Patent Publication Number: US-2009225414-A1

Title: Dark Field Objective for a Microscope

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of PCT International Application No. PCT/EP2006/066653, filed Sep. 24, 2006, which application published in German and is hereby incorporated by reference in its entirety; said international application claims priority from German Patent Application No. 10 2005 047 847.6, filed Oct. 5, 2005 which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an objective for a microscope for dark field microscopy having alternating illumination with grazing incidence. 
     BACKGROUND OF THE INVENTION 
     An objective for dark field microscopy is known according to German Patent No. DE 199 03 486 C2. In the known objective, an annular beam bundle is led around the objective lens system and deflected concentrically at an angle onto the sample in the area of the sample-side end of the objective lens system. 
     Illumination for dark field microscopy having alternating illumination with grazing incidence according to the so-called AGID method (alternating grazing incidence) is known according to B. Brodermann et al.: “Alternating grazing incidence dark field scanning optical microscopy for dimensional measurements,” Proc. of SPIE 4277:352-361 (2002). In this method, the sample is illuminated alternately from two opposite directions perpendicular to a sample structure with grazing incidence. The illumination occurs laterally from the objective. 
     The AGID method presumes a main structure direction on the sample, such as printed conductors of a wafer. The sample is oriented having its main structure direction perpendicular to the illumination directions. The conductors to be examined are illuminated in sequence from one side and from the opposite side perpendicular to the main structure direction, a separate image being recorded for each illumination procedure. Two images of the same recording area result in each case. One illumination direction emphasizes one edge side, and the other illumination direction emphasizes the other edge side. The two images are analyzed individually for the position of the corresponding edges and subsequently the analyzed images are superimposed. It is thus possible to resolve structure widths smaller than half of the light wavelength. The illumination light is preferably polarized in such a way that the electric field is oriented parallel to the edge of the structure. 
     The known prior art has the disadvantage that it is either unsuitable for the AGID method or requires a complicated construction. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is therefore based on the object of specifying an illumination device for a microscope for dark field microscopy having alternating illumination with grazing incidence, which is simple and compact. 
     This object is achieved by the dark field objective specified herebelow. Advantageous embodiments of the invention are specified in the further description herebelow. 
     According to the invention, the object is achieved in a dark field objective for a microscope having a front lens for receiving light from a sample and having a dark field illumination device for guiding illumination light onto the sample in that the dark field illumination device comprises at least one pair of light decoupling elements, which are each situated opposite to the optical axis around the front lens for alternating, counter parallel illumination of the sample. The junction of objective and counter parallel illumination allows the execution of the illumination to be designed simply and compactly. 
     The light decoupling elements are expediently offset by 180° in pairs. This implements the effect intended in the AGID method most favorably. 
     Decoupling elements are preferably prisms. These are simpler to position than mirrors and permit the objective to be designed having its terminating face encapsulated. 
     The prisms and the front lens favorably end on the probe side in the area of a joint plane. This results in an especially compact construction. 
     The prisms and the front lens ideally end on the probe side in a joint plane for contact on an immersion liquid film. The AGID method using a simple objective is thus accessible for microscopy using immersion liquid. 
     According to one embodiment, the dark field illumination device guides the illumination light in at least one beam pair through the objective. The top side of the objective may thus simultaneously be used for the exit of the imaging beam and for the entry of the illumination beams. The construction of the objective thus becomes especially compact, like that of the entire microscope. 
     According to a preferred embodiment, the dark field illumination device guides the illumination light through the objective at least partially parallel to the optical axis. This implements especially simple beam guiding. 
     According to a further embodiment, the dark field illumination device receives the illumination light perpendicular to the optical axis from an illumination source. The coupling may thus occur directly from the side into the objective. A coupling mirror above the objective may thus be saved. 
     The dark field illumination device and the front lens are especially advantageously enclosed by a shared housing. An especially compact and robust construction is thus achieved. The configuration is less susceptible to misalignment. 
     According to a refinement of the invention, the dark field illumination device comprises two pairs of light decoupling elements, which are situated crossed in pairs. It is possible through this configuration to apply the AGID method to a sample having a crossed structure. 
     Ideally, the dark field illumination device is provided for decoupling the illumination light onto the sample at an angle of 65° to 89°, in particular 75° to 80° to the optical axis. It has been shown that the specified angle ranges result in especially good imaging. 
     These and other objects and advantages of the present invention will be readily appreciable from the following description of preferred embodiments of the invention and from the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       In the following, the invention is explained in greater detail on the basis of schematic illustrations of an exemplary embodiment. Identical reference numerals in the individual figures identify identical elements. The figures include: 
         FIG. 1  shows an objective having decoupling prisms; 
         FIG. 2  shows an objective having decoupling mirrors; 
         FIG. 3  shows an immersion objective having decoupling prisms; 
         FIG. 4  shows an objective according to the invention from the bottom side; 
         FIG. 5  shows an assigned illumination device; and,  FIG. 6  shows a schematic sketch of the assigned microscope. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects. 
     Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described. 
       FIG. 1  shows an objective  20  according to the invention over a sample  10  on a sample support  11 . The objective comprises a front lens  30  and a left decoupling prism  43  and a right decoupling prism  44 . The front lens  30  and the prisms  43  and  44  terminate at a joint plane  32 . The front lens and the prisms are partially enclosed by the housing  22  of the objective. The prisms  43  and  44  form a part of the dark field illumination device  40 . The alternately incident left and right illumination beams  41  and  42 , respectively, are shown simultaneously visible in all figures for simplification. The illumination beams  41  and  42  are respectively incident on the prisms  43  and  44  parallel to the optical axis  21  inside the housing  22 . These prisms deflect the illumination beams in a grazing manner onto the sample  10 . The imaging beams  31  are received by the front lens  30  from the sample  10 . 
       FIG. 2  shows an objective  20  according to the invention analogous to the objective of  FIG. 1 . Instead of the prisms, a left and a right decoupling mirror  45  and  46 , respectively, are provided here. The decoupling mirrors are located in the area of the plane  32  of the sample-side end of the front lens. They may also be situated slightly above or also beneath. 
       FIG. 3  shows an objective  20  according to the invention for use with immersion media, again analogous to the objective from  FIG. 1 . The sample is fixed here using a sample cover  12 . The immersion liquid  13  is located between the sample cover and the sample-side end of the front lens and the sample-side end of the prisms  43  and  44 . This liquid is a liquid having an index of refraction in the range of the front lens  30 . The illumination beams  41  and  42  respectively incident on the prisms  43  and  44  parallel to the optical axis are reflected here inside the prisms  43  and  44  in the direction of the sample  10  and exit largely non-refracted from the prisms into the immersion liquid. The prisms are implemented here as parallelograms for this purpose. 
       FIG. 4  shows the bottom side of a further objective according to the invention analogous to the objective of  FIG. 1 . The housing  22  comprises 4 pairs of decoupling elements  48  here. The prism pair  43 ,  44  shown in  FIGS. 1  or  3  is shown here. In addition, a prism pair offset crossed to the prism pair  43 ,  44  is also shown by solid lines. The analysis of two structures of the sample perpendicular to one another is especially well possible through this further prism pair. Furthermore, two further prism pairs offset by 45° in relation to the above-mentioned two prism pairs are shown by dashed lines. The analysis of samples having structures of different directions is thus well possible. 
       FIG. 5  shows an illumination source  50  of the illumination device  40 . The illumination source comprises a laser  51  for linearly polarized light of a wavelength of approximately 500 nm. The laser radiates through a Pockels cell  52  onto a polarization beam splitter  53 . This splits the beam into the left illumination beam  41  and, via the mirror  54  and a half-wave plate  55 , into the right illumination beam  42 . The Pockels cell is driven in such a way that it periodically rotates the polarization and thus forms an optical toggle switch together with the polarization beam splitter  53 . The half-wave plate  55  is used for orienting the polarization of the right illumination beam  42 . The illumination beams  41  and  42  are ideally oriented in such a way that the electric fields of the illumination beams on the sample  10  are perpendicular to the incidence direction and parallel to the sample structure. The illumination source shown is designed for an objective according to  FIGS. 1 through 3 . The illumination source is to be expanded analogously for an objective according to  FIG. 4 . 
       FIG. 6  shows the principle of a microscope  60  having the objective  20  according to the invention analogous to  FIGS. 1 through 4 . Two pairs of decoupling elements  48  are implemented here. A control and analysis unit  62  controls, via a connection  65 , the illumination source  50  for alternating illumination of the sample  10  via one decoupling element  48  at a time on one hand and the recording of the sample  10  by the camera  61  via a connection  64  on the other hand. The control and analysis unit  62  synchronizes the illumination and the recording and performs the analysis of the individual images and the superposition of analyzed individual images. 
     Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, which modifications are intended to be within the spirit and scope of the invention as claimed. It also is understood that the foregoing description is illustrative of the present invention and should not be considered as limiting. Therefore, other embodiments of the present invention are possible without departing from the spirit and scope of the present invention. 
     LIST OF REFERENCE NUMERALS 
     
         
           10  sample 
           11  sample support 
           12  sample cover 
           13  immersion liquid 
           20  objective 
           21  optical axis 
           22  housing 
           30  front lens 
           31  imaging beams 
           32  plane of the sample-side end of the front lens 
           40  dark field illumination device 
           41  left illumination beam 
           42  right illumination beam 
           43  left decoupling prism 
           44  right decoupling prism 
           45  left decoupling mirror 
           46  right decoupling mirror 
           47  deflection mirror 
           48  decoupling elements 
           50  illumination source 
           51  laser 
           52  Pockels cell 
           53  polarization beam splitter 
           54  deflection mirror 
           55  half-wave plate 
           60  microscope 
           61  camera 
           62  control and analysis unit 
           64  connection to camera 
           65  connection to illumination source