The Study on Charge-trapping Mechanism in Nitride Storage Flash Memory Device
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The Study on Charge-trapping Mechanism in Nitride Storage Flash Memory Device Jia-Lin Wu1, Hua-Ching Chien1, Chi-Kuang Chang1, Chien-Wei Liao1, Chih-Yuan Lee1, Je-Chuang Wang1, Yung-Fang Chen2, and Chin-Hsing Kao1 1
Semiconductor Laboratory, Chung-Cheng Institute of Technology, National Defense University, No.190, Sanyuan 1st St., Tahsi, Taoyuan, 335, Taiwan 2
Department of Physics, National Taiwan University, No. 1 Sec. 4 Roosevelt Rd., Taipei, 106, Taiwan ABSTRACT In this work, the charge-trapping distributions of polysilicon-oxide-nitride-oxide-silicon (SONOS) structure are studied. The trapping energy level of SiNx films with different composition ratio deposited by low-pressure chemical vapor deposition (LPCVD) were first characterized by photoluminescence (PL) measurement. Moreover, using F-N/CHE program and charge pumping techniques, the vertical location and the lateral distribution of programmed charges are investigated in the nitride films with different composition ratio. The study offers strong evidence that the density of charge-trapping levels in the Si-rich nitride is higher than the standard nitride. A simple qualitative model and calculation explains that the trapping level distributions in the SiNx films are shallower by increasing relative Si-content. Furthermore, we have observed the nitride trap vertical location was changed by adjusted Si/N composition ratio. And the lateral distribution of hot electron programmed charges in the modified nitride is broader than that in the standard nitride because it offered more charge-trapping sites and shallower charge-trapping levels. In summary, the study can help researchers to understand the nitride charge-trapping mechanism and the analysis of optical/electrical characteristics. INTRODUCTION The nitride storage flash memory with SONOS structure has received much interest recently due to lower operation voltage, simpler fabrication process, higher density, and multi-bit operation. However, there are some technical problems, such as insufficient sensing window, low P/E speed, poor endurance and retention, remain to be solved in developing high-performance memories. Contrary to the floating gate device where charges are uniformly stored, the charges are discretely trapped in the nitride thin film. Therefore, the distribution of nitride traps is very significant in understanding the trapping efficiency of flash memory devices. In our previous studies, a new configuration of nitride trapping layer in SONOS device by band-gap engineering has been proposed [1]. In this work, the charge-trapping distributions of SiNx films with different
composition ratio deposited on Si substrate were characterized by photoluminescence (PL) measurement, transient analysis and charge pumping method. The results shall be valuable in pushing the next generation of the SONOS memory cells. DEVICE FABRICATION and MODEL DESCRIPTION Figure 1 shows the proposed SONOS memory cell based on (100) p-type substrate. The test sample was fabricated by a standard CMOS process
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