US Patent 7622049 Passivation for cleaning a material

A contact is defined by an opening etched into borophosphosilicate glass (BPSG) down to a silicon substrate. In a contact cleaning process designed to remove native oxide at the bottom of the contact with little effect on the BPSG, the contact is dipped in an etch retardant before being dipped in a cleaning solution containing both the etch retardant and an etchant. The dip in etch retardant modifies the surface of the BPSG, thereby lessening the enhanced etching experienced during the initiation of the dip into the etchant/etch retardant cleaning solution. Results of a etchant/etch retardant clean, both with and without the prepassivation, can be illustrated on a graph depicting the change in contact diameter as a function of dip time. Specifically, the results define “best fit” lines on that graph. For a given dip time, a first line representing the prepassivation+etchant/etch retardant clean is 30 to 40 Angstroms lower than a second line representing the prior art clean using the etchant/etch retardant without prepassivation. Accordingly, the first line's Y-axis intercept, representing the “zero dip time,” is also 30 to 40 Angstroms lower than the second line's zero dip time.