Radio Frequency Integrated Circuit Design - Webs

The Use and Design of Passive Circuit Elements in IC Technologies
of the 90° bends adds unnecessary resistance to the structure, and as the structure
is made circular, the performance will improve.
Some guidelines for optimum layout will now be provided. These rules
are based on considerations of the effect of geometry on the equivalent model
shown in Figure 5.12.
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1. Line spacing: At low frequencies (2 GHz or less), keep the line spacing
as tight as possible. At higher frequencies, due to coupling between
turns, larger spacing may be desirable.
2. Line width: Increasing metal width will reduce the inductance (fewer
turns in a given area as well as less inductance per unit length) and
will decrease the series resistance of the lines at low frequencies. Large
inductance area means bigger capacitance, which means lower selfresonance,
and more coupling of current into the substrate. Therefore,
as W goes up, inductance comes down and the frequency of Q peak gets
lower (and vice versa). Line widths for typical 1- to 5-nH inductors
in the 2- to 5-GHz range would be expected to be from 10 to 25 �m.
3. Area: Bigger area means that more current is present in the substrate,
so high-frequency losses tend to be increased. Bigger area (for the
same line width) means longer spirals, which means more inductance.
Therefore, as the area goes up, inductance goes up, and the frequency
of Q peak gets lower (and vice versa).
4. Number of turns: This is typically a third degree of freedom. It is usually
best to pick fewer rather than more turns, provided that the inductor
does not get to be huge. Huge is, of course, a relative term, and it is
ultimately up to the designers to decide how much space they are
willing to devote to the inductor layout. Inner turns add less to the
inductance but more resistance, so it is best to keep the inductor hollow.
By changing the area and line width, the peak frequency and inductance
can be fine-tuned.
5.18 Isolating the Inductor
Inductors tend to be extremely large structures, and as such they tend to couple
signals into the substrate; therefore, isolation must be provided. Typically, a
ring of substrate contacts is added around each inductor. These substrate contacts
are usually placed at a distance of about five line widths away from the inductor.
The presence of a patterned (slotted) ground shield, discussed in the next section,
may also help in isolating the inductor from the substrate.