# Electron Spin Resonance Measurement with Microinductor on Chip

Authors: Akio Kitagawa
Journal: Journal of Sensors
 (2011)

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## Abstract

The detection of radicals on a chip is demonstrated. The proposed method is based on electron spin resonance (ESR) spectroscopy and the measurement of high-frequency impedance of the microinductor fabricated on the chip. The measurement was by using a frequency sweep of approximately 100 MHz. The ESR spectra of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH) dropped on the microinductor which is fabricated with CMOS 350-nm technology were observed at room temperature. The volume of the DPPH ethanol solution was 2μL, and the number of spins on the micro-inductor was estimated at about 1014. The sensitivity is not higher than that of the standard ESR spectrometers. However, the result indicates the feasibility of a near field radical sensor in which the microinductor as a probe head and ESR signal processing circuit are integrated.


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## Body

## 1. Introduction

Electron spin resonance (ESR) spectroscopy or electron paramagnetic resonance (EPR) spectroscopy is a widely used way of measuring and characterizing electronic materials [1, 2]. It is also used in biomedical analysis and imaging [3, 4] because radicals can be identified and quantitatively determined. The ESR phenomenon occurs based on the Zeeman effect of the unpaired electron in radicals, which can be observed by the resonance absorption of the magnetic wave at the specified frequency. The ESR frequency of the magnetic wave is proportional to the applied DC magnetic field. The ESR spectrum is provided by a frequency sweep of the magnetic wave or a sweep of the DC magnetic field strength. However, sweeping the DC magnetic field is widely used in most cases because the resonator which is designed to make resonance at the specific frequency is used to detect the small absorption of the electromagnetic power in the resonance [5]. In ESR spectroscopy on the frequency axis, the control system of the DC magnetic field can be eliminated, and this enables development of the radical sensors which integrate the ESR measurement circuit.In this paper, we demonstrate ESR measurement with a microinductor to show the possible integration of ESR signal processing on a chip. The ESR spectroscopy was done by a frequency sweep of around 100 MHz in the fixed magnetic field.


## 2. Principle of Measurement

The configuration of the measurement system is shown in Figure1. The high-frequency current in the microinductor generates a high-frequency magnetic wave. The DC magnetic field is perpendicularly applied to the microinductor. The magnetic permeability of the sample put on the microinductor is affected by the ESR, and the variation of the magnetic permeability is then detected as a variation of the impedance of the microinductor. The high-frequency magnetic field generated by the microinductor includes the vertical and the horizontal component against the DC magnetic field on the surface of the microinductor. The magnetic permeability is strongly affected by the perpendicular component of the high-frequency magnetic field [6], which is concentrated in the proximity of the metal wire to constitute the microinductor. The spatial range of the measurement depends on the intensity of the high-frequency current and the width of the metal wire.Figure 1
Structure of measurement system.The relative permeabilityμ(ω) is expressed as a function of the frequency as shown by:(1)μ(ω)=μr(ω)-jμi(ω),
where j is an imaginary unit, μr and μi are a real and an imaginary part of the magnetic permeability. The impedance of the microinductor affected by the paramagnetic materials is expressed as follows (2):(2)Z(ω)=R0+jω{1+αμ(ω)}L0,
where R0 and L0 are a resistance and an inductance of the microinductor, which is not affected by the sample material. Here, α is a filling factor which is a parameter of the contribution of the sample material to the magnetic permeability. From (1) and (2), (3a) is derived,(3a)Z(ω)=(R0+ωαμiL0)+jω(L0+αμrL0),(3b)≝(R0+RESR)+jω(L0+LESR).Here, RESR and LESR are a resistance component and an inductance component which are derived from the permeability of the sample material and proportional to the inductance of the microinductor. The impedance Z(ω) can be calculated from the measurement of the S-parameters of the microinductor by using(4)Z(ω)=Z01+S11-S12S21/(1+S22)1-S11+S12S21/(1+S22).
Here, Z0 is the characteristic impedance of the ports of the measurement instruments. The characteristic impedance of the commercially available measurement instruments and connectors is normally 50 ohm.


## 3. Measurement Results

The microinductor of a 2-port configuration was fabricated with CMOS 350 nm technology. This microinductor was designed to have a higher self-resonance frequency than a measurement band. The wire metal was aluminum with a thickness of 570 nm and covered by the Si3N4 passivation layer. A photograph and specification of the microinductor are shown in Figure 2 and Table 1, respectively. The measured resistance (R0) and inductance (L0) are 16.2 ohm and 15.8 nH at a frequency of 100 MHz, respectively.Table 1
Specification of microinductor.
ShapeInner diameter [μm]Number of turnsLine width [μm]Line spacing [μm]Rectangular spiral2406101Figure 2
Photograph of microinductor.The measurement results for di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH) are shown in Figure3. DPPH is the popular standard of the position (g-marker) and intensity of EPR signals. The RESR and LESR were calculated from the measured S-parameters by using (3b) and (4). The 2-μL DPPH ethanol solution (0.1 mg/L) was dropped on a microinductor and dried at room temperature. The number of spins on the microinductor was estimated at about 1014. The magnetic field was 4.2 mT and the center frequency of the signal was 117.6638 MHz. From these results, the g-factor of the ESR line was estimated at 2.0016. The estimated g-factor is smaller than the reported value (g=2.0023) [7]. As the magnetic field is not measured accurately enough to estimate the g-factor, calibration by another g-marker is needed to estimate the g-factor more precisely.Figure 3
Spectra ofRESR and LESR for DPPH at room temperature. Magnetic field was 4.2 mT.


## 4. Conclusions

The ESR measurement with a microinductor fabricated on a chip is demonstrated to show the possible integration of ESR signal processing on a chip. The ESR spectra of DPPH are observed at room temperature. The number of spins on the microinductor was estimated at about 1014. While sensitivity is not higher than that of the standard ESR spectrometers, the measurement system with a microinductor has several advantages. First, it is of small size, simple, and compact. Second, frequency scans can be done without the need for a magnetic field sweep. Third, the instrumentation is well developed in the very high frequency range (from 30 MHz to 300 MHz) and the ultrahigh frequency range (from 300 MHz to 3 GHz). Fourth, the probe head can be manufactured with CMOS technology. This instrument may be used to construct near-field ESR apparatuses with high spatial resolution.



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*Source: 813636-2011-09-12.xml*