Self-Healing Effect of Wet-Processed Organic SAM Diffusion Barrier Layer Using Polymer Capped Palladium Nanoclusters
Authors: Wang Wei-Yen , Wei Tzu-Chien 
Published: February 2020

In ultra-large scale integrated circuits (ULSIs), aluminum (Al) has been replaced by copper (Cu) as the interconnect material due to Cu has higher resistance to electromigration and lower electrical conductivity [1]. Unfortunately, Cu has high mobility in silicon (Si) and silicon dioxide (SiO2), thus impacting interconnect reliability and leading to inferior electrical performance or device failure. Therefore, it is necessary to form a good copper diffusion barrier between Si/SiO2 and Cu layer. In the past few decades, inorganic materials such as tantalum (Ta) or tantalum nitride (TaN) based compounds or metal-alloys has been successfully introduced as the diffusion barrier industrially. However, from the ULSI roadmap revealed by the International Technology Roadmap for Semiconductors report (ITRS), the necessary diffusion barrier thickness for the line-width of 22 nm is less than 2 nm [2], which challenges the current Ta or TaN technology by using the conventional vapor-phase deposition. Recently, a few reports use the organo-silane compounds such as 3-aminopropyltrimethoxysilane (APTMS) as the diffusion barrier to substitute inorganic Ta or TaN barrier layer [3-5], and show promising result. However, forming SAM is known to very sensitive to process conditions, particularly when SAM is prepared by wet-soaking. Imperfect SAM cannot block Cu diffusion efficiently and thus fails to compete with inorganic counterparts.
In this study, we developed an all-wet, self-healing process to converge the copper diffusion blocking property on a Cu/SAM/SiO2 substrate. In particular, a 3-[2-(2-aminoethylamino)ethylamino] propyl-trimethoxysilane (ETAS) layer was deposited on SiO2 surface via wet soaking process. Then a 200 nm Cu film was electroless plated onto ETAS-treated SiO2 with the help of a home-made poly(vinyl alcohol)-capped palladium (PVA-Pd) nanoparticle catalysts. By varying ETAS soaking time and the ratio of PVA:Pd in the process, the sheet resistance of the Cu/SAM/SiO2 substrate after rapid thermal annealing (RTA) was investigated. As shown in Fig.1, it can be seen the sheet resistance increased rapidly after RTA over 300 oC in the case of no ETAS treatment, indicating the formation of copper silicide because of the absence of diffusion barrier layer. It is also found in the case of 30 minutes ETAS treatment (approaching SAM[6]) and 1:1 PVA to Pd ratio (ETAS-30-1xPVA-Pd), no copper silicide formation up to 500 oC. Interestingly, in 1 minute ETAS (ETAS-1) soaking samples, which is considered islandish ETAS coating rather than SAM, the PVA:Pd ratio affects the effectiveness of Cu diffusion barrier significantly. The more the PVA, the better Cu diffusion blocking capability is. Finally, in the sample processed by 1 minute islandish ETAS treatment and PVA:Pd ratio of 2 (ETAS-1-2xPVA-Pd), the Cu diffusion blocking capability is identical to the case of ETAS SAM, evidencing PVA can mend the imperfect SAM layer and thus converging the overall performance. Our study provides a simple and controllable process to utilize organic compound as the alternative for Ta or TaN diffusion barrier layer in fine line ULSI fabrication.
REFERENCES
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[6] C. W. Hsu, W. Y. Wang, K. T. Wang, H. A. Chen and T. C. Wei, Sci. Rep., 7 (2017) 1.

Figure 1