CCD and CMOS Imaging Array Technologies: Technology Review

This paper provides an overview of both CCD (charged coupled device) and CMOS (complimentary metal oxide semiconductor) imaging array technologies. CCDs have been in existence for nearly 30 years and the technology has matured to the point where very large, consistent (low numbers of defects) devices can now be produced. However, CCDs suffer from a number of drawbacks, including cost, complex power supplies and support electronics. CMOS imaging arrays, on the other hand, are still in their infancy, but are set to develop rapidly and offer a number of potential benefits over CCDs. This review provides an overview of both CCD and CMOS imaging technology, and includes explanations of how images are captured and read out from the imaging arrays. Also covered are issues such as performance characteristics, cost considerations and the future of imaging arrays. This review does not provide details of colour sensors, colour filter arrays and colour interpolation, etc., as these will be the subject of a separate report. Introduction to CCDs The Charged Coupled Device (CCD) was invented in 1970 by Willard Boyle and George Smith at Bell Laboratories, USA [Sharma97]. The idea originated from research into magnetic bubble memories, and as with many great inventions, Smith is quoted as saying “[we] invented charged-coupled devices in an hour” [Lucent96]. In the intervening twentyeight years, CCDs have found their way into a huge range of products including fax machines, photocopiers, cameras, scanners and even children’s toys. CCDs consist of thousands (or millions) of light sensitive cells or pixels that are capable of producing an electrical charge proportional to the amount of light they receive. Typically, the pixels are arranged in either a single line (linear array CCDs) or in a two-dimensional grid (area array CCDs). The particular application will, in general, dictate the type of CCD that is used. Flatbed scanners, for example, use linear array CCDs and, in this case, it is necessary to progressively move the CCD over the object being imaged (or vice versa) while capturing multiple one-dimensional images in order to build up the final two-dimensional image. Digital cameras, on the other hand, normally use area array CCDs, thus allowing the full twodimensional image to be captured within a single exposure. One of the fundamental parameters of a CCD is resolution, which is equal to the total number of pixels that makes up the light sensitive area of the device. One of the first area array CCDs, manufactured by Fairchild in 1974, had a resolution of 100x100 [Oregon97]. Today, the largest commercially available device is approximately 9000x7000 or roughly 63 million pixels [Pixelv97]. Other parameters that characterise CCDs will be discussed in more detail later.