MIL- STD-883F 2004 TEST METHOD STANDARD MICROCIRCUITS - 第611页
MIL-STD-883F METHOD 5004.11 18 June 2004 21 APPENDIX A ATTACHMENT 1 Ex ample 2 - Reliab ility Sc enario : Charact erizat ion of par tic les at a gate oxide precl ean operati on showed that the part icl es cont ribut ed b…
MIL-STD-883F
METHOD 5004.11
18 June 2004
20
APPENDIX A
ATTACHMENT 1
Example 1 - Quality Scenario
:
A defect characterization has been performed on an LPCVD operation. The primary defect mechanism was found to be
particles. These particles were quantified using a laser surface-scanning tool. The results show that the particles fell into
three size distributions: 1) <0.3 microns randomly distributed from wafer to wafer and within a wafer, 2) about 1.0 microns
with a higher density near the pump end of the deposition tube, and 3) greater than 6.0 microns that appeared heavily on
some wafers but did not appear at all on others. The defects in the 1.0 micron or less categories were found to be relatively
small, dark particles when viewed with an optical microscope. The larger particles (>6 microns) appeared as large, black
particles that appeared to be on the wafer surface. A compositional analysis of particles from the three distributions showed
that the first two types (<0.3 microns and about 1.0 microns) were composed of Si and O, essentially the same composition
as the deposited film. The large particles were composed of primarily Fe and Ni.
Wafers containing defects from the smaller size distributions were processed through the subsequent patterning
operations. The 1.0 micron particles were observed to have an affect on the subsequent pattern when they occurred
adjacent to the patterned lines. The <0.3 micron particles had no observable effect. Both defects were characterized using
optical microscopes and an automated pattern inspection system. After resist strip, the 1.0 micron particles were gone, with
only their effects on the patterning operation being visible. The <0.3 micron defects were still observable after resist strip.
After a subsequent LTO deposition, the <0.3 micron particles appeared to "telescope" in size to about 1 micron due to the
conformal nature of the LTO film. The "telescoped" particles had a noticeable effect on the next patterning operation.
Observation of both particle types using an SEM (scanning electron microscope) showed that the 1.0 micron particles
appeared to be incorporated into the film, whereas the <0.3 micron particles appeared to be under the film. This was
consistent with the defect behavior observed during subsequent processing.
The signal from the large particles suggested contamination from a stainless steel source. Observation of the defect with
an SEM showed that the defects were on top of the deposited film. The defects were found to be coming from the unload
arm of the LPCVD system. The unload arm was occasionally striking another piece of the load/unload assembly, generating
metal particles each time it did this.
The characterization of particle defects from this LPCVD operation resulted in the following monitoring plan: 1) The
alignment of the unload arm was found to be most affected by the preventive maintenance procedure performed on the
load/unload assembly once each week. As a result, a bare silicon particle monitor is run after each PM, before any product
wafers can be run on the system. The monitor is set to look for 6 micron and larger particles with the expectation that no
such particles should be present if the unloader is working properly. 2) The source of the 1.0 micron particles is unknown.
What is known is that these defects are always worse near the pump-end of the tube. As a result, the monitor for this
particle source is run at the pump end of the tube, with a door end monitor run simultaneously as a "control". Different
action limits exist for each monitor. 3) The small particles were found to be very difficult to monitor at the LPCVD operation
since they fell into the "noise" caused by limitations in the particle detection equipment. However, they are easily monitored
in a "look-back" fashion after the subsequent patterning operation using the automated pattern inspection system. As a
result, this defect is monitored at the post-patterning inspection step with action limits initiating feedback to the LPCVD
operation.
MIL-STD-883F
METHOD 5004.11
18 June 2004
21
APPENDIX A
ATTACHMENT 1
Example 2 - Reliability Scenario
:
Characterization of particles at a gate oxide preclean operation showed that the particles contributed by the operation tend
to be small (0.2 microns) and vary in concentration from 0.02 d/cm
2
to 0.8 d/cm
2
depending on how heavily the station is
utilized. Defect density increased as the number of wafers processed through the station increased.
Wafers from this operation were selected such that some of them had low defect densities (approximately 0.3 d/cm
2
) and
the remainder had high defect densities (approximately 0.8 d/cm
2
). These wafers were processed through the line and the
die from these wafers subjected to high voltage stress testing. The results of the tests were that the low and moderate
defect density groups showed levels of gate leakage consistent with the historical process baseline. The high defect density
die show gate leakage that was 3 times that of the historical baseline and resulted in barely acceptable failure rates.
As a result of this characterization, a particle monitor was implemented at gate oxide preclean with an upper limit of 0.6
d/cm
2
to allow some safety margin from the gate leakage problems seen at 0.8 d/cm
2
. However, due to resource limitations,
this monitor can only be run once every shift (approximately every 12 hours). It is likely that the movement of material in the
line will lead to the station occasionally exceeding its control limits between monitors. A second preclean station is
scheduled to be installed in about three months. This station will provide enough capacity to prevent wafer-volume related
out-of-control particle conditions at the gate preclean operation. In order to ensure that no material with bad gate oxide is
shipped during the interim period (before the new station comes on-line), a manufacturer imposed screen (high-voltage
stress test) is used on all material processed between a failing monitor and the last known good monitor at this operation.
In order to show that the screen is effective, particle monitors are processed through the station with every lot of wafers.
This test is done for a period of time sufficient to yield multiple lots at various defect densities. Die from each of these lots
are processed through the high-voltage screen. The results show that the screen is 100 percent effective at detecting the
lots with defect densities greater that 0.6 d/cm
2
. The results show a solid correlation between gate oxide preclean defect
densities and gate oxide leakage levels. The screen is then used to augment station particle level data and remains in place
until the second station is installed and qualified.
MIL-STD-883F
METHOD 5004.11
18 June 2004
22
APPENDIX A
ATTACHMENT 2
EXAMPLE OF DEFECT DETECTION FOR KEY PROCESS STEPS:
PROCESS STEP
PRODUCT MONITOR EQUIPMENT MONITOR RELIABILITY
MONITOR
Wafer start Incoming Si QA NA NA
EPI Laser surface particle Gas flow/pressure, NA
scan. chamber temp
Start Oxide Oxide thickness, laser Tube temp profile, Oxide integrity
surface particle scan. CV, thermocouple cal, test wafers
gas flows, tube particle
checks using laser surface
scan.
Patterning/Well UV light particle insp, Exposure dose, reticle/ LYA
Implant optical pattern insp, pellicle inspection,
e-test parametrics. stepper stage checks
implant dose processor
and voltage calibration,
DI water resistivity.
Particle checks of stepper,
implanter, coat/develop
tracks using laser surface
particle scan.
Active Region Alignment check, optical Exposure dose, reticle/ Oxide integrity
Patterning/Gate Oxide inspection, automated pellicle inspections, test wafers,
(no 2010 equiv.) pattern inspection, UV light stepper stage checks, tube comb/serpentine
and laser surface particle temp profile, CV thermo- test structures,
inspections, in-line SEM CD couple cal, gas flows, DI LYA.
measurement, e-test water resistivity. Particle
parametrics. checks of stepper, diffusion
tube, coat/develop tracks
using laser surface particle
scan.
Poly Dep/Patterning Alignment check, optical and Dep tube pump/vent speed, Comb/serpentine
automated pattern inspection, MFC calibration, gas flows, test structures,
laser surface particle pressures, temperature. buried contact
inspections, in-line SEM CD Expose dose, reticle/pellicle check, LYA.
measurement, e-test checks, stepper stage checks.
parametrics. DI water resistivity.
Particle checks on poly tube,
stepper and coat/develop
tracks using laser surface
particle scan.