Patent Publication Number: US-9903708-B2

Title: Method and apparatus to fold optics in tools for measuring shape and/or thickness of a large and thin substrate

Description:
PRIORITY 
     The present application claims the benefit under 35 U.S.C. § 120(pre-AIA) of U.S. patent application Ser. No. 13/561,377, filed Jul. 30, 2012, issued as U.S. Pat. No. 9,279,663, which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention is directed generally toward semiconductor wafer processing and more particularly toward measuring the shape and thickness of semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     In a typical tool for measuring the shape and thickness of a silicon wafer, two channels of interferometers are employed to measure both surfaces of the wafer. Each interferometer usually comprises lenses that image the wafer to a video camera. This way the whole wafer can be measured by the camera with millions of pixels, eliminating the need to mechanically scan the wafer, and the throughput is dramatically improved compared to scanning systems. 
     One disadvantage of this method is that the size of the measuring tool is large due to the size of imaging optics. As the semiconductor industry shifts to larger wafers (for example from 300 mm to 450 mm) the size of the measuring tool may increase significantly. Simply scaling up existing measuring tools designed for a 300 mm wafer to accommodate a 450 mm wafer would result in a measuring tool much more expensive and fifty percent larger in every direction. At that size, the measuring tool may not physically fit in a space currently designated for such measuring tools. 
     Consequently, it would be advantageous if an apparatus existed that is suitable for measuring the shape and thickness of a silicon wafer with a compact optical arrangement. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel method and apparatus for measuring the shape and thickness of a silicon wafer with a compact optical arrangement. 
     In one embodiment of the present invention, a measuring device includes mirrors for directing the optical path along an axis parallel to an axis normal to the surface of the wafer. Such configuration allows utilization of the space along the length of the measuring tool. 
     In another embodiment of the present invention, a method for measuring semiconductor wafers includes reflecting an interferometric image from a first axis normal to a surface of the semiconductor wafer to a second axis parallel to the first axis where the interferometric image is captured by a camera. By this method, the optical path from the wafer to the image is extended concurrently with the length of the measuring tool. Increased length allows for larger optical components and therefore imaging larger semiconductor wafers. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a perspective view of a measuring tool; 
         FIG. 2  shows a perspective view of a measuring tool according to the present invention; and 
         FIG. 3  shows a flowchart of a method for measuring a semiconductor wafer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , a perspective view of a measuring tool is shown. The measuring tool may include one or two reference flats  102 ,  120 . Reference flats  102 ,  120  are the references for measuring surface flatness of a semiconductor wafer  100 . The measuring tool may also include one or two collimators  104 ,  122 . Collimators  104 ,  122  narrow or collimate light from a semiconductor wafer  100 . The measuring tool may also include one or more series of folding mirrors  106 ,  108 ,  110 ,  124 ,  126 ,  128 . 
     Each series of folding mirrors  106 ,  108 ,  110 ,  124 ,  126 ,  128  may reflect an interferometric image of a semiconductor wafer  100  from a collimator  104 ,  122  to one or more optical elements including a camera  118 ,  136 . Folding mirrors  106 ,  108 ,  110 ,  124 ,  126 ,  128  allow an interferometric image of a semiconductor wafer  100  to be redirected so that the measuring tool may be made more compact and stable. 
     A first or large folding mirror  106 ,  124  may reflect an interferometric image from a collimator  104 ,  122  along a path substantially perpendicular to the path of the interferometric image when it exits the collimator  104 ,  122 . A second or mid-folding mirror  108 ,  126  may reflect the interferometric image from the large folding mirror  106 ,  124  along a path substantially perpendicular to both the path of the interferometric image when it exits the collimator  104 ,  122  and the path of the interferometric image when it is reflected by the large folding mirror  106 ,  124 . A third or small folding mirror  110 ,  128  may reflect the interferometric image from the mid-folding mirror  108 ,  126  along a path substantially parallel to the path of the interferometric image when it is reflected by the large folding mirror  106 ,  124 . In this configuration, an interferometric image of a semiconductor wafer  100  may be extended over a necessary distance defined by the parameters of the optics used to transmit the interferometric image and the size of the semiconductor wafer  100 . 
     The small folding mirror  110 ,  128  may reflect the interferometric image from the mid-folding mirror  108 ,  126  to a series of optics. The series of optics may include a λ/4 plate  112 ,  130 , a polarized beam splitter  114 ,  132 , a relay lens  116 ,  134  and a camera  118 ,  136 . The λ/4 plate  112 ,  130  is an optical device that alters the polarization state of the beam from circularly polarized to linearly polarized so that the beam passes through the polarized beam splitter  114 ,  132  with minimum loss. The relay lens  116 ,  134  may be either a lens or group of lenses that re-constructs the interferometric image at the camera  118 ,  136 . 
     The polarized beam splitter  114 ,  132  is also the point at which an illumination source (not shown) is introduced. Light from an illumination source may enter the polarized beam splitter  114 ,  132  from one side, become linearly polarized after reflected by the polarized beam splitter, and then pass through the λ/4 plate  112 ,  130  where the light may become circularly polarized. The light may then reflect off each of the folding mirrors  106 ,  108 ,  110 ,  124 ,  126 ,  128  to enter the collimator  104 ,  122  and reference flat  102 ,  120 . The light may then illuminate the semiconductor wafer  100 . An interference pattern may be formed between the reference flat  102 ,  120  and the semiconductor wafer  100 . The interference pattern is the image delivered to the camera  118 ,  136 . 
     Semiconductor inspection facilities may be configured to accommodate a measuring tool designed for 300 mm semiconductor wafers. A 450 mm semiconductor wafer may require a measuring tool with corresponding larger optics. For example, a measuring tool suitable for a 450 mm semiconductor wafer may include one or two reference flats  102 ,  120  fifty percent larger than reference flats  102 ,  120  suitable for a measuring tool designed to accommodate a 300 mm semiconductor wafer  100  (a 450 mm semiconductor wafer  100  being fifty percent larger than a 300 mm semiconductor wafer  100 ). The size of the semiconductor wafer  100  being inspected may dictate the distance that the light from an illumination source needs to travel in order to expand and illuminate the entire semiconductor wafer  100 . Likewise, the same distance may be necessary to focus the interferometric image. 
     In a measuring tool according to  FIG. 1 , all of the dimensions of the measuring tool may need to be extended to accommodate a 450 mm semiconductor wafer. Such dimensions may increase the size of the measuring tool beyond the space available in existing 300 mm semiconductor processing facilities. Furthermore, optics such as mirrors may be prone to vibration and gravitational distortion. Increasing the size of certain optical components may make such optical components more prone to these effects. 
     Referring to  FIG. 2 , a perspective view of a measuring tool according to the present invention is shown. The measuring tool may include one or two reference flats  202 ,  220  and one or two collimators  204 ,  222 . The measuring tool may also include one or more series of folding mirrors  206 ,  208 ,  210 ,  224 ,  226 ,  228 . 
     Each series of folding mirrors  206 ,  208 ,  210 ,  224 ,  226 ,  228  may reflect an interferometric image of a semiconductor wafer  200  from a collimator  204 ,  222  to one or more optical elements including a camera  218 ,  236 . Folding mirrors  206 ,  208 ,  210 ,  224 ,  226 ,  228  allow an interferometric image of a semiconductor wafer  200  to be redirected so that the measuring tool may be made more compact and stable. 
     A first or large folding mirror  206 ,  224  may reflect an interferometric image from a collimator  204 ,  222  along a path toward a second or mid-folding mirror  208 ,  226 . The large folding mirror  206 ,  224  may be oriented with respect to an axis normal to the reference flats  202 ,  220  such that the angle of incidence of light exiting the collimator  204 ,  222  may be less than 45°; an angle of incidence less than 45° may allow the large folding mirror  206 ,  224  to comprise a smaller, lighter mirror as compared to the prior art. The mid-folding mirror  208 ,  226  may reflect the interferometric image from the large folding mirror  206 ,  224  toward a third or small folding mirror  210 ,  228 . The mid-folding mirror  208 ,  226  may be positioned relative to the large folding mirror  206 ,  224  so as to redirect the light to travel between the large mirror and the collimator  204 ,  222 . 
     The small folding mirror  210 ,  228  may reflect the interferometric image from the mid-folding mirror  208 ,  226  along a path substantially parallel to the path of the interferometric image when it exited the collimator  204 ,  222 . In this configuration, a dimension of the measuring tool corresponding to an axis normal to a semiconductor wafer  200  being measured may define a distance that may be utilized multiple times to focus the interferometric image. Elongated the measuring tool along that dimension may increase the distance traveled by the interferometric image by some multiple of the actual elongation, and thereby reduce the scaling factor when processing 450 mm semiconductor wafers  200  as compared to 300 mm semiconductor wafers  200 . Furthermore, the compact nature of the measuring tool may reduce the potential for vibrational distortion. 
     The small folding mirror  210 ,  228  may reflect the interferometric image from the mid-folding mirror  208 ,  226  to a series of optics. The series of optics may include a λ/4 plate  212 ,  230 , a polarized beam splitter  214 ,  232 , a relay lens  216 ,  234  and a camera  218 ,  236 . The λ/4 plate  212 ,  230  is an optical device that alters the polarization state of the beam from circularly polarized to linearly polarized so that the beam passes the polarized beam splitter  214 ,  232  with minimum loss. The relay lens  216 ,  234  may be either a lens or group of lenses that re-constructs the interferometric image at camera  218 ,  236 . 
     The polarized beam splitter  214 ,  232  is also the point at which an illumination source (not shown) is introduced. Light from an illumination source may enter the polarized beam splitter  214 ,  232  from its side, be reflected by it towards λ/4 plate  212 ,  230 . At this point, it becomes linearly polarized. After passing through the λ/4 plate  212 ,  230 , the light may become circularly polarized. The light may then reflect off of each of the folding mirrors  206 ,  208 ,  210 ,  224 ,  226 ,  228  to enter the collimator  204 ,  222  and reference flat  202 ,  220 . The circularly polarized light may then illuminate the semiconductor wafer  200 . An interference pattern may be formed between the reference flat  202 ,  220  and the semiconductor wafer  200 . The interference pattern is the image to be delivered to the camera  218 ,  236 . 
     Semiconductor inspection facilities may be configured to accommodate a measuring tool designed for 300 mm semiconductor wafers. One potential advantage of the present invention is the ability to operate a measuring tool for 450 mm semiconductor wafers in a facility designed for 300 mm semiconductor wafers. A 450 mm semiconductor wafer may require a measuring tool with correspondingly larger optics. A measuring tool suitable for a 450 mm semiconductor wafer may include one or two reference flats  202 ,  220  fifty percent larger than reference flats  202 ,  220  suitable for a measuring tool designed to accommodate a 300 mm semiconductor wafer  200  (a 450 mm semiconductor wafer  200  being fifty percent larger than a 300 mm semiconductor wafer  200 ). The size of the semiconductor wafer  200  being inspected may dictate the distance light from an illumination source needs to travel in order to expand and illuminate the entire semiconductor wafer  200 . Likewise, the same distance may be necessary to focus the interferometric image. A measuring tool with folding mirrors according to the present invention  206 ,  208 ,  210 ,  224 ,  226 ,  228  may conform to size restrictions imposed by a facility designed for 300 mm semiconductor wafers  200  even when the measuring tool is configured to inspect 450 mm semiconductor wafers  200 . One skilled in the art may appreciate that the concepts set forth herein are equally applicable to semiconductor wafer measuring tools of all sizes, and that the example of 450 mm semiconductor wafers is exemplary and should not be considered a limitation. 
     Referring to  FIG. 3 , a flowchart of a method for measuring a semiconductor wafer is shown. The method may include illuminating  300  a semiconductor wafer with an illuminator. The light may pass through a reference flat prior to illuminating the semiconductor wafer to produce an interference pattern. The interferometric image may provide important measurement information about the semiconductor wafer. The image may need to travel a certain distance to be reduced in size to be captured by a camera. Such distance, if applied in any one direction, may be impractical for a measuring tool in a semiconductor production facility. The distance may be distributed along various dimensions within the measuring tool. 
     The interferometric image may travel along a path substantially normal to the surface of the semiconductor wafer, and may then be reflected  302  by a large folding mirror. The interferometric image may be reflected  302  by the large folding mirror so as to direct the interferometric image along one or more first alternative dimensions of the measuring tool; furthermore, the interferometric image may be reflected  302  by the large folding mirror at an angle less than 45°. 
     The interferometric image may be reflected  304  by a mid-folding mirror along a path defined by one or more second alternative dimensions of the measuring tool. The interferometric image may then be reflected  306  by a small folding mirror along a path substantially parallel to a path defined by a line normal to the surface of the semiconductor wafer. The interferometric image may then be captured  308  by a camera suitable for capturing an interferometric image of a semiconductor wafer. 
     By this method, any increase in the length of the measuring tool along an axis corresponding to a line normal to the surface of a semiconductor wafer may be reflected multiple times in distance travelled by the interferometric image. Furthermore, a measuring tool such as the one shown in  FIG. 2  may measure both sides of a semiconductor wafer simultaneously. In that case, optics dedicated to each side of the semiconductor wafer may be configured to utilize different areas of the measuring tool so as to achieve the necessary distance traveled by each interferometric image. Two separate interferometric image paths may utilize one dimension of the measuring tool multiple times such that any increase in the length of the measuring tool along an axis corresponding to that dimension may be reflected multiple times in distance travelled by each interferometric image. The interferometric image may thereby pass the same space, for example the space between the collimator and the large mirror, multiple times, reducing the size of the measuring tool overall and placing components of the measuring tool in proximity to increase stability. 
     These systems and methods may allow for measuring tools capable of measuring larger semiconductor wafers that prior art measuring tools cannot. They may also allow for the use of smaller optics and improved stability. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.