Structural and surface potential characterization of annealed HfO2 and (HfO2)x(SiO2)1-x films

Abstract

The evolution of the microscopic electrical properties of hafnium oxide and hafnium silicate films with high temperature anneals and their relationship to morphological changes is reported. The as-grown, amorphous 3nm thick HfO2 and 2.2nm thick Hf0.78Si0.22O2 layers were deposited ex situ on Si(100) by atomic layer deposition and metal organic vapor deposition, respectively, and annealed in situ in ultra high vacuum. A noncontact atomic force microscope operating in the electrostatic force mode was used to image the topography, contact potential difference (CPD), and differential capacitance. The as-grown and annealed films essentially retained their smoothness even after undergoing crystallization; rms roughness of ∼0.13nm for HfO2 and 0.077nm for the 900°C annealed Hf0.78Si0.22O2 layer were measured. These values compare favorably with state-of-the-art rapid thermal oxidation and nitrided SiO2 gate oxides. CPD or surface potential fluctuations of up to 0.3–0.4V were observed in images of area 200×200nm2; 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 2 larger than for SiO2 gate oxides. Differential capacitance images reflected mainly topographic surface features, as the high κ inhibits image contrast in the images for small to moderate changes in κ. Nevertheless, for the Hf0.78Si0.22O2 sample annealed at 900°C, which exhibited the lowest roughness, increase in differential capacitance could be attributed to microstructures of high-κ material, most likely HfO2, which phase separated during the anneal. Because of screening, the high κ dielectric also tends to suppress contributions of isolated charges to the CPD image. A spherical tip model is presented that supports these observations.