Phase-change Random Access Memory Cell Research Articles

Investigation on Sb-rich Sb–Se–Te phase-change material for phase change memory application

Sb-rich Sb65Se6Te29 film was investigated for phase change random access memory (PCRAM) application. The crystallization temperature of the Sb65Se6Te29 film is 174°C and the crystalline activation energy is about 2.7eV. The 10-years' failure temperature for the Sb65Se6Te29 film is about 87°C, sufficient for most consumer applications. The results of UV/visible/NIR spectrophotometer measurements show that the optical gap of the Sb65Se6Te29 film decreases as it transforms from amorphous phase to crystalline phase. Compared with the Ge2Sb2Te5 based PCRAM cell, the Sb65Se6Te29 based PCRAM cell has the advantages of lower threshold voltage and larger resistance contrast. Furthermore, as short as 10ns electrical pulse can achieve Reset operation with a Reset voltage of 2.9V.

Multi-level phase change memory devices with Ge2Sb2Te5 layers separated by a thermal insulating Ta2O5 barrier layer

This work investigates multi-level phase change random access memory (PCRAM) devices comprising two Ge2Sb2Te5 (GST) layers sandwiching a thermal insulating Ta2O5 barrier layer. 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 GST (NGST) layer on a thin Ta2O5 barrier layer on an undoped GST layer. It is demonstrated that one of the phase change layers in the GST stack can be selectively amorphized by using a voltage pulse. This enables multi-level resistance switching. The differences in resistivities, as well as the different melting and crystallization temperatures of both the NGST and GST layers, contribute to the multi-level switching dynamics of the PCRAM device. The thermal conductivity of Ta2O5 with respect to GST is also another factor influencing the multi-level switching. Extensive electrical characterization of the PCRAM devices was performed. Thermal analysis was used to examine the physics behind the multi-level switching mechanism of these devices.

Sb-rich Si–Sb–Te Phase-Change Material for Phase-Change Random Access Memory Applications

An Sb-rich (48 at.%) Si-Sb-Te phase-change material with moderate Si content (24 at.%) is proposed for phase-change random access memory (PCRAM) applications. The real time amorphous to crystalline transformation was studied by in situ transmission electron microscopy observations and in situ resistance measurements. The results of time-dependent resistance measurements show that the Si24Sb48Te28 phase-change material has data retention of 10 years at about 382 K, suggesting a more stable amorphous state than the usual Ge2Sb2Te5 (GST) phase-change material. The reversible set-reset ability of the PCRAM cell based on the Si24Sb48Te28 phase-change material is much better than that of the device employing GST. The programming cycles can reach 2.2 × 104 under a set pulse of 1.5 V/1000 ns with a 30-ns falling edge and a reset pulse of 3.5 V/400 ns, whereas the resistance contrast retains a value of as large as two orders of magnitude.

Ultrafast switching in nanoscale phase-change random access memory with superlattice-likestructures

Phase-change random access memory cells with superlattice-like (SLL)GeTe/Sb2Te3 were demonstrated to have excellent scaling performance in terms of switching speed andoperating voltage. In this study, the correlations between the cell size, switching speed andoperating voltage of the SLL cells were identified and investigated. We found that smallSLL cells can achieve faster switching speed and lower operating voltage compared to thelarge SLL cells. Fast amorphization and crystallization of 300 ps and 1 ns were achieved inthe 40 nm SLL cells, respectively, both significantly faster than those observed in theGe2Sb2Te5 (GST) cells of the same cell size. 40 nm SLL cells were found to switch with lowamorphization voltage of 0.9 V when pulse-widths of 5 ns were employed, which ismuch lower than the 1.6 V required by the GST cells of the same cell size. Theseeffects can be attributed to the fast heterogeneous crystallization, low thermalconductivity and high resistivity of the SLL structures. Nanoscale PCRAM with SLLstructure promises applications in high speed and low power memory devices.

Microstructures corresponding to multilevel resistances of In3Sb1Te2 phase-change memory

The origin of multilevel resistances of In3Sb1Te2 (IST) phase-change random access memory cell has been investigated with high-resolution transmission electron microscopy (HR-TEM). The HR-TEM indicates that the microstructure of IST in the programming volume changes from amorphous to InSb and amorphous at the first state, and InSb and InTe at the second state, and IST at the third state, which are fairly consistent with four different levels of resistance. The resistance difference between the amorphous and the IST is about four orders of magnitude.

Si–Sb–Te materials for phase change memory applications

Si–Sb–Te materials including Te-richSi2Sb2Te6 andSixSb2Te3 with different Si contents have been systemically studied with the aim of finding the mostsuitable Si–Sb–Te composition for phase change random access memory (PCRAM) use.SixSb2Te3 shows better thermalstability than Ge2Sb2Te5 or Si2Sb2Te6 inthat SixSb2Te3 does not have serious Te separation under high annealingtemperature. As Si content increases, the data retention ability ofSixSb2Te3 improves. The 10 years retention temperature forSi3Sb2Te3 film is ∼ 393 K,which meets the long-term data storage requirements of automotive electronics. In addition, Si richerSixSb2Te3 films also show improvement on thickness change upon annealing and adhesion onSiO2 substrate comparedto those of Ge2Sb2Te5 or Si2Sb2Te6 films. However, the electrical performance of PCRAM cells based onSixSb2Te3 filmswith x > 3.5 becomes worse in terms of stable and long-term operations.SixSb2Te3 materialswith 3 < x < 3.5 are proved to be suitable for PCRAM use to ensure good overall performance.

SET/RESET properties dependence of phase-change memory cell on thickness of phase-change layer

The thermal properties of a phase-change random access memory (PCRAM) cell are dominated by the phase-change recording material. The SET/RESET resistances, the minimum width of the SET/RESET pulse, the threshold voltage and the maximum temperature of PCRAM cells are impacted by the thickness of the phase-change layer. In this paper, a PCRAM cell with different Ge–Sb–Te phase-change layer thickness are fabricated, and the electrical properties of the PCRAM cells are tested. The SET/RESET properties dependence of PCRAM cells on the thickness of the phase-change layer is investigated and compared for the low-power consumption of PCRAM.

Specific Contact Resistance of Phase Change Materials to Metal Electrodes

For phase change random access memory (PCRAM) cells, it is important to know the contact resistance of phase change materials (PCMs) to metal electrodes at the contacts. In this letter, we report the systematic determination of the specific contact resistance (ρc) of doped Sb2Te and Ge2Sb2Te5 to TiW metal electrodes. These data are reported for both the amorphous and the crystalline states of these PCMs. The temperature and voltage dependences of ρc are also studied. A detailed understanding of these contacts is essential for the scaling, design, device modeling, and optimization of PCRAM cells.

Sb 2Te 3–Ta 2O 5 nano-composite films for low-power phase-change memory application

Sb 2Te 3–Ta 2O 5 nano-composite films were deposited by the cosputtering of Sb 2Te 3 and Ta 2O 5 targets using radio frequency magnetron sputtering system at room temperature. A phase-change random access memory (PCRAM) device based on the Sb 2Te 3–Ta 2O 5 films was successfully fabricated. Compared to a pure Ge 2Sb 2Te 5 based PCRAM cell, the reset voltage of the Sb 2Te 3–Ta 2O 5 based cell was obviously reduced, which was attributed to the reduced thermal conductivity and lower melting point of the Sb 2Te 3–Ta 2O 5 films. In addition, the device with the Sb 2Te 3–Ta 2O 5 layer could work with much shorter pulse widths for both SET and RESET, suggesting that the Sb 2Te 3–Ta 2O 5 based compounds are promising candidates for low-power and fast-speed PCRAM application.

Growth Rate Determination through Automated TEM Image Analysis: Crystallization Studies of Doped SbTe Phase-Change Thin Films

A computer-controlled procedure is outlined here that first determines the position of the amorphous-crystalline interface in an image. Subsequently, from a time series of these images, the velocity of the crystal growth front is quantified. The procedure presented here can be useful for a wide range of applications, and we apply the new approach to determine growth rates in a so-called fast-growth-type phase-change material. The growth rate (without nucleation) of this material is of interest for comparison with identical material used in phase-change random access memory cells. Crystal growth rates in the amorphous phase-change layers have been measured at various temperatures using in situ heating in a transmission electron microscope. Doped SbTe films (20 nm thick) were deposited on silicon nitride membranes, and samples with and without silicon oxide capping layer were studied. The activation energy for growth was found to be 3.0 eV. The samples without capping layer exhibit a nucleation rate that is an order of magnitude higher than the samples with a silicon oxide capping layer. This difference can be attributed to the partial oxidation of the phase-change layer in air. However, the growth rates of the samples with and without capping are quite comparable.

Elevated-Confined Phase-Change Random Access Memory Cells

A new elevated-confined phase-change random access memory (PCRAM) cell structure to reduce power consumption was proposed. In this proposed structure, the confined phase-change region is sitting on top of a small metal column enclosed by a dielectric at the sides. Hence, more heat can be effectively sustained underneath the phase-change region. As for the conventional structure, the confined phase-change region is sitting directly above a large planar bottom metal electrode, which can easily conduct most of the induced heat away. From simulations, a more uniform temperature profile around the active region and a higher peak temperature at the phase-change layer (PCL) in an elevated-confined structure were observed. Experimental results showed that the elevated-confined PCRAM cell requires a lower programming power and has a better scalability than a conventional confined PCRAM cell.

Simulation of SET Operation in Phase-Change Random Access Memories with Heater Addition and Ring-Type Contactor for Low-Power Consumption by Finite Element Modeling

A three-dimensional finite element model for phase change random access memory (PCRAM) is established for comprehensive electrical and thermal analysis during SET operation. The SET behaviours of the heater addition structure (HS) and the ring-type contact in bottom electrode (RIB) structure are compared with each other. There are two ways to reduce the RESET current, applying a high resistivity interfacial layer and building a new device structure. The simulation results indicate that the variation of SET current with different power reduction ways is little. This study takes the RESET and SET operation current into consideration, showing that the RIB structure PCRAM cell is suitable for future devices with high heat efficiency and high-density, due to its high heat efficiency in RESET operation.

Achieving Multiple Resistance States in Phase-Change Memory Cell

Ge2Sb2Te5 (GST)-based phase-change random access memory (PCM) cells were successfully fabricated by a standard 0.18 µm complementary metal–oxide–semiconductor (CMOS) process. The bottom electrode contacts (BECs) of the PCM cells are tungsten heaters with hollow columns filled with SiO2 and Ge. An approach of multiple resistance states (MRSs) was demonstrated. It was revealed and simulated that the observed MRS, in fact, derives from the hollow tungsten BEC architecture. The variance of electrical resistance between the MRSs can be as high as two orders. With a proposed model, our research demonstrated a simple but applicable concept of building a more stable multiple storage (MS) PCM cell by simply tuning the contact area between the BEC and GST.

Influences of partial melting and overheating on amorphization of Ge2Sb2Te5 during the reset process

AbstractIt is very important to observe the change in the atomic structure of melt‐quenched amorphous Ge2Sb2Te5 (GST) to discover the failure mechanism of phase‐change random access memory (PRAM) devices during the reset process. We fabricated the phase‐change devices and measured the initial resistance changes with various reset pulse amplitudes. For the observation of the atomic structure changes of GST thin films, we designed the specimens for transmission electron microscopy (TEM) and annealed them below and above the melting temperature of GST (Tm = 650 °C) before quenching. The annealed‐and‐quenched GST at 630 °C has a hexagonal structure with a grain size of about 10 nm after the partial melting. The annealed‐and‐quenched GST at 750 °C also has a hexagonal structure because of the latent heat. The observed atomic structures of GST are in accordance with the resistance changes inferred from the fabricated edge‐contact type of PRAM cell. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Simulation of Phase-Change Random Access Memory with Ring-Type Contactor for Low Reset Current by Finite Element Modelling

A three-dimensional finite element models for phase change random access memory (PCRAM) is established to simulate thermal and electrical behaviours during RESET operation. The RESET behaviours of the conventional structure (CS) and the ring-type contact in bottom electrode (RIB) are compared with each other. The simulation results indicate that the RIB cell has advantages of high heat efficiency for melting phase change material in cell, reduction of contact area and lower RESET current with maintaining good resistance contrast. The RESET current decreases from 1.26mA to 1.2mA and the heat consumption in GST material during programming increases from 12% to 37% in RIB structure. Thus the RIB structure PCRAM cell is suitable for future device with high heat efficiency and smaller RESET current.

Core Circuit Technologies for PN-Diode-Cell PRAM

Phase-change random access memory (PRAM) chip cell phase of amorphous state is rapidly changed to crystal state above 160 Celsius degree within several seconds during Infrared (IR) reflow. Thus, on-board programming method is considered for PRAM chip programming. We demonstrated the functional 512Mb PRAM with 90㎚ technology using several novel core circuits, such as metal-2 line based global row decoding scheme, PN-diode cells based BL discharge (BLDIS) scheme, and PMOS switch based column decoding scheme. The reverse-state standby current of each PRAM cell is near 10 pA range. The total leak current of 512Mb PRAM chip in standby mode on discharging state can be more than 5 mA. Thus in the proposed BLDIS control, all bitlines (BLs) are in floating state in standby mode, then in active mode, the activated BLs are discharged to low level in the early timing of the active period by the short pulse BLDIS control timing operation. In the conventional sense amplifier, the simultaneous switching activation timing operation invokes the large coupling noise between the VSAREF node and the inner amplification nodes of the sense amplifiers. The coupling noise at VSAREF degrades the sensing voltage margin of the conventional sense amplifier. The merit of the proposed sense amplifier is almost removing the coupling noise at VSAREF from sharing with other sense amplifiers.

3-D Finite Element Simulation of a Phase-change Random Access Memory Cell with a Self-insulated Structure

AbstractIn this paper, we proposed a phase-change random access memory (PCRAM) cell with a self-insulated structure (SIS), which is expected to have better thermal efficiency than the conventional structures. 3-D finite element simulation is used to study the most power consuming RESET process for both SIS and conventional normal bottom contact (NBC) cells driven by a MOSFET. Instead of programming current, power consumption is investigated to give a more fundamental comparison between the two structures. Thermal proximity effect for both kinds of cells is directly analyzed by simulating a 3×3 device array. The potential slow-quenching issue of SIS is also discussed.

발열 전극에 따른 상변화 메모리 소자의 전자장 및 열 해석

PRAM (Phase change random access memory) is one of the most promising candidates for next generation non-volatile memories. However, the high reset current is one major obstacle to develop a high density PRAM. One way of the reset current reduction is to change the heater electrode material. In this paper, to reduce the reset current for phase transition, we have investigated the effect of heater electrode material parameters using finite element analysis. From the simulation. the reset current of PRAM cell is reduced from 2.0 mA to 0.72 mA as the electrical conductivity of heater is decreased from ) to ). As the thermal conductivity of heater is decreased, the reset current is slightly reduced. But the reset current of PRAM cell is not changed as the specific heat of heater is changed.

Three-dimensional simulation model of switching dynamics in phase change random access memory cells

Switching dynamics associated with reset and set operations of vertical phase change random access memory (PCRAM) cells are studied using a three-dimensional simulation model. Based on a finite difference method, the numerical algorithm simulates the electrical, thermal, and phase change dynamics in the PCRAM device during switching operations taking into account electrical and thermal percolation characteristics of the phase change material. Toward a better understanding of switching operations and the optimization of cell designs, the obtained simulation results provide unprecedented insight into temporally and spatially resolved kinetics of device temperature, current densities, and phase transitions. Threshold conditions for reset and set operations are identified in close agreement to existing experimental data, and the scaling ability of the investigated vertical PCRAM cell design to a minimum feature size of at least 40 nm is demonstrated.

Simulation Study on Heat Conduction of a Nanoscale Phase-Change Random Access Memory Cell

We have investigated heat transfer characteristics of a nano-scale phase-change random access memory (PRAM) cell using finite element method (FEM) simulation. Our PRAM cell is based on ternary chalcogenide alloy, Ge2Sb2Te5 (GST), which is used as a recording layer. For contact area of 100 x 100 nm2, simulations of crystallization and amorphization processes were carried out. Physical quantities such as electric conductivity, thermal conductivity, and specific heat were treated as temperature-dependent parameters. Through many simulations, it is concluded that one can reduce set current by decreasing both electric conductivities of amorphous GST and crystalline GST, and in addition to these conditions by decreasing electric conductivity of molten GST one can also reduce reset current significantly.