The Investigation of Active VC and EBAC Analysis
Utilization on Test Structure
Link Chang, Kuo Yu Wang, Simon TC Wang, Yu Pang Chang,
United Microelectronic Corporation, Ltd
No. 18`20, Nake 2nd Rd., Tainan Science Park, Sinshih Dist., Tainan City, Taiwan, R.O.C
Phone: 888-6-5054888 ext:11399 Fax: 886-6-5051220 Email: link_chang@umc.com
Abstract-As the founded of nano-probing technique on Scanning
Electron Microscope (SEM) base system, the Active Voltage
Contrast (AVC) and electron beam absorbed current (EBAC)
technique are easier to apply on failure analysis. Both these two
can isolate failure location accurately, and high resistance site also
can be located by these powerful tools. Based on passed analysis
experience, we will investigate and present the utilization of these
two techniques to develop the better condition. By the investigating
on test circuit of metal/via chain, the better effective analysis skill
can be applied to the related high resistance failure analysis.
I. Introduction
Voltage Contrast is a common and effective analysis in
failure analysis field, especially, the Passive Voltage Contrast
(PVC) checking is a standard procedure for some failure
analysis cases. But, passive VC is not so powerful to isolate
some high resistance issue. So that, the Active Voltage Contrast
(AVC) [1] have alternative to dig out this kind of failure. As the
Figure 1 shown, the external voltage applied to a test chain to
charge a potential difference. If there is a faulty like partial
connected via, it can be appearance of voltage contrast. It is very
easy to apply the voltage on the test chain base by a
nano-probing system. We also can tune the accelerated voltage
of SEM or change voltage between two nodes of test chain to
find the best sensitivity of contrast.
EBAC [2-4] image analysis is another alternative and more
powerful tool on contemporary integrated circuit failure
analysis. On conventional analysis technique, Infar-red Optical
Beam Induce Current (IR OBIRCH) or Thermal-laser Induced
Voltage Alteration (TIVA) are common methodologies to locate
failure for high resistance site, but, the resolution of optical
microscope is too low to isolate failure site accurately.
EBAC image system is equipped with nano-probing and
SEM. When primary electron beam penetrate sample surface,
electron can be absorbed at abnormal site via probe to amplifier
and EBAC image formation. The penetration depth of electron
is depended on the acceleration voltage of SEM and sample
material. The effective depth can be predicted by the Monte
Carlo simulation for analysis [5] necessary. The electron beam
is an injection current source and the current flow into metal/via
structure was measured by the probe. If the metal/via chain is
broken or opened, the open position can be localized by the
change of EBAC contrast image that superimpose with SEM
image of test structure. Even some of short or leakage site are
also can be found by EBAC analysis technique. But, for some
higher resistive (partial connected via), by one single probe
978-1-4799-9928-6/15/$31.00 ©2015 IEEE
cannot localize this kind of defect, therefore, it need two probes
of EBAC technique as referred Resistive Contrast Imaging(RCI)
[6] to dig out the failure site. Two probes method induce the
different voltage potential that was caused high resistance
failure between one probe connected to current-amplifier and
one probe connected to ground.
II. Active Voltage Contrast Analysis Method
Beyond nano-probing system development, AVC is easier to
execute in FA laboratory. Furthermore, active voltage contrast is
more powerful than PVC, especially for some partial connection
metal/via chain failures. Because this kind of failure cannot
have obvious voltage level among metal/via chain, so the
secondary electron (SE) yield discrepancy was not appearance
during electron beam scan on the sample surface. It was charged
by external voltage source and will increase voltage level
between good site and failure node, so that, it will reveal an
obvious contrast level on SE image. By optimizing the external
voltage, we will obtain the best contrast level on SE image.
Fig.1. The scheme of Active VC revealed contrast different at high resistance of
via chain.
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Fig.2. The scheme of EBAC principle.
IV. Active VC on test structure case study
III. Electron Beam Absorbed Current Analysis Method
As the Figure 2 shown, the injection electron-beam Ii cause
scattered current Is and absorbed current Ia. A high gain EBAC
amplifier is connected to the nano-probe system. The
nano-probe makes contact with a conductive structure (metal
line or via) at the sample surface. Electrons are absorbed by
features (metal lines) at the surface which are connected to the
conductive structure. The EBAC image is presented by
amplifying these absorbed electrons and can be superimposed
with SEM image to localize the failure site.
When perform EBAC analysis, the E-Beam injection Energy
must penetrate deep enough into the sample to interact with the
conductive sub-surface feature (metal lines) for it to be imaged
by the EBAC nano-probe computer. Figure 3 shows the depth of
electron ejection and E-beam energy. The depth can be
calculated by Monte Carlo simulation. The Figure 4(a) and
Figure 4(b) shows the scheme of EBAC analysis with single/two
probe.
In this case study, we will introduce a test structure of Via
chain and suffered high resistance (not open) at WAT (wafer
acceptance test) test. The test structure is especially designed to
monitor the defect density for manufacturing process
continuously. It is also called a DD (defect density) chip. At first,
we use PVC method to observe any bright/dark VC, but, no
obvious bright/dark VC was observed. Then, we use the AVC
method by nano-probe system. Figure 5 shows the nano-probe
hardware in the chamber. As mentioned before, an external bias
to one side of the Via chain was added, the different bright/dark
contrast was detected, shown as Figure 6. The X-S FIB
investigation revealed Via Cu loss, shown as Figure 7. By this
result, it show high resistance but not fully open. So, the
bright/dark contrast cannot obviously be detected by PVC.
Fig.3. The depth of electron ejection and the depth can be calculated by Monte
Carlo simulation
Fig.5. The nano-probe hardware in the chamber.
Fig.4(a).The scheme of EBAC analysis with one probe.
Fig.4(b).The scheme of EBAC analysis with two probes, one needle land on the
right side of test structure, one land on the left side and set as ground.
Fig.6. Active VC revealed different bright/dark contrast at resistance defect site
by applied external voltage at nano-probing system
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when E-Beam inject charge to the sample surface and
nano-probe absorbed the electron.
After the high gain current amplifier, the EBAC image was
generated, shown as the Figure 9. The high resistance site was
detected and presented with bright/ dark contrast. Then, we
superimpose the EBAC image to SEM image, the high
resistance site can be precisely localized, shown as Figure 10.
The X-S FIB investigation revealed poor connected between
metal and via, shown as Figure 11.
Fig.7. The high resistance was located by active VC and M/Via Cu loss caused
partial connection of metal/via chain was found by X-S FIB.
Because the AVC external bias can enhance the bright/dark
contrast, we can tune the different voltage to see the best
contrast condition. Figure 8(a) show one probe with different
voltage and the left image show better bright/dark contrast.
Figure 8(b) show two probes with different voltage and the
better condition is the middle image.
Fig. 9 EBAC image analysis. Left is small magnification image, right is
large magnification image. High resistance site was detected (arrow).
Fig.10. EBAC image analysis. Left is SEM image, right is the EBAC image
superimpose with SEM image. High resistance site was localized (arrow)
Fig.8(a). AVC with one probe and different conditions study.
Fig. 8(b). AVC with two probes and different conditions study.
V. EBAC on test structure case study
In this case, we are going to introduce a test structure Via
chain and suffered high resistance, not open. This wafer is
designed especially for yield improvement. We also perform the
EBAC method by nano-probe system. As mentioned before,
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Fig.11. High resistance was localized by EBAC analysis and poor connected
between metal and via was found by X-S FIB.
VI. Conclusion
As the semiconductor technology continue to shrink, the high
resistance failure has become more difficult to be detected by
conventional method, such as, IR OBIRCH or PVC. As the
above, a Via chain which suffered high resistance, but, not fully
open, the IR OBIRCH resolution is not good enough to precisely
locate a single high resistance Via. And, the PVC is hard to
observe bright/ dark contrast. The AVC and EBAC method are
the effective analysis skill and can be applied to the related high
resistance failure analysis.
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