Dual-mode VIS-IR complementary metal-oxide-semiconductor (CMOS) image sensors combine color and near-IR imaging capabilities into a single sensor. Using a dual-mode image sensor is more economical and space-efficient than using two single-mode sensors to independently capture color and IR images. Applications of VIS-IR image sensors include gesture recognition, depth analysis, iris detection and eye tracking.
A conventional color image sensor includes a pixel array, wherein each pixel includes a microlens to focus light on it and a spectral filter embedded beneath the pixel's microlens. The spectral filter is designed to transmit a specified range or ranges of visible electromagnetic radiation to its associated underlying pixel. For example, visible spectral filters based on primary colors have pass bands corresponding to the red, green, or blue (RGB) region of the electromagnetic spectrum. Visible spectral filters based on secondary colors have pass bands corresponding to combinations of primary colors, resulting in filters that transmit either cyan, magenta, or yellow (CMY) light. Since the transmission spectrum of a pixel's spectral filter distinguishes it from its neighboring pixels, a pixel is referred to by its filter type, for example, a “red pixel” includes a red filter. Herein, the transmission of a pixel refers to the transmission spectrum of its spectral filter.
FIG. 1 shows representative transmission spectra of visible and IR spectral filters used in CMOS image sensors. Transmission spectra 145, 155, and 165 have peak transmission in the blue, green, and red regions of the electromagnetic spectrum, respectively. Accordingly, transmission spectra 145, 155, and 165 are representative of the spectral sensitivity of blue, green, and red pixels, respectively, of both conventional color image sensors and VIS-IR image sensors.
Herein the terms “IR light,” “IR electromagnetic radiation,” and “IR wavelengths” refer to electromagnetic energy at wavelengths between λ≈0.75 μm and λ≈1.1 μm. The upper limit of λ≈1.1 μm corresponds to the band gap energy of the silicon in CMOS image sensors. Similarly, the terms “visible light,” “visible electromagnetic radiation,” and “visible wavelengths” refer to electromagnetic energy at wavelengths between 0.40 μm and 0.75 μm.
Visible spectral filters used in CMOS image sensors also transmit IR wavelengths, which results in image distortion known as “IR contamination.” For example, absent green light incident on a pixel with a “perfect” green filter (non-zero transmission for green light only), the pixel produces no photocurrent beyond its dark current. However, a typical green filter associated with a green pixel has a transmission spectrum similar to transmission spectrum 155, FIG. 1. Within the visible spectrum, transmission spectrum 155 has a peak transmission near λ≈550 nm, corresponding to green light. Since the transmission spectrum 155 exceeds 80% above λ≈800 nm, the green pixel will produce a photocurrent in response to incident IR light, which results in IR contamination. To prevent such distortion, the conventional color image sensor also includes a single continuous IR-cut filter, or IR-blocking filter, disposed across the entire pixel array.
VIS-IR image sensors include both visible spectral filters and IR spectral filters—the latter having a transmission spectrum similar to transmission spectrum 191. Since devices employing VIS-IR image sensors depend on detecting IR wavelengths, a single continuous IR-cut filter covering the entire pixel array cannot be used in VIS-IR sensors as they are for color image sensors. Since transmission spectra 145, 155, and 165 have significant transmission in the IR region, removing IR contamination requires pixel-level filtering: where an IR-cut filter operates only on color pixels while not impeding IR light from reaching IR pixels.
FIG. 2 is a cross-sectional view of a representative prior-art VIS-IR image sensor 200 with reduced IR contamination. FIG. 2 is adapted from Kawada et al, IEEE Conf. on Sensors, 2009. Image sensor 200 includes a blue pixel 204, a green pixel 205, a red pixel 206, and an IR pixel 211. Pixels 204, 205, and 206 include visible spectral filters 245, 255, and 265, respectively, and each of visible spectral filters 245, 255, and 265 is disposed on an IR-cut filter 291. In Kawada et al, IR-cut filter 291 is a multilayer stack of SiO2 and TiO2 thin films, and is not included in a transparent region 295 of an IR pixel 211.
One method for fabricating image sensor 200 requires masking IR pixels 211 when forming IR-cut filter 291 within color pixels 204, 205, and 206. An alternative method of fabricating image sensor 200 involves forming IR-cut filter 291 within all pixels 245, 255, 265, and 211 (visible and IR), and then removing regions of IR-cut filter 291 above the photodiode regions of IR pixels 211 using, for example, a photolithographic patterning process. However, as pixel dimensions become smaller the processes involved with photolithographic patterning become more challenging.