A Selective Grafting Method of Polyacrylic Acid Insulation Films on Silicon and Aluminum Surfaces for TSV-CIS Package

With the development of miniaturization of electronic devices, the demand for through-silicon via (TSV) technology has become increasingly urgent. Insulation technology with function of preventing copper diffusion is a key issue for TSV quality and reliability. In this paper, a one-step selective grafting method was reported for covalently grafting polyacrylic acid (PAA) insulation films only on silicon (Si) surface without grafting on aluminum (Al) surface. This method is implemented in an acidic aqueous solution with 4-nitrobenzene diazonium tetrafluoroborate (NBD), hydrofluoric acid (HF), sodium tripolyphosphate (STPP) and acrylic acid (AA) monomers. The existence of STPP in the solution can not only make Si more susceptible to corrosion and further facilitate grafting reaction of PAA films on Si substrate, but also form a layer of sodium fluoroaluminate precipitated film on the Al surface and further avoid the grafting reaction caused by NBD and corrosion of Al by HF. In addition, the effect of surfactants on PAA film formation was studied. Experiments shows that the combination of Sodium dodecyl sulfate (SDS) and sodium lauryl sulfate (SLS) as surfactant can obtain PAA films with great compactness and uniform thickness. This one-step selective grafting method on Si and Al surfaces will have great application prospects for CMOS image sensors (CIS) package when the aspect ratio of TSV increases.