Vlsi btech subject notes important for stud

Therse are important notes of vlsi.. These are very helpful for btech ece student's

1. •The fabrication of circuits on a wafer requires a process by which specific
patterns of various materials can be deposited on or removed from the wafer's
surface.
•The process of defining these geometrical patterns from mask to the
wafer is known aslithography.
•Lithography is the process of transferring patterns of geometric shapes in a
mask to a thin layer of radiation-sensitive material (called resist) covering the
surface of a semiconductor wafer.
•Figure illustrates schematically the lithographic process employed in IC
fabrication.
•As shown, the radiation is transmitted through the clear parts of the mask and
makes the exposed photoresist insoluble in the developer solution, thereby
enabling the direct transfer of the mask pattern onto the wafer.
•After the patterns are defined, an etching process is employed to selectively
remove masked portions of the underlying layer.
1
Lithography

2. 2
Lithography

3. •Theperformanceofalithographicexposureisdeterminedbythree
parameters:resolution,registration,andthroughput.
•Resolutionisdefinedtobetheminimumfeaturedimensionthat
canbetransferredwithhighfidelitytoaresistfilmona
semiconductorwafer.
•Registration/DepthofFocusisameasureofhowaccurately
patternsonsuccessivemaskscanbealignedoroverlaidwith
respecttopreviouslydefinedpatternsonthesamewafer.
•Throughputisthenumberofwafersthatcanbeexposedperhour
foragivenmasklevelandisthusameasureoftheefficiencyofthe
lithographicprocess.
3
Lithography

4. Lithography Processes
•Surface Preparation
•Coating
•Soft baking
•Alignment
•Exposure
•Resist Development
•Hard Baking
•Removal of exposed photo resist
•Etching of mask film
•Removal of unexposed resist
4
Lithography

5. TherearefollowingtypesofLithography:
•OpticalLithography(PhotoLithographyorUltraViolet
Lithography)
•ElectronBeamLithography:directwriting
•X-rayLithography
•InterferenceLithography
5
Types of Lithography

6. •ThemeaningofPhotoLithographyinLatinislight-stone-writing.
•OpticalLithographyreferstoalithographicprocessthatuses
visibleorultravioletlighttoformpatternsonthephotoresist
throughprinting.
•Opticallithographyisaphoton-basedtechniquecomprisedof
projectinganimageintoaphotosensitivephotoresistcoatedonto
asubstratesuchasasiliconwafer.
•Itisthemostwidelyusedlithographyprocessinthehighvolume
manufacturingofnanoelectronicsbythesemiconductorindustry.
•Printingistheprocessofprojectingtheimageofthepatternsonto
thewafersurfaceusingalightsourceandaphotomask.
•Therearethreetypesofprinting-contact,proximity,and
projectionprinting.
6
Optical Lithography (Photo Lithography)

7. •Patternedmasks,usuallycomposedofglassorchromium,areused
duringprintingtocoverareasofthephotoresistlayerthatshould
notgetexposedtolight.
•Developmentofthephotoresistinadevelopersolutionafterits
exposuretolightproducesaresistpatternonthewafer,which
defineswhichareasofthewaferareexposedformaterial
depositionorremoval.
7
Optical Lithography (Photo Lithography)

8. 8
Optical Lithography (Photo Lithography)

9. 9
Optical Lithography (Photo Lithography)
•Photolithography is the process of transferring geometric shapes on a mask
to the surface of a silicon wafer.
•The steps involved in the photolithographic process are following
•Wafer cleaning, Barrier layer formation, Photo resist application
•Soft baking
•Mask alignment, Exposure
•Development
•Hard-baking.

10. 10
Optical Lithography (Photo Lithography)
Wafer cleaning, Barrier layer formation, Photo resist application
•In the first step, the wafers are chemically cleaned to remove particulate
matter on the surface as well as any traces of organic, ionic, and metallic
impurities.
•After cleaning, silicon dioxide, which serves as a barrier layer, is deposited
on the surface of the wafer.
•After the formation of the SiO2 layer, photo resist is applied to the surface
of the wafer.
•High-speed centrifugal whirling of silicon wafers is the standard method
for applying photo resist coatings in IC manufacturing.
•This technique, known as "Spin Coating," produces a thin uniform layer of
photo resist on the wafer surface.
•Spin Coating is shown in the following figure.

11. 11
Optical Lithography (Photo Lithography)

12. 12
Optical Lithography (Photo Lithography)
Positive and Negative Photoresist
•There are two types of photoresist: positive and negative.
•For positive resists, the resist is exposed with UV light wherever the
underlying material is to be removed.
•In these resists, exposure to the UV light changes the chemical structure of
the resist so that it becomes more soluble in the developer.
•The exposed resist is then washed away by the developer solution, leaving
windows of the bare underlying material.
•In other words, "whatever shows, goes."
•The mask, therefore, contains an exact copy of the pattern which is to
remain on the wafer.

13. 13
Optical Lithography (Photo Lithography)
•Negative resists behave in just the opposite manner.
•Exposure to the UV light causes the negative resist to become polymerized,
and more difficult to dissolve.
•Therefore, the negative resist remains on the surface wherever it is
exposed, and the developer solution removes only the unexposed portions.
•Masks used for negative photo resists, therefore, contain the inverse (or
photographic "negative") of the pattern to be transferred.
•The figure below shows the pattern differences generated from the use of
positive and negative resist.

14. 14
Optical Lithography (Photo Lithography)

15. 15
Optical Lithography (Photo Lithography)
Soft-Baking
•Soft-baking is the step during which almost all of the solvents are removed
from the photoresist coating.
•Soft-baking plays a very critical role in photo-imaging.
•The photo resist coatings become photosensitive, or image able, only after
soft baking.
•Over soft-baking will degrade the photosensitivity of resists by either
reducing the developer solubility or actually destroying a portion of the
sensitizer.
•Under soft baking will prevent light from reaching the sensitizer.
•Positive resists are incompletely exposed if considerable solvent remains in
the coating.
•This under soft-baked positive resists is then readily attacked by the
developer in both exposed and unexposed areas, causing less etching
resistance.

16. 16
Optical Lithography (Photo Lithography)

17. 17
Optical Lithography (Photo Lithography)
Mask Alignment and Exposure
•One of the most important steps in the photolithography process is mask
alignment.
•A mask or "photo mask" is a square glass plate with a patterned emulsion
of metal film on one side.
•The mask is aligned with the wafer, so that the pattern can be transferred
onto the wafer surface.
•Each mask after the first one must be aligned to the previous pattern.
•Once the mask has been accurately aligned with the pattern on the wafer's
surface, the photo resist is exposed through the pattern on the mask with a
high intensity ultraviolet light.
•There are three primary exposure methods : contact, proximity, and
projection.

18. Contact Printing
•In contact printing, the resist-coated silicon wafer is brought into physical
contact with the glass photo mask.
•The wafer is held on a vacuum chuck, and the whole assembly rises until the
wafer and mask contact each other.
•The photo resist is exposed with UV light while the wafer is in contact
position with the mask.
•Because of the contact between the resist and mask, very high resolution is
possible in contact printing (e.g. 1-micron features in 0.5 microns of positive
resist).
•The problem with contact printing is that debris, trapped between the resist
and the mask, can damage the mask and cause defects in the pattern.
18
Optical Lithography (Photo Lithography)

19. Proximity Printing
•The proximity exposure method is similar to contact printing except that a
small gap, 10 to 25 microns wide, is maintained between the wafer and the
mask during exposure.
•This gap minimizes (but may not eliminate) mask damage.
•Approximately 2-to 4-micron resolution is possible with proximity printing.
19
Optical Lithography (Photo Lithography)

20. Projection Printing
•Projection printing, avoids mask damage entirely.
•An image of the patterns on the mask is projected onto the resist-coated
wafer, which is many centimeters away.
•In order to achieve high resolution, only a small portion of the mask is
imaged.
•This small image field is scanned or stepped over the surface of the wafer.
•Projection printers that step the mask image over the wafer surface are called
step-and-repeat systems.
•Step-and-repeat projection printers are capable of approximately 1-micron
resolution.
20
Optical Lithography (Photo Lithography)

21. 21
Optical Lithography (Photo Lithography)

22. Development
•One of the last steps in the photolithographic process is development.
•The figure below shows response curves for negative and positive resist after
exposure and development.
•At low-exposure energies, the negative resist remains completely soluble in the
developer solution.
•As the exposure is increased above a threshold energy Et, more of the resist film
remains after development.
•At exposures two or three times the threshold energy, very little of the resist film
is dissolved.
•For positive resists, the resist solubility in its developer is finite even at zero-
exposure energy.
•The solubility gradually increases until, at some threshold, it becomes completely
soluble.
•These curves are affected by all the resist processing variables: initial resist
thickness, prebake conditions, developer chemistry, developing time, and others.
22
Optical Lithography (Photo Lithography)

23. 23
Optical Lithography (Photo Lithography)

24. Hard-Baking
•Hard-baking is the final step in the photolithographic process.
•This step is necessary in order to harden the photo resist and improve adhesion of
the photo resist to the wafer surface.
24
Optical Lithography (Photo Lithography)

25. 25
Optical Lithography (Photo Lithography)

26. 26
Optical Lithography (Photo Lithography)
Advantages of Photo Lithography
•Versatile
•High density
•Large array size
•Mass production

27. 27
Optical Lithography (Photo Lithography)
Disadvantages of Photo Lithography
•High up-front cost
•Time consuming
Applications of Photo Lithography
•IC patterning process
•Printed electronic board
•Printed name plate
•Printed printer plate

28. •Electron Beam Lithography is a specialized technique for creating
extremely fine patterns (~ 50nm).
•Derived from the early scanning electron microscopes, the technique
consists of scanning a beam of electrons across a surface covered with a
resist film sensitive to those electrons.
•Thus depositing energy in the desired pattern in the resist film.
•It is a high resolution patterning technique in which high energy electrons
(10-100 k eV) are focussed into a narrow beam & are used to expose
electron sensitive resists.
28
Electron Beam Lithography

29. 29
Basic Set up of Electron Beam Lithography

30. 30
Construction of Electron Beam Lithography
•A typical EBL system consists of the following parts –
•Electron source
•Electron column for shaping & focussing the beam
•A mechanical stage that positions the wafer under the electron beam
•A wafer handling system for automatically feeding & unloading the wafer
•A computer system for controlling the equipment.
•Additionally, a vacuum system is needed to maintain the appropriate
vacuum level throughout, while a set of control electronics supplies power
& signals to the various parts of the machine.

31. 31
Working of Electron Beam Lithography System
•The source of the beam is the electron gun, positioned at the top of the
column.
•There are two basic types of electron guns: thermionic (lower resolution)
and field emission (higher resolution).
•A thermionic source is a heated wire from which electrons are given
thermal energy to overcome the work function of the source, combined
with an electric potential to give the newly free electrons direction and
velocity.
•A field emission gun consists of a sharply pointed tungsten tip held at
several kV negative potential relative to a nearby electrode, so that there is
a very high potential gradient at the tungsten tip.
•The electron beam is then focused with several magnetic lenses and finally
the beam hits the sample.

32. 32
Working of Electron Beam Lithography System
•The way in which an electron beam is used in microelectronic fabrication is
to create a mask in resist that can be employed in any of several ways to
create a pattern on the substrate (e.g., a silicon wafer or a mask blank).
•The electron beam exposes the resist where it strikes, i.e. the electrons
break the molecules of the resist and so locally change its characteristics in
such a way that subsequent development can either remove selectively the
exposed part (positive resist) or remove the unexposed part (negative
resist).

33. 33
Working of Electron Beam Lithography System

34. 34
Working of Electron Beam Lithography System

35. 35
Working of Electron Beam Lithography System

36. 36
Proximity Effect in Electron Beam Lithography System

37. 37
Proximity Effect in Electron Beam Lithography System

38. 38
Advantages of Electron Beam Lithography System
Three major advantages of electron-beam lithography are
•Its ability to register accurately over small areas of a wafer.
•Low defect densities.
•Direct generation of patterns from circuit design data.

39. 39
Disadvantages of Electron Beam Lithography
System
The principal drawback of electron-beam lithography are
•Its low throughput
•High capital cost

40. 40
Applications of Electron Beam Lithography System
•Applications of e-beam lithography span a wide range of nano structured
devices including but not limited to electronic devices, opto-electronic
devices, quantum structures, meta materials, transport mechanism studies
of semiconductor/superconductor interfaces, micro electromechanical
systems, optical, and photonic devices.
•E-beam lithography can be also used formask making and direct writing
on non-planar substrates.

41. 41
Electron Beam Lithography
•https://www.youtube.com/watch?v=vPDUSdm1CAY

42. 42
Photo Masks
•A mask, or photomask, is a glass or quartz plate coated on one side with
chrome.
•Conventional Mask pattern generator uses step-and-repeat and contact
printing photolithography method to transfer the circuit layout designs from
CAD into the photo mask for circuit requiring minimum line width of <
1mm.
•Modern mask pattern generation systems use precision lasers or electron
beams to image the design of one layer of an integrated circuit (IC), or
chip, onto the mask especially for circuit requiring minimum line width of
<1mm.
•After the design has been exposed on the mask, the pattern is etched into
the chrome, and the mask is inspected.

43. 43
Photo Masks

44. 44
Photo Masks

45. 45
Conventional Photo Masks Fabrication
•Circuit Layout
•Data Digitising
•Photo mask Coat Materials and Sizes
•Pattern Generation
•Step & repeat reduction into master copy mask plate
•Contact print working mask plate

46. 46
Conventional Photo Masks Fabrication
Circuit Layout
•In the circuit design process, patterns which represent the circuits are
created by the chip designer.
•These patterns are then sent via magnetic media or electronically to the
mask shop where the pattern data is prepared for mask manufacturing.
Data Digitising
•Early photo masks were cut by hand in a material called ruby lith, a
sandwich of a clear backing layer and a thin red layer of Mylar.
•The red layer was cut with a stylus and peeled off, leaving the desired
pattern in red.
•The original ruby lith copy of the mask was 100 to 1000 times larger than
the final integrated circuit and was photographically reduced to form a
reticle for use in a step-and-repeat camera.
•Today, computer graphics systems and optical pattern have largely
supplanted the use of ruby lith.
•An image of the desired mask is created on a computer graphics system.

47. 47
Conventional Photo Masks Fabrication
Photo mask Coat Materials and Sizes
•The primary material used to make a mask is a quartz substrate that has a
layer of chrome on one side.
•The chrome layer is covered with an antireflective coating and a
photosensitive resist.
•Mask sizes range from three to nine inches square, but most masks
produced today are five or six inches square.

48. 48
Conventional Photo Masks Fabrication
Pattern Generation
•Once the image is complete, files containing the commands needed to drive
a photolithography pattern generator are created on magnetic tape or disks.
•A pattern generator consists of a light source and a series of motor-driven
shutters.
•The pattern generator uses 4 flash lamps to expose the series of rectangles
composing the mask image directly onto a blank photographic plate called
the reticle.
•The chrome-covered mask or reticle coated with a layer of photo resist is
moved under the light source as the shutters are moved and opened to allow
precisely shaped patterns of light to shine onto the resist creating the
desired pattern.
•The reticle or mask is processed through the development, develop inspect,
chrome etch, resist stripping.

49. 49
Conventional Photo Masks Fabrication
The final inspection steps that transfer the pattern permanently into the chrome
layer as follow:
•Etch and resist stripping
•Defect Inspection
•Repair
•Metrology Inspection
•Cleaning
•Final Inspection

50. 50
Conventional Photo Masks Fabrication
Step & repeat reduction into master copy mask plate
•After the creation of the reticle of the circuit image, the reticle pattern is
transferred to a new resist-coated mask blank by a step-&-repeat camera in
order to reduce the reticle images into the final size on a master copy mask
plate.
•After each of the exposure steps, the reticle or mask is processed through
the development, develop inspect, chrome etch, resist striping, & final
inspection that transfer the pattern permanently into the chrome layer of the
master copy mask plate.
•This process gives a master copy of the actual circuit features.

51. 51
Conventional Photo Masks Fabrication
Contact print working mask plate
•The master copy mask plate is then used to create multiple working mask
plates in a contact printer using photolithography.
•This tool brings the master copy mask into contact with a resist-coated
mask blank and has a UV light source for transferring the image from the
master copy plate into the working copy plate.
•After the exposure steps the reticle or mask is processed through the
development, develop inspect, chrome etch, resist stripping, & final
inspection that transfer the pattern permanently into the chrome layer of the
working mask plate.
•Inspections are again very critical since any undetected mistake or defect
has the potential of creating thousands of scrap wafers.

52. 52
Etching
•Simply removing unwanted materials from the surface to form a required
pattern.

53. 53
Types of Etching
There are two types of Etching process.
•Wet Etching
•Dry or Plasma Etching
•Wet Etching is donebyliquid chemicals
•Unmasked areas are etched away by the chemical reactions (Oxidation and
Reduction)
•Wet chemical etching is used for products with feature sizes greater than
2 mm.
•Dry or Plasma Etching is done exposing the material into bombardment of
ions inPlasma.

54. 54
Physical Mechanism of Wet Etching
•Etchants: KOH, HF, BF6,BCl3
•SiO
26HFH
2SiF
62H
2O

55. 55
Physical Mechanism of Wet Etching
•Wet chemical etching of a solid in a solution is a heterogeneous process.
•Contain mixture of oxidising agent and reducing agent where by etching
reactions involve oxidation-reduction mechanisms.
•Oxidising agent will oxidise the wafer material and the reducing agent will
dissolve the oxide product.

56. 56
Physical Mechanism of Wet Etching
There are following three major process stages:
•The diffusion of the reacting ions or molecules from etchant solution
towards the exposed film on the wafer surface through the boundary layer.
•The formation of a soluble or/and gaseous by-products through the
chemical reaction between the etchant and the exposed film.
•The diffusion of the reaction by-product from the surface of the wafer
through boundary layer into the bulk of the etchant solution.

57. 57
Methods of Wet Etching
•Immersion wet etching
•Spray wet etching
•Silicon Wet Etching
•Anisotropic Silicon Wet Chemical Etching
•SiO
2wet etching
•Silicon Nitride Wet Etching
•Metal Wet Etching

58. 58
Immersion wet Etching
For Silicon, Silicon dioxide, Silicon nitride and Aluminium steps are
following:
•Wafers are immersed in a tank of an etchant solution for a specific
time for the wet chemical reaction between the etchant and etched
material to occur.
•Transferred to a rinse station for acid removal.
•Transferred to a station for final rinse and a spin dry step.
•Etching uniformity and process control can be enhanced by the
addition of heaters and agitation devices.
Advantages:
•The simplest and most economical techniques
•Good selectivity
•Particle contamination can be flittered

59. 59
Spray wet etching
For Silicon, Silicon dioxide, Silicon nitride and Aluminium steps are
following
•Etchants are sprayed onto the surface of the etched film in surplus,
covering all surface area to be etched for a specific time.
•DI water is then sprayed onto the surface of the etched film for etchant
removal.
•Final rinse and spin dry.
Advantages over immersion etching
•Added definition gained from the mechanical pressure of the spray
•Spray etching also minimises contamination form the etchants
•More controllable (Can be removed instantly)
•Better etch uniformity (Fresh etchant is constantly supplied)
•Uses less amount of chemicals
•Faster etching process than immersion technique.

60. 60
Spray wet etching
Disadvantages :
•System cost
•Safety considerations associated with caustic etchants in a pressurised
system
•Requirement of etch-resistant materials used for the systems to prevent
the deterioration of the machine.

61. 61
Silicon Wet Etching
•The Well Known isotropic Etchant for Silicon is HNO3 (70% as
Oxidising agent) and HF (49% as reduction agent).
•Water or acetic acid can be used to reduce etching rate.
•Oxidisation : Si+2HNO3-->SiO2+HNO2
•Reduction: SiO2+6HF-->H2SiF6+2H2O--->H2+SiF6+2H2O
•Etch stop material: Boron doped silicon p-type
•Method to make a Si membrane.
Role of acetic acid:
•Acetic acid is frequently substituted for water as the dilutent.
•Acetic acid has a lower dielectric constant than water.
•This produces less dissociation of the HNO3 and yields a higher
oxidation power for the etch.
•Acetic acid is less polar than water and can help in achieving proper
wetting of slightly hydrophobic Si wafers.

62. 62
Anisotropic Silicon Wet Chemical Etching
•The anisotropy is obtained through the different etch rates that selected
chemicals exhibit against different crystalline planes.
•Atoms lying on (111) planes appear more densely packed than those on
the (110) and (100) plane.
•As a consequence, certain etching formulations are favoured in removing
atoms from (110) and (100) planes but not from the (111) planes.

63. 63
SiO
2wet etching
Thermal oxide:
•The most common etched layer is a thermally grown silicon dioxide.
•The basic etchant is hydrofluoric acid (HF) which gives isotropic etch
of silicon dioxide.
•HF is able to dissolve silicon dioxide without attacking silicon because
the etch selectivity of silicon oxide to silicon is 100:1. For silicon nitride
at 1 nm/min
•Etch stop material Silicon Nitride
•The full strength HF (49%) has an etch rate of about 30 nm/s or 1800
•nm/min at room temperature.
•In pratice, the HF is diluted with ammonium fluoride and water, which
give a typical etch rate of 100 nm/min at room temperature
•Bubble problem

64. 64
SiO
2wet etching
Deposited Oxide
•One of the final layers on a wafer is a SiO2 film deposited over the Al
metallisation pattern.
•These films are known as vapox or silox film.
•The HF content attacks the underlying Al pads, causing bonding
problems in the packaging process.
•Etchant: 1 NH4F: 2 CH3COOH, 100 nm/min
•Bubble problem
•NH4F<--->NH3+HF
•SiO2+6HF--->H2SiF6+2H2O--->H2+SiF6+2H2O

65. 65
Silicon Nitride Wet Etching
•Etchant is a hot (140-200ºC) phosphoric acid at 10 nm/min.
•Etching process must be done in a closed reflux container equipped with
a cooled lid to condense the vapours.
•Bubble problem
•10:1 Silicon dioxide; 30:1 Silicon.

66. 66
Metal Wet Etching
•Metal wet etching is also used to pattern metal lines.
•Polycrystalline metal such as aluminium is commonly used and the
etched Al metal always has ragged edges.
•Etchant: 4H3PO4: 1H2O:1 HNO3:4CH3COOH at 35 nm/min.
•Selectivity: Do not attack the Si, SiO2, Silicon Nitride.

67. 67
Advantages of Wet Etching
•Damage-free finish to wafer surface where surface morphology is
typically smooth and shiny
•fast etch rate especially for blanket etch
•simple and direct etching process since simple resist can be used as etch
mask
•process occur at atmospheric environment
•cheaper cost
•high etch selectivity easily available for etchants, resist and etched
materials
•good etch uniformity across wafer

68. 68
Disadvantages of Wet Etching
•Isotropic etching
•No control for precision etching
•Excessive particle contamination is possible
•Bubbles can grow during etching that act as localised mask
•The resist cumming when the resist is incompletely exposed or
insufficiently developed and this residual resist act as etching mask.

69. 69
Deposition Processes of Poly silicon
•Films of various materials are used in VLSI.
•In addition to being parts of the active devices, deposited thin films
provide conducting regions with in a device, electrical insulation
between metals, and protection from the environment.
•The most widely used thin films in microelectronics are polycrystalline
silicon or polysilicon, doped or undoped silicon dioxide, stoichiometric
or plasma-deposited Silicon nitride.

70. 70
Deposition Processes of Poly silicon
•Polysiliconisdepositedbypyrolyzingsilanebetween575°Cand650°C
inalow-pressurereaction:SiH
4(g)-->Si(s)+2H
2(gas)
•Eitherpuresilaneor20to30%silaneinnitrogenisbledintotheLPCVD
systematapressureof0.2to1.0torr.
•Forpracticaluse,adepositionrateofabout10to20nm/minisrequired.
•ThepropertiesoftheLPCVDpolysiliconfilmsaredeterminedbythe
depositionpressure,silaneconcentration,depositiontemperature,and
dopantcontent.
•Amorphoussiliconcanbepreparedbytheglowdischargedecomposition
ofsilane.
•Processingparameterssuchasdepositionrateareaffectedbydeposition
variablessuchasthetotalpressure,reactantpartialpressure,discharge
frequencyandpower,electrodematerials,gasspecies,reactorgeometry,
pumpingspeed,electrodespacing,anddepositiontemperature.
•ThehigherthedepositiontemperatureandRFpower,thehigheristhe
depositionrate.

71. 71
Deposition Processes of Poly silicon
•Polysiliconcanbedopedbyaddingphosphine,arsine,ordiboranetothe
reactants(in-situdoping).
•Addingdiboranecausesalargeincreaseinthedepositionratebecause
diboraneformsboraneradicals,BH3,thatcatalyzegas-phasereactions
andincreasethedepositionrate.
•Incontrast,addingphosphineorarsinecausesarapidreductioninthe
depositionrate,becausephosphineorarsineisstronglyadsorbedonthe
siliconsubstratesurfacetherebyinhibitingthedissociative
chemisorptionofSiH
4.
•Despitethepoorerthicknessuniformityacrossawaferwhendopantsare
incorporated,uniformitycanbemaintainedbycontrollingpreciselythe
flowofreactantgasesaroundthesamples.

72. 72
Deposition Processes of SiO2
•Several deposition methods are used to produce silicon dioxide.
•Films can be deposited at lower than 500C by reacting silane, dopant
(phosphorus in this example), and oxygen under reduced pressure or
atmospheric pressure.
SiH
4(g) + O
2(g) --> SiO
2(s) + 2H
2(g)
4PH
3(g) + 5O
2(g)-->2P
2O
5(s) + 6H
2(g)
•The process can be conducted in an APCVD or LPCVD chamber.
•The main advantage of silane-oxygen reactions is the low deposition
temperature allowing films to be deposited over aluminum metallization.
•The primary disadvantages are poor step coverage and high particle
contamination caused by loosely adhering deposits on the reactor walls.

73. 73
Deposition Processes of SiO2
•Silicondioxidecanbedepositedat650°Cto750°CinanLPCVD
reactorbypyrolyzingtetraethoxysilane,Si(OC2H5)4.Thiscompound,
abbreviatedTEOS,isvaporizedfromaliquidsource.Thereactionis
•Si(OC2H5)4(l)SiO
2(s)+by-products(g)
•Theby-productsareorganicandorganosiliconcompounds.
•LPCVDTEOSisoftenusedtodepositthespacersbesidethepolysilicon
gates.
•Theprocessoffersgooduniformityandstepcoverage,butthehigh
temperaturelimitsitsapplicationonaluminuminterconnects.
•SilicondioxidecanalsobedepositedbyLPCVDatabout900°Cby
reactingdichlorosilanewithnitrousoxide:
•SiCl
2H
2(g)+2N
2O(g)SiO
2(s)+2N
2(g)+2HCl(g)
.

74. 74
Deposition Process of SiO2
•Thisdepositiontechniqueprovidesexcellentuniformity,andlike
LPCVDTEOS,itisemployedtodepositinsulatinglayersover
polysilicon.
•However,thisoxideisfrequentlycontaminatedwithsmallamountsof
chlorinethatmayreactwithpolysiliconcausingfilmcracking.

75. 75
Deposition Process of Silicon Nitride
•Stoichiometricsiliconnitride(Si3N4)canbedepositedat700°Cto800°C
atatmosphericpressure:
3SiH
4(g)+4NH
3(g)-->Si
3N
4(s)+12H
2(g)
•UsingLPCVD,siliconnitridecanbeproducedbyreactingdichlorosilane
andammoniaattemperaturebetween700°Cand800°C:
3SiCl
2H
2(g)+4NH
3(g)Si
3N
4(s)+6H
2(g)
•Thereduced-pressuretechniquehastheadvantageofyieldinggood
uniformityandhigherwaferthroughput.
•Hydrogenatedsiliconnitridefilmscanbedepositedbyreactingsilaneand
ammoniaornitrogeninplasmaatreducedtemperature:
SiH
4(g)+NH
3(g)-->SiN:H(s)+3H
2(g)
SiH
4(g)+N
2(g)-->2SiN:H(s)+3H
2(g)

76. 76
Deposition Process of Silicon Nitride
•Plasma-assisteddepositionyieldsfilmsatlowtemperaturebyreacting
thegasesinaglowdischarge.
•Twoplasmadepositedmaterials,plasmadepositedsiliconnitride
(SiNH)andplasmadepositedsilicondioxide,areusefulinVLSI.
•Onaccountofthelowdepositiontemperature,300°Cto350°C,plasma
nitridecanbedepositedoverthefinaldevicemetallization.
•Plasma-depositedfilmscontainlargeamountsofhydrogen(10to35
atomic%).
•HydrogenisbondedtosiliconasSi-H,tonitrogenasN-H,andto
oxygenasSi-OHandH
2O.