The physical mask layout of any circuit to be manufactured using a particular process must conform to a set of geometric constraints or rules, which are generally called layout design rules.

Observing the layout design rules significantly increases the probability of fabricating a successful product with high yield.

The design rules specifies the minimum dimension that can be safely transferred to semiconductor material. They provide communication link between circuit designer and process engineer during manufacturing phase.

If Interlayer registration is too close then it –

a. May merge and lead to short between circuit networks

b. May be affected by vertical topology of a process

Design rules primarily addresses two issues

a. The geometrical reproduction of features that can be reproduced by the mask making and lithographical process

b. The interaction between two layers

There are two types of design rules –

1. Micron rules

a. Micron rules, in which the layout constraints such as minimum feature sizes and minimum allowable feature separations, are stated in terms of absolute dimensions in micrometers.

b. They are given as a list of minimum feature sizes and spacing for all masks required in a given process.

c. Industry uses micron rules to ensure that the circuits built are as small as possible.

d. However, the layouts are not portable and are complex especially for deep submicron designs.

2. Lambda rules

a. Since design rules differ from company to company porting of design is a challenging issue. Therefore scalable design rules as a function of single parameter called lambda λ allows linear and proportional scaling of all geometric constraints.

b. Lambda specifies the maximum alignment of a feature from its intended position on the wafer. Therefore, maximum misalignment of two features on different mask layer is 2λ.

c. The purpose of designing lambda property is to make the design itself independent of both process & fabrication.

d. Typically feature size of process is set to 2λ. For eg. 1.2 μm process, λ=0.6 μm