Microfabricated coils for Nuclear Magnetic Resonance

Microfabricated coils for Nuclear Magnetic Resonance

C. Massin, F. Vincent

Introduction

The Microsystems Design group at EPFL is developing miniaturized coils based on planar microfabrication technology for Nuclear Magnetic Resonance (NMR) spectroscopy and imaging applications at the micrometer-scale level. The coils can be monolithically integrated with microfluidics for NMR analysis in Micro Total Analysis Systems, accomodating sample volumes between 1-100 nanoliters.

General Description

To achieve sufficient signal-to-noise ratio (SNR), the electrical performance of the coil is critical. The self-resonance frequency should lie well above 1 GHz and the series resistance should be minimized. This has lead us to develop a new fabrication process based on copper electroplating in an SU-8 mold. SU-8 is a negative-tone photoepoxy which allows to achieve high-aspect ratio structures. To minimize parasitics, the coils are fabricated on a glass substrate, with inner diameter ranging from 100 um to 2 mm (Fig.1). The fabrication is performed in-house at the Center of Microtechnolgy at EPFL. The coils can be contacted to the macro world using standard wire bonding (Fig. 2).


Fig. 1: 500 um inner diameter planar coil with 35 um thick copper spiral.	Fig. 2: 500 um diameter coil after wire bonding

Electrical Characteristics

Directly after fabrication, individual coil impedance can be measured on wafer using coplanar RF probes (Fig. 3). Typically, the coils have inductance between 1-50 nH, series resistance < 1 Ohm, and self-resonance frequency >> 1 GHz. This results in quality factors up to 40 at 800 MHz (Fig. 4).


Fig. 3: The electrical performance of the coils are measured on-wafer using coplanar RF probes.	Fig. 3: The electrical performance of the coils are measured on-wafer using coplanar RF probes.

Integration with Microfluidics

The coils can be fabricated on top of a micromachined microfluidic substrate, giving birth to a monolithic NMR-TAS. The integration on a glass substrate chip fabricated in the group of Prof. de Rooij at the University of Neuchatel is shown below (Fig. 5). The chamber below the coil accomodates approximately 30 nL of sample (Fig. 6).


Fig. 5: Monolithic NMR chip with integrated coils and microfluidic channels.	Fig. 6: Detail of an electroplated coil on a glass substrate containing an etched flow cell (30 nL).

NMR Measurements

1H NMR spectra have been obtained using the NMR-TAS chip at 7 T (300 MHz). The microcoil is used both for sample excitation and signal detection. The achieved SNR allows to obtain spectra of nanoliter-volume samples in a very short acquisition time, as is demonstrated for ethanol in Fig. 8. The spectrum linewidth is relatively large compared to that obtained using conventional high-resolution probes (30 Hz vs < 1 Hz) and prevents observing spin coupling. This is linked to probe geometry and materials, and current work is targeted at improving this critical specification.


Fig. 7: 1H NMR spectrum of 30 nL water obtained at 7 T with the integrated NMR chip.	Fig. 8: 1H NMR spectrum of 30 nL ethanol obtained at 7 T with the integrated chip (3 acquisitions).

Publications

[1] C. Massin, G. Boero, F. Vincent, J. Abenhaim, P.-A. Besse, R. S. Popovic,"High-Q factor RF planar microcoils for micro-scale NMR spectroscopy", Sensors and Actuators A: Physical, in press.

[2] C. Massin, A. Daridon, F. Vincent, G. Boero, P.-A. Besse, E. Verpoorte, N. F. de Rooij, and
R. S. Popovic, “A Microfabricated Probe with Integrated Coils and Channels for On-chip NMR spectroscopy”, 5th International Conference on Miniaturized Chemical and Biochemical Analysis Systems, Monterey, CA, USA, October 21-25, 2001, pp. 438-440.

[3] C. Massin, F. Vincent, P.-A. Besse, R. S. Popovic, "High Quality Factor Microfabricated RF Inductors", EAST-forum 2001, Ouranoupolis, Greece, 10-13 October 2001.

[4] C. Massin, G. Boero, P. Eichenberger, P.-A. Besse, R. S. Popovic, "High-Q factor RF planar microcoils on glass substrates for NMR spectroscopy", TRANSDUCERS'01, Eurosensors XV, Munich, Germany, June 2001, pp. 784-787.

For more information, contact charles.massin@epfl.ch