Thermo-mechanical failure criterion at the micron scale in electronic devices

Thermo-mechanical failures may occur in the passivation layer of micro-electronic devices during the fabrication process. These are in form of cracks which initiate at keyhole corners. In order to predict and eventually prevent these cracks a failure criterion is presented, based on an average value of the elastic strain energy in the vicinity of a reentrant corner of any angle. The proposed strain energy density (SED) failure criterion is validated by a test including 24 full size wafers which have been fabricated with different parameters: the interconnects (metal lines) height, the passivation thickness, and the passivating plasma power which was shown to correlate with the mechanical properties of the passivation layer. For each wafer, a FE model has been constructed, and the SED computed. It has been clearly shown, that above the critical value of SED_cr[R = 0. 15μm] ≈ 1000 [J/m³], all wafers manufactured were cracked. The SED criterion seems to correlate well with the empirical observations, and may be used as a standard tool for the mechanical design of failure free micro-electronic devices.