thin film deposition using physical vapor deposition

7/30/2019 thin film deposition using physical vapor deposition

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THIN FILM DEPOSITION

TECHNIQUES

PHYSICAL VAPOR DEPOSITION


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INTRODUCTION

a thin film is a low dimensional material created by

condensing, one by one, atomic/molecular/ionic speciesof matter. The thickness is typically less than severalmicrons.

Thin - less than about one micron (10,000 A0, 1000 nm)

Film - layer of material on surface. If no substrate it isfoil.

Thin film materials are key elements of continuedtechnological advances made in the fields ofoptoelectronic, photonic, and magnetic devices.

The processing of materials into thin films allows easyintegration into various types of devices. The propertiesof material significantly differ when analysed in the formof thin films.


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PVD

Physical method covers the depositiontechniques which depend on the evaporationor ejection of the material from a source, i.e.

evaporation or sputtering. The deposition is obtained by physically

transporting the atoms from source tosubstrate in gas phase

Three main techniques : evaporation ,sputtering and MBE


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EVAPORATION Used for deposition of metals (Al , Ag) , dielectrics (SiO2) and

semiconductor (Si)

Energy for deposition is in the form of heat and material fordeposition is in solid form

CELL HOLDER

SUBSTRATE OR SOLAR CELL

RESISTIVE OR EBEAM

HEATING

PELLET HOLDER

SOURCE MATERIAL

PELLETS

CHAMBER

UNDER VACUUM


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Thermal evaporation involves three steps : transition of phase(solid to gas), transport of vapor from source to substrate andcondensation of vapor on substrate.

Mo,W and Ta having high melting point are used to hold thesource.

Heat for melting source is by thermal or E beam heating or archeating , laser etc.

Vacuum of 10-4 to 10-6 torr ambience is required (higher

vacuum gives higher mean free path)Methods for

evaporating

multicomponent films

include (a) single

source evaporation,(b) multisource

simultaneous

evaporation and (c)

multisource sequential

evaporation


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Gas impingement rate

Where

T = temperature of the sourcePvap = vapor pressure (Torr)

M = molecular weight

cm2 => area of source

can convert this to mass evaporation rate

at Pvap = 10-2 torr, mass flux = 10-4 grams/cm2sec


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since dM/dAs depends on r, , so does film thickness (d)

consider flat substrate, perpendicular to source

point source:

surface source:surface source has

slightly poorer thickness

uniformity


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Film thickness uniformity for point and surface sources. (insert) geometry

of evaporation onto parallel plane substrates


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The principal requirement for successful thin-film

growth is that the mean-free path of the atoms

must be greater than the distance between the

source and substrate. The mean free path of a

molecule in a gas is

where d is the diameter of the gas molecules, and P

is the pressure of the gas.

Advantage : relatively high deposition rates, rate

and thickness control in real time, and better

evaporant stream directional control for

applications such as lift off processing to obtain

direct patterned coatings.


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the ejection of surface atoms from anelectrode surface by momentum transfer frombombarding ions to surface atoms.

an etching process, in fact, used as such forsurface cleaning and for pattern delineation.Since sputtering produces a vapor of electrodematerial, it is also (and more frequently)usedas a method of film deposition similar toevaporative deposition.

SPUTTERING


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TARGET1 kV to 5 kV

Ar+ Ar+e -

e -

PLASMA

TARGET

ATOMS

SUBSTRATE

GAS

INLET PUMP

Target is made cathode and

substrate is anode. Inert gas,Ar is

used to create plasma consisting

of ionised gas, uionized gasmolecules and electrons. Due to

high voltage electrons in chamber

get accelerated hits Ar atom and

dislodge e- to create Ar+ which

transfers momentum to Targetatom to dislodged a target atom

resulting in generation of

secondary e- from target

maintaining the plasma bycreating more Ar+. The dislodged

target atom condesned on

substrate and thin film deposition

on substrate is achieved.


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Diagram of a typical MBE system growth chamber


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Molecular beam epitaxy(MBE) is performed withdifferent types of semiconducting materialslike:

i) Group IV elementalsemiconductors like Si, Ge, and C

ii) III-V-semiconductors: arsenides

(GaAs, AlAs, InAs), antimonides

like GaSb and phosphides like InP

iii) II-VI- semiconductors: ZnSe, CdS,

and HgTe


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RHEED Gun setup for MBE growth

MBE: Working Conditions

The mean free path (l) of the particles > geometrical size of thechamber (10-5 Torr is sufficient)

Ultra-high vacuum (UHV= 10-11Torr) to obtain sufficiently clear

epilayer.

Gas evalution from materials has to be as low as possible.Pyrolytic boron nitride (PBN) is chosen for crucibles (Chemically

stable up to 1400C)

Molybdenum and tantalum are widely used for shutters.

Ultrapure materials are used as source.


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Molecular Beam Epitaxy: Process

Ultra-pure elements are heated in separate

quasi-knudson effusion cells (e.g., Ga and As)until they begin to slowly sublimate.

Gaseous elements then condense on the wafer,

where they may react with each other (e.g.,GaAs).

The term beam means the evaporated atoms

do not interact with each other or with othervacuum chamber gases until they reach thewafer.


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MBE growth mechanism.

Atoms arriving at the substrate surface may undergo

absorption to the surface, surface migration,

incorporation into the crystal lattice,

thermal desorption.

depends strongly on the temperature of the substrate..


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Advantages Disadvantages

Clean surfaces, free of an oxide layer Expensive (106 $ per MBE chamber)

In-situ deposition of metal seeds,

semiconductor materials, and dopants

ATG instability

Low growth rate (1m/h) Very complicated system

Precisely controllable thermal evaporation Epitaxial growth under ultra-high vacuum

conditions

Seperate evaporation of each component

Substrate temperature is not high

Ultrasharp profiles

thin film deposition using physical vapor deposition

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