Abstract:
The autofocus module possesses, between the detector element ( 43 ) and the optical means, a cylindrical lens ( 42 ) that, for determination of the focus position, generates a line on the detector element ( 43 ). In addition, the detector element ( 43 ) is pivotable about an axis in such a way that it is inclined with respect to a plane defined by the surface of the specimen ( 20 ), and its inclination is adjustable. All the optical components of the autofocus module are combined in a housing ( 25 ) that can be quickly flange-mounted onto an existing microscope-based system ( 1 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of the German patent application 102 34 756.5 which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention concerns an autofocus module for a microscope-based system. The invention concerns in particular an autofocus module for a microscope-based system having an objective that defines an image beam path which is perpendicular to a surface of a specimen and can be focused thereonto, and having an illumination beam path that encompasses a light source for illumination of the specimen. 
     BACKGROUND OF THE INVENTION 
     German Patent DE 32 19 503 discloses an apparatus for automatic focusing onto specimens to be viewed in optical devices. After reflection at the surface of a specimen, the reflected measurement light beam bundle passes through a pinhole after reflection at a splitter mirror. A portion of the measurement light beam bundle is reflected out by means of a fully mirror-coated surface, and after passing through a slit aperture is directed onto a differential diode. In the focused state, the focus is located between the two diodes. In the event of defocusing, the measurement spot drifts onto one of the two diodes, which are connected to corresponding control means. The control means adjust optical or mechanical means of the microscope in order to bring the measurement spot back between the two diodes, and thus reestablish the focus position. 
     European Patent Application EP-A-0 124 241 discloses a microscope having an automatic focusing device. The microscope encompasses a memory device for saving the data from the objectives that are used in the microscope. Also provided is a control device which monitors and regulates the various microscope functions. Another of the tasks of the control device is to move the focusing stage. A CCD element, which receives an image from the particular selected objective and, together with a computation unit, determines the image sharpness based on optimum contrast, is provided as the image acquisition device. The objective data of the objective presently in use must be taken into account when determining the optimum sharpness. Those data are, as mentioned above, stored in a memory. 
     German Unexamined Application DE 41 33 788 furthermore discloses a method for autofocusing of microscopes, and an autofocus system for microscopes. The image of a specimen or of a pattern reflected onto the specimen is conveyed to two areas on a detector or to two different detectors; in the focused position, one image is produced in front of one detector, and one image behind the other detector. The image sharpness states on the detectors are converted into electronic signals, whose difference is used to focus the objective. The distances of the image or of the respective pattern from the respective detectors are adjustable. Deliberate offset settings, as well as “IR offset” correction settings, can be implemented. 
     A problem in the context of automatic focusing in microscopes, for the examination of specimens having several focal planes, is that the autofocus system does not know which plane it should sharply focus onto. The autofocus system can focus onto only one of the planes, and it can easily happen that the autofocus system loses the focal plane and jumps to another as it corrects. The multiple planes result, for example in the semiconductor industry, in the context of different topological steps or multiple photoresist layers. In conventional microscopy and in confocal microscopy, there are also multiple layers that can be focused on. In samples equipped with coverslips, these can be the upper side of the coverslip with the interface to air, or the underside of the coverslip with the interface to the sample. 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of the present invention to create an autofocus module for a microscope-based system with which multiple focal planes can be detected at one time. In addition, it is possible ultimately to focus on a selected focal plane. 
     The object is achieved by way of an autofocus module for a microscope-based system comprising:
         an objective that defines an image beam path which is perpendicular to a surface of a specimen   an illumination beam path that encompasses a light source for illumination of the specimen,   a light source for generating a measurement light bundle for determining a focus position;   an optical means for splitting the measurement light bundle in such a way that an eccentrically extending measurement light beam bundle is created;   a first dichroic beam splitter is provided in the image beam path of the microscope-based system, which couples the measurement light beam bundle eccentrically into the microscope-based system and directs it onto the surface of the specimen;   the optical means directs onto a detector element a measurement light beam bundle remitted from the microscope-based system; and   a cylindrical lens between the detector element and the optical means.       

     The autofocus module has the particular advantage that there is provided between the detector element and the optical means a cylindrical lens that, for determination of the focus position, generates a line that is imaged onto the detector element. In addition, means are provided which pivot the detector element about an axis, so that the detector element is inclined with respect to a plane defined by the surface of the specimen, and its inclination is adjustable. 
     Furthermore, the detector element can be pivoted or adjusted exclusively and only about an axis that is parallel to the X axis of a coordinate system. The optical components of the autofocus module are combined in a housing. The optical elements in the module are thereby pre-aligned, and it can be quickly attached to an existing microscope-based system. The detector element can be embodied as a two-dimensional area sensor or constituted by at least two linear sensors arranged parallel to one another. A laser light source that emits IR light as the measurement light is used in the autofocus module as the light source. This is advantageous because the specimen is not influenced thereby. A computer or a control system are connected to the microscope-based system and the module, serving to control the detector element in the module and the microscope-based system. Control is of the inclination of the detector element is important because means are provided which perform an adjustment of the inclination of the detector element. In addition, the inclination of the detector element can be modified during measurement; this results in a change in the sensitivity of the measurements performed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and advantageous embodiments are the subject matter of the description below of the Figures, in whose presentation accurately scaled reproduction was dispensed with in favor of clarity. In the individual drawings: 
         FIG. 1  is a partial side view of the beam paths of an optical device or a microscope-based-system to which an autofocus module is attached; 
         FIG. 2  is a detail view of the measurement light beam bundle striking the measurement sensor; 
         FIG. 3  shows a further embodiment of the detector element; 
         FIG. 4  shows the autofocus module in the focused state, the measurement beam being focused onto one spot; 
         FIG. 5  shows the use of a cylindrical lens which generates, from the measurement spot, a sharp line that is imaged onto the detector element; 
         FIG. 6  shows that by tilting the detector element, the line is imaged sharply at multiple spots if multiple focal planes are present on the specimen; 
         FIG. 7   a  shows one (or more) transparent films, one above another, which constitute the different focal planes; 
         FIG. 7   b  shows a stepped structure on the specimen which is small compared to the measurement spot on the specimen; 
         FIG. 8   a  shows an embodiment of a further optical element that additionally widens the measurement light beam bundle; and 
         FIG. 8   b  shows an additional embodiment of a further optical element that additionally widens the measurement light beam bundle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts a vertically extending image beam path  10  of a microscope-based system  1 . Microscope-based system  1  comprises a light source  6  which emits light into illumination beam path  11 . Microscope-based system  1  serves to illuminate a specimen  20  which defines a surface  21  that is located in the focal plane of microscope-based system  1 . The light of illumination beam path  11  first passes through an objective  2  and strikes surface  21  of specimen  20 . A certain portion of the light is reflected from surface  21  of specimen  20  and passes first through an objective pupil  3 . The beam reflected from surface  21  of specimen  20  passes, in image beam path  10 , through a dichroic splitter mirror  12  that has a 50:50 ratio in the visible region and high reflectivity in the IR. The light of the image beam path then traverses a tube lens  4 , and an image of specimen  20  is generated in intermediate image plane  5 . The light in image beam path  10  then travels to an eyepiece (not depicted). 
     In the situation depicted, illumination beam path  11  of the microscope-based system extends horizontally. Light of illumination beam path  11  emerges from a light source  6 . After leaving an optical system  7 , the light passes through an aperture stop  8  in whose plane is arranged a pinhole slider (not depicted) that contains at least two pinholes of differing dimensions. With this pinhole slider, an aperture stop  8  adapted to the measurement with the microscope-based system can be inserted, in manual or motorized fashion, with position response. 
     The light of illumination beam path  11  then passes through a second dichroic beam splitter  13  which has the greatest possible transmissivity for the visible light coming from light source  6 , and the greatest possible reflectivity for IR light. A mark whose function is explained below is located in the plane of field diaphragm  9 . After passage through a lens  14 , the light of illumination beam path  11  strikes first dichroic beam splitter  12 , from which the reflected portions are deflected toward specimen  20 . 
     A laser autofocus system, which in the exemplary embodiment depicted here is combined with all the necessary optical components into a module  30 , is provided for adjusting the focus. Module  30  is surrounded by a housing  25  that is depicted symbolically in  FIG. 1  as a dashed-line box. Module  30  can be inserted, for example, into an existing optical illumination system such as the one described for incident-light microscopes e.g. in German Utility Model 79 17 232, snap-lock means known per se ensuring accurately aligned positioning of module  30  in illumination beam path  11 . 
     A (preferably pulsed) laser light proceeds from a laser light source  31  that, in the embodiment depicted, is embodied as a laser diode. Advantageously, IR light is used as the measurement light, since it does not have a disruptive influence on the microscopic image of specimen  20 . A measurement light bundle  32  is directed, via a stationary lens  33  and then via a lens  34  that can be displaced in manual or motorized fashion in the axial direction as defined by dashed double arrow  35 , onto second dichroic splitter mirror  13 , which is arranged at the optical interface of the beam paths of measurement light bundle  32  and illumination beam path  11 . An image of laser light source  31  is generated in the intermediate image plane in which field diaphragm  9  is positioned. 
     Laser light source  31  is imaged onto surface  21  of specimen  20  in a measurement spot  16 . One half of a pupil  37  is covered so that in the event of defocusing, measurement spot  16  drifts on surface  21  of specimen  20 . The geometric covering of one half of measurement light bundle  32  is achieved using a combined optical component, for example a deflection prism  38 , which is inserted halfway into measurement light bundle  32  at the level of pupil  37 . Deflection prism  38  contains a fully mirror-coated prism surface  19 . The portion of measurement light bundle  32  that is not prevented from propagating by the arrangement of deflection prism  38  is labeled in  FIG. 1  with the reference character  32   a . With eccentrically extending measurement light bundle  32   a  of illumination-side measurement light bundle  32 , eccentrically extending portion  32   a  therefore proceeds into objective pupil  3  (substantially) parallel to image beam path  10  of microscope-based system  11 . 
     After reflection at surface  21  of specimen  20 , remitted measurement light beam bundle  32   b  passes lens  14 , and after reflection at second dichroic splitter mirror  13 , the light of remitted measurement light bundle  32   b  strikes fully mirror-coated prism surface  19  of deflection  38 . Remitted measurement light beam bundle  32  is then reflected out by fully mirror-coated prism surface  19  of deflection prism  38 . After total reflection at a prism surface  40 , and after passing through optical system  41 , the beam is widened by a downstream cylindrical lens  42  before striking a detector element  43 . Detector element  43  can be embodied as a two-dimensional area sensor, e.g. as a CCD or CMOS sensor. Detector element  43  can be tilted. Provided for that purpose are displacement means  46  and  47 , which incline detector element  43  with respect to a plane defined by the surface of specimen  20 . The plane thus defined by inclined detector element  43  is oblique (not perpendicular) with respect to the incident remitted measurement light beam bundle  32   b , so that in the focused state, remitted measurement light beam bundle  32   b  is imaged sharply only at the center. A computer  80  or a control system are connected to microscope-based system  1  and to module  30 . Computer  80  serves to control detector element  43  in the module and microscope-based system  1 , and to acquire corresponding data, and also to perform the displacement or tilting of detector element  43 . Adjustment of the focus can also be performed by computer  80  via a motor  23  which actuates the Z drive of a microscope stage  22 . Motor  23  receives from computer  80  the correspondingly processed signals from detector element  43 . These signals serve for control purposes and to adjust the focus, and are conveyed to motor  23 . 
     In order additionally to achieve a beam widening of measurement light beam bundle  32 , a further optical element is provided, between deflection prism  38  and lens  34 , which additionally widens measurement light beam bundle  32 . As depicted in  FIG. 8   a , the optical element can be an axicon  53 . A first and a second axicon  53   a  and  53   b  are mounted on a glass plate  54  as support. First axicon  53   a  acts on the eccentrically extending measurement light beam bundle  32   a , and second axicon  53   b  acts on remitted measurement light beam bundle  32   b . The exemplary embodiment of  FIG. 8   b  shows a toroidal lens  57  as the optical element. Toroidal lens  57  is a lens element that is curved into a ring. The opening of the ring is equipped with an opaque stop  58 . The effect of toroidal lens  57  is comparable to that of axicon  53 . 
       FIG. 2  is a detail view of measurement light beam bundle  32   b  striking detector element  43 . Cylindrical lens  42  normally converts measurement light beam bundle  32   b , at the focus, into a line (see FIG.  4 ). In the depiction in FIG.  2 . detector element  43  is pivoted about an axis parallel to the X axis of the coordinate system. Plane  44  spanned by the X and Y coordinate axes is parallel to surface  21  of specimen  20 . Tilting of detector  43  about the X axis causes the line ( FIG. 4 ) to be imaged sharply at only one spot  45  (only one plane of sharpness present in specimen  20 ). When specimen  21  is displaced in the direction of the Z axis, measurement light beam bundle  32   b  shifts on detector element  43 . The direction of the shift is indicated by double arrow Px. In contrast to a conventional laser autofocus system, the tilting of detector element  43  still produces a single spot on detector element  43  despite the change in the position of surface  21  of specimen  20 , and in the defocused state the distance between surface  21  of specimen  20  and the focal plane can be ascertained by analyzing the position of spot  45  on detector element  43 . If detector element  43  is tilted, with respect to the plane defined by the surface of the specimen, by an angle that is attainable by displacement of the detector element about the one axis parallel to the X axis and the one parallel to the Y axis. With a detector element  43  tilted in this fashion, measurement light beam bundle  32   b  extends along double arrow Px, and the spot or spots extend along a double arrow Py. 
       FIG. 3  shows a further embodiment of detector element  43 . Detector element  43  comprises at least two linear sensors  48  and  49  arranged parallel to one another. Each of the linear sensors comprises a plurality of linearly arranged photodiodes  52 . 
       FIG. 4  shows the laser autofocus system for microscope-based system  1  in the focused state. In this example, cylindrical lens  42  is omitted and detector element  43  is not tilted. Remitted measurement light beam bundle  32   b  is, in this case, focused onto a single spot  50  determined by optical system  41 . 
       FIG. 5  shows the result when cylindrical lens  42  is added. Remitted measurement light bundle  32   b  is, in this case, focused onto a by optical system  41  and cylindrical lens  42  into a single sharp line  60 . Note in this context that the plane defined by detector element  43  is parallel to plane  44  spanned by the X and Y coordinate axes. Plane  44  corresponds to surface  21  of specimen  20 . 
       FIG. 6  depicts the situation in which surface  21  of specimen  20  has a structure that displays a differing vertical profile (see  FIGS. 7   a  and  7   b ) As already depicted in  FIG. 2 , measurement sensor  43  is pivoted about the axis parallel to the X axis of the coordinate system. Plane  44  spanned by the X and Y coordinate axes is parallel to surface  21  of specimen  20 . In the present case, tilting of detector element  43  about the X axis yields several spots  70 , of which each individual spot represents a specific surface on specimen  20 . The capability thus exists for focusing onto multiple planes. 
       FIGS. 7   a  and  7   b  depict two exemplary embodiments of the surface structure of a specimen  20 .  FIG. 7   a  depicts the situation in which one or more transparent films  61  are applied one above another onto surface  21  of specimen  20 . The autofocus system can focus simultaneously onto the different films. In the example shown in  FIG. 7   b , a stepped structure  62 , which itself can in turn be applied on a transparent film  61 , is configured on surface  21  of specimen  20 . Stepped structure  62  is small compared to the measurement spot on specimen  20 . The result for the measurement spot is therefore on the one hand an optimum focus position on top surfaces  63  of stepped structure  62 , and on the other hand a further optimum focus position for trenches  64  of stepped structure  62 . Depending on the configuration of stepped structure  62  or the number of different films  61 , several optimum focus positions are obtained that can be sharply focused onto. In the plurality of focus positions, each of these focus positions or planes generates a spot  70  on detector element  43 . By analyzing the position of these spots  70  with respect to one another, it is possible to identify the various planes of the specimen and then bring the desired plane into focus. 
     The invention has been described with reference to a particular exemplary embodiment. It is self-evident, however, that changes and modifications can be made without thereby leaving the range of protection of the claims below.