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    Thin Film Epitaxy

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    IntroductionEpitaxy comes from Greek words:

    Epi: upon

    Taxis: orderedEpitaxial growth: single crystal growth of a material in which a substrate

    serve as a seed

    2 types of epitaxy:

    Homoepitaxy material is grown epitaxially on a substrate of thesame material. E.g. grow of Si on Si substrate

    Heteroepitaxy a layer grown on a chemically different substrate.E.g. Si growth on sapphire

    Similar crystal structures of the layer and the substrate, BUT

    The shift of composition causes difference in lattice parameters

    Limit the ability to produce epitaxial layers of dissimilar materials

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    Film deposited on a

    oriented wafer

    orientation

    The presence of SiO2Layer cause depositing

    atoms have no

    structurepolysilicon

    Epitaxial and polysilicon film growth

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    Applications of epitaxial layersDiscrete and power devices

    Integrated circuits

    Epitaxy for MOS devices

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    1. Discrete and power devicesTechnology change: junction transistors diffused planar

    structure

    Requires a material structure that are not achieved by diffusion ofdopants from the surface

    Si epitaxy was developed to enhance the electricalperformance of discrete bipolar transistors

    Breakdown voltage of the discrete transistor was limited bythe field avalanche breakdown of the substrate materialUse higher resistivity substrates produced higher breakdown

    voltages but increased collector series resistance

    Structure needed: thin, lightly doped and single crystallayer of high perfection upon more heavily doped SisubstrateBut, the use of a more heavily doped substrate reduces the

    collector series resistance while the base-collector breakdownvoltage is governed by the lighter doping in the near surfaceregion

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    Epitaxial deposition of a lightly doped P+ epitaxial layer on

    a N+ substrate make the desired properties are

    achievable

    Epitaxial grows also allows accurate control of doping

    levels and advantages which arises from a generally low

    oxygen and carbon levels in epitaxial layer

    Epitaxial technique was developed to 2 and 3 layers

    epitaxial structure

    For lightly doped area of collector

    Based region was also grown epitaxially

    E.g. of multilayer structures: Si-Controlled Rectifier (SCR),

    Triac, high voltage or high power discrete products

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    Mesa discrete transistor fabricated in an epitaxial

    layer on a heavily doped N+ substrate

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    Transistors

    Diodes

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    2. Integrated circuit (IC)

    Development of planar bipolar IC caused the requirement for

    devices built on the same substrate to be electrically isolated

    The use of opposite typed substrate and epitaxial layer met partof the requirement

    Device isolation was completed by the diffusion of isolation

    region through the epitaxial layer to contact the substrate

    between active areas In planar bipolar circuits, common to employ a heavily doped

    diffused (or implanted) region under the transistor

    Usually called buried layer or DUF for diffusion under film

    The buried layer

    serves to lower the lateral series resistance between collector area below

    the emitter and the collector contact

    produce uniform planar operation of the emitter, avoiding current crowding

    which leads to hot spots near edges of the emitter

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    Integrated circuits

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    (a) A junction isolated bipolardevice fabricated as part of an

    integrated circuit using a buried

    layer subcollector and a lightly

    doped n-epitaxial layer

    (b) An N-Well CMOS structure

    fabricated in a lightly doped p-epitaxial layer

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    3. Epitaxy for MOS devices

    Unipolar devices such as junction field-effect

    transistors (JFETs), VMOS, DRAMs technologyalso use epitaxial structures

    VLSI CMOS (complimentary metal-oxide-

    semiconductor) devices have been built in thin(3-8 micron) lightly doped epitaxial layers on

    heavily doped substrates of the same type (N or

    P) That epitaxial structure reduces the latch up of high

    density CMOS IC by reducing the unwanted

    interaction of closely spaced devices

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    Advantages of epitaxy

    Ability to place a lightly oppositely doped

    region over a heavily doped region Ability to contour and tailor the doping

    profile in ways not possible using diffusionor implantation alone

    Provide a layer of oxygen free material

    that is also contained low carbon

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    Techniques for silicon epitaxy

    1. Chemical Vapour Deposition (CVD)

    2. Molecular Beam Epitaxy (MBE)3. Liquid Phase Epitaxy (LPE)

    4. Solid phase regrowth

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    1. Chemical Vapour Deposition (CVD)

    The most common technique in Si epitaxy

    In the CVD technique Si substrate is heated in a chamber: sufficient heat to

    allow the depositing Si atoms to move into position to

    Reactive Si containing gaseous compounds areintroduced

    Gaseous react on the hot surface of the substrate and

    deposit a Si layer The deposit will take on Si substrate structure if the

    substrate is atomically clean and the temperature is

    sufficient for atoms to have surface mobility

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    Schematic drawing of a simple horizontal flow, cold

    wall, CVD reactor

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    CVD reactions

    1. Pyrolysis: chemical reaction is driven by heat alone, e.g. silanedecomposes with heating

    SiH4 Si + 2H2

    2. Reduction: chemical reaction by reacting a molecule withhydrogen, e.g. silicon tetrachloride- reduction in hydrogen ambient

    to form solid silicon

    SiCl4 + 2H2 Si + 4HCl

    3. Oxidation: chemical reaction of an atom or molecule withoxygen, e.g. SiH4 decomposes at lower temperatureSiH4 + O2 SiO2 + 2H2

    4. Nitridation: chemical process of forming silicon nitride byexposing Si wafer to nitrogen at high temperature e.g. SiH2Cl2readily decomposes at 1050C

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    CVD

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    CVD film growth steps

    1. Nucleation

    Dependent on substrate quality

    Occurs at first few atoms or molecules deposit on a surface

    2. Nuclei growth

    Atoms or molecules form islands that grow into larger islands

    3. Island coalescence The islands spread , and coalescing into a continuous film

    This is the transition stage of the film growth, thickness several

    hundreds Angstroms

    Transition region film possesses different chemical and

    physical properties for thicker bulk film

    4. Bulk growth

    Bulk growth begins after transition film is formed

    CVD film growth steps

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    CVD film growth steps

    Types of film

    structure

    Amorphous

    Polycrystalline

    Single crystal

    Basic CVD subsystem

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    2. Molecular Bean Epitaxy (MBE)

    Uses an evaporation method

    MBE is carried out at a lower temperature than 1000-

    1200C (typical CVD temperature)

    Reduces outdiffusion of local areas of dopant diffused

    into substrates and reduce autodoping which is

    unintentionally transfer of dopant into epitaxial layer

    MBE is favourable

    preparation of sub-micron thickness epitaxial layers or

    high frequency devices requiring hyper-abrupt transition in the

    doping concentration between the epitaxial layer and the

    substrate

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    In MBE, Si and dopant(s) are evaporated in an ultra high vacuum (UHV)

    chamber

    The evaporated atoms are transported at relatively high velocity

    in a straight line from the source to the substrate They condense on the low temperature substrate

    The condensed atoms of Si or dopant will diffuse on the surfaceuntil they reach a low energy site that they fit well the atomic

    structure of the surface The adatom then bonds in that low energy site, extending the

    underlying crystal by a vapour to solid phase crystal growth

    Usual temperature range of the substrate is 400-800C. Higher

    than 800C is possible but it will increase outdiffusion or lateraldiffusion of dopants in the substrate

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    Schematic drawing of a molecular beam

    epitaxial system

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    Insitu cleaning of the substrate Can be done by high temperature bake at 1000-1250C for

    several minutes under high vacuum to decompose the nativesurface oxide and to remove other surface contaminants

    Other technique is by using a low energy beam of inert gas tosputter clean the substrate

    Difficult to remove carbon but will decrease at the surface bydiffusion into the substrate during short anneal at 800-900C

    Wider range of dopants for MBE than CVD epitaxy: Typical dopants: Antimony, Sb (N-type), aluminum, Al or gallium

    (Ga) for P-type

    N-type dopant: As and P, evaporate rapidly even at 200C.

    Difficult to control P-type dopant: Boron, evaporate slowly even at 1300C

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    Schematic drawing of a multiple chamber MBE

    system

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    MBE Equipment

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    Liquid Phase Epitaxy (LPE)

    LPE technique is widely used for preparation of epitaxiallayers on compound semiconductors and for magnetic

    bubble memory films on garnet substrate In films growth by LPE from solution melts, low cooling

    rates, when the surface reaction (growth)

    Kinetics are rapid compare to the mass transport of Si tothe seed, epitaxial layer thickness will vary in proportionto the temperature drop

    Increase cooling rates, mass transport rate will increase

    and the growth rate will increase with cooling rate untilgrowth rate becomes limited by surface reaction kinetics

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    Growth rate increases with

    cooling rate up to about 1

    degree/min while growth rate

    above 2 degree/min occurred

    under kinetically limited

    conditions

    LPE growth rate increasing with

    cooling rate up to about 1 micron per

    minute

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    Schematic drawing of a typical silicon liquid

    phase epitaxy (LPE)

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    Schematic of fabrication steps in the fabrication vertical field effect transistors by etch and LPE refill techniques

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    4. Solid Phase Re-growth

    i. Re-growth of amorphous layers

    Surface layers subjected to high dose ionimplants are in amorphous structure due to theheavy damage inflicted on the lattice as theenergetic ions are absorbed

    Annealing above 600C amorphous layerre-crystallize

    Re-crystallisation occurs from interface moves

    toward the surface and results in solid phaseepitaxial re-growth

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    ii. Re-crystallisation of thin films

    Involves re-crystallisation of a deposited amorphous orpolysilicon film

    Si film is deposited on a Si substrate or more commonly SiO2

    heated using a strip heater passed over the surface or by a

    scanned pulsed laser to crystallise the film to single crystal orlarge grain polysilicon

    This fabrication technique is used to produce a stacked n-channel

    device in re-crystallised polysilicon on a thermally grown ordeposited oxide

    Oriented epitaxial growth can be obtained by making series of

    holes in the oxide to allow points of contact between the

    underlying substrate and the deposited polysilicon The contact points become seeds areas for establishing re-

    growth orientation

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    Re-crystallisation solid phase

    epitaxy using a moving strip heater

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    A stacked MOS structure over an

    insulating oxide fabricated in a re-

    crystallised polysilicon layer

    S f

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    Structure and defects in epitaxial layer

    Surface morphology of Silicon epitaxial deposits isaffected by growth and substrate parameters

    Growth parameters: Temperature

    Pressure

    Concentration of Si containing gas

    Cl : H2 ratio

    Substrates parameters Substrate orientation

    Defects in the substrate Contaminants on the surface of the substrate

    T i l d f t i it i l l

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    Typical defects in epitaxial layers

    1. Substrate orientation effects

    2. Spikes and epitaxial stacking faults

    3. Hillocks and pyramids in epitaxial layers

    4. Dislocations and slip

    5. Microprecipitates (S-pits)

    1 S b t t i t ti ff t

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    1. Substrate orientation effect

    Growth of smooth epitaxial films can be obtained on

    (100) and (110) oriented Si substrates

    Epitaxial growth on substrate surface on oriented on(111) plane results in facetted alligator skin surface

    (111) surfaces contain no atomic steps to provide a density of

    growth sites Without atomic steps, the growth produces pyramids and

    terraces

    Misorientation of the surface by

    0.5 degreeintroduces a sufficient density of steps for growth of

    smooth planar films

    2 S ik d it i l t ki f lt

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    2. Spikes and epitaxial stacking faults

    i. Growth spike

    Originate from Si particle on the surface not removed

    by the pre-epitaxial cleaning process

    Si Chips may expose faster growing crystal planes

    than the plane of the substrate

    Chips nucleate and produce polysilicon nodule. The

    chips then protrude above the substrates surface into

    a region of richer supply of gaseous reactants

    Results in nodule grows at 2-10 times the rate ofepitaxial film on the substrate.

    May be removed mechanically before the next step

    but will leave a region unusable for functionalmaterials

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    ii. Epitaxial stacking faults

    Crystallographic in nature and arise from defects in

    atomic arrangement during film growth

    Could result from an extra atomic layer (extrinsic fault)

    or a missing atomic layer (intrinsic fault) along {111}

    type plane

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    Epitaxial growth spike Stacking fault on Si

    3 Hillocks and pyramids in epitaxial layers

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    3. Hillocks and pyramids in epitaxial layers

    Hillocks: Small oval mounds on the surface of the

    epitaxial

    Pyramids: Faceted regions on the epitaxial surface

    Density of hillocks and pyramids is dependent on

    growth parameters such as type and concentration of

    Si source and deposition temperature

    4 Dislocations and slip

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    4. Dislocations and slip

    Non-uniform heating of a substrate results in non-uniform thermal

    expansion of the substrate which produces elastic stresses

    The thermal stress can cause bowing which may lift the edge ofthe substrate away from the substrate in response to the thermal

    stress

    At lower temperature (< 900C) the yield point of the Si lattice is

    sufficiently high that the substrate behaves elastically. Duringcooling, the thermal stress is removed and the substrate returns

    to its original shape

    If the stress exceeds a critical values, the substrate will yield

    plastically occurs due to generation and motion of dislocationswhich are atomic level line defects which glide along slip planes

    of the crystal

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    The passage of one dislocation offsets the material above andbelow the slip plane by a unit known as Bergers vector of thedislocations

    Dislocations normally propagate from near the edge of thesubstrate (highest stress), and glide towards the centre of thesubstrate and produce plastic deformation of the substrate whichrelieves the thermal stress

    Dislocation motions is slow because dislocation moves to aregion of lower shear stress

    The continuous slow motion of the dislocations produces creepdeformation of the crystal

    Device impact from slip normally comes from rapid pipediffusion of dopant along the core of the dislocations

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    Typical wafer edge slip as a

    result of excessive within

    wafer temperature

    gradients during heating or

    during epitaxial film growth

    Crystal slip

    5 Microprecipitates (S pits)

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    5. Microprecipitates (S-pits)

    Microprecipitates may come from metallicelements such as copper, nickel, iron and

    chromium This is due to their solubility in Si at high

    temperatures and fast diffusion rates through

    the Si The metal contaminants may exist in thestarting substrates or being pick up duringhandling in the loading operation or from metalparts or susceptors within the epitaxial reactoritself