Omnivision Explains Image Ghosting Causes

Image Sensors World        Go to the original article...

Omnivision's patent application US20140117485 "Negatively charged layer to reduce image memory effect" by Howard Rhodes, Dajiang Yang, Gang Chen, Duli Mao, and Vincent Venezia talks about rarely mentioned but commonly observed ghost image effects: "The electrical signature of an image with high brightness levels that falls onto a complementary metal oxide semiconductor (CMOS) image sensor may remain embedded in subsequently read out electrical signatures of subsequently acquired images. The electrical signature of a previously sensed image remaining in the image sensor has been called a “ghost artifact” or a “memory effect.” This unwanted effect can be exacerbated by repeated exposure of static images, especially high intensity or bright images, to the image sensor. The retention of ghost images represents noise that obscures subsequently acquired images and reduces the signal to noise ratio and may cause blur if there is movement being imaged.

The memory effect problem has been found to be especially present in CMOS image sensors that have been fabricated using advanced fabrication technologies, particularly those employing measures to maximize metal interconnect density. For instance, those fabrication technologies employing so-called “borderless contacts” have been found to be associated with the root cause of this problem.
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Omnivision explains: "The deposition of the contact etch stop layer is a fabrication technique that may be utilized when providing borderless contacts, which may be employed to increase metal interconnect density in pixel array."

Contact etch stop layer 322 is deposited over passivation layer 320, which is deposited over the pinning surface layers 313 included in example pixel array 302

"In one example, contact etch stop layer 322 may include a silicon nitride based dielectric including for example, silicon oxynitride, silicon carbide, or the like... The mobile charges in the PECVD silicon nitride and/or silicon oxynitride of contact etch stop layer 322 can be moved by electrical forces such as electrical fields placed across contact etch stop layer 322, which can cause unwanted effects in nearby semiconductor regions, such as photodiode regions 312 and/or the pixel circuitry included in the pixels of pixel array 302. For example, the source to drain resistance of a transistor included in the pixel circuitry included in the pixels of pixel array 302 may be affected by the mobile charge in the overlying PECVD silicon nitride of contact etch stop layer 322 by altering the depletion characteristics of an underlying lightly doped source or drain region.

Furthermore, it is noted that net positive charges can be induced directly in the PECVD silicon nitride and/or silicon oxynitride of contact etch stop layer 322 by exposure to visible light that may pass through contact etch stop layer 322, especially when photodiode regions 312 of pixel array 302 are illuminated with bright light when imaging. In particular, the energy associated with the phonon modes of the Si—Si and Si—H crystal structures may participate in the optical excitation of the electrical carriers. Consequently, memory effect is caused by the generation of positive charges in, for example, the SiON film of contact etch stop layer 322 that overlies the photodiode region 312 under the strong light illumination.
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"To illustrate, FIG. 3A shows light 315 illuminating photodiode region 312, which therefore illuminates and passes through contact etch stop layer 322 as shown. This may occur when photodiode region is capturing an image. As a result of this illumination with light 315, positive charge 317 is induced in contact etch stop layer 322, which induces electrons 319 at the surface of the photodiode region 312 surface as shown.

FIG. 3B shows that after light 315 is no longer present and photodiode region 312 images a darker scene or is in a low light condition after having been illuminated with bright light 315 and after the image has been captured, the induced electrons 319 at the surface of the photodiode region 312 are injected into the photodiode region 312, causing the unwanted memory effect. In other words, when the pixel including photodiode region 312 images a darker scene, the induced electrons 319 at the surface of the photodiode region 312 that were a result of the previously captured image are injected into photodiode region 312, which generates localized dark current causing an unwanted “ghost image” of the previously captured image to appear as a memory effect in pixel array 302.
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A nice explanation of the positive after-image. Indeed, many CMOS sensors have it positive. In some sensors the after-image is negative though - probably a different effect.

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