Tem Analysis of Interfacial Reactions Between TiWn, Wn Gate Metallizations and GaAs in Mesfet Devices

In an effort to improve the uniformity of the threshold voltage, the Schottky barrier height, and the ideality factor, Ti 0.29 W 0.52 N 0.19 and W 0.81 N 0.19 gate metal depositions have been investigated as a function of annealing conditions for GaAs based MESFET devices. Crosssectional TEM samples were made of each device. The results of these studies indicate there is a systematic and significant reaction occurring between the gate metal and the GaAs upon 850°C and 900°C rapid thermal annealing. These reactions take different forms depending on whether Ti is present or not. If Ti is absent (i.e. WN gates) then the interfacial roughness between the gate and the GaAs is less than 30A indicating the metallization is very unreactive. The WN contacts for some gates show void formation indicative of GaAs decomposition also for some devices an amorphous layer is observed at the interface. Selected area diffraction patterns indicate only the alpha-W and beta-W 2 N phases are present. For the TiWN gates the interface roughness is as large as 200A upon 900°C 10 second RTA. However no voids or interfacial amorphous layers were observed. Again only alpha-W and beta-W 2 N were observed in the bulk of the gates. For both systems, beta-W 2 N appears to form at the interface however the morphology of the beta-W 2 N grains are much larger for the TiWN gates. Electrical results indicate the TiWN gates have a lower ideality factor (near 1.1) and greater uniformity across the wafer compared to the WN gates. It is proposed that the presence of Ti in the gate metal aids in reducing any surface oxides thus improving the ideality and uniformity of the gate metal/GaAs contact.