CMOS Technology

CMOS Image Sensor Technology

CMOS (Complementary Metal Oxide Semiconductor) is a mainstream semiconductor technology, used to fabricate a wide range of integrated electronic circuits. Image sensors based on CMOS technology can take full advantage of the fast pace of innovation in the semiconductor industry. Some of these advantages are reduction in feature size, increased operating frequencies, and integration of complex components. In addition, due to the advances in Very Large Scale Integration (VLSI), System on Chip Integration (SOC), and automated high-volume production CMOS sensor technology has improved its imaging performance.

Pixel technology

AWAIBA works with leading CMOS establishments to further develop its own CMOS sensor features, such features include buried photo-diodes, buried channel MOS devices and fully pinned pixels. AWAIBA develops it’s on pixel IP be it for small size rolling shutter pixel or for high speed global shutter pixels. Figure 1 shows a classical 3 Transistor pixel and figure 4 a typical implementation of a pixel with pinned photo-diode, bearing major advantages in terms of dark current and electrical read noise. AWAIBA adds its own architectures to those topologies for global-shutter pixels, low noise high sensitivity pixels or extremely small pixels.

Active Pixel CMOS image sensors. APS
4T Pinned pixel principle.

Global Shutter

While for some applications sensors based on the above pixel architectures provide good results’ their “rolling shutter” behavior may lead to major distortions in other applications. This is evident when pulsed illumination is used or fast moving scenes are to be captured. For these applications, AWAIBA provides the solution with its unique intellectual property on truly global shutter pixel architectures.

Image of rotating fan with rolling and with global shutter image sensor.

High Dynamic Range

High in Scene dynamic range imaging is not limited to the automotive industry. Applications such as On board vision for driver assistance and safety are not the only applications that utilize this technology. Industrial inspection environments, such as laser welding, measurement of reflecting surfaces or through silicon inspection require the use of high dynamic range imaging capability. Even if resolution of high in scene dynamic range is not necessary, the artifacts created by high illumination over load is, in many computer vision applications, mission critical. Due to AWAIBA’s proprietary pixel architecture their high dynamic range sensors are free from artifacts such as blooming, image lag and contrast inversion (corona effects).

Blooming artefacts of CCD image sensors
High dynamic scene without saturation and artefacts acquired by CMOS image sensor. The image shows a human eye under inspection with a slit lamp.

Compressed film of an AWAIBA CMOS HDR Sensors with 750 x 400 Image points (10MB). Download this film: tunnel_video_mpeg4

Spectral response

AWAIBA has also developed techniques to enlarge the spectral response of standard CMOS image sensors slightly, in both directions: towards the UV and the IR. A typical spectral response can be found in next figure.

Typical quantum efficiency versus wavelength of CMOS photo diode in AWAIBA technology

Area scan image sensors and line scan image sensors

Area scan image sensors make up for the vast majority of image sensors, they are made of a two dimensional matrix of pixels and acquire a two dimensional pixel at one and the same integration. AWAIBA develops a wide range of area scan image sensors, mainly for very high speed and high resolution image sensing, such as for crash test cameras or ultimate speed machine vision processing over 1000 images / second. Further AWAIBA specializes in development of ultra small area scan image sensors, where not only the pixel size but also the chip periphery and package is reduced to absolute minimum. Such sensors are used in minimally invasive surgery for disposable endoscopes and locater devices.

Line scan image sensors

As opposed to area scan image sensors, line scan image sensors acquire only a single line of pixels at a time. This permit’s in imaging of fast moving objects to concentrate the illumination source on a fine line, achieving much higher illumination levels compared to illumination of a whole area. Thus line scan image sensors can operate at much shorter integration times. Further line scan image sensors can read out high resolution images with much higher bandwidth compared to area scan image sensors, since there is no need to access a single line out of a full matrix for readout as in area scan image sensors. Another advantage of line scan sensors is in the inspection of endless goods, since the data can be processed directly on a line by line basis we can treat high resolution data with reduced memory requirement. Any motion blur, due to jitter in the transport speed, can be compensated by adequate triggering.

State of the art packaging

Not only the silicon of the image sensor is mission critical for high performance imaging applications, but also the packaging. This means electrical and mechanical assembly of the sensor into the optical application, has a major impact on over all performance. AWAIBA employs several cutting edge packaging technologies depending on the needs of the applications. The sensors for the inspection of large areas place a priority on the mechanical stability and planarity of the CMOS sensor. These specifications are usually achieved by means of a customized sensor board. Endoscopic and other medical applications, in contrast, demand an entirely different approach. There the solution lies in less integration and more towards miniaturized optics, as the size of the sensor is of greater importance in such applications. AWAIBA partners with leading packaging and micro machining technology providers world-wide to be able to offer optimized solution packages as per each project’s needs.