Annealing behavior of atomic layer deposited HfO₂ films studied by synchrotron x-ray reflectivity and grazing incidence small angle scattering

Abstract

New results are presented for the annealing behavior of ultrathin complementary-metal-oxide-semiconductor (CMOS) gate dielectric HfO2 films grown by atomic layer deposition (ALD). A series of ALD HfO2 dielectric films has been studied by a combination of x-ray reflectivity (XRR) and grazing-incidence small-angle x-ray scattering (GISAXS) measurements. By using these techniques together, we have shown that the surface, interfaces, and internal structure of thin ALD films can be characterized with unprecedented sensitivity. Changes in film thickness, film roughness, or diffuseness of the film/substrate interface as measured by XRR are correlated with the corresponding changes in the internal film nanostructure, as measured by GISAXS. Although the films are dense, an internal film structure is shown to exist, attributed primarily to ≈2 nm "missing island" porosity features close to the substrate; these are most likely associated with coalescence defects as a result of initial ALD growth, as they are not observed in the upper regions of the film. Some 8-9 nm heterogeneities are also present, which may indicate a widespread modulation in the film density pervading the entire film volume, and which likely also give rise to surface roughness. Comparison of the data between different scattering geometries and among a carefully designed sequence of samples has enabled important insights to be derived for the annealing behavior of the ALD HfO2 films. The main effects of single, brief, high temperature excursions to above 900 °C are to anneal out some of the fine voids and reduce the mean roughness and interfacial diffuseness of the film. These changes are indicative of densification. However, depending on the film thickness, the annealing behavior at temperatures between 650 and 800 °C is quite different for single excursion and cyclic anneals. Particularly for thin, just-coalesced films, XRR indicates marked increases in the film thickness and in the mean roughness/diffuseness dimension for cyclic anneals. GISAXS also shows an increase, rather than a reduction, in the void microstructure under these conditions. These changes in the film microstructure appear sufficient to overcome the expected film densification at elevated temperatures with implications for the gate dielectric performance of the films after extended high temperature exposure and cycling, as may occur during gate dielectric fabrication. © 2009 American Institute of Physics.