Layout design rules
Introduction
 Layout rules is also referred as design rules.
 It is considered as a prescription for preparing photomasks.
 Provides a link between circuit designer and processor engineer during
manufacturing phase.
 Design rules specify geometric constraints on the layout artwork.
• Objective:
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To obtain a circuit with optimum yield.
To minimize the area of the circuit.
To provide long term reliability of the circuit.
Design rules represent the best compromise between performance and yield:
• More conservative rules increase yield.
• More aggressive rules increase performance.
• Design rules represent a tolerance that ensures high probability of correct
fabrication - rather than a hard boundary between correct and incorrect
fabrication.
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Layout or Design Rules
• Two approaches to describing design rules:
• Lambda-based rules: Allow first order scaling by linearizing the resolution
of the complete wafer implementation.
 To move a design from 4 micron to 2 micron, simply reduce the value of
lambda.
 Worked well for 4 micron processes down to 1.2 micron processes.
 However, in general, processes rarely shrink uniformly.
 Probably not sufficient for submicron processes.
• Micron rules: List of minimum feature sizes and spacings for all masks,
e.g., 3.25 microns for contact-poly-contact (transistor pitch) and 2.75
micron metal 1 contact-to-contact pitch.
 Stated at some micron resolution, alpha() and beta ( ) rules. Basic feature
size is defined in terms of  while minimum grid size is described by .  and
 may related by a constant factor.
 Normal style for industry.
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Layout or Design Rules
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Lambda-based p-well rules
 These rules are only representative and are the result
of averaging a large number of processes.
 The rules are defined in terms of:
 Feature sizes
 Separations and overlaps.
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Mask No. 1:Thinox
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Mask No. 2: P-well
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Mask No. 3: Poly
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Mask No 4: P-plus
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Mask No 5: Contact
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Mask No 5: Contact
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Contact No 6: Metal
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CMOS Design Rules
 A layout resembles the top view of the IC
 The layout design is 2-dimentional from the viewpoint of an IC designer.
 in fact, designers normally do not control the depth dimension of an IC.
The depth of a transistor source or the thickness of a metal wire is
determined by the fabrication process.
 The layout designer only decides the dimensions and locations of the
transistors and interconnects them into the target circuit.
 The minimum feature size of a technology is typically denoted by the
narrowest width of a polysilicon wire that it can produce.
 For instance, if the narrowest polysilicon wire in a technology is 1m wide,
it is called 1 m technology.
 Design rules capture the physical limitations of a fabrication process.
 Design rules release designers from the details of fabrication so they can
concentrate on the design instead.
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CMOS Design Rules
 In -based design rules (developed by Mead and Conway) the minimum
feature size is 2.
 For a 2m technology, =1m
 Different fabrication technologies apparently require different rules.
 Drawbacks of scalable design rules: the layout produced according to
scalable design rules are often larger than necessary.
 For example, a minimum spacing of 4m may be ordered although in
reality 3.2m is sufficient.
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MOSIS Design rules
 Scalable -based design rules are used in MOSIS projects.
 http//:www.mosis.org
 A subset of MOSIS scalable CMOS (SCMOS) design rules is used to provide
an overview of their use.
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Layout Examples
Example 1: symbolic layout for and inverter
According to -based design rules, the smallest transistor channel is 2 long and 3
wide (the minimum width of diffusion region). However, in the following figure the
width of transistor has been increased to 4 so that a diffusion contact (4 X 4
required) can be readily made. This 2 X 4 transistor is referred as the minimum
size transistor.
An inverter formed of
minimum size transistors
is called a minimum size
inverter.
Area: 42 X 15= 6302
Stick diagram
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Layout Examples
Example 3.1 :Alternative layouts for inverter
Area of both: 40 X 18= 7202
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