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Materials Science at Oregon State University

Novel TEM applications with precession electron diffraction: orientation imaging at nm scale, strain analysis, and solving structures ab-initio

Date: Thursday, Jan 25th
Presenter: Robert Stroud, NanoMEGAS USA

Abstract


Precession electron diffraction (PED) in TEM is an emerging technique, using electron diffraction patterns collection very close to kinematical condition (like in x-ray diffraction), useful in solving ab-initio crystal structures of nanocrystals.

Precession diffraction based applications have been developed for TEM based phase and orientation maps for nanocrystal (EBSD-SEM like). A TEM precession interface may perform a scanning through a sample area (typical area 5x5 µm2), collecting a large number of PED patterns which are compared one by one by cross-correlation techniques with a series of generated diffraction patterns (templates) of all possible orientations of known phases existing on the sample scanned area. Such orientation/phase maps may be produced very fast, making the technique highly attractive for high throughput TEM based orientation imaging analysis. Several application examples will be presented including metals, thin films, semicinductors, nanoparticles, and even organic crystals.

Measurement of strain with high spatial resolution and high precision in semiconductor devices is critical to monitor designed and unintended strain distributions. In this regard, strain mapping using nanobeam spot diffraction patterns in the transmission electron microscope is particularly interesting due to relative ease of its implementation compared to other TEM methods combined with a spatial resolution better than 5 nm on modern TEMs. In the past, strong dynamical effects in electron diffraction have limited the use of spot patterns for strain mapping. This particular challenge can be overcome by combining nanobeam diffraction with beam precession. With precession the incident beam is tilted and rotated at a high frequency, so the dynamical effects are effectively averaged out. Precession electron diffraction (PED) patterns are less sensitive to specimen thickness variations and local bending. Also, PED patterns have more spots which improves the sensitivity of strain measurements. The strain level is determined by comparing individual strained diffraction patterns with a reference pattern taken from an unstrained area. In recent work, we have shown the application of PED strain mapping on various structures including blanket Si/SiGe, PMOS, NMOS and a commercial processor. Another application of PED is on orientation and phase mapping of nanocrystalline materials. For PED orientation and phase mapping, individual PED spot diffraction patterns are compared with pre-calculated templates and the best matching template will be assigned to the diffraction pattern via a cross correlation algorithm.

TEM based 3-D diffraction tomography technique consists of a collection of a series of randomly oriented diffraction patterns, in precession mode of the same crystal through the whole TEM angular range, usually from -45º to +45º, at 1º angular intervals. The resulting 3-D PED set of reflections can be visualized as a 3-D picture of the reciprocal cell of the crystal enabling direct cell and structure determination from the measuring hkl reflection intensities. Many diverse structures have been solved using 3-D diffraction tomography, the last few years, dealing with nm size crystals of complex minerals, complex zeolites, MOFs, organic and pharmaceutical compounds. Other important application examples will be presented.