Doctoral Dissertations
Date of Award
8-1989
Degree Type
Dissertation
Degree Name
Doctor of Education
Major Professor
A. J. Pedraza
Committee Members
James R. Thompson, Thomas T. Meek, D. H. Lowndes, J. E. Spruiell
Abstract
A study is presented on the adhesion enhancement obtained when metal films were laser-treated on ceramic substrates using a pulsed XeCl excimer laser. Copper films in the thickness range of 80nm and were sputter-deposited on thoroughly cleaned sapphire substrates and laser-treated in the energy density range of 0.2 to 3.5 J/cm2. Results indicate that film damage and separation from the substrate is minimal when the film thickness is restricted to 80nm. This implies that a multi-stage deposition and laser irradiation process is necessary to generate a thick copper film on sapphire. Strongly adherent 300nm and 1.08/μm films of copper were grown on sapphire substrates by sequential sputter deposition alternating with pulsed laser treatment.
During the earliest stages of the sequential process, the laser-melted film breaks into small clusters. Closely spaced finer clusters result from smaller film thicknesses. The process of cluster formation is driven by surface energy gradient generated by lateral thermal gradients in the melt. The thermal gradients in the liquid result from the early establishment of isolated regions of good bonding with the substrate. This good bonding is maintained through successive meltings of the region as is apparent from their one-to-one correspondence with the cells in the fully grown film.
Mechanical testing of the films showed at least a three-fold increase in the interfacial strength of the laser-treated films when compared with as-deposited ones. Transmission electron microscopy of the near-interface region revealed that a thin sapphire layer was melted along with the copper film. Two well differentiated regions could be observed in the modified layer. The region closer to the unmodified substrate consisted of epitaxially regrown alumina with crystallites misoriented up to 10 degrees relative to the substrate sapphire orientation, while precipitate particles could be seen closer to the resolidified copper. The nature of the precipitates generated in the second region was dependent on the atmosphere present during the treatment. A trirutlie-like compound was obtained in air, whereas a different compound with hexagonal structure closely related to sapphire was observed when the treatment was carried out in a reducing atmosphere.
300nm-thick films of nickel and niobium were built on sapphire substrates using sequential deposition and laser treatment. Interfacial strength was found to have increased similar to the case of copper-sapphire couple. In the case of niobium-sapphire couple, softening of the substrate was observed when an 80nm-thick niobium film was deposited on sapphire and laser-treated with an energy density of 0.5J/cm2.
The effect of interfacial thermal conductivity and laser energy density on the laser processing was studied using two different substrates. 80nm-thick copper and niobium films were deposited on sapphire and quartz substrates and laser-irradiated with energy densities in the range of 0.2 to 3.5J/cm2. The change in film morphology and thickness as a function of laser energy density as well as substrate nature was analysed by scanning electron microscopy and energy dispersive x-ray spectroscopy. Four regimes can be established as a function of the laser energy density, for low energy densities up to a critical value the film is mechanically removed either completely or in part. As the energy density increases above the critical value, larger portions of the film remain attached to the substrate. It is in this regime where adhesion enhancement occurs. Further increase in the energy density results in film evaporation. Finally, an intermediate compound results in the high range of energy densities. The amount of film removal as well as the energy density range for each regime is a function of both the substrate nature and the film. The results are correlated with a computer model of heat flow during laser processing of metal-ceramic couples.
Recommended Citation
Godbole, Mukund J., "Adhesion enhancement of metal-ceramic couples using pulsed excimer laser. " PhD diss., University of Tennessee, 1989.
https://trace.tennessee.edu/utk_graddiss/11651
