WO1996006693A1

WO1996006693A1 - Photo reactive cleaning of critical surfaces in cd manufacturing

Info

Publication number: WO1996006693A1
Application number: PCT/US1995/010929
Authority: WO
Inventors: David J. Elliott; Richard F. Hollman; Francis M. Yans; Daniel K. Singer
Original assignee: Uvtech Systems, Inc.
Priority date: 1994-08-29
Filing date: 1995-08-29
Publication date: 1996-03-07
Also published as: AU3374195A

Abstract

A process for cleaning critical surfaces used in manufacturing of compact disks. Such cleaning is required, for example, to remove photoresist from a master disk after plating. Cleaning of compact disk stampers is also required periodically to remove polycarbonate residue from injection molding, and contamination from handling. A high intensity pulsed beam of radiation (34) is swept across the surface (30) to be cleaned, while a reactive gas (36) is flowed across the surface (30) where the beam (34) strikes it. The beam of radiation (34) causes the unwanted material to react with the gas, resulting in gaseous products (40) which are drawn away from the surface. Cleaning of a compact disk stamper can be accomplished by rotating the stamper under a beam which extends from its inner diameter to its outer diameter. Cleaning may be accomplished by multiple scans of the surface, or by scanning with multiple beams. This cleaning process is an improvement on conventional wet chemical methods, in part because it can remove contaminants that conventional techniques cannot.

Description

PHOTO REACTIVE CLEANING OF CRITICAL SURFACES IN CD MANUFACTURING

Background This is a continuation-in-part of United States Patent Application Serial Number 08/298,023, filed August 29, 1994.

This invention relates to the production of compact disks (CD's) . CD' s have become a widespread product in recent years, with applications in audio recording, computer data storage, and other areas. The CD market has grown rapidly, with current annual production of over l billion disks, and a projected yearly growth rate of 30% over the next 3 years.

As shown in Figure 1, the manufacturing process begins with a glass plate 14 coated with photoresist 10. The data is recorded as a series of dots exposed in the photoresist using a laser with a high speed shutter (not shown) . A thin layer of silver (not shown) is deposited over the developed photoresist containing the dot pattern. Following the silver deposition, nickel 12 is electroplated to a thickness of approximately 400 microns. The nickel is then peeled from the glass plate, carrying the silver and photoresist with it. The photoresist is removed (and in some cases, the silver as well) from the nickel disk. The nickel disk, or master, may be used in some cases as a stamper for the final product, but is more likely to be used to electroform intermediate disks, referred to as "fathers'* and "mothers" (not shown) , which are in turn used to electroform stampers. Stampers are nickel disks, larger in diameter than the product to be stamped, which contain the data pattern in relief on one surface. Stampers are used in a press for injection molding of polycarbonate, to transfer the dot pattern to the surface of the plastic disk (CD) which is formed. The surface of the plastic disk containing the data is coated with a thin layer of aluminum for reflectivity, then covered with an additional protective layer of plastic. Finally, the disk is printed with a label and packaged. There are several technical problems in this process which the present invention can address. One problem is stripping the photoresist from the nickel master after peeling it from the glass plate. There are several methods for doing this currently, such as plasma ashing, caustic reaction bath, or a solvent rinse. Current methods are costly either because the process is time consuming, or because the quantities of chemicals required to obtain a clean surface are costly to use and to dispose.

As seen in Figure 2, a more serious problem in compact disk manufacturing is the contamination of stampers 18 during use. In the course of stamping disks, a layer of polycarbonate residue 20 builds up on the outer rim of the stamper. This layer can cause the stamping process to fail by compromising the seal on the stamper. In some cases, the data area 22 of a stamper is contaminated by small particles of polycarbonate 26, resulting from a small "spray" of the liquefied plastic. Other types of contamination include oil and other organic materials 24 from handling, and oxidation stains 28 of the nickel surface. All of these factors might cause a stamper to fail. In current compact disk manufacturing, there is a limited capability for cleaning stampers, but this procedure uses large volumes of organic solvents or acids, and does not address all of the varieties of contaminants. Summary In general, the invention features a system for cleaning a critical surface used in the manufacture of compact disks, by providing a flowing gas, including a reactant, and delivering pulsed radiation to the critical surface in the presence of the flowing gas, such that the contamination on the critical surface is reacted and removed.

Implementations of the invention may include the following features. The radiation may be in the wavelength range 150 nm to 405 nm. The radiation source may be an excimer laser. The radiation source may be a solid state laser, such as a frequency-multiplied Nd.YAGlaser. The radiation may be optically shaped into a blade, and relative motion may be induced between the optics and the surface to be cleaned so that the blade scans the entire surface. The relative motion may be rotary or linear. The intensity of a pulse of radiation at the surface to be cleaned may be between 10 mJ/cm2 and 500 mJ/cm2. The angle of incidence of the beam may be other than 90 degrees. Multiple beams may be used. The surface to be cleaned may be heated. The cleaning may take place in an enclosed chamber, in which pressure or vacuum is applied for the purpose of accelerating or improving the efficiency of the reaction. The surface to be cleaned may be that of a CD master. The material to be removed may be photoresist or other organic materials, and the flowing gas may include oxygen or ozone. The material to be removed may be silver, and the gas may include a halogen-containing species. The surface to be cleaned may be that of a glass plate, or of a nickel "stamper", "mother", or "father". The material to be removed may be polycarbonate or other organic materials, and the flowing gas may include oxygen or ozone. The material to be removed may be nickel oxide and the flowing gas may include hydrogen. The surface may be cleaned by a sequence of steps in which different gases are used to remove different contaminants. If the surface to be cleaned is that of a "stamper", the beam may be shorter than the annular width of the stamper, and the entire surface may be scanned by rotating the stamper while the beam is moved in a linear fashion from the inner rim to the outer rim of the stamper. The surface to be cleaned may be immersed in a liquid while being scanned by the radiation. The surface may be cleaned by a sequence of steps, and the surface to be cleaned may be immersed in a liquid during one or more of these steps.

Among the advantages of the invention are the following. The invention applies to surface processing of a variety of materials used in the manufacturing of compact disks. This includes stripping of photoresist from masters, and stripping of organic and metal residues from glass plates following the separation of the master disk. It also includes cleaning of stampers by removing polycarbonate residue, other organic contamination, and particles. Finally, it includes the cleaning of masters, stampers and other disks by removing metal oxide films from their surface. The entire critical surface is scanned by a high intensity pulsed beam of radiation while a reactive gas is flowed across the surface.

Contamination is ablated or otherwise activated by the radiation to cause it to react with the flowing gas, leaving only gaseous reaction products which are carried away by the gas flow. The contamination is removed without damaging the critical surface.

Other advantages and features will become apparent from the following description and from the claims. Description Figure 1 shows the cross section of master disk after it is peeled from the glass plate.

Figure 2 is a schematic view of a compact disk stamper showing typical locations and types of contaminants.

Figure 3 is a representation of the laser cleaning process, indicating the ablation of foreign material by the radiation and reaction of the ablated material in the flowing gas.

Figure 4 shows the linear scanning scheme for compact disk masters.

Figure 5 shows the rotary scanning scheme for compact disk stampers, in which the beam of radiation extends from the inner to the outer diameter of the stamper.

Figure 6 shows a spiral-track scanning scheme combining linear translation of the beam-forming optics with rotary motion of the workpiece. Figure 7 describes the use of multiple beams, produced using a beamsplitter.

Figure 8 is a block diagram of a laser cleaning system for cleaning compact disk stampers.

Figure 9 is a schematic of an enclosed process chamber for use with gases such as H2.

As shown generally in Figure 3, foreign material 32 on the surface of a workpiece 30 is processed to form a reaction product 40, by the combination of providing a directed flow of a gas 36, including a reactant, in the vicinity of the foreign material, and delivering a beam of radiation 34 to aid the reactant to react with the foreign material to form the reaction product. The beam may be deep ultraviolet radiation in the wavelength range from 155 nm to 375 nm. The source of the radiation may be an excimer laser, for example a KrF excimer producing radiation at 248 nm wavelength, or an ArF excimer producing radiation at 193 nm wavelength. Alternatively, the source may be a solid state laser such as a frequency-quadrupled Nd.YAG laser, producing radiation at 266 nm wavelength, a frequency-tripled Nd.YAG at 355 nm, or a frequency-tripled Alexandrite laser producing radiation with a tunable wavelength in the range 240 nm to 266 nm. Other wavelengths of light may be present in addition to the deep ultraviolet light. For example, the beam from a frequency-quadrupled Nd:YAG laser may combine light of the fundamental wavelength 1064 nm and the second harmonic 532 nm along with the fourth harmonic 266 nm.

The reactant may be a gas flowing at a velocity preferably between 20 mm/sec and 500 mm/sec. The gas may include one or more members of the group of oxidants consisting of oxygen, fluorine and chlorine, and molecules containing oxygen, fluorine and chlorine. When the foreign material includes organic material, the reactant may include an oxidant and the beam may be ultraviolet radiation. When the foreign material is nickel oxide, the gas may include hydrogen or a molecule containing hydrogen.

The beam may be delivered by receiving a source laser beam and focusing the cross-sectional size of the beam in one dimension and broadening the cross-sectional size in another dimension. The size of the beam in the dimension which is broadened may be at least as great as the width of the object to be cleaned. Alternatively, if the object is annular in shape (for example, a stamper) , the size of the beam may be greater than the radial distance from the inner to the outer edge. The size along the other dimension may be chosen to provide an energy density insufficient to damage the surface. Relative motion may be caused between the surface and the beam. Relative motion may be linear, and may be accomplished by placing the object on a translating stage.

Linear motion may alternatively be obtained by mounting the optical components used to form the beam on a translating stage, while keeping the object to be cleaned stationary. In the case of an annular object such as a stamper, the relative motion may be rotary. Rotary motion may be accomplished by placing the object on a turntable. This rotary motion of the workpiece may be combined with linear translation of the optics, for example to scan a small beam across the surface in a spiral path.

The beam may be directed to the surface at an angle other than 90 degrees. In certain cases, a non-normal incidence allows a larger beam energy to be directed to the contaminant without causing damage to the surface to be cleaned.

The beam may be swept in several passes over the entire surface, for thorough cleaning in cases where the foreign material is not completely removed by a single pass. Multiple passes of the beam across the surface may be accomplished by several physical scans (linear or rotary),or by splitting the beam, using a diffraction grating or beamsplitters, into several closely spaced focused beams at the surface. The steps of (a) scanning a beam of radiation across the surface and (b) flowing a reactant gas across the surface where the beam strikes it can be used to (1) clean the surface of a compact disk master by stripping the exposed and developed photoresist, (2) clean the surface of the glass plate used in forming a compact disk master by stripping traces of photoresist and adhesion promoter, metals such as nickel and silver, and their oxides (3) clean the surface of a compact disk stamper by removing polycarbonate residue, other organic contamination and particles, and (4) clean the surface of a compact disk stamper by removing a film of metal oxide.

An example of a system for cleaning compact disk stampers is shown in Figure 8. The light source 76 is an excimer laser producing pulsed deep UV radiation of 248 nm wavelength from a KrF gas fill. A suitable laser for this task would have a pulse energy of 100 millijoules, a pulse rate of 100 Hz and output beam divergence less than 3 milliradians. The output beam 82 is typically rectangular, with dimensions approximately 7mm by 20 mm. The laser is fired under the control of a computer 98 which coordinates the laser firing with the motion of the rotary stage 90. The laser is mounted in a cabinet 102 below the work area, for containment of the UV beam, and to minimize system footprint.

To bring the output beam to the work area, beam steering mirrors 78 and 80 direct the beam upwards through a hole in the table top, then direct the beam horizontally to the beam forming optics head 84. The beam path must be laid out carefully to insure that the horizontal beam enters the optics head in the correct orientation; an additional "jog" may be necessary to accomplish this. For safety reasons, the beam path should be enclosed, for example in aluminum or UV-opaque plastic tubing, from the laser output to the optics head.

The optics head contains one or more diverging cylindrical lenses (not shown) , to spread out the beam in its long axis. These lenses should be fabricated from fused silica for usable transmission in the deep UV wavelength range, and should preferably have antireflective coatings to maximize the overall transmission efficiency of the system. Typically, there will be a single diverging lens with a focal length of -125 mm. The optics head also contains a converging cylindrical lens to bring the beam to a focus in the short axis of the beam, at the stamper surface. The converging lens should also be fabricated from fused silica and have antireflective coatings, and would typically have a focal length of approximately 200 mm.

The optics head additionally contains a mirror set at 45 degrees, to direct the beam downwards to the stamper surface. The lenses and mirror are contained in an enclosed box with openings at the beam input and output locations. This enclosure keeps dust and contamination from the optical elements, and shields the surroundings from any scattered UV light from within the optics head. The stamper 88 lies on a rotary stage 90, which turns at a rate in the range 0.5 to 4 RPM. The stamper should be supported from its edge on a conical surface, or a set of three small cones, so that the supports do not touch the critical surface of the stamper. The rotary stage operates under computer control, and has an indexer so that it can repeatedly stop at a specified angle for loading and unloading, i.e. so that the supports will not block the path of a transfer arm 94.

A gas nozzle 92 is mounted in such a way that it will blow the reactive gas across the surface in the region where the beam strikes. This nozzle should have an elongated output to provide reasonably uniform gas velocity over the reaction region. The gas will typically be provided from a cylinder, which may be located remotely from the system. Before reaching the nozzle, the gas passes through a mass flow controller, which is controlled by the system computer. Typically the gas will be oxygen, with a flow rate in the range 20 to 100 standard cubic feet per hour (SCFH) . In the example shown in Figure 5, the stamper 48 is mounted on a rotary stage 50. The beam 52 is formed and directed in such a way that it forms an intense stripe extending from the inner diameter to the outer diameter of the stamper. By rotating the stamper while pulsing the beam at a high rate (20 to 200 Hz) , the entire stamper surface can be exposed to the beam in a continuous scanning motion.

In another example, as shown in Figure 6, the beam 54 projected onto the stamper surface does not extend from the inner to the outer diameter. This may be the case if the pulse from the light source does not have sufficient total energy to provide the required intensity over this distance. In this case, the beam forming optics 58 are mounted on a linear translation stage 60. The incoming beam 56 is collimated and directed along the axis of the linear stage so that the properties of the beam projected onto the stamper surface do not materially change as the beam forming optics are translated. As the optics are translated, the beam projected onto the stamper surface moves continuously from the inner diameter of the stamper to the outer diameter. The stamper is rotated continuously while the optics are translated. In this way the beam covers the entire stamper surface in a spiral path.

In some cases it may be desirable to split the incoming beam to provide two or more beams on the work surface. Referring to Figure 7, this may be accomplished by placing a beamsplitter 66 in the path of the incoming beam 64. A portion of the beam is directed toward the work surface 62, while the remaining portion of the beam continues towards mirror 68, which directs this portion to a different location on the work surface. Both portions may pass through a single focusing lens 70, or alternatively there may be a separate lens for each portion. An alternative system may be used for cleaning workpieces other than stampers, where the workpiece is not annular in shape, for example masters or glass disks. Referring to Figure 4, the workpiece 42 is mounted on a linear translation stage 44. The beam 46 projected onto the work surface is preferably at least as wide as the workpiece. As the workpiece is translated, the beam is pulsed at a rapid rate, so that the entire surface of the workpiece is exposed to the beam. For removing certain types of contaminants, it may be necessary to perform the processing in an enclosed chamber. This might be necessary if the process is performed at a pressure above or below atmospheric pressure, or if the process requires precise control of the gas composition, or if a hazardous gas is used. For example, stampers may be processed in a hydrogen atmosphere to remove nickel oxide from the surface.

Figure 9 illustrates an enclosed chamber for laser Photo reactive cleaning of stampers. The enclosure 106 is strong enough to withstand the pressure differential between the interior and the ambient, and is leaktight. A transparent window 104 is provided to admit the beam 86. The window is preferably composed of a deep UV-transmitting grade of fused silica. The stamper 88 is mounted on a rotary stage 90. A gas nozzle 92 directs a flow of the reactive gas to the stamper surface. An exhaust port 108 provides an exit path for the gaseous reaction products. The exhaust port leads to a vacuum pump (not shown) to maintain the enclosure at a constant internal pressure. An electrically actuated door 110 opens for loading and unloading the stamper, which is moved into and out of the enclosure by a transfer arm 94. Other embodiments are also within the scope of the following claims. What is claimed is:

Claims

1. A system for cleaning a critical surface used in the manufacture of compact disks, by providing a flowing gas, including a reactant, and delivering pulsed radiation to the critical surface in the presence of the flowing gas, such that the contamination on the critical surface is reacted and removed.

2. The system in claim 1 where the radiation is in the wavelength range 150 nm to 405 nm.

3. The system in claim 2 where the radiation source is an excimer laser.

4. The system in claim 2 where the radiation source is a solid state laser, such as a frequency-multiplied Nd:YAGlaser.

5. The system in claim 1 where the radiation is optically shaped into a blade, and where relative motion is induced between the optics and the surface to be cleaned so that the blade scans the entire surface.

6. The system in claim 5 where the relative motion is rotary.

7. The system in claim 5 where the relative motion is linear.

8. The system in claim 2 where the intensity of a pulse of radiation at the surface to be cleaned is between 10 mJ/cm2 and 500 mJ/cm2.

9. The system in claim 1 wherein the radiation is delivered at an angle other than 90 degrees to the surface.

10. The system in claim 1 wherein the pulsed radiation is delivered as multiple beams.

11. The system in claim 1 where the surface to be cleaned is heated.

12. The system in claim 1 where the cleaning takes place in an enclosed chamber, in which pressure or vacuum is applied for the purpose of accelerating or improving the efficiency of the reaction.

13. The system in claim 1 where the surface to be cleaned is that of a CD master.

14. The system in claim 13 where the material to be removed is photoresist or other organic materials, and the flowing gas includes oxygen or ozone.

15. The system in claim 13 where the material to be removed is silver, and the gas includes a halogen-containing species.

16. The system in claim 1 where the surface to be cleaned is that of a glass plate.

17. The system in claim 1 where the surface to be cleaned is that of a nickel "stamper", "mother", or "father".

18. The system in claim 17 where the material to be removed is polycarbonate or other organic materials, and the flowing gas includes oxygen or ozone.

19. The system in claim 17 where the material to be removed is nickel oxide and the flowing gas includes hydrogen.

20. The system in claim 1 where the surface is cleaned by a sequence of steps in which different gases are used to remove different contaminants.

21. The system in claim 1 where the surface to be cleaned is that of a "stamper", the beam is shorter than the annular width of the stamper, and the entire surface is scanned by rotating the stamper while the beam is moved in a linear fashion from the inner rim to the outer rim of the stamper.

22. The system in claim 1 where the surface to be cleaned is immersed in a liquid while being scanned by the radiation.

23. The system in claim 1 where the surface is cleaned by a sequence of steps, and the surface to be cleaned is immersed in a liquid during one or more of these steps.