Chapter 5: Lithography
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Introduction
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The mechanism to print 2-D patterns to a thin film layer
on the wafer surface.
Masks are glass plates (soda lime or quartz glass) that
contain the patterns.
The patterns are first transferred from the mask to
photoresist (PR), a light-sensitive polymer.
After opening windows in the PR, the pattern is
transferred to the thin film using etching techniques.
Complexity of a fabrication process is often measured
by the number of photolithographic masks used in the
process.
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Introduction
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The concept is simple
– Spin on a thin layer of light-sensitive photoresist
– Selectively expose it to UV light
Causing chemical bonds to either form or break
– Develop to selectively remove the lighter weight PR
The resist may be used as a mask for either etching or
for ion implantation
Because of constraints of resolution, exposure field,
accuracy, throughput, and defect density, the
implementation is not so simple
– Very expensive
– Very complex
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Introduction
•
Steps in the mask fabrication process:
Designing 2D
layout using CAD
tools
Transfer data to
pattern generator
(mask maker)
Only Glass
Pattern generation
on the mask plate
coated with Cr&PR
Etching PR and
then Cr
Inspection
Glass plate with Cr
Stripping PR
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Introduction
Mask Maker
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Introduction
•
Steps in the photolithography
Clean wafer
Hard Bake
deposit film (oxide,
nitride, metal, …)
Etch the deposited
film
Coat with PR
Remove PR
Typical for 1800 Series PR:
Soft Bake: 110°C for 1min on a hotplate
Hard Bake: 110°C for 3min on a hotplate
PR1813  1.3µm @ 4krpm & 30sec
PR1827  2.7µm @ 4krpm & 30sec
Soft bake
Align masks
Expose Pattern
Develop PR
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Introduction
Hotplate
Spinner
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Introduction
Mask Aligner
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Introduction
UV
1
5,6
2
6
3,4
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Introduction
Some common PRs:
1800 series (for thin)  will be developed in MF 319
9200 series (for thick)  will be developed in AZ 400
7,8
9
Be aware that there are two different types of PR:
Positive PR: exposed areas will be developed
Negative PR: exposed areas will not be developed
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Alignment Markers
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Once a photolith process is done, the pattern
developed is used to perform some additional
process selectively on the wafer
– Etching trenches in Si or SiO2
– Making metalization runs
– Implantation of dopants
Then the wafer will come back for another
photolith step
Alignment markers are registration patterns
that mate from one mask to another so that
the multiple pattern sets match one another.
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Introduction
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Positive resists
provide better
controllability for
small features.
Positive resists are
easier to work with
and use less corrosive
developers and
chemicals.
Positive resists are
the dominant type of
photoresists today.
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Clear Field and Dark Field Masks

Most photolith engineers prefer clear field
masks when possible
– Easier to detect pattern on the wafer itself
as there is more clear glass in the mask
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Introduction
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Demands placed on this process for
– Resolution: smaller device structures
– Exposure field: ever-increasing chip sizes
– Placement accuracy: aligning with existing
layers
– Throughput: manufacturing cost
– Defects: yield and cost
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NTRS Lithography Requirements
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Introduction
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The National Technology Roadmap for Semiconductors
defines the future needs
Note especially
– The driving force is the reduction of feature size
– For every factor of two in reduction of area, there is
a reduction of 0.7 in the linear dimensions
– The reduction is required every three years
– The most commonly quoted feature size is not as
small as isolated MOS gate lines
– Critical dimension (CD) control must improve (about
10% of minimum feature size)
– Alignment accuracy must be about 1/3 of minimum
feature size
– The printing area increases with time since we must
print one full die at a time
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Introduction

About 1/3 of the cost of a wafer cost (about
$1000 for an 8-inch wafer) is associated with
lithography; we have only a few hundred
dollars per wafer to spend
– Optical lithography is used down to
0.13 m (130nm) generations
– For smaller dimensions, X-ray, direct ebeam, or extreme UV (EUV) processes are
used.
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Basic Concepts
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We generally separate lithography into three parts
– The energy source (photons or electrons)
– The exposure system
– The resist
The exposure tool, which includes the light source and
the exposure system, creates the best image possible
on the resist (resolution, exposure field, depth of focus,
uniformity and lack of aberrations)
– Optimization of the photoresist with the settings on
the exposure tool transfers the aerial image from the
mask to the best thin film replica of the aerial image
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Light Source
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Historically, light sources have been arc lamps
containing Hg vapor
A typical
emission
spectra from
a Hg-Xe lamp
Low in DUV
(200-300nm)
but strong in
the UV region
(300-450nm)
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Light Source
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A much smaller set of wavelengths used to
expose the resist
– to minimize optical distortion associated
with the lens optics.
– to match the properties of the resist
Pick the wavelength that is heavily
absorbed and causes changes in resist
chemical properties
Two common monochromatic selections are
the g-line at 436 nm and the i-line at 365 nm.
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UV Light Sources

To expose < 250nm wide lines, we need to use shorter
wavelength light
– Two excimer lasers (KrF at 248 nm and ArF at 193
nm)
– These lasers contain atoms that do not normally
bond, but if they are excited the compounds will
form; when the excited molecule returns to the
ground state, it emits UV light
– These lasers must be continuously strobed (several
hundred Hz) or pulsed to pump the excitation; can
get several mJ of energy out
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Excimer Lasers
Low reliability due to etching of the electrodes
and the optical windows by the energitic F ions
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E-beam Source
Field Emission Gun (3), which provides the source of
the electron beam, is a W or LaF6 filament.
Condenser Lens (7) are pairs of electromagnets that
are used to collimate the beam of electrons.
Beam Booster, composed of Anode (5), Vacuum
Tube (6), Apertures (8), Alignment Coils (9a, b, c),
Stigmator (13), and Isolating Valve (15) is used to
determine the energy of the electrons and to remove
the electrons moving off-axis.
Objective Lens (10,11) is another set of
electromagnets that focuses the electron beam onto
the specimen (12), also containing the Deflecting
System (14), which is another set of electromagnetics
that sweep the electrons across the field of view and
off of the sample .
http://cmi.epfl.ch/metrology/img/LEO1550/LEOColumn.gif
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X-Ray Source
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High energy electrons collide with a metal.
The transfer of energy results in the release of
x-rays (short wavelength photons).
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Exposure System
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There are three classes of exposure systems
– Contact
– Proximity
– Projection
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Exposure System
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Contact printing is the oldest and simplest
The mask is put with the absorbing layer face
down in contact with the wafer
This method
– Can give good resolution
– Machines are inexpensive
– Cannot be used for high-volume due to
damage caused by the contact
– Still used in research and prototyping
situations
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Wafer Exposure Systems
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Proximity printing solves the defect problem
associated with contact printing
– The mask and the wafer are kept about
5 – 25 m apart
– This separation degrades the resolution
– Cannot print with features below a few
microns
– The resolution improves as wavelength
decrease. This is a good system for X-ray
lithography b/c of the very short exposure
wavelength (1-2 nm).
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Projection/Step and Repeat
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For large-diameter wafers, it is impossible to achieve
uniform exposure and to maintain alignment between
mask levels across the complete wafer.
– Masks are now called reticules
Projection printing is the dominant method today
– They provide high resolution without the defect
problem
– The mask is separated from the wafer and an optical
system is used to image the mask on the wafer.
– The resolution is limited by diffraction effects
– The optical system reduces the mask image by 4X to
5X
– Only a small portion of the wafer is printed during
each exposure
– Steppers are capable of < 0.25 m
– Their throughput is about 25 – 50 wafers/hour
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Optics Basics
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We need a very brief review of optics
If the dimensions of objects are large
compared to the wavelength of light, we can
treat light as particles traveling in straight
lines and we can model by ray tracing
When light passes through the mask, the
dimensions of objects are of the order of the
dimensions of the mask
We must treat light as a wave
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Snell’s Law and Reflectivity
n1 sin(q1) = n2 sin(q2)
1 = T+R+A, where
T is transmission
R is reflection
A is absorption
http://scienceworld.wolfram.com/physics/SnellsLaw.html
If q1 = p/2, q2 = sin-1(n1/n2)
R = [(n1-n2)/(n1+n2)]2
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Refractive index of SiO2
Transmission through
two air-glass surfaces
is less than 93.1%.
R = 3.5
in air
l=
365nm
http://www.mellesgriot.com/products/o
ptics/images/fig5_12.gif
http://www.ioffe.ru/SVA/NSM/nk/Oxides/Gif/sio2.gif
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Snell’s Law/Antireflective Coatings
 n1n2  n 

R
 n n  n2 
 1 2

2
when the layer thickness,t, is
t = (m+1)l/4; m = 0,1,2…
n1
n2
n
t
http://en.wikipedia.org/wiki/File:Optical-coating-1.png
R = 0 when n = (n1n2)1/2
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Young’s Single Slit Experiment
sinq = l/d
http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html
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Amplitude of largest
secondary lobe at
point Q, eQ, is given
by:
eQ = a(A/r)f(c)d
where A is the amplitude
of the incident wave, r is
the distance between d
and Q, and f(c) is a
function of c, an
inclination factor
introduced by Fresnel.
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http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html
Young’s Double Slit Experiment
http://micro.magnet.fsu.edu/optics/lightandcolor/interference.html
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Diffraction of Light
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Diffraction of Light

The Huygens-Fresnel principle states that
every unobstructed point of a wavefront at a
given time acts as a point source of a
secondary spherical wavelet at the same
frequency
– The amplitude of the optical field is the sum
of the magnitudes and phases
For unobstructed waves, we propagate a
plane wave
For light in the pin-hole, the ends
propagate a spherical wave.
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Diffraction of Light
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Basic Optics
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Basic Optics
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Information about the shape of the pin hole is
contained in all of the light; we must collect all
of the light to fully reconstruct the pattern
– If only part of the diffraction pattern is
collected and focused on the substrate, the
image created is not identical to the one on
the mask.
The light diffracted at higher angles
contains information about the finer
details of the structure and are lost
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Basic Optics
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The image produced by this system is
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Basic Optics
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The diameter of the central maximum is given by
Diameter of central maximum 
1.22lf
d
d  focusing lens diameter
f  focal length
λ  wavelengt h of light
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Note that you get a point source only if d  
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Basic Optics
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There are two types of diffraction
– Fresnel, or near field diffraction
– Fraunhofer, or far field diffraction
In Fresnel diffraction, the image plane is near the
aperture and light travels directly from the aperture to
the image plane.
In Fraunhofer diffraction, the image plane is far from
the aperture, and there is a lens between the aperture
and the image plane.
Fresnel diffraction applies to contact and proximity
printing while Fraunhofer diffraction applies to
projections systems
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Fraunhofer Diffraction
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We define the performance of the system in
terms of
– Resolution
– Depth of focus
– Field of view
– Modulation Transfer Function (MTF)
– Alignment accuracy
– throughput
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Fraunhofer Diffraction
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Imagine two sources close together that we
are trying to image (two features on a mask)
– How close can these be together and we
can still resolve the two points?
The two points will each produce an Airy disk.
– Lord Rayleigh suggested that the minimum
resolution be defined by placing the
maximum from the second point source at
the minimum of the first point source.
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Fraunhofer Diffraction
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Fraunhofer Diffraction

With this definition, the resolution becomes
1.22 lf
1.22 lf
0.61l


d
n2 f sin a  n sin a
n  index of refraction of the material between the object and lens
a  maximum half angle of the diffracted light
R
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For air, n=1
a is defined by the size of the lens, or by an
aperture and is a measure of the ability of the
lens to gather light
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Fraunhofer Diffraction

This is usually defined as the numerical
aperture, or NA
NA  n sin a
0.61l
l
R 
 k1
NA
NA
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Defined only for point sources as the point
source Airy function was used to develop the
equation
A more generalized equation replaces 0.61 by
a constant k1 which lies between 0.6 and 0.8
for practical systems.
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Fraunhofer Diffraction
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From this result, we see that we get better
resolution (smaller R) with shorter
wavelengths of light and lenses of higher
numerical aperture
We now consider the depth of focus over
which focus is maintained.
We define  as the on-axis path length
difference from that of a ray at the limit of the
aperture. These two lengths must not exceed
l/4 to meet the Rayleigh criterion
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Depth of Focus
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Depth of Focus

From this criterion, we have
l / 4     cosq

For small q
  q 2 
q2
l / 4   1  1    
2 
2
 
q  sin q 
d
 NA
22 f
 DOF    
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l
2 NA
2
 k2
l
NA2
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Fraunhofer Diffraction
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From this we note that the depth of focus
decreases sharply with both decreasing
wavelength and increasing NA.
The Modulation Transfer Function (MTF) is
another important concept
This applies only to strictly coherent light, and
is thus not really applicable to modern
steppers, but the idea is useful
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Fraunhofer Diffraction
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Because of the finite aperture, diffraction
effects and other non-idealities of the optical
system, the image at the image plane does
not have sharp boundaries, as desired
If the two features in the image are widely
separated, we can have sharp patterns as
shown
If the features are close together, we will get
images that are smeared out.
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Modulation Transfer Function
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Fraunhofer Diffraction
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The measure of the quality of the aerial image is given
by
MTF 
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I MAX  I MIN
I MAX  I MIN
The MTF is really a measure of the contrast in the aerial
image
The optical system needs to produce MTFs of 0.5 or
more for a resist to properly resolve the features
The MTF depends on the feature size in the image; for
large features MTF=1
As the feature size decreases, diffractions effects casue
MTF to degrade
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Change in MTF versus Wavelength
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Contact and Proximity Systems
These systems operate in the Fresnel regime
– If the mask and the resist are separated by some
small distance “g” and a plane wave is incident on
the mask, light is diffracted at the aperture edges.
– As shown in next slide, there is
1. Small maximum at the edge from constructive
interference
2. Ringing caused by constructive and destructive
interference

To minimize effects, multiple wavelengths of
light may be used to expose PR
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Fresnel Diffraction
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Fresnel Diffraction

As g increases, the quality of the image
decreases
– The aerial image can be computed accurately when
lg
W2
l
where W is the feature size
– Within this regime, the minimum resolvable feature
size is:
Wmin  lg
– Proximity aligner with a 10 m gap and an i-line
source can resolve ~ 2 m features.
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Resolution

A more exact solution for the theoretical resolution for
proximity or contact aligners is given by:
3 
z
R
l g  
2 
2

Where l is the wavelength of light used to exposure the
pattern, g is the distance between the bottom of the
mask and the top of the photoresist, z is the thickness
of the photoresist (typically 0.8-1.2m).
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Fresnel Number
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Fresnel diffraction when F ≥ 1
Fraunhofer diffraction when F << 1
2
W
F
lg
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Depth of Focus
http://www.research.ibm
.com/journal/rd/411/hol
m1.gif
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Summary of the Three Systems
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Photoresists

Parameters that determine the usefulness of
the resist include:
– Sensitivity: a measure of how much light is
required to expose the resist - typically
100mJ/cm2
– Resolution where the effects of exposure,
baking, developing should not degrade the
quality of the image
– Chemical and physical properties: it must
withstand chemical etching, mild
temperature excursions, ion implantation
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Photoresists
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Photoresists usually contain three components
– Inactive resin (usually a hydrocarbon which
forms the base material)
– Photoactive compound (PAC)
– Solvent which is used to adjust the viscosity
The most common g- and i-line resists use
– Diazonaphthoquinones (DNQ) as the PAC
– Novolac as the resin
– Propylene glycol monomethyl ether acetate
(PGMEA) as the solvent (this has replaced
Cellosolve acetate, which is a toxic hazard)
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Basic Structure of Novolac
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Novolac is a polymer containing hydrocarbon rings with
2 methyl groups and 1 OH group
The basic ring structure is repeated to form a long chain
polymer
Novolac readily dissolves in developer at about 15 nm/s
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Diazoquinone

The photoactive part of the molecule is the
part above the SO2
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Diazoquinone

The function of the PAC is to inhibit the
dissolution of the resin in the developer
– DNQ is essentially insoluble in developer
prior to exposure to light
– When dissolved in the resin, DNQ reduce
the resist dissolution rate from ~ 15nm/s to
1-2 nm/s
When the resist is exposed to light, the
diazoquinone molecule changes
chemically and increases the dissolution
rate to ~100nm/s.
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Properties and Characteristics of Resists

Two parameters are used to define the
properties of photoresists
– Contrast
– Critical modulation transfer function (CMTF)
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Contrast

The ability of the photoresist to distinguish
between various levels of light intensities.
– It is experimentally determined by exposing
the resist to differing amounts of light,
developed for a fixed time and measuring
the thickness of resist remaining after
developing.
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Photoresist Contrast
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Photoresist Contrast
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

For positive resists, material exposed to low light will
not be attacked by the developer; material exposed to
large doses will be completely removed
Intermediate doses will result in partial removal
The contrast is the slope of this curve and is given by
g
1
log 10

Qf
QO
Typical g- and i-line resists will achieve a contrast of
g = 2-3 and Qf values of 100 mJ/cm2
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Photoresist Contrast


The contrast is not a constant, but depends on
process variables such as
– development chemistry,
– bake times,
– temperatures before and after exposure,
– wavelength of light, and
– underlying structure
It is desirable to have as high a contrast as
possible in order to produce the sharpest
edges in the developed pattern
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Photoresist Contrast
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Modulation Transfer Function (MFT)


Defined in two points of the lithographic system.
– MTF: Measure of the dark versus light intensities in
the aerial image produced by the projection system
– CMTF: Measure of the exposed versus unexposed
regions in the high contract image focused on the PR
Q f  Q0 101/ g  1
CMTFresist 
 1/ g
Q f  Q0 10  1
The CMTF is the minimum optical transfer function
necessary to resolve a pattern in the resist
– For g- and i-line resists, CMTF  0.4
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Effect of Resist Thickness

Resists usually do not have uniform thickness on the
wafer
– Edge bead: The build-up of resist along the
circumference of the wafer
- There are edge bead removal systems
– Step coverage
Centrifugal Force
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Effect of Resist Thickness


The resist can be underexposed where it is thicker and
overexposed where it is thinner
– This can lead to linewidth variations
Light intensity varies with depth below the surface due
to absorption
I ( x)  I 0 exp( ax)

where a is the optical absorption coefficient
– Thus, the resist near the surface is exposed first
A process called bleaching in which the exposed
material becomes almost transparent (i.e., a decreases
after exposure)
– Therefore, more light goes to deeper layers after
bleaching the near surface layer of PR
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Photoresist Absorption


If the photoresist becomes transparent and if
the underlying surface is reflective, reflected
light from the wafer will expose the
photoresist in areas we do not want it to.
– This leads to the possibility of standing
waves (due to interference), with resultant
waviness of the developed resist
We can solve this by putting an antireflective
coating on the surface of the substrate before
spinning the photoresist  increases process
complexity
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Standing Waves Due to Reflections
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Standing Waves Due to Reflections
http://www.lithoguru.com/scientist/lithobasics.html
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Removal of Standing Wave Pattern
(a)
(b)
(c)
Diffusion during a post-exposure bake (PEB) is often used to
reduce standing waves.
Photoresist profile simulations as a function of the PEB
diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm.
http://www.lithoguru.com/scientist/lithobasics.html
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Mask Engineering

There are two ways to improve the quality of
the image transferred to the photoresist
– Optical Proximity Correction (OPC)
– Phase Shift Masks (PSM)

We note that the lenses in projections systems
are both finite and circular but most features
on the mask are square.
– The high frequency components of the pattern are
lost and the “squareness” of the corners of the
pattern disappear.
– Can be taken into account by adjusting feature
dimensions and shapes in the masks
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Mask Engineering
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Phase Shift Masks


In a projection system the amplitudes at the wafer add
so that closely spaced lines interact; the intensity at the
wafer is smeared
– If we put a material of proper index of refraction on
part of the mask, we can retard some of the light and
change its phase by 180 degree and the two portions
of light interfere and cancel out.
The thickness of the PS layer is
d
l
2n  1
n is the index of refraction of the phase shift material
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Phase Shift Masks (PSM)
Intensity
pattern is
barely
sufficient
to resolve
the two
patterns.
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Scanning Projection Aligners


Projection aligners have been industry
standard for about 20 years
– It is easier to correct for aberrations in
small regions than in large
Scan a small slit across the mask while the
wafer is simultaneously scanned
– Scanning projection aligners must use 1:1
masks
Pattern on the mask is the same size as
the one imaged on the wafer.
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Scanning Projection Printer
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Scanning Projection Systems

Cost effective and has high throughput
– Linewidth control for smaller devices is
difficult
– As chips became larger, it is more difficult
to produce good full wafer masks
– With ULVI and WSI, this system could not
scale and was replaced by systems that
exposed only a single die at a time
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Step-and-Repeat Projection Aligners


Exposed a limited portion of the wafer at a
time
– The image on the wafer is 4-5 times smaller
than the image on the mask or reticule.
– Masks thus are much larger, and thus
repairable to some extent
Steppers also allow better alignment because
they align on the exposure field rather than
for the entire wafer
– Wafer can be moved vertically to keep
image plane at some location as the PR
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Off-Axis Illumination

By changing the angle of incidence of the light
on the mask, change the angle of the
diffracted light
– Although some of the diffracted light is lost
in this scheme, much of the higher order
diffraction is captured
– As the resolution is decreased, it is harder
to make these optics work
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Off-Axis Illumination
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Step and Scan
A hybrid has been developed called a “step-and-scan”, but is very complex
and very expensive.
https://www.chiphistory.org/product_content/lm_asml_pas5500-400_step&scan_system_1990_intro.htm
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DNQ/Novolac Resist Process
The details of the process are
more complex that described
earlier
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DNQ/Novolac Resist Process

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
We first must consider adhesion
– There can be one or more operations
depending on what is under the resist
The wafer must be clean before resist is
applied
It may need to be heated to a few hundred
degrees to drive off water
Adhesion to Si is not as good as to metals and
silicon dioxide
– Adhesion promoter, Hexamethyldisilane
(HMDS), may be needed
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DNQ/Novolac Resist Process


Dispensing the resist can be done either with a
stationary or a slowly spinning wafer
The solvent evaporates rapidly after
dispensing the resist and during the spin
– Generally more uniform resist thicknesses
are obtained the faster the wafer is
accelerated.
– The faster the final speed, the thinner the
resist.
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DNQ/Novolac Resist Process

Exposure times and source intensity are
reciprocal—one can reduce exposure times
with more intense sources
– Exposure time is increase by increasing the bake
temperature (due to decomposition of the PAC and
thus decreased sensitivity)


Post-exposure bake is often done before
development because the PAC can diffuse and
this will eliminate the standing wave pattern
Post-development bake is done to remove
standing wave pattern by flowing resist (90100oC) or increase chemical/mechanical
strength of resist (120-150oC)

Long UV exposure can also be used to cross-link the
polymer chains in the remaining photoresist
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http://www.research.
ibm.com/journal/rd/
411/holm4.gif
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Measurement Methods



Measurement of
– Mask Features and Defects
– Resist Patterns
– Etched Features
– Alignment
Measure resist pattern after development
– The aerial image is not generally
measurable
Because of the complexity of the masks, the
inspection must be fully automated—manual
observation under a microscope is not possible
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Mask Inspection System
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Measurement of Mask Features and Defects


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Here, light is passed through the mask and collected by
an image recognition system
Solid state detectors are used to collect the light
The information is compared against the database of
the mask design or with an identical mask
The inspection process is more difficult if the mask
contains OPC or is a PSM
Often, defects found in this process can be corrected
– Lasers can burn off excess Cr or Fe oxide.
– Adding absorber to clear areas is harder
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SEM Measurement
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State-of-the-Art



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Capable of exposing down to ~ 10nm
– E-beam lithography
– X-ray lithography
– Extreme UV lithography
E-beam and EUV are performed under vacuum
– Throughput is very slow
New resist families are required
– Most are very difficult to remove after use
Research needed on mask material for x-ray and EUV
– Glass absorbs
– Thickness of metal needed to block x-rays is very
thick (20-50m)
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