Transistors in a modern microprocessor are so small that some of the critical film layers in them are only a few atoms thick and well over a million transistors would fit inside the period at the end of this sentence. ALD is one technique for making these infinitesimal structures that is becoming increasingly common.
The ALD process builds up material directly on the surface of the chip, a fraction of a monolayer at a time, to produce the thinnest, most uniform films possible. The self-limiting nature of the process and the related capacity for conformal deposition are the basis for its importance as a scaling and 3D enabler. The self-limiting surface reaction makes atomic-scale deposition control possible: film thickness depends only on the number of reaction cycles performed. Surface control makes the films extremely conformal and uniform in thickness, both essential properties in emerging 3D device designs.