Future Trends of Imaging Sensors and Systems
jan.andersson@acreo.se
The Imaging Department at ACREO works with all parts of electronic imaging systems, in particular detector arrays for infrared and X-ray, design of CMOS circuitry, optically based image processing, modulator arrays, image quality, and visual ergonomics of computer displays.
Throughout the history of mankind there has been the desire of humans to record scenes observed by the human eye. The earliest way of performing this was the artist carefully painting or sculpturing scenes or objects. Nowadays imaging systems such as cameras do the job for us. Generally speaking, an imaging system comprises collection of light by focussing optics, detection of the optical image at the focal plane, image processing and storage, and finally displaying the image. It should be noted that for displaying devices, such as computer monitors or hard copies on paper, meant to be perceived by a human observer, the human being also becomes an integral part of the imaging system. In film based cameras photographic film both detects, processes and stores the image information in a chemical format.
The major technological trend today is R&D as well as extensive marketing of electronic imaging systems such as digital cameras, where optoelectronic semiconductor devices are key components. This adds completely new features to the systems as well as higher performance and larger flexibility. The role of the photographic film is taken by an imaging sensor that converts the optical image to electronic format. A common design is a detector array consisting of a two-dimensional array of detector pixels. High-resolution detector arrays for visible, infrared and X-ray radiation are already on the market today, but there is an enormous developmental activity going on throughout the world in the field aiming at novel products at larger volumes. The image may be displayed on a computer display such as a CRT (cathode ray tube) or an LCD (liquid crystal display), to mention a few.
Turning now to the development trends of imaging sensors one could first conclude that a golden rule of semiconductor technology says that silicon should be used if it can do the job for you. Over a long time period an extensive infrastructure has been developed especially for silicon CMOS (complementary metal oxide semiconductor). Today numerous foundries exist where device fabrication can be carried out, and customers are encouraged to use their own designs. The CCD (charge coupled device) technique is an alternative to CMOS when making imaging sensors. Drawbacks of CCDs are higher price, fewer foundries, and that signal processing electronics is not easily implemented. Advantages are low noise and high pixel resolution. However, CMOS technology is successively improving and is now competitive to CCDs for most applications except where the highest performance is needed. A clear trend is that the older CCD technique is successivley replaced by the flexible and cheaper CMOS technique.
Detector arrays can be monolithic or hybrid. Monolithic arrays, which are built from a single chip, are typically CMOS based and are usually the cheapest alternative at least for large volumes. It is expected that image sensors operating at visible wavelengths will in the long run be almost completely CMOS based.
For long wavelength (>3 µm) infrared sensors, used for night vision or thermal imaging, CMOS will also dominate but here the process either needs to be modified or hybrid solutions are needed. A recently developed infrared sensor concept is the microbolometer array which is able to operate at room temperature in contrast to conventional infrared sensors which need cooling to liquid nitrogen temperatures. Uncooled infrared sensors offer much cheaper infrared camera systems especially for very large volumes, which makes it the technology of choice for future night vision systems in cars. Microbolometers are typically fabricated by combining a CMOS process by a post-process based on micromachining (MEMS).
When monolithic sensors cannot be made one has to resort to hybrid ones, where several chips are combined into a unit. The reason is usually that detectors cannot be based on silicon or materials compatible with silicon, and that more exotic materials are needed such as gallium arsenide, indium antimonide and mercury cadmium telluride for nfrared, and cadmium telluride or gallium arsenide for X-ray detection. In this hybrid case a chip consisting of the detector pixels are usually flip-chip bonded to a silicon CMOS chip, consisting of a readout integrated circuit (ROIC) where multiplexing and conditioning of the electronic signals take place. An example is the quantum well infrared photodetector (QWIP) array that is developed and produced at ACREO.
The general trend for electronic imaging sensors is certainly a higher performance at a lower cost. This puts requirements from the application’s side, such as detecting and transmitting imaging information at higher speed and more efficiently, and a higher pixel resolution. Today pixel counts of a million or more are standard. High sensitivity, but just enough for the application, is another prerequisite. Furthermore, detection at non-standard wavelengths is achievable such as ultraviolet detection, and simultaneous detection in several different wavelength bands. Compact, user friendly components or interfaces is another important trend.