Spin Coat Theory
Spin coating has been used for several decades as a method for applying thin films. A typical process involves depositing a small puddle of a fluid material 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 material on the surface. Final film thickness and other properties will depend on the nature of the fluid material (viscosity, drying rate, percent solids, surface tension, etc.) and the parameters chosen for the spin process. Factors such as final rotation speed, acceleration, and fume exhaust affect the properties of the coated films. One of the most important factors in spin coating is repeat-ability, as subtle variations in the parameters that define a spin-coating process can result in drastic variations in the coated film.
Spin-Coating Process Description
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.
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 Â±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 Â±5 rpm at all speeds. Typical performance is Â±1 rpm. Also, all programming and display of spin speed is given with a resolution of 1 rpm.
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.
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.
Bake Plate Process Theory
Hotplate Bake Process Theory
Hotplate bake processing has increased in popularity since the early 1980s. Previously the most common technique for film drying and curing as the convection oven. Hotplates offer several advantages in the form of increased throughput, increased uniformity and reproducibility and decreased particle contamination. In a typical bake process the substrate is placed into contact with a heated surface of known temperature. The substrate quickly rises to a peak temperature slightly lower than the hotplate surface temperature. Drying and curing steps generally take about 1 minute. This is in contrast to traditional oven processes taking 30 minutes or more.
Hotplate Bake Variables and Methods
A typical bake process consists of preheating the surface to a known temperature, loading the substrate onto the surface for a specific length of time and removing it promptly at the end of the cycle. The selection of the temperature and time values used as well as the bake method employed all affect the overall performance of the process.
The bake temperature used is dependent on several factors. The material and substrate being baked as well as the results desired are key factors to be considered in developing a bake process.
In general hotplate baking will be performed at temperatures slightly higher than those used in oven bake processes. The film being baked will reach a temperature somewhere between the temperature of the hotplate and the ambient air above the film. As an example, with a hotplate surface temperature of 115°C, a layer of photoresist on a silicon wafer will reach a final temperature of about 105°C after a few seconds. Thicker substrates and/or substrates with lower coefficients of thermal conductivity will require even higher temperatures to compensate for this phenomenon.
Another reason for using higher temperatures is to increase process throughput. In oven processes there is a problem commonly known as the "skin effect". This is a result of the outer exposed layer of the film drying and forming a skin before all of the solvents in lower layers have evaporated. Most oven processes are adjusted to use lower temperatures and bake times measured in minutes and hours to prevent this. During a hotplate bake process the film is baked from the bottom up thus preventing the formation of a skin over the surface. Because of this it is possible to increase temperatures and adjust bake times to be measured in seconds without danger of blistering or cracking in the film.
The selection of the bake time parameter plays an important role in the reproducibility of the bake process. Substrate thermal properties and the choice of bake method greatly affect the amount of time necessary for the substrate and therefore film temperature to stabilize during the bake. Thicker substrates and the use of proximity bake methods will increase the time necessary for the film to reach its final temperature. It is important that most of the baking action in the film takes place after this temperature is reached. A silicon wafer will reach a stable temperature within a few seconds and so it is traditional to adjust a photoresist bake processes to be completed in 60-90 seconds with an appropriate bake temperature.
For thicker substrates such as photomasks and ceramic modules the increased time necessary to heat the larger mass of the substrate results in bakes times approaching five minutes. It should be noted that these substrates can be processed with higher temperature and much shorter bake times but reproducibility may suffer. If the bake time is too short then a significant amount of the actual bake process will take place during the loading and unloading steps as well as while the substrate is cooling after removal from the hotplate. This is an unstable condition since it is very difficult to exactly reproduce conditions during these steps.
In general the temperature-time relationship in a bake process can be taken as a "dose" of the (temperature) x (time) product. Increasing the bake temperature results in a need for decreasing bake time. The limits for both of these parameters can be considered to be reached when the process is no longer reproducible or when the physical temperature limitations of the resin or substrate have been reached.
Another important factor is the method of bake. Cee® hotplates allow for three distinct bake methods. These are Proximity, Soft contact and Hard contact. The choice of bake method is programmable and up to three consecutive bake steps may be programmed into the onboard microcomputer.
In a hard contact bake the substrate is clamped onto the hotplate surface by the application of vacuum to the underside of the substrate. Small holes are machined into the hotplate surface in a pattern which optimizes vacuum distribution without the formation of cold spots or warping of the substrate. This method is usually preferred for silicon and other flat substrates where back side contact is not a problem.
Soft contact baking uses gravity alone to hold the substrate onto the hotplate. This method generally offers less uniformity since the substrate-hotplate thermal interface is not as efficient as in hard contact baking and can be somewhat random in variation.
Proximity baking is accomplished by forcing heated gas (usually nitrogen) through the same ports in the hotplate surface that are used for vacuum in the hard contact method. This forces the substrate to float at a distance of one to four mils (25-100Âµm) above the hotplate surface. Proximity baking allows a slower warm-up than contact bake methods and can be advantageous when baking thick films where blistering would otherwise be a problem.
Another advantage of proximity baking in this manner is that in many cases cambered or warped substrates can be baked with a high degree of uniformity. This is usually not possible with the contact methods since it is not possible to achieve a vacuum under a substrate that is not flat to start with. Processing cambered substrates with the soft contact method creates hot spots where the substrate touches the hotplate and cold spots where it does not. It should be noted as well that this type of proximity process is "self-leveling" in that the substrate will tend to form a uniform gap to the hotplate surface. This is a significant advantage not found in "pin lift" type systems.
Proximity baking also offers the unique advantage of allowing hotplate processing without touching the bottom side of the substrate. An example of this application is photomask processing. In processing these relatively thick glass plates it is important that the back side of the glass not directly touch the hotplate since this causes micro-fractures in the glass itself from rapid heating. By performing the entire bake process in the proximity mode the integrity of the substrate is not endangered and the uniformity is excellent.