Semiconductor image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals. A back side illuminated (BSI) image sensor device is one type of image sensor devices. These BSI image sensor devices are operable to detect light from its backside.
The conventional sensor, called the “front side illumination (FSI)” image sensor for these CMOS chips, is constructed in a fashion similar to the human eye, and has a lens at the front, layers of metal having wiring in the middle, and photo detectors on a silicon substrate (which absorbs the light) at the back. These metal layers may not only deflect the light on the sensor, they could also reflect it, reducing the incoming light captured by the photo detectors. By contrast, the back side illuminated sensor has the same elements as FSI, but orients the wiring behind the photo detectors layer by flipping the silicon wafer during manufacturing and then thinning its reverse side so that light will hit the silicon first, and the photo detectors layer without passing through the wiring layer. This change can improve the chance of an input photon being captured from about 60% to over 90%, and the sensitivity per unit area to deliver better low-light shots.
A BSI image sensor device typically has a radiation-absorption region, a periphery region or a radiation-blocked region, and a bonding pad region. The radiation-absorption region has a silicon substrate that includes an array or grid of pixels formed inside for sensing and recording an intensity of electromagnetic radiation or wave (such as light) entering the substrate from the backside, and some circuitry and input/outputs adjacent the grid of pixels for providing an operation environment for the pixels and for supporting external communication with the pixels. After the grid of pixels and the circuitry and input/outputs are formed within the substrate, the substrate is thinned from its backside to a desired thickness, the backside of the substrate in the radiation-absorption region is covered by one or more bottom anti-reflective coating (BARC) layers or films and a buffer oxide layer or film. The radiation-blocked region includes devices that need to be kept optically dark such as an application-specific integrated circuit (ASIC) device, a system-on-chip (SOC) device, a logic circuit, or a reference or calibration pixel that is used to establish a baseline of an intensity of light. For that, a conductive layer including a metal grid and a metal shield ground is formed over the BARC layers. The metal grid blocks external radiation from entering the substrate in this radiation-blocked region, and the metal shield ground releases charges on the metal grid collected and/or induced during wafer processes to the substrate, which is grounded.
BARC layers are made, in some cases, of high dielectric constant (“K”) material and mostly have accumulated charges (mostly negative but in some case positive). The charge accumulated layer of the BARC films play an important role in improving dark current, white pixel, and dark image non-uniformity (DINU) quality issues. Such an undesired neutralization of the BARC layers by the excess charges on the metal shield ground causes. When the BARC layers have negative (positive) charges accumulated, they attract positive (negative) charges in the substrate to the BARC/substrate interface to form electric dipoles. And the electric dipoles play the role of a charge barrier, trapping the imperfections or defects such as dangling bonds.
In that regard, the performance of a metal ground in effectively releasing charges collected on the metal grid is highly important for quality control and cross-talk improvement. The problem, however, is that imperfections in the interface inevitably present between the metal ground and the substrate damage the performance. When the performance of the metal ground is not good enough, the excess charges accumulated on the metal ground tend to neutralize the negative charges present in the adjacent BARC layers. When the electric dipoles become destroyed by such neutralization of the BARC layers, the un-trapped imperfections or defects contribute to dark currents, cause DINU failures, and impairs quality of the device.
Therefore, in order to have the BARC layers in a BSI image sensor device efficiently trap the imperfections or defects, and thereby reduce dark currents and DINU and enhance the quality and performance of the device, it is desirable to provide a system and a method of preventing the undesired neutralization of the accumulated charges in the BARC layers by the excess charges accumulated on a metal shield ground.