Microvia Filling Research Articles

Fast Filling of Microvia by Pre-Settling Particles and Following Cu Electroplating

Microvia interconnectors are a critical element of 3D packaging technology, as they provide the shortest interconnection path between stacked chips. However, low efficiency of microvia filling is a long-standing problem. This study proposed a two-step method to enhance the electroplating filling efficiency by pre-setting metal particles in microvias and later electroplating the Cu to fill the gaps among the pre-settled particles. Since these particles occupy a certain volume in the microvia, less electroplating Cu is needed for microvia filling, leading to a shorter electroplating period.

A Mini Review on the Microvia Filling Technology Based on Printed Metal Nano/Microparticles

Microvia filling is a core interconnection technique in electronic manufacturing. Electroplating is the primary method used in the industry for filling microvias but also has the drawbacks of cumbersome procedures and toxic by-products. New microvia filling technology through printing of metal particle–based conductive fillers was then developed, with the advantages of simpler process, higher efficiency, better compatibility, and eco-friendliness. Here, we review the up-to-date research findings on the microvia filling technology based on printed metal nano/microparticles from the perspectives of material development and filling performance. Some key questions about this technology are also discussed. We hope the outlook presented by this review could help the further studies on this topic. The review also identifies some key remaining issues to be resolved.

Function of Sulfhydryl (–HS) Group During Microvia Filling by Copper Plating

In this work, 3-mercapto-1-propanesulfonate (MPS) with one sulfhydryl (–HS) group and 2,3-dimercapto-1-propanesulfonate (DMPS) with two –HS groups were used as model additives to reveal the function of –HS group during copper plating. Chronopotentiometry was employed to characterize the different electrochemical behaviors of the two model additives. It indicated that DMPS showed stronger accelerating effect than MPS in the plating bath containing Cl−, which could be attributed to its enhanced adsorption caused by two –HS groups. The pre-adsorption and desorption experiments demonstrated the synergistic effects and competitive adsorption between Cl− and the model additives. Moreover, the accelerating mechanism was investigated by cyclic voltammetry combined with rotating ring-disk electrode (RRDE), demonstrating MPS could accelerate the rate-determining step (RDS) of copper deposition only if Cl− was present, whereas DMPS could enhance the RDS whether Cl− existed or not. Finally, microvia filling experiments were carried out to evaluate the filling performance of the two model additives. The results indicated that in the plating bath containing PEG and Cl−, MPS showed good filling performance, while DMPS did poorly. However, after the introduction of JGB, the filling performance of DMPS was greatly improved, which was resulted from the competitive adsorption between DMPS and PEG being balanced by JGB.

Effects of Accelerator Alkyl Chain Length on the Microvia Filling Performance in Copper Superconformal Electroplating

In addition to the commercialized accelerator bis-(sodium sulfopropyl)-disulfide (SPS), we designed and synthesized two other disulfide compounds, namely, bis-(sodium sulfohexyl)-disulfide (SHS) and bis-(sodium sulfoethyl)-disulfide (SES), to investigate the effect of their alkyl chain lengths on the microvia filling performance. The galvanostatic measurements show that the addition of Cl− to different accelerator-containing solutions leads to the same synergistic effects at a current density of 20 mA/cm2, indicating that the interactions between accelerators and Cl− are independent of their alkyl chain lengths. However, when both Cl− and a suppressor are present, the alkyl chain length in accelerators strongly impacts their anti-suppressor strength, which increases with the growth of the alkyl chain length, reaches a maximum (SPS) and then decreases. A possible adsorption and interaction model was proposed to explain the difference of anti-suppressor effects, and microvia filling plating experiments were carried out to evaluate the effects on the filling performance. It was found that both SES and SPS, which have appropriate molecular lengths and relatively strong antagonistic effects against polyethylene glycol (PEG) and Cl− ensemble, achieved complete microvia filling. For SHS, due to the overlong alkyl chain and weak antagonistic effect against PEG and Cl−, no microvia was completely filled.

Communication—Localized Accelerator Pre-Adsorption to Speed Up Copper Electroplating Microvia Filling

A practical fast microvia filling method in copper electroplating is developed with the aid of bottom-localized pre-adsorption of bis(3-sulfopropy)-disulfide (SPS). The pretreatment initiates with accelerator adsorbed at all copper surface. Subsequently, the adsorbed SPS on board surface is broken down by sodium persulfate, but the function of adsorbed SPS at microvia bottom is mostly retained due to the mass transfer limitation. The pre-adsorbed SPS at microvia bottom improves the accumulation of SPS at microvia bottom in electroplating filling, resulting in the speeding up of microvia filling.

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 By Using Insoluble Anodes in an Acidic Copper Planting Bath with a Novel Accelerator

To develop copper plating formula for high-speed via filling, a more stable insoluble anode, namely dimensionally stable anode (DSA), was used at high current density. We choose a novel accelerator, 3-(N,N-dimethylthiocarbamoyl)-thio propanesulfonic acid (DPS), which has better acceleration performance on copper superfilling than Bis(3-sulfopropyl) disulfide (SPS) in the DSA plating system. This work investigates the influence of experimental conditions on filling performance and also use numerical simulation method to quantify the filling performance. According to the results, DPS can exhibit excellent acceleration on copper deposition in the DSA plating system only. NMR result shows that DPS is oxidized by the DSA to be more active products which bears -SH and -SO3 - groups, which can accelerate on copper deposition. Putting the DSA in a filter bag can block oxygen which was produced by water electrolysis at the anodes to enter the plating solution and reduce DPS oxidation on the surface of the DSA. As a result, via bottom and top deposition rate was different from that without the filter bag. From the results of microvia filling and numerical simulation method, the inhibition of complex inhibitor (PEG-Cl-) and the filling performance was positive in correlation with convection in the blind via. In a plating system with a strong forced convection and a low temperature exhibited better filling performance. On the other hand, the filling performance was Cu(II) mass-transfer controlled in the weakly forced convection system, so that the filling performance is not good.

Structural Effect of Polymers on Their Microvia Filling Performance as Suppressors During the Copper Electroplating

Structural Effect of Polymers on Their Microvia Filling Performance as Suppressors During the Copper Electroplating

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.

Microvia Filling by Copper Electroplating Using a Modified Safranine T as a Leveler

In this study, a modified Safranine T (mST) was used as a leveler in Cu electroplating for blind microvia filling. A perfect super-conformal filling effect was achieved by a combination of additives: polyethlene glycol (PEG), bis (3-sulfopropyl) disuldide (SPS) and mST. The filling time of the PEG-mST-SPS electrolyte was only half as long as that of the conventional PEG-JGB-SPS and the deposition thickness on the surface was one-tenth of that of the PEG-JGB-SPS. By electrochemical analysis, it was found that the composite suppressor of PEG-mST had a much stronger adsorption competitiveness against SPS on the surface, but a much weaker competitiveness against SPS at the microvia bottom. Based on that, it was deduced that the PEG-mST would occupy the most of the locations on the surface and the SPS would occupy the most of locations at the microvia bottom, which might account for such a perfect bottom-up filling mode, an ultra-thin surface thickness and an ultra-fast filling speed. But it brought about a risk of overfilling bump phenomenon under high mST concentration and strong convection force. This study proposes that the modified hydroxy in mST molecular structure might play a key role in this filling performance.

Tetraoxa-Diphosphaspiro Derivative as Suppressor for Microvia Filling by Copper Electroplating in Acidic Solution

In this work, a novel suppressor, tetraoxa-diphosphaspiro derivative (MPTD) was applied in copper electroplating solution for microvia filling. The suppression function of MPTD and its synergistic effect with other additives during copper electroplating were investigated by means of galvanostatic measurements, cyclic voltammetric and EIS. The surface morphology of electrodeposition experiments was examined using a scanning electron microscope (SEM). The filling performance was evaluated by observing the obtained cross-sectional views of the microvias after electroplating. The galvanostatic measurements results revealed the strong synergistic inhibitory effect on both MPTD and chloride ions. In addition, the results from cyclic voltammetry (CV) measurements indicates that the concentration of the plating additives (such as suppressor, accelerator and leveler) at fixed relative proportion required an optimal Cl− concentration in the plating bath to reach the strongest suppression strength. The negative values of ΔG0ads suggest that the synergistic inhibitory effect of Cl− onto the cathode surface is a spontaneous process. The values of the standard free energy of adsorption reveals that Cl− adsorbed on Cu surface involves both chemisorption and physisorption. This improved copper plating solution composed of CuSO4, H2SO4, MPTD, SPS, chloride ions, and JGB is optimal and capable to perform bottom-up copper filling of microvias.

Synergistic Effect of Different Additives on Microvia Filling in an Acidic Copper Plating Solution

Microvia filling by copper electroplating was carried out using a plating bath with a triblock copolymer comprised of propylene oxide (PO)-ethylene oxide (EO)-propylene oxide (PO) (named PEP) and bis– (3–sulfopropyl) disulfide (SPS) as the suppressor and the accelerator respectively, the suppression function of the triblock copolymers suppressors and their synergistic effect with other additives during copper electroplating were studied by galvanostatic measurements (GMs) and the cyclic voltammetry (CV), the results of GMs revealed that the suppression of PEP on copper electrode position strongly depended on the chloride ion (Cl−). In addition, the CV measurements indicated that a given Cl− in the plating bath required an optimal suppressor concentration to realize the strongest suppression strength. The negative values of the standard free energy of adsorption (ΔG0ads) suggest that the adsorption of SPS onto the cathode surface was a spontaneous process and physisorption was dominant in the plating formula. An optimal copper plating solution composed of CuSO4, H2SO4, PEP, SPS, chloride ions, and Janus Green B (JGB) was developed and used to perform bottom-up copper filling of microvias.

Microvia Filling in an Acidic Copper Planting Bath with Insoluble Anodes

Microvia filling of a printed circuit board (PCB) by acidic copper electroplating has been widely used in electronic industry. The filling mode of copper electrodeposition in microvia must be bottom-up form. Moreover, bottom-up filling phenomenon is believed to be related to the properties and interaction of plating additives. Especially, the interaction of three (accelerator, suppressor, leveler) components have significantly effect on copper filling performance. It was confirmed that soluble anodes (i.e., P-doped Cu anode) have disadvantage when they were used in the acidic copper plating bath. For instance, the shape of the soluble anodes are changeable with operating time, which may causes non-uniform current distribution to form unequal copper thickness on substrate. Therefore, a special functional insoluble anode was used to replace the soluble anode and conventional dimensionally stable anode (DSA) to maintain better bath stability. The disadvantage of soluble anode can be improved by using the novel DSA. In this work, a planting formula which was composed of chloride ions, suppressor (polyethylene glycol), leveler (quaternary ammonium species) and a novel accelerator instead of bis(3-sulfopropyl) disulfide (SPS). SPS is not appropriate for DSA system because of easy breakdown; therefore, we develop a novel accelerator, ACC, which has a longer lifetime and better performance for acceleration on copper deposition than SPS. Using DSA instead of P-doped Cu anode has another important contribution, that is, SPS reacts with the copper anode to produce Cu(I)-thiolate. The Cu(I)-thiolate is a strong accelerator in the presence of chloride ion. Cu(I)-thiolate will be accumulated in the copper plating bath to destroy the synergy between the suppressor and accelerator. DSA has no this issue because DSA has no copper. Herein, we will show that ACC combining with DSA can perform excellent copper fill in microvia.

Microvia Filling with Nickel-Tungsten Alloy to Decrease the Coefficient of Thermal Expansion of Electronic Circuit Interconnections

A nickel-tungsten alloy plating formula was developed to electrochemically fill the microvias of printed circuit boards and the through-silicon vias (TSVs) of wafers. The plating solution was composed of Ni(SO3NH2)2, citric acid, sodium citrate, Na2WO4, chloride ions, and 2-mercapto-5-benzimidazolesulfonic acid. A void-free Ni-W superfilling of a microvia and a TSV were achieved. The tungsten content in the filled alloy varied from 1.5 atom% to 5.5 atom%, depending on the plating temperature. The coefficient of thermal expansion of the filled Ni-W was theoretically calculated according the tungsten content, which was lower than that of copper. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any

Microvia Filling in an Acidic Copper Planting Bath with Insoluble Anodes

Microvia filling of a printed circuit board (PCB) by acidic copper electroplating has been widely used in electronic industry. The filling mode of copper electrodeposition in microvia must be bottom-up form. Moreover, bottom-up filling phenomenon is believed to be related to the properties and interaction of plating additives. Especially, the interaction of three (accelerator, suppressor, leveler) components have significantly effect on copper filling performance. It was confirmed that soluble anodes (i.e., P-doped Cu anode) have disadvantage when they were used in the acidic copper plating bath. For instance, the shape of the soluble anodes are changeable with operating time, which may causes non-uniform current distribution to form unequal copper thickness on substrate. Therefore, a special functional insoluble anode was used to replace the soluble anode and conventional dimensionally stable anode (DSA) to maintain better bath stability. The disadvantage of soluble anode can be improved by using the novel DSA. In this work, a planting formula which was composed of chloride ions, suppressor (polyethylene glycol), leveler (quaternary ammonium species) and a novel accelerator instead of bis(3-sulfopropyl) disulfide (SPS). SPS is not appropriate for DSA system because of easy breakdown; therefore, we develop a novel accelerator, ACC, which has a longer lifetime and better performance for acceleration on copper deposition than SPS. Using DSA instead of P-doped Cu anode has another important contribution, that is, SPS reacts with the copper anode to produce Cu(I)-thiolate. The Cu(I)-thiolate is a strong accelerator in the presence of chloride ion. Cu(I)-thiolate will be accumulated in the copper plating bath to destroy the synergy between the suppressor and accelerator. DSA has no this issue because DSA has no copper. Herein, we will show that ACC combining with DSA can perform excellent copper fill in microvia.

Optimization of the Copper Plating Process Using the Taguchi Experimental Design Method: I. Microvia Filling by Copper Plating Using Dual Levelers

Copper electroplating formulas composed of CuSO4, H2SO4, chloride ions, polyethylene glycol (PEG), bis (3-sulfopropyl) disulfide (SPS), and different levelers for microvia filling of a printed circuit board (PCB) were studied. The influence of the copper electroplating parameters, accelerator and leveler concentrations and cathodic current density on the microvia filling performance was explored using the Taguchi experimental design method. An L9 orthogonal array with four controlling factors at three levels was employed in the experimental design method. Variance analyses of the mean filling performance and the signal-to-noise ratio of the controlling factors showed that the most significant factor for the microvia filling performance was the leveler concentration. The other factors exhibited little to no effect on the microvia filling performance. The contribution of levelers to the filling performance was characterized using galvanostatic measurement and linear sweep voltammetry (LSV) with a working electrode at different rotational speeds. The experimental results indicated that the inhibiting effect of the levelers on the copper deposition was related to forced convection, which is a key physicochemical interaction for exhibiting good filling performance.

An Effective N, N-Diethylthiourea as Accelerator for Microvia Filling via Copper Electrodeposition

The effect of N, N-diethylthiourea (DET) upon the microvia filling by Cu electrodeposition was investigated by cross-sectional images using optical microscopy. The bottom-up filling of the electroplating bath was achieved with an addition of DET. The electrochemical study indicated that the polarisation on the cathode was increased with an addition of DET. The results present DET as an accelerator which is beneficial for microvias filling for high density interconnections printed circuit board.

Use of 3,3-Thiobis(1-propanesulfonate) to Accelerate Microvia Filling by Copper Electroplating

The most widely used accelerators for microvia filling by copper electroplating are 3-mercapto-1-propanesulfonate (MPS) and bis(3-sulfopropyl) disulfide (SPS). In this study, a new accelerator, 3,3-thiobis(1-propanesulfonate) (TBPS), which has neither a thiol group (e.g., MPS) nor a disulfide group (e.g., SPS) but rather contains a thioether group and two sulfonic acid groups, was investigated and formulated to perform microvia filling by copper electroplating. The accelerating effect of TBPS on copper electrodeposition and its chemical interaction with chloride ions and H2SO4 were characterized by galvanostatic measurements. Electrochemical analyzes showed that the acceleration of TBPS on copper electrodeposition strongly depended on the chloride ion and H2SO4 concentrations. An optimal copper plating solution composed of CuSO4, polyethylene glycol (PEG), TBPS, chloride ions, and H2SO4 was developed and used to perform bottom-up copper filling of microvias. Although TBPS contains a thioether group instead of a thiol group, a trace amount of TBPS can strongly accelerate copper electrodeposition in the presence of proper chloride ion and H2SO4 concentrations.

Triblock Copolymers as Suppressors for Microvia Filling via Copper Electroplating

In this work, ethylene-propylene-ethylene (EPE) oxide triblock copolymers were found to be effective suppressors for microvia filling during copper electroplating. The studied EPE copolymers were divided into two groups according to their solubility and compositions. The first group was composed of EPE 1000, EPE 2000, and EPE 3500, and the second group contained EPE 2450, EPE 2900, and EPE 8000. In particular, the function of the triblock copolymer suppressors (e.g. EPE 2900) and their synergistic effect with other additives during copper electroplating were studied by galvanostatic measurements, suggesting that the suppression strength was greatly dependent on Cl−. In addition, cyclic voltammetry (CV) measurements indicated that a given suppressor in the plating bath required an optimal Cl− concentration to realize the strongest suppression strength. Then the filling performance of the plating bath using different suppressors was systematically compared by galvanostatic measurements and filling plating experiments in the plating bath with a fixed Cl− concentration (60 ppm), an optimal Cl− concentration, and a wide operation window of Cl− respectively. These experimental analyzes indicated that EPE 2000 and EPE 2900 were the best suppressors in the first and second group, respectively. The relevant mechanism of triblock copolymer suppressors for microvia filling was discussed.

Accelerator Screening by Cyclic Voltammetry for Microvia Filling by Copper Electroplating

A cyclic voltammetry (CV) screening method is proposed in this work to identify an effective accelerator for the copper superfilling of microvias of a printed circuit board (PCB). Five typical organosulfides are used as model additives to evaluate the performance of the screening method. The five organosulfides are 3-mercapto-1-propanesulfonate (MPS), bis(3-sulfopropyl) disulfide (SPS), 3-mercaptopropionic acid (MPA), 3-mercapto-1-propanol (MPO), and 3,3-thiobis(1-propanesulfonate) (TBPS), where MPS, MPA, and MPO contain a thiol head-group, SPS contains a disulfide group, and TBPS contains a thioether group. MPS, SPS, and TBPS have one or two terminal groups of sulfonates, whereas MPA has a terminal group of carboxylic acid, and MPO has a terminal group of alcohol. The CV screening method revealed that MPS, SPS, and TBPS are accelerators for copper electrodeposition and that MPA and MPO are not. A specific copper deposition peak, specifically, the α peak, that appears in the CV patterns is an indicator for accelerator screening. This study is the first to indicate that TBPS with a thioether group rather than a thiol group can accelerate copper electrodeposition and can be used for the copper superfilling of the microvias of a PCB.