  Tearing patterns affecting flat field frames in CCDs are a visually striking
obstacle to performing Pixel Response Non-Uniformity corrections. These
patterns can be explained by lateral field distortions, caused by the
non-uniform distribution of holes in the channel stops between sensor columns.
Over the course of LSST camera development, a number of practical fixes have
been suggested to get rid of tearing. But applying these fixes to our
16-channel Teledyne-e2v sensors leaves at best a distortion pattern at the
vertical edges of every segment. Our working hypothesis is that the origin of
the tearing is the parallel clocking itself, which moves the holes that are
present in the channel stops regions. The efficiency of these transfers depends
strongly on the details of the clocking operations, resulting in the observed
variety of distortion patterns. Removal of most of the distortion patterns can
therefore be achieved by executing a purge operation, which flattens back the
hole distribution in the channel stops, immediately before acquiring a frame. A
more effective solution is to switch all clocking operations to use a bipolar
voltage set.
