Ultra-Uniform Copper Deposition in High Aspect Ratio Plated through Holes via Pulse-Reverse Plating

Round 1

Reviewer 1 Report

The manuscript ID: coatings-1723858, entitled “Ultra-uniform copper deposition in high aspect ratio plated through holes via pulse-reverse plating” was carefully reviewed, and provided comments are listed as below. The manuscript reports the microstructure of the copper deposition, through pulse electrodeposition, on the high aspect ratio substrate. There are good connections between reported data and discussion part. However, the manuscript needs further improvement and I recommend the publication of this manuscript after modifying based on the provided comments.

1. Authors need to specify the main reason for using electroplating of copper, while chemical vapor deposition (CVD) is a powerful manufacturing technique for the fabrication of highly conformal and uniform coatings?. In the introduction section, the authors need to provide a sentence or two for other manufacturing techniques for the fabrication of ultrathin highly conformal coating. Below article, which covers all different methods of CVD for fabrication of various highly conformal thin films, needs to be mentioned in the introduction section:

“L. Sun, et al., Chemical Vapor Deposition, Nature Reviews Methods Primers, 1, 5, (2021).”

2. Page 4, Line 119. “where I(hkl) and I0(hkl) are the diffraction intensities of each (hkl) lines, respectively”? Please correct this sentence by further explaining what I0(hkl) is.
3. Figure 5. There is no evidence of conformal and uniform coating after modifying the electroplating conditions. Please insert a better image if it is possible to show providing uniform coating.
4. Figure 8. Please describe and provide a detailed discussion about the reason for changing Texture Coefficient as a function of electroplating parameters.

Author Response

1. Authors need to specify the main reason for using electroplating of copper, while chemical vapor deposition (CVD) is a powerful manufacturing technique for the fabrication of highly conformal and uniform coatings?. In the introduction section, the authors need to provide a sentence or two for other manufacturing techniques for the fabrication of ultrathin highly conformal coating. Below article, which covers all different methods of CVD for fabrication of various highly conformal thin films, needs to be mentioned in the introduction section:

“L. Sun, et al., Chemical Vapor Deposition, Nature Reviews Methods Primers, 1, 5, (2021).”

Reply:Thanks very much for your positive comments on our work.Among various methods of copper thin film deposition onto the substrate, such as PVD,CVD, and Sputtering, the electrochemical methods have proven to be least expensive, highly productive, readily adoptable and the most generally employed to obtain metallic films of adequate thickness, porosity-free structure and good adhesion.( https://doi.org/10.4028/www.scientific.net/MSF.510-511.942) .The plated through hole (PTH) has become a commonly used and well documented metallization process for PCB fabrication. And Pulse-reverse (PR) electroplating method is one of the practical technologies for the high AR PTH metallization.

Revision:

With the miniaturization of modern electronics, the multi-layer, thin and high-density wiring printed circuit boards (PCBs) are the research and development direction^[1]. The high-density interconnected printed circuit boards (HDI-PCBs) offers significantly advantage in improving the device performance and reliability^[2]. Hole metallization is the core technology in HDI-PCBs to achieve the high quality interlayer connections^[3]. Copper is the most widely used metal in the hole metallization due to the superior comprehensive properties of high ductility, low cost, high electrical and thermal conductivity^[4]. Among various copper deposition methods , such as PVD, CVD^[5], and sputtering, the electrochemical methods have proven to be costless, highly productive, readily adoptable and the most generally route employed to obtain metallic films of adequate thickness, porosity-free structure and good adhesion. In the development of HDI-PCBs integration, the aspect ratio of through holes increases significantly, where the aspect ratio (ε) is defined as the depth (D) of the hole divided by its width (W). In high aspect ratio plated through holes, The diffusion of the reactants would be constricted by the very narrow and deep holes, leading to the concentration polarizations of copper ions, which would impede the copper deposition process^[5-7][6-8]. Although many strategies have been developed to address the above mentioned limitations^[8-11][9-12], it is still challenge to realize the uniformly copper superfilling of the plating through hole with high aspect ratio.

1. Page 4, Line 119. “where I(hkl) and I0(hkl) are the diffraction intensities of each (hkl) lines, respectively”? Please correct this sentence by further explaining what I0(hkl) is.

Reply:Thanks very much for your positive comments on our work. Preferred orientation was also studied by using the relative texture coefficient (RTC, TC_(hkl)), which is given by the following expression.

 

where I(hkl) and I0 (hkl) are the diffraction intensities of (hkl) lines in the XRD patterns of the deposits and randomly oriented copper powder sample (JCPDS No. 02-1225), respectively. (https://doi.org/10.1016/S1003-6326(18)64846-0)

Revision: Preferred orientation was also studied by using the relative texture coefficient (RTC, TC_(hkl)), which is given by the following expression.

 

where I_(hkl) and I⁰_(hkl) are the diffraction intensities of each (hkl) lines in the XRD patterns of the deposits and randomly oriented copper powder sample (JCPDS No. 02-1225), respectively.

1. Figure 5. There is no evidence of conformal and uniform coating after modifying the electroplating conditions. Please insert a better image if it is possible to show providing uniform coating.

Reply:Thanks very much for your positive comments on our work. The uniformly copper  coating has good filling property in the hole, could be completely filled with the electroplating time without no defects such as void.The filling mechanism are butterfly-shape of the filled copper in the through hols（DOI：10.1149/2.003305eel）and the “V” shape growth in blind vias（DOI: 10.1149/2.073405jes）. At the same time, the throwing power (TP) is defined as the ability of an electroplating solution/method to uniformly deposit metal on the cathode. （https://doi.org/10.1016/j.microrel.2019.04.004）（DOI: 10.1149/1.1560942）. The larger the TP value, the uniformity of copper are better.

We prepared the copper under different parameters, the growth mode of the copper in the hole is from the hole wall to the middle.In experiments a1, a2, and c1, the through holes are completely filled. We think the copper is uniformly grown in the plating growth process. In experiment a4, the thickness of copper is not uniform. In experiments a3 and b1, large holes appeared in the coating. We think that this defect is also caused by the uneven growth of the coating. In experiment c3, the thickness of copper in the orifice is larger than that in the hole. This growth mode will lead to excessive deposition of the orifice, and the completely filled cannot be achieved, which is also a kind of uneven growth of the plating layer. We calculated TP values of copper with different parameters, which show that the uniformity of the copper was different.

Revision:

 

Figure 5. Cross-sectional observations of the filled PTH by PPR plating.

1. Figure 8. Please describe and provide a detailed discussion about the reason for changing Texture Coefficient as a function of electroplating parameters.

Reply:The difference of the texture coefficients of copper is related to the orientation of the substrate, the kinds of additives and the parameters of pulse-reverse plating. ( https://doi.org/10.1016/j.jcrysgro.2017.09.009)For DC plating, the current does not change, and the crystal plane orientation is determined by the orientation of the substrate , the current density of the components and the plating solution.For pulse-reverse plating, there are forward growth and reverse dissolution processes, Ton and Toff also affect the growth of the coating. stresses accumulate during Ton and relax during Theories have suggested that Toff from recrystallization, thereby creating twins.The difference in grain orientation of the copper coating has a great relationship with the formation of twins during the growth process.Twin formation is one mechanism by which new crystal directions appear on the oriented substrates. The period pulse reverse deposit a barrier layer at points of high current density. This inhibits Cu deposition at these points and reduces,compared to DC plating, the required amount of organic additives.Furthermore, the PPR plating favors grain nucleation and increases the number of grains per unit area.The reverse pulse can be the dissolution of the cathode surface, which leads to possible changes in the orientation of the crystal planes of the substrate and ultimately affects the texture coefficient of the crystal planes. Therefore, different reverse pulse parameters will affect the texture coefficient of the crystal planes.

 Revision: Figures 7 6 and 8 table 3 show the XRD data and the texture coefficients of different crystal planes of copper prepared by different experimental parameters as shown in Table 1. It can be seen from Figure 6 that copper prepared with different reverse pulse parameters all have copper standard XRD diffraction peaks, and the (111) plane is the preferred orientation, which benefits the interconnection performance of copper. When the reverse duty cycle was 0.4, the TC value of (111) plane reaches maximum as 68.21%, and the TC value of (111) plane decreased with the increase of reverse duty cycle. The copper prepared with different reverse pulse frequency were all with high portion (111) textured copper. The reverse pulse frequency has little effect on the TC value of (111) plane, the same trend was observed in the copper uniformity. The TC value of (111) plane decreased when reverse pulse frequency was 1500Hz. When the reverse pulse frequency was 100 Hz, the TC value of (111) plane reaches its peak. According to the texture coefficients data, TC value of (111) plane decreased when reverse current density is either too high or too low. When the reverse current density was 0.2 A/dm², the TC value of (111) plane was the highest. The result indicates that the reverse pulse affects both the growth of copper (111) plane and its texture coefficient. For DC plating, the current does not change, and the crystal plane orientation is determined by the orientation of the substrate, the current density of the components and the plating solution. For pulse-reverse plating, there are forward growth and reverse dissolution processes, T_on and T_off also affect the crystal growth . The difference in grain orientation of the copper coating has a closerelationship with the formation of twins during the growth process. Twin formation is one mechanism by which new crystal directions appear on the oriented substrates. The period pulse reverse deposit a barrier layer at points of high current density. This inhibits Cu deposition at these points and reduces, compared to DC plating, the required amount of organic additives. Furthermore, the PPR plating favors grain nucleation thus increases the number of grains per unit area. The reverse pulse can dissolve the cathode surface, which leads to possible changes in the orientation of the crystal planes of the substrate and ultimately affects the texture coefficient of the crystal planes. Therefore, different reverse pulse parameters will affect the texture coefficient of the crystal planes. With different reverse pulse parameters, all prepared samples possess high-portion (111) textural copper, which is beneficial to the stability of copper and the interconnect performance of plating. Based on the TC values, the optimal reverse pulse parameters are 100 Hz, -0.2 A/dm²和40%, which also have higher TP value and copper uniformity.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

This review paper deals with Ultra-uniform copper deposition in high aspect ratio plated through holes via pulse-reverse plating. Its review is very interesting work in a related field for potential readers. However, there is not clear something for publication and I recommend the author should improve the manuscript.

1. In the Introduction, your research’s motivation or originality should be shown in the Introduction.

2. How about the uniformity of films that were prepared via PPR plating? And how much thickness of copper?

3. In Figures 7 and 8, I recommend that authors would show summaries with tables including the film’s XRD data such as grain size, planes, orientation, etc.

4. In Figures 10-12, why are the changed surface morphologies of copper deposited by PPR plating?

5. As various film fabrication parameters, is any film’s conductivity changed?

6. In Conclusions. I recommend you would summarize the conclusions with your originality.

Author Response

1. In the Introduction, your research’s motivation or originality should be shown in the Introduction.

Reply:Thanks very much for your positive comments on our work.

Revision: Numerous research of period pulse reverse plating in plating through hole has been carried out, such as comparison of period pulse reverse plating and direct current plating for the uniformity of copper deposition in plated through holes^[24-26], microstructure of copper deposition with various pulse reverse frequency^[27]. However there is still lack of comprehensive study on a battery of pulse reverse parameters^[28]. In order to further understand the effect of reverse pulse parameters on the uniformity and the microscopic properties of copper electrodeposition in PTH. In this paper, a comprehensive study has been performed to investigate the effect of pulse reverse parameters, including pulse reverse frequency, duty cycle and current on lattice strut size, morphology, surface structure and internal porosity. On this basis, the mechanism of the reverse pulse in the electrodeposition behavior was discussed. This mechanism study enables the subsequent adjustable craft for the ultra-uniform copper deposition in  high aspect ratio PCB plating through hole.

2. How about the uniformity of films that were prepared via PPR plating? And how much thickness of copper?

Reply:Thanks very much for your positive comments on our work. The uniformly copper  coating has good filling property in the hole, could be completely filled with the electroplating time without no defects such as void.The filling mechanism are butterfly-shape of the filled copper in the through hols（DOI：10.1149/2.003305eel）and the “V” shape growth in blind vias（DOI: 10.1149/2.073405jes）. At the same time, the throwing power (TP) is defined as the ability of an electroplating solution/method to uniformly deposit metal on the cathode. （https://doi.org/10.1016/j.microrel.2019.04.004）（DOI: 10.1149/1.1560942）. The larger the TP value, the uniformity of copper are better.

We prepared the copper under different parameters, the growth mode of the copper in the hole is from the hole wall to the middle. In experiments a1, a2, and c1, the through holes are completely filled. We think the copper is uniformly grown in the plating growth process. In experiment a4, the thickness of copper is not uniform. In experiments a3 and b1, large holes appeared in the coating. We think that this defect is also caused by the uneven growth of the coating. In experiment c3, the thickness of copper in the orifice is larger than that in the hole. This growth mode will lead to excessive deposition of the orifice, and the completely filled cannot be achieved, which is also a kind of uneven growth of the plating layer. We calculated TP values of copper with different parameters, which show that the uniformity of the copper was different.

The thickness of copper are shown in table 2.

	a1	a2	a3	a4	b1	b2	b3	b4	c1	c2	c3	c4
coppr inside the hole/μm	136.39	142.71	126.26	82.22	114.69	136.39	36.46	33.26	135.38	136.39	42.18	24.71
copper of surface/μm	77.14	60.21	73.31	64.31	88.39	77.14	54.58	52.42	88.30	77.14	40.06	43.07

 

Revision:

Table 2. The thickness of copper and the TP vaule.

	a1	a2	a3	a4	b1	b2	b3	b4	c1	c2	c3	c4
coppr inside the hole/μm	136.39	142.71	126.26	82.22	114.69	136.39	36.46	33.26	135.38	136.39	42.18	24.71
copper of surface/μm	77.14	60.21	73.31	64.31	88.39	77.14	54.58	52.42	88.30	77.14	40.06	43.07
TP/100%	176.80	237.03	172.23	127.84	129.75	176.80	66.80	63.45	153.31	176.80	105.28	57.37

 

3. In Figures 7 and 8, I recommend that authors would show summaries with tables including the film’s XRD data such as grain size, planes, orientation, etc.

Reply:Thanks very much for your positive comments on our work. The TC vaule of copper are shown in table 3.

TC/100%	(111)	(200)	(220)	(311)
a1	68.21	13.34	6.93	11.52
a2	59.23	16.10	12.03	12.64
a3	64.92	16.08	8.80	10.20
a4	51.23	17.09	15.6	16.08
b1	66.66	15.42	6.72	11.20
b2	68.21	13.34	6.93	11.52
b3	56.20	15.47	14.13	14.20
b4	54.01	18.00	13.69	14.30
c1	59.90	16.25	9.92	13.93
c2	68.21	13.34	6.93	11.52
c3	66.66	10.28	7.26	15.80
c4	51.91	17.92	14.06	16.11

Revision:

Table 3. The TC vaule of copper.

TC/100%	(111)	(200)	(220)	(311)
a1	68.21	13.34	6.93	11.52
a2	59.23	16.10	12.03	12.64
a3	64.92	16.08	8.80	10.20
a4	51.23	17.09	15.6	16.08
b1	66.66	15.42	6.72	11.20
b2	68.21	13.34	6.93	11.52
b3	56.20	15.47	14.13	14.20
b4	54.01	18.00	13.69	14.30
c1	59.90	16.25	9.92	13.93
c2	68.21	13.34	6.93	11.52
c3	66.66	10.28	7.26	15.80
c4	51.91	17.92	14.06	16.11

4. In Figures 10-12, why are the changed surface morphologies of copper deposited by PPR plating?

Reply:Thanks very much for your positive comments on our work.During the electrodeposition process, the composition of solution, the type of power supply (DC、PPR plating), stirring and other external conditions have an impact on the growth of crystal. （https://doi.org/10.1016/j.electacta.2020.137391）

The PPR plating has both the deposition process and the dissolution process,which influence the surface morphologies. The cathode peak current density of plating, while increasing the deposition rate creates more densely packed nanotwins.The reverse pulse carries out the copper dissolution process. The reverse pulse can make the surface of the cathode supplemented by copper ions, and different copper ion concentrations will affect the process of copper deposition, and then affect its surface morphology. In addition, the reverse pulse can directly act on the surface of the copper crystal to change the surface morphology. Different reverse pulse parameters will affect the dissolution process of copper, such as dissolution rate and area. Therefore, reverse pulse parameters will change surface morphologies of copper.

Revision: Figures 10 to 12 show the SEM images of copper surface prepared by different experimental groups as shown in Table 1. When the reverse pulse frequency was 100Hz, the reverse duty cycle was 20% or 40%, and the reverse current density was 0.1 A/dm² or 0.2 A/dm², the flat and dense surface is obtained. This result was consistent with the previous uniformity of copper, TP value and XRD texture analysis. The dense copper surface is closely arranged of uniform and small spherical particles clustering. In high magnification SEM, planar morphology was a hexagonal pyramid with a clear step-and-terrace structure. The hexagonal pyramid structure at the surface of (111) copper is owing to screw dislocation growth^[31]. When the reverse pulses increase to 1000 Hz and 1500 Hz, the planar morphology of the copper is the accumulation of copper crystals with a longer hexagonal pyramid structure, with larger particles. Some pores were observed as the copper was not tightly arranged. And the spherical hexagonal pyramid structure of the copper transformed into a long needle-like hexagonal pyramid structure. That morphology would deteriote the stability of copper and the interconnect performance of copper. A similar situation occured when the reverse duty cycle was 60% or 80%, with the current density of 0.4 A/dm². In addition, when the reverse pulse current density increases to 1 A/dm², the copper surface structure was mostly a large block crystal, and large gaps appears. The PPR plating has both the deposition process and the dissolution process, which influence the surface morphologies. The cathode peak current density of plating, while increasing the deposition rate creates more densely packed nanotwins. The reverse pulse carries out the copper dissolution process. The reverse pulse can make the surface of the cathode supplemented by copper ions, and different copper ion concentrations will affect the process of copper deposition, and then affect its surface morphology. In addition, the reverse pulse can directly act on the surface of the copper crystal to change the surface morphology. Different reverse pulse parameters will affect the dissolution process of copper, such as dissolution rate and area.Therefore, different reverse pulse parameters have an effect on the surface state of copper, which in turn affects its interconnect performance.

 

5. As various film fabrication parameters, is any film’s conductivity changed?

Reply: Thanks very much for your positive comments on our work. Copper is a metal with good conductivity, and different preparation methods have little effect on the conductivity. In our previous work of preparing copper in PTH, the resistivity of copper of different preparation conditions is 7.32 uΩ•cm or 4.72 uΩ•cm, and there is little difference.

6. In Conclusions. I recommend you would summarize the conclusions with your originality.

Reply: Thanks very much for your positive comments on our work. We have added the relevant comments on the conclusions.

Revision: This work investigated the effect of reverse pulses parameters on copper uniformity in high aspect ratio through holes. The uniformity, copper texture and surface morphologies of the copper platings were characterized via SEM, XRD and EBSD.The reverse pulse frequency can be tailored in a wide range of 100~1000Hz. The reverse current density and the reverse duty cycle are more sensitive parameters. To achieve the high uniformity of the through-hole plating, the reverse current density was preferably 10% of the forward current density, and the reverse duty cycle should be 20%-40%. Different reverse pulse parameters have an effect on the surface state of copper, which in turn affects its interconnect performance. The surface state of the copper is consistent with the uniformity of the copper. The copper with good surface state also has good uniformity by the same reverse pulse parameter. The optimal reverse pulse parameters are 100Hz, -0.2 A/dm² and 40%, copper with ultra-uniformity and strong (111) crystallographic orientation were successfully prepared. The refined crystal grains and the large-angle grain boundaries enables the good stability of filling copper.

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Dear Authors,

I have read your manuscript with great attention and interest. The article falls within the scope of the journal and is sufficiently original, and I have a remark, so I recommend publishing after MINOR REVISIONS.

1. Section 2 should be supplemented with the technique for making cross sections of holes in printed circuit boards and indicate which fill was used to make the cross sections.
2. Please indicate in 2^nd section the manufacturer of the research equipment as well as the city and country where the equipment was manufactured.
3. The thickness of the resulting electrochemical copper coating is not specified.
4. The thickness of the chemical copper sublayer of 10 microns is not Modern technologies assume a sublayer with a thickness of up 1 micron.
5. In paragraph 2.1 the aspect ratio of 150/30 microns (depth/diameter) is given, which corresponds to 15 mm/0.3 mm, apparently, the authors were mistaken by an order of magnitude. In addition, the adequacy of model with RDE to the copper plating conditions of a real board by researching the smallest of the 0.2 mm holes should have considered.
6. A note about the provided chronopotentiograms. The author’s claim about the defragmentation of PEG molecules by the SH110 additive and the increase in the number of inhibitor fragments, that leads to an increase in polarization, is substantiated by nothing. Perhaps the authors meant “desorption”, not “fragmentation”, and there is an inaccuracy of translation. The low quality of some photographs of cross sections (Fig. 5, a1, a2, b2, c1, c2) does not permit to judge the correctness of the conclusions.

7. A note concerning the experiment on the effect of the parameters of the reverse pulse on the uniformity of electrodeposited copper.            The authors have declared that the duration of the experiment in the forward direction was 100 ms, and in the reverse direction has changed. The data shown on Fig. 2 does not correspond to this. Judging by the figure, the duration of work in the cathode direction is 30 ms, which consist of 3 separate pulses of 4 ms and 3 pauses between them of 6 ms. The authors claim that they have determined the optimal parameters of the reverse pulse. It is difficult to agree with this, since only a small part of the possible options have been investigated, including in combination with various other parameters of cathode pulses.
8. During electrodeposition of copper on a rotating printed circuit board, the hydrodynamic mode in the holes depends on their distance to the axis of rotation. Neither the size of the printed circuit board nor the specified distances are given in the article. It would have been more correct to provide the linear velocity of the hole in question. In addition, the hydrodynamics will be influenced by the size and shape of the electrochemical cell.

Author Response

1. Section 2 should be supplemented with the technique for making cross sections of holes in printed circuit boards and indicate which fill was used to make the cross sections.

Reply:Thanks very much for your positive comments on our work.

Revision: The microvia fabricated by mechanical drilling had an aspect ratio of 150μm/30μm 1.5mm/0.3mm(depth/diameter).Electroplated copper was used to make the cross sections.

1. Please indicate in 2^nd section the manufacturer of the research equipment as well as the city and country where the equipment was manufactured.

Reply:Thanks very much for your positive comments on our work.

Revision: After being precise polished, the filled microvia cross-section’s morphology of were observed under field emission scanning electron microscopy(FE-SEM, Thermo ThermoFisher Scientific Aperos S, USA)to calculate the TP.

All electrochemical measurements were performed in a three-electrode system using a Gamry electrochemical workstation(Gamry, USA) at 24 ± 1°C.

The XRD pattern was recorded through a X’Pert PRO Dy2198  X-ray diffractometer(X’Pert PRO, Dy2198, Spectris. Pte. Ltd, Netherlands) with Cu Kα radiation.

The microstructures of the Cu electro deposition in the PTHs were examined using the Electron back-scatter diffraction (EBSD, Oxford Instrument, UK) analysis system and AztecCrystal, software (version 7.0 published by EDAX, Inc.).

1. The thickness of the resulting electrochemical copper coating is not specified.

Reply:Thanks very much for your positive comments on our work. The thickness of copper are shown in table 2.

	a1	a2	a3	a4	b1	b2	b3	b4	c1	c2	c3	c4
coppr inside the hole/μm	136.39	142.71	126.26	82.22	114.69	136.39	36.46	33.26	135.38	136.39	42.18	24.71
copper of surface/μm	77.14	60.21	73.31	64.31	88.39	77.14	54.58	52.42	88.30	77.14	40.06	43.07

 

Revision:

Table 2. The thickness of copper and the TP vaule.

	a1	a2	a3	a4	b1	b2	b3	b4	c1	c2	c3	c4
coppr inside the hole/μm	136.39	142.71	126.26	82.22	114.69	136.39	36.46	33.26	135.38	136.39	42.18	24.71
copper of surface/μm	77.14	60.21	73.31	64.31	88.39	77.14	54.58	52.42	88.30	77.14	40.06	43.07
TP/100%	176.80	237.03	172.23	127.84	129.75	176.80	66.80	63.45	153.31	176.80	105.28	57.37

 

1. The thickness of the chemical copper sublayer of 10 microns is not Modern technologies assume a sublayer with a thickness of up 1 micron.

Reply:Thanks very much for your positive comments on our work.The thickness of the chemical copper sublayer was not tested.Therefore, we revised the article to: An approximately 10-μm thick thin Cu film was predeposited on the hole wall by electroless (ELS) copper deposition.

Revision:An approximately 10-μm thick thin Cu film was predeposited on the hole wall by electroless (ELS) copper deposition.

1. In paragraph 2.1 the aspect ratio of 150/30 microns (depth/diameter) is given, which corresponds to 15 mm/0.3 mm, apparently, the authors were mistaken by an order of magnitude. In addition, the adequacy of model with RDE to the copper plating conditions of a real board by researching the smallest of the 0.2 mm holes should have considered.

Reply:Thanks very much for your positive comments on our work. This is the mistaken in the article.It should be :The microvia fabricated by mechanical drilling had an aspect ratio of 150μm/30μm 1.5mm/0.3mm(depth/diameter).

The RDE model is mainly used to study the electrochemical behavior of additives under strong and weak convection, it cannot fully represent the convection on the cathode surface during the actual electroplating process. This experimental method has certain limitations.

Revision: The microvia fabricated by mechanical drilling had an aspect ratio of 150μm/30μm 1.5mm/0.3mm(depth/diameter).

1. A note about the provided chronopotentiograms. The author’s claim about the defragmentation of PEG molecules by the SH110 additive and the increase in the number of inhibitor fragments, that leads to an increase in polarization, is substantiated by nothing. Perhaps the authors meant “desorption”, not “fragmentation”, and there is an inaccuracy of translation. The low quality of some photographs of cross sections (Fig. 5, a1, a2, b2, c1, c2) does not permit to judge the correctness of the conclusions.

Reply:Thanks very much for your positive comments on our work.During the process of plating, SH110 would decompose into inhibitor and accelerator fragments, which has been proved in relevant research. (https://doi.org/10.1080/00202967.2019.1636581) ( https://doi.org/10.1038/s41598-021-91318-9) We did not prove this, just quoted the conclusions of relevant researchers.

Revision: Figure 4 shows the results of galvanostatic measurements of PEG10000 and SH110 injection during copper electroplating. When 200 mg/L PEG was injected to the base plating solution, the cathode potential shifted to negative rapidly, indicating that PEG was adsorbed on the cathode surface as an inhibitor, increasing the cathode polarization. So, the growth rate of copper crystals decreased accompanied with the increase of nucleation rate, and there was a certain grain refinement. After 1 mg/L SH110 was injected, it acted as an accelerator and exhibit a competitive adsorption with the inhibitor PEG adsorbed on the cathode surface, and the absorptive strength was higher than PEG. So, the addition of SH110 enables the depolarization of copper deposition. With the SH110 concentration increase, the concentration of inhibitor fragments decomposed by SH110 gradually increased, and the cathodic polarization increased^{[30, 31]}. Different potential changes were gradually displayed under strong and weak convection. When 10 mg/L SH110 was injected, the polarization of cathode surface decrease from 1000 rpm to 100 rpm. So, the electrodeposition rate under strong convection at the orifice was lower than that in the weak convection inside the hole, thus leading to high TP. When 20 mg/L SH110 was injected, the concentration of the decomposed inhibitor fragments further increased. Under the strong convection at 1000 rpm, the higher polarization of the cathode leads to a negatively shift of the cathodic potential. The cathode potential at 100 rpm had smaller negative shift than at 1000 rpm. Therefore, SH110 exhibits both acceleration and inhibition effect, and the concentration should be moderate, and 10 mg/L was found to be the optimized value in this system.

 

1. A note concerning the experiment on the effect of the parameters of the reverse pulse on the uniformity of electrodeposited copper.            The authors have declared that the duration of the experiment in the forward direction was 100 ms, and in the reverse direction has changed. The data shown on Fig. 2 does not correspond to this. Judging by the figure, the duration of work in the cathode direction is 30 ms, which consist of 3 separate pulses of 4 ms and 3 pauses between them of 6 ms. The authors claim that they have determined the optimal parameters of the reverse pulse. It is difficult to agree with this, since only a small part of the possible options have been investigated, including in combination with various other parameters of cathode pulses.

Reply:Thanks very much for your positive comments on our work.The schematic diagrams of the forward- and reverse-pulse currents are different under different experimental conditions.Figure 2 only describes the parameters of the period pulse reverse power supply, to make the reader understand the definition and connection of each parameter. It does not represent the period pulse reverse power supply parameter settings in the actual electroplating process.

The optimal reverse pulse parameter means the relatively optimal parameter in this research system that fixed other external conditions and forward pulse parameters. It is not the best optimal parameter of the whole plating system. The best optimal parameter should also consider the effect of the forward pulse parameters and external conditions, such as plating temperature, hydrodynamics and so on.

1. During electrodeposition of copper on a rotating printed circuit board, the hydrodynamic mode in the holes depends on their distance to the axis of rotation. Neither the size of the printed circuit board nor the specified distances are given in the article. It would have been more correct to provide the linear velocity of the hole in question. In addition, the hydrodynamics will be influenced by the size and shape of the electrochemical cell.

Reply:Thanks very much for your positive comments on our work.The overall size of the electroplating tank is 9cm×9cm×8cm.We placed 4 identical anode plates inside the plating tank, and the thickness of the anode plate is 0.2cm. When the cathode and anode are facing each other, the distance is approximately 4.3cm.

Revision: The PCBs were composed of FR-4 epoxy resin and copper. The microvia fabricated by mechanical drilling had an aspect ratio of 150μm/30μm 1.5mm/0.3mm (depth/diameter). Electroplated copper was used to make the cross sections.Before metallization, a desmearing process was done to remove the smear formed by the mechanical drilling on the hole wall of the THs. An approximately 10-μm thick thin Cu film was predeposited on the hole wall by electroless (ELS) copper deposition. The electroless copper acts as the catalytic seed layer of copper plating.The electroless copper was also an important step of the current PCB through-hole metallization process, which affects the final uniformity of the through-hole plating and the device performance.The phosphor copper plate is used as the anode and the PCB is used as the cathode. In our procedure,four anode plates were adopted and the PCB board was located in the middle of the rectangle enclosed by the anode plates. The overall size of the electroplating tank is 9cm×9cm×8cm.We placed 4 identical anode plates inside the plating tank, and the thickness of the anode plate is 0.2cm. When the cathode and anode are facing each other, the distance is approximately 4.3cm.The cathode was rotated, and the speed was 100 revolutions/min. The electroplating bath temperature was 40℃, and the SMD-30 was used as the period pulse reverse power supply. The four-anode system is adopted, and the rotation of the cathode was adpted to improve the uniformity of PTH. The experimental device was illustrated in Figure 1.

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

All the comments were replied sufficiently and this manuscript is in the good shape for being published.