Low-temperature co-fired ceramic-based coils in probe heads for nuclear magnetic resonance spectrometry

Abstract

Over the last few decades, nuclear magnetic resonance (NMR) has had a tremendous impact on research in physics, chemistry, biology and medicine. Although this invaluable technique for quantitative structure analysis, NMR spectrometry suffers from an inherent lack of sensitivity compared to other popular analytical techniques like fluorescence and mass spectrometry or electron spin resonance. The sensitivity of the NMR spectrometer is defined through the signal-to-noise ratio (SNR) and is dependent on sample conditions and spectrometer parameters. One decisive Parameter for the sensitivity improvement of the NMR experiment is the quality of the sensing element of the spectrometer - a coil in the probe head. Low temperature co-fired ceramics (LTCC) offer prototyping and rapid development of various passive electronic components and, thanks to an inherent low dielectric loss of the ceramic and applicability of highly conductive silver, it is a Technology of choice for the development of high quality radio frequency inductors. The possibility to fabricate multi-layered three dimensional structures, the remarkable electrical and thermo-mechanical properties of LTCC make this material desirable for the production of the coils suited for NMR experiments. This thesis deals with the optimization of the coil design for given NMR samples. The optimization starts with a mathematical description of the field generated by the coil and an attempt to get the end result - the SNR of the experiment in an analytical expression. Further analysis by means of finite element method simulations delivers a deeper insight in coil-s properties as for instance inductance, quality factor, field homogeneity and self-resonant frequency. All these features are used to get the geometrical parameters of the coil in terms of diameter, track width, or number of windings for which the coil yields the best possible NMR sensitivity. After thorough analysis the various sets of coils are produced in the Laboratory for Ceramic Technology, characterized electrically and employed in the NMR spectrometer. The production of the coils tested the limits of established LTCC technology for both large and small dimensioned structures were successfully fabricated. Coil characterization and NMR measurements confirmed the predictions from theoretical analysis. A high frequency coil with a nano-liter sample volume was tested in the NMR spectrometer and yielded an SNR convincingly higher than the copper wire coil that is usually used for this purpose. The extreme working conditions, such as temperatures as low as 80K and voltages of few kilovolts have proven superiority of LTCC for this application.