A fast and reliable method used to investigate the size-dependent retention lifetime of a phase-change line cell

In this article we use a novel method to assess the retention properties of a PCM line cell having a fast-growth chalcogenide material. We monitor the cell resistance during temperature-ramp experiments carried out after reset programming of the cells. The temperature-dependent resistance is modeled assuming the crystallization proceeds by the crystal growth of the amorphous/crystal interfacial front. The results show good reproducibility, excellent match with the measurements, and consistency with investigations using isothermal experiments. For a single temperature-ramp experiment this fast method allows the extraction of the activation energy for growth as well as the extrapolated temperature for 10 years retention lifetime. We use the method to investigate the influence of the length and thickness of the programmed amorphous mark on the retention lifetime. The results point to improved retention for thicker and longer amorphous marks, however the extrapolation of the results down to short marks suggests a limited reduction of the retention lifetime with the cell downscaling.     

Improving the data retention of phase change memory by using a doping element in selected Ge_2Sb_2Te_5

The crystallization characteristics of a ubiquitous T-shaped phase change memory(PCM) cell, under SET current pulse and very small disturb current pulse, have been investigated by finite element modelling. As analyzed in this paper, the crystallization region under SET current pulse presents first on the corner of the bottom electron contact(BEC) and then promptly forms a filament shunting down the amorphous phase to achieve the low-resistance state, whereas the tiny disturb current pulse accelerates crystallization at the axis of symmetry in the phase change material. According to the different crystallization paths, a new structure of phase change material layer is proposed to improve the data retention for PCM without impeding SET operation.This structure only requires one or two additional process steps to dope nitrogen element in the center region of phase change material layer to increase the crystallization temperature in this confined region. The electrical-thermal characteristics of PCM cells with incremental doped radius have been analyzed and the best performance is presented when the doped radius is equal to the radius of the BEC.     

Modeling of Programming and Read Performance in Phase-Change Memories—Part II: Program Disturb and Mixed-Scaling Approach

The scaling analysis of phase-change memory (PCM) cells is an essential step toward validation as a competitive technology in terms of array density and current consumption. While the current scaling has been addressed in a companion paper, we focus here on the thermal crosstalk, namely, the temperature increase in 1 bit in the array while an adjacent cell is being programmed by a high-current reset pulse. This parasitic heating may lead to partial crystallization in the amorphous phase and to a consequent resistance decrease after cycling. Our analysis shows that the thermal crosstalk strongly depends on the scaling approach used, e.g., isotropic or nonisotropic scaling. A new mixed-scaling option for PCM cells is proposed, which provides the maximum decrease of programming current compatible with the reliability requirements deriving from the thermal crosstalk. The effects of this new scaling approach on the programmed volume and data retention are finally addressed.     

Modeling of SET seasoning effects in Phase Change Memory arrays

The experimental analysis of the electrical behavior of Phase Change Memory arrays evidenced a seasoning effect both on SET and RESET state. The previous modeling efforts on this issue were addressed only towards RESET operation. This work presents a SET seasoning model implemented within a numerical simulator starting from the extraction of the characteristic Erase operation kinetic parameters, in order to complete the picture of seasoning modeling in PCM. The analysis of such phenomenon, that is not detrimental to the memory, allows a better comprehension of the transition dynamics from the amorphous to the crystalline phase. The model developed reproduces experimental data obtained with writing waveforms featuring different crystallization approaches.     

Impact of Ge-Sb-Te compound engineering on the set operation performance in phase-change memories

The phase-change memory (PCM) technology is considered as one of the most attractive non-volatile memory concepts for next generation data storage. It relies on the ability of a chalcogenide material belonging to the Ge-Sb-Te compound system to reversibly change its phase between two stable states, namely the poly-crystalline low-resistive state and the amorphous high-resistive state, allowing the storage of the logical bit. A careful study of the phase-change material properties in terms of the set operation performance, the program window and the electrical switching parameters as a function of composition is very attractive in order to enlarge the possible PCM application spectrum. Concerning the set performance, a crystallization kinetics based interpretation of the observed behavior measured on different Ge-Sb-Te compounds is provided, allowing a physics-based comprehension of the reset-to-set transition.     

Analysis by simulation of amorphization current in phase change memory applied to pillar and GST confined type cells

In this work comparative simulations and analysis of amorphization current are presented for pillar type and GST confined type PCRAM structures. The simulations are realized with the PCM model of Sentaurus Device using an analytical phase transition model coupled with a drift-diffusion electro-thermal model for the transport. The objectives of this work are the selection of optimized cell structures for reset conditions. It is confirmed that regarding I_reset the GST confined cell is more efficient than the pillar type. Our study points out that a compromise has to be found in some devices where the conditions of an optimized amorphous current correspond to a lowered resistivity contrast between the amorphous and crystalline states. A compromise has also to be found between optimal structures as designed by simulation and technological constraints associated to their fabrication.     

Explanation of programming distributions in phase-change memory arrays based on crystallization time statistics

In order to validate phase change memory (PCM) technology, the programming reliability, in terms of reading window between the programmed and erased state, must be guaranteed at array level with an error less then 1 part-per-billion. The reset distribution is significantly influenced by the quenching operation [Mantegazza D, Ielmini D, Pirovano A, Gleixner B, Lacaita A L, Varesi E, et al. Electrical characterization of anomalous cells in phase change memory arrays. IEDM Tech Dig 2006:53–56]. In this paper this phenomenon is explained in terms of PCM active material crystallization statistics. A significant spread in the crystallization times among PCM cells is detected both in the write-reset operation from the melted-amorphous state (quenching) and in the erase-set operation from the solid-amorphous state. At statistical level, a correlation between set at high and low temperatures and quenching behavior of cells is found, allowing to describe the programming distributions uniquely in terms of crystallization times statistics.     

Modeling of the intrinsic retention characteristics of FLOTOXEEPROM cells under elevated temperature conditions

A model for the intrinsic retention characteristics of FLOTOX EEPROM cells is presented, which is based on the temperature dependence of the Fowler-Nordheim emission current. This model which has been successfully tested on single-poly-FLOTOX EEPROM cells, enables the device lifetime to be calculated for given memory operating conditions, instead of being extrapolated as is usually done. The sensitivity of the retention characteristics to several technological parameters is also investigated. It is expected that this intrinsic retention model (with minor modifications) will also be applicable to FLASH EEPROM cells     

Fast Reversible Phase Change Silicon for Visible Active Photonics

Both amorphous and crystalline silicon are ubiquitous materials for electronics, photonics, and microelectromechanical systems. On‐demand control of Si crystallinity is crucial for device manufacturing and to overcome the limitations of current phase‐change materials (PCM) in active photonics. Fast reversible phase transformation in silicon, however, has never been accomplished due to the notorious challenge of amorphization. It is demonstrated that nanostructured Si can function as a PCM, since it can be reversibly crystallized and amorphized under nanosecond laser irradiation with different pulse energies. Reflection probing on a single nanodisk's phase transformations confirms the distinct mechanisms for crystallization and amorphization. The experimental results show that the relaxation time of undercooled silicon at 950 K is 10 ns. The phase change provides a 20% nonvolatile reflectivity modulation within 100 ns and can be repeated over 400 times. It is shown that such transformations are free of deformation upon solidification. Based on the switchable photonic properties in the visible spectrum, proof‐of‐concept experiments of dielectric color displays and dynamic wavefront control are shown. Therefore, nanostructured silicon is proposed as a chemically stable, deformation free, and complementary metal–oxide‐semiconductor compatible (CMOS) PCM for active photonics at visible wavelengths.     

Numerical modeling of an amorphous-silicon-based p-i-n solar cell

A simulation program for amorphous-silicon-based p-i-n solar cells which allows for accurate calculation of single-junction or multijunction cell response under monochromatic or global AM1.5 illumination is discussed. The device model is based on a complete set of Poisson and current continuity equations describing the amorphous intrinsic and microcrystalline or amorphous n⁺ and p⁺ contacts. It predicts solar cell behavior with uniform and nonuniform optical (mobility) bandgaps, spatially dependent doping densities, and various layer thicknesses, as demonstrated by the very good agreement between the experimental and simulated current-voltage characteristics of single cells, with the bandgaps in the range of 1.75 to 1.47 eV. The material parameters used in the simulation have been obtained from experimental results reported in the literature. The possibility of obtaining higher efficiencies using novel cell designs has also been investigated. Calculations have been carried out on cell structures in which the bandgap of the intrinsic layer is profiled to help hole transport. The most efficient structure, also confirmed by recent experimental data, incorporates normal profiling throughout the bulk of the intrinsic layer with a thin graded buffer at the p⁺ -intrinsic junction     

An SPICE model for phase-change memory simulations

Along with a series of research works on the physical prototype and properties of the memory cell,an SPICE model for phase-change memory(PCM) simulations based on Verilog-A language is presented.By handling it with the heat distribution algorithm,threshold switching theory and the crystallization kinetic model,the proposed SPICE model can effectively reproduce the physical behaviors of the phase-change memory cell.In particular,it can emulate the cell’s temperature curve and crystallinity profile during the programming process,which can enable us to clearly understand the PCM’s working principle and program process.     

A Reliable Technique for Experimental Evaluation of Crystallization Activation Energy in PCMs

This letter investigates the extraction of activation energy for the crystallization of an amorphous chalcogenide material in phase-change memories. It is demonstrated for the first time that the critical resistance, which is the value of resistance defining the crystallization time for the chalcogenide material, has a major impact on the extraction of the activation energy for crystallization. Applying a statistical Monte Carlo model for crystallization coupled with an electrothermal model for both the amorphous and crystalline phases, we analyzed the standard methodology for the extraction of the activation energy. It is shown that a careful choice of the critical resistance is mandatory and a new accurate technique is proposed, resulting in a reliable value for the crystallization activation energy of 2.6 eV in the Ge₂Sb₂Te₂-based devices.     

一个集成电路工艺诊断实例

介绍了一种计算机自动PCM(process control module)工艺参数分析系统,可用于集成电路制造过程中的工艺诊断和分析.在具体处理上应用了主成分分析方法,能够从大量数据中提取其统计特征,根据这一特征可找出造成工艺波动的关键因素.从所给出的具体诊断实例来看,该方法能够得出一般人工诊断所不能得出的诊断结论,效果较好,是实施严格生产控制的有效工具.     

一种用于相变存储器仿真的SPICE模型

本文提出了一种用于相变存储器仿真的SPICE模型,该模型基于Verilog-A描述语言,通过对相变存储单元在编程过程中的热分布、阈值效应以及结晶理论的分析,该模型系统地概括了影响相变存储单元编程过程的各种因素。模拟结果表明,该模型的仿真结果与实验数据基本符合,同时,该模型还能直观地反映出相变存储单元在编程过程中的温度特性和结晶特性,能更直观地反映出相变存储器的工作原理和工作过程。     

An empirical model to determine the grain size of metal-induced lateral crystallized film

Thin-film transistors (TFTs) have been fabricated using the nickel-seeded metal-induced lateral crystallization (MILC), in which an amorphous silicon is crystallized to form a large grain polysilicon film. Single crystal SOI, solid phase crystallization (SPC), and MILC TFTs were fabricated and the carrier mobilities extracted. Different types of devices have different variations in electrical properties. An empirical model based on the presence of the grain boundaries is proposed to explain the experimental results. The experimental data was used to extract the model parameters and the number of grains and grain size present in the device channel. The results can be further used to optimize the crystallization process and the device design.     

Recovery and Drift Dynamics of Resistance and Threshold Voltages in Phase-Change Memories

The electronic behavior of the chalcogenide material used in phase-change memory (PCM) plays a key role in defining the operation voltages and times of the memory cell. In particular, the threshold voltage for electronic switching of the amorphous chalcogenide determines the boundary between programming and readout operation, while its resistance allows the recognition of the bit status. This paper present a time-resolved analysis of threshold voltage and resistance in a PCM. Both dynamics of threshold voltage and resistance display a fast transient, named recovery behavior, in the first 30 ns after programming. A slower, nonsaturating drift transient is found for longer times. The two transients are discussed referring to electronic and structural rearrangements in the amorphous chalcogenide. Finally, the impact on the device level is considered     

Status and challenges of phase change memory modeling

Phase change memory (PCM) is the most performing non-volatile memory-technology alternative to FLASH. In order to design optimized cells, in term of geometry and materials, for present and future technological nodes, physics-based models for readout, programming and data retention are required. In this work, transport and phase-change modeling in PCM cell is reviewed and used to discuss (1) the programming-current reduction through geometry optimization, (2) the trade-off between programming current and readout resistance and (3) the program disturb phenomenon. In particular, scaling of PCM is extensively investigated, comparing the impact of isotropic and non-isotropic scaling on cell programming-current and program-disturb. Recent developments and open issues in PCM physical modeling are finally presented.     

Dynamic Resistance—A Metric for Variability Characterization of Phase-Change Memory

The resistance of phase-change-memory (PCM) cells measured during RESET programming (dynamic resistance, $R_{d}$) is found to be inversely proportional to the amplitude of the programming current, as $R_{d} = [A/I] + B$. We show that parameters $A$ and $B$ are related to the intrinsic properties of the memory cell, and demonstrate by means of experimental data that they could be used to characterize the cell-to-cell process-induced variability of PCM cells.      

Erratic erase in flash memories - part I: basic experimental and statistical characterization

This paper presents experimental results and statistics about the erratic erase in Flash Memories, setting the basis for any physical modeling of the phenomena and data comparison. Statistical parameters like the reliability function and the failure rate have been measured and modeled by analytical functions showing that all cells of an array may potentially exhibit erratic events. By mapping the physical position of each erratic bit in a sector and using an equivalent cell approach, it has been possible to establish a correlation between the erratic phenomena and the intrinsic amorphous nature of SiO/sub 2/. Tail bits of the erased distribution have been shown to be caused by erratic events suggesting a unique physical cause for the two phenomena. The relation between positive and negative shifts has also been discussed and overerase risks caused by erratic behaviors have been estimated.     

相变异质结(PCH)存储材料相变行为的原位观测

相变存储器(PCM)具有速度快、寿命长等一系列优点.传统的相变存储器在相变过程中会产生蘑菇状的相变层,导致沿电流方向的元素迁移,使器件在多次循环之后失效.相变异质结(phase-change heterostructure,PCH)存储材料通过在相变材料Sb2Te3(ST)中嵌入约束层TiTe2(TT),能够有效降低其...