(8 2) Film Preparation PVD[1]

8/8/2019 (8 2) Film Preparation PVD[1]

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Physical Vapor Deposition (PVD)

Vacuum evaporation


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Vacuum evaporation

Material is heated to attain gaseous state

Carried out under high vacuum (10-7 torr, or 10-4 ~10-5Pa)

Advantages

Films can be deposited at high rates (~0.5 m/min)

Low energy atoms (~0.1 eV) leave little surface damage

Little residual gas and impurity contamination due to high vacuum

No substrate heating

Inexpensive

Limitations

Difficult to control alloy compounds

Poor step coverage

Nonuniformity of coverage over wafer or multiple wavers

Resistive heating evaporation

Simple, Robust, Inexpensive

Can only reach temperatures of1800C Uses W, Ta, or Mo filaments to heat sources

Typical filament currents are 200-300A

Exposes substrate to visible and IR radiation

Typical rates are 0.1-2 nm/sec

Materials

Au, Ag, Al, Sn, Cr, Ti, Cu


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Evaporation system requirements

Vacuum:

10-6 torr for medium quality films

Cooling water

Hearth

Bell Jar

Mechanical Shutter

Evaporation rate is set by temperature of source, can

not be turned on and off rapidly. A mechanical shutter

allows control of start and stop times.

Electrical Power

Either high current or high voltage: typically 1-10kW

Evaporation support materials

Metals Tungsten (W): MP = 3380C Tantalum (Ta): MP = 3000C Molybdenum (Mo): MP = 2620C

Ceramics Graphitic Carbon (C): MP = 3700C Boron Nitride (BN): MP = 2500C Alumina (Al2O3) MP = 2030C


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Resistive heating elements

Electron beam heated evaporation source


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Electron beam properties

More complex than resistive heating but extremely

versatile

Can achieve temperatures > 3000C Uses evaporation crucibles in a copper hearth

Typical emission voltages: 8-10 kV can produce

x-rays

Typical deposition rates 1-10 nm/sec

Evaporant Materials:

Everything resistance heating uses plus

Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo

Al2O3, SiO, SiO2,SnO2, TiO2, ZrO2

Adsorption

Adsorption is the sticking of a particle to the surface

Physisorption:

The impinging molecule loses kinetic energy to thermal

energy within some residence time, and the lower energy of

the molecule does not allow it to overcome the threshold thatis needed to escape

Chemisorption:

The impinging molecule loses its kinetic energy to a

chemical reaction which forms a chemical bond between it

and other substrate atoms.


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Condensation

The molecules impinging on the surface may:

Adsorb and permanently stick where they land (rare)

Adsorb and diffuse around the surface to find an

appropriate site

Adsorb and desorb after some residence lifetime.

Immediately reflect off the surface

Incident vapor molecules normally have a kinetic energy

much higher than kT of the substrate surface

Whether they stick depends on how well it can equilibrate

with the substrate surface giving up enough energy so that

it does not have enough to escape

Condensation control

Control of condensation of the evaporant is achieved

through control of the substrate temperature

Higher substrate temperature

Increases thermal energy of adsorbed molecules

Shortens the residence time

Increases surface diffusivity of adsorbed molecules

Performs annealing of deposited film

Substrate heaters

IR lamps from frontside

Heater coils from backside


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Kinetic theory of gases

PV=nRT n=PV/RT concentration of gas

At STP n ~ 2.7 x 1019 molecules/cm3

Standard pressure

1 atm = 760 mmHg = 760 torr = 1.013 x 105 Pa

Mean Free Path (): mean distance a moleculetravels before colliding with another molecule

for 10-4 Pa = 60m Line of sight travel no collisions

ndpd

kT

22

707.0

2 ==

Flux

Can calculate the bombardment rate of molecules on the surface

(Flux) # of molecules per area per second

Used to estimate the deposition rate

sec)/(1063.2

2

220

== cmmoleculesMT

P

mkT

P

M: molecular weight

P: Pressure (Pa)

T: Temp in Kelvin

K: Boltzman Coef.

m: mass of molecule

Pressure : P

P

mkTNNt

ss 2=

=

assumes each molecule sticks

t: time to form monolayer

Ns: surface density (molecules/cm2)


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Evaporation Source Position

Molecular Beam Epitaxy (MBE)

Insert sampleand heatmaterialsources

Open shutters

Monitorcondensationandsublimation

Closeshutters

Removesample


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Actually Evaporation, not CVD

Requires Ultra-High vacuum (10-10 torr)

Shuttered effusion cellscontain very pure samples ofthe target material.

Shutters are opened and exposed to an electron beamwhich vaporizes the target material.

Wafer surface is heated to promote epitaxial filmgrowth.

Wafer is rotated to improve uniform film growth.

MBE allows for the creation of very specialized devices:

- mono-atomic sandwiched layers are possible

A favorite toy of research laboratories, but is also usedin the mass-production of gallium arsenide devices.


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Physical Vapor Deposition (PVD)-- Sputtering

Uses high energy particles (plasma) to dislodge atoms

from source surface

Carried out in low-medium vacuum (~10-2 torr)

Advantages Can use large area targets for uniformity of film

Easy thickness control via time

Easy to deposit alloys and compounds

Good step coverage

No x-ray damage

Sputter Station

Magnetic field used to confine plasma and electric field used to

accelerate

DC plasma used for conductive metals

RF plasma used for nonconductive dielectrics

Several targets can mix or due layers without breaking vacuum


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RF Sputter System

Magnetron Sputter Deposition

Sputter Deposition Systems

E

DC Sputter Deposition

Physical Vapor Deposition (PVD)-- DC Sputtering

Uses plasma to sputter target, dislodging atoms whichthen deposit on wafers to form film. Higher pressures than evaporation - 1-100 mtorr. Better at depositing alloys and compounds thanevaporation.


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Physical Vapor Deposition (PVD)-- RF Sputtering

For DC sputtering, target electrode is conducting.

To sputter dielectric materials use RF power source.

Due to slower mobility of ions vs. electrons, the plasma biases

positively with respect to both electrodes. (DC current must be zero.) continuous sputtering. When the electrode areas are not equal, the field must be higher at thesmaller electrode (higher current density), to maintain overall currentcontinuity

Physical Vapor Deposition (PVD)-- RF Sputtering

Thus by making the target electrode smaller,sputtering occurs "only" on the target. Waferelectrode can also be connected to chamberwalls, further increasing V1/V2.

The wafer electrode can be separately biased(RF), which allows cleaning or controlledsputtering of the wafer with Ar+ ions (bias-sputterdeposition).

This can allow more conformal depositionbecause the ions are highly nondirectional andsputter selectively.


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Comparison of evaporation and sputtering

EVAPORATION

Low energy atoms

High vacuum path

Few collision

Line-of-sight deposition

Little gas in film

Larger grain size

Fewer grain orientations

Poorer adhession

SPUTTERING

High energy atoms

Low vacuum, plasma path

Many collision

Less line-of-sight deposition

Gas in film

Smaller grain size

Many grain orientations

Better adhession

Pulsed Laser Deposition System

Target: metals, semiconductors

Laser: UV, 10 ns pulses

Vacuum: Atmospheres to ultrahigh vacuum

Film thickness: typically 100-200 nm.

Deposition rate: 0.1 nm/pulsehttp://www.physandtech.net/


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Advantages of Pulsed Laser Deposition

Flexible, easy to implement

Growth in any environment

Exact transfer of complicated materials (YBCO)

Variable growth rate

Epitaxy at low temperature

Resonant interactions possible (i.e., plasmons in metals,absorption peaks in dielectrics and semiconductors)

Atoms arrive in bunches, allowing for much more controlleddeposition

Greater control of growth (e.g., by varying laserparameters)

Uneven coverage

High defect or particulate concentration

Not well suited for large-scale film growth

Mechanisms and dependence on parameters not well understood

Disadvantage:

PLD with ultrafast (


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Optimization of PLD Parameters

PLD technique is one of the most popular and effectivetechniques used in the present days for the deposition of thinfilms. In this technique, a pulsed laser is directed on a solidtarget. The nanosecond laser pulse is focused to give anenergy density sufficient to vaporize a few hundredangstroms of surface material in the form of neutral or ionicatoms and molecules with kinetic energies of a few eV, whichthen get deposited onto the substrate.

The plasma temperature is high (~ 103 K) and the evaporantsbecome more energetic when they pass through the plume.This affects the film deposi tion in a positive manner due toincrease in the adatom surface mobility.

Use of short pulses helps to maintain high laser pow erdensity in a small area of the target and produces congruentevaporation.

Deposition parameters: substrate temperature, laser fluence,pulse repetition rate, and target substrate distance.

Glancing Angle Deposition (GLAD)

GLAD UHV deposition system

Kevin Robbie et al.,J. Vac. Sci. Technol. A15 (1997) 1460; B16 (1998) 1115.

Rev. Sci. Instrum. 75 (2004) 1089.


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GLAD is based on thin film deposition, by evaporation or sputtering, and employs

oblique angle deposition flux and substrate motion to allow nanometer scale control

of structure in engineered materials.


Stationary substrate

Tangent rule: tan() = (1/2) tan () (when is small; poor when > 50)(Nieuwenhuizen and Haanstra, Philips Tech. Rev.27 (1966) 87.)

Tait relationship: = asin ((1-cos())/2) (when is large)(Tait, Smy, Brett, Thin Solid Films 226 (1993) 196.)


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Staionary substrate, one evaporation source, Cr films


Stationary substrate, two evaporation sources, SiO2 films

Independent control of column angle and film porosity. The porosity is constant, the

column angle is controlled between the inclined and vertical angles.


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Various nanostructures obtained in GLAD thin films


Various nanostructures obtained in GLAD thin films


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GLAD is based on thin film deposition, by evaporation or sputtering, and employs

oblique angle deposition flux and substrate motion to allow nanometer scale control

of structure in engineered materials.

flux arrived from

the right for the

entire deposition

the direction of arrival

of the flux was

alternated from the

left and right 12 times

during deposition

the substrate

was rotated

continuously

during deposition

a combination of the

techniques used in b)

and c). The substrate

was rotated in 90

degree steps during

deposition.


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2/1

1 )/( gUch =

h: film thickness;

U: substrate speed;

: liquid viscosity;

: liquid density;

c1: ~0.8 for Newtonian liquid

2/11/6

LV

2/3 )(/)(0.94 gUh =When U and are not high enough,


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2/12

0

2

0 )3/41/()( thhth +=h0: initial thickness; t: time : liquid density;

: angular velocity; : liquid viscosity;

3/1

2

m3

AAfinal ))(/-(1 2A

=h AAoff-spinoff-spinfinal /mhtt +=


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Nonhydrolytic Sol-gel


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Nonhydrolytic Sol-gel

Documents

(8 2) Film Preparation PVD[1]