Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C—APPARATUS FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05C3/00—Apparatus in which the work is brought into contact with a bulk quantity of liquid or other fluent material
  • B05C3/02—Apparatus in which the work is brought into contact with a bulk quantity of liquid or other fluent material the work being immersed in the liquid or other fluent material
  • B05C3/09—Apparatus in which the work is brought into contact with a bulk quantity of liquid or other fluent material the work being immersed in the liquid or other fluent material for treating separate articles
  • B05C3/109—Passing liquids or other fluent materials into or through chambers containing stationary articles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
  • B05D1/00—Processes for applying liquids or other fluent materials
  • B05D1/18—Processes for applying liquids or other fluent materials performed by dipping

Definitions

• the present invention relates generally to a new method for uniformly coating a structure and an apparatus for doing the same.
• the structure to be uniformly coated is completely immersed in the coating fluid and the coating fluid is drained in a controlled manner.
• ⁇ n apparatus is also provided that has at least one side wall that may be sloped or curved in a unique way to control the draining of the coating fluid in order to provide the structure with the desired uniform coating configuration.
• a different or a combination of coating methods or processes may be employed.
• the coating on the surfaces of large area flat substrates with a coating material, such as, organic polymers, is usually accomplished by one of the following techniques, 1) dip coating, 2) roller coating, 3) spin coating or 4) spray coating.
• the dip coating and spray coating methods provide coatings with extreme thickness variation which can be as much as 100 percent.
• the roller coating method does provide coatings of good uniformity but equipment is complex, costly and limited in its ability to coat, and handle thin flexible unsupported substrates.
• the spin coating method has similar limitations.
• Processes for coating films from solvent based systems are usually produced by one of the following methods: 1) dip coating, 2) roller coating, 3) spin coating or 4) spray coating.
• methods of coating such as, spin, spray and roller coating are impractical for the following reasons.
• Roller coating equipment is not designed to handle thin delicate substrates especially when both surfaces must be coated. And, usually both surfaces of a substrate cannot be coated simultaneously.
• Spin coating for example, can only do one side at a time; when the substrate is turned over to coat the other side there is a chance of damaging the coated side as it is being held against the chuck.
• Spray coating does not produce films of uniform thickness.
• the process requires expensive equipment that must satisfy environmental and industrial hygiene requirement, and usually each side to be coated is coated separately.
• Conventional dip coating is the most practical method for the simultaneous coating of both surfaces in a manufacturing environment.
• state of the art equipment is not capable of producing films of uniform thickness. Typically, these films are thicker at the lower portion of the substrate than in other areas.
• This phenomenon is caused by the flow of the wet film as it is being wi hdrawn and partially cures or dries forming what is referred to as a wedge and the coating material that adheres to the bottom edge of the coated structure forms a bead.
• a wedge the coating material that adheres to the bottom edge of the coated structure forms a bead.
• One way is to keep the coating tank still while the substrate or the structure to be coated is dipped and pulled out of the coating fluid.
• the other method is to keep the substrate still while the coating tank is moved away from the substrate.
• a third method is where both the coating tank and the substrate to be coated are held stationary and the coating fluid is by force made to go past the substrate.
• a fourth process within the dip coating industry has now been discovered, by the present inventors, which heretobefore has not been appreciated is a process of coating a structure where both the coating tank and the structure are kept still while the coating fluid is drained.
• U. S. Patent No. 2,515,489 (Borushko) is an example where the substrate is immersed and withdrawn from a reservoir. A hot solution of polymer is used to heat a substrate. The heated, coated substrate is suspended in a coating chamber above the tank which condenses the solvent vapors and returns them to the solution. The resulting film on the substrate will be nonuniform and as such has limited use, especially in the electronics area where finer geometries are being required signifying the need for a more optimum control and uniformity of film coatings.
• U. S. Patent No. 4,004,045 (Stelter) is an example of a single sided coating system where the substrate is moved and the coating tank is kept stationary.
• U. S. Patent No. 4,275,098 (Gunji, et al.) is another example where the substrate moves and the coating tank is kept stationary. Due to its conventional design and use of dipping as a means of coating, the resulting film will be nonuniform, especially for highly flat and close tolerance structures.
• U. S. Patent No. 4,341,817 (Tozier, et al.) is a typical example of coating a structure while keeping the coating tank in a stationary position.
• the coating deposition rate is varied by varying the speed at which the bulb is coated. This produces a film of varying thickness; with the thicker, protective coating being where it is most needed.
• U. S. Patent No. 4,438,159 (Weber) is another example of inserting the item to be coated into a coating medium and taking it out while the fluid and the coating tank remain stationary.
• Multifocus optical lenses are immersed in coating solutions and allowed to reach the temperature of the solution. The lenses are withdrawn and are held in an atmosphere saturated with coating solvent until the excess material flows off the object surface, but due to the physics of dip coating will result in a "wedge shaped" coating.
• U. S. Patent No. 4,840,821 (Miyazaki, et al.) basically shows another method of dip coating, i.e., in which one of the flat surface of the substrate is held parallel to the coating fluid and the surface of the coating fluid is raised to make a direct contact with the flat surface of the substrate while the tank is held stationary.
• U. S. Patent No. 4,085,010 (Ishimori et al.) describes the third method that is currently used in the dip coating industry where the object to be plated is held in an upright position, in an apparatus for electroplating in which a powdery material is uniformly dispersed in the plating solution.
• the plating solution is introduced at the bottom of the tank by a pump from one of three different storage tanks, the solution flows upwardly and overflows into a recycling tank surrounding the cylindrical plating tank.
• the dried film is not uniform in thickness.
• the thickness profile of the dry coating takes the shape of a tapered wedge with the thinnest part at the top of the substrate and the thickest part at the bottom of the substrate. This variation can be as much as twofold.
• This wedge condition exists when the wet polymer flows as the solvent evaporates, thus causing the evaporation rate to change with the thickness variation.
• surface tension and viscosity increase witli loss of solvent, the flow slows, thus leaving greater amounts of polymer at the lower part of the substrate.
• This invention demonstrates a practical approach that will produce coated films of extreme uniformity and integrity over large areas. Furthermore, the coating rate can be varied by the profile of the tank containing the solution and/or the drain rate of the fluid.
• structure as used herein means any item that is capable of being coated, regardless of its shape, size or form.
• the structure to be coated should preferably be held in a stationary position, and using the dynamics of the tank design and fluid characteristics, the fluid is fed and withdrawn at a predetermined rate to coat the structure.
• One advantage of using the teaching of this invention is that the uniform coating of a structure can be performed without the use of a complex electrical-mechanical control system. This is also important because electronic circuits that can cause an explosion or fire in the presence of coating fluids, such as, flammable liquids, photoresist, are not being used.
• Another advantage of this invention is that for a given geometric tank shape, the fluid withdrawal rate profile is repeatable from cycle to cycle, and will thus result in uniform coating of the structure. Any desired linear motion of the coating fluid across the structure to be coated can be achieved, from constant to varying, by simply changing the geometric shape of the tank.
• a second way would be to control and/or vary the draining of the coating fluid from the coating tank into the holding or the gravity tank. This could be done, for example, by a pumping system, which could be an electrical, mechanical or pneumatic system. The pumping system could also be a part of a filtering system that controls the flow of the coating fluid out of the coating tank into the holding or gravity tank. Similarly, gravity could also be used to allow the coating fluid to flow into the holding tank.
• This invention also provides the means for uniform, repeatable deposits of a coating film on any structure or on any electronic component, such as, foils, masks, PC boards, substrates, semiconductor wafers, to name a few.
• the coating fluid for the coating layer or film is a polymeric material dissolved in solvents or aqueous media.
• a specific material could also be used using this invention. For example, applying a coating of a fluid, such as photoresist on a metal mask or metal foil.
• the structures to be coated can be made in almost any shape or size and may have applications in different industries, their major application is expected to be in structures that require a very uniform coating.
• An example of such an application is the electronics industry where, in some cases, it is critical that the structure, such as a substrate, has a uniform coating.
• the shape of the structure to be coated could be selected from a group comprising a circular shape, a rectangular shape, a triangular shape or polygonal shape, to name a few. Additionally, the method and the apparatus of this invention provides the ability to coat ultra uniform film thickness across large area planar substrates.
• the basic reason for the elimination of the bead is the constant dissolution of the thicker film that is formed at the bottom of the substrate, which is caused by the sagging or flow of the film as it dries and sets.
• An objective of this invention is to provide a technique that will apply a uniform film of coating material on both surfaces of planar substrates. In particular, this process works well with delicate large area planar substrates such as thin metal foils. It is a further objective of this invention to describe the equipment that will produce such films with extreme uniformity.
• the equipment described in this invention is devoid of complexity and by virtue of its simplicity will provide repeatability.
• One aspect of this invention discloses a process for coating a structure comprising the steps of:
• Another aspect of this invention discloses an apparatus for coating a structure, comprising a coating tank having a coating fluid, the coating tank has a bottom wall, a cover and at least three side walls, wherein at least one side wall is not at a right-angle with respect to the bottom wall, and wherein a means is provided for the removal of the coating fluid while the structure is being coated.
• the invention also encompasses an apparatus for coating a structure, comprising a coating tank having a coating fluid, the coating tank has a bottom wall, a cover and at least three side walls, wherein at least one side wall is at a right-angle with respect to the bottom wall, and wherein a means is provided for the removal of the coating fluid while the structure is being coated.
• the invention also includes an apparatus for coating a structure, comprising a coating tank having a coating fluid, the coating tank has a bottom wall, a cover and at least one curved side wall, and wherein a means is provided for the removal of the coating fluid while the structure is being coated.
• the apparatus of this invention could also be provided with a holding or a gravity tank for the storage or removal of the coating fluid from the coating tank. This removal could be done by gravity or pumping type system.
• the apparatus could be provided with a temperature control means to control the heating or cooling of the coating fluid. At least a portion of the structure could be coated with the coating fluid by submerging at least a portion of the structure into the coating fluid.
• Figure 1A is a side view of an apparatus made according to the teachings of the present invention.
• Figure IB is a side view of a structure coated using the method or an apparatus made according to the teachings of the present invention.
• Figure 2 is a side view of another embodiment of an apparatus made according to the teachings of the present invention.
• FIG 3 is a side view of still another embodiment of an apparatus made according to the teachings of the present invention.
• Figure 4 is a side view of yet another embodiment of an apparatus made according to the teachings of the present invention.
• Figure 5 is a side view of another embodiment of an apparatus made according to the teachings of the present invention.
• Figure 6 is a graph comparing the coating obtained from a straight walled apparatus of Figure
• This invention describes a novel method and an apparatus for the uniform coating of a structure.
• a chain driven conveyor dips parts in and out of a reservoir of a fluid, such as a photoresist, at the same rate at which the parts are pre-baked (soft bake) .
• This invention discloses a practical approach to coating, such as resist coating, that will produce uniform coating of films over a large surface area.
• Bernoulli's Equation Using a fundamental relation in fluid mechanics, referred to as "Bernoulli's Equation,” one is able to calculate the fluid withdrawal rate as a function of liquid height in a sloped tank. Bernoulli's equation is as follows for a steady, incompressible flow
• P 2 - P l P 9 (Y 2 - Y l>
• P Ingredient and P refer to the static pressure of a solution confined to a tank at the bottom and top of the tank, respectively;
• p is the density of the fluid in the tank,
• g is the gravitational force;
• Y and Y refer to the difference in height of the solution at the top and bottom of the tank, respectively.
• the above relation indicates a pressure difference for a difference in height of a solution. Therefore by applying Bernoulli's relation, and designing a tank's shape to fit the particular physical properties of a coating solution will enable one to uniformly coat a flat substrate.
• FIG 1A shows the preferred apparatus for carrying out the invention.
• the coating apparatus 5 comprises of a coating tank 10 , having a bottom wall 12, an oblique side wall 14, and a straight side wall 16.
• the end walls (not shown) can be straight or oblique or curved.
• a holding or gravity tank 30, having a fluid level 26, is connected to the coating tank 10, through either the bottom wall 12, or any of the side walls.
• a pipe 31, having a conventional control valve 32 connects the coating tank 10, to the holding tank 30.
• the holding or gravity tank 30, acts not only as a reservoir for the coating fluid 22, to provide all the required coating fluid into the coating tank 10, but also as a holding or gravity tank once the fluid is drained from the coating tank 10 , during the structure coating process.
• a filtering system could also be provided.
• the filtering system could also include a pumping system to extract fluid or liquid from the coating tank 10.
• Figure 1A shows a filtering system 35, interposed between the coating tank 10, and the holding tank 30, via pipes 33 and 37, having control valves 34 and 36, respectively.
• a pipe 39, with a pump 40 and a control valve 38 is provided in order to replenish the fluid 22, into the coating tank 10, a pipe 39, with a pump 40 and a control valve 38, is provided.
• the coating tank 10 is filled to the desired level with a coating liquid 22, and a structure to be coated or substrate 20, through a substrate holder 21, is dipped into the coating fluid 22.
• the portion of the substrate 20, that has to be coated must be below the fluid or liquid level 24. Therefore, if the whole structure 20, is to be coated then the whole substrate 20 , must be completely submerged into the fluid 22, below the fluid level 24.
• the coating system is set-up, then either the coating fluid 22, in the coating tank 10, is drained by gravity into the holding tank 30, or it is pumped out at a fixed rate in a controlled manner via the pump and filtering system 35. In either case the coating tank 10, and the substrate 20, remain stationary.
• the coating fluid 22, into the holding tank 30, can also be controlled via any of the control valves 32, 34 or 36.
• additional coating layers can be formed depending upon film material properties, it may be feasible to form a thicker coat through buildup of successive coatings by elevating the substrate 20, filling the coating tank 10, again with the coating fluid 22, and dipping the substrate 20, back into the coating fluid 22. This step can be repeated as many times as desired. It would be obvious to make sure that sufficient time has lapsed between each coating layer to insure that the previous coat has dried before a subsequent coating layer is formed. This approach is possible only if the coating material does not have the property of partially or completely redissolving upon subsequent immersions.
• ⁇ coated substrate 20, which results from a single layer of coating 26, is illustrated in Figure IB.
• the substrate 20 could have multiple layers of coating 26, of either the same material or different coating material. If a substrate with only a single sided coating is desired then it would be very easy for a person skilled in the art to remove the coating from the undesired areas, such as, by masking off the areas where a coating is not desired.
• FIG. 2 illustrates another embodiment of the coating tank structure.
• the coating tank 50 has a bottom wall 12, and a cover 18, which is similar to the coating tank 10, except now the coating tank 50, has side walls 54 and 56, that are oblique.
• the structure to be coated is oblique.
• a heating or cooling means 58 could be provided with coating tank 50, or with any other coating tank that utilizes this invention.
• the coating tank 60 has two side walls 64 and 66, that are oblique where the surface area of the cover 18, is greater or larger than the surface area of the bottom wall 12.
• a pipe 62 is used to drain the fluid 22, into a gravity or holding tank 30, either directly or via a pump and filtering system 35. Shown is an odd shaped substrate 61, to be coated being fully immersed in the coating fluid 22.
• Figure 4 discloses that the shape of the coating tank 70, could be different and still result in uniform coating of the substrate 20.
• the side wall 74 and 76, of the coating tank 70 are curved.
• the end walls (not shown) may or may not have a curved profile.
• a stirring means 78 could also be provided to the coating tank 70, to insure that the consistency of the coating fluid 22, is maintained. This stirring of the coating fluid also maintains homogeneity of the coating fluid.
• the stirring action from the stirring means 78, as shown in Figure 4 should be kept to a minimum agitation to insure that the liquid or fluid movement does not adversely effect the structure that is being coated.
• the curved side walls 74 and 76 could have, a circular or an elliptical profile, to name a few. As shown, the substrate 20, has been partially coated with a layer
• the coating tank 80 has straight side walls 84 and 86, with a bottom wall 12, and a cover
• the substrate 20, as shown has been coated on all sides with a coating layer 26.
• the level of the fluid 24, has dropped below the coated substrate 20, because the fluid 22, is being drained out of the tank 80, into a holding tank 30, via the pipe 82.
• the fluid 22, could be taken into the holding tank 30, via gravity or via a pumping and filtering system 35.
• the coating of large planar substrates can be done uniformly, expediently, and in a highly controlled manner.
• One of the key elements in providing the uniform coating of this invention is the draining of the coating fluid from the container at a constant flow rate (rather than extracting the structure to be coated from the container) while controlling the coating integrity by the profile of the coating tank.
• the laver or film made usin ⁇ the coating Drocess of this invention could also be made to form composite layers of material on the structure. This would be done by first forming a first uniform layer and allowing the first layer to completely dry, and then repeating the process for the desired number of additional uniformly coated layers given that this method is compatible with the film material properties.
• Two 10 inch (25.4 cm) structures which were thin molybdenum masks were selected and their physical characteristics, such as dimensions and thickness were recorded.
• One of the mask was placed inside a straight-walled coating tank, similar to the one shown and described in Figure 5, and the second mask was placed inside a coating tank that had one side-wall that had a 15 degree slope, which was similar to the one shown and described in Figure 1A.
• Both of the coating tanks were filled with the same coating fluid, namely, a photoresist, and both of the masks were fully and completely submerged in the coating fluid.
• a valve was opened to allow the coating fluid from both of the coating tanks to flow out by gravity, into a gravity or holding tank.
• the rate of the flow from both of the tanks was controlled at a rate of 16.4 ml/minute.
• the straight-walled tank gives a "wedge" shaped profile, because the coating thickness at the top of the mask was 3.0 microns, and the coating thickness at the bottom of the 10 inch mask was 4.0 microns, whereas the sloped-wall tank gives an almost uniform profile with no significant thickness difference from top to bottom of the structure.
• the straight-walled tank there was a coating thickness variation of 1 microns, but for the sloped-wall tank under the same conditions the coating thickness variation between the top and bottom of the 10 inch mask was only 0.1 microns.
• the straight-walled coating tank similar to the one shown in Figure 5, did provide a "wedge", but, using the prior art methods one would not only get a wedge that in most cases is two fold in thickness, but, also a bead at the bottom edge of the mask. Whereas, using the method of this invention a slight "wedge” did result when a straight-walled coating tank was used, but there was no formation of the bead. This is due to the fact that the draining fluid pulled the boundary layer or meniscus as it went passed the bottom edge of the 10 inch mask, and prevented the formation of the bead.

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• 1990
  • 1990-11-21 WO PCT/US1990/006852 patent/WO1992003232A1/en not_active Application Discontinuation
  • 1990-11-21 JP JP3502510A patent/JPH0794028B2/ja not_active Expired - Lifetime
  • 1990-11-21 EP EP91902268A patent/EP0660759A1/de not_active Withdrawn

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