Patent Number: 046410333
Section: summary

TECHNICAL FIELD The present invention relates to an apparatus and method for heating an optical system so as to prevent a loss of or a reduction in the transmittance of radiant energy because of radiation induced degradation of one or more of the elements of an optical system. BACKGROUND OF THE INVENTION In the development of modern optical systems, efforts were concentrated initially on developing optical materials of high transmittance and coatings for optical elements that would reduce reflection from element surfaces of the wavelengths to be transmitted by the optical system. It was later recognized that radiation-induced defects in silica and other optical materials could also interfere with the transmittance of desired wavelengths of the radiation. The study of radiation-induced defects in silica based glasses intensified with the advent of fiber optics and the use of photolithography in the manufacture of semi-conductor chips and other electronic devices. Thus, the performance of high purity, highly-transparent glasses can be significantly reduced by absorption bands developing as a result of the inherent incidence of radiation on the glass materials from which the lenses of an optical system may be made. The principal effect of radiation on highly-transparent glasses of silica or similar optical materials is the creation of molecular or atomic defects such as the creation of electron vacancies or "holes" which may become trapped at certain trapping sites present in glass. Trapping sites might involve atomic vacancies, interstitials sites, strained bonds, multivalent ions, and the like. In addition, high doses of ionizing radiation of sufficiently high energies may serve to create additional trapping sites, particularly an atomic vacancy or an atomic impurity (interstitial). Unpaired electrons also may comprise a radiation induced optical defect. Either unpaired electrons or holes trapped in a silica material may result in optical absorption bands at energies lower than the intrinsic band gap of the material. Radiation generated defects in high-purity fused silica have been referred to as "oxygen-associated trapped-hole centers" (OHC's). While not wishing to be bound by any one theory, it is postulated that such absorption defects may be caused by electrons trapped at interstitial vacancies within the ordered structure of the silica. Thus, the term "trapped hole centers". Because of the long optical path-lengths inherent in fiber geometry, light absorbing defect centers, even at very low concentrations, can seriously degrade optical fiber performance. Such radiation induced defect centers in high purity, fused silica are also of particular interest in many applications other than fiber optics because silica is a prototype for many glassy radiation transmissive materials. One such application is photolithography which in recent years has facilitated more effective and inexpensive manufacture of semi-conductor devices, such as transistors and integrated circuit wafers. In the practice of photolithography, a pattern in an optical mask, which corresponds to the features of the integrated circuit to be manufactured, is imaged onto a semi-conductor wafer with radiant energy such as electron beams, gamma rays, X-rays or ultraviolet light. The wafer is coated with a radiation sensitive photoresist composition, which changes chemically during exposure to the radiation over areas determined by the pattern in the mask. After exposure, the photoresist coating is developed, and the semi-conductor wafer is further processed by etching away areas determined by the imaged pattern. The process may be repeated on the wafer until the desired integrated circuit has been fabricated. Such semi-conductor devices are the building blocks of virtually all consumer, industrial and military electronic apparatus today, such as computers, calculators, automated equipment, and communications equipment, including televisions, radios, and stereos. One radiation source which may be used for conventional photolithography is ultraviolet light which may be provided by an electrode arc lamp generating UV wavelengths of about 260-460 nanometers (nm). In the fabrication of integrated circuits, it is desirable to reduce the size of circuit features as much as possible so that more circuit components may be included on a single integrated circuit wafer of a given size. However, as the resolution of imaged lines approaches one micrometer in width, the conventional UV wavelengths are too long and result in defraction effects which impair effective imaging. This is because at such narrow circuit line widths, the slits allowing the radiation to pass through the mask have dimensions that are relatively close to the wavelength of the UV radiation being used, which significantly influences the behavior of the radiation as it passes through the slits of the mask. One solution to this problem has been the use of an imaging radiation medium having a shorter wavelength than conventional ultraviolet. While several approaches have been proposed, including the use of X-rays and electron beams, the most promising approach has been the use of deep ultraviolet light having wavelengths in the range of 190-260 nm. Accordingly, a suitable deep UV photoresist known as polymethyl methacrylate (PMMA) has been developed and currently is in use. Molecular bonds of this resist material are broken by exposure to deep UV light so that exposed portions of PMMA coated on a substrate can be removed from substrate by an etching solution or the like. However, one disadvantage of this solution which has kept deep UV from realizing its full potential for providing integrated circuits of greater density has been that the spectral output of optical systems for deep ultraviolet light has deteriorated with age due to the development of a radiation induced absorption band centered at about 215 nm. In order for an ultraviolet illuminator to be effective for deep UV photolithography, it must expose the photoresist coating to a certain minimum dose per unit area. In addition to producing a high total dose, the source of deep UV radiation must also produce a certain minimum brightness (light flux) for efficient optical transfer to the photoresist area of the wafer. Radiation degradation of the spectral output of a deep UV optical system increases the on-line time required for exposure of the photoresist coating of each wafer. This in turn may result in unacceptable long processing times and consequently low yields per unit time of completed semi-conductor devices. For example, the degradation of an optical system having lenses made of quartz by exposure to deep ultraviolet radiation for a period of about 1,000 hours can double the on-line exposure time required for each semi-conductor wafer. Such degradation may also cause the level of light flux to fall below the minimum irradiance required. Although high temperature annealing for several hours has been investigated for its effects upon radiation-induced defect centers in high purity fused silicas, these investigations have been for the purpose of developing a hypothetical model of the defect structure and have not suggested a method of heat treatment for commercial application. In addition, significant differences have been observed in the annealing behavior of defect centers in different silica compositions and these differences are not well understood. In some cases, the amount of absorption of certain optical bands has increased with annealing and in others the amount of absorption has decreased with annealing. It has also been suggested to heat optical elements in instruments such as telescopes, television cameras, periscopes, bombsights and similar sighting and/or recording devices to prevent the condensation of moisture on these optical elements. Such condensation may result in fogging of the optical elements whereby visibility is impaired. Such heating devices have been suggested where the temperature of the optical instrument is lower than the dew point temperature of the ambient atmosphere so as to prevent water condensation on the cooler surfaces of the lenses or other optical elements. Heating components for conventional optical systems include placing electrical resistance heating rings or coatings in direct contact with a lens surface or between different layers of a sandwich-like lens structure. Lenses also have been heated by heating air around or adjacent to the lenses. DISCLOSURE OF THE INVENTION The purpose of the present invention is to provide a method and apparatus for preventing significant losses of transmittance because of radiation induced degradation of highly transmissive elements in optical systems for coupling a source of radiation to a target. More particularly, a means is provided for heating one or more optical elements to a temperature sufficiently high to prevent significant losses of transmittance by reason of increased absorption of at least one wavelength by radiation-induced defect centers. The optical materials with which this invention is concerned are those in which the population of defect centers can be reduced by heating the optical material to an elevated temperature above ambient for "annealing" out these defects. As used in this specification, "annealing" refers to maintaining an optical element at an elevated temperature for only a relatively short period, namely, 1 to 10 hours, preferably 3 to 5 hours. Where an absorption band can be decreased by annealing, it was suggested initially that, upon resumption of irradiation, the absorption band would reappear at about the same rate of degradation as observed with a new, unexposed optical element. To the contrary, the results of tests leading up to the present invention demonstrate that the rate of absorption degradation is much more rapid than was anticipated from a review of existing literature. It was thus discovered that after annealing, the amount of absorption increases very rapidly so that the same level of absorption is reached after only a fraction of the dose required to reach this level of absorption during the first time of exposure. Through use of the present invention, the increase of radiation absorption with time, by either a new or a used optical element, is substantially eliminated so that the performance of fiber optics will not be impaired with time and the length of time required for photolithographic exposure of semi-conductor wafers can be maintained substantially constant. Additionally, the invention overcomes other problems and disadvantages associated with radiation induced degradation of high purity fused silica and equivalent materials. Further objects of the invention are described in the paragraphs below. A principal object of the present invention is to provide a method and apparatus for preventing deterioration of an optical system upon exposure to a wide range of radiation wavelengths, including gamma radiation, X-ray radiation, electron radiation, and both near and deep ultraviolet radiation. Another object of the invention is to provide a method and apparatus for preventing radiation induced degradation of fiber optic waveguides so as to ensure the integrity of fiber communications systems and data links, even where the optical path links exceed many kilometers. A further object of the invention is to provide a method and apparatus for performing deep ultraviolet photolithography in which the instantaneous light flux per unit area (irradiance) and the total amount of deep ultraviolet energy delivered to the target (dose) do not significantly decrease with time because of radiation induced degradation of the optical system of the photolithographic UV illuminator. Another object of the invention is to provide a method and apparatus which are capable of printing narrow lines in the photoresist coating of semi-conductor wafers short exposure times that do not increase significantly in length over periods of use much greater than has heretofore been possible. A further object of the invention is to provide a radiant energy illuminator having an output spectrum which does not deteriorate significantly with the energy of the radiation to which the optical system of the illuminator is exposed over long periods of use. Another object of the invention is to provide a deep ultraviolet illuminator having an output spectrum which does not deteriorate significantly with age. Yet another object of the invention is to provide novel heating arrangements for maintaining the lenses of an optical system at a temperature sufficiently high to prevent development of radiation induced absorption of one or more wavelengths within the band of radiation to be transmitted by the optical system during operating of the optical instrument of which it is a part. The above objects and advantages are realized by the present invention which comprises a source of radiation, a target to be exposed to radiation from this source, an optical system for transmitting this radiation from the source to the target, and a heating means for maintaining the transmissive elements of the optical system while in use at a temperature sufficiently high to prevent radiation induced degradation of the output of the optical system. The optical system includes at least one transmissive element of a material which upon exposure to the radiation in the absence of the heating means would become degraded by an increase in its absorption of at least one wavelength of the radiation. Preferably, the material of the element also is such that the radiation induced absorption is reversible upon annealing the element at an elevated temperature. The radiation transmitted by the optical system may be any type that induces increased absorption of at least one wavelength of the radiation by the material of the transmissive element(s). The types of radiation known to cause such degradation include gamma radiation, X-ray radiation, electron beam radiation, and near and deep ultraviolet radiation. The transmissive material may be any material of relatively high transmittance which undergoes degradation by increased absorption of at least one wavelength upon continuing exposure to a band of radiation including this wavelength. The lens preferably comprises a high purity fused silica, more preferably quartz, and most preferably a "wet" synthetic quartz having an OH radical content of at least about 1500 ppm. Synthetic quartzes of this type are available as Spectrasil from Thermal American Corporation of Montville, N.J.; as Corning Quartz 7940 from Corning Glass Corporation of Corning, N.Y. and as Suprasil from Hereaus-Amersil of Seyreville, N.J. Although the transmissive element may be a fiber optic, it is preferably a lens used in an optical system comprising an optical coupling means between a source of radiant energy and a target to be irradiated by this energy. The lens material altered by irradiation may be either the body of the lens proper or a coating on the lens body. Coatings may be thin films used to improve lens transmittance by reducing reflection, namely, antireflective coatings. Where such coatings contain a crystalline material, such as silica, alumina or an equivalent material, irradiation may create defect centers in the same manner as in a lens body of the same types of materials. Although lens containing optical systems may be employed in a wide variety of devices, one of the preferred devices employing such an optical system is an illuminator for irradiating a target. A wide variety of target structures may be exposed to the radiation from the illuminator for a wide variety of purposes. One such purpose is to expose a radiation sensitive coating on a substrate which may then be developed to bring out the pattern of the exposure. One such target is a semi-conductor wafer coated with an ultraviolet sensitive photoresist composition which changes chemically during exposure to ultraviolet radiation over areas determined by the pattern of an optical mask. The pattern of the optical mask may correspond to the features of an integrated circuit to be imaged onto the wafer. After exposure to the ultraviolet radiation, the photoresist is developed and the semi-conductor wafer is further processed by etching away areas determined by the imaged pattern. Although a conventional ultraviolet light source providing UV wavelengths of 260-460 nm may be used, a special ultraviolet light source providing deep ultraviolet radiation having wavelengths between about 190 nm and about 260 nm is preferred. Such a deep UV light source is described in copending U.S. patent application Ser. No. 362,825 filed Mar. 29, 1982, the entire contents of this copending application being incorporated herein by reference. One embodiment of an optical system employing the invention includes multiple lenses for transmitting and adapting deep UV radiation for its desired end use in the manufacture of integrated circuit wafers. At least one of these lenses upon exposure to the radiation becomes degraded by an increase in the absorption of at least one wavelength of the radiation, such as 215 nm. The material of the degraded lens is such that this increase in absorption is reversible, at least to some extent, by heating the lens to a significantly higher temperature for at least one hour, preferably two to five hours (annealing). The lens material comprises a silica, preferably a wet, fused silica such as quartz. The invention further includes means for heating the entire optical system and/or the degradable lense(s) itself so as to maintain the lens during its use for transmission of the radiation at a sufficiently high temperature to prevent a significant increase in the absorption of the wavelength(s) concerned. The heating means preferably maintains the lense(s) at a temperature of at least 280.degree. C., more preferably about 300.degree. C.-400.degree. C., and most preferably about 300.degree. C.-350.degree. C., at all times during exposure of the lense(s) to radiation of the wavelength(s) concerned. The maximum temperature that may be employed in practicing the invention depends upon the thermal stability of the lens mounting, as well as the lens and/or its coating, and generally should be at least about 100.degree. C. or more below the temperature at which the most sensitive of these materials would become unstable. A number of different apparatuses may be used as the heating means. A preferred heating means comprises mounting the optical elements in a chamber of metal or other heat conductive material and heating the walls of the chamber with a heating device employing an electrical resistance wire or coating so that the optical elements are heated by irradiation. Where the chamber contains a fluid, such as air, the fluid in the chamber also is heated and this fluid in turn heats the optical elements by convection. This heating means is particularly effective where lenses of an optical system are made of a glass having relatively poor heat conductive characteristics. Thus, the lenses are immersed in hot air or another gas so that the outer surfaces of the lenses are heated directly by contact with this heated medium. Such irradiation and convection heating can be supplemented by conduction heating from a heat source in contact with a lens mounting of heat conductive material. A preferred heating device is a ceramic band heater available as model CCX 363 from Tempco, Inc., of Franklin Park, Ill. Heating tape containing an electrical resistance wire or layer also may be used. Another preferred method of lens heating is to attach an electrical resistance heating means directly to each lens to be heated. For example, a metallic resistance coating or electrical resistance tape may be placed along a peripheral portion of the lens so as not to interfere with the optical path of the optical system. Heating means of the types described above can be found in the prior art in connection with devices to prevent water condensation on the optics of cameras and the like. An example of a heating chamber for a camera lens is described in U.S. Pat. No. 2,442,913 to Abrams, et al., the entire contents of which are incorporated herein by reference. An example of a resistance element for heating a mounting in conductive contact with a lens is described in U.S. Pat. No. 1,791,254 to Von Brockdorff, the entire contents of which are incorporated herein by reference. Examples of metallic layers or other electrical resistance coatings for conductive heating of optical lenses are described in U.S. Pat. No. 3,495,259 to Rocholl, et al., and U.S. Pat. No. 4,355,861 to Sebald, the entire contents of these two patents being incorporated herein by reference. An example of a heated glass sandwich for optical instruments is described in U.S. Pat. No. 3,111,570 to Strang, et al., the entire contents of which are incorporated herein by reference. The invention has utility in any field utilizing transmissive lenses of glass or other materials that undergo optical degradation in the presence of radiation. The invention is especially useful in the fields of fiber optics and photolithography. The various heating means described are applicable to both of these applications. For example, fiber optic waveguides can be heated either by hot convection gases such as air or by direct contact with an electrical resistance tape or coating. Similarly, the lenses of photolithographic illuminators may either be mounted in a heated chamber containing hot convection gases or contacted directly with electrical resistance coatings or tape. A more particular application of the invention is for illuminators used in performing deep ultraviolet photolithography for the manufacture of integrated circuits on semi-conductor wafers and other substrates.