Simultaneous formation of electrical connection, mechanical support and hermetic seal with bump-less cu-cu bonding for 3D wafer stacking

Wafer-on-wafer stacking is demonstrated successfully using bump-less Cu-Cu bonding for simultaneous formation of electrical connection, mechanical support and hermetic seal. The mechanical strength of the bonded Cu-Cu layer sustains grinding and chemical etching. Daisy chain of at least 44,000 contacts at 15μm pitch is connected successfully. Cu-Cu hermetic seal ring shows helium leak rate >;10X lower than the reject limit without under-fill. This provides robust IC-to-IC connection density of 4.4 × 10⁵ cm^-2 suitable for future wafer level 3D integration.