X-ray Photoelectron Spectroscopy (XPS) Investigations to Monitor the Surface Chemistry during Palladium-Free Colloidal Copper Activation

Colloidal copper-based activators, which are free of palladium, are emerging as a viable alternative to traditional ionic palladium systems for electroless copper plating. Recent advancements demonstrate that such colloidal copper activators can effectively initiate electroless copper deposition on various materials, including SAP-relevant substrates, with performance comparable to ionic palladium. Notably, good copper cleanliness is demonstrated post-activation and reliability testing reveals no interconnect defects (ICDs). These promising findings motivate further research to comprehensively understand the surface chemistries involved in the complete colloidal copper activation process.To this end, we have recently used sequential X-ray photoelectron spectroscopy (XPS) analyses to monitor the chemical changes at the surface of copper during consecutive chemical treatment steps. By drawing from our previous studies, we will first present a review highlighting the efficacy of XPS analysis for monitoring the cleanliness of copper surfaces during the colloidal copper pretreatment and activation process. In our previous XPS investigations, we emulated the surfaces of copper interconnects, within through holes (TH), or target pads, within blind micro vias (BMVs), by using copper-coated silicon wafers as a substrate. XPS analysis was performed after each consecutive cleaning, conditioning, and colloidal copper treatments (i.e. cleaner, conditioner, acid rinse, selector, colloidal copper activator). It was shown that N 1s spectra can be used to identify the functionality of various organic components on copper surfaces after each treatment step. We also present how the Cu LMM Auger spectra can be used to track the changes in the speciation of copper at the surface. The combination of surface treatment and XPS analysis has enabled us to better understand the role of the different treatment chemistries. For example, we found that an acid rinse treatment step after the conditioning step is most effective at removing the conditioner chemistry from copper surfaces. However, the acid cleaner also drives copper oxidation and hydrolysis reactions. Subsequently, by combining the acid rinse with an additional selector treatment step, we find from the XPS analysis that the selector can remove unwanted copper oxides from the surface and functions to stabilize metallic Cu
⁰
from substantial oxidation and hydrolysis by oxygen in air and rinse water. In these previous investigations we established the specific roles of each pretreatment step and their impact on the surface chemistry of copper at nanometer resolutions. This provided us with an advantage to optimize each treatment step and ultimately strive towards enhancing the reliability of palladium-free electroless copper activation processes.We have continued this approach, and in this study, we utilized XPS analysis to investigate the surface chemistry and air stability of the palladium-free colloidal copper activator after deposition, rinsing, and air exposure, exclusively. To conduct these investigations, we used gold-coated silicon wafers treated with our colloidal copper activator. As previously shown, we used the Cu LMM Auger spectra to track the time-dependent changes in the relative quantities of copper species (oxidation states) following consecutive intervals of air exposure. We identify that the surfaces of the colloidal copper activators are predominantly composed of metallic Cu
⁰
after rinsing and 6 hours of air exposure. However, with prolonged air exposure (24 to 48 hours) significant copper oxidation and hydrolysis occurs, which slowly converts the colloidal copper surface from mostly metallic Cu
⁰
to predominantly Cu(OH)
2
. As a comparison we prepared samples using a conventional ionic palladium-based activator system (Neoganth MV series). Here we analyzed the activator surface chemistries, before and after the reducer treatment. In this case, we used the Pd 3d spectra to monitor the reduction of ionic Pd
²⁺
ions to Pd
⁰
metal following reducer treatment. In this case the palladium remained metallic even after prolonged air exposure (24 hours). From these XPS analyses we can predict the relative lifetime and stability of copper nanoparticles aft.er surface activation. We can now leverage these new findings to further improve the handling and industrial-scale utilization of electroless copper activators based on colloidal copper.