Radio Frequency Integrated Circuit Design - Webs

The Use and Design of Passive Circuit Elements in IC Technologies
For 1-�m-thick aluminum, a typical value for maximum current would
be 1 mA of dc current for every micrometer of metal width. Similarly, a
2-�m-thick aluminum line would typically be able to carry 2 mA of dc current
per micrometer of metal width. The ac current component of the current can
be larger (a typical factor of 4 is often used). We note that other metals like
copper and gold are somewhat lower in resistance than aluminum; however,
due to better metal migration properties, they can handle more current than
aluminum.
Example 5.4 Calculating Maximum Line Current
If a line carries no dc current, but has a peak ac current of 500 mA, a 1-�mthick
metal line would need to be about 500 mA/4 mA/�m = 125 �m wide.
However, if the dc current is 500 mA, and the peak ac current is also 500 mA
(i.e., 500 ± 500 mA), then the 500-�m-wide line required to pass the dc current
is no longer quite wide enough. To cope with the additional ac current, another
125 �m is required for a total width of 625 �m.
Current limitations have implications for inductors as well (these will be
considered next). Integrated inductors are typically 10 or 20 �m wide and can
therefore handle only 10 to 40 mA of dc current, and up to 160 mA of ac
current. This obviously limits the ability to do on-chip tuning or matching for
power amplifiers or other circuits with high bias current requirements.
5.7 Poly Resistors and Diffusion Resistors
Poly resistors are made out of conductive polycrystalline silicon that is directly
on top of the silicon front end. Essentially, this layer acts like a resistive metal
line. Typically, these layers have a resistivity in the 10 �/� range.
Diffusion resistors are made by doping a layer of silicon to give it the
desired resistivity, typically 1 k�/� or more, and can be made with either p
doping or n doping as shown in Figure 5.5. If n doping is used, then the
structure can be quite simple, because the edge of the doping region will form
a pn junction with the substrate. Since this junction can never be forward
biased, current will not flow into the substrate. If, however, p doping is used,
then it must be placed in an n well to provide isolation from the substrate.
5.8 Metal-Insulator-Metal Capacitors and Poly Capacitors
We have already discussed that metal lines have parasitic capacitance associated
with them. However, since it is generally desirable to make capacitance between
metal layers as small as possible, they make poor deliberate capacitors. In order
to improve this and conserve chip area, when capacitance between two metal
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