Layout Design

Lecture 418-322 Fall 2003

Textbook: Design Methodology Insert A

[Portions adapted from J. P. Uyemura “Introduction to VLSI Circuits and Systems”, Wiley 2001.]

Roadmap

Today: Basic CMOS Layout: “design in the small”Thursday: Layout Verification & “design in the large”Next week:

Transistor sizingWires

Homework 1: Due ThursdayHomework 2: Out ThursdayLab 2: This week

Today’s Overview

Physical structure of ICsDesign rulesBasic gates layout

Stick diagrams Basic rulesExamples

Cadence (Virtuoso)

Review: MOSFETs

Gate (G)No connection

G = 0Open switchSource Drain

Sourcelayer

Drainlayer

Gate layerConduction layer

G = 1Closed switch

G is responsible for the absence or presence of the conduction region between the drain and the source regions

Review: Controlling Current Flow (nFET)

0V

p

n+ n+

No electronsL

insulatordrain diffsource diff

VG

n+ n+ W

L

Top viewSide view

+

p

n+ n+

electrons n+ n+

electron channel

Review: Manufacturing

2D top-down view

How design engineers seethe chip.

3D “cross-section” view

How process engineers seethe chip.

Design Rules

Interface between designer and process engineerClean separation between the process during wafer fabrication and the designeffort ⌧ Permissible geometries -> DESIGN RULES

• Width rule, space rule, overlap rule, etc.

Ways to do design rules“Scalable Design Rules”Absolute measures

Scalable Design Rules

CMOS scalesImplement something now, shrink it laterExpress all design rules in terms of a unit dimensionChange the actual dimension of the unit, and the whole design shrinksMead and Conway

Unit dimension: Minimum line width (2λ)In 1978, λ = 1.5 µm (a.k.a. 3 micron technology)In 2003, λ = 0.065 µm (a.k.a. 0.13 micron technology)

Important Intellectual idea, not used in industry (but we will)

Transistor Layout

1

2

5

3

Tran

sist

or

Well boundary

L

W

The choice of geometry determines transistor parameters!

All distances are expressed in λ

poly

Transistor Layout

4λ

L

W2λ

2λ

5λ

λ

λ

Source Drain2λSource to

gate shortcirc

Non-catastrophicmisalignment

2λ

AS = AD = 5λWλ = 0.5µm -> A = 12.5µm2

Absolute Design Rules

It is hard to scale every aspect of design linearlyThe elegance of scalable CMOS isn’t worth the costSpecify all dimensions in real units (µm or nm)

Currently (0.13 micron), there are THOUSANDS of design rules

CMOS Process Layers

Layer

Polysilicon

Metal1

Metal2

Contact To Poly

Contact To Diffusion

Via

Well (p,n)

Active Area (n+,p+)

Color Representation

Yellow

Green

RedBlue

MagentaBlack

BlackBlack

Select (p+,n+) Green

Inverters

Vin Vout

VDD

GND

Layout of a NOT gate

Vin Vout

GND

VDD

Vin Vout

VDD

GND

Alternate layout of a NOT gate

Transistor sizing determines inverter fundamental properties!

Series/Parallel Connections

A B

n+ n+ n+

A B

n+ n+ n+

A B

n+ n+ n+

p

Devices can share patterned regions; this may reduce the layout area or complexity!

A BX

Y

A B

XX X

X

Y X

poly Redn+/p+ Greenmetal Bluecontact Black

NAND2

V DD

GND

AB NOT(AB)

GND

A

B

V DD

NOT(AB)

Question: How About AND2?

V DD

GND

AB NOT(AB) A and B

VDD

GND

GND

A

B

V DD

NOT(AB)

NOR2

B

A

V DD

GND

NOT(A+B)

The output here is connected to one p-trans drain and two n-trans drains.

V DD

GND

A

BNOT(A+B)

NOR2 (alternate layout)

B

A

V DD

GND

NOT(A+B)

The output here is connected to one p-trans drain and one n-trans drain.

V DD

GND

A

BNOT(A+B)

This is better! Less drain area connected to the output . This results in a faster gate.

Complex Logic Gates: OAI Gates

ABCD

FA

B C D

1

2

A

B C D

1

2

#1 #2

F= NOT(A(B+C+D))

OAI Gates: Sharing S/D (option 1)

A B C D

A

B C D

1

2

OAI Gates: Sharing S/D

A B C D

VD D

GND

A

B C D

1

2

OAI Gates: Sharing S/D

A B C D

F

VD D

GND

A

B C D

1

2

The output here has four output drain capacitances.

Capacitance: Friend or Foe???

Foe: Slows down the output:

Friend: Stabilizes the Power Supply

Big CapacitanceMore charge toto change voltageSLOWER!

Big CapacitanceMore charge toto change voltageMore stable supplyvoltage!

OAI Gates: Sharing S/D (option 2)

A B C D

A

B C D

1

2

#2

OAI Gates: Sharing S/D

A B C D

F

VDD

GND

A

B C D

1

2

OAI Gates: Sharing S/D

A B C D

F

V DD

GND

Wrong

A B C D

F

V DD

GND

Right

The output here has two output drain capacitances.

Gate Design Procedure

Run VDD and GND in metal at top and bottomRun vertical poly for each gate inputOrder gates to allow maximum source-drain abuttingPlace max number of n-diffusions close to GNDPlace max number of p-diffusions close to VDDMake remaining connections with metal

Minimize metal usage

Question: How About TGs?

Overview

Physical structure of ICsDesign rulesBasic gates layout

Stick diagrams Basic rulesExamples

Cadence (Virtuoso)

Stick Diagrams

•Introduced by Mead & Conway in the ‘80s

•Every line of a conduction material layer is represented by a line of a distinct color

nFET and pFET Representations

Basic Rules (1)

Basic Rules (2)

Basic Rules (3)

Logic Gates Design

Examples

Complex Functions

OUT = ABC + D

OUT

VDD

C

CB

D

A

B

A

D

X X XX

X

X

X X

ABC D

VDD

GND

OUT

Summary

Discussed Design rulesBasic gates layout Stick diagrams

Need more practice on Stick diagramsLayout (mostly in the lab)

Preview: The 18-322 Flow

Boolean function Transistor Schematic

Schematic Simulation

ComponentDesign

Extracted Simulation

Layout (w/ DRC)1st part

of the Thursday’s

LectureLVS Check

Preview: Modern ASIC Design

Designer Productivity is a big problemIn 1978, people could draw transistors, now there are 100s of millions per chip…New abstractions necessary:

Masks LayoutDesign

Design R

ules

Cell Libraries

Std CellDesign

18-322