Tantalum oxide thin films for high-k dielectric applications: Crystallization, anisotropy and three dimensional imaging.

摘要

High dielectric constant (high-k) materials have been drawing much attention for applications such as gate oxides for transistors and dynamic random access memory (DRAM) capacitors. Tantalum pentoxide (Ta2O5) is a possible replacement because of its relatively high dielectric constant and process compatibility.; The microstructure of Ta2O5 thin films, deposited on Si substrates by atomic layer deposition, was investigated primarily using transmission electron microscopy (TEM). The kinetics of the crystallization and evolution of a complicated subgrain structure were studied by in-situ TEM heating experiments carried out at nominal temperatures of 790°C, 820°C and 850°C. It was found that the crystallization behavior could be modeled by a standard kinetic approach, using the Avrami equation, and differences from conventional bulk behavior are discussed. The growth and overall crystallization activation energies extracted from the in-situ TEM results were 4.2 eV and 6.3 eV, respectively.; The tantalum oxide films used in this study were found to have two predominant crystallographic orientations. In order to evaluate the possible anisotropic properties of the two types of grains, micro-capacitors were fabricated on each type using a combination of a focused ion beam machine (FIB) and the TEM. Plan-view TEM samples were prepared to identify and locate grain orientations followed by electron beam assisted chemical vapor deposition of a Pt top electrode using the local deposition capability of the FIB. Electrical measurement of the micro-capacitors was carried out using a micromanipulator inside the FIB. The dielectric properties of the two types of grains were directly compared from the measurements.; A further application of FIB combined with TEM allows novel specimen preparation for electron tomography. Accordingly, we can address the three dimensional imaging of Ta2O5 thin films deposited on hemi-spherical grain silicon (HSG) structures. The post-shaped sample preparation method using a FIB and the full rotation holder were designed to achieve complete dataset acquisition. The results show that the reconstructed image from a complete dataset eliminates the artifacts caused by missing data wedges, demonstrating the successful application of electron tomography to the imaging of Ta 2O5 thin films on HSG.