Electronic Design

Brighten Your CMOS Image Sensor Prospects By Resolving Dark Current

Implemented with matrix-addressed photodiode arrays, CMOS image sensor (CIS) solutions take full advantage of the economies of scale afforded by a highly developed semiconductor-manufacturing infrastructure. The chip industry's steady march to finer submicron nodes, combined with adding more features on a per-pixel basis, continue to push CIS solutions ahead of charge-coupled devices (CCDs).

In fact, CIS technology makes it possible to integrate imaging, timing, and readout functions all on the same device. This leads to practical system-on-a-chip solutions that serve an expanding array of display-centric applications. While the noise levels of both technologies are comparable, CIS technology has much more saturation capacity than CCD technology.

Recent CMOS-processing advances have boosted CISs, particularly the virtual elimination of "dark current." Dark current is produced by a photodiode array in the absence of light. Such unwanted current is caused by volatile organic compounds (VOCs) in air, also known as "photochemical smog." VOCs can adversely impact the ability of CIS photodiode arrays to convert incident photon flux into high resolution digital images.

To overcome image degradation caused by dark current, a team from Dongbu took a three-pronged attack at the CMOS substrate level. First, it built up the P+ region at the junction of shallow trench isolation (STI) and the photodiode, eliminating "dark current" resulting from defects caused by a non-uniform silicon interface (Fig. 1). Second, it built up mask polysilicon to create a protection pole, enabling uniform colors up to 10 Mpixels (Fig. 2). Third, it removed "image lagging" by infusing tilt ions (Fig. 3).

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