To understand the mechanism underlying the fast, reversible, phase transformation, information about the atomic structure and defects structures in phase change materials class is key. PCMs are investigated for many applications. These devices are chalcogenide based and use self heating to quickly switch between amorphous and crystalline phases, generating orders of magnitude differences in the electrical resistivity. The main challenges with PCMs have been the large power required to heat above crystallization or melting (for melt-quench amorphization) temperatures and limited reliability due to factors such as resistance drifts of the metastable phases, void formation and elemental segregation upon cycling. Characterization of devices and their unique switching behavior result in distinct material properties affected by the atomic arrangement in the respective phase. TEM is used to study the atomic structure of the metastable crystalline phase. The aim is to correlate the microstructure with results from electrical characterization, building on R vs T measurements on various thicknesses GST thin films.
To monitor phase changes in real-time as a function of temperature, thin films are deposited directly onto Protochips carriers. The Protochips heating holders provides controlled temperature changes while imaging in the TEM. These studies can provide insights into how changes occur in the various phase transformations even though the rate of temperature change is much slower than the PCM device operation. Other critical processes such as void formation, grain evolution and the cause of resistance drift can thereby be related to changes in structure and chemistry. Materials characterization is performed using Tecnai F30 and Titan ETEM microscopes, operating at 300kV. Both the microscopes can accept the same Protochips heating holders. The K2 direct electron detector camera equipped with the ETEM allows high-speed video recording (1600 f/s) of structural changes occurring in these materials upon heating and cooling. In this presentation, we will describe the effect of heating thin films of different thickness and composition, the changes in crystallinity and grain size, and how these changes correlate with changes in the electrical properties of the films. We will emphasize that it is always important to use low-dose and/or beam blanking techniques to distinguish changes induced by the beam from those due to the heating or introduction of an electric current.
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PCM Materials and Devices: High Speed and Low-dose TEM Imaging
Phase-change memory (PCM) materials are being developed for faster, non-volatile & high-density memory that can facilitate more efficient computation as well as data storage. The materials used for these PCM devices are usually chalcogenides that can be switched between their amorphous and crystalline phases thus producing orders of magnitude difference in the electrical resistivity [1, 2]. The operation of such devices is limited by elemental segregation and void formation, which occurs as a result of the extensive cycling. After crystallization, the structure gradually transforms from fcc to hexagonal. In the present work, we are studying these different phase changes in-situ as they occur in PCM materials basically using TEM imaging. The aim is to correlate device modeling and electrical
characterization in order to improve the models and enable accurate, predictive simulations. The thin film materials and devices can be directly deposited onto Protochips devices, allowing controlled temperature changes while imaging in the TEM. Although the temperature change rate achievable is too slow as compared to the fastest PCM-device operation, these rates can provides valuable insights into the various property changes in the material and phase transformations as well. Both a Cs-image corrected Titan ETEM and a Tecnai F30 have been used for this study. The ETEM is equipped with a K2 direct electron detector camera allowing high-speed video recording of these PCM materials.
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- Award ID(s):
- 1710468
- NSF-PAR ID:
- 10105349
- Date Published:
- Journal Name:
- International Microscopy Congress
- ISSN:
- 2178-728X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Understanding and possibly recovering from the failure mechanisms of phase change memories (PCMs) are critical to improving their cycle life. Extensive electrical testing and postfailure electron microscopy analysis have shown that stuck–set failure can be recovered. Here, self‐healing of novel confined PCM devices is directly shown by controlling the electromigration of the phase change material at the nanoscale. In contrast to the current mushroom PCM, the confined PCM has a metallic surfactant layer, which enables effective Joule heating to control the phase change material even in the presence of a large void. In situ transmission electron microscope movies show that the voltage polarity controls the direction of electromigration of the phase change material, which can be used to fill nanoscale voids that form during programing. Surprisingly, a single voltage pulse can induce dramatic migration of antimony (Sb) due to high current density in the PCM device. Based on the finding, self‐healing of a large void inside a confined PCM device with a metallic liner is demonstrated for the first time.
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