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.
