Effects of Material and Processing Parameters on the Dielectric Strength of Thermally Grown SiO[sub 2] Films

Abstract

The influence of several material and processing parameters on the dielectric strength of thermally grown SiO,, films on silicon has been assessed. This was accomplished by statistically analyzing the breakdown characteristics of a large number of MOS capacitor structures, which had been fabricated in various ways. Although the results of this investigation are only qualitative, they clearly demonstrate that the effective breakdown strength of these films is strongly dependent on: SiO2 purity, structural perfection and thickness; the presence of a passivating phosphosilicate glass layer; the presence and reactivity of the metal electrode; and, the duration of the postmetallization heat treatment. The morphology of certain micron-size defects which develop in the MOS structure during annealing is described in some detail, since these faults are apparently responsible for oxide shorting in some instances.Since the oxide layer under the gate electrode in MOSFET's is subjected to very large electric fields (up to 2 MV-cm-1), its dielectric strength has an important bearing on the reliability of these devices. Although interest in electrical breakdown phenomena in such films has increased in recent years, along with the rapid development of MOSFET technology, the effect of various process and material variables on the effective dielectric strength of thermally grown SiO~ films has not yet been well characterized. This ~-s due to the extreme sensitivity of this property to changes in oxide composition and structure, which are, in turn, quite dependent on device fabrication procedures. An additional uncertainty is introduced by the dependence of the breakdown characteristics on the particular testing technique employed.The dielectric strength of thermally grown SiO2 has been reported by Deal (1) to increase with the density of the insulating film, going from 4.8 to 5.5 MV-cm -1 as the density was raised from 2.04 to 2.22 g-cm -3. Breakdown strength values of 4 to 6 MV-cm -1 for 2000A thick SiO2 films on silicon were found by Ainger (2), who also observed that lower values were encountered on oxide films grown on more heavily doped, p-type silicon. A more thorough investigation of breakdown phenomena in A1-SiO2-Si structures has been made by Klein (3,4), who found that a succession of breakdown events could be initiated at low fields (< 2 MV-cm -1) when the applied voltage was slowly increased. These events produced visible spark discharges which created individual macroscopic holes in the capacitor, due to localized evaporation of oxide and metal. Such events were labeled "single-hole" or "self-healing" because the test capacitor was still operational after they occurred. When the applied field reached the 5 to 9 MV-cm -1 range, the dielectric failure was accompanied by arcing or by propagation of single-hole events to adjacent sites. This resulted in permanent destruction of the capacitor. Consequently, Klein's data indicate that the effective breakdown strength of SiO2 films does not have a well-defined value but incr...

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