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Copper RF Inductors

(by Dr. Brian H. W. Toh)

Integrated inductors are desirable for high performance radio frequency and microwave front-end integrated circuits such as voltage controlled oscillators (VCOs), low noise amplifiers (LNAs), filters and interstage and output impedance matching networks in power amplifiers. The inductance of an inductor is based on the combination of self inductance of a wire and the mutual inductance between a pair of wires. Spiral inductors were fabricated in copper to take advantage of its low resistivity. Single-layered square inductors, with number of turns varying from 2 to 7, were contacted by underpasses. Measured inductances were in the range of 2.8 to 8.5nH.

The performance of different inductor geometries was studied. Varying track spacing 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 was increased from 0.8 to 2µm. Acceptable Q values of 9.5 to 12 were obtained at a frequency of 1.3GHz.  The measured inductances were in the range of 2.8 to 8.5nH, when the coils were varied from 2 to 7 turns. However, the value of Q reduces as the number of turns increases, because of the increased series resistance.

Dependence of inductance with number of turns.
Dependence of Q-factor with varied inductor turns.

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.