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    Crystal Growth and WaferPreparation

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    Advantages of Si over Ge

    Si has a larger bandgap (1.1 eV for Si versus 0.66 eV

    for Ge)

    Si devices can operate at a higher temperature (150oC

    vs 100oC)

    Intrinsic resistivity is higher (2.3 x 105 -cm vs 47 -

    cm)

    SiO2 is more stable than GeO2 which is also water

    soluble

    Si is less costly

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    The processing characteristics and some material propertiesof silicon wafers depend on its orientation.

    The planes have the highest density of atoms on the

    surface, so crystals grow most easily on these planes and

    oxidation occurs at a higher pace when compared to other

    crystal planes.

    Traditionally, bipolar devices are fabricated in

    oriented crystals whereas materials are preferred for

    MOS devices.

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    Defects

    Any non-silicon

    atoms incorporated

    into the lattice ateither a substitutional

    or interstitial site are

    considered pointdefects

    Point defects are important in the kinetics of diffusion and

    oxidation. Moreover, to be electrically active, dopants must

    occupy substitutional sites in order to introduce an energy level in

    the bandgap.

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    Dislocations are line defects.Dislocations in a lattice are

    dynamic defects. That is, they

    can diffuse under applied

    stress, dissociate into two or

    more dislocations, or combine

    with other dislocations.

    Dislocations in devices are

    generally undesirable, because

    they act as sinks for metallicimpurities and alter diffusion

    profiles.

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    Defects Two typical area or planar defects are twins and grain

    boundaries

    Twinning represents a change in the crystal orientation

    across a twin plane, such that a mirror image exists across

    that plane

    Grain boundaries are more disordered than twins and

    separate grains of single crystals in polycrystalline silicon

    Planar defects appear during crystal growth, and crystals

    having such defects are not considered usable for IC

    manufacture and are discarded

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    Precipitates of impurity or dopant

    atoms constitute the fourth class of

    defects. The solubility of dopantsvaries with temperature, and so if an

    impurity is introduced at the

    maximum concentration allowed by

    its solubility, a supersaturatedcondition will exist upon cooling. The

    crystal achieves an equilibrium state

    by precipitating the impurity atoms in

    excess of the solubility level as asecond phase.

    Precipitates are generally undesirable

    as they act as sites for dislocationgeneration. Dislocations result from

    the volume mismatch between the

    precipitate and the lattice, inducing a

    strain that is relieved by theformation of dislocations.

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    Electronic Grade Silicon

    Electronic-grade silicon (EGS), a polycrystalline material of high

    purity, is the starting material for the preparation of single crystal

    silicon. EGS is made from metallurgical-grade silicon (MGS) which

    in turn is made from quartzite, which is a relatively pure form ofsand. MGS is purified by the following reaction:

    Si (solid) + 3HCl (gas)

    SiHCl3 (gas) + H2 (gas) + heat

    The boiling point of trichlorosilane (SiHCl3) is 32oC and can be

    readily purified using fractional distillation. EGS is formed by

    reacting trichlorosilane with hydrogen:

    2SiHCl3 (gas) + 2H2 (gas) 2Si (solid) + 6HCl (gas)

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    Czochralski Crystal Growth

    The Czochralski (CZ) process, which

    accounts for 80% to 90% of worldwide

    silicon consumption, consists of dipping

    a small single-crystal seed into moltensilicon and slowly withdrawing the seed

    while rotating it simultaneously.

    The crucible is usually made of quartz

    or graphite with a fused silica lining.

    After the seed is dipped into the EGSmelt, the crystal is pulled at a rate that

    minimizes defects and yields a constant

    ingot diameter.

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    Impurity Segregation

    Impurities, both intentional and unintentional, are introduced into the silicon ingot.Intentional dopants are mixed into the melt during crystal growth, while

    unintentional impurities originate from the crucible, ambient, etc.

    All common impurities have different solubilities in the solid and in the melt. An

    equilibrium segregation coefficient ko

    can be defined to be the ratio of the

    equilibrium concentration of the impurity in the solid to that in the liquid at the

    interface, i.e. ko

    = Cs/C

    l. Note that all the values shown in the table are below

    unity, implying that the impurities preferentially segregate to the melt and the

    melt becomes progressively enriched with these impurities as the crystal is

    being pulled.

    Impurity Al As B C Cu Fe O P Sb

    ko 0.002 0.3 0.8 0.07 4x10

    -6

    8x10-6

    0.25 0.35 0.023

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    Impurity Distribution

    The distribution of an impurity in the grown crystal can be

    described mathematically by the normal freezing relation:

    1)1(

    = okoos

    XCkC

    Xis the fraction of the melt solidified

    Co

    is the initial melt concentration

    Cs

    is the solid concentration

    ko

    is the segregation coefficient

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    Ingot

    Weight = M

    Weight = dMDopant conc. = C

    s

    Melt

    S = dopant remaining in melt

    Consider a crystal being grown from

    a melt having an initial weight Mo

    with an initial dopant concentration

    Co

    in the melt (i.e., the weight of the

    dopant per 1 gram melt).

    At a given point of growth when a

    crystal of weightMhas been grown,the amount of the dopant remaining

    in the melt (by weight) isS.

    For an incremental amount of the crystal with weight dM, the corresponding

    reduction of the dopant (-dS) from the melt is Cs

    dM, where Csis the dopant

    concentration in the crystal (by weight): -dS = Cs

    dM

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    The remaining weight of the melt is Mo

    - M, and the dopant concentration in

    the liquid (by weight),C

    l, is given by

    Combining the two equations and substituting

    Given the initial weight of the dopant, , we can integrate and obtain

    Solving the equation gives

    C SM M

    l

    o

    =

    C C ks l o

    =

    dS

    Sk

    dM

    M Mo

    o

    =

    C Mo o

    dS

    Sk

    dM

    M MC M

    S

    o

    oo

    M

    o o

    =

    C k CM

    Ms o o

    o

    ko

    =

    1

    1

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    Impurity concentrationprofiles along the silicon

    ingot (axially) for different

    ko with Co = 1

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    CZ-Si crystals are grown

    from a silicon melt contained

    in a fused silica (SiO2)

    crucible. Fused silica reacts

    with hot silicon and releases

    oxygen into the melt giving

    CZ-Si an indigenous oxygen

    concentration of about 1018

    atoms/cm3.

    Although the segregation coefficient of oxygen is

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    Oxygen in Silicon

    Oxygen forms a thermal donor in silicon

    Oxygen increases the mechanical strength

    of silicon

    Oxygen precipitates provide gettering sitesfor unintentional impurities

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    Thermal Donors

    Thermal donors are formed by the polymerizationof Si and O into complexes such as SiO4 in

    interstitial sites at 400oC to 500oC

    Careful quenching of the crystal annihilates these

    donors

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    Internal Gettering

    Under certain annealing

    cycles, oxygen atoms in

    the bulk of the crystal

    can be precipitated as

    SiOx

    clusters that act as

    trapping sites toimpurities.

    This process is called internal gettering and is one of the mosteffective means to remove unintentional impurities from the

    near surface region where devices are fabricated.

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    Float-Zone Process

    The float-zone process has someadvantages over the Czochralski

    process for the growth of certain

    types of silicon crystals.

    The molten silicon in the float-zone

    apparatus is not contained in a

    crucible, and is thus not subject to

    the oxygen contamination present in

    CZ-Si crystals.

    The float-zone process is also

    necessary to obtain crystals with a

    high resistivity (>> 25 W-cm).

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    Characterization

    Routine evaluation of ingots or boules

    involves measuring the resistivity,evaluating their crystal perfection, andexamining their mechanical properties, such

    as size and mass

    Other tests include the measurement ofcarbon, oxygen, and heavy metals

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    Resistivity

    MeasurementResistivity measurements are made

    on the flat ends of the crystal by the

    four-point probe technique.

    A current, I, is passed through the

    outer probes and the voltage, V, is

    measured between the inner probes.

    The measured resistance (V/I) is

    converted to resistivity (W-cm)

    using the relationship:

    = (V/I)2S

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    The calculated

    resistivity can be

    correlated withdopant concentration

    using a dopant

    concentration versusresisitivity chart

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    Wafer Preparation

    Gross crystalline imperfections are detected visually anddefective crystals are cut from the boule. More subtle defectssuch as dislocations can be disclosed by preferential chemical

    etching

    Chemical information can be acquired employing wet

    analytical techniques or more sophisticated solid-state andsurface analytical methods

    Silicon, albeit brittle, is a hard material. The most suitablematerial for shaping and cutting silicon is industrial-gradediamond. Conversion of silicon ingots into polished wafersrequires several machining, chemical, and polishing

    operations

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    Slicing determines four wafer parameters:

    Surface orientation (e.g., or )

    Thickness (e.g., 0.5 0.7 mm, depending on wafer

    diameter)

    Taper, which is the wafer thickness variations from one

    end to another

    Bow, which is the surface curvature of the wafermeasured from the center of the wafer to its edge

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    Finished Wafers

    The wafer as cut varies enough in thickness to warrant an additional lapping

    operation that is performed under pressure using a mixture of Al2O

    3and glycerine.

    Subsequent chemical etching removes any remaining damaged and contaminated

    regions.

    Polishing is the final step. Its purpose is to provide a smooth, specular surface on

    which device features can be photoengraved.

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    Typical Specifications for Silicon Wafers

    Parameter 125 mm 150 mm 200 mm 300 mm

    Diameter (mm) 125+1 150+1 200+1 300+1

    Thickness (mm) 0.6-0.65 0.65-0.7 0.715-

    0.735

    0.755-

    0.775

    Bow (m) 70 60 30

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    Epitaxy (epimeans "upon" and taxis means "ordered") is a term applied to

    processes used to grow a thin crystalline layer on a crystalline substrate.The seed crystal in epitaxial processes is the substrate. Unlike the

    Czochralski process, crystalline thin films can be grown below the melting

    point using techniques such as chemical vapor deposition (CVD), molecularbeam epitaxy (MBE), etc.

    When a material is grown epitaxially on a substrate of the same material,

    the process is called homoepitaxy. On the contrary, if the layer and

    substrate are of different materials, such as AlxGa

    1-xAs on GaAs, the

    process is termed heteroepitaxy. Naturally, in heteroepitaxy, the crystal

    structures of the layer and the substrate must be similar in order to achieve

    good crystalline integrity.

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    Advantages of epitaxy:

    (1) Doping profiles that are

    not attainable through other

    conventional means such asdiffusion or ion implantation

    (2) Physical and chemicalproperties of the epitaxial

    layers can be made different

    from the bulk materials.

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    256Mbit DRAM

    (buried strap trench)

    Cross section of a trench DRAM

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    Alpha-particles originating from packagingmaterials and the environment can cause electron-

    hole pairs in the bulk of the wafer. If these charges

    migrate to the storage cell of a DRAM (dynamic

    random access memory) structure, the data stored

    can be wiped out.

    A heavily doped substrate increases the rate of

    electron-hole pair recombination and the DRAM is

    less prone to alpha-particle soft errors.

    Non-Volatile Flash Memory

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    Non-Volatile Flash Memory

    Writing Erasing

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    Vapor Phase Epitaxy

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    The Reynolds number, Re, characterizes the type of fluid flow in a

    reactor:R

    e= D

    rv/

    where Dr

    denotes the diameter of the reaction tube, v is the gas velocity,

    represents the gas density, and stands for the gas viscosity. Valuesof D

    rand v are generally several centimeters and tens of cm/s,

    respectively. The carrier gas is usually H2, and using typical values for

    and , the value ofRe is about 100. These parameters result in gasflow in the laminar regime. That is, the gases flow in a regular,

    continuous, and non-turbulent mode and in a specific direction.

    Accordingly, a boundary layer of reduced gas velocity will form abovethe susceptor and at the walls of the reaction chamber. The thickness of

    the boundary layer, y, is defined as: wherexis the distance

    along the reactor.

    2/1

    =

    e

    r

    R

    xDy

    Boundary Layer Formation

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    Boundary Layer Formation

    (Horizontal Reactor)

    Reactants are transported to the substrate surface and reaction by-products

    diffuse back into the main gas stream across the boundary layer

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    The fluxes of species going to and coming from the wafer

    surface are complex functions of the temperature, pressure,reactant, concentration, layer thickness, etc. By convention, the

    flux, J, is defined to be the product ofD and dn/dy, and is

    approximated as:

    y

    nnDJ

    sg )( =

    where ng and ns are the gas stream and surface reactant

    concentrations, respectively, D is the gas-phase diffusivity,

    which is function of pressure and temperature, y is the

    boundary layer thickness, and J is the reactant flux of

    molecules per unit area per unit time.

    In steady state the reactant flux across the boundary layer is equal

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    In steady state, the reactant flux across the boundary layer is equal

    to the chemical reaction rate, ks

    , at the specimen surface.

    Therefore,

    J= ksns

    D

    yk

    nn

    s

    g

    s

    +=

    1

    The quantity D/y is called the gas phase mass-transfer coefficient,

    hg.

    In the limiting case when ks >> hg, ns approaches zero, thereby

    implying that the overall reaction is limited by transport of reactant

    across the boundary layer. Conversely, ifks

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    Growth Chemistry

    The most common starting chemical is silicon tetrachloride

    (SiCl4) as it has a lower reactivity with respect to oxidizers in

    the carrier gas than the other silicon hydrogen chloride

    compounds, such as SiH4, SiHCl3, etc. The overall reaction is:

    SiCl4

    (gas) + 2H2

    (gas) Si (solid) + 4HCl (gas)

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    Experimental results indicate

    the presence of many

    intermediate chemical

    species. In particular, at a

    reaction temperature of

    1200oC, four species have

    been observed using FTIR.

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    Doping

    Autodoping Zone A is due to solid-state out-

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    diffusion from the substrate, and

    can be approximated by thecomplementary error function if

    the growth velocity is less than

    2(D/t)1/2, where D is the dopant

    diffusion constant and t denotes the

    deposition time.

    Zone B originates from gas-phase autodoping.

    Because the dopant evaporating from thewafer surface is supplied from the wafer

    interior by solid-state diffusion, the flux of

    dopant from an exposed surface decreases

    with time.

    When autodoping diminishes, the

    intentional doping predominatesand the profile becomes flat.

    Autodoping thus limits the

    minimum layer thickness thatcan be grown with controlled

    doping as well as the minimum

    dopant level.

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    Defects

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    Defects

    (1) Line (or edge) dislocation initially present in the substrate andextending into the epitaxial layer

    (2) Epitaxial stacking fault nucleated by an impurity precipitate on the

    substrate surface

    (3) Impurity precipitate caused by epitaxial process contamination

    (4) Growth hillock

    (5) Bulk stacking faults, one of which intersects the substrate surface,

    thereby being extended into the layer

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    The crystal perfection of an epitaxial layer never exceedsthat of the substrate and is frequently inferior.

    Generally, defects can be reduced by a higher growthtemperature, reduced gas pressure, lower growth rate, and

    cleaner substrate surface.

    A typical pre-epitaxy substrate cleaning process consists

    of a wet clean followed by a dilute HF dip and an in-situ

    HCl, HF, or SF6 vapor etch.

    Selective Epitaxy Growth (SEG)

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    Selective Epitaxy Growth (SEG)

    Selective epitaxy is a

    technique by which single-crystal silicon is fabricated

    in a small designated area

    SEG is usually accomplished at reduced partial pressure of the

    reactant in order to suppress the nucleation of silicon on the

    dielectric film, thereby resulting in nucleation only on theexposed silicon surface

    L T t E it (LTE)

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    Low Temperature Epitaxy (LTE)

    Low-temperature epitaxy (LTE) of Si produces epitaxial growth

    at temperature of 550o

    C or less, much lower than that inconventional epitaxial processes. A low temperature is required

    to minimize thermal diffusion and mass-transport-controlled

    processes.

    CVD and molecular beam epitaxy (MBE) are the most popular

    methods. The success of these techniques relies on both an

    ultra-clean growth environment and a unique Si surface-

    cleaning process.

    Molecular Beam Epitaxy (MBE)

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    Molecular Beam Epitaxy (MBE)

    Molecular beam epitaxy,

    which utilizes evaporation,

    is a non-CVD epitaxial

    growth process. MBE istherefore not complicated

    by boundary-layer transport

    effects, nor are there

    chemical reactions to

    consider. The essence of

    the process is evaporation

    of silicon and one or moredopants.

    Silicon MBE is performed under ultra-high vacuum (UHV)

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    S p g (U V)

    conditions of 10-8 to 10-10 Torr, where the mean free path of the

    atom is given by 5x10-3/P where P is the system pressure in

    Torr. At a typical pressure of 10-9 Torr, L is 5x106 cm,

    transport velocity is dominated by thermal energy effects

    Lack of intermediate reactions and diffusion effects, coupled

    with relatively high thermal velocities, results in film properties

    changing rapidly with any change of the source

    Typical growth temperature is between 400oC and 800oC in

    order to reduce out-diffusion and autodoping. Growth rates are

    in the range of 0.01 to 0.3 m/minute

    Despite the slow growth rate and relatively expensive

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    Despite the slow growth rate and relatively expensive

    instrumentation, MBE offers several advantages overconventional CVD for VLSI

    MBE is a low-temperature process that minimizes dopantdiffusion and autodoping

    MBE allows more precise control of doping and layer

    thickness, because CVD is limited by reactant introduction andpumping time constants

    These advantages are not exploited extensively in silicon ICtechnology, but MBE has found tremendous usages inmicrowave and photonic devices made of III-V semiconductors

    Rapid Thermal Processing (RTP)

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    Chemical and physical processes applied to silicon wafers aregenerally thermally activated. Typical silicon-based processesuse batch furnaces for thermal fabrication steps, where a batch

    consists of 20 to 100 wafers that are simultaneously processed ina single system

    Processing of wafers requires tight control of contamination,process parameters, and reduced manufacturing costs, and someproducers are now using single-wafer processing in some steps

    Using transient lamp heating or a continuous heat source (verticalfurnace), a single wafer can be heated very quickly to reduce thethermal cycle and mitigate undesirable effects such as dopantdiffusion

    p g ( )

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    Rapid thermal

    processing (RTP)system that is

    optically heated

    Conventional

    batch-furnacethat is resistively

    heatedContinuous heat

    source, vertical

    furnace RTP system

    The most important feature of a rapid thermal annealing

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    The most important feature of a rapid thermal annealing

    processing system consisting of tungsten-halogen lamps isits generation and quick delivery of radiant energy to thewafer (large dT/dt) in a wavelength band of 0.3 to 4.0 m

    Because of the optical character and wavelength of theenergy transfer, the quartz walls do not absorb lightefficiently, whereas the silicon wafer does

    The wafer is not in thermal equilibrium with the cold wallsof the system, allowing for short processing times (seconds

    to minutes) compared to minutes to hours for conventionalfurnaces. The reduction in temperature-time exposureafforded by RTP is dramatic

    Rapid heating with large temperature gradients can cause

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    Rapid heating with large temperature gradients can cause

    wafer damage in the form of slip dislocations induced bythermal stress and heating can be laterally non-uniform

    across the wafer

    Conventional furnace processes bring with them significant

    problems such as particle generation from the hot walls,

    limited ambient control in an open system, and a largethermal mass that restricts controlled heating times to tens

    of minutes

    Requirements on contamination, process control, cost, and

    space are driving a paradigm shift to RTP

    RTP demands on the growth of high-purity epitaxial Si include

    ambient purity (oxygen and water concentrations in the parts per

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    billion range), optimization of gas flow patterns, minimum wall

    deposition, and vacuum compatibility.

    The deposition

    process comprises a

    mass-transport process

    with a weak

    temperaturedependence and a

    sequential surface-

    reaction process that is

    exponentiallydependent on wafer

    temperature.

    Silicon-on-Insulator (SOI)

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    Silicon device structures have inherent problems that are associated

    with parasitic circuit elements arising from junction capacitance.These effects become more severe as device dimensions shrink. A

    viable means to circumvent the problem is to fabricate devices in small

    islands of silicon on an insulating substrate.

    SOI Fabrication Techniques

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    Traditional approach is to fabricate such a structure in a silicon

    epitaxial thin film grown on sapphire (Al2O3)

    The lattice parameters of silicon and sapphire are quite similar,

    high quality SOS (silicon-on-sapphire) epitaxial layers can be

    fabricated

    The high cost of sapphire substrates, low yield, and lack of

    commercially viable applications limit the use of SOS to

    primarily military applications

    SOI Fabrication Techniques

    SIMOX (separation by implantation of oxygen) utilizeshigh dose blanket o gen ion implantation to form a

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    high dose blanket oxygen ion implantation to form a

    sandwiched buried oxide layer to isolate devices from thewafer substrate

    Wafer bonding utilizes Van der Waals forces to bond twopolished silicon wafers, at least one of which is coveredwith thermal oxide, in a very clean environment at about1000oC. Mechanical or electrochemical thinning has

    achieved 1 m thickness with 0.1 m deviations

    More recent approaches include the combination of wafer

    bonding and layer cleavage using hydrogen or helium ionimplantation (ion-cut) as well as epitaxial growth on poroussilicon and wafer bonding

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    Oxidation

    Roles of SiO2

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    Roles of SiO2

    Mask against implant or diffusion of dopant

    into silicon Surface passivation

    Device isolation

    Component in MOS structures (gate oxide)

    Electrical isolation of multi-level

    metallization systems

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    Oxide Growth

    Si (solid) + O2 (gas) SiO2 (solid)Si (solid) + 2H2O (gas) SiO2 (solid) + 2H2 (gas)

    During the oxidation process, oxygen or water molecules diffuse through the

    surface oxide into the silicon substrate, and the Si-SiO2

    interface migrates into the

    silicon. Thermal oxidation of silicon results in a random three-dimensional

    network of silicon dioxide constructed from tetrahedral cells. Since the volume

    expands, the external SiO2

    surface is not coplanar with the original silicon surface.

    For the growth of an oxide of thickness d, a layer of silicon equal to a thickness of

    0.44dis consumed.

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    F1

    can be approximated to be proportional to the difference in

    concentration of the oxidizing species in the gas phase and on the oxide

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    concentration of the oxidizing species in the gas phase and on the oxide

    surface: F1

    = hG

    (CG

    - CS

    )

    where hG

    is the gas-phase mass-transfer coefficient, CG

    is the oxidant

    concentration in the gas phase, and CS

    is the oxidant concentration

    adjacent to the oxide surface. Substituting C=P/kT,

    F1

    = (hG/kT)(P

    GP

    S).

    Henry's Law states that, in equilibrium, the concentration of a species

    within a solid is proportional to the partial pressure of that species in the

    surrounding gas. Thus, Co

    = HPS

    , where Co

    is the equilibrium

    concentration of the oxidant in the oxide on the outer surface, H is the

    Henry's Law constant, andPS

    is the partial pressure of oxidant in the gas

    phase adjacent to the oxide surface.

    We denote the equilibrium concentration in the oxide, that is, the

    concentration which would be in equilibrium with the partial pressure in

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    the bulk of the gasPGby the symbol C*, andC*=HP

    G

    C*- Co

    =H(PG

    - PS

    )

    F1

    = (hG/HkT)(C*- C

    o) = h (C*- C

    o)

    where h = hG/HkT is the gas-phase mass-transfer coefficient in terms of

    concentration in the solid.

    Oxidation is thus a non-equilibrium process with the driving force

    being the deviation of concentration from equilibrium. Henry's Lawis valid only in the absence of dissociation effects at the gas-oxide

    interface, thereby implying that the species diffusing through the

    oxide is molecular.

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    In order to calculate the oxide growth rate, we define N1 as thenumber of oxidant molecules incorporated into a unit volume of the

    oxide layer. If oxygen is the reactant, N1

    = 2.2 x 1022 atoms/cm3

    because the density of SiO2 is 2.2 x 1022

    cm-3

    . If water is used, N1becomes 4.4 x 1022 cm-3 as two H

    2O molecules are incorporated into

    each SiO2

    molecule. The differential equation for oxide growth is

    given by

    D

    dk

    h

    k

    CkCk

    dt

    ddN

    oss

    s

    is

    o

    ++==

    1

    )( *

    1

    With an initial condition ofdo(t= 0) = d

    i, the solution is

    d 2 + Ad = B (t + )

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    do

    Ado

    B (t )

    whereA 2D [1/kS

    + 1/h],B 2DC*/N1, and (d

    i2 +Ad

    i) /B.

    The quantity represents a shift in the time coordinate to account for the

    presence of the initial oxide layerdi. Solving ford

    oas a function of time

    gives1

    4/

    1

    2/

    2/1

    2

    ++=BA

    t

    A

    do

    For long oxidation times, i.e., t>> and t>> A2/4B, do2 Bt. B is

    therefore called theparabolic rate constant. For short times, i.e., (t+ )

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    Oxidation

    temperature

    (oC)A (m)

    Parabolic rate

    constant

    B (m2/h)

    Linear rate

    constant

    B/A (m/h)(h)

    1200 0.05 0.720 14.40 0

    1100 0.11 0.510 4.64 0

    1000 0.226 0.287 1.27 0

    920 0.50 0.203 0.406 0

    Rate constants for dry oxidation of silicon

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    Oxidation

    temperature

    (oC)A (m)

    Parabolic rate

    constant

    (m2/h)

    Linear rate

    constant

    B/A (m/h)(h)

    1200 0.040 0.045 1.12 0.027

    1100 0.090 0.027 0.30 0.076

    1000 0.165 0.0117 0.071 0.37

    920 0.235 0.0049 0.0208 1.40

    800 0.370 0.0011 0.0030 9.0

    Oriental Dependence

    The rate of oxidation depends on the availability of reaction sites on the

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    Orient-

    ation

    Area of

    unit cell(cm2)

    Si atoms

    in area

    Si bonds

    in area

    Bonds

    available

    Available

    bonds,N

    (1014 cm-2)

    Nrelative

    to

    4 8 4 9.59 1.000

    2 4 3 11.76 1.227

    a2 2 4 2 6.77 0.707

    p y

    silicon substrates. Hence, as the surface areal density of atoms isdependent on crystal orientation, oxidation rates are expected to be

    orientation dependent. Oxidation on the crystal plane occurs at

    a higher rate because there are a higher number of surface atoms, i.e.

    reaction sites or chemical bonds, when compared to a plane.

    2 2a

    232

    1a

    Rate constants for silicon oxidation in H2O (640 Torr)

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    OxidationTemp (oC)

    Orientation A (m) Parabolicrate constantB (m2/h)

    Linear rateconstant

    B/A(m/h)

    B/A ratio/

    900

    0.95

    0.60

    0.143

    0.151

    0.150

    0.252

    1.68

    950

    0.74

    0.44

    0.231

    0.231

    0.311

    0.524

    1.68

    1000

    0.480.27

    0.3140.314

    0.6641.163

    1.75

    1050

    0.295

    0.18

    0.413

    0.413

    1.400

    2.307

    1.65

    1100

    0.175

    0.105

    0.521

    0.517

    2.977

    4.926

    1.65

    Average 1.68

    Oxide thickness versus oxidation time for silicon

    in H2O at 640 Torr

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    Effects of Impurities

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    Moisture much higher oxidation rate with traces of

    water in the ambient

    Boron (segregation into oxide) enhanced diffusionthrough the weakened bonds

    Phosphorus (segregation into silicon) - concentrationdependence observed only at lower temperature,

    where the surface reaction becomes important. This

    dependence may be the result of phosphorus beingsegregated into the silicon

    Oxidation of boron-doped silicon in wet oxygen as

    a function of temperature and boron concentration

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    a function of temperature and boron concentration

    Oxidation of phosphorus-doped silicon in wet oxygen as a

    function of temperature and phosphorus concentration

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    High Pressure Oxidation

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    Plasma Oxidation

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    Anodic plasma oxidation has all the advantages associated with

    the high-pressure technique and also offers the possibility of

    growing high-quality oxides at even lower temperatures. Plasma

    oxidation is a low-pressure process usually carried out in a pure

    oxygen discharge. The plasma is sustained either by a high-

    frequency or DC discharge. Placing the wafer in the uniform

    density region of the plasma and biasing it slightly negatively

    against the plasma potential allows it to collect active charged

    oxygen species. The oxidation rate typically increases with

    higher substrate temperature, plasma density, and substrate

    dopant concentration.

    Rapid Thermal Oxidation

    Rapid thermal oxidation (RTO) is increasingly used in the growth of thin,

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    p ( ) g y g ,

    high-quality dielectric layers. The primary issues that differentiate RTO

    from conventional thermal oxidation are the more complex chamber

    design, radiation source, as well as temperature monitoring. From the

    point of view of oxide-growth kinetics, RTO may be influenced by both

    thermally activated processes and a non-thermal, photon-induced process

    involving monatomic O atoms generated by UV and creating a parallel

    oxidation reaction that dominates at lower temperature.

    RTO growth kinetics exhibit activation energies differing from those

    measured in conventionally grown oxides. In the initial stage (on theorder of 20 seconds), the RTO growth rate is linear followed by nonlinear

    growth. The duration of the linear region is hardware dependent,

    particular the heating source.

    Oxide Properties

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    A silicon dioxide layer can provide a selective mask against thediffusion of dopant atoms at elevated temperature, a very useful

    property in IC processing. For it to work, the dopant diffusion rate

    in the oxide must be slow with respect to that in silicon, so that thedopant does not diffuse through the oxide in the masked region

    into the silicon. The masking oxide thickness must also be large

    enough to prevent it from reaching the silicon substrate.

    The often used n-type impurities as well as boron have very small

    diffusion coefficients in oxide and are compatible with oxidemasking. However, this is not true for gallium, indium, and

    aluminum.

    Diffusion constants in SiO2

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    Dopants Diffusion constants at 1100oC (cm2/s)

    B 3.4 x 10-17 to 2.0 x 10-14

    Ga 5.3 x 10-11

    P 2.9 x 10-16 to 2.0 x 10-13

    As 1.2 x 10-16 to 3.5 x 10-15

    Sb 9.9 x 10-17

    Oxide Charges

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    Various charges and traps exist in thermally grown

    oxide films. If a charge is present close to theSi/SiO2 interface, it can induce a charge of the

    opposite polarity in the underlying silicon, therebyaffecting the ideal characteristics of the device, such

    as the threshold voltage of a MOS capacitor.

    Mobile ion charges (Qm) are attributed to

    alkali ions such as Na, K, and Li, as well as

    negative ions and heavy metals. They

    Oxide-trapped charges (Qot) may be

    positive or negative, due to holes or

    electrons being trapped in the bulk of

    the oxide. They can be annealed out by

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    Interface-trapped charges (Qit) can interact with the underlying silicon. They

    originate from structural defects related to the oxidation process, metallic impurities,

    and bond-breaking processes. A low temperature hydrogen anneal at 450oC

    effectively neutralizes most interface-trapped charges.

    Fixed oxide charges (Qf) are located in theoxide within approximately 3 nm of the

    SiO2 / Si interface. Qfcannot be charged or

    discharged easily.

    originate from processing materials,chemicals, ambient, or handling. Common

    techniques employed to minimize Qminclude cleaning the furnace tube in a

    chlorine ambient, gettering with

    phosphosilicate glass (PSG), and using

    masking layers such as silicon nitride.

    low-temperature treatment.

    Dopant Redistribution

    D i h l id i h i f d i h ili

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    During thermal oxidation, the interface advances into the silicon

    substrate, and doping impurities will redistribute at the interface

    until its chemical potential is the same on each side of the

    interface. The ratio of the equilibrium concentration of the

    impurity in silicon to that in SiO2 at the interface is called the

    equilibrium segregation coefficient. Two additional factors that

    influence the redistribution process are the diffusivity of the

    impurity in the oxide (if large, the dopant can diffuse through the

    oxide rapidly, thereby affecting the profile near the Si - SiO2interface) and the rate at which the interface moves with respect to

    the diffusion rate.