Patent Number: 046410333
Section: description

DESCRIPTION OF BEST MODE AND OTHER EMBODIMENTS FOR CARRYING THE INVENTION FIG. 1 illustrates an exemplary illuminator 10 using an optical system in accordance with the present invention. Referring to this figure, an electrodeless lamp 11 comprises a microwave chamber 12 containing a lamp bulb 13 excitable by microwave energy 14. Microwave chamber 12 has a circular aperture 15 covered by a planar circular mesh 16 which is secured to the spherical wall of chamber 12. Both the spherical and mesh portions of chamber 12 are made of a conductive material such as copper or aluminum. Additionally, a portion of the inner surface of the chamber wall opposite to aperture 15 may be coated with a deep UV reflecting material. The spherical wall of chamber 12 also has a rectangular slot 18 in the position shown for coupling microwave energy to the lamp bulb. The envelope of lamp bulb 13 is preferably spherical in shape and is disposed at the center of spherical chamber 12. The envelope is made of high purity, high OH (wet) quartz, which is a highly transmissive material for deep UV radiation. Bulb 13 has a quartz stem 20 for mounting the envelope in the chamber. In order to provide cooling of bulb 13 during its operating, the bulb is rotated by an electric motor 22 while streams of compressed air are directed at the bulb by nozzles 21 and 23 which are connected by appropriate conduits to a source of compressed air 24. Bulb stem 20 is in effect an extension of motor shaft 26. Microwave energy 14 is generated by a magnetron 30 which is energized by a power supply 32. Microwave energy 14 is fed from magnetron 30 to chamber 12 through slit 18 by a rectangular waveguide 34. Lamp bulb 13 is filled with a plasma forming medium, such as mercury dispersed in a noble gas, and the microwave energy passing through slot 18 excites the plasma substantially throughout the volume of the bulb envelope. This causes the bulb envelope to emit ultraviolet radiation which is directed through the ultraviolet transmissive window formed by circular mesh 16. It has been found that the radiation that is emitted by such electrodeless lamps is much richer in the deep UV part of the radiation spectrum than the radiation omitted by conventional UV arc lamps. The spherical envelope of lamp 13 therefore effectively emits a uniform stream of ultraviolet radiation 36 in the direction of a lens array in the form of an optical assembly 37. Assembly 37 forms part of an optical train for coupling the ultraviolet radiation exiting from mesh 16 to wafer 38 as efficiently as possible. A mask 40 for providing an irradiation pattern to a photoresistive coating on wafer 38 is disposed in contact with the wafer coating. The system illustrated in FIG. 1 is therefore known as a contact or proximity photolithographic system. However, as indicated above, the invention is applicable to a wide variety of other types of optical systems and apparatuses, including those for merely transmitting, e.g., fiber optics, and also projecting types having a projector instead of an illuminator. Optical assembly 37 may be comprised of a series of lens elements, such as lenses 42, 44, 46 and 51 shown diagrammatically in FIG. 1. Lenses 42, 44 and 46 interact to form a condensor array. Downstream of lens 46 is a collimating lens 51. The optical train further includes a filter mirror 52 which reflects the longer ultraviolet wavelengths and the visible and infrared components of the radiation while transmitting deep ultraviolet wavelengths in the range of 190 to 260 nm. The UV light transmitted by optical assembly 37 to filter mirror 52 is fed through a shutter 54 which controls the duration (amount) of the ultraviolet radiation to which semi-conductor wafer 38 is to be exposed. Shutter 54 is electronically controlled in conjunction with power supply 32 for magnetron 30 by a controller 56 which controls the shutter speed and lamp bulb intensity in response to an ultraviolet senser 58 so as to provide the desired dose of radiation to the coating on semi-conductor wafer 38. The final element of the optical train is a collimating lens 60 which transmits a uniform UV radiation field to mask 40. The collimated field is large enough to fill the required diameter of the wafer surface with ultraviolet light of the required irradiance as formed by the optics of optical assembly 37. Because of its high transmittance of deep UV radiation, high purity, high OH content quartz is a preferred material for the multiple lenses of optical assembly 37 and collimator lens 60, as well as for the envelope of bulb 13. One or more of these lenses also may be coated with an optical coating composition to provide a thin film of anti-reflective material. In accordance with the present invention, the lenses of optical assembly 37 are each heated to and maintained at a temperature of at least about 280.degree. C., preferably a temperature in the range of about 300.degree.-400.degree. C., more preferably a temperature in the range of about 300.degree. C.-350.degree. C., during operation of the illuminator. Heating is preferably accomplished by ceramic heating bands 62 containing a resistance wire 63 connected to a source of electricity (not shown). The bands may be wound in spiral coils around a cylindrical lens housing 64, which is preferably of aluminum, steel or some other heat conductive material. The heating bands may be enclosed in a casing 65 clamped around housing 64. Housing 64 preferably contains a gas, such as air, which also is heated and transfers heat from the walls of the housing to each of the lenses by gas connection. In the absence of such heating of optical assembly 37, the time required to deliver the desired dose of radiation to the surface of semi-conductor wafer 38 can double in about 1,000 hours of operation due to ultraviolet degradation of the lenses of the optical train. This degradation is in the form of significant increases in the absorption band which develops at and on either side of about 215 nm as previously described. This degradation has been found to be particularly pronounced in lenses occupying the position of lens 46 in FIG. 1. The heating of collimator lens 60 may be optional because this lens is subjected to much lower levels of ultraviolet radiation than the lenses of optical assembly 37. However, in many applications, it also may be desirable to maintain the temperature of collimator lens 60 at the elevated temperatures specified above during operation of the illuminator. Lens 60 may be heated by wrapping coils of a heating band or tape around a conductive lens mounting 70 in a manner similar to the application of heating bands 62 around housing 64 of optical assembly 37. As an alternative, the collimator lens 60 may be provided with an annular heating tape 72 in conductive contact with the lens material. Heating tape 72 is preferably located outside of the optical path near the periphery of lens 60 so as not to interfere with the useful optical area of the lens or otherwise impair its transmission capabilities. The annular shape of tape 72 is interrupted at one location by a narrow radial gap 74 so that resistance wire 76 within the tape can be connected to a source of electrical energy (not shown). As previously indicated, a thin layer of metal or other electrically resistive material may be coated directly onto the body of the lens to provide an annular ring-like structure similar to that of tape 72. Such coatings also are capable of conductively heating an optical lens. The provision of an electrically resistive coating on a lens is described in U.S. Pat. No. 3,495,259 which already has been referred to above. Direct conductive heating of lenses by heating tape or other coatings also may be used for each of the lenses in optical assembly 37 as an alternative or supplement to the heating of housing 64 by heating bands 62. According to the invention, therefore, radiation degradation of optical elements, such as lenses and fiber optics waveguides, is prevented by maintaining these optical elements at a temperature above that at which the lens material could be annealed to remove defect centers or other causes of selective absorption of certain wavelengths which develop upon prolonged irradiation of these elements. Such heating may be achieved by a number of different techniques, including irradiation, hot air convection and/or direct contact with a heating member such as a heating tape or a thin layer of metal or other coating placed on one or more surfaces of the optical elements. Where a heating member is in direct contact with an optical element, it is positioned so as not to interfere with the optically useful area of the element or the equipment in which it is located. The width, thickness and length of a heating member in direct contact with the material of an optical element are selected to achieve the level of sustained heating desired, and depend on the size of the optical element to be heated and the available power supply. The amount of electrical resistance necessary for a required heat output can be determined by conventional means. If the central portions or other optically useful area of a lens cannot be heated sufficiently on account of the temperature gradient between the heated periphery and the optically useful area, an increase in the heating temperature of the resistance member may be necessary. Alternatively, supplemental heating may be provided by direct radiative heating and/or convection heating with hot gases such as air, particularly where the optical elements are enclosed within a housing of heat conductive material. In such optical embodiments, heating of the housing causes direct radiative heating of the lens and also will heat air within the housing for convection heating of the lenses. Heat also may be transmitted to the optical elements by conduction through lens mountings of conducting material. Degradation in the presence of deep ultraviolet radiation may first become noticeable after about 200 hours of exposure at an irradiation level of about 150 milliwatts per square centimeter. Accordingly, the total amount (dose) of ultraviolet energy accumulated after 200 hours at this irradiation level is about 100 kilojoules per square centimeter. The amount of absorption in the wavelength band around 215 nm increases with further irradiation and becomes particularly pronounced after about 800 hours, which exposure time at 150 milliwatts per square centimeter is equivalent to a total accumulated dose of about 400 kilojoules per square centimeter. This phenomenon of increased ultraviolet absorption with increased time of exposure is illustrated in FIG. 3 for different lenses which have been exposed to about 150 milliwatts per square centimeter of deep ultraviolet radiation for different lengths of time. Thus, line 77 represents the level of absorption by a new lens, line 78 the absorption of a lens exposed for 300 hours, and line 79 the absorption of a lens exposed for 800 hours. The lenses measured in developing the data for FIG. 3 were those occupying the position of lens 46 of FIG. 1 taken from three different optical assemblies, one being new and the other two being exposed for the times indicated. The transmittance of each of these lenses was measured separately with a laboratory set-up using a deuterium lamp producing a deep ultraviolet continuum in the range of 200-250 nm. The output from the deuterium lamp was directed through a small, round hole in a diaphragm to create a narrow beam of deep ultraviolet radiation through the middle of the lens. This narrow beam was then passed through a diffuser and into the slit of a monochromator apparatus for measuring the intensity of radiation as a function of its wavelength. The output of the monochromator was then fed to a computer for storage and subsequent print-out of the graphs making up FIG. 3. The same laboratory set-up used for FIG. 3 also was used in developing the data of FIGS. 4-6 discussed below. Referring to FIG. 4, this figure illustrates the change in transmittance of a lens element at 215 nm with the time of exposure without the application of heat (line 80). This figure further illustrates that where another lens element is maintained continuously at 300.degree. C., the radiation degradation represented by line 80 does not occur even after 1,000 hours of exposure (line 82). In FIG. 5, there is shown the graph of two lenses that are each irradiated without heating for the first 1,000 hours of operation and accordingly undergo degradation as indicated by the single line 83 starting at about 95% transmittance and decreasing to about 70% transmittance. Both lenses were then annealed at about 400.degree. C. for about 3 hours. Irradiation was then resumed for about another 100 hours of operation with a continuation of heating to at least about 300.degree. C. of the lens represented by line 84 and a discontinuance of heating of the lens represented by line 86. The dotted line 85 between lines 84 and 86 is a representation of what had been predicted from prior literature on the possible effects of annealing radiation degraded slicia. As indicated by the rapid divergence of lines 85 and 86, the unheated lens after being annealed returned to its degraded state at a rate far exceeding its rate of degradation as a new lens. The extremely rapid degradation of an annealed lens as shown by line 86 was an entirely unexpected result of testing related to the present invention. In marked contrast, the annealed lens of which heating was continued did not exhibit any such degradation but maintained its annealed transmittance at a level which is only about 3-5 percent less than that of a new lens. The broken portions of lines 84 and 86 beyond 1,100 hours are not based on actual measurements but represent an extrapolation of the performance of these lenses based on the test results described. FIG. 6 is similar to FIG. 5 except that the graphs represent a summation of the transmittance of all lenses in optical assemblies corresponding to optical assembly 37 of FIG. 1, instead of the transmittance of a single lens such as lens 46. Thus, line 88 represents an optical assembly 37 that is not heated for 1,000 hours but is then wrapped with a heating tape and thereafter heated to about 300.degree. C. for the remainder of its exposure to deep ultraviolet radiation. For comparison, line 90 is representative of an optical assembly 37 which is annealed only for about 3 hours at about 400.degree. C. and thereafter heating is discontinued upon further exposure of the assembly to deep ultraviolet radiation. Again, the unexpectedly rapid degradation of the annealed assembly was observed and is illustrated by the rapid divergence of line 90 from a dotted line 89 representing what had been predicted from prior literature on the possible effects of annealing radiation degraded silica. FIGS. 7 and 8 represent a comparison of the total output of an illuminator of the type shown in FIG. 1 without use of the heating band 62 and after annealing for about 2 hours with use of the heating band 62, respectively. In this illuminator, each of the lenses were coated with an antireflective coating to decrease reflection and thereby increase transmittance of the deep ultraviolet radiation. The test set-up for these measurements was basically that shown in FIG. 1 except that the wafer 38 and mask 40 were eliminated and the UV sensor 58 moved into the plane vacated by wafer 38. In this case, the ultraviolet sensor was the detector element of an irradiation measuring apparatus known as a "Mimir" which is capable of measuring the level of irradiation over relatively short intervals or bands of deep ultraviolet radiation. Thus, the irradiation in milliwatts per square centimeter was measured for each band width of 5 nm from 200 nm to 280 nm. These band width intervals and the measured irradiation by these wavelengths are shown in the first two columns of the table portions of FIGS. 7 and 8. The last column of these tables is a summation of the second column to give the integrated amount of radiation incident upon the UV sensor over the entire range from 200 nm to 280 nm. The graph portion of FIGS. 7 and 8 is a plot of the first two columns of the table, with the total area under the curve from 200 nm to 280 nm in increments of 5 nm being given by the third column of the table. The illuminator tested had been operated for 2,400 hours, which was the time of exposure of the optical system to deep UV irradiation and is indicative of the total dose delivered to the optical elements. Prior to making each set of measurements, the ultraviolet lamp 13 was operated for about 20 minutes in order to achieve stability within the optical system. The nomenclature of the table portion of FIGS. 7 and 8 is further described as follows. The tests were run on Mar. 22, 1984. There is then given the serial number of the lamp bulb, the illuminator unit number, and the optical assembly number. The optical assembly providing the data for FIG. 7 had no heating band throughout 2,400 hours of illuminator operation. A heating band was then applied to the optical assembly of FIG. 7 and was activated to heat this optical assembly to about 300.degree.-350.degree. C. for about 2 hours in order to provide the data for FIG. 8. The headings of the table have already been explained above. The nomenclature of the graph portion of FIGS. 7 and 8 is further described as follows. To show particularly that the invention eliminates degradation at wavelengths on either side and at about 215 nm, a band of from 210 nm to 240 nm was selected and the irradiation measured by the Mimir instrument was integrated over this range. The integrated irradiation without heating over this band width was 9.1 milliwatts per square centimeter as compared to an irradiation of 15.1 milliwatts per square centimeter where the optical system had been heated to more than 300.degree. C. for an annealing period of about 2 hours. Annealing invention thereby resulted in an increase of over 60% in the level of UV irradiation delivered by the illuminator to the semi-conductor wafer in the wavelength range of 210 nm to 240 nm. The remaining nomenclature associated with the graph portions of FIGS. 7 and 8 indicate that the lamp was calibrated in February at a power supply level of 650 milliamperes and 3.4 kilovolts, and that the ultraviolet sensor generated a maximum signal of 0.16 volts for the unheated optical system as compared to a maximum signal of 0.22 volts for the heated optical system. Although it could not be confirmed by experimental data, some of the measured degradation indicated by FIG. 7 may have been due to radiation degradation of the composition with which the lenses were coated. In this regard, the invention is also applicable to optical coatings which upon exposure to radiation become degraded by an increase in absorption by the coating of one or more wavelengths of the radiation to which the coating is exposed. This degradation must then be reversible upon annealing the coating. In other words, coatings may also contain silica and other materials that can be degraded by exposure to radiation and then annealed to temporarily remove such degradation. Maintaining these coatings at a temperature above the annealing temperature throughout exposure of the coating to radiation will avoid such radiation degradation in the same manner that degradation of a silica lens is avoided by application of the invention. There thus has been disclosed a method and apparatus for preventing radiation degradation of optical elements and systems employed in a wide variety of applications, such as fiber optics and deep ultraviolet photolithography. The numerous advantages realized by practicing the invention have heretofore been discussed in detail. The heating methods and apparatuses disclosed for practicing the invention may find use in numerous processes other than fiber optics and photolithography. Accordingly, while specific preferred embodiments have been illustrated and described, many variations of these embodiments will fall within the scope of the invention which is defined only by the claims below.