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Microstructure evolution in electrodeposited copper thin films for advanced microelectronic applications Alshwawreh, Nidal Khalaf

Abstract

Copper interconnects in advanced integrated circuits are manufactured by processes that include electrodeposition, chemical mechanical polishing and annealing. The as-deposited copper is nano-crystalline and undergoes a microstructure evolution at room temperature (self-annealing) or during an annealing step. During this process, significant changes in resistivity and grain size are observed. In this work, the microstructure evolution in 0.5-3 μm-thick electrodeposited copper thin films was studied. Resistivity measurements were used to quantify the role of deposition conditions on the microstructure evolution rate. In-situ electron backscatter diffraction (EBSD) was employed to observe self-annealing at the film surface. The resistivity-microstructure correlation during self-annealing was examined. A phenomenological model using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach was developed to describe recrystallization during isothermal and continuous annealing treatments. The microstructure evolution in copper-silver alloys and films produced by variable deposition rates was investigated. Phase-field model was applied to simulate self-annealing and the effect of deposition current density. The results show that the drop in resistivity during self-annealing is accompanied by significant changes of the microstructure at the film surface. Different criteria were developed to assess self-annealing rate from EBSD maps including grain size, image quality and local orientation spread. Adopting a grain size threshold, it was found that there is a reasonable correlation between resistivity and microstructure during self-annealing. The recrystallization in copper thin films appears to be thermally activated with an activation energy of 0.89-0.93 eV. Adopting the principle of additivity, it was found that the recrystallization rate during continuous annealing can be described by the JMAK model using the isothermal resistivity profiles. A method was proposed to accelerate recrystallization based on a capping layer deposition. No recrystallization was observed when silver was co-deposited with copper in the absence of chloride (even when annealed at 100 °C for 5 hours). Phase-field model was able to describe self-annealing and the effect of deposition current density. The results in this thesis are of significance to the microelectronic industry where recrystallization is a crucial step in the fabrication of copper interconnects for the high performance integrated circuits.

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