Patent Publication Number: US-2002005958-A1

Title: Non-contact thickness-measuring device

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a thickness-measuring device for determining the thickness of a workpiece such as a semiconductor wafer.  
       [0003] 2. Related Arts  
       [0004] Referring to FIG. 4, a grinding machine  50  is used in grinding a semiconductor wafer to a desired thickness. As shown in the drawing, a pair of rails  53  is laid on an upright wall  52 , which stands upright on the base  51  of the grinding machine  50 . A slider  54  is driven on the rails  53  by an associated stepping motor  55  to be raised or lowered, carrying a grinding unit  11 . A chuck table  10  is placed on the base  51 , and a workpiece to be ground is positively held by the chuck table  10 .  
       [0005] The grinding unit  11  comprises a spindle housing  12 , a spindle  13  rotatably supported by the spindle housing  12 , a grinding wheel  16  fixed to the tip of the spindle  13  via an associated mount  14 , and a grind stone  15  fixed to the grinding wheel  16 . When the spindle  13  rotates, the grindstone  15  rotates accordingly.  
       [0006] When a semiconductor wafer W is ground, it is fixedly held by the chuck table  10  so that it may be put under the grinding unit  11 . The spindle  13  is rotated, and the grinding unit  11  is lowered until the grind stone  15  rotating at a high speed has been pushed against the semiconductor wafer W, thereby grinding the surface of the semiconductor wafer W.  
       [0007] As seen from the drawing, a thickness-measuring device  61  uses two needle-like sensors  60   a  and  60   b  in determining the thickness of the semiconductor wafer W. The lower sensor  60   a  is put on the surface of the chuck table  10  whereas the upper sensor  60   b  is put on the surface of the semiconductor wafer W. The thickness of the semiconductor wafer W can be determined in terms of the difference between the lower and upper levels at which the lower and upper sensors  60   a  and  60   b  extend.  
       [0008] Such thickness-measuring device, however, has the defect of injuring semiconductor wafers W with its needle-like sensors, and hence there is a fear of lowering the qualities of semiconductor wafers when their thickness is measured.  
       SUMMARY OF THE INVENTION  
       [0009] One object of the present invention is to provide a non-contact thickness-measuring device capable of determining the thickness of a workpiece in non-contact way, thus assuring that the workpiece is prevented from being injured.  
       [0010] To attain this object a non-contact thickness-measuring device for determining the thickness of a workpiece to be machined according to the present invention comprises: a laser light projecting means for projecting a ray of laser light to the top surface of the workpiece at a predetermined angle of incidence relative to the top surface of the workpiece; an imaging means for capturing the first ray of laser light reflected from the top surface of the workpiece and the second ray of laser light passing through the thickness of the workpiece and reflecting from the bottom surface of the workpiece; and an arithmetic means for determining the thickness of the workpiece from the distance between the first point at which the first ray of laser light falls on the imaging means and the second point at which the second ray of laser light falls on the imaging means.  
       [0011] The thickness of the workpiece “t” is given by the following equation: 
         t =( a /2 sin θ 1 )·tan θhd  2   
       [0012] where “a” stands for the distance between the first point and the second point; “θ 1 ” stands for the predetermined angle of incidence; and “θ 2 ” stands for the angle of refraction at which the ray of laser light goes in the workpiece.  
       [0013] The laser projecting means may comprise an infrared laser; the imaging means may comprise an infrared-sensitive camera and the workpiece may be a semiconductor wafer.  
       [0014] According to such thickness-measuring device of the present invention constructed as above, thickness of a workpiece is determined by capturing the first ray of laser light reflected from the top surface of the workpiece and the second ray of laser light passing through the thickness of the workpiece and reflecting from the bottom surface of the workpiece, thus assuring that the workpiece is prevented from being injured.  
       [0015] Further, in a case where the workpiece is neither transparent nor translucent, advantageously thickness of the workpiece can be measured without being injured as same in a case where the workpiece is transparent or translucent by using the infrared laser, since the infrared rays can pass through even such workpiece. 
     
    
    
     [0016] Other objects and advantages of the present invention will be understood from the following description of a non-contact thickness-measuring device according to one preferred embodiment of the present invention, which is shown in accompanying drawings.  
     BRIEF DESCRIPTION OF THE DRAWING  
     [0017]FIG. 1 is a perspective view of a grinding machine equipped with a non-contact thickness-measuring device according to the present invention;  
     [0018]FIG. 2 illustrates the principle according to which the thickness of a workpiece can be determined;  
     [0019]FIG. 3 illustrates the part of a cutting machine to which a non-contact thickness-measuring device according to the present invention is attached; and  
     [0020]FIG. 4 is a perspective view of a grinding machine equipped with a conventional thickness-measuring device. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
     [0021] Referring to FIGS. 1 and 2, in which same parts as appear in FIG. 4 are indicated by same reference numerals, a workpiece  40  is fixedly held by the chuck table  10 , and the grind stone  14  is fixed to the tip of the spindle  13  via the mount  14 , and the spindle  13  is rotatably supported by the spindle housing  12 .  
     [0022] A laser light projecting means  21  is so placed in the vicinity of the workpiece  40  that the ray of laser light may be thrown to the workpiece  40  obliquely, and a imaging means  22  is so placed that the ray of laser light may fall on the imaging means  22  after being reflected on the top surface of the workpiece  40 . The imaging means  22  is connected to an arithmetic means  23  for determining the thickness of the workpiece from the distance “a” between the first point at which the first ray of laser light  30   a  falls on the imaging means  22  and the second point at which the second ray of laser light  30   b  falls on the imaging means  22 .  
     [0023] In FIG. 2, the ray of laser light projecting means  21  throws a ray of laser light  30  to the top surface  40   a  of the workpiece  40  at a predetermined angle of incidence θ 1 , which is smaller than 90 degrees.  
     [0024] A division of ray of laser light is reflected from the top surface  40   a  of the workpiece  40 , and the remaining division of ray of laser light is refracted at the top surface  40   a  of the workpiece  40  to go in the workpiece  40 , and then, the refracted ray of laser light is reflected from the bottom surface  40   b  of the workpiece  40 , traveling to the top surface  40   a  of the workpiece  40 , where the refracted ray of laser light is refracted again. Finally the refracted ray of laser light falls on the imaging means  22  as the second ray of laser light  30   b.    
     [0025] As seen from FIG. 2, the first division of the ray of laser light  30  from the ray of laser light projecting means  21  is reflected on the top surface  40   a  of the workpiece  40  at the same angle θ 1  as the angle of incidence θ 1 . The remaining second division of-the ray of laser light  30  is refracted on the top surface  40   a  of the workpiece  40  at an angle of refraction θ 2  to go in the thickness of the workpiece  40 , and then, the second division of ray of laser light  30   b  is reflected on the bottom surface of the workpiece  40  to go to the top surface  40   a  of the workpiece  40 , where the second division of ray of laser light  30   b  comes out at the same angle as the angle of incidence θ 1 . Thus, the second ray of laser light  30   b  travels in parallelism with the first ray of laser light  30   a , leaving the distance “a” therebetween. Finally these rays of laser light fall in the imaging means  22 . The second division of ray of laser light comes out from the top surface of the workpiece  40  at the distance “b” apart from the point at which it goes in the workpiece. The distance “b” can be given by the following equation: 
       b=a /sin θ 1   (1) 
     [0026] The thickness “t” of the workpiece  40  is given by the following equation: 
       t=b· tan θ 2 /2  (2) 
     [0027] By substituting Equation (1) for “b” in Equation (2) the following equation results: 
       t= ( a /2 sin θ 1 )·tan θ 2   (3) 
     [0028] In case a CCD camera is used as the imaging means which is composed of, for instance, 256 times 256 pixels, the distance “a” can be measured in terms of the number of pixels existing between the first point which the first ray of laser light  30   a  falls on and the second point which the second ray of laser light  30   b  falls on the camera&#39;s exposure plane.  
     [0029] As above mentioned, thickness of a workpiece can be determined by computation with capturing the first ray of laser light  30   a  and the second ray of laser light  30   b  without contact to the workpiece. Accordingly, the workpiece can be prevented from injuring, thus preventing from lowering the quality.  
     [0030] In a case where a workpiece is transparent or semi-opaque or translucent, the infrared laser need not be used, but in a case where a workpiece such as a silicon semiconductor wafer is neither transparent nor translucent, advantageously the infrared laser can be used. The infrared rays can pass through the semiconductor wafer, and then an infrared camera can be used as the imaging means  22 .  
     [0031] The thickness-measuring device can be installed in a machining apparatus other than the grinding machine. Referring to FIG. 3, a cutting machine  45  has a cutting blade  47  attached to its spindle  46 . The cutting blade  47  is lowered while rotating at a high speed, thus notching a workpiece  48  which is held by the chuck table  49 . The thickness of the workpiece  48  must be watched constantly to assure that a “V”-shaped cut reaches short of the bottom of the workpiece  48 .