Patent Publication Number: US-7900478-B2

Title: Method of manufacturing an ultrathin optical element

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 11/009,705 filed Dec. 13, 2004, now U.S. Patent No. 7,308,185, issued Dec. 11, 2007, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to ultra-thin high-precision glass optics for use, for example, in photolithography tools, and methods of manufacturing ultra-thin high-precision glass optics. 
     BACKGROUND OF THE INVENTION 
     Photolithography systems/tools are used to print features on a substrate in a variety of manufacturing applications. Typically, in operation, a reticle having patterned features is exposed by an illumination source such as, for example, a laser to form images of the features. An optical system projects images of the features onto the substrate. 
     Conventional photolithography systems control the amount of illumination light that reaches the substrate, for example, by light beam alignment, homogenizers, mechanical trimming, and/or diffractive elements. These conventional techniques disrupt the quality of light that reaches the substrate. 
     What is needed is a means for attenuating light in a photolithography system, as well as other optical systems, that does not disturb the quality of light. 
     SUMMARY OF THE INVENTION 
     The present invention provides an ultra-thin high-precision glass optic and method of manufacturing the same. It also provides an attenuation system that includes the ultra-thin high-precision glass optic, which is useful for example in photolithography systems/tools. 
     The ultra-thin high-precision glass optic has an axial thickness that is less than 1,000 microns. A pattern and/or coating is disposed on a surface of the glass optic to provide controlled attenuation of light intensity in an optical system. In one embodiment, the glass optic has a pattern disposed on a first surface that attenuates light. An optional anti-reflective coating covers the patterned surface and/or a second parallel surface. In another embodiment, the glass optic has a dielectric coating rather than the pattern on the first surface that attenuates light. It is a feature of the glass optic that it minimizes secondary effects of aberrations, focal shift, black border effects, scattered light, et cetera. 
     The pattern comprises geometric shapes such as, for example, squares, dots, gratings et cetera that randomly or pseudo-randomly cover the surface. The pattern can be formed, for example, using chrome. Alternatively, a dielectric coating can be used in place of the pattern. In embodiments, the pattern or dielectric coating attenuates incident light between five percent and twenty-five percent per unit area. Other attenuation factors can also be achieved. 
     An optional anti-reflective coating covers the patterned surface and/or a second parallel surface through which light exits the glass optic. The coating improves light transmission through the glass optic by about four to seven percent for each surface coated. 
     In an embodiment, the ultra-thin high-precision glass optic is manufactured by disposing a pattern on a surface of an optical blank or reticle. The patterned surface is coated with a covering to protect the pattern and glass optic. Individual glass optics are cut from the optical blank so that each optical element includes a portion of the pattern. The glass optic is thinned by removing material from the glass optic until it has an axial thickness of less than 1,000 microns. The glass optic is cleaned after thinning and coated with an optional anti-reflective covering. During selected steps, wax may be used to fixture the optical blank and/or optical elements. 
     In a non-patterned embodiment, the ultra-thin high-precision glass optic is manufactured by coating a first surface of the optical blank or reticle with a protective covering. Individual glass optics are cut from the optical blank. The glass optic is thinned by removing material from the uncoated surface of the glass optic until it has an axial thickness of less than 1,000 microns. The glass optic is cleaned after thinning and covered with a dielectric coating on the first surface. An optional anti-reflective coating is applied to the surface through which light exits the glass optic. 
     Further embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
         FIG. 1  is a schematic diagram of a photolithography system that includes an attenuation system having ultra-thin high-precision glass optics. 
         FIGS. 2A-D  are schematic diagrams of attenuation systems that illustrate the ultra-thin high-precision glass optics. 
         FIG. 3  is a schematic diagram of a single ultra-thin high-precision glass optic. 
         FIG. 4  is a schematic diagram illustrating the light transmission profile of an ultra-thin high-precision glass optic. 
         FIG. 5  is a schematic diagram of an optional mask for an attenuation system having ultra-thin high-precision glass optics. 
         FIG. 6  is a flowchart of a method for manufacturing an ultra-thin high-precision glass optic. 
         FIGS. 7A-B  illustrate a flowchart of a more detailed method for manufacturing an ultra-thin high-precision patterned glass optic. 
     
    
    
     The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an ultra-thin high-precision glass optic and method of manufacturing the same. It also provides an attenuation system that includes the ultra-thin high-precision glass optic, which is useful for example in photolithography systems/tools. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. Persons skilled in the art(s) will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to persons skilled in the pertinent art(s) that this invention can be employed in a variety of other applications. 
       FIG. 1  is a schematic diagram of an example photolithography system  100  that includes an attenuation system having ultra-thin high-precision glass optics according to an embodiment of the present invention. Photolithography system  100  includes an illumination source  102 , illumination optics  104 , a pattern  106 , projection optics  108 , and a substrate  110 . 
     Illumination source  102  can be any source that emits light, including ultraviolet light. In an embodiment, illumination source  102  is a laser. Light emitted from illumination source  102  enters illumination optics  104 . 
     Illumination optics  104  condition the light emitted by illumination source  102 . Various optical elements that can be used to condition light such as, for example, diffusing optics, grating optics et cetera are known in the relevant art(s). 
     Illumination optics  104  also include an attenuation system  105 . Attenuation system  105  has multiple ultra-thin high-precision glass optics that attenuate light without disturbing its quality. Attenuation systems according to the present invention are described in more detail below with reference to  FIGS. 2-5 . 
     Pattern  106  is typically a reticle that has a chrome pattern disposed on it or a spatial light modulator that produces a pattern. In the case of a reticle, light from illumination optics  104  passes through the reticle and reproduces the pattern disposed on the reticle on substrate  110 . In the case of a spatial light modulator, light is reflected, for example, off of mirrors to produce a desired pattern on substrate  110 . 
     Projection optics  108  focus the light from pattern  106  onto substrate  110 . In an embodiment, projection optics  108  also reduce the size of pattern  106  in the process of focusing the light on substrate  110 . 
       FIGS. 2A-D  are schematic diagrams that illustrate various embodiments of an attenuation system having ultra-thin high-precision glass optics. These embodiments can be used with photolithography system  100 . 
       FIG. 2A  is a schematic diagram of an attenuation system  200 . Attenuation system  200  includes several ultra-thin high-precision glass optics  202  that extend into a light path or light beam  204 . Each of the ultra-thin high-precision glass optics  202  has an axial thickness that is less than 1,000 microns. In embodiments, the axial thickness is on the order of 0.500 mm, 0.300 mm, 0.200 mm, 0.150 mm, and 0.100 mm. 
     In one embodiment, a pattern is disposed on a surface  203  of each of the glass optics  202  to provide controlled attenuation of light passing through the glass optics  202 . The amount of attenuation is determined by the pattern. An optional anti-reflective coating covers the patterned surface and/or the surface opposite the patterned surface through which light exits glass optic  202 . In another embodiment, each glass optic  202  has a dielectric coating on surface  203 , rather than the pattern, that attenuates light. 
     In the patterned embodiment, the pattern comprises geometric shapes such as, for example, squares, dots, gratings et cetera that randomly or pseudo-randomly cover surface  203 . The pattern can be formed, for example, using chrome. 
     In an embodiment, each of the ultra-thin high-precision glass optics  202  of attenuation system  200  is movable in the plane containing the ultra-thin high-precision glass optics  202 . The glass optics  202  are moved, for example, using actuators (not shown) coupled to the ultra-thin high-precision patterned coated glass optics  202 . By appropriately positioning of the ultra-thin high-precision patterned coated glass optics  202 , it is possible to optimize the uniformity of the light intensity that exposes substrate  110  without disturbing the light quality. 
     The glass optics of attenuation system  200  can be oriented for example either vertically, so that gravity acts perpendicular to the optical axis of each glass optic  202 , or horizontally, so that gravity acts parallel to the optical axis of each glass optic  202 . 
     More details regarding glass optics  202  are provided below with reference to  FIGS. 3-4 . 
       FIG. 2B  is a schematic diagram of an attenuation system  210 . The ultra-thin high-precision glass optics  202  of attenuation system  210  are separated into two groups  202   a  and  202   b . As shown in  FIG. 2B , the two groups  202   a  and  202   b  extend into light path  212  from opposite directions. In an embodiment, each of the ultra-thin high-precision glass optics  202  is movable in the plane containing the ultra-thin high-precision glass optics  202 . The glass optics  202  can be moved either closer to the center of light path  212  or closer to an edge of light path  212  by actuators (not shown) coupled to the ultra-thin high-precision glass optics  202 . In embodiments, attenuation system  210  is installed, exchanged, replaced and/or relocated in photolithography system  100  as a unit or separately. This is also the case for the other attenuation systems described herein. 
       FIG. 2C  is a schematic diagram of an attenuation system  220  that includes ultra-thin high-precision glass optics  202  and an optional mask  222 . As shown in  FIGS. 2C-D , the ultra-thin high-precision glass optics  202  are separated into two groups  202   c  and  202   d . Each of the ultra-thin high-precision glass optics  202  that form a part of these two groups is cantilevered into the light path. 
     In an embodiment, each of the ultra-thin high-precision glass optics  202  of attenuation system  220  is movable in the plane containing the ultra-thin high-precision coated glass optics  202 . The glass optics  202  can be moved either closer to the center of mask  204  or closer to an edge of mask  204 , as illustrated in  FIG. 2D , for example, either during mounting or optionally by actuators (not shown) coupled to the ultra-thin high-precision glass optics  202 . By appropriately positioning of the glass optics  202 , it is possible to optimize the uniformity of the light intensity that exposes substrate  110  without disturbing the light quality. In embodiments, attenuation system  220  is installed, exchanged, replaced and/or repositioned in an optical system, such as photolithography system  100 , as a unit or separately. 
       FIG. 3  is a schematic diagram of a single ultra-thin high-precision glass optic  202 . As shown in  FIG. 3 , in an embodiment ultra-thin high-precision glass optic  202  has a length L, a width W, and a thickness T. The length L is selected so that glass optic  202  is sufficiently long enough to reach into an optical path of light to be attenuated. The width W is selected based on the frequency of light to be corrected. In embodiments, W is on the order of 4 mm, 8 mm, and 12 mm. The thickness T is less than 1,000 microns. In embodiments, T is on the order of 0.500 mm, 0.300 mm, 0.200 mm, 0.150 mm, and 0.100 mm. Due to the thinness of glass optic  202 , ripples, shadows, and light scattering effects are reduced. Other embodiments have different dimensions and/or shapes. 
     In an embodiment, a short end  304  of ultra-thin high-precision glass optic  202  is perpendicular to its long end  306 , as shown for example in  FIG. 3  and  FIGS. 2A-B . In other embodiments, short end  304  is not perpendicular to long end  306 , so that the glass optics  202  can be oriented at an angle as shown in  FIG. 2D . Orienting the glass optics  202  at an angle aids in controlling, for example, exposure of substrate  110  due to any gaps between adjacent glass optics  202 , particularly in a step-and-scan type photolithography tool. 
     A surface  302  of ultra-thin high-precision glass optic  202  has a pattern or dielectric coating disposed on it that attenuates light. In embodiments, the attenuation is between five percent and twenty-five percent. Other amounts of attenuation can also be achieved by varying, for example, the number of geometric shapes of the pattern on surface  302  per unit area or the thickness of the dielectric coating on surface  302 . 
     In patterned embodiments, the pattern is typically composed of several million geometrical shapes such as, for example, squares, dots, gratings et cetera randomly or pseudo-randomly (uniformly) distributed across surface  302 . The geometrical shapes can vary in size. In one embodiment, for example, the pattern is formed using small square chrome islands approximately 1.0-50 microns in size. 
     Ultra-thin high-precision patterned coated glass optic  202  is optionally covered with an anti-reflective coating. In embodiments, the anti-reflective coating improves light transmission through glass optic  202  by about four to seven percent for each surface coated (i.e., coating both the surface where light enters glass optic  202  and the surface where light exits glass optic  202  will improve light transmission by about eight to fourteen percent.). 
       FIG. 4  is a schematic diagram illustrating an example light transmission profile  402  for an embodiment of ultra-thin high-precision patterned coated glass optic  202 . In the embodiment shown, the transmission factor for the entire length of ultra-thin high-precision patterned coated glass optic  202  is 0.85. The amount of attenuation is thus fifteen percent. The value 0.85 is only an example value, and it is not intended to limit the present invention. Other embodiments of the present invention have other transmission profiles. 
       FIG. 5  is a schematic diagram of optional mask  222 . Optional mask  222  has a length L and a width W. The length and width of optional mask  222  are selected, for example, based on the illumination cross section present at pattern  106  (or reticle) of photolithography system  100 . 
     A surface of optional mask  222  is covered with several chrome lines  502 . Chrome lines  502  reduce non-uniformities at substrate  110  that can result from gaps between adjacent ultra-thin high-precision glass optics  202 . Chrome lines  502  are oriented at an angle θ with respect to an edge of mask  222 . The value of angle θ is selected to match the orientation of ultra-thin high-precision glass optics  202  (e.g., the chrome lines are positioned to block light passing between gaps in adjacent glass optics  202 ). In an embodiment, the angle θ shown in  FIG. 5  is equal to zero. In other embodiments, angle θ is greater than zero. 
       FIG. 6  is a flowchart of a method  600  for manufacturing an ultra-thin high-precision glass optic according to an embodiment of the present invention. Method  600  has six steps. 
     In step  602 , a pattern is disposed on an optical blank. In an embodiment, the pattern is composed of several million geometrical shapes such as, for example, squares, dots, gratings et cetera randomly or pseudo-randomly distributed across a surface of the optical blank. The sizes of the geometrical shapes can vary. In one embodiment, the pattern is formed using chrome and a conventional reticle manufacturing process. In embodiments, the number of geometric shapes of the pattern per unit area is varied to achieve a desired transmission or attenuation factor. 
     The optical blank can be any suitable glass optic such as, for example, commercially available reticles. 
     In step  604 , the patterned side of the optical blank is covered with a thin protective coating. The purpose of this coating is to protect the pattern and glass from small chips and other debris produced during step  606 . Any suitable coating that protects the pattern and glass, without harming it, can be used. 
     In step  606 , the patterned optical blank is cut to form optical elements. This cutting step can be performed using any known cutting process such as, for example, saw cutting, machine grinding, hand grinding, milling, et cetera. Step  606  results in the production of optical elements having a desired shape and dimensions, except for axial thickness. 
     In step  608 , the optical elements are thinned, for example, by grinding to a desired axial thickness of less than 1,000 microns. In embodiments, the optical elements are thinned for example to an axial thickness of 0.500 mm, 0.300 mm, 0.200 mm, 0.150 mm, or 0.100 mm. 
     In step  610 , the optical elements are cleaned. This cleaning step preferably includes both mechanical cleaning and chemical cleaning. The cleaning should not interfere with the patterns on the optical elements and/or the glass surface. 
     In step  612 , the optical elements are coated with an optional second coating. In embodiments, this optional coating is an optical coating such as, for example, an anti-reflective coating. As used herein, an optical coating refers to any coating intended to effect the optical properties of the optical elements. Optical coatings include, for example, coatings that stop light reflections at the glass surface or that block or facilitate the transmission of certain wavelengths of light. 
     As will be understood by persons skilled in the relevant art(s), method  600  can be used to manufacture optical elements that use a dielectric coating, rather than a pattern, to attenuate light with certain modifications. For example, when manufacturing optical elements that use a dielectric coating to attenuate light, step  602  is not performed. This is because such optical elements do not have a pattern. When manufacturing such optical elements, the dielectric coating that attenuates light is applied to the optical elements in step  612 . 
       FIGS. 7A and 7B  present a flowchart of a more detailed method  700  for manufacturing an ultra-thin high-precision optic according to an embodiment of the present invention. Method  700  has eight steps. 
     In step  702 , a chrome pattern is disposed on a surface of a reticle blank. In an embodiment, the reticle blank is a commercially available 6″×6″ reticle blank. The pattern is composed of several million geometrical shapes such as for example, squares, dots, gratings and/or other geometrical shapes randomly or pseudo-randomly distributed across the surface of the reticle blank. In embodiments, the number of geometric shapes of the pattern per unit area is varied to achieve a desired transmission or attenuation factor. The pattern is formed using a conventional reticle manufacturing processes. 
     In step  704 , the patterned surface of the reticle blank is coated with a protective covering. In one embodiment, the protective covering is photoresist and enamel paint. Applying a thin 1 micron coating of photoresist (or enamel paint, optical tape, or wax) is typically sufficient to protect the pattern and glass from small chips and other debris produced during cutting and grinding of the reticle blank. The enamel paint is used to aid in fixturing the reticle blank in wax in step  706 . Generally, wax will not stick to photoresist. Other covering can be used. 
     In step  706 , the reticle blank is fixtured in wax. This is achieved by sinking the reticle blank in hot wax and allowing the wax to cool and harden. 
     In step  708 , the reticle blank is saw cut, ground and/or milled to form several optical elements. These optical elements are formed to size, except for their axial thicknesses. 
     Depending on the setup and equipment used to cut and grind the reticle blank, the reticle blank and/or optical elements formed may have to be removed from the wax, cleaned, and refixtured one or more times in step  708  in order to properly size and finish each of the surfaces of the optical elements. 
     After step  708 , the optical elements are preferably checked to verify they are within their required specifications prior to proceeding with step  710 . In an embodiment, specifications which can be checked include the length, width, and end or side angles of the optical elements, plus optical properties. Additionally, the parallelism of the sides of the optical elements can be checked. Other specifications that can be checked for other embodiments will become apparent to persons skilled in the relevant art(s) given the description herein. 
     In step  710 , the optical elements formed from the reticle blank are fixtured in wax for thinning. In an embodiment, several optical elements are fixtured together in a wax having a higher than normal melting point so that they can be thinned together in step  712 . Fixturing multiple optical elements together helps improve the stiffness of the fixtured structure. The higher than normal melting point of the wax prevents the heat generated during thinning from softening and/or melting the wax. Thinning several optical elements together also helps ensure uniformity. 
     In step  712 , the optical elements are thinned to a desired thickness. The optical elements are typically thinned to a thickness of 1,000 microns or less by grinding and polishing. In embodiments, the optical elements are thinned to a thickness on the order of 0.500 mm, 0.300 mm, 0.200 mm, 0.150 mm, or 0.100 mm. During thinning, wax can be removed from between the optical elements, while leaving sufficient wax to hold the optical elements, in order to facilitate polishing the optical elements to a final desired thickness. A surface plate and laser ruler are preferably used to verify the thickness of the optical elements. 
     In step  714 , the optical elements are cleaned. The optical elements are preferably cleaned initially with diatomatious earth and a soapy solution such as Valtron while still fixtured. The optical elements are then removed from the fixture and cleaned with toluene and acetone, or other commercial solvents, to dissolve the wax. At this point, one can check the optical elements for flatness using an interferometer. The optical elements are also cleaned using an acid bath to remove organic materials including the photoresist coating applied in step  704 . The optical elements are also cleaned with methanol in preparation for covering with an anti-reflective coat in step  716 . 
     In step  716 , the optical elements are coated with an anti-reflective coating. This coating improves the transmission of the optical elements. 
     In an embodiment, the optical elements are laid in a metal recess that matches the shape and thickness of the optical elements, and the anti-reflective coating is allowed to flow over the exposed surface of the optical elements. This process ensures a complete and uniform covering of each optical element. If a recess is not used, or if the depth of the recess does not match the thickness of the optical elements, the ends of the optical elements may not be properly coated. 
     As noted herein, various specifications are checked to ensure the optical elements perform as expected. The dimensions checked will depend on the shape and/or size of a particular optical element. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.