Circuit Layout

Table of Contents - 1 2 3 4 5 6 7 8 9 R S

Stick diagram

A "stick" diagram is a simplified layout form which does contain the information related to each of the process steps, but does not contain the actual size of the individual features. Instead the features are represented by simple lines hence the name "stick" diagram.

The stick diagram can easily be drawn by hand and is a handy intermediate form between the circuit diagram and the physical layout since it can easily be modified and corrected. It can therefore be used to anticipate and avoid possible problems when laying out the circuit.

The circuit diagram and stick diagram of a gated R/S flipflop implemented using an enhancement load technology is provided below as an example:

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Fig.9.1

Circuit diagram (left) and the corresponding "stick" diagram (right) of a gated R/S flipflop

Shown are the six transistors which can be identified by the red squares which indicate the presence of a gate oxide. The diffused areas are show in green and the aluminum metal is shown in blue. Contact between the aluminum metal and the diffused regions is made by a via hole as indicated by a black circle. The color code used is listed in the table below:

Description	Color/shape
Diffusion	Green lines
Gate oxide	Red squares
Via hole	Black circles
Aluminum metal	Blue lines

Physical layout using a metal gate process

The physical layout of the circuit requires the physical dimensions of each feature. The objective is typically to layout the circuit using a minumal area. To this end one first lays out an invertor or gate which is frequently used in the circuit. Multiple input NAND gates will take the most vertical space and will determine the spacing between the power supply line (Vdd) and ground (GND). Once the individual invertors and gates are put in place the electrical connections can be made using the aluminum wiring level and diffusion/aluminum cross-overs.

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Fig.9.2

"Stick" diagram (left) and the corresponding physical layout (right) of a gated R/S flipflop

The physical layout was layed out rather generously to avoid overlapping lines. A more compact layout is possible. The layout also contains two cross-overs; one to connect the two inverters into a flipflop and another one to connect the output of the flipflop to a large area contact pad.

A compressed version of the same circuit is shown in the figure below as well as a design using filled shapes rather than contours only.

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Fig.9.3

A compressed physical layout (left) and layout with filled shapes (right) of a gated R/S flipflop

A description of the color code used for the layout using filled shapes is provided in the table below:

Color	Description
Blue	Aluminum metal only
Black	Via hole between a diffused region and aluminum metal
Red	Gate oxide with aluminum metal on top
Red violet	Gate oxide on a diffused region with aluminum metal on top
Green	Diffused region
Dark green	Diffused region with aluminum metal on top