A new CMY camera technology using Al-TiO2-Al nanorod filter mosaic integrated on a CMOS image sensor
Xin He, Y. Liu, P. Beckett, H. Uddin, A. Nirmalathas, R. R. Unnithan
AA new CMY camera technology using Al-TiO -Al nanorod filtermosaic integrated on a CMOS image sensor X. He , Y. Liu , P. Beckett , H. Uddin , A. Nirmalathas , and R. R. Unnithan Department of Electrical and Electronic Engineering, The University of Melbourne,Melbourne, VIC, 3010, Australia. School of Engineering, RMIT University, Melbourne, VIC, 3000, Australia. Melbourne Centre for Nanofabrication, Australian National Fabrication Facility,Clayton, VIC, 3168, Australia.October 23, 2020
Corresponding author: R. R. Unnithan (e-mail: [email protected])
Abstract
A CMY colour camera differs from its RGB counterpart in that it employs a subtractive colourspace of cyan, magenta and yellow. CMY cameras tend to performs better than RGB cameras inlow light conditions due to their much higher transmittance. However, conventional CMY colourfilter technology made of pigments and dyes are limited in performance for the next generation imagesensors with submicron pixel sizes. These conventional filters are difficult to fabricate at nanoscaledimensions as they use their absorption properties to subtract colours. This paper presents a CMOScompatible nanoscale thick CMY colour mosaic made of Al-TiO -Al nanorods forming an array0.82 million colour pixels of 4.4 micron each, arranged in a CMYM pattern. The colour mosaicwas then integrated onto a MT9P031 monochrome image sensor to make a CMY camera and thecolour imaging demonstrated using a 12 colour Macbeth chart. The developed technology will haveapplications in astronomy, low exposure time imaging in biology and photography. Keywords:
CMY image sensor, Plasmonics, nanophotonics
In a conventional CMOS based image sensor, colour imaging relies on the integration of filters on top ofthe photodetector array. These filters typically cover the three primary colours (bands): red (around 650nm), green (550 nm) and blue (450 nm) (RGB), predominately in a Bayer pattern [1-9]. As the humaneye is more sensitive to green light than either red or blue, the widely used Bayer filter mosaic is formedwith twice as many green as red or blue filters. RGB colour space uses an additive colour mixing of red,green, and blue that combine to create a white output.In contrast, CMY (cyan, magenta, and yellow) is a subtractive colour mixing scheme where colourfilters are used to remove certain wavelengths of white light. For example, cyan is obtained when the redis subtracted from the image. Similarly, magenta and yellow are obtained by subtracting the green andblue respectively. In RGB space, a red filter transmits only about 1/3 of the visible light as the remaininglight is absorbed in the filter (only the red light passes through the filter). In contrast, the correspondingcyan filter in CMY colour space transmits about 2/3 of the spectrum because only the red is subtracted,and the remaining is transmitted through the filter. In general, CMY colour filters pass approximatelytwice the spectral power as their corresponding RGB filters [11] and so are promising candidates forlow-light imaging applications. Examples of such applications include astronomical imaging of nebulaand the like that are characterized by dim objects against a dark background. Furthermore, astronomicalimages are often required to be captured with short exposure times, lest rotation effects blur the image(for example, images of Jupiter). These requirements are best served by high transmission filter schemessuch as CMY.Conventional CMY colour filters are based on the absorption properties of organic dye-based materialsor pigments to subtract selected wavelengths from the incident light. However, as the pixel size in the1 a r X i v : . [ phy s i c s . i n s - d e t ] O c t ensor is reduced to submicron dimensions [8], conventional CMY colour filters start to suffer fromcolour cross talk as their performance deteriorates at nanoscale thicknesses [1-9]. Further, existing CMYtechnology must be fabricated in several steps, which presents severe challenges when trying to accomplishsubmicron-scale alignment. These issues demand the development of new CMY color filter technologiesthat can be fabricated easily at nanoscale thickness, using CMOS compatible materials, and that willsupport the creation of millions of filter pixels within a colour mosaic.Colour filters based on plasmonic effects [1-9, 11-27] are suitable candidates due to their ability tobe precisely tuned using nanoscale thick films. Colour filters based on localized surface plasmons aresuperior to those based on surface plasmons as the former are angle independent. As a result, thetransmitted colours in LSP based filters are same for any angle of incidence, an important requirementfor image sensor applications. Recently, gold (Au) nano-disk based CMY filters have been demonstratedwith useful characteristics such as polarization independence and angle insensitivity [14]. However, Aubased plasmonic filters are limited in their colour tuning capability, especially below 550 nm as Au doesnot readily support plasmonic resonance peaks below that wavelength [28].CMY filters with high transmission coefficients have been reported based on silver (Ag) nano-slits[15], nano-disk [17] and Si meta-surfaces [18-19]. However, Ag oxidizes quickly in air and this degradesthe optical properties, thereby mandating additional protective coatings that can further affect the filtercharacteristics. CMY filters operating in reflection mode based on surface plasmon polaritons (SPP) aredemonstrated in [21-24], but the reflection mode is not suitable for image sensor applications.In this paper, we present a colour filter mosaic built from a hexagonal array of Al-TiO -Al nanorodson a quartz substrate that is derived from a subtractive MDM (metal-dielectric-metal) nanohole arraystructure. The structure exhibits a high transmission efficiency and narrow bandwidth to produce supe-rior colour separation. Colour tuning is achieved by varying the rod radius across the array, while keepingthe base thickness constant, thereby forming a mosaic of CMY filters across the substrate. Further, wehave applied this colour mosaic to develop a CMY camera. The 4mm x 4mm optical filter mosaic en-compasses 0.82 million pixels, each pixel being 4.4 µ m square and arranged in CMYM pattern. Thefilter has been integrated onto a monochrome image sensor (MT9P031) to create a CMY camera and itsimaging capabilities demonstrated using a standard 12 colour Macbeth chart. To date, there has beenno prior demonstration of a full camera system using a nanoscale thick CMY mosaic integrated onto animage sensor that illustrates its feasibility for imaging applications. This is the first such demonstration. Fig. 1a shows the proposed structure of the CMY colour filter, which is formed from Al-TiO -Al nanorodsfabricated on a quartz substrate and then embedded in a SOG (spin on glass [33]) matrix for refractiveindex matching. The optical characteristics of this structure can be considered to arise from a Fanoresonance in which the top and bottom metal disk operate together as a coupled dipole, enhancingthe magnetic field between them to produce both destructive and constructive (i.e., in-phase and anti-phase) interference modes. Choosing an appropriate radius and period for the structures results in anarrow reflection peak and a deep transmission valley that are sensitive to the symmetry of the dipoleelements [32]. It has been shown in previous work related to nanodisks [27, 32] that these structuresexhibit a broad (in-phase) resonance at smaller wavelengths and a narrow anti-phase resonance at largerwavelengths. This can be described in terms of a Q factor, which is proportional to the ratio of theresonant frequency to the FWHM. It is also clear that, while introducing asymmetries can strengthenthe peaks (in particular, the anti-phase resonance), it introduces an undesirable sensitivity to the angleof incidence [32]. As a result, our work has kept the thickness and radius of both the top and bottommetal disks the same, which has the added advantage of simplifying its fabrication.The CMY filters were computationally investigated in 3D using finite element methods (FEM) im-plemented in COMSOL MULTIPHYSICS. The simulation model consists of a unit cell on a semi-infinitethick quartz substrate consisting of a single nanorod at the centre and one-quarter of a nanorod at eachcorner as shown in Fig. 1a (red rectangle). Each rod consists of an Al-TiO -Al stack with thickness of 40nm, 90 nm and 40 nm, respectively. The simulated unit cell on the semi-infinite glass substrate is coveredwith spin on glass. The 3D simulation geometry has been truncated using perfect matched layer (PML)and periodic boundary condition (PBC). The PML is applied on the top and at the bottom, PBCs areapplied on the four sides of the cell as highlighted in the red block of Fig. 1a. For calculating the peakwavelengths, the refractive index of glass is set to 1.5, spin on glass 1.45 and the wavelength dependent2igure 1: Simulation results of Al-TiO -Al nanorod based CMY (cyan, magenta, yellow) filter mosaic:(a) Al-TiO -Al nanorods in hexagonal array on a quartz substrate covered with spin-on-glass, (b) Thenormalized electric field at valley wavelength for the filters: yellow (470 nm), magenta (570 nm) andcyan (670 nm). Electric field at 470 nm, (c) The normalized electric field of cross section (red dottedline) at valley wavelength for the filters, (d) Numerically simulated transmission spectra of the CMYcolour filters. The wavelength is swept from 300 nm to 800 nm (e) CIE chart of simulated CMY coloursin the filter mosaic.Table 1: Parameters of Al-TiO -Al Nanorod (layer thickness, period, rod diameter)Parameters Cyan Magenta YellowTop Al thickness (nm) 40 40 40TiO thickness (nm) 90 90 90Bottom Al thickness (nm) 40 40 40Period (nm) 500 430 350Diameter (nm) 60 90 40refractive index of Al was taken from Rakic’s data [34]. The refractive index of TiO (i.e., 2.1) wasobtained experimentally by measuring values from a TiO film deposited with an E-beam evaporator.This value was also used in the simulation model. The transmission coefficient has been obtained fromS-parameters using the port boundary conditions.The wavelength of light was swept from 300 nm to 800 nm to find the valley transmission [35], keepingthe thickness values constant while varying the rod radius from 20 nm to 30 nm. The normalised electricfield of each colour filter-yellow (470 nm), magenta (570 nm) and cyan (670 nm)—at their resonantwavelength (i.e., the valley wavelength) is shown in Figure 1b. The corresponding normalized electricfield across each CMY pillar is shown Fig. 1c while Fig. 1d shows the simulated transmission spectrumfor CMY filters. The layer parameters (thickness, period and rod diameter) are summarised in Table1. Fig. 1e plots the simulated CMY filter wavelengths on the CIE chromaticity chart, illustrating thatthese fall in an appropriate part of the colour space, thereby demonstrating their suitability for imagingapplications.As the refractive index of the middle (insulator) layer has substantial influence on the optical prop-erties of the filter, we also investigated a number of alternative dielectric materials for this layer. Inour simulation model, we applied SiO , TiO , Si (assuming an ion assisted deposition with E-beamevaporator) and Ge with refractive index values of 1.45, 2.1, 2.8 and 4, respectively. These materialswere used to simulate a cyan colour filter and the results are summarised in Figure 2. Note that thesedata are measured using ellipsometry (J.A. Woollam M-2000DI). As shown in Fig. 2, the transmissionspectrum exhibits the largest transmission contrast and smallest linewidth (to produce a more vivid cyan3igure 2: Comparison of transmission spectra from Al-X-Al, where X represents the middle dielectricmaterial: SiO , TiO , Si and Ge.Figure 3: Strategy for fabricating the Al-TiO -Al nanorod based CMY colour filter mosaic.colour) with TiO compared to SiO , Si and Ge so this material was selected for the remainder of ourexperiments.We fabricated a 4 mm x 4 mm mosaic onto a quartz substrate using standard nanofabrication tech-niques [36-39]. The mosaic comprises repeating CMYM units in both the horizontal and vertical di-rections to form a colour array. The fabrication strategy using a PMMA-MMA bilayer is described asfollows (see Fig. 3).Firstly, a 1 mm thick quartz substrate was cleaned with acetone under sonication for 5 minutesfollowed by IPA and DI water rinse. The substrate was then preheated at 80 ° C for 10 minutes and a thinEL9 (MMA) layer spin-coated onto the quartz at 3000 RPM for 1.5 minutes and baked at 180 ° for 15minutes. A thin PMMA A2 layer was spin-coated onto the sample at 3000 RPM for 1.5 minutes and bakedat 180 ° C for 5 minutes. To ameliorate problems with charging of the quartz substrate under ElectronBeam Lithography (EBL), a 30 nm Cr layer was deposited on the sample by conformal sputtering tofinish the sample preparation.After patterning with EBL (Vistec EBPG5000plusES), the Cr layer was removed using 1 minute in aCr etchant, stopped by 5% H SO . The sample was then developed with diluted MIBK (MIBK:IPA=1:3)for 1 minute and stopped by IPA and DI water. It can be seen in Fig. 3 that the MMA exhibits a bowlshape after development, which makes subsequent lift off easier due to the smaller contact area betweenthe Al-TiO -Al nanorod and the quartz.Finally, the Al-TiO -Al nanorod was deposited by E-beam evaporator (Intlvac Nanochrome II). Thedeposition rate for the TiO was 0.1 nm/s. After MMA/PMMA lift-off, the filter was completed by4able 2: Optical Characteristics of the CMY Filter MosaicCyan Magenta YellowTransmission Characteristic Sim Expt Sim Expt Sim ExptValley Wavelength (nm) 660 670 580 580 470 480Maximum Contrast (%) 60 45 70 47 80 50Maximum Efficiency (%) 90 90 100 92 100 90Figure 4: Integration of the CMY colour mosaic on the image sensor. (a) CMOS image sensor, MT9P031integrated with 4 mm x 4 mm CMY colour mosaic using flip chip bonder for alignment; (b) CMY cameradeveloped with optics ( f number 1.4) for imaging; (c) an Australian 50c coin as a reference to show thesize of the CMY camera; (d) the CMY colour mosaic under optical microscope; (d) magnification × ×
40; (f) SEM image of the filter mosaic made of Al-TiO -Al nanorods from top view(cyan is shown); (g) Reflected RGB colours from (e); (h) Experimental transmission spectra of the CMYcolor filters from the color mosaic; (i) CIE chromaticity chart of the CMY filters in the mosaic fromexperimental data.spin-coating with SOG (Desert NDG-2000) at 2000 RPM for 20 seconds and baking at 210 ° C for 15minutes.Fig. 4d shows an image of the CMYM mosaic in transmission mode under an optical microscope(Olympus BX53M) with 20 × magnification and an enlarged section of this is shown in Fig. 4e. ASEM image of part of a cyan pixel (top view) is shown in Fig. 4f. Note that, as the sample surface isnon-conductive, 7nm of Cr was deposited prior to performing the SEM imaging, which altered the shapeof the nanorods. Finally, it can be seen from the reflection mode image (Fig. 4g) that the reflected lightfrom the sample (i.e., Fig. 4e) is RGB.The transmission spectrum of the fabricated mosaic was measured using a CRAIC spectrometer(Apollo Raman Microspectrometer). It can be seen from Fig. 4h that the maximum transmittance ofthe individual Cyan, Magenta and Yellow cells is around 90%. The spectrum displays high transmissionwithout any secondary resonances from UV to Near-IR wavelengths. Figure 4i positions these exper-imentally measured CMY data on the standard CIE chromaticity chart. The detailed results for theCMY filter are collated in Table II.As is evident from Table 2, generally good agreement was observed between the simulated and exper-imental data in the case of the valley wavelength and efficiency values. However, the peak transmissioncontrast is consistently smaller (by almost 40% for yellow). This difference may be caused by the surface5igure 5: Integration of CMY filter mosaic on the CMOS image sensor MT9P031: (a) flip chip bonder,(b) display showing the alignment of filter colour pixels on the sensor pixels, (c) placement of the filtermosaic on the sensor (d) CMY filter aligned on the image sensor after being glued by PMMA powderdiluted with anisole, (e) logo of the University of Melbourne (f) CMY filter with the logo behind it toshow its high transmission efficiency, (g) CMY colour filter after being diced.roughness of the deposited TiO . It has been observed in prior work [27] that defects and surface rough-ness in the metal film can result in inhomogeneous broadening of the resonances without affecting theFWHM and can even eliminate the anti-phase mode entirely. In order to achieve a relatively uniformmetal oxide layer, and therefore a high refractive index, the deposition rate of the E-beam evaporatorneeds to be set as high as possible. This resulted in a surface roughness of around 3-5 nm in our work(measured by Atomic Force Microscopy). In the absence of an additional planarization step, the topAl disk becomes similarly rough, potentially affecting its behavior. However, this reduction in contrastmay be offset in part by the high transmission efficiency, which increases the received brightness. Thecontrast can also be corrected by subsequent image processing. The CMYM mosaic was integrated onto a commercial CMOS sensor using a flip-chip bonder (Fineplacer@Lambda) and raw images captured for further analysis as shown in Fig. 5. The CMOS image sensorused in this experiment was a MT9P031 monochrome array with a pixel size of 2.2 µ m. Fig. 5(a-d)shows the CMY mosaic aligned and glued onto the image sensor. The glue was made in-house usingPMMA powder diluted with anisole solution and the filter was pressed onto the image sensor with justsufficient force to exclude most of the air between them. The high transmission efficiency of the CMYfilter is demonstrated using the University logo on white paper which can be clearly seen through filter(Fig. 5(e-g)). The overall filter mosaic size is 4 mm x 4 mm and each colour cell covers a 2 x 2 patch toincrease the light absorption and decrease the spatial crosstalk. Therefore, the pixel size in this mosaicis 4.4 µ m square. The image sensor integrated with the mosaic is then fitted with optics ( f number =6igure 6: Demonstration of colour imaging by the CMY camera: (a) 12-bit raw image of the 12 colourMacbeth chart captured by CMY camera. (b) Three bands (cyan, magenta and yellow) extracted fromthe raw image. (c) The three bands (three channel combination) are recombined to get a CMY colorimage. (d) The CMY colour image is converted into RGB and applied color correction and white balance.(e) Standard image of the 12 colour Macbeth color chart. (f) The plot shows signals from three pixelnumbers along the vertical red dotted line in the raw image.1.4) for imaging as shown in Fig. 4b. The compact size of the CMY camera is shown by reference to anAustralian 50c coin (Fig. 4c).The CMY camera was then used in an example imaging application shown in Fig. 6. Firstly, a rawimage of the standard 12-colour Macbeth chart (Figure 6e) was taken with the CMY camera (Fig. 6a).The recorded 12-bit raw image data was then transmitted to a laptop for processing using MATLAB.The raw data was first analyzed for saturation and signal intensity variation. Fig. 6f shows a plot ofthe signals from the 12-colour Macbeth chart pixels across the transect indicated by the red dashed line.The results show that the intensity variations are present in the raw image. Then, the individual CMYpixel data were grouped to form separate images for each of cyan, magenta and yellow pixels (Fig. 6b).Fig. 6c shows the reconstructed colour image. Next, a CMY to RGB conversion algorithm: R=Y+M-C;G=Y+C-M; B=C+Y-M [10], was applied to transform the CMY image to RGB for display. Lastly,colour correction and white balance were applied to recover the Macbeth chart as shown in Fig. 6e.These results demonstrate that colours can be retrieved from the 12-bit raw image data and illustratecorrect operation of the CMY camera. In conclusion, a CMY camera is demonstrated using a nanoscale CMY colour filter mosaic made of CMOScompatible Al-TiO -Al nanorods with high transmission. The overall filter mosaic is 4 mm x 4 mm witheach pixel occupying 4.4 µ m square in a CMYM pattern and is integrated onto a MT9P031 CMOS imagesensor using flip-chip bonding and an in-house prepared glue. The performance of the filter mosaic itselfwas first characterised and then the correct operation of the integrated CMY camera was illustrated usinga 12 colour Macbeth chart as an object. The colours were retrieved using image processing algorithms.The use of more powerful image processing algorithms (as employed by commercial camera systems)will further improve the colour performance. This nanorod technology will overcome the limitationsimposed by conventional colour filter technology for creating CMY colour image sensors and cameraswith submicron pixel sizes. As a result, this technology is likely have applications ranging over astronomy,low exposure time imaging in biology and general photography. A 12 sided coin measuring 31.65 millimetres between flat sides. onflict of Interest The authors declare that they have no conflict of interest.
Acknowledgment
This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the VictorianNode of the Australian National Fabrication Facility (ANFF). The authors acknowledge financial supportthrough ARC Discovery Project: DP170100363.
Author Contributions
X. H. and R. R. U conceived the idea. X. H carried out theoretical design, fabricated the CMY mosaicfilter and the optical measurements. X. H. Y. L performed the integration of the mosaic filter withimage sensor. X. H. Y. L. and P. B. processed the image data and analyzed the results. X. H and R.R.Uwrote the paper. R. R. U, H.U and A.N supervised the project. All authors discussed the results andcommented on the manuscript.