Characterization of generation currents in solid-state imagers

The study of dark current in CCD's is difficult because of the complexity of the process and because generation can come from a variety of sources. For scaled devices, generation from channel-stop sidewalls is particularly important, since the sidewall scales as a perimeter. Relatively little attention has been paid in the past to this source of generation. In this paper, analytical techniques are described for profiling interface states along channel-stop sidewalls. These techniques rely on changes in generation current caused by expansion of the surface depletion region. Two methods of determining the extent of surface depletion are discussed. The first relies heavily on two-dimensional modeling, while the second uses experimental measurements of interelectrode capacitance to avoid certain limitations associated with model parameters. These techniques are used to show that the generation-current density peaks strongly in the birdsbeak region of LOCOS isolation, resulting in a significant contribution to dark current during CCD operation, even for channel-stop spacings as far apart as 12 µm. These techniques are also used to compare the behavior of different regions of the oxide interface of CCD imagers as a function of post-oxidation annealing, including rapid thermal annealing.