Chapter 9 Thin Film Deposition. Introduction The layers on top of the silicon substrate are usually...

Chapter 9Chapter 9Thin Film DepositionThin Film Deposition

IntroductionIntroduction

The layers on top of the silicon substrate The layers on top of the silicon substrate are usually depositedare usually deposited– DielectricsDielectrics

Silicon oxide, silicon nitrideSilicon oxide, silicon nitride

– SemiconductorsSemiconductors poly-Si or a-Sipoly-Si or a-Si

– MetalsMetals 95% Al/5% Si95% Al/5% Si Ti or W clad copperTi or W clad copper Silicides (metal-silicon molecule)Silicides (metal-silicon molecule) CarbonCarbon

Characteristics of Characteristics of DepositionDeposition

Quality of depositionQuality of deposition– Composition of the filmComposition of the film– Contamination levelsContamination levels– Defect densityDefect density

Pinholes, step coveragePinholes, step coverage

– Mechanical propertiesMechanical properties StressStress

– Electrical propertiesElectrical properties ConductivityConductivity

– Optical propertiesOptical properties ReflectivityReflectivity


CompositionComposition– May vary with deposition method and May vary with deposition method and

parametersparameters– Composition control is very important when the Composition control is very important when the

material can have a range of compositionsmaterial can have a range of compositions Ratio of alloys and multilayer stacks of materials can Ratio of alloys and multilayer stacks of materials can

change the chemical, electrical, optical, and change the chemical, electrical, optical, and mechanical properties of film.mechanical properties of film.

ContaminationContamination– Unwanted moisture, undesired metals, Unwanted moisture, undesired metals,

incorporation of oxygen and halogensincorporation of oxygen and halogens

IntroductionIntroductionDefectsDefects

– Pinholes and other structural defects must be Pinholes and other structural defects must be minimizedminimized often result from particles on the surface of the waferoften result from particles on the surface of the wafer


Other quality considerationsOther quality considerations– Films must be stableFilms must be stable

Particularly if there are further thermal or Particularly if there are further thermal or chemical procedures to be carried out on chemical procedures to be carried out on the wafer.the wafer.

– They must adhere to the substrateThey must adhere to the substrate They must have minimum stressThey must have minimum stress


Uniformity of ThicknessUniformity of Thickness– The films must be uniform across the wafer and The films must be uniform across the wafer and

from wafer to waferfrom wafer to wafer

– Variations in thickness as in (b) can lead to high Variations in thickness as in (b) can lead to high electrical resistance and localized heatingelectrical resistance and localized heating Can lead to cracking from thermal cycling and Can lead to cracking from thermal cycling and

electromigrationelectromigration

Step CoverageStep Coverage

Coverage of the side of the stepCoverage of the side of the step– The ratio of the minimum thickness The ratio of the minimum thickness

deposited on the side of the step divided deposited on the side of the step divided by the thickness deposited on the top by the thickness deposited on the top horizontal surfacehorizontal surface

Conformal step coverageConformal step coverage

Refers to a step coverage of unityRefers to a step coverage of unity– Usually desired, but there are processes Usually desired, but there are processes

that rely on a step coverage of zerothat rely on a step coverage of zero

http://www.hitech-projects.com/dts/docs/pecvd.htm

Conformal step coverage of PECVD SixNy

Aspect RatioAspect Ratio

Deep, narrow features with high ARs are Deep, narrow features with high ARs are harder to fillharder to fill

w

h

feature ofwidth

feature ofheight AR

http://openlearn.open.ac.uk/mod/resource/view.php?id=257298

PVD tantalum barrier layer with ~60% step coverage

SEM image showing poor step coverage (breadloafing) of metal 1 into a silicon contact. (Courtesy Analytical Solutions) http://www.semitracks.com/reference/FA/die_level/sem/semxsc04.htm


Space-filling propertiesSpace-filling properties– Via hole or contact hole filling with Via hole or contact hole filling with

metalmetal– Filling spaces or gaps in shallow Filling spaces or gaps in shallow

trenches or between metal linestrenches or between metal lines– Voids in the film itself or between film Voids in the film itself or between film

and semiconductorand semiconductor High contact or sheet resistance High contact or sheet resistance Voids can lead to cracking of dielectricsVoids can lead to cracking of dielectrics

Two main categories Two main categories of thin fim deposition of thin fim deposition

They are:They are:– Chemical vapor deposition (CVD)Chemical vapor deposition (CVD)– Physical vapor deposition (PVD)Physical vapor deposition (PVD)

Wafer is placed in a chamber and the Wafer is placed in a chamber and the constituents of the film are delivered in constituents of the film are delivered in the gas phase to the surface where they the gas phase to the surface where they form a filmform a film

Chemical Vapor DepositionChemical Vapor Deposition

Reactant gases are introduced to the Reactant gases are introduced to the chamberchamber– One or more than one gas may be used One or more than one gas may be used

plus carrier gases (nonreactive gases)plus carrier gases (nonreactive gases)– In some cases, there is no gas source for In some cases, there is no gas source for

a particular material so an inert carrier a particular material so an inert carrier gas (Ar, Ngas (Ar, N22) is bubbled bubble through a ) is bubbled bubble through a liquid source and the vapor is liquid source and the vapor is transported into the chamber.transported into the chamber.


The system is designed so that the chemical The system is designed so that the chemical reactions between the gases takes place on reactions between the gases takes place on or very close to the wafer surface and not in or very close to the wafer surface and not in the gas stream to produce the filmthe gas stream to produce the film– Particles produced in the gas stream rain down on Particles produced in the gas stream rain down on

the wafer surface and cause pinholes or low the wafer surface and cause pinholes or low density filmsdensity films

– CVD is used to deposit Si and dielectrics because CVD is used to deposit Si and dielectrics because of good quality films and good step coverageof good quality films and good step coverage


There are several variants of the processThere are several variants of the process– Atmospheric pressure (APCVD)Atmospheric pressure (APCVD)– Low pressure (LPCVD)Low pressure (LPCVD)– Plasma-enhanced (PECVD)Plasma-enhanced (PECVD)

Most processes take place at elevated Most processes take place at elevated temperatures (250-650temperatures (250-650ooC)C)– Increase reaction rateIncrease reaction rate– Provide kinetic energy to allow reaction Provide kinetic energy to allow reaction

products to move along wafer surfaceproducts to move along wafer surface Increases film density and reduces pinholes and voidsIncreases film density and reduces pinholes and voids

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

Issues in APCVDIssues in APCVD Release of the reactants or reaction Release of the reactants or reaction

product from the surfaceproduct from the surface– Defined by the “sticking coefficient”Defined by the “sticking coefficient”

Composition of surface changes sticking coefficientComposition of surface changes sticking coefficient

– Re-emission is important in coverage and fillingRe-emission is important in coverage and filling Reaction on the chamber wallsReaction on the chamber walls

– cold wall versus hot wall processescold wall versus hot wall processes Wafer surface topologyWafer surface topology

– surface diffusion of reactants and byproductssurface diffusion of reactants and byproducts

Model for APCVDModel for APCVD

Simple model for the two important Simple model for the two important processesprocesses– Mass transfer of reactants to wafer surfaceMass transfer of reactants to wafer surface– Surface reactionsSurface reactions

Equate these two steps under steady state conditionsEquate these two steps under steady state conditions

The model looks very much like the model The model looks very much like the model we developed for oxidationwe developed for oxidation

APCVDAPCVD

The problem can be set up as followsThe problem can be set up as follows

There are two fluxes of atoms: FThere are two fluxes of atoms: F11 and F and F22

APCVDAPCVD

Flux from the gas phase is driven by Flux from the gas phase is driven by the concentration gradient from the the concentration gradient from the flowing gas to Si surface through a flowing gas to Si surface through a stagnant boundary layerstagnant boundary layer– Laminar flow conditionLaminar flow condition– It is given (in molecules/cmIt is given (in molecules/cm22/s) by/s) by

hhGG is the mass transfer coefficient through is the mass transfer coefficient through the boundary layerthe boundary layer

SGG CChF 1

APCVDAPCVD

Flux that is consumed by the Flux that is consumed by the reaction at the surface is if the reaction at the surface is if the reaction is a first order reaction.reaction is a first order reaction.

kkSS is the chemical reaction rate at the is the chemical reaction rate at the surface (cm/s)surface (cm/s)

SSCkF 2

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

DGS 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

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

Chapter 9 Thin Film Deposition. Introduction The layers on top of the silicon substrate are usually...

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