FINAL MICROFABRICATION

8/7/2019 FINAL MICROFABRICATION

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MICROFABRICATION

YASH V. KANKAL

ROLL NO.-32


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Definition:-y Microfabrication is the term that describes processes

of fabrication of miniature structures, of micrometresizes and smaller.

y Historically the earliest microfabrication processes wereused for integrated circuit fabrication, also known as

"semiconductor device fabrication," "semiconductormanufacturing,VLSI technology, microelectronicfabrication".


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Fields of use:-Microfabricated devicesinclude:-

yFabrication ofintegrated circuits

y Microelectromechanical systems (MEMS),

y microfluidic devices

y solar cells

y Flat Panel Displaysy Sensors (micro-sensors) (biosensors, nanosensors)

y PowerMEMSs, fuel cells, energy harvesters/scavengers


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Origins:-

y Microfabrication technologies originate from themicroelectronicsindustry, and the devices are usuallymade on silicon wafers even though glass, plastics andmany other substrate are in use .


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Microfabrication Techniques :-


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Microfabrication Processes:-Microfabrication is actually a collection of technologies

which are utilized in making microdevices.

y Substrates

y Deposition or Growth

y Patterning

y Etchingy Micro cutting / microfabrication

y Cleanlinessin wafer fabrication


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1. Substrates:-y Microfabricated devices are not generally freestanding

devicesbut are usually formed over or in a thicker supportsubstrate.

y The substrate enables easy handling of the micro devicethrough the many fabrication steps.

y For electronic applications,semiconducting substratessuch assilicon waferscan be used.

y For optical devices or flat panel displays, transparentsubstratessuch as glass or quartz are common.


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2. Deposition or Growth:-Microfabricated devices are typicallyconstructed using oneor more thin films.

Deposition techniquesinclude:y Thermal oxidationy chemical vapor deposition (CVD)

y APCVDy LPCVDy PECVD

y Physical vapor deposition(PVD)y Sputter depositiony evaporative deposition

y Electron beam PVD


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[a].Thermal oxidation:-

y In microfabrication,thermal oxidation is a way toproduce a thin layer of oxide (usuallysilicon dioxide) on the

surface of a wafer.

y The technique forces an oxidizing agent to diffuse into the

wafer at high temperature and react with it.

y Most thermal oxidation is performed in furnaces, at

temperaturesbetween 800 and 1200C.


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Furnaces used for diffusion and thermal oxidation

yVertical furnacesstand higher thanhorizontal furnaces,so they may not fitinto some microfabrication facilities.However, they help to prevent dust

contamination.

yVertical furnaces also eliminate non-uniformity of grown oxide across the

wafer.

yVertical furnaces have wafer sittinghorizontally, and then having the gas

flowin the furna

ce flow

ing from top tobottom.


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[b]. Chemical vapor deposition (CVD):-y Chemical vapor deposition (CVD) is a chemical process

used to produce high-purity, high-performance solidmaterials.

y The processis often used in the semiconductor industry toproduce thin films.

y In a typical CVD process, the wafer (substrate) is exposedto one or more volatile precursors, which react and/ordecompose on the substrate surface to produce the desired

deposit.


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Types of chemical vapor depositiony Atmospheric pressure CVD (APCVD) :- CVD processes at

atmospheric pressure.

y Low-pressure CVD (LPCVD) :-CVD processes atsubatmospheric pressures.

Reduced pressures tend to reduce unwanted gas-phase reactions and improve film uniformity across the

wafer.

y Plasma-Enhanced CVD (PECVD) :- CVD processes thatutilize plasma to enhance chemical reaction rates of theprecursors.


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[c].Physical vapor deposition (PVD) :-

y Physical vapor deposition (PVD) is a variety of

vacuum deposition.

y It is a general term used to describe methods todeposit thin filmsby the condensation of a vaporized

form of the material onto varioussurfaces.


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I.Sputter deposition :-

y Sputter deposition is aphysical vapor deposition(PVD) method ofdepositing thin filmsbysputtering, that isejecting, material from a"target," that issource,

which then deposits ontoa "substrate," such as asilicon wafer.


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yResputtering is re-emission of the deposited material during thedeposition processbyion or atom bombardment.

ySputtered atoms ejected from the target have a wide energy

distribution.

yThe sputtered ions (typically only a small fraction order 1% of the ejected particles are ionized) can ballistically fly from the

target in straight lines and impact energetically on the substratesor vacuum chamber (causing resputtering).

yThe sputtering gasis often an inert gassuch as argon.


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II.Evaporation (deposition) :-

Evaporation machine used for metallization


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y Evaporation is a common method of thin film deposition.

y The source material is evaporated in a vacuum.

y The vacuum allows vapor particles to travel directly to thetarget object (substrate), where theycondense back to asolid state.

y Evaporation involves two basic processes: a hot sourcematerial evaporates and condenses on the substrate.


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y Evaporation takes place in a vacuum. In high vacuum (with

a long mean free path), evaporated particlescan traveldirectly to the deposition target without colliding with thebackground gas.

y

Evaporated atoms that collide with foreign particles mayreact with them; for instance,if aluminium is deposited inthe presence of oxygen,it will form aluminium oxide.

y They also reduce the amount of vapor that reaches thesubstrate, which makes the thickness difficult to control.

y Evaporated materials deposit nonuniformlyif thesubstrate has a rough surface.


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III. Electron beam physical vapor deposition:-

Electron Beam Physical Vapor Deposition


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y Electron beam physical vapor deposition, however, yields a

high deposition rate from 0.1 m / min to 100 m / min atrelatively low substrate temperatures, with very highmaterial utilization efficiency.

y In an EBPVD system, the deposition chamber is evacuatedto a pressure

y The material to be evaporated isin the form ofingots.

y Electron beamscan be generated by thermionic emission,or the anodic arc method.


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y The generated electron beam is accelerated to a highkinetic energy and focused towards the ingot.

y 85% of the kinetic energy of the electronsisconverted into

thermal energy as the beam bombards the surface of theingot.

y The surface temperature of the ingot increases resulting in

the formation of a liquid melt.

y The ingot itselfis enclosed in a copper crucible, which iscooled by water circulation.


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3.Patterning :-y It is often desirable to pattern a film into distinct features

or to form openings (or vias) in some of the layers.

y These features are on the micrometer or nanometer scale.

y patterning techniquesinclude :-

Photolithography


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Photolithography :-

y Photolithographyis a process used in microfabrication tosele

ctively remove part

sof a th

in f

ilm or the

bulk of asubstrate.

y It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", on the

substrate.

y Photolithographyiscarried out in a sequence as givenbelow in tree-diagram -


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Clean wafer

Deposit barrier layer

SiO2, Si3N4, metal

Coat with photoresist

Soft bake

Align masks

Substrate

SiO2

Substrate

SiO2

PR

Substrate

SiO2

PR

Light

Photolithography


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Expose pattern

Develop photoresist

Hard bake

Etch windows in

barrier layer

Remove

photoresist

Substrate

SiO2

PR

Substrate

PR

SiO2

Substrate

SiO2


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4.Etching :-y Etching is the removal ofsome portion of the thin film or

substrate .

y The substrate is exposed to an etching which chemically or

physically attacks the film until it is removed.

y Etching techniquesinclude:

1. Dry etching (Plasma etching)

2. Wet etching or Chemical Etching


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I.Dry etching (Plasma etching) :-y Plasma etching is a form of plasma processing used to

fabricate integrated circuits.

y It involves a high-speed stream of glow discharge (plasma)of an appropriate gas mixture being shot at a sample.

y

The plasma

sour

ce,

known as

etchspe

cies,

can

be e

ithercharged (ions) or neutral (atoms and radicals).


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II.Wet etching :-

y The wafer can be immersed in a bath of etchant, which

must be agitated to achieve good processcontrol.

y Buffered hydrofluoric acid (BHF) is used commonly toetch silicon dioxide over a silicon substrate.


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5.Micro cutting / microfabrication :-

yMicro cutting is an alternative to lithographic techniques,by downscaling macro processessuch ascutting andforming, to tool sizes under 100 m in diameter.


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6.Cleanliness in wafer fabrication :-

y Microfabrication iscarried out in cleanrooms.

y Where air hasbeen filtered of particle contamination andtemperature, humidity, vibrations and electricaldisturbances.

y Smoke, dust,bacteria and cells are micrometersin size, andtheir presence will destroy the functionality of amicrofabricated device.


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THANK YOU

FINAL MICROFABRICATION

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