In-Situ Characterisation of Precipitation in Al-Cu thin films
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Abstract The precipitate formation occurring in Al-Cu thin foils with copper concentrations of either 1.15 at.% or 0.3 at.%, has been studied. In-situ X-ray diffraction analysis and differential scanning calorimetry are applied to determine the phases formed and the enthalpy changes in the same samples. Both X-ray diffraction and differential scanning calorimetry indicate that the precipitation behaviour of thin films (about 500 nm thickness) differs significantly from that of bulk material. In the films studied the precipitation of AI2Cu occurs at a much lower temperature than expected on the basis of the (bulk) phase diagram. Moreover, no intermediate phases are observed prior to Al 2Cu precipitation. Also the amount of Cu in solid solution (0.20 at%Cu) observed by electron-probe micro-analysis after slowly cooling from 500'C to room temperature, exceeds the solubility of bulk Al-Cu. Introduction Aluminum-copper (Al-Cu) alloys are known to form reliable interconnects. The addition of copper improves the electromigration performance of the interconnects compared to pure aluminum [1]. Apart from the electromigration performance, a great concern is the mechanical response to thermally induced stresses in the material. These stresses are induced during thermal cycling, as a consequence of the different thermal expansion coefficients of the metal, the silicon substrate and surrounding dielectrics. One possible way to determine stresses during thermal cycling, in thin metal films on a silicon substrate is the wafer-curvature technique [2 and references therein]. It is known that the addition of small amounts of copper to the aluminum will drastically change the stress-temperature dependence of the thin film [3, 4]. To interpret the stresstemperature curves of Al-Cu thin films it is necessary to characterise the behaviour of the copper in the aluminum matrix. From the bulk Al-Cu phase diagram [51 it is known that small amounts of copper can be dissolved in the aluminum matrix at high temperatures. Upon cooling, having a fixed amount of copper in the film, at a certain temperature the copper solvus is crossed and AI 2Cu phase formation occurs. It is the intention of this paper to determine the onset precipitation temperature in Al9Cu, and A199. 7 Cu 03 thin
films. To this end, two independent techniques are employed to determine in-situ the precipitation temperature during cooling. Firstly, X-ray diffraction analysis (XRD) is used. This technique provides direct identification of the phases present in the thin film. Secondly, the onset temperatures are also determined by differential scanning calorimetry (DSC). Furthermore, DSC is used to measure the energy flux involved in the precipitate formation. The presented data imply that the phase diagram for Al-Cu thin films deviates substantially from the phase diagram known fore bulk Al-Cu alloys.
Experimental Details Samples for in-situ XRD and in-situ DSC are prepared in the following way. Silicon
wafers of 100 mm diameter are covered with a photoresist layer of 1 microm
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