Study on the Copper-Based Resistive Random-Access-Memory (RRAM) Devices
研究中提出以氫氧化鉀非等向性蝕刻出金字塔結構之矽基板沉積以金屬銅作為金屬性憶體單元之底電極，並明顯地改善其電阻切換特性。相比於傳統的平面銅電極元件，本金字塔結構電極之樣品達成了1 V/ 0.6V之低編碼/抹除電壓，並具有優異之電阻切換耐力，可以短如1微秒的-5/+3V編碼/抹除脈衝，進行長約2400個切換週期之操作。在本研究中所呈現之優越性能，可以歸因其樣品之金字塔結構電極尖頂具有較高的區域電場，因此生成了較細的導電絲，從而反映出較低的操作電壓以及較好的元件耐力。
In this thesis, five methods to enhance the resistive switching characteristic of metallic conductive types RRAM have been proposed, including pyramid-structured copper active electrodes, oxidized copper active electrodes, Cu-Ti alloy active electrodes, oxidized Cu-Ti alloy active electrodes, and Cu-doped TiO2 resistive switching layers with inert electrodes. The pyramid structure fabricated with the potassium hydroxide (KOH) anisotropically etched (100) silicon substrate has been deposited with a copper film as the bottom electrode of the programmable metallization cell (PMC) memory to significantly improve the resistive switching characteristics. As compared with the conventional flat copper electrode, this pyramid-structured electrode exhibited the set/reset voltage as low as 1/0.6 V and superior endurance of 2400 cycles at the set/reset voltages of −5/+3 V for the voltages pulsewidth of 1 μs. The high performance of this PMC could be attributed to high local electrical fields at the tips of the pyramid structure, resulting in the formation of the narrower conductive filaments that facilitate the lower operation voltage and better endurance. In order to reduce the copper content in the resistive switching layers, the RRAM cells with the oxidized copper has been deposited as the electrode. The sample possessed the better stability and larger memory window as compared to the conventional non-oxidized copper electrode ones. It was conjectured that the diluted copper atomic concentrations of the oxidized copper electrodes were favorable to reduce the excess residual copper atoms in the TiO2 layers. Therefore, the oxidized copper electrode samples could exhibited the lower OFF-state current with the larger on/off state current ratio (memory window) of 3 orders (103) and the better endurance as high as 1000 cycles than the conventional non-oxidation copper electrode ones of 1.7 orders and 400 cycles. Furthermore, the programmable metallization cell (PMC) memory devices with Cu-Ti alloy films as the bottom electrodes have been shown to exhibit a larger memory window of 2.3 orders and better endurance of 1000 cycles as compared to the conventional pure copper electrode ones. It was conjectured that the Cu-Ti alloy electrodes could obtain the appropriate amount of copper atoms to format and rupture the conductive filaments in the resistive switching layer. Moreover, the devices with oxidized Cu-Ti alloy bottom electrodes have been also shown to achieve the a superior memory windo as high as 3 orders and endurance of 3000 cycles as compared to conventional pure copper and non-oxidized Cu-Ti alloy electrodes ones. It was conjectured that the oxidized Cu-Ti alloys could control the Cu cations from the Cu and Cu2O to the appropriate amount to achieve the most favorable resistive switching characteristics. In addition, the resistive RRAM cells with Cu-doped TiO2 film between two Pt inert electrodes were produced in this work, and these devices could achieve a lower required programming voltage of –0.7 V and higher endurance of about 1000 cycles at the programming/erasing voltage of –5 V/+3 V for the pulse width of 1 μs, as compared with the conventional Pt/TiO2/Cu ones. It was conjectured that the distribution of Cu sources in the Cu-doped TiO2 (TiO2 : Cu) resistive switching film facilitated the formation of sharp and narrow conductive filaments since the Cu sources were redox more easily to form the filaments. The proposed Pt/TiO2:Cu/Pt sample could thus achieve a lower DC programming voltage than the conventional one. Moreover, it was conjectured that the better endurance of the Pt/TiO2:Cu/Pt sample was due to the lower amount of residual Cu atoms in the TiO2 layer from the ruptured narrower filaments during the erasing process. This was because the increase in the amount of Cu atoms was limited by the inert Pt electrode when there were uniformly distributed Cu sources and there was no Cu electrode. Finally, the summary and conclusions as well as the future prospects for the further research were also proposed.