Thin-Film Resistor Fabrication for InP Technology Applications

In this study we evaluated both NiCr and TaN thin-film resistor material for use with our InP technology. Thermal stability, sensitivity to oxidation, temperature coefficients, and patterning techniques were compared for the two materials. The film stiochiometry was determined using RBS. E-beam evaporation of NiCr (80:20) resulted in films that were rich in Cr (38%), due to a higher vapor pressure of Cr. Thus, it was preferentially evaporated from the source. The TaN, on the other hand, was stiochiometric to within a few atomic %. This was due to better composition control with sputter deposition as opposed to e-beam evaporation from an alloy source. The NiCr showed a 5% increase in Rs when exposed to O2 plasma, and was stable for anneal temperatures of up to 300°C. Alternatively, the TaN was stable for O2 plasma exposure, but had an increase in Rs of 5% or 10% when annealed at 300°C in N2 or air ambients, respectively. From a fabrication point of view, the NiCr was more stable during bake cycles. Howev r, it must be encapsulated to protect it from O2 plasma exposure. It can, therefore be used earlier on in the process. On the other hand TaN was more stable during plasma exposure, however, it must not be baked at high temperatures. It can thus be used at the back-end of the process. The NiCr thickness required to obtain an Rs of 50W/£ was around 250Å, as opposed to 800 for the TaN. Thus, the TaN material may be more reliable for higher current density applications. In addition, the TaN would be better in applications where resistors with higher Rs are required, such as 200W/£.