B.S.T.J. Briefs: Blooming Suppression in Charge Coupled Area Imaging Devices

Blooming Suppression in Charge Coupled Area Imaging Devices By C. H. SEQUIN (Manuscript received June 23, 1972) An intense spot of light projected onto the photo-sensitive surface of an imaging device can cause this device to saturate locally. Excess carriers generated by the light source can diffuse into the neighboring area which may also be driven into saturation. In the display the light source will then appear as a white area that can be considerably larger than its image in the true geometrical proportions. This effect, known as blooming, is present in most TV camera tubes, and demands special care by the operator to avoid bright objects in the scene being imaged. For the Pictureyhone® camera, which often has to operate in less than ideal conditions, the design of a camera with limited blooming is thus more than desirable. The camera tube presently used in the Picturephone station set has a silicon diode array target scanned by an electron beam. Blooming is produced by the diffusion of carriers in the bulk silicon, leading to a circular spreading of the saturated area. In solid-state imaging devices, blooming can take on even more objectionable forms. The complicated potential distribution at the silicon surface can cause excess carriers to move along a preferred axis, generating quite irregular blooming patterns in the display. In a recently demonstrated 128 X 106-element charge coupled array 1 the excess charge spills preferentially in the vertical direction. In this n-channel frame transfer 2 device the isolation between adjacent CCD channels is achieved by a p-type channel stopping diffusion which keeps the potential at the Si-Si02 interface close to zero.