Plasmonics (2012) 7:695–699 DOI 10.1007/s11468-012-9360-6
A CMOS Image Sensor Integrated with Plasmonic Colour Filters Q. Chen & D. Das & D. Chitnis & K. Walls & T. D. Drysdale & S. Collins & D. R. S. Cumming
Received: 16 December 2011 / Accepted: 16 March 2012 / Published online: 27 March 2012 # Springer Science+Business Media, LLC 2012
Abstract Multi-pixel, 4.5×9 μm, plasmonic colour filters, consisting of periodic subwavelength holes in an aluminium film, were directly integrated on the top surface of a complementary metal oxide semiconductor (CMOS) image sensor (CIS) using electron beam lithography and dry etch. The 100×100-pixel plasmonic CIS showed full colour sensitivities across the visible range determined by a photocurrent measurement. The filters were fabricated in a simple process utilising a single lithography step. This is to be compared with the traditional multi-step processing when using dyedoped polymers. The intrinsic compatibility of these plasmonic components with a standard CMOS process allows them to be manufactured in a metal layer close to the photodiodes. The incorporation of such plasmonic components may in the future enable the development of advanced CIS with low cost, low cross-talk and increased functionality. Keywords CMOS image sensors . Colour filters . Surface plasmons . Subwavelength structures
Introduction In solid-state digital imaging, complementary metal oxide semiconductor (CMOS) image sensors are the leading mass market technology. A CMOS image sensor (CIS) has many Q. Chen : K. Walls : T. D. Drysdale : D. R. S. Cumming (*) School of Engineering, University of Glasgow, Rankine Building, Glasgow G12 8LT, UK e-mail:
[email protected] D. Das : D. Chitnis : S. Collins Department of Engineering Science, University of Oxford, Oxford OX1 3PJ, UK
advantages including low voltage operation, low power consumption, compatibility with standard CMOS technology and the realisation of integrated functionality. In the past decade, significant improvement in the performance of CIS has been obtained with the scaling of feature sizes in CMOS technology [1]. However, with the downscaling of pixel size to the sub-2-μm range, conventional dye-doped polymer colour filters for CIS suffer from performance degradation, such as colour cross-talk [2, 3]. Furthermore, these filters have to be fabricated successively in several process steps. Continuous development of new applications for CIS requires that they are able to be manufactured at low cost. Because of the encroaching difficulties of cross-talk and the cost of manufacture using the dye-doped polymer method, it is desirable to find new ways to make colour filters for CIS. Alternative techniques such as silicon subwavelength gratings [4], photonic crystal colour filters based on a multilayer stack [5] and multiple Fabry–Pérot (FP) cavities using metal mirrors [6] have been investigated but the practical applications are limited due to poor performance and complicated fabrication. An alternative method is to use surface plasmon resonance (SPR) in thin metal films patterned with nanostructures; these metallic structures offer a suitable candidate for making filters [7–10]. The incident light couples with the surface plasmon in a way determined by the nanostructures in the metal film. Varying the nanostructures enables tuning of the resonant wavelength and thus achieves the light filtering. The technique has the potential to produce all the required colour filters in a single layer. This would reduce both cost and colour cross-talk because plasmonic colour filters can be integrated in lower metal layers so that they are very close to the CMOS photodiodes. Furthermore, the great flexibility of SPR could provide a high degree of optical functionality on the CIS. There have been several reports of results from plasmonic filters; however, to the
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best of our knowledge, a CIS with plasmonic components has not been reported previously. Catrysse et al. showed the potential use of plasmonic filters with detailed numerical simulations and preliminary experimental results [11, 12]. Lee et al. integrated a visible pass-band filter based on aluminium gratings in a CIS to suppress infrared illumination [13]. Tang et al. demonstrated an increase of the photocurrent in a CMOS detector with a C-shaped nanoaperture due to the effect of SPR [14]. More recently, large area single-pixel CMOS photodetectors with plasmonic colour filters were demonstrated working in the visible range [15]. The aluminium nanostructures that comprised the filter were fabricated on to the plasmonic CIS (pCIS) surface using electron beam lithography (EBL) covering a photodiode area of approximately 1 mm2. In this paper, we demonstrate a 100×100-pixel pCIS with a pixel size of 4.5×9 μm. The CIS was manufactured using a microelectronics foundry process (in particular the AMS 0.35 μm process). Back-end-of-line processing was then carried out in our laboratories to integrate plasmonic components on to the CIS. The filters were made on a glass calibration sample and on the CIS to enable comparison of the spectra. The glass samples were characterised in a microscope spectrophotometer, and the pCIS was characterised using the integrated photocurrent from the photodiodes. The 100×100-pixel plasmonic CIS showed full colour sensitivities across the visible range.
Plasmonic Colour Filters on Glass Ebbeson et al. investigated a periodic subwavelength hole array in a silver film and observed unexpected optical properties such as enhanced transmission of light through the holes and wavelength filtering due to the excitation of SPR [7]. The peak position, lmax, of the transmission spectrum of a subwavelength hole array in a triangular lattice at normal incidence can be approximated by rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a "m "d lmax ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1Þ " þ "d 4 2 2 m ð i þ ij þ j Þ 3 where a is the period of the array, εm and εd are the dielectric constants of the metal and the dielectric material in contact with the metal, respectively, while both i and j are integers that determine the scattering orders of the array [16]. In this work, aluminium, a metal commonly used in standard CMOS processes, was used instead of noble metals such as gold and silver. Initially, the designs of these filters were tested by creating plasmonic filters on a glass substrate. The first step in creating these filters was to evaporate a 150-nm aluminium film on a glass substrate in a Plassys MEB 400S electron beam evaporator. ZEP520A electron
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beam resist was then spin-cast on to the sample and exposed using a Vistec VB6 UHR EWF EBL system. The pattern transfer was done with a SiCl4 dry etch in a Plasmalab System 100 after development in o-xylene. An additional top SiO 2 layer of 200 nm was deposited by plasmaenhanced chemical vapour deposition to enhance the SPR coupling at either side of the metal film and thus increase the transmission. A SEM image of a patterned hole array at a period of 410 nm in aluminium is shown in Fig. 1a. The colour images of plasmonic filters with different nanostructures on the same sample were examined using an Olympus BX51 microscope fitted with a broadband halogen lamp. Figure 1b shows the colour squares with a size of 50× 50 μm consisting of aluminium holes in a triangular lattice with different periods. Spectral measurement was carried out on a microscope spectrophotometer TFProbe MSP300 that can sample the signal from a minimum area of 10× 10 μm. An unpolarized light beam from a halogen lamp was launched normally on to the back side of the sample. The transmission spectra from the sample in the visible range from 400 to 700 nm are shown in Fig. 1c, where transmittances are between 20 and 40%, with full width at half maximum between 70 and 110 nm. Loss mechanisms include reflection, re-radiation and absorption within the metal due to the non-zero imaginary component of the permittivity [17]. As predicted in Eq. (1), Fig. 1d shows that the wavelength at which transmission through each filter peaks was found to linearly increase with the period of the holes in the metal layer. This demonstrates that a complete colour filter set can be reliably made using a single lithographic step.
Integration of Plasmonic Colour Filters on a CIS To study the performance of the plasmonic filters in the application of digital imaging, we integrated them on to a CIS [18]. The CIS used in this work had a 100×100-pixel array. As shown in Fig. 2a, the pixel size is 10×10 μm, and each pixel contains a 4.5×9-μm photodiode with an interphotodiode gap of 0.7 μm between some pixels. The topography of the pixels has a 1.1-μm vertical step between the photodiode and the circuit regions within each pixel due to the top metal layer in the AMS 0.35 μm process as shown in Fig. 2b. The fabrication procedure must be modified from that used to make plasmonic filters on glass. To increase the transmission of the colour filters integrated on the CIS, we deposited a layer of SiO2 on top of the SiNx surface passivation layer of the CIS before depositing the aluminium. This SiO2 layer also protected the bond pads of the chip during processing. Because the CMOS chips use aluminium, it is difficult to register accurately to the pixels using our 100-kV EBL system that has a backscatter detector. We therefore deposit approximately positioned gold electron
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Fig. 1 Performance of various plasmonic colour filters fabricated on glass. a A SEM image of a plasmonic colour filter with a hole array in aluminium that has a period of 410 nm. b Images of various plasmonic colour filters taken in microscope transmission mode under a white light illumination. c Transmissive spectra of the filters shown in (b). d Wavelengths at which transmission through each peaks versus the period of the plasmonic nanostructures
beam markers around the CIS using a standard lift-off process. A registration EBL step is then used to write a dummy pattern on to the pixel array. With this pattern, we determine the positional error between our gold markers and the pixel array using a high-resolution SEM Hitachi S4700 Fig. 2 a A microscope image of the photodiode pixel array on an unprocessed chip. b An AFM image of the topography of pixels before processing. c A microscope image of the photodiode pixel array with plasmonic colour filters fabricated on top. d A SEM image of the plasmonic colour filters on top of a CIS. The inset is an enlarged image of a filter with the period of 230 nm
SEM. This data is then used in a third EBL step to write the final pattern on a 150-nm aluminium film evaporated on the photodiode pixel array. Of course, only one lithography step would be needed if this process was implemented in the manufacturing flow of the foundry. A final mask and etch
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Fig. 3 a Photocurrent distribution across the 100×100-pixel photodiode array. b Normalised photocurrents of photodiodes with various plasmonic colour filters
step was required to reopen windows over the bond pads of the integrated circuits. A microscope image of the processed pixels is shown in Fig. 2c, where a series of varying plasmonic components were repeated across the photodiode pixel array. In this reflection image, it is possible to see the various colours of the pixels due to the SPR generated by differently patterned nanostructures. It is also possible to see black rectangles that correspond to reference pixels, from which the aluminium film above the photodiodes has been totally removed. A SEM image of the pixels with plasmonic filters is shown in Fig. 2d, where we can see a good alignment of the plasmonic structure to each pixel. The inset in Fig. 2d shows a hole array with a period of 230 nm. This filter transmits a dark blue colour. To test the pixel array, a tungsten bulb and a monochromator were used to illuminate the array with wavelengths in the range between 400 and 800 nm. The slits used within the monochromator were chosen to limit the range of wavelengths during each experiment to 5 nm, whilst the centre wavelength was changed in 2-nm increments. At each centre wavelength, the response of each pixel was measured using a data acquisition system, specifically a NI USB-6218. In order to correct for fixed pattern noise, the response of each pixel was determined from the change in the output voltage after an integration time of 125 ms. In addition, to compensate for the effects of the currents that flow within each pixel even in the absence of light, the response of each pixel after an integration time of 125 ms in the absence of light was subtracted from the response at each centre wavelength. The response distribution across the pixel array when it is illuminated by light with a wavelength of 550 nm is shown in Fig. 3a to demonstrate that, with the exception of a very few defective pixels, there is a good uniformity to the plasmonic components. The spectral responses we present have been determined by averaging the responses of all the pixels with the same plasmonic filter across the whole pixel array. Since it is impossible to directly measure the transmission spectrum of the filters, the performance of the filters has been assessed by normalising the average response of each pixel design using the average response of the unpatterned
reference pixels. Results for pixels with five different plasmonic filters (Fig. 3b) show that the filters act as bandpass filters with different centre wavelengths. The normalised responses of these filters are above 50 % of the signal measured from an unprocessed pixel, and the full width at half maxima are between 110 and 150 nm. One obvious feature of these results that was not seen in the results in Fig. 1c is the oscillations in all the responses. The fact that these oscillations were previously observed in the singlepixel plasmonic CMOS photodetector [15] and that they are also observed in the responses of pixels before any backend-of-line processing has occurred leads to the conclusion that they are attributable to FP resonances in the CIS dielectric stack. The resonances occur because the layers in the dielectric stack have different permittivities; hence, there is a reflection at each interface. This result is not unexpected since the CMOS process we used has not been optimised to reduce this effect, as is usual for commercial CIS. Average results of three groups of pixels with the peak response wavelengths close to red (600 nm), green (555 nm), blue (445 nm), according to the 1931 International Commission on Illumination 2° standard observer colour matching functions [19], are shown in Fig. 4. The transmission
Fig. 4 Normalised photocurrents of three sets of photodiodes with peak responses close to 445, 555 and 600 nm. Transmission spectra of the same colour filters on glass are shown for comparison
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spectra of the same filters on glass are shown for comparison. All photodiode pixels show a blue shift of their peak responses compared to the peak transmission wavelengths of the plasmonic colour filters on glass. Two factors contribute to this phenomenon; one is the smaller hole sizes of the filters integrated on chip than those on glass [9], and the other is the highindex substrate loading effect [20] in the case of CMOS chips. This effect can be eliminated by calibration of the process for colour matching. In addition, the transmission bands of the filters on the CIS seem to be wider than the equivalent filters when they are made on glass. The phenomena that could be contributed to this broadening include the possibility that a wider range of angles of incidence of light occur when the pixels are tested and cross-talk caused by the large vertical separation, 8 μm, between the filters and the photodiodes underneath. These effects mean that the results obtained are not necessarily representative of those that will be obtained in an optimised manufacturing process, especially if the plasmonic colour filters are integrated in lower metal layers in a standard CMOS process. In addition, it is anticipated that the broadening will be reduced by using narrow band filters such as low loss silver filters. The optoelectronic efficiency can be further improved if these plasmonic nanostructures are fabricated close enough to the photodiodes, where the localized field can be greatly enhanced due to the SPR effects [14, 21]. It is anticipated that in the future, state-of-the-art CMOS technology (ITRS roadmap 2010) the half pitch in metal 1 will scale down to 32 nm in 2012, enabling mass manufacture of suitable SPR structures.
Summary In conclusion, a 100×100-pixel CIS incorporating plasmonic colour filter has been presented as an alternative to the conventional pigment dye filters that are commonly implemented in commercial CIS. The plasmonic CIS was successfully fabricated using back-end-of-line processing including electron beam lithography and dry etch. Plasmonic colour selectivity was demonstrated by the photocurrent characterisation of photodiode pixels with various plasmonic filters. The normalised photocurrents of the integrated plasmonic colour filter on a CIS agree well with the transmission of the same filters fabricated on glass. Further work is required to study the effect of colour cross-talk. This research can aid in the development of advanced CIS with reduced cost, low cross-talk and without the need for multilayer processing of dye-based filters. Acknowledgments The authors thank James Grant for chip packaging. This work was supported by a UK EPSRC research grant.
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