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A bilayer metal structure has been demonstrated to adjust the gate work function over the Si band gap. The underlying tuning mechanism is believed to be due to metal interdiffusion based on comparison of work function behavior under different anneal conditions. In this paper, we conduct physical characterization on bilayer metal gates and successfully verify that the interdiffusion is the cause of the work function tuning. Furthermore, we find that metal interdiffusion significantly slows down after an initial anneal, resulting in a stable work function. A diffusion model involving the annealing out of fast diffusion paths is proposed to explain the work function results.

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Low-temperature Aluminum-Germanium (Al-Ge) eutectic bonding has been investigated for monolithic threedimensional integrated circuits (3DIC) applications. Successful bonds using Al-Ge bilayer films as thin as 157 nm were achieved at temperatures as low as 435 °C, when applying 200 kPa downpressure for 30 minutes. The liquid phase of the eutectic composition ensured a seamless and void-free bond. The fracture energy of the Al-Ge bond (630 nm thick) was measured to be G c = 50.5 ± 12.7 J/m 2 , using double cantilever beam thin-film adhesion measurement technique. An array of silicon islands was attached onto an amorphous SiO 2 wafer using low-temperature Al-Ge bonding. These islands could be used to form devices on upper layers of monolithically integrated 3DICs.Index Terms-wafer bonding, monolithic integration, Al-Ge eutectic, 3DIC.

Low-temperature aluminum-germanium (Al-Ge) bonding has been investigated for monolithic three-dimensional integrated circuit (3DIC) applications. As upper layer devices of a monolithic 3DIC are fabricated in situ, a suitable technique for providing high-quality semiconducting material without inflicting damage to underlying circuits below is needed. Here, the authors demonstrate a method of attaching high-quality single-crystal Si (100) and Ge (100) islands (3-3000 lm in size) onto amorphous SiO 2 substrates using both eutectic (435 C) and subeutectic (400 C) Al-Ge bonding. The 30 min, 3DIC compatible process utilizes Al-Ge bilayer films as thin as 157 nm to form void-free bonds strong enough to withstand SmartCut V R hydrogen splitting of the donor wafer. The fracture energy of the Al-Ge bond was measured to be G C ¼ 50.5 6 12.7 J/m 2 , as measured by the double cantilever beam thin-film adhesion measurement technique.