Achieving process

DEFECT REDUCTION AND YIELD ENHANCEMENT STRATEGIESFOR SEMICONDUCTORS AND ADVANCED MICROELECTRONICS

understanding andreal-time fault detection
on a PVD tool
Suradej Promreuk, INFICON

T

he increasing demand for fewer
defects, higher throughput, and cost reductions in semiconductor processing
has sparked steady interest in advanced
process control (APC). Many companies

engineers to characterize previously unknown details of the process and prevented wafer misprocessing through active fault detection.
The heart of the APC system in thisinstallation was the FabGuard control package
from INFICON (East
Syracuse, NY). FabGuard collects data from
integral as well as addon sensors and analyzes
the data using advanced
statistical and modelingtechniques. By comparing an active process with a model developed from previous runs, the system
can detect excursions from acceptableprocessing, detecting faults with minimal false alarms. At the fab where thisstudy was conducted, the control package was installed on an Endura 5500
from Applied Materials (Santa Clara,CA) through the tool’s secondary SECS

A control system attached to a PVD tool can be used
to customize recipes during process development,
perform process monitoring to detect misprocessedwafers, and optimize PM cycles.
are evaluating APC’s potential to increase capacity while investing little
capital. This article focuses on how amajor Asian semiconductor facility increased its process understanding of aphysical vapor deposition (PVD) tool.
The study involved a control package
on a PVD tool that was equipped with
six sensors. This setup enabled company

TAKING CONTROL
cess parameter profiles fromone of the degas chambers. The
box on the top right of thescreen (“Select Bins to View”)
allows users to plot data from a
specific sensor and provides the
key to the data in the chart.
Items labeled “TDS” indicate
data coming from the tool controller. Figure 1 demonstrates
how the water peak (represented by the blue line) increased
approximately two decades
each time the loadlock doors
opened (represented by thedark green and dark brown vertical lines). Opening the cooling chamber slit had a similareffect (represented by the ocean
blue vertical lines).
Once the engineers hadcompleted the fingerprinting
process and established baseline values, they could begin to
Figure 1: Degas data collected from a 25-wafer production lot illustrate normal processcompare those values with subparameter profiles. The data demonstrate how the water peak (blue line) increased
sequent processing results.
approximately two decades each time the loadlock doors opened (dark green and dark
During the relatively short pebrown vertical lines).
riod in which this study was
conducted, they were able toport. The tool was equipped with two aluminum-silicon- distinguish between incoming wafer types and chambers,
copper metal deposition chambers, a preclean chamber, and reduce overall processing time, and detect and stop processtwo degas chambers. Each of these chambers had active resid- es when resist removal was incomplete.
ual gas analysis (RGA) sensors, and the preclean chamberalso had a particle monitor. After performing a series of wafer Experimental Results
runs to characterize the system, the engineers had a picture
The Effect of Wafer Substrate Type. One of the engineers’
first findings was that thermal performance differed between
runs. After several runs, it was apparent that that difference
was related to substrate type: epitaxial wafers demonstrated
significantly higher levels of thermal oscillation than nonepitaxial substrates under the same degas recipe conditions. Examining the data more closely, the engineers discovered thatthe epitaxial substrates experienced an overshoot of approximately 30°C while the nonepitaxial substrates experienced anovershoot of only 10°C, as shown in Figure 2. Although thethermal plots from both substrate types oscillated, they osof the process, which they could compare with process im- cillated at different frequencies and amplitudes. The epitaxprovements and which they could use as a baseline to detect ial substrates oscillated 8°–12°C every 20–25 seconds, while
process excursions.
the nonepitaxial substrates oscillated only about 1°C and at
a higher frequency.
Establishing Baseline Values
These detailed data enabled the engineers to investigate
the impact of the different substrates and customize the
To provide a baseline for controlling the PVD process, the temperature-control coefficients for each substrate type toengineers first took a fingerprint of each chamber. They ran achieve the highest yield. As a result of this investigation, theseveral production lots through each chamber and used the engineers created a new recipe for degassing epitaxial substrates.
control package to collect data from the tool sensors, RGA
Characterizing Chamber Differences. Using the control
sensors, and particle sensor. Figure 1 shows the normal pro- package, the engineers were able to compare the two degas

The system distinguishedbetween wafer types andchambers and stopped
processes when resist
removal was incomplete.

substrate or other preprocessing
differences, the engineers split a
lot and ran half through one
degas chamber and half throughthe other. With all set points thesame, the thermal response of
the two chambers differed significantly. This difference also affected the RGA data from thetwo chambers. The control package corroborated the differences
between the two chambers by
analyzing the ion currents formasses 55, 77, and 91.
The control package also allowed the engineers to investigate why the two chambersfunctioned differently. By analyzing the lamp current profiles
at the bottom of the chart (purple and green traces), it was determined that the lamps cycled
at different times throughout
Figure 2: The thermal response of epitaxial substrates differs from that of nonepitaxial the process.
substrates using the same degas recipe. The epitaxial substrates (oscillating profile)
Preventing Wafer Misprocessexperienced an overshoot of approximately 30°C, while the nonepitaxial substrates ing. Combined with RGA sen(straight profile) experienced an overshoot of only 10°C.
sors, the control package canidentify wafers that have undergone incomplete photoresist removal and prevent them from entering the PVD chamber. The
package looks for masses that
correspond to the organic compounds in photoresist. When oneof these compounds is identified,
CHAMBER E
the system activates an alarmand sends a stop-processing signal
CHAMBER F
to the tool controller.
To test the control package’s
ability to detect photoresist andtest for false alarms, the engineers conducted an experimentusing completely ashed wafers,
wafers that had undergone
90% photoresist removal, andwafers that had undergone only
50% photoresist removal. A
clean TEOS wafer was processed between each contaminated wafer run.
The engineers investigatedFigure 3: Comparison of data from two different degas chambers using the masses 15, 48, 77, and 91. Whensame recipe.
the signal intensity of mass 15
was >5 × 10–10, mass 48 was >8–11–11chambers. Figure 3 shows the thermal response from the two × 10 , mass 77 was >2 × 10 , and mass 91 was >2 × 10–11 A
degas chambers using the same recipe of 350° for 200 seconds. for five data points between 90.2 and 182.87 seconds into
To be certain they were examining chamber differences and not the run, the control package activated a yellow (moderate)

alarm. When the signal intensity of mass 15 was >1 × 10–9,
mass 48 was >2 × 10–10, mass 77
was >5 × 10–11, and mass 91 was>5 × 10–11 A for five data points
between 90.2 and 182.87 seconds into the run, the control
package activated a red (critical) alarm. These signal intensity limits were calculated fromthe product wafer data stored
in the database. The analysis
was activated between 90.2 and
182.87 seconds into the run because the wafer must be heatedlong enough so that the relatively large molecules present inphotoresist organics can be released from the wafer surface.
Figure 4 presents the datafrom three wafers in the experiment. The first run (represented by the data in the first thirdof the graph) involved a clean Figure 4: Comparison of the performance of three wafers. The first run involved a
TEOS wafer. Since the signal in- clean TEOS wafer, which did not activate an alarm. The second involved a 50% ashedtensity of three of the masses re- wafer, which activated a critical alarm. The third was a clean TEOS wafer, whichmained below the alarm limits, activated a moderate alarm because residual resist gases remained in the chamber.
no alarm was activated. The
second run (represented by thedata in the middle third of the graph) involved the 50% ashed
A common degas recipe used at the fab where this study
wafer. The control package activated the red alarm 109 sec- was conducted heated the wafer at 350°C for 200 seconds. To
onds into the degassing step, because the signal intensity of all maintain that temperature, represented by the red line in Figure 5, the lamp cycled on and off, as represented by the green
line. Excluding events caused when the loadlock and coolingchamber slit valves opened, the water and hydrogen profiles
were virtually flat beyond the initial 90 seconds of processing.
After determining that only 109 seconds were required todetect the presence of photoresist in the chamber and only 90
seconds were required to complete the degas step, the engineers decided that the full degas recipe of 200 seconds was exfour masses exceeded the red alarm limit. After the 50% ashed cessive. Consequently, the step was reduced to 120 seconds
wafer was processed, a clean TEOS wafer was run (repre- with no adverse affect—a 40% reduction in degas time that
sented by the data in the last third of the chart). Residual re- contributed to an overall throughput improvement.
sist gases in the chamber caused the signal intensity of all
Verifying Pumpdown Performance in the Preclean Chamfour masses to exceed the yellow alarm limit. These results in- ber. Monitoring pumpdown in any chamber can help to endicate that the control package detects cross-contamination sure that maintenance is performed at correct intervals. By
during normal processing.
monitoring for the presence of hydrogen, for example, engiDegas Process Optimization. Lamp power and time are neers can determine when cryopump performance begins to
the two critical parameters in a degas recipe. Both are relat- degrade, indicating the need to regenerate the pump.
ed to each other and should be optimized. Lamp power must
Figure 6 demonstrates that particle counts (represented by
be set high enough to heat the wafer to the target temperature, the green lines) fell slowly when pressure (represented by the
but it must not be set too high, since rapid heating can alter red line) decreased during the initial pumpdown. After thethe electrical characteristics of previously fabricated layers. initial pumpdown, the chamber was purged with nitrogen
Degas time must be long enough for moisture and hydrocar- for 15 cycles. Each time the nitrogen came on, there was a corbons to be desorbed from the wafer surface, but once mois- responding particle spike (center of the chart). After the niture and hydrocarbons have been desorbed, extra degas time trogen purge cycle was complete, however, particle counts
lowers tool throughput.
fell significantly.

The control system was used
to reduce the degas recipefrom 200 to 120 seconds
with no adverse effect.

TAKING CONTROL
11 wafers had particles. In fact,
one wafer had as many as 50.
After preventive maintenance,
particles were detected on only
9 wafers out of the lot, noneof which had more than 10
particles.
Optimizing PVD Bake-Out
Times. After preventive maintenance is performed on a PVD
chamber, the chamber must be
baked to expel the moistureand contaminants that collectin it while it is exposed to theatmosphere. Chamber bakeouts, including bake and cooldown cycles, typically last six
to eight hours. Any reduction
in bake-out times would significantly improve tool use.
Consequently, the engineers
decided to use the RGA datacollected from the PVD chamFigure 5: Data showing a common degas recipe, in which wafers are heated at 350°C for ber to investigate what hap200 seconds. The water (blue) and hydrogen (purple) profiles were virtually flat pened when the PVD chambers were baked for a specified
beyond the initial 90 seconds of processing.
length of time at a specified
temperature, and to determine
what could be done to reducebake-out times.
First, the engineers reduced
the sampling rate of the control
package to every two seconds inorder to compress the six-hourprocesses on the screen. Figure
7 shows the data from a sixhour bake-out at 100°C. Thevery slow decrease in the water
signal (represented by the blueline) indicates that the temperature was not high enough to
expel the water from the chamber. A six-hour bake-out at
100°C would still leave a significant amount of water in thechamber. However, additional
testing indicated that shorter
bake-outs at higher temperatures can remove most water
Figure 6: Data demonstrating that particle counts (green lines) fell slowly when from the chamber.
Second, the engineers invespressure (red line) decreased during the initial pumpdown. During nitrogen purge,
tigatedhow the cooling waterparticle counts increased intermittently (green spikes at center of chart). But after theaffected
the bake-out. Datanitrogen purge cycle was complete, particle counts fell significantly.
from the control package
The engineers also were able to investigate particle counts demonstrated that the chamber outgases more rapidly when
before and after preventive maintenance was performed. In the cooling water is left off during the bake-out process.
the 25-wafer lot processed before preventive maintenance, Additional experiments should make it possible to furtherreduce bake-out times, perhaps
to as little as 1.5 hours. Because
the control package can monitor residual gas profiles, it canstop the bake-out when the
profiles show that the chamberis qualified to specifications.
This monitoring helps to reduce preventive maintenancetimes, thereby improving machine uptime.

HEATER WAS SHUT OFF

Conclusion
The control package discussed in this article can be used
to perform a range of functions
in semiconductor fabrication.
During process development,
the package enabled engineers
to customize degas recipes toobtain the same temperature
profile for epitaxial and nonepitaxial wafers, as well as for dif- Figure 7: RGA data showing the results of a six-hour bake-out at 100°C. The slow
ferent devices and layers. These decrease in the water signal (blue line) indicates that the temperature was not highrecipes can be improved to pro- enough to expel the water from the chamber.
vide increased throughput
without risk of reduced wafer quality. The engineers also in- downtime without sacrificing equipment reliability. The inivestigated performance differences between different degas tial work presented in this article can lead to process enchambers. Because the temperature and duration of the degas hancements, effective equipment-troubleshooting techniques,
process can affect the physical nature of wafers, control strate- and cost reductions.
gies are required to understand and control these process
Suradej Promreuk is a semiconductor applivariables.
cations engineer, working on in situ sensor
In a production setting, the control package performed proautomation and fault detection systems forcess monitoring to detect misprocessed wafers before they could
cause cross-contamination. It may be possible to set alarms to
APC applications at INFICON (Watertown,
MA). He received a BS in chemical engineerdetect faults associated with specific product wafer types.
ing from the University of Wyoming inFurther tests were performed to optimize preventive maintenance cycles. Using control software, maintenance engi- Laramie and an MS in chemical engineering from Lamarneers can adjust schedules based on actual system perfor- University in Beaumont, TX. (Promreuk can be reached atmance and improve maintenance procedures to shorten 617/924-2112 or suradej.promreuk@inficon.com.)

Reprinted from MICRO, March 2002 • Copyright © 2002 Canon Communications LLC