The patent's assignee for patent number 8650912 is
News editors obtained the following quote from the background information supplied by the inventors: "This invention relates to a burner for use in the manufacture of synthetic quartz glass ingots useful as the stock material for excimer laser synthetic quartz glass optical members and large-diameter synthetic quartz glass ingots useful as the stock material for liquid crystal-related large-size photomask substrates. More particularly, it relates to a burner for use in the manufacture of synthetic quartz glass ingots having optical-grade high homogeneity and a minimal change of light transmittance and useful as optical members such as lenses, prisms, mirrors, windows and photomask substrates in excimer laser systems, especially ArF excimer laser systems. The invention also relates to a method for the manufacture of synthetic quartz glass ingots.
"To meet the recent trend of LSI toward higher integration, the photolithography of defining an integrated circuit pattern on a wafer requires an image exposure technique on the order of submicron units. For finer line width patterning, efforts have been made to reduce the wavelength of a light source of the exposure system. In the lithography, a KrF excimer laser (wavelength 248 nm) took over the prior art i-line (wavelength 365 nm) as the mainstream light source in steppers; and the practical use of an ArF excimer laser (wavelength 193 nm) has recently started. Then, the lens for use in steppers is required to have homogeneity, improved UV transmission, and resistance to UV irradiation.
"In order to avoid contamination with metal impurities which cause UV absorption, the synthesis of quartz glass is generally carried out by introducing the vapor of a high purity silicon compound such as silicon tetrachloride directly into an oxyhydrogen flame. Flame hydrolysis takes place to form silica fines, which are directly deposited on a rotating heat-resistant substrate such as quartz glass while being melted and vitrified thereon. In this way, transparent synthetic quartz glass is produced.
"The transparent synthetic quartz glass thus produced exhibits satisfactory light transmittance in the short-wavelength range down to about 190 nm in the LSI field. It has been utilized as a material capable of transmitting UV laser light, specifically i-line and excimer laser light such as KrF (248 nm), XeCl (308 nm), XeBr (282 nm), XeF (351 and 353 nm) and ArF (193 nm), and the four-fold harmonic wave (250 nm) of YAG.
"The absorption of light in the UV region that is newly created by irradiating synthetic quartz glass with UV light having great energy as emitted by an excimer laser is deemed to be due to the paramagnetic defects formed through photo-reaction from intrinsic defects in synthetic quartz glass. Many light absorption bands due to such paramagnetic defects have been identified by ESR spectroscopy, for example, as E' center (Si.) and NBOHC (Si--O.).
"The paramagnetic defects generally have an optical absorption band. When UV light is irradiated to quartz glass, the problematic absorption bands in the UV region due to paramagnetic defects in quartz glass are, for example, at 215 nm due to E' center (Si.) and 260 nm, which has not been accurately identified. These absorption bands are relatively broad and sometimes entail strong absorption. This is a serious problem when quartz glass is used as a transmissive material for ArF and KrF excimer lasers.
"Intrinsic defects in synthetic quartz glass which cause paramagnetic defects arise from structures other than SiO.sub.2 such as Si--OH and Si--Cl and oxygen-depleted or enriched structures such as Si--Si and Si--O--O--Si. As the approach for suppressing paramagnetic defects, it is proposed in JP-A 6-199532 to use a chlorine-free alkoxysilane such as tetramethoxysilane as the silane compound for preventing Si--Cl, one of paramagnetic defects, from being incorporated in glass.
"It is also known that if hydrogen molecules are present in quartz glass in a concentration above a certain level, few defects of E' center (Si.) which are oxygen defects are formed, leading to improved durability to laser damage. Since ArF excimer laser light causes several times serious damages to quartz glass as compared with KrF excimer laser light, the quartz glass for ArF laser application must have several times higher a hydrogen molecule concentration than the quartz glass for KrF laser application.
"It is proposed in JP-A 6-305736 to control the hydrogen molecule concentration in synthetic quartz glass. Depending on the energy using conditions of an ArF laser, the hydrogen molecular concentration in glass is adjusted.
"Now that the efforts to reduce the wavelength of light source have reached excimer laser light having extremely greater energy than the traditional i-line light, active research works have been made on the laser durability of glass.
"Exposure apparatus using such shorter wavelength light include many optical parts such as lenses, windows, prisms, and photomask-forming quartz glass substrates. With respect to projection lens materials among these optical parts used in exposure apparatus, the recent progress is toward a higher NA, the diameter of lens is annually increasing, and the optical homogeneity of lens material is required to be of higher precision. Especially for the ArF excimer laser, it is required that the initial transmittance of quartz glass, specifically the transmittance at wavelength 193.4 nm over the entire surface of an optical member be close to the theoretical value, the theoretical value at wavelength 193.4 nm being computed to be 99.85% by taking into account multiple reflection. Since the optical system in the exposure apparatus is composed of several to several tens of lenses, it is important that setting an initial transmittance of quartz glass even a little higher restrains the absorption of optical energy within the bulk of quartz glass, thereby minimizing a possibility that the light energy once absorbed is converted to thermal energy to incur a change of density and in some cases, a change of refractive index. In addition to the essential uniformity of refractive index, a reduction of birefringence becomes a crucial problem.
"As stated above, in order to avoid contamination with metal impurities which cause UV absorption, the synthesis of quartz glass is generally carried out by introducing the vapor of a high purity organosilicon compound such as silicon tetrachloride directly into an oxyhydrogen flame. Flame hydrolysis takes place to form silica fines, which are directly deposited on a rotating heat-resistant substrate such as quartz glass and melted and vitrified thereon to form transparent synthetic quartz glass. The synthetic quartz glass ingot thus produced is sliced perpendicular to its growth direction whereupon a distribution of transmittance at wavelength 193.4 nm is determined in a plane of the growth direction. Then, the slice has an in-plane distribution, typically with the tendency that transmittance decreases from the center to the periphery. If the value required for the initial transmittance is at least 99.7% as an internal transmittance, for example, an effective portion of the synthetic quartz glass ingot that can be utilized, generally known as percent yield, is determined by this value.
"Apart from the LSI application, large-size quartz glass substrates for photomasks are now used in the liquid crystal display (LCD) application. It is required to form synthetic quartz glass ingots for use as the stock material therefor to larger diameters, particularly when the percent yield of the manufacturing process of large-size glass substrates is considered. While the mainstream of conventional synthetic quartz glass substrates for IC use is of 6 inch square size, large-size glass substrates have already been required to have one side of 1 meter or longer. In fabricating large-size quartz glass substrates, as opposed to the synthetic quartz glass ingot for IC use which must have a diameter of about 100 to 140 mm, for example, an ingot which is of a conventional ingot diameter must be increased in length in order to insure a certain product weight. Shaping must be repeated many times until the size is tailored to a desired profile. The situation is detrimental in production yield and efficiency."
As a supplement to the background information on this patent, NewsRx correspondents also obtained the inventors' summary information for this patent: "An object of the invention is to provide a burner for use in the manufacture of synthetic quartz glass ingots which serve as the stock material from which synthetic quartz glass members having high optical homogeneity useful as optical parts such as lenses, prisms, windows and photomask-forming quartz glass substrates in excimer laser systems can be readily obtained or synthetic quartz glass members for liquid crystal-related large size glass substrates can be efficiently obtained. Another object is to provide a method for the manufacture of synthetic quartz glass ingots using the burner.
"In the manufacture of synthetic quartz glass ingots by vapor phase hydrolysis or oxidative decomposition of a silica-forming compound with the aid of an oxyhydrogen flame, the burner structure for forming a flame is important. The prior art burner is of the structure including a central triple-tube assembly, a tubular shell surrounding the triple-tube assembly, a plurality of nozzles disposed between the triple-tube assembly and the tubular shell, the foregoing components forming a main burner, and a tubular jacket disposed around the tubular shell and at the forward end of the main burner. Replacing the prior art burner by a burner for the manufacture of synthetic quartz glass comprising a main burner comprising at least a central triple-tube assembly, a tubular shell surrounding the triple-tube assembly, and a plurality of nozzles disposed between the triple-tube assembly and the tubular shell and within the confine of the tubular shell, and a double-tube assembly disposed around the main burner, the present invention has succeeded in manufacturing synthetic quartz glass ingots from which synthetic quartz glass having high optical homogeneity and synthetic quartz glass members for liquid crystal-related large size glass substrates are obtainable.
"The inventors have intended to extend the effective portion over the entire region of the synthetic quartz glass ingot. Factors of the manufacturing process that substantially dictate the initial transmittance of a synthetic quartz glass ingot include a burner (structure and set conditions) which is an important constituent of the direct flame process, as well as a starting material or silane compound, a combustible gas (typically hydrogen) and a combustion-supporting gas (typically oxygen) fed thereto, and a balance of these gases. It has been found that the manufacturing process largely depends on the structure of burner among other factors.
"In one aspect, the present invention provides a burner for use in the manufacture of synthetic quartz glass, comprising
"a main burner comprising a multi-tube assembly of a three or more tube construction including a center tube for feeding a silica-forming compound, a first enclosure tube surrounding the center tube for feeding a combustion-supporting gas, and a second enclosure tube surrounding the first enclosure tube for feeding a combustible gas; a tubular shell surrounding the multi-tube assembly for feeding a combustible gas; and a plurality of nozzles disposed within the tubular shell for feeding a combustion-supporting gas, the main burner defining a forward opening, and
"a double-tube assembly surrounding at least the forward opening of the main burner.
"In a preferred embodiment, the double-tube assembly includes an outer tube and an inner tube disposed within the outer tube. The outer tube surrounds the forward opening of the main burner and projects further forward. The inner tube has a forward end which is disposed in register with or backward of the forward opening of the main burner.
"In a preferred embodiment, the double-tube assembly defines therein a passage for a combustion-supporting gas. Typically, the combustion-supporting gas passage is defined between the outer and inner tubes.
"In a preferred embodiment, the total cross-sectional area of gas discharge ports of the plurality of nozzles disposed in the tubular shell accounts for 5% to 20% of the cross-sectional area of a gas discharge region between the multi-tube assembly and the tubular shell.
"Another aspect of the invention provides a method for the manufacture of a synthetic quartz glass ingot using the burner defined above, comprising the steps of placing the burner to face a quartz glass target mounted on a rotating support; feeding a silica-forming compound to the center tube, a combustion-supporting gas to the first enclosure tube and the nozzles, a combustible gas to the second enclosure tube and the tubular shell, and a combustion-supporting gas to the double-tube assembly; forming an oxyhydrogen flame from the combustion-supporting gas and the combustible gas for subjecting the silica-forming compound to vapor phase hydrolysis or oxidative decomposition to form silica fines; depositing the silica fines on the target; and melting and vitrifying the deposited silica into quartz glass.
"In a preferred embodiment, the silica-forming compound is a silane or siloxane, the combustion-supporting gas is oxygen, and the combustible gas is hydrogen. The silica-forming compound and oxygen are fed to the burner such that the molar amount of the silica-forming compound is at least 1.3 times the stoichiometry of oxygen. The molar ratio of the amount of actually fed oxygen to the stoichiometry of oxygen necessary for the silica-forming compound and hydrogen fed to the burner is from 0.6 to 1.3. Typically, the combustion-supporting gas is fed through the double-tube assembly at a flow velocity of 0.5 to 1.3 m/sec.
"Most often, the ingot has a diameter of at least 150 mm.
"Using the burner of the invention, it becomes possible to manufacture synthetic quartz glass ingots which serve as the stock material from which are manufactured synthetic quartz glass members having high optical homogeneity for use in excimer laser systems, especially ArF excimer laser systems, optical members having high laser resistance, and optical members associated with light sources such as excimer lasers, and optical fibers for ultraviolet radiation."
For additional information on this patent, see:
Keywords for this news article include: Gases, Silicon, Elements, Hydrogen, Chalcogens, Inorganic Chemicals,
Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC
Most Popular Stories
- Obama Administration Releases Proposal to Regulate For-Profit Colleges
- Apple, HP, Intel May Take a Hit from Slowdown in Smartphone Sales Growth
- Elizabeth Vargas' Husband Marc Cohn Addresses Rumors
- FDIC Files Lawsuit on Behalf of Banks Allegedly Hurt by Libor Scandal
- Keurig Adds Peet's coffee, Alters Starbucks deal
- Motley Crue's Nikki Sixx Marries Model Courtney Bingham
- U.S. to Relinquish Gov't Control Over Internet
- Chinese e-Commerce Giant Alibaba Gears for IPO in U.S.
- Some California Cities Seeking Water Independence
- Will Missing Malaysian Jet Prompt Aviation System Change?