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Effect of Resist Thickness
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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
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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( x)
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where  is the optical absorption coefficient
Thus, the resist near the surface is exposed first
– We have good fortune. There is a process called
bleaching in which the exposed material becomes
almost transparent
i.e.,  decreases after exposure to light
- Therefore, more light goes to deeper layers
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exposed = B and unexposed = A+B
C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. 27(23), Dec. 1, 1988,
pp. 4913-4919.
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Photoresist Absorption
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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.
However, 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 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
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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
Most features on the mask are square
We lose the high frequency components of the
pattern
We thus lose information about the
“squareness” of the corners
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Mask Engineering
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The effects are quite predictable
We can correct them by adjusting feature
dimensions and shapes in the masks
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Mask Engineering
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Phase Shift Masks
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In a projection system, the amplitudes of the diffracted
light at the wafer add
– 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 degrees
– Properly done, the amplitudes interfere
The thickness of the PS layer is
d
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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
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The wafer is simultaneously scanned, thus printing the
mask on the wafer
Systems of the type shown on the next page are cost
effective, but they must use 1:1 masks
– The concept is that it is easier to correct for
aberrations in small regions than a large area
– Integration of a focusing laser and vertical positioner
allowed adjustment of imaging plane to maximize
resolution
– Technology became obsolete as wafer size increased
and linewidths became smaller
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Scanning Projection Printer
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Step and Repeat Projection Systems
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Steppers expose a limited portion of the wafer
at a time
– Features on the masks (reticules) are 4-5X
the size of the features exposed on the
wafer
Steppers also allow better alignment because
they align on the exposure field rather than
for the entire wafer
– Integration of a focusing laser and vertical
positioner
Laser is also used to read information that is
scribed on wafer prior to processing
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Projections Systems
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Key features of steppers include
– Kohler illumination
– Off-axis illumination
Kohler illumination focuses the light at the
entrance pupil of a projection lens, rather than
on the photoresist
This setup allows the projection lens to
capture the diffracted light from any features
on the mask
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Kohler Illumination
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Off-Axis Illumination
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By changing the angle of incidence of the light
on the mask, we also 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
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Off-Axis Illumination
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Step and Scan Aligners
https://www.chiphistory.org/product_content/lm_asml_pas5500-400_step&scan_system_1990_intro.htm
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Step and Scan
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DNQ/Novolac Resist Process
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Hexamethyldisilane (HMDS) is often used as an
adhesion promoter
– As a liquid, drops are deposited on the wafer and then
spread by spinning at 3000 – 6000 rpm for 30 s
– Sometime HMDS is applied from the vapor
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The surface chemistry is that the silane end of
the molecule bonds with the Si while the other
end bonds with the resist
Resist is then spun on
immediately following HMDS
http://bmrc.berkeley.edu/courseware/ICMfg92/images/gif/spin-on.gif
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DNQ/Novolac Resist Process
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Prebake is usually done on a hot plate at 90—100oC
– Infrared or microwave heating can also be used
This step:
– Evaporates the last of the solvent
Solvent content in the photoresist film decreases
from 25% to 5%
– Adhesion is improved because heat strengthens the
bonds between the resist and HMDS
– Stresses in the resist caused by spinning are
thermally relieved
The resist flows slightly
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DNQ/Novolac Resist Process
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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)
The postexposure bake is often done before
development because the PAC can diffuse and
this will eliminate the standing wave pattern
The developer is a basic solution such as
TMAH, NaOH, or KOH and is applied by
immersion, or spraying
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DNQ/Novolac Resist Process
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Rinsing in H2O stops the development process
The rate for developing depends strongly on
temperature, developer concentration, and the exposure
and bake procedures
– The chemistry is the dissolution of the carbolic acid
The final step is postbake (typically 10—30 min at
100—140 C)
This hardens the resist and improves etch resistance
The resist flows a bit during the process, and all
remaining solvents are driven off
Many of the steps described above are done in a single
system called a wafer track system
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DNQ/Novolac Resist Process
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A SITE system ESVG
86 track and coat
system
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Measurement Methods
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Measurements are made to determine
– Mask Features and defects
– Resist Patterns
– Etched Features
– Alignment Accuracy (x-y-theta)
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Measurement of Mask Features and Defects
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Because almost all production now uses
reducing steppers, a defect in a mask will
produce a defect in every die
– Therefore the mask must be “perfect”
Because of the complexity of the masks, the
inspection must be fully automated
– manual observation under a microscope is
not possible
The process is illustrated in the next slide
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Mask Inspection System
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Measurement of Mask Features and Defects
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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 (optical proximity correction) or is a PSM
(phase shift mask)
Often, defects found in this process can be corrected
– Lasers can burn off excess Cr
– Adding Cr to clear areas is harder
Done using chemical vapor deposition
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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 (20-50mm)
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