Recent Advances in Barrier Layer of Cu Interconnects

Round 1

Reviewer 1 Report

This review presents a detailed discussion of state-of-the-art developments of barrier layers used for Cu interconnects in microelectronics fabrication.  The review is well written and covers latest developments, including the use of 2D materials, self-assembled monolayers and high-entropy alloys, and will therefore be a valuable resource for researchers in this field.  I recommend publication of this work in Materials after the following minor issues have been addressed:

• The paper is already classified as "Review", so that this word can probably be omitted from the title: "Recent advances in barrier layers for Cu interconnects".
• Line 86 mentions "terminal effect" as an unpleasant phenomenon, but does not explain what it means.  If space allows, please include a technical explanation of the effect - I also recommend including the following reference, where this effect and other essential knowledge are explained in a tutorial manner: R. Akolkar, Current Status and Advances in Damascene Electrodeposition, in Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier 2018, pp. 24-31. DOI: 10.1016/B978-0-12-409547-2.14058-2
• In Figure 3, the bright yellow (and especially white text within it) is poorly visible against a white background.  I recommend using the same colour for Cu as in Figure 1.
• In the last paragraph of section 4, please add that the possibility of Cu intercalation during electrodeposition on 2D materials has been suggested in the case of hexagonal boron nitride (Mertens, Electrochim. Acta 246, 2017, 730–736).

Author Response

This review presents a detailed discussion of state-of-the-art developments of barrier layers used for Cu interconnects in microelectronics fabrication.  The review is well written and covers latest developments, including the use of 2D materials, self-assembled monolayers and high-entropy alloys, and will therefore be a valuable resource for researchers in this field.  I recommend publication of this work in Materials after the following minor issues have been addressed:

The paper is already classified as "Review", so that this word can probably be omitted from the title: "Recent advances in barrier layers for Cu interconnects".

Author reply: We thank the reviewer for his/her evaluation of our manuscript and the recommendation of accepting our manuscript. We have changed the title.  

Line 86 mentions "terminal effect" as an unpleasant phenomenon, but does not explain what it means.  If space allows, please include a technical explanation of the effect - I also recommend including the following reference, where this effect and other essential knowledge are explained in a tutorial manner: R. Akolkar, Current Status and Advances in Damascene Electrodeposition, in Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier 2018, pp. 24-31. DOI: 10.1016/B978-0-12-409547-2.14058-2

Author reply: We thank the reviewer for his/her suggestions. We have provided a brief explanation of “terminal effect” on Page 3 from line 101 to 104. We have also added the suggested reference.

In Figure 3, the bright yellow (and especially white text within it) is poorly visible against a white background.  I recommend using the same colour for Cu as in Figure 1.

Author reply: We have made changes accordingly.

In the last paragraph of section 4, please add that the possibility of Cu intercalation during electrodeposition on 2D materials has been suggested in the case of hexagonal boron nitride (Mertens, Electrochim. Acta 246, 2017, 730–736).

Author reply: We have added a sentence to point out that Cu intercalation into h-BN may occur during electrochemical deposition of Cu, and included the suggested reference as ref. 146.

Reviewer 2 Report

This review surveys the frontier of new barrier materials of Cu interconnects. The Cu interconnects support modern IC technologies, significantly contributing to the downsizing of devices. The barrier layer plays an important role in the prevention of Cu diffusion. This review is well organized and is worth for publishing after addressing the issue listed below.

• I recommend the authors to give a Table or a Figure, which explains the comparison of some properties between new barrier materials (PGM based materials, 2D materials, SAMs, and HEAs) and widely used Ta/TaN.   

Author Response

This review surveys the frontier of new barrier materials of Cu interconnects. The Cu interconnects support modern IC technologies, significantly contributing to the downsizing of devices. The barrier layer plays an important role in the prevention of Cu diffusion. This review is well organized and is worth for publishing after addressing the issue listed below.

I recommend the authors to give a Table or a Figure, which explains the comparison of some properties between new barrier materials (PGM based materials, 2D materials, SAMs, and HEAs) and widely used Ta/TaN.  

Author reply: We thank the reviewer for his/her evaluation of our manuscript and the valuable suggestions. A comparison between new barrier materials and Ta/TaN is now provided on page 4 from line 125 to 133, as a new table (table 1).

Reviewer 3 Report

Review attached. 

Comments for author File: Comments.pdf

Author Response

The review article by Zhi Li et al. presents an overview of the recent technological advances regarding the barrier layer of Cu interconnects. It is a well-articulated review paper that includes a

brief history of the development of various barrier layers of Cu interconnects, along with their synthetic methods. Additionally, this review article discusses on the research directions and highlights the drawbacks associated with the current barrier materials. While this article is outlined in a well-organized and detailed manner, there are some minor points (below) in the manuscript that need to be addressed before it can be accepted for publication.

1. In the introduction, the authors mention that electrochemical deposition (ED) is a moreefficient way to super-fill damascene features without defects compared to PVD and CVD. While PVD and CVD result in defects, atomic layer deposition (ALD) is well known method to deposit uniform films without defects. I would recommend adding aparagraph that explicitly discusses on the uniqueness and advantages of ALD to obtain defect-free films.

Author reply: We thank the reviewer for his/her evaluation of our manuscript and the valuable suggestions. We have added a paragraph describing the uniqueness and advantages of ALD to deposit uniform films without defects on page 2 from line 51 to 58.   

2. On page 2, it is mentioned that there are no self-passivation oxide layers for preventingcorrosion of Cu under chip fabrication process. However, there is abundant literature on developing passive oxide layers on transition metals using wet-chemistry and vapor phase depositions techniques for wide-ranging applications, especially batteries. It is recommended to discuss about those ultrathin barrier layers that prevent active metals from corrosion.

Author reply: We apologize for the misleading description. We were meant to demonstrate that unlike Al, Cu cannot form an intact passive oxide layer by itself for anti-corrosion, so it is fragile under chip processing conditions. We have no doubt that passive oxide layers can be developed on transition metals using wet-chemistry and vapor phase deposition for anti-corrosion. Anyway, we have now included examples and references regarding the use of Ru oxide and Ir oxide layers as metal anti-corrosion barrier layers, on page 2 from line 74 to 80.

3. On page 3, the authors state that the PVD, CVD, and ALD cause undesired over-hangwhen the seed layers are deposited. This reviewer is curious to know if there is any when the seed layers are deposited. This reviewer is curious to know if there is anyparticular reference that demonstrates this issue of seed layer over-hang with ALD. 

Author reply: We apologize for the inappropriate description. ALD is without doubt an excellent technique for the fabrication of defect-free thin films. What we meant to say is that the extra Cu seed layer was normally the cause of forming overhang at the opening of trench. We have rewritten that section and specified that extra seed layer generated by different deposition protocol is responsible for the overhang.

4. In the last paragraph on page 3, it is specified that effective barrier layers are yet to be developed for preventing Cu corrosion and diffusion. As I mentioned earlier, I believethere are reports regarding these ultrathin barrier layers to prevent Cu diffusion.

Author reply: Yes, indeed. We have modified the false statement.

5. On page 4, they discuss about different vapor-phase methods of Ru. Can they authors discuss if there is literature on depositing Ru barriers using cost-effective sol-gel, spin coating, and dip-coating methods.

Author reply: Yes, indeed. Cost-effective protocols such as sol-gel, spin coating, and dip-coating were usually used to deposit metal oxide layers. Of course, Ru oxide layers can be prepared using those methods, and RuO₂ thin films are well-known metal corrosion resistant, but a RuO₂ layer would significantly increase the resistivity of the electrode, as ρ_RuO2 is as high as 35.2μΩ·cm. Therefore, a RuO₂ layer would arrest further direct electroplating of Cu. In addition, a Ru oxide layer weakens the adhesion between Cu and the substrate, and that lowers the nucleation density of Cu. (L. Burke, N. Naser, R. Sharna, Journal of Applied Electrochemistry, 2008, 38, 377-384.) As such, thin films of metal, rather than metal oxide, are preferred as barrier layers for electrochemical deposition of Cu. We have added this part in the main text (at the end of section 3 from line 225 to 231) to demonstrate the possibility of using Ru oxide as an anti-corrosion barrier, but it is not suitable for direct electroplating of Cu. 

 

6. On page 11, there is a typo in the sentence “….mechanical stability of HEAs and HEAs, they have been proven….” Please fix these errors.

Author reply: We thank the reviewer for his/her careful reading of our manuscript. We have corrected the typo.

Once the above-mentioned points are adequately addressed, this manuscript can be accepted for publication as a review article in Materials.