Type of Culminating Activity
Doctor of Philosophy in Electrical and Computer Engineering
Electrical and Computer Engineering
William B. Knowlton, Ph.D.
Hafnium oxide (HfO2) is replacing silicon dioxide (SiO2) as the gate dielectric in metal oxide semiconductor (MOS) structures driven mainly by need to reduce high leakage currents observed in sub-2nm SiO2. The high dielectric constant of HfO2 (~25) compared to SiO2 (3.9 bulk) allows a thicker HfO2 layer to be used in place of the thinner SiO2 layer thereby reducing the gate leakage current in MOS devices while maintaining the same capacitive coupling provided by the thinner SiO2. However, incorporating HfO2 into MOS devices produces a SiO2 interfacial layer between the Si substrate and HfO2 interface. The increased complexity of the multilayer dielectric gate stack and introduction of new materials requires knowledge of the carrier transport mechanisms for accurate modeling and process improvement.
A large temperature dependence of the leakage current in HfO2 gate dielectrics are observed compared to SiO2, indicating temperature dependent leakage current measurements maybe well suited to understand the transport mechanism of
HfO2-based gate dielectrics. The leakage currents are measured for two different titanium nitride (TiN) metal gate stacks composed of either 3nm or 5nm HfO2 on 1.1nm SiO2 interfacial layer over temperatures ranging from 6K to 400K. For gate biases that yield equivalent electron energy barriers for the 3nm and 5nm HfO2gate stacks, the 5nm stack shows orders of magnitude less current and an order of magnitude larger increase in the gate leakage current with respect to temperature from 5.6K to 400K.
Knowledge of the energy band structure is crucial in determining what carrier transport mechanisms are plausible in multilayer dielectric stacks. Important parameters, necessary for modeling different transport mechanisms, can be extracted from accurately constructed energy band diagrams such as electric fields and barrier heights. An existing program developed by the author is further modified to incorporate image charge effects, multilayer dielectrics, and transmission coefficient calculations for use in this study.
Results indicate that the widely used Poole-Frenkel and Schottky conduction mechanisms for HfO2 dielectrics can only explain a narrow electric field and temperature range and fail to explain the observed thickness dependence. Modeling the temperature dependence of 3nm and 5nm HfO2/1.1nm SiO2 n/pMOSFETs with a combination of a temperature independent term, variable range hopping conduction, and Arrhenius expression (e.g., nearest neighbor hopping) describes the entire measured temperature range (6K to 400K). Additionally, HfO2 defect densities can be extracted using the proposed model and provide densities in the range of ~1019 to ~1021 cm-3 eV-1, which correlate well with defect densities reported in the literature. Defects in the HfO2 are likely a result of oxygen vacancies.
Southwick, Richard G. III, "An Investigation of Carrier Transport in Hafnium Oxide/Silicon Dioxide MOS Gate Dielectric Stacks from 5.6-400K" (2010). Boise State University Theses and Dissertations. 149.