By Christophe
Maleville Process
Engineering
Manager, Soitec www.soitec.com
Monitoring defects using
low thresholds is key to
manufacturing yield. For
inspecting SOI wafers,
UV light overcomes the
limitations of visible light.
Here’s why.
With visible-light inspection tools, the
scattering behavior of defects on SOI
structures depends on silicon and oxide
thicknesses. Because of buried interfaces,
transmitted visible light is sent back to the
surface after coherent reflection, and can
interfere with incoming light. Phase shift
of the reflected beam is driven by silicon
and buried oxide thickness, resulting in
constructive or destructive interferences,
increasing or decreasing reflectivity
compared to a reference bulk-silicon
reflectivity. Scattering intensity on the
surface is proportional to the apparent
illumination, and depends on structure
thicknesses.
Therefore, the only way to implement
robust defectivity monitoring using visible
light sources is to generate calibration
curves for each product generation and
SOI thickness. But this is not sufficiently
aggressive for sub-90nm technologies.
Using UV light, on the other hand, the
transmitted light has to be absorbed
before interfering with incoming light at the
surface, resulting in constant reflectivity
regardless of SOI thickness. Bulk siliconlike
metrology can then be implemented on
SOI, without additional calibration work for
specific SOI films thickness combinations.
With UV defectivity inspection, SOI
behaves like bare silicon regardless of
the silicon and oxide layer thicknesses.
Aggressive thresholds are demonstrated,
closing the gaps with industry roadmaps.
SOI substrates can be inspected using
standard inspection strategies. Similar
yields, which have been reported for
microprocessor device processing on SOI
and bulk, can then be confirmed when
benchmarking similar defectivity levels on
SOI and epi material, using the same high
sensitivity recipe.
