Radio Frequency Integrated Circuit Design - Webs

The Use and Design of Passive Circuit Elements in IC Technologies
diffuse into the silicon. When metals such as copper diffuse into silicon, they
cause junctions to leak, seriously impairing the performance of transistors. A
contact layer is used to connect this tungsten layer to the active circuitry in the
silicon. Higher levels of metal can be connected to adjacent layers using conductive
plugs that are commonly called vias. Whereas metal can be made in almost
any shape desired by the designer, the vias are typically limited to a standard
square size. However, it is possible to use arrays of vias to reduce the resistance.
Higher metal layers are often made out of aluminum, as it is much less
resistive than tungsten. In some modern processes, copper, which has even lower
resistance than aluminum, may be available. The top level of metal will often
be made much thicker than the lower levels to provide a low resistance routing
option. However, the lithography for this layer may be much coarser than that
of underling layers. Thus, the top layers can accommodate a lower density of
routing lines.
5.3 Sheet Resistance and the Skin Effect
All conductive materials can be characterized by their resistivity � or their
conductivity �. These two quantities are related by
� = 1
�
97
(5.1)
Resistivity is expressed as ohm-meters (�m). Knowing the geometry of a
metal and its resistivity is enough to estimate the resistance between any two
points connected by the metal. As an example, consider the conductor shown
in Figure 5.2. To find the resistance along its length, divide the resistivity of
the metal by the cross-sectional area and multiply by the length.
R = �L
Wt
Figure 5.2 Rectangular conductor with current flowing in the direction of L.
(5.2)