A low temperature bonding metal such as Sn with a melting temperature of 232°C [1] can be used to reduce the process temperature of metal bonding in hybrid bonding. In the presented work, ultra-thin Cu - Sn bilayer stack in a 5 µ m x 5 µ m × 0.8 µ m SiO2 pad has been fabricated using an immersion electroless deposition of Sn on Cu. Intermetallic growth of this stack has been studied at room temperature (RT) for many days i.e., 0 - 84 days and at different annealing temperatures i.e., 80°C-250°C for 10 seconds. Stacks with three different Cu to Sn thickness ratios of 1.5, 2 and 2.5 have been considered for this investigation. Sn thicknesses of230 nm, 270 nm and 320 nm have been electroless plated for this study. Intermetallic growth study of the top surface of these pads has been done to understand how much Sn is left (not transformed into intermetallic compounds (IMCs)) for the following bonding process. After 84 days of RT aging, it has been observed that 55.2 % of the pad's top surface (i.e., 13.8 µ m2 of 25 µ m2) reacted to form IMC in sample with 230 nm thick Sn. Whereas 48.8 % of the pad's top surface was consumed into IMC in sample with 320 nm thick Sn. Here, after 84 days of RT aging, Cu6 Sn5 IMC is observed. In the case of higher temperatures annealing, negligible change in intermetallic growth has been observed for the samples annealed at 150°C as compared to that of 0°C for all the three above mentioned Sn thicknesses. However, nearly 90 % of the pad's top surface reacted to form IMCs for all Sn thicknesses when annealed at 250°C for 10 seconds. Cu6 Sn5 IMC along with pure Sn have been observed in samples annealed at 0°C for 10 seconds. In the samples annealed at 25 0°C for 10 seconds, most of the Sn is consumed into layers of Cu6 Sn5 and Cu3 Sn IMCs.