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Spin Coating Process

Author:Glichn Date:8/21/2011 8:55:43 AM
Spin Coating Process
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 exibility for complex spin coating requirements.
 
Spin Speed
Spin speed is one of the most important factors in spincoating. The speed of the substrate (rpm) affects thedegree of radial (centrifugal) force applied to the liquidresin as well as the velocity and characteristic turbulenceof the air immediately above it. In particular, the highspeed spin step generally defines the final film thickness.
Relatively minor variations of ±50 rpm at this stage cancause a resulting thickness change of 10%. Film thicknessis largely a balance between the force applied to shearthe fluid resin towards the edge of the substrate and thedrying rate which affects the viscosity of the resin. As theresin dries, the viscosity increases until the radial forceof the spin process can no longer appreciably move theresin over the surface. At this point, the film thicknesswill not decrease significantly with increased spin time. All Cee spin coating systems arespecified to be repeatable to within ±5 rpm at all speeds. Typical performance is ±1rpm. Also, all programming and display of spin speed is given with a resolution of 1 rpm.
 
Acceleration
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 isimportant to accurately control acceleration. In some processes, 50% of the solvents inthe 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. Inmany 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 accelerationthat provides a twisting force to the resin. This twisting
aids in the dispersal of the resin around topographythat might otherwise shadow portions of the
substrate from the fluid. Acceleration of Cee spinnersis 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.
 
Fume Exhaust
The drying rate of the resin fluid during the spinprocess 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 dampcloth will dry faster on a breezy dry day than during damp weather, the resin will drydepending on the ambient conditions around it. It is well known that such factors asair temperature and humidity play a large role in determining coated film properties. Itis also very important that the air flow and associated turbulence above the substrateitself be minimized, or at least held constant, during thespin process.
All Cee spin coaters employ a “closed bowl” design.While not actually an airtight environment, the exhaustlid allows only minimal exhaust during the spin process.
Combined with the bottom exhaust port located beneaththe spin chuck, the exhaust lid becomes part of asystem to minimize unwanted random turbulence. Thereare two distinct advantages to this system: sloweddrying of the fluid resin and minimized susceptibility toambient humidity variations.
The slower rate of drying offers the advantage of increased film thickness uniformityacross the substrates. The fluid dries out as it moves toward the edge of the substrateduring the spin process. This can lead to radial thickness nonuniformities since the fluidviscosity changes with distance from the center of the substrate. By slowing the rate ofdrying, it is possible for the viscosity to remain more constant across the substrate.
Drying rate and hence final film thickness is alsoaffected by ambient humidity. Variations of only a
few percent relative humidity can result in largechanges in film thickness. By spinning in a closed
bowl the vapors of the solvents in the resin itselfare 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 liftedto 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 variationsin air flow around the spinning substrate. In a typical clean room, for instance, there is aconstant downward flow of air at about 100 feet per minute (30m/min). Various factorsaffect the local properties of this air flow. Turbulence and eddy currents are commonresults of this high degree of air flow. Minor changes in the nature of the environmentcan create drastic alteration in the downward flow of air. By closing the bowl with asmooth lid surface, variations and turbulence caused by the presence of operators andother equipment are eliminated from the spin process.