Abstract:
A device for noncontact determination of the edge profile at a thin disk-shaped object helps determining the edge profile at semiconductor wafers in which exact image recording is not impaired by specular reflections of the edge profile. A plurality of light sources in the form of laser radiation sources each emitting a line-shaped light bundle are arranged so as to be coplanar in a common plane representing a measurement plane oriented orthogonal to a base plane of the object and are directed from different directions to a common intersection of the laser radiation sources in the edge region of the object. A light sheet is formed in the measurement plane and at least one base camera is directed in the base plane lateral to the measurement plane to capture scattered light proceeding from a light line generated by the light sheet when impinging the object edge region.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a device for noncontact determination of the edge profile of a thin disk-shaped object comprising a turntable for rotating the disk-shaped object around an axis of rotation and a measuring arrangement for radial positioning of at least one light source for illuminating an edge region of the disk-shaped object in virtually radial direction to the axis of rotation thereof, and at least one camera for recording the illuminated edge region. The invention is particularly suitable for reliable and highly precise characterization of the edge profile of a wafer. 
         [0002]    In semiconductor fabrication, wafers are machined sequentially in a multitude of process steps during the fabrication process. With increasingly higher integration density of the semiconductor structures, the requirements for the quality of the wafers also increase. In the fabrication of wafers enormous expenditures on material and technology can sometimes result at the end of the value chain. It is therefore meaningful and legitimate to subject wafers to extensive testing before processing, i.e., at the beginning of the value chain, so that the wafers can be selected based on the highest reliability and fullest possible utilization of material surfaces. This testing also includes inspection of the outer circumferential edge of a wafer to check for suitable shape and integrity of the circumferential edge. A number of checking devices able to perform these inspections has already been suggested. 
       BACKGROUND OF THE INVENTION 
       [0003]    DE 10 2007 024 525 B4 describes a device in which three cameras are used to perform a visual assessment of defects in the edge region of a wafer. For recording defects, one camera is positioned opposite the edge region above the wafer and one camera is positioned opposite the edge region below the wafer. A third camera is disposed opposite the edge region of the wafer in radial direction. The edge region of the wafer captured by the cameras is illuminated by a homogeneous, diffusely radiating illumination, and pictures are taken by the camera and displayed to a user of the device on a monitor for visual evaluation. Thus it is possible when capturing and using the exact position of a defect on the wafer that subjective assessments of the defects can also be made. An objective, qualitative assessment of defects is not possible in this device. 
         [0004]    In an edge checking device disclosed in DE 11 2008 000 723 T5, test results are displayed depending on the edge information acquired from the inspected wafer. For this purpose, the circumferential surface of a wafer is captured from three recording directions by at least one CCD line camera. The axes of the three image recordings intersect in the center plane of the wafer at a point close to the circumferential surface of the wafer so that one viewing direction is directed to the outer circumferential surface and the other two image recordings are directed, respectively, to the beveled circumferential edges of the wafer. The configuration of the illumination needed for image capture was not disclosed. The captured images are displayed on a display device for manual evaluation and the position-dependent edge information is stored in a storage unit. Further, edge information is stored in a storage unit depending on position based on changes in shading in the captured image content. The acquired data are preferably displayed visually in the form of a profile curve on the basis of which a statistical evaluation of the edge information is made possible so that a trend in the overall shape can be determined therefrom. This has the drawback that image defects caused, for example, by reflected light at trouble spots or by improperly angled edge regions when making photographic recordings of a small segment of the edge region of a wafer by means of CCD line cameras can lead to erroneous interpretations of the actual edge shape, can corrupt measurement results or even render measurement impossible. 
       SUMMARY OF THE INVENTION 
       [0005]    Therefore, it is the object of the invention to find a novel possibility for determining an edge profile at thin disk-shaped measurement objects (e.g., semiconductor wafers) which makes it possible during image recording of the edge profile to substantially suppress specular reflections which impede or degrade determination of the edge profile. 
         [0006]    In a device for determination of the edge profile at a thin disk-shaped object comprising a turntable for rotating the disk-shaped object around an axis of rotation and a measuring arrangement for radial positioning of at least one light source for illuminating an edge region of the disk-shaped object in virtually radial direction to the axis of rotation thereof, and at least one camera for recording the illuminated edge region, wherein the camera is arranged in a base plane extending parallely and medially between the plane faces of the disk-shaped object, the above-stated object is met in that there is provided a plurality of light sources in the form of laser radiation sources with line-shaped beam profile which each emit a line-shaped light bundle, in that the line-shaped light bundles of the laser radiation sources are arranged so as to be coplanar in a common plane representing a measurement plane oriented orthogonal to the base plane and are directed from different directions to a common intersection of the laser radiation sources in the edge region of the object, wherein a light sheet composed of the line-shaped light bundles of the laser radiation sources is formed in the measurement plane, and in that the at least one camera, as base camera, is directed in the base plane lateral to the measurement plane so that it records scattered light proceeding from a light line illuminated by the light sheet in the edge region of the object. 
         [0007]    The laser radiation sources are advantageously arranged in such a way that the line-shaped light bundles thereof illuminate the edge region of the disk-shaped object so as to surround it in a U-shaped manner. In this respect, it is useful to arrange three laser radiation sources in such a way that a base laser radiation source is arranged in the base plane and two further laser radiation sources are arranged (symmetric to the two sides of the base laser radiation source) in the measurement plane at an irradiation angle of equal size but different mathematical sign and are directed to the common intersection. 
         [0008]    For alignment of the base camera, it is advantageous that an observation angle between the base camera and the base laser radiation source in the base plane is adjustable in the range between 30° and &lt;90°. 
         [0009]    In addition to the base camera, it is advisable that two further cameras are directed lateral to the measurement plane and to the intersection of the optical axes of the laser radiation sources, preferably at the same pitch angle perpendicular to or under the base plane in each instance, to improve the resolution of the image recording. 
         [0010]    Further, it proves advantageous to provide a notch camera in addition to the base camera, the optical axis of the notch camera being arranged in the base plane at a latitude angle to the base laser radiation source that is substantially smaller than the observation angle of the base camera to the base laser radiation source. 
         [0011]    To adjust the measuring arrangement to different diameters of disk-shaped objects and to compensate for eccentricity in a rotating edge profile, a linear guide is advisably provided for moving the measuring arrangement orthogonal to the axis of rotation of the turntable. 
         [0012]    A centering camera oriented perpendicular to the base plane is advantageously provided for detecting an eccentric position of the edge region of the disk-shaped object relative to the axis of rotation of the turntable and is arranged outside the measurement plane defined by the laser radiation sources. The radial position of the centering camera can be adjusted to a diameter of the object that is known beforehand, and the centering camera is arranged opposite a diffuse illumination unit. For this purpose, it is advisable that the angular position of the centering camera to the measurement plane, which angular position is adjusted in the base plane, is provided for calculating a tracking movement of the measuring arrangement which compensates for eccentricity. 
         [0013]    For vibration-decoupled measurement, a solid base plate is advantageously used as a component carrier for a table system with the turntable, for a linear guide and a supporting system for the measuring arrangement and for additional elements of the device. 
         [0014]    The invention is based on the fundamental consideration that because of interfering reflections a purely optical generation and observation of images of the edge profile leads to a flawed acquisition of at least some portions of the edge profile of wafers. The invention solves this problem by selecting a camera arrangement which records exclusively scattered light from the object edges and in that the scattered light is captured lateral to an illumination plane generated by line-shaped illumination. The illumination is preferably carried out by means of line lasers which impinge from different directions so as to generate a thin planar light sheet (light curtain) into which the profile of the object to be measured intrudes and is moved orthogonally through the latter. The line lasers generate a homogeneous laser line on the measurement object, which laser line is illuminated by line lasers impinging in a coplanar manner orthogonally on the edge profile to be measured. As a result of this light curtain impinging “on all sides”, virtually every point of the edge region of the measurement object is illuminated orthogonally and an intensive, narrowly spatially defined fringe of light is generated around the end profile during lateral image recording by the camera arrangement so that the edge profile is progressively imaged planewise due to the orthogonal movement of the edge profile through the light curtain. 
         [0015]    The images which are successively recorded by the camera arrangement and which have no superposition errors or distortion in spite of a plurality of light sources allow a more precise measurement of edge(s) compared to previously known solutions. This happens because when the edge profile penetrates into the light sheet, a uniform intensive laser line is generated along the edge profile and a light fringe thereof which is generated by scattered light is recorded by the camera arrangement lateral to the light sheet and can be objectively evaluated by means of software. It should be noted that the light fringe of the light sheet impinging on the measurement object “on all sides” is also referred to herein interchangeably as “light line” to simplify the description of the image recordings of the profile of the measurement object. 
         [0016]    The device makes it possible to determine the edge profile at thin disk-shaped measurement objects quickly and reliably, and a reflection-free, highly precise recording of the edge profile is achieved even when trouble spots or improperly angled object edges are found in the edge profile. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be described more fully in the following with reference to embodiment examples. The drawings show: 
           [0018]      FIG. 1  a schematic construction of the device according to the invention; 
           [0019]      FIG. 2  one specific embodiment form of the device according to the invention in full elevation (right-hand side) and a fragmentary view of the back (left-hand side); 
           [0020]      FIG. 3  a schematic construction of the device according to the invention in a preferred embodiment with four cameras for edge recording and with an additional unit for detecting eccentricity; and 
           [0021]      FIG. 4  a schematic illustration of the generation of the light sheet in the region of a wafer edge profile. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    According to  FIG. 1 , the device has a measuring arrangement  3  including a base laser radiation source  31 , at least two further laser radiation sources  32  and at least one base camera  35 . The optical axis  34  of the base laser radiation source  31  and the optical axis  36  of the base camera  35  are arranged substantially orthogonal to one another in a preferably horizontally oriented common base plane  41  and meet at an intersection  42 . The further laser radiation sources  32  are arranged with their optical axes  34  symmetric to both sides of the base laser radiation source  31  in a measurement plane  43  at an irradiation angle  45  of the same size but different sign relative to the base laser radiation source  31  and are likewise directed into intersection  42 . The laser radiation sources  31  and  32  are preferably line lasers of identical construction and have line-shaped beam profiles whose light bundles  33  collectively form a light sheet  4  inside the measurement plane  43 . The light sheet  4  has an orthogonal orientation to the base plane  41 . 
         [0023]    In order that a profile to be measured at a measurement object, which in this case—without limiting generality—is the edge profile  21  of a wafer  2 , can be aligned with the components (laser radiation sources  31  and  32  and at least the base camera  35 ) of the measurement arrangement  3  which are exactly aligned with one another, a table system  1  is arranged at a defined distance from the measuring arrangement  3 . The wafer is movably supported by the table system  1  and can be moved through the light sheet  4  along the edge profile  21  to be measured. 
         [0024]    The table system  1  according to  FIG. 2  is outfitted with a turntable  11  for the wafer  2  which is provided in this example as measurement object. The turntable  11  has a horizontal support surface supporting the wafer  2 . The axis of rotation  12  of the turntable  11  is oriented orthogonal to the base plane  41 . 
         [0025]    According to  FIG. 2 , a linear guide  51  is provided on a base plate  5  for receiving the measuring arrangement  3 . The linear guide  51  is oriented in such a way that the measuring arrangement  3  is arranged with its intersection  42  of the optical axes  34  and  36  of laser radiation sources  31 ,  32  and of base camera  35 , respectively, displaceably in an orthogonal direction relative to the axis of rotation  12  of the turntable  11  in the base plane  41 . The optical axis  34  of the base laser radiation source  31  is arranged parallel to the movement direction of the linear guide  51  so that the line-shaped light bundle  33  of the base laser radiation source  31  is oriented substantially in a radial plane to the axis of rotation  12  of the turntable  11 . 
         [0026]    To achieve the highest possible accuracy with the device, a solid granite block with a moment of inertia adapted to the maximum acceleration forces of the turntable  11 , linear guide  51  and table system  1  is used as base plate  5 . The base plate  5  is supported so as to be decoupled from vibrations relative to the substrate at the installation site. 
         [0027]    As is shown in  FIG. 2 , the wafer  2  with an edge profile  21  to be inspected is placed so as to be as centered as possible with one of its plane faces on the support surface of the turntable  11 . The support surface has a smaller diameter than the wafer  2  to be measured so that the entire edge region  22  of the wafer  2  freely projects beyond the edge of the turntable  11 . The support surface of the turntable  11  can be adapted to commercial wafer sizes in a corresponding manner for optimal accommodation of various wafer sizes. 
         [0028]    The wafer  2  can be set in rotation with the turntable  11 . Inaccuracies in the positioning of the wafer  2  resulting in an eccentricity between the wafer axis and the axis of rotation  12  of the turntable  11  are captured by a centering camera  13 . For this purpose, as is shown in  FIG. 3 , the centering camera  13  is positioned above the support surface of the turntable  11  over the wafer edge region  21 . A telecentrically radiating illumination unit  14  which is arranged below the support surface of the turntable  11  radiates a diffuse light in direction of the centering camera  13 . With the wafer edge region  22  arranged therebetween, a silhouette of the outer edge  23  of the wafer  2  is generated opposite the centering camera  13 . Based on the silhouette, the cyclical movements of the outer edge  23  of the wafer occurring during the rotation of an eccentrically positioned wafer  2  can be captured by the centering camera  13  depending on the angle of rotation and stored. The values acquired in this way are used to control the linear guide  51  in the process of determining the edge profile so as to compensate for the eccentric position of the wafer  2  relative to the intersection  42  of the optical axes  34  and  36  of the measuring arrangement  3  so it is not necessary to correct the eccentric position of the wafer  2  on the turntable  11 . To correlate the rotational angle-dependent positional variations of the outer edge  23  of the wafer  2  relative to the intersection  42  of the measuring arrangement  3 , it is merely necessary to know the angle formed by the axis of rotation  12  between the intersection  42  and optical axis  36  of the centering camera  13 . 
         [0029]    A holder  15  shown in  FIG. 2  is provided for fastening the centering camera  13  which is situated on the optical axis  34  of the illumination unit  14 . Adjustment of the position of the centering camera  13  to the different diameters of commercial wafer sizes is ensured in that the holder  15  is displaceable relative to the turntable  11  in a radial direction relative to the axis of rotation  12 . 
         [0030]    After acquiring the eccentricity of the wafer  2  and, therefore, the rotation angle-dependent position of the edge profile  21 , the measuring arrangement  3  can be moved by means of the linear guide  51  in direction of the turntable  11  out of an idle position at the greatest distance from the turntable  11  into a ready position which is determined based on wafer size. In accordance with the previously measured eccentricity of the supported wafer  2 , a rotation angle-dependent signed offset is applied to this ready position. By summing the ready position and offset, the measuring arrangement  3  reaches an inspection position in which the intersection  42  of the optical axes  34  and  36  of the measuring arrangement  3  is always held in a constant position relative to the outer edge  23  of the wafer  2 . 
         [0031]    As is shown in  FIG. 4 , the light sheet  4  is formed as measurement plane  43  in orthogonal orientation to the base plane  41  owing to the line-shaped beam profile of the light bundles  33  proceeding from the laser radiation sources  31  and  32 . The irradiation angle  45  of the further laser radiation sources  32  can have a value ranging between 10° and 90° to the base laser radiation source  31  depending upon requirements. Therefore, the light bundles  33  of the further laser radiation sources  32  always impinge on the edge region  22  of the wafer  2  from a position arranged below and above the plane faces of the wafer  2  so that a light line  44  enclosing the edge profile  21  in a U-shaped manner is illuminated on the edge region  32  of the wafer  2  when the wafer  2  penetrates the light sheet  4 . If the irradiation angle  45  of the laser radiation sources  32  is in the range of 45° or less, the base laser radiation source  31  can be dispensed with. 
         [0032]    The scattered light proceeding from the light line  44  can be captured in the form of a light fringe by the base camera  35  which is arranged in the base plane  41  and which operates telecentrically. This light fringe “seen” by the base camera  35  is shown in  FIG. 4  in an enlarged section (upper right) as a stylized image recording  49  of the base camera  35 . 
         [0033]    By capturing the scattered light emanating from the light line  44  at the edge profile  21  and through a procedure which is already known from light section methods as they are called, the surfaces of the edge region  22  of the wafer  2  and especially the outer edge  23  of the wafer  2  can be inspected and any anomaly, e.g., divergent shape or mechanical damage, can be recorded. In order to capture the edge profile  21  with high spatial resolution, the light sheet  4  has a thickness, and therefore the light line  44  has a width, between 1 μm and a maximum of 25 μm. 
         [0034]    To capture the scattered light of the light line  44 , the base camera  35  with a high-resolution objective is secured in the measuring arrangement  3 . Its optical axis  36  is arranged in the base plane  41  at an observation angle  46  to the optical axis  34  of the base laser radiation source  31 . The working distance of the base camera  35  is selected in such a way that the light sheet  4  is located exactly in the depth of focus range of the objective of the base camera  35 . Since as a general rule there are no further elements in the edge region  22  of the wafer  2  which mask the scattered light in the base plane  41 , the observation angle  46  between the base camera  35  and the base laser radiation source  31  can be selectively adjusted within a very wide range between 30° and &lt;90°. 
         [0035]    In order to achieve a more compact construction of the measuring arrangement  3 , the base camera  35  can also be arranged perpendicularly as is shown in  FIG. 2 ; for this purpose, a deflecting prism  39  is arranged in front of the objective of the base camera  35 . In this case, to capture the scattered light of the light line  44  the deflecting prism  39  is arranged exclusively in the base plane  41  in order to direct the angled optical axis  36  of the base camera  35  in the intersection  42  tangential to the outer edge  23  of the wafer  2 . 
         [0036]    The edge profile  21  of the wafer  2  rotating by means of the turntable  11  continuously passes through the light sheet  4 . The reflections of the light line  44  projected on the edge profile  21  are acquired by the base camera  35  only in the form of a scattered light distribution. The corresponding rotation angle of the wafer  2  is captured at the same time based on the position of the turntable  11 . In this way, the captured scattered light distribution can also be associated with an unambiguous position on the edge region  22  of the wafer  2 , the local edge profile  21  can be acquired by assessing the characteristic features of the scattered light distribution, and every trouble spot on the edge profile  21  of the wafer  2  can be recorded and stored. 
         [0037]    If the scattered light of the light line  44  is observed at a defect-free wafer edge region  22 , the greatest intensity of scattered light that is recorded corresponds to a perspective edge profile  21  within the radial plane of the wafer  2  through the intersection  42  defining the measurement plane  43 . Every profile deviation or damage to the edge region  22  of the wafer  2  changes the extent, structure and intensity of the scattered light and therefore provides information about characteristic surface changes deviating from an expected standard shape. 
         [0038]    By means of the known observation angle  46  between the base camera  35  and base laser radiation source  31  and the known rotation angle of the wafer  2  on the turntable  11 , the position and magnitude of changes to the requisite edge profile  21  can be detected in a very precise manner. The position data which are determined in this way are converted into a digital blank profile and used to determine the edge profile  21  by applying appropriate algorithms. The data of the edge profile  21  can be evaluated within the framework of quality assurance or sent to appropriate machines for carrying out subsequent edge machining. 
         [0039]    With highly reflective surfaces such as are found in polished metals or semiconductor substrates, reflections may occur during the inspection of an edge profile  21  which interfere with a reliable detection of the scattered light by an individual base camera  35 . In order to achieve a reliable detection of the edge profile  21  of a wafer  2  in the edge region  22  thereof also under reflective surface conditions of this kind, further cameras  37  can be used in addition to the base camera  35 . 
         [0040]    For this purpose, as is shown in  FIG. 3 , two additional cameras  37  are arranged above and below the base plane  41  in a tangential plane extending through the optical axis  36  of the base camera  35  and oriented orthogonal to the base plane  41  and are directed to the intersection  42 . The two additional cameras  37  have the same pitch angle  47  and, therefore, a symmetrical arrangement with respect to the base plane  41 . The pitch angle  45  is preferably 45° but can also be adjusted in the range between 10° and 90° in principle. 
         [0041]    To identify the crystal orientation in silicon wafers, the edge region  22  of the wafer  2  is usually provided with at least one notch  24 . As a result of the standardized notch  24 , when traversing the light sheet  4  neither the base camera  35  nor the additional cameras  37  can capture portions of the light line  44  at the deeper points of the notch  24  because they are partially concealed by the regular edge profile  21  of the edge region  22  of the wafer  2 . It is useful to employ an additional notch camera  38  so that the edge profile  21  of the outer edge  23  of the wafer  2  can also be fully captured in this area as well. 
         [0042]    To this end, the notch camera  38  is arranged with its optical axis  36  in the base plane  41  and in a latitude angle  48  of preferably 45° to the optical axis  34  of the base laser radiation source  31 . The latitude angle  48  can also be adjusted so as to diverge from 45° provided the notch camera  38  can still capture the scattered light of the light line  44  uninterruptedly in the entire region of the notch  24 . The precisely acquired position of the notch  24  can also be used in combination with the angle of rotation of the turntable  11  as a reference point for associating the angle of rotation with the successively acquired image recordings of the light line  44  of the edge profile  21  of the wafer  2 . 
         [0043]    The objectives of the base camera  35 , of all of the additional cameras  37  and of the notch camera  38  are configured confocally, i.e., the focal points thereof lie exactly in the light sheet  4  at the intersection  42  of the optical axes  34  and  36  of the base laser radiation source  31  and base camera  35  and accordingly correspond to the desired point of incidence of the base laser radiation source  31  on the outer edge  23  of the wafer  2 . As is shown in  FIG. 2 , the alignment of the cameras  35 ,  37  and  38  and of the laser radiation sources  31  and  32  is carried out by means of precisely adjustable fastening elements  52  which are arranged at a supporting system  53  for the measuring arrangement  3 , this supporting system  53  being moved by means of the linear guide  51 , and the cameras  35 ,  37  and  38  and laser radiation sources  31  and  32  of the measuring arrangement  3  can be adjusted and fixed in a defined manner relative to one another by means of these fastening elements  52 . As a result of this arrangement and the known angles between the light sheet  4 , base plane  41  and camera positions for defining the measurement plane  43 , the recordings of the scattered light of the light line  44  made by the individual cameras  35 ,  37  and  38  along the edge profile  21  of the wafer  2  are superposed without distortion, and a very precise edge profile  21  of the edge region  22  of the wafer  2  can be calculated therefrom. This makes possible a reliable and precise characterization of the edge profile  21  of a wafer  2 . 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  table system 
           11  turntable 
           12  axis of rotation 
           13  centering camera 
           14  illumination unit 
           15  holder 
           2  wafer 
           21  edge profile 
           22  wafer edge region 
           23  outer edge of the wafer 
           24  notch 
           3  measuring arrangement 
           31  base laser radiation source 
           32  additional laser radiation source 
           33  light bundle 
           34  optical axis (of the light source) 
           35  base camera 
           36  optical axis (of the base camera) 
           37  additional camera 
           38  notch camera 
           39  deflecting prism 
           4  light sheet 
           41  base plane 
           42  intersection 
           43  measurement plane 
           44  light line 
           45  irradiation angle 
           46  observation angle 
           47  pitch angle 
           48  latitude angle 
           49  image recording (of the base camera) 
           5  base plate 
           51  linear guide 
           52  fastening element 
           53  supporting system (of the measuring arrangement)