Additive-Free Copper Deposition of Interconnects

by Yuxiu Liu

Department of Chemical Engineering, University of Rochester, Rochester, NY

 

Introductions


Electroplating of copper in microelectronic technology is achieved by using organic additives, which create problems in terms of cost and monitoring since they are consumed and incorporated during deposition. The incorporation of deposit gives rise to complicated processing control where the concentration of additives has to be regularly monitored and the lost additives have to be regularly replenished. Therefore, an additive free process is highly desirable for the industry. This work focuses on electroplating trenches/vias in the absence of additives from a copper sulfate bath containing a complexing ligand, acetonitrile (CH3CN). Samples prepared with presence of complexing ligand and without complexing ligand were compared by several techniques to determine the effect of presences of cuprous complex .

Sample Preparation

The samples prepared by the above methods were then mounted on sample stubs for analysis using the SEM. The cross section samples were prepared by cutting the samples with diamond knife. The following techniques were used during the analysis.

Images obtained from each of these techniques will be discussed in the results part.

Results
1. Samples prepared without complexing ligand

Figure 1 and 2 are SE SEM images of original sample from top view and cross section. The via width is around 0.36um. Figure 3 is the cross section of sample 1, which is prepared without complexing ligand, in CuSO4 + H2SO4 solution only. It is obvious that only a small amount of copper deposited into the vias in the upper part. Most of the copper deposited on the surface of the sample.

Figure 1 Top View of Original Sample

Figure 2 Cross Section of Original Sample Figure 3 Cross Section of sample 1

2. Samples prepared with complexing ligand

2.1 Potentiostatic Mode

Under potentiostatic mode, with presence of CH3CN, copper was deposited into vias. Figure 4 showed the Back scattered and secondary electron SEM images of sample 3, respectively.

Figure 4 Images of Sample 3 under Potentiostatic mode

2.2 Galvanostatic Mode

Experiments were performed under galvanostatic mode to compare its characteristics of deposition to potentiostatic mode of operation. Figure 5 showed images of sample 2 prepared under galvanostatic mode. There are some defects in the right side image, which were caused by cutting the sample. The copper was pulled out of the vias.

Figure 5 Images of Sample 2 under Galvanostatic mode

Comparing the images in Figure 4 and 5, it is indicated that in the case of this experiment, the deposition under both modes looks very similar to each other.

3. EDAX

Figure 6 EDAX analysis of sample 2

The intermediate cuprous complex product that is formed due to complexation of cupric ions and acetonitrile is converted back to its reactant species either by oxidation or reaction with dissolved oxygen in the presence of acid. EDAX analysis was performed on samples deposited using CH3CN as a complexing ligand to detect deposits for impurities. The impurities could have been in the form an insoluble cuprous acetonitrile complex. Figure 6 shows EDAX scans for samples 2. This indicates that all samples deposited using acetonitrile as the complexing ligand are free of it. The peaks for Si and O are present due to the interaction between electrons and the parts other than copper inside of vias. The scale of vias is only 0.36um, while the interaction volume is much larger than it at 20 kV accelerating voltage. This can be further explained by the Electron Flight Simulation results.

4. Mapping

To further understand the distribution of copper deposited, Figure 7 gives the element distribution images. The green one is the image of Cu L-series, the purple represents Cu K-series, and the blue shows the distribution of Si.

Figure 7 Mapping of Sample 2

5. Electron Flight Simulation

As mentioned in the results of EDAX, Si and O peaks are present in the X-ray spectrum of deposited copper. The explaination is that the interaction volume is much larger than one via, signals produced by the interaction of electrons and Si, O are also collected. Using Electron Flight Simulation software, it is obvious that at 20 kV a.c. the interaction with Si is not negligible, shown in Figure 8. Figure 9 is the corresponding X-ray spectrum.

Figure 8 Electron Flight Simulation

Figure 9 X-ray Spectrum corresponding to Electron Flight Simulation

6. Colorization

Each image has two major colors. The green part is Silicon, and the other is copper.

Figure 10 Colorized image of original sample

Figure 11 Colorized image of sample 2

Figure 12 Colorized image of sample 3

Conclusions

The results of this entire work indicate that copper could be electroplated on silicon wafers with copper seed layer using acetonitrile (CH3CN) as complexing ligands to obtain superfilling. The deposition under potentiostatic and galvanostatic modes looks very similar to each other. EDAX result shows all samples deposited using acetonitrile as the complexing ligand are free of impurity.

Acknowledgements
I would like to thank Brian McIntyre for his guidance throughout this project. I also wish to thank Palash Bharadwaj for his great help. Thanks Prof. Jacob Jorne for providing me the samples.