Capacitor re-design overcomes the rotation rate limit of MACS resonators

The magic angle coil spinning (MACS) technique has provided a breakthrough in enhancing sensitivity in magic angle spinning (MAS) NMR. However, efforts in improving the MACS detector for higher spinning speeds have been lacking. One published MACS construction technique is to solder a handwound solenoidal coil to a commercial non-magnetic capacitor and subsequently centering the detector inside the MAS rotor. An alternative method to realize these detectors is by using MEMS fabrication at the wafer scale, potentially capable of achieving reproducible MACS detectors. However, it is also important that the performance of the sensors does not deteriorate as a result of microfabrication constraints. The footprint of the detectors is a limiting factor in achieving higher spinning speeds. One of the key elements of a micro-resonator is its tuning capacitor, whose geometry has a significant influence on its electrical and mechanical performance. The quality factor of the capacitor, along with the induced eddy currents, are the key performance parameters considered. The article addresses these concerns by presenting a study of microfabricated on-chip capacitors for magic angle coil spinning (MACS) detectors. The capacitors are juxtaposed with commercially available capacitors and the most suitable fit to be integrated with a micro-coil is established.