Physical Vapor Physical Vapor DepositionDeposition

PVDPVD

Physical methods produce the atoms Physical methods produce the atoms that deposit on the substratethat deposit on the substrate– EvaporationEvaporation– Sputtering Sputtering

Sometimes called vacuum deposition Sometimes called vacuum deposition because the process is usually done in because the process is usually done in an evacuated chamberan evacuated chamber

PVD is used for metals. PVD is used for metals. Dielectrics can be deposited using specialized Dielectrics can be deposited using specialized

equipment equipment

EvaporationEvaporation Rely on thermal energy supplied to the crucible or Rely on thermal energy supplied to the crucible or

boat to evaporate atomsboat to evaporate atoms Evaporated atoms travel through the evacuated Evaporated atoms travel through the evacuated

space between the source and the sample and space between the source and the sample and stick to the samplestick to the sample– Few, if any, chemical reactions occur due to low Few, if any, chemical reactions occur due to low

pressurepressure– Can force a reaction by flowing a gas near the crucibleCan force a reaction by flowing a gas near the crucible

Surface reactions usually occur very rapidly and Surface reactions usually occur very rapidly and there is very little rearrangement of the surface there is very little rearrangement of the surface atoms after stickingatoms after sticking– Thickness uniformity and shadowing by surface Thickness uniformity and shadowing by surface

topography, and step coverage are issuestopography, and step coverage are issues

EvaporationEvaporation

http://www.ee.byu.edu/cleanroom/metal.parts/vaporpressure.jpg

Mean Free PathMean Free Path

~ 63% of molecules undergo a collision in a ~ 63% of molecules undergo a collision in a distance less than distance less than and ~0.6% travel more and ~0.6% travel more than 5 than 5

– where dwhere doo is the diameter of the evaporatant and is the diameter of the evaporatant and n is the concentration of gas molecules in the n is the concentration of gas molecules in the chamberchamber

)in torr(

05.0

Law) Gas (Ideal

2

12

P

nRTPV

ndo

EvaporationEvaporation The vacuum is usually < 10The vacuum is usually < 10-5-5 torr torr

– 4x104x10-6-6 torr, torr, = 18 inches = 18 inches The source heater can beThe source heater can be

– Resistance (W, Mo, Ta filament)Resistance (W, Mo, Ta filament) Contaminants in filament systems are Na or K because they Contaminants in filament systems are Na or K because they

are used in the production of Ware used in the production of W– E-beam (graphite or W crucible)E-beam (graphite or W crucible)

E-beam is often cleaner although S is a common E-beam is often cleaner although S is a common contaminant in graphitecontaminant in graphite

– Top surface of metal is melted during evaporation so there is Top surface of metal is melted during evaporation so there is little contamination from the cruciblelittle contamination from the crucible

More materials can be evaporated (high melting-point More materials can be evaporated (high melting-point materials)materials)

A downside of e-beam is that X-rays are produced when the A downside of e-beam is that X-rays are produced when the electron beam hits the Al meltelectron beam hits the Al melt

– These X-rays can create trapped charges in the gate oxideThese X-rays can create trapped charges in the gate oxide– This damage must be removed by annealingThis damage must be removed by annealing

Thermal EvaporationThermal Evaporation

http://www.lesker.com/newweb/Deposition_Sources/ThermalEvaporationSources_Resistive.cfm

E-beam EvaporationE-beam Evaporation

http://www.fen.bilkent.edu.tr/~aykutlu/msn551/evaporation.pdf

PVDPVD

At sufficiently low pressure and At sufficiently low pressure and reasonable distances between source reasonable distances between source and wafer, evaporant travel in and wafer, evaporant travel in straight line to the waferstraight line to the wafer– Step coverage is close to zeroStep coverage is close to zero– If the source is small, we can treat it as If the source is small, we can treat it as

a point sourcea point source– If the source emission is isotropic, it is If the source emission is isotropic, it is

easy to compute the distribution of easy to compute the distribution of atoms at the surface of the wafer atoms at the surface of the wafer

PVDPVD

PVDPVD

For a source that emits only upwards, For a source that emits only upwards, = = 22– The number of atoms that hit the area AThe number of atoms that hit the area Akk of the of the

surface issurface is

– The deposition velocity is the above expression The deposition velocity is the above expression divided by the density (N) of the materialdivided by the density (N) of the material

kk

RF cos

r2

evap

kN

Rv cos

r2

evap

EvaporationEvaporation

– PPee is the equilibrium vapor pressure of the melt (torr) is the equilibrium vapor pressure of the melt (torr)

– m is the gram-molecular massm is the gram-molecular mass– T is the temperature (K)T is the temperature (K)– As is area of sourceAs is area of source

The vapor pressure depends strongly on the The vapor pressure depends strongly on the temperature (Claussius-Clapeyron equation)temperature (Claussius-Clapeyron equation)– In order to have a reasonable evaporation rate (0.1-1 In order to have a reasonable evaporation rate (0.1-1

m/min), the vapor pressure must be about 1-10 mtorrm/min), the vapor pressure must be about 1-10 mtorr

eS PT

mAR

2/12

evap 1083.5

PVDPVD

PVDPVD

The velocity can be normalized to the The velocity can be normalized to the velocity at the center of the wafervelocity at the center of the wafer

PVDPVD

Corrections can be applied if the source is Corrections can be applied if the source is a small, finite areaa small, finite area– If we now move the center of the wafer from If we now move the center of the wafer from

the perpendicular position, but tile it with the perpendicular position, but tile it with respect to the source, an extra term must be respect to the source, an extra term must be addedadded

ikN

Rv coscos

r2

evap

PlanetariesPlanetaries

Wafer holders that rotate wafer Wafer holders that rotate wafer position during deposition to increase position during deposition to increase film thickness uniformity across film thickness uniformity across wafer and from one wafer to another.wafer and from one wafer to another.– Wobbling wafer holders increase step Wobbling wafer holders increase step

coveragecoverage

PVDPVD

Nonuniformity of evaporatant can Nonuniformity of evaporatant can occur when angular emission of occur when angular emission of evaporant is narrower than the ideal evaporant is narrower than the ideal sourcesource– Crucible geometryCrucible geometry– Melt depth to melt area ratioMelt depth to melt area ratio– Density of gas atoms over the surface of Density of gas atoms over the surface of

the meltthe melt

EvaporationEvaporation

Evaporating alloys is difficult Because of Evaporating alloys is difficult Because of the differing vapor pressures. the differing vapor pressures. – Composition of the deposited material may Composition of the deposited material may

very different from that of the target materialvery different from that of the target material The problem can be overcome by The problem can be overcome by

– Using multiple e-beams on multiple sources Using multiple e-beams on multiple sources This technique causes difficulties in sample This technique causes difficulties in sample

uniformity because of the spacing of the sourcesuniformity because of the spacing of the sources

– Evaporating source to completion (until no Evaporating source to completion (until no material is left)material is left) Dangerous to do in e-beam systemDangerous to do in e-beam system

EvaporationEvaporation

Compounds are also hard to Compounds are also hard to evaporate because the molecular evaporate because the molecular species may be different from the species may be different from the compound compositioncompound composition– Energy provided may be used to Energy provided may be used to

dissociate compound.dissociate compound.– When evaporating SiOWhen evaporating SiO22, SiO is deposited. , SiO is deposited.

Evaporation in a reactive environment Evaporation in a reactive environment (flowing O2 gas near crucible during (flowing O2 gas near crucible during deposition) helps reconstitute oxide.deposition) helps reconstitute oxide.

EvaporationEvaporation

AdvantagesAdvantages– Little damage to the Little damage to the

waferwafer– Deposited films are Deposited films are

usually very pureusually very pure– Limited step Limited step

coveragecoverage

DisadvantagesDisadvantages– Materials with low Materials with low

vapor pressures ae vapor pressures ae very difficult to very difficult to evaporatedevaporated Refractory metalsRefractory metals High temperature High temperature

dielectricsdielectrics

– No No in situin situ precleaning precleaning– Limited step Limited step

coveragecoverage– Film adhesion can be Film adhesion can be

problematicproblematic

Step-coverageStep-coverage

Evaporation technique is Evaporation technique is very directional due to the very directional due to the large mean free paths of gas large mean free paths of gas molecules at low pressure.molecules at low pressure.

Shadowing of patterns and Shadowing of patterns and poor step coverage can poor step coverage can occur when depositing thin occur when depositing thin films.films.

Rotation of the planetary Rotation of the planetary substrate holder can substrate holder can minimize these effects.minimize these effects.

Heating substrate can Heating substrate can promote atom mobility, promote atom mobility, improve step coverage and improve step coverage and adhesion.adhesion.

Shadow masking and lift-off Shadow masking and lift-off are processes where poor are processes where poor step coverage is desirable. step coverage is desirable.

Other PVD TechniquesOther PVD Techniques

Other deposition techniques includeOther deposition techniques include– Sputter deposition (DC, RF, and Sputter deposition (DC, RF, and

reactive)reactive)– Bias sputteringBias sputtering– Magnetron sputteringMagnetron sputtering– Collimated and ionized sputter Collimated and ionized sputter

depositiondeposition– Hot sputter depositionHot sputter deposition

SputteringSputtering Sputter deposition is done Sputter deposition is done

in a vacuum chamber in a vacuum chamber (~10mTorr) as follows:(~10mTorr) as follows:– Plasma is generated by Plasma is generated by

applying an RF signal applying an RF signal producing energetic ions.producing energetic ions.

– Target is bombarded by Target is bombarded by these ions (usually Arthese ions (usually Ar++).).

– Ions knock the atoms from Ions knock the atoms from the target.the target.

– Sputtered atoms are Sputtered atoms are transported to the substrate transported to the substrate where deposition occurs.where deposition occurs.

SputteringSputtering Wide variety of materials can be Wide variety of materials can be

deposited because material is deposited because material is put into the vapor phase by a put into the vapor phase by a mechanical rather than a mechanical rather than a chemical or thermal process chemical or thermal process (including alloys and insulators).(including alloys and insulators).

Excellent step coverage of the Excellent step coverage of the sharp topologies because of a sharp topologies because of a higher chamber pressure, higher chamber pressure, causing large number of causing large number of scattering events as target scattering events as target material travels towards wafers.material travels towards wafers.

Film stress can be controlled to Film stress can be controlled to some degree by the chamber some degree by the chamber pressure and RF power.pressure and RF power.

http://www.knovel.com

Deposition conditionsDeposition conditions

Temperature: Room to higherTemperature: Room to higher Pressure: 100mtorrPressure: 100mtorr

– compromise between increasing number compromise between increasing number of Ar ions and increasing scattering of Ar of Ar ions and increasing scattering of Ar ions with neutral Ar atoms ions with neutral Ar atoms

PowerPower– Heating of target materialHeating of target material

Low temperature metals can melt from Low temperature metals can melt from temperature rise caused by energy transfer temperature rise caused by energy transfer from Ar ionsfrom Ar ions

Sputter sourcesSputter sources MagnetronMagnetron

– Magnetic field traps freed electron near targetMagnetic field traps freed electron near target– Move in helical pattern, causing large number of Move in helical pattern, causing large number of

scattering events with Ar gas – creating high density of scattering events with Ar gas – creating high density of ionized Arionized Ar

Ion beamIon beam– Plasma of ions generated away from target and then Plasma of ions generated away from target and then

accelerated toward start by electric fieldaccelerated toward start by electric field Reactive sputteringReactive sputtering

– Gas used in plasma reacts with target material to form Gas used in plasma reacts with target material to form compond that is deposited on wafercompond that is deposited on wafer

Ion-assisted depositionIon-assisted deposition– Wafer is biased so that some Ar ion impact its surface, Wafer is biased so that some Ar ion impact its surface,

density the deposited film. May sputter material off of density the deposited film. May sputter material off of wafer prior to deposition for in-situ cleaning.wafer prior to deposition for in-situ cleaning.

SputteringSputtering Advantages

– Large-size targets, simplifying the deposition of thins with uniform thickness over large wafers

– Film thickness is easily controlled by fixing the operating parameters and simply adjusting the deposition time

– Control of the alloy composition, step coverage, grain structure is easier obtained through evaporation

– Sputter-cleaning of the substrate in vacuum prior to film deposition

– Device damage from X-rays generated by electron beam evaporation is avoided.

Disadvantages– High capital expenses are

required– Rates of deposition of

some materials (such as SiO2) are relatively low

– Some materials such as organic solids are easily degraded by ionic bombardment

– Greater probability to introduce impurities in the substrate because the former operates under a higher pressure 

SalicideSalicide

http://www.research.ibm.com/journal/rd/444/jordansweet.htmlhttp://www.research.ibm.com/journal/rd/444/jordansweet.html