Sputtering 1

8/3/2019 Sputtering 1

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Sputtering

Eyal Ginsburg

WW46/02


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Contents

Metallization structure

PVD System Overview

Sputtering: yield, conditioning,methods

Film nucleation and growth


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Contact & Metal Lines - SEM

M3

M2

M1

W PlugVia 2

Silicon substrate


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Glue Layer (Cont. 1)


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Aluminum - General

Al-alloys thin films were selected forthe first 30 years of the IC industry.

They continue to be the most widelyused materials, although copper.

Al has low resistivity (=2.7-cm),and its compatibility with Si and SiO2.

Al forms a thin native oxide (Al2O3) onits surface upon exposure to oxygen,and affect the contact resistance.


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Aluminum - General (cont.)

Al thin films can also suffer fromcorrosion (ex. Al dry etch may leavechlorine residues on Al surface andlead to formation of HCl and thenattack the Al).


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Aluminum interconnects

The material used in interconnects is notpure aluminum, but an aluminum alloy.Usually with Cu (0.5-2%), sometimes with

Si. The Cu in Al-alloy slows the

electromigration (EM) phenomenon. Sislows EM slightly, used in contact level toprevent spiking.

Al-alloys decrease the melting point,increase the resistivity and need to becharacterized (ex. Dry etch).


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Metal line stack

Usually the metal line contains 4-5layers:

Al - This layer makes the contacts withthe Tungsten plugs. It is the primarycurrent carrier.

TiN Layer - Creates a barrier between

the Al/Cu and the Titanium layersbecause of the increasing temperatureat a downstream process will increasethe rate of the reaction of Al with Ti.


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Metal stack (Cont. 1)

Titanium Layer - Provides an alternatecurrent path (shunt) around flaws inthe primary current carrier. And thusimproves electromigrationcharacteristics.


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Metal stack (Cont. 2)

TiN ARC Layer - This is ananti-reflecting coating which aideslithography to keep control of criticaldimensions and to absorb light duringthe resist exposure. It also functionsas a hillock suppressant.


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Metal stack - SEM

ILD

Metal line

W- Via2

Metal line

Al

TiN

Ti

TiN


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PVD System Overview(Endura)


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Endura PVD system


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Endura standard mainframe


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Mainframe Components Preclean Ch. Applies a light. Non selective plasma

etch to the wafer before the PVD process.

Cooldown Ch. Cools the wafer after the PVDprocess.

Expansion Ch. (C&D) Optionally configured for PVD

or other processes such as etch. Wafer orienter/degas Ch. Orients the wafer flat to a

designated angle and degasses the wafer to removewater vapor before the preclean process.

PVD Ch. DC magnetron sputter depositionchambers for depositing materials used ininterconnects metalization (ex. Al, Ti, TiN, TiW).

Cassette loadlocks The starting point for wafertransfers. Accept 1 cassette with 25 wafers.


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Vacuum system

PVD system uses Ultra-High Vacuum(UHV) to reduce particulates andprovide purer film qualities.

The tool uses staged vacuum regimesto achieve UHV.


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Pressure regions and vacuum stages


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PVD chambers and pumps


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Sputter deposition for

ULSI


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Sputtering General

Sputtering is a term used to describe themechanism in which atoms are ejectedfrom the surface of a material when that

surface is stuck by sufficiency energeticparticles.

Alternative to evaporation.

First discovered in 1852, and developed as

a thin film deposition technique byLangmuir in 1920.

Metallic films: Al-alloys, Ti, TiW, TiN,Tantalum, Nickel, Cobalt, Gold, etc.


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Reasons for sputtering

Use large-area-targets which givesuniform thickness over the wafer.

Control the thickness by Dep. timeand other parameters.

Control film properties such as step

coverage (negative bias), grainstructure (wafer temp), etc.

Sputter-cleaned the surface invacuum prior to deposition.


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Sputtering steps

1. Ions are generated and directed at atarget.

2. The ions sputter targets atoms.3. The ejected atoms are transported to

the substrate.

4. Atoms condense and form a thinfilm.


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SputteringCoating process

that involves thetransport ofmaterial from thetarget to thewafer. Atomsfrom the target

are ejected as aresult ofmomentumtransfer betweenincident ions andthe target. Theparticles traversethe vacuumchamber and aredeposited on thewafer.


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Application of Sputtering

Thin film deposition:

Microelectronics

Decorative coating

Protective coating

Etching of targets:

Microelectronics patterning

Depth profiling microanalysis

Surface treatment:

Hardening

Corrosion treatment


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The billiard ball model

There is a probability that atom C willbe ejected from the surface as a result

of the surface being stuck by atom A. In oblique angle (45-90) there is

higher probability for sputtering,

which occur closer to the surface.


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Sputter yield

Defined as the number of atoms ejectedper incident ion.

Typically, range 0.1-3.

Determines the deposition rate.

Depends on:

1. Target material.

2. Mass of bombarding ions.

3. Energy of the bombarding ions.4. Direction of incidence of ions (angle).

5. Pressure


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1

2

Sputter yield (Cont. 1)Target materials:

Al/Cu(0.5%)

Grain size: 45m

Grain size: 200m


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Sputter yield (Cont. 2) Molecule sizeneed to be about the same

size as the sputtered material: too big cause layer deformation and yield a lot of

material.

too small cause layer deformation w/o ejecting atoms.Target deformation = Less uniform dep.


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Sputter yield (Cont. 3)

Ion energy Vs. sputter yield:


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Sputter yield (Cont. 4)

Sputter yield peaks at


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Sputter yield (Cont. 5) Pressure reductionallow better

deposited atoms/molecules flux flowtowards the substrate. Expressed by Meanfree path which is the average distance anatom can move, in one direction without

colliding at another atom.


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Process conditions

Type of sputtering gas. In purelyphysical sputtering (as opposed toreactive sputtering) this limits to noblegas, thus Argon is generally thechoice.

Pressure range: usually 2-3 mTorr (by

glow discharge).

Electrical conditions: selected to givea max sputter yield (Dep rate).


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Sputter deposition film growth

Sputtered atoms have velocities of3-6E5 cm/sec and energy of 10-40 eV.

Desire: many of these atomsdeposited upon the substrate.

Therefore, the spacing is 5-10 mm.

The mean free path is usually


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Sputter dep. film (Cont. 1)

The sputter atoms may therefore:

1. Arrive at surface with reduce energy (1-2eV).

2. Be backscattered to target/chamber.

The sputtering gas pressure canimpact on film deposition parameters,such as Dep rate and composition ofthe film.


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Sputtering additional methods

Reactive sputtering

RF sputtering

Magnetron sputtering Collimated sputtering

Hot sputtering


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Reactive sputtering Reactive gas is introduced into the

sputtering chamber in addition to theArgon plasma.

The compound is formed by the elementsof that gas combining with the sputter

material (Ex. TiN). The reaction is usually occurs either on the

wafer surface or on the target itself.

As you add more reactive gas at some

point the reaction rate exceeds thesputtering rate.

At this point the target surface switchesfrom clean metal to compound over a short

time.


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Reactive sput. (Cont. 1)

The transition in target chemistry changessputtering conditions dramatically !


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Reactive sput. (Cont. 2)

Typical compounds deposited by reactivesputtering:

Target Reactive Gas Compound

Al O2

Al2O

3Al N2 AlN

Ti O2 TiO2Ti N2 TiN

Si N2 Si3N4Ta O2 Ta2O5Zn O2 ZnO

In-Sn O2 In2O3-SnO2


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RF sputtering

DC sputter deposition is not suitable forinsulator deposition, because thepositive charge on the target surfacerejects the ion flux and stop thesputtering process.

RF voltages can be coupled capacitivelythrough the insulating target to theplasma, so conducting electrodes are notnecessary.

The RF frequency is high enough tomaintain the plasma discharge.


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RF sputtering (Cont. 1)

During the first few complete cycles moreelectrons than ions are collected at eachelectrode (high mobility), and cause tonegative charge to be buildup on theelectrodes.

Thus, both electrodes maintain a steady-state DC potential that is negative withrespect to plasma voltage, Vp.

A positive Vp aids the transport of theslower positive ions and slow down thenegative electrodes.

RF sputtering (Cont 2)


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The induced negative biasing of the targetdue to RF powering means that continuous

sputtering of the target occurs throughoutthe RF cycle.

But it is also means that this occurs at bothelectrodes.


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The wafer will be sputtered at thesame rate as the target since thevoltage drops would be the same atboth electrodes for symmetric system.

It would thus be very difficult todeposit any material in that way.

Smaller electrode requires a higher RFcurrent density to maintain the sametotal current as the larger electrode.


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By making the area of the targetelectrode smaller than the otherelectrode, the voltage drop at thetarget electrode will be much greaterthan at the other electrode.

Therefore almost all the sputtering will

occur at the target electrode.

RF i (C 5)


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We also use RF sputtering to clean out bottoms

of Contacts and Vias before the actualdeposition. Remove native oxides and etch residues from

Contacts/Vias.

During this step, a controlled thickness of

surface material is sputtered off the wafer,removing any contaminants or native oxide.

A film can then be sputter depositedimmediately afterward without breaking thevacuum.

This process was done in the pre-cleanchamber.

This may also be done by BIAS SPUTTERING(reversing the electrical connections).


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Magnetron sputtering Here magnets are used to increase the

percentage of electrons that take part inionization events, increase probability ofelectrons striking Ar, increase electronpath length, so the ionization efficiency

is increased significantly. Another reasons to use magnets:

Lower voltage needed to strike plasma.

Controls uniformity.

Reduce wafer heating from electronbombardment.

Increased deposition rate


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Magnetron sputtering (Cont. 1) Lower voltage:

Magnets produce magnetic field Magnetic field make an electron go in curved path

(helix)

Curved paths are longer more collisions More collisions make more ions easier to strike

plasma.

Controls uniformity: Electrons paths are more curved near stronger

magnetic field.

More ions collide with target in regions of high

magnetic field. More ion collisions lead to more target atoms

sputtering.

More magnets near edge/center makes edge/centerthick deposition.


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Magnetron sputtering (Cont. 2)

A magnetic field is applied at rightangle to electric field by placing largemagnets behind the target.

This traps the electrons near thetarget surface, and causes them tomove in spiral motion until the collidewith an Ar atom.

Dep rate increases up to 10-100 timesfaster than without magnetronconfiguration.


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Magnetron sput (Cont. 3)

Magnetron sputtering can be done in either DC or

RF modes, but the former is more common. Target erodes rapidly in the ring region resulting in

a deep groove in the target face, which cause to

non-uniformity film.

C lli t d tt i


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Collimated sputtering During the PVD process, metal atoms are sputtered

at all angles. The standard process deposits metalon all areas of the process kit and at various angleson the wafer.

A small range of arrival angles during depositioncan cause nonuniform film.

This leads to poor bottom coverage of smallgeometry, high aspect ratio contacts and vias asthe holes seal off at the top before filling up atthe bottom.

C lli d (C )


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Collimated sput. (Cont. 1) One way to improve this by having a narrow range

of arrival angles, while atoms arrivingperpendicularly to the wafer.

This method called collimated sputtering (firstproposed in 1992).

A hexagonal holes plate is placed between thetarget and the wafer.


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Collimated sput. (Cont. 2) As the sputtered atoms travel through the

collimator toward the wafer, only those withnearly normal incidence trajectory will continueto strike the wafer.

The collimator thus acts as a physical filter to

low angle sputter atoms.


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Collimated sput. (Cont. 3)

70-90% of atoms are filtered and thereforethe Dep rate is significantly reduced.

In addition the collimator should be

cleaned and replaced, resulting additionaldowntime of the tool = COST.

Suitable for contact and barrier layerswhere lot of material is not needed to bedeposited.

Benefit with cover the bottom of Vias.


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Collimated sput. (Cont. 4)

The next figure shows the bottomcoverage of collimated sputteringcompared to conventional versus contactaspect ratio.


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Hot sputtering Hot sputtering is a method used to fill

spaced during deposition as well as toimprove overall coverage.

The basic idea is to heat the substrate to450-500C during deposition.

Surface diffusion is significantlyincreased so that filling in spaces,smoothing edges and planarization areaccomplished, driven by surface energy

reduction. The temperature in Via planarization

processes is generally lower than that incontact to protect previously deposited

Al layers.


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Hot sputtering (Cont. 1)

The lower power in the hot aluminum stepincreases the length of time that the Alatoms can diffuse, increasing the distancethat they travel before they stop.

Usually, a thin cold deposition is donefirst with substrate at room temperature,which has better adhesion to theunderlying material.

Then is followed by hot PVD deposition.

Main drawbacks is the relatively high temp.(reaction, thermal-budget, etc).


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Film Nucleationand Growth


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Things affect film structure

The things that control grain structureare:

Substrate

Base pressure (or contamination level)

Deposition temperature

Deposition rate

Later processing temperature

Process pressure (#collisions)

Fil i t t


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Film microstructure The film microstructure gives a graphic representative of

how changing process pressure and wafer temperature

affects the structure of a PVD film.

Grain size


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Grain sizeAl grains - AFM photos.

What is the reason for the differences between these pictures ?

A. B.

Wh t h d t thi Ti t t ?


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What happened to this Ti target ?

Target malfunction


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Target malfunction Ti target was warped near the edge of the target

The root cause: the flatness of the backing plates,being out of specification. The epoxy did not adhereto the blank. During sputtering, the area where theepoxy did not adhere to the blank experienced hightemperatures that could no longer be dissipated bythe backing plate due to the minimal contact to theblank. Thus, as the area in question became hotter,the more likely that assembly warped.


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The crystal structure of Ti:

HCP up to 882 C

BCC above 882 C

D t V KWHR


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Dep rate Vs. KWHR


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Where to Get More Information

S. Wolf, Silicon Processing for the VLSI era,Vol 1-2.

Peter Van Zant, Microchip Fabrication.

Stephen A. Campbell, The science andengineering of microelectronic fabrication.

J. D. Plummer, M. D. Deal and P.B. Griffin,

Silicon VLSI technology. J.L. Vossen and W. Kern, Thin film

processing II.

Documents

Sputtering 1