HfO2/metal stacks: determination of energy level diagram, work functions & their dependence on metal deposition

We report the following results for metal/HfO₂/oxide stacks, (i) The energy level band diagram of HfO₂/SiO₂ stacks is experimentally determined for the first time; the conduction band offset between HfO₂ and interfacial SiO₂ is estimated to be 2.05 eV. (ii) Work functions of W, Re and TaSiN are measured for HfO₂/SiO₂/Si and SiO₂/Si stacks: work functions exhibit no Fermi pinning effect in HfO₂, unlike previous report by Schaeffer et al in 2004. (iii) The impact of metal gate deposition on its work function and the oxide charge density is investigated. Measurements show that the tungsten work function is independent of deposition time and method (CVD vs. sputtering). However, oxide charge density (Q_0X) depends both on the deposition time and method: Q_0X is positively charged for CVD and negatively charged for sputtered depositions. Also, Q_0X increases with W deposition time.