Introduction Amorphous arrangement of atoms means that there is a possibility that multiple Si atoms will be connected Amorphous arrangement of atoms means

IntroductionIntroduction

Amorphous arrangement of atoms means Amorphous arrangement of atoms means that there is a possibility that multiple Si that there is a possibility that multiple Si atoms will be connectedatoms will be connected Thin ‘wires’ of Si within SiOThin ‘wires’ of Si within SiO22 layer enables layer enables

leakage currents to flowleakage currents to flow When films get this thin, quantum When films get this thin, quantum

mechanical effects including tunneling mechanical effects including tunneling become importantbecome important Finite probability that an electron can Finite probability that an electron can

penetrate through an energy barrierpenetrate through an energy barrier Tunneling is usually undesirable, but some Tunneling is usually undesirable, but some

devices are now built using this phenomenon devices are now built using this phenomenon (nonvolatile memory)(nonvolatile memory)

Basic ConceptsBasic Concepts

Oxide grows by diffusion of Oxide grows by diffusion of oxygen/Hoxygen/H22O through the oxide to the O through the oxide to the Si/SiOSi/SiO22 interface interfaceThus, a new interface is continuously Thus, a new interface is continuously

growing and moving into the Si wafergrowing and moving into the Si wafer

The process is known as:The process is known as:Dry oxidation when oxygen only is used.Dry oxidation when oxygen only is used.Wet oxidation when water vapor (with or Wet oxidation when water vapor (with or

without oxygen) is used.without oxygen) is used.


Basic ConceptsBasic Concepts The process involves an expansion The process involves an expansion

the density of an equal volume of Si occupies less the density of an equal volume of Si occupies less space than a volume of oxide containing the same space than a volume of oxide containing the same number of Si atomsnumber of Si atoms

Nominally, the oxide would like to expand by 30% Nominally, the oxide would like to expand by 30% in all directions; but it cannot expand sideways in all directions; but it cannot expand sideways because it is constrained by the Si atomsbecause it is constrained by the Si atoms

Thus, there is a 2.2 Thus, there is a 2.2 expansion in the expansion in the vertical directionvertical direction In figure 6-4, note the growth of the LOCOS (In figure 6-4, note the growth of the LOCOS (LocLocal al

OOxidation of xidation of SSilicon) oxide above the surfaceilicon) oxide above the surface Also note the “bird’s beak” of oxide under the Also note the “bird’s beak” of oxide under the

nitride layer – a stress-induced rapid growth of nitride layer – a stress-induced rapid growth of oxideoxide




If there are shaped surfaces where oxide If there are shaped surfaces where oxide must grow, this expansion may not be so must grow, this expansion may not be so easily accommodatedeasily accommodated

The oxide layers are amorphous (i.e., there The oxide layers are amorphous (i.e., there is only short range order among the atoms)is only short range order among the atoms) There are no crystallographic forms of SiOThere are no crystallographic forms of SiO22 that that

match the Si lattice match the Si lattice The time required for transformation to a The time required for transformation to a

crystalline form at device temperatures is very crystalline form at device temperatures is very very long very long

Basic ConceptsBasic Concepts The oxide that grows is in compressive stressThe oxide that grows is in compressive stress

This stress can be relieved at temperatures above 1000This stress can be relieved at temperatures above 1000ooC by C by viscous flowviscous flow

There is a large difference in the TCE (There is a large difference in the TCE (tthermal hermal ccoefficient of oefficient of eexpansion) between Si and SiOxpansion) between Si and SiO22

This increases the compressive stresses in the oxide and This increases the compressive stresses in the oxide and results in tensile stresses in the Si near its surfaceresults in tensile stresses in the Si near its surface

Si is very thick while the oxide is very thinSi is very thick while the oxide is very thin Si can usually sustain the stressSi can usually sustain the stress Since the wafer oxidizes on both sides, the wafer remains flat; Since the wafer oxidizes on both sides, the wafer remains flat;

if you remove the oxide from the back side, you will see a if you remove the oxide from the back side, you will see a warping of the waferwarping of the wafer

The stress can be measured by measuring the warp of the The stress can be measured by measuring the warp of the waferwafer

Basic ConceptsBasic ConceptsThe electrical properties of the Si/SiOThe electrical properties of the Si/SiO22

interface have been extensively studiedinterface have been extensively studiedTo first order, the interface is perfectTo first order, the interface is perfect

The densities of defects are 10The densities of defects are 1099 – 10 – 101111 /cm /cm22 as compared to Si atom density of 10as compared to Si atom density of 101515 /cm /cm22

Most defects are associated with Most defects are associated with incompletely oxidized Siincompletely oxidized Si

Deal (1980) suggested a nomenclature Deal (1980) suggested a nomenclature that is now used to describe the various that is now used to describe the various defectsdefects

Defect NomenclatureDefect Nomenclature


There are four type of defectsThere are four type of defects1.1. QQff is the fixed oxide charge. is the fixed oxide charge.

– It is very close (< 2 nm) to the Si/SiOIt is very close (< 2 nm) to the Si/SiO22 interfaceinterface

– Surface concentration of 10Surface concentration of 1099 –10 –101111/cm/cm22

– Related to the transition from Si to SiORelated to the transition from Si to SiO22

– Incompletely oxidized Si atomsIncompletely oxidized Si atoms

– Positively charged and does not change Positively charged and does not change under normal conditionsunder normal conditions

Defect NomenclatureDefect Nomenclature2.2. QQitit is the interface trapped charge is the interface trapped charge

– Appears to incompletely oxidized Si with Appears to incompletely oxidized Si with dangling bondsdangling bonds

– Located very close to the interfaceLocated very close to the interface– Charge may be positive, neutral, or negativeCharge may be positive, neutral, or negative

– Charge state may change during device operation Charge state may change during device operation due to the trapping of electrons or holesdue to the trapping of electrons or holes

– Energy levels associated with these traps Energy levels associated with these traps are distributed throughout the forbidden are distributed throughout the forbidden band, but there seem to be more near the band, but there seem to be more near the valence and conductions bandsvalence and conductions bands

– Density of traps is 10Density of traps is 1099—10—101111 cm cm-2-2 eV eV-1-1


3.3. QQmm is the mobile oxide charge is the mobile oxide charge– It is not so important today but was very It is not so important today but was very

serious in the 1960’sserious in the 1960’s– It results from mobile NaIt results from mobile Na++ and K and K++ in the in the

oxideoxide– Shift in VShift in VTHTH is inversely proportional to C is inversely proportional to COXOX

and thus, as oxides become thinner, we and thus, as oxides become thinner, we can tolerate more impuritycan tolerate more impurity

OX

oOXOX

OX

M

OX

fAs

fFBTH

t

AC

C

qQ

C

qNVV

22


4.4. QQotot is charge trapped anywhere in the is charge trapped anywhere in the oxideoxide– Broken Si-O bonds in the bulk oxide well away Broken Si-O bonds in the bulk oxide well away

from the interfacefrom the interface– by ionizing radiation or by some processing steps by ionizing radiation or by some processing steps

such as plasma etching or ion implantationsuch as plasma etching or ion implantation– Metal ions from surface of Si or introduced Metal ions from surface of Si or introduced

during growthduring growth– Fe, Mn, Cr, CuFe, Mn, Cr, Cu

– Normally repaired by a high-temperature Normally repaired by a high-temperature annealanneal

– They can trap electrons or holesThey can trap electrons or holes– This is becoming more important as the electric field This is becoming more important as the electric field

in the gate oxide is increasedin the gate oxide is increased

– They result in shifts in VThey result in shifts in VTHTH


All four types of defects have All four types of defects have deleterious effects on the operation deleterious effects on the operation of devicesof devices

High temperature anneals in Ar or NHigh temperature anneals in Ar or N22 near the end of process flow plus an near the end of process flow plus an anneal in Hanneal in H22 or forming gas at the or forming gas at the end of process flow are used to end of process flow are used to reduce their effectreduce their effect

Manufacturing MethodsManufacturing Methods

Furnace capable of 600 – 1200 Furnace capable of 600 – 1200 ooC with a C with a uniform zone large enough to hold several uniform zone large enough to hold several waferswafers

Gas distribution system to provide OGas distribution system to provide O22 and and HH22OOGenerally, HGenerally, H22 is burnt with O is burnt with O22 at the entrance of at the entrance of

the furnace to create water vaporthe furnace to create water vaporTCA or HCl may be used to remove metal ionsTCA or HCl may be used to remove metal ions

Control system that holds the temperatures Control system that holds the temperatures and gas flows to tight tolerances (and gas flows to tight tolerances (0.5 C)0.5 C)

PRODUCTION FURNACESPRODUCTION FURNACES

Commercial furnace showing the furnace with Commercial furnace showing the furnace with wafers (left) and gas control system (right).wafers (left) and gas control system (right).


Close-up of furnace with wafers.Close-up of furnace with wafers.


ModelsModels

The first major model is that of Deal The first major model is that of Deal and Grove (1965)and Grove (1965)This lead to the linear/parabolic modelThis lead to the linear/parabolic model

Note that this model cannot explain Note that this model cannot explain the effect of oxidation of the diffusion ratethe effect of oxidation of the diffusion rate the oxidation of shaped surfacesthe oxidation of shaped surfaces the oxidation of very thin oxides in mixed the oxidation of very thin oxides in mixed

ambientsambients

The model is an excellent starting place The model is an excellent starting place for the other more complicated modelsfor the other more complicated models

CHEMICAL REACTIONSCHEMICAL REACTIONS

Process for dry oxygenProcess for dry oxygen

Si + OSi + O22 SiO SiO22

Process for water vaporProcess for water vapor

Si + 2HSi + 2H22O O SiO SiO22 + 2H + 2H22

OXIDE GROWTHOXIDE GROWTHSi is consumed as oxide grows and oxide expands. Si is consumed as oxide grows and oxide expands.

The Si surface moves into the wafer.The Si surface moves into the wafer.

54%

46%SiO2

Siliconwafer

Originalsurface

MODEL OF OXIDATIONMODEL OF OXIDATIONOxygen must reach silicon interfaceOxygen must reach silicon interface

Simple model assumes OSimple model assumes O22 diffuses diffuses through SiOthrough SiO22

Assumes no OAssumes no O22 accumulation in SiO accumulation in SiO22

Assumes the rate of arrival of HAssumes the rate of arrival of H22O or OO or O22 at the oxide surface is so fast that it can at the oxide surface is so fast that it can be ignoredbe ignoredReaction rate limited, not diffusion rate Reaction rate limited, not diffusion rate

limitedlimited

Deal-Grove Model of Deal-Grove Model of OxidationOxidation

Fick’s First Law of diffusion states that Fick’s First Law of diffusion states that the particle flow per unit area, J (particle the particle flow per unit area, J (particle flux), is directly proportional to the flux), is directly proportional to the concentration gradient of the particle.concentration gradient of the particle. We assume that oxygen flux passing through We assume that oxygen flux passing through

the oxide is constant everywhere.the oxide is constant everywhere.

FF11 is the flux, C is the flux, CGG is the concentration in the is the concentration in the gas flow, Cgas flow, CSS is the concentration at the is the concentration at the surface of the wafer, and hsurface of the wafer, and hGG is the mass is the mass transfer coefficienttransfer coefficient

)(1 SGG CChF

J

Distance from surface, x

N

No

Ni

Silicondioxide

Silicon

SiO2 Si

Xo

J D N N xi ( ) /0 0


Assume the oxidation rate at Si-SiOAssume the oxidation rate at Si-SiO22 interface is proportional to the Ointerface is proportional to the O22 concentration:concentration:

Growth rate is given by the oxidizing flux Growth rate is given by the oxidizing flux divided by the number of molecules, M, of divided by the number of molecules, M, of the oxidizing species that are incorporated the oxidizing species that are incorporated into a unit volume of the resulting oxide: into a unit volume of the resulting oxide:

J k Ns i

skDxM

DN

M

J

dt

dx

0

00


The boundary condition isThe boundary condition is

The solution of differential equation isThe solution of differential equation is

AD

kB

DN

M

x

B

x

B As

o i i 2 2 2

ixtx 00

AB

x

B

xt 0

20


xxox ox : final oxide thickness: final oxide thickness

xxi i : initial oxide thickness : initial oxide thickness

B/AB/A : linear rate constant: linear rate constant

B : parabolic rate constantB : parabolic rate constant

There are two limiting cases:There are two limiting cases:Very long oxidation times, Very long oxidation times, t >> t >>

xxoxox2 2 = B t= B t

Oxide growth in this parabolic regime is Oxide growth in this parabolic regime is diffusion controlled.diffusion controlled.

Very short oxidation times, (Very short oxidation times, (t + t + ) << ) << AA22/4B/4B xxoxox

= B/A ( t + = B/A ( t + ) )Oxide growth in this linear regime is Oxide growth in this linear regime is

reaction-rate limited.reaction-rate limited.


Documents

Introduction Amorphous arrangement of atoms means that there is a possibility that multiple Si atoms will be connected Amorphous arrangement of atoms means