chapter 6 photolithography - ntut fabrication_ga/ic... · photolithography is : ... polymer...
TRANSCRIPT
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Chapter 6
Photolithography
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Objectives•List the four components of the photoresist•Describe the difference between +PR and PR•Describe a photolithography process sequence•List four alignment and exposure systems•Describe the wafer movement in a track-stepper
integrated system.•Explain relationships of resolution and depth of
focus to wavelength and numerical aperture.
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IntroductionPhotolithography is :•Temporarily coat photoresist on wafer and
Transfers designed pattern to photoresist•Most important process in IC fabrication•To consume 40 to 50% total wafer process
time•Determines the minimum feature size, e.g.
0.18um technology in 2000, 70nm technology in2004
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Applications of Photolithography
•Main application: IC patterning process•Other applications: Printed electronic board,
nameplate, printer plate, and et al.
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IC Fabrication Flow
e-Beam or Photo
EDA PR Chip
Photolithography
Ion ImplantMask orReticle Etch
EDA: Electronic Design Automation
PR: Photoresist
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Photolithography Requirements
•High Resolution•High PR Sensitivity•Precision Alignment, say within 10% of
minimum feature size
•Precise Process Parameters Control•Low Defect Density
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Photoresist
•Photo sensitive material, sensitive to ultraviolet(UV) but to visible light
•It’s why we use yellow light to illuminate and call“yellow room”
•Transfer design image on it through exposure anddevelopment
•Very similar to the photo sensitive coating on thefilm for camera
•Positive and negative types
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PhotoresistNegative Photoresist
•Becomes insolubleafter exposure
•When developed,the unexposed partsdissolved.
•Cheaper with poorresolution
Positive Photoresist
•Becomes solubleafter exposure,(photosolubilization)
•When developed, theexposed partsdissolved
•Expensive with betterresolution
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Mask/reticle
Exposure
AfterDevelopment
NegativePhotoresist
UV light
PositivePhotoresist
Substrate
Substrate
Substrate
Photoresist
Negative and Positive Photoresists
SubstratePhotoresist
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Photoresist Composition
•Polymer•Solvents•Sensitizers•Additives
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Polymer
•Solid organic material•Transfers designed pattern to wafer surface•Changes solubility due to photochemical
reaction when exposed to UV light.•Positive PR: from insoluble to soluble•Negative PR: from soluble to insoluble
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Solvent•Dissolves polymers into liquid•Allow application of thin PR layers by spinning•75% of PR before spin coating•Acetate-type solvent for positive PR; xylene
(C8H10) for negative PR
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Sensitizers•Controls and/or modifies photochemical
reaction of resist during exposure.•Determines exposure time and intensity
Additives•Various added chemical to achieve desired
process results, such as dyes to reducereflection.
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Negative Resist
•Most negative PR are polyisoprene type•Exposed PR becomes cross-linked polymer•Cross-linked polymer has higher chemical
etch resistance.•Unexposed part will be dissolved in
development solution.
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Negative Photoresist
Mask
Expose
Development
NegativePhotoresist
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Negative Photoresist
Disadvantages•Polymer absorbs the development solvent•Poor resolution due to PR swelling•Environmental and safety issues due to the
main solvents xylene.
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Comparison of Photoresists
PR
Film+ PR
Film
Substrate Substrate
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Positive Photoresist
•Exposed part dissolve in developer solution•Image the same that on the mask•Higher resolution•Commonly used in advanced IC fabs
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Question
•Positive photoresist can achieve much higherresolution than negative photoresist, whydidn’t people use it before the 1980s?
•Positive photoresist is much more expensivetherefore negative photoresist was used untilit had to be replaced when the minimumfeature size was shrunk to smaller than 3 m.
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Chemically Amplified Photoresists
•To pattern a small feature, a shorter wavelengthlight source is required
•For deep ultraviolet (DUV), 248 nm or 193 nm•Light source: excimer lasers•Light intensity is lower than I-line (365 nm) or G-
line (436 nm) from high-pressure mercury lamp•Need different kind of photoresist
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Chemically Amplified Photoresists
•Catalysis effect is used to increase the effectivesensitivity of the photoresist
•A photo-acid is created in PR when it exposes toDUV light
•During PEB, head-induced acid diffusion causesamplification in a catalytic reaction
•Acid removes protection groups•Exposed part will be removed by developer
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Requirement of Photoresist•High resolution
–Thinner PR film has higher the resolution–Thinner PR film, the lower the etching and ion
implantation resistance
•High etch resistance•Good adhesion•Wider process latitude
–Higher tolerance to process conditions like spinrate, baking temperature and exposure flux
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Photoresist Physical Properties
•Photoresist must be able to withstandprocess conditions•Coating, spinning, baking, developing.•Etch resistance•Ion implantation blocking
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Photoresist Performance Factors :•Resolution•Adhesion•Expose rate, Sensitivity and Exposure Source•Process latitude•Pinholes•Particle and Contamination Levels•Step Coverage•Thermal Flow
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Resolution Capability
•The smallest opening or space that canproduced in a photoresist layer.
•Related to particular processes including exposesource and developing process.
•Thinner layer has better resolution.•Etch and implantation barrier and pinhole-free
require thicker layer•Positive resist has better resolution due to the
smaller size of polymer.
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Photoresist CharacteristicsSummary
Parameter Negative Positive
Polymer Polyisoprene Novolac Resin
Photo-reaction Polymerization Photo-solubilization
SensitizerProvide free radicalsfor polymer cross-link
Changes filmto base soluble
Additives Dyes Dyes
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Photolithography Process
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Basic Steps of Photolithography
1. Photoresist coating2. Alignment and exposure3. Development
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Basic Steps, Old Technology
•Wafer clean•Dehydration bake•Spin coating primer and PR•Soft bake•Alignment and exposure•Development•Pattern inspection•Hard bake
PR coating
Development
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Basic Steps, Advanced Technology
•Wafer clean•Pre-bake and primer coating•Photoresist spin coating•Soft bake•Alignment and exposure•Post exposure bake•Development•Hard bake•Pattern inspection
PR coating
Development
Track-stepperintegratedsystem
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Wafer Clean
P-Well
USGSTIPolysilicon
Gate Oxide
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Pre-bake and Primer Vapor
P-Well
USGSTIPolysilicon
Primer
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Photoresist Coating
P-Well
USGSTIPolysilicon
Photoresist
Primer
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Soft Bake
P-Well
USGSTIPolysilicon
Photoresist
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Alignment and Exposure
P-Well
USGSTIPolysilicon
Photoresist
Gate Mask
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Alignment and ExposureGate Mask
P-Well
USGSTIPolysilicon
Photoresist
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Post Exposure Bake
P-Well
USGSTIPolysilicon
Photoresist
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Development
P-Well
USGSTIPolysilicon
PR
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Hard Bake
P-Well
USGSTIPolysilicon
PR
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Pattern Inspection
P-Well
USGSTIPolysilicon
PR
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Wafer Clean
•Remove contaminants•Remove particulate•Reduce pinholes and other defects•Improve photoresist adhesion•Basic steps
–Chemical clean–Rinse–Dry
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•Older ways–High-pressure nitrogen blow-off–Rotating brush scrubber–High-pressure water stream
Photolithography Process, Clean
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DryDryChemical Clean Rinse
Wafer Clean Process
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•Dehydration bake•Remove moisture from wafer surface•Promote adhesion between PR and surface•Usually around 100 °C•Integration with primer coating
Photolithography Process, Prebake
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•Promotes adhesion of PR to wafer surface•Wildly used: Hexamethyldisilazane (HMDS)•HMDS vapor coating prior to PR spin coating•Usually performed in-situ with pre-bake•Chill plate to cool down wafer before PR coating
Photolithography Process, Primer
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Primer Vapor CoatingDehydration Bake
Wafer
Prep Chamber Primer Layer
Pre-bake and Primer Vapor Coating
Wafer
Hot Plate Hot Plate
HMDSVapor
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Wafer Cooling
•Wafer need to cool down•Water-cooled chill plate•Temperature can affect PR viscosity
–Affect PR spin coating thickness
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Spin Coating
•Wafer sit on a vacuum chuck•Rotate at high speed•Liquid photoresist applied at center of
wafer•Photoresist spread by centrifugal force•Evenly coat on wafer surface
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Photoresist Spin Coater
Vacuum
PR
EBR
Wafer
Chuck
WaterSleeve
Drain Exhaust
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Viscosity
•Fluids stick on the solid surface
•Affect PR thickness in spin coating
•Related to PR type and temperature
•Need high spin rate for uniform coating
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Relationship of Photoresist Thicknessto Spin Rate and Viscosity
Thi
ckne
ss(m
m)
Spin Rate (rpm)
0 7k2k 3k 4k 5k 6k
0.5
1.0
1.5
2.0
2.5
3.0
3.5100 cst
50 cst
27 cst20 cst
10 cst
5 cst
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Dynamic Spin Rate
Time
Spin
r at e
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PR Spin Coater
Photoresist spread on spinning wafer surface
Wafer held on a vacuum chuck
Slow spin ~ 500 rpm
Ramp up to ~ 3000 - 7000 rpm
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Photoresist Applying
Spindle
PR dispensernozzle
Chuck
Wafer
To vacuumpump
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Photoresist Suck Back
Spindle
To vacuumpump
PR dispensernozzle
Chuck
PR suck backWafer
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Photoresist Spin Coating
Spindle
To vacuumpump
PR dispensernozzle
Chuck
PR suck backWafer
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Photoresist Spin Coating
Spindle
To vacuumpump
PR dispensernozzle
Chuck
PR suck backWafer
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2005/10/18 58
Edge Bead Removal (EBR)
•PR spread to the edges and backside•PR could flakes off during mechanical
handling and causes particles•Front and back chemical EBR
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Edge Bead Removal
Spindle
To vacuumpump
Chuck
WaferSolvent
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2005/10/18 60
Optical Edge Bead Removal
•After alignment and exposure•Front-side wafer edge expose (WEE)•Exposed photoresist at edge dissolves
during development
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Optical Edge Bead Removal
Spindle
Chuck
Wafer
Photoresist
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Spindle
To vacuumpump
Chuck
Wafer
Patterned photoresist
Developer Spin Off
Edge PR removed
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Soft Bake•Evaporating most of solvent (> 80%) in PR•Solvents help to make a thin PR but absorb radiation
and affect adhesion•Soft baking time and temperature are determined by PR
types and specific process•90~110°C for 30 min. in oven; 10~15 min. for hotplate•Over bake: polymerized, less photo-sensitivity•Under bake: affect adhesion and exposure
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Baking Tools
Heater
Vacuum
Wafer
Heater
Heated N2
Wafers
MW Source
VacuumWafer
Photoresist
Chuck
Hot plate Convection oven Microwave oven
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Hot Plates
•Widely used in theindustry
•Back side heating, nosurface “crust”
•In-line track systemHeater
Wafer
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Wafer Cooling•Need to cool down to ambient temperature
after baking•Water-cooled chill plate•Silicon thermal expansion rate: 2.5106/C•For 8 inch (200 mm) wafer, 1C thermal
change causes 0.5 m difference indiameter
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Alignment and Exposure
•Most critical process for IC fabrication•Most expensive tool (stepper) in an IC fab.•Most challenging technology•Determines the minimum feature size•Currently 0.18 m and pushing to 0.13 m
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Alignment and Exposure Tools
Contact printerProximity printerProjection printerStepper
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Contact Printer•Simple equipment. Widely used before mid-
70s•Resolution: capable for sub-micron•Use of UV light source•Image ratio 1:1•Direct mask-wafer contact, limited mask
lifetime•Particle contamination issue
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Contact Printer
Light Source
Lenses
Mask
PhotoresistWafer
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Proximity Printer
•10 ~ 20 m distance from wafersurface. No direct contact
•Use of UV light•Image ratio 1:1•Less particles and longer mask
lifetime•Resolution: > 2 m
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Proximity PrinterLight Source
Lenses
Mask
PhotoresistWafer
~10 m
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Projection Printer•Works like an overhead projector•Mask to wafer ratio, 1:1•Resolution to reach at 1 m•The scanning projection exposure system
- the mask and wafer stage move synchronously, allowing UVlight source scanning across the mask to refocus and exposePR across the wafer
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Light Source
Lenses
Mask
PhotoresistWafer
Projection System
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Light Source
Lens
Mask
Photoresist
Wafer
Scanning Projection System
Synchronizedmask and wafermovement
Slit
Lens
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Stepper•Most popular used photolithography tool in the
advanced IC fabs•Reduction of wafer image gives high resolution•Use of deep UV light•Reticle-to-wafer ratio ~ 10:1•A reticle with 1.25 m min. feature size say can achieve
0.125 m min. feature size on wafer•Very expensive ! (extremely complicated and precise)
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Q & A
Q : Why does the 5:1 shrink ratio is morepopular than the 10:1 shrink ratio?
A : 10:1 image shrink has better resolutionthan 5:1 image shrink. However, it onlyexposes a quarter of the area, which meanstotal exposure time will be quadrupled. Atrade-off between resolution andthroughput.
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Step-&-Repeat Alignment/Exposure
Wafer Stage
ProjectionLens
LightSource
Reticle
Wafer
ProjectionLens
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Step&Repeat Alignment System
Wafer Stage
InterferometerMirror Set
Alignment Laser
Projection Lens
Wafer
InterferometerLaser
X
Y
Reticle Stage
Reference MarkLight Source
Reticle
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Exposure Light Source
Should have :–Short wavelength–High intensity–Stability
Includes :High-pressure mercury lampExcimer laser
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Spectrum of the Mercury Lamp
G-line(436)
H-line(405)
I-line(365)
300 400 500 600Wavelength (nm)
Inte
nsity
(a.u
)
Deep UV(<260)
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Photolithography Light SourcesName Wavelength (nm) Application feature
size (m)
G-line 436 0.50
Mercury Lamp H-line 405
I-line 365 0.35 to 0.25
XeF 351
XeCl 308
Excimer Laser KrF (DUV) 248 0.25 to 0.15
ArF 193 0.18 to 0.13
Fluorine Laser F2 157 0.13 to 0.1
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Exposure Control•Exposure light flux is controlled by production
of light intensity and exposure time•Very similar to the exposure of a camera•Intensity controlled by electrical power•Adjustable light intensity•Routine light intensity calibration is required.
Intensity, I, measured in mW/cm2
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Standing Wave Effect
•Interference of the incident and reflection lights•Due to constructive and destructive interferenceat different depth•Periodically overexposure and underexposure•Affects photolithography resolution.
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Standing Wave IntensityL
ight
Inte
nsity
Surfacethe of PR
Surface ofthe substrate/nPR
ConstructiveInterference,Overexpose
AverageIntensity
DestructiveInterference,Underexpose
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Standing Wave Effect on Photoresist
Photoresist
/nPR
Substrate
OverexposureUnderexposure
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Post Exposure Bake (PEB)
•Photoresist’s glass transition temperature, Tg
•Baking temperature is higher than Tg
•Induce thermal movement of photoresistmolecules
•Rearrangement of the overexposed andunderexposed PR molecules
•Average out standing wave effect,•Smooth PR sidewall and improve resolution
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PEB (cont.)
•For DUV chemical amplified photoresist, PEBprovides the heat needed for acid diffusionand amplification.
•After the PEB process, the images of theexposed areas appear on the photoresist, dueto the significant chemical change after theacid amplification
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Post Exposure Bake Steps
•PEB normally uses hot plate at 110 to 130 Cfor about 1 minute.
•For the same kind of PR, PEB usually requiresa higher temperature than soft bake.
•Insufficient PEB will not completely eliminatethe standing wave pattern,
•Overbaking will cause polymerization andaffects photoresist development
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Development
•Developer solvent dissolves the softenedpart of photoresist
•Transfer the pattern from mask or reticle tophotoresist
•Three basic steps:–Development–Rinse–Dry
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Development: Immersion
Spin DrySpin DryDevelop Rinse
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Development –to make etch orimplantation perfect
PR
PR PR
PR
Substrate Substrate
Substrate Substrate
Film Film
FilmFilm
Mask
Exposure
DevelopmentEtching
PR Coating
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Development Profiles
PR PR
Substrate Substrate
PR
Substrate
PR
Substrate
Normal Development
Under Development Over Development
Incomplete Development
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Developer Solutions
Positive PR Negative PR
Developer TMAH Xylene
Rinse DI Water n-Butylacetate
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Hard Bake
•Evaporating all solvents in PR•Improving etch and implantation resistance•Improve PR adhesion with surface•Polymerize and stabilize photoresist•PR flow to fill pinhole
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PR Pinhole Fill by Thermal Flow
PR
Substrate Substrate
PR
Pinhole
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Hard Bake (cont.)
•Hot plate is commonly used•Can be performed in a oven after inspection•Hard bake temperature: 100 to 130 C•Baking time is about 1 to 2 minutes•Hard bake temperature normally is higher than
the soft bake temperature for the same kind ofphotoresist
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Improper Hard Bake
•Under-bake–Photoresist is not filly polymerized–High photoresist etch rate–Poor adhesion
•Over-baking–PR flow and bad resolution
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Photoresist Flow
PR
Substrate Substrate
Normal Baking Over Baking
•Over-baking can causes too much PR flow,which affects photolithography resolution.
PR
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Pattern Inspection
•Inspection, stripped PR and rework–Photoresist pattern is temporary–Etch or ion implantation pattern is permanent.
•Photolithography process can rework•Can’t rework after etch or implantation.•Scanning electron microscope (SEM) for
small feature size (< 0.5 um)•Optical microscope for large feature size
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Q & A
•Why can’t optical microscope be used forthe 0.25 m feature inspection?
•Because the feature size (0.25 m = 2500Å) is smaller than the wavelength of thevisible light, which is from 3900 Å (violet)to 7500 Å (red)..
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Pattern Inspection
•Overlay or alignment–run-out, run-in, reticle rotation, wafer rotation,
misplacement in X-direction, and misplacementin Y-direction
•Critical dimension (CD) loss•Surface irregularities such as scratches, pin
holes, stains, contamination, etc.
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Misalignment Cases
Run-out
Run-in
Reticle rotationWafer rotation
Misplacement in x-direction
Misplacement in y-direction
Run-out
Run-in
Reticle rotationWafer rotation
Misplacement in x-direction
Misplacement in y-direction
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Critical Dimension
Good CD CD Loss Sloped Edge
PR PR
Substrate
PR
Substrate Substrate
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Track-Stepper System or Photo Cell
•Integrated process system of photoresistcoating, exposure and development
•Center track robot•Higher throughput•Improves process yield
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Wafer In
Hot PlateDeveloperdispenser
Track
Hot Plate Spin Station
Stepper
Track Robot
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Stacked Track System
Hot PlatesChill Plates
Prep Chamber
Developers
SpinCoaters
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Future Trends
•Smaller feature size•Higher resolution•Reducing wavelength•Phase-shift mask
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Optical Lithography
•Optics•Light diffraction•Resolution•Depth of focus (DOF)
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Light Diffraction Without Lens
Diffracted light Mask
Intensity of theprojected light
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Diffraction Reduction
•Short wavelength waves have less diffraction•Optical lens can collect diffracted light and
enhance the image
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Light Diffraction With Lens
Diffracted lightcollected by thelens
Strayedrefracted light
Lens
Ideal lightIntensity pattern
Less diffraction afterfocused by the lens
Mask
ro
D
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Numerical Aperture
•NA is the ability of a lens to collect diffractedlight
•NA = 2 r0 / D–r0 : radius of the lens–D : the distance of the object from the lens
•Lens with larger NA can capture higher orderof diffracted light and generate sharper image.
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(Optical) Resolution•The achievable, repeatable minimum
feature size•Determined by the wavelength of the light
and the numerical aperture of the system.The resolution can be expressed as
NAK
R1
K1 : the system constant, is the wavelength of the light,NA = 2 ro/D, the numerical aperture
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Exercise 1, K1 = 0.6
RG-line 436 nm 0.60 ___ mI-line 365 nm 0.60 ___ mDUV 248 nm 0.60 ___ m
193 nm 0.60 ___ m
NAK
R1
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To Improve Resolution•Increase NA
–Larger lens, could be too expensive and unpractical–Reduce DOF and cause fabrication difficulties
•Reduce wavelength–Need to develop light source, PR and equipment–Limitation for reducing wavelength–From UV to DUV, to EUV, and to X-Ray
•Reduce K1
–Phase shift mask (PSM)
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Wavelength and Frequency ofElectromagnetic Wave
RF MW IR UV X-ray
Visible
104 106 108 1010 1012 1014 1016 1018 f (Hz)
104 102 100 102 104 106 108 1010 (meter)
-ray
1012
1020
RF: Radio frequency; MW: Microwave; IR: infrared; and UV: ultraviolet
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Depth of focus
•The range that light is in focus and canachieve good resolution of projected image
•Depth of focus can be expressed as:
22
)(2 NAK
DOF
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Depth of Focus
)(22
NAK
DOF
2
Focus
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Depth of Focus•Smaller numerical aperture, larger DOF
–Disposable cameras with very small lenses–Almost everything is in focus–But, with bad resolution
•Prefer to reduce wavelength than increaseNA to improve resolution
•High resolution, small DOF•Focus at the middle plane of PR layer
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Focus on the Mid-Plain toOptimize the Resolution
PhotoresistSubstrate
Depth of focusCenter of focus
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Surface Planarization Requirement
•Higher resolution requires
–Shorter
–Larger NA.
•Both reduces DOF
•Wafer surface must be highly planarized.•That’s why CMP is significantly required for
0.25 m feature patterning.
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I-line and DUV•Mercury i-line, 365 nm
–Commonly used in 0.35 m lithography
•DUV KrF excimer laser, 248 nm–0.25 m, 0.18 m and 0.13 m lithography
•ArF excimer laser,193 nm–Application: < 0.13 m
•F2 excimer laser 157 nm–Still in R&D, < 0.10 m application
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Silica and DUV
•SiO2 strongly absorbs UV when < 180 nm•Silica lenses and masks can’t be used•157 nm F2 laser photolithography
–Fused silica with low OH concentration, fluorinedoped silica, and calcium fluoride (CaF2),
–With phase-shift mask, even 0.035 m is possible
•Further delay next generation lithography
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Future Trends
1.5
1.0
0.8
0.50.35
0.250.18 0.13
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
84 88 90 93 95 98 01 04Year
Feat
ure
Size
(mm
)Fe
atur
eSi
ze(m
m)
07 10
0.10 0.07
Photolithography
Next GenerationLithography
Maybe photo-lithography
14
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Phase Shift Mask
Quartz substrate
Chrome patternPellicle Phase shift coating
d
nf
d(nf 1) = /2
nf : Refractive index of phase shift coating
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Phase Shift Mask
Quartz substrate
Chrome patternPellicle
d
Phase-shifting etch
ng
d(ng 1) = /2
ng: refractive index of the quartz substrate
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Phase Shift Mask Patterning
SubstratePR
SubstratePR
Total LightIntensity
Final Pattern
Designed Pattern
SubstratePR
Designed Pattern
SubstratePR
Final Pattern
Total LightIntensity
Phase shiftcoating
Normal Mask Phase Shift Mask
DestructiveInterference
ConstructiveInterference
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Next Generation Lithography (NGL)
Extreme UV (EUV) lithographyX-Ray lithographyMaskless lithography - electron beam or ion beam Immersion lithography
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EUV•= 10 to 14 nm•Short wavelength and reduced NA•Mirror basis due to strong absorption at
short wavelength•Use a mask with Pd/C and Mo/Si
multilayer coatings•For 0.1 m technology and beyond•Still in development (support from Intel)
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EUV Lithography
Mask
Mirror 2 Mirror 1Wafer
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X-ray lithography
•Similar to proximity printer•Difficult to find pure X-ray source
(synchrotron radiation facility)•Challenge on mask making (1:1)•Very expensive! unlikely will be used in
production
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X-ray Printing
Photoresist
Substrate
X-rayBeryllium
Gold
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Optical Mask and X-ray Mask
X-ray Mask
Beryllium
Gold
Glass
Chromium
Photo Mask
Aspect ratio < 1:5 Aspect ratio > 1:1
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E-Beam•Used for making mask and reticles•Smallest geometry achieved : 0.014 m•Direct print possible, no mask is required
–Low throughput
•Scattering exposure system (SCALPEL) lookspromising–Tool development–Reticle making–Resist development–Very similar to stepper lithography
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Electron Beam Lithography System
Wafer
Blanking Plate
Lens
Lens
Lens
Electron Gun
DeflectionCoils
Stigmator
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SCALPEL
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Ion Beam Lithography•Can achieve higher resolution
–Direct writing and projection resist exposing–Direct ion implantation and ion beam sputtering
patterned etch, save some process steps
•Serial writing, low throughput•Unlikely will be used in the mass production•Appropriate for mask and reticle repairing•IC device defect detection and repairing
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Immersion Lithography•Fill DI water between light source and wafer•Reach higher DOF
Kw : refractive index of water (1.43)
•Applied in 193 nm or 248 nm systems•Likely to push further to 90 or beyond if
refractive index increased•TSMC proved result in 90 nm product with
ASML
2)(2 NAK
DOF w