Improved Flash Memory Characteristics with High Mobility C-Axis Aligned Crystal IGZO (CAAC-IGZO) and High-k Thin Film

Abstract

ABSTRACT

NAND flash memory is a nonvolatile memory that can store data even when power is not supplied. Among various flash memories, a high-k thin film is used in the charge trap layer instead of the conventional SONOS structure to enable deep trapping and a large band-offset with the tunneling layer. We fabricated the flash memory device with Al2O3/HfO2/Al2O3 dielectric stack using high-k materials as blocking layer (BL), charge trap layer (CTL) and tunneling layer (TL). The dielectric stack was deposited on each layer using an atomic layer deposition (ALD) process. Both the amorphous (a-IGZO) and c-axis aligned crystalline IGZO (CAAC-IGZO) channels were deposited using a sputter. The channel was post-deposition annealed (PDA) at 300 ℃ for 1 hour. In addition, the aluminum (Al) source/drain (S/D) and tantalum (Ta) capping layer of CAAC-IGZO were also deposited by sputtering. It was analyzed by the X-ray diffraction (XRD) to confirm the crystallization of the CAAC-IGZO channel. In order to analyze the cross section of the device, the atomic composition was confirmed by the transmission electron microscope (TEM) analysis and the energy dispersive spectrometer (EDS) mapping. The field effect mobility (µFE) was improved through the CAAC-IGZO channel thin film transistor (TFT). The program/erase (P/E) characteristics of the device showed that the CAAC-IGZO (ΔVTH = 1.0 V) channel device has larger than the a-IGZO (ΔVTH = 0.5 V). The retention characteristics of the device were measured up to 104 s, and the endurance characteristics were measured up to 103 cycles. Applying the CAAC-IGZO channel material may solve many problems of the conventional poly-Si channel, which can contribute to the development of V-NAND technology in the future.