Experimental evaluation of melting point depression in AlSi/AlN nanomultilayer system

Joanna Lipecka 1Mariusz Andrzejczuk 1Malgorzata Lewandowska 1Jolanta Janczak-Rusch 1,2Gunther Richter 3Lars Jeurgens 2

1. Warsaw University of Technology, Faculty of Materials Science and Engineering, Woloska 141, Warsaw 02-507, Poland
2. Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Joining Technologies and Corrosion (EMPA), Überlandstrasse 129, Dübendorf 8600, Switzerland
3. Max Planck Institute for Intelligent Systems, Central Scientific Facility Thin Film Laboratory (MPI), Heisenbergstrasse 3, Stuttgart 70569, Germany


The aim of the study was to evaluate the structural changes in AlSi/AlN nanomultilayers (NML) upon annealing for low temperature joining applications. It is known that due to relatively high thermal sensitivity of nanometals, standard methods of joining, e.g. furnace brazing or welding, are not suitable as they cause e.g. grain coarsening. To overcome the limitations of conventional brazing technologies based on a bulk eutectic alloy approach, proposed research strategy aims to employ the size-dependent melting behaviors of metals confined in a nanostructured multilayer geometry. The use of such a nanoarchitectured configuration can reduce the processing temperature, thus allowing benign joining of heat sensitive nanometals. Nevertheless, before the step for application the basic understanding of the melting point depression occurred in such systems, its significance and possible mechanisms need to be deeply investigated, which is the main goal of this study.

The system investigated was produced by magnetron sputtering and consisted of Al-Si(12%) braze filler metal layers (bulk Tm=577°C) with a thickness of 4 nm alternated by aluminium nitride diffusion barrier layers with a constant thickness of 3 nm. The bilayer of AlN/AlSi was repeated 10 times on the Si substrate and covered with the final AlN layer on the top. In order to investigate the melting behavior, the system was heat treated at various temperatures and observed with the use of Scanning Electron Microscope (SEM). Further, the cross section FIB-lamellae of the as-deposited and the heat treated state directly under the droplet were cut and examined using a Scanning Transmission Electron Microscope (STEM) and Transmission Electron Microscope (TEM).

The TEM observations of the as-deposited state showed that the AlSi/AlN system consists of policrystalline layers of AlN and AlSi, where the layers possess very fine structure with the grain sizes similar to the corresponding film thicknesses. The chemical analysis using EDX revealed a homogenous distribution of both Al and Si throughout the AlSi layers, which suggests that the structure is single phase. The annealing at 300°C did not cause any significant changes in the multilayer structure, while the annealing at 400°C brought about a visible phase separation within the Al-Si layers, where the supersaturated solid solution transformed into Al reach and Si reach regions.

The SEM observation of metal-containing features created on the top of the nanomultilayer revealed their first appearance at a temperature of 400°C. The droplets had irregular shape and were randomly distributed on the surface. The chemical analysis on the cross section of a droplet showed that it consisted of Si and Al indicating that the droplets formed as a result of liquid metal outflow from the nanomultilayer. The STEM observations of the central region under the droplet revealed that the NML was partially damaged and became thinner. Some of the AlN layers were deformed and created bumps due to the accumulation of the AlSi alloy under them, while others were broken and removed and their fragments were embedded in a solidified AlSi droplet above the system.

Presented results demonstrate that the melting point depression in AlSi/AlN NML system reached the value of 177°C. The mechanism responsible for melting at depressed temperature consisted of two stages. In the first stage, at a temperature of 400°C phase separation within the AlSi layers occured. This led to the creation of privileged sites at the interface between AlSi and AlN layers, where the liquid Al-Si alloy was cumulated. In the second stage, the upper AlN layers became bent and beyond their bending strength, AlN layers started to deform and break, which allowed liquid metal to freely outflow and create a two-phase flower-like structures on the NML surface.

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Presentation: Poster at Nano PL 2014, Symposium A, by Joanna Lipecka
See On-line Journal of Nano PL 2014

Submitted: 2014-09-11 14:40
Revised:   2014-09-11 16:31
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