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3.2 Diffusion in Polysilicon

  In recent years, the usage of polysilicon layers in modern IC fabrication became quite popular. Many important applications in microelectronics, such as gate electrodes, interconnects, load resistors and diffusion sources are realized by using polysilicon. Self-aligned techniques for the fabrication of high-performance bipolar devices became important in high frequency applications [vdV89] [Shi91] [Bur89]. The morphology and surface topography of these polysilicon layers can strongly influence the performance of thin-film layered device structures. Unfortunately, the morphological structure of polysilicon depends on the applied thermal treatment. Therefore, thermal processing of polysilicon is hard to control with respect to reliability and reproducibility. But these drawbacks are accepted, because of its exciting heat resistance properties. Compared to metals, the low thermal expansion coefficient and the temperature resistivity ensure less heat dissipation in the device. Polysilicon is nowadays one of the materials at the forefront of process development.

In this section we give an overview on the methods to fabricate doped or undoped polysilicon layers, followed by investigations on the complex morphological transformation during thermal treatment of polysilicon layers. All this information is needed to model the diffusion behavior of polysilicon consistently. Finally, we present the major diffusion mechanisms responsible for the extraordinary high diffusivity of dopants in polysilicon.

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