Surface inversion and back side bias in large area X-ray CCDs.

Our group is currently testing large format low noise CCD deep-depletion detectors developed by MIT Liconln Laboratory for future X-ray missions, such as STAR-X, AXIS, Lynx and others. Imaging section of those large CCDs has correspondingly large area and capacitance, which has serious consequences when it is necessary to run such devices at high speed of parallel transfers. While it is well known that running the CCD gates in the inversion mode can significantly reduce dark current by suppressing surface traps, we have found that in such mode parallel charge transfer at higher speed becomes problematic. This can be explained by larger gate-to-substrate capacitance in the inversion regime and delayed signal propagation along the gates in the imaging section. Surprisingly, we discovered that changing substrate bias (voltage applied to the back side of the CCD in order to achieve deep depletion) can also affect parallel charge transfer efficiency, seemingly in correlation with the state of surface inversion. Studying dark current behavior as a function of substrate voltage we established that substrate voltage indeed can move buried channel surface in and out of inversion and we attribute this to the fact that channel stops inside the array are not firmly connected to an external contact. More negative voltages (around -15V - -20V) can pull device out of inversion and significantly improve parallel transfer quality. If inversion mode is necessary for dark current reduction, one possible solution is to keep surface inverted during integration only, and avoid inversion during parallel transfer. In any case, a discovered interaction of back side bias with the surface inversion is an important trade space that needs to be taken in consideration for future applications of large X-ray CCD arrays.