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ESE370: Circuit-Level Modeling, Design, and Optimization for Digital Systems. Day 12: October 4, 2010 Layout and Area. Today. Layout Transistors Gates Design rules Standard cells. 2. Transistor. Side view. Perspective view. 3. Layout. Sizing & positioning of transistors

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Ese370 circuit level modeling design and optimization for digital systems l.jpg

ESE370:Circuit-Level Modeling, Design, and Optimization for Digital Systems

Day 12: October 4, 2010

Layout and Area


Today l.jpg
Today

Layout

Transistors

Gates

Design rules

Standard cells

2


Transistor l.jpg
Transistor

Side view

Perspective view

3


Layout l.jpg
Layout

Sizing & positioning of transistors

Designer controls W,L

tox fixed

Sometimes thick/thin oxide “flavors”

4


Nmos geometry l.jpg
NMOS Geometry

L

W

Top view

Perspective view

5


Nmos geometry6 l.jpg
NMOS Geometry

Color scheme

Red: gate

Green: source and drain areas (n type)

Where is tox?

L

S

G

D

W

Top view

6


Slide7 l.jpg
tox

Transistors built by depositing materials

Constant rate of deposition (nm/min)

Time controls tox

Oxides across entire chip deposited at same time

Same interval

So, thickness is constant

Process engineer sets value to maximize:

Yield

Performance

7


Nmos vs pmos l.jpg
NMOS vs PMOS

Mostly talked about NMOS so far

PMOS: “opposite” in some sense

NMOS built on p substrate, PMOS built on n substrate

Name refers to when channel is inverted

Rabaey text, Fig 2.1

8


Pmos geometry l.jpg
PMOS Geometry

Color scheme

Red: gate

Orange: source and drain areas (p type)

Green: n well

NMOS built on p wafer

Must add n material to build PMOS

L

S

G

D

W

n well

9


Body contact l.jpg
Body Contact

“Fourth terminal”

Needed to set voltage around device

PMOS: Vb = Vdd

NMOS: Vb = GND

At right: PMOS (orange) with body contact (dark green)

Side view: R.R. Harrison, ECE 5720 notes (Utah)

10


Interconnect l.jpg
Interconnect

How to connect transistors

Different layers of metal Intermediate layers

“Contact” - metal to transistor

“Via” - metal to metal

Rabaey text, Fig 2.7k

11



Masks l.jpg
Masks

Define areas want to see in layer

Think of “stencil” for material deposition

Use photoresist (PR) to form the “stencil”

Expose PR through mask

PR dissolves in exposed area

Material is deposited

Only “sticks” in area w/ dissolved PR

13


Masking process l.jpg
Masking Process

Goal: draw a shape on the substrate

Simplest example: draw a rectangle

Mask

Silicon wafer

14


Masking process15 l.jpg
Masking Process

First: deposit photoresist

Silicon wafer

Mask

photoresist

15


Masking process16 l.jpg
Masking Process

Expose through mask

UV light

16


Masking process17 l.jpg
Masking Process

Remove mask and develop PR

Exposed area dissolves

This is “positive photoresist”

Negative photoresist?

17


Masking process18 l.jpg
Masking Process

Deposit metal through PR window

Then dissolve remaining PR

Why not just use mask?

Masks are expensive

Shine light through mask to etch PR

Can reuse mask

18


Logic gates l.jpg
Logic Gates

How to build?

Connect NMOS, PMOS using metal

HW4, part 6: reverse engineer layouts into gates

19



Inverter layout example21 l.jpg
Inverter Layout Example

Start with PMOS, NMOS transistors

Space for interconnect

21



Inverter layout example23 l.jpg
Inverter Layout Example

Add body contacts

Connect gates of transistors

23



Inverter layout example25 l.jpg
Inverter Layout Example

Add contacts to source, drain, gate, body

Connect using metal (blue)

25


Design rules l.jpg
Design Rules

Why not adjacent transistors?

Plenty of empty space

If area is money, pack in as much as possible

Recall: processing imprecise

Margin of error for process variation

26


Design rules27 l.jpg
Design Rules

Contract between process engineer & designer

Minimum width/spacing

Can be (often are) process specific

Lambda rules: scalable design rules

In terms of  = 0.5 Lmin

Can migrate designs from similar process

Limited scope: 45nm process != 1m

27


Design rules some examples l.jpg

2

2

3

1.5

6

6

contact

Legend

metal 1

n doping

via

gate

p doping

metal 2

Design Rules: Some Examples

2


Layout revisited l.jpg
Layout Revisited

How to “decode” circuit from layout?

29


Layout to circuit l.jpg
Layout to Circuit

1. Identify transistors

30

Penn ESE370 Fall2010 -- DeHon


Layout to circuit31 l.jpg
Layout to Circuit

2. Add wires

31


Layout to circuit32 l.jpg
Layout to Circuit

2. Add wires

32

Penn ESE370 Fall 2010 -- Townley (DeHon)


Layout to circuit33 l.jpg
Layout to Circuit

2. Add wires

33

Penn ESE370 Fall 2010 -- Townley (DeHon)


Layout to circuit34 l.jpg
Layout to Circuit

2. Add wires

34

Penn ESE370 Fall 2010 -- Townley (DeHon)



Standard cells l.jpg
Standard Cells

Lay out gates so that heights match

Rows of adjacent cells

Standardized sizes

Motivation: automated place and route

EDA tools convert HDL to layout

36


Standard cell area l.jpg
Standard Cell Area

All cells

uniform

height

inv

nand3

Width of

channel

determined

by routing

Cell area

Identify the full custom and standard cell regions on 386DX die

http://microscope.fsu.edu/chipshots/intel/386dxlarge.html


Admin l.jpg
Admin

  • HW4 – slight update online to clarify Q1

  • HW3 – trickier than intended in places

    • Our guess is 1, 2, maybe 7

    • (let us know if that’s not where)

    • Office hours to clear up any remaining confusion?

  • Andre back for Wed. Lecture


Big idea l.jpg
Big Idea

Layouts are physical realization of circuit

Geometry tradeoff

Can decrease spacing at the cost of yield

Design rules

Can go from circuit to layout or layout to circuit by inspection

39