“Sub-Angstrom-Resolution Analysis With Electron Beam: Challenges and Prospects”
Se Ahn Song, Samsung Advanced Institute of Technology
With the advent of aberration corrector for electron optics and the adoption of monochromator for smaller energy spread of the electron beam, revolutionary new transmission electron microscopes fitted with such capability of spatial and energy resolution better than 1 angstrom and 0.2 eV at high electron energy up to 300 keV has become to be available in the last decade.
We have set up a probe Cs-corrected and monochromised scanning and transmission electron microscope (FEI Titan 80-300 S/TEM) and have used it since 2006 for materials research. Here we present some application to phase-change memory materials.
As one of the next-generation semiconductor memory candidates including FRAM and MRAM, PRAM (phase-change random access memory) has gained the greatest interest because of its potential in scalability, speed and nonvolatility. Samsung announced the development of 512Mb PRAM device in 2006 and plans to produce it soon. To develop such a memory device there have been a lot of challenges and barriers to overcome for microscopy analysts to probe the phase-change materials and to evaluate the fab-out devices with respect to its electrical performance. Imaging and analysis capability with very high resolution together with new advanced techniques such as FIB-TEM sampling, STEM/ EDS/ EELS, nanodiffraction RDF and electron tomography have been essential.
The rapid switching between SET(0) and RESET(1) in PRAM is based on the remarkably large change of resistivity between crystal (~kohm) and amorphous (~Mohm) phases of chalcogenide alloys such as Ge2Sb2Te5. This switching process is monitored by electrical measurements, meanwhile the need to visualize the switching in the cell by TEM has been enormous. We first reported the visualization of the programmed amorphised Ge2Sb2Te5 volume in the CMOS-based PRAM device at APEM 2004.
Ge2Sb2Te5 (GST) is used as a rapid phase change alloy in PRAM. The structural changes and the mechanism that underpin this rapid switch between the amorphous and the crystalline phase have been largely unknown. We obtained the reduced density function (RDF) of an amorphous thin film of GST from an energy filtered nano-area electron diffraction pattern and employed reverse Monte Carlo (RMC) modeling to extract the partial distribution functions (PDFs). Then we retrieved a plausible atomic structure of the amorphous Ge2Sb2Te5 phase. In-situ high resolution heating experimentation has been carried out to observe the phase transition process at atomic level in the transmission electron microscope in real time.
As the PRAM device size becomes smaller (down to sub-100nm cell size) and its geometrical and chemical structure becomes complicated, the characterization of the device materials and process becomes extremely difficult. High quality TEM sample preparation with specific nano-area positioning is increasingly important together with even, very thin and non-damaged sampling. Electron tomography approach in combination with such TEM sampling greatly enhances the failure analysis capability of the PRAM devices.State-of-the-art high resolution microscopy techniques, including the aberration correction enabling resolution better than 1 angstrom, are necessary for developing next-generation new memory devices. But there remain many challenges to overcome.