Istituto dei materiali per l'elettronica ed il magnetismo     
Watts B. E., Attolini G., Bosi M., Frigeri C. A novel mechanism to explain wafer bending during the growth of SiC on Si. In: COST 539 Action ELENA - Workshop Fabrication, Properties & Applications of Electroceramic Nanostructures / European Co-operation in the Field of Scientific and Technical Research (Genova, 26-28 giugno 2008). Abstract, pp. 45 - 45. COST, 2008.
Wafer bending is a severe problem that limits the application of silicon carbide films in both electronic and MEMS applications. Photolithography may be impossible and in severe cases the wafer can shatter. Additionally, compressive stresses can make it difficult to form membranes from the SiC film. Hence, it is important to understand the causes of wafer distortion. The origins of the wafer distortion that have been proposed so far are: (i) Misfit strain arising from different thermal expansion of the silicon and SiC film (ii) Misfit strain due to the different lattice parameters of Si and SiC. However, estimates of the magnitude of the bending that one might suggest that neither of these mechanisms can account for the magnitude of the curvature. Another phenomenon that can cause distortion is interdiffusion [1]. These diffusion phenomena and the associated effects, deformation, strain and voids, are related to the Kirkendall effect, commonly observed in metal couples. In the case of silicon carbide a single component diffuses towards the free surface leaving vacancies in the substrate, in a manner similar to the formation of oxide scales. Jaenicke et al [2] present various possible regimes of stress and subsequent bending that can be present at a surface reacting with ambient oxygen. One such case is presented in Fig. 2 where the upper layer of the substrate is under tension bending the substrate and forcing the growing film under compression. Interdiffusion induces large plastic deformations and could easily account for the bending. Si diffuses out of the substrate to react with the carbon during the initial stages of growth and as proceeds a large number of vacancies may be created in the substrate which, in some cases, condense to form voids. Stresses may then be generated in the substrate in where bowing is concave but the SiC film under compression. Clearly, when the system is complex the stress may have different signs in different regions and other mechanisms may come into play, such as thermal mismatch. However, the similarities between the growth features of SiC on silicon and those observed during diffusion in bimetallic couples and surface scales are compelling. This novel mechanism of wafer warp will be illustrated with examples from growth experiments and the processes leading to the plastic deformation of the wafer will be discussed. References 1. Daruka I., Szabo I. A., Beke D. L., Cserhati C., Kodentsov A., Van Loo F. J. J., Acta Materialia, 44 (1996) 4981. 2. Jaenicke W., Leistikow S., Stadler A., J. Electrochem. Soc., 111 (1964), 1031
Subject SiC, growth, Bending

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