Remote detection of the conductivity of current-carrying metallized traces

The weak magnetic fields generated by a current-carrying metallized traces are detected in view of their applications in bionic systems and neural-electrode interface technologies. The traces are formed by femtosecond laser processing of the surface of polydimethylsiloxane polymer substrate and further functionalization by electroless metallization. The measurements are performed by means of magneto-optical spectroscopy involving two optical beams, serving as pump and probe, where the magnetic field sensor is 87Rb atoms confined in a paraffin-coated optical cell. The experimental results show the feasibility of remote detection of the conductivity of metallized nickel traces.


Introduction
Recently, current-carrying metallized conductive traces deposited onto flexible biopolymer carriers have been widely used in neural-electrode interface technologies and bionic systems, as they allow fine limb movements.Polydimethylsiloxane (PDMS) polymer has remarkable properties such as elasticity and mechanical flexibility, high chemical stability and high dielectric breakdown threshold, cyto-and bio-compatibility and stability, which leads to long-term implant life without disruption of the soft tissues (brain, muscles).The laser processing of PDMS elastomer is an easy and powerful method for surface modification without altering its bulk properties.
In this study, we measure the weak magnetic field (MF) generated by the current flowing through traces realized by surface modification and activation of optically-transparent biocompatible polymers.The process includes femtosecond (fs) pulsed laser processing of the surface of PDMS elastomer and subsequent electroless metallization of the activated surface by nickel (Ni).
To detect remotely the conductivity of the current-carrying metallized micro-traces, we use optical magnetometry based on zero-field level crossing magneto-optical resonances in a paraffin-coated sensor cell containing Rb vapor.A solenoid provides scanned around zero precisely calibrated magnetic field.Pump and probe beams collinear to the scanned magnetic field are applied to prepare high-contrast magneto-optical Electromagnetically Induced Absorption (EIA) resonances in the transmission of the probe beam.The scheme is described in more detail in [1,2].Additional magnetic field introduced at the sensor region leads to modification of the parameters of the obtained resonances (shift of the resonance center for longitudinal MFs and broadening of its linewidth for transverse MFs).It should be noted that the resonance has a complex shape: it consists of a narrow component superimposed on a pedestal of a broader one [3].Further in the text we discuss the parameters of the narrow structure only, since namely it is used for magnetic field estimation.
A similar measurement was reported in [4], where a transverse magnetic field of (3.8 ± 0.4) mG was measured on a micro-wire prepared by nanosecond (ns) pulsed laser structuring and activation of the surface of PDMS elastomer and electroless platinum (Pt) metallization.It has been shown that it is possible to discriminate between operating and nonoperating mode of the micro-wire.Ni has a lower resistivity than that of the noble metal Pt, but as a cheaper material with similar behaviour in the electroless deposition process, it is interesting to test its potential for measuring weak magnetic fields for more affordable applications.
The aim of the present work is to demonstrate the possibility of remotely detecting the conductivity of current-carrying metallized Ni traces in view of their applications in independently operating devices.

Materials and methods
For our study, the sample preparation involves surface modification and functionalization of PDMS elastomer.The first stage of the preparation of this type of microelectrode traces is the structuring and activation of the PDMS polymer surface by ablation with fs laser radiation.The operating laser parameters are: wavelength 1055 nm, pulse duration 300 fs and repetition rate 33 Hz.A detailed description of the sample preparation procedure is given in [5].The second stage of the sample processing is its functionalization by autocatalytic (electroless) deposition of Ni on the activated areas, as described in [6].The width of the metallized traces is about 200 µm, and their length -about 6 mm, with a thickness of about several nanometers.For convenience, in our measurements the polymer was placed on a glass substrate, which limited the distance between the conductive trace and the window of the rubidium cell to about 10 mm.The inset in figure 1 presents a photograph of the sample used in this test experiment after connecting the wires at the ends (the scaling bar is 1 mm).We use the experimental setup described in [7], which is shown schematically in figure 1.The linearly polarized laser beam, 4 mm in diameter (d beam ), of a single mode 794.76 nm DFB diode laser is in resonance with the D 1 line of Rb and passes through the optical cell (with a length of 2.5 mm and a diameter of 2 mm) filled with 87 Rb and coated with paraffin to increase the time of interaction of the alkali atoms with the laser beam.By means of a polarizing beam splitter (PBS), the beam is split into two orthogonally polarized beams.The pump beam ⃗ (with a horizontal polarization, along the Y axis in figure 1) passes through the cell and aligns the ground-state level of the Rb atoms.The probe beam ⃗ has an orthogonal polarization (along the X axis in figure 1) and propagates in direction opposite to the pump beam to probe the state prepared by it.The transmission of the probe beam is registered depending on the dc magnetic field scanned at a frequency 0.33 Hz around zero value.
The sample was placed in front of the cell window (figure 1) and supplied with a voltage of 1 V.The current flowing was 0.15 mA, giving a resistivity value (on a thin film) of about 0.2 Ω/sq.The same sheet resistivity for the Pt micro-trace tested in [4] is estimated to about 6 Ω/sq.It should be borne in mind that under normal operating conditions, it is not possible to check the operation of the circuit by plugging an ammeter into it, which prompted us to test the capabilities for remote operation monitoring.

Results and discussions
The sensitivity of the measurement method strongly depends on the ability to accurately determine the resonance parameters, which are tightly related to the width of the resonance.Therefore, careful optimization of the operational parameters is required.

EIA resonance dependence on the 87 Rb vapor density
First, the resonance width (full width at half maximum FWHM) with respect to the cell temperature was optimized, and the results are shown in figure 2. Here, and in the following figures, the solid lines are guides for the reader's eye only.It can be seen that up to about 40 ⁰С the width slightly increases, after which it saturates in the region of 40-50 ⁰С.We did not apply heating to higher temperatures because the paraffin anti-relaxation coating of the cell walls does not allow temperatures above 60 ⁰C.In turn, the amplitude of the signal shows a similar behaviour: it also increases with cell temperature, reaching a plateau in the region of 40-50 ⁰С, where it is about twice as high as the amplitude of the signal at room temperature (24 ⁰С).

EIA resonance dependence on the pump and probe laser beam intensities
The influence of the pump and probe beam powers on the resonance parameters obtained in a cell with anti-relaxation wall coating was also evaluated.The results of these measurements are presented in figure 3.As can be seen from the graphs, the optimal power of the pump beam is 500 μW, which provides a sufficiently narrow width (black triangle points), but at the same time increased signal amplitude (red circular points), thus greatly increasing the contrast and sensitivity of the signal.Regarding the power of the probe beam, a plateau is observed in the region of 5-9 μW, where at the same time the signal is narrow enough, defining this range of powers to be used for the further measurements.
For the final system for optimal signal, we have chosen: cell operating temperature 45-50 ⁰С, pump beam power 500 μW and probe beam power 6-8 μW.These parameters were also used for the temperature optimization measurements.
After positioning the sample in our setup, the voltage was applied to the sample.The current flowing through the micro-wire creates a magnetic field that changes the width and position of the magneto-optical resonance.With the live micro-wire, we register no shift of the resonance center, but rather a broadening of the resonance signal, confirming that the magnetic field created by the sample in the chosen position is orthogonal to the scanning magnetic field.At a voltage of 1 V, the obtained linewidth of the signal was (3.2 ± 0.3) mG.Using the calibration curve presented in [4], we estimate the corresponding additional transverse dc magnetic field in the Y direction to be (1.5 ± 0.4) mG.The measured magnetic field proves the conductivity of the metallized trace.In this way, its integrity can be tested during bending and twisting to which it will be subjected during implantation.It should be emphasized that the signal was obtained at a relatively large distance between the sample and the sensor (due to the glass substrate), which shows the applicability of the method and our developed system for non-contact detection of the working state of conducting micro-wires.

Conclusions
We have shown that applying an optical method based on coherent magneto-optical spectroscopy allows measurement of the magnetic field generated by microscaled Ni metallized traces.The magneto-optical resonance obtained in the rubidium sensor cell was used in a scheme with two counter-propagating pump and probe light waves with orthogonal linear polarizations.We have measured the very weak magnetic field generated by the current flowing through traces realized by a two-step process consisting of femtosecond pulsed laser processing of the surface of PDMS elastomer and subsequent electroless metallization of the activated surface by Ni.The application of 1 V supply to the micro-wire placed at a distance of 10 mm from the Rb vapor sensor allowed us to measure a transverse magnetic field of (1.5 ± 0.4) mG created by the sample.The possibility of utilizing such high-sensitivity optical methods for MF measurement is of extreme importance for the remote contactless control of the operation of neural implants.It is also worth noting that for bulk material, the resistivity of Ni (normal conditions) is ( ) , and for Pt it is ( ) .This gives a ratio of ( ) ( ) .On the other hand, for the sheet resistivity of the practically 2D metallic trace we obtain ( ) ( ) .We attribute this to the fact that the sheet conductivity in our case depends on the substrate morphology, which differs in the femtosecond and nanosecond laser processing, respectively.
Further work is in progress for a detailed study of the modification of the electrical properties of thin metal sheets due to laser structuring and metal layer dimensions.

Figure 1 .
Figure 1.Geometry of the experimental setup for remote detecting of the conductivity of metallized Ni traces.The position of the sample relative to the 87 Rb cell sensor is shown.Inset (right): photo of the sample (the metallized Ni trace) after connecting the wires at its ends.

Figure 2 .
Figure 2. Temperature dependence of the signal linewidth.

Figure 3 .
Figure 3. Dependence of the linewidth (black triangle points, left axis) and amplitude (red circular points, right axis) of the narrow resonance component as a function of the pump (a) and probe (b) laser beam power.For the measurements in (a) the power of the probe laser beam was 8 μW and for (b) the power of the pump laser beam was 0.5 mW.