Migration of Gold and Nickel Ions in Thin Films of Silicon Dioxide

Reversible migration of contaminant or electrode metal ions in insulating films has been blamed for short to medium term instability in the electrical behavior of tunneling, surface-controlled, and planar devices. Ultimate failure of devices in many cases is attributed to the irreversible migration of ions. The present work is the irreversible migration of ions. The present work is concerned with the irreversible migration of electrode metal ions in silicon dioxide films. Primary emphasis has been on the development of techniques for obtaining concentration profiles of Ni and Au in thermally grown films of SiO2 before and after the induction of ion migration. Mass spectroscopy, employing a variant of the spinning electrode spark source, has been used to detect nickel migration. Radio-tracer counting, combined with oxide etch-back, has given specific, sensitive and quantitative results in the case of gold migration. For the nickel migration studies, mechanically polished silicon wafers were prepared with 3000 Å thick oxides. Thin nickel film (< 200 Å) were vacuum deposited and the oxides on the unpolished surfaces were removed. For sensitive mass spectroscopic analysis, large area samples are required. Pin-hole and localized breakdown effects during drift were avoided in the following way: A nickel coated sample was sandwiched against a clean oxidized specimen which served as an insulated field plate. Ion drift was induced by a field of 8 × 105 v/cm while the sandwich was held between heated electrode blocks for 4 hrs at 600°C.