Semiconductor Surface with Strain Control

Hiroo Omi, David Bottomley, and Toshio Ogino
Device Physics Laboratory

    Looking to the long term future of Si semiconductor technology, we have proposed, fabricated and demonstrated strain distribution control on the planar Si wafer scale for advanced nanostructure self-assembly. Self-assembled growth attracts much attention because it is free from the optical lithography minimum feature size limit. In most research to date on nanostructure formation on Si surfaces, the substrate was not subjected to advanced preparation procedures. Preparation typically consisted of chemical etching followed by thermal treatment in ultra high vacuum. The size variation of Ge or GexSi1-x alloy nanostructures grown coherently on the surface was appreciable, with no localization control. As a result the structures are difficult to utilize effectively in commercial applications.
    Here we apply commercially-viable industrially-proven processes to prepare the Si wafer for advanced nanostructure fabrication. Pre-deposited layers on the surface of the wafer are patterned using optical lithography; oxygen ions are implanted; the sample is subjected to a 1325oC anneal, and finally the pattern is removed in a chemical etch. The oxide inclusions are completely stable up to at least 1325oC and are therefore compatible with further fabrication processes. The O ion implantation and annealing approach is a spinoff from the silicon on insulator (SOI) field, specifically the separation by implanted oxygen (SIMOX) process invented 23 years ago at NTT.
    Strained epitaxial growth of Ge on the Si(001) substrate surface at 550oC in ultra high vacuum produces three dimensional islands whose location and size distribution are well-controlled (Fig. 1). Ge grows forming three dimensional islands surrounded by a wetting layer. The growth mode occurs to reduce the combined surface energy and elastic energy of the system. Growth on a line pattern produces aligned, closely-spaced islands with a highly uniform size distribution (Fig. 1(a)). In Fig. 1(b), the islands have self-organized into a ring at the centre of the implanted region.

Fig. 1. Atomic force microscopy images of the surface after growth of Ge (a) on a 700 nm implantation width line pattern (b) on a 2 μm implantation diameter hole pattern.

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