Fabrication of high aspect ratio subwavelength gratings based on X-ray lithography and electron beam lithography
Highlights
► Two kinds of high aspect ratio subwavelength gratings are manufactured. ► These gratings can be used as mold for nano-imprinting technology. ► Fast atom beam (FAB) etching is used for effective fabrication. ► A coupling layer is introduced to keep the gratings from conglutination
Introduction
So far, there are a number of reported techniques for fabrication of diffractive optical elements (DOE), yet the height of those gratings is not sufficient [1], [2], [3]. Grating is a very important optical component that has been widely used in optical measurement, integrated optics, optical information processing, sensors, lab-on-chip, filters and other areas [4], [5]. A single fiber Bragg grating, 1.5 cm in length, embedded on a cantilever, is capable of simultaneously measuring and discriminating strain and temperature over wide ranges with sensitivities of 3.38 pm/μm and 19.05 pm/°C, respectively [6]. For photodetectors, nanoscale diffraction gratings in active region can trap light, significantly modifying their optical absorption, reflectance and transmission properties and thus resulting in enhanced photoresponse [7]. A high-contrast subwavelength grating is integrated into a surface emitting laser instead of conventional Bragg-reflectors [8]. Subwavelength gratings can operate as waveguide couplers, subwavelength antireflection structures, resonance structures, gratings can be used in distributed-feedback and distributed-Bragg-reflector laser configurations, and wavelength-division multiplexing [9], [10], [11], [12], [13].
Nowadays, there is a growing need for a class of optical components integrated with diffraction gratings [14], [15]. And the grating period is required to be not more than1 μm, such as X-ray transmission gratings (TGs) [16]. Subwavelength phenomenon has been exploited for many years in free-space optics. A broad-band mirror with reflectivity more than 98.5% has been achieved by Mateus et al. [17]. And recently, it has been found that surface plasmons can concentrate and channel light with subwavelength grating, leading to miniaturized photonic circuits [18].
Section snippets
Nano-manufacturing of subwavelength gratings
These years great interest is focused on laser based nano-manufacturing technologies [19]. Scanning Near-field Photolithography (SNP) using laser coupled Near-field Scanning Optical Microscopy (NSOM) can achieve 20–55 nm resolution in surface patterning and the laser etched depth is 20–100 nm [20], [21]. Immersion laser interference lithography is developed at Max-Planck Institute of Microstructure Physics, Germany [22]. Line patterns with a period less than 100 nm and width of 45 nm are
Experimental work
A schematic of the X-ray lithography fabrication process is shown in Fig. 1. It starts from the coupling layer which is the interlayer between PMMA and Si substrate, spin-coated to the silicon substrate, 300 nm in thickness (Fig. 1(b)). Since, the coupling layer is very important, we tried several kinds of interlayer materials, and NUC APZ6633 has the best performance. It can keep gratings from conglutination. Secondly, PMMA is spin-coated to the interlayer, 2300 nm in thickness (Fig. 1(c)). And
Results and analysis
The SEM micrograph of PMMA nano-gratings obtained by the processes is shown in Fig. 4; the inset is a magnified view of the gratings. It's easy to see that the side walls are very flat and straight. The structure is line gratings, 250 nm in width, 2000 nm in depth, and 500 nm in period. It is area is 2 mm2, and aspect ratio is about 8, which has not been achieved before. A schematic of the test equipment is shown in Fig. 5. After the TM light beam is diffracted by the sample, it is diffracted into
Conclusions
In summary, two subwavelength grating's fabrication methods are successfully developed. One subwavelength grating is directly fabricated on PMMA photoresist using X-ray lithography, obtaining a high aspect ratio which is approximately 8. Especially, the introduction of interlayer eliminates the phenomenon of conglutination. The other one is prepared using EBL and FAB techniques on Si (100) substrates. Their uniformities of grating pitches and shapes are confirmed. Both of them can be directly
Acknowledgment
The authors acknowledge financial support by “Shanghai Pujiang Talent Plan” Sponsorship (no. 09PJ1406200) and International Cooperation Project from Ministry of Science and Technology of China (2009DFB10330).
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