity led to the formation of the ~70 nm height Gaussian shaped structure profile (the red dotted line is the Gaussian function fit) given in Fig. 3(e). On the other hand, higher (~110 nm height) structures with the circle shaped profile were fabricated by 1.6 times higher (0.91 GW/cm²) pulse peak intensity (the green dotted line is the circle function fit) as demonstrated in Fig. 3(f). As mentioned before, fabrication using the highest (7.55 GW/cm² and 18.11 GW/cm²) pulse peak intensities resulted in the dimple-like submicron structure formation shown in Fig. 3(c, d). In this case, the spaces between each periodic structure were too narrow for AFM scanning tip to fit in and come into contact with the substrate. Therefore, height profiles plotted in Fig. 3(g, h) are relative to the lowest measured level. Consequently, the absolute heights of structures in these arrays are unknown. Nevertheless, it was possible to measure the depths of formed dimples, which were 5 nm, for structures fabricated by 7.55 GW/cm² pulse peak intensity as seen in Fig. 3(g), and 8 nm, for structures fabricated by 18.11 GW/cm² pulse peak intensity as depicted in Fig. 3(h). Polymer arrays fabricated by intermediate (1.51 GW/cm² and 4.53 GW/cm²) pulse peak intensities are not shown here because their height profiles are similar and have no particular shape.

 

Fig. 3 3D AFM amplitude images of the fabricated polymer arrays by different pulse peak intensities: a) 0.57 GW/cm²; b) 0.91 GW/cm²; c) 7.55 GW/cm²; d) 18.11 GW/cm²; and their single structure height profiles, respectively (e-h). The dimensions of AFM images are 5x5 μm².

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The influence of the shrinkage effect on the geometry of fabricated structures is assumed to be negligible in this case because the used photopolymer exhibits ultra-low shrinkage [35]. Furthermore, it was estimated that SZ2080 shrinkage is only significant near the polymerization threshold, whereas using the ~2 times higher intensity than polymerization threshold results in almost zero linear shrinkage strain [37].

4. Conclusions

In this work, we have demonstrated the approach of using the laser interference lithography to fabricate significantly different geometries of the submicron polymer structures by simultaneously changing the pulse duration and the pulse peak intensity. The height profiles of the fabricated structures varied from the Gaussian and circle function shapes to the concave shape with a few nanometer-sized dimples. The results of this research are important for understanding the polymer structure shape development as the laser pulse peak intensity is gradually increased and shows the ability to control the geometry of periodic polymer nanostructures by inducing different polymerization kinetics. This approach might lead to the fabrication of unique, dimple-like, geometry nanostructures for the development of diffraction-based optical elements and anti-reflective surfaces, or modulation of surface wetting and tribological properties.