Degradation of Micron-Sized Silicide Lines on Polycrystalline Silicon
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DEGRADATION OF MICRON-SIZED SILICIDE LINES ON POLYCRYSTALLINE SILICON
J.RANDALL PHILLIPS*, LUNG-RU ZHENG**, and JAMES W.MAYER* *Cornell University, Dept. of Materials Science and Engineering, Ithaca, NY 14853 **Eastman Kodak Corp., Corporate Research Laboratories, Rochester, NY 14650
ABSTRACT The thermal stability of silicide fine lines on undoped CVD polycrystalline silicon was investigated. Heat treatments were in vacuum at temperatures up to 950 0 C. We observed that fine silicide lines on undoped polysilicon degrade during vacuum annealing. Voids and hillocks are formed as silicon diffuses out from the fine grained poly-Si, undergoes long-range transport through the silicide, and recrystallizes into large grains. The phenomenon is similar to that seen previously in planar samples, but fine lines were found to degrade at lower temperatures and especially along line edges. Lines of two refractory metal silicides, TiSi 2 and CrSi 2 , and one near-noble silicide, CoSi 2 were examined.
INTRODUCTION Reactions between polycrystalline silicon (poly-Si) and silicides are of practical interest because silicides are used on poly-Si in contacts in integrated circuits.[l,2] These structures need to be stable at high temperatures that are applied to them during circuit fabrication. Although in applications the silicides are used in small features such as lines, small structures have received little attention in studies of silicide interactions and stability. Previous work has shown that for thin metal (Al, Ag, or Au)[3,4] or silicide (5] planar films in contact with undoped fine-grained poly-Si, Si crystallites form within the metal or silicide layer when samples are annealed in vacuum. For silicides, the reaction temperatures for 30 minute anneals were around 0.6 of the melting temperature of the silicide. We later observed that for laterally discontinuous structures of CrSi 2 , degradation by hillock growth took place preferentially at pattern edges.r61 We discuss here the results for TiSi 2 and CoSi 2 lines along with the results from CrSi 2 . A micrograph of a portion of a typical pattern after annealing is shown in Fig.l. This micrograph illustrates the greater susceptibility to degradation of fine silicide lines relative to large area silicide films.
Mat. Res. Soc. Symp. Proc. Vol. 106. 9 1988 Materials Research Society
156
EXPERIMENTAL PROCEDURES Poly-Si 300 nm to 500 nm thick was deposited on thin SiO2 layers by chemical vapor deposition at 630 0 C. Metal lines and pads were created on the poly-Si using electron beam patterning of two-layer resist, followed by pla3ma and acid cleaning, electron beam evaporation of 60 nm to 100 nm of metal, and lift-off of the unwanted metal. Each sample was annealed in two steps: first, a formation anneal at 550 0 C to 750 0 C, depending on and then a second anneal at some higher the silicide, All anneals were in vacuum of lx10- 7 Torr or temperature. better, and all anneals were for 30 minutes. Samples were examined using a scanning electron microscope, usually with the beam
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