Monday, July 22, 2019

In Situ Cu Surface Cleaning with Anhydrous Hydrazine highlighted at AVS ALD 2019 by University of Texas at Dallas and RASIRC

Copper replaced Aluminum for interconnects in the semiconductor industry due to its low resistivity, high resistance to electromigration, low temperature coefficient of resistance, and good thermal stability [1].

Due to the lack of volatile copper compounds, copper could not be patterned by the techniques of photoresist masking and plasma etching that had been used for aluminum. The inability to plasma etch copper meant that the whole metal patterning process had to be redesigned and the result was a process referred to as an additive patterning, also known as a "Damascene" or "dual-Damascene" process by analogy to a traditional technique of metal inlaying. [2]

However, the exposed Cu interconnects during via-opening and post CMP process are vulnerable to oxidation with water rinse and exposure to air, resulting in reliability degradation [3]. Therefore, additional process for reduction of copper oxide should be required. The cleaning of copper can be achieved by either physical Ar sputtering or chemical reduction process [4]. Recent demonstration of chemical-based cleaning of Cu interconnects is expected to overcome disadvantages of physical Ar sputtering process, such as chamfering and re-deposition on vias and trenches. A number of studies on vapor-based reduction of copper oxide under ambient pressure conditions and at temperatures below 350 °C using hydrogen, ammonia, carbon monoxide, forming gas, acetic acid, formic acid, and ethanol as reducing agents have been reported [5,6]. On the other hand, Hydrazine (N2H4) can be used in the reduction of copper oxide due to its higher reduction capability [7].

Inspired by hydrazine’s unique characteristics, University of Texas at Dallas and RASIRC have explored the feasibility of vapor-phase reduction of copper oxide using anhydrous N2H4 to achieve an ideal metallic Cu film in an ALD environment.

Due to the lack of volatile copper compounds, copper could not be patterned by the techniques of photoresist masking and plasma etching that had been used for aluminum. The inability to plasma etch copper meant that the whole metal patterning process had to be redesigned and the result was a process referred to as an additive patterning, also known as a "Damascene" or "dual-Damascene" process by analogy to a traditional technique of metal inlaying. [2]

However, the exposed Cu interconnects during via-opening and post CMP process are vulnerable to oxidation with water rinse and exposure to air, resulting in reliability degradation [3]. Therefore, additional process for reduction of copper oxide should be required. The cleaning of copper can be achieved by either physical Ar sputtering or chemical reduction process [4]. Recent demonstration of chemical-based cleaning of Cu interconnects is expected to overcome disadvantages of physical Ar sputtering process, such as chamfering and re-deposition on vias and trenches. A number of studies on vapor-based reduction of copper oxide under ambient pressure conditions and at temperatures below 350 °C using hydrogen, ammonia, carbon monoxide, forming gas, acetic acid, formic acid, and ethanol as reducing agents have been reported [5,6]. On the other hand, Hydrazine (N2H4) can be used in the reduction of copper oxide due to its higher reduction capability [7].

Inspired by hydrazine’s unique characteristics, University of Texas at Dallas and RASIRC have explored the feasibility of vapor-phase reduction of copper oxide using anhydrous N2H4 to achieve an ideal metallic Cu film in an ALD environment.


Figure 1. Schematic of (a) RTALD system, (b) Process sequence, and (c) representative time sequence of stop valve process.

In summary, it could be shown that following an ozone treatment (Figure 1) a N2H4 treatment could effectively reduce the Cu2O to metallic Cu(0) from 150 – 200 oC. In addition, there was no detection of intermediate materials (e.g. Cu3N, Cu(OH)2, CuH, etc.). The following possible thermodynamic reaction is given CuO + Cu2O + N2H4 à 3Cu + 2H2O(g) + N2(g)

The details of the study will be presented at AVS ALD2019 and future work will be on potential application to Ru and Co cleaning/reduction, which have become important interconnect metals for 14/16 nm Logic and below, especially at the highly scaled lower metallization levels (M0 to M4).

References

1. R. P. Chaukulkar, N. F. W. Thissen, V. R. Rai, and S. Agarwal, J. Vac. Sci. Technol. A, 32, 01A108 (2014).
2. Copper interconnects, Wikipedia LINK: https://en.wikipedia.org/wiki/Copper_interconnects
3. Y.-L. Cheng, C.-Y. Lee, and Y.-L. Huang, in Noble and Precious Metals-Properties, Nanoscale Effects and Applications, M. Seehar and A. Bristow, Editors, p. 216–250, Intechopen (2018).
4. C. K. Hu et al., Microelectron. Eng., 70, 406–411 (2003).
5. L. F. Pena, J. F. Veyan, M. A. Todd, A. Derecskei-Kovacs, and Y. J. Chabal, ACS Appl. Mater. Interfaces, 10, 38610–38620 (2018).
6. Y. Chang, J. Leu, B.-H. Lin, Y.-L. Wang, and Y.-L. Cheng, Adv. Mater. Sci. Eng., 2013, 1–7 (2013).
7. D. M. Littrell, D. H. Bowers, and B. J. Tatarchuk, J. Chem. Soc. Faraday Trans. 1 Phys. Chem. Condens. Phases, 83, 3271–3282 (1987).


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