Processing Hundreds of Nanometres Thick Electrografted P4VP for High Aspect Ratio TSV Insulation
Authors: Dequivre Thomas , Bérubé Michael , Brisard Gessie M , Charlebois Serge A 
Published: February 2020

In 3D integration of microsystems, electrical insulation of High Aspect Ratio Through Silicon Via (HAR TSV) is a major challenge. This insulation is traditionally realized through the deposition of a thin SiO2 layer. Electrografted poly-4-vinylpyridine (P4VP) has been demonstrated to be considered as an alternative to SiO2 dielectric, traditionally used, for HAR TSVs insulation1. Electrografted P4VP has comparable electrical performances than SiO2 based dielectric and is capable to smoothly cover the scalloped side walls of the TSV with better coverage uniformity than most dielectrics considered for Via-Middle or Via-Last processes2.
Electrografting methods are electro-initiated processes. The commercial electrografting solution contains organic reactants in an aqueous acidic media: 4-nitrobezen diazonium (NBD) and monomers of 4 vinylpyridine (4VP). The electrografting mechanism of P4VP onto Si in aqueous media through reduction of diazonium salts is complex. However, a mechanism based on electrochemical initiation followed by a purely chemical polymerization has been proposed. In a first electro-initiated step, aryl radicals must be generated by the electrochemical reduction of NBD on the Si electrode. This can be achieved using different technics such as electrochemical reduction. The generated aryl radicals can covalently bond to the silicon surface through an electron transfer or can initiate radical polymerization of 4VP monomers into P4VP. Most of the published work about electrografting on silicon substrate is realized on oxide free Si-H functionalized surface. Such experimental condition do not usually permit an electrografted polymer to grow up to hundreds of nanometres, which is not suitable to meet electrical requirement for TSV insulation.
Experimentally, reverse potential pulse conditions applied on p-type Si-OH functionalized sample allow the electrografting process of P4VP to grow faster and the grafted layer to be thicker. This phenomenon is enhanced if the process is realized under illumination (with the proper wavelength source with the respects of the silicon band gap). One of the hypotheses to explain this increase of kinetic and thickness could be the rapid inversion of the surface charge during the reverse pulse: the polymer structure would stay open by the creation of channels in the film, through desorption of physisorbed species, allowing the reactive species to diffuse more easily from the electrografting solution to the silicon surface. This phenomenon could facilitate the generation of aryl radicals for the 4VP polymerization process to be maintained. And thus, for the P4VP electrografting technic to be a versatile method for HAR TSV electrical insulation.
We will present and discuss the electrografting experimental conditions and the impact of the surface preparation on the P4VP to understand their role on the kinetic and the thickness of the grafted P4VP.
Acknowledgement
Université de Sherbrooke, Natural Sciences and Engeeniring Research Council of Canada (NSERC), Institut National de la Recherche Scientifique (INRS), Teledyne Dalsa and aveni are gratefully acknowledged for their help and their financial support.
References
1. T. Dequivre, E. Al Alam, J. Plathier, A. Ruediger, G. Brisard, and S. Charlebois, ECS Trans., 69, 91–97 (2015).
2. C. Truzzi, F. Raynal, and V. Mevellec, 2009 IEEE Int. Conf. 3D Syst. Integr., 1–6 (2009).