銥奈米晶體於非對稱穿隧能障結構之非揮發記憶特性研究

Abstract

近來非揮發性奈米晶體記憶體被用來廣泛的研究來克服傳統的浮動閘極記憶體的極限。利用奈米晶體被用來克服傳統浮動閘極元件在微縮時遇到電荷流失的問題、容許更薄的穿隧氧化層、更低的操作電壓，及更好的容忍度和電荷保存能力。比較半導體奈米晶體和金屬奈米晶體兩者，金屬奈米晶體作為浮動閘極有許多優點：對於電容特性改變量、較多種可供利用並設計的功函數、在費米能階周圍有高的狀態密度以及不易受載子侷限效應所引起能階擾動等等。 本論文研究利用非對稱結構與一般單層結構的差異，比較兩者的操作電壓與耐用度，可以藉由非對稱結構在寫入與抹除跟資料保存度之間得到好的補償，加上金屬的高功函數，可以看到銥奈米晶體在不同的穿隧氧化層上置入後的電容特性以及晶體顆粒大小與密度多寡。在相同操作電壓下(+/-5 V)，非對稱(SiO2/Si3N4)結構ΔVFB ≒4.2 V，而單層(SiO2)結構ΔVFB ≒1.5 V，每顆粒子所儲存的電荷在非對稱結構約為4個電子或電洞，在單層結構約為2個電子或電洞。在資料維持度的量測，經過104秒，非對稱結構剩下50%，單層結構剩下55%，資料維持度沒有更好，但是得到了低電壓的操作與運作速度的提升。此外，在非對稱結構電容的可靠度分析比較也能維持好的耐用度。

Recently, nonvolatile memory with nanocrystals (NCs) has been widely studied to overcome limitations of conventional floating gate memory. The use of NCs as distributed floating gates minimized the problems of charge loss encountered in conventional floating-gate devices, allowing thinner tunnel oxide and, thereby, a lower operating voltage, better endurance and retention, and faster program/erase (P/E) speed. Compared to the semiconductor NCs, metallic NCs as floating gates possesses several advantages, such as larger change of electric capacity, stronger coupling with the conduction channel, a wide range of available work functions, higher density of states around the Fermi level, and a smaller energy perturbation due to carrier confinement. In this thesis, it used the difference between asymmetric tunnel barrier (ATB) and a single layer structure. Both compared operating voltage and endurance. It can take the better tradeoff between the programming/erasing and retention characteristic by ATB structure. And Iridium has high work function and good thermal stability, we can demonstrate Iridium nanocrystals embedded in different tunneling oxide layer for capacitor characteristic and find different nanocrystals’ diameter and density. At the same operating voltage(+/-5 V), ATB(SiO2/Si3N4) structure ΔVFB ≒4.2 V and single layer(SiO2) structure ΔVFB ≒1.5 V. Each Ir-NCs stored 4 electrons or holes in ATB structure decive and 2 electrons or holes in single-layer structure device. The charge remaining of ATB memory device was 50% at 104 s, and 55% for single-layer memory device. Although, there is no improvement in data retention ATB device do lower the operating voltage and increase higher P/E speed.