Patent Application
20080280539
1. Field of the Invention
The present invention is directed generally to optic fabrication. More particularly, the present invention relates to the fabrication of optical components, such as mirrors, for use in lithographic processing.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a an array of individually controllable elements, a mask, a reticle, or the like, can be used to generate a circuit pattern corresponding to an individual layer of an IC, flat panel display, or other device. This pattern can be transferred onto all or part of the substrate (e.g., a glass plate, a wafer, etc.), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. The imaging can include the processing of light through a projection system, which can include optical components such as mirrors, lenses, beam splitters, and the like.
The optical components used in lithographic processing can be fabricated out of a variety of materials. However, many materials are not currently chosen for manufacturing reasons or due to recent performance needs. Two materials currently used widely in the lithography industry are Zerodur® (produced by SCHOTT Corporation) and Ultra Low Expansion (ULE®) glass (produced by Corning Inc.). Each of these materials exhibits a near-zero coefficient of thermal expansion (CTE). An optical component made of either of these materials will not change shape appreciably during exposure, which is of particular importance for certain types of lithographic processing, such as extreme ultra-violet (EUV) processing.
A cause of image flare is Mid-Spatial Frequency Roughness (MSFR), i.e., periodic surface errors found in optical components, which can scatter light near the intended image. In projection systems, EUV projection systems in particular, it is desirable to reduce image flare as much as possible. This is mainly to reduce the image flare's impact on contrast. Another level of spatial frequency roughness in optical components, known as High-Spatial Frequency Roughness (HSFR), can impact transmission by scattering light outside of the image field. Having MSFR and/or HSFR that are too high can cause clarity, resolution, and background light issues, among many other problems. It is ideal to have MSFR and HSFR be as low as possible for each optical component. The ranges of MSFR and HSFR for a particular optical component are dependent on the component size. Therefore, what is considered MSFR and HSFR will vary among the optical components of a particular system. Currently, MSFR is typically on the order of millimeters (mm) to micrometers (μm) and HSFR is typically on the order of micrometers (μm) to nanometers (nm).
Although both Zerodur® and ULE® glass exhibit otherwise ideal properties for lithographic (particularly, EUV) processing, they each have intrinsic material properties that make achieving the desired MSFR and HSFR extremely challenging. For example, Zerodur® is a multiphase material. Some optics manufacturing processes, such as ion beam figuring (IBF), affect phases at different rates, which essentially limits the achievable MSFR/HSFR. ULE® glass is a multilayer material. Some optics manufacturing processes work differently on the layers, causing striae, thereby creating another challenge for meeting the desired MSFR/HSFR specification(s).
Currently in industry, thin film coatings are sometimes applied to polished optical surfaces for the purpose of creating a resultant smoother surface, in other words, reducing MSFR and/or HSFR. However, the methods currently used in industry do not guarantee that the necessary ranges of MSFR and/or HSFR are reached during fabrication.
Therefore, what is needed is a method for producing an optical component, and the optical component produced, exhibiting desired ranges of MSFR and HSFR, which are conducive to providing low scatter and low image flare when used during lithographic processing.
In one embodiment of the present invention, a method of fabricating or preparing an optical component, such as a mirror, using an amorphous oxide coated substrate is presented. An amorphous oxide coating, such as SiO2 or SiO, for example, is applied to an optical substrate. An assessment of surface roughness of the coated surface is performed. The coated surface is then polished based on the assessment. Initial assessments can be conducted and polishing can be performed based on those initial assessments prior to applying the coating in order to better prepare the surface for the coating. Each of the assessments can assess the surface's Mid-Spatial Frequency Roughness (MSFR), High-Spatial Frequency Roughness (HSFR), or both. An initial polish of the surface can be conducted first to provide aspherization, for example. The performing of the assessments, polishing and/or the coating can be computer-controlled. An optical component fabricated in this manner is also presented.
This process is ideal in the fabrication of an optical component (a mirror, in particular) that is formed from a substrate with a near-zero coefficient of thermal expansion. Such a substrate can be made of a multiphase material, such as Zerodur® (a glass ceramic) or of a multilayer material, such as Ultra Low Expansion (ULE®) glass, for example.
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.
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.
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.
While specific configurations, arrangements, and steps are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations, arrangements, and steps can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications.
It is noted that references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it would be within the knowledge of one skilled in the art to incorporate such a feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.
The optical components 118/120 are commonly made from a variety of materials with a non-amorphous structure, and of those, silicon carbide and beryllium are typical. However, it is desirable that the optical components used are of a material having a near-zero coefficient of thermal expansion so as not to appreciably change shape during exposure (e.g., when light beam 114 is illuminated). Examples of materials having a near-zero coefficient of thermal expansion include Zerodur® and ULE® glass, as discussed above.
A simplified block diagram of a conventional manufacturing system 200 for fabricating an optical component 118/120 is shown in
Other methods are depicted in the following references: Spiller et al., “Smoothing of mirror substrates by thin-film deposition,” SPIE Conference on EUV, X-RAY, and Neutron Optics and Sources, Jul. 21, 1999, Denver, Colo., Proceedings of SPIE Vol. 3767, edited by Carolyn A. MacDonald, Kenneth A. Goldberg, Juan R. Maldonado, Huaiyu H. Chen-Mayer, and Stephen P. Vernon, November 1999, pp. 143-153; Braun et al., “Carbon buffer layers for smoothing substrates of EUV and X-ray multilayer mirrors,” SPIE Conference on Testing, Reliability, and Application of Micro- and Nano-Material Systems II, Mar. 15, 2004, San Diego, Calif., Proceedings of SPIE Vol. 5392, edited by Norbert Meyendorf, George Y. Baaklini, and Bernd Michel, SPIE, Bellingham, Wash., July 2004, pp. 132-140; and Kleineberg et al., “Bufferlayer and Caplayer Engineering of Mo/Si EUVL Multilayer Mirrors,” SPIE Conference on Soft X-Ray and EUV Imaging Systems II, Jul. 31, 2001, San Diego, Calif., Proceedings of SPIE Vol. 4306, edited by Daniel A. Tichenor and James A. Folta, December 2001, pp. 113-120, all of which are incorporated by reference herein in their entireties. See also U.S. Pat. Appl. Pub. No. 2003/0057178 A1, to Michael Goldstein, entitled “Method for Making a Mirror for Photolithography,” published on Mar. 27, 2003 and filed Sep. 26, 2001, and European Pat. Appl. No. EP 0 955 565 A2, to Murakami et al., entitled “Mirror for Soft X-Ray Exposure Apparatus,” published on Nov. 10, 1999, both of which are incorporated by reference herein in their entireties.
The exemplary embodiments of the invention described herein facilitate the fabrication of optical components (in particular, mirrors) that are made from multiphase or multilayer materials such as Zerodur® or ULE® glass, respectively, which are extremely critical to the EUV processing due to their near-zero coefficients of thermal expansion. These embodiments are advantageous because the potential for reaching the desired MSFR (currently less than 0.14 nm rms for EUV applications) is possible with a substrate coated according to embodiments described herein, but is unlikely to be achieved with a bare substrate.
Although specific reference is made in this text to a fabrication process for optical components used for lithographic processing, it should be understood that the fabrication process described herein can be used to fabricate optical components for use in any application in which optical components are used. Further, specific reference is made herein to EUV processing-related advantages of the invention. However, it should be understood that other processing techniques would benefit from the use of optical components fabricated according to embodiments of the present invention. These processing techniques include, but should not be limited to, ultra smooth optics and ultra precise optics, for example. In addition, although the optical materials Zerodur® and ULE® glass are highlighted as ideal materials to use for the optical component fabrication as described herein, other materials having low coefficients of thermal expansion may also be suitable, as would be understood by those of ordinary skill in the relevant art(s).
It is to be appreciated that these above-noted embodiments can be used in conventional mask-based lithography, maskless lithography, immersion lithography, interferemetric lithography, or other types of optical systems that include a similar functioning optical system.
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 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.
Further, the purpose of the foregoing Abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is not intended to be limiting as to the scope of the present invention in any way.
