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Thin-Film Resistor Fabrication for InP Technology Applications

01 January 2002

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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 O sub 2 plasma, and was stable for anneal temperatures of up to 300 degrees C. Alternatively, the TaN was stable for O sub 2 plasma exposure, but had an increase in Rs of 5% or 10% when annealed at 300 degrees C in N sub 2 or air ambients, respectively. From a fabrication point of view, the NiCr was more stable during bake cycles. However, it must be encapsulated to protect it from O sub 2 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 50 ohms/sq was around 250 angstroms, as opposed to 800 angstroms 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 200 ohms/sq.