Nano-structured transmissive spectral filter matrix based on guided-mode resonances
In this work, a nanostructured guided-mode resonance filter matrix with high transmission efficiency and narrow bandwidth is demonstrated. The developed nano-filter arrays have various usages, e.g., combined with the CMOS image sensors to realize compact spectrometers for biomedical sensing applications.
In order to optimize the filter performance, the spectral responses of filters with different structural parameters are carefully studied based on the variable-controlling method. A quality factor is carried out for quantitative characterization.
In this case, a high fill factor of 0.9 can strongly suppress sidebands, while buffer layer thickness can be adjusted to mainly control the bandwidth. The transmission peaks shift from 386 nm to 1060 nm with good linearity when periods vary from 220 nm to 720 nm. The incident angle dependence is simulated to be ~ 1.1 nm/degree in ±30° range. The filters are then fabricated and characterized. The results obtained from both simulations and experiments agree well, where the filters with the period of 352 nm exhibit simulated and measured transmission peaks of 564 nm and 536 nm, the FWHM of 13 nm and 17 nm, respectively. In terms of metal material, besides aluminum, silver is also investigated towards optimization of the transmission efficiency.
The transmission spectra of designed filters have high transmission and low sideband; its peaks cover the whole visible and near infrared range. These characteristics allow them to have the possibility to be integrated into image sensors for spectrometer applications.
KeywordsTransmissive color filter matrix Guided-mode resonance
Atomic force microscopy
Atomic layer deposition
Full width at half maximum
Plasma enhanced chemical vapour deposition
Scanning electron microscopy
Surface imaging system
Surface plasmon resonance
Super sharp silicon
Highly compact spectrometers can be envisioned by combining CMOS image sensors with pixelwise spectral filters . Ideally, the transmission function of the filter is as sharp as possible and should change from pixel to pixel, whereas the pixel distance of modern CMOS chips is in the range of a few micrometers [2, 3]. Since each pixel refers to a separate wavelength, the image of the CMOS sensor can then be transferred directly into an optical spectrum [4, 5]. In addition to the high spatial resolution , in order to allow e.g. multi-spectral imaging in biology, the transmission wavelength of such filters needs to be adjustable across the visible and infrared spectrum . Such novel compact spectrometers will not focus on the super high resolution, but on the light information located at certain wavelengths. Some human body parameters such as blood oxygen saturation can be noninvasively determined via red and infrared light beams . Other hemoglobin levels and health indicators are measurable with the same principle but using light beams at other wavelengths. Moreover, the accuracy of oxygen saturation determination can be also improved by using multispectral sensing method.
A possible realization of such a pixel spectral filter is a matrix of absorbers like organic pigments or dyes . Besides the problem of depositing different dyes with micrometer resolution, those filters usually have a low transmission and a relatively broad absorption spectrum. Since they are based on organic molecules, they often suffer from degradation or aging, in particular when exposed to ultraviolet radiation. In contrast to that, light can also be filtered by interference effects occurring in periodic thin film superlattices. By adjusting its parameters (e.g., layer thickness  or periodicity [10, 11]), the filter function of such resonant filters can be controlled. Therefore, they are now becoming more popular in various applications such as color displays [12, 13], image sensors , and biosensors . Some dielectric nanostructure-based color printings with high spatial resolution have been reported by other groups [16, 17], but most of them work on the reflection mode, such transmissive filters are still absent.
For both chemical color filters (e.g., Bayer filters used on many commercial image sensors ), and resonant filters based on the variation of thin film thickness, separate photolithography steps need to be performed to laterally control the filter function. As a matter of fact, to obtain three different filter functions (i.e., for red, green and blue colors ), three separate photolithography steps have to be used. For spectrometers, however, the number of different wavelengths, and consequently the number of photolithography steps, is so large that this approach cannot be used. In contrast to that, filters with a lateral periodicity only require one lithography step and are thus much more convenient in terms of fabrication effort. In this case, the filter function would be controllable from pixel to pixel in a single patterning step.
There are two basic concepts for spectral filters with lateral periodicity, namely metallic nanohole array filters and guided-mode-resonance (GMR) spectral filters.
The filtering effect of arrays of nanoholes in thin metal films is due to the excitation of surface plasmon resonances (SPR) . Such filters can be patterned in triangular  or square lattice [11, 21, 22] with different shapes of metallic holes (i.e., circular [11, 21], triangular , square  and annular holes ) and have been both theoretically and experimentally reported by many groups. The spectral response can be tuned by changing their lattice period. On one hand, these filters have the advantage of being fabricated by straightforward technological steps, and can then be directly integrated with image sensors. However, on the other hand, they yield low transmission efficiency, which is normally about 25–50%, with a spectral line width of the filter function (i.e., full width at half maximum (FWHM)) over 100 nm, which would be too large for a CMOS spectrometer. Yokogawa et al. have reported CMOS image sensors integrated with circular hole array color filters in different sizes from (1 × 1) μm2 to (5 × 5) μm2 . Although they have realized the filter arrays with low crosstalk effect, their transmission efficiency of 40–50% could still not be further improved.
In this work, we aim at developing resonant filters based on lateral periodicity, which can be integrated into CMOS image sensors. In this case, GMR filters based on the coupling of the incident wave to an adjacent leaky lateral waveguide mode can achieve high transmission efficiency with very narrow linewidth . Sharp resonance phenomena can take place only if the phases of external diffracted wave and the structural waveguide mode match. Since 1995, after Magnusson et al. introduced the first transmissive multilayer GMR filter , more advanced transmissive filters with various structures or thin-film materials were reported. To enhance the interaction of light and nanostructure, metal gratings are used instead of dielectric gratings in many works. The surface plasmon modes of the metal grating are excited and coupled to the waveguide mode of dielectric waveguide, that the electromagnetic field can be further extended into the waveguide . However, most of these works only focus on RGB (red, green, and blue) filters . Besides, those filters still have large sidebands; their bandwidth can be further optimized [27, 28]. Thus, for realizing single peak transmissive GMR filter cross whole visible and near infrared range on the micron scale with high efficiency, narrow bandwidth, and low sidebands, a comprehensive simulation of the influence of different parameters like periodicity, film thickness, and other material properties like e.g. index of refraction on the filter properties is prerequisite.
Results and discussion
where A1 and A2, TP and SP are the field area of 1 and 2, and transmission and sideband peak amplitude, respectively. An ideal transmission spectrum requires higher A2 to (A1 + A2) ratio, a larger TP and SP difference as well as a lower FWHM. The largest Q value is searched during the whole simulation study.
At first, fill factor of the aluminum grating is varied from 0.7 to 0.95 in 0.01 steps (fill factor lower than 0.7 results even larger sideband by transmission spectrum), while other parameters were kept constant at P = 500 nm, W = 100 nm, B = 50 nm, and M = 30 nm. The simulation results are illustrated in Fig. 2a. At the maximum of the transmission peaks, a transmission of about 90% is achieved when the fill factor is lower than 0.85. But in this case, there are relatively high sidebands. Higher fill factors, equivalent to a larger fraction of the surface covered with metal, cause lower transmission but can strongly suppress the sidebands. Thus, according to the requirements of the envisioned application, the fill factor can be adjusted to obtain a suitable compromise between high transmission and low sideband contributions. In our case, for multispectral sensing applications, higher fill factor F = 0.9 was chosen for the followed studies, with peak transmission sacrifice, and lower sideband.
where j = (− 1)1/2, k0 = 2π/λ, with λ as the wavelength in free space, pwi = [εw – (βi/k)2]1/2, pbi = [εb,eff – (βi/k)2]1/2, and psi = [εs – (βi/k)2]1/2, with i as the integer number labelling the diffracted orders (i = 0, ±1, ±2, …), εw, εs and εb,eff are the relative permittivity of the waveguide layer, substrate and effective permittivity of the buffer layer with the metal grating, respectively. βi is the propagation constant along the boundary layer. According to this equation, a larger approximate resonant wavelength λres is calculated by a thicker waveguide layer thickness dw for a constant periodicity. A proportional relationship between center wavelength and waveguide layer thickness is also obtained from Fig. 2b, which is consistent with the equation solution. Depending on this relationship, Qian et al. have reported a tunable filter consisting of a wedge-shaped waveguide layer Ta2O5 with 50 nm increment of thickness resulting in a resonance peak shift from 684.2 nm to 725.3 nm . The ratio of wavelength increment to thickness is much higher than our work, which is mainly because of the different waveguide material. In this work, we optimize the waveguide layer thickness of 100 nm to avoid the high sidebands at thinner or thicker waveguide layer.
The additional buffer layer in our filter design serves to reduce the bandwidth of the resonance. The buffer layer reduces the interaction of the waveguide mode with the metal grating, which causes loss. This can be observed from the 2D plot of the magnetic field distribution at transmission peak wavelengths with buffer layer thickness of 150 nm (Fig. 2e) and 0 nm (Fig. 2f). The magnetic field is no longer confined at the aluminum and dielectric layer interface when the buffer layer exists. This phenomenon is due to the presence of surface plasma mode, and the buffer layer could lead to a narrow plasmonic bandgap [30, 31]. Moreover, the transmission spectrum of the filter with ITO as buffer layer has a FWHM around 3 times larger than that of SiO2 as buffer layer with the same structural parameters. The explanation is that, a more symmetry dielectric environment for the waveguide layer will be created when the buffer layer is thicker, and thus it will cause a sharper resonance. Therefore, a larger buffer layer leads to a narrower resonance bandwidth . Based on this influence, we can control the filter bandwidth by tuning the buffer layer thickness. Figure 2c shows transmission spectra with buffer layer thickness ranging from 0 nm (without buffer layer) to 150 nm. As can be seen from the figure, the FWHM of the filter becomes narrower when the buffer layer thickness increases. The FWHM of a GMR filter without buffer layer is about 55 nm, which can be reduced to 25 nm when a 50 nm thick buffer layer is used.
with nb as buffer layer refractive index. Thus, the minimum λ with P = 500 nm at normal incident is calculated at 730 nm, which agrees with simulated result.
Moreover, filter with the finite field (i.e., containing 5, 15, 25 or 35 grating lines) was also investigated using simulation tool. Compared with the filter with infinite gratings, the transmission spectrum of filter with only 5 gratings has two peaks instead of one single peak. A similar transmission spectrum as the infinite one (with peak shift less than 7 nm) can be achieved by filters with more than 15 grating lines.
For the optimized parameter set, the angle dependence was studied. Simulations of incident angles ranging from − 85° to 85° in 5° steps with optimized structural parameters of F = 0.9, W = 100 nm, B = 50 nm, and M = 30 nm at P = 500 nm were performed and the results are plotted in Fig. 3b. Figure 3c is the angle dependence of simulated transmission spectra up to ±20° in 1° steps. The transmission peak shifts about 34 nm if the incident angle increases from 0° to 30°, resulting in the shift rate of about 1.1 nm per degree. Consequently, for GMR filters in CMOS based spectrometers the peak wavelength shifts as a function of incident angle and has to be taken into account and the elements have to be tailored for a specific angle of incident. Normal incidence light will be mainly used in our future biomedical applications, to avoid the incident angle influences.
Moreover, as stated in some previous works [30, 31], the metal grating based GMR filter is polarization dependent. The filter matrix can be thus fabricated containing both vertical and horizontal gratings with the same periods, to compensate the polarization dependency.
A 400 μm thick quartz substrate was prepared and cleaned using nitrogen gun. The first layer of waveguide was then deposited on top of the substrate by plasma enhanced chemical vapor deposition (PECVD). In this case, 100 nm thick Si3N4 layer was deposited within 5 min with a deposition rate of about 20 nm/min.
Subsequently, a 50 nm thick SiO2 layer was conformally applied with atomic layer deposition (ALD) technique. The SiO2 was deposited at a temperature of 250 °C using 500 ALD cycles. Silane (SiH4) was carried by N2 with 1.6 s pulse time and 8 s purge time. O2 was carried by Ar with 13.5 s pulse time and 4 s purge time. Ellipsometry (SIE EP4 LDXe+L, Accurion GmbH, Göttingen) was used to determine the layer thickness. The measured layer thickness and refractive indices of Si3N4 and SiO2 layers are 84.5 nm with n ~ 2 and 49.2 nm with n ~ 1.46, respectively, which are in good agreement with literature data of refractive indices of the employed materials .
The sample was then spin-coated using a PMMA (AR-P 671.02, Allresist GmbH, Strausberg) resist layer with a thickness of 130 nm.
Afterward, the patterning step was followed using electron beam lithography tool Raith EBPG 5200 with an acceleration voltage of 100 keV and a base dose of 300 μC/cm2. The solvent of a mixture of 10 ml Methylethylketon (MEK) plus 247.5 ml Methylisobutylketon (MIBK) was used as the developer, and 742.5 ml Isopropanol (IPA) as stopper. The development and stopper (IPA) time were both 30s.
A 40 nm thick aluminum layer was then deposited on the top layer.
Then the sample was immerged in acetone overnight (> 12 h) for lift-off preparation. After sprayed with acetone and with IPA, blown dry with nitrogen gun, the whole nanofilter fabrication procedure ended up with photoresist removal.
Overall, the proposed GMR filter is able to achieve a single transmission peak instead of multi-peaks  over a large wavelength range. Compared with previous works , we use a higher fill factor to realize the low sideband spectra by reasonable scarifying the transmission efficiency, but still with narrow bandwidth. This helps our future biomedical application to collect the light information at certain wavelengths in both visible and near infrared range . However, a higher fill factor means the smallest filter metal grating gap has to be fabricated down to around 30 nm in our case, which is quite challenging. This can be done by using an optimized fabrication process (e.g., two-step EBL), or other fabrication methods, (e.g., step and flash imprint lithography ), which will be discussed in our future work.
In our future biomedical sensing system, the filter polarization dependency can be overcome by either using a polarized light source or pattering both vertical and horizontal gratings within one matrix element. Furthermore, as mentioned previously, the minimization of filter dimension is limited by the required grating number in one filter (minimum 15 grating lines in a single filter). Thus, to integrate the filter matrix in CMOS sensors with smaller pixel size, more pixels (2 × 2 or 3 × 3 pixels) can be settled under one filter.
Metal-based GMR spectral nanofilters have been fabricated in size as small as (10 × 10) μm2 with robust filter properties. The experimentally determined filter functions have been simulated to obtain a set of parameters like grating periods, thickness of the waveguide, buffer layer, and filling factor. The optimum parameter set is: F = 0.9, W = 100 nm, B = 50 nm, and M = 30 nm. Hence, sharp transmission functions with wavelengths in the whole visible and near IR range can be achieved. The structural parameter studies demonstrated that the position of the central transmission peak is mainly controlled by grating period, whereas the FWHM of the transmission peak is predominantly influenced by the buffer layer thickness. Both simulated and measured FWHM were under 20 nm. Moreover, simulations of GMR nanofilters with different metals indicate that silver can be more suitable than aluminum for the grating. Overall, it could be demonstrated that such GMR filters have a very high potential to be integrated into commercial image sensors even when pixel sizes are in the order of micrometers.
The authors thank Juliane Breitfelder, Feng Yu, Andrey Bakin, Jan Gülink, Carol Rojas-Hurtado, Peter Thiesen and Rainer Macdonald for valuable discussion and technical support. J.D. Prades acknowledges the support of the Serra Húnter program. We gratefully acknowledge the support of the Braunschweig International Graduate School of Metrology B-IGSM and the DFG Research Training Group GrK1952 “Metrology for Complex Nanosystems (NanoMet)”.
WW, JDP, HSW and AW conceived and designed the simulations and experiments; WW and LW performed the simulations, WW, PH and TW manufactured the filter matrix, SK and BB designed and performed the transmission spectra measurements; TD performed the AFM measurement; WW analyzed the data; WW wrote the paper. All authors read and approved the final manuscript.
This work is funded in part by the Lower Saxony Ministry for Science and Culture (N-MWK) within the group of “LENA-OptoSense” and in part by the European Union’s Horizon 2020 research and innovation program within the project of “ChipScope – Overcoming the Limits of Diffraction with Super-Resolution Lighting on a Chip” under grant agreement no 737089.
The authors declare that they have no competing interests.
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