東北大学
学際科学フロンティア研究所

島津グループ（先端学際基幹研究部）

「2014 4th IEEE International Workshop on Low Temperature Bonding for 3D Integration, (LTB-3D 2014), Best Presentation Award」を受賞

受賞日／2014年7月16日

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題目："Room Temperature Bonding of Wafers in Air using Au-Ag Alloy Films"

受賞者名： 今 一恵，魚本 幸，島津武仁（先端学際基幹研究部，情報・システム領域）

受賞日： 平成26年7月16日

先端学際基幹研究部の島津武仁教授の研究グループが，IEEE（米国電気学会）の国際ワークショップ「3次元集積化のための低温接合」にて,”Best Presentation Award”を受賞いたしました．本賞は，このワークショップにおける優秀な発表を表彰するものであり，第4回となる今回（2014年7月15, 16日，東京）は，本件を含む4名が表彰されました．

【受賞発表のアブストラクト】 Bonding of two flat wafers using Au films [1,2] is a promising candidate process to achieve wafer bonding at room temperature in air along with surface-activated bonding using Au film surfaces [3,4]. At room temperature, Au is not oxidized. The high atomic diffusion coefficient obtained at the grain boundaries and film surfaces, in addition to a large surface energy, enables bonding at room temperature in air using Au films. Bonding of wafers with mirror-polished metals, which is important for enhancing cooling efficiency of electrical power devices, was also achieved [2]. Moreover, the Ag thermal conductivity is higher than that for Au. Furthermore Ag is much less expensive than Au, underscoring Ag benefits for mass production. However, the free energies of formation of oxide compounds from the metals at room temperature, G, for Ag is slightly negative (Table1), although Ag shows an almost equal self-diffusion coefficient to that for Au. The bonding performance in air for Ag films is lower than that for Au films [5]. This study assessed room temperature bonding with Au-Ag alloy films, and the performance was discussed according to Ag composition. Experimental results show that Au-Ag alloy films with Ag contents up to 70–80 at% have almost equal bonding performance in air to that with Au films: bonding of two films starts with nucleation of recrystallization, immediately followed by propagation of the recrystallization. No substrate heating was conducted during the bonding process. A TEM image for Ag80Au20 films is similar to that for Au films. Bonded films show almost no interface corresponding to the original film surface, indicating that the recrystallization occurred at the original surfaces. However, large vacancies were observed for wafers bonded using Ag films [5]. Moreover, loading pressure of more than 0.5 MPa was necessary for bonding Ag-Ag films over the entire bonded area. Bonding in air using thin Au-Ag alloy films is a promising method for bonding wafers to fabricate thin film devices and particularly for enhancing the heat dissipation efficiency.

[1] T. Shimatsu, and M. Uomoto, “Room temperature bonding of wafers with thin nanocrystalline metal films,” ECS Transactions, vol. 33, no. 4, pp. 61–72, 2010.

[2] H. Kon, M. Uomoto, and T. Shimatsu, “Room Temperature Bonding of Wafers and Polished Metals using Thin Au Films for Enhancing the Heat Dissipation Efficiency,” Conference on Wafer Bonding for Microsystems and Wafer Level Integration, P18, Stockholm, Sweden, Dec. (2013).

[3] R. Takigawa, E. Higurashi, T. Suga, S. Shinada, and T. Kawanishi, “Low-temperature Au-to-Au bonding for LiNbO3/Si structure achieved in ambient air,” IEICE Trans. on Electronics, vol. E90C, pp.145–146 (2007).

[4] E. Higurashi, T. Imamura, T. Suga, and R. Sawada, “Low-temperature bonding of laser diode chips on silicon substrates using plasma activation of Au films,” IEEE Photonics Technology Letters, vol. 19, pp. 1994–1996 (2007).

[5] H. Kon, M. Uomoto, and T. Shimatsu, “Room Temperature Bonding of Wafers using Ag Films in Air,” The 28th Annual Conference of the Japan Institute of Electronics Packaging, 7C-08, Tokyo, March (2014).