臨場蒸氣產生技術(ISSG)應用於鎢奈米點非揮發性記憶體之研究

Abstract

近幾年，具有奈米點(nanocrystals)儲存單元的非揮發性記憶體元件被廣泛的提出來克服傳統浮動閘極記憶體在操作上及可靠度上的問題 。良好的記憶體元件需要具備有低功率耗電、操作速度快、及良好的元件耐力、保存時間長等特性。在眾多奈米點記憶體元件中，金屬奈米點因具有更高的能態密度、強通道耦合能力、可調變金屬功函數以及不易受載子侷限效應所引起的能階擾動 等優點，所以金屬奈米點的記憶體受到廣泛的研究而有機會成為新一代記憶體元件結構 。而鎢金屬因具有許多優點，如較大的功函數(4.6eV)、極高的熔點 (3411°C)、不易和氧化層產生反應、及在現今超大型積體電路技術廣泛應用等，已被大量使用在奈米點非揮發性記憶體的元件製作上。 大多數製作奈米點的方法都需要長時間的高温熱氧化時間，這會大大影響現階段半導體的熱預算及造成摻雜再分佈。 在本論文中，一種新穎的溼式快速熱氧化製程叫作臨場蒸氣產生技術(ISSG)，被應用來降低熱預算及抑制摻雜再分佈。另外，臨場蒸氣產生技術(ISSG)氧化過程中會導入少許氫氣參與反應，已有許多文獻證實氫氣的增加可以幫助產生更多的氧自由基(O*)，增加臨場蒸氣產生技術(ISSG)的氧化速率，而且氧自由基(O*)對氧化層的缺陷有修補作用，以臨場蒸氣產生技術(ISSG)所製作的氧化層比傳統的爐管方式有較好的氧化層品質及可靠度。 本論文即是以臨場蒸氣產生技術(ISSG)氧化方法來製作鎢金屬奈米點， 並研究臨場蒸氣產生技術(ISSG)氧化的温度、時間及氫氣含量對鎢奈米點的形成的影響，並探討此奈米記憶體元件之物性與電性特性。

In recent years, nonvolatile memory with nanocrystals cell have been widely studied to overcome difficulties occurring to of operation and reliability of conventional floating gate memory. Desirable electrical characteristics of memory devices include the following criteria : low power consumption, high-speed operation, good endurance, and long retention time. Nonvolatile memory devices, using metal nanocrystals cells have received extensive search because of many advantages, such as a high density of states around the Fermi level, stronger coupling with the conduction channel, a wide range of available work functions and a smaller energy perturbation due to carrier confinement, and become a potential candidate for future flash memory devices. Because tungsten has many attractive material properties, such as a high work function (4.6eV), an ultra high melting point(3411°C), being chemically stable with silicon oxide and being compatible with VLSI technology nowadays, it is widely used for the fabrication of the metal nanocrystals in nanocrystal nonvolatile memory devices. Most methods applied for the nanocrystals fabrication need thermal treatments at high temperatures and a long duration which cause high energy consumptions and impurity redistribution. In this thesis, a new rapid thermal wet oxidation technology called “In-Situ Steam Generation (ISSG)”, is adopted for the tungsten nanocrystal fabrication to reduce the thermal budget and prevent impurity redistribution. In addition, during the ISSG oxidation process, hydrogen is introduced into the reaction chamber, thereby increasing the concentration of oxygen radicals. Oxygen radicals present in the ISSG process can reduce intrinsic defects in the oxide layer. ISSG oxides have much better reliability than conventional furnace oxides. We fabricated tungsten nanocrystals under various ISSG process conditions, including the oxidation temperature, the process time and the hydrogen concentration, and studied material and electrical characteristics of the tungsten nanocrystal nonvolatile memory devices.