X-ray diffraction analyses of electrolytic and electroless copper films for circuit boards

Author

Yun Lu

Date of Award

5-1991

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Anthony J. Pedraza

Committee Members

J. E. Spruiell, C. Brooks

Abstract

Electrolytic and electroless deposited copper films from different baths and with different polymeric substrates (ABS, polyimide, and FR-4) were analyzed by x-ray diffraction (XRD). The sin²φ method was employed for determining macrostresses in those films. Separate contributions to line broadening from crystallite size, microstrains, stacking faults, twins and dislocations were determined by a Fourier analysis method developed by Warren and Averbach. Phase identification and preferred orientation analyses were also performed on various samples.

The deposition conditions generate internal stresses that develop into macro stresses in the film. The mismatch in the thermal coefficient of expansion (TCE) between film and substrate is another source of macrostresses when the films are cooled to room temperature. Film macrostresses result from both intrinsic stress and thermal stress effects. The balance between deposition conditions and substrate nature controls both sign and magnitude of the generated stresses.

High macrostresses were present in the film despite little TCE difference between the film and the substrate. It is believed that the thermal stresses are not the only source of residual macrostresses. Impurities incorporated into the film during deposition are considered to be another source for generating film macrostresses.

Compressive stresses in the copper films were found to increase with longer exposure of the substrate to chemical etching prior to film deposition. Cu₂O detected at the interface region of copper/polymer couples was found to be associated with high tensile stresses in the films. Since the Cu₂O layer has the effect of mechanically decoupling the film from the substrate, the presence of this layer eliminates the thermal stress effect and influences stress relaxation behavior at the interface region directly.

X-ray line broadening analyses showed that as-received electroless copper films have extremely small crystallite size, large microstress, many stacking fault and twins, and high dislocation densities. In general, the XRD lines of electroless deposits are broader than the lines of electrolytic deposits. Particle size of the electroless copper films is smaller than that of electrolytic deposits, whereas their microstrains are higher. The ductility of as-received electroless copper is lower than that of electrolytic copper, while its yield strength is higher. These facts can be explained from the line broadening results directly.

As demonstrated by XRD line broadening and preferred orientation analyses, the recrystallization temperature of high strength (HS) electroless copper is 100° higher than that of high elongation (HE) copper. Clearly, a large difference in impurity content is the main cause for the large difference in recrystallization temperature between HS and HE materials.

The microstresses in HE electroless copper decrease rapidly at the early stage of recrystallization. By contrast, the microstresses in HS copper show an appreciable increase, even after over 50% of the material has recrystallized. Hydrogen filled cavities in HS copper deposits have been suggested as the source of these microstresses. The dramatic improvement in ductility of HE copper after annealing correlates with the sharp decrease in line broadening.

Recommended Citation

Lu, Yun, "X-ray diffraction analyses of electrolytic and electroless copper films for circuit boards. " Master's Thesis, University of Tennessee, 1991.
https://trace.tennessee.edu/utk_gradthes/12461