A Study on Defect and Strain Engineering to Improve Ferroelectric Properties of TiN/Hf0.5Zr0.5O2/TiN Devices

Abstract

With the advent of the fourth industrial revolution, an explosive increase in the Internet of Things (IoT) devices is expected. The amount of data to be handled in real time has also increased exponentially, and high-speed data processing is required. In addition, most IoT devices require a low-power operation and high-speed memory semiconductor devices under low-power has become more critical than ever. DRAM and NAND flash memory are the most representative of semiconductor memory and storage devices; however, have difficulty in technology development owing to their density limitations. Although NAND flash is an extremely promising non-volatile memory device capable of implementing a high degree of integration, its high drive voltage and power are undesirable for mobile devices and sensors that require low power consumption. In addition, the approximately three orders of magnitude higher latency of NAND flash than that of DRAM can lead to a degradation in the overall system performance. Therefore, storage-class memory (SCM) is introduced to mitigate the latency gap between NAND flash and DRAM. Ferroelectric random access memory (FeRAM) is a promising SCM device owing to its fast operation speed (~ns), complementary metal-oxide-semiconductor (CMOS) compatibility, scalability, high endurance, and the same architecture as DRAM (one transistor-one capacitor memory cell). Since the introduction of HfO2- based ferroelectric thin films, intensive studies have been conducted to utilize HfO2-based thin films for FeRAM. In particular, Hf0.5Zr0.5O2 (HZO) thin films are the most desirable materials for ferroelectric devices with strong ferroelectricity. This dissertation focuses on understanding the critical parameters affecting the ferroelectricity of an HZO thin film and utilizing the parameters for achieving strong ferroelectric properties of such film. Two methods are introduced to improve the performance of TiN/HZO/TiN devices. First, the relationship between the thermal expansion coefficient (TEC) values of the capping electrode materials and ferroelectric phase formation through a re-capping process is demonstrated. Tungsten (W), which has a relatively low TEC, is used as a capping electrode instead of titanium nitride (TiN) during rapid thermal annealing (RTA). The W-capped capacitor exhibits improved ferroelectric properties and reduced leakage current. In addition, the interface regions between the top electrodes and HZO film are investigated in detail. Second, a high-pressure oxygen annealing (HPOA) process is proposed to optimize the device performance by controlling the defect concentration in an HZO film. The optimized device performance and reduced wake-up effect can be elucidated based on the appropriate amount of oxygen vacancies and reduced carbon contaminants. In addition, the impact of the oxygen vacancy and the carbon concentration in an HZO film are thoroughly investigated using electrical and structural measurements.