Radio Frequency Integrated Circuit Design - Webs

100 Radio Frequency Integrated Circuit Design
5.4 Parasitic Capacitance
Metal lines, as well as having resistance associated with them, also have capacitance.
Since the metal in an IC technology is embedded in an insulator over
a conducting substrate, the metal trace and the substrate form a parallel-plate
capacitor. The parasitic capacitance of a metal line can be approximated by
C = � o � r A
h
(5.5)
where A is the area of the trace and h is the distance to the substrate.
Since metal lines in ICs can often be quite narrow, the fringing capacitance
can be important, as the electric fields cannot be approximated as being perpendicular
to the conductor, as shown in Figure 5.3.
For a long line, the capacitance per unit length, taking into account
fringing capacitance, can be determined from [2]
C = �o �r� W
h
W
+ 0.77 + 1.06� h � 1/4
+ 1.06� t
h� 1/2
� (5.6)
We note that the terms in the square brackets are unitless; the final
capacitance has the same units as � o (� o is 8.85 × 10 −12 F/m, � r for SiO 2 is
3.9). The first term accounts for the bottom-plate capacitance, while the other
three terms account for fringing capacitance. As will be seen in Example 5.2,
wider lines will be less affected by fringing capacitance.
Note there is also capacitance between lines vertically and horizontally. A
rough estimate of capacitance would be obtained by using the parallel-plate
capacitance formula; however, this omits the fringing capacitance, so would be
an underestimate. So what is the effect of such capacitance? For one, it can
lead to crosstalk between parallel lines, or between lines that cross over. For
parallel lines, crosstalk can be reduced by further separation, or by placing a
ground line between the two signal-carrying lines.
Figure 5.3 Electric field lines showing the effect of fringing capacitance.