新式相變化材料開發與改質

Development and Modifications of Novel Phase-change Materials

張博欽(Po-Chin Chang)

指導教授 ： 張士欽 ； 金重勳

國立清華大學/工學院/材料科學工程學系/博士(2013年)

https://doi.org/10.6843/NTHU.2013.00520

摘要

相變記憶體是先進非揮發性記憶體中重要的一員。本論文探討摻雜異質元素對相變化材料的影響及評估新穎材料在相變化記憶體的應用。從具有優異熱穩定性的三元鎵銻碲合金出發，利用添加矽進行改質，成功開發出具超高熱穩定性的四元相變化材料。結晶行為屬成長控制型的二元鎵銻合金雖有相分離的潛在缺點，但可被用於多階儲存用途。此二元鎵銻合金更進一步利用矽進行改質，發現其仍具有高速操作特性，同時熱穩定性大幅被提升。最後評估具有共晶組成的鍺鋁二元合金以及全固相相轉變的共析鋼用於相變化記憶體的可行性。
第一主題為鎵銻碲三元合金薄膜開發，包含熱性質、結晶構造、結晶動力學等基礎物性研究，並進行記憶體元件的電性測試。鎵銻碲薄膜展現出高度熱穩定性的高結晶溫度(Tx: 253 °C)及高結晶活化能(Ea: 5.8 eV)，非晶薄膜更可在201 °C下保存長達10年以上時間。薄膜具有單相結晶結構，結晶化過程具有9 %的體積收縮率。其記憶體元件可以在10奈秒的電壓脈衝進行寫入/擦拭的動作，在優化的操作條件下可循環操作超過109次。
第二主題為矽添加鎵銻碲四元合金薄膜的開發，其中組成為Si29(Ga2TeSb7)71的合金具有非常優異的熱穩定性，結晶溫度達340 °C，10年資料保存溫度為254 °C。利用此組成製成的元件其高阻狀態可承受超過25分鐘的250 °C恆溫加熱。非晶化過程可以在10奈秒完成，唯結晶化過程需要20微秒，此結果與我們利用結晶動力學預測的結果相符。元件可以重複進行寫入/擦寫動作超過106次。化學電子能譜儀分析顯示當矽含量超過10 at.%會形成矽碲鍵結。利用化學電子能譜儀分析的鍵結型式，我們可以較為準確的預測薄膜的結晶溫度以及玻璃轉換溫度。
第三主題為以二元鎵銻合金為基底，利用矽進行改質。Ga19Sb81合金具有合適的熱穩定性，其結晶溫度為228 °C，十年資料保存溫度為156 °C。元件測試結果現顯示Ga19Sb81合金可於10至100奈秒內完成寫入/擦寫的動作。更重要的發現為其具有多階儲存的能力。矽添加後的Ga19Sb81合金熱穩定性依矽含量有著顯著的提升，結晶溫度為236至284 °C，十年資料保存溫度為 171至217 °C。加Si 4 at.%的組成可於40奈秒內完成寫擦動作，而添加9 at.% Si的合金為100奈秒。耐久度測試以組成S9為最佳，目前可達104次。
第四主題探討共晶合金以及共析鋼於相變記憶體的應用。鍺鋁共晶合金可分別於10奈秒及80奈秒完成寫擦動作，耐久度測試達104次。鎳添加共析鋼可於10奈秒完成寫擦，耐久度達105次。其他共晶系合金諸如矽基以及銻基也被提出，這些共晶系合金具有與目前半導體製程相容的利基。共析合金具有全固相相變機制，可以藉此提升元件的可靠度。

關鍵字

相變化記憶體 ； 結晶動力學 ； 多階儲存 ； 相變化材料

並列摘要

Phase-change random access memory (PCRAM) is one of the most important emerging non-volatile memories. The purpose of this dissertation is to study the effect of modification on new phase-change materials for applications in PCRAM. We worked out Ga-Te-Sb and Si-doped Ga-Te-Sb alloys for highly thermal stable PCRAM and Ga-Sb for multi-level storage applications. Meanwhile, Si-doped Ga-Sb alloys are suitable for high-speed operation while retain excellent thermal stability. Eutectic Ge-Al alloy for PCRAM was also evaluated. Finally, solid-state phase transformation based on Fe-C alloy is proposed.
The Ga2TeSb7 films exhibit a high crystallization temperature (Tx: 253 °C), high temperature corresponding to 10-year data retention (T10y: 201 °C) and high activation energy of crystallization (Ea: 5.76 eV) hence providing outstanding thermal stability. X-ray diffraction (XRD) analysis showed that amorphous Ga2TeSb7 crystallizes into single R3m Sb-structure. The density change of Ga2TeSb7 film is 8.9 %, which is smaller than GST (9.5 %). Ga2TeSb7 memory cells demonstrate fast SET/RESET operation within 10 ns and excellent cycling endurance up to 109 cycles.
Si-doped Ga2TeSb7, Si29(Ga2TeSb7)71, showed further improved thermal stability (Tx: 340 °C, T10y: 254 °C, Ea: 5.2 eV). Data retention of Si29(Ga2TeSb7)71 memory cells is beyond 1500 s at 250 °C. The memory cell can be operated using set and reset pulses at 20 μs and 10 ns, respectively, over 106 cycles. X-ray photoelectron spectroscopy (XPS) results showed the formation of Si-Te bonds as Si content is greater than 10 at.%. Based on bonding status revealed by XPS, prediction of glass-transition temperature (Tg) and Tx can thus be more dependable.
Tx, T10y and Ea of Ga19Sb81 thin films are 228 °C, 156 °C and 4.7 eV, respectively. Ga19Sb81 memory cell can be operated within 10-100 ns. Most importantly, it shows multi-level capability (3 bits/cell). Tx, T10y and Ea of Si-doped Ga19Sb81 films (Si at.% : 1~14) are 236-284 °C, 171-217 °C and 5.3-9.1 eV, respectively. The memory cells of Si4(Ga19Sb81)96 film show reversible switching within 40-200 ns and that of Si9(Ga19Sb81)91 film is 100 ns. The endurance for Si4(Ga19Sb81)96 and Si9(Ga19Sb81)91 memory cells are ~103 and 104, respectively. All of our developed materials showed remarkable thermal stability than the benchmark material Ge2Sb2Te5 (Tx: 176 °C, T10y: 93 °C, Ea: 3.1 eV) while having satisfactory operation speed. They have the chance to replace NOR Flash even the DRAM technology.
Ge-Al film with eutectic composition can be set and reset in 10 ns and 80 ns, respectively. Cycling times can be up to 104. Fe-Ni-C film can be set and rest within 10 ns while showing reversible switching over 105. Others such as Si-based and Sb-based films also show potential applications in PCRAM.

並列關鍵字

Phase-change memory ； Crystallization Kinetics ； Multi-level memory ； Phase-change materials

參考文獻

[73] A. T. H. Chuang, Y. C. Chen, Y. C. Chu, P. C. Chang, K. F. Kao, C. C. Chang, K. Y. Hsieh, T. S. Chin, and C. Y. Lu, "Temperature Robust Phase Change Memory Using Quaternary Material System Based on Ga2TeSb7," in Memory Workshop (IMW), 2011 3rd IEEE International, 2011, pp. 1-3.

[175] G. X. Wang, Q. H. Nie, X. Shen, R. P. Wang, L. C. Wu, Y. G. Lv, J. Fu, T. F. Xu, and S. X. Dai, "Advantages of Zn1.25Sb2Te3 material for phase change memory," Materials Letters, vol. 87, pp. 135-138, Nov 2012.

[11] D. Loke, T. H. Lee, W. J. Wang, L. P. Shi, R. Zhao, Y. C. Yeo, T. C. Chong, and S. R. Elliott, "Breaking the Speed Limits of Phase-Change Memory," Science, vol. 336, pp. 1566-1569, Jun 2012.

[101] X. Q. Liu, X. B. Li, L. Zhang, Y. Q. Cheng, Z. G. Yan, M. Xu, X. D. Han, S. B. Zhang, Z. Zhang, and E. Ma, "New Structural Picture of the Ge2Sb2Te5 Phase-Change Alloy," Physical Review Letters, vol. 106, p. 025501, Jan 2011.

[156] J. Shen, C. Xu, B. Liu, Z. T. Song, L. C. Wu, S. L. Feng, and B. Chen, "Reversible resistance switching of GeTi thin film used for non-volatile memory," Japanese Journal of Applied Physics Part 2-Letters & Express Letters, vol. 46, pp. L1-L3, Jan 2007.