COATING METHOD AND APPARATUS

Abstract: An apparatus and method for coating a substrate with a thin film of material. The coating material is converted to a mist by means of an ultrasonic nebulizer and transported by a carrier gas to a chamber which contains the substrate where the mist is allowed to settle onto the substrate under the force of gravity. Sonic means are also provided to increase the settling rate of the mist droplets to lessen the settling time. Means are also provided to maintain the volume and concent­ ration of the coating material in the nebulizer chamber over successive coatings.

Description: Coating Method and Apparatus In the fabrication of micro circuits a photoresist coated silicon wafer is eXposed to ultra-violet light through a mask containing the circuit patterns to be formed on the wafer. The photoresist is developed or otherwise partially removed leaving the circuit pattern outlined on the wafer. During actual buildup of a complete circuit on the wafer, the wafer may have to be repeatedly coated with the photoresist material. The mask and, therefore, the wafer has a plurality of identical circuits arranged in rows and columns and once the process is completed the wafer is cut into rows and columns to form individual circuits. A critical part of the foregoing described process is in the provision of silicon wafers coated with the light sensitive photoresist material. As the state of the art in the fabrication of micro circuits advances, it becomes more and more critical that the silicon wafers have a film of photoresist deposited thereon which is highly uniform and lacking defects such as striations. The technique most widely in use for the coating of silicon wafers with photoresist is known as spinning. This method entails placing a drop of liquid photoresist material on a wafer which is spun. Due to the aerodynamic and centrifugal forces the photoresist eventually covers the entire wafer in a thin film. This method has the disadvantage of poor uniformity in coating thickness. For example, spin coating causes intrinsic and induced striations which are grooves or channels formed in the surface of the coating. Intrinsic striations are caused by the nature of spin coating itself due to the outward radial movement of the liquid photoresist, Induced striations are due to specks such as dust or foreign matter in the photoresist or on the wafer, as well as previously etched patterns on a wafer. These striations, which are areas of differing thickness of the photoresist layer, result in vaxyint degrees of exposure sensitivity which lead to unreliable photoresist development such as removing too little or too much photoresist during development. This ultimately results in unreliable or failed circuit elements in the finished micro circuits. Another disadvantage of the spin method of coating is that it is wasteful of the photoresist, with losses typically exceeding 9W%o of the material. The present invention relates to a method and apparatus for coating wafers with photoresist material which overcomes the foregoing disadvantages and provides more economical coatings of uniform thickness free of the striations encountered in spin coating. The present invention relates to a method and apparatus for providing a thin uniform coating of material on a flat surface. More particularly the invention relates to a method and apparatus for providing a uniform thin coating of photoresist on a silicon wafer. A liquid photoresist is converted into a mist by means of ultrasonic nebulization and transported to a chamber contnining a silicon wafer to be coated. The mist is then allowed to settle by gravity onto the silicon wafer thereby providing a uniform thin coating thereon. The present invention also includes sonic means to significantly shorten the settling time. The sonic technique uses a transducer mounted at the top of the settling chamber to provide resonant sonic waves within the chamber while the mist settles. In edition to providing coatings of highly uniform, striation-free coatings the present invention greatly reduces loss of photoresist during the coating process. Nebulization itself has the inherent advantage of elimination from the photoresist. Nebulization also ensures that the mist is made up of droplets of uniform size which is an advantage over spray coating techniques. Figure 1 illustrates in schematic form a preferred embodiment of the present invention. Referring now more particularly to Figure 1 there is shown the coating apparatus of the present invention. The coating apparatus of the present invention comprises a nebulizer 10 which includes a chamber 11 formed by cylinder 12. Attached to the bottom of the cylinder 12 forming one end thereof is a flexible membrane or diaphragm 13. The diaphragm 13 may be made of any sound transmissive material such as flexible plastic or mylar. The diaphragm 13 and a portion of the end of the cylinder 12 are immersed in a liquid 14 held in tank 15. A transducer 16 is disposed at the bottom of the tank 15 directly beneath the diaphragm 13. An oscillator 17 is connected to the transducer 16 via an amplifier 18. The chamber 11 has at its bottom end a quantity of liquid photoresist 19 deposited therein. The upper surface of the photoresist 19 is maintained below the surface of the liquid 14. The upper end of the chamber 11 is connected to chamber 20 through conduit 22. The chamber 20 comprises a cylinder 21 which is significantly longer than the chamber cylinder 12. The conduit 22 is angled upwardly so that it communicates with the chamber 20 near the upper end of the cylinder 21. Use of an angled conduit 22 permits photoresist on the walls of conduit 22 to fall back into chamber 11 for reuse. The chamber 20 is vented to atmosphere by means of the vent tube 24. A transducer 25 which may be a speaker is disposed at the top end of the cylinder 21. Transducer 25 is circular in cross-section having a diameter substantially the same as the diameter of cylinder 21 and forms an air tight closure for the upper end of the cylinder 21. Transducer 25 is connected to audio oscillator 26 through amplifier 27. The cylinder 21 is supported by base 28 which forms an air tight seal with the bottom end of the cylinder 21 at surface 29 of the base 28. A wafer 30 is disposed at the bottom of the chamber 20. The wafer is held by carrier 31 which is slidable perpendicular to the plane of the drawing within the recess 32 formed in the base 28. In this way the wafer 30 may be easily inserted into and removed from the chamber 20. If desired, the carrier 31 may be dimensioned to hold more than one wafer to permit removal of a coated wafer while another is being coated. l A conduit 33 has one end connected to the chamber 11 near the bottom end of the cylinder 12. The other end of the conduit 33 is connected to a pressurized source of gas 34 through toggle valve 35, flow valve 36, flowmeter 37, and regulator 38. The apparatus is designed so that a mist of a liquid photoresist which is created within chamber 11 is transported to chamber 20 where it is allowed to settle onto the surface of wafer 30. The formation of a mist within a chamber 11 is accomplished by a process known as nebulization. In this process a liquid such as the liquid photoresist 19 is converted to a mist or aerosol by means of ultrasonic waves produced by the transducer 16 and transmitted through liquid 14 to the photoresist 19 through the diaphragm 13. The transducer 16 is energized by oscillator 17 via the amplifier 18. During the initial mist formation the chamber 11 is isol ated Z-om the pressurized gas from gas tank 34 by means of toggle valve 35 being in the closed position. During generation of the mist within the chamber 11 the transducer 16 is driven at its mechanical resonance which in one practical example is 1.25 megahertz by means of RF oscillator 17 and power amplifier 18. The ultrasonic waves are transmitted through the liquid 14. The ultrasonic waves pass through the liquid photoresist 19. The mist or aerosol is formed at the surface of the photoresist. The liquid 14 is used to transfer the ultrasonic energy from the transducer 16 to the photoresist 19. Water is a good medium for the transfer. If a liquid were not used, the transducer 16 would have to be placed closely adjacent or contiguous with the diaphragm 13 which might erode the transducer 16 or cause excessive wear on the diaphragm 13. Additionally, the liquid 14 provides a thermal mask which permits the liquid 14 to be maintained at a predetermined temperature which because of its mass does not respond to rapid changes in ambient temperatures. Further, the liquid 14 maintains the liquid photoresist which is immersed therein at a stable constant temperature. The temperature of the liquid 14 and therefore the mist may be maintained by a heater 39 wrapped around the tank 15 and controlled by a thermocouple 40 for switching a source of power 41 to the heating tape when the temperature falls below a desired value. In a practical embodiment for a photoresist of a given viscosity, e.g. Shipley 1350B, it has been found desirable to maintain the mist created within the chamber 11 at 1550 Fahrenheit. Since deposition times and coating thicknesses may vary with mist temperature depending on the viscosity of the photoresist 19 plus other variables, it is important for reproducibility considerations that mist temperature be maintained constant. It has been found that acceptable coating of a liquid photoresist such as Shipley 13503 photoresist nay be obtained by maintaining the mist between 1300 and 1900 Fs#brenheit. Also acceptable coatings are obtained when power at which transducer is driven is within 10 to 40 watts. As aforesaid, the temperature and drive power conditions apply to 13503 Shipley photoresist. Conditions will vary depending upon the particular photoresist used. Once a sufficient amount of mist is generated within the chamber 11 it is caused to be transferred into chamber 20 via conduit 22. The mist within the chamber 20 is then allowed to settle by precipitation onto the exposed surface of the wafer 30. A refill chamber 42 contnining liquid photoresist 45 is connected to chamber 11 via tube 43 and valve 44 for the purpose of adding photoresist to chamber 11 to compensate for loss after a predetermined number of coatings. The concentration (resin:solvent) of the photoresist added to chamber 11 to compensate for the losses is lower than the concentration of the photoresist in chamber 11. This is due to the fact that the loss of solvent is greater than the loss of resin for each coating. In a practical embodiment for Shipley 13503 photoresist it has been found that the concentration in the refill chamber should be between 1:2 and 1:3, where concentration is defined as the ratio of 1350B photoresist to the solvent AZ thinner. It has been discovered by applicant that by causing an acoustic resonance within the chamber 20 the settling rate of the mist onto the wafer 30 is significantly increased. Acoustic resonance is provided by means of acoustic transducer 25 powered by oscillator 26 via amplifier 27 at a frequency equal to the resonant acoustic frequency of the chamber 20 at a relatively low power, for example, I to 5 watts. In a practical embodiment it has been found that a coating of one micron thickness of photoresist on the wafer 30 without acoustic resonance takes between one and two minutes whereas with acoustic resonance the settling time is reduced to 15 seconds. The specific frequency used is 310fly, the fundamental acoustic resonance of chamber 20. The next resonance at 620Hz is equally effective, but is more irri- tating to the human ear. Higher frequency resonances are less effective due to their decreasing Q. The manner in which the mist is transferred from chamber 11 to chamber 20 is described hereinbelow. As aforesaid during the initial mist formation, the chamber 11 is isolated from the pressurized source of inert gas 34 by the toggle valve 35. On the other side of flow valve 36 gas pressure is at a higher pressure than atmosphere established by the gas regulator 38. Flow regulator 38 is adjusted so that the gas flow after the toggle valve 35 and flow valve 36 are opened reaches a low steady state value. When the temperature in the chamber 11 reaches the desired value, for example, 1550 Fahrenheit as measured by a thermocouple 40 which indicates that sufficient thermocouple mist has formed, toggle valve 35 is opened. The high gas pressure existing in the conduit 33 between the toggle valve 35 and the flow valve 36 causes an initial rush of gas into the photoresist chamber via conduit 33. This initial rush pushes the mist from chamber 11 through the conduit 22 into the chamber 20. This displaces the air within the chamber 20 which escapes through vent 24. The mist then settles onto the wafer 30. As aforesaid settling time is decreased by use of speaker 25 causing acoustic resonance within the chamber 20. It could be noted that wafer 30 may be introduced into chamber 20 after transfer of the mist thereto as well as before the mist is transferred to chamber 20. when the mist is in the chamber 20, the flutter valve 23 may be closed along with valves 35 and 36 to permit formation of more mist in the chamber 11 for coating the next wafer. when the wafer 30 is coated, the carrier 31 may be slid out and the wafer 30 removed and replaced with a new wafer to be coated. As aforesaid the carrier 31 may be elongated or long enough to hold two or more wafers so that when wafer 30 is removed another wafer is automatically placed in position for coating. Alternatively, the process may be automated with carrier 31 in effect forming a conveyor which is automatically moved to place a next wafer within the chamber 20 after the preceding wafer is coated. Also, the entire process may be automated by timing techniques for turning on power to the transducer 16, speaker 25 and opening and closing valves 35 and 36 all based on time requirements for coating a wafer to a desired coating thickness. It should be noted that means other than that shown may be used for transferring a mist from chamber 11 to chamber 20. For example, a bellows arrangement may be used for puffing the mist into the chamber 20. The bellows could use any convenient gas, such as air. Also, it should be noted that the average cost in photoresist material for coating a four inch wafer to one micron thickness using the present invention is one cent as opposed to an average cost of fourteen cents for the spinning method. Other modifications of the present invention are possible in light of the above specification which should not be deemed as limiting the invention beyond those limitations placed in the claims which follow.

Claims: Claims: 1. An apparatus for coating the surface of an object with a material, comprising in combination a first chamber containing the object to be coated, a second chamber communi- cawing with said first chamber and containing the coating material, first means associated with said second chamber for forming a mist of said coating material within said second chamber, second means for directing said mist to said first chamber, and third means associated with said first chamber increasing the rate of settling of the mist onto the object. 2. An apparatus according to claim 1, wherein said first means comprises transducer means for transmitting ultrasonic waves into said coating material. 3. An apparatus according to claim 2, wherein said second chamber comprises a diaphragm closing one end of said second chamber, said one end of said second chamber containing the coating material positioned above said transducer means. 4. An apparatus according to claim 3, wherein said first means further comprises a tank containing a liquid, said one end of said second chamber immersed in said liquid, and said transducer means disposed within said tank directly beneath said diaphragm. 5. An apparatus according to claim 4, wherein said third means comprises acoustic wave generating means for providing acoustic waves within said first chamber at a resonant frequency of said first chamber. 6. An apparatus according to claim 5, wherein said acoustic wave generating means comprises acoustic transducer disposed at one end of said first chamber and audio oscillator means connected to said transducer. 7. An apparatus according to claim 6, further including base means closing the other end of said first chamber supporting the object to be coated within the chamber. 8. An apparatus according to claim 7, wherein said base means comprises a base supporting said first chamber, and carrier means disposed within a recess formed in said base and roveable relative to said base. 9. An apparatus according to claim 8, wherein said second means comprises fourth means providing a gas flow, first conduit means connecting said fourth means to said second chamber near said one end such that said mist is carried from said second chamber to said first chamber, and valve means in said first conduit means for opening and closing said first conduit means. 10. An apparatus according to claim 9, wherein said fourth means comprises a source of pressurized gas and regulator means for controlling the rate of gas flow through said conduit. 11. An apparatus according to claim 9, further including second conduit means connecting the other end of said second chamber to said first chamber near said one end of said first chamber, said first chamber being substantially longer than said second chamber, and said second conduit means extending upwardly from said second chamber to said first chamber. 12. An apparatus according to claim 11, wherein said coating material is a liquid photoresist and said object to be coated is a silicon wafer. 13. An apparatus according to claim 12, further comprising fifth means associated with said second chamber for replenishing said photoresist in said second chamber after a predetermined number of coatings. 14. An apparatus according to claim 15 wherein said carrier means is formed to support a plurality of said wafers whereby when one wafer is coated the nat wafer to be coated may be moved into position for coating. 15. A method of coating a surface with a material comprising the steps of generating a mist of the material, transporting the mist to a volume over the surface, holding the mist within the volume until the mist settles onto the surface by force of gravity, and increasing the settling rate of the mist droplets after the mist is transported to the volume. 16. A method of coating a surface with a material according to claim 15, wherein the step of generating a mist includes the step of ultrasonically nebulizing the material. 17. A method of coating a surface with a material according to claim 16, wherein the step of increasing the settling rate comprises, providing acoustic waves of a predetermined frequency within the volume. 18. An apparatus for coating the surface of an object with a liquid photoresist comprising in combination a first chamber containing the object to be coated, a second chamber communicating with said first chamber and containing the coating material, first means associated with said second chamber for forming a mist of said liquid photoresist within said second chamber, second means for directing said mist to said first chamber, and third means for maintaining the temperature of the mist within a predetermined range. 19. An apparatus according to claim 18, wherein said first means comprises transducer means for transmitting ultrasonic waves through said photoresist and said transducer being driven at a power between ten and forty watts. 20. An apparatus according to claim 19, wherein said second chamber comprises a diaphragm closing one end of said second chamber, said one end of said second chamber containing the liquid photoresist positioned above said transducer means. 21. An apparatus according to claim 19, wherein said first means further comprises a tank containing a liquid, said one end of said second chamber immersed in said liquid, and said transducer means disposed within said tank beneath said diaphragm. 22. An apparatus according to claim 21, further including acoustic wave generating means for providing acoustic waves within said first chamber at a resonant frequency of said first chamber. 23. An apparatus according to claim 22, wherein said acoustic wave generating means comprises a speaker closing one end of said first chamber, and audio oscillator means connected to said speaker. 24. An apparatus according to claim 23, further including base means closing the other end of said first chamber supporting the object to be coated within the chamber. 25. An apparatus according to claim 24, wherein said base means comprises a base supporting said first chamber, carrier means disposed within a recess formed in said base and moveable relative to said base, said carrier means formed to support a plurality of the objects to be coated whereby when one object is coated the next object to be coated may be moved into position for coating. 26. An apparatus according to claim 25, wherein said second means comprises fourth means providing a gas flow, first conduit means connecting said fourth means to said second chamber near said one end such that said mist is carried from said second chamber to said first chamber, and valve means in said first conduit means for opening and closing said first conduit means. 27. An apparatus according to claim 26, wherein said fourth means comprises a source of pressurized gas and regulator means for controlling the rate of gas flow through said conduit. 28. tt apparatus according to claim 26, further including second conduit means connecting the other end of said second chamber to said first chamber near said one end of said first chamber, said first chamber being substantially longer than said second chamber, and said second conduit means extending upwardly from said second chamber to said first chamber.

Inventors: AUSSCHNITT, CHRISTOPHER PERRY, HUCHITAL, DAVID A.
Applicants: THE PERKIN-ELMER CORPORATION
Application year: 1980
Main classification: G03F7
IPC codes: B05D1, B05B17, G03F7