Page 290 - A Practical Guide from Design Planning to Manufacturing

260 Chapter Eight

There has been more success in automating the placement and rout-
ing of custom-drawn layout cells. Synthesis and cell-based design flows
allow layout from custom-drawn libraries to be placed and connected by
automated tools. The library of cells itself must still be drawn by hand,
but once created the effort of using this layout is greatly reduced. Still,
the overall density achieved by using these tools is typically less than
that could be achieved by full custom layout. The time and effort saved
in layout can come easily at the cost of a larger die area. It seems likely
that there will always be a strong need for experienced mask designers
and hand-drawn custom layout.
Layout is the final design step before chips are manufactured. When
the layout is complete, it is used to create the photolithography masks,
and production of prototype chips begins. This date when the layout is
complete is called tapeout, since in the past the layout of the chip would
be copied on to magnetic tape to be sent to the fabrication facility. Tapeout
is one of the most important milestones in any chip design, but it is by no
means the last. The following chapters will describe the chip manufac-
turing, packaging, and testing and debugging, which must happen before
shipping customers.

Key Concepts and Terms
Bit pitch Power grid
Contact and via Physical, temporal, and logical
Drawn, schematic, and layout Shielding
transistor count Tapeout
Electromigration Transistor legging
Half and full shielding Well taps
Layout design rules

Review Questions

1. What is the difference between a contact and a via?
2. What does layout density measure? Why is layout density important?
3. Why should wide transistors be legged?
4. What are the uses of drawn transistor count and layout transistor
count?
5. What is the difference between device-limited and wire-limited layout?
6. What is the difference between physical, logical, and temporal
shielding?
7. [Bonus] Use colored pens or pencils to draw the layout for a two-input
NOR gate. Use a different color for each different material.