1. Field of the Invention
The present invention relates to a method for manufacturing an optical element, an optical element, an optical system using the optical element, an optical apparatus and exposure apparatus using the optical system, and a method for manufacturing a device. In particular, the present invention relates to a diffractive optical element suitable for semiconductor equipments and a method for manufacturing the diffractive optical element, an optical system and optical instrument using the diffractive optical element, and a device and a method for manufacturing the device.
2. Description of the Related Art
Refractive optical elements such as a lens and prism are frequently used for optical systems constituting optical instrument. Particularly, the diffractive optical element has been used as an optical element for converting a incident wavefront into a desired wavefront. This diffractive optical element has characteristics that are not found in the refractive optical element such that, for example, it has an inverse dispersion value to that of the refractive optical element, and the optical system can be compactly arranged.
The diffractive optical element has been manufactured by mechanical grinding, or by using a mold manufactured by mechanical grinding. However, the optical element is desirably manufactured with pitches as fine as possible, in order to endow the diffractive optical element with a large power as an optical element. Accordingly, applying a semiconductor manufacturing process has been considered.
A semiconductor manufacturing technology is usually applicable when the diffractive optical element assumes a binary shape. This technology enables fine pitches to be manufactured with high precision as compared with conventional mechanical grinding method. Consequently, a binary type diffractive optical element in which blazed shapes are approximated by stepped shapes has been actively studied in recent years.
The binary optics will be described in detail hereinafter with reference to FIGS. 7A, 7B and 8.
In FIG. 7, the reference numeral 50 denotes a Fresnel lens, the reference numeral 51 denotes a blazed shape, the reference numeral 52 denotes a binary optics, and the reference numeral 53 denotes a stepped shape. In FIG. 8, the reference numeral 55 shows a whole view of the diffractive optical element.
Ideally, the Fresnel lens 50 as a diffractive optical element should have a cross section of the blazed shape 51 as shown in FIG. 7A, which enables light diffraction ratio against the design wavelength to reach almost 100%. However, since machining into a perfect blazed shape 51 is actually difficult, the blazed shape 51 is approximated by quantization to form a binary optics 52 having a stepped cross section 53 as shown in FIG. 7B. Although the binary optics 52 is an approximation of the Fresnel lens 50, it ensures a diffraction efficiency of the primary diffraction light of 90% or more.
The minimum line width of the diffractive optical element should be as fine as possible in order to enhance the degree of approximation and to endow the diffractive optical element with high power as an optical element. Accordingly, a lithography process that has been experienced in the production of semiconductors is used for obtaining a high performance diffractive optical element.
The semiconductor manufacturing apparatus as used herein is designed on the premise that a wafer with a thickness of less than 1 mm is handled. Consequently, the diffractive optical element manufactured by using the semiconductor lithography process is formed as a wafer-shaped optical member.
Assume that the diffractive optical element obtained by the manufacturing method as described above is employed as the optical system of a semiconductor exposure apparatus using an eximer laser such as a KrF, ArF or F2 laser. FIG. 4 shows a schematic drawing of the exposure apparatus for manufacturing semiconductors.
In FIG. 4, the letter A denotes a light source of an eximer laser such as a KrF, ArF or F2 laser. A light flux C from the light source is guided to an illumination optical system D by a mirror B, and the light flux after passing through the illumination optical system illuminates the surface of a reticle E as a first block. The light flux carrying a line of reticle information is projected onto a light-sensitive substrate (a wafer) G through a reductive projection optical system F. The letter H denotes a wafer stage, which adjust the wafer G at a focal point by means of the wafer stage H.
While an oxide such as alumina and silica glass, or a fluoride such as calcium fluoride and magnesium fluoride may be used as a material of the optical element using a light source of the eximer laser such ad KrF, ArG or F2 laser, silica glass is mainly used since it is suitable for machining, homogeneous and has a low thermal expansion coefficient. As disclosed in Japanese Patent Publication No. 6-48734, the concentrations of OH group and hydrogen molecules are further controlled in a high purity synthetic silica glass (synthetic silica glass) prepared by controlling the content of impurities in order to use the material as silica glass on which a laser beam having a wavelength of 300 nm or less is irradiated from the eximer laser source such as the KrF, ArF or F2 laser, thereby permitting such silica glass to be used as a material having high resistivity against a long term laser irradiation.
However, the quality of silica glass as disclosed in Japanese Patent Publication No. 6-48734 has been controlled on the premise that it is used for a refractive optical element commonly called as a lens. Japanese Patent Laid-Open No. 10-330120 discloses a method for controlling the hydrogen concentration in the silica glass for irradiating with the eximer laser by annealing, wherein discussions have been mainly related to the silica glass having a minimum thickness of 10 mm or more by adjusting the thickness of the silica glass before annealing to be larger than 10 mm or more as compared with the finally required thickness.
In manufacturing the diffractive optical element through the lithography process, the semiconductor manufacturing apparatus to be used for this purpose is designed within a range of the standard of the wafer, and the highest accuracy is obtained within the range. While a variety of standards of the wafer are known, the outer diameter is defines to be 150 mm (6 inches), 200 mm (8 inches) and 300 mm (12 inches), and the range of thickness is also defined depending on each size. The thickness of the wafer is 1 mm or less, forming a substrate far more thinner than the conventional optical elements. However, remodeling of the semiconductor manufacturing apparatus so as to be used out of the Si wafer standard makes it difficult to maintain its accuracy.
Resist patterning and etching steps are included in the detailed lithography process. In the resist patterning steps, an organic resist is coated, the coating pattern is exposed to a light through a reticle on which a planar shape to be printed is formed, and a resist pattern having a desired planar shape is formed after baking and development steps. Since an apparatus called a spinner, which allows the substrate to rotate at a high speed to coat the resist with a uniform film thickness, is used for coating the resist, the heavier substrate gives so much load on the spinner that control of rotation turns out to be difficult.
While a hot plate is used in the baking step with a strict temperature control in a unit of second, temperature control of a substrate such as silica glass having a low heat conductivity and large thickness is quite difficult. Meanwhile, although the substrate is etched with chemicals using the resist pattern as a mask, or the substrate is processed with a dry etching machine using a plasma, the dry etching method is mainly used since its accuracy is high. While the dry etching machine requires a device for cooling the substrate, temperature control of a substrate such as silica glass having a low heat conductivity and large thickness is quite difficult as in baking step.
The diffractive optical element preferably has a thickness close to the thickness defined in the standard of the Si wafer that is thinner than 1 mm, when the diffractive optical element is manufactured using the semiconductor manufacturing process. The process is significantly different from the process for manufacturing the refractive optical element comprising only the grinding process. The material is coated with an organic compound, heated or cooled at a desired temperature, exposed to a chemical solution, heated in vacuum, and subjected to plasma implantation in each step for manufacturing the optical element by the semiconductor manufacturing process.
Japanese Patent Application No. 2000-56113 discloses a method in which the optical element is directly bonded to a different thick material, in order to prevent deformation by the weight of the optical element, by holding the optical element, and by the effect of atmospheric pressure changes that adversely affect the optical performance of the element. However, a heating treatment is necessary for improving the bonding strength, sometimes requiring a long period of heating at a temperature range of 200 to 400xc2x0 C., or at a temperature range of as high as 400 to 1000xc2x0 C.
The diffractive optical element manufactured by the process as described above involves the following two problems. One problem is that hydrogen molecules doped before processing are discharged by exposing the thin plate to various environment to change the hydrogen molecule content, thereby inducing deterioration of silica glass (such as decrease of transmittance and change of refractive index) by a long period of irradiation of the laser beam.
The other problem is that the surface of silica glass is denatured by heating the element while an organic substance is coated on its surface, or by implantation with plasma.
The present invention for solving the problems as hitherto described provides a method for manufacturing an optical element comprising the steps of processing a high purity synthetic silica glass by lithography, and allowing the silica glass to contain hydrogen, wherein the lithography process comprises resist process, exposure, development and etching.
The silica glass may be treated with a chemical solution containing hydrofluoric acid after the lithography process.
Each of the silica glass and the optical element preferably contains hydrogen molecules in a concentration of (5E+16 molecules/cm3) to (5E+19 molecules/cm3).
Preferably, each of the silica glass and the optical element has a thickness of 0.5 to 10 mm.
The optical element may be used for an eximer laser.
The optical element may be irradiated with an eximer laser with an intensity of 0.01 to 1 mJ/cm2/pulse, and contains hydrogen molecules in a concentration of (5E+16 molecules/cm3) to (5E+18 molecules/cm3). The optical element may be also irradiated with an eximer laser with an intensity of 1 to 100 mJ/cm2/pulse, and contains hydrogen molecules in a concentration of (5E+18 molecules/cm3) to (5E+19 molecules/cm3).
The optical element may be a diffractive optical element, each of the silica glass and optical element has a thickness within a range of 0.5 mm to 10 mm, and the optical element contains hydrogen molecules in a concentration of (5E+16 molecules/cm3) to (5E+19 molecules/cm3).
The optical element is manufactured by the method according to the present invention, and the optical system preferably comprises at least one optical element.
Preferably, substantially all the high purity synthetic silica glass placed on the optical path of the optical system is manufactured by the method according to the present invention.
The present invention also provides an optical instrument constructed by using the optical system according to the present invention.
The present invention also provides an exposure apparatus comprising an optical system for illuminating the pattern with a light from a light source, and a projection optical system for projecting the light from the pattern onto a light-sensitive substrate, wherein at least one of the illumination optical system and projection optical system is the optical system according to the present invention.
The present invention also provides a method for manufacturing a device using the exposure apparatus according to the present invention.
The present invention provides a method for manufacturing an optical element comprising the steps of processing a high purity synthetic silica glass by lithography, and allowing the silica glass to contain hydrogen before processing the silica glass.
In the present invention for manufacturing an optical element, a high purity synthetic silica glass is processed by lithography followed by a dry etching step, and the dry etching step is carried out in an etching gas added hydrogen gas.
Further objects, features and advantages of the present invention will become apparent from the following descriptions of the preferred embodiments with reference to the attached drawings.