HfSiO/SiO2- and SiO2/HfSiO/SiO2-gate stacks for non-volatile memories

The use of hafnium silicate (HfSiO) as a charge trapping layer in charge-based non-volatile memory cells will be discussed. First, fundamental properties (e.g., dielectric constant, trap depths/levels) of the hafnium silicate layers, deposited from a single-source precursor, are deduced from capacitance-voltage and current density-voltage measurements. The oxide trap density of the analyzed HfSiO layers can be tuned to exceed that of silicon nitride. At the same time, a significant reduction of the write voltage is achieved due to a reduced effective oxide thickness. The erase operation, however, is hampered by the lower electric field at the HfSiO layer due to its high dielectric constant. Measurements also indicate that HfSiO exposed to a higher thermal budget during device fabrication results in fewer trapping centers. Retention measurements show that information can be reliably stored in memory cells with a trapping layer of HfSiO for more than 10 years similar to their silicon nitride counterparts. But the thickness of the top and bottom oxides must be increased for compensation of additional charge losses which are due to lower trap depth and free electron mass in HfSiO.