BF2 implanted polycrystalline silicon gates with and without CoSi2: Microstructure and work functions

Abstract

The microstructure and metal-silicon work function was evaluated for polycrystalline Si (poly-Si) films ( -3500 A) implanted with BF,, with and without a CoSi, superlayer. The poly-Si implant doses were 2, 4, and 6 x lOI cm -*, activated at 800-950 "C, for various times. The work function is observed to be dependent upon the BF, dose, the anneal time, the anneal temperature, and independent of the presence of a thin CoSi, superlayer subsequently sintered on the poly-Si. The work function is not affected by additional implants into a CoSi, superlayer. The measured work function for this material is shifted to more positive values by penetration of boron from the gate, into the oxide and possibly into the substrate during a GZ 900 "C activation cycle. The major correlation of CoSi,/poly-Si interface roughness is with poly-Si grain size. The interface becomes smoother as the poly-Si grain size becomes smaller. Similarly, the maximum extent of CoSi, asperities is less for Co sintered on BF, implanted poly-Si when the poly-Si is activated at 900 "C (smaller grain size) rather than at 950 "C. Within statistical limitations of cross-section transmission electron microscopy and sensitivity limitations of Rutherford backscatterings, CoSi, asperities do not extend close enough to the (4000-A-thick) poly-Si/SiO, interface to affect complimentary metal-oxide-semiconductor device characteristics.1510