lecture 7 optical lithography
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ECE416/516ICTechnologies
ProfessorJamesE.MorrisSpring2012
4/23/2012 ECE416/516ICTechnologies Spring2011
Chapter7OpticalLithography
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Figure7.1SimplifiedICdesignprocessflowdiagram.3
Lecture topics:
A. Light sources
B. Wafer exposure systems
1. Optics
2. Projection
3. Proximity/contact
4. Immersion
C. Photoresist (also lecture 8) D. Simulation
E. Sub-wavelength lithography
F. Interference effects
G. Minimum feature sizes
LectureObjectives:
Can calculate resolution limits
and line width errors due to
optical limitations
Can calculate PR exposure
variations due to standing wave
effects
Lecture emphasis:
Optical Limits
o Proximity Printing
o Projection Printing
Technology Comparisons
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- Process of image transfer to wafer
- Origin pattern drawn N x
- Images control diffusion, oxidation, & metallization sequences
- expose parts, mask others
-Masking levels refer to each mask used
-Lithographic Sequence:
1. Draw mask 100-2000x final size
2. Photographically reduce to 10x final size (glass)
3. Step & repeat -> [1 x final size] x [ matrix of images ] (glass)
4. Spin coat substrate with photoresist
- thickness (spin rate)-1/2
5. Expose PR through mask & develop to dissolve unwanted PR, etc.
Registration tolerance --> design rules
f1
f2 Mask 1ideal
pos iti on
Mask 2 ideal
po sit ion
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Estimated final tolerance:
- T 3[( f1/2 )2 + (f2/2 )2 + r2]1/2
where r is registration errors Gaussian distribution if independent
For f1 ~ f2 ~ r~ 0.15m- T ~ 0.6 m- limits how densely features can be packed
Ideal feature separation mask 1 to mask 2> 1/2 (f1 + f2)
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Figure7.2Excerptoftypicaldesignruleset.Thisportiondealswithfirstmetalrulesforaparticulartechnology.
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Figure7.3 Typicalphotomasksincluding(fromleft)a1 plateforcontactorprojectionprinting,a10 plateforareductionstepper,anda10 platewithpellicles.
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Figure7.4 Projectedlithographyrequirementsshowingoverlayaccuracy(rightaxis)andresolutionrequirements(leftaxis).Datatakenfrom2005InternationalTechnology RoadmapforSemiconductors.
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LITHOGRAPHY
0.7X in linear dimension every 3 years. Placement accuracy 1/3 of feature size. 35% of wafer manufacturing costs for lithography. Note the ??? - single biggest uncertainty about the future of the roadmap.
Year of Production 1998 2000 2002 2004 2007 2010 2013 2016 2018
Technolo gy Node (half pitch) 250
nm
180 nm 130 nm 90 nm 65 nm 45 nm 32 nm 22 nm 18 nm
MPU Printed Gate Length 100 nm 70 nm 53 nm 35 nm 25 nm 18 nm 13 nm 10 nm
DRAM Bits/ Chip (Sampl ing) 256M 512M 1G 4G 16G 32G 64G 128G 128G
MPU Transistors/C hip (x106) 550 1100 2200 4400 8800 14,000
Gate CD Control 3 (nm) 3.3 2.2 1.6 1.16 0.8 0.6
Overlay (nm) 32 23 18 12.8 8.8 7.2
Field Size (mm) 22x32 22x32 22x32 22x32 22x32 22x32 22x32 22x32 22x32
Exposure Tech nology 248
nm
248 nm 248 nm
+ RET
193nm
+ RET
193nm +
RET
193nm
+ RET
+ H2O
193nm
+ RET
+ H2O
157nm??
??? ???
Data Volume/Mas k lev el (GB) 216 729 1644 3700 8326 12490
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Historical Development and Basic Concepts
Patterning process consistsof mask design, maskfabrication and waferprinting.
CAD System Layout Simulation Design Rule Checking
ElectronGun
Focus
Deflection
Mask
Mask Making Wafer Exposure
LightSource
Mask
Wafer
CondenserLens
ReductionLens
AerialImage
(Surface)
N Well P Well
P+ N+ N+P+
P
LatentImage
in Photoresist
TiN LocalInterconnect Level
(See Chapter 2)
It is convenient to divide thewafer printing process into threeparts A: Light source, B. Waferexposure system, C. Resist.
Aerial image is the pattern ofoptical radiation striking the topof the resist.
Latent image is the 3D replicaproduced by chemical processesin the resist.
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A. Light Sources
Decreasing feature sizes require the use of shorter .
Traditionally Hg vapor lamps have been used which generate many spectrallines from a high intensity plasma inside a glass lamp.
(Electrons are excited to higher energy levels by collisions in the plasma.Photons are emitted when the energy is released.)
g line - i line - (used for 0.5 m, 0.35 m)
Brightest sources in deep UV are excimer lasers
436 nm 365 nm
Kr NF3 energy KrF photon emission (1)
KrF - (used for 0.25 m, 0.18m, 0.13 m) ArF - (used for 0.13m, 0.09m, . . . ) FF - (used for ??)
248 nm 193 nm 157 nm
Issues include finding suitable resists and transparent optical components atthese wavelengths.
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Figure7.14 Linespectraoftypicalmercuryarclampshowingthepositionsofthetwolinesmostcommonlyusedinlithography.
Figure7.13Typicalhighpressure,shortarcmercurylamp(courtesyOsramSylvania).HighTelectronsgray body radiation, (absorbed by silica glass)
Hgatoms
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Figure7.15Schematicofatypicalsourceassemblyforacontact/proximityprinter(afterJain).15
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Figure7.17Opticaltrainforanexcimerlaserstepper(afterJain).16
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B. Wafer Exposure Systems
Three types ofexposure systemshave been used.
MaskPhotoresist
Si Wafer
LightSource
OpticalSystem
Contact Printing Proximity Printing Projection Printing
Gap
1:1 Exposure Systems Usually 4X or 5XReduction
Contact printing is capable of high resolution but has unacceptable defectdensities.
Proximity printing cannot easily print features below a few m (except forx-ray systems).
Projection printing provides high resolution and low defect densities anddominates today.
Typical projection systems use reduction optics (2X - 5X), step and repeat or stepand scan mechanical systems, print 50 wafers/hour and cost $10 - 25M.
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Usually x, y, & control to align mask to substrate
- alignment marks
mask
PR
wafersContact
Proximity
Projection
Simultaneous XY scan
of mask & wafer
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Figure 7.5 Schematic of a simple lithographic exposure system.
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Figure7.6 Huygens'sprincipleappliedtotheopticalsystemshowninFigure7.5.Apointsourceisusedtoexposeanapertureinadarkfieldmask.
dRR
eAjR
erEr
RRjk
rj
),(
),(
0
)(
)(),(
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B1. Optics - Basics and Diffraction (Huygens-Fresnel principle)
Ray tracing (assuming light travels in straight lines) works well as long as thedimensions are large compared to .
At smaller dimensions, diffraction effects dominate.
LightSource
ApertureImagePlane
If the aperture is on the order of , the light spreads out after passing through
the aperture. (The smaller the aperture, the more it spreads out.)
a). b).
2121
2
1
2
1
0101
2
000
*
cos
sourcespoint2for0,)(
,)(
2121
EEEE
eEeEeEeEILWb
EeEeEILWa
jjjj
jj
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Diffracted LightCollected Light
ImagePlaneAperture
PointSource
CollimatingLens FocusingLens
f
d
A simple exampleis the image formedby a small circularaperture (Airy disk).
Note that a point imageis formed only if ,
or .
If we want to image the apertureon an image plane (resist), we cacollect the light using a lens andfocus it on the image plane.
But the finite diameter of the lensmeans some information is lost(higher spatial frequencycomponents).
1.22 f/d 0f 0 d
Diffraction is usually described in terms of two limiting cases: Fresnel diffraction - near field. Fraunhofer diffraction - far field.
wheref=focallength,d=focusinglensdiameterDiameterofcentralmaximum=1.22f/d
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Figure 7.8 Typical far field (Fraunhofer) image.
ndiffractioFraunhoferIf222 rgW
g
yL
g
yL
I
g
xW
g
xW
IIIg
LWoIyxI yxyxe
2
2sin
,2
2sin
,)2)(2(
)(),( 222
B2
Projection
printing
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Figure 7.21 Schematic for the optical train of a simple projection printer.
Projection
NA=nsinRefractiveindex n=1forairRayleighsresolutioncriterion Wmin =k1/NA
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B2. Projection Systems (Fraunhofer Diffraction)
These are the dominant systems in use today. Performance is usually describedin terms of: resolution
depth of focus field of view modulation transfer function alignment accuracy throughput
ImagePlane
PointSources d
A
BA'
B'
EntranceAperture
Consider this basic optical projectionsystem.
Rayleigh suggested that a reasonablecriterion for resolution is that thecentral maximum of each point sourcelie at the first minimum of the Airypattern.
With this definition,
sin
61.0
sin2
22.122.1
nfn
f
d
fR (2)
The numerical aperture of the lens is by definition, NA represents the ability of the lens to collect diffracted light.
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R 0.61 NA k 1 NA (4) k1 is an experimental parameter which depends on the lithography system
and resist properties ( 0.4 - 0.8).
Obviously resolution can be increased by: decreasing k1 decreasing increasing NA (bigger lenses)
However, higher NA lenses also decrease the depth of focus. (See next slidefor derivation.)
DOF 2 NA
2 k2
NA2
(5)
k2 is usually experimentally determined.
Thus a 248nm (KrF) exposure system with a NA = 0.6 would have a resolutionof 0.3 m (k1 = 0.75) and a DOF of 0.35 m (k2 = 0.5).
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d
Entranceaperture
Imageplane
Limitoffocus
f
RayleighcriterionforDOF:
222
22
)()(2
2/sin
2211
4:smallFor
4/cos
NAk
NADOF
NAf
d
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Another useful concept is the modulation transfer function or MTF, defined
as shown below.
MTF IMAX IMIN
IMAX IMIN (6)
Intensityat Mask
Intensityon Wafer
0
1
0
1IMAX
IMIN
Aperture
LightSource
CondenserLens
Objective orProjection
Lens
Photoresiston Wafer
Mask
Position Position
Note that MTF will be afunction of feature size
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Figure 7.9 Far field image for a diffraction grating.
3
2
15
15
shownpatterngrating
ndiffractiofore.g.
image,anfor
:Define
minmax
minmax
MTF
II
II
MTF
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Figure7.10
Plot
of
dose
versus
position
on
the
wafer.
Dose
is
given
by
the
intensity
of
thelightintheaerealimagemultipliedbytheexposuretime.TypicalunitsaremJ/cm2.
DD100 PRwillcompletelydissolveindeveloperD0
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IdealPRdevelopsforD>Dcr only ActualPRpartiallydevelopsforD0>D>D100
>Partiallydissolves
1
0
1
0
Dose
Dose
Ideal PR
Actual PR
DCr
D100%
D0%
Rayleighresolutionlimit:ResolvableseparationSr =0.6 /NA
Examplesforobjectsequalsize&spacing
Notesquarepatternfromsineintensityallowsopticalprinttofinerlinesthanoriginallypredicted(duetoclippingactionofnonlinearPR)
Notealsocoherenceeffects(CampbellFig.7.18)
Image
intensities
Developed
patterns
optically
resolvable
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Finally, another basic concept is thespatial coherence of the light source.
LightSource
MaskCondensorLens
s d
Practical light sources are not pointsources.the light striking the mask will not beplane waves.
The spatial coherence of the system isdefined as
S light source diametercondenser lens diameter
sd
(7)
or often as S NA condenser
NA projection optics (8)
Typically, S 0.5 to 0.7 in modern systems.
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Figure7.22 Modulationtransferfunctionasafunctionofthenormalizedspatialfrequencyforaprojectionlithographysystemwithspatialcoherenceasaparameter.
S=(Sourceimagediameter)/(Pupildiameter)
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Illumination System EngineeringAperture Projection
Lens
"Captured"Diffracted
Light
"Lost"Diffracted
Light
"Captured"Diffracted
Light
"Lost"Diffracted
Light
Wafer
Off-axis illumination also allowssome of the higher order diffractedlight to be captured and hence canimprove resolution.
Aperture
PointSource
CollimatingLens
ProjectionLens
Photoresiston Wafer
Kohler illumination systems focus thelight at the entrance pupil of the objectivelens. This captures diffracted lightequally well from all positions onthe mask.
Advanced optical systems using Kohlerillumination and/or off axis illumination
are commonly used today.
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Figure7.23Schematicfortheoperationofascanningmirrorprojectionlithographysystem(courtesyofCanonU.S.A.).
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Figure7.26ConfigurationofStepandScan193nmsystem.Thelaserisattheleft.Thereticleisattheupperright,whilethewaferisatthelowerright(photocourtesyASML).
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Figure7.7 Typicalnearfield(Fresnel)diffractionpattern.
ndiffractioFresnelIf 222 rgW
ForWverylarge ray tracing
Image width on waferW=W(g/D)
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B3. Contact/proximity systems
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B3. Contact and Proximity Systems (Fresnel Diffraction)
Contact printing systems operate in the near field or Fresnel diffraction regime. There is always some gap g between the mask and resist.
WaferResistMaskAperture
IncidentPlaneWave
Light Intensityat Resist Surface
g
W
The aerial image can be constructed by imagining point sources within theaperture, each radiating spherical waves (Huygens wavelets). Interference effects and diffraction result in ringing and spreading outside
the aperture.
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Fresnel diffraction applies when g W2 (9)
Within this range, the minimum resolvable feature size is Wmin g (10) Thus if g = 10 m and an i-line light source is used, Wmin 2 m.
Summary of wafer printing systems:
WaferResistMaskAperture
IncidentPlaneWave
Light Intensityat Resist Surface
W
Contact
Proximity Projection
Separation Dependson Type of System
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Maskclosetowafer/resistminimizediffraction/divergence.
Separation ~m Fresnel(nearfield)diffraction Minimumlinewidth:
Wm =(d )1/2
(d=maskwafer
separation)
10
1
.11 10 100
Mask-wafer spacing (m)
Line width (m) Wm
= 36 nmuv = 249 nm
Deep uv
1 nmx-ray
D g
ForW2>> (g2+r2)1/2
W=W(g/D)
r
mask
pt source
2 W
2 L
wafer
wafer
position
Intensity
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Penumbraeffectonlinewidth: W=2dtanwhere angleoffcollimation
Wellcollimatedlightminimizespenumbra. Wellcollimatedlightmaximizesdiffraction. ~ fewdegreesforoptimumbalance.
w
mask
wafer
d
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Figure7.19Typicalcontactexposuresystem(courtesyofKarlSuss).
Contactexposure
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Figure7.20 Intensityasafunctionofpositiononthewaferforaproximityprintingsystemwherethegapincreaseslinearlyfromg = 0tog = 15m(afterGeikasandAbles).
Contact proximity
Gapg
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Figure7.24 Setupofanimmersionsystemusingsurfacetension(fromSwitkesetal.,reprintedfromtheMay2003editionofMicrolithographyWorld.Copyright2003byPennWell.)
B4.Immersion
gratinguniformafor2WPwhere.
andO,Hfor43.1
)/(1
)/(
DOF
DOFand
)2/(sin.4
P2
2
22
1(air)n
n(liq)
2
Pnn
P
Pn
nDOF
P
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Figure7.25 NozzlesystemusedbyNikontoputthewaterdownandsuctionitupforeachstage(fromGeppert,reprintedbypermissionIEEE.)
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C. Optical Intensity Pattern in the Resist (Latent Image)
Aerial Image Ii(x,y)
Exposing Light
P
P WellN Well
PNP+ P+ N+ N+
Latent Image inResist I(x,y,z)
The second step in lithographysimulation is the calculation of thelatent image in the resist.
The light intensity duringexposure in the resist is a functionof time and position because of Light absorption and bleaching. Defocusing. Standing waves.
These are generally accounted for by modifying Eqn. (21) as follows:
I x, y, z I i x, yI r x ,y , z (22)
where Ir(x,y,z) models these effects.
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Microns0 0.8 1.6 2.4
Microns
0
0.4
0.8
1.2
Example of calculation of light intensitydistribution in a photoresist layer duringexposure using the ATHENA simulator.A simple structure is defined with aphotoresist layer covering a silicon substrawhich has two flat regions and a slopedsidewall. The simulation shows the [PAC]calculated concentration after an exposureof 200 mJ cm-2. Lower [PAC] valuescorrespond to more exposure. The colorcontours thus correspond to the integratedlight intensity from the exposure.
C. Photoresist Exposure
Neglecting standing wave effects (for the moment), the light intensity in theresist falls off as
dI
dz I (23
(The probability of absorption is proportional to the light intensity and theabsorption coefficient.)
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The absorption coefficient depends on the resist properties and on the [PAC].
resist Am B (24)where A and B are resist parameters (first two Dill parameters) and
m PACPAC
0
(25)
m is a function of time and is given by
dm
dt C Im (26)
Substituting (24) into (23), we have:
IBAmIdz
dI (27)
Eqns. (26) and (27) are coupled equations which are solved simultaneously byresist simulators.
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[PAC] = photoactive compound concentration
C is 3rd Dill parameter
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The Dill resist parameters (A, B and C) can be experimentally measuredfor a resist.
LightSource
CondenserLens
Photoresiston Transparent
SubstrateFilter to Select
Particular
LightDetector
TransmittedLight
D
Transmittance
Exposure Dose (mJ cm-2)
1
0.75
0.5
0.25
200 400 600
T
T0
By measuring T0 and T,
A, B and C can be extracted.
2
120
1200
0
1
11where|
)1(
ln1
ln1
resist
resistE
n
nT
dE
dT
TTAT
BAC
TD
B
T
T
D
A
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Figure7.11Contourplot(left)and10slicethroughthecenterofprojectedintensityasafunctionofpositionforagratingwith1.6mpitch,exposedatR=436nmandNA= 0.43.
D.Simulation
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Figure 7.12 Same 1D plot for a grating with a pitch of 0.8 m exposed on the same system.
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Models and Simulation
Lithography simulation relies on models from two fields of science: Optics to model the formation of the aerial image.
Chemistry to model the formation of the latent image in the resist.
A. Wafer Exposure System Models
There are several commercially available simulation tools that calculate theaerial image - PROLITH, DEPICT, ATHENA. All use similar physical models.
We will consider only projection systems.
Light travels as an electromagnetic wave.
W, t Re U Wejt where U W C Wej P P, t C Wcos t (t)
or, in complex exponential notation,
(13)
(14)
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Consider a genericprojection system:
Aperture
LightSource
Condenser
Lens
Objective orProjection
Lens
Photoresiston Wafer
Mask
z
x
y
x1y1 Planex'y' Plane
x y Plane
The mask is considered to havea digital transmission function:
After the light is diffracted, it is
described by the Fraunhoferdiffraction integral:
where fx and fy are the spatialfrequencies of the diffractionpattern, defined as
t x1 , y 1 1 in clear areas0 in opaque areas
(15)
x',y' t x1 , y1e2j fxxfy ydxdy (16)fx x'
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(x,y) is the Fourier transform of the mask pattern. fx ,fy F t x1 , y 1 (17)
The light intensity is simply the square of the magnitude of the field, so thatI fx ,fy fx ,fy 2 F t x1 , y 12 (18)
Example - consider a longrectangular slit. The Fouriertransform of t(x) is in standardtexts and is the sin(x)/x function.
Mask
t(x)
F{t(x)}
I(x')
w/2
x
z
Aperture
LightSource
CondenserLens
Objective orProjection
Lens
Photoresiston Wafer
Mask
z
x
y
x1y1 Planex'y' Plane
x y Plane
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But only a portion of the light is collected. This is characterized by a pupil function:
P fx ,fy 1 if fx2 fy2 NA
0 if fx2 fy2 NA
(19)
The objective lens now performs the inverse Fourier transform.
x, y F1 fx ,fy P fx , fy F1 F t x1 , y1 P fx ,fy (20)
resulting in a light intensity at the resist surface (aerial image) given by
I i x, y x, y2 (21)
Summary: Lithography simulators performthese calculations, given a mask design and
the characteristics of an optical system.These simulators are quite powerful today.Math is well understood and fast algorithmshave been implemented in commercial tools.These simulators are widely used.
MaskTransmittance
t(x1,y1)
Far FieldFraunhofer
Diffraction Pattern Pupil FunctionP(fx,fy)
Lens Performs InverseFourier Transform
Light Intensity
(x,y) = F-1{(fx,fy)P(fx,fy)} Ii(x,y) = (x,y)2
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Microns
Microns
0
1
2
3
-1
-2
-3 0 1 2 3-1-2
-3
Microns0 1 2-1-2
Microns
0
1
2
-1
-2
Microns
0
1
2
-1
-2
Microns0 1 2-1-2
Exposure system: NA =0.43, partially coherentg-line illumination( = 436 nm). Noaberrations ordefocusing. Minimumfeature size is 1 m.
ATHENA simulator (Silvaco). Colors correspond to optical intensity in theaerial image.
Same example except thatthe feature size has beenreduced to 0.5 m. Notethe poorer image.
Same example except thatthe illuminationwavelength has now beenchanged to i-lineillumination ( = 365 nm)and the NA has beenincreased to 0.5. Note the
improved image.
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E. SubWavelength Lithography
Beginning in 1998, chip manufacturers began to manufacture chips withfeature sizes smaller than the wavelength of the light used to expose thephotoresist.
This is possible because of the use of a variety of tricks- illumination system optimization- optical pattern correction (OPC), and- phase shift mask techniques.
E. Mask Engineering - OPC and Phase Shifting
Optical Proximity Correction (OPC) can be used to compensate somewhat for
diffraction effects. Sharp features are lost because higher spatial frequencies are lost due to
diffraction. These effects can be calculated and can be compensated for.
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Figure 7.27 Basic concept of phase shift masksas described by Levenson et al. [49].
Creating phase shift masks involves massive numerical calculations and oftenthe implementation involves two exposures - a binary mask and a phase shiftmask - before the resist pattern is developed.
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Another approach uses phase shifting to sharpen printed images.
Mask
Amplitudeof Mask
Amplitudeat Wafer
Intensityat Wafer
180 Phase Shift
A number of companies now provide OPC and phase shifting software services. The advanced masks which these make possible allow sharper resist images
and/or smaller feature sizes for a given exposure system.
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Figure 7.28 Selfaligned method of phase shifting the radiation at the edges of a pattern.
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Figure7.29 Lightfromtheexposedregionscanbereflectedbywafertopologyandbeabsorbedintheresistinnominallyunexposedregions.
Here,inthelaterstages,withsurfacetopography
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Reflection & Scattering
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Figure7.30 Resistlinesforpatterningthemetalrunningdiagonallyfromthelowerleft.OntherightisanexpandedviewofthegeometrydescribedinFigure7.31(afterListvanetal.).
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Figure7.31Micrographsofresistlinesonreflectivemetalwithoutandwitha2700antireflectinglayerundertheresist(afterListvanetal.).
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Figure7.32Theformationofstandingwavesintheresist.67
F.InterferenceEffects
EI =E0 cos(t - z)ER =RE0 cos(t + z) =2 (n i k)/R=reflectioncoefficient=r2
=[(n1n2)/(n1+n2)]2
n2 =Sicomplexrefractiveindexn1 =PRcomplexrefractiveindexk 0forPRoroxide, 2 n/
EI +ER =E0[cos(t z)+Rcos(t + z)]>StandingWave
Es=Eso sint sin((2 nz)/ + )Maxima/Minimaseparations /2nMinimawhen(2 nz/ )+ =m
m=0,1,2,...Forr= 1(Si,GaAs,metals), =0.minimaatz=m/2n,m=0,1,2&exposureduetointensityI=I0 sin2 z
Notes:(1)Poordevelopmentatsurface(2)PRonoxide>opticalpropertiessimilar,soSWcontinuousacrossinterface;
adjustthicknessofoxideZ0 togetantinodeatPR/oxideinterface.
Z0 =(2m+1) /4n(3)EliminateSWformation:
(a)thinabsorbinglayerbetweenSi&PR
(b)dyeinPRforlightscatter(4)PlasmaetchremovesPR
residue
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1988Technologies: Optical Proximity
Full Wafer Projection
Optical Stepper
Deep UV contact
X-Ray
Electron beam & ion beam
Minimum Feature Size
0.1 1 10 m
Light
Developed
resist
Focussed
image
edge
- 0 +
Exposure Energy
mJ/cm
W (m)0.1
-0.1
-0.2
-0.3
-0.4
0
20 807060504030
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W (m)
Light
Developed
resist
Focussed
image
edge
- 0 +
Ratio of Water to Developers
1 65432
0
-0.8
-0.6
-0.4
-0.2
+0.2
Light
Developed
resist
Focussedimage
edge
- 0 +
1 5432 60
0.05
0.1
W (m)
Time Delay between exposure and Development
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.8 1.0 1.2 1.4 1.6 m
Light
Developed
resist
Focussed
image
edge
- 0 +
-0.2
-0.1
0
W (m)
Resist Thickness
Increasingexposuretimedecreasesintensityforthreshold.
Light
Developed
resist
Focussed
image
edge
- 0 +
Light intensity
mask
Diffraction
No change
Defocus
Incr expos energy
-0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2 1.5
m0
0.2
0.4
0.6
0.8
1.0
1.2
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Light
Developed
resist
Focussedimage
edge
- 0 +
W (m)
Defocus
Exposure times
8.3 sec
5.3 sec
0 5 10 15 20 m
-0.2
-0.1
0
+0.1
+0.2
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Figure7.33Twotypicalregistrationerrors.Temperaturechanges
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Reliesonfractureofmetalfilm
PR
metal
Less soluble PR
(surface treated)
Highly soluble PR
metal
metal
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Some Final Thoughts
Lithography is the key pacing item for developing new technology generations.
Exposure tools today generally use projection optics with diffraction limitedperformance.
Lithography simulation tools are based on Fourier optics and do an excellent jobof simulating optical system performance. Thus aerial images can be accuratelycalculated.
A new approach to lithography may be required in the next 10 years.
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Problems: 6.1 7.46.2 7.56.5 7.6/7
Midtermcourseevaluation:Summarizewhatyoulikeaboutthecourse,andwhatyoudontlike.
(Suggestion:Identify5each.)Considerlectures,textbook,assignments,notes,videos,etc.(Weightedas2problems)
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Phasemasking Specificdesignexample
Opticalproximitycorrection Specificdesignexample
Throughsiliconvias (TSVs) Solidcopper&barrel
Planarization Chemical/MechanicalPolishing
Onchip
resistors
Materials/fabrication/properties
Onchipinductors Spiralandferrite
Waferthinning Thinwaferapplications
Cryopumping Theoryandpractice
Unbalancedmagnetron Sources/deposition
Thinmetalfilmresistivity (Continuous)thicknessvariation
Stiction avoidance Includingsurfacestructuring
Electroless deposition Theoryandtechniques
Atomiclayerdeposition Theoryandtechniques
MEMSmotorfabrication Linearandrotational
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