Conversion of Silicon Nitride Films to Anodic SiO[sub 2]

Abstract

An introduction is given to the principles underlying the anodization of pre‐existing dielectric films. In the case of amorphous silicon nitride films on a silicon substrate it is shown that conversion of the nitride film to anodic SiO2 occurs by oxygen ions reacting with the nitrogen, the latter being evolved from the film as N2 or as a nitrogen oxygen compound. The process has been applied to the opening of windows in normalSi/SiO2/Si3N4 structures. Nitride films up to 1000Aå thick can be rendered soluble by a single anodization step to 350v; thicker films require a repeated anodization. The electric field applied during anodization causes a shift in surface potential of the silicon substrate. The original surface potential can be restored by a bake in forming gas at 500°C. High‐resistivity silicon tends to passivate under anodic bias; growth of an oxide film occurs only at very large current densities.

Properties of Anodic Oxide Films Formed in the Anodization of Silicon Nitride

“…The etch solution was renewed for each etch step to reduce effects of depletion. On reaching the SisN4 interface, the etch rate changes almost abruptly and an abrupt silicon nitride oxide interface is indicated in agreement with previous infrared transmission results (1). The break in the etch curves ( Fig.…”

“…One of the questions raised in the anodization of silicon nitride concerns the nature of the silicon nitride oxide interface. In the original work, Schmidt and Wonsidler concluded that the interface was abrupt from infrared absorption evidence (1). Later Tripp corroborated this evidence but, claiming that the IR method was insensitive, opted for a graded oxy-nitride interface on the basis of oxide solubility data (3).…”

“…Based on prior evidence (1,3), the expected optical model consists of two layers on silicon where the outer layer, presumably anodic silicon dioxide, grows at the expense of the silicon nitride layer during anodization (Fig. 1).…”

“…Anodization of silicon nitride is of practical importance because it converts silicon nitride to a more soluble form and thus can be used in the localized etching of silicon nitride films passivating semiconductor devices (1). Areas can be selectively anodized while the remaining area is protected from anodization usually by a greater dielectric thickness in the form of silicon dioxide.…”

“…This poses certain difficulties in determining the properties of the oxide produced. In the past, the conversion of silicon nitride to anodic oxide has been qualitatively followed by infrared interferometry (1), and the oxide thickness has been measured by destructive techniques (3). More recently, nuclear microanalysis has been used to determine the constituents of the oxide (4).…”