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Abstract
Antireflection sub-wavelength structures (SWSs) on ZnS were designed and ZnS SWSs with HfO2 protective film were prepared, and their properties in long-wave infrared applications were examined and compared to AR coatings. The SWS has good antireflection performance and exhibits less polarization sensitivity than the AR coating. At temperatures above 500 °C, the SWS with HfO2 protective film has a better thermal endurance property than the multilayer AR coating. Moreover, the HfO2 protective film significantly improved the mechanical properties of the ZnS SWS and was similar to HfO2 covered AR coating when the HfO2 film was not broken. This study shows that the ZnS SWS with HfO2 protective film has promising application prospects in infrared optical windows.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Infrared optical windows are essential components of infrared detection equipment and typically used in harsh and thermal shock environments. Zinc sulfide (ZnS) is a widely used material for long-wave infrared optical windows with good thermal and chemical properties and low absorption in the wavelength range of 8–12 μm with [1]. However, the high refractive index (approximately 2.2, at a wavelength of 10 μm) of ZnS frequently results in large Fresnel reflection losses (approximately 25%), which can reduce the performance of optical devices [2]. As a result, antireflection coatings on ZnS substrates are frequently required to reduce Fresnel reflection [3,4]. Because of the large wavelength and limited refractive index of the coating materials available, traditional antireflection (AR) coatings for long-wave infrared optical windows typically have complex multilayer structures and large thickness, resulting in several shortcomings, such as high cost, large thermal stress and thermal mismatch, limited optical angular range, polarization sensitivity, and limited scalability [5,6].
An alternative approach to reduce Fresnel reflection is the direct patterning of antireflection structures on the surfaces of optics [7]. Intensive studies have created various low-reflectance AR structures on the surfaces of Si, GaAs, sapphire, glasses, and many other optical materials [8–12]. AR structures have significant advantages over traditional AR coatings in many areas, such as spectral acceptance angles and polarization sensitivity, thermal mismatch, material selection, and thickness control [13–16]. Thus, the AR structures have development potential in ZnS infrared optical windows.
Existing studies have proposed novel efficient fabrication processes to prepare a variety of AR structures on ZnS, such as interference lithography [17–19], nanosphere lithography [20], self-assembled metallic droplets [21,22] and direct laser writing [23,24]. However, the majority of existing research focuses on the fabrication process. The practical advantages of ZnS antireflection structures in infrared applications over traditional AR coatings, such as thermal performance and broadband wide-angle antireflection properties, have yet to be studied. Moreover, because ZnS infrared optical windows is typically used in harsh environments, and because of the relatively low hardness of ZnS material, the ZnS antireflection structures must be covered with high-hardness protective films [25–27]. Therefore, the behavior of protective films on ZnS AR structures in comparison to flat AR coatings is also worth studying.
In this work, first high-performance AR structures on ZnS were designed and the AR structures on ZnS were prepared using holographic lithography and reactive ion etching processes with an atomic layer deposition HfO2 protective film to improve the mechanical properties. Subsequently, the comprehensive properties of the ZnS AR structures in long-wave infrared applications compared with traditional AR coatings were characterized and analyzed, including broadband wide-angle antireflection performance, thermal endurance, and mechanical properties.
2. Design
Based on their microscopic morphologies, the most commonly used antireflection surface structures can be divided into two categories . One is a sub-wavelength structure (SWS) with column-shaped morphology that can be equivalent to a single-layer coating with variable refractive index and thickness [28]. The other type of structure is a moth-eye structure with a tapered morphology that can provide a gradual change in the refractive index for light propagating from air into the substrate [29,30]. Two types of typical antireflection structures were designed on the ZnS substrates of the two categories. Figure 1(a) and (c) show the microscopic morphology of SWS with two-dimensional periodic cuboid columns and moth-eye structures with two-dimensional periodic cones. To obtain a high antireflection property at a wavelength of 8–12 μm on a ZnS substrate, the electromagnetic behaviors of the antireflection structures were calculated using a 3D finite difference time-domain (FDTD) simulation with Lumerical software.
Fig. 1. (a) Schematic diagram and (b) simulation reflectance of column-shaped SWS with a fill factor of 0.47 and a height of 1.7 μm, (c) schematic diagram and (d) simulation reflectance of cone-shaped moth-eye structures with close packing and different height, and (e) simulated reflectance of the designed AR coating.
Because high-order diffracted light increases stray light and decreases transmittance, the period of the microstructures must be sufficiently small to ensure that there is only zero-order diffracted light. The maximum period can be calculated using the grating formula as follows:
where d represents the period, m represents the diffraction order, λ represents the wavelength, n and n0 represent the refractive indices of the exit and incident medium, respectively, and θm and θ0 represent the diffraction and incident angles, respectively. To ensure that only zero-order diffracted light exists, the period must be set to a sufficiently small value, as expressed in the following formula, to ensure that the grating formula is valid only for m=0For ZnS antireflection structures working at a wavelength of 8–12 μm, the periods of the two types of antireflection structures were both set to 2.5 μm to meet the requirement.
The column-shaped SWS shown in Fig. 1(a) can be equivalent to a single-layer coating, and the best antireflection property appears when the effective refractive index of the SWS is the square root of the refractive index of the substrate, and the structure height is a quarter-wave optical thickness at the effective index. Therefore, the optimum values of the fill factor and height of the SWS were calculated. The optimized SWS has a fill factor of 0.47, and a height of 1.7 μm. The simulated reflectance of the double-sided antireflection SWS is shown in Fig. 1(b). Under ideal conditions the theoretical reflectance is less than 5% at a wavelength of 8–12 μm, which is nearly the best antireflection property that column-shaped SWS can achieve. For the cone-shaped moth-eye structures shown in Fig. 1(c), the antireflection property increases with a larger fill factor and height. The maximum fill factor was achieved with close packing. Figure 1(d) shows the variation in reflectance with different structure heights (H) for double-sided antireflection moth-eye structures. The reflectance decreases as the height of the moth-eye structures increases, and with a large height, the theoretical reflectance can be very low. However, the moth-eye structures require a much greater height to achieve the same antireflection property as the SWS. When the reflectance is less than 5%, the moth-eye structures require a height of approximately 3.7 μm, which is more than twice the height of the SWS, increasing the difficulty of preparation and reducing the fracture resistance. For the application of long-wave infrared optical windows that is typically used in harsh environments, the durability and mechanical properties are vital requirements; thus, the SWS was used in ZnS substrates.
3. Experimental process
Antireflection SWS was prepared on a hot-pressed polycrystalline ZnS substrate using holographic lithography and a three-step reactive ion etching (RIE) process. The fabrication process is shown in Fig. 2. First, a thin SiO2 film was deposited on the ZnS substrate through an electron beam evaporation (EBE) process. Then, a photoresist grating mask with a designed period and fill factor was fabricated above the SiO2 film using conventional holographic exposure and development. With the photoresist mask, the exposed SiO2 film was etched using a reactive ion etching system (Beijing Zhongke Tailong Electronic Technology Co. Ltd RIE-100) with CHF3 gas to obtain a SiO2 grating mask. The gas flow rate of CHF3 was 80 sccm, and the RF power was 300 W. Then, ZnS was etched using a H2/CH4 mixture gas using the RIE system to obtain ZnS SWS. The gas flow rate of H2/CH4 were 80/20 sccm, and the RF power was 300 W. Subsequently, the residual SiO2 film was removed by repeating the CHF3 based etching process using the RIE system. Finally, an HfO2 protective film with a thickness of 90 nm was deposited on the SWS using atomic layer deposition (ALD) process, covering any surface of the ZnS SWS with HfO2 film, thus effectively improving the mechanical properties of ZnS SWS.
For comparison, a typical multilayer antireflection coating composed of a high-refractive-index germanium (Ge) material and a low-refractive-index ZnS material was used. The AR coating was a four-layer coating consisting of Ge and ZnS materials that was deposited on a ZnS substrate using the EBE process. For comparison, the AR coating was also coated with the same 90 nm ALD HfO2 film. The arrangement and thickness of the film layers in the AR coating were optimally designed as follows:
where H and L represent Ge and ZnS, respectively. The total thickness of the AR coating was 2.27 μm. The theoretical reflectance was less than 8% at a wavelength of 8–12 μm, as shown in Fig. 1(e). Although many more film layers can be used to further reduce reflectance, the total coating thickness will increase significantly, resulting in high preparation difficulty and low environmental stability.After preparation, the antireflection performance, thermal properties, and mechanical properties of the ZnS SWS and AR coatings for long-wave infrared applications were characterized. Scanning electron microscopy (SEM) was used to characterize the surface morphology and uniformity of the ZnS SWS patterns. Spectral performance was measured using a Fourier transform infrared spectrometer (FTIR). Thermal endurance was measured using FTIR with a heating device. A nanoindentation tester was used to determine the hardness and mechanical properties of the samples.
4. Results and discussion
SEM images of the ZnS SWS before and after HfO2 film deposition are shown in Fig. 3. Figure 3(a) and 3(b) show a representative overhead view and cross-sectional of the ZnS SWS, respectively. The uniformity of the SWS was evident, with features exhibiting straight walls with a well-defined rectangular cross-section. However, the shape of the structures was rhomboid rather than a perfect two-dimensional square column, which may result in a polarization effect and deviation of the fill factor. The height was estimated to be approximately 1.64 μm, the grating periodicity and the width were approximately 2.5 μm and 1.7 μm, respectively, which conformed to the design value (smaller than the design). Figure 3(c) shows a representative overhead view, and Fig. 3(d) shows a cross-sectional view of the ZnS SWS with HfO2 protective film. It can be observed that the surface morphology of the SWS is almost unchanged after the ALD HfO2 film is deposited.
Figure 4 shows the transmittance and reflectance spectra of the SWS and AR coatings measured by FTIR, a polarizer is added behind the light source to achieve linearly polarized light and measure the spectra of S and P polarizations. The transmittances of the SWS and AR coatings are clearly higher than those of bare ZnS at wavelengths of 8–12 μm. The average transmittances of the ZnS SWS with and without HfO2 film are both approximately 92.7% at a wavelength of 8–12 μm, which is 19% higher than that of bare ZnS. The average transmittance of the AR coating is approximately 93.4%, which is 19.7% higher than that of bare ZnS. The average transmittance improvement of the SWS is close to that of the AR coating at a wavelength of 8–12 μm. The transmittance of ZnS SWS with HfO2 protective film does not change significantly compared to that without the HfO2 film at a wavelength of 8–12 μm. The HfO2 film has absorption above the wavelength of 12 μm, reducing the transmittance at that wavelength. For the HfO2 protected ZnS SWS, the highest transmittance is approximately 97% at 8.4 μm wavelength. The transmittance spectrum of the SWS is blue-shifted compared to the design value, and the relative deviation could be caused by the experimental errors of the model grating features.
Fig. 4. (a) Transmittance of SWS and AR coating, reflectance spectrum for (b) S-polarization light and (c) P-polarization light of SWS and AR coating at 30° and 45° incident angle, respectively.
From the spectra of bare ZnS, it can be observed that the hot-pressed polycrystalline ZnS substrate has clear absorption around the wavelength of 11 μm, which is commonly used in polycrystalline ZnS substrates owing to impurities and defects. At a wavelength of approximately 11 μm, the transmittance of the SWS is clearly lower than that of the AR coating. However, the reflectance spectrum of the SWS is similar to that of the AR coating at a wavelength of approximately 11 μm for both the S-polarization and P-polarization. The reflectance of the SWS with HfO2 film did not increase significantly at small incident angles. This suggests that the SWS has larger absorption than the AR coating at a wavelength of approximately 11 μm. The reason for the increase in SWS absorption could be that the etching process increases impurities and defects in polycrystalline ZnS, increasing material absorption. Because coating materials have less absorption, the absorption of the AR coating does not increase significantly.
From the reflectance spectrum shown in Fig. 4(b) and (c), the SWS demonstrates better wide-angle broadband antireflection properties than the AR coating, particularly for S-polarized light. For the S-polarized light, all of the reflectance of SWS, AR coating, and bare ZnS increased when the incident angle increased, and the average reflectance of the ZnS SWS was obviously lower than that of the AR coating. For the P-polarized light, the reflectance of bare ZnS decreases with an incident angle close to the Brewster angle. The reflectance of the SWS and AR coatings did not change significantly. The lowest reflectance of the AR coating was lower than that of the SWS. However, the SWS showed a wider antireflection broadband than the AR coating. The average reflectance of ZnS SWS with HfO2 protective film was approximately 7% for S-polarization and 5% for P-polarization at a wavelength of 8–12 μm when the incident angle increased to 45 °, which is lower than that of bare ZnS. The average reflectance of the AR coating was approximately 13% for S-polarization and 4% for P-polarization at a wavelength of 8–12 μm when the incident angle increased to 45 °. That is, the reflectance of ZnS SWS shows less obvious polarization sensitivity than that of the AR coating. Therefore, it is reasonable to conclude that the ZnS SWS with HfO2 protective film possesses excellent optical properties for broadband wide-angle antireflection.
The thermal properties were measured using FTIR with a heating device. High temperatures can result in thermal shock and increased material absorption. Figure 5 shows the extinction ratio change of the bare ZnS, the SWS with HfO2 protective film, and the AR coating under high temperature conditions. For the multilayer AR coating, the extinction ratio increased slightly at 300 °C. However, cracking and delamination of the AR coating were identified at 500 °C. For a multilayer AR coating with a large thickness, the crystalline states of the films change during heating, inducing large thermal stress and thermal mismatch, resulting in cracks in the film [31,32]. For the SWS with HfO2 protective film, the extinction ratio also slightly increased as the temperature increased. The average increase in the extinction ratio at a wavelength of 8–12 μm is approximately 3% when the temperature increases to 500 °C, which is similar to the average extinction ratio increase of the bare ZnS. The optical property does not change significantly, and cracks do not appear when the temperature rises to 300 and 500 °C. Because the SWS is directly patterned on the bulk material surface, the thermal property of the SWS is similar to that of the hot-pressed polycrystalline ZnS substrate, which has a strong thermal shock resistance. The ALD HfO2 protective film is too thin to produce significant thermal or thermomechanical effects. This results in good thermal endurance of the ZnS SWS.
Fig. 5. Extinction ratio spectrum of (a) bare ZnS, (b) SWS with HfO2 protective film and (c) AR coating at different temperature.
The hardness and mechanical properties of the samples were characterized using a nanoindentation tester. The load-depth curves of SWS with and without the HfO2 protective film and the AR coating are shown in Fig. 6. It can be observed that the displacement into the surface of the SWS with HfO2 protective film was significantly reduced compared with the SWS without HfO2 film. This indicates that the HfO2 film provides good protection for SWS. The load-depth curve of the SWS with HfO2 film is similar to that of the AR coating when the depth is less than 200 nm. The hardness values of unprotected SWS, HfO2 protected SWS and HfO2 protected AR coating are about 0.34 GPa, 3.17 GPa and 3.13 GPa at the depth of 200 nm respectively. This indicates that the hardness and adhesion of ALD HfO2 on SWS are similar to those on a flat film. However, when the depth was more than 200 nm, the hardness of HfO2 protected SWS decreased significantly and was lower than that of the AR coating. The reason is that the HfO2 film is damaged when the indentation depth is much larger than the thickness of HfO2 film, and results to a decrease of the hardness of HfO2 protected SWS. For the AR coating sample, the ALD coating is also broken at indentations above 200 nm. However, the Ge/ZnS multilayer coating has better mechanical property than the ZnS pillars, thus the hardness of AR coating does not decrease significantly. This indicates that the mechanical properties of the HfO2 protected ZnS SWS were similar to those of the flat AR coating when the HfO2 film was not broken. In addition, it is natural to think that one can increase the thickness of HfO2 film to protect the ZnS SWS better, however, the optical performance and thermal property will decrease.
5. Conclusion
In summary, we designed and fabricated a ZnS antireflection SWS with a HfO2 protective film was designed and fabricated and the comprehensive properties of SWS in long-wave infrared applications compared with traditional AR coatings were characterized. Two common microscopic morphologies of AR structures were calculated on ZnS, and it was illustrated that the cone-shaped moth-eye structures require much higher structures to achieve the same theoretical antireflection property as the column-shaped SWS. The prepared ZnS SWS with HfO2 protective film has an average transmittance of 92.7%, which is approximately 19% higher than that of bare ZnS at a wavelength of 8–12 μm, and the highest transmittance reached approximately 97%. The SWS with HfO2 protective film has an average reflectance of approximately 7% for S-polarization and 5% for P-polarization at a wavelength of 8–12 μm when the incident angle increases to 45 °, and shows better wide-angle broadband antireflection properties and less polarization sensitivity than the AR coating. The optical properties of the ZnS SWS with HfO2 protective film do not change significantly at high temperatures, and they have good thermal endurance properties compared to the multilayer AR coating. Furthermore, the ALD HfO2 protective film provides good protection for the ZnS SWS, and the mechanical properties of the HfO2 protected ZnS SWS are similar with HfO2 covered flat AR coating when the HfO2 film is not broken. This study shows that the prepared ZnS SWS with HfO2 protective film has improved broadband wide-angle antireflection performance, thermal endurance, and mechanical properties, which make it suitable for application in infrared optical windows.
Funding
National Natural Science Foundation of China (61621001, 61925504, 62061136008); National Key Research and Development Program of China (2016YFA0200900); Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-07-E00063).
Acknowledgements
The authors would like to thank Huoyao Chen from the University of Science and Technology of China for the holographic lithography process.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper can be obtained from the authors upon reasonable request.
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