Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B1/00—Processes of grinding or polishing; Use of auxiliary equipment in connection with such processes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B13/00—Machines or devices designed for grinding or polishing optical surfaces on lenses or surfaces of similar shape on other work; Accessories therefor
  • B24B13/06—Machines or devices designed for grinding or polishing optical surfaces on lenses or surfaces of similar shape on other work; Accessories therefor grinding of lenses, the tool or work being controlled by information-carrying means, e.g. patterns, punched tapes, magnetic tapes
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B19/00—Other methods of shaping glass
  • C03B19/14—Other methods of shaping glass by gas- or vapour- phase reaction processes
  • C03B19/1453—Thermal after-treatment of the shaped article, e.g. dehydrating, consolidating, sintering
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B19/00—Other methods of shaping glass
  • C03B19/14—Other methods of shaping glass by gas- or vapour- phase reaction processes
  • C03B19/1469—Means for changing or stabilising the shape or form of the shaped article or deposit
• • C—CHEMISTRY; METALLURGY
  • C03—GLASS; MINERAL OR SLAG WOOL
  • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
  • C03B23/00—Re-forming shaped glass
  • C03B23/02—Re-forming glass sheets
  • C03B23/023—Re-forming glass sheets by bending
  • C03B23/025—Re-forming glass sheets by bending by gravity
  • C03B23/0252—Re-forming glass sheets by bending by gravity by gravity only, e.g. sagging

Definitions

• the present invention relates to extreme ultraviolet (EUV) mirrors for EUV lithography. More particularly, it relates to methods for manufacturing EUV mirrors.
• EUV extreme ultraviolet
• EUVL Extreme ultraviolet lithography
• EUVL uses extreme ultraviolet (EUV, also called soft X-ray) radiation with a wavelength in the range of 10 to 14 nanometers (nm) to perform the imaging.
• EUV extreme ultraviolet
• optical lithography has been the lithographic technique of choice in the high-volume manufacture of integrated circuits (IC).
• IC integrated circuits
• NGL Next-Generation Lithographies
• EUVL is similar to the optical lithography.
• the basic optical design for an EUVL system is similar to that of an optical lithographic system. It comprises a light source 1, a condenser 2, a mask (reticle) 4 on a mask stage 5, an optical system 6, and a wafer 7 on a wafer stage 8.
• Both EUV and optical lithographies use optical systems (cameras) to project images on the masks onto substrates which comprise silicon wafers coated with photo resists.
• the apparent similarity stops here. Because EUV is strongly absorbed by virtually all materials, EUV imaging must be performed in vacuum, which is achieved by enclosing the system in a chamber 3.
• the chamber 3 might be further partitioned into different compartments 10 and 20, which have their own vacuum systems. Because EUV is absorbed by most materials, there are no suitable lenses to focus and project an image on mask 4 onto a substrate (wafer) 7. As a result, it is necessary to operate EUVL in a reflective mode instead of a transmissive mode. In the reflective mode, light is reflected from mirrors (not shown; inside the optical system 6), instead of being transmitted through lenses. Even with reflective optics, there are not many materials capable of reflecting EUV. In order to achieve reasonable reflectivities at near normal incidence (i.e., an incident beam landing on the surface of a mirror at an angle close to normal to the surface), the surface of a mirror is typically coated with multilayer, thin-film coatings. These multilayer, thin- film coatings reflect EUV in a phenomenon known as distributed Bragg reflection.
• the multilayer coatings for the reflective surfaces in EUVL imaging system consist of a large number of alternating layers of materials having dissimilar EUV optical constants. These multilayers provide a resonant reflectivity when the period of the layers is approximately ⁇ /2, where ⁇ is the wavelength.
• the most promising EUV multilayers are coatings of alternating layers of molybdenuum (Mo) and silicon (Si). These layers are deposited with magnetron sputtering. Each layer of Mo or Si is coated to a thickness of ⁇ /4 of the EUV light so that it will have a periodicity of ⁇ /2. In this type of reflector, a small portion of the incident light is reflected from each silicon surface.
• the thickness of the layers causes the reflected light waves to interfere constructively. The more layers there are, the more light will be reflected. However, imperfections in the surface coating will eventually diminish the reflectivity return of more coatings.
• most mirrors in EUVL systems have around 40 alternating layer pairs of Mo:Si. Furthermore, most of these Mo:Si multilayers are optimized to function best with wavelengths at around 13.4 nm, which is the wavelength of a typical laser plasma light source.
• a typical EUVL optical system or camera (see 6 in FIG. 1) consists of several mirrors (e.g., a four-mirror ring-field camera shown in FIG. 2).
• the mirrors that comprise the camera must have a very high degree of perfection in surface figure and surface finish in order to achieve diffraction limited imaging. It is predicted that the surface figure (basic shape) of each mirror must be accurate to 0.25 nm rms (root mean square) deviation, or better. In addition to surface figure, stringent requirement must also be placed on the finish of the surfaces.
• the challenge for a fabricator of optics for EUVL is to achieve the desired levels of surface figure accuracy and surface finish simultaneously.
• FIG. 2 illustrates a typical prior art four-mirror optical system for EUVL application.
• Such an optical system is used to project and reduce an image from a mask onto a wafer.
• the reduction achieved by the optical system permits the printing of a image smaller than that on the mask onto a wafer.
• the projection operation is typically carried out in a step-and-scan process.
• a light beam from a light source (see 1 and 2 in FIG. 1) is used to scan the image on the mask.
• the light beam L reflected from the mask is further reflected by four mirrors Ml, M2, M3 and M4 in succession to project and reduce the image from the mask onto the wafer.
• mirrors should be substantially invariant to environmental changes, e.g., temperature changes.
• these mirrors be made of light weight materials with very low coefficients of thermal expansion (CTE).
• CTE coefficients of thermal expansion
• ULE glass has a CTE of about 0+30 ppb/°C over the temperature range of 5 to 35 °C.
• the CTE in ULE glass is a function of the titanium concentration.
• ULE glass typically contains about 6 to 8 wt. % of TiO 2 . Compositions containing about 7 wt. % TiO 2 have near zero CTE.
• ULE glass is also unique in that it hAs no crystalline phase. In other words, ULE glass is completely amorphous.
• ULE glass is a high temperature glass which makes it unsuitable for manufacturing by conventional means. Instead of being poured, it is fabricated by a flame hydrolysis fused glass process which is similar in scope to chemical vapor deposition.
• ULE glass is formed in layer deposits. This means ULE glass inherently has striae, though these striae are not apparent and do not affect most applications. Although ULE glass has been polished to 0.5 A rms (root mean square) surface roughness, the striae may present problems for stringent applications like EUV mirrors. For example, it can create a mid frequency surface structure that would cause image degradation in mirrors used in the projection systems for EUV microlithography.
• FIG. 3A illustrates a piece of ULE " glass 31, in which striae 32 are shown.
• FIG. 3B shows a cylinder of ULE glass 33 with its top ground to give a convex surface. It is apparent that different layers of striae planes are cut across, leaving approximately concentric circles of striae edges 34. This is not a problem for applications where the source light is in the range of visible to infrared. However, in EUV lithography, which uses lights with wavelengths around 13 nanometer (nm), these striae edges 34 may manifest themselves as small ridges. These can cause aberrations which would degrade any images projected within the optical train.
• ULE glass Although this problem is illustrated with ULE glass, the problem is not unique to this glass.
• the same problem due to striae will be encountered in any material that is prepared by gradual deposition of newly formed material onto the materials, like the flame hydrolysis process in the formation of ULE glass.
• Such materials which inherently have striae, will be generally referred to as the ULE M glass-like materials herein.
• Embodiments of the invention relate to methods for manufacturing mirrors for use in EUV lithography. Some embodiments include sagging a plate of a glass material to produce a mirror blank; and polishing a top face of the mirror blank to produce a polished mirror. Other embodiments include grinding a top face of a piece of a glass material; sagging a plate of the glass material over the top face of the piece to produce a mirror blank; and polishing a top face of the mirror blank to produce a polished mirror.
• the glass material may be ultra low expansion glass material with a coefficient of thermal expansion of no more than 30 parts per billion per degree Celsius in a temperature range of 5 to 35 °C.
• FIG. 1 is a diagram of a prior art EUVL system.
• FIG. 2 is a diagram illustrating a prior art four-mirror optic system for an
• FIG. 3 A is a diagram illustrating a piece of a ULE glass-like material having striae.
• FIG. 3B illustrates a cylindrical piece of a ULE glass-like material. The top of the cylinder has been ground to a convex configuration.
• FIG. 4A is a diagram illustrating a side view of a thin plate of a ULE glasslike material showing striae on the side.
• FIG. 4B shows the same side view after the thin plate was sagged to have a curved surface.
• FIG. 5A is a diagram illustrating a cylinder of a ULETM glass-like material having striae planes perpendicular to the axis of the cylinder.
• FIG. 5B is a convex mirror blank prepared from the cylinder shown in FIG. 5A, showing the concentric striae ridges on the convex surface.
• FIG. 6k shows a top plate made of a ULETM glass-like material about to be attached to a mirror base with a convex top face.
• FIG. 6B shows a mirror blank with the top plate annealed to the base.
• Embodiments of the invention relate to methods for manufacturing EUV mirrors from the ULE glass-like materials.
• the invention significantly reduces the striae effects which might degrade image qualities.
• Some embodiments relate to methods of manufacturing thin-plate mirrors.
• FIG. 4A illustrates a side view of a thin plate 41, showing striae 42 on the side.
• the striae planes are parallel with the surfaces of the plate. If the surface of this plate were to be ground to the desired curvature, the grinding would cut across striae planes, and the above-mentioned problem will be unavoidable.
• the thin plate 41 is "sagged" to the desired curved surface to produce a mirror blank 43, in which the striae planes 44 remain substantially parallel to the sagged, curved surface of the mirror. Because this sagged, curved surface has a near net shape of the final mirror, only minor polishing and grinding are required to produced a finished mirror.
• Sagging refers to the process of thermal deformation of a plate-like glass material.
• the polishing or lapping of a finished mirror may be achieved by any method known in the art.
• coating of molybdenum (Mo) and silicon (Si) can be accomplished with either magnetron sputtering, ion-beam sputtering, or other suitable methods.
• magnetron sputtering ion-beam sputtering, or other suitable methods.
• these mirrors should be coated with multiple alternating layers of Mo and Si. Polishing as used herein may include the Mo:Si coating step.
• FIG. 1 Another embodiments of the invention relate to methods of manufacturing thick mirrors. With thick mirrors, the above-described sagging method is not applicable. When a mirror is sagged over a form, the material flows. This makes it difficult to maintain enough material to have a desired thickness. This difficulty can be overcome by using a combination of sagging and grinding.
• FIG. 5A illustrates a thick cylindrical plate/block 53 showing that striae planes 54 are perpendicular to the axis of the cylinder.
• this cylindrical block 53 is first ground to provide a top face with a shape near the net shape of the mirror.
• a thin plate may be annealed (sagged) to this base.
• a thin plate 61 with striae planes 62 parallel with the surfaces of the plate may be sagged over the base 63, which is like the one shown in FIG. 5B.
• the mirror base 63 has a top face with a near net shape and striae ridges 64 due to cut through of striae planes.
• Mirror 65 has a top face devoid of striae ridges because this top face is the top face of the thin plate 61 as shown in FIG. 6A.
• annealing the top late 61 to the base 63 include thermal fusion, frit fusion, and annealing using an adhesive (i.e., adhesion).
• thermal fusion the top plate 61 and the base 63 are fused (joined) by applying thermal energy to melt the regions at the joint.
• frit fusion low-melting frits (powders) of a glass material are added to the joint to "glue" the pieces together when heated.
• U.S. Patent No. 6,048,811 which was assigned to the same assignee herein, discloses a frit fusion process suitable for this process.
• the annealing may be achieve by adhesion, i.e., by applying suitable adhesive materials at the joint.
• suitable adhesive materials include, but not limited to, epoxies, silicone adhesives, and solder or bonding materials which, upon heating, will melt and form a bond between the base and the faceplate.
• the faceplate is constructed of a material having a CTE closely equivalent or identical to that of the base, the adhesive should ideally match the CTE.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Organic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Manufacturing & Machinery (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Optical Elements Other Than Lenses (AREA)
• Exposure Of Semiconductors, Excluding Electron Or Ion Beam Exposure (AREA)
• Glass Melting And Manufacturing (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

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Cited By (16)

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Citations (5)

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• 2001
  • 2001-08-30 US US09/943,252 patent/US6776006B2/en not_active Expired - Lifetime
  • 2001-09-18 EP EP01979254A patent/EP1324857A4/en not_active Withdrawn
  • 2001-09-18 WO PCT/US2001/029100 patent/WO2002032622A1/en not_active Ceased
  • 2001-09-18 JP JP2002535847A patent/JP5142444B2/ja not_active Expired - Lifetime

Patent Citations (5)

Non-Patent Citations (1)

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