In our study, we spun a negative photoresist layer on top of a plasmonic lens which was formed by adopting a metallic ring structure with a nano-scale width opening. We recorded the beam shape of the Bessel-like beam emitting from the plasmonic lens which formed a high aspect ratio structure. We found that the high aspect ratio structure was higher after exposure as the inner and outer diameter had increased. In addition, we used an oblique incidence on the negative resist metallic ring structure to produce an inclined micro-structure. Different exposure results were obtained with the two different metal thicknesses. Therefore, in our study, we not only proved that it is possible to record the shape of a Bessel-like beam, but we also demonstrated that it is possible to create a plasmonic lens which is capable of creating a high aspect ratio structure through exposure.
©2009 Optical Society of America
When a light beam is incident on a subwavelength featured metallic structure and the momentums match, a surface plasmon (SP) will be generated. The SP can be emitted to far field through interaction with the subwavelength featured surface [1–4]. For the case of a metallic coaxial waveguide, cylindrical surface plasmon (CSP) will be present to enhance the optical transmission . Previous research [6–8] has confirmed that a subwavelength annular aperture structure in a metallic film excites and focuses the electromagnetic energy of a SP to a near-field region due to constructed interference. The silver subwavelength annular aperture structures can manipulate the transmission light at the metal surface . Furthermore, a coaxial waveguide also emits a light beam with a small focal spot and ultra-long depth-of-focus (DOF) in far-field . This phenomenon matches known Bessel beam behavior. A Bessel-like beam can be used in photolithography as its long working distance allows us to develop a photolithography machine which can operate without precision control along the beam propagating direction.
In recent years, the generation of a Bessel beam has been widely studied and has gained importance in applications such as optical manipulation  and laser machining . Typically, a Bessel beam can be generated by using a large ring and a convex lens. However, the potential to use a Bessel beam to fabricate a high aspect ratio (HAR) structure has not been explored. The mode of optical waves in a coaxial waveguide is TEm1 mode . Using a ray model to study the behavior of a waveguide  led to the observation that an optical incident ray can be reflected in a normal and radial direction of a coaxial waveguide. For an incident ray at a higher angle, a CSP can be produced. In this paper, we investigated the possibility of using the above-mentioned plasmonic lens to fabricate a HAR structure. Our results show that as the metal film thickness increases, the ratio of a higher diffraction order to total transmission light also increases. This can be clearly seen by looking at the HAR structure formed within the negative photoresist. Our result is consistent with previously reported experimental results which found that light beam transmission efficiency decreases as the metal film thickness increases .
Much previous research has been done on using an optical microscope to observe the behavior of Bessel beams. However, this method only captures a cross-sectional picture of a Bessel beam. In this paper, we use negative photoresist to record the entire shape of a Bessel-like beam generated by a metallic ring structure. We also demonstrate that this kind of nondiffracting beam can be used to fabricate a HAR structure. We can also show that the oblique incident light will produce a beam of an inclined shape which demonstrates that a Bessel-like beam can be used to manipulate the subwavelength structure shape generated on a negative photoresist.
2. Fabrication process
The process of using the above-mentioned plasmonic lens to fabricate a HAR structure can be seen in Fig. 1. First, we deposited silver film onto the glass substrate using a DC sputtering machine. We then used a focused ion beam (FIB) to fabricate a metallic ring structure with a different inner and outer diameter onto the substrate directly. A commercially available negative resist (SU-8 2100, MicroChem Corp.) was spun-coated onto the silver film at 700rpm for 15 seconds then at 4200rpm for 35 seconds. It was followed by a soft bake at 65 deg C and then at 95 deg C for 5 and 20 minutes, respectively. The result was an 85μm thickness resist layer. Next, we used a linearly polarized light emitted from a solid state laser (λ = 406nm) as the exposure light source. The 33mW light source was incident to the back side (glass substrate) of the metallic ring structure along either the normal or the oblique direction. After exposure, the sample underwent a post exposure bake at 65 deg C for 5 minutes and then at 95 deg C for 10 minutes. In order to obtain a clear HAR structure and to strip the uncured photoresist, the sample was developed using a SU-8 developer for over two hours. The HAR structure then underwent a capillary force phenomenon when the structure was rinsed in the liquid . More specifically, the HAR structure often collapsed after the development process due to the capillary force. To circumvent this problem, we removed the sample while it still contained some developing solution immediately after the development process. We then set the still wet sample into a methanol solution which gradually allowed the methanol to replace the SU-8 developing solution. Finally, we placed the sample into an oven at 120 deg C to gradually evaporate the methanol. We found the surface tension between the methanol and the HAR structure was significantly reduced during the evaporation process.
3. Experimental results and discussions
Figure 2 shows a scanning electron micrograph of the HAR structure. We fabricated rings with different inner and outer diameters on the same substrate. The rings were then used to fabricate the HAR structures. The varying inner diameter rings were from 12μm to 4μm with an interval of 2μm. All rings were characterized by a 150nm opening and a silver thickness of 140nm. The results clearly demonstrate that the plasmonic lens can be used to fabricate HAR structures. From Fig. 2, we show for the first time, the entire shape of a Bessel-like beam and it appears it possesses a long DOF. The height and size of the apex of the HAR structure were 42μm and 850nm respectively (see Fig. 2(d)). The resolution and the aspect ratio achieved from this HAR structure exceeded the specifications of the SU-8 developer even when the exposure wavelength was near the UV region . It means that the emitted light from a metallic ring possesses additional excellent optical properties. We also found that the height of the HAR structure increases as the inner and outer diameter increases. The results match our previously reported results  which found that the light emitted from a metallic ring possesses different focal lengths and DOF in free space for different inner and outer diameters. The principle behind this is that the diffraction angles of the emitted light from different rings will remain identical if the opening width of all the rings remains unchanged. As the inner and outer diameters of ring increases, the surrounding region used to form the emitted light will become larger. It thus leads to a different focal length and different DOF. In other words, the optical properties of Bessel-like beams can be tuned by changing its inner and outer diameters.
From Fig. 2, we see that the height of HAR structure is larger than our previously reported results . The reason lies in the fact that the light emitting from a ring will be focused at the certain position away from the metal surface. Furthermore, the propagation length  of a SP at the surface for a 406nm incident wavelength will be very short (1.4μm) and cannot provide enough surface energy to cure the negative photoresist when the exposure time is short. As such, only the photoresist area located farthest away from the metal surface will be cured. During the development process, the HAR structure, developed by the metallic ring plasmonic lens will collapse since no substrate exists to support it. To obtain the results shown in Fig. 2, we must over-expose the sample such that a small amount of leaked energy near the surface can accumulate to cure the photoresist. This approach can guarantee that the HAR structure fabricated can stand up well on the metal surface and as a result extend the height of the HAR structure. It should be noted that even though we created the above-mentioned over-exposure process to make sure the HAR structure developed stands up well, the metallic ring plasmonic lens by itself makes a good tool to use when required to fabricate a HAR structure since a Bessel-like beam possesses a long DOF.
From Fig. 3 we can see the symmetrically inclined microstructure. The dimensions of the metallic ring structures were identical to that of those in Fig. 2. We set the oblique incident angle to be 33 deg and then rotated the sample for a double exposure. We obtained a V-shaped micro-structure. It means that the emitted light from the metallic ring structure propagated along the inclined direction as the incident light shone on the ring structure obliquely. As a result of the double exposure, the bottom of the HAR structure became broader. Although the inclined angle of the structure (21.5 deg) obtained was not the same as the obtained oblique incident angle (33 deg), it can be explained by using Snell’s Law. As the light incident onto the surface was from free space, the transmission light at the other side was refracted and the refracted angle can be estimated using Snell’s Law where n1sin (θ1) = n2sin (θ2). More specifically, the inclined angle of the HAR structure can also be obtained by Snell’s Law. Since the refraction index of the SU-8 2100 was 1.62, the inclined angle of the HAR structure was calculated to be around 20 deg. It matched well with our experimental results.
To observe how the emitted light of higher diffraction influences exposure results, we compared it to the case of a ring structure with a thicker metal thickness. We fabricated a 210nm thick metal film. Figure 4 shows the experimental results. From Fig. 4(a) to Fig. 4(c), it clearly demonstrates that for different exposure times, there will be different HAR structures. In our experimental set-up we also found that a HAR structure can be created by using less exposure time which can collapse the structure (see Fig. 4(a) and Fig. 4(b)). Figure 4(c) shows that a structure can stand well on the surface with more exposure time.
From Fig. (2), we know that an exposure time of 13 minutes will be enough time to allow the HAR structure to stand well on the surface of a metal thickness of around 140nm. The thin metal thickness allows the light to pass through the plasmonics structure easily. By comparing Fig. 4(c) and Fig. 2(a), it appears that both structures stand well on the surface. It means that the curing energy created by both the plasmonic lenses was enough. Nevertheless, it should be noted that their exposure shapes are different even though the inner and outer diameters of the rings were identical. More specifically, it seems that the distribution of the emitted light from the metallic ring was not the same for the two different metal film thicknesses. We used FDTD to simulate the distribution of the emitted light from the ring at different metal thicknesses. In Fig. 5, we found that the intensity of the emitted light at far-field decayed gradually as the metal thickness increased. Comparing Fig. 5(b) with Fig. 5(c), we found that the emitted light from the ring on the thin metal film had good focusing properties and which possessed a good focus contrast. In Fig. 5(c), the distribution of the emitted light possessed a higher diffraction order. The exposure results of a thick metal (210nm thickness) showed some interference features when compared to that of a thin metal (140nm thickness) due to the fact that the ratio of higher diffraction order to total intensity became larger. The exposure results (Fig. 2(f) and Fig. 4(d)) are consistent with the simulation results (Fig. 5(b) and Fig. 5(c)).
As the metal film thickness increased, the ratio of a higher diffraction order versus the total transmitted light also increased. Therefore, a micro-structure fabricated by using metallic ring structure with a thicker metal thickness will have interference features as the emitted light of higher diffraction orders around the structure (Fig. 4(d) and Fig. 4(e)). This shows that the emitted light from the ring on the thick metal film has a worse focus contrast. Also, the total emitted light from the ring structure with thicker metal film and side lobes is not a good characteristic feature when converting plasmonic lens to a photolithography application. From Fig. 4(d) and Fig. 4(e), it appears that we can obtain the shape of a Bessel-like beam with interference features. The main beam of a Bessel-like beam can be attributed to two emitted light with constructed interference. In addition, the location of the curing photoresist by the two emitted light corresponds to a polarization direction of the incident light. The shape of Bessel-like beam cannot be easily captured using an optical microscope individually, but we can obtain information such as it entire shape.
In conclusion, we used a metallic ring structure to successfully fabricate a HAR structure and then observed the propagation process of the Bessel-like beam in a negative photoresist. We found that the exposure results can be altered as the metal film thickness of the metallic ring structure changes. As the metal thickness decreases, a HAR structure can be obtained by adopting a metallic ring structure. Such HAR structures can be useful for applications such as microfluidic systems to mix fluids, to taper the waveguide of plasmonics [19,20] and as tips of future atomic force microscopes.
This work was supported by the Materials & Chemical Research Laboratory of the Industrial Technology Research Institute (ITRI) and Taiwan’s National Science Council under Grant No. 97-2221-E-002-159-MY3.
References and links
1. H. J. Lezec, A. Degiron, E. Devaux, R. A. Linke, L. Martin-Moreno, F. J. Garcia-Vidal, and T. W. Ebbesen, “Beaming light from a subwavelength aperture,” Science 297(5582), 820–822 (2002). [CrossRef] [PubMed]
2. L. Martín-Moreno, F. J. García-Vidal, H. J. Lezec, A. Degiron, and T. W. Ebbesen, “Theory of highly directional emission from a single subwavelength aperture surrounded by surface corrugations,” Phys. Rev. Lett. 90(16), 167401 (2003). [CrossRef] [PubMed]
3. C. K. Chang, D. Z. Lin, Y. C. Chang, M. W. Lin, J. T. Yeh, J. M. Liu, C. S. Yeh, and C. K. Lee, “Enhancing intensity of emitted light from a ring by incorporating a circular groove,” Opt. Express 15(23), 15029–15034 (2007), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-15-23-15029. [CrossRef] [PubMed]
4. D. Z. Lin, C. K. Chang, Y. C. Chen, D. L. Yang, M. W. Lin, J. T. Yeh, J. M. Liu, C. H. Kuan, C. S. Yeh, and C. K. Lee, “Beaming light from a subwavelength metal slit surrounded by dielectric surface gratings,” Opt. Express 14(8), 3503–3511 (2006), http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-14-8-3503. [CrossRef] [PubMed]
5. Y. Poujet, J. Salvi, and F. I. Baida, “90% Extraordinary optical transmission in the visible range through annular aperture metallic arrays,” Opt. Lett. 32(20), 2942–2944 (2007). [CrossRef] [PubMed]
7. J. M. Steele, Z. Liu, Y. Wang, and X. Zhang, “Resonant and non-resonant generation and focusing of surface plasmons with circular gratings,” Opt. Express 14(12), 5664–5670 (2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-12-5664. [CrossRef] [PubMed]
8. C. K. Chang, D. Z. Lin, C. S. Yeh, C. K. Lee, Y. C. Chang, M. W. Lin, J. T. Yeh, and J. M. Liu, “Experimental analysis of surface plasmon behavior in metallic circular slits,” Appl. Phys. Lett. 90(6), 061113 (2007). [CrossRef]
9. Z. Liu, J. M. Steele, H. Lee, and X. Zhang, “Tuning the focus of a plasmonic lens by the incident angle,” Appl. Phys. Lett. 88(17), 171108 (2006). [CrossRef]
10. D. Z. Lin, Z. H. Chen, C. K. Chang, T. D. Cheng, C. S. Yeh, and C. K. Lee, “Subwavelength nondiffraction beam generated by a plasmonic lens,” Appl. Phys. Lett. 92(23), 233106 (2008). [CrossRef]
12. Y. Matsuoka, Y. Kizuka, and T. Inoue, “The characteristics of laser micro drilling using a Bessel beam,” Appl. Phys., A Mater. Sci. Process. 84(4), 423–430 (2006). [CrossRef]
13. F. I. Baida, A. Belkhir, and D. V. Labeke, “Subwavelength metallic coaxial waveguides in the optical range: Role of the plasmonic modes,” Phys. Rev. B 74(20), 205419 (2006). [CrossRef]
14. A. W. Synder and J. D. Love, Optical Waveguide Theory (Chapman & Hall, 1995), Chap. 2.
15. M. I. Haftel, C. Schlockermann, and G. Blumberg, “Role of cylindrical surface plasmons in enhanced transmission,” Appl. Phys. Lett . 88(19), 193104 (2006). [CrossRef]
16. A. D. Campo and C. Greiner, “SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography,” J. Micromech. Microeng. 17(6), R81–R95 (2007). [CrossRef]
17. Y. Y. Yu, D. Z. Lin, L. S. Huang, and C. K. Lee, “Effect of subwavelength annular aperture diameter on the nondiffracting region of generated Bessel beams,” Opt. Express 17(4), 2707–2713 (2009), http://www.opticsinfobase.org/abstract.cfm?uri=oe-17-4-2707. [CrossRef] [PubMed]
18. W. L. Barnes, “Surface plasmon-polariton length scales: a route to sub-wavelength optics,” J. Opt. A, Pure Appl. Opt. 8(4), S87–S93 (2006). [CrossRef]
20. E. Verhagen, A. Polman, and L. K. Kuipers, “Nanofocusing in laterally tapered plasmonic waveguides,” Opt. Express 16(1), 45–57 (2008). http://www.opticsinfobase.org/oe/abstract.cfm?uri=OE-16-1-45. [CrossRef] [PubMed]