Apparatus and method for measuring two opposite surfaces of a body

Abstract

An apparatus and a method are provided which allow two opposite plane surfaces of a body to be interferometrically measured simultaneously using light from a single light source. From a parallel light beam (P) produced by a light source ( 1 ) partial light beams (A, B) having positive and negative diffraction angles are produced using a beam splitter ( 8 ) in the form of a diffraction grating. The partial light beams strike the respective surfaces ( 90, 91 ) of the body ( 9 ) to be measured and are reflected thereat. The reflected partial light beams (A, B) are interfered with the throughgoing partial light beam (P) having an order of diffraction of zero and the thus produced interference patterns are digitized and subtracted from each other, whereby the parallelism of both surfaces ( 90, 91 ) of the body can be determined.

1 . Apparatus for measuring two opposite surfaces of a body, comprising 
 a light source ( 1 ) producing a light beam;    a positioning device ( 25 ,  19 ,  50 ) for positioning the body ( 9 ) into the path of the light beam (P);    a beam splitter device ( 8 ) disposed between the light source ( 1 ) and the position of the body ( 9 ) for splitting the light beam (P) for branching-off a partial light beam (A, B) from the light-beam (P), the partial light beam striking one of the surfaces ( 90 ,  91 ) to be measured at an angle and being reflected at the surface, and    a detector device ( 16 ) for producing an interference pattern between the light beam (P) and the reflected partial light beam (A, B),    characterized in that the positioning device ( 25 ,  19 ,  50 ) comprises a holding device ( 50 ) supporting the body ( 9 ) in such a manner that the surfaces ( 90 ,  91 ) to be measured are arranged substantially parallel to the light beam (P) and are freely accessible to the measurement, and that the beam splitter device ( 8 ) is designed so as to simultaneously produce a first partial light beam ( 8 ) striking the first surface ( 90 ) and a second partial light beam (B) striking the second surface ( 91 ).    
     
     
         2 . Apparatus according to    claim 1   , characterized in that the holding device ( 50 ) comprises a device for putting up the body ( 9 ) in the beam path, whereby the surfaces ( 90 ,  91 ) to be measured are aligned in a substantially vertical direction.  
     
     
         3 . Apparatus according to    claim 1    or    2   , characterized in that the beam splitter device ( 8 ) comprises a diffraction grating disposed in the beam path, whereby the first partial light beam (A) striking the first surface ( 90 ) comprises waves having a positive diffraction angle and the second partial light beam (B) striking the second surface ( 91 ) comprises waves having a negative diffraction angle.  
     
     
         4 . Apparatus according to any of the    claims 1    to    3   , characterized in that a calibration device ( 20 ) with a reference body ( 21 ) comprising at least one plane surface ( 24 ) is provided.  
     
     
         5 . Apparatus according to    claim 4   , characterized in that the calibration device ( 20 ) comprises a positioning device ( 23 ,  18 ) for positioning the reference body ( 21 ) in the beam path in place of the body ( 9 ) to be measured, whereby the plane surface ( 24 ) is vertical.  
     
     
         6 . Apparatus according to    claim 5   , characterized in that the positioning device ( 20 ) comprises a traveller ( 23 ) for positioning the reference body ( 21 ) in the beam path.  
     
     
         7 . Apparatus according to any of the    claims 4    to    6   , characterized in that the calibration device ( 21 ) comprises means for turning the body by 180° around an axis parallel to the plane surface ( 24 ).  
     
     
         8 . Apparatus according to any of the    claims 1    to    7   , characterized in that a receiving device ( 25 ) for the body ( 9 ) to be measured is provided, that the body can be put into the receiving device so that its surfaces ( 90 ,  91 ) to be measured are arranged in a substantially horizontal direction, and further that a tilting device ( 26 ) is provided for tilting the body into its measuring position.  
     
     
         9 . Apparatus according to any of the    claims 1    to    8   , characterized in that the detector device ( 16 ) comprises two detectors ( 16 ) each measuring the interference of the partial light beams (A, B) deflected at the surface ( 90 ,  91 ) to be measured with the reference light beam (P).  
     
     
         10 . Apparatus according to    claim 9   , characterized in that the detectors ( 16 ) are formed as CCD cameras.  
     
     
         11 . Apparatus according to    claim 9    or    10   , characterized by an image processing device ( 30 ) for producing digitized phase patterns ( 160 ) on the basis of the interferences measured by each detector ( 16 ) for each surface ( 90 ,  91 ) to be measured.  
     
     
         12 . Apparatus according to    claim 11   , characterized by an evaluating device ( 40 ) for evaluating the phase patterns ( 160 ).  
     
     
         13 . Apparatus according to any of the    claims 1    to    12   , characterized by a phase shifter ( 17 ) for varying the phase of the light beam (P) by a defined amount.  
     
     
         14 . Apparatus according to any of the    claims 1    to    13   , characterized by a beam collector ( 10 ) disposed in the beam path between the position of the body ( 9 ) and the detector device ( 16 ).  
     
     
         15 . Apparatus according to    claim 14   , characterized in that the beam collector ( 10 ) is formed as a diffraction grating.  
     
     
         16 . Apparatus according to any of the    claims 1    to    15   , characterized in that a parabolic mirror ( 7 ) for producing a parallel light beam (P) from the light beam leaving the light source ( 1 ) is provided.  
     
     
         17 . Apparatus according to any of the    claims 1    to    16   , characterized in that the light source ( 1 ) is formed as a laser.  
     
     
         18 . Apparatus according to any of the    claims 13    to    17   , characterized in that the phase shifter ( 17 ) comprises a piezoelectric actuating element for displacing the diffraction grating of the beam splitter ( 8 ).  
     
     
         19 . Apparatus according to any of the    claims 1    to    18   , characterized by a control device ( 60 ) for controlling the positioning devices ( 22 ,  19 ,  50 ) for the body ( 9 ) to be measured and for the reference body ( 21 ).  
     
     
         20 . Method for measuring two opposite surfaces ( 90 ,  91 ) of a body ( 9 ), whereby each surface ( 90 ,  91 ) is measured using interference between a light beam (A, B) delivered from a light source ( 1 ) and reflected at the surface ( 90 ,  91 ) with a light beam (P) delivered from the light source ( 1 ) and travelling substantially parallel to the surface ( 90 ,  91 ), 
 characterized in that a single light source ( 1 ) is used for measuring both surfaces ( 90 ,  91 ) and that the body ( 9 ) is disposed in the beam path of the light source ( 1 ) so that both surfaces ( 90 ,  91 ) to be measured are simultaneously accessible to the interference measurement.    
     
     
         21 . Method according to    claim 20   , characterized in that the body ( 9 ) is arranged in the beam path so that the surfaces ( 90 ,  91 ) to be measured are aligned substantially in a vertical direction.  
     
     
         22 . Method according to    claim 20    or    21   , characterized in that the light beam from the light source ( 1 ) is split into three partial light beams (A, B, P) by means of a beam splitter ( 8 ), whereby two of the partial light beams (A, B) are deflected each on one of the surfaces ( 90 ,  91 ) to be measured and reflected thereat and the third partial light beam (P) is used as a reference beam for the interference measurement.  
     
     
         23 . Apparatus according to    claim 22   , characterized in that the partial light beams (A, B) striking the surfaces to be measured are produced by diffraction at a grating, whereby the partial light beam (A) having a positive diffraction angle is used for measuring the one surface ( 90 ) and the partial light beam (B) having a negative diffraction angle is used for measuring the other surface ( 91 ), and whereby the partial light beam having a diffraction angle of zero is used as a reference beam (P).  
     
     
         24 . Method according to any of the    claims 20    to    23   , characterized in that several phase-shifted interference patterns ( 160 ) are produced of each surface ( 90 ,  91 ) to be measured.  
     
     
         25 . Method according to    claim 24   , characterized in that the phase patterns ( 160 ) are digitized.  
     
     
         26 . Method according to any of the    claims 20    to    25   , characterized in that in the measurement the phase of the partial light beams (A, B, P) is shifted by a defined phase angle using a phase shifter ( 17 ) for producing moving phase patterns for determining protuberances or depressions in the surfaces ( 90 ,  91 ) to be measured.  
     
     
         27 . Method according to any of the    claims 24    to    26   , characterized in that the phase patterns ( 160 ) of both surfaces ( 90 ,  91 ) are subtracted from each other for determining the parallelism of the surfaces ( 90 ,  91 ).  
     
     
         28 . Method according to any of the    claims 20    to    27   , characterized in that a calibration measurement is made, whereby a reference body ( 21 ) having at least one plane surface ( 24 ) is brought into the measurement position and the plane surface ( 24 ) is measured.  
     
     
         29 . Method according to    claim 28   , characterized in that the reference body ( 21 ) is turned by 180° around an axis parallel to the plane surface ( 24 ) and the same surface ( 24 ) is again measured.  
     
     
         30 . Method according to any of the    claims 20    to    29   , characterized in that two plane surfaces ( 90 ,  91 ) of a semiconductor wafer ( 9 ) are measured.