A. Transport of Reactions to A. Transport of Reactions to Wafer Surface in APCVDWafer Surface in APCVD

1.1. Transport of reactants by forced convection to the deposition Transport of reactants by forced convection to the deposition regionregion

2.2. Transport of reactants by diffusion from the main gas stream to Transport of reactants by diffusion from the main gas stream to the wafer surfacethe wafer surface1.1. Turbulent flow can produce thickness nonuniformitiesTurbulent flow can produce thickness nonuniformities2.2. Depletion of reactants can cause the film thickness to decrease in Depletion of reactants can cause the film thickness to decrease in

direction of gas flowdirection of gas flow

3.3. Adsorption of reactants on the wafer surfaceAdsorption of reactants on the wafer surface

APCVDAPCVDB. Chemical reactionB. Chemical reaction

1.1. Surface migration Surface migration 2.2. Site incorporation on the surface Site incorporation on the surface 3.3. Desorption of byproductsDesorption of byproducts

C.C. Removal of chemical byproductsRemoval of chemical byproducts1.1. Transport of byproduct through the boundary Transport of byproduct through the boundary

layerlayer2.2. Transport of byproducts by forced convection Transport of byproducts by forced convection

away from the deposition regionaway from the deposition region

APCVDAPCVD

At steady state – if two fluxes are equalAt steady state – if two fluxes are equal

The growth rate of the film, v (cm/s), is The growth rate of the film, v (cm/s), is

– Where N is the number of atoms Where N is the number of atoms incorporated into the film per unit volumeincorporated into the film per unit volume For single composition film, this is the densityFor single composition film, this is the density

1

1

G

SGS h

kCC

N

C

hk

hk

N

Fv G

GS

GS

Mole fractionMole fraction

The mole fraction in incorporating The mole fraction in incorporating species in the gas phasespecies in the gas phase

where Cwhere CTT is the concentration of all is the concentration of all molecules in the gas phase molecules in the gas phase

pressure gas Total

gasesreactant theof pressure Partial

T

G

C

CY

Two limiting cases for APCVD Two limiting cases for APCVD modelmodel

Surface reaction controlled case Surface reaction controlled case (k(kSS<<h<<hGG))

Mass transfer or gas-phase diffusion Mass transfer or gas-phase diffusion controlled casecontrolled case (h(hGG<<k<<kSS))

YkN

Cv S

T

YhN

Cv G

T

APCVDAPCVD Both cases predict linear growth ratesBoth cases predict linear growth rates

– but they have different coefficientsbut they have different coefficients There is no parabolic growth rateThere is no parabolic growth rate

Surface reaction rate constant is Surface reaction rate constant is controlled by Arrhenius-type equation controlled by Arrhenius-type equation (X=X(X=Xooee-E/kT-E/kT))– Quite temperature sensitiveQuite temperature sensitive

Mass transfer coefficient is relatively Mass transfer coefficient is relatively temperature independenttemperature independent– Sensitive to changes in partial pressures and total Sensitive to changes in partial pressures and total

gas pressuregas pressure

APCVDAPCVD

Epitaxial deposition of SiEpitaxial deposition of Si

Epitaxial deposition of SiEpitaxial deposition of Si Slopes of the reaction-limited graphs Slopes of the reaction-limited graphs

are all the sameare all the same– activation energy of about 1.6 eVactivation energy of about 1.6 eV

This implies the reactions are similar; just the This implies the reactions are similar; just the number of atoms is differentnumber of atoms is different

There is reason to believe that desorption of HThere is reason to believe that desorption of H22 from the surface is the rate limiting stepfrom the surface is the rate limiting step

In practiceIn practice– epitaxial Si at high temperatures (mass epitaxial Si at high temperatures (mass

transfer regime) transfer regime) – poly-Si is deposited at low temperatures poly-Si is deposited at low temperatures

(reaction limited, low surface mobility)(reaction limited, low surface mobility)

Deposition of SiDeposition of Si Choice of gas affect the overall growth rateChoice of gas affect the overall growth rate

Silane (SiHSilane (SiH44) is fastest) is fastest

SiClSiCl44 is the slowest is the slowest

Growth rate in the mass transfer regime is Growth rate in the mass transfer regime is inversely dependent on the square root of inversely dependent on the square root of the source gas molecular weightthe source gas molecular weight

Growth rate is dependent on the Growth rate is dependent on the crystallographic orientation of the wafercrystallographic orientation of the wafer

(111) surfaced grow slower than (100)(111) surfaced grow slower than (100) Results in faceting on nonplanar surfacesResults in faceting on nonplanar surfaces

APCVDAPCVD

In the preceding theory, assumed hIn the preceding theory, assumed hGG and and CCss were constants were constants

Real systems are more complex than thisReal systems are more complex than this Consider the chamber where wafers lie on Consider the chamber where wafers lie on

a susceptor (wafer holder). a susceptor (wafer holder). – Stagnant boundary layer, Stagnant boundary layer, SS, is not a constant, , is not a constant,

but varies along the length of the reactorbut varies along the length of the reactor

– CCss varies with reaction chamber length as varies with reaction chamber length as reaction depletes gasesreaction depletes gases

SGG CChF 1

APCVDAPCVD

APCVDAPCVD

EffectsEffects

Changes the effective cross section of the Changes the effective cross section of the tube, which changes the gas flow ratetube, which changes the gas flow rate– Increasing the flow rate reduces the thickness Increasing the flow rate reduces the thickness

of the boundary layer and increases the of the boundary layer and increases the mass transfer coefficientmass transfer coefficient

– Reduces gas diffusion lengthReduces gas diffusion length To correct for the gas depletion effect, the To correct for the gas depletion effect, the

reaction rate is increased along the length reaction rate is increased along the length of the tube by imposing an increasing of the tube by imposing an increasing temperature gradient of about 5—25temperature gradient of about 5—25ooCC

APCVDAPCVD

Sometimes we wish to dope the thin films Sometimes we wish to dope the thin films as they are grown (e.g. PSG, BSG, BPSG, as they are grown (e.g. PSG, BSG, BPSG, polysilicon, and epitaxial silicon).polysilicon, and epitaxial silicon).– Addition of dopants as gases for reactionAddition of dopants as gases for reaction

AsHAsH33, B, B22HH66, or PH, or PH33..

Surface reactions now include Surface reactions now include – Dissociation of the added doping gasesDissociation of the added doping gases– Lattice site incorporation of dopantsLattice site incorporation of dopants– Coverage of dopant atoms by the other atoms Coverage of dopant atoms by the other atoms

in the filmin the film

APCVDAPCVD

Another problem, common in CMOS Another problem, common in CMOS production, is unintentional doping of production, is unintentional doping of lightly doped epitaxial Si when depositing lightly doped epitaxial Si when depositing them on a highly doped Si substrate.them on a highly doped Si substrate.

Occurs by diffusion because of the high deposition Occurs by diffusion because of the high deposition temperatures (800—1000temperatures (800—1000ooC)C)

Growth rate of the deposited layers is Growth rate of the deposited layers is usually much faster than diffusion rates usually much faster than diffusion rates (vt >> √Dt), the semi-infinite diffusion (vt >> √Dt), the semi-infinite diffusion model can be appliedmodel can be applied

Dt2

xerfc

2,

CtxC

APCVDAPCVD

Mass transport on to deposited Mass transport on to deposited filmsfilms

Atoms can outgas or be transported by carrier Atoms can outgas or be transported by carrier gas from the substrate into the gas stream and gas from the substrate into the gas stream and get re-deposited downstreamget re-deposited downstream– The process is called autodopingThe process is called autodoping

Empirical expression to describe autodopingEmpirical expression to describe autodoping

– CC**S S is an effective substrate surface concentration is an effective substrate surface concentration

and L is an experimentally determined parameterand L is an experimentally determined parameter– As film grows in thickness, dopant must diffuse As film grows in thickness, dopant must diffuse

through more film and less dopant enters gas phase.through more film and less dopant enters gas phase.

L

xCC S exp*

autodoping

AutodopingAutodoping

Autodoping from the backside, edges, or Autodoping from the backside, edges, or other sources usually results in a other sources usually results in a relatively constant level.relatively constant level.

This is because the source of dopant does This is because the source of dopant does not diminish as quickly but is at a much not diminish as quickly but is at a much lower level.lower level.

APCVDAPCVD

The left part of the The left part of the curve arises from curve arises from the out-diffusion the out-diffusion from the substratefrom the substrate

The straight line The straight line part arises from part arises from the front-side the front-side autodiffusionautodiffusion

The background The background (constant) part is (constant) part is from backside from backside autodopingautodoping

In APCVDIn APCVD– It is critical to deliver the same gas flows to all the It is critical to deliver the same gas flows to all the

wafers in order to produce the same growth rateswafers in order to produce the same growth rates Wafers placed side-by-side Wafers placed side-by-side

In LPCVDIn LPCVD– Mass transfer coefficient is higher as the diffusion Mass transfer coefficient is higher as the diffusion

distance of reactants is increased.distance of reactants is increased. the boundary layer thickness slightly increased as P the boundary layer thickness slightly increased as P

decreasesdecreases

– Reactions are limited by kReactions are limited by kss where adsorption on where adsorption on wafer surface is key limiting step.wafer surface is key limiting step. Wafers can be stacked parallel to one anotherWafers can be stacked parallel to one another

– Note that this arrangement will provide a higher throughputNote that this arrangement will provide a higher throughput

P

Dh

S

GG

1

LPCVDLPCVD

PECVDPECVD

There are some cases where temperature There are some cases where temperature requirements (thermal budget) will not requirements (thermal budget) will not allow high-temperature depositionsallow high-temperature depositions– E.g., depositing SiE.g., depositing Si33NN44 or SiO or SiO2 2 after Al after Al – APCVD and LPCVD do not produce good quality APCVD and LPCVD do not produce good quality

films ~ 450films ~ 450ooCC

Reaction produced in a plasma ignited Reaction produced in a plasma ignited using an inert gas between two electrodesusing an inert gas between two electrodes– The sample may be heatedThe sample may be heated

Usually between 200-450Usually between 200-450ooCC

PECVDPECVD

PECVDPECVD

Ideally suited to rapidly varying the Ideally suited to rapidly varying the film composition and properties during film composition and properties during depositiondeposition– Apply a potential and an AC signal (13.56 Apply a potential and an AC signal (13.56

MHz) across a low pressure of the inlet MHz) across a low pressure of the inlet gasesgases The processes that occur are complicated and The processes that occur are complicated and

very difficult to modelvery difficult to model The resultant products are very far from The resultant products are very far from

thermodynamic equilibriumthermodynamic equilibrium Generates high concentration of particulatesGenerates high concentration of particulates

– Pinhole density is a problem if chamber is not Pinhole density is a problem if chamber is not routinely cleaned. routinely cleaned.

PECVDPECVD

Microfluidic Channels formed by PECVDMicrofluidic Channels formed by PECVD

OxideOxideOxynitrideOxynitride

CVDCVD

LPCVD

PECVD

PolysiliconPolysilicon LPCVDLPCVD

– Deposited by thermal decomposition of silane (SiH4)Deposited by thermal decomposition of silane (SiH4) Deposition temperature range 580-650°CDeposition temperature range 580-650°C

– SiH4 (vapor) = Si (solid) + 2H2 (gas)SiH4 (vapor) = Si (solid) + 2H2 (gas)

– A typical set of deposition parameters: A typical set of deposition parameters: Temperature: 620°CTemperature: 620°C Pressure: 0.2-1.0 torrPressure: 0.2-1.0 torr SiHSiH44 flow rate ~ 250sccm flow rate ~ 250sccm Deposition rate = 8-10nm/minDeposition rate = 8-10nm/min

– Doping of filmDoping of film In-situ (during deposition) by the addition of dopant gases In-situ (during deposition) by the addition of dopant gases

such as phophine, arsine, and diboranesuch as phophine, arsine, and diborane Often doped after deposition by diffusion or ion implantationOften doped after deposition by diffusion or ion implantation

– Typically highly doped to achieve low resistance Typically highly doped to achieve low resistance interconnectionsinterconnections

– 0.01-0.001 ohm-cm can be obtained in diffusion-doped 0.01-0.001 ohm-cm can be obtained in diffusion-doped polysilicon.polysilicon.

Silicon dioxideSilicon dioxide Variety of methods (PECVD, LPCVD, APCVD)Variety of methods (PECVD, LPCVD, APCVD)

– Index of refraction is used to determine the quality.Index of refraction is used to determine the quality.– Can be doped or undopedCan be doped or undoped

Oxide doped with 5-15% by weight of various dopants can be Oxide doped with 5-15% by weight of various dopants can be used as a diffusion source. used as a diffusion source.

– PECVD oxidePECVD oxide SiHSiH44 (gas) + 2N (gas) + 2N22O (gas) O (gas) SiO SiO22 (solid) + 2N (solid) + 2N22 (gas) + 2H (gas) + 2H22

(gas)(gas) Temperature: 200-400°CTemperature: 200-400°C Deposition rate of 900nm/min is achievableDeposition rate of 900nm/min is achievable Not a good step coverageNot a good step coverage Controllable film stress (compressive)Controllable film stress (compressive) Contains hydrogen (SiOH)Contains hydrogen (SiOH) Used for deposition over metals Used for deposition over metals

– LPCVD oxide using dichlorosilaneLPCVD oxide using dichlorosilane SiClSiCl22HH22 + 2N + 2N22O O SiO SiO22 + 2N + 2N22 + 2HCl + 2HCl Temperatures ~ 900°CTemperatures ~ 900°C Good step coverageGood step coverage Compressive stress used to stress-compensate LPCVD nitrideCompressive stress used to stress-compensate LPCVD nitride

OxideOxide– LPCVD oxide using tetraethylorthosilicate LPCVD oxide using tetraethylorthosilicate

(TEOS) (liquid source)(TEOS) (liquid source) Si(OCSi(OC22HH55))44 SiO SiO22 (solid) + 4C (solid) + 4C22HH44 (gas) + 2H (gas) + 2H22O (gas)O (gas) Deposited at temperatures between 650-750°CDeposited at temperatures between 650-750°C Excellent uniformity and step coverageExcellent uniformity and step coverage

Silicon nitrideSilicon nitride Variety of methods (PECVD, LPCVD, APCVD)Variety of methods (PECVD, LPCVD, APCVD)

– Can be used as an oxidation mask (LPCVD)Can be used as an oxidation mask (LPCVD)– An excellent barrier to moisture and sodium An excellent barrier to moisture and sodium

contamination (PECVD)contamination (PECVD)– PECVD nitridePECVD nitride

SiHSiH44 (gas) + NH (gas) + NH33 (gas) (gas) Si SixxNNyyHHzz (solid) + H (solid) + H22 (gas) (gas) Deposition temperature: 200-400°CDeposition temperature: 200-400°C Deposition rate of 20-50nm/min Deposition rate of 20-50nm/min Not a good step coverageNot a good step coverage Controllable film stressControllable film stress

– Changes after high temperature anneal due to HChanges after high temperature anneal due to H22 out-diffusion out-diffusion– LPCVD nitride using dichlorosilaneLPCVD nitride using dichlorosilane

2SiCl2SiCl22HH22 (gas) + 4NH (gas) + 4NH33 (gas) (gas) Si Si33NN44 (solid) + 6H (solid) + 6H22 (gas) (gas) Deposition temperatures: 700-800°CDeposition temperatures: 700-800°C Good step coverageGood step coverage Tensile stressTensile stress Better resistivity (10Better resistivity (1016 16 ohm-cm) and dielectric strength (10 ohm-cm) and dielectric strength (10

MV/cm) compared to PECVD nitride films (10MV/cm) compared to PECVD nitride films (1066-10-1015 15 ohm-cm ohm-cm and 1-5MV/cm)and 1-5MV/cm)