Application of Metastable Phase Diagrams to Silicate Thin Films for Alternative Gate Dielectrics

Using the concept of metastable phase diagrams, we discuss the microstructure evolution during annealing of amorphous ZrO₂–SiO₂ and HfO₂–SiO₂ thin films for gate dielectric applications. These systems are characterized by a low solid solubility, a liquid miscibility gap and a kinetic barrier to the formation of the complex, crystalline silicate. We show that phase partitioning is expected for most compositions. Compositions within the metastable extensions of the spinodal are unstable and will spontaneously unmix in the amorphous phase upon heating. Hafnia- or zirconia-rich phases will subsequently crystallize to form HfO₂ or ZrO₂. Most compositions outside the metastable extensions of the liquid phase miscibility gap must phase separate above the crystallization temperature by nucleation of crystalline HfO₂ or ZrO₂ out of an amorphous silica-rich matrix. We present calculations of the metastable extensions of the miscibility gap and spinodal. The calculations predict that SiO₂-rich compositions, investigated for gate dielectric applications, will show spinodal decomposition if they contain less than ∼90 mol% SiO₂ at the typical device processing temperature of 1000°C. Experimental studies of Hf-silicate films with three different compositions, between ∼40 and 80 mol% HfO₂ that lie inside and outside the miscibility gap, respectively, are presented. All three compositions show phase separation. Despite the different pathways of microstructure evolution, the final phase separated microstructures are similar. Experimental verification of the pathways that lead to these microstructures requires further studies.