Convection-dependent Adsorption Research Articles

1-Dodecyl-2-methyl-3-benzylimidazolium chloride as a novel leveler: Towards simultaneously both the microvia void-free filling and through hole conformal thickening

In this work, an organic additive, 1-Dodecyl-2-methyl-3-benzylimidazolium chloride (DMBI), is explored as a novel leveler with excellent property in simultaneous microvia and through hole copper metallization. Studies of galvanostatic measurements (GM) and cyclic voltammetry stripping (CVS) demonstrate that DMBI exhibits the necessary fundamental leveler’s characteristics of convection-dependent adsorption, and the stronger inhibition effect on copper ions electroreduction at a higher electrode rotating rate. Moreover, the combination of DMBI and SPS behaves a stronger convection diffusion effect than that of DMBI and PEG. In situ attenuated total reflectance-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) spectroscopic analysis illustrates that DMBI affects the electroreduction of copper ions by competing with bis-(sodium sulfopropyl) disulfide (SPS), and polyethylene glycol (PEG) can slightly weaken the decrease of DMBI adsorption cross section on copper surface. Based on the outstanding characteristics of DMBI as a novel leveler, we develop an advanced DMBI-based formula for Cu electronic plating, and achieve the microvia void-free filling and through hole conformal thickening simultaneously in a printed circuit board (PCB).

Tributyl(hexyl)phosphonium chloride as a new leveler for microvia copper superconformal electronic plating

A phosphorus-containing new compound of tributyl(hexyl)phosphonium chloride (TBHP), but not the commonly used nitrogen-containing heterocycles and quaternary ammonium salts, is successfully applied for the superconformal copper filling of microvia in acid sulfate electroplating bath for the application of electronic industry. The molecular interaction mechanisms of TBHP combined with suppressor (polyethylene glycol, PEG) and accelerator (bis-(sodium sulfopropyl)-disulfide, SPS) are investigated in detail. Galvanostatic measurements at platinum rotation disk electrode indicate that the leveler of TBHP among with the suppressor and accelerator behaves the key characters of inhibition and convection-dependent adsorption during copper ions electroreduction for the bottom-up filling of microvia. Cyclic voltammetry measurements and in-situ FTIR spectroscopy further reveal that TBHP synergistically interacts with the suppressor to inhibit copper ions electroreduction at the mouth of microvia; while TBHP competes for adsorption with the accelerator, behaving as TBHP gradually replaced by the accelerator at the bottom. Based on the sufficient theoretical supports, the practical copper electroplating is conducted for the verification of microvia void free filling. In the developed novel copper plating formula containing TBHP, PEG and SPS, the void free filling is successfully achieved and the filling percentage (FP) is high in 95.5%.

Molecular structure impacts of tetrazole derivatives on their diffusion and adsorption behaviors for microvia copper void-free filling

Optimizing a high-quality leveler has become the core task in developing the microvia copper filling technology. In this work, two nitrogen-containing heterocyclic compounds, iodonitrotetrazolium chloride (INT) and thiazolyl blue tetrazolium bromide (MTT) with a similar tetrazolyl group are selected as the levelers to investigated the molecular structure impacts on their microvia copper filling performance. Both INT and MTT inhibit the electroreduction of copper ions. The inhibiting effect of INT is greatly enhanced by convection while MTT is independent of convection, indicating a bottom-up filling performance of INT and a conformal filling performance of MTT. The microvias are supper-filled and poor-filled in electrolytes containing INT and MTT, respectively, as predicted by electrochemical analysis. Molecular dynamic simulation demonstrates that INT adsorbs in a flat orientation with its iodine and nitro group's two oxygen atoms interacting on the Cu(111) plane surface. MTT adsorbs on Cu(111) surface via positively charged tetrazole ring and its adsorption ability is much stronger than that of INT. The weak adsorption ability of INT on copper surface and the slow mass transfer rate result in its convection-dependent adsorption behavior and the excellent microvia copper filling performance.

Experimental and Theoretical Study of the New Leveler Basic Blue 1 during Copper Superconformal Growth.

A novel chlorinated functional group-modified triphenylmethane derivative leveler BB1 is used to achieve superconformal electrodeposition in microvias. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are performed to study the suppressing effect of BB1, while the convection-dependent adsorption of BB1 on the copper surface is analyzed by galvanostatic measurement, and a BB1 concentration window between 100 and 200 mg/L is beneficial for superfilling. The interactions among BB1, bis-(sodium sulfopropyl) disulfide (SPS), and poly(ethylene glycol) (PEG) are also investigated. Density functional theory (DFT) calculation and in situ Raman spectroscopy are coupled to study the suppression mechanism and synergistic suppression mechanism, namely, the adsorption effect between BB1 and copper substrate, as well as the coordination effect between the modified chlorinated functional group and Cu2+, is proposed. The copper layer becomes smoother and more compact with an increase in BB1 concentration, according to scanning electron microscopy (SEM) and atomic force microscopy (AFM), while X-ray diffraction (XRD) analysis shows that the introduction of BB1 is conducive to the formation of the copper (220) plane. Besides, the solution wettability is boosted by BB1. A copper interconnecting layer with high quality is achieved with 150 mg/L BB1, while the surface deposition thickness (SDT) is about 34 μm and filling percentages (FPs) for microvias with diameters of 100, 125, and 150 μm are 81.34, 82.72, and 81.39%, respectively.

Effect of leveler on performance and reliability of copper pillar bumps in wafer electroplating under large current density

This work investigated the effects of levelers on the electroplating performance and reliability of copper pillar bumps (CPBs) on wafer. Under large current density, the usage of Janus green B (JGB) or a commercial leveler (L1) could obtain a satisfied performance in flatness, co-planarity and growth rate. It was found that this qualified leveler possessed a remarkable electrochemical characteristic, i.e., strong convection dependent adsorption (CDA). The void growth at the plated CPBs/Sn-Ag soldering interface during thermal aging was greatly accelerated when the leveler was applied during electroplating. The mass spectrum analysis showed that the level of impurities in the plated CPBs using JGB was much higher than that plated without JGB. The usage of leveler could produce a high level of impurities in the plated CPBs, which accounted for the boosting of voids at the soldering interface during thermal aging.

Strong Convection-Dependent Adsorption of Phenothiazine Skeleton Levelers and Their Application in Copper Electroplating

The design and synthesis of high-performance levelers remain a major challenge to date. Herein, a series of phenothiazine (PTZ) derivatives were designed and synthesized, and they had a butterfly conformation, and their adsorption capacities were easier to modulate. To check out the inhibition capabilities among these eight PTZ derivatives, electrochemical measurements and theoretical calculations were carried out, and PTZ-8 indicated the strongest inhibition capabilities among these PTZ derivatives. Besides, galvanostatic measurements were also carried out to evaluate the overall performance of the levelers. The results showed that PTZ-8 had a potential oscillation, but PTZ-6 had strong convection-dependent adsorption. Due to the potential oscillation of PTZ-8, PTZ-6 as the representative leveler was selected for through-hole electrodeposition. In addition, field-emission scanning electron microscope images and X-ray diffraction results displayed the surface morphology, which indicated that the addition of PTZ-6 could lead to a fine copper deposit and cause the preferential orientations of copper deposits to change from [220] to [111]. This study might lay the foundation for the design of high-performance levelers.

Investigation of novel leveler Rhodamine B on copper superconformal electrodeposition of microvias by theoretical and experimental studies

Achieving void-defect copper filling by electrodeposition on microvias is essentially important. Rhodamine B (RhB) is of excellent suppressing ability based on electrochemical analysis. Convection-dependent adsorption analysis demonstrates that 100 mg/L was the optimum value to achieve bottom-up growth. The interaction among sodium 3,3′-dithiodipropane sulfonate, polyethylene glycol, and RhB was investigated and competition adsorption between RhB and SPS is observed. Meanwhile, copper electrodeposition obeys 3D diffusion-controlled progressive nucleation process at low potential but obeys 3D diffusion-controlled instantaneous nucleation process at high overpotential. Density functional theory and in situ Raman spectra were used to study the adsorption behavior of RhB. The calculated results indicates that RhB can effectively increase the energy barrier during copper electrodeposition. The surface morphology is improved and surface roughness is lowered with proper RhB concentration. The influence of RhB on orientation and grain size was analyzed and proper RhB (100 mg/L) is conducive to the formation of Cu (111) surface and lowering the grain size. Besides, the introduction of RhB can improve the wettability, which is conducive to the bottom-up growth. The electroplating process is optimized and void-defect copper interconnect line with high quality is obtained.

The influence of leveler Brilliant Green on copper superconformal electroplating based on electrochemical and theoretical study

Finding proper additive to achieve copper superconformal electrodeposition is significantly important. Brilliant Green exhibits excellent suppressing ability to the copper electrodeposition according to electrochemical analysis. Meanwhile, 100 mg/L was selected as the optimum value based on the convection-dependent adsorption behavior analysis of Brilliant Green. The interaction among three different additives was also investigated by applying chronopotentiometry as well as Brilliant Green compete to absorb on the cathode surface with SPS. Besides, the introduction of Brilliant Green can improve the transport of cupric ions (Cu2+). The nucleation and growth of copper deposition is 3D diffusion-controlled instantaneous growth process at high overpotential but 3D diffusion-controlled mixing growth process at low overpotential. The synergistic suppressing mechanism was proposed to explain strong suppressing effect of BG on reduction of Cu2+ and the reaction pathways was studied theoretically. Copper interconnect layer with high FP values (80.52 % to 84.38 %) was obtained with low SDT (about 28 μm) after electroplating process was optimized. The surface morphology under the influence of Brilliant Green is compact and uniform and the grain size is reduced by Brilliant Green.

Benzyl-containing quaternary ammonium salt as a new leveler for microvia copper electroplating

Recently, quaternary ammonium salt is considered as an effective leveler because of its easy adsorption on the cathode surface. However, quaternary ammonium salt compounds usually have a variety of functional groups with different structures, which may bring different adsorption effects. In this paper, three kinds of benzyl-containing quaternary ammonium compounds with different side chains, i.e., benzyl trimethyl ammonium chloride, tetradecyl dimethyl benzyl chloride ammonium chloride and benzethonium chloride, were selected as potential levelers for microvia overfilling of PCB with high-density interconnect. The wettability and the electrochemical behavior of the three levelers on copper deposition were studied through contact angle measurements and cyclic voltammetry striping tests. It was found that among the three kinds of quaternary ammonium salts, benzethonium chloride (BZC) with three side chains of ether bond, alkyl and benzene ring has the most significant inhibitory effect on copper deposition. Electrochemical tests including cyclic voltammetry striping (CVS) and linear sweep voltammetry (LSV) performed at two different speeds that simulates the microvia filling environment were then used to analyze the effect of the addition concentration of BZC as leveler on the deposition behavior of copper to make sure an optimal concentration. Subsequently, the effect of BZC as leveler on the electrodeposition behavior of copper under the synergistic action of conventional accelerator, suppressor and chloride ion was further analyzed by comparing with traditional leveler based on GM and LSV. The results show that under the synergistic action of other additives, this BZC produces greater inhibition of copper deposition than the traditional JGB, has more obvious convection-dependent adsorption behavior and forms a larger potential difference to meet the super filling. Filling experiment further shows that BZC as a leveler had an excellent filling performance. In addition, Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) were used to analyze the surface morphology and crystal structure of microvia filled copper at different concentrations of BZC. The present results show that benzyl ammonium chloride with ether bond, alkyl and benzene ring is an effective leveler for the microvia filling of PCB.

Structural Influence of Terminal Functional Groups on TEG-Based Leveler in Microvia Filling

We synthesize new triethylene glycol-based levelers containing quaternary ammonium groups with different terminal functional groups (allyl, propyl, benzyl, and naphthylmethyl) to investigate the structure-property relationship of the levelers. Though all the synthesized levelers indicate the convection-dependent adsorption behavior through the onset potential in linear sweep voltammetry, the inhibition strength depends considerably on the functional groups. The exchange current density of 0.40 mA cm−2 for Cu reduction in the naphthylmethyl case is the lowest among our levelers, owing to the π-cloud interactions of a large aromatic naphthylmethyl group with Cu. The naphthylmethyl leveler maintains the inhibition layer that is not deactivated even by the addition of bis(3-sulfopropyl) disulfide (SPS) unlike the other functional groups in chronopotentiometry. The complex inhibition layer composed of the leveler and a suppressor is retained, displaying a high potential difference of –157 mV between 100 and 1000 rpm. By using three-additives containing the naphthylmethyl leveler, microvias show the filling ratio of 100% within 50 min and a thin top thickness of only about 15 μm. These results provide the effectiveness of the naphthylmethyl terminal functional group for enhancing the inhibition of leveler and can help the new design of electroplating additives.

Convection-Dependent Competitive Adsorption between SPS and EO/PO on Copper Surface for Accelerating Trench Filling

In the absence of halogen ion, the convection-dependent competitive adsorption between bis(3-sulfopropy)-disulfide (SPS) and poly(ethylene glycol)-block-poly(propylene glycol) copolymer (EO/PO) on unpowered (in open circuit) copper surface is found in sulfuric acid solution. Galvanostatic measurements and electrochemical impedance spectroscopy were employed to analyze this particular adsorption behavior, revealing that the adsorption of EO/PO is enhanced by increased local convection intensity. The molecular dynamic simulation demonstrates that the increase of EO/PO adsorption decreases the adsorption of coexisting SPS on copper surface, supporting that the enhanced EO/PO adsorption in strong convection condition leads to the decrease of coexisting SPS adsorption on copper surface with the increase of convection intensity. The study of this convection-dependent competitive adsorption behavior enriches the understanding for the convection-dependent adsorption (CDA) behavior on electrolytic copper ‘bottom-up’ filling. Besides, the competitive adsorption behavior serves as a powerful handle to manipulate the accumulation of SPS at via or trench bottom before electroplating for improving the efficiency of via or trench filling. For example, the silicon wafer trench filling by copper electroplating is significantly accelerated with the introduction of convection-dependent competitive adsorption. Furthermore, a possible mechanism for accelerating trench filling by pre-adsorption process of SPS together with EO/PO is proposed.

Communication—Triphenylmethane-Based Leveler for Microvia Filling in Copper Super-Conformal Electroplating

Levelers exhibiting high microvia-filling power and wide operating-concentration ranges are required for the development of additives in copper super-conformal electroplating. In this study, a novel structure based on triphenylmethane (TPM-1) was synthesized and investigated as a leveler for microvia-filling electroplating. The galvanostatic measurements and cyclic voltammetry stripping experiments show that TPM-1 is a stronger inhibitor than Janus Green B for copper ion reduction and is essential in creating convection-dependent adsorption behavior. Copper electroplating experiments were conducted, and TPM-1 showed superior microvia-filling performance at a relatively wide concentration range.

Microvia Filling with Copper Electroplated with Quaternary Ammonium-Based Leveler: The Evaluation of Convection-Dependent Adsorption Behavior of the Leveler

In this study, we synthesized a leveler that contained two quaternary ammonium groups attached to hexaethylene glycol. The leveler enabled void- and seam-less Cu filling of the microvia. The convection-dependent adsorption of the leveler was assessed through electrochemical analyses such as linear sweep voltammetry and chronoamperometry on a rotating disk electrode, and the effect of convection on the adsorption of the leveler is interpreted in terms of adsorption diffusion model. In a Koutecký–Levich plot, the enhanced adsorption and inhibition of the leveler at the higher convection was clearly observed with the synthesized leveler.

2-Mercaptopyridine as a new leveler for bottom-up filling of micro-vias in copper electroplating

In order to achieve a perfect bottom-up electroplated Cu filling with a minimal surface thickness, 2-mercaptopyridine (2-MP) was investigated as a new leveler for replacing Janus Green B (JGB) for bottom-up copper filling. Electrochemical impedence results indicate that 2-MP has a stronger suppression for Cu deposition than JGB. With the addition of 2-MP, the filling capability of the electroplating solution is improved significantly with the Cu thickness on surface decreasing from ∼16μm to ∼10μm. The interaction mechanisms of 2-MP, bis(3-sulfopropyl) disulfide (SPS), Cl− and tri-block copolymer of PEG and PPG with ethylene oxide terminal blocks (EPE) in the plating solution are studied by galvanostatic measurements (GMs). The acceleration effect of SPS and the inhibition effect of 2-MP on copper deposition occur in the presence of EPE, and the convection-dependent adsorption (CDA) behavior of additives usually occurs with the injection of four additives at optional concentrations. Further, it was found that when 1.0ppm 2-MP, 1.0ppm SPS and 200ppm EPE were injected into the basic electrolyte, the potential difference (Δh) value of the electrolyte became positive, and the bottom-up electroplated copper filling was obtained in the electrolyte in absence of Cl−. The interaction mechanisms of three additives for bottom-up filling have been investigated by GMs.

Tetrazole Derived Levelers for Filling Electroplated Cu Microvias: Electrochemical Behaviors and Quantum Calculations

Three tetrazole derivatives (TDs), 1-(3-Acetamide) phenyl-5-mercaptotetrazole (ACET), 1-Phenyl-5-mercaptotetrazole (PMT) and 1-Methyl-5-mercaptotetrazole (MMT), have been studied as novel levelers for filling electroplated Cu microvia. It is found that the Cu deposition potential decreases with the addition of TDs into the Cu plating solutions, with the lowest deposition potential observed for MMT and the largest deposition potential change (Δη) observed for ACET. The ACET exhibits the strongest convection-dependent adsorption behavior with SPS, Cl−, and OP/OE, and the thickness of copper surface layer can be thinned to ∼15μm in the ACET concentration range of 1.0∼10.0ppm. The electrochemical impedance spectroscopy studies suggest that both the charge transfer resistance (Rct) and the adsorbed layer resistance (Rad) increase with the addition of TDs. Quantum chemical calculations indicate that the energy gaps (ΔE) of TDs correlate negatively with Δη, and the most active reaction sites for surface adsorption are the N3 in the thiol forms and the N4 in the thione forms of the TDs through Fukui functions.

Effect of External Factors on Copper Filling in 3D Integrated Through-Silicon-Vias (TSVs)

Except for the internal factors such as properties of additives, adsorption or desorption mechanism, the external factors such as forced convection and environment temperature play an important role in achieving the void-free copper filling. In this paper, the effect of the external factors on the 3D integrated Cu-TSVs filling was investigated. For large dimension via (Ø 50 μm × 150 μm, AR∼3), forced convection factor decreases the via filling efficiency due to the convection-dependent adsorption (CDA) effect. While for the relatively small dimension via (Ø 20 μm × 120 μm, AR∼6), a modest forced convection is vitally necessary for the void-free filling. Too low or too high convection rate would cause voids during filling due to the lack of accelerator replenishment in the via bottom or the existence of the beak structure in the via opening, respectively. The inactive behavior of accelerator is the main reason for the filling model changes from V-shape model to the near-ideal bottom-up filling model. With an increase of the electrolyte temperature, elastic modulus of Cu-TSVs decreases sharply, while the hardness varies slightly. The hardness of the Cu-TSVs plated in a large range of electrolyte temperature (4–45°C) is significantly higher than that of the bulk copper.

Effect of Convection-Dependent Adsorption of Additives on Microvia Filling in an Acidic Copper Plating Solution

Microvia filling of a printed circuit board by acidic copper electroplating was performed to evaluate whether the filling performance of an acidic copper plating solution could be quantified and monitored. The acidic copper plating solution was composed of polyethylene glycol, chloride ions, bis(3-sulfopropyl) disulfide and Alcian Blue. The electrochemical methodology for quantifying the filling performance of the copper plating solution is based on the adsorption and mass transfer of chloride ions. The potential difference that was obtained from galvanostatic measurements at two different rotating speeds of a copper working electrode was confirmed to be an effective indicator of filling performance. A larger potential difference reflected better filling performance. The individual contribution of each additive in the potential difference was explored in detail. The leveler and chloride ion concentrations dominated the potential difference, i.e., the filling performance. When the potential difference was larger than a critical value, i.e., 11 mV, the filling performance was higher than 80%.

Filling mechanism in microvia metallization by copper electroplating

This work explores the mechanism of microvia filling by copper electroplating using a printed circuit board (PCB) with a specific pattern design. The microvias employed in this work had no sidewall copper layer. The outer and inner copper layers of these microvias that had no sidewall copper layer were together connected to the cathode during electroplating in order to clarify the mechanism of bottom-up filling. A plating formula that was composed of CuSO 4, H 2SO 4, polyethylene glycol (PEG), bis(3-sulfopropyl) disulfide (SPS), Cl − and Janus Green B (JGB) was employed as a model formula for studying the filling mechanism. The results showed that bottom-up filling stemmed from two crucial factors. One was the sidewall growth of the microvia, increasing the surface coverage of an accelerator; the other was the convection-dependent adsorption (CDA) of additives, leading to different copper deposition rates on the outer and inner copper layers. When a leveler was present in the plating solution, CDA behavior dominated the filling mechanism, regardless of whether a sidewall copper layer was present. On the other hand, the mechanism of coverage accumulation of the accelerator was dominant only when the microvia possessed a sidewall copper layer and no leveler was present in the plating solution.

Evaluating the Filling Performance of a Copper Plating Formula Using a Simple Galvanostat Method

This work presents a feasible galvanostat method using a copper working electrode that was individually operated at two different rotation speeds to accurately predict the filling performance of a copper plating formula for microvia filling. This approach is based on the convection-dependent adsorption (CDA) of additives. Of six copper-plating formulas, formulated to examine the applicability of this method, four were effective for bottom-up filling and two were ineffective. In contrast with the microvia cross sections, it was demonstrated that this approach is effective to evaluate the workability of a plating formula in microvia filling. A relationship between the filling performance and the leveler concentration of a plating formula that was composed of polyethylene glycol, bis(3-sulfopropyl) disulfide, and Janus Green B was also examined by using the galvanostatic measurement. The final results showed that the variation trend of the filling performance with the leveler concentration coincided with that of the potential difference obtained from the galvanostatic measurement.

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-1-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, , , 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.