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12/16/2014

1

VLSI Physical Design Automation

Prof. David Pan

[email protected]

Office: ACES 5.434

Lecture 2. Review of Device/VLSI/Algorithm



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Objective of this Lecture

To review the materials used in fabrication of VLSI

devices.

To review the structure of devices and process

involved in fabricating different types of VLSI circuits

To review the basic algorithm concepts

To level-set everyone so that we can get into serious

Physical Design topics in the next lecture



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Wafer, Die and Package



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Fabrication Materials

copper



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Electron and Holes

Holes travel as do electrons

Material can be enriched in holes or electrons by introducing impurities

Holes in crystals can be enriched by embedding some boron atoms

Electrons in crystals can be enriched by embedding phosphorus atoms

Recent breakthroughs: strained silicon (IBM) to stretch silicon such that

electrons experience less resistance and flow up to 70% faster

Free

Electron

Silicon

atom

+Ion

Holehttp://researchweb.watson.ibm.com/resources/press/strainedsilicon/



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The Three Regions in a n-p Junction

A mask is a specification of geometric shapes that need tobe created on a certain layer. Masks are used to create a

specific patterns of each material in a sequential manner

and create a complex pattern of several layers

Electron

richInterface Hole rich

Carrier-depletion

zone

MaskSilicon dioxideinsulator

Phosphorous

Depletionzone

Substrate( a ) ( b ) ( c )

Formation of a Diffused Junction



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A nMOS Transistor

Enhancement Mode

Source Gate Drain Channel

Gate

Source Drain

Vg<Vt Vg Vt

Vs Vs Vd Vd

( c )

( a ) ( b )



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Fabrication of VLSI Circuits

1. Create

2. Define

3. Etch

Material formation by deposition,diffusion or implantation

Pattern definition by

photolithography

Etch

8 to 10 iterations

Silicon wafers



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Details of Fabrication Processes

Crystal growth & wafer preparation

Epitaxy

Dielectric & polysilicon film deposition

Oxidation

Diffusion

Ion implantation

Lithography

Etching

Packaging



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Basic Design Rules

1. Size Rules2. Separation Rules

3. Overlap Rules

Basic nMOS Design Rules

Diffusion Region WidthPolysilicon Region WidthDiffusion-Diffusion SpacingPoly-Poly Spacing

Polysilicon Gate ExtensionContact ExtensionMetal Width

2

2

3

2

2

3



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Size and Separation Rules

Incorrectly and Correctly Formed Channels

Diffusion

Short

Poly

Incorrectly formed

Channel

Correctly formed

Metal

Diffusion Poly



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Overlap Rules for Contact cuts

( a ) ( b )



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Layout of Basic Devices

nMOS Inverter

CMOS Inverter

nMOS NAND Gate

CMOS NAND Gate

nMOS NOR Gate

CMOS NOR Gate

Complicated devices are constructed by using basic devices



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A CMOS Inverter



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A CMOS NAND Gate



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A CMOS NOR Gate



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Additional Fabrication Factors

Scaling

Parasitic Effects

Yield Statistics and Fabrication Costs

Delay Computation

Noise and Crosstalk

Power Dissipation



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Mini Summary

The three types of materials are insulators, conductors andsemiconductors

A VLSI chip consists of several layers of different materials on

a silicon wafer.

Each layer is defined by a mask

VLSI fabrication process patterns each layer using a mask

Complex VLSI circuits can be developed using basic VLSI

devices

Design rules must be followed to allow proper fabrication

Several factors such as scaling, parasitic effects, yieldstatistics and fabrication costs, delay computation, noise and

crosstalk and power dissipation play a key role in fabrication

of VLSI chips



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Complexity of

VLSI circuits

Full custom

Performance Size Cost Market time

Standard Cell Gate Array FPGA

Different design styles

Cost, Flexibility, Performance

Design Styles



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Full Custom Design Style

Pad Metal Via Metal 2

I/OData Path

ROM/RAM

PLA

A/D ConverterRandom logic



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Standard Cell Design Style

VDD

Metal 1Cell

Metal 2Feedthrough GND

D C C B

A C C

D C D B

C C C B

Cell A

Cell C

Cell B

Cell DFeedthrough cell



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Gate Array Design Style

A

B

C

A

B

C

VDD Metal1 Metal2

Structured ASICs (hot topics nowadays) are essentially gate array



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FPGA Design Style



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Programmable logic

Programmable interconnects

Programmable inputs/outputs

Field-Programmable Gate-Arrays (FPGAs)



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Comparisons of Design Styles

full-custom standard cell gate array FPGA

cell size variable fixed height * fixed fixed

cell type variable variable fixed programmable

cell placement variable in row fixed fixed

interconnections variable variable variable programmable

* uneven height cells are also used

style



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Area

Performance

Fabrication

layers

style

full-custom standard cell gate array FPGA

compact

high

compact

to moderatemoderate large

high

to moderatemoderate low

ALL ALLrouting

layers none

Comparisons of Design Styles



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Printed Circuit Board

PCB

Multi-Chip Module

MCM

Wafer Scale Integration

WSI (SOC)

Packaging

Area

Performance, cost

The increasing complexity and density of the semiconductor devices

are driving the development of more advanced VLSI packaging and

interconnection approaches.

Packaging Styles



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History of VLSI Layout Tools

Year Design Tools

1950 - 1965

1965 - 1975

1975 - 1985

1985 – 1995

1995 – 2002

2002 - present

Manual Design

Layout editorsAutomatic routers( for PCB)Efficient partitioning algorithm

Automatic placement toolsWell Defined phases of design of circuitsSignificant theoretical development in all phases

Performance driven placement and routing toolsParallel algorithms for physical designSignificant development in underlying graph theoryCombinatorial optimization problems for layout

Interconnect layout optimization, Interconnect-

centric design, physical-logical codesign

Physical synthesis with more vertical integrationfor design closure (timing, noise, power, P/G/clock,manufacturability)



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Now You Need Algorithms

• To put devices/interconnects together into VLSI chips

• Fundamental questions: How do you do it smartly?

• Definition of algorithm in a board sense: A step-by-step procedure for solving a problem. Examples: – Cooking a dish

– Making a phone call

– Sorting a hand of cards

• Definition for computational problem: A well-defined

computational procedure that takes some value asinput and produces some value as output



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• Input: An array of n numbers D[1]…D[n].

• Output: An array of n numbers E[1]…E[n] such that

E[1] E[2] … E[n].

• Algorithm:1. For i from 1 to n do

2. Select the largest remaining no. from D[1..n].

3. Put that number into E[i].

Example: Selection Sort



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Some Algorithm Design Techniques

• Greedy

• Divide and Conquer

• Dynamic Programming

• Network Flow

• Mathematical Programming (e.g., linear programming,integer linear programming)



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Reduction

• Idea: If I can solve problem A, and if problem B can betransformed into an instance of problem A, then I cansolve problem B by reducing problem B to problem Aand then solve the corresponding problem A.

• Example: – Problem A: Sorting

– Problem B: Given n numbers, find the i-th largest numbers.



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Analysis of Algorithm

• There can be many different algorithms to solve thesame problem.

• Need some way to compare 2 algorithms.

• Usually run time is the most important criterion used – Space (memory) usage is of less concern now

• However, difficult to compare since algorithms may beimplemented in different machines, use differentlanguages, etc.

• Also, run time is input-dependent. Which input to use?

• Big-O notation is widely used for asymptotic analysis



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Big-O Notation

• Consider run time for the worst input=> upper bound on run time.

• Express run time as a function input size n.• Interested in the run time for large inputs.

• Therefore, interested in the growth rate.• Ignore multiplicative constant.• Ignore lower order terms.

• 3n2+6n+2.7 is O(n2).

• n1.1+10000000000n is O(n1.1).• n1.1 is also O(n2), but to be more precise, it is O(n1.1)



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Effect of Multiplicative Constant

0

100

200

300

400500

600

700

800

0 10 20 n

R u n t i m

e

n2

10n



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Growth Rates of some Functions

n

nnn

nn

ncc

nOnO

OOO

OnO

cOnO

nOnO

nOnOnnOnnO

nOnOnOnO

!

432

2

constantanyfor2

loglog

loglog

2loglog

log

43

25.12

2

E x p on en t i al

F u

n c t i on s

P ol y n omi al

F un c t i on

s



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Problem of Exponential Function

• Consider 2n

, value doubled when n is increased by 1.

• If you borrow $10 from a credit card with APR 18%, after40 yrs, you will own $12700!

n 2n 1ms x 2n

10 103 0.001 s

20 106 1 s

30 109 16.7 mins

40 1012 11.6 days

50 1015 31.7 years

60 1018 31710 years



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NP-Complete

• The class NP-Complete is the set of problems whichwe believe there is no polynomial time algorithms.

• Therefore, it is a class of hard problems.

• NP-Hard is another class of problems containing the

class NP-Complete.

• If we know a problem is in NP-Complete or NP-Hard,there is no hope to solve it efficiently.



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NP-Complete

• I can't find an efficientalgorithm, I guess I'm just toodumb.

• I can't find an efficientalgorithm, but neither can allthese famous people.

• I can't find an efficientalgorithm, because no suchalgorithm is possible.

Source: Computers and Intractibility by Garey and Johnson



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Solution Type of Algorithms

• Polynomial time algorithms

• Exponential time algorithms

• Special case algorithms

• Approximate algorithms

• Heuristic algorithms



Before Next Class

• Refresh your Algorithms: – C. J. Alpert, D. P. Mehta, S. S. Sapatnekar, Handbook of

Algorithms for Physical Design Automation, AuerbachPublications, 2008

– T. H. Cormen, C. E. Leiserson, R. L. Rivest, C. SteinIntroduction to Algorithms, MIT Press, 2009 (3rd edition)

• Circuit partitioning in the next class

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