Phase-change Random Access Memory Cell Research Articles

Use of Eutectic Effects in the Possible Creation of PCM Memory Cells on the Basis Ag–Cu Nanoclusters

Abstract—An attractive direction in the development of nanoelectronics is the development of a new generation of non-volatile storage devices, namely, electric phase memory or PC-RAM (Phase Change Random Access Memory). However, there are a number of unresolved problems here, such as: the stability of the amorphous phase, high power consumption, long information recording time, etc. In order to resolve these contradictions, a new approach was proposed, which consists in the use of Ag–Cu binary alloy nanoparticles as PC-RAM cells. To this end, the molecular dynamics method was used to study the processes of structurization of nanoparticles of this alloy with a size D = 2–10 nm of various target compositions with a variation in the rate of removal of thermal energy. Criteria for the stability of the amorphous and crystalline structure were evaluated, and conclusions were drawn about the target composition and size of nanoparticles suitable for creating phase-change memory cells. It was shown that in the case of the use of nanoparticles of the binary Ag–Cu alloy, it is possible to reduce the size of one cell to 6–8 nm, reduce the time of recording information to 2.5 ns, and, for the first time, based on the eutectic approach, achieve the stability of the amorphous and crystalline structure at different rates of thermal energy removal.

Boolean Logic Function Realized by Phase-Change Blade Type Random Access Memory

Phase-change random access memory (PCRAM) has been considered as one of the most promising candidates for future nonvolatile storage applications. Recent advent of the blade-type PCRAM further shed a light on its commercial prospect due to its much smaller energy consumption than the conventional Lance-type PCRAM. However, the logic computing capability of the blade-type PCRAM remains vague up-to-date. To deeply study its function versatility, we developed a comprehensive three-dimensional model based on the classical nucleation-growth theory to mimic the crystallization and amorphization processes of the blade-type PCRAM device. According to this well-established model, the device resistance after phase transformation was calculated with respect to different crystallization and amorphization pulses. By pertaining high and low pulse magnitudes and device resistances to appropriate input and output logic levels, a variety of Boolean logic functions, including “ NAND,” NOR,” AND,” OR,” and “ NOT,” were successfully achieved on one single PCRAM cell. The feasibility of simultaneously realizing storage and logic computing functions on the designed PCRAM device was therefore demonstrated. Such intriguing merit, combined with its superb storage traits, makes the proposed blade-type PCRAM cell suitable for current in-memory computing and neuromorphic circuits as the key components of future digital computers operated on non-von Neumann mode.

Excellent thermal stability attributed to Cr dopant in Sb2Te phase change material

Herein, Cr was selected as the dopant to improve the performance of Sb2Te in phase change random access memory (PCRAM). The thermal properties, crystal structure and chemical bonding state of the as-prepared Cr0.39Sb2Te (CST) were investigated. The CST containing 11.5At% Cr-dopant exhibited excellent performances including high crystallization temperature (218.6℃), good data retention (10 years @136.5 ± 1.0℃) and high stability with low-density change rate (2.8%). The switching speed up to 10 ns and the endurance nearly 1 × 105 cycles were obtained for the CST based PCRAM cells. All these findings indicated that the CST material is a potential candidate for universal memory devices.

Thermoelectric Effects on Amorphization Process of Blade-Type Phase Change Random Access Memory

Commercial prospect of phase-change random access memory (PCRAM), considered as an encouraging option for future “universal” memory, is however challenged by its large programming current that severely impairs device scalability. The recent advent of the blade-type PCRAM configuration can effectively address this issue by shrinking the heated region. However, the thermoelectric (TE) effects that play a key role in write performances of conventional Lance-type PCRAM have not been systematically studied for this blade-type case to date. The influence of TE effects on the amorphization process of the blade-type PCRAM cell is therefore investigated here through the establishment of a comprehensive 3-D electrothermal and phase-transformation model. Two main typical TE effects, i.e., Thomson heat inside the Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (GST) bulk region and Peltier heat at the GST-TiN heater interface, have been mimicked and discussed in detail. Simulation results show that both TE effects depend on the polarities of the programming currents, and the calculated Thomson heat is more than 20 times as much as Peltier heat. Accordingly, a novel skutterudite-based heater that exhibits more pronounced TE effects than TiN heater has been proposed. The practicality of reducing programming current by 13.8% and peak power by 17.9% is demonstrated using the improved version of the PCRAM cell.

Influence of Thomson effect on amorphization in phase-change memory: dimensional analysis based on Buckingham’s П theorem for Ge2Sb2Te5

To evaluate the Thomson effect on the temperature increase in Ge2Sb2Te5 (GST)-based phase-change random access memory (PCRAM), we created new dimensionless numbers based on Buckingham’s П theorem. The influence of the Thomson effect on the temperature increase depends on the dominant factor of electrical resistance in a PCRAM cell. When the effect is dominated by the volumetric resistance of the phase-change material the dimensionless evaluation number is where ρ c is the contact resistance, Δx is the thickness of PCM, σ and k are the electrical and thermal conductivities, μ T is the Thomson coefficient, and Δϕ is the voltage. When the contact resistance cannot be ignored, the evaluation number is B/(1 + C). The characteristics of hexagonal-type crystalline GST in a PCRAM cell were numerically investigated using the defined dimensionless parameters. Although the contact resistance of GST exceeded the volumetric resistance across the temperature range, the ratio of contact resistance to the whole resistance reduced with increasing temperature. Moreover, increasing the temperature of GST enhanced the influence of the Thomson effect on the temperature distribution. At high temperatures, the Thomson effect suppressed the temperature increase by approximately 10%–20%.

A physics-based compact model of phase change for the design of cross-point storage-class memories

A physics-based compact model for phase-change random access memory (PcRAM) was proposed considering the ratio of vertical-to-lateral crystal growth rate (α), and it was incorporated into HSPICE via Verilog-A. The proposed model was verified using the experimental results taken from the 256 × 256 cross-point (X-point) PcRAM cell array with the Ge2Sb2Te5 20–22 nm ITRS technology node. The proposed compact model successfully reproduced the measured PcRAM cell resistance (RC) depending on the SET pulse width and amplitude after a background RESET, which is a challenging issue that holds back the X-point PcRAM as a promising candidate for a modern storage-class memory in perspective of the write latency and power consumption, without heavy computational burden while capturing the essence of physical meaning via the multidomain simulation which includes the threshold switching, electrical, thermal, and phase-change modules. The extracted α value was 1.55. Furthermore, it was found that the SET pulse-dependent abrupt/gradual change of RC is sensitive to α. This suggests that α should be carefully optimized for PCM-based neuromorphic applications

The crystallization mechanism of zirconium-doped Sb2Te3 material for phase-change random-access memory application

Sb2Te3 (ST) as phase-change material has the advantage of high speed, but very poor thermal stability, which cannot be directly used for phase-change random-access memory (PCRAM). In this study, Zr1.5(Sb2Te3)98.5 (ZST) material was investigated for PCRAM application. Zr dopant can efficiently improve the thermal stability of ST alloy, stabilizing its amorphous state at room temperature. During annealing process, amorphous ZST film firstly transfers to face-centered cubic structure with small grain size, and following the second switching to hexagonal phase, it is delayed to 225 °C, which is more than 100 °C higher than ST alloy, confirming by in situ heating transmission electron microscopy. Furthermore, ZST-based PCRAM cell has good endurance up to 1.5 × 104 electrical cycles, a high amorphous resistance larger than 106 Ω and a resistance ratio of about 1.5 orders of magnitude. The reversible phase transition can be realized by a pulse of 100 ns.

Scandium doping brings speed improvement in Sb2Te alloy for phase change random access memory application

Phase change random access memory (PCRAM) has gained much attention as a candidate for nonvolatile memory application. To develop PCRAM materials with better properties, especially to draw closer to dynamic random access memory (DRAM), the key challenge is to research new high-speed phase change materials. Here, Scandium (Sc) has been found it is helpful to get high-speed and good stability after doping in Sb2Te alloy. Sc0.1Sb2Te based PCRAM cell can achieve reversible switching by applying even 6 ns voltage pulse experimentally. And, Sc doping not only promotes amorphous stability but also improves the endurance ability comparing with pure Sb2Te alloy. Moreover, according to DFT calculations, strong Sc-Te bonds lead to the rigidity of Sc centered octahedrons, which may act as crystallization precursors in recrystallization process to boost the set speed.

Contact resistance change memory using N-doped Cr2Ge2Te6 phase-change material showing non-bulk resistance change

Phase-change random access memory (PCRAM) is enabled by a large resistance contrast between amorphous and crystalline phases upon reversible switching between the two states. Thus, great efforts have been devoted to identifying potential phase-change materials (PCMs) with large electrical contrast to realize a more accurate reading operation. In contrast, although the truly dominant resistance in a scaled PCRAM cell is contact resistance, less attention has been paid toward the investigation of the contact property between PCMs and electrode metals. This study aims to propose a non-bulk-resistance-dominant PCRAM whose resistance is modulated only by contact. The contact-resistance-dominated PCM exploited here is N-doped Cr2Ge2Te6 (NCrGT), which exhibits almost no electrical resistivity difference between the two phases but exhibits a typical switching behavior involving a three-order-of-magnitude SET/RESET resistance ratio owing to its large contact resistance contrast. The conduction mechanism was discussed on the basis of current–voltage characteristics of the interface between the NCrGT and the W electrode.

Incremental Bitline Voltage Sensing Scheme With Half-Adaptive Threshold Reference Scheme in MLC PRAM

Research on phase-change random access memory (PRAM) for multilevel cells (MLCs) has been actively conducted owing to the advantages of PRAM cells, such as large resistance margin and fast read/write access time. However, the resistance drift (R-drift), which increases the resistance of the PRAM cells with time, should be overcome to achieve MLC PRAM operation. In this paper, we introduce sensing methods with R-drift tolerance, namely, drift-resilient cell-state metric and incremental bitline voltage (IBV), and compare these sensing methods in terms of the sensing margin and read access time. In addition, we propose a sensing scheme for IBV (IBVSS) with a half-adaptive threshold reference scheme (H-ATRS) to achieve high-R-drift tolerance in severe R-drift conditions with a small layout area for the reference cell. Verification of the IBVSS with H-ATRS is performed by HSPICE simulation using the 0.25- $\mu \text{m}$ model parameters used in the peripheral circuit of Samsung’s 20-nm PRAM technology. From the simulation, we find that the IBVSS with H-ATRS achieves more than 1 V of sensing margin under severe R-drift conditions, which ensures stable read operation in the MLC PRAM with 304 ns of sensing time.

Enhanced Wear-Rate Leveling for PRAM Lifetime Improvement Considering Process Variation

The limited write endurance is one of the major obstacles for phase-change random access memory (PRAM)-based main memory. Traditionally, wear-leveling (WL) techniques were proposed to enhance its lifetime by balancing write traffic. However, these techniques do not concern the endurance variation in PRAM chips. When different PRAM cells have distinct endurance, balanced writes results in lifetime degradation due to the weakest cells. In this paper, we first define a new metric-wear rate (i.e., writes/endurance) considering both the write traffic and endurance distribution from application and hardware, respectively. After investigating the writing behavior of applications and endurance variation, we propose an architecture-level leveling mechanism to balance wear rate of cells across the PRAM chip. Hardware and algorithm to support the proposed leveling mechanism are presented. Moreover, there is an important tradeoff between endurance improvement and swapping data volume. To co-optimize endurance and swapping, this situation is formulated as a maximum weight perfect matching problem in bipartite graph. Thereafter, a novel algorithm that minimizes wear-rate and swapping by employing Kuhn–Munkras algorithm is proposed to maximize PRAM lifetime and minimize performance degradation. The experimental results show $\sim 17 \times $ lifetime improvement over prior WL.

Lattice Distortion in In3SbTe2 Phase Change Material with Substitutional Bi.

Sb atoms in In3SbTe2 (IST) are partially substituted by 3.2–5.5 at.% of Bi atoms. As a result, the NaCl crystal structure of IST is slightly distorted. The distorted inter-planar angles observed with fast Fourier transformation of the lattice images are within the maximum range of interplanar angles calculated by density functional theory. When the Bi content is increased, the crystallization temperature becomes relatively lower than that of IST, the activation energy decreases from 5.29 to 2.61 eV, and the specific heat and melting point are obviously reduced. Consequently, phase change random access memory (PRAM) fabricated with Bi-doped IST (Bi-IST) can operate with lower power consumption than pure IST PRAM. The set and reset speeds of PRAM cells fabricated with Bi-IST are both 100 ns with 5.5 at.% Bi, which are obviously faster than the switching speeds of PRAM cells fabricated with IST and Ge2Sb2Te5 (GST). These experimental results reveal that the switching speed is closely related with the thermal properties of the distorted lattice structure.

Microstructure-dependent DC set switching behaviors of Ge–Sb–Te-based phase-change random access memory devices accessed by in situ TEM

Phase-change random access memory (PCRAM) is one of the most promising nonvolatile memory devices. However, inability to secure consistent and reliable switching operations in nanometer-scale programing volumes limits its practical use for high-density applications. Here, we report in situ transmission electron microscopy investigation of the DC set switching of Ge–Sb–Te (GST)-based vertical PCRAM cells. We demonstrate that the microstructure of GST, particularly the passive component surrounding the dome-shaped active switching volume, plays a critical role in determining the local temperature distribution and is therefore responsible for inconsistent cell-to-cell switching behaviors. As demonstrated by a PCRAM cell with a highly crystallized GST matrix, the excessive Joule heat can cause melting and evaporation of the switching volume, resulting in device failure. The failure occurred via two-step void formation due to accelerated phase separation in the molten GST by the polarity-dependent atomic migration of constituent elements. The presented real-time observations contribute to the understanding of inconsistent switching and premature failure of GST-based PCRAM cells and can guide future design of reliable PCRAM. Using in situ electron microscopy, researchers in Korea have discovered why phase-change memory devices can fail during switching operations. Phase-change materials, such as germanium–antimony–tellurium (Ge–Sb–Te) alloys, rapidly switch between crystalline and amorphous phases on electrical heating. Researchers have used this to make memory chips that are hundreds of times faster than flash devices, but inconsistent performance currently limits practical, high-density applications. Sang Ho Oh from Pohang University of Science and Technology and colleagues observed the phase changes of Ge–Sb–Te memory cells in real time by applying direct-current voltages through a transmission electron microscope. They found that the device lifetime critically depended on the atomic microstructure around the active switching site. Ironically, higher degree of crystallinity of this passive zone can cause more excessive heat dissipation, leading to eventual melting and evaporation of cells. Applying in situ TEM techniques to GST-based vertical PCRAM cells, we directly observed the DC set switching process of real devices for the first time. The results show that the microstructure of crystalline GST matrix is an important structural parameter determining the local temperature distribution. In the case of highly crystallized GST matrix, the device failure occurred via two-step void formation due to the polarity-dependent electromigration.

Investigation of Cr0.06(Sb4Te)0.94 alloy for high-speed and high-data-retention phase change random access memory applications

The effects of Cr doping on the structural and electrical properties of Crx(Sb4Te)1−x materials have been investigated in order to solve the contradiction between thermal stability and fast crystallization speed of Sb4Te alloys. Cr0.06(Sb4Te)0.94 alloy is considered to be a potential candidate for phase change random access memory (PCM), as evidenced by a higher crystallization temperature (204 °C), a better data retention ability (137.6 °C for 10 years), a lower melting point (558 °C), a lower energy consumption, and a faster switching speed in comparison with those of Ge2Sb2Te5. A reversible switching between set and reset states can be realized by an electric pulse as short as 5 ns for Cr0.06(Sb4Te)0.94-based PCM cell. In addition, Cr0.06(Sb4Te)0.94 shows good endurance up to 1.1 × 104 cycles with a resistance ratio of about two orders of magnitude.

Thermal effect of Ge2Sb2Te5 in phase change memory device

In the fabrication of phase change random access memory (PRAM) devices, high temperature thermal processes are inevitable. We investigate the thermal stability of Ge2Sb2Te5 (GST) which is a prototypical phase change material. After high temperature process, voids of phase change material exist at the interface between Ge2Sb2Te5 and substrate in the initial open memory cell. This lower region of Ge2Sb2Te5 is found to be a Te-rich phase change layer. Phase change memory devices are fabricated in different process conditions and examined by scanning electron microscopy and energy dispersive X-ray. It is found that hot-chuck process, nitrogen-doping process, and lower temperature inter-metal dielectric (IMD) deposition process can ease the thermal impact of line-GST PRAM cell.

Study and Improvement on Tungsten Plug Corrosion in CMP Process for PCRAM

To reduce the reset current for developing reliable high density phase change random access memory (PCRAM), small bottom electrode contact (BEC) size formation is a critical process. One of the failure mode for the process is the corrosion of tungsten plug, which is caused by tungsten chemical mechanical planarization (CMP) process. In this paper, this CMP process was analyzed. The tungsten polishing step process was characterized by the coefficient j and it shows good performance in tungsten polishing process. The alkali and acidic buff slurry effect on tungsten plug performance were studied. The result shows that the recess free tungsten plug had been fabricated with acidic buff slurry. The electric results confirm that it can fulfill the set operation of PCRAM cells.

The effect of titanium doping on the structure and phase change characteristics of Sb4Te

As a growth-dominated phase change material, Sb4Te (ST) has fast crystallization speed while thermal stability is very poor, which makes it unsuitable for application in phase change random access memory (PCRAM). After doping Ti, the crystallization temperature is greatly improved to 210.33 °C, which is much higher than that of conventional Ge2Sb2Te5 (∼150 °C), and the melting point is reduced to 540.27 °C. In addition, grain size of crystalline Ti-doped Sb4Te (TST) film is significantly decreased to nanoscale. Ti atom is believed to occupy the lattice site of Sb atom in TST. With good thermal stability, TST-based PCRAM cell also has fast crystallization rate of 6 ns. Furthermore, the energy consumption is also lower than that of Ge2Sb2Te5-based one. Endurance of exceeding 2E5 cycles is obtained with a resistance ratio of one order of magnitude. Therefore, Ti doping seems to be a good way to solve the contradiction between thermal stability and fast crystallization speed of Sb-Te alloys.

Intrinsic threshold mechanism of phase-change memory cells by pulsed current–voltage characterization

A short pulsed current–voltage (I-V) measurement method is proposed for phase-change random access memory (PCRAM) to investigate the self-heating behavior. The pulse widths and periods are indispensable parameters to describe pulsed I-V characteristics of PCRAM cells. By comparing the difference between direct current I-V curves and pulsed I-V curves, the threshold voltages of pulsed I-V are much higher. It implies the existence of self-heating and energy accumulation. Assume that the heating of the active region causes the change of the electronic barrier and the electronic activity, the physical model dominated by the self-heating in PCRAM cells is proposed.

Investigation of CuSb4Te2 alloy for high-speed phase change random access memory applications

The thermal stability of amorphous Sb2Te film can be significantly improved by the addition of Cu. CuSb4Te2 alloy is considered to be a potential candidate for phase change random access memory (PCRAM), as evidenced by a higher crystallization temperature, a better data retention ability, and a faster switching speed in comparison with those of Ge2Sb2Te5. A reversible switching between set and reset states can be realized by an electric pulse as short as 7 ns for CuSb4Te2-based PCRAM cell. In addition, CuSb4Te2 shows endurance up to 1.5 × 105 cycles with a resistance ratio of about two orders of magnitude.

Multi-Level Phase Change Memory Cells with SiN or Ta2O5 Barrier Layers

This work compares the effects of SiN and Ta2O5 barrier layers in a multi-level phase change random access memory (PCRAM) cell. The PCRAM cell comprises a phase change material stack between a top and a bottom electrode. The phase change material stack comprises a nitrogen-doped Ge2Sb2Te5 (NGST) layer on top of a thin barrier layer on an undoped GST layer. The thermal conductivity and electrical resistivity of the barrier layer affect multi-level switching performance in terms of endurance as well as power consumption. Extensive electrical characterization was performed on these PCRAM multi-level devices. Thermal analysis was also performed to investigate the thermal efficiency of each barrier layer. It was observed that for a constant barrier layer thickness of 1.5 nm, the endurance of the multi-level device with the SiN barrier layer was better than that with the Ta2O5 barrier layer; however, the multi-level device with the Ta2O5 barrier layer had a lower power consumption than that with the SiN barrier layer.