wafer flats characterization

8/9/2019 Wafer Flats Characterization

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Silicon Crystal growth


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Single Crystal Growth

Czochralski or crucible growncrystals ( CZ crystals).Float zone or FZ crystals.


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Float zone

Produces crystals with the highest purity, but isnot easily used at large diameters. 150 mmcrystals are already quite difficult to make and

nobody so far has made a 300 mm crystal thisway.Float zone crystal growth, while the main method

at the beginning of the Si age, is now only usedfor some specialities.


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Czochralski method

The Czochralski method, invented bythe Polish scientist J. Czochralski in1916 , is the method of choice for highvolume production of Si single crystalsof exceptional quality

The pulling rate (usually a few mm/min)and the temperature profile determinesthe crystal diameter


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Float Zone (FZ) Process

A seed crystal is brought into contact withthe top end of the rod.The seed crystal determines the crystalorientation of the boule.

A small RF coil provides power, which

generates large currents in the silicon andlocally melts it.The "floating" melt zone is about 2 cmwide.Surface tension and levitation due to RF

field keep the system stable. Atoms from the liquid phase bond to thesingle-crystal solid material plane byatomic plane (As in the CZ process).No crucible is used, which reduces

impurity levels.


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Float-Zone (FZ) Process

FZ material is usedprimarily today inapplications whichrequire highresistivities, low oxygencontent, such asdetectors and powerdevices (switches orrectifiers).Melt is not held in acontainer, it is float,thus the name floatzone.


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Doping in FZ growth

(Doping can of course be achieved if the starting material poly-crystalline Sirod is doped. But due to zone-refining, the doping is not uniform along theboule)Gas doping:Dopants are introduced in gaseous form during FZ growth.

n-doping: PH 3 (Phosphine), AsCl 3p-doping: B 2H6 (Diborane), BCl 3Good uniformity along the length of the boule.Pill doping:

Drill a small hole in the top of the EGS rod, and insert the dopant.If the dopant has a small segregation coefficient, most of it will be carriedwith the melt as it passes the length of the boule.Resulting in only a small non-uniformity.

Ga and In doping work well this way.


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Dopant Incorporation duringCrystal GrowthDoping is required in CMOS process with specified type (P or N) andconcentration.Dopants are incorporated into the crystal during growth simply by addingdopants to the melt.It is very important to be able to predict the dopant concentration in thepulled crystal. This is not as straightforward as might be because of

segregation.The impurities segregate between the liquid and solid phase at the interfacebetween them.The concentrations of the impurity are C S in the solid and C L in the liquid, Asegregation coefficient k O is defined as:

Segregation occurs due to different solubilities of impurity in two phases.

L

so C

C k =


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Typical Sizes ofSemicoductor Wafers

1 inch or 25mm2 inch or 50mm3 inch or 75mm4 inch or 100mm5 inch or 125mm6 inch or 150mm8 inch or 200mm

12 inch or 300mmThe Diameter of a wafer is measured throughits center and not through any flats


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Silicon crystal plane

Silicon, exhibit anisotropicetching in certain chemicals.

Anisotropic etching in contrast toisotropic etching means differentetch rates in different directionsin the material. The classicexample of this is the crystal plane sidewalls thatappear when etching a hole in a silicon wafer in achemical such as potassiumhydroxide (KOH). The result is apyramid shaped hole instead ofa hole with rounded sidewallswith a isotropic etchant.


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Measurement of wafercharacteristics

dark field and bright field detection


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Wafer Flats

Purpose and Function1. Orientation for automatic equipment2. Indicate type and orientation of crystal.Primary flat The flat of longest lengthlocated in the circumference of the wafer. Theprimary flat has a specific crystal orientationrelative to the wafer surface; major flat.Secondary flat Indicates the crystalorientation and doping of the wafer. Thelocation of this flat varies.


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Ingot grinding


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Traditional method of slicing

Wafer slicing

The saw blade itself is about 400 m thick, togetherwith the loss at the seed and tail end of the crystal,only 50% of the boule ends up in wafer form.

After slicing, mechanical lapping and wet chemicaletching is performed before final chemicalmechanical polishing. The wet etching is typically:

3Si + 4HNO 3 + 18HF 3H2SiF6 + 4NO + 8H 2O

Wire saw for large wafers


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Wafer polishing

Chemical mechanical polishing

The rotation andpressure generatesheat that drives achemical reaction inwhich OH - radicalsfrom the NaOHoxidize the silicon.The SiO 2 particlesabrade the oxideaway.

Slurryconsists ofnano-particles(10nm SiO 2or Al2O3) andchemicals(NaOH).


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Common (not always) wafer surface orientation

{100} wafer usually breaks along {110} plane (actually Si cleaves naturally along{111} plane, which meet the surface at an angle of 54.7 o, the angle between and ).Sometimes (not often) {100} wafers break along {100} plane. ({100} =(100)+(010)+(001))

Another flatconfiguration for {100}n-type wafer

Shouldbe{110 }plane


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Flats

P type No secondary FlatP type 905 Clockwise fromPrimary FlatN type 455 Clockwise fromPrimary FlatN type 1805 Clockwise fromPrimary Flat


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Wafer Flats

flat at 180 deg for n-type and 90 deg for p-type.

flat at 45 deg for n-type, no secondary for p-type.


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Segregation coefficient

The segregation coefficient in thermodynamicequilibrium gives the relation between theconcentration of impurity atoms in the

growing crystal and that of the melt. It isusually much lower than 1 because impurityatoms "prefer" to stay in the melt.

solubility of impurity atoms in the melt islarger than in the solid


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Impurities, both intentional and unintentional,are introduced into the silicon ingotIntentional dopants are mixed into the melt

during crystal growth, while unintentionalimpurities originate from the crucible,ambient

All common impurities have differentsolubilities in the solid and in the melt.


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An equilibrium segregation coefficient ko canbe defined to be the ratio of the equilibriumconcentration of the impurity in the solid tothat in the liquid at the interface, i.e. ko =

Cs / Cl All segregation coefficients for some commonimpurities and dopants are below unity,implying that the impurities preferentiallysegregate to the melt and the melt becomesprogressively enriched with these impuritiesas the crystal is being pulled.


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Dopant Behavior During CZCrystal Growth

For dopants likeantimony where k O


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The crystal will be cleaner than the liquid,crystal growing is simultaneously apurification method.

Discard the last part of the crystal where allthe impurities are now concentrated.What was in the melt must be in the solid

after solidification - only the distribution maynow be different.


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one reason why practically only As , P ,and B is used for doping! Theirsegregation coefficient is close to 1which assures half-way homogeneousdistribution during crystal growth.

Achieving homogeneous doping with Bi,on the other hand, would beexceedingly difficult or just impossible.


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Oxygen in Silicon

CZ-Si has an indigenous oxygenconcentration of approximately 10 18atoms/cm 3. Although the segregationcoefficient of oxygen is


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Oxygen

oxygen atoms in the bulk ofthe crystal can beprecipitated as SiOx clusters

that act as trapping sites toimpurities. This process, iscalled internal gettering andis one of the most effective

means to removeunintentional impurities fromthe near surface regionwhere devices are fabricated.


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Characterization

Routine evaluation of ingots or boulesinvolves measuring the resistivity,evaluating their crystal perfection, andexamining their mechanical properties,such as size and mass. Other lessroutine tests include the measurementof oxygen, carbon, and heavy metalcontents.


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Four-point probemeasurement on crystal end

Resistivity measurements aremade on the flat ends of thecrystal by the four-point probetechnique.

A current I passes through theouter probes and the voltage Vis measured between the innerprobes. The measuredresistance ( V / I ) is converted toresistivity (-cm) using therelationship = ( V / I )2 S whereS is the probe spacing incentimeters


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Conversion between resistivityand dopant density in silicon.


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Hot Point ProbeThe hot probe technique is used to determine thetype of dopant in a wafer N or P.

A voltmeter placed across the probes will measure apotential difference whose polarity indicates whetherthe material is N or P type.For N-type sample

At the hot probe, the thermal energy of the electronsis higher than at the cold probe so the electrons willtend to diffuse away from hot probe.

As the electrons diffuse away from the hot probe,they leave behind the positively charged, while, thecold probe will be negatively charged


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Hot probe measurement of carrier type

The hot probe technique is used to determine the type of dopant in awafer.

It relies on the generation of the Seebeck voltage (open circuit), i.e. thediffusion of free carriers (electrons or holes) as a result of a temperaturegradient.

The sign of the voltage tells the carrier type. Alternatively, one can measure the current direction (short circuit). The current that flows due to the majority carrier is given byPn is thermoelectric power, negative for electrons, positive for holes.

V m

HotCold

e-

n-typewafer

dxdT

pqn J nnn =


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Electrical measurement techniques four point

probe

The four point probe method measures the resistance/resistivity of awafer.Using values of carrier mobility, one can calculates the carrierconcentration.Use four points (rather than two) to eliminate the effect of contactresistance.

V

I

S t

d

=(e n) -1, is mobility, n is carrier density.

wafer flats characterization

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