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Overview of MCI
Magnetic Current Imaging Revisited
Dave Vallett, PeakSource Analytical, LLC
[email protected]
F
ault isolation using magnetic current imaging
(MCI) based on scanning superconducting
quantum interference device (SQUID) and giant magneto-resistive (GMR) microscopy has been
around for almost 15 years. The original technique
and its derivatives are widely published.[1-5] For those
new to MCI or unfamiliar with its full capabilities or
latest developments, this review article will summarize basic background and theory, magnetic sensors
and their performance capabilities, and examples
and applications. Readers already familiar with the
technology will find a new discussion of ultrahigh
sensitivity and its benefits to low-power fault isolation
of subtle defects, as well as a previously unpublished
example of backside submicron die-level localization
using a GMR.
Background and Introduction
The process for imaging currents from magnetic
fields first appeared in 1989.[6] With the development
of the scanning SQUID microscope in the 1990s it was
possible to “see,” for the first time, buried current
paths on the surface of multilayered microelectronic
devices through multiple conductive and insulating layers.[7] The method was well suited for fault
isolation and failure analysis and became known as
“magnetic current imaging,” or MCI.
At first MCI was used for localization of short
circuits in die-level and packaged ICs, discretes, and
printed circuit boards (PCBs).[8] Since then a versatile
suite of techniques has evolved. Isolation of resistive
opens soon became possible by precise alignment
and image subtraction between a known-good and
a failing device. Imaging of current paths and defects
down to submicron dimensions followed with the
addition of the GMR. Later, methods for localizing
completely open circuits using a radio-frequency
(RF) SQUID and a process for 3-D reconstruction of
buried currents appeared. Finally, MCI also provides
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Electronic Device Failure Analysis
a means of comparative analysis useful for investigating counterfeit devices, imaging power-distribution
networks, and verifying design.
Magnetic Imaging Theory
The Biot-Savart law defines the relationship between magnetic fields and current for an infinitesimal
length of current element as:
where B is the magnetic field (Tesla), Id is an element
of current (amperes), the constant µ0 is the permeability of free space (4π × 10−7 Tesla ∙ meter/amperes),
and r (meters) is the distance between the current and
the sensor. A properly oriented sensor scanned parallel to a surface detects the bipolar components, Bz , of
the magnetic field lines normal to that surface (Fig. 1).
Fig. 1 Magnetic field about a current-carrying conductor with its
vertical components detected by a horizontally-oriented magnetic sensor
The two-dimensional current density in the x,y
plane of the device is then derived from the magnetic
field, knowing only the approximate separation between the current and the sensor. This is accomplished
by solution of the “magnetic inverse problem,” which
transforms the field components Bz(x,y) into Fourier
space, applies a low-pass spatial filter, and inverts
the data back to the x,y plane in the form of the current density elements Jx(x,y).[9] The entire process is
completed automatically in seconds, with current
from individual conductors and defects on numerous planes beneath the surface able to be observed.
Attributes of Magnetic
Current Images
An important aspect of the inversion process is
that it results in current density. Thus, the width of
conductors carrying current modulates the intensity
of the signal—an important feature that creates easily recognized signatures for locating point-source
short-circuit and leakage defects. Another distinction
is that magnetic fields from buried conductors pass to
the surface unperturbed (Fig. 2) through all common
insulating, conducting, and semiconducting materials used in microelectronics. (While these materials
each possess a finite magnetic permeability, their
magnitudes are close enough to that of the free-space
constant, µ0, to be negligible.)
So, unlike other energy forms used in image-based
fault isolation (i.e., heat, light, electron and ion beam)
that are impacted by intervening materials, magnetic
Fig. 2 Magnetic field from a buried defect penetrating multiple layers of a packaged IC
fields are unaffected by properties such as absorption,
thermal conductivity, diffraction, doping, dielectric
constant, and so on. Thus, current from buried (or, of
course, surface) conductors or defects can be imaged
without sample preparation up to millimeters above
die/wafer/substrate, packaged device, or finished
assembly surfaces, from the frontside, backside, or
edges (although minimizing sensor-to-sample separation by thinning or removing overlying materials
improves resolution and sensitivity).
Finally, two additional characteristics prove useful
as diagnostic aids: determining relative current direction from the measured polarity of the magnetic field,
which helps identify current sources and sinks in a
circuit; and calculating current depth using the known
geometry of the field, the current magnitude, and
the separation distance between sensor and sample
(without the need for reference circuits or devices,
or material constants such as thermal conductivity,
doping, velocity factor, etc.). This can be accomplished
with accuracy sufficient to differentiate simple isolated wiring paths between and within layers of a
package or die.[10]
MCI Sensors
Magnetic microscopes for fault isolation employ
two magnetometers, each with specific advantages.
The SQUID is a small superconductor loop containing two Josephson junctions, which are structures
separated by a thin insulating layer acting as a tunnel
barrier. When current-biased, the voltage across the
junctions is proportional to the magnetic field passing through the loop. SQUIDs used in fault isolation
are made from high-temperature superconductors
(HTSs) that operate at <90 K (usually 72 to 78 K). Only
the SQUID need be cooled so the sample remains
at ambient temperature (or may be independently
cooled or heated as necessary). The SQUID scans
over the surface at the closest possible distance to
obtain the greatest resolution and sensitivity. In this
manner the localization resolution (i.e., placement of
the MCI signal on the optical image of the sample) of
current density peaks due to short circuits is better
than ±3 µm.
The extreme sensitivity of the HTS SQUID, approximately 20 pico-Tesla/√Hz, is one of its strongest assets. (Magnetic resonance imaging produces
single-Tesla fields; the Earth’s magnetic field is 30
to 60 micro-Tesla; and the human brain generates
fields in the femto- to pico-Tesla range).[11,12] An
HTS SQUID can detect and image currents to below
500 nano-amperes acpp, up to approximately one-half
millimeter away. (For low noise and high sensitivity,
MCI measurements are usually made with an ac
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Magnetic Current Imaging Revisited
signal to the sample that is referenced to a lock-in
amplifier.) At such low currents, imaging is possible
down to femto-watts of incidental power, depending
on resistance.
The GMR’s advantage is high resolution. It contains
multiple nonmagnetic and ferromagnetic layers arranged so the stack resistance perpendicular to the
surface varies linearly with magnetization. Operating
in ambient conditions, the active portion is at the end
of a sharply tapered tip and scans extremely close to
features such as wire bonds or probes or inside small
cavities. Without the need for cooling or vacuum, it
can also be scanned directly on the surface, placing
it in direct proximity to current paths. As a result, a
GMR can achieve submicron resolution (Fig. 3).[13] Its
sensitivity is below 20 micro-amperes acrms, allowing
it to image current paths and defects dissipating only
pico- to nano-watts of power.
Sample Configuration and
Electrical Excitation
Samples are scanned on a stage moving beneath
the magnetic sensors and then under a visible/nearinfrared microscope. The current density signal is
automatically aligned to and overlaid with the optical image for navigation and registration. Current is
supplied as necessary through wire bonds, soldered
wires, wire-wrapped pins, PCB connectors, or microprobes. For more complex test conditions, the sample
may be mounted on a test card and cabled to a tester
or personal computer. When using microprobes, the
sample platform also holds the manipulators with
their probes placed using the optical imaging system.
The entire assembly then raster-scans while maintaining contact to package lands or pads, leads, ball grid
array solder balls, wire bonds, I/O pads, metal wiring
traces, and so on, and even focused ion beam pads
down to approximately 10 µm2.
To help achieve high sensitivity while suppressing
noise, an ac bias is supplied to the sample. This signal is referenced to a lock-in amplifier that processes
magnetic signals preferentially at the same frequency,
generally between 5 and 100 kHz for shorts, leakages,
and resistive opens, depending on the sensor. The
ac signal may also be offset positively or negatively,
creating a varying excitation current “riding” on a
dc operating point. For devices with low capacitive
reactance (in that frequency range) that may shunt
too much current away from the defect, a straight
dc bias can be used with the poweredoff magnetic field subtracted from the
powered-on state (albeit trading off some
sensitivity). When imaging the location of
fully open circuits, a single 50 to 100 MHz
signal (with no need for grounding other
pins) is applied to the open connection on
the device.
Applications
Fig. 3 GMR capabilities. (a) Spatial resolution of a frontside-acquired current density path
superimposed on an SEM image of a metal serpentine. Courtesy of Neocera, LLC. (b)
Peak resolution of a backside-acquired current density signal from a flip-chip power
supply short overlaid on a near-infrared optical image
Fig. 4 Common current density signatures for short circuits and leakage currents. (a)
Point-source defect between large power and GND network. (b) Narrow current path
shorted to larger GND network overlaid on an optical image. (c) Short circuit between
two narrow current paths superimposed on an x-ray image of a package lead frame
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Electronic Device Failure Analysis
Short Circuits and
Leakage Currents
Short circuits and leakage currents exhibit three recognizable MCI signatures
(Fig. 4). Point-source defects tend to be
narrower than the designed-in conductors
that source and sink current to and from
them. Here, a relatively small conductive path (the defect) connects two larger
conductors (e.g., a power and a ground
plane), resulting in a sharp spike in current
density as current “squeezes” through the
narrow pathway. Another common and
well-recognizable signature is a narrow
path (e.g., a signal trace) shorting to a
large power or ground network, whereby
a strong current density abruptly diffuses
(continued on page 32)
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Magnetic Current Imaging Revisited
(continued from page 28)
(much like river current disperses into a
larger body of water at its mouth). Finally,
when neighboring conductors of similar
width are shorted (e.g., parallel traces in
a die or package), the defect location is
simply the point where the current paths
meet, usually seen as a sharp bend.
An important consideration is that
MCI needs only a finite current of 500
nano-amperes acpp or more to produce
a signal. (The voltage required to attain
that current and the resulting power is
incidental.) This provides two advantages
over other methods. The first is in finding large-area, low-resistance leakage
currents and short circuits. The thermal
mass of such defects can limit their heat
and power densities to undetectable levels; their large area makes them difficult
to heat with a laser sufficient to generate
a change in resistance; and their very
low resistance limits voltage across them
to levels that preclude light emission.
Magnetic current imaging simply maps
the current density distribution.
Fig. 5 Power vs. current for a range of resistance values, illustrating the equivalent power
sensitivities of MCI at its lowest current sensitivity
The second advantage is for small highresistance defects. The power dissipated
at defect sites (I2R) during analysis is a
critical but frequently overlooked concern. Shorts tend to be much smaller than
their surrounding native conductors. The
power density and local temperature rise
that may be tolerated by nondefective
conductors can easily damage small, subtle defects (e.g., filaments, dendrites, thin
films, contaminants, etc.), causing them to
overheat, deform, or even become open.
Subsequent root-cause analysis then becomes impossible, difficult, inconclusive,
or, at the least, highly questionable, with
little to no useful evidence of the original
defect remaining.
At its highest sensitivity, a SQUID
measurement of <500 nano-amperes acpp
is equivalent to a root mean square (rms)
ac current (its effective heating value) of
<177 nano-amperes. So, for a 1 k-W short,
for example, just 31 pico-watts are generated. For low-resistance defects, say 1 Ω,
only <31 femto-watts are produced (Fig.
5). Hence, the highly sensitive SQUID
is quite well suited to detect very weak
currents from relatively large distances
and find low-resistance shorts as well as
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Fig. 6 Space-domain reflectometry open-circuit isolation. (a) Surface rendering of magnetic
field from decaying standing wave. (b) Current density signal from decaying standing wave with open-circuit location highlighted. (c) Open-circuit location overlaid
on optical image. (d) Optical micrograph of highlighted region in (b) showing the
open-circuit defect. Courtesy of Neocera, LLC
Electronic Device Failure Analysis
small, subtle voltage or heat-sensitive defects while
preserving their shape and morphology.
Complete Opens
Applying an RF signal between 50 and 100 MHz
to an open connection gives rise to a standing sinewave and a corresponding reflection from the open
location. This creates a standing voltage wave and
a corresponding current that is then imaged by the
SQUID (also operating now in the RF spectrum). The
wavelength at this frequency range is many times
longer than the open trace within the device, such
that the standing wave portion along the length of
the conductor is essentially linear. Projecting this
linear decay along the path of the signal and the trace
locates the open (Fig. 6). The technique is known as
space-domain reflectometry, with a demonstrated
resolution of 25 µm in the z-direction and 30 µm in
the x,y directions.[14]
3-D Current Path Imaging
Unfortunately, the general magnetic inverse problem discussed earlier has no truly unique solution.
For a given 3-D field distribution there are an infinite
number of possible current paths. Without such a
solution or approximation, MCI is restricted to twodimensional mapping of surface fields and their
associated current paths. However, with a thorough
investigation and analysis of the magnetic field signal
under a few reasonable constraints typical of microelectronic devices (i.e., current confined to vertical layers; layers interconnected vertically; and Manhattan
geometry restrictions on current paths), an algorithm
has been developed that allows the conversion of this
ill-posed problem into a tractable one having a solution approximated with good results (Fig. 7).
The “3-D solver” allows the extraction of threedimensional current paths and critical information
such as current depth, which can be used to localize
the vertical layer where the failure occurs, as well
as separation between layers of current. Vertical
resolution better than 50 µm, lateral resolution of
3 µm, and capability to differentiate dice in a stacked
configuration have been demonstrated.[15] This innovation suggests MCI can be extended into a true,
noninvasive 3-D fault-isolation technique.
Conclusions
Magnetic current imaging provides
electrical fault isolation for a wide range of
defect types, including short circuits, leakage currents, resistive opens, and complete
opens, all on a single platform. Because of
the unimpeded propagation of magnetic
fields through all common microelectronic
materials, MCI can be performed nondestructively from any surface of a device,
including dice, wafers, 2-/2.5-/3-D packaged ICs, discrete components, and PCBs.
The capability to determine current direction and depth adds to its versatility.
Fig. 7 3-D current path reconstruction. (a) Stacked-dice sample configuration. (b) 2-D
MCI surface current density image using a GMR. (c) Computer-aided design drawing of same daisy-chain structure showing top die (red lines) and bottom die (blue
lines) segments and their connecting through-silicon vias (blue squares) with site of
inversion of top/bottom sequence (red arrow). (d) 3-D solver results showing reconstructed current paths in bottom die (green line) and top die (blue line). Courtesy of
Neocera, LLC
The extreme sensitivity of the SQUID
allows isolation of short circuits and leakages to below 200 nano-amperes acrms with
effective power dissipations down to a
few femto-watts, enabling localization
to ±3 µm without excessive heating and
possible destruction of subtle defects. The
GMR provides imaging of current paths
spaced only hundreds of nanometers
apart and current density peaks to below 300 nm, with less than 20 microamperes acrms and power levels of pico- to
nano-watts.
Space-domain reflectometry enables isolation of completely open circuits to within
30 µm using only one probe and without
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Magnetic Current Imaging Revisited
the need for material constants or reference devices.
Finally, 3-D current path reconstruction has emerged
as a valuable and critical solution for pinpointing buried defects and current paths with a vertical resolution
under 50 µm and lateral resolution of 3 µm. These are
especially critical developments as fault isolation using optical, electron, and ion beam methods becomes
intractable on 2.5- and 3-D assemblies with stacked
chips embedded in thick layers of opaque or highly
absorptive materials.
Acknowledgments
The author acknowledges Jan Gaudestad, Antonio
Orozco, and Vladimir Talanov of Neocera, LLC, and
Lee Knauss of Booz Allen Hamilton, Inc. for their
significant technical contributions and numerous
insightful discussions.
References
1. S. Chatraphorn, E.F. Fleet, F.C. Wellstood, L.A. Knauss, and
T.M. Eiles: “Scanning SQUID Microscopy of Integrated
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2. S. Hsiung, K.V. Tan, A.J. Komrowski, D.J.D. Sullivan, J.
Gaudestad, A. Orozco, E. Talanova, and L.A. Knauss:
“Failure Analysis on Resistive Opens with Scanning SQUID
Microscopy,” Reliab. Phys. Symp. Proc., IEEE Reliability
Society, 2004, pp. 611-12.
3. S.I. Woods, N.M. Lettsome, Jr., A.B. Cawthorne, L.A.
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4. W. Qiu, S.C. Tan, M.Y. Tay, J. Gaudestad, V. Talanov, and
M.S. Wei: “Non-Destructive Open Fault Isolation in FlipChip Devices with Space-Domain Reflectometry,” 20th
IEEE Int. Symp. Phys. Fail. Anal. Integr. Circuits (IPFA), IEEE
Electron Device Society and IEEE Reliability Society, 2013,
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5. A. Orozco, J. Gaudestad, N.E. Gagliolo, C. Rowlett, E. Wong,
A. Jeffers, B. Cheng, F.C. Wellstood, A.B. Cawthorne, and F.
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ASM International, 2013, pp. 189-93.
6. B.J. Roth, N.G. Sepulveda, and J.P. Wikswo, Jr.: “Using
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7. E.F. Fleet, S. Chatraphorn, F.C. Wellstood, S.M. Green, and
L.A. Knauss: “HTS Scanning SQUID Microscope Cooled by
a Closed-Cycle Refrigerator,” IEEE Trans. Appl. Supercond.,
1999, 9(2), pp. 3704-07.
8. L.A. Knauss, A.B. Cawthorne, N. Lettsome, S. Kelly,
S. Chatraphorn, E.F. Fleet, F.C. Wellstood, and W.E.
Vanderlinde: “Scanning SQUID Microscopy for Current
Imaging,” Microelectron. Reliab., 2001, 41(8), pp. 1211-29.
9. J.P. Wikswo: “The Magnetic Inverse Problem for NDE,”
SQUID Sensors: Fundamentals, Fabrication and Applications,
H. Weinstock, Ed., Kluwer Academic Publishers, The
Netherlands, 1996, pp. 629-95.
10.B.D. Schrag, X. Liu, J.S. Hoftun, P.L. Klinger, T.M. Levin,
and D.P. Vallett: “Quantitative Analysis and Depth
Measurement via Magnetic Field Imaging,” Electron. Dev.
Fail. Anal., 2005, 7(4), pp. 24-31.
11. L.A. Knauss, S.I. Woods, and A. Orozco: “Current Imaging
Using Magnetic Field Sensors,” Microelectronics Failure
Analysis Desk Reference, 6th ed., R.J. Ross, Ed., ASM
International, Materials Park, OH, pp. 301-09.
12.“Orders of Magnitude (Magnetic Field),” Wiki­
p e d i a , h t t p : / / e n . w i k i p e d i a . o rg / w i k i / O rd e r s _
of_magnitude_(magnetic_field).
13.D. Vallett, C. Richardson, and J. Gaudestad: “High
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Contour-Milled Ultrathin Die,” Proc. 40th Int. Symp.
Test. Fail. Anal. (ISTFA), ASM International, 2014 (to be
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14.J. Gaudestad, A. Orozco, V.V. Talanov, and P.C. Huang:
“Open Failure Detection in 3D Device Non-Destructively,”
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Institute of NANO Testing.
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About the Author
Dave Vallett provides magnetic current imaging services at his company, PeakSource
Analytical LLC (www.peaskourcevt.com), in Fairfax, Vermont. Prior to this he was with the IBM
Microelectronics Division in Essex Junction, Vermont, for 31 years as an engineer and laboratory
manager in semiconductor characterization, analytical technology development, fault isolation,
and reliability failure analysis. He has performed SQUID and GMR magnetic microscopy for
over 11 years on a wide variety of die, substrate, package-level, and PCB yield and reliability
failures and customer returns. Since 2005 Mr. Vallett has also provided professional technical
education in fault isolation and failure analysis to companies and individuals in the United
States, Europe, and Asia for ASM International, CEI Europe, Semitracks, and Spyro Technology. He is a former
general chair (2008) and regular contributor to ISTFA, is widely published, and holds 27 U.S. patents in the field.
Mr. Vallett is a member of EDFAS and a Senior Member of the IEEE. He received a B.S. degree in electrical engineering from the State University of New York at Buffalo.
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