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

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.