Ming-Yao Yen

Influence of Molecular Weight of Polyethylene Glycol on Microvia Filling by Copper Electroplating

The influence of the molecular weight (Mw) of polyethylene glycol (PEG) on the microvia filling by copper electroplating was demonstrated and examined by cross-sectional images using an optical microscope. The electrochemical behavior of PEG of different molecule weights in the copper electroplating was characterized by galvanostatic measurement. In the presence of excess Cl - , the surface coverage of PEG of various Mw adsorbed on the copper surface was characterized by observing the size and distribution of CuCl precipitates using a scanning electron microscope. As PEG Mw was increased, the best filling performance of plating formula was obtained when the PEG Mw ranged from 6000 to 8000 g/mol. Only large PEG amounts whose Mw exceeds 2000 g/mol can effectively polarize the cathode, in turn inducing the catalytic effect of bis(3-sulfopropyl) disulfide on copper deposition, resulting in a synergistic interaction between the suppressor and accelerator on the microvia filling.

Influence of Convection-Dependent Adsorption of Additives on Microvia Filling by Copper Electroplating

Influences of forced convection during acid copper electroplating on microvia fill of printed circuit boards were studied. The plating formula was composed of polyethylene glycol (PEG), chloride ion, 3-mercapto-l-propanesulfonate, and Janus green B (JGB). The filling performances under various plating conditions were examined with scanning electron microscopy and optical microscopy. Chemical and physical interactions between these additives and fluid dynamics were characterized by cyclic linear sweep voltammetry, steady-state current-potential measurements, linear sweep voltammetry, and galvanostatic measurements using different rotation speeds of working electrode. Experimental results indicated that the potential-dependent adsorption of chloride ion on the copper surface determined and governed the electrochemical effect of these organic additives on the cathode, resulting in a convection-dependent adsorption of inhibiting reagents and an asymmetrical fill of copper deposit in the microvias. A synergistic inhibition effect on the copper deposition caused by a composite suppressor composed of PEG, Cu + , Cl - , and JGB was demonstrated. The dominance of the competitive adsorption between the accelerator and the composite suppressor was shown to depend significantly on the chloride ion concentration and on the forced convection.

Roles of Chloride Ion in Microvia Filling by Copper Electrodeposition II. Studies Using EPR and Galvanostatic Measurements

The interactions among additives employed in acidic copper plating solution for microvia filling are characterized by galvanostatic measurement (GM) and electron paramagnetic resonance (EPR), respectively. These additivesinclude polyethylene glycol (PEG), chloride ion, 3-mercapto-1-propanesulfonate (MPS), and bis-(3-sulfopropyl) disulfide (SPS). EPR patterns show that MPS can reduce Cu 2 + to Cu + before electrochemical reduction. However, the GMs show that MPS is an inhibiting reagent for copper electrodeposition. An inhibition mechanism of the MPS is proposed according to the EPR patterns. GMs also revealed that the combination of MPS and Cl - could result in a strongly catalytic effect on the Cu 2 + reduction rate. This synergistic effect between MPS and Cl - on enhancing copper electrodeposition is attributed to an inner sphere electron transfer net constructed by a competitive coordination of MPS and Cl - on Cu 2 + complexes. SPS also exhibits similar electrochemical behavior to MPS. However, the response speed of SPS in enhancing the Cu 2 + reduction rate is slower than that of MPS. GM results obtained by adopting two different rotating speeds of the working electrode demonstrate that PEG competes with MPS to grab Cl - through Cu + , and that the competitive adsorption between PEG-Cl - and MPS-Cl - on the cathodic surface is strongly convection-dependent.

Through-Hole Filling by Copper Electroplating

Through holes (THs) with different shapes were formed by laser drilling on a printed circuit board to evaluate the filling capability of two copper plating formulas. The shapes of these THs were cylindrical, V- and X-shaped. Two copper plating formulas, accelerator-free formula (AFF) and accelerator-containing formula (ACF), were employed in this work. The AFF contained only one organic additive and the ACF was composed of multiorganic additives. The electrochemical characteristics of the AFF were investigated by cyclic voltammetry, which could be utilized to explain the results of filling plating. The plating results showed that the cylindrical TH could be fully filled using AFF. However, the V- and X-shaped THs could be fully filled using ACF. TH and microvias could be simultaneously filled in one plating bath using the AFF. A filling mechanism based on an adsorption/ consumption/diffusion mode was proposed to explain these plating results.

Microvia Filling by Cu Electroplating Over a Au Seed Layer Modified by a Disulfide

A plating process for microvia filling by Cu electroplating, carried out in a plating bath without an accelerator but with a suppressor only, is proposed in this work. The seed layer of microvia used for subsequent Cu-filling plating is Au formed by electroless plating. The surface of the Au seed layer is modified in a solution containing bis(3-sulfopropyl)-disulfide (SPS) and various supporting electrolytes. This pretreatment is similar to the self-assembly monolayer (SAM) of a thiol molecule on a Au substrate. The coverage density of the adsorbed thiolate strongly depends on the presence or the absence of a supporting electrolyte and it crucially determines the filling performance of the plating process. The plating results demonstrate that the thiolate adlayer which is initially formed on the Au seed layer is transferable onto the surface of the plated Cu and then interacts with chloride ions to further facilitate Cu nucleation and growth. According to the results of the filling plating and the electrochemical analysis, an accelerating mechanism of SPS-SAM for copper electrodeposition and its transferring mechanism are proposed in this work.

Practical Monitoring of Filling Performance in a Copper Plating Bath

A method for monitoring the filling performance of a copper plating solution for microvia metallization was examined by a continuous operation in a pilot plating bath. The monitoring method was carried out using a simple galvanostat measurement. A potential difference between two polarization curves obtained from the galvanostatic measurements at different flow rates can serve as an effective indicator of filling performance, because a linear function can correlate the potential difference with the filling performance. The shift of the polarization curve of the working solution after a period of operation can be regarded as the occasion of accumulation of by-products.

Studies of microvia filling mechanism and a novel Cu plating formula

Microvia filling by copper electroplating has been a key process for fabrication of high density interconnection (HDI) of advanced PCBs or IC substrates. Currently, conventional copper plating formulas used for the microvia filling typically contain at least two or three components, such as suppressor, accelerator, and leveler. A specific synergy between these additives leads to a bottom-up deposition of copper in the microvia, so that the microvia can be fully filled by the copper deposit without a void [1-6]. However, copper is also deposited simultaneously on the board surface during plating, when the conventional copper plating formula is employed. Consequently, a pattern with fine lines formed by an etching process is not easily obtained due to the thick copper film plated on the board surface. Therefore, we have developed a novel copper plating formula herein in order to achieve microvia filling without an increase in copper thickness on the board surface after the copper electroplating. In other words, this novel copper plating formula can offer a highly selective filling for microvia metallization, that is, the whole copper was exclusively deposited in the microvia. A comparison of cross-sections of copper filled microvias using two different plating formulas, one conventional and the other novel, is shown in this work. A basic cyclic voltammetry (CV) analysis was carried out to explain the filling results. A simple mathematical simulation of current density distribution is also presented to explain the filling mechanism.

Advanced thin copper electroplating process for HDI microvia filling application

Electrolytic copper microvia filling is an enabling technology, prominently used in todays manufacture of high density interconnect (HDI) and packaging substrate applications for better reliability, increased circuit densification, design flexibility and thermal management. To meet these needs, seemingly incompatible objectives must be met. Thinner and more uniform surface copper deposits have to be produced; increasingly difficult microvia geometries must be filled, while maintaining plating rates capable of delivering production throughputs. This paper describes a new panel and pattern-plate, direct current (DC) copper electroplating process designed for packaging substrate and HDI applications. Microvia filling performance, surface distribution and product reliability as a function of a variety of physical processing variables is discussed.