Formation of SOI by oxidation of silicon with engineered porosity gradient

A method of making a silicon-on-insulator substrate, comprising:

• a) epitaxially growing a first highly doped silicon-containing layer onto a major surface of an underlying semiconductor region of a substrate, the first highly-doped epitaxial layer having a p-type dopant concentration greater than about 5×1018 cm−3 and substantially higher than the p-type dopant concentration of the underlying semiconductor region;
• b) epitaxially growing a non-highly doped p-type silicon-containing layer onto a major surface of the first highly-doped epitaxial layer to cover the first highly doped epitaxial layer, the non-highly doped epitaxial layer having a dopant concentration substantially lower than the dopant concentration of the first highly-doped epitaxial layer;
• c) epitaxially growing a second highly-doped epitaxial layer overlying the non-highly doped epitaxial layer, the second highly-doped epitaxial layer having a p-type dopant concentration greater than about 5×1018 cm−3 and substantially higher than the dopant concentration of the non-highly doped epitaxial layer;
• d) with a major surface of the second highly-doped epitaxial layer fully exposed, subjecting the substrate with the layers thereon to anodization to selectively form first and second coarse porous silicon-containing layers from the first and second highly-doped epitaxial layers and a fine porous silicon-containing layer from the non-highly doped layer;
• e) subjecting the substrate with the layers thereon to oxidation to oxidize the first and second coarse porous silicon-containing layers, and then annealing the substrate with the layers thereon to form a buried oxide layer from the first coarse porous silicon-containing layer, a monocrystalline semiconductor layer from the fine porous silicon-containing layer, the monocrystalline semiconductor layer overlying the buried oxide layer, and an overlying oxide layer from the second coarse porous silicon-containing layer, the overlying oxide layer having a fully exposed major surface;
• and
• f) then removing the overlying oxide layer to expose the monocrystalline semiconductor layer.