Microwave integrated electronics for ESR spectroscopy

In this study, we deal with the design and implementation of microsystems for electron spin resonance (ESR) applications. Three different microsystems are designed with different approaches for microwave magnetic field generation, ESR detection and sample handling. To the best of our knowledge, these microsystems are the first ever realized fully integrated solutions providing X-band ESR spectroscopy functionality on a single silicon chip. The first microsystem implements absorption mode ESR detection by means of a differential approach. It consists of two pairs of planar excitation/detection coils, a voltage controlled oscillator (VCO), a low noise amplifier (LNA) and a mixer. The excitation/detection coil pairs are identical. The excitation coil has a diameter of 300 microns, and it also functions as the LC-tank inductor of the VCO. The detection coil, which is placed in the middle of the excitation coil, has a diameter of 100 microns. The ESR sample is placed over one of the detection coils. This coil functions as the sample coil, while the empty coil functions as the reference coil. The excitation coils generate the microwave magnetic field acting on the detection coils. In the presence of ESR, the induced voltage on the sample coil differs from the induced voltage on the reference coil. This difference is amplified by the LNA and down-converted by the mixer. Two such microsystems with operating frequencies at 6 and 8 GHz are prototyped on a single standard CMOS chip with an area of 8.6 mm2. The ESR spin sensitivities achieved with the 6 and 8 GHz microsystems are 6.7×1010 and 3.9×1010 spins/GHz1/2, respectively. The second microsystem is an improved version of the first microsystem, with a slightly different topology. Furthermore, LNAs are implemented with SiGe bipolar transistors for higher gain with lower noise figure. It is also prototyped on a single silicon chip with an area of 4 mm2. The ESR spin sensitivity achieved with this microsystem is 3×1010 spins/GHz1/2. The third microsystem implements dispersion mode ESR detection by means of a differential approach in frequency domain. It consists of two VCOs, a mixer, a high frequency buffer, and a frequency divider. The VCO LC-tank inductor has a diameter of 100 microns, and it functions both as the excitation and detection coil. The two coils are placed perpendicularly in order to prevent injection locking. The VCO outputs are multiplied by the mixer, and a difference frequency output is generated. This output is amplified by the high frequency buffer. Its frequency is then divided by the frequency divider, and a 1-bit digital frequency output is generated. In the presence of ESR, the center frequencies of one of the VCOs changes slightly. This change is observed at the output in the form of frequency. This microsystem is prototyped on a standard CMOS chip with an area of 1 mm2. The ESR spin sensitivity achieved with this microsystem is 2×1010 spins/GHz1/2. The realized microsystems already have performances comparable to commercial systems. Their performances can further be improved; and they may be used in different micro scale applications of ESR spectroscopy and imaging.