Applications of CoSi 2 to VLSI and ULSI
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ABSTRACT Silicides have found application as high conductivity, high temperature, and corrosion resistance materials that form good electrical contacts to silicon and good low resistivity cladding on polysilicon films used as gate metal. Of various silicides investigated in past CoSi, offers several advantages including lowest resistivity, self-aligned formation, low lattice mismatch with silicon, stability in presence of dopants and on SiO 2 , Si 3N4, or Sioxynitrides, and reliability to process temperatures _5900'C even when used in thicknesses as thin as 50-60 nm. Thus, CoSi2 has found an application in VLSI and ULSI. In this paper, the properties, formation and processing, reliability, and applicability of CoSi2 will be reviewed. It will be shown that CoSi 2 is only silicide that offers properties and reliability for continued use in sub-0.25 pm VLSI and ULSI integrated circuits. 1.
Introduction
First CoSi2 films were formed by reactions of cobalt films with underlying silicon substrates."' It was shown that C0 2Si was the first silicide to form, followed by CoSi, and finally CoSi2. However, the resistivities of these silicides were not recorded until much later.(2) Polycrystalline films of CoSi2 was reported to have a resistivity of -15-18 PfDcm making it among the two lowest resistivity silicides (TiSi 2 is the other one). Around the same time it was realized that CoSi2 has an excellent lattice match (-1.2% mismatch) with silicon and could be grown epitaxially using MBE techniques opening the door to the growth of heteroepitaxial Si-silicide and Si-silicide-Si structures."3 ) Several attempts have since then been made to make use of the low resistivity CoSi2 as contact material to Si and as low resistance strap on polysilicon gate. 45, ) Also attempts have been made to create Sisilicide-Si metal-base-transistors (MBT) with a potential of ballistic electron device behavior."'7 ) The epitaxial silicide was also shown to have a resistivity of -10 pf'cm.(9 ) Since these findings, a great interest has developed in the use of CoSi 2 in microelectronics and related applications. Most importantly it was shown that CoSi2 can be formed in a selfaligned manner"°"') on Si or polysilicon thus eliminating the need of multicomponent alloy deposition and etching techniques to define necessary patterns of the silicide contacts or strapped CoSi. on polysilicon gate/interconnections.( 5"'" The simplicity of processing, low contact resistance, and thermodynamically favorable stability have brought attention to the use of CoSi 2 in submicron devices and integrated circuits. Table 1 makes a comparison of three most-tauted self-aligned silicides. It is obvious that CoSi 2 offers most features desirable for applications in sub-micron applications. It has now been used in 0.75 and 0.5 pm design rule circuits in manufacturing."7) More recently 0.1 pm design rule CMOS
3 Mat. Res. Soc. Symp. Proc. Vol. 320. ©1994 Materials Research Society
technology has been reported using CoSi2, where a comparison with TiSi 2 and PtSi showed a cl
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