Ellipsometric study of the polysilicon/thin oxide/single-crystalline silicon structure and its change upon annealing

Abstract

Polysilicon/thin oxide/single-crystalline silicon structures used as emitters of bipolar transistors were measured using spectroscopic ellipsometry. The thin SiO x layer was deposited on the substrate in a rapid thermal processing chamber, then polysilicon was deposited, implanted with As, and annealed. During annealing the SiO x layer dissolves to islands retarding the diffusion of As, which results in a shallow p-n junction. The process - and as a consequence, the device performance - depends sensitively on the thickness of the oxide layer. We developed optical models to measure the thickness of the SiO x layer at each process step, i.e., after SiO x deposition, after polysilicon deposition, and after annealing. The structure, the surface quality, and the homogeneity of the polysilicon layers were obtained from the same optical model. The thickness of the initially 0.76-0.86 nm SiO x layer decreased to the detection limit of about 0.2 nm during annealing, together with a significant crystallization of the deposited amorphous Si resulting in a perfectly smooth surface with a native oxide layer. The uncertainty of the measurement of the SiO x layer after annealing was typically 0.2 nm even below the polysilicon layer of about 150 nm. Our results show that ellipsometry is a proper tool for the measurement of the thickness of the buried SiO x boundary layer, which can be applied as an in line or in situ measurement as well. © 2002 American Institute of Physics. © 2002 American Institute of Physics.