Abstract:
An illumination device for a DUV microscope has an illumination beam path, proceeding from a DUV light source in which are arranged a condenser and a reflection filter system which generates a DUV wavelength band and comprises four reflection filters. At these, the illumination beam is reflected in each case at the same reflection angle α, the illumination beam path extending coaxially in front of and behind the reflection filter system. According to the present invention, the reflection angle α=30° and the DUV wavelength band λ DUV +Δλ has a half-value width of max. 20 nm and a peak with a maximum value S of more than 90% of the incoming light intensity. The resulting very narrow half-value width of the DUV wavelength band makes it possible for the DUV objectives of the DUV microscope to be very well-corrected.

Description:
FIELD OF THE INVENTION 
     The invention refers to an illumination device for a DUV microscope. Furthermore, the invention refers to a DUV microscope having an illumination device. 
     BACKGROUND OF THE INVENTION 
     Illumination devices for DUV microscopes must make available a narrow DUV wavelength band from the DUV wavelength region (DUV=deep ultraviolet, approx. 200 to 300 nm) for the illumination for which the microscope optical system is corrected. The DUV wavelength band is characterized by the spectral peak value position and the half-value width of its peak. Both, narrow band transmission filter systems and reflection filter systems, are known for generation of a DUV wavelength band of this kind. These filter systems are inserted into the illumination beam path and filter out of the light spectrum of a light source a desired DUV wavelength band as the usable light. The microscope image is visualized with a DUV-sensitive TV camera. 
     Narrow-band transmission filter systems in the DUV yield very narrow peaks, but the maximum transmission and thus the maximum value of the peak is only approximately 20% of the light intensity before to the narrow-band transmission filter system. Such weak intensities, however, require particularly sensitive and therefore technologically complex and very expensive cameras. The essential disadvantage of these cameras, however (for example, in the case of so-called “frame transfer CCD imagers”), is that they have very long exposure and image readout times, so that “live” observation of the object is impossible. This is particularly problematic if the object stage with the object upon it needs to be shifted. 
     Known reflection filter systems comprise a plurality of reflective filters at each of which the light from the light source arrives and is reflected at an angle of incidence (α=45°. With these known reflection filter systems, the maximum value of the peak is well over 90%. Their disadvantage, however, is the fact that their half-value width is approximately 30 to 50 nm. It is, however, extremely difficult and complex to correct microscope objectives with a short focal length, i.e. high magnification, in the DUV wavelength region for such a broad DUV wavelength band. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the invention to describe an illumination device for a DUV microscope which makes available a DUV wavelength band with maximum transmission and a narrow half-value width. 
     This object is achieved by an illumination as claimed in independent Claim  1 . Advantageous embodiments of the illumination device are recited in the dependent claims. 
     It is a further object of the invention to provide an illumination device for a DUV microscope which allows switching between at least two different illumination wavelength bands. Furthermore, one of the wavelength bands is a DUV wavelength band having maximum transmission and a narrow half-value width. 
     This object is achieved by an illumination device as claimed in independent Claim  10 . Advantageous embodiments of the illumination device are recited in the dependent claims. 
     It is another object of the invention to provide a DUV microscope with an illumination device which makes available a DUV wavelength band with maximum transmission and a narrow half-value width and which allows switching between at least two different illumination wavelength bands. 
     This object is achieved by a DUV microscope with an illumination device as claimed in independent Claim 25. Advantageous embodiments of the DUV microscope are recited in the dependent claims. 
     The invention is based on the idea that the large half-value widths are caused essentially by polarization effects in the reflection filter systems. These are much less for smaller reflection angles. A reflection filter system having small reflection angles and spectrally adapted reflection filters is therefore used in a DUV illumination system according to the present invention. 
     An illumination system according to the present invention for a DUV microscope has a light source from which an illumination beam path proceeds. Arranged in the illumination beam path are a condenser and a reflection filter system for generating a desired DUV wavelength band. The reflection filter system comprises four reflection filters at which the illumination beam is reflected, at the same reflection angle α in each case. The illumination beam extends coaxially in front of and behind the reflection filter system. According to the present invention, the reflection angle α at the individual reflection filters is α≦30°. 
     The reflection filter system supplies a very narrow DUV wavelength band λ DUV +Δλ with a half-value width of max. 20 nm. It has a peak with a maximum value S of more than 90% of the incoming light intensity. Depending on the reflection filters used, a maximum value S of more than 98% of the incoming light intensity can be achieved with a reflection filter system using small angles of incidence. For that purpose, reflection filters matched specifically to the desired DUV wavelength band λ DUV +Δλ are used in the reflection filter system of the illumination device according to the present invention. 
     As a result of the transition to the small reflection angles α=30° according to the present invention in the reflection filter system, polarization effects at the reflection filters can be reduced and narrow half-value widths of this kind can be achieved. Appropriately corrected DUV microscope objectives can be computed for half-value widths of the desired order of magnitude. At the same time, a DUV microscope having an illumination device according to the present invention possesses sufficiently high intensities in the illumination beam to dispense with complex special DUV-sensitive cameras, for example the aforesaid frame transfer CCD cameras with their slow image buildup. 
     The subject matter of the invention is described in more detail with reference to the embodiments shown in the schematic drawings, 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a DUV illumination device for a microscope according to the invention; 
     FIG. 2 shows wavelength-dependent reflection in a reflection filter system with a small angle of incidence α=22.5, divided into polarization components; 
     FIG. 3 shows wavelength-dependent reflection at a single reflection filter for an angle of incidence α=15° and a reference wavelength λ 0 =303.5 nm; 
     FIG. 4 shows wavelength-dependent reflection at a single reflection filter for an angle of incidence α=22.5° and a reference wavelength λ 0 =308 nm; 
     FIG. 5 shows wavelength-dependent reflection at a single reflection filter for an angle of incidence α=30° and a reference wavelength λ 0 =314 nm; 
     FIG. 6 shows the reference wavelength λ 0  as a function of angle of incidence α; 
     FIG. 7 shows wavelength-dependent reflection for three reflection filter systems adapted to different DUV wavelengths, with an angle of incidence α=22.5° in each case; 
     FIG. 8 shows a DUV microscope having a switchable illumination device for selectable VIS or DUV illumination, having two separate VIS and DUV light sources; 
     FIG. 9 shows the location of the lamp images for VIS, UV, and DUV in a DUV illumination device having a universal DUV-VIS light source; 
     FIG. 10 shows a switchable illumination device for selectable VIS or DUV illumination with a universal DUV-VIS light source, in its “DUV illumination” setting; 
     FIG. 11 shows the switchable illumination device of FIG. 10 for selectable VIS or DUV illumination with a universal DUV-VIS light source, in its “VIS illumination” setting; 
     FIG. 12 shows the linear slider for the switchable illumination device of FIGS. 10 and 11, with three switchable sections. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a microscope  1  having objectives  2  and an object  4  resting on an object stage  3 . The DUV light source  5  used is, for example, a high-pressure mercury lamp with spectral components in the VIS, UV, and DUV wavelength regions (definition of spectral regions: VIS=visible; UV=ultraviolet; DUV=deep UV). An illumination beam  6  emerges from light source  5  and passes through a condenser  7  and a reflection filter system  9  made up of four reflection filters  8 . From there, illumination beam  6  passes through an illumination optical system (not depicted), is deflected at a beam splitter  10 , and is focused by objective  2  into object  4 . The imaging beam proceeding from object  4  passes through objective  2  and beam splitter  10  to TV camera  11 . 
     According to the present invention, reflection filters  8  in reflection filter system  9  are arranged so that illumination beam  6  encounters all reflection filters  8  at the same small angle of incidence α. An angle of 30° is depicted here. Smaller angles are possible according to the present invention, but reflection filter systems with angles of incidence α&lt;15° are difficult to implement due to the tight geometry. 
     In the example depicted, reflection filters  8  adapted to the desired DUV wavelength region are vacuum-deposited films that are applied onto black glass. The desired DUV component of the lamp light is reflected. The components of the lamp light that pass through the vacuum-deposited films, i.e. the remaining DUV component and the UV and VIS components, are absorbed in the black glass. This yields an illumination having only the desired DUV wavelength region. 
     FIG. 2 shows the reflection in a reflection filter system with a small angle of incidence α=22.5°, as a function of wavelength. The polarization components with vertical (“V pol”) and parallel (“P pol”) polarization directions resulting from polarization during reflection are shown. These two curves lie very close to one another, since the polarization effect is very small due to the small angle of incidence. The curve labeled “Sum pol” for the summed polarization of the two polarization directions therefore also has a very low half-value width λ with a value of approx. 20 nm. The DUV wavelength band λ DUV  produced with a reflection filter system having a small angle of incidence is therefore sufficiently narrow that the DUV optical system of the microscope can be corrected for it. 
     FIGS. 3 through 5 show the wavelength-dependent reflection at different individual reflection filters designed for different angles of incidence α. 
     The reflection filters possess a layer structure as indicated in Table 1. Only the thicknesses of the layers are corrected, in accordance with the desired angle of incidence α, by way of a reference wavelength λ 0  that is incorporated into the calculation of the layer thicknesses. The associated reference wavelengths λ 0  are indicated in the Figures. As a result of the adaptation of the layer thicknesses, the reflection filters possess qualitatively identical reflection characteristics even for different angles of incidence. FIG. 6 depicts the reference wavelength λ 0  as a function of the angle of incidence α. It is evident that the reference wavelength λ 0  needs much more correction for changes toward greater angles of incidence. 
     FIG. 7 shows the wavelength-dependent reflection for three reflection filter systems, adapted to three different DUV wavelengths, whose reflection filters are each designed for the same angle of incidence α=22.5°. The layer system from Table 1, for example, is once again suitable. It is also possible, however, to imagine other layer systems, for example using SiO 2  and AlO 3 . It is important in this context to use materials with the smallest possible differences in their refractive indices. With the layer system of Table 1, it is possible to describe a reflection filter system that, at least for a DUV wavelength between 180 and 300 nm, matches a narrow DUV wavelength band in each case. 
     Two exemplary embodiments of an illumination device that can be switched over to different wavelength regions will be explained below. 
     FIG. 8 shows a DUV microscope having 1 a switchable illumination device for selectable VIS or DUV illumination that is equipped with two separate light sources for two wavelength regions. The configuration corresponds, in principle, to that of FIG. 1, and identical components have identical reference characters. An additional VIS light source  12  for the visible spectral region is added to the previously described illumination device of FIG. 1 having DUV light source  5 . This VIS light source  12  is, for example, a mercury bulb that radiates spectral components only in the VIS wavelength region, but none in the UV or DUV wavelength region. 
     The light from this VIS light source  12  passes through a condenser  13  and is coupled perpendicularly into illumination beam path  6  only when an additionally installed pivotable mirror  14  in illumination beam path  6  is in position “b”. The VIS light then passes along the subsequent portion of illumination beam path  6  to object  4 . In order to select DUV illumination, pivoting mirror  14  is brought into position “a”. The light from DUV light source  5  then arrives at object  4 , as shown in FIG.  1 . 
     The configuration just described does require a second light source, but is easy to implement by way of pivoting mirror  14 . In addition, the configuration greatly expands the usability of the microscope. For example, DUV light source  5 , e.g. a mercury arc lamp, also radiates in the UV and VIS wavelength region. But it possesses only a small arc, which in the case of weaker VIS objectives cannot fill the pupil and thus cannot provide illumination for these objectives. DUV light source  5  is also not dimmable. 
     The VIS light source, on the other hand, for example a halogen incandescent bulb, possesses a filament of sufficient size for adequate illumination even with weaker VIS objectives, and is dimmable. Its spectrum has no UV or DUV components. With the configuration described, it is thus possible to equip a DUV microscope with all the capabilities of VIS examination. 
     If it is desirable to implement a configuration having only one light source, the shifting of the image of the light source for various wavelengths must be taken into account. FIG. 9 depicts the different positions of the images of a universal light source for the VIS and UV and DUV wavelength regions in a DUV illumination device. The light from light source  15  passes through a condenser  16  and a reflection filter system  9  that is made up of four reflection filters  8 . Reflection filters  8  are vacuum-deposited onto black glass. 
     The various positions of image  17  of light source  15  for the DUV wavelength 248 nm, the so-called “i-line” at 365 nm, and the visible VIS wavelength region are depicted schematically. In order to retain image  17  of light source  15  in one position, light source  15  would need to be displaced in the direction of illumination beam path  6  for the different spectral illumination methods. 
     An alternative to this is shown in FIGS. 10 and 11. These each depict a switchable illumination device for selectable VIS or DUV illumination that operates with only one universal DUV-VIS light source. DUV light source  5 , which for example is a mercury arc lamp, is used for this. Proceeding from DUV light source  5  is an illumination beam path  6  in which a condenser  7  and a reflection filter system  9  having four reflection filters  8  are arranged. The four reflection filters  8  are in this case individually labeled  8   a ,  8   b ,  8   c , and  8   d  in order to distinguish their spatial positions. In this example, reflection filters  8   a ,  8   b ,  8   c , and  8   d  are vacuum-deposited onto a support material that is transparent at all wavelength regions, i.e. quartz glass. 
     FIG. 10 shows the above-described illumination device in the “DUV illumination” setting. In this, a light stopper  18  is inserted between reflection filter  8   a  and reflection filter  8   d . This can be an opaque plate of any kind, for example made of metal. This blocks the component of the lamp light passing through reflection filter  8   a , so that only the reflected DUV wavelength band is used for illumination. Position  17  of the image of light source  5  is indicated. 
     FIG. 11 shows the above-described illumination device in the “VIS illumination” setting. In this, a combination of lenses  19  and at least one filter  20 , selected for the VIS region, are inserted between reflection filter  8   a  and reflection filter  8   d . Lenses  19  shift the position  17  of the VIS image of the light source to the position of the DUV image of the light source. Filter  20  can be a color filter, neutral-density filter, etc. A light stopper  18  is inserted between reflection filter  8   b  and reflection filter  8   d  so as to exclude any DUV component from the illumination during VIS illumination. 
     FIG. 12 shows a linear slider  21  with which the two settings of FIGS. 10 and 11, and a further setting, can be implemented. Linear slider  21  comprises three switching sections  22 ,  23 ,  24  for different spectral illumination variants. Linear slider  21  is inserted perpendicularly into illumination beam path  6  into reflection filter system  9 , so that its upper half is always located between reflection filter  8   b  and reflection filter  8   c , and its lower half always between reflection filter  8   a  and reflection filter  8   d . Only one of switching sections  22 ,  23 ,  24  at a time is selectably inserted into illumination beam path  6 . Position  28  of the reflection filter system is depicted by a dashed border. In FIG. 12, switching section  23  is therefore currently inserted into the reflection filter system. 
     Switching section  22  generates DUV illumination, i.e. corresponds to FIG. 10. A clear opening  25  is arranged in its upper half in order to allow DUV light to pass. A light stopper  18  is arranged in its lower half, in accordance with FIG.  10 . 
     Switching section  23  generates VIS illumination, i.e. corresponds to FIG. 11. A light stopper  18  is arranged in its lower half in order to block out the DUV light. In accordance with FIG. 11 a lens/filter combination  26  made up of lenses and at least one filter, selected for the VIS region, is inserted into its lower half. The lenses correct the position of the image of the light source. The filters can be selected as desired, e.g. color filters, neutral-density filters, fluorescence filters, etc. 
     With switching section  24 , a further illumination with wavelengths outside the DUV wavelength band can be established. Once again, a light stopper  18  is arranged in its upper half in order to block out DUV light. Any desired lens/filter combination  27 , for example one selected for the so-called “i-line” at 365 nm, is inserted in its lower half. 
     By displacing linear slider  21 , it is easy to switch between the various spectral illumination variants (DUV, VIS, and, for example, “i-line”). A further advantage in terms of eye safety is achieved here, given the very high light intensities of light source  5 , for example in the form of a mercury arc lamp. In the “DUV” position, so little light energy is in any case allowed to pass that eye safety is attained. In all the positions that do not implement DUV illumination, however, all of the remaining light energy outside the narrow DUV wavelength band is allowed to pass through reflection filter  8   a . In these positions, however, filters  20  are always arranged in illumination beam path  6 . Because sufficient intensity damping takes place in this context by appropriate selection of filters  20 , consistently eye-safe operation of microscope  1  is easily ensured. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Layer configuration of a reflection filter 
               
               
                 L = Al 2 O 3 ; S = SiO 2 ; λ o  = 308 nm; α = 22.5° 
               
             
          
           
               
                 No. 
                 Thickness [λ o /4] 
                 Material 
                 No. 
                 Thickness [λ o /4] 
                 Material 
               
               
                   
               
             
          
           
               
                 1 
                 0.25 
                 L 
                 29 
                 0.92 
                 L 
               
               
                 2 
                 0.94 
                 S 
                 30 
                 0.72 
                 S 
               
               
                 3 
                 0.66 
                 L 
                 31 
                 0.92 
                 L 
               
               
                 4 
                 0.72 
                 S 
                 32 
                 0.72 
                 S 
               
               
                 5 
                 0.92 
                 L 
                 33 
                 0.92 
                 L 
               
               
                 6 
                 0.72 
                 S 
                 34 
                 0.72 
                 S 
               
               
                 7 
                 0.92 
                 L 
                 35 
                 0.92 
                 L 
               
               
                 8 
                 0.72 
                 S 
                 36 
                 0.72 
                 S 
               
               
                 9 
                 0.92 
                 L 
                 37 
                 0.92 
                 L 
               
               
                 10 
                 0.72 
                 S 
                 38 
                 0.72 
                 S 
               
               
                 11 
                 0.92 
                 L 
                 39 
                 0.92 
                 L 
               
               
                 12 
                 0.72 
                 S 
                 40 
                 0.72 
                 S 
               
               
                 13 
                 0.92 
                 L 
                 41 
                 0.92 
                 L 
               
               
                 14 
                 0.72 
                 S 
                 42 
                 0.72 
                 S 
               
               
                 15 
                 0.92 
                 L 
                 43 
                 0.92 
                 L 
               
               
                 16 
                 0.72 
                 S 
                 44 
                 0.72 
                 S 
               
               
                 17 
                 0.92 
                 L 
                 45 
                 0.92 
                 L 
               
               
                 18 
                 0.72 
                 S 
                 46 
                 0.72 
                 S 
               
               
                 19 
                 0.92 
                 L 
                 47 
                 0.92 
                 L 
               
               
                 20 
                 0.72 
                 S 
                 48 
                 0.72 
                 S 
               
               
                 21 
                 0.92 
                 L 
                 49 
                 0.92 
                 L 
               
               
                 22 
                 0.72 
                 S 
                 50 
                 0.72 
                 S 
               
               
                 23 
                 0.92 
                 L 
                 51 
                 0.92 
                 L 
               
               
                 24 
                 0.72 
                 S 
                 52 
                 0.72 
                 S 
               
               
                 25 
                 0.92 
                 L 
                 53 
                 0.92 
                 L 
               
               
                 26 
                 0.72 
                 S 
                 54 
                 0.67 
                 S 
               
               
                 27 
                 0.92 
                 L 
                 55 
                 0.68 
                 L 
               
               
                 28 
                 0.72 
                 S 
                 56 
                 1.75 
                 S 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Microscope 
               
               
                 2 
                 Objective 
               
               
                 3 
                 Object stage 
               
               
                 4 
                 Object 
               
               
                 5 
                 DUV light source 
               
               
                 6 
                 Illumination beam (path) 
               
               
                 7 
                 Condenser 
               
               
                 8 
                 Reflection filter, also 8a, 8b, 8c, 8d 
               
               
                 9 
                 Reflection filter system 
               
               
                 10 
                 Beam splitter 
               
               
                 11 
                 TV camera 
               
               
                 12 
                 VIS light source 
               
               
                 13 
                 Condenser 
               
               
                 14 
                 Pivotable mirror 
               
               
                 15 
                 Universal DUV-VIS light source 
               
               
                 16 
                 Condenser 
               
               
                 17 
                 Image of the light source 
               
               
                 18 
                 Light stopper 
               
               
                 19 
                 Lenses 
               
               
                 20 
                 Filter 
               
               
                 21 
                 Linear slider 
               
               
                 22 
                 Switching section for DUV illumination 
               
               
                 23 
                 Switching section for VIS illumination 
               
               
                 24 
                 Switching section for a further illumination 
               
               
                 25 
                 Clear opening 
               
               
                 26 
                 Lens/filter combination for VIS illumination 
               
               
                 27 
                 Lens/filter combination for a further illumination 
               
               
                 28 
                 Position of the reflection filter system