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GMI Output Stability of Glass-coated Co-based Microwires forSensor Application

Jing-Shun Liu1, Xiao-Dong Wang1, Fa-Xiang Qin2, and Fu-Yang Cao1, Da-Wei Xing1,Hua-Xin Peng2, Xiang Xue1, and Jian-Fei Sun1

1School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China2Advanced Composite Center for Innovation and Science (ACCIS), University of Bristol

University Walk, Bristol BS8 1TR, United Kingdom

Abstract— We report a study here on the technique of Cu electro-plated wire-connecting forstabilizing the GMI output of Co68.15Fe4.35B15Si12.5 glass-coated amorphous microwires at themagnetic field ranging from 0 Oe to 4.2 Oe. The GMI output stability was characterized bya precision impedance analyzer and in a magnetically shielded space (MSS). The results showthat this method could reduce the emission of RF electro-magnetic wave and driving signalattenuation, minimize the disturbance of stray capacity and parasitic capacity, and suppresshigh frequency destabilization and concussion at the relatively high frequency (≥ MHz), henceeffectively improve the GMI output stability for high-resolution magnetic sensor application.

1. INTRODUCTION

Glass-coated amorphous magnetically microwires have recently attracted much attention because oftheir potential industrial applications: they can be used as GMI and GSI (giant stress impedance)sensor elements in the CMOC IC circuitry to detect weak magnetic fields, small weights and slightvibrations. They find wide applications particularly in the car industry, biomedicine and navigationfields [1–4].

For the weak GMI and GSI magnetic sensors, researchers tend to enhance their stability, preci-sion, repeatability and linearity by optimizing the design of electrical circuit, in that even a slightfluctuation of impedance could induce voltage signal magnified by further amplification circuit [5].But little effort has been given in the wire-connecting perspective. Therefore, it is significant tostudy on the effect of wire-connecting on GMI output stability of the glass-coated Co-rich amor-phous microwires. In general, for the as-cast glass-coated microwires, it is very challenging toachieve a stable electrical interconnection between amorphous wire and microelectronic circuit.The issue can be tackled from the following two aspects: (i) to remove the insulating glass thor-oughly using hydrofluoric acid to assure a total exposure of the metal core. (ii) to seek for novelwire-connecting technique of improving wetting characteristic of two-terminal surface of the wire.Copper electro-plating, with higher electrical and thermal conductivity, good wettability with solderand weak destabilization of high frequency signal, is an effective way to ensure the bondability andreliability in wire-connecting process applied for packaging of chips with wire interconnects [6–8].In this context, we employed the Cu-plating technique to stabilize GMI output.

This paper aims to achieve a stable interconnection between Co-based glass-coated wires andsolder by controlling copper electro-plating process of wire-end, and explore the best conditionsto maximize its GMI output stability. The results presented here are useful for high-performanceGMI sensor applications.

2. EXPERIMENTAL DETAILS

Soft magnetic amorphous glass-coated microwires of nominal composition of Co68.15Fe4.35B15Si12.5

(the metallic nucleus diameter, d = 32 µm, the glass-coating thickness, t = 1.5µm) was preparedusing an experimental facility based on the modified Taylor-Ulitovskiy method [2, 3]. The fabricatedthin microwires possessed excellent soft magnetic properties owing to the nearly zero or negativemagnetostriction constant (λ ≤ −10−6). We selected the samples with whole length of 24 mm,and glass-coated layers of the two terminals (each about 4 mm long) were removed by hydrofluoricacid (HF) for 180 s, then Cu electro-plating was employed by minitype device. The improvedelectrolyte include copper sulphate, sulfuric acid, sodium chloride, glucose and sulfocarbolate acidwith proper proportion. In order to obtain good electro-plating layer, process parameters were

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Magnetically Shielded Space

(MSS)

Hex

Four-probe PCB Board

DC Solenoid

Stabilized Current Supply

(0.1nT)

Flux-gate Magnetometer Impedance Analyzer

(Aligent 4294A)

Figure 1: Schematic illustrations of magneto-impedance measurement system and magnetically shieldedspace (MSS). The unidirectional arrow represents the axial applied external magnetic field.

controlled effectively, (such as temperature is about 40◦C, hyperpure copper is used as anode) [9].The surface morphology of the studied microwires and electro-plated layers were measured byscanning electron microscopy (SEM). Video-based contact angle measurement device (OCA 20LHT)was employed to measure the dynamic contact angles based on the analysis of the drop shape ofthe different types of interfaces in a pipe-still heater with 10−3 torr vacuum. Figure 1 illustratesimpedance measurement system and magnetically shielded space (MSS).

The magnetoimpedance was measured by an Aligent 4294A impedance analyzer in the frequencyrange of 40Hz–110 MHz. The glass-covered microwire with two-end electro-plated was connectedinto PCB board adopting the four-probe connecting method. The composition of solder (SAC) istin (96.5%), silver (3.0%), copper (0.5%) in weight percent, respectively. The magnetoimpedanceratio ∆Z/Zmax is defined as [10]:

∆Z

Zmax(% ) =

[Z (Hex)− Z(Hmax)

Z (Hmax)

]× 100% (1)

where Hex denotes the dc axial external magnetic field, which is supplied by a solenoid; Hmax de-notes the maximum field along the wire axis, which is 4.2 Oe in the present work. All measurementswere performed at room temperature.

3. RESULTS AND DISCUSSION

As was shown in Figure 2, the morphology and thickness (precisely measured from the SEM images)of copper electroplated layer depend on the current density of cathode and electro-plating time.The uniform electro-plating layer is critical for the final stable wire-connecting, as well as impedanceoutput stability. At the cathode current density of 147.12 A/dm2, at the initial stage of electro-plating, the thickness of wires obviously increases to 0.77µm in 45 s, the microstructure of Cuelectro-plated layer is uniform, dense, slightly coarse and of no macroscopic irregularity. In 75 sand 120 s, hydrogen liberation occurred in these stages. The surface of microwires becomes rougherand porous, resulting in stress concentration in some regions because of different plating rate,thereby forming some long cracks. The thicknesses in 75 s and 120 s are up to 1.08 µm, 1.31 µm,respectively. In 240 s, copper particles grow rapidly, and the brittleness of Cu layer with 2.08µmthick increases. The microstructure becomes much more coarse, with the occurrence of abnormaldeposited regions, making the wire unsuitable for wire-connecting. Further, the relations betweenthickness (T ) of copper layer and electro-plating time (t) can be fitted by the following expressionof parabola type with small error of fitting: T = 0.063 + 0.014t− 0.000024t2. Thus, the thicknessof copper electro-plated layer can be effectively controlled by the above expression. Overall, theuniform layer and proper layer thickness are essential for wire-connecting. With the comparison ofabove presented morphology for different times, the following parameters (147.12A/dm2, 45 s) canbe chosen as the final choice for wire-end electroplating in the present work.

In order to evaluate the wetting characteristic of copper electro-plated layer at wire ends, thedynamic wetting morphologies of as-cast glass-removed microwires with two typical interfaces areshown in Figure 3. Contact angles of non-electroplated interface (a), θa, is 85.54◦, and the contact

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0 50 100 150 200 250

0.0

0.5

1.0

1.5

2.0

2.5

Th

ick

ne

ss

(u

m)

Electro-plating Time (s)

Figure 2: Variation in the surface morphology and thickness of Cu electroplated layer at the current densityof 147.12 A/dm2 with different electro-plating time: (a) 45 s, (b) 75 s, (c) 120 s, (d) 240 s.

Cu Electro-plated layer

Glass-coated layer

CoFeBSi Alloy

per

SAC

CoFeBSi

Nonelectroplating

SA

C CoFeBSi

Cu layer

Cu electroplating

15µm

(a)

(b)

(c)

Figure 3: The wetting morphology of as-cast glass-coated amorphous microwires with two typical interfaces:(a) non-electro-plated interface [8] and (b) Cu electro-plated interface. The OCA images (a) and (b) indicatecontact angles of θa and θb are 85.54◦ and 30.28◦, respectively. (c) is a SEM image of surface morphology ofthe glass-coated wire with total diameter of around 32µm.

angles of Cu electro-plated interface (b), θb, is 30.28◦ For the latter, it indicates that copper electro-plated layer as a transition medium interconnecting CoFeBSi alloy and solder has good wettabilityfor wire-connecting and that the solder has small surface tension. Therefore, wire-connecting ofcopper electro-plated layer at wire ends exhibits stability and reliability even in unfavorable workingconditions.

The microwire with copper electro-plated two-terminal is connected into PCB board, placed inmagnetically shielded space (MSS) to avoid the disturbing of geomagnetic or other magnetic field,to analyze impedance output stability of wires accurately at the same ambient temperature (25◦C)and the driving current amplitude (20 mA) [11]. The impedance stabilities and the correspondingGMI output stabilities dependence of the magnetic field (0 Oe–4.2 Oe) at the frequency range of0.1MHz–15 MHz are shown in Figure 4. For non-electro-plated at wire-terminal (as seen in Fig-ures 4(a) and (b)), the impedance variation has some obvious inhomogeneous variation regions(IVRs) at 0.1 MHz–15MHz, especially, at 3 MHz–6 MHz and 8 MHz–13 MHz. From GMI ratio(∆Z/Zmax) output stability variation, impedance fluctuation almost runs through all of frequencyregion, and the magnified meshes in partial magnifications (as shown in Figure 4(b)) present flec-tional variation, or macroscopic fluctuant variation. For copper electro-plated at wire-terminal (asseen in Figures 4(c) and (d)), as mentioned above, the IVRs are improved effectively, the impedanceand its GMI output stability vary stably and smoothly as the magnetic field and frequency are in-

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(a) (b)

(c) (d)

Non-

electro

-plated

at wire-

terminal

Copper

electro

-plated

at wire-

terminal

Inhomogeneous Variation Region

(IVR)

8MHz

Figure 4: The impedance output stabilities ((a) and (c)) and their GMI ratio ∆Z/Zmax output stabilities ((b)and (d)) vs. frequency (0.1 MHz–15MHz) and magnetic field. The red ellipses indicate the inhomogeneousvariation regions (IVRs) of impedance. The partial magnifications (imaginary purple line) in (b) and (d)are shown in order to illustrate the variation in GMI ratios with increasing applied field (0 Oe–4.2 Oe) atdifferent frequencies.

creased, even at relatively high frequency (≥ 8MHz), and the maximum GMI ratio [∆Z/Zmax]max

is up to 124%. In a word, copper electro-plating at wire-terminal is an effective approach to im-prove the GMI output stability of glass-coated microwires. In conclusion, electro-plated copper isan excellent conductor with good wettability interconnecting solder. As a result, it reduces thecontact resistance of spot weld, and help to achieve stable wire-connecting. Meanwhile, this typewire-connecting also can avoid the disturbance of stray capacity and parasitic capacity in spot weldand reduce the emission of RF electro-magnetic wave and driving signal attenuation, more impor-tantly, suppress destabilization and concussion resulting from contact instability of wire-connectingends at the relative high frequency (≥ MHz).

4. CONCLUSIONS

The wire-connecting with Cu electro-plated terminal has been demonstrated to have the signifi-cant effects of stabilizing the GMI output of glass-coated Co-based wires. A uniform and densegranular structure of Cu electro-plated layer obtained from optimized electro-plating process at147.12A/dm2 for 45 s has smaller contact angle (30.28◦) and lower contact resistance. Comparedwith non-electro-plated wire-terminals, Cu electro-plated wire-connecting reduced the fluctuationof impedance and enhanced effectively the GMI output stability at different magnetic field and fre-quency. This is of significant importance for the development of high performance GMI magneticsensors.

ACKNOWLEDGMENT

The authors would like to express their appreciation to Prof. G. H. Tu, Nanjing Normal University,China, for useful discussion and advice.

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