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Jan 31, 2017 - A series of DPP derivatives bearing quaternary ammonium salt centers with different lengths of carbon chains have been designed and ...
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Synthesis of Quaternary Ammonium Salts Based on Diketopyrrolopyrroles Skeletons and Their Applications in Copper Electroplating Biao Chen,† Jie Xu,† Limin Wang,* Longfeng Song, and Shengying Wu* Shanghai Key Laboratory for Functional Materials Chemistry, East China University of Science and Technology, Meilong Road, Shanghai, 200237, China S Supporting Information *

ABSTRACT: A series of DPP derivatives bearing quaternary ammonium salt centers with different lengths of carbon chains have been designed and synthesized. Their inhibition actions on copper electroplating were first investigated. A total of four diketopyrrolopyrrole (DPP) derivatives showed different inhibition capabilities on copper electroplating. To investigate interactions between metal surface and additives, we used quantum chemical calculations. Static and dynamic surface tension of four DPP derivatives had been measured, and the results showed DPP-10C (1c) with a faster-decreasing rate of dynamic surface tension among the four derivatives, which indicated higher adsorption rate of additive on the cathode surface and gives rise to stronger inhibiting effect of copper electrodeposition. Then, DPP-10C (1c) as the representative additive, was selected for the systematic study of the leveling influence during microvia filling through comprehensive electroplating tests. In addition, fieldemission scanning electron microscope images and X-ray diffraction results showed the surface morphology, which indicated that addition of DPP derivative (1c) could lead a fine copper deposit and cause the preferential orientations of copper deposits to change from [220] to [111], which happened in particular at higher concentrations. KEYWORDS: diketopyrrolopyrrole, quaternary ammonium salts, leveler, electroplating, microvia

1. INTRODUCTION

presence of chlorine ions. Also, the brighteners can enhance Cu deposition at the bottom of the microvia.6 For the levelers, they mainly adsorb at the opening of microvia and inhibit copper deposition there.7 The synergic effects of these additives give rise to a superfilling in the microvia.8 Former studies showed that leveler played a determined role in the brightener− inhibitor−leveler system.9−13 Adding leveler can significantly improve the filling yield. It will also extend the operative range of the brightener concentration.14 As is well-known, the commonly used levelers in copper electrodeposition are nitrogen-containing or quaternary ammonium compounds including high-molecular-weight molecules (e.g., JGB),15 quaternary ammonium surfactants,16 and

Copper electrodeposition from acidic electrolytes plays a key role in printed circuit board (PCB) manufacture and integrated circuit (IC) application.1,2 Copper electroplating resulting in filling is an extensively adopted method for the interconnection metallization of IC chips3 and PCBs.4 Copper electrodeposition progress mode in the microvia filling has to be bottom-up to ensure a void-free filling. This means the deposition rate of copper will be highest at the bottom of the microvia during electroplating. Overplating may even occur after electroplating.4,5 The uneven local current density contribution, however, produces void microvia filling. To meet the requirement of void-free filling, brighteners (i.e., bis[3sulfopropyl] disulfide, SPS), inhibitors (i.e., poly[ethylene glycol], PEG), and levelers (i.e., Janus Green B, JGB) must be added into the copper-plating bath.3,4 Inhibitors can commonly inhibit Cu deposition on the board surface in the © XXXX American Chemical Society

Received: December 1, 2016 Accepted: January 31, 2017 Published: January 31, 2017 A

DOI: 10.1021/acsami.6b15400 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Synthetic Route of 1

polymers.17 It is a very time-consuming work to develop additives which can be used for microvia filling. Till now, only few additives used for microvia filling were reported.18 Diketopyrrolopyrroles (DPPs) were discovered recently and are organic pigments.19 DPPs have a planar structure and can cause strong intermolecular H-bonding and π−π stacking. Due to their some unique properties, such as relatively simple synthesis, intense color, and excellent stability, DPPs have been adopted at a rapid pace for numerous industrial applications. They have attracted considerable research interest in the past few years.20 Besides these well-studied properties and functions, it also showed several other interesting properties in the previous reports.21 For example, (i) the reactive centers, such as the aryl rings in DPP molecules, can undergo electrophilic reactions and will let the DPP derivative adsorb easily on the surface of the cathode; (ii) the planar structure that the DPP molecule possess would be helpful for the DPP derivatives adsorbed on the cathode surface to block the Cu2+ ion in the solution reaching the surface of cathode and inhibit the copper electrodeposition; (iii) secondary amine (NH) groups could undergo structural modification. Therefore, DPP could be a potential leveler for copper electroplating. To the best of our knowledge, there has been no attention paid thus far to using DPP derivatives as a leveler for copper electroplating. Therefore, we herein designed and synthesized a new family of DPP derivatives (compounds 1a−1d) with different carbonchain lengths (Scheme 1) as levelers in copper electroplating. In this work, quaternary ammonium groups was introduced to increase the solubility of the DPP derivatives and promote the adsorption of DPP derivatives on the cathode surface, respectively.10,22 The electrochemical behaviors of these DPP derivatives were also characterized by a series of electrochemical tests. To differ from the inhibiting ability of each derivative on copper electrodeposition, quantum chemical calculation and surface tension measurement have been carried out. The electronic properties and molecule orbital information on the DPP derivatives were determined to further assess the interaction between additives and metal surfaces. In these measurements, DPP derivative 1c had the best performance as a leveler in copper electroplating. Thus, 1c was chosen for microvia filling, and its influence on the surface morphology and crystalline structure was also investigated by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD), respectively.

referenced to Me4Si (0 ppm) to assess the structure of the compounds. A Micromass GCTTM mass spectrometer was used to record the mass (see the Supporting Information) spectra. 2.1.1. Electrochemical Analyses. A three-electrode cell was used for all electrochemical tests. A PGSTAT302N (Auto-Lab) was used. We used a 3 mm diameter platinum rotating disk electrode (Pt-RDE) as a base for the working electrode (WE). A Pt stick with a diameter of 2 mm was used as counter electrode (CE), and a silver/silver chloride electrode (Ag/AgCl) was served as the reference electrode (RE). The composition of the base electrolyte was 0.88 M CuSO4·5H2O and 0.54 M H2SO4 for all electrochemical tests. Polyethylene glycol (PEG, MW = 10 000, SIGMA), NaCl (Fisher, Certified ACS), and bis(3sulfopropyl) disulfide (SPS; Raschig GmbH, Germany) were additives. The four DPP derivatives used as levelers studied in this paper were 1a, 1b, 1c, and 1d, respectively. The molecular structures of these DPP derivatives were shown in Scheme 1. A constant scan rate of 50 mV/s was used for cyclic voltammetry experiments. The domain of study ranged from 1.57 to 0.20 V. A thin copper layer with a thickness of 500 nm was predeposited onto the PtRDE in a predeposition bath before each potentiodynamic polarization test and galvanostatic measurements (GMs). The predeposition bath only contained 0.88 M CuSO4·5H2O and 0.54 M H2SO4 to prepare a Cu-RDE. A constant scan rate of 20 mV/s was used in the potentiodynamic polarization test, which scanned from 0 to 0.8 V. The GMs were carried out using the Cu-RDE at a current density of 1.5 A/dm2. 2.1.2. Electroplating Experimental. Test samples for the filling plating were 10 × 8 cm2 PCB fragments with microvias. The depth and diameter of the microvia were 80 and 120 μm, respectively. The phosphorus-containing ball was used as the anode. It was placed directly in the electroplating bath with a 5000 mL working volume. The PCB fragments were plated with a current density of 1.5 A/ dm2 at 25 °C for 70 min. Constant agitation was produced by continuous air bubble flow with a flow rate of 5.0 L h−1 during electroplating. The base electrolyte formulated for electroplating was the same as used for the electrochemical analysis. Cross-section imaging obtained from an optical microscope (OM, 9 Lecia DM LM) was used to evaluate the filling performance of a plating formula. 2.1.3. Quantum Chemical Calculations. B3LYP/6-31+G* methods were used to fully optimize the geometries of all the complexes under study.23,24 Frequency calculations were carried out at the same theoretical levels to make sure that all the structures were genuine minima on the potential energy surface. We used the molecular orbital analyses of the complexes. The contour surfaces of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) were represented. The Gaussian 09 suite of programs25 was used for all of the calculations. The Multiwfn 3.3 program26 was used to calculate the regional Fukui function, which is a well-known local descriptor for electron gain and donation. The VMD package27 was used for visualization. 2.1.4. Physical Properties of Copper Deposits. FE-SEM (Zeiss Supra 40VP) was used to examine the surface morphologies of the copper deposits formed from different plating solutions. The structure orientation of the copper deposits was determined with XRD (Bruker D8) using Cu Kα1 radiation (λ = 1.5405 Å). The diffractograms were

2. EXPERIMENTAL DETAILS 2.1. Materials and Methods. An X-4 apparatus was used to determine melting points. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded with a Bruker Avance 400 spectrometer using CDCl3 or CD3OD as a solvent. Chemical shifts were internally B

DOI: 10.1021/acsami.6b15400 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces obtained in the in 2θ range from 40° to 85° with a scanning speed of 6°/min and a step size of 0.02°. 2.1.5. Measurements of Static and Dynamic Surface Tension. A JK99c automatic tensiometer was used to measure the static surface tension. Platinum plates were cleaned with deionized water and flaming. Glassware was rinsed with tap water and deionized water. We measured the dynamic surface tension with the maximum bubble pressure method (SITA 60 from Sita Messtchnik GmbH, Germany).28 The temperature of the measuring cell was kept constant at 25 °C with a volume of 30 mL. The diameter of capillary used was 3 mm, and it was cleaned with deionized water before each test. 2.2. Synthesis procedure. Compound 1 was synthesized from the commercial available Pigment Red 254 via two steps by the procedure depicted in Scheme 1. 2.2.1. General Procedure for the Synthesis of Compound 3a−3d. A mixture of Pigment Red 254 (3.56 g, 10 mmol), potassium tertbutoxide (3.36 g, 30 mmol), tetrabutyl-ammonium bromide (0.322 g, 1 mmol), and purified CH3CN (100 mL) was stirred for 10 min at room temperature under N2 atmosphere. Next, dibromo-alkane 2 (50 mmol) was added to the mixture by heating gradually to 80 °C for 6 h. The unreacted starting material was filtered off when the reaction stopped, and then it was washed with dichloromethane. The solvent was removed under reduced pressure. Column chromatography was used for purification on a silica gel with petroleum ether and dichloromethane (10:1−2:1, v/v) as eluent to achieve an orange or red solid 3 with 15−51% yield. 2.2.2. General Procedure for Synthesis of Compound 1a−1d. Trimethylamine hydrochloride (0.382 g, 4 mmol), 3 (1 mmol), sodium bicarbonate (0.42 g, 5 mmol), and purified CH3CN (30 mL) were added into a Schlenk tube and heated to 80 °C for 12 h under N2 atmosphere. After filtration and washing with methanol, the solvent was removed under lowered pressure, and the residual crude solid product was purified with neutral alumina column chromatography with dichloromethane and methanol (100:1−20:1, v/v) as an eluent to produce a red or orange solid 1 with 56−80% yield.

inhibiting effect on copper electrodeposition. The result implied that the differences in the inhibiting capabilities of these derivatives were probably due to the change of the length of alkyl chains or molecular weight.29−31 3.2. Interaction between DPP Derivatives and the Copper Surface. The interaction between DPP derivatives and the copper surface was assessed with a series of electrode derivatization experiments. The Cu-RDE electrode was derivatized by immersion in a 0.54 M H2SO4 solution with and without 2 μmol/L various DPP derivative for 120 s at an applied potential of −0.150 V versus Ag/AgCl electrode. Meanwhile, the Cu-RDE electrode without derivatization treatment was used as blank. Then the derivatized Cu-RDE was transferred to base electrolyte and GM experiments were performed at a constant current density of 1.5 A/dm2. Similar electrode derivatization experiments have been used to explore the impact of additive on the copper deposition reaction by other researchers.32,33 The effect of the various DPP derivative pretreatments on the Cu electrodeposition in the base electrolyte was shown in Figure 2.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Evaluation. Initially, the effect of without or with DPP derivatives 1a−1d (2 μmol/L) on the cyclic voltammograms responses, respectively, has been investigated (Figure 1). Clearly, the copper deposition rate was inhibited with DPP derivative added into the base electrolyte. Figure 1 also displayed that each DPP derivative exerted a different inhibiting effect toward copper electrodeposition. The DPP derivative 1c, having a relatively longer alkyl chain (C10), exerted a greatest inhibiting effect, and 1a, containing a shortest alkyl chain (C4), had relatively weakest

Figure 2. Influence of pretreatment on the potential behavior during galvanostatic deposition (1.5 A/dm2) in electrolyte.

Figure 2 disclosed that the potential behavior observed for electrodeposition with and without DPP derivative pretreatment was significantly different. The observed start potential difference Δφ, defined by Δφ = E(start potential of blank) − E(start potential with additive), was evaluated form the GM. Normally, the potential difference observed from GM can be used as a direct measurement for the suppressing capability of the additive on the electrode surface.34 For the Cu-RDE pretreated with DPP derivative, the potential of the copper deposition started at a “low” value (high overpotential) and then increased gradually to its steady-state value when the GMs were carried out. In contrast, this type of behavior was not observed in Figure 2 curve a (no pretreatment) and curve b (pretreatment with DPP derivative-free H2SO4 solution). It indicated that the copper electrodeposition was inhibited when the GMs were carried out by the Cu-RDE pretreated with DPP derivatives. Previous studies have demonstrated that the inhibition effect of those levelers on copper electrodeposition was caused by absorption of those leveler molecule on the cathode surface.9,22,31 Hence, it could be concluded that the inhibiting effect observed in Figure 2 was due to the formation of DPP derivatives adsorption layer on the cathode surface. The

Figure 1. Cyclic voltammograms in base electrolyte containing no additives and 2 μmol/L DPP derivatives 1a−1d. C

DOI: 10.1021/acsami.6b15400 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces adsorbed DPP derivatives could form a barrier to copper deposition and inhibited the copper electrodeposition during the electroplating. The gradual increase in potential observed from Figure 2 curve c to f corresponding to a decrease in the overpotential or polarization may arise from either incorporation of the adsorption layer into the growing Cu deposit or desorption into the electrolyte.33,35 The value of Δφ achieved and the time reached the steadystate value varied with the different DPP derivatives used to pretreat the Cu-RDE. The difference of inhibiting capabilities of these DPP derivatives on copper electrodeposition could be reflected by these two values. The maximum value of Δφ and longest rebound time were achieved for the Cu-RDE treated by 1c (Δφ = 69.8 mV, shown in Table 1). It suggested that DPP Table 1. Orbital Energies (EHOMO, ELUMO, and ΔE) of Different DPP Derivative compound

EHOMO/ eV

ELUMO/ eV

ΔE = (ELOMO − EHOMO)/ eV

Δφ/ mV

1a (C4) 1b (C6) 1c (C10) 1d (C12)

−8.907 −8.139 −7.268 −6.986

−6.018 −5.268 −4.457 −4.215

2.889 2.871 2.811 2.771

37.6 44.3 69.8 55.4

Figure 3. Localization of the HOMO and LUMO of DPP derivatives. (a) DPP derivative 1a; (b) DPP derivative 1b; (c) DPP derivative 1c; and (d) DPP derivative 1d.

derivative (1c) could adsorb to the electrode surface more strongly and exert a relatively strongest inhibiting capability among these DPP derivatives (1a−1d), and the smallest Δφ value occurred for DPP derivate 1a (Δφ = 37.6 mV) with a shortest alkyl chain (C4). This illustrated that the absorb ability or inhibiting capability of these DPP derivatives was affected by the length of the alkyl chain. The result was agreed with the findings in Figure 1. 3.3. Quantum Chemical Calculations. To further understand the different inhibiting effect of these four DPP derivatives exerted during Cu electrodeposition process, the quantum chemical calculations were employed to obtain some orbital information and electronic properties. The localization of HOMO and LUMO of these four DPP derivatives were shown in Figure 3. The formation of a transition state is produced by an interaction between frontier orbitals (HOMO and LUMO) of reacting species according to the frontier molecular orbital theory.36 Based on the previous studies,37,38 the higher energy of the HOMO (EHOMO) and lower energy of the LUMO (ELUMO) are often linked with a stronger electron donating ability and electron accepting ability, respectively. The adsorption ability of the organics to the metal surface should improve with an increase in EHOMO and a decrease in ELUMO. Therefore, an energy gap, ΔE (ΔE = ELUMO − EHOMO), has been used as an indicator to portray the adsorption ability of the organics to the metal surface. It has also been used as a direct measurement of the inhibiting capability of the additive on Cu electrodeposition.39 The adsorption of DPP derivatives on the copper surface inhibited copper deposition, which resulted in a decrease of the start potential of the copper deposition (Figure 2). Therefore, the adsorption ability of DPP derivatives could be correlated with the absolute value (Δφ) of the start potential positive shifted. The values of EHOMO, ELUMO, and the ΔE of these four DPP derivatives calculated by the orbital energy and Δφ value were listed in Table 1. The ΔE values decreased with increasing of the length of hydrophobic alkyl chain that the DPP derivatives bearing

(Table 1), while the Δφ values of 1a−1d were increased and then decreased, and 1c displayed the highest value. The DPP derivative (1a) possesses the maximum ΔE (2.889 eV) among these four DPP derivatives, which illustrated that the most unstable adsorption of 1a (Δφ = 37.6 mV) on the cathodic surface and the weakest inhibition on copper electrodeposition.40 In comparison to 1a, the DPP derivative 1b (Δφ = 44.3 mV) has a minor ΔE (2.871 eV) and showed a relatively stronger inhibition on copper electrodeposition. The theoretical calculation result was consistent with the cyclic voltammetry and GM results. The DPP derivative 1d possessed the minimum ΔE (2.771 eV); it would exhibit the strongest adsorption ability and inhibition capability on copper electrodeposition among these four DPP derivatives according to the frontier molecular orbital theory. However, the cyclic voltammetry and GM results showed that 1c, which has a relatively larger ΔE (2.811 eV) compared to 1d (ΔE = 2.771 eV), possessed the strongest adsorption ability and inhibition capability on copper electrodeposition. The reason may be resulted by the atmosphere variation between the calculation and the realistic tested system.41,42 The theoretical calculations or simulations were carried out in a vacuum, whereas the electrochemical measurements were performed in acidic aqueous solution. Furthermore, it also could be found from Table 1 that the small variation of ΔE will cause a relative larger difference of inhibiting ability (Δφ value) among these four DPP derivatives. A similar phenomenon has also been reported by other researchers.39 It was also shown (Figure 3) the HOMO and LUMO of these DPP derivatives were mainly distributed on the aromatic ring region of DPP derivative molecule. These results indicated that the aromatic ring region of DPP derivatives was the likely reactive site for the adsorption of DPP derivatives on the copper surface. D

DOI: 10.1021/acsami.6b15400 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces The Fukui function of the DPP derivatives was calculated to continue to localize the nucleophilic and electrophilic sites (susceptible regions) in the molecular structure of DPP derivatives. The Fukui function f − and f + were described the ability for electrophilic and nucleophilic attack, respectively. It has been demonstrated that the atom with the highest value of f − and f + in the molecular structure was the most susceptible site for the electrophilic and nucleophilic attacks, respectively.43 The f − and f + of N, O, and C atoms located in the DPP parent structure (numbers of atoms are shown in Figure S1) were calculated by Fukui function, and the results are listed in Table S1. According to calculation results, the O atoms in aromatic ring region of DPP derivatives should be susceptible sites for the electrophilic attacks because they presented the highest f − value (Table S1). On the other hand, Table S1 shows that the atoms 7C and 8C in DPP derivatives were the most susceptible sites for nucleophilic attacks due to these sites presenting higher values of f +. The adsorption of organic materials on metal surfaces is largely led by the electrophilic attack. We therefore concluded that “O” moiety of DPP derivatives was the most-active reaction site. This was different from conventional nitrogen active sites in quaternary ammoniums. The adsorption of DPP derivatives on the copper surface may happen directly by the sharing of electrons between the “O” moiety and copper atoms. 3.4. Static and Dynamic Surface Tension Measurement. The static surface tensions of 2 μmol/L DPP derivatives (1a−1d) aqueous solutions were measured. The results were listed in Table 2.

Figure 4. Dynamic surface tension measurement at 25 °C with and without 2 μmol/L DPP derivatives by a maximum bubble pressure tensiometer: (a) pure water; (b) 1a; (c) 1b; (d) 1c; and (e) 1d.

have to pass through hydrophilic zones of adsorbed molecules.50 With longer hydrophobic alkyl chains of nonadsorbed molecules, higher energies are needed to pass the hydrophilic zones.50 Therefore, the larger molecular size and steric hindrance, which are caused by the longer chains, make movement more difficult and hardly migrate to the liquid−solid interface in a very short time.51,52 It will hinder the dynamic surface tension further decrease with increasing length of alkyl chain. These factors resulted in that 1c exhibited the lowest dynamic surface tension and maximum rate of surface tension decreasing with time among these four DPP derivatives, even if the alkyl chain length of 1c is less than 1d. The rate of surface tension decreased with time, which was caused by 1c (C10), and it was significantly faster than the other three DPP derivatives. Surface tension decreasing with time was suggested to be due to the dynamic adsorption behavior of the surface-active molecules at the interface.53 Most important was that 1c could reduce the surface tension considerably right at the beginning. It implied that the adsorption of 1c could occur in very short time (0.03s). The ability to lower interfacial tension under dynamic conditions is of great importance for electroplating processes.54 At the zero time of a new surface generation, at the instant a coating is deposited on a substrate, the concentration of surface active molecules at the interface in the same as in the bulk solution. Surfactant molecules will then adsorb at the newly created liquid−solid interface and diffuse to it. Only when the additive molecules in the bulk phase adsorb onto the liquid− solid interface can the additive exert an inhibition effect on copper electrodeposition.16 The faster the rate of dynamic surface tension decreasing with time, the faster the adsorption rate of surface-active molecules at the interface. Hence, the higher the adsorption rate of additives will give rise to stronger inhibiting effect on copper electrodeposition. Combination with the electrochemical measurement results, it can conclude that the adsorption rate of additive molecules on the interface will play a determined role on additives’ inhibition ability in Cu electrodeposition. The results of dynamic surface tension also provide a further explanation on the deviation between electrochemical analysis and quantum chemical calculations. 3.5. Electroplating Tests and Potentiodynamic Polarization Curves. In electrochemical tests and electroplating

Table 2. Static Surface Tension for DPP Derivatives 1a−1d compound

surface tension (mN/m)

/ 1a 1b 1c 1d

72.5 56.2 54.1 50.1 47.1

The test results showed that the newly designed and synthesized DPP derivatives were surface active. It also was effective in reducing surface tension to varying degrees. The static surface tension method measured only the equilibrium surface tension. The dynamic surface tension, as opposed to the static surface tension, which governs many important industrial applications,44−46 could provide some information on how rapidly surfactant molecules that are present in the solution can diffuse to and adsorb at a surface.47,48 In the plating process, an equilibrium surface tension is never reached, and a new interface is continuously formed.49 Hence, the dynamic surface tension method was used to investigate the dynamic adsorption behavior of these DPP derivatives in this work. The variation of dynamic surface tension measurement results were shown in Figure 4. It could be found from Figure 4 that the variation tendency of dynamic surface tension of these four DPP derivatives was similar to the results observed in GMs and cyclic voltammetry experiments but differed with the static surface tension and ΔE. In general, similar to static surface tension, the dynamic surface tension will decrease with the increasing the length of alkyl chain.50 However, in a surfactant solution, to adsorb on the interface, the hydrophobic chain of nonadsorbed molecules E

DOI: 10.1021/acsami.6b15400 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Typical optical microscope images of cross-sections of microvias obtained from the bath: (a) basic bath, (b) bath 1, (c) bath 2, (d) bath 3, and (e) bath 4. Via diameter is 120 μm. Via depth is 80 μm.

experiments described below, DPP derivative (1c) was selected as the representative structure type for further comprehensive tests because the cyclic voltammetry experiment indicated that 1c had relatively best electrochemical performance among those four DPP derivatives (1a−1d). Filling results (Figure 5) were achieved by using different formulas of the electroplating baths (Table 3). A large void in Table 3. Formulae of the Microvia Electroplating Baths CuSO4·5H2O (M) H2SO4 (M) Cl− (ppm) SPS (ppm) PEG (ppm) 1c (ppm)

basic bath

bath 1

bath 2

bath 3

bath 4

0.88 0.54 60 0 0 0

0.88 0.54 60 1 200 0

0.88 0.54 60 1 200 1

0.88 0.54 60 1 200 3

0.88 0.54 60 1 200 5

Figure 6. Potentiodynamic polarization curves of different formulas measured at 1000 rpm.

the filled microvia was observed in Figure 5a when the basic electroplating bath was used. A void appeared in the microvia could be attributed to uneven local current density distribution when no additives were present in bath solution.55,56 Though this phenomenon was overcome with addition of 1 ppm of SPS and 200 ppm PEG in the electroplating bath (Figure 5 b), it did not lead to bottom-up filling (BUF) but to conformal deposition. The BUF occurred only when the 1c was added into the bath solution (Figure 5c−e). Even a small amount of 1c could effectively expand the filling performance of the plating formula. In other words, the addition of 1c could play a positive role in improving the filling performance of the microvia. The filling performance varied with the increasing 1c concentration and the best filling performance was obtained when the 1c concentration was 5 ppm. To further analyze the function of 1c in the additive system, potentiodynamic polarization curves of different formulas measured at 1000 rpm were given in Figure 6. It could be seen that reductive current appeared at about −0.02 V versus Ag/AgCl electrode using the basic bath. In contrast, reductive currents appeared at about −0.20 V versus Ag/AgCl electrode when the electroplating baths contained SPS, PEG, and Cl− additives. The cathodic polarization of the electroplating bath containing additives was superior to that of the basic electroplating bath. This inhibition mainly comes from PEG-Cl−.10

With addition of DPP derivative, the reductive currents shift to more negative value (−0.22 V). This showed that the addition of a DPP derivative could increase the cathodic polarization even when SPS and PEG were added to the bath. The DPP derivative could inhibit copper electrodeposition, which was indicated by the increased cathodic polarization. According to the results as described in section 2, this inhibition should be attributed to the adsorption of DPP derivatives on the copper surface. Additionally, the cathodic polarization increased with the increasing of DPP derivatives concentration used in this work. It can be considered that larger concentration of DPP derivative gives rise to a more adsorption on the copper surface. These characteristics indicated that these new synthetic DPP derivatives may be used as a leveler for the microvia filling electroplating. 3.6. Galvanostatic Measurements at Different Rotation Speeds. To further understand the influence of DPP derivative concentration on the filling performance, the GM experiments were carried out at 1.5A/dm2, going with the addition of additives one by one at 100 and 1000 rpm (Figure 7). The low rotation speeds correlated with the bottom of the microvia, while the high rotation speeds corresponds to the outside of the microvia.57 Next, 1000 and 100 rpm were used to simulate the convection of the electroplating bath at the F

DOI: 10.1021/acsami.6b15400 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. GMs of plating solutions at the current density of 1.5 A/dm2. Rotation speeds of Cu-RDE are 100 and 1000 rpm, respectively. The basic bath is composed of 0.88 M CuSO4·5H2O, 0.54 M H2SO4, and 60 ppm of Cl−. PEG, SPS, and 1c were injected into the basic bath at designated time. (a) 5 ppm 1c was injected into bath at 3000 s. (b) The injective sequence of additives was the same as those in panel a except for the concentration of 1c injected into bath at 3000 s; the concentration of 1c was 1, 3, and 5 ppm, respectively.

Figure 8. FE-SEM images of copper deposits obtained in electrolyte: (a) base electrolyte and (b) 1, (c) 3, (d) 5, and (e) 10 μmol/L 1c.

previous literatures, using Δη as an indicator to assess the filling performance of microvia filling is condition in that a leveler must be present in the plating solution.59 A poor filling performance may be obtained without a leveler present in the plating solution, even though a positive Δη value correspondingly results. The result (Figure 5b) of this study agreed with suggestion highlighted in the literature references above. The depolarization activity proceeding with the plating time was immediately inhibited when the DPP derivatives was added at 3000 s, as recorded in Figure 7a, and the steady-state cathodic potentials were immediately shifted from the range −0.180 to −0.190 V to the range −0.245 to about −0.270 V with the addition of 5 ppm 1c. Meanwhile, the larger positive Δη value was obtained. This was attributed to the adsorption of 1c on the cathode surface. The positive value of Δη showed that the rate of the copper deposition was inversely proportional to the strength of the forced convection.58,59 The Δη value obtained from the bath solution containing 1c was larger than that got from 1c-free solution. It meant that the inhibition at 1,000 rpm increased again, compared to 100 rpm when 1c was added into the bath. In other words, the stronger convection produced stronger adsorption of 1c on the cathodic surface and stronger inhibition of copper electrodeposition. The convection at the opening of the microvia was stronger

opening and the bottom of the microvia, respectively. The potential difference at different rotating speed, Δη, defined by Δη = η1 (100 rpm) − η2 (1000 rpm) can be used to characterize the filling performance.58 If the value was positive, a stronger convection resulted in less copper deposition.58 Therefore, the plating solution may be effective for BUF of microvia. However, it was ineffective for BUF when the value of Δη was zero or negative.58 It could be seen from Figure 7a that the Δη value equaled zero when no organic additive presented in electrolyte. The cathodic potential reduced sharply when 200 ppm PEG was added at 1000 s. There was no potential difference until SPS was injected into the plating solution, which showed the copper deposition was not diffusion controlled in the presence of PEGCl−. When 200 ppm PEG was present in the bath, the addition of 1 ppm of SPS decreased the polarization and the Δη value become positive (Δη = +5 mV). However, a significant continuous depolarization was seen before the DPP derivative was added to the plating solution, which meant the plating solution was brightener dominated under this circumstance. The synergy between inhibitor and brightener was destroyed when the SPS concentration was 1 ppm. Hence, the conformal deposition observed in Figure 5b occurred. A similar result also was reported by W. P. Dow et al.58 in 2006. According to the G

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orientation peaks was obviously decreased due to the addition of 1c. 3.8. Inhibition Ability Comparison between 1c and Janus Green B. Previous studies8,9 have indicated that commercially available Janus Green B is an effective leveler for copper electroplating, and we have identified that DPP derivatives could inhibit the copper deposition. Herein, some unique properties of 1c were discussed in comparison to JGB. Potentiodynamic polarization test and GMs experiments performed with base electrolyte were used to determine the effect of JGB and 1c on copper deposition, respectively. The influences of 1c and JGB on the potentiodynamic polarization behavior during Cu electrodeposition from base electrolyte are shown in Figure 10. It was clear that the electroreduction potential of Cu2+ ions underwent a clear negative shift because of the addition of 1c along with the reduction of the cathodic current density even at 1c concentration as low as 1 μmol/L. Also, the cathodic polarization of the electrolyte containing 1c was higher than that of the base electrolyte, and the polarization increased with increasing concentration of 1c. The reductive currents shift to more negative value (−0.125 V versus Ag/AgCl electrode) when the concentration of 1c in electrolyte was 5 μmol/L. It can be considered that a large concentration of 1c gives rise to a denser adsorption layer on the copper surface. In contrast, no obviously cathodic polarization was observed with addition of JGB. The reduction current density is little different from that observed in base electrolyte (Figure 10b) even the concentration of JGB increased to 5 μmol/L. A similar result has also been reported by W. P. Dow in his research.63 In a copper-plating bath, the inhibition ability of JGB will be enhanced by form a composite suppressor by synergistic interaction with PEG and then inhibit the copper deposition rate at a high-density area.58,63 Furthermore, the inhibition effect of JGB on the coper deposition also dependent on the chloride ion concentration in electrolyte.63 The result of Figure 10a showed that the inhibition ability of 1c was stronger than JGB in base electrolyte; it also showed that 1c could exert a significant inhibition effect on copper deposition, even with no PEG and Cl− presented in the electrolyte. The properties that 1c exhibited may be attributed to the surface activity that 1c possessed. The existence of amphiphilic groups in the molecule structure can enrich at the interface easily and form a hemimicelle barrier.47,64,65 This barrier can block the active sites on the metallic surface and consequently promote inhibition.65 GMs measured at a current density of 1.5 A/dm2 with the addition of JGB or 1c at 100 and 1000 rpm were used to analyze the electrochemical activities of JGB and 1c at different rotation rates. It could be seen from Figure 11a that the cathodic potentials moved to a more negative value when 1c (3 μmol/L) was added at 1000 s. Furthermore, similar to that observed in Figure 7a, the positive Δη value was obtained immediately with the addition of 1c (3 μmol/L). This indicated that the deposition rate changed to CDA-controlled from Cu (II) diffusion-controlled quickly when the 1c was added in the electrolyte because the adsorption layer formed on the cathode surface promptly. The cathodic potentials dropped when 3 μmol/L JGB was injected into the base electrolyte at 1000 s. However, different from Figure 11a, the cathodic potential increased gradually and depolarization behavior appeared (Figure 11b) with the electrodeposition proceeding, which was caused by the

than at the bottom of the microvia. Therefore, the adsorption of 1c at the opening was stronger than at the bottom of the microvia. This led to slow copper deposition at the opening of the microvia and a relatively rapid copper deposition at the bottom of the microvia. Hence, the microvia filling performance was improved (Figure 5e). At same time, the Δη value increased constantly with the increases in the concentration of the injected DPP additives, as can be seen from Figure 7b. This could be considered that higher concentration of 1c gives rise to a denser adsorption layer on the copper surface. The larger the Δη value, the better filling performance (Figure 5c−e), which might be explained by the convection-dependent adsorption (CDA) of additives.58−60 3.7. Effects of DPP Derivative on Surface Morphology and Crystalline Orientation of Cu Deposition. The influence of DPP derivative (1c) on the surface morphology of copper deposit was investigated using FE-SEM, and the results were shown in Figure 8. The surface micrograph of copper deposits that came from the DPP free base electrolyte was rough and granular and consisted of relative large and coarse grains (Figure 8a). With various concentration of 1c (1, 3, 5, and 10 μmol/L) was added to base electrolyte, an improvement in surface morphology can be observed (Figure 8b−e). The grain size progressively decreases and the copper deposit become uniformity and flatness with increasing the concentrations of 1c. Similar results for copper electrodeposition were reported for a polymeric leveler61 and a branched quaternary ammonium surfactant as leveler.62 These results indicate that the DPP derivative have a inhibiting effect for copper electrodeposition, and the presence of the 1c in the electrolytes may produce regular and relative leveling morphologies of copper deposits. To characterize the influence of 1c on the crystalline orientation of the copper deposits, XRD of copper deposits were performed (Figure 9). The intensity of [220] orientation

Figure 9. XRD patterns of plated Cu films obtained in base electrolyte without or with different concentration of 1c.

peak significantly decreased with the addition of 1c. On the contrary, the intensity of [200] and [111] orientation peak gradually increased with increasing concentration of 1c. It indicated that the 1c could inhibit the growth in the direction of the [220] planes and promote the [200] and [111] orientation. When the 1c concentration reaches 10 μmol/L, it is the [111] orientation rather than the [220] orientation that became dominant. In addition, the intensity of the whole H

DOI: 10.1021/acsami.6b15400 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 10. Potentiodynamic polarization curves using 1c and JGB as the leveler in base electrolyte. The electrode rotation speed was 1000 rpm.

Figure 11. GMs at the current density of 1.5 A/dm2 using 1c and JGB as leveler at different rotation speeds in base electrolyte.

decomposition of JGB.15 Also, the cathodic potential at 100 rpm was more negative than that at 1000 rpm, and the Δη value obtained was negative before 1200 s. This suggested that the inhibition of copper electrodeposition on the cathode at 100 rpm was stronger than that at 1000 rpm, which is a negative factor for obtaining a void free filling in the via.15 This phenomenon was reversed after 1200 s, and a positive Δη value was obtained, which is caused by the adsorption of JGB on the cathodic surface. The result of GMs implied that 1c could exert leveling function faster than JGB in base electrolyte.

additives according to electrochemical analysis. The electrochemical behaviors of DPP derivatives indicated that 1c could be used as an effective leveler. Moreover, surface morphology observed in FE-SEM images and XRD results indicated that the addition of 1c could not only lead a fine copper deposit but also make the preferential orientations of copper deposits changed from [220] to [111], especially at higher concentrations. In comparison to JGB (a widely reported leveler), 1c have a stronger inhibition ability and could exert leveling function faster.

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4. CONCLUSIONS In summary, four DPP derivatives (1a−1d) bearing different length alkyl chain were synthesized and their action during electrodeposition process were investigated in this work. It was found all those four DPP derivatives showed inhibitive function for Cu electrodeposition, and their inhibition ability was closely related to the length of the alkyl chain that DPP derivatives bearing. Molecule orbital and Fukui function results indicated that the “O” was the most susceptible moiety anchored on Cu surface. The results of dynamic surface tension, cyclic voltammetry, and electrode derivatization experiments indicate that the adsorption rate of additive molecules on the interface will play a determined role on additives’ inhibition ability in Cu electrodeposition. In microvia-filling experiments, it was found that DPP derivative could play a positive role in improving the filling performance. The reasons for achieving BUF in the plating solution were attributed to the CDA behavior of

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15400. Detailed Fukui functions of the selected atoms, distribution isodensity plots, and 1H NMR, 13C NMR, and HRMS results of all compounds. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Limin Wang: 0000-0002-4025-5361 I

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†

All authors contributed to the writing of the manuscript. All authors gave approval to the final version of the manuscript. B.C. and J.X. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (grant nos. 21272069 and 20672035) and the Fundamental Research Funds for the Central Universities and Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences.

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