Microstructure and He Bubble Effects on Al-Cu Thin Films
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Microstructure and He Bubble Effects on Al-Cu Thin Films C.S. Camacho1, P.F.P. Fichtner2 , F.C. Zawislak1 , G. Feldmann3 1 Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil 2 Departamento de Metalurgia, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil 3 Departamento de Física e Matemática, UNIJUI, 98700-000 Ijui, RS, Brazil ABSTRACT The effects of film morphology (mosaic- or bamboo-like grain structures) and of He bubbles on the redistribution of Cu, as well as on the formation of Al-Cu precipitates in 200 nm thick Al/SiO2 films similar to microelectronic device interconnects, are investigated using Rutherford backscattering spectrometry, elastic recoil detection analysis and transmission electron microscopy. As-deposited and pre-annealed Al films were implanted with Cu and/or He ions forming concentration profiles located 100 nm below the surface and with peak concentrations of about 3 at.%. It is shown that grain boundaries and/or He bubbles can affect the vacancy fluxes inside the grains and reduce or even inhibit the Cu redistribution as well as the nucleation and growth of θ and θ′ Al-Cu precipitates during post-implantation annealings at temperatures from 473 to 553 K. It is also shown that mosaic-like grain structures allow the control of grain size distribution within the 25 to 1500 nm size range, thus providing an additional microstructure engineering tool to improve device reliability against electromigration failures. INTRODUCTION The disruption of metallic interconnects is a rather common event limiting the lifetime of microelectronic devices. The drift of atoms in response to electric current (i.e. electromigration) leads to the formation of regions of atom accumulation towards the anode and regions of vacancy accumulation towards the cathode. It is the vacancy condensation leading to nucleation and growth of voids that finally causes the interconnect disruption. The failure probability increases with the increase of device integration because it leads to higher current densities within the interconnects. The failure probability also increases with the increase of the device operation frequency because it leads to higher operation temperatures, thus enhancing atomic mobility and therefore the electromigration effects. In the case of Al interconnects, the characteristics of the microstructure in terms of grain sizes, distribution of solute atoms and/or the formation and distribution of dispersed second phase precipitates have been considered in order to prevent electromigration failures [1,2]. It was established that small additions of Cu (0.2 to 4 at%) improves the lifetime of the interconnects. One explanation for this effect assumes that, during electromigration, the Al atoms begin to be depleted only after the preferential depletion of Cu in solution [3,4]. The present work reports a systematic study on the microstructure evolution of Al films deposited on SiO2 and implanted with Cu and He ions. The idea of He co-i
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