A TRANSMISSION ELECTRON-MICROSCOPE INVESTIGATION OF THE DOSE DEPENDENCE OF THE MICROSTRUCTURE OF SILICON-ON-INSULATOR STRUCTURES FORMED BY NITROGEN IMPLANTATION OF SILICON

Transmission electron microscope studies have been made of (100) silicon wafers implanted at 500-degrees-C with 200-keV N-14+ ions to doses of either 0.25, 0.75, or 1.4 X 10(18) cm-2. For all of these specimens, the as-implanted wafers contained a buried amorphous layer with a damaged upper single-crystal silicon layer. For the 1.4 X 10(18) cm-2 specimen, the amorphous layer contained bubbles. Wafers subsequently annealed at 1200-degrees-C in order to form silicon-on-insulator structures showed the following. For the 0.25 X 10(18) cm-2 specimen, there was a buried discontinuous polycrystalline alpha-Si3N4 layer, and an upper silicon layer with no observable defects. For the 0.75 X 10(18) cm-2 specimen, there was a buried continuous polycrystalline alpha-Si3N4 layer containing small silicon islands, and an upper silicon layer either without defects or with microtwins adjacent to the nitride/silicon interface. For the 1.4 X 10(18) cm-2 specimen, there was a buried multilayer structure with the middle layer consisting of substantially single-crystal alpha-Si3N4 free from silicon islands but containing bubbles; and an upper silicon layer with microtwins and threading dislocations. For the 0.25 and 0.75 X 10(18) cm-2 specimens, the alpha-Si3N4 had often grown epitaxially in the single-crystal silicon. For the 0.75 X 10(18) cm-2 specimen, such epitaxy had less often occurred. For the 1.4 X 10(18) cm-2 specimen, such epitaxy was not observed. These structural results are correlated with the implantation conditions and nitrogen depth profiles obtained by secondary ion mass spectrometry. The mechanisms responsible for producing the structures are discussed.