An integrated phase change memory cell with dual trench epitaxial diode is successfully integrated in the traditional 0.13μm CMOS technology.By using dual trench isolated structure in the memory cell,it is feasible to employ a Si-diode as a selector for integration in a crossbar structure for high-density phase change memory even at 45 nm technology node and beyond.A cross-point memory selector with a large on/off current ratio is demonstrated,the diode provides nine orders of magnitude isolation between forward and reverse biases in the SET state.A low SET programming current of 0.7mA is achieved and RESET/SET resistance difference of 10000×is obtained.
Ge2Sb2Te5 gap filling is one of the key processes for phase-change random access memory manufacture. Physical vapor deposition is the mainstream method of Ge2Sb2Te5 film deposition due to its advantages of film quality, purity, and accurate composition control. However,the conventional physical vapor deposition process cannot meet the gap- filling requirement with the critical device dimension scaling down to 90 nm or below. In this study, we find that the deposit-etch-deposit process shows better gap-filling capability and scalability than the single-step deposition process, especially at the nano-scale critical dimension. The gap-filling mechanism of the deposit-etch-deposit process was briefly discussed. We also find that re-deposition of phase-change material from via the sidewall to via the bottom by argon ion bombardment during the etch step was a key ingredient for the final good gap filling. We achieve void-free gap filling of phase-change material on the 45-nm via the two-cycle deposit-etch-deposit process. We gain a rather comprehensive insight into the mechanism of deposit-etch-deposit process and propose a potential gap-filling solution for over 45-nm technology nodes for phase-change random access memory.
We report on the investigation of the origin of high oxide to nitride polishing selectivity of ceria-based slurry in the presence of picolinic acid. The oxide to nitride removal selectivity of the ceria slurry with picolinic acid is as high as 76.6 in the chemical mechanical polishing. By using zeta potential analyzer, particle size analyzer, horizon profilometer, thermogravimetric analysis and Fourier transform infrared spectroscopy, the pre- and the post-polished wafer surfaces as well as the pre- and the post-used ceria-based slurries are compared. Possible mechanism of high oxide to nitride selectivity with using ceria-based slurry with picolinic acid is discussed.
Sb rich Ge_(2)Sb_(5)Te_(5) materials are investigated for use as the storage medium for high-speed phase change memory(PCM).Compared with conventional Ge2Sb2Te5,Ge_(2)Sb_(5)Te_(5) films have a higher crystallisation temperature(~200℃),larger crystallisation activation energy(3.13 eV),and a better data retention ability(100.2℃ for ten years).A reversible switching between set and reset states can be realised by an electric pulse as short as 5 ns for Ge_(2)Sb_(5)Te_(5)-based PCM cells,over 10 times faster than the Ge_(2)Sb_(5)Te_(5)-based one.In addition,Ge2Sb2Te5 shows a good endurance up to 3×10^(6) cycles with a resistance ratio of about three orders of magnitude.This work clearly reveals the highly promising potential of Ge_(2)Sb_(5)Te_(5) films for applications in high-speed PCM.
A three-dimensional finite element model for phase change random access memory is established to simulate electric, thermal and phase state distribution during （SET） operation. The model is applied to simulate the SET behaviors of the heater addition structure （HS） and the ring-type contact in the bottom electrode （RIB） structure. The simulation results indicate that the small bottom electrode contactor （BEC） is beneficial for heat efficiency and reliability in the HS cell, and the bottom electrode contactor with size Fx=80 nm is a good choice for the RIB cell. Also shown is that the appropriate SET pulse time is lOOns for the low power consumption and fast operation.
GONG Yue-Feng SONG Zhi-Tang LING Yun LIU Yan LI Yi-Jin
Phase-change line memory cells with different line widths are fabricated using focused-ion-beam deposited C-Pt as a hard mask. The electrical performance of these memory devices was characterized. The current^oltage （I-V） and resistance-voltage （RV） characteristics demonstrate that the power consumption decreases with the width of the phase-change line. A three-dimensional simulation is carried out to further study the scaling properties of the phase- change line memory. The results show that the resistive amorphous （RESET） power consumption is proportional to the cross-sectional area of the phase-change line, but increases as the line length decreases.
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.
Simulation of the heat consumption in phase change random access memories （PCRAMs） is investigated by a three-dimensional finite element model. It is revealed that the thermal conductivity and electrical conductivity of the buffer layer are crucial in controlling the heating efficiency in RESET process. The buffer layer mater/Ms W, TiN, WOa, Ti02 and poly-germanium （poly-Ge） are applied in the simulation respectively, and compared with each other. The simulation results show that limitation of electrical conductivity is effective on heating efficiency and the limitation of thermal conductivity is important on the reliable RESET process.
