Kinetic Studies of Nanoscale Crystallization in Electronic Materials
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C. HAYZELDEN*t and J. L. BATSTONE** *Division of Applied Sciences, Harvard University, Cambridge, MA 02138 tNow at UltraTest International, Inc., MicroLabs Analysis Division, San Jose, CA 95131 **IBM T. J. Watson Research Center, P. 0. Box 704, Yorktown Heights, NY 10598 ABSTRACT
We report a kinetic analysis of low-temperature NiSi 2 -mediated crystallization of amorphous Si by in situ transmission electron microscopy. The initiation of crystallization by formation of crystalline Si on buried NiSi 2 precipitates is shown to have an activation energy of 2.8±0.7eV. Crystallization of the amorphous Si via migrating precipitates of NiSi 2 occurs with an activation energy of 2.0±0.2eV. The significance of these activation energies is discussed in terms of possible atomistic mechanisms of crystalline Si initiation and subsequent growth. Amorphous Si is reported to crystallize at temperatures as low as 450°C. INTRODUCTION
Transition metal silicides have a wide range of applications in electronics., With decreasing device dimensions and increasingly complex metallization layers; the formation of self-aligned metal contact regions, buried interconnects and low-temperature silicide-mediated crystallization of amorphous Si (a-Si), considerable interest has been generated in the cubic metal silicides. Metal induced crystallization of a-Si has been known for some time. 2 However, buried precipitates of NiSi 2 formed by Ni implantation of a-Si, first reported by Cammarata et al., 3 which lead to crystallization of a-Si at 500°C, can provide low-temperature, high quality crystalline Si (c-Si). Nickel disilicide, which has the CaF2 structure and a lattice parameter mismatch with c-Si of only -0.4% at room ambient temperature, can form epitaxially on c-Si in the [100], [110] and [111] orientations. 4 Using in situ transmission electron microscopy (TEM), Hayzelden and Batstone, and
Hayzelden et al. 5 directly observed the initiation of c-Si formation on NiSi 2 precipitates and the subsequent NiSi 2 -mediated crystallization of a-Si. Single c-Si regions, typically with a texture were formed by the migration of NiSi 2 precipitates through the a-Si thin film trailing needles of high-quality epitaxial Si. A suppression of the initiation of Si crystallization (though not of subsequent NiSi2-mediated growth) at an accelerating voltage of 300KeV was attributed to knock-on displacement damage of c-Si above the threshold energy of 185KeV. 5 Early observations indicated that the rate of Si crystallization appeared to be dependent on the thickness of the NiSi 2 in the growth direction, indicating diffusion controlled growth5 ,6 and effective diffu7 sivities through the NiSi 2 were determined. Ex situ high resolution electron microscopy (HREM) studies 5 -7 of partially crystallized specimens cooled to room ambient temperature appeared to show a few atomic layers of c-Si at the leading NiSi2/a-Si interface. However, dynamic in situ HREM studies of NiSi2-mediated crystallization by the present authors revealed no c-Si at the leading NiSi 2 /a-
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