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Improving CMOS sensor performance

Posted: 01 Jul 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Failop Chu? Kodak Image Sensor Solutions Group? Pixelux? cmos? sensor?

CMOS image sensors have long offered intrinsic power, system integration, form factor and overall system cost advantages. Adoption of CMOS technology has been hampered historically by image quality issues. Thus, camera designers have primarily relied on CCD technology to provide the image quality required for consumer and professional imaging applications.

Today, design engineers have an innovative, new CMOS image sensor technology for devices incorporating imaging technology such as camera phones, web cameras and DSCs. This option is the use of CMOS image sensors incorporating Kodak Pixelux technology.

This technology leverages three key design elements: a four-transistor (4T) pixel architecture, use of a pinned photodiode and a shared pixel design. These design elements combine to enhance image quality through improved photosensitivity and reduced noise, and to offer novel charge-binning modes of operation that improve image quality with reduced resolution.

The predominant architecture used in the design of today's CMOS image sensors is based on a three-transistor (3T) pixel design. A typical 3T pixel consists of a photodiode, reset gate (RG), row select and source follower. To read the charge at a given pixel, the pixel is first reset to the voltage level VDD using the RG. This pixel is then left to integrate light, decreasing the voltage across the photodiode as photons are converted to charge, with the final voltage level stored on the common column line. Next, the pixel is reset a second time. This time, the reset voltage is measured and also stored on the column line. The column circuit then samples the signal and then the reset value, and subtracts these two values to obtain the final pixel value. The pixel measurement, in turn, is the difference between the voltage levels at reset and after light integration.

While this subtraction would appear to provide an accurate measure of the signal integrated by the pixel, this is not always the case for the 3T architecture. This is because each time the photodiode is reset, it resets to a slightly different voltage. Since the reset levels before and after the integration are slightly different, an error is introduced into the subtraction. This error is a random noise source, sometimes referred to as kTC noise, in reference to the equation that determines its magnitude. Since the reset level is measured after the pixel has been integrated, the reset and pixel values are not truly correlated. Besides kTC noise, several other noise sources are intrinsic in 3T pixels:

  • The sense node where the photodiode and RG meet on a 3T pixel typically incurs a high dark current due to process-induced damage and produces unwanted noise to the image.

  • The output response of a 3T pixel is nonlinear because the photodiode's capacitance is voltage-dependent. As the photodiode fills up and the signal increases, the charge-to-voltage conversion factor gets lower. This can lead to objectionable color artifacts in the image.

  • Residual charge on the photodiode brings about image lag in fast-changing dark-to-light settings, which can cause ghost images.

Pixelux architecture
The 4T design adds an additional transistor, the transfer gate (TG), and a floating diffusion (FD) to the standard 3T CMOS pixel. Here, a pixel is reset when the RG and TG are turned on simultaneously, setting both the FD and the photodiode to the VDD voltage level. Next, the TG is turned off (disconnecting the photodiode and FD), and the photodiode is left to integrate light.

After integration, signal measurement occurs. First, the reset transistor is turned on and off to reset the FD. Immediately after this, the reset level is sampled from the FD and stored in the column circuit. Next, the TG is turned on and off, which allows charge on the photodiode to transfer to the FD. Once charge transfer is complete, this charge (the photodiode signal level plus the FD reset level) is measured and stored in the column circuit.

These two stored voltages are then differenced to determine the photodiode signal level. This design allows for true correlated double sampling (CDS) operation to occur, as the reset level used to determine the absolute pixel level is now measured before the signal level and the same reset level is referenced throughout the measurement. By mimicking the operation of a CCD, the 4T design improves on the performance of the standard 3T architecture, reducing both read noise and image lag.

Besides the 4T design, Pixelux technology also uses a pinned photodiode. Through a patented ion implantation process, a potential barrier is createdit covers up damage at the surface of the photodiode and buffers the vital signal electrons from that area. This suppresses the dark current and lowers the noise in the image. Pinning the surface potential enables readout of all electrons from the photodiode and helps eliminate the pixel's "memory" of previous integrations, thus reducing image lag.

Pixelux technology also incorporates a novel architecture that uses a common read-out circuit for multiple pixels.

Benefits of Pixelux
Two specifications determine the sensitivity of an imaging device. First is the sensor's capability to convert light into signal, usually expressed as its quantum efficiency. The second is the sensor's noise floor, which limits the ability to capture a useful image in low-light conditions.

Pixelux architecture increases quantum efficiency because of an increased fill factor, the portion of the pixel devoted to capturing light. The presence of an additional transistor relative to a 3T design appears to reduce the overall fill factor of the pixel, an issue particularly noticeable for small-pixel devices.

However, by sharing common readout elements across multiple pixels, the overall fill-factor of the pixel can be increased in a 4T4S design, improving performance.

Sensitivity is also improved by reducing noise. The pinned photodiode lowers the noise associated with the dark current of the photodiode, and the 4T architecture allows for true CDS to eliminate kTC noise. These factors combine to improve the overall SNR of the Pixelux pixel relative to the standard 3T pixel. All these considerationshigh quantum efficiency, low dark current and low read noisetranslate into higher image quality, especially in poorly lighted environments.

- Failop Chu
Applications Engineer, Kodak Image Sensor Solutions Group

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