New results are presented for the annealing behavior of ultrathin complementary-metal-gate-semiconductor (CMOS) gate-dielectric (high-) 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 and grazing-incidence 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 X-ray reflectivity, are correlated with 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 layers of the film. Some 6 8 nm structures are also present that may indicate a widespread modulation in the film density that could pervade the entire film volume. 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 to reduce the mean interfacial diffuseness of the film. These changes are indicative of densification. However, depending on the film thickness, the annealing behavior at temperatures above 650 °C to 800 °C is quite different for extended or repeated high-temperature cycling. Particularly for thin, just-coalesced films, X-ray reflectivity indicates marked increases in the film thickness and in the mean diffuseness for repeated or extended high-temperature cycling. 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 on extended high-temperature exposure and cycling.
Citation: Journal of Applied Physics
Pub Type: Journals
Gate dielectrics, HfO2, atomic layer deposition, x-ray reflectivity, grazing incidence x-ray scattering, film growth, film microstructure, density