INDEX
Spin
coating has been used for several decades for the application
of thin films. A typical process involves depositing a
small puddle of a fluid resin onto the center of a substrate
and then spinning the substrate at high speed (typically
around 3000 rpm). Centripetal acceleration will cause
most of the resin to spread to, and eventually off, the
edge of the substrate, leaving a thin film of resin on
the surface. Final film thickness and other properties
will depend on the nature of the resin (viscosity, drying
rate, percent solids, surface tension, etc.) and the parameters
chosen for the spin process. Factors such as final
rotational speed, acceleration,
and fume exhaust contribute to
how the properties of coated films are defined.
One of the most important factors in spin coating is repeatability.
Subtle variations in the parameters that define the spin
process can result in drastic variations in the coated
film. The following is an explanation of some of the effects
of these variations.
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A typical spin process consists of a dispense step in
which the resin fluid is deposited onto the substrate
surface, a high speed spin step to thin the fluid, and
a drying step to eliminate excess solvents from the resulting
film. Two common methods of dispense are Static dispense,
and Dynamic dispense.
Static
dispense is simply depositing a small puddle of fluid
on or near the center of the substrate. This can range
from 1 to 10 cc depending on the viscosity of the fluid
and the size of the substrate to be coated. Higher viscosity
and or larger substrates typically require a larger puddle
to ensure full coverage of the substrate during the high
speed spin step. Dynamic dispense is the process of dispensing
while the substrate is turning at low speed. A speed of
about 500 rpm is commonly used during this step of the
process. This serves to spread the fluid over the substrate
and can result in less waste of resin material since it
is usually not necessary to deposit as much to wet the
entire surface of the substrate. This is a particularly
advantageous method when the fluid or substrate itself
has poor wetting abilities and can eliminate voids that
may otherwise form.
After the dispense step it is common to accelerate to
a relatively high speed to thin the fluid to near its
final desired thickness. Typical spin speeds for this
step range from 1500-6000 rpm, again depending on the
properties of the fluid as well as the substrate. This
step can take from 10 seconds to several minutes. The
combination of spin speed and time selected for this step
will generally define the final film thickness.
In general, higher spin speeds and longer spin times create
thinner films. The spin coating process involves a large
number of variables that tend to cancel and average out
during the spin process and it is best to allow sufficient
time for this to occur.
A separate drying step is sometimes added after the high
speed spin step to further dry the film without substantially
thinning it. This can be advantageous for thick films
since long drying times may be necessary to increase the
physical stability of the film before handling. Without
the drying step problems can occur during handling, such
as pouring off the side of the substrate when removing
it from the spin bowl. In this case a moderate spin speed
of about 25% of the high speed spin will generally suffice
to aid in drying the film without significantly changing
the film thickness. Each program on a Cee spin coater
may contain up to ten separate process steps. While most
spin processes require only two or three, this allows
the maximum amount of flexibility for complex spin coating
requirements.
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Spin speed is one of the most important factors in spin coating.
The speed of the substrate (rpm) affects the degree of
radial (centrifugal) force applied to the liquid resin
as well as the velocity and characteristic turbulence
of the air immediately above it. In particular, the high
speed spin step generally defines the final film thickness.
Relatively minor variations of ±50 rpm at this
stage can cause a resulting thickness change of 10%. Film
thickness is largely a balance between the force applied
to shear the fluid resin towards the edge of the substrate
and the drying rate which affects the viscosity of the
resin. As the resin dries, the viscosity increases until
the radial force of the spin process can no longer appreciably
move the resin over the surface. At this point, the film
thickness will not decrease significantly with increased
spin time. All Cee spin coating systems are specified
to be repeatable to within ±5 rpm at all speeds.
Typical performance is ±1 rpm. Also, all programming
and display of spin speed is given with a resolution of
1 rpm.
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The acceleration of the substrate towards the final spin
speed can also affect the coated film properties. Since
the resin
begins to dry during the first part of the spin cycle,
it is important to accurately control acceleration. In
some processes, 50% of the solvents in the resin will
be lost to evaporation in the first few seconds of the
process.
Acceleration also plays a large role in the coat properties
of patterned substrates. In many cases the substrate will
retain topographical features from previous processes;
it is therefore important to uniformly coat the resin
over and through these features. While the spin process
in general provides a radial (outward) force to the resin,
it is the acceleration that provides a twisting force
to the resin. This twisting aids in the dispersal of the
resin around topography that might otherwise shadow portions
of the substrate from the fluid. Acceleration of Cee spinners
is programmable with a resolution of 1 rpm/second. In
operation the spin motor accelerates (or decelerates)
in a linear ramp to the final spin speed.
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The drying rate of the resin fluid during the spin process
is defined by the nature of the fluid itself (volatility
of the solvent systems used) as well as by the air surrounding
the substrate during the spin process. Just as a damp
cloth will dry faster on a breezy dry day than during
damp weather, the resin will dry depending on the ambient
conditions around it. It is well known that such factors
as air temperature and humidity play a large role in determining
coated film properties. It is also very
important that the airflow and associated turbulence above
the substrate itself be minimized, or at least held constant,
during the spin process.
All Cee spin coaters employ a "closed bowl"
design. While not actually an airtight environment, the
exhaust lid allows only minimal exhaust during the spin
process. Combined with the bottom exhaust port located
beneath the spin chuck, the exhaust lid becomes part of
a system to minimize unwanted random turbulence. There
are two distinct advantages to this system: slowed drying
of the fluid resin and minimized susceptibility to ambient
humidity variations.
The slower rate of drying offers the advantage of increased
film thickness uniformity across the substrates. The fluid
dries out as it moves toward the edge of the substrate
during the spin process. This can lead to radial thickness
non-uniformities since the fluid viscosity changes with
distance from the center of the substrate. By slowing
the rate of drying, it is possible for the viscosity to
remain more constant across the substrate.
Drying
rate and hence final film thickness is also affected by
ambient humidity. Variations of only a few percent relative
humidity can result in large changes in film thickness.
By spinning in a closed bowl the vapors of the solvents
in the resin itself are retained in the bowl environment
and tend to overshadow the affects of minor humidity variations.
At the end of the spin process, when the lid is lifted
to remove the substrate, full exhaust is maintained to
contain and remove solvent vapors.
Another advantage to this "closed bowl" design
is the reduced susceptibility to variations in air flow
around the spinning substrate. In a typical clean room,
for instance, there is a constant downward flow of air
at about 100 feet per minute (30m/min). Various factors
affect the local properties of this air flow. Turbulence
and eddy currents are common results of this high degree
of air flow. Minor changes in the nature of the environment
can create drastic alteration in the downward flow of
air. By closing the bowl with a smooth lid surface, variations
and turbulence caused by the presence of operators and
other equipment are eliminated from the spin process.
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These charts represent general trends for the various
process parameters. For most resin materials the final
film thickness will be inversely proportional to the spin
speed and spin time. Final thickness will also be somewhat
proportional to the exhaust volume although uniformity
will suffer if the exhaust flow is too high since turbulence
will cause non uniform drying of the film during the spin
process.
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Spin coater
As explained previously, there are several major factors
affecting the coating process. Among these are spin speed,
acceleration, spin time and exhaust. Process parameters
vary greatly for different resin materials and substrates
so there are no fixed rules for spin coat processing,
only general guidelines. These are explained in the "Spin
Coating Process Description" section. Following
is a list of issues to consider for specific process problems.
Film too thin
|
Spin speed too high
|
Select lower speed
|
|
Spin time too long
|
Decrease time during high
speed step
|
|
Inappropriate choice of resin
material
|
Contact resin manufacturer
|
Film too thick
|
Spin speed too low
|
Select higher speed
|
|
Spin time too short
|
Increase time during high
speed step
|
|
Exhaust volume too high
|
Adjust exhaust lid or house
exhaust damper
|
|
Inappropriate choice of resin
material
|
Contact resin manufacturer
|
|
Air
bubbles on wafer surface
Air bubbles in dispensed fluid (resin)
Dispense tip is cut unevenly or has burrs or defects
|
 |
|
Comets,
streaks or flares
Fluid velocity (dispense rate) is too high
Spin bowl exhaust rate is too high
Resist sits on wafer too long prior to spin
Spin speed and acceleration setting is too high
Particles exist on substrate surface prior to dispense
Fluid is not being dispensed at the center of the
substrate surface
|
 |
|
Swirl pattern
Spin bowl exhaust rate is too high
Fluid is striking substrate surface off center
Spin speed and acceleration setting is too high
Spin time too short
|
 |
|
Center
circle(Chuck Mark)
If the circle is the same size as the spin chuck,
switch to a Delrin spin chuck
|
 |
|
Uncoated
Areas
Insufficient dispense volume
|
 |
|
Pinholes
Air bubbles
Particles in fluid
Particles exist on substrate surface prior to dispense
|
 |
Poor reproducibility
|
Variable exhaust or ambient conditions
|
Adjust exhaust lid to fully closed
|
|
Substrate not centered properly
|
Center substrate before operation
|
|
Insufficient dispense volume
|
Increase dispense volume
|
|
Inappropriate application of
resin material
|
Contact resin manufacturer
|
|
Unstable balance in speed / time
parameters
|
Increase speed / decrease time
or visa versa
|
Poor film quality
|
Exhaust volume too high
|
Adjust exhaust lid or house exhaust
damper
|
|
Acceleration too high
|
Select lower acceleration
|
|
Unstable balance in speed / time
parameters
|
Increase speed / decrease time
or visa versa
|
|
Insufficient dispense volume
|
Increase dispense volume
|
|
Inappropriate application of
resin material
|
Contact resin manufacturer
|
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