Structural and Surface Potential Characterization of Annealed HfO2 and (HfO2)x(SiO2)1-x Films

Copyright © (2004) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

The evolution on a microscopic scale of the surface morphology and contact potential differences (CPD) with high temperature anneals in ultra high vacuum to beyond the crystallization temperatures is reported for amorphous, as-grown 3 nm thick HfO2 and 2.2 nm thick Hf0.78Si0.22O2 layers on Si(100). The films were grown ex situ by atomic layer deposition and metal organic vapor deposition, respectively. A non-contact atomic force microscope operating in the electrostatic force mode was used to image the topography, surface potential and differential capacitance. The as-grown and annealed films essentially retained their smoothness even after undergoing crystallization; rms roughness of ~0.13 nm for HfO2 and 0.077 nm for the 900 degrees C annealed Hf0.78Si0.22O2 layer were measured. These values compare favorably with state-of-the-art RTO and nitrided SiO2 gate oxides. CPD fluctuations of up to 0.3-0.4 V were measured for 200x200 nm 2 images, values that did not change appreciably with annealing. A lack of correlation between topographic and CPD image features for the as-grown amorphous samples changed dramatically once the films crystallized, with higher CPD values associated with grain boundaries for both oxide and silicate layers. CPD variations were about a factor of two larger than for SiO2 gate oxides. Differential capacitance images reflected mainly topographic surface features, as the high-k inhibits image contrast in the images for small to moderate changes in k. . Nevertheless, for the Hf0.78Si0.22O2 sample annealed at 900 degrees C, which exhibited the lowest roughness, increases in differential capacitance could be attributed to microstructures of high-k material, most likely HfO2, which phase separated during the anneal. Because of screening, the high k dielectric also tends to suppress oxide charge contributions to the CPD image. A spherical tip model is presented that supports these observations.

By: R. Ludeke, E. Gusev

Published in: Journal of Applied Physics, volume 96, (no 4), pages 2365-73 in 2004


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