Eddy current probes have been used for many years for numerous sensing applications including crack detection in metals. However, these applications have traditionally used the eddy current effect in the form of a physically wound single or different probe pairs which of necessity must be made quite large compared to microelectronics dimensions. Also, the traditional wound probe can only take a point reading, although that point might include tens of individual cracks or crack arrays; thus, conventional eddy current probes are beset by two major problems: (1) no detailed information can be obtained about the crack or crack array; and (2) for applications such as quality assurance, a vast amount of time must be taken to scan a complete surface. Laboratory efforts have been made to fabricate linear arrays of single turn probes in a thick film format on a ceramic substrate as well as in a flexible cable format; however, such efforts inherently suffer from relatively large size requirements as well as sensitivity issues. Preliminary efforts to fully extend eddy current probing from a point or single dimensional level to a two dimensional micro-eddy current format on a silicon chip, which might overcome all of the above problems, are presented

Dury, Joseph D.

Henderson, H. Thurman

Kartalia, K. P.

English

NASA Technical Reports Server

N93-B1560
AN INTEGRATED EDDY CURRENT DETECTION AND IMAGING
SYSTEM ON A SILICON CHIP
H. Thurman Henderson, K.P. Kartalia, and Joseph D. Dury
University of Cincinnati
Cincinnati, Ohio 45221
I. INTRODUCTION
Eddy Current probes have been used for many years for numerous sensing applications
including crack detection in metals. However, these applications have traditionally used the
eddy current effect in the form of a physically wound single or differential probe pairs which
of necessity must be made quite large (order of a tenth of an inch) compared to
microelectronics dimensions. Also, the traditional wound probe can only take a point reading,
although that point might include tens of individual cracks or crack arrays, thus conventional
eddy current probes are beset with two major problems: (1) no detailed information can be
obtained about the crack or crack array -- simply that local inhomogeneties exist in the
region (which just as well could be other surface defects) and (2) for applications such as
quality assurance, a vast amount of time must be taken to scan a complete surface, even if
robotic applications are used for a physical scan of the surface.
Laboratory efforts have been made to fabricate linear arrays of single turn probes in a thick
film format on a ceramic substrate (hybrid technology) as well as in a flexible cable format,
however such efforts inherently suffer from relatively large size requirements as well as
sensitivity issues.
This paper describes PRELIMINARY efforts to extend eddy current probing from a point or
single dimensional level, fully to a two-dimensional micro-eddycurrent format on a silicon
chip which might overcome all of the above problems. Moreover, using a serial readout of
rows and columns one might be able to achieve eddy current ima._in_. Thus the scanning
(within the area of the chip) would be electronic rather than physical, resulting in an
essentially instantaneous scan when compared to the physical scan method. Moreover,
although the demonstration vehicle herein described is only a 10 x 10 array on a silicon chip
of the order of .25 to .5 inch square, the ultimate chip could consist of an array covering a
complete wafer of pizza size (four to eight inches in diameter). Also, in sufficiently thin
silicon materials, the surface can conform to non-planar topographies. Also, in sufficiently
thin silicon materials, the surface can conform to non-planar topographies.
If successful, the pixel size (based upon integrated eddy current coil size) might range of the
order of a thousandth of an inch or smaller, although in our test the coil the spacing is of the
order of 5 to 15 thousandths of an inch. This opens up the possibility of a resolution within
the typical width of a crack, thus allowing identification of much smaller cracks, early onset
of material fatigue from microcracks, classification of cracks and even imaging. Furthermore,
with sufficient sensitivity it potentially becomes possible to resolve such things as surface
smoothness, grain boundaries and material homogeneity, as fine structure or super fine
structure on the signal response. Such a device might also be placed permanently for ongoing
health monitoring at critical fatigue points in the vehicle.
There is no other technology but the integrated circuit lithographic technique which allows
such miniaturization, resolution and replication uniformity. For example, the masks used for
fabricating the geometries were generated by electron beam with a submicron resolution and
line uniformity of the order of 0.1 micrometers.
51
https://ntrs.nasa.gov/search.jsp?R=19930022371 2020-03-17T05:09:53+00:00Z
A variety of chip sizes have been made, each consisting of a 10 x 10 array as shown in
Fig. 1. Each of the 100 pixels or coil combinations shown in Fig. 2 consists of two flat coils
in a "Manhattan" geometry (rectangular) for maximum area utilization, a primary and a
secondary. Figure 3 is a scanning electron micrograph of a similar coil, showing a better
prespective of the topography. The flat coils are aluminum, separated by insulating silicon
dioxide. Figure 4 is an expanded view of the center region of one coil pair.
The aluminum thin film (about a micron thick and typically about 6 to 10 microns
wide) lies on two levels, including a return path under the coils, again insulated by a silicon
dioxide layer. The silicon substrate simply serves as a convenient substrate into which the
initial silicon dioxide layer can be thermally grown using conventional integrated circuit
techniques. The other silicon dioxide layers were formed using conventional CVD (chemical
vapor deposition) techniques where silane is oxidized at an elevated temperature. The
resulting devices (chips) were mounted in a conventional in-line integrated circuit package as
shown in Fig. 5. (However it is envisioned that ultimately, vastly larger arrays will be used,
requiring special packages.) The chips were encapsulated in a high grade commercially
available polymer.
Commercially-available finite-element magnetic field software (MSG-MAGGIE) is
being used to model the system and to test improved designs beyond the present
demonstration vehicle. Devices have been tested using standard cracks 0.003 inches wide and
a few mils deep. These have resulted in a signal above noise of about five percent, using a
driving signal at one to ten megahertz.
A rastering circuit was developed for this system, but it was ultimately determined to be
more useful to use commercially-available computerized impedance meters (such as those
manufactured by Hewlett-Packard and others) which can be programmed to vary such factors
as sampling time, settling time, frequency, differential signal between any selected set of
coils, etc. However, an interface circuit was built using meter-actuated reed relays and
multiplexing, to allow activation and indexing of each coil set (one at a time) in order to
avoid inhomogenous mutual impedances and magnetic field couplings over the area of the
array. These studies are presently in progress and the results look promising.
Actual imaging has not yet been attempted. Rather we are at this point simply plotting
out the serial row-column signals and correlating these with crack sizes and other parameters.
III. BASIS OF THE DESIRED EDDY CURRENT EFFECT
According to Ampere's circuital law,
_; H • dL = NI
where the arbitrary closed line integral of the dot product of the magnetic field H with the
vector differential length dL is equal to the ampere turns NI enclosed. Thus in a solonoid,
whether of the conventional spreaded-out multi-layer type, or the present single-level coil
52
type, the magnetic field generated in the central core is based upon the current I times the
number of linked turns N. Thus for a given configuration, one need only increase the drive
current for increased field for a proportional increased sensitivity.
The generated magnetic field from the primary coil is made to penetrate an adjacent
electrically conducting material where cracks (or other inhomogeneties) are being examined,
such as a space engine material or structural element where imperfections or fatigue cracks
are of interest. According to Lenz's law, if the magnetic field is varying in time, eddy
currents will be induced in the various possible conducting paths of the subject material in
such directions as to generate magnetic fields opposing the change.
These circulating eddy currents will cause a reflected impedance in the secondary coil of
the device (wound inside the primary) which will in general consist of an induced or reflected
resistance (R), inductive reactance (XL) and capacitative reactance (Xc) or an overall complex
impedence Z kO at a polar angle O. We are presently attempting to correlate these electrical
parameters with fault analysis.
Of course each interwoven coil set (primary/secondary) will have its own resting
impedence when contacted to air or other uniform material, therefore the first issue is to
record in memory the reference impedance of each coil (with its associated external
wiring/circuit impedance) when in contact with the reference or "perfect" material. Then a
deviation from this reference should be indicative of the existence of a crack or other
material imperfection. With sufficient resolution one should be able to distinguish various
types of imperfections -- even to the extent of eddy current imaging.
One of the major problems which we have found is not so much the construction of
identical on-chip devices, but rather the differing stray impedances of the individual wiring
harness lines to the off-chip signal processing. Of course the ultimate answer lies in doing all
preliminary signal processing on-chip, with appropriate buffering to the outside world. This
level of integration, however, awaits a planned future level of effort after the present
demonstration vehicle is adequately proven. We have every reason to believe, however, that
complete on-chip processing of the row/column serial data can be done using bucket brigade
or other standard techniques in the same silicon substrate (which at this point merely serves as
a rather arbitrary physical substrate).
With elimination of external and wiring harness impedances, one should be able to
eliminate the necessity for initial material referencing. It should then be sufficient to
program the differential comparison of adjacent reflected impedances in order to sense the
existence of local cracks. However for coil spacings of the order of and less than crack sizes.
it will probably be necessary to add an integral term to reference the changing background at
this level of resolution (unlike present eddy current probes which are much larger than the
crack sizes being detected) to exist somewhere between the two coil sets. In this way one
might distinguish the "forest from the trees". In fact, with sufficient chip uniformity and on-
chip processing, absolute local values should be sufficiently meaningful. Fortunately, with
our flexible software-based approach, we can arbitrarily vary the comparison formats among
x-y elements in order to arrive at the most meaningful approacl_es.
Also, the HMTC sensors group has access to several picture and imaging processing experts:
the results are not dependent on the originating signals being eddy currents versus
conventional optical signals.
53
IV. DETECTABLE PARAMETERS
At this point integrated coil impedances run in the range of tens of ohms at one to ten
megahertz. A crack in the range of a few mils wide will result in a change of impedance of
the order of several percent when the sample is set against the packaged sensor array, which
we estimate to be separated from the reference metallic (crack-containing) material by
approximately 0.020 inch of encapsulating polymer. Since the field drops off at a rate
somewhere between the square and cube of the distance, one should be able to increase the
sensitivity by one to two orders of magnitude through closer proximity (say 2 or 3 mils). On
the other hand, we wish to minimize the placement sensitivity.
Although the beginning reflected impedance of a given coil set ranges in the tens of
ohms, we have found that we can obtain meaningful systematic changes in impedance at least
out to the third decimal place when bringing an object in from some distance (say a few
tenths of an inch away). Thus one can speculate that one might even be able to resolve such
material properties as surface smoothness from grinding or processing, as well as more gross
defects such as cracks, merely by segmenting gross changes from the fine and superfine
structure in the reflected impedances.
As one would suppose, as one brings a metallic object into the proximity of the micro
eddycurrent array, the reflected resistance and inductive reactance increase. However if one
uses, for example, the human finger, the resistance component increases also (as one would
expect, since eddy currents are being induced in the conducting skin), but the increasing
reactive component is capacitive, i.e. AO changes size. We interpret this to be due to the
basic dielectric nature of the human skin, and the group (along with some physicians and
biologists from the University of Cincinnati Medical School Perinatal Research Institute) is
contemplating the possible use of this effect for characterizing the very sensitive skin of pre-
mature infants for heat and vapor loss characteristics.
Finally, we hope to also ultimately be able to add a third dimension to the crack
probing technique by "chirping" the frequency when sampling each coil. The ordinary
(inanimate) skin effect, particularly well known to all in the microwave field, would
increasingly concentrate the eddy currents toward the surface of materials with increasing
frequency. Thus a scan of frequency at each point during column and row rastering, could in
principle provide a depth profile of the crack or defect for three-dimensional imaging.
V. POSSIBLE IMPROVEMENTS
Although we are only in the preliminary phases of this work, we can envision several
ways to improve the sensitivity up to several orders in magnitude. These includes relatively
simple issues such as (l) closer spacing of the coil array to the measurement surface, (2)
higher drive current and dense multiple level spiral coils, as well as more involved but
"doable" changes such as (3) on-chip processing or even (4) fabrication with high temperature
superconductors (which we are contemplating in connection with our High T c Superconductor
Lab). Finally, it should be pointed out that the eddy currents are also induced in the silicon
substrate, which merely represent a power loss. The use of deep impurities to increase the
resistivity would suppress this loss.
54
V[. CONCLUSIONS
The present project is vet. in its preliminary phases, but real devices have been built and
are being evaluated in the form of a demonstration vehicle, particularly for crack detection in
metals commonly used for aerospace propulsion systems and structures.
Crack detection results show impedance changes only in the range of a few percent, but
improvements are theoretically possible in the range of orders in magnitude through
reconfiguration.
It is felt that the potential exists here for three-dimensional imaging, as well as
applications to an extended range of possibilities such as surface finishes and perhaps even
grain boundaries and other material inhomogeneties.
VII. REFERENCES
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Volumes 1 & 2, Ronald Press, New York, NY, 1959.
2 Institut Dr. Forster, "Applications of Eddy Current
Testing in Aircraft Maintenance and Engine Overhaul with
Forster Defectoscop S 2.830," Application Notes of
Institut Dr. Forster. (1985).
3 S.A.Schelkunoff, "The Impedance Concept and Its
Application to Problems of Reflection, Refraction,
Shielding, and Power Absorption," Bell System Technical
_VoI.27 pg.17-48_ (1938).
4 Y.D.Krampfer & D.D.Johnson, "Flexible Substrate Eddy
Current Coil Arrays," Material Evaluation. Voi.46, pg.471-
478 (198a).
5 C.V.Dodd, W.E.Deeds, J.W.Luquire, & W.G.Spoeri, "Some Eddy
Current Problems and Their Integral Solutions," Q_
Oak Ridge National Lab., (1969).
6 T.G.Kincaid, "Eddy Current Printed Circuit Probe Array:
Phase IIA," SIGNAMETRICS Report No.10 (Internal Memo}.
(1988).
7 M.J.Moore & F.J.Dodd, "Real-Time Signal Processing in an
Ultrasonic Imaging System," Material Evaluation. Vol.40
pg.976-981, (1982).
8 S.Middelhoek & A.C.Hoogerwerf, "Smart Sensors: when and
where?," Sensors and Actuators,Vol.8, pg.39-48 (1985).
9 "Detection Theory and Applications," Special Issue of
Proceedinus of IEEE_ Voi.58_ (1970).
55
i0 R.Hochschild, "The Theory of Eddy Current Testing in One
(Not-So-Easy) Lesson," Nondestructive Testing. Voi.12,
pg.31-40 (1954).
11J.Vine, "Impedance of a Coil Placed near a Conducting
Sheet," Journal of Electronic Control. Voi.16, pg.569-577
(1964).
12 C.V.Dodds & W.E.Deeds,
Current Coil Problems,"
pg.2829-2838, (1968).
"Analytical Solutions to Eddy
Journal of A9911ed Physics. Voi.39
13 Z.Haznadar & J.Matjan, "Numerical Calculation of Skin
Effect in Systems of Straight Conductors," Elektrotehnika_
Zagreb Vol.5. (1970).
14 F.R.Bareham, "Choice of Frequency for Eddy-Current
Testing," British Journal of Applied Physics. Vol.ll
pg.218-222_ (1960).
15 T.G.Kincaid, "Eddy Current Probe Analysis," SIGNAMETRICS
Report No.7 (Internal Memo). (1987).
16 P.M.HalI, "Resistance Calculations for Thin Film
Patterns," Thin Film Solids, Elsevier, Amsterdam, 1968.
17 J.M.Janicke & E.S.Sealey, Magnetics, The Magnetics
Handbook, RFL Industries, Inc., Boonton, NJ, 1982.
18 B.A.Auld,F.Muennemann, and D.K.Winslow, "Eddy Current
Probe Response to Open and Closed Surface Flaws,"
Nondestr_G_ive Evaluation. Vol.2 pg.l-21, (1981).
19 H.L.Libby, Introduction to Electromagnetic Methods, Wiley
Press (Interscience), New York, MY, 1971.
20 H.W.Ghent, "A Novel Eddy Current Surface Probe," IL_
Energy of Limited, AECL-7518. Chalk River, Ontario, Canada
(1981).
21 D.A.Daly, S.P.Knight, M.Coulton, & R.Ekholdt, "Lumped
Elements in Microwave Integrated Circuits," IEEE
Transactions of Microwave Theory. Voi.15 Dg.713-721_
(1967).
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to Nondestructive Testing," Material Evaluation. Voi.24
pg.373-377_ (1966).
56
ORIGINAL P-_,_-_L
BLACK At_;D WHITE PHOTOGRAPH
Fig. 1 10 x 10 eddy current coil array on a silicon chip, approximately 1/4
inch square. Each flat coil set consists of a primary for micro
eddycurrent generation and a secondary for sensing reflected complex
impedance. Each row and column may be externally addressed. Future
models will have all signal conditioning on-chip to avoid the necessity
for lead compensation.
57
ORIGINAL PAGE IS
OF POOR QUAt,/T'¢
OR}GIN_L PA'SE
:_-",r;,_ A_') W.L_,TE r'L_OTOGRAPH
Fig. 2 Expanded view on one of the coil pairs (the two coils have been
connected in series in this case for test purposes). The dark lines
are aluminum leads, six microns wide in this case, separated by four
microns of silicon dioxide.
58
BLACK Af';.,3 Wr '''_
,,.. E _ '4,'-JT('IGRAPH
Fig. 3 Scanning electron micrograph of Fig. 2, except in this case the two
coils are separated in the form of a primary and a secondary. At the
center, each coil connects to a separate aluminum bus (insulated from
the top metallization by a silicon device layer) so that all primary
and secondary coils can be individually addressed.
59
OF POL_,_ _-!J.'-WY
ORIGINAL PAGE
tJL.ACK AND WHITE PHOTOGRAPH
Fig. 4 An expanded view of the center region of Fig. 4, showing the connection
of the top side six-micron aluminum leads connected to the lower leads
through an intermediate silicon dioxide (about one micron) insulator.
Edge definition generally shows a uniformity of two or three tenths of
a micron. Note that the last loop can only link about twenty percent
of the generating field, but there are five complete loops in the
symmetrical primary/secondary coils.
60
t_I..AC_ AND WHI]E. i_HOTOG_APH
FIg.5 Sensor chip array mounted in a conventional in-line integrated circuit
package.
61



An integrated eddy current detection and imaging system on a silicon chip

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