Time‐zero and time‐dependent dielectric breakdowns for thermally grown 60‐ through 70‐A˚ SiO2films were studied in two kinds of Si‐gate metal‐oxide‐semiconductor (MOS) capacitors, where Al/polycrystalline silicon (poly‐Si) connecting contact hole was formed over the thin gate oxide (type A) and the thick field oxide (type B). Almost the same time‐zero dielectric breakdown was obtained in types A and B MOSs annealed at 450 °C for metal sintering. In type A samples, however, pinholes increased for the higher sintering temperature. Time‐dependent dielectric breakdown on a type‐A MOS depended on the sintering anneal temperature and four orders of magnitude shorter lifetime than that for type‐B MOS was observed under 450 °C annealed samples. Auger measurements for a type‐A MOS yielded that (i) Al precipitated at the poly‐Si/SiO2interface, caused by Al diffusion through poly‐Si film at the Al/poly‐Si connecting contact hole area during low temperature anneal for metal sintering, and (ii) Al precipitates were extremely enhanced at the contact hole edge. Unannealed type‐A samples showed almost the same good reliability as for type‐B MOS. The inferior reliability in a type‐A MOS with a metal sintering anneal was concluded to be caused by the Al precipitates at the poly‐Si/SiO2interface.