含氧化鉿薄膜之電阻式記憶體特性研究

Abstract

電阻式記憶體（Resistive Random Access Memory，RRAM）被視為取代快閃記憶體的次世代記憶體之一。本研究利用濺鍍法製作以鋁（Al）為上電極、鉑（Pt）為下電極、含氧化鉿（HfO2）之RRAM，探討其電阻轉換（Resistive Switching）性質及改善方法。研究發現HfO2薄膜厚度會影響了Forming電壓（Forming Voltage，VForm），過大的VForm會導致元件的永久性破壞，較薄的HfO2薄膜則因為之後的熱處理使表面粗糙化，而造成電性劣化；電極的面積大小不會影響低電阻態（Low Resistance State，LRS）之電阻，但高電阻態（High Resistance State，HRS）電阻會因電極面積減小而增加。 將HfO2薄膜在大氣氣氛下做300C、500C、700C熱處理30分鐘，以X光電子能譜儀（X-ray Photoelectron Spectroscopy，XPS）及原子力顯微鏡（Atomic Force Microscopy，AFM）分析熱處理對HfO2薄膜的補氧效果及表面粗糙度，並以掃描式電子顯微鏡（Scanning Electron Microscopy, SEM）確認元件的結構。大氣氣氛、500C以下之退火處理對RRAM元件的電性質影響不大，700°C的熱處理則使RRAM之HRS電阻及Vset趨向穩定，且有較佳的循環壽命（Endurance）及資料保存能力（Retention）； X光繞射（X-ray Diffraction，XRD）分析顯示700C退火處理使HfO2從非晶態（Amorphous）轉換為單斜（Monoclinic）複晶結構，推論應為晶界（Grain Boundary）提供穩定的導電細絲（Conduction Filament）形成路徑所致；但退火亦減小HfO2薄膜和電極金屬界面的缺陷，造成不穩定且偏高之操作初始HRS電阻。

Resistive random access memory (RRAM) has been widely recognized as the next-generation nonvolatile memory to replace conventional flash memory. This study investigates the resistive switching properties of RRAM containing aluminum (Al) as the top electrode, platinum (Pt) as the bottom electrode and hafnium oxide (HfO2) as the insulator layer prepared by sputtering deposition. Electrical measurement indicated that the thickness of HfO2 layer affects Forming voltage (VForm) of RRAM and too large VForm would permanently damage the device. As to the devices containing thin HfO2 layers, subsequent annealing treatment caused the rough surface and degraded the electrical performance. The area of electrode negligibly affected the resistance of low resistance state (LRS) whereas the resistance of high resistance state (HRS) increase with the decrement of electrode’s area. For the HfO2 layers annealed at 300C, 500C, and 700C for 30 min in the atmospheric ambient, the effect of heat treatment on the remedy of the oxygen deficiency in HfO2 layer and surface roughness were analyzed by using x-ray photoelectron spectroscopy and atomic force microscopy, and the structure of devices were confirmed by using scanning electron microscopy. Moreover, the annealing treatments at temperatures less than 500°C insignificantly affected the electrical performance of samples. When annealing temperature was raised to 700C, the sample exhibited stable resistance of HRS and VSet as well as improved endurance and retention properties. In such a sample, amorphous HfO2 transformed to polycrystalline monoclinic structure as revealed by x-ray diffraction analysis. The improvement of electrical performance was hence ascribed to the presence of grain boundaries which provide stable formation routes of conduction filament in HfO2. However, annealing treatment seemed to reduce the interface traps at the interface of HfO2 and electrode, leading to high and unstable resistance of HRS at the initial stage of operation.