Atomic layer deposition builds amorphous materials; atomic layer epitaxy (ALE) builds crystalline materials. Start with a protected (passivated) surface: every available bond has a hydrogen atom. If you deprotect the surface, removing the hydrogen, then you can deposit a layer of atoms. If you choose the right precursor gas, you add only one monolayer which is protected as it's added. Then you can deprotect and add exactly one more layer of atoms. There are a number of precursor gases available. There are literally hundreds of systems to grow things with atomic precision in one dimension.
if you combine this with the ability to deprotect the surface in selected locations... With a scanning tunneling microscope, you can remove single hydrogen atoms with atomic precision. Several groups have demonstrated this. This is "the limit of a thin resist" - a monolayer of hydrogen.
Differences from mechanosynthesis:
1) Building blocks don't have to be captured by the tool tip.
2) The tool tip can be used to inspect both deprotection and assembly.
3) You can do large areas (fast) or atomic resolution, depending on mode.
4) This is a very general technique.
5) All you need is an atomic-resolution STM tip - don't need anything else with atomic resolution.
You need an atomically precise, invariant tip. ALIS has built such a tip. A reproducible atomic structure at the end of a tungsten wire.
They're trying to develop a dual-material process, silicon and germanium, so that you can make releasable structures. (They think they can deal with lattice mismatch.)
One possible product is a nano-imprint template. They expect atomically precise tools to be the most valuable product. They expect to enable productive nanosystem factories.
Question: Hydrogen migrates at normal temperatures. Is that compatible with the deposition technologies? A: We believe (after careful study) that the hydrogen is stable on a silicon surface, up to 200-300 degrees C. We think we can get epitaxy to work in that window. Cryogenic temperatures are not necessary. You do get motion on a single dimer, but no long-range motion.