Processes During Annealing of Ti—Al—Ni and Ti—Al—Ni—Au Contact Metallization Systems

The Ti/Al/Ni/Au metallization system which is widely used in the technology of GaN based devices has a very important disadvantage: after annealing in nitrogen atmosphere for 30 sec. at temperature 850 оС it has rough surface with 300 nm hillocks. This creates troubles for lithographic processes. The aim of this work is to investigate the mechanism that generates the roughness of this surface and ways to minimize this disadvantage. We have studied the formation of rough surface in Ti/Al/Ni and Ti/Al/Ni/Au multilayer metallization systems. The resistivity of the metallization sheet increases with an increase of annealing temperature. This can be attributed to the mutual diffusion of metals and their active interaction with the formation of intermetallic phases. X−ray analysis proved the appearance of the following basic intermetallic phases: NiTi, Al3Ti, и Ni2Al3 in the metallization systems. After annealing the surface of metallization system Ti/Al/Ni becomes rougher; however, large hemispherical convexes (as in the Ti/Al/Ni/ Au metallization system) are not generated. Thus, the hypothesis of balling−up of molten Al−Ni alloy on the surface of metallization system Ti/Al/Ni has not been confirmed. 

To decrease the amount of Au–Al liquid phase that causes the rough surface of Ti/Al/Ni/Au metallization we reduced the thickness of the Au layer to 50 nm. At this Au layer thickness the surface morphology of metallization became much better: roughness reduced from 300 nm to 80 nm and the surface became specular. 

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