Patent Application
20080125014
The invention is directed to a method of manufacturing LCD (“liquid crystal display’) image masks that meet a flatness requirement of less than 40 μm; and in particular the invention is directed to manufacturing high aspect ratio LCD image masks.
The task of obtaining the flatness required for LCD mask is difficult to achieve; particularly in comparison to IC (“integrate circuit”) masks. In the case of LCD masks the problem of obtaining a sub-40 μm flatness specification is compounded by the aspect ratio of the part and the amount of bow or warp it experiences due to its own weight and geometry. For example, for a standard IC mask of fused silica with dimensions of 152.4×152.4×6.35 mm, the mask sees a maximum deflection of 0.18 μm when held horizontally by its edges (see
For the IC mask exemplified above, attaining a specified flatness in the range of 0.5-1.0 μm is a relatively simple issue of conforming the part to a worktable of equal or higher flatness, and uniformly removing material. The back-side support surface does not need to have the flatness of the worktable due to limited deformation of the part during processing. Any non-uniform surface/subsurface damage and related stresses do not significantly act to deform the part due to its aspect ratio being relatively low and the part thus being relatively stiff.
In contrast to the IC mask, the extreme aspect ratio of the LCD image mask described above (e.g., an aspect ratio of 140/1, 1846 mm diagonal) can impact the process of attaining the specified flatness due in part to deflection during grinding, lapping, and polishing. If the back-side support surface is not flat, the part will conform to that surface and uniform material removal will not be achieved no matter how flat the worktable itself may be. As a result of non-uniform material removal, surface/subsurface damage (along with stresses incurred in the part as a result of surface/subsurface damage) is typically not uniform across the part and results in additional deformation due to the fact that the part is so thin that is can bow to alleviate these stresses.
As a result, the standard approach for attaining sub-40 μm flatness for high aspect ratio parts such as LCD image masks is to single-side lap on large planetary tables, allowing the part to rest under its own weight and promoting higher material removal at locations of higher stress (initial contact locations dictated by the part's initial geometry). However, this process is exceedingly slow and offers no means for correction of parts that do not meet specification after initial processing. Conversely, double-side lapping and polishing can be employed but limits attainable flatness due to the part being pressed flat during abrasive material removal, subsequently imparting a non-uniform stress across the part to maintain part contact with the table, with the lapped/polished surface resulting in “springback” once the part is removed from the table.
Although the industry standard for LCD flatness is sub-40 μm, there is a target for the production of final polished flatness levels of 10-20 μm. Since flatness is lost during polishing, a ready-for-polish flatness target of 2-10 μm is desired to enable a manufacturer to attain the 10-20 μm final flatness target. The present invention is directed to a method for producing image masks having a final flatness in the 10-20 μm range of sub-aperture deterministic polishing, lapping and grinding.
In one aspect the invention is directed to a method for manufacturing LCD image masks having a final finished flatness of less than 40 μm. In one embodiment, the invention is directed to LCD image masks having a flatness in the 10-20 μm range. To attain the final polished flatness of 10-20 μm, the method is further directed to manufacturing LCD image masks that have a ready-to-polish flatness in the of 2-10 μm.
The method of the invention is further directed to the use of an optical non-contact instrument that measures the flatness of LCD image masks up to 1200×1400 mm in size and 8-13 mm in thickness. In a preferred embodiment the optical non-contact instrument is a laser interferometer. After measurement, the LCD image mask is ground, lapped and polished as necessary using a CNC (“computer numerical controlled”) instrument that utilizes the interferometric data to grind, lap and polish the surface of the LCD mask to remove high spot and other imperfections to form a LCD image mask surface having a final finished flatness of <40 μm. In preferred embodiments the LCD image mask surface has a flatness in the range of 2-10 μm before final finishing (that is, before any grinding, lapping and polishing) and final finished flatness of <20 μm. In one particular embodiment the final finished flatness is in the range of 10-20 μm. In another embodiment the final finished flatness is <10 μm.
In a further aspect the invention is directed to a method of making very large LCD image masks having a final finished flatness of <40 μm, the method having at least the steps of obtaining a glass article having a length, a width and a thickness suitable for making LCD image masks, wherein the article has a first or front face and a second or back face; suspending the article in the vertical position so that it own weight does not bend the article; imaging both the first and the second face using an optical interferometer and storing the imaging data in algorithmic form; placing the glass article on a flat table with the first face in the upward or top position and the second face is in contact with the table and holding the article in place by its own weight or preferably by application of vacuum to the second or bottom face; grinding/lapping/polishing in a surface profile as calculated by use of the interferometric date obtained for both faces such that the first face, after grinding/lapping/polishing and release from the table, and being re-suspended in the vertical position, has a first face that is flat as may optionally be determined by interferometry. The glass article is then returned to the flat table, this time with the first face in contact with the flat table and second face in the top position, and the article is then again held in place by its own weight or preferably by application of vacuum to the first face; grinding/lapping/polishing the second face in a surface profile as calculated by use of the interferometric data obtained for both faces such that the second face, after grinding/lapping/polishing and release from the table, and being re-suspended in the vertical position, has a second face that is also flat. After both the first and second faces have been ground/lapped/polished, the faces are interferometrically rescanned to determine the flatness of the first and second faces. If sufficient flatness has not been achieved then the steps can be repeated using the new interferometric data to achieve the target degree of flatness. Application of the method of the invention results in a glass LCD image mask having a final flatness of <40 μm. In one preferred embodiment the final flatness is <20 μm. In yet another embodiment the final flatness is <10 μm.
The invention is also directed to LCD image masks having a length, width and thickness of which the length and width are each, independently of the other, greater than 400 mm and the thickness is less than 20 mm. In one embodiment the length and width is each, independently, greater then 800 mm. In a further embodiment the length and width is each, independently, greater than 1000 mm. In another embodiment the length and width is each, independently, greater then 1200 mm. In further embodiments the thickness of the LCD image mask is less then 15 mm. In additional embodiments the thickness is less than 10 mm. In all the foregoing embodiments the LCD image masks of the invention have final flatness of >40 nm, preferably <20 nm. In yet another embodiment the foregoing LCD image masks have a final flatness of <10 nm. Any glass suitable for LCD image masks can be used in practicing the invention. Preferred glasses are fused silica glass, high purity fused silica glass and silica-titania glass containing 5-10 wt. % titania. An example of high purity fused silica glass is a glass meeting or substantially meeting the specifications of the HPFS® brand high purity fused silica sold by Corning Incorporated.
a-3d is a schematic of the LCD image mask processing using a sub-aperture, deterministic tool.
The invention is directed to LCD image masks and to a method for manufacturing LCD image masks that meet flatness requirements of sub-40 μm for part sizes as large as 1220×1400 mm, and even larger as may be needed. While the material presently used for LCD image masks are fused silica and high purity fused silica glass, other glass materials such an ultra-low expansion glass containing 5-10 wt. % TiO2 doped silica (SiO2) may offer advantageous material properties for future applications, either existing or new.
Compared to IC masks, the issue of attaining a flatness specification is confounded by the aspect ratio of the part and the amount of bow or warp the part sees due to its own weight and geometry. For a standard fused silica IC mask with dimensions of 152.4×152.4×6.35 mm, the mask sees a maximum deflection of 0.18 μm when held horizontally by its edges (see
For IC masks, attaining a specified flatness in the range of 0.5-1.0 μm is a relatively simple issue of conforming the part to a worktable of equal or higher flatness, and uniformly removing material. The back-side support surface does not need to have the flatness of the worktable due to limited deformation of the part during processing. Any non-uniform surface/subsurface damage and related stresses do not significantly act to deform the part due to its aspect ratio being relatively low and thus being relatively stiff.
However, for the LCD image mask, for example, one with dimensions of 1220×1400×13 mm, the extreme aspect ratio of the mask (140/1 for the foregoing mask having a 1846 mm diagonal) can impact the process of attaining the specified flatness due to deflection of the mask (also called a “part” herein) during grinding, lapping and polishing. If the back-side support surface is non-flat, the part will conform to that surface and uniform material removal will not be achieved no matter how flat the worktable is. As a result of non-uniform material removal, surface/subsurface damage (along with stresses incurred in the part as a result of surface/subsurface damage) are not typically uniform across the part and result in added deformation again due to the fact that the part is so thin that is can bow to alleviate these stresses.
As a result, the standard approach for attaining sub-40 μm flatness for high aspect ratio parts such as LCD image masks is to single-side lap on large planetary tables, allowing the part to rest under its own weight and promoting higher material removal at locations of higher stress (initial contact locations dictated by the part's initial geometry). However, this process is exceedingly slow and offers no means for correction of parts that do not meet specification after initial processing. Conversely, double-side lapping and polishing can be employed but limits attainable flatness due to the part being pressed flat during the use of abrasive materials, subsequently imparting a non-uniform stress across the part to maintain contact with the table, with the lapped/polished surface resulting in “springback” once the part is removed from the table.
The invention at hand relates to the use of sub-aperture deterministic micro-grinding in combination with large-scale interferometric techniques to topographically map and correct bulk flatness for high aspect ratio glass parts. Utilizing the invention, one can obtain final finished flatness of <20 μm and also overcome other difficulties typically encountered in handling large, high aspect ratio parts. For example, traditional grinding/lapping/polishing procedures are exceedingly time consuming for larger parts, offer no opportunity to correct out-of-specification parts, and may not be a manufacturing-sound approach for generating high-aspect ratio parts due to stress-induced warp. The invention overcomes the disadvantages of traditional methods by combining deterministic material removal with high resolution topographical mapping of the work piece.
In the first step according to the invention, the LCD image mask having a first or front face 20 and a second or back face 30 (See
a-3d are a schematic illustrating LCD image mask 10 processing using a sub-aperture deterministic tool and the interferometric data previously obtained.
The side view of
Using the interferometric data, the first face 20 of the mask is ground, lapped and polished to a concave shape 20′ as illustrated in
The grinding, lapping and polishing can be done using methods known in the art and a CNC instrument that utilizes the interferometric data. Such methods include ion milling, magneto-rheological finishing, and deterministic polishing. Deterministic grinding and/or polishing are preferred, including options such as that provided by Zeeko Limited (http:\\//www.zeeko.co.uk/). Articles have appeared in the technical literature describing polishing using the new type of instrumentation such as the Zeeko instruments. Exemplary of this literature include D. D. Walker et al, “The Zeeko/UCL Process for Polishing Large Lenses and Prisms”, Proc. SPIE, Vol. 4411 (2002), pp. 106-111; D. D. Walker et al, “Commissioning of the First Precessions 1.2m CNC Polishing Machines for Large Optics”, Proc. SPIE Vol. 6288 (2006), 62880P-1 to 8. [Paper 62880, pages 1-8); Graham Peggs et al, “Dimensional metrology of mirror segments for extremely-large telescopes”, Proc. SPIE Vol. 5382 (2004), pp. 224-228; D. D. Walker et al, “Recent development of Precessions polishing for larger components and free-form surfaces”, Proc. SPIE Vol. 5523 (20040, pp. 281-289; D. D. Walker et al, “New Results from the Precessions Polishing Process Scaled to Larger Sizes”, Proc. SPIE Vol. 5494 (2004), pp 71-80; and H. Pollicove et al., “Deterministic Manufacturing Processes for precision Optical Surfaces”, Key Engineering Materials Vols. 2383-239 (2003), pp. 533-58.
Deterministic grinding polishing is best described as the use of a CNC tool with a contact head significantly smaller than the workpiece. The tool face can be any traditional polish surface including but not limited to metal, abrasive particles imbedded or otherwise mounted into a metal or resin, polyurethane with or without imbedded abrasive, Teflon, flexible resin-based films with or without imbedded abrasive, or pitch. Abrasive-filled fluids/slurries, water, or other liquids can be used as carrier fluids for removing heat and/or grinding/lapping/polishing debris from the tool/workpiece interface. The surface profile machined into the surface is determined (selected) based on interferometric data recorded during analysis of the given workpiece surface when held in a zero-stress state.
The options for the deterministic polishing step include (but are not limited to) the following technologies, all of which utilize interferometric data to identify highpoints on the work piece requiring removal to attain the desired surface geometry.
The invention is also directed to LCD image masks having a length, width and thickness of which the length and width are each, independently of the other, greater than 400 mm and the thickness is less than 20 mm. In one embodiment the length and width is each, independently, greater then 800 mm. In a further embodiment the length and width is each, independently, greater than 1000 mm. In another embodiment the length and width is each, independently, greater then 1200 mm. In further embodiments the thickness of the LCD image mask is less then 15 mm. In additional embodiments the thickness is less than 10 mm. In all the foregoing embodiments the LCD image masks of the invention have final flatness of >40 μm, preferably <20 μm. In yet another embodiment the foregoing LCD image masks have a final flatness of <10 μm. Any glass suitable for LCD image masks can be used in practicing the invention. Preferred glasses are fused silica glass, high purity fused silica glass and silica-titania glass containing 5-10 wt. % titania. An example of high purity fused silica glass is a glass meeting or substantially meeting the specifications of the HPFS® brand high purity fused silica sold by Corning Incorporated.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
