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This work demonstrates a promising method to fabricate periodic nanovein structures, which can be served as templates for fabricating photonic crystals possessing a large complete photonic bandgap. First, the fabrication of a one-dimensional grating structure connected with nanolines is demonstrated by controlling the exposure dosage of the second exposure of the two-exposure two-beam interference technique. Secondly, by using the same interference technique but setting each exposure under the same exposure dosage, two-dimensional periodic structures with nanovein connections were fabricated. These structures were obtained by using either a pure negative photoresist with very low concentration of photoinitiator or a mixing of a negative and a positive photoresists. The fabricated structures are not, as usual, a duplication of the interference pattern but are constituted by square or triangular rods connecting with narrow veins. They can be used as templates for fabricating photonic crystals with very large complete photonic bandgap.
©2009 Optical Society of America
1. Introduction
After the pioneering work of Yablonovitch and John [1, 2], there has been considerable interests in the fabrication of two- and three-dimensional (2D and 3D) photonic crystals (PhCs), which consist of periodic dielectric structures [3]. These structures exhibit “forbidden” frequency region (photonic bandgap - PBG) where electromagnetic waves cannot propagate along any direction inside the structures. Various techniques have been proposed to fabricate templates of PhCs such as self-assembly of colloidal particles [4], holographic lithography (HL) [5–7], and two-photon polymerization direct laser writing (TPP-DLW) [8, 9], etc. HL, in particular, is a very promising and inexpensive technique to fabricate large-area and defect-free PhC templates [5–7]. The working principle of HL technique is following: a photoresist is placed in the multiple-beams interference region and the interference pattern is transferred into the photoresist with a corresponding matter distribution. The photoresist is subsequently developed and the PhC template is then obtained. There exists typically two kinds of photoresists, i.e., negative and positive. Negative photoresists exhibit a certain exposure dosage threshold, above which photoresists become insoluble in the developing process. For positive photoresists, the unexposed regions remain unchanged after development. In most cases, one expects that the PhC template obtained by HL technique is an exact duplication (negative photoresist) or inverse-duplication (positive photoresist) of the interference pattern. However, in this simple argument, some factors may cause distortions of periodic structures are ignored. Indeed, previous researches have shown that final structures obtained by HL technique strongly depend on the developing process [10], the absorption of photoresists [11], and also the mass transport effect [12]. In addition, using long exposure TPP-DLW technique, recently it was demonstrated that nanofibers can be formed between main structures due to diffusion of photoacid of negative photoresist [13]. Employing this effect, one can use a laser micro-beam to “guide” the photoacid diffusion and create nanolines connections between main structures [14]. This idea was then used to fabricate many interesting nano structures such as nanofibers, nanotips, or even nano PhCs [13–17]. Until now most nano structures were obtained via TPP-DLW technique and no report about using HL technique to fabricate nanovein structures has been published yet.
This work shows that large area periodic structures containing with nanovein connections can be fabricated by using HL technique. The use of HL technique provides many fabrication advantages such as simple, rapid, and low cost. To obtain nanovein structures, the polymerization speed needs to be reduced, which can be achieved by either decreasing the concentration of photoinitiator of a negative photoresist or mixing a negative and a positive photoresists. Moreover, we demonstrate the expectation, that the fabricated structure is a duplication of the interference pattern, is not valid under slow polymerization speed.
This paper is organized as follow. In Section 2, we first demonstrate the idea of using a one-dimensional (1D) holographic grating structure to guide the diffusion of photoacid and form nanolines between main frames. In Section 3, we show the simulation results of a double-exposure two-beam interference pattern and compare them with the fabricated structures using a pure SU8 photoresist with high concentration of photoinitiator. Section 4 shows the fabrication of a new periodic structure employing a pure negative SU8 photoresist with very low concentration of photoinitiator. In Section 5, we demonstrate a new idea of using a mixing of a negative and a positive photoresists to fabricate a periodic structure containing with nanovein connections. We summarize the conclusions in the last Section.
2. Creation of nanoline connections in one-dimensional periodic structure
Recently, it has been demonstrated [14] that the generation and diffusion of photoacid, which acts as a cross-linking agent, depend on the fabrication conditions such as exposure intensity, exposure time, post-exposure bake (PEB), etc. During exposure process, photoinitiator molecules release photoacids in light exposure regions, and the PEB process accelerates the diffusion of photoacids and induces cationic polymerization of epoxy groups in a negative photoresist. Obviously, with high concentration of photoinitiator molecules and high exposure dosage, it is easy to result in a dense polymerized pattern, corresponding to a large size solid structure. Decreasing the concentration of photoinitiator in the photoresist is a simple way to reduce the size of the solid structure. Therefore, a commercial pure negative photoresist SU8-2002 [MicroChem Corp.] was mixed with a commercial photoinitiator H-Nu470 [Spectra Group Limited] with very low concentration (0.6wt.%) to get low absorption at laser exposure wavelength 514nm. It needs very high exposure dosage to obtain a large and solid structure.
In this work, a simple HL technique, based on a double-exposure of two-beam interference pattern [7], was employed to fabricate large size 1D periodic structures containing with periodic nanoline connections. Two exposures were made at 0° and 90°. The first exposure was made with a larger exposure dosage (1910mJ/cm2) to form a 1D main frame structure. The second exposure was made with a smaller exposure dosage (>795.8mJ/cm2) to create periodic nanoline connections normal to the 1D main frame structure. Figure 1(a) shows the schematic diagram for the fabrication process. The whole structure is a circle with a diameter of 6mm, corresponding to the size of the laser beam. The period of the 1D main frame structures is 3μm, which can be decreased to as small as 300nm by increasing the angle between two beams. Figures 1(b-d) show scanning electronic microscope (SEM) pictures of 1D periodic structures containing with (c and d) and without (b) nanoline connections, obtained by changing the exposure dosage of the second exposure. The diameters of nanoveins increase with the exposure dosage of the second exposure. For exposure dosage lower than 477.4mJ/cm2, nanolines, similar to the result reported in Ref [13], could be created but were randomly distributed between main frames (not shown in Fig. 1). As exposure dosage of the second expose was higher than or equal to 477.4mJ/cm2, periodic nanolines, like the result reported in Ref [14], could be obtained as desired. Figures 1(c) and 1(d) show the results obtained with exposure dosages of the second exposure were set at 477.48mJ/cm2 and 795.8mJ/cm2, respectively. The typical diameters of nanolines in Figs. 1c and 1d are about 70nm and 158nm, respectively. After inspecting the nanolines, we found that they are in contact with the substrate, which is different to that obtained by TPP-DLW technique [14]. The next question is what kind of structure can be obtained in low photoinitiator concentration photoresist, if it is exposed twice with the same dosage? To answer this question, we will review the fabrication of a 2D structure by a double-exposure two-beam interference technique using a commonly used pure SU8 photoresist in next section.
Fig. 1. Creation of 1D periodic structures containing with nanoline connections. (a) Illustration of the fabrication process: a double-exposure two-beam interference was used to fabricate such structures. The first exposure was realized with a larger exposure dosage to fabricate a solid and large size 1D main frame structure, while the second exposure dosage was lower to form nanoline connections. (b)-(d) show the experimental results obtained by fixing the first exposure dosage = 1910mJ/cm2 and setting the second dosages for: 168.2mJ/cm2 (b); 477.4 mJ/cm2 (c); 795.8mJ/cm2 (d). The distance between lines is 3μm.
3. Recording a 2D interference pattern on a pure negative photoresist SU8
A simple double-exposure of two-beam interference pattern was used to fabricate 2D periodic structures into a pure SU8 photoresist [7, 12]. Two exposures were realized at 0° and 90° directions and with the same dosage, e.g., same exposure time. Figure 2(a) shows the theoretical calculation of the iso-intensity distribution of the interference pattern. The iso-intensity values are varied from 0.4 to 1.6 (minimal and maximal intensities are 0 and 2, respectively), corresponding to the change from high to low dosage exposure. By changing the iso-intensity, the shape of the square lattice structure changes from cylinder (iso = 1.6) to air-hole (iso = 0.4). When the iso-intensity is equal to 1 (half of maximal intensity), a special square lattice structure composed of square rods is obtained. This is the only one exception that the obtained square lattice structure has a square rod shape as evidently seen in Fig. 2(a). To verify this prediction, we recorded the interference pattern (obtained by using He-Cd laser at 325nm, power of each beam = 260μW, exposure time varied) into a pure SU8 negative photoresist. Figures 2(b-d) show SEM images of three typical square lattice structures, corresponding to cylinders (each exposure dosage = 0.92 mJ/cm2), square rods (each exposure dosage =1.84 mJ/cm2), and air-holes (each exposure dosage = 2.76 mJ/cm2). These fabricated structures almost agree with theoretical predictions. Note that although the absorption of the pure SU8 photoresist at 325 nm is high, the absorption effect along normal direction of photoresist was neglected in the calculation because the film thickness was only 1.5μm. In addition, no developing effect was found on the structures fabricated with SU8 photoresist.
The above result agrees with a commonly used assumption in HL technique; the final structure on the photoresist is a duplication of the recorded interference pattern. Most researchers usually use the interference pattern as a final fabricated structure to calculate the optical properties of a PhC fabricated by HL technique. However, in next section the broken of this commonly used assumption will be shown, when the interference pattern is recorded into a photoresist with low concentration of photoinitiator or a mixing of negative and positive photoresists is used.
Fig. 2. (a) Iso-intensity distribution of the two-beam interference pattern, obtained with a double-exposure at 0° and 90°. The iso-intensity values are varied from 0.4 to 1.6 (minimal and maximal intensities are 0 and 2, respectively), corresponding to the change from high to low dosage exposure. (b)-(d) show the experimental results obtained by increasing the exposure dosage from 0.92 to 2.76mJ/cm2. Cylinder structures in (b) were obtained after exposed with 0.92mJ/cm2 ° 0.92mJ/cm2 and air-hole structures in (d) were obtained after exposed with 2.76 mJ/cm2 ° 2.76mJ/cm2.
4. Fabrication of a 2D structure containing with nanovein connections using SU8/HNu40 photoresist
The double-exposure two-beam interference technique was applied to fabricate a 2D periodic structures containing with nanovein connections using SU8 photoresist mixed with low concentration H-Nu470-photoinitiator. The laser source used was an Argon laser emitting at 514nm. The exposure dosage was fixed 1273.2mJ/cm2 for each exposure, while the concentration of the HNu470 photoinitiator was changed. This change allowed a control of the quantity of photoacid in each exposed region and after PEB solid nano structures were formed according to the distribution of photoacid. Figure 3 shows SEM images of different 2D periodic square structures obtained with different concentrations of H-Nu470-photoinitiator. It is clear that a 2D square structure with nanovein connections was obtained with a concentration of 0.575wt.%. The size of nanoveins is about 60nm, while the period of structure is 3μm. The optimum concentration of HNu470 photoinitiator to obtain a reliable and repeatable nanostructure is about 0.6wt.%. Further increase of photoinitiator concentration of photoresist resulted in the same results as those obtained in pure SU8 presented in section 3.
By fixing the concentration of the HNu470 photoinitiator at, for example, 0.6wt.% and choosing the appropriate exposure dosage, we could also obtain periodic nanovein structures. Figure 4 shows SEM images of three different 2D periodic square lattice structures obtained with different dosages. 2D square structure with nanovein connections was obtained, when the dosage of each exposure was about 1314mJ/cm2. Higher dosage would induce 2D structures with large size connections (not shown) as those obtained in pure SU8 photoresist.
Fig. 3. SEM images of 2D periodic square lattice structures obtained with a double-exposure of a two-beam interference pattern at 0° and 90° into a pure SU8 negative photoresist with low photoinitiator concentration. The exposure source used was an Argon laser emitting at 514nm. The exposure dosage was fixed at 1273.2mJ/cm2 for each exposure, while the concentration of the HNu470 photoinitiator was changed. 2D square structure with nanovein connections was obtained with a concentration of 0.575wt.%.
Either by changing the photoinitiation concentration or exposure dosage, a new kind of 2D periodic structure, i.e., square lattice structure constituted by square rods connecting with very narrow veins, was obtained by HL technique for the first time. This structure is totally different to that obtained with pure SU8, and does not agree with the theoretical prediction as shown in iso-intensity distribution graph; the fabricated structure is not a duplication of the interference pattern.
Fig. 4. SEM images of 2D periodic square structures obtained with a double-exposure at 0° and 90° of a two-beam interference pattern into a pure SU8 negative photoresist with low photoinitiator concentration. The concentration of the HNu470 photoinitiator was fixed at 0.6wt.% while the exposure dosage for each exposure was changed from 526 to 1314 mJ/cm2. 2D square structure with nanovein connections was obtained when the dosage of each exposure was about 1314mJ/cm2.
In next section we will introduce another way to fabricate the same kind of nanostructure by using a mixing of a negative and a positive photoresists. The adding of positive photoresist can reduce the concentration of negative photoresist and thus slow its polymerization speed.
5. Fabrication of 2D nanovein structures using a mixing of negative and positive photoresists
We mixed S1818, a kind of positive photoresist (Shipley), with SU8 negative photoresist. The fabrication process of using this kind of mixed photoresist is similar to that of pure SU8. We exposed the sample twice, at 0° and 90°, with a two-beam interference pattern (He-Cd laser at 325nm). After exposure, the sample was PEB and then developed by Acetone. The developer washed out all S1818 monomers and polymers, and SU8 unpolymerized monomers, and the final result was a solid SU8 structure. The fabricated structures obtained with different exposure dosage are shown in Fig. 5. We first found that the fabricated structures were solid and smooth, just as those obtained by pure SU8 resist. Second, the exposure dosage required for obtaining solid structure was much higher than that of pure SU8. Indeed, depending on the volume ratio (VR = 2, 1, 0.5, etc.) between SU8 and S1818, the exposure dosage needed could be varied from ten seconds to few minutes. VR = 2 was found to be the optimized ratio to achieve nanovein structures. Further increase of VR would result in a structure similar to that obtained in pure SU8 photoresist; on the other hand, random nanostructures were created for VR < 2 because the concentration of SU8 photoresist was too low to form correct structures. The increase of the minimum required exposure dosage for solid structure may be due to the decrease of concentration of SU8 monomers and photoinitiator. However, the polymerization processes in the mixing photoresists of S1818 and SU8 are complicated and the formation mechanism of structures is still unclear. In this work, we focus only on the characteristics of the fabricated structures but not on the formation mechanism of these structures. The results shown in Figs. 5(a-f) were obtained, when the dosage of each exposure was set for 5.66, 7.06, 7.78, 4.24, 8.48, and 9.90mJ/cm2, respectively. With 5.66mJ/cm2 and 9.90 mJ/cm2 (or larger exposure dosage), cylinders and air-holes structures were obtained, respectively, just as those obtained with pure SU8. However, when the exposure dosage was about 7.78mJ/cm2, new square lattice structures constituted by square rods connecting by very narrow veins were obtained (Fig. 5(c-e)). The structures are totally different to that obtained in pure SU8, and do not agree with the theoretical prediction on iso-intensity distribution. This result could be due to low concentration of photoinitiator and SU8 monomer. The narrow veins were resulted from high irradiation dosage of each individual exposure (1D) and the square rods were due to high irradiation dosage of the combination two exposures (cross-points of two 1D structures). Using the mixing of negative and positive photoresists, we have created a new periodic structure, i.e., a square lattice structure constituted by square rods connected by narrow veins. This novel structure should be useful for PhCs applications.
Fig. 5. SEM images of 2D periodic square structures obtained with a double-exposure at 0° and 90° of a two-beam interference pattern into a SU8/S1818 (ratio=2/1) mixed photoresist. The exposure source is a He-Cd laser emitting 325nm, and the exposure dosage for each exposure from (a) to (f) are 5.66, 7.06, 7.78, 4.24, 8.48, and 9.90 mJ/cm2, respectively. Square rods are connected with narrow veins, when the exposure dosage of one exposure is about 7.78 mJ/cm2.
Moreover, as demonstrated in Ref. 7, using double-exposure two-beam interference technique, we could also fabricate other kinds of 2D periodic structure. Indeed, with two exposures at 0o and 60o, triangle lattice structures with nanovein connections were obtained by using the mixing photoresist. Figure 6 shows SEM images of triangle lattice structures obtained with different exposure dosages. Triangle lattice structures with rectangular “atoms” connected by nanoveins were obtained with an exposure dosage about 12.44mJ/cm2 for each exposure. Such structure cannot be obtained by using pure SU8 photoresist even by using different HL fabrication parameters.
Fig. 6. SEM images of 2D periodic hexagonal structures obtained with a double-exposure at 0° and 60° of a two-beam interference pattern into a SU8/S1818 (ratio=2/1) mixed photoresist. From (a) to (c), the exposure dosages for each exposure are 10.76, 12.44, and 13.58mJ/cm2, respectively.
Note that the 2D periodic structures with nanovein connections could be very useful for opening a large complete PBG. Using 2D plane-wave expansion simulation, we found that our fabricated square lattice structures possess a wide complete PBG with a gap ratio of about 15% by assuming: (1) transferring them to a high index GaAs material (dielectric constant ε =11.8), (2) square rods with diagonal width R = 0.8a and vein diameter D = 0.04a (a is the lattice constant). Recently some theoretical works have shown similar structures allow to open a complete PBG and widen its gap width [19–21]. However,although designed structures shown in Refs. 19 and 20 can produce a complete PBG, the fabrication of those structures is still not realized and even cannot be realized by HL technique.
6. Conclusions
We investigated in detail the formation of nano periodic structures by using negative photoresist with low concentration of photoinitator. Nanolines can be fabricated periodically into a negative SU8 photoresist by using a holographic guiding. The nanoline size is smaller than 100 nm for a length of 3μm. 2D periodic structures with nanovein connections were created by using either SU8/HNu470 (low photoinitiator concentration) or SU8/S1818 (low photoinitiator and monomer density) mixing. The fabricated new periodic structure is constituted by square (or rectangular) “atoms” connecting by narrow veins and its shape is different to the theoretical prediction based on interference intensity distribution. We demonstrated for the first time the commonly used assumption in HL technique, that the fabricated structure is a duplication of the interference pattern, is not valid under slow polymerization speed condition. The idea of mixing a negative and a positive photoresist for fabrication of nanovein periodic structures provides a new way to fabricate nanophotonics devices.
Acknowledgments
This work is supported by the National Science Council, Taiwan, under grant Nos. NSC 95-2112-M194-014 and NSC 95-2120-M194-003. N. D. Lai acknowledges the support of postdoctoral fellowship from National Science Council, Taiwan.
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