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Considerable effort is usually expended on optimizing the layout of semiconductor devices to minimize their area. This strategy boosts the number of die per wafer and hence minimizes the unit die cost. CMOS image sensors are subject to different economics because a lens train and a protective cover for the imager die are integral to solid state camera modules and their assembly processes. These considerations dictate that the silicon layout must be compromised in order to achieve the lowest cost and smallest form factor camera modules.
Solid state optical sensors
Solid state optical sensors are finding application in an ever-widening variety of products. The largest markets by volume are camera modules for mobile phones, optical mice and digital cameras (see Figure 1). Solid state optical sensors are also utilized in large quantities in Web cams, document copiers, bar code readers, camcorders and positional control systems. These are large markets, which are growing rapidly, many exhibiting compound annual growth rates in double digits [Prismark, 2006]. More than two million cameras are made daily purely for inclusion in mobile phones.
Figure 1: The five largest markets for solid state image sensors in 2005.
The majority of solid state image sensors are based on complementary metal-oxide semiconductor (CMOS) technology as this provides a more integrated solution than competing approaches like CCD (Charged-Coupled Device). A CMOS image sensor comprises a 2D array of solar cells that provide the electro-optic conversion function, together with additional electronics for picture and power management. Each light-sensitive area on a chip with its associated electronics is referred to as a picture element, or "pixel" (see Figure 2).
Figure 2: One pixel of a solid state image sensor. The light-sensitive area is smaller than the pixel dimensions to allow for the integrated electronics.
On-chip real estate is required to house the electronics necessary to provide functionality to the pixel. Consequently, the dimensions of the solar cell in each pixel have to be decreased in proportion, which in turn adversely affects the ability of the image sensor to function in conditions where the light intensity is low. In order to compensate for the light loss, a micro lens is placed on each pixel. Micro lenses are pseudo-hemispherical in shape and serve to focus all of the light that impinges on each pixel onto the light-sensitive area, as illustrated in Figure 3. In a modern solid state image sensor, the pixels will measure about 2µm on a side, so the micro lenses are of similar height.
Figure 3: Micro lenses are used to focus the incident light on each pixel on to the light-sensitive area, increasing the low light sensitivity of the image sensor.
However, these micro lenses add significant constraints in terms of processing and packaging the image sensors. First, to function accurately, an air space must exist directly above the lenses, as any material in direct contact with these micro lenses will alter their optical performance. Second, the micro lenses are very susceptible to particulate contamination. Micro lenses are generally composed of soft, polymeric materials, so foreign particles can easily damage them or become lodged on the surface of the lenses [Chowdhury, 2006]. Third, the micro lens manufacturing process often contains a melting step to produce the curved features, resulting in structures with limited temperature stability that restricts the range and duration of subsequent thermal excursions. Nevertheless, the optical benefits outweigh the other limitations, and all quality CMOS image sensors are provided with micro lenses as standard procedure.
Next: Solid state camera modules
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