Evolution of Pore Reorganization of Porous Silicon

To obtain a closed and smooth surface of a porous Si layer for an epitaxial growth of a silicon wafer, the porous Si must be reorganized. To understand the reorganization process, the evolution of the pores is investigated in this contribution. The reorganization process is interrupted at different stages and cross-sectional SEM images are taken to evaluate the change in pore size and shape during reorganization. Additionally, the stress is measured using XRD. At the beginning of the reorganization of the pores, the as-etched columnar pores with diameters of about 5 nm become coarser with larger diameters. At temperatures above 900°C, the surface closes and thus becomes smooth. In addition, the form of the pores starts to change from columnar to round with diameters between 55-65 nm. When the highest temperature of 1100°C is reached, the columnar pores have disappeared and the pores are round and faceted. With an additional plateau time of up to 20 min, there is no visible change in pore faceting. The as-etched sample shows tensile stress and the sample annealed at 700°C is nearly unstressed. When the temperature is increased beyond 900°C, the stress is compressive and no further change is visible.