Integrated inductors are typically realized as metal spirals. Owing to the mutual coupling between every two turns, spirals exhibit a higher inductance than a straight line having the same length. To minimize the series resistance and the parasitic capacitance, the spiral is implemented in the top metal layer which is the thickest.

Consider the three turns as AB, BC, CD denoting their individual inductances by $L_1, L_2 $ and $L_3$ respectively. Also, let the mutual inductance between $L_1$ and $L_2$ be $M_{12}$, etc. Thus, the total inductance is given by -

$L_{tot} = L_1 + L_2 + L_3 + M_{12} + M_{13} + M_{23}$

The equation suggests that the total inductance rises in proportion to the square of the number of turns.

Two factors that limit the growth rate are :

1. due to the geometry's planar nature, the inner turns are smaller and hence exhibit lower inductances.

2. the mutual coupling factor is only about 0.7 for adjacent turns, falling further for non-adjacent turns.

A two dimensional square spiral is fully specified by four quantities: the outer dimension, $D_{out}$, the line width, W, the line spacing, S, and the number of turns, N. The inductance primarily depends on the number of turns and the diameter of each turn, but the line width and spacing indirectly affect these two parameters.

Our qualitative study of the square spiral inductors reveals some degrees of freedom in the design, particularly the number of turns and the outer dimension. But there are many other inductor geometries that further add to the design space. Figure shows a collection of inductor structures encountered in RF IC design. We can observe:

(1) The structures in (a) and (b) depart from the square shape,

(2) The spiral in (c) is symmetric,

(3) The stacked geometry in (d) employs two or more spirals in series,

(4) The topology in (e) incorporates a grounded shield under the inductor and

(5) The structure in (f) places two or more spirals in parallel.