Physics / mechanism
ALD deposits material one atomic monolayer at a time via sequential, self-limiting surface reactions. A precursor gas chemisorbs onto the substrate; excess is purged; a co-reactant (oxidant, plasma, or reducing agent) reacts with the chemisorbed layer; excess purged again. One cycle = ~0.1-2 Å growth. The self-limiting nature gives exceptional conformality (>99% step coverage in high-aspect-ratio structures) and sub-Ångström thickness control; the price is throughput (cycle times 0.5-5s), which is the permanent industrial constraint. Variants: thermal ALD, plasma-enhanced (PEALD, lower temperature), spatial ALD (substrate moves through zoned precursor curtains, the throughput answer), batch furnace ALD, and area-selective ALD (inhibitor chemistry blocks growth on chosen surfaces, so the film patterns itself). Key materials: Al2O3, HfO2, TiN, ZnO, Ru, and now Mo. Because ALD is CVD chemistry cut into self-limiting half-reactions, it inherits the same precursor constraint: the molecule defines what can be grown, and hydrogen rides in on most C and N precursors (CVD (Chemical Vapor Deposition)).
The forcing-function precedent (why ALD matters to every deposition thesis)
Market structure
Where value is capturable (routing)
The tool oligopoly and the ASD stack (tools + inhibitors + integration) are incumbent-owned: AMAT/Lam/ASM plus imec plus Merck/EMD. Primary route is public equity (ASM.AS is the pure ALD instrument; AMAT, LRCX, 8035.T; materials MRK.DE, AI.PA, ENTG). The only early-stage door any venture route has found is the novel precursor/inhibitor molecule out of an EU lab, as a chemistry-IP-and-exit play; through Jun 2026 repeated searches found no venture-backed ASD/precursor pure-play raising a Series A (theme predictions #2/#6 unfalsified).