These micrographs show a cross-section of a two-wafer stack. Each wafer has five metal circuitry layers and a series of copper bondpoints on its face. The wafers are joined face-to-face. Each photo shows one tungsten SuperContact™ in the top wafer.
The bright horizontal features across the center of most of the pictures are the copper bondpoints, joined face-to-face. Note the precise alignment of the two wafers! Finer horizontal features are the aluminum in the metal layers. The one bright vertical feature in each picture is a SuperContact. Some photos also also show the tiny vertical connections between the metal layers.
If a third wafer were to be added to this stack, copper bondpoints would be added to the top of the two-wafer stack. One of those bondpoints would cover the end of the SuperContact shown here. Adding a third wafer, face-down, would connect the two sets of copper bondpoints, providing vertical electrical connectivity throughout the stack. This could be repeated for additional wafers.
(All pictures taken in February, 2005)
Ten pairs of connected copper bondpoints mark the boundary between the two wafers. Five layers of aluminum are visible on each wafer. Vertical interconnects between the aluminum layers can be seen at the lower left, upper right, and upper middle. The Super-Contact is at center top.