Wetting - Dewetting Transitions of Au/Ni Bilayer Films PDF Download

Author: Cen, Xi
Publisher:
ISBN: 9780355151527
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Languages : en
Pages :

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Book Description
Thin films deposited at low temperatures are often kinetically constrained and will dewet the underlying substrate when annealed. Solid state dewetting is driven by the minimization of the total free energy of thin film-substrate interface and free surface, and mostly occurs through surface diffusion. Dewetting is a serious concern in microelectronics reliability. However, it can also be utilized for the self-assembly of nanostructures with potentials in storage, catalysis, or transistors. Therefore, a fundamental understanding of the dewetting behavior of thin metal films is critical for improving the thermal stability of microelectronics and controlling the order of self-assembled nanostructures. Mechanisms for dewetting of single layer films have been studied extensively. However little work has been reported on multilayer or alloyed thin films. In the thesis, the solid state dewetting of Au/Ni bilayer films deposited on SiO2/Si substrates was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and aberration corrected scanning TEM (STEM). Ex-situ SEM and TEM studies were performed with in-situ TEM heating characterization to identify the mechanisms during the dewetting process of Au/Ni bilayer films. The solid state dewetting of Au/Ni bilayer films from SiO2/Si substrates exhibits both homogeneous and localized dewetting of Ni and long-edge retraction for Au under isothermal annealing condition. The top Au layer retracts up to 1 mm from the edge of the substrate wafer to reduce the energetically unfavored Au/Ni interface. In contrast, Ni dewets and agglomerates locally due to its limited diffusivity compared to Au. Film morphology and local chemical composition varies significantly across hundreds of microns along the direction normal to the retracting edge. Besides long range edge receding, localized dewetting shows significant changes in film morphology and chemical distribution. Both Au and Ni shows texturing. Despite the initial deposition sequence of the as-deposited Au/Ni/SiO2/Si interface structure, the majority of the metal/SiO2 interface is Au/SiO2 after annealing at 675°C for 1 hour. Cross-Sectional TEM revealed that void nucleation is predominantly observed at Au/Ni/SiO2 triple junctions, rather than grain boundary grooving at free surface of the metal film. The formation of Au/SiO2 interface sections is found to be energetically preferred over Ni/SiO2 due to compressive stress in the as-deposited Ni layer. In-situ cross-sectional TEM heating studies demonstrate that the dewetting transition initiates at 400°C. Au diffuses through Ni grain boundaries towards the underlying Ni layer and forms more energetically favorable Au/SiO2 interface. The adsorption of Au along Ni grain boundaries, NiO[subscript x]/Ni and SiO2/Ni phase boundaries is observed to lower the interfacial energy. The experimental observations suggest that alloying, unequal self-diffusion coefficient and metal/metal interface strongly affect the bilayer dewetting process.