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)