6.152J/3.

155J - Microelectronics Procssing Technology Lithography Lecture #1

Lithography Lecture #1
OUTLINE
• Overview of Lithography
• Optics of Lithography
–Metrics, Optics of Micro-Lithography, Aligners, Photomasks

• Photoresists
–Components of Photoresist, Metrics, Photoresist processing, Multi-layer
resist

• Advanced Lithography
–E-beam Lithography, Soft Lithography

Reading Assignment: Plummer, Chapter 5

Reference: Campbell, Chapters 7, 8 & 9
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Overview of Lithography
• Lithography is the process by which circuit or device patterns are
transferred from layout to Si wafers

ENERGY
Patterned exposure of resist
MASK + to an energy source using a
ALIGNER
mask to create an aerial
image of mask in the resist
Modified
PHOTORESIST
Un-Modified
PHOTORESIST
WAFER

Un-Modified
PHOTORESIST After development, a
pattern of resist created
WAFER
by the aerial image is
Modified PHOTORESIST
removed after development
left.

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Typical Lithography Process I

Plummer Fig. 5-1

• Layout functional blocks (or use previous designs) and use software
tools help route or wire connections between functional blocks
• Tools check for design rule violations
• Circuit and system level simulation tools predict performance
• Information from design transferred to mask making machine and
pattern written on a mask blank using scanning electron or laser beam
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Typical Lithography Process II

Plummer Fig. 5-2

• Use mask to expose the resist using a photo aligner

– Creates an aerial image of mask pattern on the resist
• Resist is then developed removing (exposed or exposed regions)
• Resist is used to transfer mask pattern onto wafer
– Ion implantation, oxide tech, metal etch, silicon etch, etc

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Lithography as Information Flow

Design Ea
ch
Mask dis step
tor ca
tin n l
Aerial Image g d os
e v e in
ice fo
Real Image pa rma
tte ti
Latent Image rn on
,
Resist Image

Device Layer
Tim Brunner, IBM
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Typical Lithography Process Sequence

• Wafers spend about 40-

APPLY 50% of the time in photo
PHOTORESIST
bay
PRE-BAKE DEPOSIT/GROW
NEW LAYER
• Majority of the steps in
MASK any process sequence are
ALIGN & EXPOSE RESIST STRIP followed by lithography
and etch/implant
DEVELOP ETCH

POST-BAKE

INSPECT &
MEASURE

PHOTOLITHOGRAPHY BAY

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Typical Pattern Transfer Steps

Coat with
photoresist

Expose
Mask

Develop

Etch*

Strip resist

*Wet etch Schmidt

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Functional Components of Lithography

ENERGY

MASK +
ALIGNER

PHOTORESIST

WAFER

• Energy —cause (photo)chemical reactions that modify resist dissolution rate

• Mask —Pattern (or direct) energy to create an aerial image of mask in resist
• Aligner —Align mask to previous patterns on wafer (to a tolerance level)
• Resist —Transfer image from mask to wafer, After development Positive resist
reproduces the mask pattern, Negative resist reproduces inverse mask pattern
• Substrate —Has previous mask patterns
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Energy Sources
Waves or Particles
• Energy sources are required to modify the photoresist.
• The energy source is aerial imaged on the photoresist.
• The imaging can be done by scanning the energy beam or by masking the
energy beam.
• Bright sources are usually required for high throughput.

Wavelength Energy

Light UV 400 nm 3.1 eV

Deep UV 250 nm 4.96 eV hc

E = hν =
X-Ray 0.5 nm 2480 eV λ

Particles Electrons 0.62 Å 20 keV

Ions 0.12 Å 100 keV

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Mask
Mask

Schmidt
• Block radiation where it is not wanted i.e. absorb
radiation
– Need opaque material at the desired wavelength
• Transmit radiation where it is needed
– Need material with high transmission at the desired
wavelength
• For Optical lithography, mask is
– Quartz glass (transparent) + Cr (opaque)
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Aligner

Schmidt

• Align pattern on mask to previous patterns on the wafer

• Exposure of photoresist to radiation pattern which is an
aerial image of mask
• Types of aligners
– Contact, Proximity & Projection
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Resist

• Viscous liquid which has a “solid” form when solvents are

driven out
• Spin coated on coated on surface to be patterned
• Exposure of resist to energy/radiation leads to (photo)
chemical reactions and changes the resist dissolution rate
in the developer
• Remaining resist is “rugged” enough to protect (mask)
underlying substrate during subsequent processing

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Semiconductor Roadmap
Minimum Feature Size
300
Isolated Lines
Dense Lines

250
Feature Size, L(nm)

200

150

100

50

0
1996 1998 2000 2002 2004 2006 2008 2010 2012 2014

Year

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Metrics of Lithography Systems

• Resolution(smallest dimension that can be printed)
– Determined by optical system, resist, etch process
– Critical Dimension (CD) control (3σ = 10% of mean)
• Registration (alignment 3σ=1/3 resolution)
– Determined by optical system and aligner
• Dimensional Control (device, die, wafer, lot uniformity)
– Determined by optical system, mask, resist, etch
process
• Throughput (how many wafers/hour)
– Determined by optical system, resist

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Optical Aligners
Exposure Systems
Comparison of three exposure systems Light intensity profile for
three exposure systems

Contact Aligner: photoresist is in intimate contact with the chrome-side of the mask at the
time of exposure. Mask image:Resist image is 1:1, not limited by diffraction
Proximity Aligner: photoresist is not in intimate contact with mask⎯separated by a few
microns. Mask image:Resist image is 1:1. Limited by near field (Fresnel diffraction) diffraction
Projection Aligner: photoresist is not in intimate contact with mask ⎯mask image is
projected onto resist by lenses. Limited by far field (Fraunoffer diffraction) diffraction
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Optics Review
What is Diffraction?

• Diffraction is the spread of radiation into un-exposed

regions
– Near-field diffraction (Fresnel Diffraction)
– Far-field diffraction (Frauhoffer Diffraction)
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Contact Printing

Incident
Plane
Wave
Light Intensity
Mask Resist Wafer at Resist Surface
Aperture

• Contact Aligner—mask is in hard contact with resist

• not diffraction limited

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Proximity Printing
Near Field (Fresnel) Diffraction

W2
λ<g<
λ
W = mask feature size
Wmin ≈ λ g

• Mask and wafer are separated by a small gap of 2-20 µm

• The resulting diffraction pattern has several features
– Intensity rises gradually near the edges producing some resist
exposure outside the mask edge
– Ringing in intensity distribution within the aperture
• As mask separation g increases, quality of image degrades
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Projection Printing

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Far Field (Frauhoffer) Diffraction

d

Rλ fλ
First minimum q1 = 1.22 = 1.22
2a d
• The analysis is for circular apertures with radius a, diameter d=2a.
• Observation of the light intensity at a distance R (usually at the focal
length f) shows the above circular diffraction pattern.
• The diffraction pattern has a diameter (of central maximum) equal to q1.

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Resolution of Point Images

• Consider two close point sources that we are trying to image as shown in figure
• Images produced will be “Airy” disks
• The Rayleigh’s criteria for resolution of the images occur when the center of
one “Airy” disc is at the first minimum of the other “Airy” disk
• Resolution (minimum distance between the two sources) is given by
λf
R = 1.22
d
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Resolution of Point Images

(∆l )min =1.22 Rλ =1.22 fλ

2a d

Rayleigh’s criteria occurs

R = f = focal length of lens
when the center of one disc
d=2a=aperture diameter
is at the first minimum of the
other disc

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What is Numerical Aperture (NA)?

• Numerical aperture of an optical system is a measure of the ability of the
collect light
• NA2 is a measure of the light gathering power
Define
NA = no sin α

n0 = 1
f
d d
d
sin α ≈ 2 =
f 2f
d
NA =
2f

α is maximum acceptance angle that can be focused by the optical system

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Resolution
(Grating)

NA=0.2
NA=0.1
2b

In general
(∆l )min = 2b =1.22 fλ = 0.61 λ
d NA
λ
d Min. Line Width = k 1
NA = NA
2f k 1 = 0.6 − 0.8

• Let 2b be the period of a grating (equally spaced lines and spaces)

• 2b is separation between two images, b is minimum linewidth
• Raleigh’s criterion corresponds to situation when the two images being
resolved have intensity reduced to 80% at minimum
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Modulation Transfer Function

I −I
Modulation = M = max min
Imax + Imin

I −I
Mmask = 1 Mimage = max min
Imax + I min

Mimage Imax − I min

MTF = =
Mmask Imax + I min
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Depth of Focus
• Depth of Focus (DOF) decreases as the numerical aperture increases
• If δ is the on-axis path length difference at the limit of focus, then path length
difference is δcosθ.
• Raleigh criteria for depth of focus (DOF) is that the two path lengths do not
differ by more than λ/4
λ
= δ − δ cosθ
4
Assuming θ is small
λ ⎡ ⎛ θ2 ⎞⎤ θ2
= δ 1 − ⎜ 1− ⎟ ≅ δ
⎢ ⎥
4 ⎣ ⎝ 2 ⎠⎦ 2
d
θ ≅ sin θ = = NA
2f
λ λ
∴DOF = δ = ± = ±k 2
2(NA ) (NA )2
2

λ
δ=
(NA)2
λ
DOF = k 2
(NA )2
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Optical Sources
Hg Lamp Spectrum

• Current lithography systems use the high pressure Hg lamp which has
several lines with high intensity.
– g-line (436 nm)
– h-line (405 nm)
– i-line (365 nm)
• The optical source being contemplated for future lithographic systems
use Excimer Lasers
– deep UV (308 nm -157 nm)
• KrF (248 nm) - current generation
• ArF (193 nm) - next generation
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Spatial Coherence
• Spatial coherence S is an indication of the angular range of light waves
incident on mask or degree to which light from source are in phase
Spatially Coherent Source
source diameter
s=
aperture diameter

NA condenser optics
s=
NAprojection optics

Partially Coherent Source

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Comparison of Aligners

• Contact Aligner—mask is in hard contact with resist; not diffraction limited

• Proximity Aligner—mask is separated from resist by a gap of 2-10 µm;
limited by near-field (Fresnel) diffraction
• Projection Aligner—mask is very far from resist; aerial image focused on
resist by lens; limited by far-field (Faunhofer) diffraction
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Comparison of Aligners
Contact Printing
• Mask is brought into physical contact with wafer
Issues
– Contact implies gap < than a few λ due to surface features ≥ 1 µm, wafer warpage
& dust particules
– Direct contact to mask results in mask damage, particulates and defects

Proximity Printing
• Small gap (2-20 µm) between mask and wafer (mask damage eliminated)
Issues
– Near-field (Fresnel) diffraction effects⇒ loss of exact mask reproduction for small L
– Shadowing may occur if light source is not collimated

Projection Printing
• Similar to photography
• Project mask image on to wafer at either 1:1, 5:1 or 10:1 reduction
Issues
– Need of good lenses need image size
– Usually print small area, then step and repeat
– Trade-off resolution with speed
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Photo-Masks
• Fabricated by e-beam direct write using a electronic database
generated by the CAD tools
– There are several substrate (transparent) types
• Quartz, low expansion glass, sodalime glass
– There are also several Opaque materials used to block light
• Chrome, emulsion, iron oxide
• Often, a master is made on quartz; then the the pattern is transferred to
less expensive L.E. glass where it is step and repeated to create several
dies
• Two polarities of masks are common
– Light field, LF (mostly clear)
– Dark field, DF (mostly dark)

Pop Quiz!!!
What is the field of the of the following masks from the lab?
Mask #1 ___________ Mask#2 ____________

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Phase Shift Mask Fused Silica

Chrome

Fused Silica

Chrome

E = E (r,t) = A cos(ωt − k • r ) Phase Shift Material

E = Re{A exp[i (ωt − k • r )]}
ϕ = ωt − k • r phase

• Extends resolution capability of current optical lithography

• Takes advantage of the wave nature of light
• PSM changes the phase of light by 180° in adjacent patterns leading to
destructive interference rather than constructive interference
• Improves MTF of aerial image on wafer
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Summary
ENERGY

MASK +
ALIGNER

PHOTORESIST

WAFER

• Components of Lithography
– Energy— Modify resist dissolution rate
– Mask—Pattern (or direct) energy to resist
– Aligner—Align mask to previous patterns on wafer
– Resist—Transfer image from mask to wafer
• Aligners
– Resolution limited by diffraction
– MTF, DOF, coherence
– Contact Printing, Proximity Printing, Projection Printing
– Photomasks, Phase Shift Mask
– Hg lamp, g-line, h-line, i-line
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Three Ways to Improve Resolution

Reduce λ

λ
Wmin = k1 •
NA

Reduce k1 Increase NA

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