In this course you will learn the following
As we studied in the last lecture, Layout rules are used to prepare the photo mask used in the fabrication of integrated circuits. The rules provide the necessary communication link between the circuit designer and process engineer. Design rules represent the best possible compromise between performance and yield.
The design rules primarily address two issues -
1. The geometrical reproductions of features that can be reproduced by mask making and lithographical processes.
2. Interaction between different layers
Design rules can be specified by different approaches
1. λ-based design rules
2. µ-based design rules
As λ-based layout design rules were originally devised to simplify the industry- standard µ-based design rules and to allow scaling capability for various processes. It must be emphasized, however, that most of the submicron CMOS process design rules do not lend themselves to straightforward linear scaling. The use of λ-based design rules must therefore be handled with caution in sub-micron geometries.
In further sections of this lecture, we will present a detailed study about λ-based design rules.
14.2 λ-based Design Rules
Features of λ-based Design Rules : λ-based Design Rules have the following features-
Guidelines for using λ-based Design Rules:
As, Minimum line width of poly is 2λ & Minimum line width of diffusion is 2λ
As Minimum distance between two diffusion layers 3λ
As It is necessary for the poly to completely cross active, other wise the transistor that has been created crossing of diffusion and poly, will be shorted by diffused path of source and drain.
λ-based Design Rules
Contact cut on metal
Contact window will be of 2λ by 2λ that is minimum feature size while metal deposition is of 4λ by 4λ for reliable contacts.
Two metal wires have 3λ distance between them to overcome capacitance coupling and high frequency coupling. Metal wires width can be as large as possible to decrease resistance.
Buttering contact is used to make poly and silicon contact.
Window's original width is 4λ, but on overlapping width is 2λ.
So actual contact area is 6λ by 4λ.
The distance between two wells depends on the well potentials as shown above. The reason for 8l is that if both wells are at same high potential then the depletion region between them may touch each other causing punch-through. The reason for 6l is that if both wells are at different potentials then depletion region of one well will be smaller, so both depletion region will not touch each other so 6l will be good enough.
The active region has length 10λ which is distributed over the followings-