원거리 플라즈마 증착법을 이용한 하프늄 옥사이드와 백금 결정의 플래쉬 메모리 적용에 대한 연구

Abstract

Nonvolatile memory devices employing semiconductor and metal nanocrystals have been extensively studied. Nanocrystal floating gate shows improved data retention and endurance characteristics compared to conventional flash memories and allows more aggressive scaling of the tunneling oxide. Among various candidates for the storage media, metal nanocrystals have many advantages over semiconductor nanocrystals such as Si and Ge. Firstly, the metal nanocrystals provide strong coupling with the conduction channel and a wide range of available work functions. In addition, there is a high density of states around the Fermi level. As a result, the effect of traps between the nanocrystals / oxide interface are negligible. Lastly, the metal nanocrystals have smaller energy perturbation due to carrier confinement compared to semiconductor nanocrystals. High-k dielectrics have been studied to replace SiO2 as a gate oxide in nanocrystal floating gate memories for further improvement of retention time and low voltage operation. Among many high-k dielectrics, HfO2 has been widely investigated due to its high dielectric constant, high density(9.68 g/cm3), large band gap(5.68 eV) and good thermal stability in contact with silicon. for these reasons, we deposited the HfO2 gate dielectric material by remote plasma atomic layer deposition(RPALD). RPALD is expected to minimize the impurities of film and damage of the substrate, increase the reactivity, and widen the process window. HfO2 films were deposited using TEMAH as the Hf precursor with Ar carrier gas. O2 plasma was used as a reactant gas and the thickness of HfO2 film was controlled by the number of process cycles. In this study, Pt nanocrystals deposited with an electron beam evaporator were used for floating gates in the flash memory structure due to its high work function and thermal stability. The formation of Pt nanocrystals was confirmed using field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). In addition, the electrical characteristics, such as high-frequency C-V plots and C-t measurements, were examined to investigate the memory window for flash memories and retention time, respectively.