Study on the Action Mechanism of a Nontraditional Thioamide-Based Leveler of 1-Phenyl 2-Thiourea for Microvia Void-Free Filling

Leveler is the key additive for achieving microvia void-free filling in the high-density integration of electronic components for miniaturization, lightweighting, and intelligent development of electronic devices. Traditional organic levelers are mostly quaternary ammonium salts and nitrogen-containing heterocyclic compounds. In this study, thiosemicarbazide (TA) and 1-phenyl 2-thiourea (PTA), which are the thioamino compounds with different substituents of amino and phenyl, are used for the first time as levelers for microvia filling in an acidic copper sulfate bath of Cu electroplating (Cu-EP). The results demonstrate that only the PTA, when used as the leveler in combination with poly(ethylene glycol) (PEG) as the suppressor and bis(sodium sulfopropyl)-disulfide (SPS) as the accelerator, can achieve microvia void-free filling. Chronopotentiometric analyses reveal that both TA and PTA accelerate the reduction of cupric ions. The accelerator of SPS with TA synergistically enhances accelerating effect, while with PTA, it behaves with competitive adsorption to weaken this effect. The suppressor of PEG can cooperate with PTA to show a strong inhibiting effect on cupric ion reduction but presents competitive adsorption with TA, weakening the inhibiting effect of PEG. Under such complex interactions of these additives, it has been found that only the combination of PTA, SPS, and PEG exhibits convection-dependent adsorption behavior, which results in a more negative reduction potential of cupric ions at the mouth of the microvia than that at the bottom of the microvia, indicating a theoretical feasibility of achieving microvia void-free filling. Cyclic voltammetry experiments further demonstrate that the PTA can accelerate the reduction of cupric ions, but this acceleration effect is weakened by SPS due to competitive adsorption. PTA synergizes with PEG to greatly enhance the inhibiting effect on cupric ions reduction. According to electrochemical properties and interactions of the additives, the essence of the PTA-based acidic Cu sulfate bath for achieving microvia void-free filling is elucidated through electroplating simulations. When the three additives coexist, the adsorption coverage of PTA-PEG gradually decreases from the mouth to the bottom. As PTA-PEG exhibits a synergistic inhibition effect, such a gradient significantly inhibits Cu deposition at the mouth; simultaneously, the adsorption coverage of SPS, which behaves with an accelerating effect, gradually increases from the mouth to the bottom, facilitating the reduction of cupric ions at the bottom. The dual gradient effects enable the microvia void-free filling.