ee 330 lecture 10 - iowa state university

EE 330

Lecture 10

IC Fabrication Technology

‒ Oxidation

‒ Epitaxy

‒ Polysilicon

‒ Planarization

‒ Contacts, Interconnect, and Metallization


• Crystal Preparation

• Masking

• Photolithographic Process

• Deposition

• Ion Implantation

• Etching

• Diffusion

• Oxidation

• Epitaxy

• Polysilicon

• Contacts, Interconnect and Metalization

• Planarization

Oxidation

• SiO2 is widely used as an insulator– Excellent insulator properties

• Used for gate dielectric– Gate oxide layers very thin

• Used to separate devices by raising threshold voltage– termed field oxide

– field oxide layers very thick

• Methods of Oxidation– Thermal Growth (LOCOS)

• Consumes host silicon

• x units of SiO2 consumes .47x units of Si

• Undercutting of photoresist

• Compromises planar surface for thick layers

• Excellent quality

– Chemical Vapor Deposition• Needed to put SiO2 on materials other than Si

Oxidation

SiO2

Thermally Grown SiO2 - desired growth

Photoresist

Patterned Edges

X

0.47 X

p- Silicon

Oxidation

SiO2

Thermally Grown SiO2 - actual growth

Photoresist

Patterned Edges

Bird’s Beaking

p- Silicon

Oxidation

Thermally Grown SiO2 - actual growth

Patterned Edges

Nonplanar

Surface

p- Silicon

Oxidation

Shallow Trench Isolation (STI)

p- Silicon

Photoresist Pad Oxide

Silicon Nitride

Oxidation


p- Silicon

Pad Oxide

Silicon Nitride

Etched Shallow

Trench

Oxidation


p- Silicon

Pad Oxide

Silicon Nitride

CVD SiO2

Oxidation


p- Silicon

Planarity ImprovedPlanarization Target

Oxidation


p- Silicon

After Planarization

CVD SiO2



• Masking


• Deposition

• Etching

• Diffusion


• Oxidation

• Epitaxy

• Polysilicon

• Planarization


Epitaxy

• Single Crystaline Extension of Substrate

Crystal

– Commonly used in bipolar processes

– CVD techniques

– Impurities often added during growth

– Grows slowly to allow alignmnt with substrate

Epitaxy

p- Silicon

Original Silicon Surface

Epitaxial Layer

epi can be uniformly doped or graded

Question: Why can’t a diffusion be used to create the same effect

as an epi layer ?



• Masking


• Deposition

• Etching

• Diffusion


• Oxidation

• Epitaxy

• Polysilicon

• Planarization


Polysilicon

• Elemental contents identical to that of single crystaline silicon– Electrical properties much different

– If doped heavily makes good conductor

– If doped moderately makes good resistor

– Widely used for gates of MOS devices

– Widely used to form resistors

– Grows fast over non-crystaline surface

– Patterned with Photoresist/Etch process

– Silicide often used in regions where resistance must be small

• Refractory metal used to form silicide

• Designer must indicate where silicide is applied (or blocked)

Polysilicon

Single-Crystaline Silicon

Polysilicon



• Masking


• Deposition

• Etching

• Diffusion


• Oxidation

• Epitaxy

• Polysilicon

• Planarization


Planarization

• Planarization used to keep surface planar

during subsequent processing steps

– Important for creating good quality layers in

subsequent processing steps

– Mechanically planarized



• Masking


• Deposition

• Etching

• Diffusion


• Oxidation

• Epitaxy

• Polysilicon

• Planarization


Contacts, Interconnect and Metalization

• Contacts usually of a fixed size

– All etches reach bottom at about the same time

– Multiple contacts widely used

– Contacts not allowed to Poly on thin oxide in

most processes

– Dog-bone often needed for minimum-length

devices

Contacts

Vulnerable to pin holes(usually all contacts are same size)

A A’

Unacceptable Contact

B

B’

Acceptable Contact

Contacts

Acceptable Contact

B

B’

Contacts

2λ

1.5λ

2λ

1.5λ

Design Rule Violation

2λ

“Dog Bone” Contact

Contacts

Common

Circuit

Connection

Standard Interconnection Buried Contact

Can save area but not

allowed in many processes

Metalization

• Aluminum widely used for interconnect

• Copper often replacing aluminum in recent

processes

• Must not exceed maximum current density

– around 1ma/u for aluminum and copper

• Ohmic Drop must be managed

• Parasitic Capacitances must be managed

• Interconnects from high to low level metals

require connections to each level of metal

• Stacked vias permissible in some processes

Metalization

Aluminum

• Aluminum is usually deposited uniformly over entire surface and

etched to remove unwanted aluminum

• Mask is used to define area in photoresist where aluminum is to be

removed

Copper

• Plasma etches not effective at removing copper because of absence

of volatile copper compounds

• Barrier metal layers needed to isolate silicon from migration of

copper atoms

• Damascene or Dual-Damascene processes used to pattern copper

Patterning of Aluminum

PhotoresistContact Opening

from Mask



after SiO2 etch


Metal Applied to Entire Surface


Photoresist Patterned

with Metal Mask


Aluminum After Metal Etch

(photoresist still showing)

Copper Interconnects

Limitations of Aluminum Interconnects

• Electromigration

• Conductivity not real high

Relevant Key Properties of Copper

• Reduced electromigration problems at

given current level

• Better conductivity

Challenges of Copper Interconnects• Absence of volatile copper compounds

(does not etch)

• Copper diffuses into surrounding

materials (barrier metal required)

Source:

Sept 13, 2017

Copper Interconnects

Practical methods of realizing copper

interconnects took many years to

develop

Copper interconnects widely used in

some processes today

Patterning of Copper


after SiO2 etch

Damascene Process


Tungsten (W)

Damascene Process

CMP Target

W has excellent conformality when formed from WF6

Applied with CVD WF6+3H2 →W+6HF

Chemical-Mechanical Planarization (CMP)

http://en.wikipedia.org/wiki/Chemical-mechanical_planarizationAcknowledgement:

• Polishing Pad and Wafer Rotate in

non-concentric pattern to thin, polish,

and planarize surface

• Abrasive/Chemical polishing

• Depth and planarity are critical


W-plug

Damascene Process

CMP TargetAfter first CMP Step

Patterning of CopperDamascene Process

Oxidation

After first CMP Step


Photoresist Patterned with

Metal Mask Defines Trench


Shallow Trench after Etch

W-plug


(Barrier metal added before copper to contain the copper atoms)

Barrier

Metal


Copper DepositionW-plug


Copper DepositionW-plug

CMP Target

Copper is deposited or electroplated (Barrier Metal Used for Electroplating Seed)


After Second CMP Step

W-plug

CMP Target

Copper


PhotoresistShallow Trench Defined

in PR with Metal Mask

Dual-Damascene Process


PhotoresistShallow Trench After Etch



PhotoresistVia Defined in PR

with Via Mask



PhotoresistVia Etch Defines

Contact Region


(Barrier Metal added before copper but not shown)


Copper Deposited on Surface


Copper is deposited or electroplated (Barrier Metal Used for Electroplating Seed)


Copper Deposited on Surface


CMP Target


Copper Interconnect


CMP Target

Copper Via



Both Damascene Processes Realize Same Structure

Damascene ProcessTwo Dielectric Deposition Steps

Two CMP Steps

Two Metal Deposition Steps

Two Dielectric Etches

W-Plug

One Dielectric Deposition Steps

One CMP Steps

One Metal Deposition Steps

Two Dielectric Etches

Via formed with metal step

Multiple Level Interconnects

3-rd level metal connection to n-active without stacked vias

Multiple Level Interconnects

3-rd level metal connection to n-active with stacked vias

Metalization

Interconnect Layers May Vary in Thickness or Be Mostly Uniform

12.5μ

Interconnects

• Metal is preferred interconnect

– Because conductivity is high

• Parasitic capacitances and resistances of concern in all interconnects

• Polysilicon used for short interconnects

– Silicided to reduce resistance

– Unsilicided when used as resistors

• Diffusion used for short interconnects

– Parasitic capacitances are high

Resistance in Interconnects

L

W

H

A

B

BA

R


L

W

HA

B

BD

R

D

ρA

LR

A=HW

ρ independent of geometry and

characteristic of the process


L

W

HA

B

D

H

ρ

W

Lρ

A

LR

H << W and H << L in most processes

Interconnect behaves as a “thin” film

Sheet resistance often used instead of conductivity to characterize film

R□=ρ/H R=R□[L / W]


R=R□[L / W]

L

W

The “Number of Squares” approach to resistance determination in thin films

1 2 3 21NS = 21

L / W=21

R=R□NS


R=R□13.25

Corners Contribute

about .55 Squares

Fractional Squares

Can Be Represented

By Their Fraction

In this example:

NS=12+.55+.7=13.25

The “squares” approach is not exact but is good enough for

calculating resistance in almost all applications

Example:

The layout of a film resistor with electrodes A and B is shown. If

the sheet resistance of the film is 40 Ω/□, determine the

resistance between nodes A and B.

10u

3u1u

A

B

10u

3u1u

A

B

Solution

NS =9+9+3+2(.55)=22.1

RAB=R□NS=40x22.1=884Ω

Resistance in Interconnects(can be used to build resistors!)

• Serpentine often used when large resistance required

• Polysilicon or diffusion often used for resistor creation

• Effective at managing the aspect ratio of large resistors

• May include hundreds or even thousands of squares

Resistance in Interconnects(can be used to build resistors!)

d1

2d

2

Area requirements determined by both minimum

width and minimum spacing design rules

End of Lecture 10

ee 330 lecture 10 - iowa state university

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