| In recent years, with the development of microelectronics and computer technology, the industry of semiconductor memories has got rapid development in our country. Among them, the flash memory has been used widely due to its excellent performance and reasonable cost. However, nowadays the traditional flash memory is tending to the physical size limitation based on its work mechanism, suffering from serious challenges such as storage density, power consumption and reliability. In order to solve these problems, various new type nonvolatile memories have drawn great attention.Nanocrystal memory device using discrete charge storage units, has become a focus in the next generation nonvolatile memory fields due to its outstanding advantages such as fast operation speed, high reliability, and large storage density. Meanwhile resistance random access memory (RRAM) is also a promising candidate of the new nonvolatile memories because of its simple cell structure, low operation voltage, superior scalability, and CMOS compatibility. Besides new type nonvolatile memory structures and storage materials, it is of very importantance to develop the fabrication processing compatible with the semiconductor technology.Atomic layer deposition (ALD) which is based on sequential self-limited and complementary surface chemisorptions reactions is a new type of thin film deposition technology using precursor vapor with large area uniformity and excellent three-dimensional conformality. Nowadays, ALD shows increasing prospect and wide application in various fields such as microelectronics, nanotechnology and new energy.This thesis deals with two kinds of new type nonvolatile memory, Ti-Al-O nanocrystal memory and HfO2/TiO2/HfO2 trilayer-structure RRAM prepared by ALD technique suitable for mass production. The deposition process, microstructure, and storage performance of memory cells have been investigated deeply.The main achievements are summarized as follows:1. A nanocrystal memory using Ti-Al-O (TAO) film as charge trapping layer and amorphous Al2O3 as the tunneling and blocking layers was fabricated on Si substrates by ALD method. The impact of annealing temperature of 700℃,800℃ and 900℃ for 3 min in N2 with a rapid thermal anneal process to form nanocrystals on interface, microstructure and charge storage performance of memory cells was characterized. The cross-sectional HRTEM images of the memory cells show that the interfaces of Al2O3/Ti-Al-O/Al2O3/Si stack annealed at 800℃ are distinct and sharp with the corresponding thickness of 11.7 nm/10.4 nm/2.9 nm. Spherical nanocrystals are observed in the charge storage layer of TAO films with the average size of 3 nm. For 900℃ sample, the interface between charge trapping layer and blocking layer cannot be recognized with thinner tunneling oxide (-2.3 nm) and larger nanocrystal size (10 nm). The corresponding electrical measurements confirm that the storage performance is related to the number of nanocrystals and interfacial microstructure of nanocrystal memory cells. Compared to the 700℃ and 900℃ samples of 0.7 V and 2.8 V, the 800℃ sample exhibits a larger memory window of 4.8 V at ±6 V sweeping. The lacking of nanocrystals in TAO layer of 700℃ sample leads to the smallest memory window. Simultaneously, the memory cell annealed at 800℃ has a higher program/erase (P/E) speed and larger charge storage density of about 1.63×1013/cm2 at ±6V sweeping. After 105 cycles of P/E pulse at room temperature and 80℃, the excellent endurance characteristic is observed. The band alignment of the Al2O3/TAO/Al2O3/Si annealed at 800℃ determined by X-ray photoelectronic spectroscopy (XPS) indicates that the large conduction band offsets of 1.49 eV and 1.15 eV between Ti-Al-O and tunneling/blocking layers of Al2O3 will enhance the retention characteristic due to the deep trap level. At present the retention property of 800 ℃ sample still needs further improving.2. The HfO2/TiO2/HfO2 trilayer-structure RRAM devices have been fabricated on Si/SiO2/Ti/Pt and Si/TiN substrates by ALD. The effect of bottom electrodes of Pt and TiN on the resistive switching properties of trilayer structure has been investigated deeply and the resistive switching mechanism has been proposed. The memory cells of Pt/HfO2/TiO2/HfO2/Pt and TiN/HfO2/TiO2/HfO2/Pt show bipolar resistive switching characteristics. At the low resistance state, the conductive mechanism follows the Ohmic law. The filament model of oxygen vacancy migration can be used to explain the switching behavior. At the high resistance state, it is dominated by the typical trap-controlled space charge limited conduction (SCLC). XPS results indictate that the oxygen vacancy concentration of HfO2 is higher than that of TiO2, which is beneficial to the formation of conductive filaments in the HfO2/TiO2/HfO2 trilayer memory cells. Meanwhile HfO2/TiO2/HfO2 trilayer-structure has the capability of modulation of electric field and reduces the randomness of conductive filament formation in the RRAM switching process with improved device stability. Both Pt/HfD2/Ti02/HfO2/Pt and TiN/HfO2/TiO2/HfO2/Pt exhibit good endurance and retention characteristics. However, the bottom electrodes of Pt and TiN have great influence on the electroforming polarity, ratio of high and low resistance and dispersion of the operating voltage of trilayer-structure memory cells. The RRAM cells with Pt bottom electrodes have a forming voltage of about +7 V, set voltage of +2 V, and reset voltage of -0.8 V with ratio of high and low resistance of>105, whereas the RRAM ones with TiN bottom electrodes have an opposite forming voltage of about -4 V, set voltage of -1.5 V, and reset voltage of +1.5 V with ratio of high and low resistance of only 102. Compared to the RRAM cells on Si/SiO2/Ti/Pt, different storage units on Si/TiN substrates and single storage unit with TiN bottom electrodes after multiple cycle operations show relatively narrow distribution of the set/reset operation voltages. It is attributed to the fact that the formation of conductive filament is significantly affected by the initial distribution of oxygen vacancy in the insulator. The TiN electrode with high oxygen affinity causes a lot of oxygen vacancies in the HfO2 film near the TiN bottom electrode, easily leading to the formation of conductive oxygen vacancy filaments with improved uniformity of resistive switching parameters. Considering the modulation effect of electrode and trilayer-structure on resistive switching performance, it provides a new device design path for future RRAM applications. |
PDF Full Text Request