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Abstract
This work mainly demonstrates the development process of an ultra-broad band antireflection (AR) coating that maximizes the optical performance of germanium optical elements. A multilayer stack exhibiting high efficient AR performance on Ge consists of thin multi-layers of aluminum oxide (Al2O3) and germanium (Ge) named as low and high refractive index layer materials. A three-layer design with a very tight thickness tolerance (3%) was optimized by Optilayer software. Al2O3 and Ge layers were deposited by a plasma assisted e-beam evaporation system. Ultra-high efficient multilayer AR coating on Ge has a base reflectance 0.005% at 3550 nm and an average reflectance of 0.256% over the entire mid wave infrared spectrum. Multilayer AR coating on Ge offers lower base and average reflectance than the works reported before. Moreover, AR coating on Ge offers a cost effective process cycle due to its number of layers and its thickness for providing ultra-broadband and high efficient optical performance for mid wave infrared electro-optical applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
After discovery of infrared (IR) radiation by T. Herschel [1], electro-optical systems for efficient imaging of IR radiation has become critical especially for defence and space applications [2]. After Herschel discovery, wavelength of emitted radiation from hot objects was described by Planck’s law of radiation
where λ, T, h, c, and k are wavelength, temperature, Planck constant, speed of light and Boltzmann constant respectively. As described in Eq. (1), mid wave infrared (MWIR) spectrum is defined as the wavelength of radiation emitted by objects which have surface temperatures up to 700 °C such as engines of vehicles, aircrafts and missiles [3].In principle, electro-optical systems are designed for imaging objects reflecting or emitting light and high precision optics are required for efficient image formation on focal plane of systems.
There are various optical elements used for image formation in electro-optical systems [4]. In those elements, transmissive ones such as lenses and prisms are two of major components. Reduction of optical losses such as absorption and back reflection while bending of light becomes critical to avoid from aberrations which cause deformation of image on focal planes [5]. To avoid that, AR coatings with customized spectral performances are applied on transmissive optical elements. However, there are few number of infrared materials which are transparent for MWIR spectrum. Therefore, development of high efficient AR coatings on infrared optical elements plays critical role to have improved optical performance.
Multilayer optical coatings are designed by using different mathematical algorithms which are based on optimization of interference phenomena in stack systems [6] so that definition of optical parameters for thin films consisting stack allows to set system matrix for having target spectral performance [7]. There are few items to be considered while designing a multilayer stack. One of the efficient consideration is to reduce number of layers by selection of high refractive index difference layer materials. Overall thickness of multilayer stack can also be reduced by providing same optical thickness with low index difference material selection case. Moreover, tight thickness tolerance of layer materials also plays critical role to exhibit improved optical performance. Therefore, accurate modelling of refractive indices for each item such as substrate and layer materials becomes important while improving performance of optical coatings [8].
In previous works, there are plenty of optical coatings which provide different AR performance on different materials [9,10,11,12]. Latest work from Yenisoy et al [13] which reports a high efficient AR coating on Silicon optics with 99.786% transmittance over MWIR spectrum. Additionally, there is only one comparable high efficient work from Bhatt et.al [14] offering 98.5% transmittance over 1300 nm spectral interval for MWIR. However, our work offers an ultra-broadband and high efficient spectral performance for AR coated Ge based optical elements with base reflectance 0.005% at 3550 nm and average reflectance 0.256% over entire mid wave infrared (MWIR) spectrum which has not reported yet. Moreover, our design consists of just 3 layers and total thickness is less than 1 micron which also offer cost effective coating process for Ge based optical elements.
2. Design principles
There has been intensive research for development of AR coatings after first work on field [15,16,17]. In its simplest case, AR phenomena is observed if following relation is satisfied
where nfilm, n0, and ns are refractive indices of coating material, ambient, and substrate respectively. In this case, quarter wave optical thickness (QWOT), which can be defined as the requirement for film thickness for destructive interference at control wavelength of incident radiation, of coating material cancels back reflections at boundary of substrate and AR phenomena is observed at a single monitor wavelength [18]. However, there have always been different AR performance required so that multilayer stack is defined by matrix method which use polarization modes of incident light and optical performance is optimized by calculation of transmittance (T) and reflectance (R) at interfaces of both substrate and altering layers [19,20].In this work, accurate refractive index modelling for Ge and thin layers were determined due to requirement of ultra-broadband and high efficient AR performance. Additionally, accurate refractive index dispersion for materials are required since they are all vary due to material grades and process parameters of evaporated thin films [21,22,23]. Optilayer modules which has advanced mathematical algorithms for thin film design and modelling of optical parameters were used for modelling refractive indices of materials by using T and R curves of ZnS, Al2O3 and Ge and optimization of design was performed by using T and R spectra of Ge substrate, Al2O3 and Ge layers.
3. Process details
Native oxide on 1” (25.4 mm) diameter and 1 mm thick Ge substrate was removed by 1% HCl and 0.1% HF aqueous solution for few seconds which was followed by DI water and alcohol rinse. After surface treatment of Ge, roughness which have direct effect on optical quality of evaporated films was measured by interferometric optical profiler (Zygo, Zegage Plus). A 2.75x Michelson objective with 3 mm x 3 mm scan area was used. Al2O3 and Ge layers were coated by plasma assisted e-beam evaporation system (Bühler Syruspro 1110). Coating temperature was 200 °C during evaporation of both Al2O3 and Ge films to enhance agglomeration rate for stiffness. Ge substrates were rotated in planetary rotation system with 50 rpm to improve uniformity of thickness on substrate. Process pressure was 1 × 10−6 mbar for Ge evaporation with 0.2 nm/s rate. On the other hand, process pressure was 5 × 10−4 mbar for evaporation of Al2O3 with the same rate. For Al2O3 evaporation, 15 sccm O2 flow diluted in 25 sccm Ar flow was applied for plasma formation. Additionally, both sides of Ge substrate were coated by the same layer order and process parameters to eliminate back surface reflection. Finally, Fourier Transform Infrared (FTIR) spectrometer with 8cm-1 spectral resolution (Perkin Elmer, Optica) was used to measure T and R spectra of Ge substrate and films evaporated.
4. Results
Surface roughness of Ge substrates were interferometric optical profiler after native oxide removal and cleaning process. Precision grade (Ra < 2 nm) roughness for optical grade substrates plays important role since surface it has direct effect on adhesion and agglomeration of films evaporated [23]. Raw interferometric data was analyzed by removing Zernike terms Z1-Z3 (piston, tilts and defocus) to evaluate roughness itself and average surface roughness (Ra) which was obtained as 1.107 nm as seen in Fig. 1.
As seen in figure, surface roughness satisfies requirement of precision grade and no additional surface treatment process was applied. After inspection of surface, T and R spectra of Ge substrate were measured by FTIR and Fig. 2 shows T and R spectra of substrate over MWIR spectrum.
Measurement of T and R spectra of Ge was used for refractive index modelling of substrate. After optimizations, refractive index model was best fitted by lowest merit function in Sellmeier dispersion formula which was defined by
After modelling refractive index of Ge, 250 nm thick Al2O3 layer was evaporated on Ge. After evaporation, T and R spectra which are shown in Fig. 4 of 250 nm Al2O3 film on Ge were measured.
Similarly, refractive index model of Al2O3 film on Ge was best fitted in Sellmeier equation which was shown in Eq. (3). Sellmeier constants for refractive index dispersion which was also shown in Fig. 5 of Al2O3 film has A0, A1, A2, A3, and A4 constants 2.561, 0.046, −5,843, 7.006, and 599.958 respectively.
Since mathematical models for refractive index dispersions of substrate and layer materials consisting multilayer stack were obtained, multilayer AR design was optimized from 3.4 to 4.8 µm where wide range of MWIR detectors have non-zero spectral response.
After multilayer optimizations, Rav and Tav of resulting design were obtained as 0.256% and 99.744% respectively. Thickness tolerance for individual layers was also kept at 3% to have high efficient spectral performance for ultra-broad spectral width as 1400 nm. In addition to those parameters, non-polarized incident beam with 0° angle of incidence (AOI) were also defined for incident beam while optimizing performance of multilayer stack. After all, resulting optical parameters for 3-layer stack are shown in Table 1. QWOT of layer materials were defined for 550 nm monitor wavelength and overall thickness of multilayer stack was optimized as 965 nm.
Table 1. Optical parameters of multilayer design
Optimized 3-layer stack was evaporated on both sides of Ge substrate by the process parameters defined before. Moreover, a blank witness sample which was masked with kapton tape was also coated by the recipe to inspect overall thickness of multilayer stack. After coating process, overall thickness of stack was measured as 972 nm by optical profiler. Thickness of multilayer stack was also extracted by the spectral data obtained from FTIR. Since multilayer stack consist of non-symmetrical thickness values, random error analysis was performed to verify individual and overall thickness of layers evaporated. Random error analysis of multilayer stack was done by obtaining layer thickness errors −3 nm, 3 nm, and 8 nm for layer 1, 2, and 3 respectively which are also shown in Fig. 6.
As shown in Fig. 6, all 3 layers were evaporated by 3% tolerance as listed in Table 2 which was defined during optimization of multilayer design.
Table 2. Thickness tolerance intervals of layers
In order to examine spectral performance of multilayer coating, T and R spectra of AR coating on Ge substrate was measured by FTIR and high efficient AR over ultra-broadband was obtained as seen in Fig. 7.
When T and R spectra of multilayer AR coating was inspected in detail as shown in Fig. 8, T and R points from 3.4 µm to 4.8 µm at ant point of MWIR spectrum were more and less than 99.5% and 0.5% respectively. Moreover, multilayer coating on Ge has base R and peak T as 0.005% and 99.995% at 3550 nm respectively.
Fig. 8. Simulated (red) and measured (black) spectra of multilayer stack for R (above) and T (below).
Multilayer coating was also exhibited 99.744% Tav and 0.256% Rav which were slightly and definitely tolerably different than the simulation results obtained after optimization of design as shown in Table 3 which is possibly due to both thickness deviations from the ideal values and difference in incident angles between design (AOI = 0°) and measurement in FTIR (AOI = 10°).
Table 3. Simulated and measured values for Tav and Rav
5. Summary
In this research work, development process of high efficient and ultra-broad band multilayer AR coating on germanium was demonstrated. Tav and Rav values obtained in this work exhibit lower optical loss than the works reported before. In addition to the high performance obtained, multilayer stack with just 3 layers and overall thickness less than 1 micron correspond the work as cost effective solution of coating process for high efficient Ge based optical elements required for MWIR electro-optical applications.
Disclosures
No potential conflict of interest was reported by the authors.
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