Material Properties of Spin‐on Silicon Oxide (SOX) for Fully Recessed NMOS Field Isolation

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Multiple applications of spin‐on silicon oxide, SOX, followed by densification at 900°C in oxygen and planarization, is potentially a practical way of realizing fully‐recessed field isolation for submicron NMOS. A three‐step SOX‐application scheme was used to show the feasibility of fabricating such structures. Trenches ∼0.4 μm deep and as much as 10 μm wide were successfully planarized. However, in order to planarize still wider regions a LOCOS mask would be necessary. Because SOX shrinks during a high‐temperature anneal, there are no silicon‐crystal defects created surrounding the trenches. Doping of SOX with 0.5 mole percent boron gives a material with wet and dry‐etching characteristics comparable to that of thermal oxide. This B‐doped material is preferable for device isolation since it allows for subsequent oxide‐regrow and wet‐strip cycles. It could also serve as a source of boron for doping of the trench sidewalls. The densified SOX exhibited a mobile charge density of >1012 cm−2 (attributed to absorption of Na+ from the furnace) that could potentially deter its use as a high‐quality isolation dielectric.