Author(s): Sherief Reda (corresponding author) 
Integrated circuits are ubiquitous in modern electronic systems. Conventionally, they consist of a single layer of electronic devices and multiple layers of electrical connections (interconnects) that wire the devices to each other. Because there is a physical limit to how small such devices can be when they are created in two dimensions, researchers have targeted the third dimension to incorporate more device layers and continue the trend of increased integration density and functionality. On page 74, Shulaker et al .  report a milestone in 3D integration. They present a fully functional prototype of a 'monolithic' 3D integrated circuit that brings together some of the most promising emerging device technologies in terms of sensing, memory and computing capabilities. To demonstrate it, the authors use the prototype to sense and distinguish between ambient gases and vapours.
Researchers began working on 3D integrated circuits almost two decades ago. In the first efforts, each stack, which contained a device layer and multiple interconnect layers, was fabricated separately. These stacks were then bonded, and wired by interconnects called through-silicon vias . The separate fabrication was necessary to allow high-temperature annealing -- rapid and precise heating of the device layers to temperatures  greater than 1,000 [degrees]C -- to activate dopants (impurities) in the devices that produce the desired electrical characteristics. Stacks could not be fabricated on top of each other because the annealing of one device layer would damage the interconnect structures of the underlying layers.
A major limitation of separate fabrication is that the diameter and pitch (separation) of the through-silicon vias need to be at least micrometre-sized. This constrains the density of the through-silicon vias that can be used, and therefore the bandwidth for data transfer in the 3D integrated circuits.
In the past few years, monolithic integration -- in which additional device layers are built directly on top of the first layer -- has gained momentum. Monolithic integration enables the fabrication of interconnects called inter-layer vias that have nanoscale diameters and pitches. This means that interconnects can be fabricated at 1,000 times the density of through-silicon vias, and allows data to be transferred through the integrated circuit at...