METHOD OF PRODUCING CONTAMINATION-FREE PELLICLES

Abstract: The method of producing contamination-free pellicles and the pellicles produced by the method are adapted to prevent particulate contamination to the interior surface of a pellicle film. The preferred embodiment of the method includes the steps of selecting an appropriate planar cover sheet, applying a layer of contact adhesive to one surface of the cover sheet and detachably installing the cover sheet on a pellicle with the adhesive surface facing inward and forming a sealed interior volume with the pellicle. The contamination-free pellicles (10) include a preselected cover sheet (12) attached to a conventional pellicle (14) including a support ring (16) and a transparent film (18). The cover sheet (12) is provided with a contact adhesive (20) on the interior surface to capture and retain particulate matter within the sealed interior volume (19). The contamination-free pellicles (10) substantially reduce the risk of particulate contamination to the film (18) and are thus extremely useful in photomask processes in the electronics industry.

Description: METHOD OF PRODUCING COt * TZ_MINATION - FREE PE__LICLES TECHNICAL FIELD The present invention relates generally to protecting surfaces from dust or other contamination and more specifically to methods for preventing contamination to the interior surface of the transparent film portion of a pellicle. ■'. The predominant current usage of the present invention relates to prevention of- contamination, during shipment and storage, of the interior film surfaces of pellicles used in the semiconductor industry. BACKGROUND ART Integrated circuits and other elements in the semicon¬ ductor industry are manufactured using pho.toilluminatioή tech¬ niques incorporating devices known as photomasks. Photomasks are transparent sheets of material having an opaque pattern deposited or otherwise placed, on one surface. The pattern presents an op- tical blockage of electromagnetic radiation which activates photoresist chemicals on the surface of the material onto which the circuit design is to be placed. The manufacture of integrated circuits requires very pre¬ cise techniques. Accordingly, very tight tolerances are necessary in the production and use of photomasks. For example, any dust adhering to the surface of the photomask may interfere with the radiation path and cause the production of a defective circuit element. Therefore, it is -very important to protect the photo¬ mask surface from dust and other contamination. One common method utilized for photomask protection is a device known as a pellicle. A pellicle includes a thin trans¬ parent film or membrane of material which is bonded to the sur¬ face of the photomask via a support element such that a gap exists between the surface of the photomask and the film. A sealed volume is created between the surfaces. The film surface protects the photomask surface from contamination. The separation between the film and the photomask permits the surface " of the film to be out of focus with respect to the surface of the mask during photo- illumination. Therefore, dust which adheres to the surface of the film will not be in focus with respect to the mask and will have a significantly lessened effect on the circuit resulting from illumination of the mask. The construction and use of the pellicles is particularly described in United States Patent No. 4,131,363, issued to V. Shea, et al. The Shea, et al patent discloses the use and manufacture of pellicles similar to those in current use in the industry. A typical pellicle includes a spacer and support ele¬ ment generally in the form of a ring having a discrete thick¬ ness. A film is stretched across the ring and bonded to one edge about the entire .circumference while the other edge of the support ring is adhered to the surface of the photomask. Increasing use and popularity of pellicles, combined with continually increasing demands for precision, have led to situations wherein pellicle application alone is insufficient contamination protection to meet industry standards. Dust contamination of the pellicle film itself may be sufficient to cause imperfections in the final product. Therefore, it has become important not only to protect the surface of the photomask from contamination, but to also protect the surfaces of the pellicle film from contamination. The present invention relates to a novel and significantly improved method of pre¬ venting contamination of the interior surface of the pellicle film, as compared with prior art methods. Typically, the prior art method of cleaning the surface of a pellicle film has involved the use of an air or gas jet. Compressed gaseous material is blown across the surface of the film in order to dislodge any dust or other material that may have become adhered to that surface. This method has been reasonably successful with regard to the exterior surface of the film. However, the interior surface of the film is sur- rounded by the ridge provided by the support ring. The ridge provides an aerodynamic barrier which causes turbulence and prevents effective cleaning of the interior film surface once it has been bonded to the support ring. Therefore, it is ex¬ tremely important to prevent any dust or contamination from becoming adhered to the interior surface of the pellicle film at any time after it has been bonded to the support ring. OMPI Yλ ^ IPO None of the prior art methods have been successful in providing a clean and uncontaminated inner surface of the pellicle such that it is completely uncontaminated at the time that the pellicle is installed upon the photomask. An effective method of protecting the pellicle during manufacture, storage, shipment and manipulation is greatly desired in the industry. DISCLOSURE OF INVENTION Accordingly, it is an object of the present invention to provide a method for packaging and sealing pellicles such that no contamination is permitted to adhere to the interior surface of the film. It is another object of the present invention to provide a method which will maintain the interior surface of a pellicle film in an uncontaminated condition from the time of manufacture until the time when the pellicle is mounted on the photomask. It is a further object of the present invention to provide a method of packaging a pellicle for shipment such that the in¬ terior film surface is completely protected from contamination. The present invention is a method of producing a protected pellicle which is free from contamination of the interior film surface. The invention is especially adapted for use during the manufacturing, shipping, and storage of pellicles and is adapted to be used continually until the time the pellicle is installed on a photomask. Briefly, the preferred embodiment of the present invention is a method for packaging and protecting pellicles from particulate contamination, particularly on the interior film surface.. The method includes construction of a peelable cover sheet adapted to adhere to the opposite edge of the support ring from that of the pellicle film. The cover, which is in the form of a sheet, has an interior surface including a uniform coating of an adhesive material. The adhesive surface of the cover is directed to the interior of the pellicle when the cover is installed. The ad¬ hesive surface of the cover is selected to be a contact adhesive which acts to capture a very significant portion of the parti¬ culate matter trapped in the volume between the cover sheet and the pellicle film. In one preferred embodiment, the cover sheet is applied to the support ring before the pellicle film is applied. The protective cover is adapted to be readily removed when desired. In the preferred method, the removal of the cover sheet occurs immediately prior to bonding the pellicle to the photomask. It is an advantage of the present invention that the sealed volume provided by the cover prevents intrusion of particulate contamination during storage and transfer. It is also an advantage of the present invention that the adhesive protective cover substantially eliminates par- ticulate contamination of the interior surface of the pellicle film. It is another advantage of the present invention that the application of the cover prior to the installation of the pellicle film on the support ring permits the pellicle film to be installed in a pristine condition and maintained in such condition until use. It is a further advantage of the present invention that the method may be accomplished using relatively inexpen¬ sive and readily available materials. It is still another advantage of the present invention that the cover sheet may be removed at the very last moment before installation of the pellicle on the photomask without danger of contamination from the cover sheet. It is still a further advantage of the present in- vention that the protective cover permits inspection and testing of the pellicle film without the necessity of removal of the cove . These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment as described herein and as illu¬ strated in the several figures of the drawing. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a bottom perspective view of a protected pellicle according to the method of the present invention; O PI * >* > Fig. 2 is an inverted cross sectional view, taken alon line 2-2, of the protected pellicle of Fig. 1. BEST MODE OF CARRYING OUT INVENTION The best currently known mode of practicing the pre- sent invention is a method for the protection of the interior surface of pellicle films from particulate contamination. The method is utilized in the integrated and semi-conductor manu¬ facturing industries in order to insure clean and uncontaminate pellicles for use with photomasks. The method produces pellicl which are protected from contamination. The preferred embodiment of a protected pellicle accord ing to the present invention is illustrated in Fig. 1 and de¬ signated by the general reference character 10. The protected pellicle 10, shown in a bottom perspective view, includes a protective cover 12 formed according to the method of the present invention and installed upon a conventional pellicle 14 The conventional pellicle 14 includes a circular suppor ring 16 having a transparent pellicle film or membrane 18 ad¬ hered to the entire circumference one axial edge thereof such that the film 18 extends across the entire aperture in the support ring 16 and seals the aperture completely. In the preferred, embodiment the protective cover 12 is bonded to the axial surface of the support ring 16 opposite the pellicle film 18. The cover extends across the entire aperture in the support ring 16 such that, together with the support ring 16 and the pellicle film 18, it forms a sealed interior volume 19. The primary purpose of the invention is to maintain the interior volume 19 and the interior surface of the pellicle film 18 free of particulate contamination. The support member 18 illustrated is in the form of a circular ring. While this is the most common form of pellicle support member other shapes are also utilized. Any planar closed loop support member is adaptable to the method of the present invention. As is also shown in Fig. 1, the interior surface of a preferred embodiment of the protective cover 12 is provided with a continuous layer of contact adhesive 20. The contact adhesive 20 operates to capture and retain particulate matter that comes into contact with it. Thus any matter which trapped in the interior volume 19 and which moves in any way within the volume 19 is likely to impact the contact adhesive 20 and remain adhered thereto. Agitation of the entire pro¬ tected pellicle 10 may help dislodge particles which are ad- hered to the film surface and cause them to flow within the interior volume 19 until they are captured by the contact ad¬ hesive 20. This may improve the contamination protection properties of the method. The edge of .the support ring 16 which engages the protective cover 12 is also provided with a layer of powerful contact adhesive 22.- The purpose of the adhesive 22 on the support ring 16 is predominantly to bond the pellicle 14 to the surface of the photomask. The powerful adhesive 22 is ordinarily too sticky to be amenable to ready removal of the cover 12. Therefore, the preferred embodiment 10 employs a spacer ring 24 which is selected to be more easily peeled from the powerful adhesive 22. Another possible method is for the contact adhesive 20 and the powerful adhesive 22 to be the same and to provide a ring of releasing agent on the cover 12 so that it will readily peel off. the ring 16 when desired. In the preferred embodiment, the protective cover 12 is provided with a grasping extension 26 along one portion of the edge thereof, such that it may be easily removed from the pellicle 14 when such removal is desired. Although it is not required, the remainder of the protective cover 12 is ordi¬ narily trimmed about the circumference of the support ring 16 for easy handling and storage. Thus the only protrusion is the grasping extension 26 placed at a preselected location. Fig. 2 illustrates the protected pellicle 10 in a cross sectional view, taken along line 2-2 of Fig. 1 and in¬ verted therefrom. In Fig. 2, which is not in proportion with respect to Fig. 1, the relative axial spacing of the elements is illustrated. The illustration of Fig. 2 shows that the exterior surface of pellicle film 18 provides an unobstructed planar surface. This surface is entirely appropriate for cleaning by application of a gas jet across the surface to sweep any debris from the film 18. However, as is shown in Fig. 2, the interior surface of the film 18 , although equally planar, "§_RE O PI is obstructed at the edges by the support ring 16. Thus the interior volume 19 is subject to turbulence and is inappro¬ priate for precise cleaning of the film surface 18 by gas jet means. Thus it is ordinarily not efficacious to remove par- ticulate matter from the interior surface of the film 18 at the time of application or use. The method of providing the protected pellicle 10 illustrated in the drawing includes a series of steps. These steps include the selection of an appropriate material for the planar cover sheet, - the application of a layer of.contact ad¬ hesive to one surface of the cover sheet, the placement of a spacer ring 24, the detachable installation of the adhesive protective cover to one surface of the support ring of the pellicle, and optionally, trimming the cover sheet to conform with the lateral dimensions of the support ring. The selection of the planar cover sheet material takes into account various necessary properties. Initially, the cover sheet must be uniformly planar on its interior surface for appropriate results. Furthermore, it must be pliable such that it is easily installed upon and detached from the support ring. Additionally, the cover sheet material must be selected to have minimal light reflection characteristics such that it does not interfere with the electromagnetic radiation inspection techniques utilized to test the pellicle film for impurities and defects. Finally, to obtain the greatest economic benefit, the most readily available and inexpensive material should be selected. The properties regarding transmittal and reflection of electromagnetic radiation are particularly important for the cover sheet material. This is true because one of the primary opportunities for particulate contamination to become attached to the inside of the pellicle film under prior art processes is during the inspection period. Therefore it is highly desirable to undertake the film inspection while the protective cover remains in place. Therefore, the protective cover must not in any way interfere with the inspection. A flat black material appears to be the most successful. OMPI The preferred embodiment of the protected pellicle according to the method of the present invention utilizes polyester printed to present a black interior surface and covered with a doublesided transfer tape as a cover sheet material. Other possible materials include transparent mylar adhesive tape and 3M Company's 3M447 adhesive sheets. Non adhesive cover sheets may also be utilized if a lower level of protection is required or a particularly contamination free manufacturing environment is available. The contact adhesive must also have various properties. Initially, it must be such that it will capture and retain par¬ ticulate matter which impacts on its surface. Furthermore, it must be capable of being spread over the cover sheet material in a reasonably even fashion. The contact adhesive must also be such that it will retain its properties over long periods of storage and transportation. The adhesive also must not have an adverse effect on the inspection process, therefore it must not reflect or interfere with the electromagnetic radiation used for inspection nor can it be degraded by expo- sure to such radiation. It is extremely important that the adhesive be selected such that no conditions which would ordinarily occur during the life of the protected pellicle will cause the adhesive ' to lose its adhesive properties or to dry up, flake or otherwise become a contaminant in itself. Desirable adhesives are those which are already present on the preferred cover sheet materials described above. One further possibility is black vinyl adhesive applied to an appropriate cover sheet material. One successful method of manufacturing and maintaining protective cover sheets prior to installation upon support rings is to utilize a peel-away cover on the sheet itself. The adhesive surface is thus prevented from contamination during storage and the adhesive surface is only exposed at the time of installation. A freshly peeled adhesive surface is virtually certain to be contamination-free and thus will pro¬ vide no intrinsic contamination. Another possible method is to manufacture the cover sheets such that the interior sur¬ faces of two sheets are juxtaposed and they may be peeled apart to reveal two uncontaminated adhesive surfaces. OMPI Since the interior surface of the preferred cover sheet includes a layer of contact adhesive, as does the support ring, it is a simple matter to install the cover sheet to the spacer ring and then onto the support ring of a pel.licle simply by contact. It is desirable that this procedure, and all other in the process, be undertaken in conditions which are as free from particulate contamination as possible. This minimizes the chance of contamination passing from other elements of the protected pellicles to the film. The presently preferred timing of the application of the protective cover to the suppor ring is in the manufacturing process of the pellicle, prior to the bonding of the transparent film to the support ring. With the adhesive cover already in position prior to bonding of the film, a sealed clean volume is created immediately upon film bonding. Since, prior to bonding, the film presents a planar surface appropriate for various types of cleaning, it may be cleaned and then bonded to the ring in a pristine state. The already installed adhesive cover thus prevents any con¬ tamination subsequent to installation. The -cover sheet may be precut to shape or trimmed to fit the circumference of the support ring after it has been installed. The precise trimming is optional and not a nec¬ essary part of the invention. However, for shipping purposes and appearance it is desirable to maintain as much symmetry as possible. It is desirable to leave a grasping extension at one or more points around the circumference of the cover such that, when the time comes to remove the protective cover and install the pellicle on the photomask, the cover may be easily peeled back. Although the present invention has been described above in terms of the best presently known mode of carrying out the method of the invention, those skilled ih the art will readily recognize numerous modifications and alterations which may be made while retaining the teachings of the invention. Accordingly, the above disclosure is not to be construed as limiting. The appended claims are therefore to be inter¬ preted as encompassing the entire spirit and scope of the invention. OMPI tøATI INDUSTRIAL APPLICABILITY The method of producing dustless protected pellicles of the present invention, and the pellicles produced there¬ by, have immediate and widespread industrial applicability. Since their introduction in the late nineteen-seventies pellicles have become a very valuable tool utilized in the integrated circuit and semiconductor manufacturing industry. It is expected that photomasks utilized without pellicles will become increasingly rare. In light of the increasing precision required in the industry it is expected that pellicles, and particularly to¬ tally uncontaminated protected pellicles, will have* wide¬ spread marketability. Accordingly, the superior characteristics of pellicles constructed and packaged according to the method of the present invention should result in significant industrial utilization of the invention.

Claims: IN THE CLAIMS 1. A method of manufacture of a contamination- protected pellicle, in steps comprising: A. selecting a planar cover sheet; 5 B. applying a layer of contact adhesive to one surface of the cover sheet; and C. detachably installing the cover sheet upon an axial edge of the support member of a pellicle so as to close the aperture and 10 form, with the pellicle, an enclosed volume, the orientation of the cover sheet being such that the layer of contact ad¬ hesive is within said volume. 15 2. The method of claim 1 wherein: the step of installing the cover sheet upon the support ' member is inserted into the pellicle manufacturing process prior to the installation of the pellicle film upon the opposite edge of 20 the support member. 3. The method of claim 1 and further including: D. trimming the cover sheet such that its lateral dimensions conform to the exterior circum¬ ference of the support member. ' 25 . The method of claim 3 wherein: a grasping extension is provided along a portion of the circumference of the cover sheet such that it may be readily peeled off from the support member. 5. The method of claim 1 wherein: the cover sheet is selected such that it does not interfere with electromagnetic radiation in¬ spection processes performed on the pellicle film during the period while the cover sheet is in- position. 6. The method of claim 5 wherein: the cover sheet and adhesive are selected to be nonreflecting. 7. The method of claim 1 wherein: said contact adhesive is selected to retain its adhesive properties over extended time per¬ iods and despite exposure to the electromagnetic radiation utilized during pellicle film inspection. 8. The method of claim 1 wherein: said contact adhesive is selected to retain complete positional integrity such that it does not fragment or migrate subsequent to application. 9. The method of claim 1 wherein: said contact adhesive is selected to capture and retain as much as possible of the particulate matter coming into contact therewith. 10. The method of claim 1 wherein: the step * of installing includes stretching the cover sheet such that it' is uniformly planar across the interior of the aperture formed by support member. 11. The method of claim 4 wherein: the cover sheet is selected such that it does not interfere with electromagnetic radiation inspection processes performed on the pellicle film during the period while the cover sheet s ^ X -E . Q * y_PI_ in position; and >~ W WiIpPoO" said contact adhesive is selected to retain its spatial and positional integrity and its ad¬ hesive properties over extended time periods, agitation and exposure to electromagnetic radiation 5 such as that utilized in pellicle film inspection, and to capture and retain all particulate matter impacting said adhesive. 12. The method of claim 11 wherein: 1.0 the cover sheet and adhesive are selected to be nonreflecting. 13. A method of producing a contamination-free pellicle, in steps comprising: 5 I. selecting a planar cover sheet; and II . detachably installing the cover sheet upon an axial edge of the support member of pellicle so as to form an enclosed volume with the pellicle film and support member. 0 14. The method of claim 13 wherein: the cover sheet is selected such that it does not interfere with electromagnetic radiation in¬ spection processes performed on the pellicle film during the period while the cover sheet is in 5 position. 15. A protected pellicle in which the interior film surface is protected from particulate contamination, comprising: a pellicle including a ringlike support 0 member and a planar transparent film stretched across said support member and bonded thereto along one axial edge; and a cover sheet, detachably bonded to the opposite axial surface of said support member 5 from said film so as to form a sealed interior volume therewith. 16. The pellicle of claim 15 wherein: the interior facing surface of the cover" sheet is provided with a layer of contact adhesive. 17. The pellicle of claim 16 wherein: said contact adhesive is selected to retain its adhesive properties over extended time per¬ iods and despite exposure to the electromagnetic radition utilized during pellicle film inspection. 18. The pellicle of claim 16 wherein: said contact adhesive is selected to retain complete positional integrity such that it does not fragment or migrate subsequent to application. 19. The pellicle of claim 15 wherein: the cover sheet is selected such that it does not interfere with electromagnetic radiation in¬ spection processes performed on the pellicle film during the period while the cover sheet is in position. 20. The pellicle of claim 16 wherein: the cover sheet is selected such that it does not interfere with electromagnetic radiation inspection processes performed on the pellicle film during the period while the cover sheet is in position; and said c ntact adhesive is selected to retain its spatial and positional integrity and its ad- hesive properties over extended time periods, agitation and exposure to electromagnetic radiation such as that utilized in pellicle film inspection, and to capture and retain all particulate matter impacting said adhesive. 21. The pellicle of claim 15 wherein: the cover sheet further includes a grasping extension along its circumference such.that it may be readily detached from the support member.

Inventors: YEN YUNG-TSAI, YEN, YUNG-TSAI
Applicants: YEN YUNG TSAI, YEN, YUNG-TSAI
Application year: 1984
Main classification: G03B27
IPC codes: G03F1, H01L21