(Invited) Electro/Electroless Grafting of Organic Dielectric Film on Silicon Surface and Its Application on through Silicon Via
Authors: Li Ming , Hang Tao , Liu Yang , Zhang Junhong , Ling Huiqin , Hu Anmin , Gao Liming 
Published: February 2020

With the rapid development of electronic industry, the packaging technology requires higher density and integration, which is a great challenge for semiconductor manufacturing. In order to continue the acknowledged “Moore's law”, the three-dimensional through silicon via (TSV) packaging of stacked chips has become the focus of attention, providing fresh ideas for the industry. The preparation of insulators is one of the key technologies for TSV. However it is difficult for conventional methods to prepare SiO2 insulators with high coverage in TSV with small size and high aspect ratio. In addition, higher performance of TSV proposes a big challenge for SiO2.

Organic polymers can replace SiO2 used as an insulator for TSV, because their lower dielectric constant can greatly improve the properties of TSV. Electrografting is a novel technology for polymer film preparation, which has the advantages of being simple, low cost and does not required special equipment.. In recent years, the modification of the Si surface with the electrochemical reduction of diazonium salts has become attractive because it can be performed in aqueous media, which is more acceptable for industrial applications. In the best understood reaction, a diazonium salt with an electrolyte such as tetrabutylammonium cations is reduced by an externally applied potential, typically at around −1 V versus the saturated calomel electrode.

In our study, the electrografting technology is used to graft polymer film onto a Si surface, and its mechanism and the effects of pulsed voltage and composition of the solution are investigated. In the process of electrografting, 4-nitrobenzene diazonium (NBD) tetrafluoroborate is reduced by electrons provided by an electric source, forming nitrophenyl radicals. These radicals covalently bond to the Si surface to form a polynitrophenyl (PNP) primer layer, and initiate the polymerization of vinyl monomers. The radical-terminated polyvinyl chains formed in the solution and nitrophenyl radicals are then added to the aromatic rings of the primer layer to form the expected polyvinyl film. Consequently the grafted film is composed of PNP and the polymer of monomers.

Furthermore, we propose an easier chemical grafting method based on the finding that diazonium salt can be spontaneously reduced at the Si surface when HF is present in the aqueous solution even without an externally applied potential., and study the influence factors of structure, morphology and component of the polymer film. This process is conducted in acidic aqueous media containing HF. From the mechanistic point of view, Si atoms at the Si surface are oxidized by HF, providing electrons for the reduction of NBD cations at the open circuit potential. The mechanism of the grafting of the polymer of monomers is the same as that of an electrografting process.

The polymer films prepared by either the electrografting or chemical grafting methods have good adhesion, dielectric properties and thermal properties. Then the polymer film is applied in TSV. As far as we know, insufficient film thickness and low coverage are still serious problems for plasma-enhanced chemical vapor deposition methods. However, both electrografting and chemical grafting overcome these problems. In addition, the narrow diameter and high aspect ratio (≥1:10) are not limitations for grafting methods. Poly(methyl methacrylate) film has a coverage of 76%, while poly(acrylic acid) film has a coverage as high as 100%.