Metal contamination analysis of the epitaxial starting material for scientific CCDs

Traces of unintentionally introduced titanium into a 3 kΩ cm float-zone epitaxial silicon at ultra low levels of 1010 cm−3 were found to be the origin of charge transfer loss in pn-CCDs. We identified and backtracked the titanium impurity. The full-depletion design of the pn-CCD, a thin entrance window at the back of the CCD and the low oxygen content of the float-zone material allow no common gettering step. Titanium is introduced into the wafer during the epitaxy process. This is independent of the reactor and the producer of the epitaxial silicon and seems to be a common epitaxy-related problem. To identify the impurity, electron emission rates of traps were measured by means of the CCD. The data were compared with emission rates of identical material obtained by standard-DLTS. The data agree well with literature data of the titanium acceptor level and the titanium donor level. An analysis of the capture cross section by means of the CCD gives a high electron capture cross section. This explains the strong effect of titanium even in very little concentrations in the pn-CCD. A two-dimensional distribution of the trap concentration was visualized on some wafers.