浮停閘極快閃記憶體之可靠度改善

Abstract

非揮發性記憶體的製作，是採用整層的複晶矽的浮停閘(Floating gate)結構，利用此層來當電子的儲存層，當電子由通道注入到這層浮停閘之後，會影響到元件的臨界電壓值(Threshold voltage)，藉由判別臨界電壓的電壓值大小，即可定義邏輯的 “0＂ 與 “1＂ 狀態。但是因為這種浮停閘結構為整層的半導體薄膜，在電子反覆的從穿隧氧化層進出這層浮停閘，會使得穿隧氧化層劣化以至於出現缺陷，當缺陷一產生之後，所有儲存的電子將會隨著這層缺陷而有了漏電路徑，導致所寫入的電子全部流失掉，無法達到記憶的效果。然而在不久的將來，會因為成本的考量，將元件尺寸縮小，在尺寸微縮的同時，穿遂氧化層的厚度也會隨之減少，以至於記憶體可靠度不佳。為了應付未來高密度記憶體的需求，科學家努力研發各種可以取代的非揮發性記憶體如 PCM、FeRAM、MRAM等…，但沒有人能確定那個種結構可以取代浮停閘記憶體。 如何利用現有的技術，帶領快閃記憶體在繼續的走下去。我們採用了高含氮量氧化層(oxynitride)來代替傳統的熱氧化層當穿隧氧化層，首先把晶片浸泡於雙氧水中，形成化學氧化層 接著在低壓的環境下用氨氣去執行氮化，最後通氧氣來完成氧化動作。如此就可以在介面上形成高氮含量的氧化層，此法製程簡單，跟目前的製程技術是相容的，而且可以提高記憶體的可靠度。

Manufacture the nonvolatile memory uses poly-silicon layer treated as election storage layer named floating gate (FG). Electron injection to this layer from channel will influence the threshold voltage. Two states threshold voltage constitute logic “0” and “1”. For FG structure, the oxide has a defect because of electron impact repetitively during the write/erase cycles. Then all of the charge stored in FG layer will be loss. In the near future, the device will be scaling down for the cost. At the same time, the thickness of tunneling oxide will decrease. It causes the reliability of the memory to be worse. In order to deal with demand for high density memory in the future, the scientists make great efforts to research and develop various kinds of nonvolatile memory to replace FG memory. For instance PCM , FeRAM , MRAM ,etc. But nobody can confirm that what kind of structure can replace FG memory. How can we use existing technology to lead Flash memory continuously scale-down. We use the oxynitride to replace the traditional thermal oxide as the tunneling oxide. First, chemical oxide as a starting oxide can provide a better controllability in film thickness. Following that, the chemical oxide was nitrided using a furnace in low-pressure NH3 ambient to transfer high-nitrogen oxynitride. The nitrided chemical oxide was then placed in atmospheric O2 ambient to form a robust oxynitride. The process proposed here is simple and fully compatible with current process technology and improve the reliability of the memory.