Design and Fabrication of Highly Selective Polarizers Using Metallic–Dielectric Gratings

Abstract

Polarization imaging has been proven as an important technique for obtaining multi-dimensional information in complex environments. As the prevalent polarizers, metal gratings are widely used especially for focal-plane detection due to their flexibility and easy integration. However, high-performance polarization gratings with high transmittance and large extinction ratios typically need a large aspect ratio in design, resulting in more difficulties in fabrication with limited practical performances. In this study, we designed and fabricated a high-performance polarizer using metallic–dielectric gratings (MDGs). Through a single CMOS-compatible procedure that included electron-beam lithography (EBL) and a collimated thermal evaporation deposition process, we achieved a high TM transmittance (~90%) and a high extinction ratio (~100:1) in the experiment. We believe that our work provides an effective approach to high-performance polarization gratings, which could contribute to the development of on-chip integrated polarization imaging.

1. Introduction

Polarization detection could provide multi-dimensional information by obtaining polarization information in different directions at each pixel [1] and has been widely used in infrared imaging and remote sensing, medical imaging, etc. [2,3,4,5,6,7,8,9,10]. Perhaps the widest way to highly select the polarization state of electromagnetic waves is to use the metal polarization gratings, which can be not only featured with high polarization performance but also flexible and easy to integrate onto a chip. For example, the focal-plane polarization detector [4] has been widely applied in the field of polarization imaging due to its miniaturization and ability to flexibly detect multiple polarization directions. The key to this is getting high performance polarization gratings with high transmittance and large extinction ratios.

There have been considerable findings on metal polarization gratings in the visible [4,11,12,13,14] and infrared [15,16,17,18] regions. Typically, a higher extinction ratio requires a greater height of the gratings [14]. Previous studies have achieved an excellent polarization performance of up to 80% TM transmittance and more than a 1000:1 extinction ratio in design [18]. However, the most prepared results present seriously degraded performance with merely ~50% transmittance and a less than 20:1 extinction ratio. This is because the metal gratings are typically fabricated through a standard lift-off lithography manufacturing process [19,20]. If metal is deposited on a photoresist grating and then the photoresist is removed, metal gratings in the blank area can remain. However, on the one hand, the lift-off process for the fabrication of large area metal gratings is unstable and inefficient, i.e., the uncertain residue of metal or photoresist may break the configuration; on the other hand, the thickness of resist gratings must be several times greater than that of the deposited metal to ensure the resist stripping, which will greatly increase the difficulty of lithography. However, although there are other less-common methods [18,21,22,23,24,25,26,27,28,29] for manufacturing the metal gratings, for example, nanoimprint [14,21,23,24,25] and chlorine etching [26], they are not widely used because of poor manufacturing results or the toxicity of chlorine. Until now, there has still been a lack of effective manufacturing methodology for the fabrication of high-performance polarization gratings.

In this study, we demonstrated a new design and fabrication process for the high-performance polarization gratings by using metallic–dielectric gratings prepared via a standard electron-beam lithography without lift-off or reactive etching. The proposed polarization grating was composed of Al and SiO₂ and designed according to effective medium theory to modulate the TM/TE transmittance for a better polarization performance in the infrared band. The metallic–dielectric grating (MDG) was fabricated on a Si substrate by electron-beam lithography (EBL) for the dielectric grating and thermal evaporation deposition technology for the metal layer. The temperature of the developing solution and the deposition angle were optimized to improve the perpendicularity and aspect ratio of the grating and reduced the presence of undesirable metal on its side walls. The fabricated gratings showed a measured result of 90% TM transmittance and a 100:1 extinction ratio, which is, to our knowledge, the highest performance so far in the reported literature and products.

2. Theoretical Design and Assessment

The focal-plane type device obtains polarization signals in different regions and collects them into one picture by integrating different polarization gratings. This means that obtaining high-performance polarization gratings is critical to polarization imaging. Meanwhile, previous studies have shown that MDG is a good solution for high-performance polarization gratings [30,31,32,33,34,35]. For this reason, we designed an MDG in the infrared band, for which TM light (the electric field is perpendicular to the grating lines) was directly transmitted, whereas TE light (the electric field is parallel to the grating lines) was fully reflected to achieve a high polarization performance.

According to effective medium theory, a sub-wavelength grating shows different refractive indexes for different polarizations [28,36,37,38,39]. Furthermore, the effective refractive index can be calculated by the following formula:

$$\begin{array}{c}{n}_{TE}={\left[f{\left(n+ik\right)}^2+\left(1-f\right)n_i^2\right]}^{\frac{1}{2}}\end{array}$$

(1)

$$\begin{array}{c}{n}_{TM}={\left[f{\left(n+ik\right)}^{-2}+\left(1-f\right)n_i^{-2}\right]}^{-\frac{1}{2}}\end{array}$$

(2)

Based on Equations (1) and (2), we calculated the effective refractive indexes for gratings made from several common metals—as per the Handbook of Optical Constants of Solids written by Palik—with a duty cycle of 0.5 and 150 nm period and 150 nm thickness, as shown in Figure 1a,b. In general, they all showed the same results: for TE, the imaginary part of the refractive index is very large, similar to a metal film, reflecting incident light; for TM, the imaginary part of the refractive index is small, similar to a dielectric film, transmitting incident light. Therefore, the sub-wavelength metal grating is the basal polarization grating. Moreover, we evaluated the polarization performance of the metal grating as shown in Figure 1c,d: the TM transmittance is about 75%, and the extinction ratio is about 10³, except that the aluminum showed an extinction ratio of 10⁴. Obviously, aluminum grating has the best polarization performance among several gratings, which also showed in the previous refractive index diagram: the imaginary part of the refractive index of aluminum grating is smaller under TM and larger under TE. However, even for the aluminum grating with the best polarization performance, the TM transmittance is not perfect with producible height, such as 150 nm. In order to improve the optical performance, we decided to use a grating layer and dielectric layer alternately to form multilayer films. Because the grating layer shows different refractive indexes in different polarization directions, we varied the thickness of different layers to modulate the TM/TE transmittance, thus enhancing the polarizations performance. According to these calculations, aluminum was chosen for our design. To achieve higher polarization performance, the thickness of the metal layer was fixed at 50 nm to ensure that the reflectivity of TE was high enough, and we optimized the thickness of the medium layers to achieve the highest TM transmittance with particle swarm optimization (PSO) of the following parameters: the learning factor was 2, the number of particles was 20, and the generation was 500. Finally, an MDG consisting of a 100 nm SiO₂, 50 nm Al/SiO₂ grating with a 150 nm period and a 0.5 duty cycle, 200 nm SiO₂, and 50 nm Al grating was designed as shown in Figure 2a, where the refractive index of silicon dioxide is 1.45 and that of silicon, which was used as the base, is 3.5. Furthermore, we simulated the MDG with a rigorous coupled wave analysis (RCWA) method by the MATLAB program written by Pavel Kwiecien, Czech Technical University in Prague, Optical Physics Group, Czech Republic. The simulation was carried out in one dimension, and the harmonic number was set to 119 to ensure the convergence of the results. The simulated results are shown in Figure 2b,c. For a plane wave with vertical incidence, the TM transmittance was more than 90%, and the extinction ratio was above 10,000, which is far better than that of a producible conventional metal grating.

Considering possible defects that might occur in actual fabrication, the duty cycle can be precisely controlled by adjusting the exposure parameters, but the metal deposition may have adhesion problems if the line width is too small. Thus, we should conduct error simulation and analysis. We simulated the TM transmittance with metals of different thicknesses covering the side walls of the grating. Figure 2d shows the calculated TM transmittance for different thicknesses. According to our calculations, the TM transmittance was reduced to 60% with just 2 nm of metal covering and was reduced more as more metal covering was added. In addition, we also calculated the magnetic field intensity distribution map of TM for MDG without or with side wall metal covering, as shown in Figure 2e,f at 1.515 μm, where the side wall metal thickness is 6 nm. The side wall metal has greatly changed the transmission mode of TM. In addition, we also calculated the absorption after adding side wall metal, which is about 20%. After the side wall metal was added, both reflection and absorption for TM light rose, which would lead to a significant reduction in extinction ratio. It is obvious that avoiding depositing melt to the side walls is necessary to get a high TM transmittance and extinction ratio, and that is exactly what we will do.

3. Fabrication Process and Results

To fabricate the MDG, a SiO₂ grating was created, and metal was deposited on it. However, the main fabrication process for a SiO₂ structure is etching SiO₂, which is quite difficult, making it necessary to find a tractable substitute. Considering that optical properties are the main factors affecting polarization performance, we found two photoresists which have similar optical properties to SiO₂: polymethyl methacrylate (PMMA) and hydrogen silsesquioxane polymers (HSQ). In addition, we analyzed their mechanical performance, we found that PMMA was so soft that it collapsed when fabricating gratings with a large aspect ratio. As for HSQ, it converts to SiO₂ after electron-beam exposure, giving it sufficient strength to support a large aspect ratio. As a large aspect ratio is exactly what we needed, HSQ was the better substitute. By adopting HSQ for the EBL process, we could easily obtain a SiO₂ grating with a large aspect ratio.

The conventional development after EBL was operated at 23 °C. However, in this condition, the contrast of HSQ was so poor that the HSQ at the bottom could not be dissolved, resulting in a lot of residues. No matter how thick the HSQ was spin coated, the grating slit was still full of residues. As shown in Figure 3b, production is a low, rounded trapezoid with many residues, making it impossible to achieve the height we needed. To avoid the undesired residues, we took a hot developing process. When the temperature was above 40 °C, the residues were dissolved, and a high aspect ratio was achieved [40]. As shown in Figure 3c, because this method was adopted, the grating had a perfect aspect ratio and perfect verticality with barely any residues, meeting our expectations.

The rest of the procedure was to deposit metal on the grating. According to our previous analysis, to achieve high TM transmittance, we needed to ensure that there was no metal on the side walls of the grating. However, the metal melted at a small target and spread to the whole vacuum chamber; thus, due to thermal motion, there was vapor in all horizontal directions when it approached the grating. For general evaporation, oblique vapor deposits directly on the side walls, as shown in Figure 3d. This resulted in the grating being completely wrapped in metal, causing the grating to show high reflectivity for both TM and TE similar to a metal film. Thus, the MDG no longer showing the polarization selection as in the simulation. Figure 3e shows an MDG fabricated by general evaporation; in the schematic, the bright area is metal, and the dark area is oxide. Almost the entire surface of the grating was covered by metal, demonstrating the harm of oblique vapor. To improve it, we reduced the target distance and added a baffle to modify the angle of vapor, as shown in Figure 3d. By using these techniques, we prevented oblique vapor from depositing on the side wall of the grating. The improved result is shown in Figure 3f, where the side walls were completely dark, indicating that metal was distributed only on the top and bottom of the grating, confirming the success of our improvement.

Therefore, we achieved an effective fabrication using hot developing and modifying the angle. The specific process was as follows: after cleaning the substrates in acetone solution followed by iso-propyl alcohol (IPA), SiO₂ was deposited on Si by electron beam evaporation at an evaporation rate of 2 nm/s, at ambient temperature and in a 10⁻⁴ Pa vacuum, and then 300 nm of HSQ was spin coated onto it. The HSQ was baked in an oven for 1 h at 60 °C. In order to guarantee the low line-edge-roughness and improve the exposure time of large-area polarization grating in the next EBL process, we should ensure that the beam spot size is less than 10 nm. Finally, the EBL process was operated with a Raith EBPG 5200 with the exposure dose set to 950 μc/cm² and the exposure beam current set to 8 nA. The grating was developed in 2.38% tetramethylammonium hydroxide (TMAH) at 45 °C for 4 min. Finally, it was rinsed in deionized water for 1 min. The metal was then deposited on the photoresist grating to obtain the MDG by thermal evaporation with a box-type vacuum coating machine (ZZS-900). The evaporation parameters were set as follows: 2 nm/s evaporation rate, target distance less than 15 cm, ambient temperature, and 10⁻⁴ Pa vacuum.

The grating was then tested based on the principle shown in Figure 4a, obtaining TM/TE light by a polarizer, and the TM/TE transmittance was measured by a sensor and computer. The actual measurement was operated using a spectrometer (UMS, Agilent, CARY7000), and the results are shown in Figure 4b,c. In the principal wavelength band of 1.2−2.2 μm, the grating achieved perfect polarization performance with 90% TM transmittance and a 100:1 extinction ratio, much higher than the existing gratings with about 80% TM transmittance and a 75:1 extinction ratio [11] in the infrared band. However, it was slightly lower than the simulation, which may have been caused by the imperfect edge of the metal, a small amount of metal depositing on the side wall, the difference in thicknesses between the design and experiment, and the oxidation of metals in the depositing process, which could be improved by adjusting the duty cycle and optimizing depositing parameters. In addition, the differences between the actual materials and those used in design are also an important cause. Moreover, in the 1−1.2 μm band, the TM transmittance failed to reach a high level due to the absorption of the silicon substrate and rapidly rose to the normal level with the disappearance of absorption. In the 2−2.5 μm band, an abnormally high extinction ratio was generated due to the measuring limit of the instrument. Overall, the grating successfully achieved perfect polarization performance.

4. Discussion and Conclusions

In summary, we designed the MDG with excellent polarization performance and proposed a new effective manufacturing process for it, which is conducive to the practical application of polarization imaging. Based on effective medium theory, the MDG achieved perfect polarization performance by modulating the TM/TE transmittance to enhance its performance. We fabricated it by EBL and thermal evaporation deposition instead of by a traditional unstable lift-off process. By adopting hot developing of HSQ to improve the aspect ratio and modifying the deposition angle to avoid metal adhering to the side walls, we obtained a perfect grating as expected. Actual testing in the near-infrared band yielded 90% TM transmittance and a 100:1 extinction ratio measured by a spectrometer. Although the polarization performance reported in this paper was lower than what was designed, it can be improved by changing the duty cycle. Moreover, it can be further used for other wavelength bands, such as visible light, not just the near-infrared wavelengths for which we designed the experiment. We believe the manufacture of this polarization grating is critical for polarization imaging.