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SPIN COATING THEORY 18 Oktober 2009

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Spin coating has been used for several decades for the application
of thin lms. A typical process involves depositing a small puddle of a uid resin onto the center of a substrate and then spinning the substrate at high speed (typically around 3000 rpm). Centripetal acceleration will cause the resin to spread to, and eventually off, the edge of the substrate leaving
a thin lm of resin on the surface. Final lm 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 nalrotational speed, acceleration, and fume exhaust contribute to
how the properties of coated lms are dened.

One of the most important factors in spin coating is repeatability. Subtle variations in the parameters that dene the spin process can result in drastic variations in the coated lm.The following is an explanation of some of the effects of these variations.
Spin Coating Process Description
A typical spin process consists of a dispense step in which the resin uid is deposited onto the substrate surface, a high speed spin step to thin the uid, and a drying step to eliminate excess solvents from the resulting lm. Two
common methods of dispense are Static dispense, and Dynamic dispense.
Static dispense is simply depositing a small puddle of uid on or near the center of the substrate. This can range from 1 to 10 cc depending on the viscosity of the uid 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 uid 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 uid 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 uid to near its nal desired thickness. Typical spin speeds for this step range from1500-6000 rpm, again depending on the properties of the uid as well as the substrate.This step can take from 10 seconds to several minutes. The combination of spin speedand time selected for this step will generally dene the nal lm thickness.In general, higher spin speeds and longer spin times create thinner lms. 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 sufcient time for this to occur.A separate drying step is sometimes added after the high speed spin step to further
dry the lm without substantially thinning it. This can be advantageous for thick lmssince long drying times may be necessary to increase the physical stability of the lm 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 sufce to aid in drying the lm without signicantly changing the lm 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 exibility for complex spin coating requirements.
Spin Speed
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 denes the nal lm 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 uid 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 lm thickness will not decrease signicantly with increased spin time. All Cee spin coating systems are specied 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.
Acceleration
The acceleration of the substrate towards the nal spin speed can also affect the coated lm properties. Since the resin begins to dry during the rst 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 rst 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 uid. 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 nal spin speed. Fume Exhaust The drying rate of the resin uid during the spin process is dened by the nature of the uid 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 lm properties. It is also very important that the airow 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 uid resin and minimized susceptibility to
ambient humidity variations. The slower rate of drying offers the advantage of increased lm thickness uniformityacross the substrates. The uid dries out as it moves toward the edge of the substrate during the spin process. This can lead to radial thickness nonuniformities since the uid
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 nal lm thickness is also
affected by ambient humidity. Variations of only a few percent relative humidity can result in large changes in lm 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 ow around the spinning substrate. In a typical clean room, for instance, there is a constant downward ow of air at about 100 feet per minute (30m/min). Various factors affect the local properties of this air ow. Turbulence and eddy currents are common results of this high degree of air ow. Minor changes in the nature of the environment can create drastic alteration in the downward ow 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. Process Trend Charts
These charts represent general trends for the various process parameters. For most resin materials the nal lm thickness will be inversely proportional to the spin speed and spin time. Final thickness will be also be somewhat proportional to the exhaust volume although uniformity will suffer if the exhaust ow is too high since turbulence will cause non uniform drying of the lm during the spin process.

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