Inspection
miniscule amounts of metallic residue on the
edge of the wafer (Figure 2). During the wafer
bonding process, the thermal steps cause the
metallic residue to spread—resulting in
numerous physical defects in the top SOI layer.
This resulted in many wafers being scrapped.
None of the available inspection tools were
able to detect a signature that correlated to the
issue. However, a few SPDI scans quickly
revealed the root cause to be the metrology
system. The metrology tool was taken out of
Issue VI 2009
production and repaired, and the issue was
square4
cleared. Today, the SPDI tool is used in volume wraparound defects, further scans following Figure 4. SPDI images
.com
production to ensure that there is no recurrence process improvements indicated substantial from bare wafers show
of this problem. defect reductions. organic contamination
Follow-up analysis using TOF-SIMS at the wafer edge,
TSV edge cleaning confirmed the presence of Cu in the regions which grows more
Backside and edge cleans are increasingly used highlighted by the SPDI scans, and confirmed severe and impacts
in advanced semiconductor processes. the absence of Cu in the interior regions where more of the wafer at
Performing these cleans is necessary to prevent the SPDI scans show no residue. The the higher slot
oasiasemiconductor
cross-contamination from backside chuck visualization afforded by the SPDI image allowed positions in the FOUP
.eur
contact, edge carrier contact, and immersion the nature of the contamination problem to be
www
lithography
4
. quickly identified, and led to a rapid and
In a three-way collaboration between effective optimization of the clean step.
21
STMicroelectronics, CEA/Leti and Qcept
Technologies, the SPDI method was studied as a Carrier induced outgassing
tool to optimize backside and edge cleans by detection
minimizing such issues as extensive wrap-around Another area where SPDI scans were able to
and residual edge defectivity, which can result in provide unique insight into edge and near-edge
cross-contamination issues at subsequent defectivity was at Qimonda Dresden, where
process steps. carrier induced outgassing was characterized
The work focused primarily on the Under- directly for the first time
6
. The transport of
Bump Metallization (UBM) backside/edge clean silicon wafers in carriers such as FOUPs has been
step
5
. The first goal of the clean step is the known to be a possible contamination source for
removal of UBM elements from the backside, wafers
7
. Until now, however, this contamination
bevel, and near-edge regions to prevent could not be detected except through indirect
migration of Au, Cu, Ni and Ti into the wafer destructive studies where the contamination is
bulk, where it can degrade device performance made to crystallize so that it can be detected
and become a source of wafer-to-wafer cross optically.
contamination via handling parts. This removal With the SPDI technique (Figure 4),
must occur without impacting the desired Cu Qimonda was able to detect the build-up of
layer or undercutting the Au material above it. organic contamination on the wafers directly.
In addition, while some wraparound is necessary The left image shows contamination of the wafer
to remove residues from the near-edge and edge after long-term transportation in a FOUP.
bevel regions, too much wraparound can cause Using subsequent TOF-SIMS analysis, the NVD
residue to remain on the top surface of the signature was identified as being caused by a
active regions of the wafer, which risks damage polymeric plasticizer used in the FOUP’s
to the pattern. manufacture.
The SPDI method scanned the edge regions The wafer gallery on the right side shows
of three wafers, which were processed using increasing organic contamination of incoming
various concentrations of a dilute acid solution in wafer material. The wafers in the highest FOUP
a single wafer clean tool, followed by a slots (#22 to 25), with the shortest distance to
deionized water rinse (Figure 3). After the initial the plastic walls, exhibit the strongest
SPDI scan detected significant numbers of outgassing signatures.
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44