Page 289 - A Practical Guide from Design Planning to Manufacturing

Layout 259

with the highest current density to gradually become thinner. As the wire
thins, current density is increased, and the process accelerates. Thin spots
in the wire become even thinner until eventually a break is created. This
is an additional reason to draw supply and ground lines wider than min-
imum. The same kinds of failure happen in the vias that connect differ-
ent levels of wires. The more current carried, the more vias required for
reliability.
The need for high speed, immunity to noise, and long-term reliabil-
ity make quality layout much more than simply the layout with the
smallest area.

Conclusion
Layout is one of the most time-consuming aspects of integrated circuit
design and as a result has been the focus of many efforts at automation.
Many of these efforts have met with limited success and much of the job
of mask design is still very much a manual process. A consistent goal of
layout automation has been to automate the process of compacting from
one manufacturing generation to the next. Effort for compaction designs
would be dramatically reduced if the layout of the previous design could
be somehow automatically converted to the new process. If a new process
were created using all the same design rules as the previous generation
but with the basic feature size represented by l simply made smaller,
automatic conversion of layout would be trivial. All the dimensions of
the layout could be shrunk by the same amount to create new layout for
the new process. In reality, this is rarely the case.
Scaling all the design rules by the same amount allows the rule that
could be reduced the least to determine the scaling for all the other rules.
Inevitably, each new process generation finds some design rules more dif-
ficult to scale than others. Smaller metal 1 widths and spaces may be
more easily achieved than smaller poly widths and spaces or vice versa.
More area reduction is achieved by individually scaling each design rule
as much as is practical, but this leads to new process design rules, which
are not a simple constant scaling of the previous generation. If all the
dimensions can not be scaled by the same amount, automatic layout
compaction becomes much more difficult.
Making matters worse, new process generations often add new design
rules that were not previously needed. Perhaps to achieve better control,
a new process might specify that all poly gates must be routed in the same
direction whereas an earlier generation allowed any orientation. Perhaps
all metal 1 wires must be one of two specific widths whereas the earlier
generation allowed any width above a minimum value. These new rules
may allow the new process to achieve higher densities and higher yields,
but they can easily make automated compaction impossible.