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FAULT LOCALIZATION
TECHNIQUES I
EMT 361
SCHOOL OF
MICROELECTRONIC
ENGINEERING
KUKUM
TOPICS OF INTEREST 1
OPTICAL MICROSCOPY
MECHANICAL PROBING
LIQUID CRYSTAL HOT SPOT
DETECTION
EMISSION MICROSCOPY
SCANNING ELECTRON MICROSCOPY
Optical microscope
OPTICAL MICROSCOPE
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1/a + 1/b = 1/f
M= (Image Height)/(Object Height)
= b/a
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real image - can be viewed on a screen, recorded on film, or
projected onto the surface of a sensor such as a CCD or CMOS
placed in the image plane.
virtual image - cannot be viewed on a screen or recorded on
film.
a real image must be formed on the retina of the eye.
When viewing specimens through the eyepieces of a
microscope, a real image is formed on the retina, but it is
actually perceived by the observer as a virtual image located
approximately 10 inches (25 centimeters) in front of the eye.
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Total visual magnification = objective magnification x
eyepiece magnification.
5X objective with a 10X eyepiece = total visual
magnification of 50X
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100X objective with a 30X eyepiece = magnification of
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3000X.
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Total magnification is also dependent upon the tube
length of the microscope. Most standard fixed tube
length microscopes have a tube length of 160, 170,
200, or 210 mm- 160mm most common for
transmitted light biomedical microscopes ;
semiconductor industry, = 210mm.
Objectives typically have magnifying powers that
additional magnification factor = tube factor in the user
manuals
provided
by
most
microscope
range from 1:1 (1X) to 100:1 (100X), with the most
manufacturers. Thus, if a 5X objective is being
common powers being 4X (or 5X), 10X, 20X, 40X
used with a 15X set of eyepieces, then the total
(or 50X), and 100X
visual magnification becomes 93.75X (using a
Eyepieces, magnification factors vary between 5X
1.25X tube factor) or 112.5X (using a 1.5X tube
and 30X with the most commonly used eyepieces
factor)
having a value of 10X-15X
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The objectives and eyepieces of these microscopes
have optical properties designed for a specific tube
length, and using an objective or eyepiece in a
microscope of different tube length will lead to
changes in the magnification factor (and may also
lead to an increase in optical aberration lens
errors). Infinity-corrected microscopes also have
eyepieces and objectives that are optically-tuned to
the design of the microscope, and these should not
be interchanged between microscopes with
different infinity tube lengths.
• eyepiece/objective combination -application = eye /
photomicrography
• R = /(2NA)
(1)
• R = 0.61  /NA
(2)
• R = 1.22  /(NA(obj) + NA(cond))
(3)
Where R = resolution (the smallest resolvable distance between two
objects),
NA = numerical aperture,
 = wavelength,
NA(obj) = the objective numerical aperture, and
NA(Cond) = the condenser numerical aperture
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• Resolution and Numerical Aperture by Objective Type
Objective Type
Plan Achromat
M
N.A
R
4x
0.10
2.75
10x
0.25
1.10
20x
0.40
0.69
40x
0.65
0.42
60x
0.75
0.37
100x
1.25
0.22
N.A. = Numerical Aperture
R = Resolution (m)
M = Magnification
Plan Fluorite
N.A
R
0.13
2.12
0.30
0.92
0.50
0.55
0.75
0.37
0.85
0.32
1.30
0.21
Plan Apochromat
N.A
R
0.20
1.375
0.45
0.61
0.75
0.37
0.95
0.29
0.95
0.29
1.40
0.20
Resolution versus Wavelength
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Wavelength (nm)
360
400
450
500
550
600
650
700
Resolution (m)
.19
.21
.24
.26
.29
.32
.34
.37
Colour
UV / Deep Blue
deep Blue
Blue
Blue/Green
Green/Orange
Orange/Red
Red
Mechanical Probing
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Microprobing= probing= failure analysis technique used to achieve
electrical contact with or access to a point in the active circuitry of the
die.
microprobing station = probe station
Electrical contact - fine-tipped probe needles directly on the point of
interest, or on an area to which the point of interest is connected.
Needle tip chosen - electrical contact needed & probing area.
Micromanipulator - controlled by analyst to land the needle on the die
precisely - optical / electron microscope
Analyst employs the same thought process as when troubleshooting a
full-size circuit. Microprobing = tool for analyst to access critical nodes
on the microscopic die circuit while analyzing the behavior of the
various parts of the circuit.
Electrically pinpointing failure site = failure isolation => which requires
the analyst to identify abnormal voltages and/or currents on the die.
V and I measurements are performed by the voltmeters, curve tracers,
oscilloscopes, attached to the probe needle through micromanipulator.
Circuit excitation from voltage supplies, waveform generators, and the
like may also be supplied to the die circuit in the same manner.
Passivation - hard, impenetrable - removal =RIE, LASER
Schematic diagram & die lay-out of the device
laser cutter - metal lines to be burned open for convenient
isolation of nodes from one another
Eye and hand coordination, as well as experience
Die scratches and even chip-outs
landing four sharp probes within a 200 x 200 nm
area with less than 5 nm precision = typical
The size of the minimum landing area is dependent on
the sharpness of the probes.
With FIB sharpened probes, the minimum landing area
could be much smaller than that stated above.
ICs - 90 nm node technology to 60 and then to 45 nm
SETs?
four probes independently and linearly
operated in X, Y, and Z with <5 nm precision.
installed in a high-resolution SEM for precise
imaging and probe placement similar to the
arrangement shown
Tungsten probes with a 45-degree
bend were installed in the four positioners.
The 90 nm node IC test chip was deprocessed
to the contact level and etched for 5s in 20
parts H2O to 1 part 49% HF.
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The IDVDS curves of an integrated nchannel MOSFET
The reverse bias of the device under
test
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This comparison is one way to
determine if the device is functioning
properly. The IDVDS curves are very
similar which this shows that the
current device under investigation is
functioning as expected.
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p-channel MOSFETs, the current ID flows when the gate
voltage V GS is negative with respect to the source
voltage V DS .
Typically, p-channel MOSFETs have a higher gate threshold
voltage and lower saturation current.
These p-channel curves, show the expected lower
performance of a p-channel MOSFET in comparison with the
n-channel MOSFET.
Liquid Crystal Hot Spot
Detection
Microthermography = Hot Spot Detection = LCHD locate areas on die surface that exhibit excessive
heating - high current flow-- die defects or
abnormalities like dielectric ruptures, metallization
shorts, and leaky junctions.
Bias chosen - enough current to generate the amount
of heat needed to change the visual characteristics of
the liquid crystal
LC crystalline - 'smectic' phase and the 'nematic'
phase.
@ higher T’s - 'isotropic phase,' = molecules are
randomly located and randomly oriented
isotropic phase = a clear liquid, no distinctive optical
characteristics because of the homogeneous distribution of its
molecules
nematic phase = milky appearance & exhibits optical properties
similar to those of crystal structures
Via optical microscope - polarizing filter in illumination path
(polarizer) and a cross polarized filter in the viewing path
(analyzer) :
Isotropic films = black, since the cross polarized light is blocked
by the analyzer
Nematic films = rainbow-colored, since light reflected from
nematic films 'twist' such that they are able to pass through the
analyzer
Hots spots raise the temperature of the liquid crystal, making it
appear black at that spot. The die surface of a well-prepared
sample with a hot spot will therefore appear as rainbow-colored,
except for the hot spot which will appear as a blackened area.
correct amount.
Too thick = uniformly black=> impossible to detect nematic
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Too thin = streaks of dark and light grays=> hot spots less
visible.
perfect = general area of the die surface rainbow-colored=>
best contrast to a hot spot.
bias - defect site carries enough current to heat the liquid crystal at the hot spot,
but not enough current to heat the entire liquid crystal film.
If hot spot generates so much heat that the entire die surface is darkened even
at minimum power setting, excitation must be 'pulsed' or oscillated - locate the
hot spot, which is the point of origin and return of the oscillating color contrast on
the die surface.
Polarizing filters - adjusted to provide the best rainbow color for the liquid crystal
film.
interpreting the presence / absence of hot spots on the die = something wrong;
not always the actual failure site = good components forced to conduct high
currents by an anomaly somewhere else in the circuit.
Complement with FA techniques in order to arrive at the right conclusion.
Dielectric Shorts or Breakdowns, Metallization Shorts, Junction
Leakages,
Mobile Ionic Contamination, etc
Emission Microscopy
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• monitor VIS and NIR emissions from
ICs
- powered up
==> locating and
characterizing defects.
= TASK / APPLICATION
THEORY =
@ faulty junction / transistor,
hot charge carriers = e’s and h’s-will
often be present in the component during
operation, and may have enough energy
to result in weak visible emission during
carrier recombination. Even when they
do not, lower carrier energies may result
in NIR emission or even just local heating
ACTION =
radiation image overlaid with its
corresponding die surface image, such
that the emission spot coincides with
the precise location of the defect
TOOLS
• OPTICAL MICROSCOPE
• CCD CAMERA - VIS / NIR /
BOLOMETER / IMAGE INTENSIFIER
• PC
• IMAGE PROCESSING SOFTWARE
applications include but are not limited to :
• 1) detection of previously unknown or
undetectable electroluminescence ;
• 2) detection of avalanche luminescence from
junction breakdowns,junction defects,
currents due to saturated MOS transistors,
and transistor hot electron effects;
• 3) detection of dielectric electroluminescence
from current flow through SiO2 and Si3N4.
three common types
• simple emission microscopy
• energy-resolved emission microscopy
(EREM)
• backside die analysis
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simple emission microscopy
- bias voltage applied to exposed IC
- visible image of the device's weak
emission is recorded in the dark.
- compared to an ambient-light image of
the same area,
allowing emitting defects to be located
with great precision
energy-resolved emission
microscopy
different spectral filters/monochromator
inserted in front of the CCD camera,
quantitative comparison of the emission
intensity at several different wavelengths
provides information about the temperature of
the charge carriers,
E = h = hc/
which in turn yields greater detail about the
nature and extent of the defects.
backside die analysis
complex chips have several layers of
metallization through which defect emission
cannot be observed.
backside of the die is revealed through a hole
in the back of the chip. - package
Defect emission observed through the bulk
silicon of the die.
Si strongly absorbs VIS but NIR emission
(  1 um) can be readily imaged with this
method.
Scanning Electron Microscope
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image is formed and presented by a
very fine electron beam- focused on the
surface of the specimen.
beam is scanned over specimen in a
series of lines and frames called a
raster- accomplished by means of small
coils of wire carrying the controlling
current (the scan coils).
specimen bombarded by e’s over a very small
area.
-elastically reflected from the specimen, with no
loss of energy.
-absorbed by the specimen and give rise to
secondary electrons of very low energy,
together with X- rays.
-absorbed and give rise to the emission of
visible light (cathodoluminescence).
-give rise to electric currents within the
specimen.
All these effects can be used to produce an
image. By far the most common, however, is
image formation by means of the low-energy
secondary electrons.
• 2ndary e’s are selectively attracted to a grid at a low
(~50V) +ve potential with respect to the specimen.
• Behind the grid is a disc at ~10 kV +ve with respect
to the specimen.
• Disc consists of a layer of scintillant coated with a
thin layer of Al. 2ndary e’s pass through grid and
strike disc - emission of light from the scintillant.
• The light to PMT - photons into voltage. Strength of
this voltage depends on the number of secondary
electrons that are striking the disc.
• The voltage to electronic console - processed and
amplified to generate a point of brightness on CRT.
• Image built simply scanning e-beam across the
specimen in exact synchrony with scan of the e-beam
in CRT.
SEM does’nt have objective, intermediate
and projector lenses to magnify the image as
in the optical microscope.
magnification results from the ratio of the
area scanned on the specimen to the area of
the television screen.
Increasing mag. in SEM is achieved quite
simply by scanning the electron beam over a
smaller area of the specimen.
Image formation in SEM equally applicable to
elastically scattered electrons, X-rays, or
photons of visible light - detection systems
different in each case. Secondary electron
imaging is the most common because it can
be used with almost any specimen.
M> 300,000 X => semiconductor <3,000 X
analysis of die/package cracks and fracture surfaces, bond failures,
and physical defects on the die or package surface
energy of the primary electrons determines the quantity of secondary
electrons collected - increases as the energy of the primary electron
beam increases, until a certain limit - secondary electrons diminish as
the energy of the primary beam increases, because the primary beam
is already activating electrons
deep below the surface. Deep
electrons usually recombine before reaching the surface for emission.
emissions above 50 eV ~backscattered electrons.
Backscattered electron imaging distinguishing materials - yield of
collected backscattered electrons increases monotonically with the
specimen's atomic number. Backscatter imaging can distinguish
elements with atomic number differences of at least 3, i.e., materials
with atomic number differences of at least 3 would appear with good
contrast on the image.
For example, inspecting the remaining Au on an Al bond pad after
its Au ball bond has lifted off would be easier using backscatter
imaging, since the Au islets would stand out from the Al background.
1) The EHT must be high enough to provide a good image but
low enough to prevent specimen charging.
2) To maximize contrast due to material differences, use as low
an EHT as possible.
3) If possible, sputter-coat the specimen to prevent specimen
charging. Sputter-coating is considered destructive. Never
sputter-coat units that still need to undergo electrical testing,
curve tracing, EDX analysis, inspection, etc.
4) The probe current must be set to its default value, unless a
higher probe current is needed to focus the point of interest
properly.
Die/Package Cracks, Die Attach Failures/Defects, Bonding
Failures/Defects, Wire Defects/Fractures, Lead
Defects/Failures, Foreign Materials on
Die/Package, Die
Surface Defects, Seal Cracks/Defects, etc.
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A curve-tracer's display shows
the current-vs.-voltage plot from
tests on the RF input pin
of
the circuit shown in Figure 1.
The positive and negative
resistance changes indicate
problems with internal
components
The GaAs RF IC we investigated
could fail because of opens at R 1or
shorts at C 1or C 1.
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A
visiblelight photo of a
failed device
shows a small
spot revealed
by
liquidcrystal material.
This hot spot
indicates a short
circuit.
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An image from SEM shows the trench etched
around a defective capacitor by a laser. (The arrow
points to the defects'
location.)
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(a) The bump on a defective GaAs RF IC, as seen in this SEM image, indicated the location of
the bump in (a) to the known ESD defect—also a short circuit—shown in (b)
a short circuit. Compare
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References
• http://micro.magnet.fsu.edu/primer/anat
omy/anatomy.html
• http://www.semiconfareast.com/lem.htm