CMOS Sensor Operation

Until recently the industrial digital vision sensor market was dominated by the CCD array. However technological advances in CMOS production techniques have led to a gradual increase in the popularity of this sensor type. Like CCD arrays, CMOS sensors are also formed on a silicon substrate but the structure is more akin to that of other CMOS technology such as RAM and ROM memory devices.
The diagram below is that of an actual CMOS sensor showing the active pixel area in green and the area occupied by the on chip circuitry in yellow, which replaces that of the shuttered area on a CCD based sensor. The on chip circuitry actually converts the charge into voltage on each pixel whereas the CCD sensor shifts the charge vertically row by row, and then horizontally pixel by pixel to be converted to voltage when it reaches one or more output nodes. This gives CMOS sensors an advantage when it comes to windowing or a region of interest as the pixels can be read out randomly. CCD sensors can only limit its region of interest vertically with the resulting image always containing the data for the full image width.

The on chip active amplifier and the sampling capacitor give CMOS sensors advantages in terms of speed, full well capacities and much improved response characteristics yet introduce dark current level noise and higher black pixel content. CMOS sensors can also produce higher levels of fixed pattern noise than that of CCD, but this type of noise can be easily removed with a software filter.

The development of CMOS sensor technology has been a rapid and varied process. The initial aim of CMOS sensors was to match the imaging performance of CCD technology, with lower power requirements and at less cost. To achieve this performance it was discovered that a much greater level of manufacturing process adaptation and deeper submicron lithography were required than initially expected. This led to the desired CMOS performance but increased development costs more than anticipated.

At first the low power feature of the CMOS imaging sensors was set to be one of their distinct advantages, however the improved development of CCD sensors means that while CMOS has the advantage in this area, the margin is now much smaller.

The integration of on chip control circuitry with the CMOS imager provides the sensor with greater flexibility and integration, the downside has been the introduction of greater noise levels. Both CMOS and CCD imaging sensors still require support chips to process the image, however CMOS imagers can be produced with more functionality on the sensor chip, as shown below.

The spectral response of a CMOS sensor differs from that of the CCD sensors in that the peak response is sited at around 700Nm. Both sensors operate over the same range, typically 200Nm to 1100Nm.

The main advantages of CMOS imaging sensors still remain as faster response, increased integration flexibility and lower on-chip power demands. However the image quality has yet to match that of the CCD and the supporting chips required to increase the CMOS image quality goes some way to squander its previous advantages. Yet neither sensor is categorically superior to the other. They both have their own advantages and disadvantages and with CMOS developers working on the image quality, and CCD developers aiming to reduce power demands and increase flexibility, the existing margins in place to decide which sensor is most suitable for an application look to narrow further.