Microstructural Control Of Thin Film Si Using Low Energy, High Flux Ions In Reactive Magnetron Sputter Deposition
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Microstructural Control Of Thin Film Si Using Low Energy, High Flux Ions In Reactive Magnetron Sputter Deposition Jennifer E. Gerbi1 and John R. Abelson Coordinated Science Laboratories and the Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign Urbana, IL 1 email: [email protected] ABSTRACT Using plasma growth sources with concurrent particle bombardment, silicon thin films can be deposited with various phases and microstructures. DC Reactive Magnetron Sputtering (RMS), in particular, can produce amorphous, mixed-phase, nanocrystalline, polycrystalline, porous columnar, and epitaxial Si films. In particular, a large flux of low energy, heavy ions strongly affects the phase and microstructure, and therefore the resulting film qualities. Lowpressure (1.6 mTorr) RMS is particularly suited for this type of plasma manipulation: we bias the substrate to produce the ion energy of choice, and use an external magnetic field to control the ion/neutral flux ratio, therefore decoupling the parameters of bombardment energy and flux. In this work, we study the influence of slow (
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Figure 5. Raman scattering data of the films in Figure 4. Note the absence of the amorphous phase in the film with the ratio of 20.
Figure 4. Ellipsometry of px-Si grown directly on glass. Note the increased crystallinity of the film with the ion/neutral flux ratio of 20.
Figure 6. Raman scattering data for µc-Si:H films grown with different substrate biases. Note the FWHM increase of the crystalline (522cm-1) peak with substrate voltage, indicating smaller crystallites.
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CONCLUSIONS Reactive magnetron sputtering produces dense, smooth films, despite the ~1 sticking probability of the Si neutrals, due to energetic specie bombardment. The low pressure nature of magnetron sputtering in particular enables these species to reach the growing film before they undergo many gas-phase collisions [7]. The process of film densification and smoothening by these energetic species is enhanced in this work with the application of an external field to increase the low-energy ion flux to the growing film. This flux likely increases the surface mobilities of adspecies through collisions on the surface, as proposed in similar work on TiAlN [8]. This would encourage larger grain sizes in px-Si, as we describe, and may overcome nucleation barriers to begin crystalline growth, decreasing any a-Si interface layer, as we also observe. Enhanced surface mobilities result in film changes similar to those from increasing the substrate temperature, which are consistent with the effects observed here; thus, we may be able to further lower the substrate temperature. Our directly deposited, relatively large grained px-Si at 375oC appears fully crystalline
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