Using an E-beam method and
line monitoring to perform in-line inspection
Desmercieres and Gilles Roy, Altis Semiconductor;
and Pierre Lefebvre, KLA-Tencor
complexity of advanced wafer processes has increased the risk of unique
defects such as buried electrical defects and optically transparent residues.
Using its existing inspection toolset, Altis Semiconductor (an IBM-Infineon
joint venture in Corbeil-Essonnes, France) was unable to detect such defects
before final test, risking yield losses. To support its yield-ramp and
yield-monitoring strategies, the company turned to E-beam inspection (EBI).
With its unique voltage contrast inspection capability, EBI can ensure
that critical (killer) defects are detected before the end of line, resulting
in considerable cost savings and increasing learning cycles during technology
installed its first EBI tool, an eS20XP from KLA-Tencor (San Jose), in
April 2001. The system was used in engineering analysis and line-monitoring
applications to monitor production lots in-line, enabling the company
to react more effectively to process changes and defect excursions.
article demonstrates that EBI has been key to optimal yield improvement
and monitoring on Altis's copper logic production line. By using the technology,
the fab's yield management team was able to identify and monitor buried
electrical defects in-line. In addition, the article discusses the requirements
for implementing EBI in order to perform volume production monitoring.
Three case studies show how EBI was used to characterize and monitor buried
electrical defects and defects that could not be detected in-line using
conventional optical inspection. In all three cases, EBI resulted in yield-improvement
cycles that were three to six weeks faster than those achieved using traditional
methodologies that depend on in-line and final test results.
Engineering Analysis to Line-Monitoring Applications
requirements and toolsets vary from the product ramp phase to production.
During ramps, the line must be highly sensitive to all defect types, since
priority is placed on characterizing defects in preparation for line monitoring.
Bright-field and EBI inspection technologies provide the required sensitivity.
is used in an engineering mode to identify critical process layers, perform
systematic scanning electron microscopy (SEM) defect review, characterize
critical defect types through failure analysis, and correlate defects
with final wafer test to determine their electrical impact. The E-beam
tool's contrast binning is augmented by in-line automated defect classification
(iADC) to provide quick, accurate defect classification.
yield has been brought up to production levels, the focus shifts to monitoring
the line for yield-impacting defect excursions. A larger array of inspection
technologies—including dark-field inspection, blanket checks, bright-field
inspection, and EBI—is used during production than during product ramps.
During the product-monitoring phase, EBI is employed to detect defect
excursions caused by process or tool drifts. When an excursion is detected,
the offending tool is stopped. Analysis is performed to identify the problem,
an action plan is defined, and the tool is restarted when the problem
has been resolved.
for Implementing EBI Line Monitoring
Altis established EBI as part of its line-monitoring strategy, engineers
spent time learning about the tool's capabilities. Not only did they become
proficient at creating effective inspection setups, but they also used
characterization and failure analysis at final wafer test to study the
defect types that they were detecting. To optimize EBI in production,
the fab defined and implemented a strategy for throughput, stability,
and defect review. In addition, it devised a sampling plan as a function
of tool capacity and lot cycle time.
During the engineering phase, the target throughput was defined as 50
wafers per week. This target was based on estimated wafer inspection time
(a function of inspection setup parameters) and available engineering
time (8 hours a day). The iADC feature was used to minimize review time
and improve throughput. In addition, the inspection setup was modified
to concentrate on the failed area of the die. After the inspection care
area, pixel size, and sample plan were modified, throughput was improved
by 50%, resulting in a final throughput of 75 wafers per week.
setups are typically a compromise between lot cycle time, setup parameters,
review, and sampling strategy. While engineering usually requests that
the entire wafer be inspected, the resulting lot cycle time often renders
that impractical. Sampling strategies are derived based on the inspection
goal, the requested areas to be inspected, and the resulting throughput.
In some cases, requests from the process integration team can be fulfilled.
However, care areas and/or sample plans have to be readjusted most of
the time to achieve an acceptable throughput (1 to 1.5 hours per wafer)
while maintaining meaningful line monitoring. Meaningful line monitoring
includes finding defects of interest and their wafer signatures. Additionally,
over a period of time, Altis implemented software upgrades to improve
factory automation integration and flexibility during production.
Monitoring the line and identifying defect excursions reliably to stop
the right tool at the right time depends on a stable defect baseline.
The robustness of the inspection setup as well as the reliability and
stability of the inspection tool are critical to establishing a stable
baseline. To create a robust inspection regimen at Altis, setup parameters
were optimized to achieve a high capture rate for the defect of interest,
while an acceptable nuisance rate was maintained. Because of baseline
issues, critical tool parameters were monitored to ensure tool stability.
Weekly equipment preventive maintenance was modified to include the monitoring
of the beam current, aperture current, and spot size. As a result of these
measures, the baseline was stabilized and a procedure was put in place
for maintaining reliable trend charts.
Strategy. The quality and speed of defect classification significantly
affects the effectiveness of in-line monitoring inspection. The implementation
of iADC improved classification accuracy and maximized tool utilization
and throughput. Those improvements, in turn, guaranteed the availability
of faster, more-accurate information on killer defects of interest and,
therefore, better monitoring capability.
E-beam tool uses contrast binning to classify defects as either bright
or dark. While useful, this breakdown does not provide enough information
to classify defects on some layers. As shown in Figure
1, iADC can distinguish between cheezing, corrosion, dark, bright,
and physical defects such as surface particles, increasing the number
of classification categories from two to five or six. In addition, it
automatically transfers an image patch grabbed during inspection to the
yield management system, where it can be reviewed at any time. The system's
off-line review capacity is faster than on-line review, improving lot
cycle times and inspection tool capacity.
Inspection Paretos. Line-monitoring sampling is a function of
tool capacity and lot cycle times. Altis dedicated 79% of its EBI to line
monitoring and 21% to new products engineering. It focused 77% of inspections
on 0.18-µm copper technology, 16% on 0.13-µm copper technology,
and 7% on 0.22-µm aluminum technology. In the 0.18-µm area,
inspections focused on the post-tungsten CMP contact and metal layers,
as illustrated in Figure 2.
2: (a) Technologies and technology nodes, and (b) layers inspected
using the E-beam tool at Altis.
Yield Improvement and Excursion Control
implementation of EBI in-line monitoring has repeatedly reduced the time
necessary to identify and resolve critical yield issues by 3 to 6 weeks.
Three examples of how this was achieved are presented here.
Design. In a posttungsten CMP contact process used to manufacture
SRAM designs, EBI was first used in engineering mode to detect a critical
electrical defect. Failure analysis identified this defect, seen by the
tool as a dark contact, as an open contact. By overlaying EBI inspection
maps with maps from final wafer test, it was determined that 80% of the
open contacts were killer defects. The remaining 20% were physical nonkiller
defects such as surface particles. EBI, with its voltage contrast capability,
can flag such electrical defects in-line.
this step, yield-excursion monitoring was implemented, allowing the investigators
to rapidly identify the root cause of the excursion. Focused ion beam
(FIB) analysis revealed several root causes, including a contamination
issue that was later determined to be caused by an incomplete etch-chamber
clean and a photo/etch issue that was caused by micromasking on reticles.
Figure 3 presents SEM and
FIB images of the open contacts. Figure 4 shows the EBI defect-yield trend
chart for three contact etch chambers. Numerous excursions occurred over
the study period. In each case, the in-line EBI monitor enabled the fab
to detect the excursion and take corrective measures approximately six
weeks earlier than if they had relied on final test.
4: EBI defect-yield trend chart for three contact etch chambers. Yields
declined as a result of tool maintenance, micromasking on reticles,
and an incomplete etch-chamber clean.
Design. In another posttungsten contact CMP process, final product
test results indicated that EDRAM contact failures were much higher than
the reference SRAM contact failures for the same die. Altis used EBI to
verify these results. The failure rate was calculated as the total count
of dark defects divided by the total count of scanned contacts. EBI results
showed that EDRAM failure rates, on average, were 3.7 times the failure
rate of the SRAM design, confirming what was observed at final test.
fab then used EBI to analyze the contact failure rates of two other products.
On these products, the failure rates of the SRAMs were only 25 and 50%
of that on the reference SRAM. Based on this analysis, the fab decided
to install an EBI monitor for the EDRAM design. By using EBI monitoring
to achieve process improvements, the EDRAM failure rate, on average, was
reduced to 1.2 times the failure rate of the reference SRAM design. As
can be seen in Figure 5,
EBI results correlated with final wafer test results, and the new process
produced much more stable results than the old process. Once again, EBI
monitoring enabled the fab to realize yield improvements six weeks earlier
than if they had relied on final test to drive process improvements.
Design. In the third case, the fab identified a shortcoming in
its ability to monitor the logic metal 3 postliner CMP process. Both in-line
test and optical defect density monitors failed to detect a yield crisis
that was only visible at final wafer test. Failure analysis revealed the
source of the yield problem to be a liner residue that caused a metal
3 short. Figures 6a–6c show a top view, cross section, and postcopper
strip view of the defect.
the defect type was understood, it became clear why existing monitoring
had been ineffective. As can be seen in Figure
6, the liner defect, while visible in the SEM images, was transparent
when viewed optically. Hence, optical inspection (Figure 6d) had been
unable to detect the defect. The liner residue was also specific to the
product structure, so that it could not be detected through in-line tests.
In order to detect the defect in-line, an EBI monitor was required, resulting
in the image in Figure 6e. For monitoring efficiency, iADC was used to
filter and bin the defect types, and then SEM review was used to verify
the classification of the liner residue.
that methodology, Altis evaluated two process changes. The first, a modification
of overpolish time during postliner CMP, had little effect on the number
of liner shorts. The second, the use of a new slurry at postoxide CMP,
proved to be the solution, eliminating the liner shorts completely. In
this case, EBI monitoring resolved the issue three weeks faster than did
wafer final test.
has shown that EBI monitoring is key to effective yield improvement and
excursion monitoring, leading to tight process and tool control in its
copper logic volume-production line. Successful sampling strategies allowed
the company to compromise between the desire to inspect the full wafer
and lot cycle-time constraints.
case studies illustrated the benefits of using EBI, contrast binning,
and iADC to perform line monitoring. EBI was used to characterize and
monitor critical defects on product wafers in-line that otherwise would
have been detectable only at wafer final test. Detecting those defects
in-line enabled the company to take corrective actions sooner than would
have been the case had it relied on final wafer test results.
authors would like to thank several colleagues at Altis Semiconductor
for their involvement in the EBI studies and the implementation of EBI
line-monitoring procedures: Philippe Bertin, defect density engineer;
Sylvie Schon, technical leader; Jean-Luc Baltzinger, defect density engineer;
Matthias Bostelmann, yield engineering; Michele Mercier, failure analysis;
Jean-Yves Nots, yield engineering; and Francesco Castelli, equipment engineer.
Desmercieres is a defect density engineer at Altis Semiconductor's
manufacturing site in Corbeil-Essonnes, France. She has been with the
company since 2001. She received an MS in surface analysis technology
from the University of Orsay, Paris, in 1999. (Desmercieres can be reached
at +33 60 900207 or firstname.lastname@example.org.)
Roy is the defectivity and line-monitoring manager at Altis Semiconductor's
manufacturing site, a position he has held since 1999. He has filled several
leading positions at the facility for 20 years. In addition, he has been
in operations and engineering management at IBM, IBM/Siemens, and IBM/Toshiba.
He received an engineering degree from the Centre d'Etudes Supérieures
Industrielles in Paris in 1993. (Roy can be reached at +33 60 885226 or
Lefebvre, PhD, joined KLA-Tencor (San Jose) as an applications
engineer in 1998. He has worked in several defect inspection areas, including
bright-field and E-beam technology. He received a diploma from the Chemical
Engineering School in Lyon, France, in 1993 and a PhD in physical chemistry
from the University of North Carolina at Chapel Hill in 1997. (Lefebvre
can be reached at +65 9832 5402 or email@example.com.)
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