Yan Liu

Three-Dimensional Numerical Simulation of Phase-Change Memory Cell with Probe like Bottom Electrode Structure

A new device structure of phase-change memory (PCM) cell with a Probe like bottom electrode (PBE) was proposed and its electrical-thermal characteristics were investigated by three-dimensional finite element analysis. The programming region of the definition (GST) layer in the PBE cell is much smaller than that in a conventional normal-bottom-contact (NBC) cell after the RESET operation. The high concentrations of electric-field density and electric-current density in the small programming region of GST layer in the PBE cell have the advantages of reducing the power consumption and increasing the heating efficiency of PCM devices. Compared with the NBC cell, the RESET threshold current of the PBE cell is reduced from 1.2 to 0.45 mA and the heating efficiency increases from 28.7 to 44.1%. Therefore, the lower programming current, the smaller molten region of GST and the higher heating efficiency in the PBE cell will be propitious for developing the PCM with low power consumption and high integration density.

Optimization of 40-nm Node Epitaxial Diode Array for Phase-Change Memory Application

A numerical model of an epitaxial (EPI) diode array for next-generation memory device application, including phase-change memory, has been presented. According to a diode array process scheme and technology computer-aided design (TCAD) simulation results, a quasi-physical model with a buried n+ layer dosage, EPI layer thickness, and breakdown voltage (BVD) correlation is proposed to improve electrical performance. From the optimal silicon-based results, a 16×16 diode array shows a drive current density of ~56.6 mA/μm2, a BVD of ~8 V, a Jon/Joff ratio of ~109, and crosstalk immunity. Additionally, this calibrated physical model can be applied in the next generation of silicon-based fabrication with parameters extraction.

Cost-Effective Schottky-Barrier Diode Array With Ni–Silicidation Accessing Low Power Phase-Change Memory

A cost-effective fabrication of Schottky-barrier (SB) diode steering element for low power phase-change memory (PCM) application is realized. While superior drivability in conventional PN diode array, SB diode array with 0.0193- μm2 (5F2), performing higher switching speed, sufficient drive current density of ~ 26.30 mA/μm2, disturbance immunity, and lower power consumption has been manufactured under 40-nm standard complementary metal oxide semiconductor technology. Simultaneously, different performance specifications, including integration scheme, JON/JOFF ratio, temperature characteristics, and scalability have been studied in detail and compared in two categories of accessing diode arrays. It manifests that the scaled SB diode array is suitable for full operation of PCM.

Programming voltage reduction in phase change cells with conventional structure

A new operation strategy has been proposed to reduce the programming voltage in RESET operation of phase change memory (PCM). The amorphous state generated after first pulse would restrict the current flowing through the phase change materials, increasing the heat efficiency. Thus, the amorphous state in the second pulse would increase. The accumulative amorphous state would cover the bottom electrode finally. Thus, RESET operation commences successfully. In new RESET operation strategy, the voltage pulse could consist of small amplitude and width voltage pulses (1V/35ns). The programming voltage is reduced with conventional structure, reducing costing on refining cell structure and power consumption.

Ge2Sb2Te5 Phase Change Memory Cell Featuring Platinum Tapered Heating Electrode For Low-Voltage Operation

Phase change random access memory (PC-RAM) with a Pt tapered heating electrode (Pt-THE), which was fabricated using a focus ion beam (FIB), was investigated. Compared with the tungsten electrode, the Pt-THE facilitates the temperature rise in phase change material, which causes the decrease of reset voltage from 3.5 to 2.4 V. The programming region of the cell with Pt-THE is smaller than that of the cell with a cylindrical tungsten heating electrode. The improved performance of the PC-RAM with a Pt-THE is attributed to the higher resistivity and lower thermal conductivity of the Pt electrode, and the reduction of the programming region, which is also verified by thermal simulation.

Three-Dimensional Simulations of RESET Operation in Phase-Change Random Access Memory with Blade-Type Like Phase Change Layer by Finite Element Modeling

An optimized device structure for reducing the RESET current of phase-change random access memory (PCRAM) with blade-type like (BTL) phase change layer is proposed. The electrical thermal analysis of the BTL cell and the blade heater contactor structure by three-dimensional finite element modeling are compared with each other during RESET operation. The simulation results show that the programming region of the phase change layer in the BTL cell is much smaller, and thermal electrical distributions of the BTL cell are more concentrated on the TiN/GST interface. The results indicate that the BTL cell has the superiorities of increasing the heating efficiency, decreasing the power consumption and reducing the RESET current from 0.67 mA to 0.32 mA. Therefore, the BTL cell will be appropriate for high performance PCRAM device with lower power consumption and lower RESET current.

Threshold switching in SiGeAsTeN chalcogenide glass prepared by As ion implantation into sputtered SiGeTeN film

A memory cell composed of a selector device and a storage device is the basic unit of phase change memory. The threshold switching effect, main principle of selectors, is a universal phenomenon in chalcogenide glasses. In this work, we put forward a safe and controllable method to prepare a SiGeAsTeN chalcogenide film by implanting As ions into sputtered SiGeTeN films. For the SiGeAsTeN material, the phase structure maintains the amorphous state, even at high temperature, indicating that no phase transition occurs for this chalcogenide-based material. The electrical test results show that the SiGeAsTeN-based devices exhibit good threshold switching characteristics and the switching voltage decreases with the increasing As content. The decrease in valence alternation pairs, reducing trap state density, may be the physical mechanism for lower switch-on voltage, which makes the SiGeAsTeN material more applicable in selector devices through component optimization.A memory cell composed of a selector device and a storage device is the basic unit of phase change memory. The threshold switching effect, main principle of selectors, is a universal phenomenon in chalcogenide glasses. In this work, we put forward a safe and controllable method to prepare a SiGeAsTeN chalcogenide film by implanting As ions into sputtered SiGeTeN films. For the SiGeAsTeN material, the phase structure maintains the amorphous state, even at high temperature, indicating that no phase transition occurs for this chalcogenide-based material. The electrical test results show that the SiGeAsTeN-based devices exhibit good threshold switching characteristics and the switching voltage decreases with the increasing As content. The decrease in valence alternation pairs, reducing trap state density, may be the physical mechanism for lower switch-on voltage, which makes the SiGeAsTeN material more applicable in selector devices through component optimization.

Optimization of J-V characteristic in diode array for phase change memory

In this paper, current density-voltage (J-V) characteristic of dual trench diode array have been investigated by both TCAD model and experimental method. It is shown that the arsenic concentration in buried N+ layer (BNL), epitaxial (EPI) layer thickness, and the dosage of P region in PN junction are expected to be the prominent factors responsible for both of the leakage and drive current performance according to TCAD simulation. By introducing the optimal siliconbased results, the 4×4 diode arrays were successfully manufactured by 40nm CMOS technology. The median values of drive and reverse leakage current densities are ~7.30×10-2 A/μm2 and 5.61×10-9 A/μm2, respectively. The breakdown voltages (BVDs) of diode array are exceeding 6V, and the Jon/Joff ratios of ~109, which can satisfy the requirements of phase change memory (PCM) applications.

Simulation of Phase-Change Random Access Memory with 35nm diameter of the TiN bottom electrode by Finite Element Modeling

A three-dimensional finite element model for Phase-Change Random Access Memory (PCRAM) is established to simulate thermal and electrical distribution during RESET operation. The establishment of the model is highly in accordance with the manufacture of PCRAM cell in the 40nm process and the model is applied to simulate the RESET behaviors of 35 nm diameter of titanium nitride (TiN) bottom electrode in the conventional mushroom structure (MS). By the simulations of thermal and electrical distribution, the highest temperature is observed in TiN bottom electrode contactor and meanwhile the voltage of the TiN bottom electrode accounts for as high as 65 percent of the total voltage. It induces high RESET current which suggests that the thermoelectric conductivity of MS is crucial in improving the heating efficiency in RESET process. Simulation results of RESET current and high resistance distribution during RESET operation are close to the data from the actual measurement. However those two values of low resistance are slightly different, probably due to the interface resistance between Ge2Sb2Te5 (GST) and other materials and the resistance caused by microstructural defects. This work reveals the importance of the thermoelectrical properties of materials in PCRAM cells and improves the quality of PCRAM simulations in industrial application.

A physical model of ovonic threshold switching effect for phase change memory based on the trap-to-band transition mechanism

A new physical conductivity model of amorphous chalcogenide is proposed, based on carriers transport theory including hopping transport and trap-to-band transition. A numerical simulator based on this model is developed, and the simulation results agree well with the measurement. The ovonic switch voltage and current with different active area and temperature are studied based on the simulator, and the results show a good consistence with the measurement.