Reduction of sidewall defect induced leakage currents by the use of nitrided field oxides in silicon selective epitaxial growth isolation for advanced ultralarge scale integration

Abstract

Defects in the near sidewall region in selective epitaxial growth of silicon have prevented its widespread use as a viable dielectric isolation technology. The main cause of these defects has been demonstrated to be thermal stress due to mismatch in the coefficient of thermal expansion between silicon and silicon dioxide. This article presents the detailed electrical characterization of these sidewall defects using P+/N junction diodes fabricated using silicon dioxide and thermally nitrided silicon dioxide as the field insulator. It is shown that the use of field oxide which was nitrided at 1100 °C for 60 min in ammonia gas ambient reduced the reverse saturation current density in the diodes by a factor of 6 and also improved the forward recombination and ideality factors when compared to standard thermal field oxide isolated diodes. The improvement of the sidewall quality was attributed to a reduction in thermal stress due to the modification of the coefficient of thermal expansion of nitrided silicon oxide.

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“…Under high temperature and in pure ammonia gas, oxygen atoms in SiO2 layer are replaced by nitrogen atoms forming an oxynitride layer [8]. The nitrogen concentration decreases with the distance from the exposed SiO2 surface [8]- [10]. The incorporation of N can reduce dangling bonds and mechanical stress, resulting in a passivated surface [11].…”

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“…The ratio of silicon thickness, x s , converted to resulting oxide thickness, x ox , is proportional to their respective densities: x s =0A6x ox . Thermal oxidation of silicon generates compressive stress in the silicon dioxide film, because silicon dioxide molecules take more volume than silicon atoms, and there is a mismatch between the CTE (coefficients of thermal expansion) of silicon (CTE: 2.6xlO" 6°C1 ) and silicon dioxide (CTE: OJxKrHr 1 ) [24]. Therefore, after thermal oxidation, an anneal process is needed to minimize the compressive stress.…”