The Ti/Al/Ni/Au multilayer metallization system is widespread in the technology of n-GaN-based devices. Herein, the mechanisms of the formation of the surface roughness of the Ti/Al/Ni/Au metallization (with 300 nm hillocks) upon annealing in a nitrogen atmosphere for 30 s at a temperature of 850°C, which creates problems in further lithographic processes, and the ways of overcoming this disadvantage have been studied. The formation of a rough surface during the annealing of a multilayer metallization, associated with the interaction of the constituent metals, has been investigated with the Ti/Al/Ni and Ti/Al/Ni/Au systems. An increase in the sheet resistance of both metallization systems as the annealing temperature is raised can be explained by the interdiffusion of metals and the growing degree of interaction between them with the formation of various intermetallic compounds, exhibiting a far higher specific resistance than that of the initial metals. The X-ray analysis has confirmed the origination of the main NiTi, Al3Ti, and Ni2Al3 intermetallic phases in the Ti/Al/Ni three-layer metallization upon annealing. The surface of the Ti/Al/Ni metallization system has become rougher upon annealing; however, large hemispherical convexes (like those in the Ti/Al/Ni/Au metallization) have not been generated. This has refuted the hypothesis of the balling-up of the molten Al–Ni alloy on the surface of the Ti/Al/Ni metallization. To reduce the amount of an Au–Al liquid phase formed during the annealing, which is the reason that renders the Ti/Al/Ni/Au metallization surface rough, the Au layer thickness was reduced to a minimum, at which the contrast of the metallization elements to the semiconductor surface is sufficient for self-aligning during electron-beam lithography. It has been found that the 50-nm-thick Au layer serves a satisfactory contrast. At such a thickness of the Au layer, the metallization surface morphology improved considerably: the roughness decreased from 300 to 80 nm, and the surface become specular.
Russian Microelectronics – Springer Journals
Published: Nov 21, 2015
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