Studies of Oxide Film Growth and Oxide-Metal Processes Using in-situ and ex-situ Analytical Techniques
Scientific Achievement

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A set of related pulsed ion beam surface analysis methods, known collectively as time of flight ion scattering and recoil spectroscopy (TOF-ISARS) have been used in conjunction with complementary X-ray photolectron spectroscopy to perform in situ studies of oxide film growth and oxide-metal interface processes with nanoscale resolution. The integrated analytical techniques provide surface compositional and structural information on a time scale that is commensurate with thin film deposition and/or surface and interface processes such as oxidation. The techniques are compatible with the geometric constraints of the deposition process and the temperatures and ambient gas pressures required by physical-vapor deposition of thin films in relatively high-pressure environments and interface processing. We have used time-of-flight mass spectroscopy of recoil ions (TOF-MSRI) and X-ray photoelectron spectroscopy (XPS) as in situ probes to characterize complex oxide heterostructures. Specifically, we studied growth and free surface oxidation of oxygen diffusion barrier layers (TiAl, TiAlN) to understand oxidation processes when exposing these layers to high temperature (200-700 C) in oxygen atmospheres. The MSRI analysis involves detection of secondary ions ejected from the surface via a single collision from probing ions. The increase in secondary ions intensity correlates with the surface oxidation. MSRI analysis revealed that amorphous TiAl and TiAlN layer do not get oxidized during free surface oxygen annealing up to 600oC. XPS confirmed the MSRI results by revealing the existence of a metallic Ti peak in Ti-Al layer. In the case of TiAlN, XPS revealed the formation of TiO2 at 650-700oC by replacement of nitrogen chemically bonded to Ti by oxygen atoms. Additional studies were performed to understand the nature of conductive La0.5Sr0.5CoO9 (LSCO) oxide / TiAl or Ti AlN layers. Electrical characterization demonstrated that the LSCO / amorphous TiAl heterostructure exhibit ohmic behavior, while the LSCO / crystalline TiAl one exhibit non-ohmic behavior, mainly due to the formation of an Al2O3 interface layer between LSCO and TiAl, induced by grain boundary segregation of oxygen through the crystalline TiAl layer.

Significance

As thin film thickness and feature size in conventional semiconductor or novel hybrid oxide/semiconductor thin film-based devices decrease, new thin film materials, device structures, and complex processing conditions will be required. The TOF-ISARS technique described in this highlight has been applied to perform nanoscale studies of critical phenomena, associated with film growth and processing conditions critical to the fabrication of a new generation of advanced semiconductor and hybrid oxide/semiconductor thin film-based devices. The integrated TOF-ISARS/XPS system developed at ANL provided unique insights into fundamental film growth and interface processes that helped to develop new materials integration strategies for the fabrication of novel devices based on ferroelectric thin films integrated with Si substrates. The severity of materials integration problems in complex, multi-material electronic devices, and the significance of TOF-ISARS as a tool for understanding them has been acknowledged by the presentation of the R. A. Bunshah Award by the American Vacuum Society in 1994, and an R&D 100 Award in 1997. There is growing interested in using the unique capabilities of TOF-ISARS to develop materials integration strategies for the fabrication of the next generation of high density non-volatile ferroelectric random access memories (NVFRAMs) and high permittivity dynamic random access memories (DRAMs), TOF-ISARS has already provided critical information for the development of low density NVFRAMs, which are the heart of smart cards currently marketed by several companies worldwide.

Performers

O. Auciello, D.M. Gruen, J.A. Carlisle, A.M. Dhote (Postdoc), R. Ramesh, (University of Maryland).