Download materials in modern communications systems

Basic Concepts
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Antireflective coating is used to prevent reflections from
the chrome coming back into the resist
– Occasionally AR coatings are deposited on wafers
also
Develop the resist and etch to remove the metal
– We get good dimensional control because the Cr is
very thin (~80nm)
It is critical that the areas beneath where the Cr is
removed be highly transparent at the wavelength of the
light used in the wafer exposure system.
Masks (reticules) for steppers (step and repeat
systems) are 4x to 5x larger than what is printed
– Relaxes minimal feature requirements on mask
Masks for steppers print usually only one or two die at a
time; any defect in the mask gets reproduced for every
die!
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Reflectivity
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At the interface of two bulk layers
 n1  n2 

R  
 n1  n2 
2
http://www.mellesgriot.
com/products/optics/im
ages/fig5_12.gif
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Antireflectivity Coatings
 nair nglass  n 2film 

R
2
n n


n
film 
 air glass
– For l/4 thick films
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Ideal index of refraction for antireflective
coating is √(nairnglass)
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Basic Concepts
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We generally separate lithography into three parts
– The light source
– The exposure system
– The resist
The exposure tool creates the best image possible on
the resist (resolution, exposure field, depth of focus,
uniformity and lack of aberrations)
The photoresist transfers the aerial image from the
mask to the best thin film replica of the aerial image
(geometric accuracy, exposure speed, resist resistance
to subsequent processing)
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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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To minimize problems in the lens optics, the
lamp output must be filtered to select on of
the spectral components.
Two common monochromatic selections are
the g-line at 436 nm and the i-line at 365 nm.
The i-line stepper now dominates the 0.35 m
market
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Light Sources
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For 0.18 and 0.13, we use 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
These lasers must be continuously strobed (several
hundred Hz) or pulsed to pump the excitation
Can get several mJ of energy out
Technical problems have been resolved for KrF and
these are used for 0.25 and 0.18 m
ArF is likely for 0.18 and 0.10 m; technical problems
remain
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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 down with the Cr 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 because of the very short
exposure wavelength (1-2 nm).
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Wafer Exposure Systems
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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.
Projection printing is the dominant method today
– They provide high resolution without the defect
problem
– The mask (reticule) 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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Optics Basics
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Diffraction occurs because light does not travel
in straight lines
Pass a light through a pin-hole; we see that
the image is larger than the hole
This cannot be explained by ray tracing
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Diffraction of Light
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Diffraction of Light
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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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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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Basic Optics
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This diffraction “bends” the light
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
The following diagram shows how the system
works
Note that the focusing lens only collects part
of the diffraction pattern
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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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 (see Figure 5-4)
In Fraunhofer diffraction, the image plane is far from
the aperture, and there is a lens between the aperture
and the image plane (see Figure 5-6)
Fresnel diffraction applies to contact and proximity
printing while Fraunhofer diffraction applies to
projections systems
There are powerful simulations systems for both cases
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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
(5-7)
Lord Rayleigh suggest that we define the
resolution 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
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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
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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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This really is defined only for point sources, as
we used the point source Airy function to
develop the equation
We can generalize by replacing the 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
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From this criterion, we have
l / 4     cosq
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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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Contrast and Proximity Systems
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These systems operate in the near field or
Fresnel regime
Assume the mask and the resist are separated
by some small distance “g”
Assume a plane wave is incident on the mask
Because of diffraction, light is bent away for
the aperture edges
The effect is shown in the next slide
Note the small maximum at the edge; this
results from constructive interference
Also note the ringing
As a result, we often use multiple wavelengths
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Fresnel Diffraction
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Fresnel Diffraction
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As g increases, the quality of the image
decreases because diffraction effects become
more important
The aerial image can generally be computed
accurately when
lg
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W2
l
where W is the feature size
Within this regime, the minimum resolvable
feature size is
Wmin  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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