Investigation Of Nanocrystallites And Nanolaminate-based Charge Trapping Flash Memory Devices

Since the advent of semiconductor nonvolatile memoreis, floating gate nonvolatile memories always dominate the maket of nonvolatile memories. As the continued scalling down of the feature sizes of semiconductor devices and the increasing of the integrated density, floating gate nonvolatile memories can not meet the reqirements of the minimization of nonvolatile memories. Especially, as the feature sizes of semiconductor devices scalling down to22nm, conventional floating gate type nonvolatile memory approach to their technical and physical limitations. Threfore, it is urgent to look for a new type of nonvolatile memory with better performance. Polysilicon-oxide-nitride-oxide-silicon (SONOS) charge trapping flash memories have been received much attention due to their advantagse such as lower power, lower operation voltage, fast speed, excellent data retention and better endurance. Conventional SONOS use Si3N4as the charge trapping layer, and charge are trapped by the discrete traps within the Si3N4charge trapping layer. Such a memory structure is robust to stress-induced leakage currents. Based on the concept that charges are stored in discrete traps within the charge trapping layer, SONOS type memory have been developed gradually. High-k materials, semiconductor or metal crystallites are also employed as charge trapping layers to improve the properties of the nonvolatile memories.In this dissertation, the memory behaviour, microstructure and reliability of the ZrO2nanocrystallites (NC)-based charge trapping memories have been systematically investigated. Meanwhile, a novel nanolaminate-based charge trapping memory structure has been also proposed, and the charge storage performance has been explored. The main results are summarized as follows:1. By employing the (Zr02)o.8(Si02)o.2(ZSO) film as charge trapping layer to replace conventional Si3N4, and amorphous Al2O3layers as the tunneling layer and blocking layer, charge trapping memory capacitors were fabricated by the atomic layer deposition and pulse laser deposition technique. X-ray diffraction (XRD) pattern demonstrated that the nanocrystalline ZrO2phase embedded in the amorphous Si-rich ZSO matrix used as the storage media were precipitated from super-saturatured pararent ZSO after rapid thermal annaling (RTA) treatment. A large memory window of7.5V of nanocrystallined ZrO2-based charge trapping memory was also obtained at sweeping gate voltage Vg=±8V. The change of the memory window was much small even after write/erase operations of105cycles at150℃. The result indicated that the memory capacitors had excellent endurance characteristics. The charge losses for the memory capacitors measured at25℃,85℃and150℃were6.6%,8.3%, and12.0%, respectively after write/erase operations of105cycles and retention time up to4x104seconds.2. The effects of the size and density of the ZrO2nanocrystallites on the memory characteristcs were systematically investigated. The as-prepared ZSO films were amorphous state. With increasing the annealing time, ZrO2nanocrystallines were precipitated from the super-saturated ZSO matrix after RTA treatment. Their density increased along with the annealing time, and reached a maximum value at60s. The size of the ZrO2nanocrystalline was increased continuously with the anealling time. The present results demonstrated that the memory cells after post-annealling for60s had optimized electrical characteristics, such as maximum memory windows4.4V, minimum charge loss5%and excellent reliability. The band alignment of the ZrO2nanocrystallites-based charge trapping memory were examined by X-ray photoelectron spectroscopy (XPS). The results showed that the conduction band offset was1.3eV between the Al2O3and ZSO, which resulted in an excellent data retendion characteristics.3. A new-type charge trapping memory structure was designed by incorporating a high-κ ZrO2/Al2O3nanolaminate as charge trapping layer to replace conventional Si3N4or high-k materials. Inorder to investigate the impact of the interfaces in the nanolaminate, a series of samples with equal charge trapping layer thickness but different lamellar number were prepared and investigated. The results showed that the memory window width was increased along with the interface number at the same sweeping gate voltage, and the interfacial numbers played an outstanding role in affecting the memory characteristics of the ZrO2/AlO3nanolaminate-based charge trapping memory devices. However, as the number of the interfaces became too high, the width of memory window decreased due to the electrostatic repulsions between the electrons trapped by the neighboring interfaces and the damaged interfacial structures after RTA treatment. The memory cell with8interfaces exhibited a larger memory window of6.3V and charge loss of7.5%after10years. The conduction band offset between ZrO2/AlO3nanolaminate was determined to be1.7eV by the XPS. The observed deep quantum wells led to the good retention performance of the devices.