Inspection
Figure 2. The donor incorporate NVD inspection and review in a way
wafer at left was that is analogous to the inspection of physical
contaminated by an defects today. One approach that enables inline
edge-handling detection of NVDs is the ChemetriQ system,
metrology tool. After which uses SPDI imaging to detect changes in
the wafer was flipped the work function and charge of materials to
and bonded to the detect NVDs
2
.
substrate, the metallic The SPDI method spins the wafer and holds
contamination a probe close to the wafer surface (Figure 1).
migrated, causing The probe detects changes in work function,
defects in the final which are the result of non-uniformities in
SOI layer that. After material composition on the wafer’s surface.
the metrology tool When the work function changes, the charge
was fixed (right), the coupling between the probe and wafer surface
yield issue also changes such that:
disappeared
iprobe = vCair(dVSPD/dx)
ring6 Process-induced charge, which can build where Cair is the capacitance of the air gap,
up on wafers during single wafer wet or v = relative velocity between the probe and
plasma cleans wafer, and VSPD is the surface potential
ring6 Carrier-induced outgassing, such as difference caused by changes in material
plasticizers and inorganic salts composition.
20
These changes are detected and mapped
NVDs are often sub-monolayer or electrical in over the wafer’s surface to form the ChemetriQ
www
nature, having a z-height of a single atom or image at the right. A simple mathematical
.eur molecule, and thus cannot be detected using integration then produces the integrated image
oasiasemiconductor
traditional optical inspection methods. Surface that shows regions of NVDs on the wafer.
Potential Difference Imaging (SPDI), however, is NVD inspection is important for all aspects
able to detect these NVDs—providing new of yield management in the fab and in the lab.
“eyes” into surface quality. NVDs must be captured in the fab to prevent
This article describes SPDI imaging and excursions that can affect line yield, and in the
Figure 3. SPDI images several case studies where SPDI-based NVD lab to assist with root cause analysis and process
.com
of the edge regions inspection helped improve processes and raise development.
of three wafers, yield. When NVDs are detected in the production
showing a progressive line, their locations can be exported to analytical
square4
Issue VI 2009
improvement in NVD for yield management tools such as TXRF or TOF-SIMS, which can then
defectivity from left With the increasing impact of NVDs on device provide a detailed summary of the materials
to right yield, yield management strategies must contained in the NVDs, much as traditional
optical inspection tools report locations of
defects that are then reviewed and classified
using SEM with EDX.
Now we will explore several examples of
NVDs impacting device yields, and the practical
implementation of NVD inspection to isolate
their source and eliminate the NVDs.
Contamination detection
At Soitec, a leading SOI wafer supplier, an NVD
issue was resolved using SPDI scans to isolate
the root cause
3
. In this case, an edge-grip
metrology system was being used to inspect
“donor” wafer quality. However, the metrology
tool itself was contaminating the wafers with
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