Innovative devices for electron spin resonance spectroscopy on small samples

Within the past few years, the field of Electron Spin Resonance has become an important tool in physical, chemical and biological research. However, in many cases its usefulness is limited by the sensitivity of the experimental setup. A typical example is the study of very limited sample quantities such as tissues, cellular structures, single cells, and also thin films in microelectronic technologies. For such volume limited samples, the typical dimensions and/or spatial variations of the properties of interest range between 1 µm and 1 mm. Present X-band ESR spectrometers are optimized for samples having a volume between (1 mm)3 and (1 cm)3, and achieve a spin sensitivity of about 1010 spins/G√Hz at room temperature. Setting up new high sensitivity ESR spectrometers would overcome this limitation and open the way to numerous new applications. The goal of the present work is to study and develop innovative devices in combination with modern continuous ESR detection method to achieve a spin sensitivity of 109 spins/G√Hz or better at room temperature, specifically conceived for samples having dimensions and/or spatial variations in the range between 1 µm and 1 mm. We mainly focus our investigation efforts on the design of ESR-setups based on miniaturized sensitive devices. Miniaturization concerns size reduction of the sensitive volume where the sample will be placed, i.e., the region in which the microwave magnetic field is generated. In this work two approaches will be investigated. Inductive transverse magnetization detection by means of planar structures based on coplanar waveguide (CPW) resonator design. Single turn coils and straight trace constrictions with different dimensions are designed as sensitive regions to accommodate the appropriate sample size. These constrictions are built with open-ended half-wavelength and shorted (or shunted) quarter-wavelength lines related to the working resonance frequency. Appropriate microfabrication technologies are also developed based on two substrate candidates, SU-8 and RO4003C, with Aluminum and Copper metals, respectively. Performance of the RO4003C based-resonators proves to be better than that for the SU-8 based ones in terms of the conversion factor describing the efficiency of conversion of microwave power to magnetic field strength. The analysis we have performed shows that the dielectric losses induced in the RO4003C substrate are much lower than those induced in SU-8 one. This improves the Q-factor by almost a factor 10. Since the conversion factor varies as Q1/2, this, in turn, enhances the power conversion efficiency by almost a factor 3. Detection efficiency is also improved by almost the same factor. As a result, a spin sensitivity of about 109 spins/G√Hz is achieved at room temperature for sensitive volumes of about (100 µm)3. This result is one order of magnitude better than that of existing X-band spectrometers. Longitudinal magnetization detection using a Hall-based probe. A resonant cavity and a CPW-resonator are used to generate the microwave field. Rather than a large rectangular cavity and its disperse field lines, the CPW-resonator offers a smaller structure. Its compact geometry is suitable for small volumes, since it concentrates the microwave magnetic field precisely at the sample. This approach eliminates several experimental difficulties of cavity-employed ESR spectroscopy and better controls the electromagnetic environment of the sample, avoiding the properties of the cavity resonator which suffer from thermal drifts and background instabilities. In addition, thanks to their greater sensitivity, the CPW-resonators require much less microwave power than waveguide cavities, to generate the same microwave magnetic field. The achieved spin sensitivity is about 109 spins/G√Hz at room temperature for sensitive volumes of about (10 µm)3 . This result is, once more, one order of magnitude better than that of conventional X-band spectrometers. The obtained results confirm the effectiveness of the combination of the CPW-resonators for small sample analysis for practical enhancement of the ESR spectrometer sensitivity at X-band frequencies. Furthermore, a key aspect of the present work is the ability to go further in miniaturization: the concept of coplanar waveguide structures is very simple and easy to implement for the required ESR measurements without too much additional design complexity and change to fabrication procedures. This should be very promising, and especially highly motivating to achieve in-vivo ESR measurements inside living organism and to perform high spatial resolution ESR-imaging.


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