Optical metrology for nanowires grown with molecular beam epitaxy

Semiconductor nanowires are important materials for quantum transport experiments and are used in research on qubits. Extended arrays of nanowires can be grown bottom-up by Molecular Beam Epitaxy (MBE). The full process involves several steps. When fabricating nanowires, a common practice is to follow a well-established recipe and only characterize the finalized materials. If the final wires are found to be flawed, the process must be repeated with new parameters. It is therefore desirable to have a characterization method to monitor the process before and after each fabrication step. Conventional characterization techniques such as SEM are time-consuming and, in some cases, damage the samples, e.g. before and after an electron beam lithography process. Scatterometry is fast, accurate, non-destructive and is already used in the semiconductor industry. In this work, it is demonstrated that the imaging scatterometry technique is capable of monitoring the MBE fabrication process of InAs-nanowire arrays during the different process steps. Relevant parameters such as thin film thickness, hole depth, and diameter, etc., are found with nm precision for a macroscopic area in a few minutes. Using this approach, we demonstrate that errors can be caught early in the process and ultimately save resources while assuring a high quality of the final material.