APPARATUS AND METHOD FOR APPLYING COATING MATERIAL

Abstract: The invention is particularly useful for applying a layer of photoresist to the surface of a semiconductor wafer. It avoids the problems of prior art techniques and is characterized by holding the wafer (11) in an inverted position, contacting the inverted face (11a) of the wafer with a precisely metered amount of a flowable photoresist (21) and rotating the workpiece in its inverted position after it has contacted the photoresist to coat the wafer with a thin, uniform layer of the flowable material. The present invention is also directed to an apparatus for coating a wafer comprising, a spindle (10) holding the wafer (11) in an inverted manner so that the face of the wafer to be coated faces downwardly towards a liquid deposit of the coating material (21) which is supported on a nozzle (17) beneath the spindle, which spindle may be moved to permit the inverted slice to contact the liquid deposit, and means (14, 15, 16) for spinning the spindle in the inverted position to spread the coating material uniformly across the face of the spinning wafer.

Description: APPARATUS AND METHOD FOR APPLYING COATING MATERIAL This invention relates to an apparatus and method for applying coating material on a workpiece and, more particularly, to the application of uniform photoresist layers on the surface of a semiconductor wafer. Well known in the art are methods of dispensing liquids onto a rotatable substrate to form useful coatings. U.S. Patent 3,695,911 teaches both a method and an apparatus for applying materials such as photoresist onto the surface of a spinning slice of semiconductor material. U.S. Patent 4,267,208 teaches the coating of a lens which is dipped into a pool of a vinyl copolymer, withdrawn from the pool, and spun to provide a uniform protective coating thereon. U.S. Patent 4,378,953 teaches the formation of a thin membrane by dispensing a polymer onto a spinning substrate to form a layer of the polymer on the substrate. U.S. Patent 4,451,507 teaches that a uniformly thick layer of a slurry of glass can be formed on the surface of a semiconductor material by rotating the semiconductor material as the glass slurry is dispensed thereon as a spiral bead. IBM Technical Disclosure Bulletin, Vol. 17, No. 11, April 1975 on pages 3211-3212 discloses a typical photoresist dispensing apparatus presently in use in the Semiconductor Industry. An article entitled, "Thickness Variance of Spun-On Photoresist, Revisited" appeared in the Proceedings of the Kodak Microelectronics Seminar, 1978, and teaches that thin layers of photoresist material may be spun on silicon slices. Liquid photoresists are, today, applied to semiconductor wafers from the top as shown in the IBM Technical Disclosure Bulletin, Vol. 17, No. 11, April 1975, pages, 3211-3212. This method causes a puddle of photoresist to be dripped onto the face of the wafer being coated, after which the wafer is spun at a high rotational speed. The centrifugal forces of the spinning overcomes the surface tension of the puddle of photoresist and results in a thin film of photoresist on the surface of the wafer. This technique although widely and commonly used has several drawbacks. For example: When levels of topography increases, the actual film thickness of the coating increases because of the effects of surface tension, gravity, substrate material properties, viscosity, spin speed, temperature, and etc. Moreover contaminants on the surface such as dust particles and the like can become trapped in pockets, trenches or the like; Also, to avoid voids, pinholes, and to assure coverage, excess amounts are puddled on the surface to be coated. Of the amount so puddled about 99% is spun off and thus wasted. For example, on a four inch wafer 1.5 to 2.0 ml of photoresist is usually used. This amount if allowed to remain on the surface would produce a uniform depth of at least 180 µ. In actual fact after spinning an average depth of 1.25 µ remains. Thus at least 140 times more photoresist is puddled on the wafer than is actually needed to coat the wafer to the desired thickness. All of this excess photoresist is lost. Since photoresist costs about $1000.00 per litre one can readily see that a large part of this cost is lost. Still further there is the problem of backside contamination where unwanted photoresist is deposited on the wafer on its reverse face. This photoresist may be deposited on the back of the wafer where it is not wanted because of air turbulence, back splatter from the walls of the surrounding bowl or from extra drips that fall on the wafer holding chuck. This backside contamination can cause serious focusing problems in photolithographic tools. The invention as claimed avoids all the problems of the above described prior art techniques by holding the workpiece in an inverted position, contacting the inverted face of the workpiece with a precisely metered amount of a flowable material and rotating the workpiece in its inverted position after it has contacted the polymer to coat the workpiece with a thin, uniform layer of the flowable material. The present invention is also directed to an apparatus for coating a substrate comprising, a spindle holding the workpiece in an inverted manner so that the face of the workpiece to be coated faces downwardly towards a liquid deposit of the coating material which is supported on a nozzle beneath the spindle, which spindle may be moved to permit the inverted slice to contact the liquid deposit and means for spinning the spindle in the inverted position to spread the coating material uniformly across the face of the spinning workpiece. Because it is performed in an inverted position, the present invention not only provides a more uniform layer of coating material on the face of the workpiece regardless of the topography of the face of the workpiece but also does so with a significantly less amount of coating material required while eliminating deposition of any of the coating material on the back surface of the workpiece. Thus, the combination of gravity, centrifugal force, and surface tension all co-act to cause the photoresist to maintain a more uniform thickness across the slice regardless of the topographical features of the slice. Additionally, a significant saving in coating material is realized because smaller, more controlled amounts can be deposited on the slice to achieve a specified thickness. These and other advantages and features of the present invention can be better appreciated from the following description of an embodiment of the invention taken in conjunction with the following drawings, in which Fig. 1 is a schematic drawing of the invention showing a workpiece being held by the apparatus and positioned over a precisely metered bead of coating material to be applied on the inverted face of the workpiece. Fig. 2 is a fragmentary view of the present invention showing the workpiece in contact with the bead of coating material, and Fig. 3 is a fragmentary view of the present invention showing the workpiece coated with the coating material. Fig. 4 is a cross-sectional view of a semiconductor wafer coated using the prior art techniques and apparatus. Fig. 5 is a cross-sectional view of a semiconductor wafer coated using the techniques and apparatus of the present invention. Shown in Fig. 1, in schematic form, is the preferred embodiment of the invention which applies a selected substance onto one face of a workpiece. In this embodiment there is illustrated an apparatus which includes a vacuum chuck or wafer pedestal 10 for holding a workpiece 11, such as semiconductor wafer or slice, in an inverted position. This wafer 11 has one face 11a which is to be coated, for example with photoresist, and has its outer face 11b supported on the flat face of the vacuum chuck 10. The shaft of chuck 10 is hollow and is adapted to be connected via a hollow tubing 13 to a suitable source of vacuum (not shown) to hold the wafer 11 on the face of the chuck. It should be understood that any suitable holding technique may be employed. The chuck 10 is driven through a transmission system 14 coupled to a motor 15. The transmission system 14 is adapted to raise and lower the chuck 10 as well as, through a belt drive mechanism 16, spin the chuck 10 and wafer 11 held thereon. The chuck 10 and wafer 11 held thereon is positioned directly over a nozzle in the form of a cup 17 which is coupled through a hollow pedestal 18 to a pump 19 and a source 20 of coating material. In this example the coating material is photoresist. Pump 19 is a metering pump that dispenses an exact amount of photoresist 21 into the cup 17. By disposing a metered amount of photoresist 21 into the cup 17 and using an inverted wafer, as shown, not only is a less than normal amount of photoresist required to uniformly coat the surface 11a of the semiconductor wafer 11 but also other advantages, as will be discussed below, are realized. In a typical cycle of operation, the operator places the semiconductor wafer 11 onto the surface of the chuck 10, turns on the vacuum source to secure the wafer onto the chuck and causes the precise, selected amount of photoresist material to be dispensed into the cup 17 and form a meniscus extending above the edge of the bowl 17. Thereafter, as shown in Fig. 2, the vacuum chuck 10 together with the wafer 11 held thereon is brought down, i.e. lowered into the interior of a shroud or bowl 12 and the surface 11a of the wafer is brought into contact with the meniscus formed by the photoresist 21 sited in the cup 17. When the wafer 11 comes in contact with the meniscus of the photoresist material 21, the material wets the surface 11a of the wafer 11. Once this contact has been made the belt drive system 16 is activated to cause the chuck 10 to rotate at a selected slow speed, say about 500 rpm. As the chuck 10 rotates centrifugal forces spread the photoresist material 21 across the surface 11a. As the material spreads across the surface 11a internal cohesion forces in the photoresist cause the photoresist to be transferred out of the cup 17 and across the entire inverted surface 11a of the wafer 11. While continuing to rotate the chuck 10, the wafer 11 is raised back to its original position where it is spun at a higher speed, say about 5,000 rpm. This insures a more uniform coating. As the wafer 11 spins excess photoresist material is thrown off the edges of the wafer onto the walls of the shroud or bowl 12 surrounding the spinning wafer. By making the walls of the shroud with a re-enterent form extending around the entire rim of the shroud 12 the spin off excess is caught. The shroud 12 is provided with an outlet drain 12b through which the excess photoresist may be drained. It should be noted that these speeds can vary. For example, the initial starting, i.e. the flow speed can be from 0 to 1,000 rpm and the higher speed can be up to 16,000 rpm. In specific tests high contact resist was used to coat silicon semiconductor wafers. The test data showed that average thicknesses of photoresist deposited over the face 11a of the wafer ranged from 1100 to 1600 nm ± 25 nm depending upon wafer speeds and rotational speeds of the apparatus at the time the material was being deposited. Additionally, variation of the deposited photoresist, i.e. waves or undulations in the photoresist, deposited on the wafer was found to be equal to or less than 20 nm. The diameter of cup 17 as well as the properties of the coating materials determines the volume required to be dispensed and the shape and height of the meniscus formed. In specific tests a 15 mm diameter nozzle with a high contrast resist formed a 3 mm high bead which contains approximately 0.5 ml of resist. As noted previously the prior art apparatus required a photoresist deposit of 1.5 to 2.0 milliters. In the present invention it was found that less than one-third of that amount, i.e. 0.5 milliters or less could be used to achieve the same result. For example, when 100 mm wafers were coated using the prior art apparatus 2.0 ml of high contact photoresist was required when the wafers spun first at 1,000 rpm and then at 5,000 rpm to provide an average thickness of 1103 nm and an average thickness variation of 21,1 nm. When identical wafers were coated in the present apparatus only 0.5 ml of high contact photoresist was required to achieve an average coating thickness of 1035 nm and an average thickness variation of 7,1 nm. This significant reduction in quantities of photoresist required to be dispensed results in significant cost savings while the reduction in average thickness variation results in a 65% better control of the line widths that can be formed and developed in the photoresist thus leading to significant density improvements in the device that can be defined in the photoresist coating. Additionally it was found that with inverted spinning no problems were encountered with resist getting on the backside of the wafer and no further processing or special equipment was required to eliminate any contamination of the backside of the wafer with photoresist. Additionally, it was found that the method coated equally well as any known prior art technique and voids and pinholes are not encountered even though significantly smaller amounts of photoresist were used. As is best demonstrated in Fig. 3, the wafer 11 has deposited thereon a substantially uniform layer of photoresist 21a which is equally disposed across the entire surface of the wafer. Additionally it has been found that the photoresist 21 provides uniform coverage of the wafer regardless of the topographical features that may have been previously created on the wafer. This can be more fully appreciated from Figs. 4 and 5. Fig. 4, shows, in cross section, a partial view of series of parallel lines 31, 32, and 33 and a number of crossing lines of which only one 34 is shown. For ease of explanation it will be assumed that the center line of the wafer is, as shown, passing vertically through line 32. When a photoresist coating 35 is deposited on this wafer from the top, in accordance with the prior art and spun around its center line the forces of gravity causes the photoresist to thin out in areas 36, 37, and 38 over the lines 31, 32, and 33 and to thicken in the valleys 39 and 40 formed between the parallel lines 36, 37, and 38. The centrifugal forces resulting from the spinning action also causes build-up of the photoresist 41 and 42 on the walls of each valley 39 and 40 furthest from the center line around which the wafer is spun. This thickening of the photoresist against the walls of the valleys as well as the thickening of the photoresist in the valleys themselves causes variation in line width and required exposure times. In the present invention these effects are minimized because the force of gravity is now helping to prevent excess material from accumulating in the valleys. This is shown in Fig. 5 where a semiconductor wafer 50 has there on a series of parallel lines 51, 52, and 53, crossed by a line 54 and coated in accordance with the present invention with a coating of photoresist 55. In this case, the forces of gravity are acting such that no accumulation occurs in the valleys 59 and 60 and the layer of photoresist 55 is of relatively constant thickness over the entire surface. Also the force of gravity acting with the centrifugal forces caused by the spinning helps eliminate build-up of the photoresist on the far slopes of the valleys and thickening of the photoresist does not occur. Additionally, because of the inversion of the chuck 10 and the wafer 11, with the photoresist 21 being applied from below the wafer, the problem of excess photoresist material leaking around the shaft and into the bearings supporting the chuck 10 and into the transmission 14 is prevented thus eliminating a serious cleaning problem which is present in the prior art apparatus when the photoresist is deposited from the top of the apparatus. The present invention thus teaches a method of substantially eliminating any resist on the reverse or back surface 11b of the semiconductor and eliminates the necessity of cleaning the back surface 11b of the wafer 11 after the front surface of the wafer 11 has been coated. Additionally the present invention minimizes the waste of resist and other expensive coating materials. Still further more conformal and more uniform coatings are formed on the surface of the wafer being coated regardless of wafer topology. Finally, by reducing the amount of material required to be dispensed the expenses of coating a wafer is substantially reduced and density improvements are realized. It should also be understood that various modifications can readily be made. For example, the chuck 10 could be held in a fixed vertical position and the pedestal 18 can be made extendable so that the shroud 12 is raised to surround the wafer 11 and the photoresist 21 in the bowl 17 is brought into contact with the force 11b of the wafer 11.

Claims: An apparatus for applying coating material on a workpiece characterized in that it comprises: a chuck (10) holding the workpiece (11) with a vacuum force in an inverted position, a bowl (17) arranged beneath said chuck for holding a deposit of coating material, the bowl having a diameter substantially smaller than the workpiece (11), means for providing a predetermined, metered deposit (21) of coating material in said bowl (17), the deposit being sufficient to cause the coating material to extend above the edges of the bowl (17) and to form a convex meniscus above the surface of the bowl, means (14) for causing the inverted workpiece to contact the meniscus of the deposit of coating material in said bowl, and means (14, 15, 16) for rotating the chuck (10) with the workpiece in the inverted position in contact with said controlled amount of coating material to spin the workpiece (11) and spread the coating material (21) across the spinning workpiece. A method of depositing coating material on a workpiece comprising the steps of: supporting the workpiece (11) on a rotatable means (10) in an inverted position and providing below the inverted workpiece a preselected volume of coating material (21) in a bowl having a diameter substantially smaller than the diameter of the workpiece, moving said inverted workpiece into contact with said coating material, rotating said workpiece in said inverted position at a first speed to spread substantially the entire preselected volume of coating material across the downwardly facing side (11a) of said spinning workpiece, selectively increasing the rotational speed of the spinning workpiece above said first speed, to cast off excess coating material and to thin out said coating material to a selected average thickness, and terminating the rotation of said workpiece after a predetermined time. The apparatus of claim 1, wherein said means for rotating the chuck includes a variable speed transmission (14) and a motor (15). The apparatus of claim 3, wherein there is further provided a shroud (12) surrounding the workpiece (11) for catching excess coating material thrown off said spinning workpiece. The apparatus of claim 4, wherein said shroud is provided with a wall having a re-enterent form, said wall extending up around said bowl and said spinning workpiece. The method of claim 2, wherein said workpiece is a semiconductor wafer and said coating material is a photoresist. The method of claim 6, wherein said wafer is initially rotated in a range of 500 to 1,000 rpm and increased up to 16,000 rpm.

Inventors: RINGER, FREDERICK CHARLES, JR.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: G03F7
IPC codes: B05C11, G03F7, H01L21