# Lithography rev1 f08

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```					       Lithography

Topics:
•Wafer exposure systems
•Photoresists
•Manufacturing Methods &
Equipment
•Measurement Methods

MSE-630
Projected Lithography Requirements

MSE-630
MSE-630
Projection systems
Proximity Printing:
•Separation results in poor
resolution
•Limited to mm-sized features

Contact Printing:
•Limited diffraction effects
•Inexpensive
results in damage – low yield
•Oldest & simplest

MSE-630
Projection systems

•Produce high resolution
w/o defects
•Resolution limited by
diffraction effects
•Mask is 4X – 5X image
size
•Sub-micron features
•50 wafer/hour throughput

MSE-630
Diffraction effects
Light passing through
aperture, with an
opening ≈l, diffracts,
creating a larger
image than that on the
Light travels as a plane wave in free
space. The spherical wavelets
combine to for a uniform front.
When it passes through an aperture,
the waves from the limited number
out in all directions.

Smaller openings mean greater
diffraction                    MSE-630
The light that diffracts at
the highest angels carries
the shape of the aperture,
is not collected passing
through the focusing lens
and is lost.

Projected intensity of light
through a circular aperture.
Rings around center bright
spot result from diffraction
D = diameter of central disk
1.22 fl
D              f = focal length
d         d = objective lens diameter
MSE-630
A Fourier series is used to describe the path of light:


                       2i ( f x x  f y y )
e( x y )  
'   '
 t ( x1 , y1 )e                             dxdy



where
1 in clear areas 
t ( x1 , y1 )                    
0 in opaque areas                                     The intensity is given by:
fx 
x'
and f y 
y'                                           I(fx,fy) = |e(fx,fy)|2 =
zl                  zl
in shorthand we write                                                           |F[t(x1,y1)]|2
e( f x , f y )  F t ( x1 , y1 

MSE-630
Resolution
1.22lf     1.22lf       0.61l       l
R                                 k1
d     n(2 f sin  ) n sin       NA
n  index of refraction
NA  n sin 

MSE-630
Depth of Field

l
    cos
4
if  is small,
l        2         2
  1  1     
4            2       2              The Rayleigh criteria for depth of
d                          focus states that the path difference
  sin       NA                     for a ray on the center line and one
2f
coming from the edge of the aperture
l              l     should not differ by more than l/4.
DOF                     k2
2 NA
2
NA2

MSE-630
Example
Estimate the resolution and depth of focus of an excimer
laser stepper using KrF light source (l = 248 nm) and
NA=0.6 Assume k1 = 0.75 and k2 = 0.5.

Solution:
R = k1*l/NA = 0.75(0.248/0.6) = 0.31 nm
DOF = ± k2*l/NA2 = ±0.5(0.248/(0.6)2) = ±0.34 mm

MSE-630
Modulation Transfer Function (MTF)
The MTF is a measure of the
quality of contrast between
features. As features move
closer together, diffraction affects
cause their Airy disks to begin to
overlap, changing the degree of
intensity between the two
features.
I m ax  I m in
MTF 
I m ax  I m in
Generally, a MTF>0.5 is
needed. Smaller values
limit the minimum feature
size
MSE-630
Aerial images
produced by contact
printing (dashed line)
proximity, and
projection systems.
g=0 in contact system
and ~25 microns in
proximity system.

The quality of the image decreases as the mask is removed from the
surface, with gap size g. The image can be calculated when the gap falls
between
l < g < W2/2
where W = mask opening width
The minimum resolvable feature size is:     Wmin ≈ √(lg)

MSE-630
corresponds to optical intensity in the
aerial image. The black borders
correspond to the mask image that is
being printed. The exposure system
simulated has an NA = 0.43, partially
coherent g-line (l = 436nm). Min
feature size is 1 mm.
Below: Same example, but
wavelength has been reduced
to 0.365 nm and NA has been
increased to 0.5. Note
improvement in image quality

Left: same example as above
except feature size has been
reduced to 0.5 mm. Note the
much poorer image quality

MSE-630
Light Coherency
A perfect point source creates
parallel beams after passing
through the condensor lens. A
light source with finite size
causes light to strike mask at a
variety of angles
Coherent                  Partially Coherent

A definition of spatial coherence of light sources is:
S = light source diameter/condenser lens diameter = s/d
Or S = NAcondenser optics/NA projection optics

As s becomes more incoherent, MTF degrades for
larger features but improves for smaller ones.

MSE-630
Photoresists
•Resist changes chemical
properties when exposed to light
•Resist may be positive or negative.
In positive resists, the exposed
areas dissolve when developed
•Resist is poured on to wafer, then
wafer is spun. Viscosity is
controlled by solvents, which
evaporate
•Following patterning and
developing, resist is baked to
Typical process for g-line and i-line   increase hardness
resists

MSE-630
Photoresists
Sensitivity is a measure of the light
required to expose a resist. In Deep
UV resists (DUV), this is 20-40
mJ/cm-2. Lower sensitivity gives
higher contrast and increased
processing latitude.

The above diagram illustrates how
diazoquinone changes when exposed
to light. The final molecule (bottom
left) is carboxylic acid, which is
Basic structure of diazoquinone,         soluble in common developers (e.g.,
a photo active compound (PAC)               KOH, NaOH, TMAH (tetramethyl
in positive resists                               ammonium hydroxide))
MSE-630
As wavelengths get shorter, photoresist has to be
reformulated to have the desired reactivity and sensitivity
In DUV resists, incoming photons
react with a photo-acid generator
(PAG) molecule, creating an acid
molecule that acts as a catalyst to
make resist molecule soluble. This
process can repeat tens or hundreds
of times during the post-exposure
bake (PEB).
Sensitivity of these resists is ~ 20-40
mJ/cm-2
This scheme is being exploited in a
new generation of resists designed
to work at short wavelengths.

MSE-630
Properties and Characterization of Resists
Resist is
characterized by its
contrast and critical
modulation transfer
function (CMTF)
Contrast is its ability
Idealized contrast curves for positive and negative resists    to distinguish light
from dark areas in
1
                             Qo = dose at which               the aerial image
 Qf                 exposure begins to have
log 
Q          
                an effect                  Resists with high
 o                     Qf = dose at which           contrast can sharpen
exposure is complete
a poor aerial image
MSE-630
The MTF was used to measure the “dark” versus
“light” intensities in the aerial image. A similar
quantity is used for the resist, called the Critical
MTF. It is the minimum optical transfer function
needed to resolve a pattern in the resist.
Example of how the quality of the
aerial image and the resist contrast
combine to produce the resist edge
profile. The left side shows a sharp
aerial image and steep resist edges.
The example on the right shows a
poorer aerial image and the
resulting gradual edges on the resist
profile.
1
Q f  Q0  10   1
CMTF resist           1
Q f  Qo 10  1
MSE-630
Resist Exposure Issues
Resist coatings are:      Highly reflective substrates
•Non-uniform thickness        create standing waves that
systematically
•Not exposed                   over/underexpose resist,
simultaneously                 resulting in “wavy” lines:
•Light absorption falls off
exponentially with
increasing depth:
I = Ioexp(-z)

MSE-630
Correction
Because high frequency
detail is lost due to
diffraction defects and
apertures and lenses are
round, square details on
the image on the right to
provide greater detail in
the projected pattern

MSE-630
In the image at left,
diffraction effects result
in a loss of contrast
where the waves
overlap.
On the right, a material
cause the light to shift
phase 180o, thereby
Phase shifting can be used to improve the quality       canceling the effect of
of the aerial image or to improve the depth of focus
of the exposure system at constant resolution by              overlap.
using a lower NA system

MSE-630
Wafer Exposure Systems
The schematic at left
illustrates a scanning
system for transferring
information to the
wafer. This requires a
Modern systems use a
and combine scanning
with a stepper to
systematically cover
small portions of the
wafer.
MSE-630
Techniques to improve
image quality
In the Kohler illumination system
(left), light is focused at the
entrance pupil of the projection
lens, which captures the diffracted
light from any features on the

Off-axis illumination captures
some of the higher-order diffracted
light which was lost in the normal
illumination process

MSE-630
Measurement Methods
compare it to another
design database
•Opaque defects: Cr where it should not be
•Clear defects: Cr isn’t where it should be
•The actual size of the feature on the mask, which is influenced by the size
of the e-beam spot used to write the mask (typ. ~0.125 mm)
•Proximity effects from electron backscattering in the resist resulting in
distortion
MSE-630
Test Structures
This is an example
of a typical test
structure built into
the edges of a
chip/sheet.
It is used to extract
the sheet resistance
The geometry is
A current, I5-6 is applied, and voltage, V3-4, is read        chosen to define
at terminals 3 & 4. The sheet resistance is then:            one square of the
 V3 4                                              material (labeled s)
s 
ln 2 I 56
Given the sheet resistance, the            I15
line width can be calculated     W  s L
from:                         V2 3
MSE-630

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