High density Cu-TSVs and reliability issues

Reliability issues such as thermo-mechanical stress, extrusion of via metal, and die-cracking caused by high density Cu-TSVs in 3D-LSI Si die/wafer after wafer thinning and bonding have been systematically investigated respectively using micro-Raman spectroscopy, laser microscopy, and optical microscopy techniques. It is inferred that (i) for the TSV pitch value of less than twice the TSV-width, the remnant stress present in the Si at the TSV space region is turned to be only compressive, i.e. in the lateral direction, the compressive stress produced by the adjacent TSVs overlapped to each other; for the TSV pitch values of greater than two times the TSV-width, the compressive stress in the Si at the vicinity of TSV is followed by the tensile stress and beyond that it becomes stress free at the TSV space region; (ii) Irrespective of the TSV shape and size, the lateral extrusion of Cu occurs at the TSV space region. The lateral extrusion becomes prominent for the larger TSV size values and the higher bonding temperatures. The lateral extrusion is larger for the 20 μm-width TSV annealed at the higher temperature (∼4 μm @ 400 °C) than for the TSV annealed at lower temperature (a maximum of only 1.5 μm @ 200 °C); (iii) Cracking of LSI die/wafer occurs at the periphery of the TSV array for very fine pitch values, and for larger pitch values cracking occurs in between TSVs.