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Strained silicon propagates thin-film defects |
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Jul 10, 2006 at 05:48 PM |
 The US National Institute of Standards and Technology (NIST) and
AmberWave Systems have jointly developed an x-ray diffraction technique
to help in understanding strained silicon defects (Black et al, Applied
Physics Letters 88, 224102, published online June 2, 2006). Strained
silicon is used at the leading edge to speed up electrons and boost
device performance. However, strain deforms the crystal lattice and one
associated problem is a bunching up of crystal defects caused by the
manufacturing process for strained-silicon films, killing hoped-for
speed improvements.
The researchers detail a high-resolution form of x-ray topography that
can distinguish individual crystal defects layer by layer on thin
strained silicon (less than 50nm). The technique combines an extremely
low-angle incident x-ray beam (glancing incidence) to increase the
signal from one layer over another and highly monochromatic x-rays
tuned to separate the contributions from each layer based on their
different lattice spacing. The (311) diffraction planes were used to
limit penetration into the sample to as little as 6nm, allowing
separation of layer images.
One technique to strain the lattice is to first grow a relatively thick
crystalline layer of silicon-germanium (SiGe) on the normal silicon
substrate wafer and then grow a thin film of pure silicon. The
difference in lattice spacing between pure silicon and SiGe creates the
strain, but also creates occasional defects in the crystal that degrade
performance. The problem is particularly bad when defects cluster
together in "pile-ups".
The results show that crystal defects initially created at the
interface between the silicon wafer and the SiGe layer propagate
through that layer and create matching defects in the strained-silicon
top layer. These defects, in turn, are notably persistent; remaining in
the strained-silicon even through later processing that includes
stripping the layer off, bonding it to an oxidized silicon wafer, and
annealing it to create strained-silicon-on-insulator (SSOI) substrates.
Although x-ray topography has been a popular technique in studying
crystal defects, up to now it has not been able to study the
interaction of defects in multiple layers such as occur in complex
Si/SiGe/Si wafer formations. The research was performed at the Argonne
US National Laboratory's Advanced Photon Source, and was supported in
part by the US Department of Energy.
Picture: X-ray topographs of three different strata of a
strained-silicon wafer show close correspondence in defects from the
base silicon layer (top) through the final strained-silicon layer
(bottom). Color has been added for contrast, one particular defect area
is highlighted. Courtesy US National Institute of Standards and
Technology.
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