Surface Passivation of Polymer Based Redistribution Layers for Area Selective Deposition of an Oxygen Barrier
Authors: Brady-Boyd Anita , Chery Emmanuel , O'Connor Robert , O'Neil Darragh , Armini Silvia 
Published: February 2024

Heterogeneous 3D integration is being hailed as the driver for new technologies due to its stacked device architecture. This increases functionality as the number and length of interconnecting wires decreases, in turn decreasing power consumption. In these advanced packaging technologies, the metal redistribution layers (RDLs) and the insulating polymer surrounding them are crucial components on which stringent reliability requirements are placed. The metal RDL’s are necessary to reroute signals from device level to packaging level. The polymer insulator, used as an alternative to traditional dielectric materials, must (i) be photosensitive to be compatible with lithography processing steps, (ii) possess high mechanical strength, (iii) exhibit good electrical performance, (iv) have a high thermal stability and glass transition temperature, and crucially (v) it must not be hygroscopic. Low or no moisture uptake of the polymer is essential as any moisture can cause mechanical and electrical reliability issues. Absorbed moisture can cause oxidation of Cu lines leading to the well-known problem of Cu diffusion resulting in voids, contact issues, delamination, electromigration and higher leakage currents.
Area selective deposition (ASD) could greatly benefit these RDL’s by preferentially depositing an oxygen barrier to stop the oxidation of the Cu lines. The proprietary polymer used in this work is part of the WPR series by JSR Corporation and is a thermosetting phenol-based polymer which displays a high mechanical stability as it contains rubber nanoparticles, 70 nm in diameter. These are added to the polymer to increase the toughness and to prevent cracks and fractures forming and propagating through the film. However, achieving ASD with this polymer is difficult as the polymer is designed to have good adhesion properties so it will easily adhere to the Cu in the metal lines, and there are two materials of interest to passivate, the polymer itself and the rubber nanoparticles.
In this work the surface is passivated by utilizing a polymerizing octafluorocyclobutane (C4F8) plasma which creates a Teflon-like film on the surface. The surface morphology and hydrophobicity are investigated to assess the best plasma exposure time, which was found to be 30 s. Surface chemistry from XPS analysis reveals surface modification through the deposition of fluorine rich groups from the 30 s C4F8 plasma exposure, Figure 1(a). Water contact angle measurements reveal an initial hydrophobic surface (~94°) but this hydrophobicity decreases over the course of several seconds. Following the 30 s C4F8 plasma exposure, the hydrophobicity increases significantly to above 110° and remains stable as shown in Figure 1(b). Surface morphology studies carried out by AFM demonstrates no increase in surface roughness following plasma exposure, Figure 1(c) and (d). ALD tests show the process to be selective to the tetrachloride family of ALD precursors. No selectivity was observed with tetrakis (dimethylamino) or isopropoxide based precursors. This process has the capability to block approximately 2 nm of TiO2 and HfO2 from these tetrachloride precursors. Studies of the same process on Cu substrates show that the metal surface is only temporarily modified, and ALD growth can still take place. Reliability stress and corrosion tests were performed to assess if the selectively deposited barriers could prevent Cu oxidation.
Figure 1(a) C 1s XPS spectra showing difference before and after plasma treatment, (b) WCA value increases following plasma treatment, and AFM images in (c) and (d) show no change in the surface following plasma exposure.

Figure 1