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ABSTRACT PECVD processes are the most common way of growing amorphous and microcrystalline silicon thin films. These low pressure plasmas are known to generate large amounts of particles in the gas phase which may be a source of contamination in the production of silicon based devices. In this report, however, we show that these particles can be incorporated profitably into a growing film, provided that their size is controlled. The Modulation of the RF power is a direct method of controlling particle size. Plasma characteristics determine the dynamics of particle formation, whereas the duration of the plasma-on period controls the particle's size and atomic structure. The size and atomic structure of particles from two different periods of the same discharge were investigated. Their main characteristics, monodisperse size distribution and ordered atomic structure, are discussed in the context of the latest findings on particle nucleation and coagulation. A semi-phenomenological model was used to show that, during the first milliseconds of the discharge, particles of 1-2 nm that are electrically neutral may leave the plasma and contribute to the growth of the film. Although these particles could not be observed on the TEM grid, their contribution to film growth might be relevant. INTRODUCTION The formation of particles in PECVD processes has been considered as a drawback in thin film deposition for many years [1]. Since the early 90's a considerable effort has been devoted to understanding the dynamics of the formation of silicon and silicon-based particles[27]. These studies established that particles grow in two steps. After fast particle nucleation, there is a coagulation stage in which particle size increases rapidly and the particle concentration drops [3-5]. Therefore, present knowledge enables particle size to be controlled by means of the technological parameters of the discharge, which determine the onset of coagulation, and by the plasma-on period in modulated discharges, which controls the duration of the coagulation process. Previous reports showed that silicon films grown with a significant contribution of particles exhibit peculiar optic and electronic properties [8-10]. However, the atomic structure of these particles was not clarified. HRTEM analyses of these films revealed small ordered domains of less than 2 nm that were attributed to particles [11]. It was suggested that the peculiar atomic arrangement of clusters of this size could be responsible for the unusual properties of these films. This paper reports an HRTEM investigation into the atomic structure of particles produced after different plasma-on periods of a discharge which is suitable for the deposition of films with particles in them. Whether or not the particles observed are the ones responsible for 499

Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society

the thin film characteristics is discussed. Smaller particles, which are able to leave the discharge while plasma is on, may make a large contribution. A semi-pheno