STM hydrogen-resist lithography (donors) fab_stm

A hydrogen-terminated Si(100) surface is the resist. The tip removes hydrogen from selected dimers by electron-stimulated desorption; phosphine adsorbs only on the bare silicon; an anneal incorporates phosphorus substitutionally; low-temperature epitaxy buries the result. Hydrogen-resist patterning predates its qubit use by two decades; UNSW's Simmons group made a single-atom transistor in 2012 D[357] and SQC — branding the flow PAQMan — an 11-qubit processor in 2025 D[192].
Attributes: engineered placement of a natural dopant, no mobility and no control step, Pauli error once fabricated, a fabrication class with no CMOS analogue.

Throughput is the floor, structurally: one tip visits one site at a time, so the process scales as O(N) in qubits where photolithography scales as O(1) in wafers. In-plane registration inside the desorption window is near a lattice constant, but the donor's final position carries ~3 nm of uncertainty from segregation and diffusion during incorporation and encapsulation: the limiting step is thermal, not lithographic. Encapsulation temperature is the real trade: hot enough for good epitaxy moves donors, cold enough to freeze them leaves defects that dephase the electron. Only parallel exposure changes the scaling law.

Dominant term: device count. Every STM-donor device is fabricated individually; no parallel process exists at any scale and no placement-yield distribution is published.

Not published: a primary measurement of the incorporated-donor position distribution behind the ~3 nm placement precision; the amounts of the Firgun Ventures investment, the SkyWater agreement and the AMD collaboration, and the split of the A$60 M NRFC total by tranche; a parallelisation scheme for STM donor placement, by SQC or any other group; and any SQC statement on IonQ's acquisition of SkyWater.

No vendor sells a turnkey donor-placement tool: the equipment (UHV STM, phosphine handling, epitaxy) is merchant, the recipe is not, so no second source can be procured. Feedstock is no longer scarce: enriched ²⁸Si from ASP Isotopes and, from 2026-07-16, US DOE silane at 99.9999% ²⁸Si P[348]. SkyWater supplies US resonators and packaging (2025-11-20) C[276], and IonQ, a competing platform vendor, closed its ~USD 1.8 B acquisition of SkyWater on 2026-07-31 G[19] — the sharpest supply risk here. Export control bites on the feedstock, not the tool: no Export Control Classification Number (ECCN) covers STM lithography, but the US rule of 2024-09-06 created ECCNs for isotopically enriched silicon and germanium — silicon qualifies below 0.08% of the nuclear-spin isotope ²⁹Si — as epitaxial layers (3C907), hydrides such as silane (3C908) and bulk material or oxide (3C909), which takes in the DOE silane; its quantum-computer ECCN, 4A906, starts at 34 qubits and, below 2,000, applies only under a two-qubit (C-NOT) error ceiling set for each size band — out of reach of an eleven-qubit device G[301]. Burden at scale is unknown — no device beyond eleven qubits exists.

The manufacturing layer of donor spins in silicon: provides the donor-spin carrier and, through it, hyperfine-gated registers and spin-to-charge readout; it substitutes for foundry CMOS at placement, trading wafer throughput for unmatched precision. Nothing else in the stack requires it — the strategic weakness: a defect here cannot be routed around, and no other node gains from progress on it. Verification: the 11-qubit fidelities come from one paper by the only group practising the process D[192] — no second fabricator, and three fidelity framings across preprint, journal and main report.

Who.
| Organisation | Role | Country | What exactly they do | Evidence |
|---|---|---|---|---|
| SQC | developer | Australia | Sole practitioner of STM donor placement | D[192] |
| SkyWater Technology | supplier | USA | Resonators and packaging | C[276] |
| IonQ | supplier | USA | Owns SkyWater since 2026-07-31 | G[19] |
| NVIDIA | supplier | USA | NVQLink interconnect, SQC among 17 builders | C[531] |
| AMD | supplier | USA | Hardware for qubit control and readout, and compute for simulation (collaboration announced 2026-04-07) | C[358] |

Money. 2025-11-06 · SQC · QBI Stage B · up to USD 15 M · DARPA · awarded fact. 2026-03-24 and 2026-06-11 · SQC · equity · A$60 M combined · Australia's NRFC · closed fact. 2026-06-11 · SQC · investment · undisclosed · Firgun Ventures · announced C[358]. 2026-07-31 · IonQ · acquisition of supplier SkyWater · ~USD 1.8 B · closed G[19].

Market & supply chain. There is no market: one operator, no merchant process. Cost per placed donor is unpublished, so unit economics cannot be checked against the 2033 target. Funds G2 and G3; G4 needs parallelisation that does not exist.

IP & standards. Foundational patents trace to the UNSW/Simmons line; no dated family count from a named database as of 4 Sep 2026, and no standard covers atom-precision placement.

Roadmaps & track record. SQC (2026-06 · commercial scale by 2033 Rfact · unmet, eleven qubits published). "Continuous scaling" and "a million qubits" appear in company material with no published mechanism C[358], so the target is unfalsifiable rather than ambitious.

Strategic reading. If a parallel-write scheme appears, a decade of recipe knowledge compounds into a real moat. Without one, the precision that makes the process attractive is what caps it, and value migrates to whoever puts donors in a foundry flow.

Confirm/demote (12–24 months): a dated parallel-fabrication announcement or a device beyond eleven qubits confirms a path past boutique scale; neither by 2028 demotes this to a physics tool. Best case 2029: above fifty donors with published yield. Worst case: a single-laboratory capability. Open: (1) is multi-tip or templated exposure feasible; (2) does SkyWater's new owner keep the relationship; (3) does anyone publish a placement-yield distribution.