Page 288 - A Practical Guide from Design Planning to Manufacturing

258 Chapter Eight

to be increased to allow more room for routing wires. Cross talk noise is
reduced at less cost by using temporal or logical shielding.
The worst-case noise will be induced in a line when both of its neigh-
bors switch in the same direction at the same time. Physical shielding
prevents this by using power lines, which never switch. Temporal and
logical shielding use signals that never switch in the same direction at
the same time. For example, temporal shielding might route, as neigh-
bors to a sensitive line, a signal that switches only during the high
phase of the clock (SigBH) and a signal that switches only during the
low phase of the clock (SigCL). Because the two neighbors never switch
at the same time, the worst-case noise on SigA is reduced. SigA is effec-
tively half shielded. Alternatively, the neighbors signal B (SigB) and its log-
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ical opposite Sig B could be chosen. These signals may switch at the same
time but always in opposite directions. The noise pulling in opposite direc-
tions will cancel each other out and SigA is effectively full shielded. Even
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if SigB and Sig B might switch at slightly different times because of dif-
ferent delay paths, the worst case of two neighbors switching in the same
direction is avoided. By allowing signal lines to be routed next to SigA, tem-
poral and logical shielding reduce sensitivity to noise without the loss of as
many wiring tracks. When drawing interconnects, in addition to noise,
reliability must be considered.
When a thin wire carries large amounts of current, it will become hot.
Too much heating causes wires to fail over time just like the filament in
a light bulb. As the wire is repeatedly heated and cooled, it expands and
contracts. Because not all the materials in an integrated circuit expand
and contract with temperature at the same rate, stress is put on all cir-
cuit elements by this heating. Eventually breaks in the wire or between
the wire and its contacts and vias are created, and the circuit will fail.
Processes that use low-K insulation material between wiring levels may
be more susceptible to these problems. Low-K insulation reduces the
effective capacitance of the wires, making circuits faster, but these mate-
rials also tend to be soft. This makes them less able to hold wires firmly
in place as they try to flex while being heated. To avoid the problem of
overheating wires, the amount of current a signal wire will carry must
be considered in sizing interconnects. An extremely wide transistor driv-
ing a long minimum width wire risks creating a failure after a very short
time. A maximum current density for each wiring layer may be given as
a guideline to mask designers in sizing interconnects.
The problem of high current densities is particularly bad in wires that
always carry current in the same direction. The atoms that make up the
wire tend to be pushed out of thin spots in the wire where current is the
strongest, just like rocks being pushed along in a fast running stream. This
phenomenon is called electromigration and causes thin spots in the wire