TCAD Simulation of Random Program Charge Induced Threshold Voltage Fluctuations in SONOS Device

• Main Authors: Yen-Chen Lin, 林彥君
• Other Authors: Tahui Wang
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/48692930445926805661

碩士 === 國立交通大學 === 電子工程系所 === 96 === Instead of using a conducting poly layer as a storage node in a conventional floating gate device, a charge trap cell utilizes discrete trapping centers of an appropriate dielectric film to achieve a similar memory function. In such structure, the channel potential becomes inhomogeneous due to the discreteness of the storage charges and thus the threshold voltage (VT) will strongly depend on how an inversion percolation path from source to drain is formed in the channel. In this work, SONOS devices with randomly distributed storage charges in a nitride layer are used and the induced VT fluctuations are then emulated by using a three-dimensional drift-diffusion simulator (ISE). In the beginning, the effects of various parameters including channel length, channel width, program window, and both top and bottom oxide thicknesses on the VT fluctuation are studied. For each case, samples of 100 microscopically different programmed cells are used. The statistics variation of VT are then collected and analyzed with respect to each variable. Basically, more programmed charges or the shrinkage of device dimensions will worsen the VT distribution. Besides, the bottom oxide shows more pronounced effect than the top oxide and reasons will be given in this thesis. Finally, to further approach the real situations of memory cells, the effect of random charges with random dopants are also investigated. As expected, the fluctuation of cells gets even worse. Adopting an epi-taxial layer with a lightly doped concentration on the Si surface is useful for suppressing the random dopant effect. But the program state threshold voltage fluctuations can not be improved significantly. Thus, the random charge effect is the main source induced the dispersion of VT.