Multiple integrated circuits (ICs) die, from different wafers, can be picked-and-placed, front-side planarized using a vacuum applied to a planarizing disk, and attached to each other or a substrate. The streets between the IC die can be filled, and certain techniques or fixtures allow application of monolithic semiconductor wafer processing for interconnecting different die. High density I/O connections between different IC die can be obtained using structures and techniques for aligning vias to I/O structures, and programmably routing IC I/O lines to appropriate vias. Existing IC die can be retrofitted for such interconnection to other IC die, such as by using similar techniques or tools.