Session 31: MMemory Technology - Phase-Change Memory, RRAM and DRAM
Wednesday, December 12, 1:30 p.m.
Co-Chairs: Hsiang-Lan Lung, Macronix International Co., Ltd. Takashi Kobayashi, Hitachi, Ltd.
Phase change memory (PCRAM) is an ideal embedded memory due to its simple BEOL process and low voltage operation. Industrial and automotive applications of PCRAM, however, have not been realized because of poor high temperature properties of the conventional Ge2Sb2Te5 phase-change material [1-3]. We have previously reported a special GexSbyTez material along the Ge and Sb2Te3 tie line that showed superior high temperature performance. In this work we have further enhanced our previous "golden" material by incorporating nitrogen and engineering the Ge/N concentration. In order to rapidly explore a range of new materials a fast method to test retention behavior by laser melt-quenching is adopted which yields retention data on blanket films consistent with device results. A new material with special Ge/N concentration with excellent high temperature retention is discovered. The new material demonstrated nearly 100% yield in a 256 Mb test chip after 160 °C, 84 hrs baking, with projected 10-year retention at 120 °C. (> 9,000 years at 85 °C.)
Grain-engineered Ge2Sb2Te5 was exploited to control the crystallization kinetics and electrical properties of PCM, resulting in a 120% higher SET speed with respect to conventional scaling. Good stability (140°C), 30% RESET power reduction, and 2X lower resistance drift were also achieved. A multilevel (4 state/2-bit) cell was further demonstrated.
A new phase change memory of 68% lower reset current is demonstrated using nano-crystalline doped-Ge2Sb2Te5 with low thermal conductivity. Endurance of >1×107 cycles and enhanced crystallization temperature (~ 215 °C) are also achieved. Nano-crystalline GST makes it possible to design 4F2 cross-point cells with simple bottom electrode structure.
In this work, for the first time at our knowledge, the improvement of chalcogenide-based CBRAM performance and reliability by Sb doping of the GeS2 electrolyte is presented. An original analysis, based on in-depth physico-chemical characterization, device electrical measurements, empirical model and first principle calculations, is shown. We argue that optimized ~10% Sb doping in the GeS2 electrolyte allows to achieve SET speed of 30ns at 2.2V (i.e. 0.66pJ SET programming power), while assuring 10 years data retention at 125°C, >105 cycling and high robustness to Sn-Pb soldering profile. Finally, the improved thermal stability of the filament in the GeS2-Sb matrix is clearly elucidated by means of molecular dynamics.
A new high density Contact RRAM (CRRAM) cell realized in pure high-k metal gate 28nm CMOS logic process with a very small 35nm x 35nm resistive contact hole has been fabricated without extra masking or process step. This study reports the first time of a manufacturable tiny resistive node of RRAM cell on a 28nm CMOS logic platform and fully compatible with high-k metal gate processes.
An open-base BJT named 'biristor', which is an n-p-n vertical silicon pillar, is demonstrated for high speed volatile memory applications. A 4F2 cross-bar memory cell array is realized by the unidirectional operation of the two-terminal biristor. Due to the gate-less structure, the biristor shows excellent endurance of up to 1016.
This paper presents, for the first time, the Active Width Modulation (AWM) technology which compensates a string resistance with the active widths of local Y selectors for the purpose of increasing the number of cells-per-string (CPS). The AWM is demonstrated using 58 nm 512 Mb PRAM with 32 CPS instead of 8 CPS , which can reduce the chip size by 4.3%. Also, the systematic variability of a program current is reduced from 17.8% to 0.82%, and that of a write energy from 47.9% to 2.0%. Both write endurance and disturbance of >1M cycles are achieved for 512 Mb PRAM. The AWM can be further applied to increase CPS to 64 or 128, together with the reduction of a reset current for sub-40 nm PRAM technology and so on.