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

The microstructure and metal-silicon work function was evaluated for polycrystalline Si (poly-Si) films (∼3500 Å) implanted with BF ₂, with and without a CoSi₂ superlayer. The poly-Si implant doses were 2, 4, and 6×10¹⁵ cm^-2, activated at 800-950°C, for various times. The work function is observed to be dependent upon the BF₂ dose, the anneal time, the anneal temperature, and independent of the presence of a thin CoSi₂ superlayer subsequently sintered on the poly-Si. The work function is not affected by additional implants into a CoSi₂ 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 ≳900°C activation cycle. The major correlation of CoSi₂/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 CoSi₂ asperities is less for Co sintered on BF₂ 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, CoSi₂ asperities do not extend close enough to the (4000-Å-thick) poly-Si/SiO₂ interface to affect complimentary metal-oxide-semiconductor device characteristics.