Mechanism of Grain Densification in Nano- and Poly-Crystalline Cu Films and Its Impact in Advanced Metallization Processes

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors! I have been delighted reviewing your worth submission "Mechanism of grain densification in nano- and poly-crystalline Cu films and its impact in advanced metallization processes". That is very good, but IMHO requires some revision. 

Boring remarks.

Why references are not in the order of appearance?

Line 32 . modified -> modifies

36. An important warpage - not clear. Please, rephrase.

122. Room crystallization -> Room T crystallization

Methods:

Please, give more details on the "stress" measurements. I recommend to  give more details on the warpage curvature measurements (your ref [4] from 1909 is honorable, but seems not up-to-date) especially in terms of measurements accuracy. Si wafers are better to be described - dia and thickness.

Also - you have XRDA, it is so simple to apply sin^2 psi method for stress measurements in Cu (EDS) - I would recommend to discuss this at least! The same - FIB-DIC method is recommended even for Cu(seed).

Please, discuss whether the use of EBSD for grain structure characterization would be of benefit?  

Results.

I dislike your Figure 11 (c) very much. Cu is fcc, while W is bcc, how do you plot Vegard line in this case? In general, the shift of XRD peaks may be related with stress rather than with solid solution formation. By the way it is not allowed by thermodynamics - phase diagram returns no solubility in Cu-W system. Moreover, Ref [León Romano-Brandt, Enrico Salvati, Eric Le Bourhis, Thomas Moxham, Igor P. Dolbnya, Alexander M. Korsunsky, Nano-scale residual stress depth profiling in Cu/W nano-multilayers as a function of magnetron sputtering pressure, Surface and Coatings Technology, Volume 381, 2020, 125142, https://doi.org/10.1016/j.surfcoat.2019.125142] gives no signs of solubility of W in Cu. Please, discuss.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper is of interest to this journal, but there are a few questions and comments.

 

Lines 23-24. "Furthermore, Copper front metal enables to consider Cu-Cu wire bonding, more reliable than Cu-Al or Au-Al systems."

A short justification for this statement would be helpful.

 

Lines 31-32. "Thermal treatment of thick Cu deposited on semiconductors' substrates modified the composed system at different scales."

What about the solid-state reaction between copper and silicon occurring at a sufficiently low temperature (on the order of 100C), see, e.g., Ref: https://doi.org/10.3390/ma15238457

 

Line 67. "Cu 200 nm/(Ti)W 300 nm/Si substrate".

1) it is necessary to give some explanation - for what purpose the intermediate layer was used, why exactly such material and such thickness.

2) it is necessary to explain what "(Ti)W" stands for

 

Lines 206-207. "... we found two out of the three main peaks of the Face-Cubic Centered 206

(FCC) polymorphism, i.e., the (111) at 2θ= 43.35◦ and the (200) at 2θ= 50.48◦""

1) what is the purpose of using the term "polymorphism" here, since copper is characterized only by a FCC lattice?

2) what is the reason why only (111) and (200) are observed and the other reflexes are not observed - preferential orientation?

 

Lines 270-273. "... stress ..."

It may make sense to consider the values of thermal coefficients of linear expansion for each layer (Cu/(Ti)W/Si) to understand the mechanism of stress formation in this system.

It might not hurt to add the Conclusions.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper titled "Mechanism of grain densification in nano- and poly-crystalline Cu films and its impact in advanced metallization processes" investigates the microstructural evolution of electrochemically deposited poly-crystalline copper films during thermal process cycles. The study focuses on the modifications induced by thermal treatment, characterized using various techniques like X-ray diffraction, scanning electron microscopy, atomic force microscopy, and dynamical warpage measurements. The paper also includes a theoretical model to understand the grain densification phenomenon in copper films and its implications for electronic device manufacturing. The comprehensive analysis provides insights into strategies for integrating the process into real device structures. In general the paper is well-written and the subject can be of interest for the readers of Crystals MDPI journal. However, some improvements are important before its acceptance:

(1) Please enhance the introduction of your paper by increasing the number of references. This will provide a more comprehensive background and context for your study.;

(2) Could you clarify the novelty of your current work in relation to existing literature? This information should be prominently highlighted in the introduction to emphasize the unique contributions of your study to the field;

(3) Regarding the statement, 'The films were fabricated by sequential sputtering processes on a Si (100) substrate using the AMAT-Endura-PVD system,' it is unclear which materials were deposited by the PVD system, and the specific process parameters (discharge power, gas used, deposition time, gas flow, substrate to target distance, etc.) for the synthesis of the thin films are not detailed. Please provide more information on these aspects. A table could be helpful for a clearer presentation of these details.

(4) It would be beneficial to include a conclusion section that not only presents the findings but also discusses their implications, complementing the discussion section of the paper.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Thank you for revising your manuscript. It is much better now, and, although I still have arguments, I agree you publish the article in the present form.