Optimization of Al interlayer thickness for the multilayer diffusion barrier scheme in Cu metallization

Abstract

The TiN(5 nm)/Al/TiN (5 nm) structure is fabricated by sequential sputtering for the diffusion barrier application against Cu diffusion. Al is used as an interlayer with an expectation that it will favor stuffing of TiN grain boundaries by reacting with O2 in the structure. In one phase of experiments, the upper TiN layer is exposed to air and preannealed in N2/O2 ambient for better stuffing. In another phase, the preannealing is not performed for the sake of reducing process steps. The Al layer thickness is varied to find out the optimized value. The diffusion barrier performance is investigated by x-ray diffraction, sheet resistance measurement, secco etching and cross sectional transmission electron microscopy after stressing the structure in temperature range of 350–700 °C at vacuum for 30 min. It is found that the barrier property is improved remarkably with the introduction of an Al layer between two TiN layers as compared to that of a single TiN layer with the same thickness. The results indicate that the barrier performance is enhanced proportionally with increasing Al thickness (from 0.5 to 2 nm) in the case that upper TiN is preannealed, showing the best result for 2 nm thick Al interlayer. On the other hand, in not preannealed upper TiN, barrier property is at its best when Al thickness is 1 nm but above this value the barrier performance deteriorates drastically.