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Investigations of High Endurance Asymmetric Phase Change Random Access Memory

Published online by Cambridge University Press:  01 February 2011

Hock Koon Lee
Affiliation:
LEE_Hock_Koon@dsi.a-star.edu.sg, Data Storage Institute, OPtical Materials & Systems Division, DSI Building, 5 Engineering Drive 1 (Off Kent Ridge Cresent, NUS), Singapore, 117608, Singapore
Luping Shi
Affiliation:
SHI_Luping@dsi.a-star.edu.sg, Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Optical Materials & Systems Division, DSI Building, 5 Engineering Drive 1 (Off Kent Ridge Cresent, NUS), Singapore, 117608, Singapore
Rong Zhao
Affiliation:
ZHAO_Rong@dsi.a-star.edu.sg, Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Optical Materials & Systems Division, DSI Building, 5 Engineering Drive 1 (Off Kent Ridge Cresent, NUS), Singapore, 117608, Singapore
Hongxin Yang
Affiliation:
YANG_Hongxin@dsi.a-star.edu.sg, Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Optical Materials & Systems Division, DSI Building, 5 Engineering Drive 1 (Off Kent Ridge Cresent, NUS), Singapore, 117608, Singapore
Kian Guan Lim
Affiliation:
LIM_Kian_Guan@dsi.a-star.edu.sg, Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Optical Materials & Systems Division, DSI Building, 5 Engineering Drive 1 (Off Kent Ridge Cresent, NUS), Singapore, 117608, Singapore
Jianming LI
Affiliation:
LI_Jianming@dsi.a-star.edu.sg, Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Optical Materials & Systems Division, DSI Building, 5 Engineering Drive 1 (Off Kent Ridge Cresent, NUS), Singapore, 117608, Singapore
Tow Chong Chong
Affiliation:
CHONG_Tow_Chong@dsi.a-star.edu.sg, Data Storage Institute, A*STAR (Agency for Science, Technology and Research), Optical Materials & Systems Division, DSI Building, 5 Engineering Drive 1 (Off Kent Ridge Cresent, NUS), Singapore, 117608, Singapore
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Abstract

Asymmetric PCRAM structure with the upper contact opening at an offset to the bottom contact opening allowed us to improve the thermal distribution within the phase change layer and lower the reset current to 50% that of a conventional symmetrical structure. In terms of endurance, asymmetric cell lasted for 1.1 × 108 cycles which is at least 10X higher than the conventional symmetrical cell. These results were based on Ge2Sb2Te5 as the phase change material.

In this paper, we used nitrogen doped Ge2Sb2Te5 [1] instead and the thickness of this phase change layer was 100 nm. During the sputtering of Ge2Sb2Te5, the Argon gas flow rate was fixed at 15 sccm while nitrogen flow rates of 0, 3, 4.5 and 6 sccm were introduced each time. Thus N2/Ar gas ratio of 0, 0.2, 0.3 and 0.4 were obtained respectively. After fabrication, the cell endurance of Asymmetric PCRAM cells incorporating Ge2Sb2Te5 doped with varying concentrations of nitrogen was tested. During testing, the PCRAM was repeatedly Reset/Set and the resistances of the two states were recorded at every 100k cycles. The cell was considered to be functioning well when its Reset/Set resistance ratio was greater than 10. From experiments, N-doped asymmetric cell with N2/Ar gas ratio of 0.2 lasted 2.4 × 1010 cycles which is 1000 times that of a conventional symmetrical PCRAM cells. The N2 doping concentration was also shown to be optimized when the N2/Ar gas ratio was fixed at 0.2. Higher doping concentrations with N2/Ar gas ratio of 0.3 and 0.4 decreased the cell endurance to 8.8 × 108 and 7.6 × 108 cycles respectively. Excessive doped nitrogen atoms might have degraded the phase change material, causing breakdown to occur sooner.

N-doped conventional symmetrical PCRAM was also fabricated and its overwrite cycles were measured only up to 1.2 × 109. With better thermal confinement, asymmetric PCRAM has proved to be better in endurance too. The above results were based on asymmetric PCRAM cells with 1 µm offset.

[1] H. Horii et al, “A Novel Cell Technology Using N-doped GeSbTe Films For Phase Change RAM”, p. 177-178, VLSI Tech. 2003

Type
Research Article
Copyright
Copyright © Materials Research Society 2008

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References

REFERENCES

[1] Lai, S. and Lowrey, T., “OUM-A 180 nm Nonvolatile Memory Cell Element Technology For Stand Alone and Embedded Applications, 36.5.1-36.5.4, IEDM 2001 Google Scholar
[2] Horii, H. et al. , “A Novel Cell Technology Using N-doped GeSbTe Films For Phase Change RAM”, p. 177178, VLSI Tech. 2003 Google Scholar
[3] Kim, S.M., “Electrical Properties and Crystal Structure of Nitrogen-Doped Ge2Sb2Te5 Thin Film for Phase Change Memory”, Thin Solid Films, 469-470, p. 322326, 2004 Google Scholar