快閃記憶體操作效能提昇之研究

一般而言，快閃記憶體都會經過數十萬次、甚或數百萬次的操作，可以想見的，在經過多次的操作之後，由於種種的因素，如氧化層的損傷等，再加上快閃記憶體元件製程品質不佳的話，將會產生開口萎縮、干擾以及儲存於懸浮閘極電子流失等元件可靠度問題，而造成寫入和抹除操作的效能呈現衰減的現象。在本論文中，我們將運用新的操作方式、導入新的快閃記憶體結構以及元件高溫特性模擬等方式來提升快閃記憶體之操作效能。經實驗證明，運用新的操作方式可靠著比較大的閘極電流，能有效改善元件的操作速度，而對元件的可靠度，如氧化層傷害、臨界電壓偏移等，都較傳統的通道熱電子注入法為佳，其對元件操作之開口萎縮、干擾現象以及資料保存特性上均有一定之改善，故可發現新寫入方法之操作效能及可靠度表現均較傳統通道熱電子注入法為佳。在導入新的快閃記憶體空乏區垂直介電層結構，因其可改變通道電子流動狀態，進而增進電子注入機率與動能，對快閃記憶體操作效能之提昇是有幫助的，其中尤以同時具有source與drain端空乏區垂直介電層結構之元件表現最佳；另改變空乏區垂直介電層之介電係數模擬發現，low-k之垂直介電層可更精進快閃記憶體元件之操作效能。在高溫效能方面，快閃記憶體元件之臨界電壓隨操作溫度之上升而略為下降，當溫度愈高時，量測及模擬之結果皆得到，快閃記憶體元件寫入及抹除操作之臨界電壓差均愈來愈大。高溫時雖然通道電子濃度較小，惟與室溫時相較差不多，快閃記憶體元件操作效能仍以高溫時為佳，可能因為在高溫時之通道表面電場及電子注入機率均較大。綜合以上之實驗與模擬結果，可看出新穎的操作方式與元件結構，可以提升快閃記憶體元件之操作特性，在高溫環境也使操作速度略增。惟仍有一些元件製程上之困難或元件量測之方式尚待解決，以期進一步增進快閃記憶體元件工作效能。

關鍵字

快閃記憶體；操作效能

並列摘要

The programming and erasing operations of flash memory device can be performed by injecting electron through the tunneling dielectric layer while a high voltage is applied. These operations possibly cause damage to the SiO2/Si interface or SiO2 itself if manufacture quality of flash memory is not good, which thereby degrades reliability and eventually causes the failure of flash memory. In this work, a novel operation method, new design of structure, and the high temperature on operation performance of flash memory devices were studied. Regarding novel programming method, experimental results show that the proposed programming method with higher gate current injection efficiency not only increases the operating speed but also improves the reliability. This reliability improvement can be attributed to the reduction of oxide trap charge generation and threshold voltage shift. As for the new flash memory structure, the simulation results show that the employment of a vertical dielectric layer in the depletion region can improve the operation performance of flash memory. The improvement can be attributed to a lower potential in the central region of device channel and the increase of the potential drop in the channel direction near drain junction. Thus, this proposed vertical dielectric layer increases the electrical field of the channel and thus the probability and the momentum of electron injection. The operation characteristics of the flash device with a vertical dielectric layer in the depletion region of source and drain are superior to those of others. In addition, it is found that a vertical dielectric layer with lower dielectric constant can enhance the operation performance of flash device even more. Regarding the high temperature effects, the threshold voltage of flash memory devices is reduced with raising temperature. When the temperature is higher, the operating window (ie threshold voltage shift) of the flash memory are larger for both measurement and simulation results. Although the electron concentration in channel is reduced with increasing temperature, the reduced amount is little in comparison with those at room temperature. The operation performance of flash device at high temperature is better because of the higher electrical field and electron injection probability at channel surface.

並列關鍵字

flash ； 0peration performance

參考文獻

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國際替代計量

快閃記憶體操作效能提昇之研究

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