铜互连结构的力学性质及基于互连结构制备的阻变存储器研究

【作者】 卢茜；

【导师】 吴晓京；

【作者基本信息】 复旦大学 ， 材料物理与化学， 2011， 博士

【摘要】 金属化是现代IC工艺的一个重要组成部分,目前铜已经取代铝而成为主流互连材料。铜互连纳米多层膜体系的力学特性,如硬度与弹性模量会对器件的可靠性产生重大影响。因此,建立一套能够在纳米尺度表征其力学性能的方法是十分必要的。新型的存储器技术首先要能够集成进入标准的半导体工艺,同时需要具有高存储密度、非破坏性读写、低功耗、耐擦写等优点。阻变式随机存储器(RRAM)由于其简单的结构、优异的性能而受到业界的广泛关注。在现有互连结构上制备的RRAM单元与后端工艺之间兼容度非常高。目前,基于铜/铝金属化制备RRAM单元已取得一定进展,然而,这种两端器件的材料体系需要丰富,器件性质的表征方法有待完善,工作机理尚不明确。本文针对Cu互连结构的力学性质以及基于互连结构制备的阻变存储器进行了研究,主要包括以下四部分内容：一、使用纳米压入技术检测Cu/Ta/SiO2/Si多层膜结构的纳米力学参量(硬度、弹性模量),并研究了外力作用下体系的形变情况。为了表征纳米压痕下的形变微观区域,采用聚焦离子束加工出压痕截面,同时进行扫描电子、扫描离子显微观察,发现样品衬底发生开裂,多层膜结构出现分层现象。TEM分析表明分层出现在’Ta/SiO2界面,分层的原因主要归结为在应力作用下,多层膜中各种材料的应变、弹性恢复能力不同。二、使用导电原子力显微分析技术(C-AFM)对CuxO阻变材料及器件进行了微区电流分布研究。CuxO薄膜采用等离子体氧化工艺在Cu互连结构表面制备。结果发现开启状态薄膜中形成了导电通道；从电流分布的均匀性出发评估了相应器件的可缩小性；结合Pt/CuxO/Cu存储单元性能测试结果,分析了它的工作机理；对宏观I-V测试中观察到的开启电压浮动、高阻值涨落、无形成过程等现象从微观角度做出解释。三、使用等离子体浸没注入技术(PⅢ)在铜互连结构上制备了氮化铜薄膜,通过调控注入条件优化薄膜质量。对以氮化铜作为中间层的M/CuxN/Cu MIM结构进行电学测试,结果表明该结构具有电阻开关特性。结合器件微区电流分布情况,认为其阻变特性是源于金属性导电细丝的形成／断裂。然后进一步研究了CuxN基RRAM存储单元的开启速度、温度稳定性、疲劳特性、非易失性以及可缩小性等器件性能。最后讨论了电极材料以及制备工艺对于器件电阻开关性质的影响。四、分别使用PⅢ技术和脉冲激光沉积技术在Al互连结构表面制备了A1N薄膜,针对不同工艺对于薄膜物性的影响进行了讨论。通过沉积上电极Cu形成Cu/AIN/Al单元,初步研究了该结构的电学性质,展望了这种A1N基存储单元的应用前景。更多还原

【Abstract】 Metallization is one of the crucial parts in modern IC technology. Nowadays, copper has been widely adopted as a substitute for aluminum in the manufacture of ultra large-scale integrated circuits (ULSI). The mechanical properties of the Cu interconnection, such as hardness and elastic modulus, also significantly influence the performance and reliability of ICs. Therefore, the characterization of nanotribological properties of Cu/Ta/SiO2/Si multilayer thin films as a system turns out to be important in the evaluation of multilevel copper interconnects and nanoscale ICs.New memory technologies must be easily integrated into standard semiconductor process. The devices should be read out non-destructively, have low power consumption and high scalability. As a promising candidate for next generation nonvolatile memories, resistive random access memory (RRAM) can provide a simple structure and outstanding performance. The interests in studying the resistance switching materials will come from both academia and industry. RRAM cells fabricated on the interconnect structures show excellent compatibility with copper/aluminum back-end-of-line (BEOL). However, the material systems of such memory cells need to be enriched; the methods of characterization remain to be improved and the switching mechanism is still a matter of debate.In this context, the nanotribological properties of Cu/Ta/SiO2/Si multilayer system as well as RRAM cells based on interconnection structure were studied respectively, the following parts were included:1. Nanoindentation was adopted to investigate the compound hardness and elastic modulus of Cu/Ta/SiO2/Si multilayer thin film system. In order to reveal the structure variance, a residual indent was cut by FIB. The cross-section of the residual indent was observed by the scanning electron microscope (SEM), as well as the scanning ion microscope (SIM). It was found that an apparent fracture occurred in Si substrate and the multilayer thin film structure delaminated. Transmission electron microscope (TEM) analysis showed that the delamination occurred at the interface between the Ta layer and SiO2 layer of the residual indent. The different strain and the ability of elastic recovery of each material under stress are supposed to be the main reason of the delamination. 2. Conductive atomic force microscopy (CAFM) analysis has been proved to be an effective method for the investigation of local conductivity of CuxO based RRAM devices. The CuxO films were grown by plasma oxidation process at room temperature. Current mapping results showed that local filaments growth and dissolution during the switching process. The scaling potential of such devices was discussed based on the density and uniformity of the filaments. The Pt/CuxO/Cu terminal memory cells did not need any forming process to activate it, however, the high resistances and the set voltages scattered in a large extent during cycling, the mechanism behind this phenomenon has been explained from microcosmic aspect.3. Copper nitride film prepared by plasma immersion ion implantation (PⅢ) was demonstrated to exhibit reversible resistance variance character. The nitrogen ion implantation process was carried out with progressively increased negative high voltage and single voltage, so as to controll the total nitrogen concentration and distribution in the CuxN film. By forming a gradually distributed nitrogen concentration in the Cu-nitride film, the nitride based memory devices show bipolar nature with a low operation voltage, forming-free characteristics, a distinguishable resistance ratio and a dependence on electrode materials. The bipolar switching behavior is attributed to the formation and rupture of conductive filaments within a cycle, which is confirmed by spreading resistance images. From the voltage step experiment, it was found that local conducting filaments might be formed by migration of Cuz+ ions under bias voltage. The properties of the Ni/CuxN/Cu memory cell, including programming speed, temperature dependence, endurance, retention and scalability, were discussed.4. AlN thin films were fabricated on Al interconnection by using PⅢand Pulse Laser Deposition (PLD), respectively. And their topography, crystal structure and chemical composition were investigated. By introducing top electrode metal Cu, a capacitor consisted of insulating layer AlN, sandwiched between Cu and Al was formed. The Cu/AlN/Al memory cells showed resistive switching behavior; however, the device performance has remained quite poor up to now and needs to be improved in the next future. Finally the application prospect of the AlN based RRAM cells are discussed.更多还原