[ISSCC] Samsung's CMOS Sensor Takes Range, RGB Images at Same Time
Samsung Electronics Co Ltd developed what it claims is the world's first CMOS sensor that can obtain a range image and a normal RGB (red, green and blue) image at the same time.
The sensor was announced at ISSCC 2012, which took place from Feb 19 to 23, 2012, in the US (thesis number 22.7).
As a method of obtaining a range image, the sensor uses the ToF (time-of-flight) method, which is commonly used. In the past, Samsung Advanced Institute of Technology (SAIT) announced a technology to integrate pixels for obtaining range images (Z pixels) and RGB pixels on one image sensor. But, due to limitations related to near-infrared filter, etc, it cannot simultaneously obtain a range image and an RGB image in a strict sense. It is just an output in a time-sharing manner.
Range image sensors are drawing attention because of the success of Microsoft Corp's "Kinect" gesture-based controller. However, the Kinect is equipped with a range image sensor using the "structured light method." And an image sensor for RGB images is required in addition to the range image sensor. Also, with a stereo method, there need to be two cameras for a parallax.
With the new technology, a normal RGB image and a range image can be obtained at the same time by using a single image sensor, enabling to reduce the sizes of gesture-based controllers, etc. Also, the technology might make it easy to add a range image measurement function to digital cameras, camcorders, etc so that they can recognize gestures.
RGB area has pixel count of 1.38 million
The pixel counts of an RGB image and a range image are 1,920 x 720 and 480 x 360, respectively. The aperture ratio is 48%. The pixel pitch of the RGB area is 2.25μm with a normal RGBG arrangement.
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The Z pixel measures 2.25 x 9.0μm and has an area that is four times larger than that of a red, green or blue pixel. The Z pixel is adjacent to RGB pixels and located below them.
Two column CDS circuits were formed on the upper and lower sides of the pixel area for the RGB pixels and the Z pixel, respectively. A part of the RGB pixels' information that is missing due to the existence of the Z pixel is interpolated by an image processing algorithm.
The operating frequency of the ToF is 20MHz. The integral time of the Z pixel is 10ms per one phase (out of four phases). The margin of error in range measurement is 1m or less for a distance of 1-5m when the output of the LED for the ToF is 0.92W. And it is 1% or less in all the measurable areas when the output of the LED is 1.35W.
Z pixel located below RGB pixels
To develop the new CMOS sensor, Samsung made some improvements so that the effective size of the Z pixel becomes three times as large as its actual size. Specifically, a potential barrier was formed in the layer below the RGB pixels so that photoelectrons generated by the near-infrared light for the ToF do not go to the RGB pixels.
The PD of the Z pixel was formed even in an area deeper than the potential barrier. Also, an epitaxial layer is used to increase quantum efficiency.
The sensor was manufactured by using 0.13μm CMOS image sensor technology. This time, Samsung used a process for FSI. But if BSI (back-side illuminated) technology is applied, it is possible to double the quantum efficiency, the company said. It is because the cross-talk to the RGB pixels by the near-infrared light for the ToF can be more easily prevented.