THE STUDY OF 28NM BEOL CU GAP-FILL PROCESS
Yu Bao, Gang Shi, Lin Gao, Yanyan Zhang, Yingming Liu, Peng Tian, Fuchun Xi, Wei Hu, Ying Gao, Zhenhua Cai, Baojun Zhao, Zhigang Yang, Jianghua Leng, Haifeng Zhou, Jingxun Fang Shanghai Huali Microelectronics Corporation, Shanghai 201203, China
baoyu@hlmc
ABSTRACT
In this paper, the influence of Copper (Cu) barrier and seed process tuning on step coverage was analyzed. TEM images show relatively thinner barrier can improve the opening CD of a metal line structure hence improve the sidewall coverage of Cu seed. Cu Seed adopts the deposition/re-sputter method to improve the step coverage, and a higher ratio of re-sputter/deposition can increase the thickness of Cu seed on the sidewall. According to the post CMP surface defects scan results, the optimization of barrier Cu seed thickness can significantly reduce the copper void defects density.
INTRODUCTION
depositionCopper has been widely used in integrated circuit as the interconnect material because of its low resisti
vity and good electro-migration (EM) resistance. However, As the CD of integrated circuit scaling down, the Cu gap filling became a big challenge. For Cu interconnect formation, Cu barrier seed process is very important, and a robust condition can provide wider process window for subsequent electro Cu plating process (ECP). [1-3] Combined with the TEM images of post-Cu barrier/seed deposition and the post-Cu-CMP surface defects data, the influence and mechanism of Cu barrier/seed process tuning on Cu-void performance were proposed. EXPERIMENT
Metal line structures were prepared with ULK/MHM scheme. Ta(N) based barrier and CuMn seed layer were deposited by PVD. To investigate the mechanism of Cu gap filling, the thickness of Cu barrier/seed were measured using TEM images. The gap fill performance was evaluated by surface defects scan at post CMP. RESULTS AND DISCUSSIONS
Barrier Layer
Bilayer Ta(N) based barrier is deposited by PVD. The whole deposition process is divided into 4 steps, including TaN deposition, Ta deposition, Ar re-sputter, and Ta Flash. TaN is the barrier layer to prevent Cu diffusion. Ta as the wetting layer for Cu seed. Ar re-sputtering can thin down the barrier at the bottom of  metal line structures by ion bombardment. The re-sputtered Ta(N) re-deposited onto th
e sidewall resulting in  better step coverage. Figure 1 showed the TEM images of barrier seed with 3 different barrier conditions.
Figure 1. The TEM images of barrier seed with different barrier. (a).Thinner barrier with less Ar re-sputter;
(b).Thinner barrier; (c).Control barrier
The step coverage of Cu barrier seed was shown in Table I. It is difficult to distinguish the boundaries between barrier and Cu seed on sidewall, so we measured the total thickness of the barrier and Cu seed. As shown in Figure 2, the thinner barrier  improved the opening CD and hence improved the sidewall coverage of Cu seed.
Table I. Normalized step coverage of barrier seed with
Figure 2. Normalized step coverage with different barrier.
(a).Overhang; (b).Sidewall
Cu Seed Layer
Copper seed works as cathode and conductive layer a    b
during Cu ECP process. The quality of Cu seed is critical to ECP. Cu Seed formation can be divided into 2 steps, deposition and Cu re-sputter. The Cu re-sputter step can  improve the coverage on the sidewall. Figure 3 showed the TEM images of Cu barrier seed with 3 different seed conditions.
Figure 3. The TEM images of barrier seed with different seed. (a).Less deposition seed; (b).Control seed;
(c).More re-sputter seed.
The step coverage of Cu barrier/seed is shown in Table II.  The higher the Cu re-sputter/deposition ratio, the higher the Cu seed coverage on sidewall.
Table II. Normalized step coverage of barrier seed with
As shown in figure 4, both less deposition and more re-sputter seed can improve sidewall coverage, and less deposition can reduce overhang. So seed with more re-sputter is preferred to gap filling.
Figure 4. Normalized step coverage with different seed.
(a).Overhang; (b).Sidewall
Defect Scan Results
Table III is the design of experiment of Cu barrier/ seed thickness. The gap fill results were evaluated by post Cu CMP void inspection.
Post-CMP defect data in Fig.5 showed thinner barrier is beneficial for ECP gap filling due to larger opening CD. However, the thinner seed split can make the Cu void worse significantly.
Figure 5. Normalized Cu void density post Cu-CMP with
various barrier/seed splits
Typical Cu void post CMP was shown in figure 6,  missing Cu can be found on the sidewall of recess structure. It seems that continuous seed on the sidewall is more important for gap filling.
Figure 6. Typical Cu void post CMP. (a).Top view;
(b).Cross section CONCLUSION
In conclusion, Cu-void performance was remarkably improved by the optimization of Cu barrier/seed scheme. Relative thinner barrier combined with thicker continuous seed layer was preferred for subsequent Cu ECP. Improved gap fill performance confirmed by post Cu-CMP defect data. Result is consistent with the good step coverage of Cu barrier/seed shown by TEM images.
a    b
ACKNOWLEDGEMENTS
The author would like to acknowledge all the 28nm process integration team members of HLMC for the technical discussions, and the failure analysis lab for supporting  TEM  work.
REFERENCES
[1]Weiye He, Beichao Zhang, Jian Kang, et. al. The
contributions of barrier resputter for BEOL integration. ECS Transactions. 44 (1) 487-492 (2012) [2]Yu Bao, Xuezhen Jing, Jingjing Tan, et. al.
Optimization of Metallization Processes for 28-nm-node Low-k /Cu Multilevel Interconnects.
ECS Transactions. 44 (1) 477-480 (2012)
[3]Xuezhen Jing, Jingjing Tan, Jiquan Liu. 32/28NM
BEOL CU GAP-FILL CHALLENGES FOR METAL FILM. CSTIC 2015

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