Nanoscale microstructural characterization of aluminum and copper bilayer thin films deposited on silicon substrate using magnetron sputtering technique

Abstract

Aluminum and copper (Al-Cu) bilayer thin films are commonly used as conductors in electronic applications. Unfortunately, the effect of thermal aging on the interfaces of Al-Cu bilayer thin films have led to the formation of intermetallic compounds (IMCs) which eventually degrades the materials. The study of diffusion reaction in Al-Cu thin film is very challenging compared to the bulk Al-Cu system. From previous related works, the compilation of data regarding the diffusion kinetics in thin film has led to discrepancies due to the complex relationship between the selected experimental parameters and the formation of the IMCs. Therefore, further study is necessary to fully understand the growth kinetics of IMCs particularly for the Al-Cu system. The present work is an attempt to contribute in the understanding of the phase formation by varying the diffusion parameters. Syntheses of the Al-Cu bilayer thin films (800 nm of total thickness) were physically achieved by magnetron sputtering. The bilayer was then aged at temperatures from 100 oC, 150 oC and 200 oC within the duration of one to six hours. The formation of IMCs of different chemical composition and crystallographic structures were characterized by using High Resolution Transmission Electron Microscope (HRTEM), X-ray diffractometer (XRD) and Energy Dispersive X-ray Spectrometer (EDX). Field Emission Scanning Electron Microscope (FESEM) was utilized to study the morphology and the integrity of the bilayer after subjected to aging. Each layer shows both columnar and non-columnar structures which eventually affects the film integrity. The combination of analytical TEM and X-ray diffraction techniques has revealed the formation of several equilibrium phases which could be identified as tetragonal ?-Al2Cu (I4mcm), monoclinic ?-AlCu (I2/m), orthorhombic ?-Al3Cu4 (fmm2) and cubic crystal structure Al4Cu9 (P-43m). Finally, electrical resistivity test on the bilayer indicates increase in resistivity due to the formation of IMC.