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.
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
Related stories:
Findings a step toward making new optical materials
Chemical engineers have developed a "self-assembling" method that could lead to an inexpensive way of making diamondlike crystals to improve optical communications and other technologies.
Assembly technique for tiny wires may help detect cancer, other diseases
Bottom-up manufacturing may hold the key to production of tiny medical devices capable of testing for multiple molecules like viruses or cancer markers, according to an interdisciplinary team of Penn State researchers.
The solution to a 7-decade mystery is crystal-clear to FSU chemist
A Florida State University researcher has helped solve a scientific mystery that stumped chemists for nearly seven decades. In so doing, his team’s findings may lead to the development of more-powerful computer memories and lasers.
Speed Bumps Less Important Than Potholes for Graphene
For electrical charges racing through an atom-thick sheet of graphene, occasional hills and valleys are no big deal, but the potholes—single-atom defects in the crystal—they’re killers.
Scientists tailor light waves to desired frequencies
The ability to control light is vital for many of today’s technologies, most notably in communications and advanced computing. For example, by using materials from mirrors to nanoparticles, scientists can alter light’s speed with refraction, use diffraction to bend light, use electric fields to rotate light’s polarization, and more.
Atoms under the mantle
French CNRS scientists have succeeded in modelling the defects of the earth’s mantle responsible for its deformation. These results, obtained using a novel approach which combines numerical calculus and quantum mechanics, constitute the first step towards modelling deformation of this layer and its effects on the mantle. They are published in the March 1st, 2007 issue of
Nature.
Disorder May Be in Order for ‘Spintronic’ Devices
Physicists at JILA are using ultrashort pulses of laser light to reveal precisely why some electrons, like ballet dancers, hold their spin positions better than others—work that may help improve spintronic devices, which exploit the magnetism or “spin” of electrons in addition to or instead of their charge. One thing spinning electrons like, it turns out, is some disorder.
Self-Aligning Liquid Crystal Technique Could Simplify Manufacture of Display Devices
A new technique for creating vertical alignment among liquid crystal molecules could allow development of less-costly flexible displays and lead to a better understanding of the factors that govern operation of the popular liquid crystal display systems.
