游離輻射對電阻式記憶體的影響之研究

Abstract

在本論文中，我們對游離輻射對電阻式記憶體 (RRAM)產生的影響進行研究，其中游離輻射包括極紫外光 (EUV)以及X-ray。 我們所使用的RRAM是以5 nm的二氧化鉿作為介電層。在RRAM的電性方面，討論切換特性、記憶窗口、資料儲存持久性以及耐久度等特性受輻射的影響。在切換特性以及記憶窗口的方面，看不到任何變化，表示經由輻射照射後所產生的電子-電洞對不會影響RRAM的操作。另一方面，資料儲存的持久性也不受輻射影響，在照射前後都顯示出良好的儲存持久性。這是因為在室溫下氧空缺擁有較低的遷移率。 此外，我們也觀察已儲存的記憶狀態經過輻射產生的變化。元件在照射前分別被設定在高阻態 (HRS)以及低阻態 (LRS)，並且在照射後馬上進行量測。對於照射前被設為低阻態的元件，其本身的電阻值經過照射沒有太大的改變；反之，在照射前被設為高阻態的元件，有部分元件在照射後阻值改變。此外，阻值改變的元件百分比隨著輻射的總劑量增加而增加。這是因為在低阻態時，氧化層中有多數的氧空缺形成電流導通路徑，以至於經由照射產生的缺陷不會影響低阻態的阻值。然而，當元件處於高阻態的狀態下時，在氧化層中只存在零星的氧空缺，所以比較容易被輻射所產生的缺陷影響，並且此影響機率會正比於照射輻射之總劑量。一部分的高阻態元件經由輻射阻值會變高，表示氧化層被輻射退火，使得氧化層內的缺陷變少，另一部分的元件經由照射後阻值變低，表示輻射產生的缺陷有可能將零星的氧空缺連成一條電流導通路徑。不過，改變阻值的高阻態元件可由正常的操作回覆到一般高阻態。此外，元件的持久性之劣化會隨著輻射的總劑量增加，這是因為在不斷切換阻態時，在氧化層中即會產生缺陷，因此，當照射產生額外的缺陷時，即有可能使氧化層產生崩潰，造成元件回不到高阻態。最後，在同樣的劑量下，X-ray所造成的傷害比EUV來的輕微。 本論文顯示RRAM有極好的抗輻射能力，因此，在元件尺寸微縮時，EUV微影技術將會是一個好的解決方案。

In this thesis, the effect of ionizing radiation, including extreme ultra-violete (EUV) irradiation and X-ray irradiation, on the characteristics of Hf-based resistive RAM (RRAM) is investigated. RRAM device with 5-nm-thick HfO2 is used. The effect of radiation on the switching characteristics, memory window, retention, and endurance performance of RRAM is studied. The stable of the switching characteristics and the memory window implies that defects generated by irradiation don’t affect the operation of RRAM. On the other hand, the retention performance exhibits good radiation hardness since the oxygen vacancies have low mobility in the dielectric layer at room temperature even after the irradiation. The change of the memory state is measured as well. The RRAM devices are set to the low resistance state (LRS) and high resistance state (HRS) before irradiation, and the memory state is measured right after irradiation. The devices in the LRS initially don’t exhibit resistance change after irradiation, while some devices in the HRS initially change their resistance after the irradiation. Besides, the percentage of failed devices increases with the increase of the total irradiation dose. This is because that there are several filaments in the oxide layer in the LRS so that the irradiation generated defects would not affect the resistance. However, when the devices are in the HRS, sporadic oxygen vacancies in the oxide layer might be affected by radiation when the total dose increases. The increase of resistance after irradiation implies the oxide layer is annealed by irradiation, reducing defects in the oxide; while the change to the lower resistance means the defects might bridge the oxygen vacancies in the oxide layer. The changed HRS states can be rewrite through a regular switching process. Moreover, the endurance performance of RRAM degrades after the X-ray irradiation, and the number of the devices having degradation in the endurance performance increases with the total irradiaiton dose. This result suggests that through lots of switching cycles, defects generated by irradiation might still enhance the possibility for oxide breakdown. Finally, the effect from EUV is stronger than that from X-ray at the same total dose. The difference between EUV and X-ray irradiation damage effect on RRAM is because the calculation method of the total dose leads to the total flux of X-ray being much less than that of EUV. This work exhibits that RRAM has good immunity against radiation Hence, EUV lithography could be a good solution for the RRAM when the device is scaling down.