CMOS Image Sensors
Table of Contents:
Advent of CMOS technology in eighties led to the phenomenal growth in semiconductor industry. Transistors have become smaller, faster, consume less power, and are cheaper to manufacture. It is CMOS technology which has enabled very high integration on the chips leading to modern high performance, miniaturized integrated circuits.
Apart from the valuable contribution in miniaturization of integrated circuits, CMOS technology found applications in sensing applications. CMOS technology has been adopted to design sensors, especially in the field of imaging. Due to the wide usage of CMOS based image sensors, CMOS sensors are often considered to be a synonym of CMOS based image sensors and have emerged as a competitor to CCD based image sensors.
Until recently, Charge Coupled Devices (CCDs) dominated most of the image sensing systems, i.e., cameras, camcorders, etc. CCDs have been in use in astronomical cameras, video camcorders and scanners. However of late, CMOS Imaging have emerged as an alternative to CCD imagers and it also offers better features.
Subsequent sections will discuss both CCD and CMOS sensor based imagers, their pros and cons, and also their applications. Further, other applications of CMOS technology in the field of sensing will be discussed.
CMOS Vs CCD
Invention of CCD marked the end of vacuum tube imagers used in television cameras as it overcame the disadvantages of vacuum tubes like chronic picture artifacts as lag and burn-in, fragility of large glass tubes or the sensitivity to shock, vibration and electromagnetic radiation, painstaking periodic alignment of tubes, etc. It also marked the beginning of a new era in imaging systems and for decades, it enjoyed quality advantages over the rival CMOS sensors. Wherever image quality was paramount, CCDs were preferred, CMOS were used mainly in applications where small size and low power were prime requirements.
With the technological development in CMOS technology, gap between CCD and CMOS sensors has narrowed; CMOS sensors can also achieve competitive quality. Choice amongst CCD and CMOS sensors has become increasingly difficult.
Both CCD and CMOS image sensors use large arrays of thousands (sometimes millions) of photo-sites, commonly called pixels. Both carry out same steps.
1. Light-to-charge conversion
Incident light is directed by the microlens (a tiny lens placed over the pixel to increase its effective size and thereby fill factor) onto the photo-sensitive area of each pixel where it is converted into electrons that collect in a semiconductor "bucket."
The bigger the pixel, the more light it can collect. Thus, big pixel sensors work best under low-light conditions. For the same number of pixels, bigger pixels results in bigger chip, this means higher cost. Conversely, smaller pixels enable smaller chip sizes and lower chip prices, as well as lower lens costs. But there are limitations on pixel size reduction. Smaller pixels are less sensitive to light, the optics required to resolve the pixels becomes expensive and requires expensive fabrication possesses.
2. Charge accumulation
As more light enters, more electrons accumulate into the bucket.
Accumulated charge must be transferred to the signal conditioning and processing circuitry.
4. Charge-to-voltage conversion
The accumulated charge must be output as the voltage signal.
Voltage signal is then amplified before it is fed to the camera circuitry.
Both CMOS and CCD perform all these tasks; however the aspect in which they differ is the order of execution of these tasks.