Thermo-mechanical behaviour of thick copper metallizations for power electronics

Stephan Bigl

    Research output: ThesisDoctoral Thesis

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    Abstract

    Copper films play a very important role for microelectronic components. Due to the increasing functional demands in the area of power electronics copper metallizations are not only used for electrical contacting but rather as active heatsinks. Within the electrical switching operations high energy densities are dissipated via such metallizations and lead to great mechanical stresses due to the different expansion coefficients. The accumulated plastic deformation caused by the cyclic switching pulses can eventually lead to failure of the device. In the presented work copper films in the thickness range of 5-20 µm on silicon substrates are studied with respect to their thermo-mechanical behaviour. Therefore, a special laser cycling device was developed which was able to heat up the Cu-Si specimens within a few tens of a seconds to temperatures up to 400°C which leads to cyclic thermo-mechanical stresses in the copper films. Via a novel characterization approach, which site specifically tracks the topographical (Atomic force microscopy) as well as the crystallographic (electron back scatter diffraction) evolution of the thermo-mechanically stress copper films, valuable influence parameters could be investigated in a closer manner. Cost-efficient deposition processes such as electroplating, lead to inorganic impurities in the ppm range. Metallizations with higher sulphur and chloride content showed a marginal grain growth and a stable twin network in combination with a constraint roughness evolution (smooth within the grain, rough at the grain boundaries) regarding thermo-mechanical cycling. Furthermore, the impurities decreased the activation energy of void formation, which would lead to crack initiation and eventually to failure of the metallization if further thermal cycling would be applied. Quantitative investigations of the impurity influence with respect to the local mechanical behaviour showed that inorganic elements (S, Cl) segregate to the grain boundaries after annealing treatments which leads to the formation of localized hard interfaces. Local nanoindentation revealed that the hardness in the grain interior is identical for pure and impure Cu films, but due to the segregated impurities at the twins and grain boundaries, an about 1.5 higher hardness was observed in the impurity enriched Cu films. The thermo-mechanical investigations with the new laser cycling device showed that the higher heating/cooling rates and therefore higher strain rates in the Cu films lead to an accelerated fatigue damage. Grain growth and roughening were significantly greater with respect to thermal cycling than in specimens with the same cycle number and temperature range but lower strain rate. The quantitative surface damage, using a newly developed analysis, showed 1/hfilm dependency of the fatigue damage for thicker Cu films (≥ 5 µm). This is in contrast to the thin film counter parts (≤ 5 µm) where the thermo-mechanical damage is proportional to the film thickness. Moreover, a transition from an orientation independent surface damage (5µm) to an orientation dependent damage for thicker Cu films (10 and 20 µm) was observed. On thinner substrates with constant Copper film thickness (5 µm) an accelerated film fatigue was observed. On a substrate thickness of 323, respectively 220 µm the determined surface roughness after 750 thermal cycles was a factor of three, respectively five greater than on standard substrates (725 µm).
    Translated title of the contributionThermo-mechanisches Verhalten von dicken Kupferschichten für Leistungselektronik
    Original languageEnglish
    QualificationDr.mont.
    Supervisors/Advisors
    • Clemens, Helmut, Assessor B (internal)
    • Kiener, Daniel, Assessor A (internal)
    Publication statusPublished - 2017

    Bibliographical note

    embargoed until null

    Keywords

    • copper fatige
    • grain growth
    • void formation
    • surface roughening
    • thermal laser cycling

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