Nanopillar array with a λ/11 diameter fabricated by a kind of visible CW laser direct lithography system
© Zhang et al.; licensee Springer. 2013
Received: 12 April 2013
Accepted: 1 June 2013
Published: 11 June 2013
Nanoscale functional structures are indispensable elements in many fields of modern science. In this paper, nanopillar array with a pillar diameter far smaller than Abbe's diffraction limit is realized by a new kind of continuous wave (CW) laser direct lithography technology. With atomic force microscopy technology, the average diameter of nanopillars on thin OIR906 photoresist film is about 65 nm and the smallest diameter is 48 nm, which is about 1/11 of the incident laser wavelength. Also, the influences of coma and astigmatism effects to the shape and size of nanopillar are numerically simulated by utilizing vector integral. As far as we know, it is the first time that nanopillar array is implemented by a donut-shaped 532-nm visible CW laser. The study presents a new, simple, inexpensive, and effective approach for nanopillar/pore array fabrication.
KeywordsNanopillar Diffraction limit Lithography Coma Astigmatism
Nowadays, nanoscale structures such as nanopillar and nanopore arrays are considered essential functional nanotexturizations for modern scientific research and application. Nanopillar arrays have been employed in the study of field emission, solar cell industry, biological sensing, micro-/nanoscale fluidics, near-field optics, and the lab-on-a-chip technology. Nanopore arrays have also been recognized as valuable structures in many advanced fields such as photovoltaic and photonic crystal research, gas detection, and especially in biological molecules detection and separation. Fitting with foregoing scientific advancements, the nanoscale fabricating methods and technologies have been made good progress. Nanopillar and nanopore arrays can be fabricated with direct growth approaches (metal-organic chemical vapor deposition, hydride vapor phase epitaxy, molecular beam epitaxy)[9–11], nanosphere-assist etching[12, 13], electronic beam lithography[14, 15], nanoimprint technology, and laser lithography.
Since the merits of fabricating speediness and cleanliness, maskless process, controllable pattern shape and size, and capability of lithograph in three dimensions[18, 19], laser direct lithography technology is one of the most attractive approaches to fabricate nanoscale functional structures as compared with the disadvantages such as expensive, heavy, or low precision of other methods. Choi's group has reported implementing 100-nm-level nanostructure arrays over a large scale by means of laser interference lithography[20–23]. Scott and Li have respectively fabricated sub-100-nm isotropic voxel and voxel with a 40-nm axial size by photo-initiation inhibiting technology. Cao has obtained a nanoline with a width of 130 nm and nanodots with a diameter of 40 nm by polymerization inhibiting, too. In Andrew's work, the nanolines with an average width of 36 nm were drawn employing absorbance modulation lithography. Tanaka and Thiel have shown fabricating spatial voxel to sub-120 nm with the two-photo-absorption technology[28, 29]. Qi got a single polymerized tip with a diameter of 120 nm with the same technical route.
However, the utilization of femtosecond laser systems makes the lithography system complex and expensive. Even, in a continuous wave (CW) laser two-photon absorption method, photoresist is tailored and the whole system is costly. Furthermore, two laser sources are required in both photo-inhibiting and absorbance modulation methods, and the photoresist materials should have particular properties that result in restrictions in choosing light sources and resist materials.
In the paper, we will report a kind of nanopillar array with a pillar diameter much smaller than Abbe's diffraction limitation by visible CW laser direct lithography technology. A 532-nm CW laser beam, which is modified by a phase mask to generate a nanolevel dark core in the focus space, is proposed and applied in nanopillar/pore fabricating. The nanopillar array is obtained when the laser beam is irradiated to the positive tone photoresist, while nanopore will be generated with a negative tone photoresist. To the best of our knowledge, this is the first time that nanopillar arrays are fabricated with a spatial donut shape, structured visible CW laser. Experimental results are measured by AFM, and the distortion and the inconsistency of nanopatterns are analyzed with theoretical simulation. This preliminary work explores a novel, easy, and effective method of maskless CW laser direct writing technology to carry out functional nanopillar/pore arrays.
In principle, with the modulation of the vortex phase-shifting plate, the circularly polarized Gaussian beam is generated as a donut-shaped pattern on the focal plane. The dimension of the dark core of the donut-shaped pattern is smaller than the diffraction limitation. During the experiment, the photoresist at the center of the pattern will not be exposed because of the null intensity point. Since the positive tone photoresist was applied in this work, a kind of nanopillar structure, whose diameter is far below diffraction limitation, could be obtained in the center of the donut-shaped pattern with appropriate input laser power.
The procedure of experiment is composed of the steps of spin coating, preexposure baking, exposing, post-exposure baking, developing, and hard baking in sequence. The obtained nanostructures are measured, characterized, and analyzed with an atomic force microscopy (AFM, Veeco Dimension 3100 AFM system, Veeco Instruments Inc., Plainview, NY, USA). To obtain the nanopatterns with high precision and consistency, the focal sphere should be accurately focused onto the surface of the photoresist. Furthermore, the motion of the scanning stage is required to be synchronized with laser exposure for fast fabricating nanopatterns.
Results and discussion
Comparing the experimental pillars in Figure 2 with the laser spot shown in Figure 1b, as well as in Figure 3, it seems that the nanopillars' location deviated a little from the center of the donut-shaped beam. Meanwhile, the entire donut-shaped pattern seems changed to an elliptical shape rather than a cylindrical donut shape. In order to fabricate large area-distributed nanopillar/pore array with high consistency with the system, the reasons of the nanoscale patterns transformed are systematical analyzed.
It is well known that the transformation of donut-shaped patterns might be caused by the laser quality, the photoresist surface roughness, the optical system errors, or laboratory personnel operational interferences. However, this phenomenon should not be caused by the laser beam quality because the laser focal spot has a symmetric donut shape on the focal plane which is shown in Figure 1b. Otherwise, the surface roughness should not be the issue that can be clarified in Figure 2c in which the coating photoresist surface is flat. During lithography, the laser beam is well aligned to expose the resist vertically; thus, shape deformation is not caused by a tilt photoresist wafer.
Besides the factors mentioned above, optical system errors can affect laser distribution. Spherical aberration, coma, and astigmatism are three primary factors of optical system errors. In general, the focal spot cannot be transformed to an irregular shape under the influence of spherical aberration. On the contrary, coma may cause one-directional deformation of the focal spot, while astigmatism can split the laser spot into two parts. There are two more factors: one is that this kind of laser lithography system is not sensitive to the influence of the spherical aberration; another is that the objective is designed as an aplanatic lens which eliminates the spherical aberration of the objective. Taking these factors into account, theoretical analysis and numerical calculation will be focused on the influences of coma and astigmatism effect. Aberration influence theory of the focal donut spot is described in the Appendix.
It makes sense to compare the results of the experiments and simulations. Their resemblances are easily found out. First, the calculated results shown in Figure 4a, b, c have similar patterns with those experimental patterns imaged in Figure 4a, b, c, respectively. The donut-shaped focal spot is a semilunar appearance in both experiment and simulation. Next, the gradual transformation of nanopillars in the experiment has the same variation tendency with the dark spots in the numerical simulation. Figure 4d, e, f illustrates the asymmetric intensity distribution on the yz plane; they explain the reasons why the two sides of the nanopillars are ruptured with different depths. Furthermore, Figure 4g, h, i has shown that the depletion of light intensity increased with the increased A c , which correctly reflects the variation of depths at the two sides of the nanopillars in Figure 4d, e, f. Thus, coma effect is the main influence factor which results in nonideal nanopillar patterns in Figures 2 and3.
We deem that the influence of coma effect caused by the ×100/1.4 objective lens is insignificant since this type of objective is aplanatic which dispels coma influence of the objective. Also, the focal spot has a well-defined symmetric shape before patterning the photoresist as is displayed in Figure 1b. In addition, the extents of coma effect, which is shown in Figure 3a, b, c, are different under the same experimental conditions. Therefore, we consider that coma effect of the laser lithography system should be caused by mechanical disturbance. In fact, the mechanical vibration during the system working may disturb the laser beam and then induce an angle of deviation between the laser beam and objective lens.
It is also meaningful to compare the experimental results shown in Figure 6 with the simulation results in Figure 7; the pattern of the marked experimental result in Figure 6a is found very similar with the simulation result in Figure 7b with A a = 0.1. It can be seen from Figure7f that the distribution is symmetric with the origin, and the light intensity is different along x = y and x = −y. These calculated results explain the laser lithography symmetric depth on the two sides of the nanopillar shown in Figure 7d, e. The widths of the longer axis and the shorter axis of the pillar top are 83 and 47 nm, respectively, which is illustrated in Figure 7d, e.
In conclusion, combining the experimental work and the numerical simulation, it can be illustrated that the nanopillar structure could be transformed by both coma and astigmatism effects. The diameter of the nanopillar is increased and the height of the nanopillar is decreased with enhanced coma value. The shape of the nanopillar is likely to be compressed into a belt form as the astigmatism influence enhanced. In the subsequent work, the effects of coma and astigmatism of the donut-shaped laser direct writing system should be carefully dealt.
Theoretically, the resolution of this laser lithography system increases when laser intensity enhances; thus, the resolution would be extremely small. However, it cannot be that small due to optical aberration effects in the system and the material utilized in the experiment. In this work, the smallest resolution that was obtained with the photoresist OIR906 is 48 nm, which is 1/11 of the incident wavelength. It is expected that the resolution should be finer with a smaller aberration influence.
The patterning speed of the lithography system is mainly determined by factors that include the scanning speed of position stage, exposure time, and pattern complexity. In this report, it takes approximately 4 min to pattern a nanopillar array within the area of 100 × 100 μm2. Furthermore, an improved lithography system, which is being built in our laboratory, is capable to reduce the fabrication time to 1 min on the same pattern.
In addition, the size of the donut-shaped pattern is related to the wavelength of the incident beam. The beam with a shorter wavelength will generate a smaller donut-shaped pattern on the focal plane. Feature sizes can be tuned by shifting the wavelength of the laser with a fixed input power. In fact, we have quantitatively simulated how the donut-shaped patterns changed with the different wavelengths such as λ = 800 and 400 nm. The results showed that the radius of the pattern is 468 nm (at 800 nm) and 234 nm (at 400 nm).
Nanopillar array has been successfully obtained on a spin-coated thin film of OIR906 photoresist, employing a kind of novel visible CW laser direct lithography system. The diameter of the fabricated nanopillar was able to be as small as 48 nm, which is 1/11 of the wavelength of the incident laser. The lithographic nanopatterns were calibrated and analyzed with AFM. Shape influences of the coma effect and astigmatism effect were simultaneously analyzed using vector integral. The simulation results explain the distortion and inconsistency of the fabricated nanopatterns well. The work has demonstrated a simple, efficient, and low-cost method of fabricating nanopillars. It could pave a new way to fabricate nanopillars/pore arrays of large area distribution for optical nanoelements and biophotonic sensors while integrated with high-speed scanning system.
Aberration theory about high NA objective
The amplitude of the Gaussian beam at the input plane is expressed as in Equation 2:
A c and A a are coefficients for coma and astigmatism, respectively. Both A c and A a multiply λ, representing the departure of the wavefront at the periphery of the exit pupil. The values for λ, n, NA and θmax adopted in simulation correspond to the practical values in the experiment. Refractive index of oil n = 1.52; γ is supposed to be 1, which means that the objective is fulfilled by the Gaussian beam.
The electrical distributions for the donut-shaped pattern affected by aberrations are carried out using Matlab software.
CZ is a Ph.D. candidate of the Institute of Photonics and Photo-technology, Northwest University, Xi'an, China, with a research direction that is concerned on laser technology and application. KW is a professor of the Institute of Photonics and Photo-technology, Northwest University, Xi'an, China. His research direction focuses on nanotechnology, nanobiophotonics, and soft matter physics. JB is a professor of the Institute of Photonics and Photo-technology, Northwest University, Xi'an, China. His main research areas are all-solid-state laser, laser devices and laser technology. SW is a lecturer of the Institute of Photonics and Photo-technology, Northwest University, Xi'an, China. His study concentrates on biophotonics and biomedical optics. WZ is a Ph.D. candidate of the Department of Mechanical Engineering, University of South Carolina, Columbia, USA. His research topics are related to applied optics and fluid dynamics. FY is a postdoc in the Department of Mechanical Engineering, University of South Carolina, Columbia, USA. He works on high resolution microscopy system and MEMS. CG is a researcher of Institute of Physics, Chinese Academy of Sciences, Beijing, China. He works in the fields of nanostructure and nanodevices. GW is an associate professor at the Department of Mechanical Engineering and is interested in nanotechnology, bioMEMS, and lab-on-chip.
Atomic force microscopy
This work was supported by the Major Research Plan of the Natural Science Foundation of China (91123030) and the International Science and Technology Cooperation Program of China (2011DFA12220).
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