Magnetic imaging for resistive, capacitive and inductive devices; …homepages.laas.fr/nolhier/ESREF2015/SESSION_C/PC_4.pdf · Magnetic imaging for resistive, capacitive and inductive

Microelectronics Reliability xxx (2015) xxx–xxx

MR-11566; No of Pages 6

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Microelectronics Reliability

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Introductory invited paper

Magnetic imaging for resistive, capacitive and inductive devices; from theory to piezoactuator failure localization

N. Courjault a,⁎, P. Perdu b, F. Infante a, T. Lebey c, V. Bley c

a Intraspec Technologies, 3 Avenue Didier Daurat, Toulouse, Franceb French Space Agency (CNES), 18 Avenue Edouard Belin, 31401 Toulouse, Francec CNRS UMR 5213 LAPLACE, Bat 3R3 118 Route de Narbonne 31062 Toulouse Cedex 9, France

⁎ Corresponding author.E-mail address: Nicolas.courjault@intraspectechnologi

http://dx.doi.org/10.1016/j.microrel.2015.06.0170026-2714/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: N. Courjault, et al., Mlocalization, Microelectronics Reliability (201

a b s t r a c t
a r t i c l e i n f o
Article history:Received 22 May 2015Received in revised form 18 June 2015Accepted 19 June 2015Available online xxxx

Keywords:Magnetic microscopyFailure analysisActuator piezoelectricPassive component

Short circuit is a knowndefect in electronic devices and several non-destructive techniques have been developedin the last two decades in order to localize them. Magnetic microscopy is very efficient to localize leakage pathsinside complex and packaged devices. Nevertheless, when the device under test has a normal inductive orcapacitive path, the dynamic current consumption can be considerably higher than the leakage itself challengingthe defective path localization.This paper will detail the possibilities given by the magnetic imaging to observe this kind of defect in conditionswhere the basic use would normally be unable to, since a capacitive and/or an inductive device would blind it. Atest vehicle has been realized in order to evaluate this new use of themagnetic imaging. Finally, a piezo-actuatorcomponent is then studied in order to prove the concept.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction and motivations

Magnetic microscopy is a failure analysis tool which uses themeasurement of the magnetic field generated by an electronic systemor component to localize defects [1,2]. In the usual working conditions,the vertical component of the magnetic field is acquired along ahorizontal plane parallel to the sample. In static or at very low frequen-cy, there is no (or very small) electric field E variation. The magneticfield B and the current density J are therefore linked by the simplifiedMaxwell–Ampère Eq. (1)

rot�!

B! ¼ μ0 J

!þ δε0 E!

δt≈μ0 J

!: ð1Þ

From this equation, it is possible to reverse Bz to Jx and Jy to image asurface current density from a magnetic scan. This technique, namelyMagnetic Current Imaging (MCI) [3], works well on samples when theleakage path impedance is low compared to good device impedance(Fig. 1).

By using this technique, it is possible to observe the magnetic fieldgenerated by the current path in the component. We can thus create amapping of the current passing through in the componentwhich bringssignificant information to localize defects in a component or electronicsystem. However, due to the use of a lock-in amplifier, the AC currentmay range from 1000 Hz to 99,000 Hz (Fig. 2).

es.com (N. Courjault).

agnetic imaging for resistive,5), http://dx.doi.org/10.1016

Wenormally use this technique to observe a resistive current. If Z is acapacitor or an inductor, its impedance can be lower than the onerelated to the leakage.

In this case, the classical use of the magnetic microscope does notallow observing the current path. In this paper we want to prove thatwe can still localize defect, like short circuit or resistive open, even inthe presence of complex impedances.

2. Electrical considerations

In parallel branches, voltage V(t) is common (same node). In the Rbranch, I(t) and V(t) are in phase:

Ir tð Þ ¼ V tð ÞR

:

In the C branch, I(t) is out of phase:

Zc ¼ 1j � C �ω ¼ N Ic tð Þ ¼ j � V tð Þ � C �ω:

In the L branch, I(t) is out of phase:

Zl ¼ j � L �ω ¼ N Il tð Þ ¼ −j � V tð ÞL �ω :

Bz and I are directly connected and have the same phase. Therefore,wewill have an in phase (R) and out of phase (C, L) behavior that can be

capacitive and inductive devices; from theory to piezo actuator failure/j.microrel.2015.06.017

http://dx.doi.org/10.1016/j.microrel.2015.06.017

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ZGooddevice ZR

Fig. 1. Impedance schematic of reference (left) and faulty device (right).

Fig. 2. Synoptic of the magnetic microscope.

Fig. 4. Test vehicle circuit.

LRC

Fig. 5.Wire side view of the RLC circuit.

2 N. Courjault et al. / Microelectronics Reliability xxx (2015) xxx–xxx

measured. To do this, we have implemented a slightmodification in ourdetection chain.

3. Use of all lock-in possibilities

The acquisition chain of themagneticmicroscope has therefore beenmodified in order to observe the different types of currents (resistive,inductive, and capacitive). The proposed modification is shown inFig. 3. The signal acquired by the magnetic sensor is amplified by the

Fig. 3.Modified acq

Please cite this article as: N. Courjault, et al., Magnetic imaging for resistive,localization, Microelectronics Reliability (2015), http://dx.doi.org/10.1016

lock-in amplifier and the result is typically presented on the X line ofthe same instrument. The Y line is now also acquired in order to observethe out of phase signal.

By using both X and Y lines it is possible to observe the magneticphase image related to the electrical phase. Indeed, X corresponds tothe in-phase part of the electrical signal whereas Y corresponds to thequadrature one. As a consequence, introducing the X and Y values inan algorithm calculating the Arc Tangent (Tan−1), it is theoreticallypossible to obtain the phase of the signal. It is also possible to computethe amplitude of the signal. The use of lock-in technique is a commonmethod in the failure analysis world. The new possibilities concern theuse of the X and Y lines combination of the lock-in which also enablethe possibilities to acquire magnetic phase and amplitude imagesdirectly with the magnetic microscope.

uisition chain.



Fig. 6.Rawmagnetic scans (up), andMCI images obtained (down) from respectively X andY line.

LRC

LRC

Fig. 8. Overlay of the MCI results with the optical view, (Up) for the X line of the lock-inoutput signal, (Down) for the Y line.

3N. Courjault et al. / Microelectronics Reliability xxx (2015) xxx–xxx

4. Test vehicle

In order to illustrate these new possibilities, a RLC circuit (Figs. 4 and5) whose resonance frequency ranges in the lock-in amplifier workingfrequencies (1000 Hz–99,000 Hz) has been realized. The characteristicsof the components are the following ones: R = 1 Ω, C = 10 μF, L =100 μH. Measurements are performed at the resonance frequency.

The magnetic microscope typical experimental conditions are thefollowing ones. The AC-generator supplies a sinusoidal signal at5333Hz synchronizedwith the lock-in amplifier at the same frequency.An auto-phase mode is activated in order to maximize the signal on theX line by adjusting the phase reference. Nevertheless, this mode is onlyefficient for a pure resistive device case. Therefore, for the case understudy it is not able to suppress the entire Y signal. The current suppliedby the generator reaches 600 μApp.

Fig. 6 shows the rawmagnetic results and the current images obtain-ed by using the MCI algorithm.

On the images, a small current path can be observed on the X line aswell as the Y line. These particularities are caused of the imperfection ofthe electrical circuit. Indeed, the inductance component owns a resistivepart on its branch which induced that its electrical phase current is not

Fig. 7. Schematic of the taken example for the amplitude and phase calculation.


perfectly orthogonal with the pure resistive branch. That explains whywe observe a small current path on the X line. On the Y line, we alsoobserve a small current on the resistive branch. Different elementscould explain this; the phase reference of the lock-in was orthogonalat ∓ 2° with the electrical circuit. Furthermore, the magnetic sensorSQUID owns a phase response which is depending of the currentfrequency of the circuit. This phase response is not perfectly knowncurrently and can cause other modification of the phase between thecurrent signal and the measuring signal.

Interesting results for the failure analysis are observed on the wireside and are presented in Fig. 8.

As previously mentioned, the X line corresponds to the real part ofthe current (flowing through the resistance) whereas the Y line depictsits imaginary part. Since the measurements are performed at theresonance frequency of the RLC circuit, the current loop corresponds

LRC

Fig. 9. Amplitude image obtained from X and Y lines of the magnetic scan. Blue depicts azero amplitude while red correspond to the highest value.



LRC

Fig. 10. Phase image obtained fromX andY lines of themagnetic scan. Blue corresponds toa phase of − π

2, white a phase of 0, and red a phase of þ π2 : (For interpretation of the

references to color in this figure legend, the reader is referred to the web version of thisarticle.)

Fig. 12. Photography of a functional piezo actuator.


to the energy transfer from the inductance to the capacitor andconversely.

LFG

Rcc

= 9.64

ZCpiezo

= 23

Rsystem

=1500

Piezo Actuator

5. Are phase, amplitude images useful?

The amplitude and phase images are given in Figs. 9 and 10. Colorscale is different, in the amplitude image, blue indicate amplitude of 0while red corresponds to the highest value. We can observe a highermagnetic field generated near the resistance in comparison with theinductance and the capacitor, for this circuit. The magnetic field, atthis frequency, is mainly generated by the circulation of current andthe amplitude image depicts where the highest magnetic field ispositioned by combining both X and Y lines images.

In the phase image, blue color indicates a phase around − π2 rad,

white color depicts a 0 rad phase, and the red one corresponds to þ π2

rad. MCI algorithm can be used on these new kinds of images but ithas not been created to work on it. We can observe some abruptinterface on the phase image. These are artifact caused by thedivergence of the Tan−1 function when X signal is near 0.

Further investigation have to be done in order to know if MCI onamplitude and phase images can improve the current path localization,bring some new information about the failure localization, or be used toquantify the current amplitude.

Succinct calculations have been performed in order to analyze thelink between amplitude and phase information according to the size

L RC

L RC

Fig. 11. Overlay of the current mapping with the optical view of the second RLC circuit.(up) Current mapping obtained from X line of a normal magnetic scan. (down) Currentmapping obtained from X line with a modified phase reference of the lock-in amplifier.


and distance separation between circuit elements. Fig. 7 shows aschematic and describes the different used variable for the calculation.

The resulted expression of the phase and amplitude are thefollowing one:

Bz w; tð Þ ¼ μ0

2πV0 sin wtð Þ

Rx

x2 þ h2� −jRCw

x−dð Þx−dð Þ2 þ h2

0@

1A

Phase w; t;xð Þ ¼ Arctg

RCwd−xð Þ

x−dð Þ2 þ h2

x

x2 þ h2� �

0BBBB@

1CCCCA:

These expressions conclude that themagnetic field can be altered bythe distance separation of the component.

These results show that it is possible to observe a phase image fromamagnetic scan and that the real and imaginary parts of the current maybe separated.

However we can note that magnetic phase image does not allow usto split R current part, C current part or L current part while the use of inphase and out of phase information can.

The phase reference of the lock-in amplifier may be changed, thusmeaning that, it is not necessary to observe permanently the real andthe imaginary part of the current, a simple variation of the phase

Device

Fig. 13. Schematic of the tested circuit of the first defective piezo device.

Fig. 14.Magnetic scans of the low resistance leakage actuator. (up) X-line, (down) Y-lineof the lock-in amplifier.



Fig. 15.Magnetic Current Imaging of the previous magnetic scans.Fig. 18.Magnetic scans of themedium resistance leakage actuator. (up) X-line, (down) Y-line of the lock-in amplifier.

5N. Courjault et al. / Microelectronics Reliability xxx (2015) xxx–xxx

reference may allow the observation of a special combination of thesecurrents depending of the sample under study.

To give an illustration, another experiment with the RLC circuit isperformed. In this case, the resistance represents a short circuit andthe inductance (of 1 μH now) is positioned to radiate directly in thedirection of the magnetic sensor. In this position, the coil concentratesthe magnetic field avoiding the observation of the current in theresistance area (Fig. 11).

Using a shift in the phase reference, a large part of the inductivecurrent, dazzling the image, is suppressed. However, the chosen phasehas not to eliminate the resistive part of the current, otherwise nocurrent would be observed in the short circuit.

I remind that an inductance owns a small resistive part and thereforethe phase of the L current will not be perfectly at þ π

2. That is why, weacquire some in phase signal on the L branch and because of thedirection of the coil, the picture is dazzled.

With these conditions it is possible to acquire a good image of themapping current without the “interferences” associated to the coil andmove the L part to the out of phase branch. Fig. 11 illustrates the rightchoice of reference phase that is adjustable in our system. We canobserve correctly the current path of the resistance which depicts thecurrent passing through a short circuit.

Short circuit

Fig. 16. Overlay of the current mapping from the X-line and the optical view.

Fig. 17. Schematic of the tested circuit of the second defective piezo device.


6. To a case study

In the previous case, the possibilities to separate the different typesof current in an electrical circuit have been demonstrated. This approachis now applied to a real case study. The defective device is a piezoelectricactuator originally designed for space application but also used inautomotive [4,5]. The device has been used for a test campaign ofhumidity resistance. The failure was researched in order to find outthe use limitation of this kind of component. Failure was created byelectrical shock after a 500 h test sequence in a wet environment,between 60% and 70%, at a voltage of 150VDC. The failed devices haveserved as cases study in this paper (Fig. 12).

This component behaves as a pure capacitor of 1.3 μF. The observedfailures were mainly the creation of a short circuit in parallel to thedielectric.

So the defective component presents the behavior of a parallel RCcircuit (Cf. Fig. 13). Several failed devices are available, each of thempresenting its own electrical characteristics for the short-circuit. Twoof them are particularly interesting. The first one has a very smallshort-circuit resistance (of few Ohm) while the second one has amedium resistance (of several kOhm).

Both results will be presented in this paper. The low resistance caseis the easier one because of a higher current level on the resistive part.Fig. 14 shows acquired magnetic scans while Fig. 15 depicts obtainedcurrent image after using the MCI algorithm.

The sample is supplied with a current of 1.471 mA distributedbetween the capacitive part and resistive part according to the imped-ance ratio. At this frequency of 5333 Hz, we got a capacitive impedanceof 23 Ω, and a short circuit resistance of 9.6 Ω. This is, theoreticallydepicting a distribution of 30% of current in the capacitive part and the

Fig. 19. Magnetic Current Imaging of the second defective piezo device. (up) X-line,(down) Y-line of the lock-in amplifier.



Short circuit

Fig. 20.Overlay of the currentmapping from the X-line and the optical view of the secondpiezo device.


other 70% in the short circuit. In others word, there are 442 μA in thecapacitive part and 1029 μA in the resistive part.

The overlay, in Fig. 16, illustrates where the current on the X line ispassing through the sample.

The resistive current flowing through the short-circuit is correctlyobserved. But the most important point is the success of the separationof the resistive current from the capacitor current as observed on the Yline of the lock-in amplifier.

The second defective piezo device also has comportment as aparallel RC circuit. However the short circuit resistance is slightly higherthan the previous one and reaches 1200 Ω. Its capacity remains astypical around the 1.25 μF. The schematic of the circuit is presented inFig. 17.

Because of the higher resistance, a lower level of current will beinjected and a lower signal to noise ratio will be present. In fact, 98%of the current go in the capacitive part of the component. The experi-mental condition was identical as the first sample and this one wasalso supplied by a current of 1.471mA. In these conditions, the resistivepart is crossed by 29 μA. This can be observed in Fig. 18.

In comparison with Fig. 14, we clearly observe a noisier image.Nevertheless, it is completely sufficient to acquire a correct currentimage by using the MCI algorithm (cf. Fig. 19).


The overlay can then be obtained in order to localize the position ofthe defect on the device (Fig. 20).

7. Conclusion & perspective

In this paper, we have presented an extension ofmagneticmicrosco-py that allows us to distinguish between in phase (R) and out of phasebranches (L or C). Using phase offset gives us extra opportunities tofind leakage paths in more complex situations where some componentcould dazzle the image and prevent us from observing the interestingarea. The ability to localize a leakage path in a piezoresistive actuatorhas been demonstrated in two study cases. However X, Y, phase andamplitude magnetic images open the door of new applications basedon faulty and golden device comparisons. Phase and amplitude imagesdo not really show help in this paper, nevertheless, further investiga-tions will be undertaken in order to define the limit of these kind ofresults.

Acknowledgment

I would like to thank Laurent Cadiergues from CNES as well as theCEDRAT TECHNOLOGIES Company for supplying us the actuator deviceswhich were necessary for the writing of this article.

References

[1] S. Chatraphorn, E.F. Fleet, F.C. Wellstood, L.A. Knauss, T.M. Eiles, Scanning SQUIDmicroscopy of integrated circuits, vol. 76, no. 162000. 2304–2306.

[2] W. Knauss, Orozco, Advances in magnetic-based current imaging for high resistancedefects and sub, micron resolution, 2004. 267–270.

[3] L.A. Knauss, S.I. Woods, A. Orozco, “Current Imaging using Magnetic Field Sensors.”,Microelectronics Failure Analysis Desk Reference Fifth Edition 2004, pp. 301–309.

[4] C. Allegranza, L. Gaillard, R.L. Letty, S. Patti, L. Scolamiero, M. Toso, Actuators for spaceapplications: state of the art and new technologies, March 2014. 23–25.

[5] G. Gnad, R. Kasper, Power drive circuits for piezo-electric actuators in automotiveapplications, IEEE Int. Conf. Ind. Technol. 159 (2006) 7–1600.


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