Passive device integration is typically done by:                1) Back side on active side die stacking with wire bond interconnection,        2) Surface Mount of the passive to the top layer of a laminate substrate, or        3) Embedding large passive components into a printed circuit board.        
The limitations of these methods prevent direct electrical connection between mother and daughter die, followed by the flip chip attach of the resulting inverted die stack onto a Ball Grid Array (BGA) or Land Grid Array (LGA) laminate substrate. Direct connection would allow for better electrical performance and printed circuit board (PCB) real estate reduction by integrating passives off the PCB and into the package.
In the past, substrate technologies were limited with respect to the ability to laser or mechanically drill a cavity into a laminate substrate having a Bismaleimide Triazine (BT) core while maintaining mechanical and reliability integrity. With the advancements in substrate manufacturing technology, it is now possible to manufacture substrates with mechanically drilled or lasered cavities.
U.S. Pat. Nos. 7,915,084 (Hong), 8,222,717 (Shim et al), and 7,835,157 (Tilmans) discuss integrated passive devices, but these are completely different die stacking and die to substrate interconnection and package types from those in the present disclosure.