Abstract:
An inspection microscope ( 1 ) having a light source ( 3 ) that emits light of a first wavelength below 400 nm for illumination of a specimen ( 13 ) to be inspected, and having an objective ( 11 ) that is composed of multiple optical components and has a numerical aperture and a focal length, and having a tube optical system ( 21 ) and an autofocus device ( 25 ) that directs light of a second wavelength onto the specimen ( 13 ), is disclosed. The inspection microscope ( 1 ) is characterized by the objective ( 11 ), which has an optical correction that eliminates the longitudinal chromatic aberrations with respect to the first and the second wavelength and whose optical components are assembled in cement-free fashion, the second wavelength being greater than 400 nm.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This invention claims priority of the German patent application 101 17 167.6-42 which is incorporated by reference herein.  
         FIELD OF THE INVENTION  
         [0002]    The invention concerns an inspection microscope. The invention further concerns an objective used with the inspection microscope.  
         BACKGROUND OF THE INVENTION  
         [0003]    In the semiconductor industry, inspection microscopes are used for the examination and inspection of wafers, masks, and semiconductor modules during the various phases of their production. Inspection microscopes are for the most part largely automated. This encompasses, inter alia, automatic transport and handling systems for the modules or wafers to be examined, as well as an automatic focusing capability.  
           [0004]    Inspection microscopes are described, for example, in the German patent documents DE 39 17 260 “Wafer inspection device” and DE 197 42 802 C1 “Microscope stand for a wafer inspection microscope.” 
           [0005]    The optical resolution capability of a microscope depends on the wavelength of the illuminating light and the numerical aperture of the objective. The smaller the feature to be resolved, the shorter the illuminating light wavelength that must be selected, since the numerical aperture of the objectives cannot be increased indefinitely. For dry objectives, numerical apertures of no more than 0.9 to 0.95 can be attained. The size of the features on wafers for highly integrated circuits necessitates the use of ultraviolet light. Illuminating wavelengths between 248 nm and 365 nm are common at present.  
           [0006]    Standard objectives are operated in the visible region of the light spectrum, i.e. in the spectral region from 400 nm to 800 nm. Standard objectives are unsuitable for applications with ultraviolet light, since the transmittance of standard objectives decreases dramatically the further into the ultraviolet the selected wavelength lies.  
           [0007]    An objective that is achromatic in both the visible and the ultraviolet region is disclosed in the Japanese Patent having publication number JP2000105340 A. This objective is made of at least three different types of glass that contain barium fluoride, the lens elements being assembled into several groups of which the first, second, and fourth have positive refractive power while the third group has negative refractive power.  
           [0008]    Irradiation with extremely short-wave ultraviolet light results in damage both to standard objectives and to special objectives that were in fact manufactured for the ultraviolet region. In standard objectives this damage is attributable, inter alia, to phototropic effects in the glass that cause a diminution in transmittance due to chemical modification of the glass structure. Damage of this kind is often reversible. Objectives designed specifically for the ultraviolet region are usually fabricated from quartz glass or calcium fluoride. Glasses made of these materials exhibit high transmittance in the ultraviolet region and are not modified by ultraviolet light. Irreversible damage nevertheless also occurs in these special objectives just as in standard objectives, becoming evident as gradual clouding, decreased transmittance, and degraded resolution. These difficulties have hitherto not been completely understood.  
           [0009]    An additional difficulty occurs when an inspection microscope is equipped with an autofocus system, in which an autofocus light beam is coupled into the beam path of the inspection microscope and focused by the objective. Focusing is then performed, for example, by ascertaining the contrast of the image of the light reflected from the component being examined, using a four-quadrant photodiode. Since the inspection microscope must be usable in both visible light and ultraviolet light, the wavelength of the autofocus light must not lie within those regions in order to prevent the measurement operation from being influenced by light of the autofocus system. Since the sensitivity of semiconductor detectors is highest in the red to infrared region of the spectrum, it is advantageous to select an autofocus wavelength in that region. The optical properties of objectives are generally different for light of differing wavelengths; this complicates evaluation of the autofocus system signals, which as a result are erroneous.  
         SUMMARY OF THE INVENTION  
         [0010]    It is therefore the object of the invention to describe an inspection microscope that has an autofocus device and that solves the problem stated above.  
           [0011]    The aforementioned object is achieved by an inspection which comprises:  
           [0012]    a light source, emitting light of a first wavelength below 400 nm for illumination of a specimen to be inspected,  
           [0013]    an objective that is composed of multiple optical components, wherein the objective has a numerical aperture and a focal length,  
           [0014]    a tube optical system,  
           [0015]    an autofocus device that directs light of a second wavelength onto the specimen, wherein the second wavelength is greater than 400 nm, wherein the objective shows an optical correction that eliminates the longitudinal chromatic aberrations with respect to the first and the second wavelength, and  
           [0016]    a cement-free mounting of all optical components.  
           [0017]    It is an additional object of the present invention to create an objective which is shows constant optical properties over the long term.  
           [0018]    The aforesaid object is achieved by an objective that transmits light of a first wavelength below 400 nm and light of a second wavelength above 400 nm and comprises:  
           [0019]    multiple optical components, wherein the objective has a numerical aperture as well as a magnification,  
           [0020]    an optical correction that eliminates longitudinal chromatic aberrations with respect to the first and the second wavelength, and  
           [0021]    a cement-free mounting of all optical components.  
           [0022]    The invention has the advantage that the inspection microscope and objective according to the present invention allow a specimen under inspection to be examined using ultraviolet and deep ultraviolet light with no occurrence of damage to the inspection microscope as a result. In addition, it is possible to use a universal autofocus device whose light is transmitted even when standard objectives are utilized.  
           [0023]    It has been recognized that the irreversible damage occurring even in special objectives is brought about as a result of chemical modifications of the optical cement between the individual lens elements upon irradiation with ultraviolet light and in particular upon irradiation with deep ultraviolet light. Objectives known from the existing art that are corrected for more than one wavelength have hitherto had lens elements cemented to one another or cemented lens element groups. The present invention completely resolves this difficulty, and moreover is configured in such a way that with collimated entry, the intersection distance for the autofocus light and for the illuminating light is at least largely identical, so that the autofocus device functions reliably.  
           [0024]    In a preferred embodiment, the optical components are lens elements that are largely transparent to the first and to the second wavelength. They are preferably fabricated from calcium fluoride or quartz glass or barium fluoride or lithium fluoride or strontium fluoride. In a particularly preferred embodiment, adjacent lens elements are produced from calcium fluoride and quartz glass, respectively.  
           [0025]    In a concrete embodiment, the first wavelength is 248 nm and the second wavelength is 903 nm. For high-resolution applications in particular, objectives having a numerical aperture that is greater than 0.8 and a high magnification are particularly advantageous. This means that the focal length of the objective is preferably less than 3.5 mm for a working distance exceeding 0.15 mm.  
           [0026]    In the inspection of masks having pellicles, in which context large unobstructed working distances are important, objectives that have an unobstructed working distance of at least 7 mm with a numerical aperture of at least 0.5 to 0.55 are very particularly advisable.  
           [0027]    It is very particularly advantageous, especially with regard to the physical size of the objective, if the aberrations are not all corrected exclusively in the objective, but rather if an overall correction is achieved by the coaction of the objective and tube optical system. Only the longitudinal chromatic aberration in terms of the first and second wavelengths must be corrected in the objective, since the beam path of the light of the autofocus device does not pass through the tube optics. The correction for light of the second wavelength is, in particular, in fact limited to the center of the image.  
           [0028]    The tube optical system preferably also contains no optical cement, and is configured in such a way that as a result of the coaction of the tube optical system and the objective, the longitudinal chromatic aberration of the objective is compensated for in the region less than 10 nm above and below the first wavelength. It is moreover particularly advantageous to configure the tube optics in such a way that as a result of the coaction of the tube optical system and the objective, the transverse chromatic error of the tube optical system is compensated for in the region less than 10 nm above and below the first wavelength.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The subject matter of the invention is depicted schematically in the drawings and will be described below with reference to the Figures, in which:  
         [0030]    [0030]FIG. 1 shows an embodiment of the inspection microscope;  
         [0031]    [0031]FIG. 2 shows the internal construction of an embodiment of the objective according to the present invention;  
         [0032]    [0032]FIG. 3 is a graphic depiction of the relative intersection distance of the objective as a function of wavelength;  
         [0033]    [0033]FIG. 4 shows the internal construction of a further embodiment of the objective according to the present invention;  
         [0034]    [0034]FIG. 5 shows the internal construction of a further embodiment of the objective according to the present invention;  
         [0035]    [0035]FIG. 6 shows the internal construction of a further embodiment of the objective according to the present invention;  
         [0036]    [0036]FIG. 7 shows the internal construction of a further embodiment of the objective according to the present invention;  
         [0037]    [0037]FIG. 8 shows the internal construction of a further embodiment of the objective according to the present invention; and  
         [0038]    [0038]FIG. 9 shows the internal construction of a tube optical system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    [0039]FIG. 1 shows an embodiment of inspection microscope  1  according to the present invention. A mercury lamp  5 , which emits an illuminating light beam  7  of a first wavelength in the region from 243 to 266 nm, serves as light source  3 . Illuminating light beam  7  is directed with the aid of a semitransparent mirror  9  to objective  11 , and there focused onto a specimen  13  to be inspected. Specimen  13  is located on an X-Y precision positioning stage  15  that is movable along the illuminating light axis for focusing and is driven by an electric motor  17 . Detected light  19  proceeding from the specimen passes via objective  11  and through semitransparent mirror  9  to tube optical system  21  which is assembled in cement-free fashion from multiple lens elements, and ultimately encounters a TV camera  23  whose image is displayed to the user on a monitor (not shown). The inspection microscope comprises an autofocus device  25  that, with the aid of a laser (not depicted), generates an autofocus light beam  27  of a second wavelength of 903 nm, which is coupled into the illuminating beam path with a dichroic beam splitter  28  and is focused by objective  11 . The autofocus light reflected from the specimen passes through objective  11  and via dichroic beam splitter  28  back to the autofocus device, where it is directed onto a four-quadrant photodiode (not shown) whose electrical signals are electronically evaluated in order to assess the position of specimen  13  relative to the focal plane of objective  11 . Autofocus device  25  controls electric motor  17  of precision positioning stage  13  in such a way that the surface of specimen  13  to be inspected lies in the focal plane of objective  11 . Objective  11  contains exclusively lens elements assembled in cement-free fashion, and possesses, with collimated light entry, the same intersection distance with respect to the first and second wavelengths. It has a 150×magnification with a numerical aperture of 0.9. The longitudinal chromatic aberration of objective  11  in the region from 238 nm to 258 nm is compensated for by the opposite longitudinal chromatic aberration in tube optical system  21 , and similarly for the transverse chromatic aberration.  
         [0040]    [0040]FIG. 2 shows the internal configuration of an embodiment of objective  11  that contains twenty-one lens elements labeled with reference characters  29  through  69 . In combination with the tube optical system of 200 mm focal length shown in FIG. 9, objective  11  has a 150×magnification and a numerical aperture of 0.9. Lens elements  29 - 69  each have two boundary surfaces  71 - 153 , and are each manufactured of quartz glass or calcium fluoride. Air gaps are present between lens elements  29 - 69 . With collimated light entry, the objective has the same intersection distance of 0.418 mm for light of wavelengths 248 nm and 908 nm. The radii of curvature of boundary surfaces  71 - 153  of lens elements  29 - 69 , and lass of the individual lens elements and their spacings from one be gathered from the following table:  
                                                                     Lens   Boundary                   element   surface   Radius/mm   Spacing/mm   Glass type                                29   71   −1.0070   1.0200   QUARTZ GLASS       29   73   −1.0070   .2000       31   75   −5.1142   .7000   QUARTZ GLASS       31   77   116.3432   .0200       33   79   116.3432   2.1000   CAF2       33   81   −3.3054   .1000       35   83   13.0257   1.1000   QUARTZ GLASS       35   85   15.2835   .0200       37   87   15.2835   2.5000   CAF2       37   89   −6.3932   .5000       39   91   −63.2383   1.4000   QUARTZ GLASS       39   93   10.4603   .0200       41   95   10.4603   2.6000   CAF2       41   97   −15.0571   .2000       43   99   −71.0504   1.5000   QUARTZ GLASS       43   101   9.8492   .0200       45   103   9.8492   2.4000   CAF2       45   105   −58.9093   .1000       47   107   23.1720   2.9000   CAF2       47   109   −9.2663   .0200       49   111   −9.2663   1.5000   QUARTZ GLASS       49   113   23.9793   .1000       51   115   11.6587   3.4000   CAF2       51   117   −12.6456   .1000       53   119   −12.6456   1.5000   QUARTZ GLASS       53   121   2610.3417   .3000       55   123   30.6023   2.7000   CAF2       55   125   −9.9195   .0200       57   127   −9.9195   1.5000   QUARTZ GLASS       57   129   34.6720   .1000       59   131   7.0048   3.5000   CAF2       59   133   41.7404   1.8000       61   135   −27.4055   1.0000   QUARTZ GLASS       61   137   3.5732   .3000       63   139   4.0071   2.8000   CAF2       63   141   −7.1920   .0500       65   143   −9.8514   1.0000   QUARTZ GLASS       65   145   30.3842   5.5000       67   147   −1.7127   1.0000   CAF2       67   149   10.6695   .0500       69   151   4.0356   1.5000   QUARTZ GLASS       69   153   −18.3130   inf.                  
 
         [0041]    [0041]FIG. 3 shows, in a graphic depiction, the relative intersection distance of objective  11  as a function of wavelength λ. It is evident that in the region a few nanometers around 248 nm, a low longitudinal chromatic aberration exists. In an inspection microscope, this longitudinal chromatic aberration is preferably compensated for using a specially designed tube optical system, so that even broad-band light used in aberration-free fashion.  
         [0042]    [0042]FIG. 4 shows the internal configuration of a further embodiment of objective  11  that contains sixteen lens elements labeled with reference characters  155  through  185 . In combination with the tube optical system of 200 mm focal length shown in FIG. 9, objective  11  has a 63×magnification and a numerical aperture of 0.55. lens elements  155 - 185  each have two boundary surfaces  187 - 249 , and are each manufactured of quartz glass or calcium fluoride. Air gaps are present between lens elements  155 - 185 . With collimated beam entry, the objective has the same intersection distance of 7.7 mm for light of wavelengths 248 nm and 903 nm. The working distance is 7.0 mm. The radii of curvature of boundary surfaces  187 - 249  of lens elements  155 - 185 , and the types of glass of the individual lens elements and their spacings from one another, may be gathered from the following table:  
                                                                     Lens   Boundary                   element   surface   Radius   Spacing   Glass type                                155   187   −31.8610   3.2000   QUARTZ       155   189   −8.9120   .3000       157   191   36.5170   5.3400   CAF2       157   193   −10.2030   .2400       159   195   −9.9220   2.0000   QUARTZ       159   197   20.1110   .2200       161   199   21.2920   6.0200   CAF2       161   201   −16.4330   .2000       163   203   44.0460   4.9000   CAF2       163   205   −16.5100   .3600       165   207   −15.6420   2.0000   QUARTZ       165   209   14.7420   .1700       167   211   15.1020   5.5000   CAF2       167   213   −29.4540   .2000       169   215   19.5190   5.0000   CAF2       169   217   −19.5190   .0800       171   219   −19.5200   2.0000   QUARTZ       171   221   8.8780   .5000       173   223   9.6970   5.8000   CAF2       173   225   −13.2430   .0700       175   227   −15.5050   2.0000   QUARTZ       175   229   plane surface   2.4010       177   231   −11.5420   1.8500   QUARTZ       177   233   48.0020   .2000       179   235   9.8410   3.2000   CAF2       179   237   plane surface   10.7500       181   239   −10.1470   1.6000   QUARTZ       181   241   −5.3040   .2300       183   243   −4.8080   1.1000   CAF2       183   245   4.8080   .0700       185   247   4.6630   1.4500   QUARTZ       185   249   7.1840   inf.                  
 
         [0043]    [0043]FIG. 5 shows the internal configuration of a further embodiment of objective  11  that contains seventeen lens elements labeled with reference characters  251  through  282 . In combination with the tube optical system of 200 mm focal length shown in FIG. 9, objective  11  has a 150×magnification and a numerical aperture of 0.90. Lens elements  251  through  283  each have two boundary surfaces  285 - 351 , and are each manufactured of quartz glass or calcium fluoride. Air gaps are present between lens elements  251  through  283 . With collimated beam entry, the objective has the same intersection distance of 0.364 mm for light of wavelengths 248 nm and 903 nm. The working distance is 0.2 mm. The radii of curvature of boundary surfaces  285 - 351  of lens elements  251  through  283 , and the types of glass of the individual lens elements and their spacings from one another, may be gathered from the following table:  
                                                                     Lens   Boundary                   element   surface   Radius   Spacing   Glass type                                251   285   −1.3020   1.2200   QUARTZ GLASS       251   287   −1.3020   .2000       253   289   −4.4850   1.8800   CAF2       253   291   −2.8770   .1000       255   293   −25.3180   2.4000   CAF2       255   295   −3.9040   .0800       257   297   −3.8810   1.2000   QUARTZ GLASS       257   299   16.8000   .0310       259   301   16.8010   3.7000   CAF2       259   303   −7.4810   .5000       261   305   9.4200   4.1500   CAF2       261   307   −14.1090   .2000       263   309   −12.9970   1.5000   QUARTZ GLASS       263   311   8.4710   .0380       265   313   8.4720   4.1800   CAF2       265   315   −10.8320   .0350       267   317   −10.8320   1.5000   QUARTZ GLASS       267   319   7.5030   .6000       269   321   10.0000   3.3500   CAF2       269   323   −12.2530   .1000       271   325   18.1250   1.5000   QUARTZ GLASS       271   327   5.8690   .1000       273   329   5.9510   4.5300   CAF2       273   331   −8.0360   .0360       275   333   −8.0360   1.5000   QUARTZ GLASS       275   335   50.6020   .1000       277   337   6.3030   4.1000   CAF2       277   339   48.0020   1.2100       279   341   −8.1590   3.1000   QUARTZ GLASS       279   343   5.4540   3.8000       281   345   −3.3030   1.0000   CAF2       281   347   5.1430   .0500       283   349   4.6190   1.2000   QUARTZ GLASS       283   351   −17.7680   inf.                  
 
         [0044]    [0044]FIG. 6 shows the internal configuration of a further embodiment of objective  11  that contains nineteen lens elements labeled with reference characters  353  through  389 . In combination with the tube optical system of 200 mm focal length shown in FIG. 9, objective  11  has a 150×magnification and an aperture of 0.90. Lens elements  353  through  389  each have two boundary surfaces  391 - 465 , and are each manufactured of quartz glass or calcium fluoride. Air gaps are present between lens elements  353  through  389 . With collimated beam entry, the objective has the same intersection distance of 0.42 mm for light of wavelengths 248 nm and 903 nm. The working distance is 0.2 mm. The radii of curvature of boundary surfaces  391 - 465  of lens elements  353  through  389 , and the types of glass of the individual lens elements and their spacings from one another, may be gathered from the following table:  
                                                                     Lens   Boundary                   element   surface   Radius   Spacing   Glass type                                353   391   −1.0070   .9600   QUARTZ GLASS           393   −.9500   .1000       355   395   −5.4908   .7000   QUARTZ GLASS           397   27.0607   .0200       357   399   27.0607   2.2000   CAF2           401   −3.2531   .1000       359   403   −10.7848   1.2000   QUARTZ GLASS           405   12.1721   .0200       361   407   12.1721   2.8000   CAF2           409   −6.8280   .4000       363   411   −64.0972   1.5000   QUARTZ GLASS           413   9.8461   .0200       365   415   9.8461   3.0000   CAF2           417   −12.9844   .2000       367   419   101.2944   3.0000   CAF2           421   −7.8870   .0200       369   423   −7.8870   1.5000   QUARTZ GLASS           425   15.6216   .2000       371   427   11.8848   4.1000   CAF2           429   −8.5251   .0200       373   431   −8.5251   1.5000   QUARTZ GLASS           433   −140.7178   .2000       375   435   20.3658   3.3000   CAF2           437   −9.4168   .0200       377   439   −9.4168   1.5000   QUARTZ GLASS           441   93.5331   .1000       379   443   6.9040   4.7000   CAF2           445   897.4094   1.9000       381   447   −11.6312   1.0000   QUARTZ GLASS           449   3.7870   .3000       383   451   4.1771   2.8000   CAF2           453   −6.0687   .0500       385   455   −6.9364   1.0000   QUARTZ GLASS           457   −35.5321   5.8000       387   459   −1.7885   1.0000   CAF2           461   5.0394   .0500       389   463   3.5153   1.7000   QUARTZ GLASS           465   −17.9020   inf.                  
 
         [0045]    [0045]FIG. 7 shows the internal configuration of a further embodiment of objective  11  that contains twenty lens elements labeled with reference characters  467  through  505 . In combination with the tube optical system of 200 mm focal length shown in FIG. 9, objective  11  has a 200×magnification and an aperture of 0.90. Lens elements  467  through  505  each have two boundary surfaces  507 - 587 , and are each manufactured of quartz glass or calcium fluoride. Air gaps are present between lens elements  467  through  505 . With collimated beam entry, the objective has the same intersection distance of 0.384 mm for light of wavelengths 248 nm and 903 nm. The working distance is 0.2 mm. The radii of curvature of boundary surfaces  507 - 587  of lens elements  467  through  505 , and the types of glass of the individual lens elements and their spacings from one another, may be gathered from the following table:  
                                                                     Lens   Boundary                   element   surface   Radius   Spacing   Glass type                                467   507   −1.3406   1.3417   QUARTZ GLASS           509   −1.2832   .2000       469   511   −4.6344   2.0000   CAF2           513   −3.0395   .1000       471   517   −19.1192   2.2000   CAF2           519   −4.1705   .0500       473   521   −4.2122   1.2000   QUARTZ GLASS           523   15.2410   .0500       475   525   15.2410   3.3201   CAF2           527   −8.9154   .5000       477   529   9.9453   4.3403   CAF2           531   −11.5633   .1570       479   533   −10.9741   1.5000   QUARTZ GLASS           535   9.4708   .0500       481   537   9.4708   4.0274   CAF2           539   −10.1508   .0500       483   541   −10.1508   1.5000   QUARTZ GLASS           543   8.4705   .5016       485   545   10.8133   3.4593   CAF2           547   −12.9233   .1000       487   549   11.9103   1.5000   QUARTZ GLASS           551   6.1198   .0500       489   553   6.1198   4.2788   CAF2           555   −9.7847   .0500       491   557   −9.7847   1.5000   QUARTZ GLASS           559   92.3864   .1000       493   561   11.5539   3.8000   CAF2           563   −62.2415   1.8263       495   565   −6.2817   1.0000   QUARTZ GLASS           567   7.3680   .0500       497   569   6.8762   2.0000   CAF2           571   −14.6005   .1151       499   573   −12.1375   1.0000   QUARTZ GLASS           575   6.8053   9.0000       501   577   −3.6630   1.5000   QUARTZ GLASS           579   −3.0606   .0500       503   581   −3.7004   1.0000   CAF2           583   6.1983   .1993       505   585   7.5116   1.7000   QUARTZ GLASS           587   −33.0000   inf.                  
 
         [0046]    [0046]FIG. 8 shows the internal configuration of a further embodiment of objective  11  that contains twenty lens elements labeled with reference characters  589  through  623 . In combination with the tube optical system of 200 mm focal length shown in FIG. 9, objective  11  has a 63×magnification and an aperture of 0.90. Lens elements  589  through  623  each have two boundary surfaces  625 - 695 , and are each manufactured of quartz glass or calcium fluoride. Air gaps are present between lens elements  589  through  623 . With collimated beam entry, the objective has the same intersection distance of 0.417 mm for light of wavelengths 248 nm and 903 nm . The working distance is 0.2 mm. The radii of curvature of boundary surfaces  625 - 695  of lens elements  589  through  623 , and the types of glass of the individual lens elements and their spacings from one another, may be gathered from the following table:  
                                                                     Lens   Boundary                   element   surface   Radius   Spacing   Glass type                                589   625   −2.1244   1.7200   QUARTZ GLASS           627   −2.1486   .1000       591   629   −4.6685   2.5000   CAF2           631   −3.2430   .2000       593   633   −8.3322   1.2000   QUARTZ GLASS           635   25.1850   .3268       595   637   43.3114   4.4000   CAF2           639   −8.1452   .3000       597   641   16.8037   5.7000   CAF2           643   −11.6783   .2968       599   645   −11.0103   1.8000   QUARTZ GLASS           647   16.4439   .0500       601   649   16.4439   6.5000   CAF2           651   −13.6015   .0500       603   653   −16.2615   1.8000   QUARTZ GLASS           655   15.3763   .0500       605   657   15.3763   5.7500   CAF2           659   −22.5022   .3000       607   661   14.5691   5.8000   CAF2           663   −21.6045   .4808       609   665   −17.9557   1.8000   QUARTZ GLASS           667   9.2065   .1088       611   669   9.3059   5.3000   CAF2           671   −38.5138   .4000       613   673   21.6588   3.3000   CAF2           675   −37.7818   .8000       615   677   −15.4376   1.9000   QUARTZ GLASS           679   356.4300   3.8480       617   681   −7.5117   1.4000   QUARTZ GLASS           683   15.5540   .1000       619   685   8.1444   2.8000   CAF2           687   16.3383   1.4000       621   689   −13.4675   1.3000   CAF2           691   8.6117   .1736       623   693   9.6434   2.5000   QUARTZ GLASS           695   −33.000   inf.                  
 
         [0047]    [0047]FIG. 9 shows the internal configuration of tube optical system  21 , which contains three lens elements labeled with reference characters  697  through  699 . Tube optical system has a focal length of 200 mm at 248 nm. Lens elements  697  through  699  each have two boundary surfaces  703 - 713 , and are each manufactured of quartz glass or calcium fluoride. Air gaps are present between lens elements  697  through  699 . Tube optical system  21  contains no cement, and is configured in such a way that as a result of the coaction of tube optical system  21  and objective  11 , the longitudinal chromatic error of objective  11  is compensated for in the region 10 nm above and below the first wavelength. In addition, tube optical system  21  is embodied in such a way that as a result of the coaction of tube optical system  21  and objective  11 , the transverse chromatic error of tube optical system  21  is compensated for in the region less than 10 nm above and below the first wavelength. The radii of curvature of boundary surfaces  703 - 713  of lens elements  697  through  699 , and the types of glass of the individual lens elements and their spacings from one another, may be gathered from the following table:  
                                                                     Lens   Boundary                   element   surface   Radius   Spacing   Glass type                                697   703   174.9820   3.0000   Quartz glass           705   −60.4200   12.1575       699   707   −44.6500   2.5000   CaF2           709   76.1030   .1000       701   711   76.1030   3.0000   Quartz glass           713   −191.4220   169.6484                  
 
         [0048]    The invention has been described with reference to a particular embodiment. It is nevertheless self-evident that changes and modifications can be made without thereby leaving the range of protection of the claims recited hereinafter.