Metal contamination analysis of the epitaxial starting material for scientific CCDs

Traces of unintentionally introduced titanium into a 3 k Omega cm float-zon e epitaxial silicon at ultra low levels of 10(10) cm(-3) were found to be t he 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 ent rance 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 an d the producer of the epitaxial silicon and seems to be a common epitaxy-re lated problem. To identify the impurity, electron emission rates of traps w ere measured by means of the CCD. The data were compared with emission rate s of identical material obtained by standard-DLTS. The data agree well with literature data of the titanium acceptor level and the titanium donor leve l. An analysis of the capture cross section by means of the CCD gives a hig h electron capture cross section. This explains the strong effect of titani um even in very little concentrations in the pn-CCD. A two-dimensional dist ribution of the trap concentration was visualized on some wafers. (C) 2000 Elsevier Science B.V. All rights reserved.