Cooperative Suppression and Accelerator Exclusion on Copper: Mechanistic Origin of Thiadiazole Leveler G1 for Blind-Microvia Superfilling in Acidic Sulfate Plating

High-density interconnection (HDI) printed circuit boards require precise defect control during copper microvia electrodeposition, where levelers play a decisive role in defining the filling window and deposit quality. This work investigates 2-amino-5-mercapto-1,3,4-thiadiazole (G1) as a leveler and compares it with the structurally analogous 2,5-diamino-1,3,4-thiadiazole (G2) to clarify the relationship between molecular structure, interfacial adsorption regulation, and microvia superfilling. Microvia plating experiments, electrochemical characterization, density functional theory (DFT), and molecular dynamics (MD) simulations were combined to reveal the distinct behaviors of G1 and G2. Plating results show that only G1 enables bottom-up superfilling of blind microvias while maintaining low surface roughness and a preferred Cu(220) orientation. Electrochemical measurements demonstrate that G1 exhibits strong cooperative suppression with the suppressor PEP and effectively resists competitive adsorption by the accelerator ZPS, forming a highly inhibiting PEP-ZPS-G1 system with a measurable convection response. In contrast, G2 shows weak synergy with PEP and lacks comparable electrochemical features, explaining its inferior filling performance. DFT calculations indicate that the asymmetric substitution of G1 produces a more heterogeneous charge distribution and enhanced active sites, strengthening its interactions with the cathodic interface and coadditives. MD simulations further confirm that G1 adsorbs more stably on Cu(111), increases PEP surface coverage, and suppresses effective ZPS adsorption, leading to selective interfacial regulation characterized by strong PEP adsorption and weak ZPS adsorption. These results establish a clear structure-function relationship for thiadiazole-based levelers and provide guidance for rational molecular design in advanced copper electrodeposition.