Nitridation of Ultrathin SiO₂ Layers in Metal-Ferroelectric-Insulator-Semiconductor Structures

Abstract

Ultrathin SiO₂ films of 7.5 nm thickness have been nitrided by Catalytic-Chemical Vapor Deposition (Cat-CVD) or by plasma nitridation and used as insulator (buffer) layers in metal-ferroelectric-insulator-semiconductor (MFIS) structures. That allows us to make the buffer layers as thin as possible while maintaining good barrier properties between ferroelectric and buffer layer while improving interface properties and the memory retention characteristic. The X-ray Photoelectron Spectroscopy (XPS) revealed a nitrogen peak only for SiO₂ treated using plasma treatment or Cat-CVD treatment at 1750°C, the peak not being detected if the treatment was performed at 1500°C. Nitrogen atom concentration in treated SiO₂ decreases gradually from the SiO₂ surface. Capacitance-voltage (C-V) and retention characteristics of the MFIS structures are clearly improved by using SiO₂ nitrided by plasma treatment or Cat-CVD. Memory window in C-V characteristics is 1.8 V for the MFIS structures with plasma-nitrided SiO₂, being twice as large as for samples with Cat-CVD-nitrided SiO₂. Memory retention is extended considerably by the nitridation and this process has been found to be very effective to improve interface properties of MFIS structure, enabling us to make the buffer layer thinner while maintaining a good barrier property between ferroelectric and buffer layer.

Keywords:

• MFIS structure
• SrBi₂Ta₂O₉ (SBT)
• plasma nitridation
• Catalytic (Cat)-CVD
• Rapid Thermal Annealing (RTA)