Microstructural engineering of metal oxide thin films deposited by remote plasma atomic layer deposition

Abstract

The functioning of our modern world depends on our effort to optimize and modify the physical, chemical, and electrical properties of a wide class of functional materials. These material properties are often strongly reliant on the nature, concentration, and arrangement of defects such as grain boundaries, impurities, and vacancies. Today, the thin films technology is indispensable to the many application areas such as semiconductor device, protective coating, photo-catalyst, and energy storage. Thus optimization and modification of thin film process is essential to satisfy the materials properties required in each application area. Of the many possible deposition methods for thin film, atomic layer deposition (ALD) has been introduced as the method to form the conformal thin films on the flat or 3 dimensional structure. The famous materials deposited by ALD process are Al2O3, HfO2, TiO2, ZrO2, and ZnO. In this study, remote plasma was chosen as high reactive reactant for ALD process. Remote plasma atomic layer deposition was developed to improve the chemical reactivity by plasma and minimize plasma induced substrate damage during the thin film process. In this dissertation, metal-oxide thin films (Al2O3, TiO2) were deposited by RPALD method, and the process parameters such as plasma density and deposition temperature in RPALD were changed to improve and optimize the required properties of thin films in each application areas. For the first work in this dissertation, we investigated the interface stability and memory properties of an Al2O3 blocking oxide deposited by RPALD with various RF powers. The plasma density was increased with an increment of RF power from 50 to 300 W. The Al2O3 films deposited at high RF power has the improved interface properties between Al2O3 and Si3N4, and this result resulted in excellent program/erase performance of MANOS-type flash memory. Moreover, we also applied the Al2O3 film deposited at high RF power as tunnel and blocking oxides in charge trap flash memory using Au nanocrystals. It also exhibited the excellent interfacial properties and memory characteristics such as large memory window and P/E speed. For the next work in this dissertation, in order to apply Al2O3 film as a passivation layer or a diffusion barrier for low temperature fabrication processes or organic applications, the Al2O3 films were deposited by a RPALD at room temperature (25 °C). The RF power for generating oxygen plasma was increased to produce enough radicals for room temperature Al2O3 deposition. Then, we observed the changes in the interfacial and bulk properties of Al2O3 film according to increasing RF power. The interfacial properties of Al2O3/Si improved with increasing plasma density. However, the bulk properties of Al2O3 film deposited above RF power of a certain standard were similar to each other. The final work of dissertation focused on understanding the changes of grain size and crystal structure in titanium oxide thin film deposited by RPALD. The grain size of anatase TiO2 phase was decreased with increasing deposition temperature. Also, the grain size of anatase TiO2 affected the transition temperature to rutile phase. The anatase TiO2 phase with large grains exhibited the increased transition temperature to rutile due to that small grain boundary volume provided low density of nucleation sites for rutile. Moreover, the photo-catalytic activities of TiO2 thin films grown by RPALD were studied as a function of grain size and crystal structure. Because the large grain boundary volume is a mainly cause of degradation in photo-catalytic activity, the photo-catalytic activity of TiO2 thin film was maximized in case of anatase TiO2 phase with large grains.