Laboratory for Joining Technologies & Corrosion , Empa, Swiss Federal Laboratories for Materials Science and Technology , Ueberlandstrasse 129 , 8600 Duebendorf , Switzerland.
ACS Appl Mater Interfaces. 2019 Feb 13;11(6):6605-6614. doi: 10.1021/acsami.8b19091. Epub 2019 Jan 29.
This study addresses the phase stability and atomic mobility of Ag-Cu nano-alloys confined by AlN in a nanomultilayered configuration during thermal treatment. To this end, nanomultilayers (NMLs) with a fixed Ag-Cu nanolayer thickness of 8 nm and a AlN barrier nanolayer with variable thickness of 4, 8, or 10 nm were deposited by magnetron sputtering on sapphire substrates and subsequently isothermally annealed for 5 or 20 min in air in the range of 200-500 °C. The microstructure of the as-deposited and heat-treated NMLs was analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy. Annealing of the thicker AlN barrier layers at T > 300 °C leads to the formation of an interconnected network of line-shaped Cu(O) protrusions on the annealed NML surface. The well-defined outflow pattern of Cu(O) originates from the thermally induced surface cracking of the top AlN barriers with subsequent fast mass transport of Cu along the Cu/AlN interfaces toward the surface cracks. The thinnest (i.e., 4 nm thick) AlN barrier layers exhibit a relatively open grain boundary structure and act as nanoporous membranes upon heating, resulting in the formation of a dense and homogenous distribution of Cu(O) and Ag droplets on the NML surface. These findings demonstrate that the microstructure (i.e., layer thicknesses, interface coherency, and texture) of hybrid nanolaminates can be tuned to provide defined pathways for fast, directional transport of the confined metal to the surface at relatively low temperatures, which might open new routes for low-temperature bonding of micro- and nano-scaled systems.
本研究探讨了在热处理过程中，Ag-Cu 纳米合金被 AlN 限制在纳米多层结构中的相稳定性和原子迁移性。为此，通过磁控溅射在蓝宝石衬底上沉积了具有固定 Ag-Cu 纳米层厚度为 8nm 和可变厚度为 4、8 或 10nm 的 AlN 阻挡纳米层的纳米多层（NML），并随后在空气 200-500°C 的范围内分别进行 5 或 20min 的等温退火。通过 X 射线衍射、扫描电子显微镜、透射电子显微镜和能量色散光谱对沉积和热处理后的 NML 的微观结构进行了分析。较厚的 AlN 阻挡层在 T>300°C 退火会导致在退火的 NML 表面上形成线形状 Cu(O) 突出物的互连网络。Cu(O) 的明确流出模式源自顶部 AlN 阻挡层的热诱导表面开裂，随后 Cu 沿着 Cu/AlN 界面快速向表面裂纹进行质量传输。最薄的（即 4nm 厚）AlN 阻挡层具有相对开放的晶界结构，并在加热时作为纳米多孔膜，导致在 NML 表面上形成 Cu(O)和 Ag 液滴的密集且均匀的分布。这些发现表明，混合纳米层压板的微观结构（即层厚度、界面相干性和织构）可以进行调整，以在相对较低的温度下为受限金属向表面的快速、定向传输提供定义的途径，这可能为微纳尺度系统的低温键合开辟新途径。
