About this document
This Technical Note sets out a process design for making Cu/PDA particles for copper sintering pastes used in power semiconductor die attach, running from acid pickling to seed growth in one wet process. It also describes how each step is to be verified. The full report is available in Korean. An English summary is provided on this page.
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Summary
Replacing silver sintering materials with copper requires nano-scale copper on the particle surface to promote sintering. Reducing copper ions loaded into a polydopamine (PDA) layer yields less than a monolayer of copper. This note covers an integrated wet process that uses the loaded Cu as seeds to grow nano copper connected to the particle body, together with the checks and comparison groups for each step.
Conclusion
The target particle has an acid-pickled copper body, a thin PDA interface layer, and nano copper on the surface bonded to the body. The process runs through acid pickling, PDA formation, copper ion loading, seed reduction, seed growth, washing and storage, and sinter bonding. It relies on stirring, washing and solid-liquid separation, so no photonic pulse equipment is needed. Each step runs in a separate bath to control surface condition and solution chemistry between steps. The key design change is the shift from reducing loaded ions to growing seeds. Assuming a 2 nm PDA layer and a copper-to-PDA repeat unit ratio of 0.5, one loading cycle gives Cu equivalent to about 0.07 nm of metal, short of a monolayer (about 0.2 nm). For flake particles, the new Cu is 0.19% of the starting mass, two to three orders of magnitude below the amount of nano copper typically blended in as a sintering aid. Using the loaded Cu as seeds and growing an equivalent 5 nm raises the new Cu in flake particles to 13.3%. All of these are design calculations based on the assumptions stated in the document. PDA formation and ion loading involve conflicting requirements, which the design separates. Growing PDA with dissolved oxygen also re-oxidizes the copper body. The process therefore uses Cu²⁺ in the bath to catalyze dopamine oxidation, keeping PDA growth going at reduced dissolved oxygen. The acceleration of polymerization by Cu²⁺ is supported by literature; the simultaneous capture of Cu²⁺ released from the body is an inference-based design. The loading bath is set at 1 mM and pH 5.5 based on a calculated Cu(OH)₂ precipitation boundary. Borane-type reducing agents are the main candidates for seed reduction because they add no sodium, a mobile-ion contaminant in power semiconductors. Verification is designed to isolate the contribution of each step. ⁶⁵Cu isotope tracing quantifies the loaded Cu independently of copper dissolved from the body, and cross-section mapping shows directly whether the reduced and grown Cu is connected to the body. Comparison groups A to G, which switch pickling, PDA, loading, reduction and seed growth on and off, separate the contribution of each step. Bonded joints are assessed for shear strength, void fraction, interfacial delamination, electrical resistance and bond-layer thermal resistance, with reproducibility checked across independent lots and after thermal cycling. The document is a process design and contains no measured bonding performance data.
Topics
- Bonding materials
- Power semiconductors
