Visible filter integrated with an image sensor fabricated by a 90-nm standard CMOS process

Abstract

A visible filter, enabling a homogeneous integration with an image sensor, was demonstrated through an advanced standard 90-nm CMOS process. The overall transmission was as high as about 85% in the visible band.Visible filters, in conjunction with a complementary metaloxide-semiconductor (CMOS) image sensor (CIS), have been extensively used as a color selecting element for such diverse applications as digital cameras and sensors [1]. However, a homogeneous integration of the visible filter with the image sensor via a CMOS process is definitely preferred to the hybrid integration, in order to accomplish prominent benefits, including a high compatibility, a low cost and a flexible integration with signal processing circuits and other optical devices.In this paper, we attempted to demonstrate a visible filter incorporating a one-dimensional (1D) subwavelength patterned grating in aluminum (Al), integrating homogeneously with a CMOS image sensor.The configuration of the proposed device is sketched in Fig. 1. The CIS was first covered with a bottom oxide film, serving as the substrate for the filter. A 1D Al grating was formed on the bottom oxide, and then overlaid with a top oxide layer. An array of microlens was formed on top of the top oxide to be used for a beam focusing. The structural parameters associated with the device are: Λ for a grating pitch, H a for a grating thickness, W for a grating width, H o for the height of the top oxide layer. A filter which incorporates a 1D metallic subwavelength grating is well known to function as a low-pass wavelength filter for a transverse electric (TE)-polarized light, when the electric field is aligned in the direction of the grating [2].A simulation tool based on the finite-difference timedomain method, OptiFDTD, was employed to analyze and design the device by taking into account the dispersion characteristics of the metallic layer. It should be especially noted that the transmission efficiency could be increased by as much as ~70% with the help of an overlay covering the Al grating having the same properties as the bottom oxide layer. This stems from the fact that the difference in the surface plasmon wave on the top and bottom side of the metallic grating may be minimized in order to help boost the light transmission [3]. The theoretical spectral response of the device is shown in Fig. 2 for a TE-polarized light. Whereas the grating width was fixed at a minimum feature size of 90 nm, the grating pitch was primarily altered to adjust the cutoff wavelength and to maximize the transmission efficiency [2]. The consequent structural parameters were: Λ=300 nm, W=90 nm, H a =170 nm, H o =150 nm. As for the calculated performance, the overall transmission efficiency in the visible band was at about 85%, and the transmission in the near-infra band was substantially suppressed below 20%. Im ag e se ns or Bo tto m ox id e la ye r Al To p oxi de lay er Visible filter H o H a Λ W Microlens Im ag e se ns or Bo tto m ox id e la ye r Al To p oxi de lay er Visible filter H o H a Λ W ...

Nanophotonic Image Sensors

“…For nanophotonic elements integration to IS, there have been several methods, such as physical attachment, post‐CMOS process, back‐end‐of‐line CMOS process . Results of both photocurrent measurement and real imaging process prove the functions.…”

“…The nanophotonic optical elements have to be optimized to fit for this process rules at a cost of performance, unless the academia's research outcome convince the industry to modify their processes. Fortunately, several industry players including CEA LETI, ST Microelectronics, Panasonic, Sony, Samsung and LG have been involved in nanophotonic spectral devices and nanophotonic IS . Considering the ultrasmall physical size of nanophotonic elements and their unique structure based properties, nanophotonic IS has a promising potential for the next generation IS with the development from both academia and industry.…”

“…In this section, recent development and progress of IS integrated with nanophotonic color filters are reviewed. They are sorted into three types, 1) IS with attached filters that are fabricated on other carriers, 2) IS with filters integrated on top via post‐CMOS process, 3) IS with filters integrated via the back‐end‐of‐line CMOS process …”

“…d) Reproduced with permission of the Copyright holder, Laurent Frey. e,f) Reproduced with permission . Copyright 2010, IEEE.…”

“…Yoon and Lee with support from Siliconfile Technologies reported one dimensional grating color filters integrated in IS . Using a 90 nm CMOS process, aluminium grating color filters with a width of 90 nm and a period of 260–360 nm were fabricated.…”