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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
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
Basic ConceptsBasic Concepts
Basic ConceptsBasic Concepts
Basic ConceptsBasic Concepts
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
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
Defect NomenclatureDefect Nomenclature
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
C
qNVV
22
Defect NomenclatureDefect Nomenclature
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
Defect NomenclatureDefect Nomenclature
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).
PRODUCTION FURNACESPRODUCTION FURNACES
Close-up of furnace with wafers.Close-up of furnace with wafers.
PRODUCTION FURNACESPRODUCTION FURNACES
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
Deal-Grove Model of Deal-Grove Model of OxidationOxidation
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
Deal-Grove Model of Deal-Grove Model of OxidationOxidation
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
Deal-Grove Model of Deal-Grove Model of OxidationOxidation
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.
Deal-Grove Model of Deal-Grove Model of OxidationOxidation