Characterisation of copper inductors fabricated by dual damascene and electroplating techniques

Abstract

Spiral inductors were fabricated in copper to take advantage of its low resistivity. The single-layered square inductors, with number of turns varying from 2 to 7, were contacted by underpasses. The measured inductances were in the range of 2.8 to 8.5 nH. Inductances, quality factors, resonance frequencies, series resistances and substrate capacitances were extracted from the measured S-parameters. The performance of different inductor geometries was studied. Varying track width affects both the inductance and the resonance frequencies. Wires with smaller cross-section area have a slightly larger inductance because they generate more magnetic flux external to the wire. Varying track spacing also had an effect on the inductor performance due to cross-talk signals caused by the close proximity of the tracks. To improve the Q performance of the inductors, the thickness of the underlying SOG dielectric layers (κ = 3) was increased from 0.8 to 2 μm. Acceptable Q values of 9.5 to 12 were obtained at a frequency of 1.3 GHz. Insertion of a blanket ground plane below the dielectric caused the Q values to drop dramatically due to eddy currents formed in the ground plane. In order to break up the formation of these eddy currents, perforated ground shields were employed. This technique proved successful and Q factors were almost restored to their original values.