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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 vender 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 £gm, 0.5 £gm and 0.3 £gm 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 £gm 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 £gm 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 £gm.
The video camera can easily detect particles smaller than 0.3£gm
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 £gm 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 £gm
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.
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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 oxidation
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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