5.4.1 Thermal budget

The thermal budget for the formation of the low resistivity NiSi is much lower compared to its CoSi ₂ counterpart. An advantage is that once the low resistivity NiSi is formed, one does not have to anneal further and reach the NiSi ₂ phase. Even when comparing the formation of the monosilicides, it is found that NiSi is much easier to form than CoSi. In FIGURE 5.3, we show a comparison of the process windows for forming the low resistance phases in both the Ni/Si and Co/Si systems using in situ resistance measurements. The traces shown in FIGURE 5.3 were measured from thin films of Ni and Co deposited on undoped poly-Si while the samples were annealed at 3 C/s in nitrogen. The resistance trace for the Ni/Si reaction has been shifted up for clarity. As the temperature increases, the film resistance is modified by the successive formation of silicide phases. From the graph, it is clear that the low resistance CoSi ₂ phase forms slightly above 600 C for these anneal conditions, whereas the formation of the low resistivity NiSi is achieved at 350 C. Note that at the temperature of formation of the CoSi ₂ phase, the resistance of NiSi is already starting to increase, showing the importance of reducing the thermal budget for the process after NiSi formation. A material change from CoSi ₂ to NiSi thus not only allows for a reduction in the thermal budget but...