Controlling the oxygen content becomes a formidable challenge when using the traditional method of copper pre-oxidation to attach copper metal to AlN ceramic substrates. The issue results in copious amounts of copper oxide (CuO) formation. Excessive CuO causes voids to form at the interfacial junction, harming the long-term reliability of the copper-bonded substrate. To address these issues, this study employed the sol-gel technique, in which a copper-containing solution was applied to the AlN substrate via spin coating, followed by thermal treatment in a nitrogen-rich atmosphere. The goal was to create thin films of Cu2-xO to improve the adhesion between copper and AlN substrates. It was observed that the surface roughness of the resulting Cu2-xO thin film after 5 spin-coating cycles was the lowest (3.48 nm). Furthermore, at a fixed thickness of the Cu2-xO oxide layer (approximately 36 nm), insufficient oxygen content in the thin film heated at 250 °C prevented the formation of sufficient Cu–O eutectic liquid at high temperatures, resulting in insufficient copper-bonded substrate bonding strength (0.47 MPa). When the thin film was heated to 400 °C, the proportion of the CuO phase in the film increased, effectively increasing the bonding strength of the copper-bonded substrate (0.84 MPa).