Nitrogen vacancies (NV) in diamond are excellent qubits for nanoscale quantum sensing due to their millisecond scale coherence times, optical spin initialization, and optical spin readout at room temperature in ambient conditions. However, methods of creating diamond based quantum devices can result in poor NV center spin and charge properties near the diamond surface, which can further be degraded by surface and subsurface damage during the microfabrication and device integration process.
Recently, a quasi-isotropic etch based process was used to fabricate a free standing micromembrane with a dense array of nanobeams utilizing sacrificial tethers to the host substrate. The etch is performed at a lower temperature and inductively coupled plasma reactive ion etching power, reducing the etch rate and enabling control of the undercut geometry. The process achieves low surface contamination, low subsurface damage, and high quality oxygen termination. This results in nanobeams that host NV centers within nanometers of the surface that have state of the art nanoscale sensing performance.
This new work was reported by Alexander C Pakpour-Tabrizi, Artur Lozovoi, Sean Karg, Tecla Bottinelli Montandon, Melody Leung, Kai-Hung Cheng, and Nathalie P. de Leon from Princeton University.
The work was performed in part using an STS-Elionix ELS-F125 electron beam lithography system at the Princeton Quantum Device Nanofabrication Lab.
For more information see https://doi.org/10.1002/adom.202503864
Image: Tilted scanning electron micrograph (left) of the micromembrane structure, with a higher magnification (right) of an individual nanobeam, 720 nm wide and 300 nm thick. (Image use is made available under the terms of the Creative Commons Attribution 4.0 International License.)
STS-Elionix Questions and Answers
A diamond nitrogen-vacancy (NV) center is an atomic-scale defect in diamond where a nitrogen atom sits next to a missing carbon atom. Rather than being an unwanted defect, this unique structure can function as an extremely sensitive quantum sensor.
Researchers can use light to initialize and read the quantum state of an NV center. Because that state responds to very small changes in its surroundings, NV centers can be used to investigate magnetic fields, electrical activity, and material properties at the nanoscale.
One particularly attractive feature of diamond NV centers is their ability to operate at room temperature, making them promising candidates for future quantum sensing and nanoscale measurement technologies.
For an NV center to detect extremely small signals, it often needs to be positioned very close to the material or device being studied. This creates a significant nanofabrication challenge: researchers must fabricate extremely small diamond structures without damaging the surface or degrading the quantum properties of the NV centers.
In this research, the Princeton team developed a low-damage fabrication process for creating free-standing diamond structures containing arrays of nanobeams. An individual nanobeam shown in the study was approximately 720 nm wide and 300 nm thick.
Importantly, the NV centers maintained strong quantum-sensing properties after fabrication. The diamond nanobeam geometry also improved light collection from the NV centers, helping researchers detect the extremely small optical signals generated by these quantum sensors.
Electron Beam Lithography (EBL) allows researchers to define extremely small and complex patterns that would be difficult or impossible to produce using conventional lithography.
In this research, an Elionix ELS-F125 Electron Beam Lithography system was used to pattern ZEP 520a electron beam resist. The resulting nanoscale pattern was transferred into a silicon nitride hard mask, which was then used during the diamond etching process.
In simplified terms, Electron Beam Lithography creates the nanoscale blueprint that determines where material will ultimately remain and where it will be removed.
This demonstrates an important role for high-resolution EBL in advanced nanofabrication and quantum-device research: precisely defining nanoscale structures that must maintain their dimensions and integrity through multiple subsequent fabrication processes.
As semiconductor devices, quantum devices, and advanced materials continue to move toward smaller dimensions, researchers increasingly need tools capable of both fabricating and measuring structures at the nanoscale.
Diamond NV centers offer a potential method for detecting extremely small magnetic and electrical signals with very high spatial resolution. Creating free-standing diamond nanostructures while preserving the quality of these quantum sensors could make them useful for investigating materials and devices at dimensions that are increasingly difficult to study using conventional measurement techniques.
Future applications could include quantum computing, semiconductor characterization, advanced-material research, nanoscale magnetic imaging, quantum sensing, and other emerging quantum technologies.
Research such as this also illustrates the relationship between Electron Beam Lithography and quantum technology: high-precision EBL provides researchers with a nanofabrication tool capable of turning nanoscale designs into physical structures that can ultimately become functional quantum devices and sensors.