标题: | 电阻式记忆体及电子突触元件之数值模拟研究 Numerical simulation of RRAM and electronic synaptic device |
作者: | 王钰芬 Yu-Fen Wang 侯拓宏 电子工程学系 电子研究所 |
关键字: | 电阻式记忆体;电子突触;数值模拟;RRAM;electronic synapse;numerical simulation |
公开日期: | 2014 |
摘要: | 在过去,元件尺寸的微缩能够直接反应在电脑系统效能的提升。然而在传统的记忆储存阶层架构下,元件的微缩造成固态记忆体和硬碟两者的存取时间相差愈来愈大,而存取时间的巨大差异使得元件微缩不再能直接提高电脑效能。为了解决存取时间差异的问题,必须在记忆储存阶层中增加储存记忆体(storage class memory),来弥补记忆体和硬碟之间的存取时间差异。另一个传统大型资讯系统的问题是,要作出能与人类智慧匹敌的电脑需要消耗大量的电力。一般人脑的功耗大约是十瓦,而超级电脑的电力消耗高达一百四十万瓦。因此低功耗且具有容错特性的电子突触(electronic synapse)元件与其在类神经计算上的应用是值得关注的。而在储存记忆体与电子突触元件发展上,电阻式记忆体(RRAM)都扮演着非常重要的关键角色。 对于发展储存记忆体而言,了解RRAM阻值转换的机制是必须的。了解阻值转换的机制才能进一步研究阻值转换变异、元件热效应、电流杂讯以及耐久性和保存时间这些特性或现象的根源,进一步帮助改善元件。对于电子突触元件来说,虽然元件变异对类神经计算系统所造成的影响较小,但研究阻值转换的机制仍然能提供元件特性解释以及改善方式,而且建立的物理模型能进一步提供类神经计算系统模拟设计使用。目前已经有不少的电子突触模型可模拟元件特性,然而大部分提出来的模型仍太过简化,无法提供时间动态的物理解释以及元件改善的建议。 本篇论文建立了两个RRAM的数值模拟模型,一个是TiN/HfO2/Pt灯丝型(filamentary)电阻式记忆体模型,另一个是Ta/TaOx/TiO2/Ti非灯丝型电阻式记忆体模型。灯丝型电阻式记忆体模型是根据渗透理论(percolation theory)所建立的。这个模型考虑了缺陷辅助穿隧电流、氧缺(oxygen vacancy)的生成与消灭、氧离子的移动以及焦耳热效应。这个模型以SET和RESET过程中缺陷的分布成功地解释阻值转换的现象。也成功地模拟出I-V特性和SET切换电压的韦伯分布(Weibull distribution)。 Ta/TaOx/TiO2/Ti非灯丝型电阻式记忆体模型是根据均值能障调变机制所建立的模型。这个模型考虑了被TaOx限制的WKB(Wentzel-Kramers-Brillouin)穿隧电流、帕松方程式(Poisson equation)以及计算氧离子移动的连续方程式。这个模型以SET和RESET过程中氧缺和氧离子的分布成功地解释阻值转换和自我整流的现象。也成功模拟出RESET造成的多阶阻态转换以及调变不同材料厚度的影响。此外,这个模型也成功模拟了电子突触的特性,像是突触连结的增强(potentiation)和抑制(depression)、STDP(spike-timing-dependent plasticity)以及PPF(paired-pulse facilitation),和实验结果相较皆有很好的一致性。 In order to continue improving performance of large-scale information systems, it is essential to revolutionize the present memory and storage hierarchy where a large access time gap exists between DRAM and hard disks. Storage class memory (SCM) is proposed to fill the access time gap and significantly improve the system performance. Another critical issue of large-scale information systems is the power consumption. To achieve human-level intelligence, the required power consumption is as high as 1.4 MW by using the most advanced supercomputer, while the power consumption of human brain is merely 10 W. Aiming for energy-efficient, fault-tolerant and high-performance information systems, electronic synaptic devices for neuromorphic computing now attract significant attention. Among many emerging devices for storage class memory and electronic synapse, it is widely believed that RRAM is an extremely competitive candidate. For SCM, a comprehensive understanding of RRAM switching mechanism is the foundation of studying resistive-switching variation, thermal effect, current noise, retention/endurance degradation, and device optimization. For electronic synaptic devices, although there have been a few models proposed to explain the device behavior, most of them are oversimplified and cannot be applied for dynamic response, device optimization strategy, and simulation of large-scale neuromorphic computing systems. In this thesis, two numerical models are constructed for RRAM physical simulation. One is TiN/HfO2/Pt filamentary RRAM, and the other is Ta/TaOx/TiO2/Ti non-filamentary RRAM. The filamentary RRAM model is constructed based on the percolation theory. The model considers trap-assisted-tunneling current, oxygen ion migration, generation/recombination of oxygen vacancies, and Joule heating. The transient defect patterns of forming, SET, and RESET are investigated to explain the resistive switching. The I-V characteristics and Weibull distribution of SET voltage are also simulated. Furthermore, the Ta/TaOx/TiO2/Ti non-filamentary RRAM model is constructed based on homogeneous barrier modulation mechanism. The model considers WKB tunneling current limited by the TaOx barrier, Poisson equation, and continuity equation of ion migration. The transient oxygen ion and vacancy profiles during SET and RESET are investigated to explain the resistive switching and self-rectifying I-V curves. The multi-level RESET and film-thickness dependence are also successfully simulated. Furthermore, the Ta/TaOx/TiO2/Ti non-filamentary RRAM model is applied to simulate various electronic synaptic characteristics, including potentiation, depression, spike-timing-dependent plasticity, and paired-pulse facilitation, and show excellent agreements with the experiment data. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT070150130 http://hdl.handle.net/11536/76099 |
显示于类别: | Thesis |