LONG-TERM RELIABILITY OF PT AND MO DIFFUSION-BARRIERS IN TI-PT-AU ANDTI-MO-AU METALLIZATION SYSTEMS FOR GAAS DIGITAL INTEGRATED-CIRCUITS

Reliability of Ti-Pt-Au and Ti-Mo-Au systems has been investigated for GaAs integrated circuit first-level metallizations on semi-insulating GaAs substrates and second-level metallizations on interlayer SiO2 fi lms using Auger depth profile analysis, residual resistance examinatio n and temperature storage step-stress testing. Auger analysis and resi dual resistance examination showed significant reaction between first- level Ti-Pt-Au and GaAs substrates during metallization processes, whi le Ti-Mo-Au system with the electron-beam evaporated Mo film showed hi gher thermal stability because the Mo film acted as a good diffusion b arrier between GaAs and Au. The second-level Ti-Pt-Au on SiO2 was foun d to be free from the reaction with GaAs substrates, and its degradati on was ascribed to interdiffusion of composite metals. The resistance increase in step-stress testing for the second Ti-Pt-Au was analyzed o n the basis of a new diffusion-controlled model, and long-term reliabi lity was estimated. A mean time to failure value of 3X10(5) h at 150 d egrees C was obtained for a failure defined as 10% increase in resista nce. Much higher reliability was estimated for Ti-Mo-Au, because the r esistance continued to decrease as long as 3000 h at 250 degrees C. Th e decrease in resistance clearly indicates defect annealing with reduc ed defect scattering in Au layers. This also shows that foreign metal diffusion into Au, acting as impurity scattering centers, is perfectly eliminated by Mo diffusion barriers.