Role of grain boundaries in the epitaxial realignment of undoped and As-doped polycrystalline silicon films
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A. Cacciato Istituto di Metodologie e Tecnologie per la Microelettronica, CNR, Corso Italia 57, I 95129 Catania, Italy
E. Rimini Dipartimento di Fisica, Universita di Catania, Corso Italia 57, I 95129 Catania, Italy G. Fallico and P. Ward SGS-Thomson, Stradale Primosole 50, I 95100 Catania, Italy (Received 16 December 1992; accepted 15 June 1993)
The early stages of the thermally induced epitaxial realignment of undoped and As-doped polycrystalline Si films deposited onto crystalline Si substrates were monitored by transmission electron microscopy. Under the effect of the heat treatment, the native oxide film at the poly-Si/c-Si interface begins to agglomerate into spherical beads. The grain boundary terminations at the interface are the preferred sites for the triggering of the realignment transformation which starts by the formation of epitaxial protuberances at these sites. This feature, in conjunction with the microstructure of the films during the first instants of the heat treatment, explains the occurrence of two different realignment modes. In undoped films the epitaxial protuberances, due to the fine grain structure, are closely distributed and grow together forming a rough interface moving toward the film's surface. For As-doped films, the larger grain size leaves a reduced density of realignment sites. Due to As doping some of these sites grow fast and form epitaxial columns that further grow laterally at the expense of the surrounding polycrystalline grains.
I. INTRODUCTION Polycrystalline silicon (poly-Si) films deposited onto bare silicon crystal (c-Si) substrates are now widely used in microelectronics technology for the fabrication of bipolar transistors. For this application the poly-Si is heavily doped and serves as a diffusion source for the formation of the emitter region as well as a contact to the emitter. Two features of the poly-Si/c-Si structure are of crucial relevance for the characteristics of the devices. The first is the morphology of the native silicon oxide film at the interface, inherent to the deposition technique.1 It is shown that under heat treatment the oxide film breaks up to reduce its surface energy.2 Depending on the annealing temperature and time, the extent of the oxide rupture ranges from small discontinuities to completely balled up into spherical beads. •"'Permanent address: Departement de Physique, Faculte des Sciences, Universite Moulay Ismail, BP 4010 Beni M'Hamed, Meknes, Morocco. 2608 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 10, Oct 1993
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The other important feature is the structural change the poly-Si layer experiences during the thermal process needed for the emitter diffusion. Under the effect of the heat treatment, the grains of the polycrystalline layer grow and epitaxial realignment with the underlying crystalline substrate can occur if the oxide film is broken.3'4 Numerous studies have already been dedicated to understanding the poly-Si epitaxial realignment.5'6 The role on the realignment transformation of para
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