Impact of the CMP process on the electrical properties of ultra low k porous SiOCH

The impact of chemical mechanical polishing (CMP) on SiOCH films (thickness = 300 nm) for the 32-45 nm node Cu-interconnect process is investigated by low-frequency dielectric spectroscopy and thermally stimulated depolarization current (TSDC). After CMP process, the dielectric permittivity is degraded of about 25% in the whole range of the investigated frequency (10⁻¹ Hz-100 kHz). In a same way, the dielectric losses tan δ increase at the lowest frequencies. An annealing (300 °C during 20 min) carried out after CMP induces a reduction of the dielectric permittivity without however reaching the value of initial as-deposited material. In agreement with other published papers focusing on the damage caused by the CMP, OH bonding and water adsorption due to surfactants explain the degradation of these dielectric properties. The identification of OH bonds and an increase in the intensity of CH_x in the 2800-3050 cm⁻¹ range after CMP seems to confirm this point. The moderate temperature of annealing, used to restore layers and to avoid the degradation of copper lines, suppresses the physisorbed water but not the chemisorbed water. TSDC measurements confirm that dipolar relaxation, due to water in the material, result in a peak of relaxation at a temperature around 175 °C.