Stress Management: X-Rays Reveal Si Thin-Film Defects

Jul 06, 2006

Pile-ups, bad on the freeway, also are a hazard for the makers of high-performance strained-silicon semiconductor devices. A sensitive X-ray diffraction imaging technique developed by researchers at the National Institute of Standards and Technology can help manufacturers avoid the latter—a bunching up of crystal defects caused by the manufacturing process for strained-silicon films.

Strained silicon is a new, rapidly developing material for building enhanced-performance silicon-based transistors. Introducing a slight tensile strain in the lattice of the silicon crystal dramatically improves the mobility of charges in the crystal, enabling faster, higher-performance devices.

The strain is achieved by first growing a relatively thick crystalline layer of silicon-germanium (SiGe) on the normal silicon substrate wafer, and then growing a thin film of pure silicon on top.

Stress Management: X-Rays Reveal Si Thin-Film Defects
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. Credit: NIST

The difference in lattice spacing between pure silicon and SiGe creates the desired strain, but also creates occasional defects in the crystal that degrade performance. The problem is particularly bad when the defects cluster together in so-called “pile-ups.”

One of the best methods for studying crystal defects is to observe the image of X-rays diffracted from the crystal planes, a technique called X-ray topography. Until now, however, it’s been impossible to study the interaction of defects in the multiple layers of these complex Si – SiGe – Si wafers. In a recent paper in Applied Physics Letters, researchers from NIST and AmberWave Systems Corporation (Salem, N.H.) detail a high-resolution form of X-ray topography that can distinguish individual crystal defects layer by layer. The technique combines an extremely low-angle incident X-ray beam (“glancing incidence”) to increase the signal from one layer over another and the use of highly monochromatic X-rays tuned to separate the contributions from each layer based on their different lattice spacings.

Their results show that crystal defects initially created at the interface between the silicon wafer and the SiGe layer become “templates” that 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.

The research was performed at Argonne National Laboratory’s Advanced Photon Source, and supported in part by the Department of Energy.

Citation: D.R. Black, J.C. Woicik, M. Erdtmann and T.A. Langdo. Imaging defects in strained-silicon thin films by glancing-incidence x-ray topography. Applied Physics Letters 88, 224102. Published online June 2, 2006.

Source: NIST

Explore further: Pseudoparticles travel through photoactive material

Related Stories

Recommended for you

Pseudoparticles travel through photoactive material

Apr 23, 2015

Researchers of Karlsruhe Institute of Technology (KIT) have unveiled an important step in the conversion of light into storable energy: Together with scientists of the Fritz Haber Institute in Berlin and ...

From metal to insulator and back again

Apr 22, 2015

New work from Carnegie's Russell Hemley and Ivan Naumov hones in on the physics underlying the recently discovered fact that some metals stop being metallic under pressure. Their work is published in Physical Re ...

Electron spin brings order to high entropy alloys

Apr 22, 2015

Researchers from North Carolina State University have discovered that electron spin brings a previously unknown degree of order to the high entropy alloy nickel iron chromium cobalt (NiFeCrCo) - and may play ...

Expanding the reach of metallic glass

Apr 22, 2015

Metallic glass, a class of materials that offers both pliability and strength, is poised for a friendly takeover of the chemical landscape.

Electrons move like light in three-dimensional solid

Apr 22, 2015

Electrons were observed to travel in a solid at an unusually high velocity, which remained the same independent of the electron energy. This anomalous light-like behavior is found in special two-dimensional ...

Quantum model helps solve mysteries of water

Apr 20, 2015

Water is one of the most common and extensively studied substances on earth. It is vital for all known forms of life but its unique behaviour has yet to be explained in terms of the properties of individual ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.