Method of forming field oxide

A method of forming field oxide, comprising:

• providing a pad oxide layer over a monocrystalline silicon substrate;
• providing a silicon nitride layer over the pad oxide layer;
• forming a patterned masking layer over the silicon nitride layer and pad oxide, the patterned masking layer covering portions of the silicon nitride layer and underlying pad oxide, and leaving other portions uncovered;
• removing the uncovered portions of the silicon nitride layer and pad oxide layer while leaving the covered portions to form masking blocks from the covered portions of the silicon nitride layer and pad oxide layer, the removing also removing portions of the substrate between the masking blocks to form a trench extending into the substrate;
• the trench having sidewalls defined by monocrystalline substrate pillars beneath the masking blocks, the masking blocks and monocrystalline substrate pillars comprising coextensive sidewalls;
• forming a polycrystalline silicon layer along the coextensive sidewalls of the masking blocks and monocrystalline substrate pillars;
• the polycrystalline silicon layer being against the monocrystalline substrate pillars and also being against the monocrystalline silicon substrate at a bottom of the trench;
• anistropically etching the polysilicon layer to form polysilicon pillars extending into the trench;
• the polysilicon pillars having bottom surfaces against the monocrystalline silicon of the substrate, and also having side surfaces against the monocrystalline substrate pillars;
• forming a second silicon nitride laver along the polysilicon pillars and spaced from the masking blocks by the polysilicon pillars;
• anisotropically etching the second silicon nitride laver to form silicon nitride pillars along the polysilicon pillars;
• oxidizing the monocrystalline silicon substrate and the polycrystalline pillars to form field oxide regions proximate the masking blocks, and silicon nitride pillars;
• and
• after the oxidizing, removing the silicon nitride masking blocks and the silicon nitride pillars from over the substrate.