Grain Growth in Polycrystalline Silicon Films

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GRAIN GROWTH IN POLYCRYSTALLINE SILICON FILMS C.V. Thompson, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.

ABSTRACT Experimental observations of recrystallization, normal grain growth and secondary grain growth in silicon films are reviewed. Normal grain growth leads to grain sizes which are approximately equal to the film thickness. Secondary grain growth can lead to larger grains with restricted crystallographic textures. These procesess are affected by the as-deposited or as-crystallized grain structures and orientations. The rate of grain growth has been shown to be higher in phosphorous or arsenic doped films. Ion bombardment, oxidation, and interactions with silicides also lead to increased grain growth rates. Grain growth enhancement has been related to increased point defect concentrations or dopant redistribution.

I.INTRODUCTION Polycrystalline silicon films are used in a variety of applications, including as gates in metal oxide semiconductor field effect transistors, as base and emitter contacts and/or diffusion sources for bipolar transistors, as resistors and as first level local interconnects. Use of polycrystalline Si in thin film transistors 1,2 and sensors 3,4 is also being explored. In all of these applications, the electronic properties of polycrystalline films are Both grain sizes and strongly affected by the nature of their microstructure. grain orientations or crystallographic textures can be of importance. 5 Grain size uniformity also affects the uniformity of device properties. Grain growth during post-deposition annealing of fully crystalline films can lead to significant changes in grain sizes, orientations and size distributions. The rate and nature of grain growth is affected not only by the annealing temperature but also by the initial film structure, the film thickness, the concentration of dopants or other impurities, oxidation, silicidation, and ion bombardment. In this paper, these phenomena will be reviewed and discussed. II.

GENERAL PHENOMENOLOGY OF GRAIN GROT JTH IN THIN FILMS

Grain growth in polycrystalline thin films proceeds through two fundamentally different processes, normal grain growth and secondary grain growth. 6 (The latter is also known as abnormal grain growth.) When the initial grain size is smaller than the film thickness, normal grain growth leads to a continuous increase in the average grain size. The grain sizes remain monomodally distributed throughout this process. Figure 1 shows a schematic illustration of normal grain growth in a thin film. When the grain size is smaller than the film thickness, grain growth is three dimensional and therefore equivalent to bulk grain growth. In this case, the average grain size increases so that the total grain boundary area in the film decreases. This lowers the energy of the film through elimination of the energy associated with the grain boundaries. During grain growth, some

Mat. Res. Soc. Symp. Proc. Vol. 106. '1988 Materials Research Society

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