Detection of sputter contamination and sputter-etch residue by ion microprobe analysis

Abstract

Metallization defects at the edges of contact windows can initiate electrical failures in beam lead silicon integrated circuits. Observations of malformed silicide and excessive reverse leakage in unguarded Schottky diodes have emphasized the role of metallic contamination in metallization failures that sometimes cause impaired circuit performance. Ion and electron microprobe analyses of a typical T 2L wafer have revealed seven metallic elements contaminating the first level of sputtered platinum after PtSi sintering: rhodium, titanium, aluminum, magnesium, sodium, potassium and calcium. The first four were cosputtered with platinum; the origin of the alkali metals is not known. Among the alkali metals, the heaviest concentration observed was for sodium which, at 10 15-10 16 cm -3, was not judged sufficient to affect device performance. The most damaging contaminant that was discovered was aluminum which was eventually shown to induce PtSi attack during the subsequent removal of unreacted platinum by etching in hot aqua regia. The silicon ion image from the device wafer showed excessive silicon buildup in a narrow area about 3 μm wide framing the sintered PtSi contacts. Subsequent observations on specially prepared experimental wafers indicated deposits of amorphous silicon residue at the edges of windows, etched in a layer of platinum which had been exposed to sputter etching. A sputter-etch residue of amorphous silicon appears to be an inevitable consequence of PtSi contact formation as it is presently being carried out. The extent to which such peripheral deposits of amorphous silicon have contributed to earlier metallization problems, such as PtSi attack and gold penetration, is only a matter for speculation at the present time, because these problems have been subsequently eliminated by process modifications.