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The rapid implementation of HDI technology has focused increased attention on the thermo-mechanical reliability of stacked microvias. Factors like base material properties, lamination and drilling parameters, de-smearing, and electroless copper activation had been extensively studied. Research by Atotech indicated that the detailed formulation of the copper electroplating process significantly influenced the nature of the metal deposited on and near the surface and that subtle differences in the co-deposition of additives played an important role in determining the integrity of the interface between stacked micro-vias. Roland Herold, Atotech senior specialist in electroplating, gave an overview of the latest findings and developments.
How could the bond be improved? His Ishikawa diagram was indeed “a fish with many bones,” but he chose to focus on the functions and purposes of essential additives: halides, brighteners, levellers, and carriers, and the extent to which these could be preferentially co-deposited at or near the interface. Clearly, the surface of the electroless copper at the target pad needed to be perfectly clean to enable the best metallic bond to be achieved by undisturbed epitaxial copper crystallization, but even when the prior processes were perfectly under control, it was possible for components of the electroplating process itself to effectively contaminate the target pad surface and thus weaken the metallic bond.
Herold described the chemistry of organic additives, which typically contained sulphur and nitrogen in combination with carbon and oxygen and were normally associated with chlorine, and discussed methods by which they could be detected and measured. Dynamic secondary ion mass spectrometry (dSIMS) had been successfully used as an analytical technique for investigating failed interfaces. He gave a detailed account of electrodeposition mechanisms to explain the interaction of brighteners, carriers, and levellers and their influence on the initiation and growth of copper crystal structure. A fascinating lesson in theoretical electrochemistry!
Investigational work at Atotech had demonstrated that leveller compounds could be co-deposited onto interfaces and into the electrodeposited copper deposit, synergistically with the brightener, and the rate of co-deposition of additives was highly dependent on their chemistry. The co-deposition of additives could be controlled to a low level by careful choice and balance of chemical composition, together with optimisation of operating temperature, current density, and reverse pulse frequency, as well as the configuration and maintenance of the anode system.
After a question and answer session, Alun Morgan thanked the presenters, the organisers, and the participants, and brought EIPC’s very successful and extremely informative second technical snapshot to a close. The next one in the series is planned for December 16.
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