Microfabricated inserts for sensitivity enhancement in MAS NMR

This thesis reports on the development and stabilization of the fabrication process of NMR micro-detectors as a part of the effort to address the gap in accessibility to miniaturized detectors for the NMR community.


The magic angle coil spinning (MACS) approach in NMR has combined the advantages offered by a micro-coil detector and magic angle spinning (MAS) to bring about considerable improvements in sensitivity and resolution in solid-state NMR experiments. The MACS strategy features co-rotation of the sample and the micro-detector while the latter is inductively coupled to the pick-up coil in the MAS probehead. The micro-detector used for this technique was conceived as handwound micro-solenoid with its ends soldered to an off the shelf capacitors to form a micro-resonator circuit tuned to the Larmor frequency of interest. This manual fabrication of the MACS detectors is not only cumbersome but also makes these devices imprecise and prone to imbalance. Moreover, the wireless inductive coupling arrangement mandates adequate electrical performance from these devices to avoid signal losses. Another drawback of the MACS technique is the sample heating due to induced eddy currents in the spinning micro-detector. ... mehr

On the other hand, utilization of MEMS fabrication techniques delivers photolithographic accuracies as well as scalability and reproducibility for these devices. Implementing the capacitor on-chip to go with the automated wirebonding process for the micro-coil would eliminate the issues associated with the manual hand fabrication method. Therefore, with this backdrop, this thesis reports on the development of microfabricated MACS devices that are fine tuned not only for their electrical performance but also provides excellent thermal performance with reduced induced eddy currents. Furthermore, the goal was also to produce devices that are reproducible, robust and easy to handle so as to seamlessly facilitate NMR measurements. Therefore, this thesis tackles the challenges associated with having a suitable design for the on-chip capacitor connected to the micro-coil as well as finer aspects of microfabrication techniques employed to manufacture the MACS detectors. These devices are also extensively characterized using standard benchmarking protocols and the feasibility of adopting these devices for metabolomic studies is also investigated.

Another major focus of this thesis is to explore the concept for the non-resonant variant of MACS detector with broadband performance across the frequency spectrum. This would overcome the limitations associated with tuned MACS devices which originate from the requirement to tune them to the target Larmor frequency to have the best NMR sensitivity. Wideband characteristics render the devices flexible with respect to either magnetic field strength and/or nuclei of interest and would also bring in added benefits for cross-polarization (CP) and heteronuclear experiments.


The broadband functionality is conceptualized by integrating the micro-coil as a part of Lenz lens arrangement instead of an on-chip capacitor. Three different designs are introduced and analyzed analytically as well as using FEM simulations. In order to implement these geometries, innovative micro-manufacturing approaches have been established keeping in focus the objective of high volume fabrication.

Thus, the major outcome of this work is an established wafer scale fabrication process to deliver micro-detectors that are cost effective and at the same time that drastically boost NMR sensitivity for mass- and volume-limited samples and offer part per billion spectral resolution for NMR metabolomics.