Potassium hydroxide surface modification for low temperature Cu/SiO2 hybrid bonding

This paper reports a novel plasma-free surface modification approach for low temperature Cu/SiO2 hybrid bonding. A 1-M KOH solution was employed, which acts as a hydroxyl group supply and further modifies the surface of SiO2 dielectric layer by the Si–OH transformation. Such a KOH solution makes the Cu surface more hydrophilic without increasing its surface roughness. We found that the contact angles significantly decreased after dipping into 0.5–3 M of KOH. A contact angle below 5° was achieved with > 1 M KOH. The X-ray photoelectron spectroscopy (XPS) results showed an increase in Si–OH (7.8% to 40.6%) and decreases in Si–O–Si (64% to 55%), Si–O and CO (28.2% to 4.4%) intensities after KOH wet treatment. Furthermore, the KOH treatment almost does not etch nor roughen the Cu surfaces. Therefore, the bonding temperature could be lowered to 200 °C while keeping high bonding strength and reliability performance. A low specific contact resistance of 5  × 10−8 Ω.cm2 was obtained. Results obtained by scanning electron microscope (SEM) and transmission electron microscope (TEM) show the excellent bonding quality of both SiO2–SiO2 and Cu–Cu interface. Additionally, energy-dispersive X-ray spectrometry (EDS) analyses show no signs of oxides and potassium contaminations at the bonding interfaces.