Title:
Polishing method for extreme ultraviolet optical elements and elements produced using the method
Kind Code:
A1


Abstract:
The invention is directed to polished glass substrates suitable for extreme ultraviolet lithography. The elements are silica-titania glass elements having a coefficient of thermal expansion of 0±30×10−9/° C. or less, and containing 5-10 wt. % titania. The polished elements have a mid-spatial frequency peak-to-valley roughness of <10 nm and a high-spatial frequency roughness of <0.20 nm average roughness. The invention is further directed to a method of for producing optical elements suitable for extreme ultraviolet lithography (“EUVL”), the method having at least the steps of providing a glass substrate in the shape of the desired optical element and polishing the shaped substrate using a high abrasive slurry flow rate of >2.0 ml/cm2/min. Generally the flow rates are in the range of 2.0-10 ml/cm2/min. Glass substrates suitable for extreme ultraviolet lithography element have a coefficient of thermal expansion of 0±30×10−9/° C. or less. A particular glass suitable for EUVL use is silica-titania glass containing 5-10 wt. % titania.



Inventors:
Arserio, Gregory John (Walworth, NY, US)
Sabia, Robert (Corning, NY, US)
Sylvia, Brian Daniel (Rochester, NY, US)
Application Number:
11/699287
Publication Date:
06/05/2008
Filing Date:
01/29/2007
Primary Class:
Other Classes:
430/4, 451/41, 501/27
International Classes:
B24B1/00
View Patent Images:
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Primary Examiner:
NGUYEN, DUNG V
Attorney, Agent or Firm:
CORNING INCORPORATED (CORNING, NY, US)
Claims:
1. An element suitable for extreme ultraviolet lithography, said element comprising a silica-titania glass having a mid-spatial frequency roughness of <10 nm and a high-spatial frequency roughness of <0.20 nm average roughness; wherein said silica-titania glass contain 5-10 wt.% titania and has a coefficient of thermal expansion of 0±30×10−9/° C.

2. The element according to claim 1, wherein said element a high-spatial frequency roughness of <0.17 nm average roughness.

3. The element according to claim 2, wherein said element is selected from the group consisting of lenses, prisms, mirror and image masks.

4. The element according to claim 1, wherein said element has a mid-spatial frequency roughness peak-to-valley value in the range of 6-8 nm.

5. An optical element suitable, said element comprising a glass selected from the group consisting of silica-titania glass having a titania content in the range of 5-10 wt.%; silica-germania glasses' silica-alumina-germania glasses; and a silica-titania glass containing one or a plurality of additional metal oxide components and having a titania content in the range of 2-20 wt. %; wherein said element has a mid-spatial frequency roughness of <10 nm and a high-spatial frequency roughness of <0.20 nm average roughness.

6. The optical element according to claim 4, wherein said element is selected from the group consisting of lenses, prisms, mirrors, display screens and masks.

7. A method of for producing optical elements suitable for extreme ultraviolet lithography, said method comprising at least the steps of: providing a glass substrate in the shape of the desired optical element; and polishing the shaped substrate using a high abrasive slurry flow rate of >2.0 ml/cm2/min. wherein said glass substrate has a coefficient of thermal expansion of 0±30×10−9/° C.

8. The method according to claim 7, wherein said abrasive slurry flow rate is in the range of 2-10 ml/cm2/min.

9. The method according to claim 7, wherein said abrasive slurry flow rate is in the range of 3.1-4.5 ml/cm2/min.

10. A method of for producing optical elements suitable for extreme ultraviolet lithography, said method comprising the steps of providing a glass substrate suitable for making extreme ultraviolet lithographic element; shaping the glass substrate into said element; and polishing the surfaces of the optical element using a high abrasive slurry flow rate, said flow rate being >2.0 ml/cm2/min.

11. The method according to claim 10, wherein said flow rate is in the range of 2.0-10 ml/cm2/min.

12. The method according claim 10, wherein the flow rate is in the range of 3.1-5.4 ml/cm2/min.

13. The method according to claim 10, wherein said slurry contains a polishing abrasive selected from the group consisting of cerium oxide, alumina, silicon carbide, colloidal silica and diamond.

14. The method according to claim 13, wherein the abrasive is cerium oxide.

15. The method according to claim 10, wherein providing a glass substrate means providing a silica-titania substrate contain 5-10 wt. titania.

16. A method of for producing optical elements suitable for extreme ultraviolet lithography, said method comprising the steps of providing boule of a silica-titania glass containing 5-10 wt. % titania, obtaining from said boule glass substrates of size suitable for forming the desired optical elements, shaping said substrates into said optical elements blanks; and grinding, lapping and/or polishing said blanks into the shape of the desired optical elements; wherein said polishing is carrier out using a high abrasive slurry flow rate, said flow rate being in the range of 2.0 ml/cm2/min.

17. The method according to claim 16, wherein said flow rate is in the range of 2.0-10 ml/cm2/min.

18. The method according to claim 16, wherein said flow rate is in the range of 3.1-4.5 ml/cm2/min.

19. The method according to claim 16, wherein polishing means polishing the surfaces of said element to a mid-spatial frequency roughness of <10 nm.

20. The method according to claim 16, wherein polishing means polishing the surfaces of said element to a high-spatial frequency roughness of <0.20 nm average roughness.

21. The method according to claim 20, wherein the high-spatial frequency roughness of <0.20 nm average roughness is 0.17 nm average roughness.

22. The method according to claim 160, wherein said abrasive slurry contains an abrasive selected from the group consisting of alumina, diamond, silicon carbide, colloidal silica and cerium oxide.

23. The method according to claim 22, wherein the abrasive in said abrasive slurry is cerium oxide.

Description:

PRIORITY

This application claims the priority of U.S. Provisional Application No. 60/861,922, filed Nov. 30, 2006, and titled “A POLISHING METHOD FOR EXTREME ULTRAVIOLET OPTICAL ELEMENTS AND ELEMENTS PRODUCED USING THE METHOD.”

FIELD OF THE INVENTION

The invention is directed to polished silica-titania glass elements, and in particular to polished silica-titania glass elements suitable for use in extreme ultraviolet lithography, and to a polishing method for making such elements.

BACKGROUND OF THE INVENTION

Glass materials such as silica-titania (SiO2—TiO2) glasses that are used for extreme ultraviolet lithography (“EUVL”) elements have been produced by various methods. In some of these methods, for example the flame hydrolysis method, striae can occur during the laydown process, resulting in discrete layers of silica-titania glass that can have a variation of ±0.1 wt. % TiO2. During cool-down from glass forming temperatures differential stress relaxation from the slightly different CTE (“coefficient of thermal expansion”) values in each layer generates stress. Although these small, localized glass chemistry deviations do not affect bulk CTE, the uniformity of material removal during polishing can be affected due to the fact that these areas of localized stress can be susceptible to differential reaction in aqueous environments. For most glasses, chemically-assisted polishing such as the methods that use cerium oxide (“ceria”) as the polishing material are beneficial in generating low roughness surfaces with negligible subsurface damage. However, materials with differential stress (for example, silica-titania and other “doped” glasses) can be sensitive to ceria's combined chemical-mechanical material removal mechanism. As a result, high surface roughness can occur.

Typically, and most economically, the silica-titania glass that is used for making optical elements for EUVL applications is made in the form of a large glass boule that is then cut, sawed, or otherwise formed into a plurality of separate pieces or parts (e.g., optical blanks) which are then formed into the desired optical elements. However, it is nearly impossible to extract or form such pieces without having at least some striae break the surface. When standard polishing processes are used to polish the piece into the desired optical element it has been found that these standard methods result in finished pieces that have mid-spatial frequency (“MSF”) topographical defects that have been found to perfectly coincide with varying TiO2 level in the glass (the difference in TiO2 between different layers), and thus the stress between discrete striae layers. If the pieces are intended as for use as a mask materials for EUVL, these striae-induced MSF defects are a limiting factor for the qualification of any polished part.

An extreme example of striae-induced mid-spatial frequency roughness is shown in FIG. 1 where a SiO2—TiO2 sample was extracted from a boule at an angle φ of 5° relative to the perpendicular to the surface 12 of the boule. After polishing as described below and analyzing the sample using a scanning white light interferometer, the sample was found to exhibit a peak-to-valley roughness of 16 nm and a RMS roughness of 2.5 nm for the following polishing regime. The SiO2—TiO2 sample was processed using the traditional glass polishing method of dripping a slurry of cerium oxide abrasive particles dispersed in water onto a polishing platen. The abrasive slurry flow was 30 ml per min for a 38 cm table, translating to a flow of 0.026 ml/cm2 per minute (ml/cm2/min). Peak-to-valley values of >20 nm have been observed for SiO2—TiO2 samples produced by flame hydrolysis methods in which material was removed perpendicular to the laydown surface (with the least amount of striae breaking the surface) and processed with a similarly low abrasive slurry flow rate polishing process which are within the standard flow rates use in the industry, such flow rates being in the approximate range of 0.02 to 0.1 ml/cm2/min. This observed striae-induced topographical effect is believed to be the result of chemically-induced sub-surface reactions that accelerate preferential material removal at locations of high stress between striae due to the variation in TiO2 content between striae layers.

Since the standard polishing methods (herein also called “conventional polishing methods”) produce parts that are not acceptable for use in EUVL processes, it is desirous to have a new polishing method that does not produce unacceptable surface roughness or sub-surface defects. The present invention is directed to a new method of polishing EUVL parts in which the flow rates of the have been greatly increased over the rates presently used in the industry and to the elements and optical element (for example, lenses, prisms, masks and display screens) produced using the method.

SUMMARY OF THE INVENTION

The invention, in one aspect, is directed to a method of polishing silica-titania glass surfaces, the method having at least the step of using a slurry or solution of a polishing agent, the slurry or solution being supplied to the glass surface(s) at a flow rate greater than 1.0 ml/cm2/min.

The invention, in another, is directed to a method of polishing one or more surfaces of complex, multi-component glasses, the method having at least the step of using a slurry or solution of a polishing agent, the slurry or solution being supplied to the glass surface(s) at a flow rate greater than 1.0 ml/cm2/min.

The invention, in a further aspect, is directed to a method of polishing silica-titania glass surfaces, the method having at least the step of using a slurry or solution of a polishing agent, the slurry or solution being supplied to the glass surface(s) at a flow rate greater than 2.0 ml/cm2/min.

In another aspect the invention is directed to a method of polishing silica-titania glass surfaces, the method having at least the step of using a slurry or solution of a polishing agent, the slurry or solution being supplied to the glass surface(s) at a flow rate in the range of 3.1-4.5 ml/cm2/min (3.8-5.0 liters per minute for a 38 cm diameter table).

In a further aspect the invention is directed to a method of polishing glass surfaces, and parts or optical elements that are suitable for EUVL applications and are made from such glass. The inventions has at least the steps of providing a glass material suitable for making EUVL parts or elements, forming such elements and polishing the light transmitting surfaces of the glass using a high flow rate of an abrasive aqueous slurry solution. The abrasive material in the slurry can be any material suitable for polishing glass; for example, diamond, silicon carbide, alumina, colloidal silica, cerium oxide and other polishing abrasives known in the art. In accordance with the invention the slurry is supplied to the surface of the piece being polished at a flow rate greater than 2.0 ml/cm2/min. In one embodiment the flow rate is greater than 2.5 ml/cm2/min. In another embodiment the flow rate is in the range of 2.0 to 10 ml/cm2/min. In a further embodiment the flow rate is in the range of 3.1-4.5 ml/cm2/min. Polishing pads, mechanical equipment, and other materials, equipment and supplies used in conventional glass polishing can be used with the invention.

An additional aspect the invention is directed to a polished optical element suitable for extreme ultraviolet lithography, the element being made of a silica-titania glass having 5-10 wt. % titania content, and the element having a mid-spatial frequency peak-to-valley roughness less than 10 nm and a high-spatial frequency of less than 0.20 nm average roughness. Preferably the high-spatial frequency of less than 0.17 nm average roughness.

An another aspect the invention is directed to a polished glass element, the element being made from any glass material that exhibits zones of varying composition that result in stress. For example, such glasses include silica-titania glass having 5-10 wt. % titania content and more complex, multi-component glass compositions that exhibit striations in composition. Examples of such complex, multi-component glasses include, without limitation, silica-germania glasses, silica-alumina-germania glasses, and a silica-titania glass with one or a plurality of additional metal oxide components and a titania content in the range of 2-20 wt. %. In all the foregoing the element has a mid-spatial frequency peak-to-valley roughness less than 10 nm and a high-spatial frequency of less than 0.20 nm average roughness. Preferably the high-spatial frequency of less than 0.17 nm average roughness. The glasses have a coefficient of thermal expansion of 0±30×10−9/° C. Examples of such elements include, without limitation, lenses, mirrors, prisms, masks (for example, EUVL masks, LCD image masks, and IC masks) and display screens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an interferogram illustrating striae-induced mid-spatial frequency roughness of a surface of a part polished using a standard (conventional) low slurry flow rate.

FIG. 2 is an interferogram illustrating striae-induced mid-spatial frequency roughness of a surface of a part polished using a slurry flow rate in accordance with the present invention.

FIG. 3 is an atomic force microscopy (“AFM”) image of the silica-titania glass sample of FIG. 2 illustrating that high-spatial frequency roughness of a surface of a the part polished according to the invention is less than 0.17 nm average roughness (“Ra”).

FIG. 4 is an illustration of a boule and a cylinder of material being cored from the boule along an axis that is at an angle of 5° from the perpendicular to the surface of the boule.

FIG. 5 illustrates how a cylinder is cut into parts, with striae breaking through the faces for exemplary purposes, for optical elements or masks.

FIG. 6 is a comparison of mid-spatial frequency peak-to-valley data for parts extracted and polished using the method of the invention (squares) and a conventional method using low slurry flow rates (diamonds).

DETAILED DESCRIPTION OF THE INVENTION

As has been mentioned above, the standard methods of polishing ultra-low expansion glass surfaces, and in particular silica-titania glass surfaces, using “low” slurry flow rates can produce surfaces that have sufficient roughness such that they are unsuitable for EUVL applications. Ultra-low expansion glasses are those having a CTE of less than 0±30×10−9/° C. (0±30 ppb/° C.) as exemplified by ULE® glass from Corning Incorporated. As used herein the term “ULE” without the trademark symbol is used to signify “ultra-low expansion glass” generally, and in particular silica-titania glass and glass surfaces, that can be polished using the invention to produce optical elements with finished surfaces having extremely low mid-spatial frequency (“MSF”) and extremely low high-spatial frequency (“HSF”) roughness values. Striae in ULE is well recognized as a limiting factor in attainment of surfaces planar in the MSF range due to preferential material removal at locations of high stress between striae layers that leads to frequency defects as shown in FIG. 1. Utilizing high slurry flow rates in accordance with the present invention and contrary to industry practice, it is possible to produce surfaces free of striae-induced defects, while further allowing for HSF range values that approximate those required for EUVL mask applications. To-date we have demonstrated ULE surfaces that have MSF peak-to-valley values equivalent to fused silica surfaces and HSF roughness values of <0.17 nm Ra. [It should be noted that fused silica does not exhibit MSF polishing defects due to its binary composition, and is thus an ideal standard for comparison to determine the absolute lowest mid- and high-spatial frequency roughness attainable for a flame hydrolysis derived glass composition. Best-effort fused silica surfaces typically exhibit MSF range peak-to-valley values of 3.5-5.0 nm and HSF range values of 0.10-0.15 nm Ra.]

The present invention describes a novel method to prepare striae-free ULE surfaces, specifically for applications (such as for EUVL masks) that call for extremely low mid- and high-spatial frequency roughness values. In accordance with the invention a high slurry flow rate is used to promote a greater degree of lubrication between the workpiece and abrasive particles/platen surface relative to the conventional industry flow rates. Such a high slurry flow rate is non-traditional and is at least one order of magnitude greater than flow rates traditionally employed for planar part processing (flow rates normalized to table size). It is believed that the increased slurry flow rate acts to prevent preferential material removal at locations of high stress in the surface caused by striae. It is further believed that the use of high slurry flow rates minimizes the stress induced by contact between the abrasive particles and the workpiece is minimized. In addition, the use of high slurry flow rates minimizes the diffusion of water into any surface defects and minimizes aqueous-based reactions that promote heterogeneous material removal at locations of different titania content. The invention trades the rate of material removal for improved surface quality in terms of preventing the formation of striae-induced topographical features as may be seen by a comparison of FIG. 1 and FIG. 2.

The invention has particular relevance to the polishing of glass materials that exhibit zones of varying composition that result in stress; for example, TiO2—SiO2 ULE glass containing 5-10 wt. % titania and more complex, multi-component glass compositions that exhibit striations in composition. Examples of such complex, multi-component glasses include, without limitation, silica-germania glasses, silica-alumina-germania glasses, and a silica-titania glass containing one or a plurality of additional metal oxide components and having a titania content in the range of 2-20 wt. %. With regard to the silica-titania glasses containing one or a plurality of additional metal oxide components and having a titania content in the range of 2-20 wt. %, the additional metal oxide components, without limitation, can be the oxides of germanium, aluminum, lanthanide metals, alkali metals and alkaline earth metals. In addition, the silica-titania glasses containing one or a plurality of additional metal oxide components and having a titania content in the range of 2-20 wt. % have a coefficient of thermal expansion is less than that of fused silica. The surfaces any glass product, whether intended for lithographic or other uses, made from any of the foregoing complex, multi-component glass compositions have, after polishing, a mid-spatial frequency peak-to-valley roughness less than 10 nm and a high-spatial frequency of less than 0.20 nm average roughness. Preferably the high-spatial frequency of less than 0.17 nm average roughness. The surfaces of any glass element or product made from the multi-component glasses described in this paragraph are polished to the foregoing roughness values using slurry flow rates according to the invention as they are described herein for ULE glass.

FIG. 1 is an interferogram showing striae-induced mid-spatial frequency roughness, after conventional polishing, on the silica-titania surface of a part that was extracted from a silica-titania boule at an angle φ of 5° relative to the surface of the boule as illustrated in FIG. 4 described below. The plane of the material laydown is along perpendicular 20 to the surface and (see FIG. 4) and the sample was extracted from the boule and formed into an optical blank as illustrated in FIG. 5. The optical blank was then further ground and lapped into shape of the desired optical element before being polished using conventional techniques. Peak-to-valley values of >20 nm were observed for silica-titania surface removed an angle φ of 5° relative to the perpendicular of the boule, and processed with a low flow rate abrasive slurry polishing process that is standard in the industry. [See FIG. 6—Conventional.) This striae-induced topographical effect is believed to be the result of chemically-induced sub-surface reactions that accelerate preferential material removal at locations of high stress between striae due to the variation in TiO2 between striae layers.

FIG. 2 is an interferogram showing striae-induced mid-spatial frequency roughness of a silica-titania surface on a part made from a cylinder of material that was extracted from a silica-titania boule at an angle φ of 5° relative to the surface of the boule as illustrated in FIG. 4 described below. The plane of the material laydown is along perpendicular 20 to the surface (see FIG. 4) and the sample was extracted from the boule and then processed described in the preceding paragraph, except that the part was polished using a high slurry flow rate according to the invention. Peak-to-valley values of <10 nm were observed for silica-titania surface removed at an angle φ of 5° relative to the perpendicular of the boule, and processed using the high flow rate abrasive slurry polishing process of the invention. Comparing FIGS. 1 and 2 clearly shows that polishing using the high flow rates of the invention greatly reduces surface roughness. The parts illustrated by FIG. 2 have a roughness comparable to fused silica parts polished by conventional methods. The part used to generate FIG. 2 has MSF peak-to-valley roughness in the range of 6-8 nm, compared to the range of 10-28 nm for the conventionally polished part as illustrated in FIG. 6.

FIG. 4 is an illustration of a glass boule 10 having a top surface 12, a bottom surface 14 and a thickness defined by side 16. The normal to the surface of the boule is represented by line 20. Also illustrated in FIG. 4 are two lines 40 that are exemplary representations of the striae that can be present in the boule. The numeral 30 indicates how a cylindrical sample of material is cored from the boule at an angle φ as indicated above. Once cored, the cylinder is cut, sawed and otherwise processed to make a sample part (blank). These samples were further processed into shaped optical elements and then polished using conventional polishing methods and the method of the invention that utilizes high slurry flow rates. The polishing results for elements polished using the convention methods and the method of the invention are shown in FIGS. 1-3. FIG. 5 illustrates how cylinder 30 is cut into parts along lines 50 for further processing into optical blanks, elements and masks. For simplicity, only two parts A and B are shown, though more parts could be made from a single cylinder depending on the thickness of the boule, the thickness of the desired cored part and the thickness of the desired part. Further, to illustrate the invention the cylinder was cut so that striae represented by lines 60 would break through the surface. In normal practice the cylinder or other form would be obtained from the boule in the direction of the perpendicular to the boule's surface to minimize striae breakthrough. In the present case the cylinder was obtained at an angle so that striae would break through the surface in order that the utility of polishing method of the invention would be clearly indicated by the interferometric results. As mentioned above, the results indicate that polishing using conventional methods result in a finished part as illustrated by FIG. 1 whereas using the high slurry flow rates of the invention produces parts with much lower mid- and high-spatial frequency roughness.

FIG. 3 is an atomic force microscopy (“AFM”) image of a ULE surface that was polished using high slurry flow rates according to the invention. The AFM image shows that HSF roughness is less than 0.17 nm average roughness (Ra) when the part is polished according to the invention. The sample used to create the AFM image is the same sample used to generate the interferometric data shown in FIG. 2.

FIG. 6 is a direct comparison of mid-spatial frequency (MSF) peak-to-valley data for ULE (silica-titania) parts extracted one block of material and polished (1) according to the invention (squares) and (2) using the conventional polishing methods such as those used for polishing fused silica parts (diamonds). As illustrated in FIG. 6, the ULE parts polished using the conventional methods have MSF peak-to-valley values in the range of 10-28 nm whereas those polished according to the invention have a MSF peak-to-valley value in the range of 6-8 nm, thus demonstrating the utility and novelty of the method invention and the parts produced using the method of the invention.

Several observations and hypotheses concerning polishing reactions and removal rates can be offered to explain the beneficial impact of increasing fluid flow between the workpiece and polishing surface on striae mitigation.

First, it has been directly observed that striae-induced topographical features in ULE (i.e., mid spatial frequency roughness, as shown in FIG. 1) is primarily impacted by chemical reactions that take place during polishing. The use of the conventional slow drip cerium oxide abrasive technique was found to promote striae-induced topography, whereas polishing using a purely mechanical abrasive (alumina) was found not to promote striae-induced topography. The reason such a purely mechanical polishing process was not implemented for ULE is due to the fact that both mid- and high-spatial frequency surface roughness were unacceptably high. Cerium oxide has been reported to promote a combined chemical-mechanical material removal mechanism, one that does not work effectively without the presence of hydroxide but can be used to produce a damage-free surface when applied in a fully aqueous environment (Cook, J. Non-Cryst. Solids, 120, pp 152-171, 1990; Sabia & Stevens, Mach. Sci. & Tech., 4, 2, pp 235-251, 2000). Proper application of cerium oxide requires the abrasive to be thermally treated by the manufacturer to be of similar hardness to the glass being processed so as to maximize the mechanical interaction without producing excessive damage.

Second, it is well understood that increasing the load applied to a workpiece during polishing increases removal rate. For those applications that do not require “ideal” surfaces, surface roughness and sub-surface damage can be sacrificed for increased material removal rate, and the exchange is viewed as being acceptable. Increased material removal is accomplished by increasing the abrasive-to-surface interactions, increasing mechanical interactions, and limiting the effectiveness of the slurry carrier fluid to remove debris. Increased sliding contact is promoted between debris and abrasive particles over the surface by minimizing the extent in which debris is carried away from the interaction zone without subsequently damaging the surface. Increased surface and sub-surface damage can further assist in accentuating material removal by propagation of damage deeper into the surface through continual abrasion, and by the presence of compressive and tensile stress zones around damage areas that promote aqueous reactions similar to those that occur during aqueous-based slow crack growth in SiO2 glasses.

Third, some authors have postulated that tensile stresses applied to a glass surface increase the rate of chemical reaction of the workpiece surface and sub-surface in aqueous environments by affecting the extent in which water can diffuse into the surface (N. Nogami and M. Tomozawa, Phys. Chem. Glasses 25, pp 82-85, 1984; N. Nogami and M. Tomozawa, J. Am. Ceramic. Soc., 67, pp 151-154, 1984). If correct, by minimizing stress between the abrasive particle and workpiece, chemical interactions are limited to the near surface and thus promote reaction with the glass surface on the atomistic scale. Cook has postulated an atomistic-level reaction mechanism for cerium oxide abrasive polishing of glass, supported in part by observations reported by Sabia and Stevens (Cook, J. Non-Cryst. Solids, 120, pp 152-171, 1990; Sabia & Stevens, Mach. Sci. & Tech., 4, 2, pp 235-251, 2000). Cook also reported experimental results that supported Nogami and Tomozawa'a theory, showing that the amount of stress applied to a surface can increase the depth of the diffusion of water into the surface during polishing, such depths being in the range of 1 to 20 nm (Cook, J. Non-Cryst. Solids, 120, pp 152-171, 1990).

Comparing the striae-induced topographical defects for ULE shown in FIG. 1 to the defect-free surface shown in FIG. 2, stress-induced aqueous-based sub-surface reactions offer an explanation the beneficial effect of the novel, high slurry flow rate. Surface stress under traditional polishing conditions (i.e., relatively low slurry flow rates) accentuates the aqueous-based reaction deep into the surface where stress from non-uniform titania content between striae layers further impacts the rate of reaction in terms of glass solubility. Conversely, as shown in FIG. 2 the novel, high slurry flow rate of the invention limits stress at the workpiece surface and minimizes water diffusion into the subsurface preventing the formation of striae. The results presented in the Figures and described herein indicate that the high slurry flow rates of the invention are successful in preventing striae-induced topographical features.

Having set forth the foregoing detailed explanation of the invention, in one aspect the invention is directed to a method for producing optical elements suitable for extreme ultraviolet lithography. The method has at least the steps of providing a glass substrate in the shape of the desired optical element and polishing the shaped substrate using a high abrasive slurry flow rate of >1.0 ml/cm2/min. In one embodiment the abrasive slurry flow rate is >2.0 ml/cm2/min. The glass substrate suitable for EUVL has a coefficient of thermal expansion of 0±30×10−9/° C. In an additional embodiment of the invention the abrasive slurry flow rate is in the range of 2-10 ml/cm2/min. In another embodiment the abrasive slurry flow rate is in the range of 3.1-4.5 ml/cm2/min.

In another aspect the invention is directed to a method for producing optical elements suitable for extreme ultraviolet lithography; the method having at least the steps of providing a glass substrate suitable for making extreme ultraviolet lithographic elements; shaping the glass substrate into an extreme ultraviolet lithographic element; and polishing the surfaces of the optical element using a high abrasive slurry flow rate, said flow rate being >2.0 ml/cm2/min. The glass substrate suitable for EUVL has a coefficient of thermal expansion in the range of 0±30×10−9/° C. or less. In one embodiment of the invention the abrasive slurry flow rate is in the range of 2-10 ml/cm2/min. In another embodiment the abrasive slurry flow rate is in the range of 3.1-4.5 ml/cm2/min. The slurry used in the method contains a polishing abrasive selected from the group consisting of cerium oxide, alumina, silicon carbide, colloidal silica and diamond. The concentration of the abrasive in the slurry is in the range used in conventional polishing techniques. While the preferred glass substrate is a silica-titania substrate containing 5-10 wt. titania, the invention can be used to polish any EUVL-suitable glass substrate, typically a glass substrate having a coefficient of thermal expansion in the range of 0±30×10−9/° C. or less can be used for EUVL elements.

In another aspect the invention is directed to a method for producing optical elements suitable for extreme ultraviolet lithography in which the method has at least the steps of providing boule of a silica-titania glass containing 5-10 wt. % titania; obtaining from said boule glass substrates of size suitable for forming the desired optical elements; shaping said substrates into said optical elements blanks; and grinding, lapping and/or polishing said blanks into the shape of the desired optical elements; wherein said polishing is carried out using a high abrasive slurry flow rate, the flow rate being in the range of 2.0 ml/cm2/min. In one embodiment the flurry flow rate is in the range of 2.0-10 ml/cm2/min. In another embodiment the slurry flow rate is in the range of 3.1-4.5 ml/cm2/min. When the element is polished using the method of the invention one can polish the element to a mid-spatial frequency roughness of <10 nm and a high-spatial frequency roughness of <0.20 nm average roughness. Preferably the high-spatial frequency roughness of <0.20 nm average roughness is 0.17 nm average roughness. The abrasive in the abrasive slurry is selected from the group consisting of alumina, diamond, silicon carbide, colloidal silica and cerium oxide. Cerium oxide is the preferred abrasive.

In another aspect the invention is directed to optical elements suitable for extreme ultraviolet lithography, the elements being made of a silica-titania glass having a titania content in the range of 5-10 wt. %. The elements have been polished to a mid-spatial frequency roughness of <10 nm and a high-spatial frequency roughness of <0.20 nm average roughness. The coefficient of thermal expansion is in the range of 0±30×10−9/° C. Exemplary optical elements, without limitation, include lenses, prisms, mirrors and image masks.

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.