Multistacked Tunneling Oxide for Nonvolatile Nanocrystal Memory Application

Abstract

In recent years, Flash memory has been used extensively in digital storage devices, such as memory cards, USB storage device, MP3 audio player, and so on [11], [12]. Moreover, portable electrical products will demand even more nonvolatile memories with high density, fast programming / erasing speed, excellent endurance and retention characteristics. Based on these demand, the market of Flash memory will increase in a foreseeable future. However, poly-silicon floating gate in conventional Flash memory is suffered from the charge leakage through the local defect in tunneling oxide. Therefore, separated charge storage elements, such as nanocrystals embedded in MOSFET, i.e. nanocrystal memory, were studied extensively in recent years [1-5]. Moreover, to further improve the programming/erasing speed without sacrificing the retention performance, a concept of “nanocrystal memory embedded with asymmetric band engineering of tunneling oxide” is proposed in this work. We set up two series of experiments, one is the Au nanocrystal memory using SiO2-HfO2-Al2O3 (namely SHA) multistacked tunneling oxide, the other is the Au nanocrystal memory using Al2O3-HfO2-Al2O3 (namely AHA) multistacked tunneling oxide with different thickness variation. The results and discussion will be described in chapter 4 and chapter 5 in detail. In these experiments, high-κ thin films were deposited using atomic layer deposition (ALD) to make sure the uniformity of our films. Subsequently, Au thin films were deposited using RF sputtering, and then annealed in N2 ambiance at 500oC and 650oC for SHA and AHA, respectively. Moreover, to increase the voltage drop on tunneling oxide to further improve the programming/erasing efficiency, ALD-Al2O3 control oxide were used in this work. Finally, Pt top electrode and bottom electrode were capped to form the MOS capacitor structure. After the fabrication of devices were accomplished, C-V curve and retention characteristics were measured by HP 4285 LCR Precision Meter, I-V curve were measured using Keithley 4200, erasing speed were measured by connecting the Agilient 81110A pulse generator and HP 4285 LCR Precision Meter, and the microstructure analysis were accomplished by HRTEM images.