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Handling
Inspection
Because of the pellicle's structure, only film
can be inspected automatically. Other components have to be
visual inspected. Defects should be categorized, i.e. killing
defect, functional defect, or cosmetic defect, and appropriate
actions should be taken. Functional defects should be well-defined
between vendor and user, while cosmetic defects should be
compared with a sample standard. To properly inspect a pellicle,
attention must be given to several components - transmission
and uniformity of the pellicle film, particle contamination
on the film, frame, and adhesive; and overall integrity of
the pellicle.
 Inspection
of Film Transmission and its Uniformity |
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To inspect transmission, a spectrophotometer
is used to measure pellicle film transmission and its
uniformity. Consideration must be given to the background
stray light noise, resolution, i.e. bandwidth of the
slit, and the angle of the measuring incident light
from the spectrophotometer to the pellicle. Although
some pellicles might occasionally have been out of specification
from some pellicle suppliers, transmission generally
presents no problem because each pellicle's transmission
is individually measured.
Transmission uniformity is typically inspected with
a monochromatic light, usually green mercury light or
a helium-neon light. With the aid of a monochromatic
light, detection of non-uniform spots on the film can
easily be found, such as those from the spin-coating
process. Similarly, a laser reflection inspection machine
has been used by MLI to inspect the coating uniformity
of uncoated wafer stepper film. Uncoated film thickness
can easily be measured by calculations from the transmission
or reflection spectra of a spectrophotometer or any
commercial film thickness measuring machine. |
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| Inspection
of Film Particles |
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Particle standards should be used for
inspection calibration. Specifically, a 1 µm,
0.5 µm and 0.3 µm standard on pellicle
surface should be used to calibrate any inspection tool.
Even for visual inspection, these standards should regularly
be shown to operators to insure accurate inspection.
Currently, there are three methods of inspecting film
contamination - human eye, laser scan, and video camera
inspection. The human eye is quite sensitive. MLI has
found that with proper lighting a particle as small
as 0.3 µm can be detected (calibrated with a
standard polystyrene bead on pellicle film and verified
by a laser scan machine). However, human-eye inspection
has a non-quantitative nature. There are several factors
involved in this inspection process, such as operator
eye sensitivity, ability to focus, inspection angle
and position, film distance from the eye, incident and
background light intensity, and the eye's pupil response
to background light. The ability to detect a particle
below 1 µm can be very different between operators.
Even inspection with machines can give irreproducible
results. With laser inspection of film particles, the
best reproducibility is approximately +/- 20%. This
limitation is primarily controlled by the scan line
overlap and scan line stability, which is affected by
the vibration limit of the scanning optics. Laser scan
can detect particles smaller than 0.3 µm.
The video camera can easily detect particles smaller
than 0.3µm with proper illumination. With current
high-speed computer video capture and processing, this
is the best method to detect particles on pellicle film.
All three inspection methods have their limitations.
Human-eye, laser scan, and video camera detection is
a detection of scattered light, not the real particle
size. The real particle size can be substantially larger
than the size its scattering light appears, sometimes
as large as 10 times that of the detected particle size.
In addition, laser detection is limited to about 2-3
mms from the frame's edge, depending on the laser scanning
angle, intensity distribution of the laser spot, height
of the frame, and light scattering of the frame edge.
Comparing human-eye, laser scan, and video inspections,
the advantage of the laser and video inspection is that
they are both a mechanical process that generates more
reproducible results, whereas human-eye inspection under
strong light scattering presents an unpleasant condition
to work. However, human-eye inspection does have the
desired sensitivity, fast inspection speed, ability
to inspect up to the frame's edge, and even the ability
to determine which side of the film the particle lies
on, at least for larger particles. |
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| Inspection
of Frame |
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A frame is usually machined,
sandblasted, and black anodized. Currently, human-eye
inspection is used under a projector light to inspect
such pellicle frames. However, an operator can not distinguish
particles of 3 µm or even larger from frame irregularities
which came from machining marks, a rough surface from
sandblasting, and a porous anodized surface with etch
pits. Therefore, it is very difficult to differentiate
between a frame surface irregularity and particle contamination.
In addition, the strong background scattered light from
the frame makes the task even more difficult.
Inspection of frame particles under a microscope has
been attempted but has not been successful. Seen under
a microscope, the irregularity of the frame will become
even more obvious and more difficult to distinguish
it from particles. Therefore, surface coating of the
frame is necessary to give the frame a smooth surface
and make automated and human-eye inspection reasonably
possible. |
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| Inspection
of Adhesive |
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Foam adhesives previously used had holes larger than
50 µm on the side wall. The irregularity of the
foam material made it almost impossible to detect small
particles on such an adhesive, only a rigorous cleaning
procedure was performed to ensure cleanliness. As the
illustration shows in Figure 6, cast-in-place, non-carrier
adhesives allow the surface to be much smoother, making
it easier to differentiate between particles and adhesive
irregularities. However, there is still difficulty detecting
particles on the edge of the frame and adhesive. |
Handling and the Environment
A pellicle has to be made clean in a cleanroom
area, inspected, stored and shipped in clean box and bag,
mounted on the photomask, stored and used for a long time
in a photomask box. It is expected that all these steps won't
create any more particles or contamination. Actually, many
steps of handling and inspection can degrade the cleanliness
of the pellicle. Usually, many different sizes and shapes
of pellicle have to be handled in one area, making automation
a difficult task and human handling the only solution. Even
holding a pellicle for inspection is a difficult task and
can be a source of contamination. Handling of a pellicle is
not a small task. Standardization of pellicle sizes might
be a good idea to solve some of our handling problems.
| Controlling
Static Charge |
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The importance of controlling static charge
during pellicle handling cannot be overemphasized. Given
that a pellicle is plastic, e.g. film, release liner,
backside cover, it is easy to generate static charge
during handling. For example, peeling the release liner
or backside cover from the mounting adhesive can generate
2000 volts of electricity and, if not neutralized quickly,
can almost instantly attract particles from the surrounding
environment onto the pellicle. Also the packaging material
such as shipping box, bag, and storage container are
all plastic. Handling of these materials in the production
line can easily generate electrostatic charge and attract
particles to the working area. Therefore, it is crucial
to control the electrostatic discharge (ESD) at the
working area. Under appropriate air flow conditions,
proper antistatic equipment should be used to reduce
ESD and allow for a decay time at least 10 seconds for
a charging plate monitor to decay from 1000 volts to
100 volts and thus minimize contamination from electrostatic
attraction. Without a clean and proper ESD environment,
contamination can be generated and lead to a defective
photomask, causing them to fail during incoming inspection,
outbound inspection, or even after repeated uses. Proper
air flow is also needed at the mounting machine and
working table to insure cleanliness. |
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| Mounting |
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Putting a pellicle onto a
photomask is called mounting. Ideally, a mounting machine
with automatic handling and automatic inspection should
do the job. In reality, only the film of a pellicle
can be automatically inspected. Mounting operations,
peeling release liner or back side cover from a pellicle
can create static charge and contamination. Therefore,
a simple, easy to keep clean, mounting machine with
human inspection before mounting is still a better solution
for mounting.
Even force has to be used in mounting the pellicle on
a photomask. The mounting fixture can damage the pellicle
frame edge and get contaminated. Mounting tools have
to be kept clean all the time because cross contamination
is possible. It is recommended a strong anti-static
environment with good surface air flow on the machine
should be used. Although mounting accuracy is typically
obtained, proper communication between pellicle manufacturer
and equipment designer is necessary for many sizes of
pellicles. MLI has invented a process of putting each
pellicle on a mounting plate and ships the whole package
to the customer.7 The customers don't have to touch
the pellicle directly in handling and can put the whole
package into a simple mounting machine after an inspection
of the pellicle. |
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| Cleaning |
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If particles are generated onto the pellicle
film, blowing may be used in an attempt to remove these
particles. Specifically, a filtered, deionized air or
nitrogen gun with a needle point blower is preferred.
Blowing is only effective in removing large particles
and might generate some small particles and contaminate
the environment in the clean area if not used carefully.
Cleaning mounting adhesive residue on a photomask after
removal the pellicle can be a challenging job in an
area using multiple vendors because different vendors
use different mounting adhesives. Different vendor may
supply different cleaning method or solution. |
Long Term Stability
Long term stability requirement of a pellicle
on a photomask depends on the expected usage life time of
the photomask and the sensitivity of the lithography process.
Most of us like the life time of pellicle on a photomask to
be infinite. Unfortunately, most of material in a pellicle
and photomask should be considered only have a limited life
time.
| Outgassing
and Crystallization |
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Long-term usage and storage of a pellicle
on a photomask provides a real challenge for pellicle
design and material selection. Ideally, a pellicle on
a photomask will seal off all outside particle and vapor
contamination. Unfortunately, the organic components
in a pellicle can lead to outgassing and may contaminate
even the area it is supposed to protect. The most susceptible
outgassing components of the pellicle are the mounting
adhesive, inside coating, and glue which are a mixture
of polymers and low molecular weight organic compounds
such as residue solvent, plasticizer, anti-oxidant and
UV stabilizer. The chrome surface on the photomask is
known as an active surface which can adsorb many organic
chemicals on it. Unprotected chrome surface made from
sputtering can adsorb material from photomask container
outgassing and make it impossible to coat a positive
photoresist on its surface after only one day. Outgassing
and crystallization can also come from the box or storage
container and easily contaminate area that is not under
pellicle protection. Environmental outgassing or vapor
can also contaminate the pellicle, reduce its transmission
or form crystal even underneath the pellicle.12 Outgassing
or vapor can easily get into the pellicle protected
area through the vent hole with a filter or through
the film directly when the film is thin and permeability
to the vapor is high.
For example, in 1986, crystals of 2,5-di (t-amyl) quinone
were found on a pellicle protected photomask surface.
In one extreme case, even within a day of mounting pellicle
on a photomask, crystals could be found on the chrome
pattern edge and the photomask had become useless. The
origin of the crystals was identified by us and it came
from the outgassing of anti-oxidant stabilizer of 3M447
- a rubber type double-sided pressure sensitive tape
used for pellicle mounting adhesive.13 This stabilizer
was 2,5-di (t-amyl) hydroquinone. Recently in 2001,
a crystal was formed on the pattern surface and identified
as a quinone. The quinone comes from the dimerization
of the antioxidant as a stabilizer in the hot melt pressure-sensitive
adhesive.
An outgassing test should be performed for any new pellicle
qualification. In addition, a lifetime test for particle
generation on the photomask under the pellicle should
be completed in a real production environment as well.
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Material stability |
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All the material used for a pellicle has
to be subjected to some photodegradation from UV light
and oxidative degradation from air and outgassing. In
addition, film thickness can change due to the residue
solvent evaporation out of film, polymer molecular rearrangement
and humidity change. Film for pellicle is always chosen
to resist UV light radiation which may degrade the film
transmission or mechanical strength. However, other
components such as film glue, mounting adhesive or frame
coating are not necessary going through a rigorous checking.
For example, one of our early mounting adhesive, i.e.,
a polyethylene vinyl acetate hot melt pressure sensitive
adhesive without anti-oxidant, lost its pressure sensitivity
in 6 months. In our laboratory, a silicone adhesive
used to mount the film to frame lost its adhesion on
a perfluoropolymer film only after a few days when it
is fully cured. Some mounting adhesive using hot melt
pressure sensitive adhesive such as SEBS or SES polymer
was found not DUV stable and not oxidation stable without
anti-oxidant.
As discussed above, the material of container, i.e.,
the box, used for pellicle and photomask has to be screened
for outgassing because some of them do contain possible
outgassing low molecular organic compounds in the plastics.
Depending on the functional requirement, not all the
degradation will result in the defect generation for
a photomask with a pellicle. However, depending on the
usage life time, sometimes it may take a long time to
find if a pellicle is suitable for the process.
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