Abstract:
A measuring system is disclosed with enhanced resolution for periodic structures on a substrate for semiconductor manufacture. Aperture structures of varying geometries are provided in the illumination beam path. The aperture structures differ regarding the transmission characteristics of light, and which adjust the intensity distribution of the diffraction orders in the imaging pupil of the optical system.

Description:
RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 2007 021 823.2, filed on May 7, 2007, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a measuring apparatus with enhanced resolution for measuring periodic structures on a substrate for semiconductor manufacture. The measuring system comprises a condenser with an entrance pupil. 
     The present invention further relates to the utilization of various apertures or aperture structures in the illumination beam path of a measuring system for measuring periodic structures on a substrate for semiconductor manufacture. 
     BACKGROUND OF THE INVENTION 
     European Patent EP 0 938 031 B1 discloses a method for the manufacture of integrated circuits using an aperture with variable transmission. Herein, an aperture comprises areas of varying transmission. The method and device proposed here are not suitable for use in a measuring system which measures the position and/or the dimension of structures on a substrate. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a measuring system with enhanced resolution for measuring certain periodic structures on a substrate for semiconductor manufacture. 
     The present object is solved by a measuring system comprising a condenser with an entrance pupil, wherein the condenser can be configured as a measuring objective and aperture structures provided in an illumination beam path, for changing the distribution of the intensities of the orders of diffraction in an imaging beam path. 
     It is a further object of the present invention to utilize an aperture structure in the measuring system in such a way that enhanced resolution can be achieved in the inspection of periodic structures on a substrate for semiconductor manufacture. 
     The above object is solved by the use of an aperture structure in a measuring system for periodic structures on a substrate for semiconductor manufacture. The measuring system has a condenser with an entrance pupil and an aperture structure is provided in the entrance pupil of the condenser. 
     The present invention is advantageous in that it provides enhanced resolution for periodic structures on a substrate for semiconductor manufacture. The measuring apparatus comprises a condenser with an entrance pupil. The measuring light is focused onto the structure to be measured through the condenser, which can be formed as the actual measuring lens, at various angles determined by the aperture of the condenser and the illumination source. The diffraction of light on the periodic structure generates an intensity distribution of the diffraction orders in the pupil of the imaging optics. This intensity distribution is dependent both on the structure and on the angle of incidence. It goes without saying for a person skilled in the art that the intensities are dependent on the aperture of the lens. This ultimately defines the order of diffraction at which diffracted light can be captured by the imaging beam path (simply illustrated e.g. in the case of vertical incidence of light in  FIG. 4A ). It is precisely this insufficient supply of light of a diffraction order greater than zero in the pupil, that leads to inadequate resolution. 
     An annular aperture structure is, for example, arranged in the entrance pupil of the condenser. The annular aperture structure comprises a circular inner area and an annular outer area, which differ according to the transmission characteristics of the light. Further known aperture types are described in various other publications as annular, dipolar, quasar or quadrupole apertures. In other publications, the illumination means is also often referred to as off axis illumination. Common to all, is that the intensity distributions of the different diffraction orders are changed in the imaging pupil, resulting in the enhanced resolution of the imaged structures. Well known from microscopy is the dependency of contrast (steep edges) on the diameter of the aperture stop. 
     The imaging process can be simply described in an exemplary manner, wherein diffracted beams interact with the edges of the transilluminated structure and carry information not only about the edges but also about the structure itself. The less information available for image creation (small aperture) or the more undiffracted light or scattered light that overlaps this portion, then the more blurred the edges become and resolution declines. 
     In one embodiment the circular inner area of the aperture can be opaque while the annular outer area is then transparent. In this way the intensity ratio of the 0 th  and ±1 st  diffraction order is modified in favor of the ±1 st  order thus enhancing resolution. 
     In a further embodiment, the transmission characteristic of the circular inner area declines toward the center in accordance with a function. The annular outer area is likewise transparent. The function describing the decline of transmission toward the center of the circular inner area can, for example, be a cosine function. 
     A further configuration of the aperture structure consists in a variable configuration of the diameter of the circular inner area and the width of the annular outer area. An LCD array structure, for example, can be used for the variable configuration of the annular aperture structure, wherein transmission for the individual arrays can be set. Alternatively it is also possible, for example, to pivot in on a mechanical slide or turret various fixed annular or circular apertures of varying transmission ranges. Depending on the periodic arrangement of the structures on the substrate for semiconductor manufacture, it is thus possible to select a aperture setting suitable to achieve enhanced resolution. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIG. 1  schematically shows a coordinate measuring device according to the prior art for measuring the position and/or dimension of structures on a substrate; 
         FIG. 2  shows a schematic arrangement of a microscope stand for incident light and transmitted light; 
         FIG. 3  shows a schematic view of a substrate on which periodic structures are deposited; 
         FIG. 4A  shows a schematic representation of an aperture structure comprising an inner area and an outer area, which differ with respect to transmission; 
         FIG. 4B  shows a schematic view of an aperture structure comprising a transparent outer area and an inner area, wherein the inner area exhibits a transmission gradient; 
         FIG. 4C  shows a schematic representation of various other aperture forms, the marked areas of which differ with respect to transmission; 
         FIG. 5A  schematically shows the illumination of a substrate with periodic structures arranged on it, whereby illumination is perpendicular to the substrate; and 
         FIG. 5B  shows the illumination of a substrate at an angle to the optical axis. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A coordinate measuring device of the type shown in  FIG. 1 , is an arrangement known from the state of the art, used also for measuring structures with a high measuring accuracy. The coordinate measuring device is an example of a device group used for measuring structures  3  on a substrate  2 . In the case of measuring device  1 , the actual measuring device itself is mounted on a vibration-damped granite block  20 . The masks or wafers are placed on the measuring stage  18  by an automatic handling system. This measuring stage  18  is supported on the surface of granite block  20  by air bearings  19 . Measuring stage  18  is motor driven and displaceable in two dimensions (X coordinate direction/Y coordinate direction). The corresponding driving elements are not shown. Planar mirrors are mounted on two mutually perpendicular sides of measuring stage  18 . A laser interferometer system  22  is used to track the position of measuring stage  18 . 
     Illumination and imaging of the structures to be measured is carried out using a high-resolution microscope optics with incident light and/or transmitted light. A CCD camera serves as a detector. Measuring signals are obtained from the pixel, positioned within a measuring window, of the CCD detector array. An intensity profile of the measured structure is derived therefrom by means of image processing, for example, for determining the edge position and width of the structure or the intersection point of two structures intersecting each other. Usually the positions of such structural elements are determined relative to a reference point on the substrate (mask or wafer). Together with the interferometrically measured position of measuring stage  18 , the coordinates of structure  3  can be derived herefrom. The structures on the wafers or masks used for exposure of the wafers only allow extremely small tolerances. Thus, to inspect these structures, extremely high measuring accuracies (currently in the order of nanometers) are required. A method and a measuring device for determining the position of such structures is known from German Patent Application Publication DE 100 47 211 A1. For details of the above position determination explicit reference is made to that document. 
     In the example of measuring device  1 , illustrated in  FIG. 1 , measuring stage  18  is formed as a frame so that sample  3  can also be illuminated with transmitted light from below. Above sample  3  is a further illumination ( 13 ) and imaging device (objective  9 , mirror  12 , tube lens  10 ), which is arranged about an optical axis  11 . The objective displaceable in the Z coordinate direction is indicated with reference numeral  9 . Reference numerals  14  and  15  indicate, for example, positions at which one or more apertures or aperture structures can be arranged. Advantageously, the form and transmission characteristics of the apertures can be automatically configured and set. By means of a motor-driven slide or turret  14   a ,  15   a  the apertures can be advantageously exchanged. Additionally a comparison of measurements carried out with different aperture forms is also advantageous. 
     A transmitted-light illumination means with a height-adjustable condenser  8  and a light source  6  is also used in granite block  20 . 
       FIG. 2  schematically shows an incident-light and transmitted-light arrangement, as used for microscope stands. The arrangement also comprises a transmitted-light lamp housing  71 , that directs light via collector  72  to mirror  73 , which then reflects the light via condenser  73   a  with entrance pupil  73   b , onto substrate  74  bearing the varying structures. Further, an incident-light illumination means  80  is provided which also couples-in light via an incident-light collector  79  by means of an incident-light coupling-in mirror  78  (not shown in drawings) into optical axis  51 , or illumination beam path  50  of the optical system. A lens  75  is provided above substrate  74  for imaging the light of incident-light illumination means  80  onto the substrate and collects the light of transmitted-light illumination means  71  and/or of incident-light illumination means emitted by substrate  74  and ultimately images it onto detector  83 , which can be either a camera, a scanner or a line scanner. Additionally, the optical system is provided with a focusing system  87 , the measuring light of which is likewise coupled into optical axis  51  of the optical system via beam splitting mirror  76 . The incident light and transmitted light collected by lens  75  passes through the various beam splitters in the illumination beam path and is directed via a tube lens optics  81  and, if necessary, an additional optics  82  to detector  83 . The adjustable, motor-driven apertures  77  can be in various embodiments, e.g. can be either a slide or a turret. The structures on the substrate  74  may be illuminated using a coherent illumination. 
       FIG. 3  shows a schematic view of a substrate  100 , on which numerous structures  103  are periodically arranged. The substrate  100  is located on a stage  18  moveable in X/Y coordinate directions. It is obvious to a person skilled in the art that the structures  103  in  FIG. 3  are not to be regarded as limiting. The shape of the structures is not restricted to an elongate form. 
       FIG. 4A  shows a schematic view of the annular aperture structure  110 , comprising an inner circular area  111  and an outer annular area  112 . The aperture structure depicted in  FIG. 4A , can be brought, for example, into the entrance pupil of the condenser, effectively enhancing the resolution and contrast of periodic structures. The depth of focus for periodic structures is improved simultaneously, which additionally enhances the reproducibility of the measuring results. In the embodiment shown in  FIG. 4 , the inner circular area  111  is opaque while the outer annular area  112  is transparent in this case. 
     A further embodiment of the aperture structure according to the present invention is shown in  FIG. 4B . The outer annular area  112  is likewise completely transparent. The inner circular area  111  is formed in such a way that transmission declines toward the center  115  of the aperture structure  110 . The transmission of the circular inner area declines in accordance with a cosine function  113 . Even in this case, an enhancement of resolution and an increase in the depth of focus can be realized for certain periodic structures. Possible fields of application are, in particular, the measurement of critical dimensions (CD) on wafers and reticle structures, as well as all optical inspection procedures reliant on optimized resolution. 
       FIG. 4C  shows a schematic representation of various other forms of apertures, wherein marked areas vary in transmission. One possible aperture structure is an annular aperture  160 . It is further possible, to use a dipole  140  as an aperture structure, wherein the dipole can be arranged in varying orientations. Yet another alternative aperture structure is a quadrupole  150 . A quasar  160  is also conceivable as an aperture structure. 
     It is also conceivable that the annular aperture structure  110  is formed as a driveable LCD. This has the advantage that the aperture structure can be adjusted freely. As a result, the diameter of the circular inner area  111  and the annular outer area can be adjusted at any time without difficulty in accordance with the structures to be inspected and their periodicity. 
     Since the aperture structure  110  is variable, the width of the transparent outer area  112  can be selected, in accordance with the illumination mode, to achieve optimized resolution.  FIG. 5A  is an idealized illustration of an illumination of substrate  100  with light beam  118 , directed from condenser  117  at right angles onto substrate  100 . Providing the aperture of the lens  120  is adequate, the 0 th  and ±1 st  order will contribute to the image. The intensities of the orders vary. A decreasing distance between the structures or the size reduction of the structures themselves, result in an enlargement of the angle between the 0 th  and 1 st  order. If the 1 st  order cannot be captured by the lens, resolution declines and the structures in the image merge together. 
       FIG. 5B  shows the illumination of substrate  100  with an ideal illumination beam path  119  directed at an angle  125  to optical axis  11 ,  51  onto substrate  100 . 
     As a general rule, the illumination always has a certain range of angles determined by the lamp, the collector and the condenser (lens). This range of angles is indicated with broken lines in  FIGS. 5A and 5B . The annular aperture structure  110  thus results in an oblique illumination which radiates symmetrical to the optical axis from various directions. Providing a suitable width is chosen for the annular outer area  112 , the zero order of diffraction  122   0  and the first order of diffraction  122   +1  are symmetrical to optical axis  11 ,  51 . As a result, the phase relationship between the wave fronts involved in imaging is more constant in the case of defocusing than in conventional illumination, resulting in an enhanced depth of focus. As mentioned above, to achieve optimal conditions for imaging, the radius of the annular aperture structure  110  must be adjusted in accordance with the grid constants or periodicity of structures  103  to be measured on substrate  100 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.