Epitaxial CoSi 2 formation by a Cr or Mo interlayer
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Epitaxial CoSi2 formation by a Cr or Mo interlayer. C. Detavernier, R.L. Van Meirhaeghe, F. Cardon Vakgroep Vaste Stofwetenschappen, Universiteit Gent, Krijgslaan 281/S1, B-9000 Gent, Belgium.
K. Maex+, B. Brijs, W. Vandervorst IMEC, Kapeldreef 75, B-3001 Leuven, Belgium. + also at E.E. Dept, K.U. Leuven, B-3001 Leuven, Belgium.
ABSTRACT We have studied CoSi2 formation in the presence of a Cr or Mo interlayer or capping layer. We shall show that, contrary to what was previously reported, Cr and Mo may be used as interlayers to grow epitaxial CoSi2. However, unlike for Ti, the thickness of the interlayer is very important. If the Cr or Mo interlayer is too thick (> 5 nm), polycrystalline CrSi2 or MoSi2 are formed first and epitaxial growth of CoSi2 becomes impossible. However, both XRD and random/channeling RBS results indicate that for a 2-3 nm interlayer of Cr or Mo, CoSi2 forms epitaxially on Si(100). For thinner interlayers, there is a preferential (220) and (400) orientation. This can be explained by the presence of Cr or Mo on the CoSi grain boundaries, which will affect the heterogeneous nucleation of CoSi2. INTRODUCTION Due to the small lattice mismatch (-1.2%), CoSi2 may be formed epitaxially on Si. However, in the standard Co/Si reaction, polycrystalline CoSi2 is formed. Various methods of obtaining epitaxial CoSi2 growth have been developed over the last decades : techniques based on molecular beam epitaxy (template methods [1], allotaxy [2]), ion beam synthesis (IBS) by high dose Co implantation [3] and solid phase epitaxy (SPE), using an interlayer in between the Co and Si substrate [4]. The presence of a Ti interlayer is known to result in epitaxial growth of CoSi2. In literature, the epitaxial growth is usually explained by (1) the ability of Ti to reduce interfacial SiO2 and by the fact that (2) the Ti interlayer acts as a diffusion barrier, mediating the Co flux towards the substrate. Inspired by the results for the Co/Ti/Si system and in an attempt to determine the mechanism responsible for solid phase epitaxy (SPE), several groups have studied the effect of other interlayer materials. It has been shown that SPE “works” for Ta, Hf, C, W and Zr interlayers, while in case of Mo [5], V and Cr [6,7] interlayers, it was reported that the solid phase reaction results in polycrystalline CoSi2. In this paper, it is shown that the previous results on Cr and Mo interlayers should not be interpreted as “Cr and Mo are bad interlayer materials to promote epitaxy” : it is shown that depending on the thickness of the interlayer, either polycrystalline or epitaxial CoSi2 can be formed. It will be shown that the amorphous character of the diffusion mediating interlayer is important to achieve epitaxial growth of the CoSi2.
C10.2.1
EXPERIMENTAL We used p-type Si(100) substrates (Na = 1013-1014 cm-3). Cr and Co were deposited by e-beam evaporation in a vacuum of 10-6 mbar. Mo was deposited by DC sputtering in argon. Deposition of the multilayer structure was done without breaking the vacuum. After deposition, the
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