We have extended the recent work1,2 on grain boundary grooving, agglomeration and islanding during the reaction of thin Co films on single crystal Si<100> substrates (source/drain contacts) to the case of poly silicon substrates3 (gate contact and local interconnects). The stability of 27 to 50 nm CoSi2 films formed via rapid thermal annealing on polysilicon substrates with and without P+ or Ge+ implantation was examined using XTEM, RBS, XRD, and Four-probe measurements. For an undoped polysilicon, partial agglomeration of a 30 nm CoSi2 film has been observed, similar to that seen on a single crystal Si substrate.2For heavily Phosphorus doped polysilicon, a transition from low resistance, stoichiometric cobalt disilicide films (700 °C, 10 sec RTA) to high resistance films at 700 °C, 30 sec RTA (600–1000 Ω per square and discontinuous suicide grain growth) has been observed. Concurrent suicide formation on preamorphized polysilicon (Ge+ I/I and no anneal) has been found to suppress agglomeration of nanoscale cobalt suicide layers, since small grain size and low grain boundary energy are favorable for improving thermal stability4. These continuous layers also exhibited the lowest sheet resistance corresponding to a resistivity of ≤ 21 μΩ-cm.