MRS Spring 2009: Working on HK+MG, solar cell efficiency, flash replacements

Tony Chiang, chief technology officer, Intermolecular

The 2009 Materials Research Society Spring meeting (April 13-17, San Francisco) 42 different symposia topics covering electronics, optics, energy, as well as bio-technology. Symposium A, “Amorphous and Polycrystalline Thin-Film Silicon Science,” had a strong emphasis on solar cell technology. Efficiently collecting light by minimizing reflection losses is an important aspect of building more efficient solar cells. One session focused on theoretical and experimental approaches to understand how texturing top and bottom interfaces improve solar cell efficiency. Papers by Li et al., Hasse et al., and Sai et al. showed the potential of efficient light trapping for improving solar cell efficiencies. Theoretical optical models provide important insight, but the best experimental performance lies in a regime where theoretical modeling is most difficult and less predictive. Thus, there is a significant need to fill this gap by collecting extensive experimental data potentially through employing combinatorial technologies that will not only help in increasing solar cell efficiency but also supplement theoretical models.

Material and integration challenges for CMOS gate-stacks were addressed in Symposium C, “CMOS Gate-Stack Scaling: Materials, Interfaces, and Reliability Implications.” J. Robertson from Cambridge University provided key insights on work function considerations for high-k/metal gate (HK+MG) development. Reviewing Fermi level pinning and the effect of capping layers, he noted that intrinsic pinning in pFET is a function of temperature, and provided an ab-initio model for flatband voltage shift observed with different capping layers ranging from La2O3 to Al2O3. Advancements in the use of ab-initio quantum mechanical (QM) calculations and density function theory (DFT) were apparent. Several authors applied these techniques to study interface and bulk states as well as the role of dopants in the substrate and dielectric.

High-throughput combinatorial methodologies have become a powerful means to accelerate the pace of research and development for a wide variety of complex material systems and devices. Symposium G, “High-Throughput Synthesis and Measurement Methods for Rapid Optimization and Discovery of Advanced Materials,” had papers that discussed high-throughput processing, high-throughput characterization, as well as advanced informatics for combinatorial materials research. Intermolecular had two talks about applying combinatorial methodologies to semiconductor R&D with site-isolated experiments.

With traditional floating gate-based flash memory approaching scaling limits, Symposium H, “Materials & Physics of Non-volatile Memories,” covered many of the technologies considered as potential replacements. Resistive memory (ReRAM), based on state change of metal oxides (e.g., NiO, TiO2, HfO2, CoOx), solid electrolytes (e.g., doped perovskites, AgGeSe), or carbon-based polymers is another area of non-volatile memory which is attracting a lot of recent attention. Because these systems can better withstand higher temperature processes than low melting point chalcogenide materials used in phase change memory, ReRAM is an attractive option for integrating into 3D-stackable architecture. However, one of the challenges with this technology has been need for better understanding of the switching mechanisms, and also achieving unipolar (same polarity) switching for ease of integration. Samsung & Seoul National University in a joint paper, modeled the ON –> OFF unipolar switching behavior based on Joule heating, for a NiO ReRAM system.

For phase-change memory, the focus was on improving integration issues to make this technology more manufacturable. Papers from Samsung, Hynix, Numonyx, and Ovonyx looked at MOCVD/ALD-based processes for depositing the chalcogenide materials in smaller cell structures with topography. With existing NAND flash facing scaling and reliability limits, it is very likely that one of the above alternate non-volatile memory technologies will soon emerge as the leading candidate, though no clear winner seems to be evident yet.

Tony Chiang is chief technology officer at Intermolecular, responsible for the company’s technology strategy and direction. Prior to joining Intermolecular in 2005, he founded Angstron Systems Inc., a venture-backed ALD start-up that was acquired by Novellus Systems, and subsequently served as Novellus’ manager of productization of the iALD technology for advanced copper-barrier and seed applications. Before that he developed several generations of advanced-ionized and self-ionized physical vapor deposition (PVD) technologies at Applied Materials. Dr. Chiang holds 52 patents in the field of advanced semiconductor processing technologies.


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