1. Introduction

Much as the development of steel girders suddenly freed skyscrapers to reach beyond the 12-story limit of masonry buildings 6, achievements in four key processes have allowed the concept of 3D integrated circuits 2, proposed more than 20 years ago by visionaries (such as Jim Meindl in the United States and Mitsumasa Koyanagi in Japan), to actually begin to become realized. These factors are (1) low-temperature bonding, (2) layer-to-layer transfer and alignment, (3) electrical connectivity between layers, and (4) an effective release process. These are the cranes which will assemble our new electronic skyscrapers. As these emerged, the contemporary motivation to create such an unusual electronic structure remained unresolved. That argument finally appeared in a casual magazine article that certainly was not immediately recognized for the prescience it offered 5.