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1. UNIT-IV Doping and Deposition
•Diffusion:Modelsofdiffusioninsolids,Fick’s1-Dimensional
diffusionequation,DiffusionofImpuritiesinSiliconandSilicon
Dioxide,DiffusionEquations,DiffusionProfiles,DiffusionFurnace,
Solid,LiquidandGaseousSources
•Ion-Implantation: Ion-Implantation Technique, Range Theory,
Implantation Equipment.
12/21/2024 1
AKTU Syllabus UNIT-IV

2. •How does diffusion occur ?
•Why is it an important part of processing ?
•How can the rate of diffusion be predicted for some simple
cases?
•How does diffusion depend on structure and temperature?
12/21/2024 2
Diffusion in Solids

3. •Two methods for introducing impurities into Si to control the
majority-carrier type and resistivity of layers.
•Diffusion
•Ion Implantation
•Diffusion: dopant atoms move from the surface into Si by thermal
means via substitutional or interstitial diffusion mechanisms.
•Ion implantation: dopant atoms are forcefully added into Si in the
form of energetic ion beam injection
12/21/2024 3
Impurity Doping

4. 12/21/2024 4
Diffusion & ion Implantation
Diffusion
Ion
Implantation

5. •Formation of pn junction and fabrication of devices duringwafer
fabrication.
•Alter the type and level of conductivity of semiconductor
materials.
•Form bases, emitters, and resistors in bipolar devices, as well as
drains and sources in MOS devices.
•Dope polysilicon layers.
12/21/2024 5
Need of Doping

6. •Movementofachemicalspeciesfromanareaofhighconcentrationtoan
areaoflowerconcentration.
•Diffusionisaprocessofintroducingcontrolledamountofdopantsinto
semiconductor
•Thediffusionprocessbeginswiththedepositionofashallowhigh
concentrationofthedesiredimpurityintheSisurfacethroughwindows
etchedintheprotectivebarrierlayer.
12/21/2024 6
Diffusion

7. There are different types of diffusion mechanism.
•Interstitial Diffusion
•Substitutional Diffusion
•Interstitial Substitutional Diffusion
•Interchange or Self Diffusion
12/21/2024 7
Types of Diffusion

8. •In this the impurity atoms move through the crystal lattice by
jumping from one interstitial site to the next.
•Interstitial diffusion requires that their jump motion occurs from
one interstitial site to another adjacent interstitial site.
•Alkali metals like Li, Na, K, and gases like Ar, He,Hare moved by
this mechanism. (Due to inactive except Li, they do not contribute
in circuit fabrication.
12/21/2024 8
Interstitial Diffusion

9. •In this the impurity atoms move through the crystal by jumping
from one lattice site to the next.
•In this way they substitute for the original host atom.
•The presence of vacancies is must for substitutional diffusion.
•Substitutional diffusion occurs at a much slower rate as compared
to interstitial diffusion since the equilibrium concentration of
vacancies is low.
•Al, B, Ga, In, At, As, P are moved by this mechanism.
12/21/2024 9
Substitutional Diffusion

10. •It is a combination of Interstitial and Substitutional Diffusion.
•In this type of diffusion the diffusing atom moves by pushing one
of the nearest substitutional neighbors into an adjacent interstitial
site and taking up the substitutional site.
•Figure (a) and (b) shows Diffusion by dissociative mechanism.
•Last figure shows Diffusion by kick out mechanism.
•Au, Pt, Ag, Co, Fe, Ni, Cu are moved by this mechanism.
12/21/2024 10
Interstitial Substitutional Diffusion

11. •Two or more atoms diffuse by an interchange process to cause
interchange diffusion.
•This is a direct process of interchange when two atoms are
involved and is a cooperative interchange when larger numbers are
involved.
Label some atoms After some time
12/21/2024 11
Interchange or Self DiffusionA
B
C
D

12. •The local rate of transfer of solute per unit area per unit time is
proportional to the concentration gradient of the solute and defines
the proportionality constant as diffusivity of the solute. The
negative sign shows the flow towards lower concentration of
solute.
•Mathematically
•J=rate of transfer of solute per unit area or diffusion flux
C=concentration of solute (function of x and t only)
•x=coordinate axis in the direction of solute flow
•t=diffusion time
•D=diffusivity (Diffusion constant)
12/21/2024 12
Fick’s First Law of Diffusion

13. •Though it describes diffusion process accurately.
•But, has no convenient measure of current density of theimpurity.
•Thus, second law developed to describe the concept with more
readily measurable quantities.
12/21/2024 13
Limitations of Fick’s First Law of Diffusion

14. •Consider a long bar of material with uniform cross-sectional area A.
For a small volume of length dx,
•(J2-J1) / dx = -∂ J / ∂ x
•J1 is the flux entering into the volume and J2 is the flux leaving the
volume.
•Then the continuity equation gives,
•A dx ∂ C / ∂ t = -A (J2-J1) = -A dx ∂ J / ∂ x
•Law of conservation of matter: change in solute concentration per
unit time= local decrease in diffusion flux in the absence of source
12/21/2024 14
Fick’s Second Law of Diffusion

15. •But according to the law of conservation of matter,
•change in solute concentration per unit time = local decrease in
diffusion flux in the absence of source
∂ C(x,t) / ∂ t = -∂ J (x,t) / ∂ x ………………(1)
Combining with Fick’s first law,
∂ C(x,t) / ∂ t = -[∂ / ∂ x] ( -D ∂C(x,t) /∂ x)
• ∂ C(x,t) / ∂ t = D [∂
2
C(x,t) / ∂ x
2
] ……………(2)
•So Fick’s second law of diffusion is given as
• ∂ C(x,t) / ∂ t = D [∂
2
C(x,t) / ∂ x
2
]
Where, C = concentration of solute,D = diffusivity
•x = coordinate axis in the direction of solute flow
•t = diffusion time
12/21/2024 15
Fick’s Second Law of Diffusion

16. •CASE : When total diffusion source concentration (Cs) is fixed
or forconstant diffusivity (D).
•Solution for constant diffusivity is done in three ways:
–Constant surface concentration
–Constant total dopant
–Sheet resistance of diffused layer
12/21/2024 16
Analytical Solution of Fick’s Law

17. •Constant Surface Concentration
•We know that Fick’s second law of diffusion is given as
∂ C(x,t) / ∂ t = D [∂
2
C(x,t) / ∂ x
2
]
•This type of solution occurs when the source is fixed at the
surface for all times greater than zero.
•This is called pre deposition diffusion.
•We shall apply the following boundary conditions.
C(x,0) = 0
C(0,t) = C
s
C(∞,t) = 0
•The solution for these equations is given by
C(x,t) = C
s erfc[ x / 2√ Dt]t > 0
12/21/2024 17
Analytical Solution of Fick’s Law

18. •Where C
sis the fixed surface concentration in atoms/ Cm
3
.
•D is the Diffusivity inCm
2
/sec.
•x is the distance in cm.
•t is the diffusion time in second.
•erfcis the complementary error function.
•√ Dt is the characteristic diffusion length.
12/21/2024 18
Analytical Solution of Fick’s Law

19. 12/21/2024 19
Impurity Distribution for constant surface
concentration

20. •Dose is measured in units of impurities per unit area (per cm
2
).
•It varies with time of diffusion.
Q
T(t) = Limit 0 to ∞ ∫ C (x, t) dx
= (2/√ᴫ) C(0,t) √ Dt
12/21/2024 20
Dose of Diffusion

21. •Constant Total Dopant
•We know that Fick’s second law of diffusion is given as
∂ C(x,t) / ∂ t = D [∂
2
C(x,t) / ∂ x
2
]
•This type of solution occurs when an initial amount of impurity
Q
Tis introduced into the wafer and diffused subject to the
boundary condition that Q
Tis fixed.
•This is called “drive in diffusion”.
•We shall apply the following boundary conditions.
C(x,t) = 0 x ≠ 0
dC(0,t) /dx = 0
C(∞,t) = 0
Limit 0 to ∞ ∫ C (x, t) dx = (Constant) Q
T
12/21/2024 21
Analytical Solution of Fick’s Law

22. •The solution of Fick’s law using these boundary conditions are
given by
C(x,t) = (Q
T/ √ᴫDt ) e
-x2/4Dt
t > 0
•By putting x = 0, the surface concentration is
C
s= C(0,t) = (Q
T/ √ᴫDt )
•In practice √Dt
pre depo << √Dt
driven in
12/21/2024 22
Analytical Solution of Fick’s Law

23. •The solution of Fick’s law using these boundary conditions are
given by
C(x,t) = (Q
T/ √ᴫDt ) e
-x2/4Dt
t > 0
•By putting x = 0, the surface concentration is
C
s= C(0,t) = (Q
T/ √ᴫDt )
•In practice √Dt
pre depo << √Dt
driven in
12/21/2024 23
Analytical Solution of Fick’s Law

24. •The solution of Fick’s law using these boundary conditions are
given by
C(x,t) = (Q
T/ √ᴫDt ) e
-x2/4Dt
t > 0
•By putting x = 0, the surface concentration is
C
s= C(0,t) = (Q
T/ √ᴫDt )
•In practice √Dt
pre depo << √Dt
driven in
12/21/2024 24
Impurity Distribution for constant total dopant

25. •For a diffused layer that forms a p-n junction, an average sheet
resistance R
s is defined as
R
s = 1 / ( q Lim 0 tox
j ∫ μC(x) dx)
R
s = 1 / ( q μ
eff Lim 0 tox
j ∫ C(x) dx)
Where
x
j isthe junction depth.
μis the carrier mobility.
C(x) is the impurity distribution.
μ
eff is the effective mobility.
12/21/2024 25
Sheet Resistance

26. •Thediffusionprofileofdopantatomsisdependentontheinitialand
boundaryconditions.
•Solutionsforonedimensionaldiffusionequationhavebeenobtained
forvarioussimpleconditions,includingconstant-surface-
concentrationdiffusionandconstant-totaldopantdiffusion.
•Inthefirstscenario,impurityatomsaretransportedfromavapor
sourceontothesemiconductorsurfaceanddiffuseintothe
semiconductorwafer.
•Thevaporsourcemaintainsaconstantlevelofsurfaceconcentration
duringtheentirediffusionperiod.
•Inthesecondsituation,afixedamountofdopantisdepositedonto
thesemiconductorsurfaceandissubsequentlydiffusedintothe
wafer.
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Diffusion Profile

27. There are the following parameters that affect the diffusion
profile.
•Solid solubility
•Diffusion temperature
•Diffusion time
•Surface cleanliness and defects in silicon crystal
12/21/2024 27
Diffusion Profile

28. Solid solubility
•In deciding which of the availability impurities can be used, it
is essential to know if the number of atoms per unit volume
required by the specific profile is less than the diffusant solid
solubility.
Diffusion temperature
•Higher temperatures give more thermal energy and thus higher
velocities, to the diffused impurities.
•It is found that the diffusion coefficient critically depends upon
temperature.
•Therefore, the temperature profile of diffusion furnace must
have higher tolerance of temperature variation over its entire
area.
12/21/2024 28
Diffusion Profile

29. Diffusion time
•Increases of diffusion time, t, or diffusion coefficient Dhave
similar effects on junction depth as can be seen from the
equations of limited and constant source diffusions.
•For Gaussian distribution, the net concentration will decrease
due to impurity compensation, and can approach zero with
increasing diffusion tunes.
•For constant source diffusion, the net Impurity concentration
on the diffused side of the p-n junction shows a steady increase
with time.
12/21/2024 29
Diffusion Profile

30. Surfacecleanlinessanddefectsinsiliconcrystal
•Thesiliconsurfacemustbepreventedagainstcontaminantsduring
diffusionwhichmayinterfereseriouslywiththeuniformityofthe
diffusionprofile.
•Thecrystaldefectssuchasdislocationorstackingfaultsmay
producelocalizedimpurityconcentration.
•Thisresultsinthedegradationofjunctioncharacteristics.
•Hencesiliconcrystalmustbehighlyperfect.
12/21/2024 30
Diffusion Profile

31. Surface cleanliness and defects in silicon crystal
•The silicon surface must be prevented against contaminants
during diffusion which may interfere seriously with the
uniformity of the diffusion profile.
•The crystal defects such as dislocation or stacking faults may
produce localized impurity concentration.
•This results in the degradation of junction characteristics.
•Hence silicon crystal must be highly perfect.
12/21/2024 31
Diffusion Profile

32. Followingpropertiescouldbeconsideredfordesigningandlayingout
ICs.
•Whencalculatingthetotaleffectivediffusiontimeforgivenimpurity
profile,onemustconsidertheeffectsofsubsequentdiffusioncycles.
•TheerfcandGaussianfunctionsshowthatthediffusionprofilesare
functionsof(x/√Dt).
•Hence,foragivensurfaceandbackgroundconcentration,the
junctiondepthx1andx2associatedwiththetwoseparatediffusions
havingdifferenttimesandtemperature.
Lateral Diffusion Effects
•The diffusions proceed sideways from a diffusion window as well as
downward.
•In both types of distribution function, the side diffusion is about 75
to 80 per cent of the vertical diffusion.
12/21/2024 32
Basic Properties of the Diffusion Process

33. •The dopants selection affects IC characteristics.
•Boron and phosphorus are the basic dopants of most ICs.
•Arsenic and antimony, which are highly soluble in silicon and
diffuse slowly, are used before epitaxial processing or as a
second diffusion.
•Gold and silver diffuse rapidly. They act as recombination
centresand thus reduce carrier life time.
12/21/2024 33
Dopants and their Characteristics

34. •Boron is almost an exclusive choice as an acceptor impurity in
silicon since other p-type impurities have limitations as
follows:
•Gallium has relatively large diffusion coefficient in Si0
2, and
the usual oxide window-opening technique for locating
diffusion would be inoperative
•Indium is of little interest because of its high acceptor level of
0.16 eV, compared with 0.01 eV for boron, which indicates that
not all such acceptors would be ionized at room temperature to
produce a hole.
•Aluminum reacts strongly with any oxygen that is present in
the silicon lattice.
12/21/2024 34
Dopants and their Characteristics

35. •The choice of a particular n-type dopant is not so limited as for
p-type materials.
•The n-type impurities, such as phosphorus, antimony and
arsenic, can be used at different stages of IC processing.
•The diffusion constant of phosphorus is much greater than for
Sb and As, being comparable to that for boron, which leads to
economies resulting from shorter diffusion times.
12/21/2024 35
Dopants and their Characteristics

36. •ThecommondopantsinVLSIcircuitfabricationareboron,
phosphorus,andarsenic.
•Phosphorusisusefulnotonlyasanemitterandbasedopant,butalso
fargetteringfast-diffusingmetalliccontaminants,suchasCuandAn,
whichcausejunctionleakagecurrentproblems.
•Thus,phosphorusisindispensableinVLSItechnology.
•However,n-p-ntransistorsmadewitharsenic-diffusedemittershave
betterlow-currentgaincharacteristicsandbettercontrolofnarrow
basewidthsthanthosemadewithphosphorus-diffusedemitters.
•Therefore,inVLSI,theuseofphosphorusasanactivedopantin
small,shallowjunctionsandlow-temperatureprocessingwillbe
limitedtoitsuseasthebasedopantofp-n-pdeviceandasagettering
agent.
•Arsenicisthemostfrequentlyuseddopantforthesourceanddrain
regionsinn-channelMOSFETs.
12/21/2024 36
Dopants in VLSI Technology

37. •Impurities are diffused from their compound sources as
mentioned above.
•The method impurity delivery to wafer is determined by the
nature of impurity source.
•Two-step diffusion is widely technique.
•Using this technique, the impurity concentration and profiles
can be carefully controlled.
•The type of impurity distribution (erfcor Gaussian) is
determined by the choice of operating conditions.
12/21/2024 37
Diffusion Systems

38. •Thetwo-stepdiffusionconsistsofadepositionstepandadrive-in
step.
•Intheformerstepaconstantsourcediffusioniscarriedoutfora
shorttime,usuallyatarelativelylowtemperatures,say,1000°C.
•Inthelatterstep,theimpuritysupplyisshutoffandtheexisting
dopantisallowedtodiffuseintothebodyofthesemiconductor,
whichisnowheldatadifferenttemperature,say1200°C,inan
oxidizingatmosphere.
•Theoxidelayerwhichformsonsurfaceofthewaferduringthisstep
preventsfurtherimpuritiesfromentering,orthosealreadydeposited,
fromdiffusingout.
•Thefinalimpurityprofileisafunctionofdiffusioncondition,suchas
temperature,time,anddiffusioncoefficients,foreachstep.
12/21/2024 38
Diffusion Systems

39. Therearetwotypesofdiffusionsystems.
•SealedTubeType
•OpenTubeType
•SealedTubeType
•Inthistheslicesanddopantareenclosedinaclean,evacuatedquartz
tubepriortoheattreatment.
•Theslicesareremovedbybreakingthetubeafterdiffusionsealed
tubesystemcanbeeasilymaintainedfreefromcontamination.
•Sealedtubesystemsoperatebythermalevaporationofthedopant
source,transportinthegasphase,adsorptiononthesurfaceofthe
semiconductorandthetubewallsandeventualdiffusionofthe
dopantintotheslices.
•Theresidualgaspressureintheampulecanalsoaffectthediffusion
process.
12/21/2024 39
Types of Diffusion Systems

40. •Pressures in excess of 10 torr can cause problems with dopant
transport, presumably by covering the surface with residual
contamination.
•Sealed tube diffusion for silicon devices are usually carried out
at temperatures in the range of 1200 –1225 C.
•Here pressures as high as 200-250 torr are used, to prevent
undue mechanical strain in the ampoule at the diffusion
temperature.
12/21/2024 40
Types of Diffusion Systems

41. Open Tube Type
•Open tube method is used in commercial diffusion system.
•In this diffusion system, slices are placed in clean, horizontal
diffusion tube made of high purity fused quartz.
•A separate diffusion furnace, slice carrier and diffusion tube are
reserved for each impurity.
•Insertion is done from one end of the tube, whereas the other
end is used to flow of gases of impurities in vapour form.
•This system is capable of handling a number of slices at one
time.
•This system is more convenient than the sealed tube system.
12/21/2024 41
Types of Diffusion Systems

42. On the basis of sources there are following types of diffusion
systems:
•Solid Source Diffusion System
•Liquid Source Diffusion System
•Gaseous Source Diffusion System
12/21/2024 42
Diffusion Systems Based on Source

43. •The circuit arrangement for the solid source diffusion system is
shown in figure.
12/21/2024 43
Solid Source Diffusion System

44. •In this system, we can use a platinum boat to hold a solid
source of dopant species upstream from the carrier with
semiconductor wafers.
•In operation, the carrier gas transports vapours from this source
and deposits them on the silicon wafer.
•Source shut off is usually accomplished by moving the dopant
source to a colder region of the furnace.
•The success of this technique depends critically on the vapour
pressure of the source.
•The source boat and the slices can be maintained at the same
temperature, avoiding the need for a two zone furnace.
12/21/2024 44
Solid Source Diffusion System

45. •Liquid source diffusion system is shown in the figure.
12/21/2024 45
Liquid Source Diffusion System

46. •Inthissystemthecarriergasisbubbledthroughtheliquidwhichis
transportedinvaporformtothesurfaceoftheslices.
•Itistypicallytosaturatethisgaswiththevaporssothatthe
concentrationisrelativelyindependentofgasflow.
•Sothesurfaceconcentrationofliquidsourcediffusionsystemis
entirelysetbythetemperatureofthebubblerandofthediffusion
system.
•Thissystemisquiteconvenientasthedopingprocesscanbereadily
initiatedorterminatedbycontrolofthegasthroughthebubbler.
•Anumberofhalogenicdopantcompoundsareavailableasliquids.
•Useofthesesourcesgreatlyreducesheavymetalcontaminationin
diffusionsystem.
12/21/2024 46
Liquid Source Diffusion System

47. •Liquidsourcesystemsareextremelyconvenientsincethedoping
processcanbereadilyinitiatedorterminatedbycontrolofthegas
throughbubbler.
•Anumberofphylogenicdopantcompoundsareavailableasliquids.
•Useofthesesourcesgreatlyreducesheavilymetalcontaminationin
diffusionsystem.
•Theamountofdopanttransportedtothesiliconwaferisrelatively
easytocontrolinthesesystems,byadjustingthetemperatureofthe
bubbler.
12/21/2024 47
Advantages of Liquid Source Diffusion System

48. •Figure shows the gas source diffusion system.
12/21/2024 48
Gaseous Source Diffusion System

49. •Gaseous sources are more convenient than the liquid source
systems.
•It is generally to use an excess dopant gas concentration, so that
these systems are relatively insensitive to gas flow rate.
•In gaseous source diffusion system provision is made for an
ambient carrier gas in which the diffusion takes place.
•A chemical trap is often incorporated to disposed of unreacted
dopant gas.
•The major drawback of the gas source diffusion system is that
it is difficult to maintain doping uniformity over all the silicon
wafers during a diffusion run and even over the surface area of
each individual wafer.
•This is due to the depletion occurs during of the reactant vapor
between wafers.
12/21/2024 49
Gaseous Source Diffusion System

50. •Spin on method can be used for obtaining uniform source
coverage over a silicon wafer.
•In this technique, a dilute mixture of a dopant compound in an
organosilance is used as the source material.
•The silicon wafer is held in a vacuum chuck and rotated at
2500-5000 rpm speed.
•A drop of the source chemical is next applied to form a thin
layer across the slice by means of centrifugal force.
•By the proper viscosity control, relatively uniform layers of
dopant can be obtained in this method.
12/21/2024 50
Spin –on Diffusion System

51. •The SiO2 layer serves as a cap during the diffusion process.
•This technique is versatile and almost any dopant for silicon
can be applied in this method, by the addition of a few drops of
source chemical to an undoped organo-silane.
12/21/2024 51
Advantages of Spin –on Diffusion System

52. •Diffusionsystemforsiliconareaoftheopentubetypewithquartz
beingthemostcommontubematerial.
•Tubesofsemiconductorgradesiliconaresometimesusedforthis
purpose.
•Theirusefullifeisverylongcomparedtoquartztubessincetheydo
notdevitrifyduringuseandareimpervioustothermalshock.
•Theexamplesforptypeandntypediffusionprocessesoffewtype
ofdiffusionsystemsarelistedbelow.
•BoronDiffusionSystem
•PhosphorousDiffusionSystem
•ArsenicDiffusionSystem
•AntimonyDiffusionSystem
•GoldDiffusionSystem
•PlatinumDiffusionSystem
12/21/2024 52
Diffusion System for Silicon

53. •SiO
2 is used for insulation and as barrier to impurity diffusion.
•Arsenic diffusivity in SiO
2 :
•In nitrogen diffusivity was higher than in oxygen.
•At concentrations above 5 X 10
20
cm
-3
,As was found to be
immobile.
12/21/2024 53
Diffusion System for Silicon Di Oxide

54. •BorondiffusioninSiO
2:
–Bdiffusessubstitutionally.
–NitrogenreducesBdiffusivitybyincreasingthediffusion
activationenergy.
–Sonitrogenisincorporatedintomostgateoxidestoprevent
Btodiffusefromp-typepolycrystallinegateelectrodesto
thechannelofPMOSdevices.
–IfBdiffusesintochannel,thethresholdvoltagechangesand
itdegradesoxidereliability.
–HydrogenincreasesBdiffusivityinSiO
2.
12/21/2024 54
Diffusion System for Silicon Di Oxide

55. •For the various types of diffusion processes a resistance-heated
tube furnace is usually used.
•A tube furnace has a long (about 2 to 3 meters) hollow opening
into which a quartz tube about 100,150 mm in diameter is
placed as shown in the figure below.
12/21/2024 55
Diffusion Furnace

56. 12/21/2024 56
Diffusion Furnace

57. •The temperature of the furnace is kept about 1000°C.
•The temperature with in the quartz furnace tube can be
controlled very accurately such that a temperature within 1/2°C
of the set-point temperature can be maintained uniformly over
a “hot zone” about 1 m in length.
•This is achieved by three individually controlled adjacent
resistance elements.
•The silicon wafers to be processed are stacked up vertically
into slots in a quartz carrier or “boat” and inserted into the
furnace tube.
12/21/2024 57
Diffusion Furnace

58. •The results of a diffusion process can be evaluated by three
parameters:
(1) Junction depth.
(2) Sheet resistance.
(3) Dopant profile.
•The junction depth is measured by techniques: (i) Staining and
(ii) Grooving.
•The sheet resistance is measured by the 4-probe method.
•The dopant profile is commonly measured by Secondary Ion
Mass Spectrometry (SIMS)
12/21/2024 58
Measure the result of Diffusion

59. •Staining
–Junctiondepthsarecommonlymeasuredonanangle-lapped
(1°to5°)samplechemicallystainedbyamixtureof100
c.c.HF(49%)andafewdropsofHNO
3.Ifthesampleis
subjectedtostrongilluminationfor1-2minutes,thep-type
regionwillbestaineddarkerthanthen-typeregion,duetoa
reflectivitydifferenceofthetwoetchedsurfaces.
–Thelocationofthestainedjunctiondependsonthep-type
concentrationlevelandsometimesontheconcentration
gradient.Ingeneral,thestainboundarycorrespondstoa
concentrationlevelintherangeofmid-10
17
atoms/cm
3
.
12/21/2024 59
Measure the result of Diffusion

60. 12/21/2024 60
Measure the result of Diffusion

61. 12/21/2024 61
Measure the result of Diffusion

62. •SecondaryIonMassSpectrometry(SIMS)
–SIMSisveryusefulinobtainingimpuritydiffusionprofiles
withexcellentdepthresolution(1-20nm)andhighsensitivity
(ppmtoppb).
–Anoxygenorcesiumionbeamofenergy1to20keVisused
tosputterasampleandthesputteredionsareseparated
accordingtotheirmass-to-chargeratios(m/e)byamass
spectrometertypicallycomposedofamagneticsectoror
quadrupole.
12/21/2024 62
Measure the result of Diffusion

63. •Ionimplantationcanbedefinedastheprocessbywhichimpurityions
areacceleratedtohighvelocityandphysicallylodgedintotarget
material.
•Ionimplantationisaprocessbywhichenergeticimpurityatomscan
beintroducedintoasinglecrystalsubstrateinordertochangeits
electronicproperties.
•Themajoradvantageofionimplantationisthatitofferstheabilityto
preciselycontroltheamountofdopantanditsdepthbelowthe
surface.
•Itisalowtemperatureprocessthateliminatesdeformationofwafer
causedathightemperatures.
12/21/2024 63
Ion Implantation

64. •Contaminationfree
•Thedominant,accurateandlowtemperaturedopingmethod
•Excellentcontrolofdosewithlargerange(10
12
to10
18
dopants/cm
2
)
•Non-equilibriumprocess.
•Goodcontrolofimplantdepth(100Å-10µm)withenergy(KeVto
MeV)
•Repairingcrystaldamageanddopantactivationrequiresannealing,
whichcancausedopantdiffusionandlossofdepthcontrol.
•Widechoiceofmaskingmaterials
12/21/2024 64
Ion Implantation

65. 12/21/2024 65
Range Theory
•Factors which affect Range in Ion-Implantation
–Ion Stopping (ions lose energy in the target material through nuclear
and electron stopping).
–Range distribution (the total path travelled is a mixture of vertical and
lateral motion and the projected range has a Gaussian distribution).
–Damage to lattice structure (a primary ion can generate secondary ions
by collision, self-annealing at high temp. can assist).
–Channeling of ions (Lattice perfections and imperfections can lead to
different paths of implanted ions).
–Recoil of the ions (In thin multi-layer structures, atoms get displaced
from one region to other).

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Ion Implantation System

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Ion Implantation System
•Ion implantation system can be divided into three parts.
Ion source
Acceleration tube
End station

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Ion source
•It starts with a feed gas that contains the desired implant
species.
•Common feed gases for use in Silicon technologies are BF3,
ASH3, PH3.
•The flow of gas can be controlled with a variable nobe
(Orifice).
•If desired implant species is not available in gaseous form , a
solid charge can be heated and the resultant vapour used as a
source.
•The basic requirement of the ion implantation system is to
deliver a beam of ions of a particular type and energy to the
surface of a silicon wafer.

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Ion source
•In figure, following the ion path on the left hand side there is a
high voltage enclosure containing many of the system
components.
•A gas source feeds a small quality of source gas into the ion
source where a heated filament causes the molecules to break
up into charged fragments.
•This ion plasma contains the desired ion together with many
other species from other fragments and contamination.
•An extraction voltage, around 20KV, causes the charged ions
to move out of the ion source into the analyzer.
•The pressure in the reminder of the machine is kept below
.000001 Torr to minimize ion scattering by gas molecules.

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Ion source
•The magnetic field of the analyzer is chosen such that only
ions with the desired charge to mass ratio can travel through
without being blocked by the analyzer walls.
•The material is heated in the oven and the vapor flows passed
in filament.
•For gaseous precursors, the oven is replaced with a simple gas
feed.
•The feed gas flows through an orifice into the low pressure
source chamber where it passes between a hot filament and a
metal plate.
•The filament is maintained at a large negative potential with
respect to the plate.

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Ion source
•Electrons boil off the filament and are acceleration towards the
plate.
•As they do so they collide with feed gas molecules transferring
some of their energy.
•If the energy is large enough, molecular dislocation can occur.
•To improve the ionization efficiency a magnetic field is often
imposed in the region of the electron come out.
•This produces a spinal path for the electrons dramatically
increasing the ionization probability.
•The positive ions are attracted to the exist side of the source
chamber.
•Chamber is biased at a large positive potential with respect to
the filament.

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Ion source
•The positive ions then exit positive source chamber through a
slit.
•The resulting ion beam at .00001 -.0000001 Torrpressure,
resulting in a stable arc between the filament and the anode
maximum ion current typically of few milliamperes.
•If we select only B+ ions from the beam and prevent the other
species from continuing down the implanter.
•This is normally done with or analyzing magnet.
•The beam enters a large chamber also maintained low
pressure.
•A magnetic field perpendicular or to the beam velocity exist in
this chamber.

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Range Theory
•In ion implantation, "range theory" refers to the study of how
far implanted ions penetrate a target material, essentially
defining the average depth at which the ions come to rest,
which is called the "range" of the ions.
•The range is a crucial parameter in determining the
characteristics of the implanted region in a semiconductor
device or other material being modified through ion
implantation.

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References
Text Books
•S. M. Sze, "VLSI Technology", McGraw Hill Publication, 2003
•S.K. Ghandhi, "VLSI Fabrication Principles", Willy-India Pvt. Ltd,
2008
ReferenceBooks:
•J.D.Plummer,M.D.DealandPeterB.Griffin,“SiliconVLSI
Technology:Fundamentals,PracticeandModeling",PearsonEducation
Publication,2009
•StephenA.Campbell,"FabricationEngineeringattheMicroand
Nanoscale",OxfordUniversityPress,2013