LAMBDA BASED DESIGN RULES

DESIGN RULESAllow translation of circuits (usually in stick diagram or

symbolic form) into actual geometry in siliconInterface between the circuit designer and process

engineerDesign rules are a set of geometrical specifications that

dictate the design of the layout masksA design rule set provides numerical values

For minimum dimensionsFor minimum line spacings

Design rules must be followed to insure functional structures on the fabricated chip

Design rules change with technological advancesRules constructed to ensure that design works even

when small fab errors (within some tolerance) occur

Design rules specifies the constraints on layout

Two types of layout constraints are: Resolution constraints Alignment/overlap constraints Resolution constraints specify: smallest width feature that can be used

eg.channel width,wire length

smallest spacings that will guarantee no shortcuts

Alignment/overlap constraints specify; alignment between the layers minimum overlaps or overhangs

Design rules can be expressed in absolute physical units… eg. Poly width .3µm

poly spacing .45µm metal width .45µm metal spacing .45µmScalable design rules express dimensions in

normalised units called lambda (λ)

The design rules are usually described in two ways :

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, or,λ

Lambda rules, which specify the layout constraints in terms of a single parameter (λ) and, thus, allow linear, proportional scaling of all geometrical constraints.

Lambda-based layout design rules were originally devised to simplify the industry-standard micron-based design rules and to allow scaling capability for various processes.

Lambda Based Design Rules Design rules based on single parameter, λSimple for the designerWide acceptanceProvide feature size independent way of

setting out maskIf design rules are obeyed, masks will produce

working circuitsMinimum feature size is defined as 2 λPrevents shorting, opens, contacts from

slipping out of area to be contacted

Lambda design rules are based on a reference metric λthat has units of um.

All widths, spacing and distances are written in the form Value = m λ

Where m is scaling multiplier. <e.g.> λ= 1um w = 2 λ=2um s = 3λ=3umThree major rules:

Wire width: Minimum dimension associated with a given feature.

Wire separation: Allowable separation.Contact: overlap rules.

Min. Width : The min. width of the line (layer)

<Example> Wpoly(min.) = 0.5um

Min. Space : The min. spacing between lines with same material

<Example> Spoly-poly(min.) = 0.5um

<Min. Extension : The min. extension over different layers

<Example> Poly-gate extension over diffusion area = 0.55um

Min. Overlap : The overlap between different layers

<Example> Poly1 overlap Poly2 min. = 0.7um

Design Rules

Minimum width of PolySi and diffusion line 2

Minimum width of Metal line 3 as metal lines run over a more uneven surface than other conducting layers to ensure their continuity

2

Metal

Diffusion

Polysilicon

3

2

Design RulesPolySi – PolySi space 2Metal - Metal space 2Diffusion – Diffusion 3 To avoid the

possibility of their associated regions overlapping and conducting current

2

Metal

Diffusion

2

3polysilicon

Design RulesDiffusion – PolySi To prevent the lines

overlapping to form unwanted capacitorMetal lines can pass over both diffusion

and polySi without electrical effect. Where no separation is specified, metal lines can overlap or cross

Metal

Diffusion

Polysilicon

Metal Vs PolySi/Diffusion Metal lines can pass over both diffusion

and polySi without electrical effect It is recommended practice to leave

between a metal edge and a polySi or diffusion line to which it is not electrically connected

Metal

Polysilicon