Page 268 - A Practical Guide from Design Planning to Manufacturing

240 Chapter Eight

a form that is used to create the photolithography masks required to pat-
tern each layer during the manufacturing process. As a result, layout
specialists are often called mask designers. The area of the entire layout
will determine the die area, which makes layout a critical step in deter-
mining the cost of manufacturing a given product.
Mask designers are often more artists than engineers. Layout is not
something that can be analyzed easily with mathematical equations or
numeric analysis. Mask designers must be able to look at two-dimensional
drawings of layout and visualize the three-dimensional structures that
they will create. Creating a single transistor requires multiple layers of
material. Making the interconnections between the transistors requires
multiple levels of wires. Mask designers must picture in their minds how
the different physical pieces of a circuit will fit together.
The layout must faithfully reproduce the circuit desired and should
use no more area than required. At each layer there are design rules that
specify the minimum widths and spaces that can be reliably manufac-
tured. As if working on a three-dimensional jigsaw puzzle, the mask
designer must fit together the pieces needed by the circuit designer
while following the design rules of the process, which determine how the
pieces are allowed to go together. This jigsaw puzzle, however, has many
different possible solutions. Even a very simple circuit can be drawn in
layout in an enormous number of different ways. How the layout is
drawn will determine not only the die area required by the circuit, but
will also impact speed and reliability. In designing the circuit, assump-
tions were made about the maximum lengths of wires and amount of
capacitance on each node. If the layout is poorly done, these assumptions
may not be met, forcing the circuit to be redesigned. This can have a huge
impact on the schedule of a project, and because it affects schedule,
cost, and performance so strongly, creating layout is a critical step for
any integrated circuit design.

Creating Layout
Creating layout begins with a circuit schematic. The circuit designer will
have determined the number of NMOS and PMOS transistors required
as well as the desired length and width of each transistor and all the
needed connections between transistors. The mask designer constructs
the layout for this circuit beginning with the lowest layers and working
up. This is also the order the layers will be created in manufacturing
with each new layer being added on top of the last. Creating the layout
for transistors requires drawing the well, diffusion, and poly layers.
Figure 8-1 shows a cross section of the three-dimensional structure
of an NMOS transistor and the corresponding layout, which shows an
overhead view. Creating a MOSFET begins with defining a well region.