Application of novel gate materials for performance improvement in flash memory devices

Abstract

The overall objective of this work is to apply novel gate materials for the performance enhancement of flash memory devices, including both floating gate-type flash memory devices and SONOS-type flash memory devices. These attempts could be of practical value for flash memory devices, especially in improving the operation speed and data retention. A novel floating gate engineering scheme using carbon doped polysilicon floating gate is proposed to overcome the scaling barrier for floating gate-type flash memory devices. It has been found that incorporating carbon into conventional n+ polysilicon floating gate will be able to significantly improve the program/erase speed, especially for devices with small coupling ratio (~0.3), which is the bottleneck for sub 30 nm flash memory technology. The data retention of such devices is also improved. All these improved properties originate from the increased conduction band offset of the floating gate caused by the incorporation of carbon. The formation of silicon carbide nano-structure is responsible for the band structure change. Adoption of the carbon doped polysilicon floating gate will result in little process modification to the current technology, and is an effective and simple solution for floating gate-type flash memory scaling. In the advanced SONOS-type flash memory devices, the application of high dielectric constant materials as the blocking oxide attracts much research interest. The feasibility of a novel rare earth high-k material, Gd2O3, as the potential candidate for the blocking layer application in SONOS-type flash memory devices is evaluated. The material properties of Gd2O3, including deposition method, leakage current performance, crystal information as well as the band structure have been studied systematically. Control of the crystal structure of Gd2O3 has been found to be the key point for a high quality dielectric film. SONOS transistors with Gd2O3 blocking layer exhibits superior performance over those with Al2O3 blocking layer in several aspects such as program/erase speed, room temperature retention, etc. Experimental results have demonstrated that Gd2O3 is a favorable blocking oxide candidate except that the retention after cycling remains problematic. Doping of Al into pure Gd2O3 is proposed for the robust data retention after cycling, since the increase in the conduction band offset is always an effective method to block the electron leakage, both for room temperature and high temperature retention. The optimized Al concentration needs to be carefully considered to balance all the following factors: dielectric constant, conduction band offset, film morphology as well as memory characteristic. All those questions will be well addressed in Chapter 4. The use of GdAlOx doped with 35% Al results in superior memory performance over those using Al2O3 blocking layers, and this material could be a promising candidate for the future blocking oxide material. In Chapter 5, structure optimization of SONOS cell with 35% Al incorporated GdAlOx blocking oxide is discussed. The study focuses on the relationship between the blocking layer thickness and long term retention reliability at room temperature, after program/erase cycles and at elevated temperature. A novel leakage current separation technique will be applied to differentiate the leakage components in SONOS memory in order to improve the retention effectively. Charge leakage mechanisms for SONOS-type flash memory devices will be discussed in this chapter as well.