A VCO-based EPR Sensor Featuring a Large 400 µm Coil to Enhance MW B₁ Homogeneity and Concentration Sensitivity

We present a voltage-controlled oscillator (VCO)-based electron paramagnetic resonance (EPR) sensor featuring an enhanced sensing area for a high concentration sensitivity and improved homogeneity compared to conventional, chip-integrated planar sensors. Prior designs have relied on VCO-array configurations to expand the sensing area, which comes at the cost of increased power consumption. Here, the relatively small diameters of the individual coil elements limit the homogeneous region in the direction perpendicular to the chip surface. To address these limitations, we propose the use of inductance-capacitance (LC) VCO-based EPR detectors with large coil diameters with a laser-fabricated hole to create a large sensitive volume with high homogeneity of the microwave B<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> field inside the coil. Despite the enlarged coil diameter and the correspondingly large inductance, the VCO-based detector achieves a low phase noise (PN) of -116 dBc/Hz at a 1 MHz offset from a center frequency of 6.75 GHz, and a 10 % frequency tuning range. The presented sensor frontend incorporates the VCO-based EPR detector inside an integer-N phase-locked loop (PLL), enabling precise phase and frequency control from an external low-frequency reference. The C-band operation of the presented VCO-based EPR detector is compatible with the static magnetic fields achievable with compact permanent magnets. The proposed sensor is electrically characterized and validated in the target EPR application using standard EPR samples of BDPA (α, γ-bisdiphenylene-β-phenylallyl) and TEM-POL (2,2,6,6-tetramethyl-4-hydroxy-piperidine-1-oxyl), achieving a spin sensitivity of 5 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> spins/(G Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup>) and concentration sensitivity of 15.6 µM/(G Hz<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup>), respectively.