Coating processes – Nonuniform coating – Mask or stencil utilized
Reexamination Certificate
1998-11-30
2003-04-15
Lamb, Brenda A. (Department: 1734)
Coating processes
Nonuniform coating
Mask or stencil utilized
C118S406000, C118S301000, C438S943000, C438S945000
Reexamination Certificate
active
06548115
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to providing precision coatings to substrates and more particularly to the coating of substrates with modular coating apparatus adapted to provide various desired results including no edge beading, no edge coating, reduced material loss, reduced cleaning solvent usage, and increased processing throughput.
BACKGROUND OF THE INVENTION
Providing coatings of a desired thickness and uniformity on a substrate has been a necessity in many industries, most notably in the processing of silicon wafers to produce integrated circuits (“chips”). For example, in the production of chips, a wafer may be coated with photoresist in order to develop a mask for controlling the introduction of dopants into or onto the wafer. However, due to the typically very large scale integration of electrical components (often in the hundreds of thousands and even in the millions) integrated into a single chip, many of which are produced from a single wafer, tolerances with respect to providing these coatings are very narrow. Likewise, as particular areas to be defined by such coatings may be very small, such as on an order of magnitude equivalent with the wavelength of light, the cleanliness and purity requirements of such coatings and coating processes are high. Moreover, the chemicals used are often very costly and/or very volatile and, therefore, it is generally desired that their use, dispensation, and handling be strictly controlled so as to not unnecessarily waste such chemicals and/or cause additional costs in their clean-up and subsequent handling in addition to environmental concerns.
The current state of the art in coating wafers is to provide a relatively large amount of coating material at or near the center of a wafer to be coated and then to rapidly spin the wafer for a precisely controlled time and at a precisely controlled speed. Such spinning, although generally providing a coating of the substrate wafer having an acceptable thickness and uniformity, results in a great deal of wasted coating material as a relatively large amount of such material, in proportion to that remaining as a coating of the substrate, is expelled from the surface of the substrate. For example, some coating materials may cost as much as $1500 to $5,000 a liter. With spin coating an amount of material is deposited which may include coating material sufficient to allow for 90% of the material to be thrown off in the spinning process.
It should be appreciated that the current state of the art, utilizing spin coating, does not adequately address the need for applying coatings of relatively viscous coating material. Because these materials are much more reluctant to migrate in response to the centrifugal forces, higher spinning speeds may be required, thus resulting in greater amounts of expelled material. Likewise, in order to provide a uniform coating, i.e., adequately disperse any center accumulation or pooling of the thicker material, longer spin times may be required, also causing additional waste.
Similarly, spin coating does not lend itself to providing thick coatings or coating surfaces with severe topography. The spinning necessary to cause migration of the deposited pool of coating material generally leaves a very thin layer of coating material remaining on the substrate. If thicker coatings are required a different process must be utilized, thus necessitating additional equipment if both thick and thin coatings are to be used at a single facility, or multiple thin coats must be applied, also introducing typically undesired characteristics such as additional processing time and a stratified end product. Moreover, many of the wafers or substrates have a fairly high degree of surface topography or surface roughness of various features for which spin coating is not particularly well suited to provide uniform coverage of those severe features.
Additionally, such spinning results in the coating of the periphery or circumferential edge of the wafer, in addition to typically causing an edge bead, or area of slightly thicker coating at the edge of the wafer, to form. These edge beads and edge coatings must typically be removed in a subsequent processing step as they may gum up the hot plates and downstream systems if the coating material, such as resist, is left on the edge or has crept around the back side of the substrate during the spin operation. Therefore, the current art of spin coating of a wafer requires additional steps in order to clean the expelled material from equipment as well as to remove the edge coating and edge bead formed. For example, current coating processes may employ another fluid dispense system that lends itself to removing this unwanted coating material from the substrate therefore introducing additional cost and/or more production facility floor space.
A further drawback of the state of the art spin coaters is that they are not adapted, or adaptable, to accommodate a variety of substrate sizes. For example, due to the precise spin speeds which must be maintained in order to produce a coating of a desired thickness, a spin chuck, and it attendant spinning mechanism, may not be acceptable for use in spinning particular size substrates. Accordingly, various sizes of substrates may require a variety of spin chucks and their associated component.
Moreover, the equipment utilized in spin coating the substrate must be adapted to handle particular sizes of substrates. For example, as spinning necessarily casts off coating material, the spin mechanism must typically include a spin bowl to catch this material. This spin bowl will define the largest substrate which may be accommodated by the mechanism. Likewise, in order to introduce the coating material at or near the center of the substrate, a delivery mechanism must be adapted to pump coating material from a reservoir to the center of the substrate surface which, as the substrate sizes to be handled increases, affects this delivery mechanism.
Other constraints on the ability to adapt to various size substrates include the actual chuck utilized to hold the substrate for coating and spinning. One chuck design generally used in spin coating is a vacuum chuck, wherein the surface of the chuck includes vacuum orifices in order to draw down a substrate placed thereon and firmly hold the substrate for spinning. Accordingly, a substrate which is smaller than this chuck will result in coating material which is expelled from the substrate surface onto the chuck surface requiring cleaning prior to engaging subsequent substrates. Additionally, the coating material may be introduced into the vacuum system which, at the least, may cause later problems in establishing a proper vacuum and, because of this material's, and the solvents used in cleaning such materials, often volatile nature, may quite likely cause much more drastic results. Therefore, although a vacuum chuck having different vacuum positions that allow you to have the different form factors on it may not be practical for particular applications and/or for a particular range of substrate sizes.
Further, spin coating primarily lends itself to coating of circular objects, such as the typical silicon wafer. This limitation is due to balancing requirements of the substrate when spun at relatively high speeds as well as the windage problems resulting from spinning an irregular or other than circular shape.
Furthermore, as different size and/or dimension form factors of substrates are used, the coating apparatus must still achieve the same level of uniformity of the coating. Spin coating necessarily requires a longer time for coating of the outer edges of a large substrate than a small substrate. Accordingly, uniformness in coating the substrate may be difficult to attain. Irregular shapes, where it is not an equal distance from the center of the substrate to each outside edge, or all portions of the outside edge, compound this problem.
A still further drawback in the current state of the art is the time required to complete the coating of the
Costa James J.
Gibson Gregory M.
Ehrlich Henry L.
FAStar, Ltd.
Lamb Brenda A.
Winstead Sechrest & Minick P.C.
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