Detectors and emerging quantum hardware rely on nanofabrication processes to transform a uniform substrate into a complex spatial architecture that endows it with its function. Despite the capabilities of these processes, it is now clear that the chemical and structural quality of the resulting structures are insufficient to meet future scientific needs. The core problem is the precision gap between material growth and removal: while methods exist to synthesize materials with near-perfect atomic quality, analogous methods to remove material do not. Typical etching methods such as wet etches or reactive ion etching leave up to nanometer-scale surface roughness and a poor quality surface region that extends tens of Angstroms into the bulk. Thus, the initial Angstrom-scale quality of the substrate is degraded by nearly two orders of magnitude, indicating a fundamental gap in precision between material growth and removal. This precision gap leads to a variety of deleterious effects that are now the primary limiting mechanism for diverse figures of merit for devices such as electromagnetic resonators.

Atomic layer etching (ALE) is a method that has potential to mitigate the nanofabrication precision gap. ALE can be roughly viewed as the inverse of atomic layer deposition in that it consists of two or more self-terminating reactions at the surface of a substrate. Unlike in ALD, though, ALE leads to the removal, rather than deposition, of a monolayer. In the simplest two-step process, the first step weakens the bonds at the top monolayer of  material, for instance by fluorination. The second step  serves to remove this modified surface compound and can be accomplished by various schemes. In directional ALE, ions in a low energy plasma are directed to the surface, dislodging primarily the modified monolayer. In isotropic ALE, another reactant volatilizes the modified surface which is subsequently purged.

We are developing atomic layer etching processes which will enable subtractive manufacturing of electronic and quantum devices with atomic precision for the first time. We are applying these methods to fabrication of various devices, including low-noise InGaAs transistor amplifiers, single-photon detectors and qubits based on patterned superconducting thin films, and quantum photonic devices based on thin-film lithium niobate.