MOUNTING PREPARATION FOR OPTICAL MEMBRANE

Abstract: A method of preparing a plural-layer optical membrane for adhesion mounting on a support frame, such as a ring. The method is employed with such a membrane whose mounting side is formed with a first layer structure and an underlying layer structure. The method involves the selective removal, in the pattern of the ring on which the membrane is intended to be mounted, of a substantially matching shape in the first-mentioned layer structure, thus to expose a portion of the higher-adhesion-affinity underlying layer structure.

Description: This invention relates to a method for preparing a plural-layer optical membrane for adhesion mounting on a support frame, such as a ring. In recent years, optical membranes called pellicles have played an important role in the manufacture of semiconductor wafers that are used in various integrated semiconductor circuits. As is well known by those skilled in the art, pellicles protect (against contamination) photo masks which are used in the various photolithography steps required in wafer preparation. The typical pellicle takes the form of an extremely thin optical membrane which is supported on (bonded to) a ring-like frame, the entirety of which is then placed over a wafer during photolithography. Obviously, it is important that the membrane be securely bonded to the frame so that it will function properly. In some instances, the membrane is prepared with plural layers, for example to include, on what might be thought of as the mounting side of the membrane, an anti-reflective coating to improve optical transmissivity. In some of these instances, the layer structure thus prepared on the mounting side in a membrane has an adhesion affinity for a typical adhesive, such as any conventional 5-minute epoxy, which is lower than that of one or more of the immediately underlying layers. This lower affinity makes adhesion-bonding of the membrane to a support ring somewhat difficult, and sometimes subject to later failure. Bonding to a support ring is also sometimes a problem where the adhesion between the multiple layers is not as good as it might be. Where such internal adhesion is weak, mounting a support ring to the outer-most layer could promote tearing away or cracking between the layers of a multi-layered membrane. A general object of the present invention, accordingly, is to provide a unique mounting preparation technique which avoids such a bonding problem in a very practical and satisfactory manner. According to a preferred method of practicing the invention, and having in mind the outline of the ring structure which is to become the membrane's support structure, a portion of the layer structure on the mounting side of the membrane is selectively removed to expose the better bonding capability of an underlying layer. The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a simplified, fragmentary elevation illustrating application of the technique of the invention to a three-layer optical membrane, where selective layer removal takes place in two of the three layers. Fig. 2 is similar to Fig. 1, except that it illustrates application of the invention to remove but one of the three layers in the same kind of optical membrane. Fig. 3 is a view on a much smaller scale, in plan, showing a support ring, with a small portion removed, bonded to the mounting side in an optical membrane, such as the membranes shown in Figs. 1 and 2. Turning now to the drawings, and referring first of all to Fig. 1, indicated generally at 10 is an optical membrane which is to become part of a pellicle. Membrane 10 includes a base layer, also referred to herein as a second layer structure, 12, an intermediate, or inner, layer 14, and thereabove (in Fig. 1) an outer layer 16. Layers 14, 16 herein are also referred to collectively as a first layer structure. In the membrane now being described, layer 12, which has a thickness of about 2.8-microns, is formed of nitrocellulose. Layer 14, which has a thickness of about 700-Angstroms is formed of an organic polymer, such as polyvinylnaphthalene. Layer 16, which collectively with layer 14 forms an anti-reflective coating for the membrane on the membrane's mounting side, is formed of an organic fluorocarbon compound, prepared as follows. 1000-ml of a product made by the Minnesota Mining and Manufacturing Company (3M), known as FC-721 was concentrated to 200-ml by low pressure evaporation. 1800-ml of another 3M product known as FC-77 was added to the concentrate. After filtration, the resulting 2000-ml mixture was ready for application, as by spin casting, to layer 14. In the assembly of membrane 10 with a support ring to form a final pellicle, it is intended that the chosen ring be bonded adhesively on the mounting side of the membrane, which, in membrane 10, is the upper side thereof in Fig. 1. In the case now being described, the particular ring to which bonding is to occur has a somewhat rectangular outline, and is illustrated at 18 in Fig. 3. Because of the specific material characteristics of layers 12, 14, 16, it turns out that layer 12 has a higher adhesion affinity for the above-indicated kind of adhesive than layer 14, and that layer 14 has a higher adhesion affinity than layer 16. Accordingly, bonding of the ring to the membrane will be most secure with the bond occurring on the face of layer 12, slightly less secure with bonding occurring on the face of layer 14, and even less secure with bonding occurring on the face of layer 16. Following the method of the present invention, and according to one way of practicing the same, portions of layers 14, 16 are removed, as will be explained, to enable bonding on the face of layer 12. According to another way of practicing the invention, a portion of layer 16 only is removed to enable bonding on the face of layer 14. Still with reference to Fig. 1, with knowledge of the mounting outline of ring 18, and using a suitable guide template, a first step takes place with the solvent-removal of a pattern in layer 16 which substantially matches the outline and configuration of ring 18. This is done preferably by guiding a suitable device, such as cotton-tipped wand, about such a template, with the wetted tip applying an appropriate solvent, like that described above, in a controlled path which thus removes material from layer 16 in the desired pattern. Such a wand is shown generally at 20 in Fig. 1. The next step in the practice of the invention with regard to what is shown in Fig. 1, is passage of the wand along the same path to apply another appropriate solvent, toluene in the case of polyvinylnaphthalene, to remove, essentially, a matching pattern from the material in layer 14. What results after this operation is that there exists a pattern of removed material, indicated by the dashed lines in Fig. 1, in layers 14, 16, which pattern, on the inside, substantially exactly matches the inside outline of ring 18. Thus, the upper surface of layer 12 in Fig. 1 is properly exposed for adhesion to the ring. One should note that, in the patterned removal of material from layers 14, 16, care need only be taken to assure that the inside outline thereof substantially exactly matches the inside outline of ring 18. Control over the outside outline is unnecessary since, ultimately, the expanse of the membrane outside the perimeter of the ring will be removed. Shifting attention for a moment to Fig. 3, here ring 18 is shown in a condition bonded to layer 12. Where a portion of the ring has been removed, the dashed lines indicate the trough of removed materials in layers 14, 16. Fig. 2 illustrates another way of practicing the invention where material is removed only from layer 16 to enable the ring to be bonded to layer 14. Obviously, this is performed by making a pass of the wand over the upper surface of the membrane in the manner described for the first step outlined above in connection with preparation of the membrane in Fig. 1. This pass of the wand removes material only from layer 16, and such is indicated by the dashed lines in Fig. 2. Thus, there is proposed by the invention a unique and very simple technique to maximise bonding between a support ring and a pellicle membrane in instances where a plural-layer membrane has, on its mounting side, a relatively poor adhesion capability.

Claims: A method of preparing a plural-layer optical membrane for adhesion mounting on a support frame having an inside mounting outline, where the membrane has a mounting side formed with a first layer structure which overlies a second layer structure, characterized in that a portion of such first layer structure is removed from the membrane's mounting side in a pattern substantially matching the frame's inside mounting outline and by such removal exposing, substantially in the shape of such pattern, an expanse of the underlaying layer. A method according to claim 1, characterized in that the removal is accomplished by solvent application to such first layer structure. A method according to claim 1 or 2, characterized in that the first layer structure includes outer and inner layers. A method according to claim 3, characterized in that a portion of the inner layer in the first layer structure is removed in a pattern corresponding to the removed portion of the outer layer. A method according to claim 4, characterized in that the removal of the inner layer is accomplished by solvent application to the inner layer. A method according to claim 5, characterized in that the removal steps are accomplished by applying a solvent-soaked wand to the portions to be removed.

Inventors: YEN, YUNG-TSAI
Applicants: YEN, YUNG-TSAI
Application year: 1986
Main classification: G03F1
IPC codes: G03F1, H01L21