THERMAL AGGLOMERATION OF THIN SINGLE-CRYSTAL SI ON SIO2 IN VACUUM

Changes in the surface structure of the single-crystal Si/SiO2/Si system during vacuum heating (950-1200 degrees C) have been investigated using atomic force microscopy and scanning Auger electron microscopy. When the top Si layer is sufficiently thin, e.g., below similar to 30 nm for heating at 1100 degrees C, holes are formed due to Si agglomeration. The buried SiO2 surface emerges at the bottom of the holes and Si atoms accumulate around them. These holes expand and coalesce with increasing temperature, and Si islands are finally formed at 1200 degrees C. This Si agglomeration starts at the points where the Si layer is locally extremely thin due to SiO2 protrusions and proceeds anisotropically; the shape of the holes is hexagonal for the (111) surface and square for the (001) one. In both cases, their sides are in the [110] directions. The driving force of agglomeration in a single-crystal film is discussed.