Abstract:
A system and a method for thickness measurement that comprises providing a first confocal microscope, emitting a first light beam from the first confocal microscope in a first direction, focusing the first beam at a first focal plane, providing a second confocal microscope, emitting a second light beam from the second confocal microscope in a second direction substantially opposed to the first direction, focusing the second beam at a second focal plane, and adjusting the relative position of the first and second microscopes by overlapping the first and second focal planes.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to optical measurement and, more particularly, to a system and method for measuring the cell gap of a liquid crystal cell. 
   With the advent in semiconductor processes, electronic products are increasingly required to be lightweight, compact and low profile. Consequently, the fabrication of liquid crystal display (“LCD”) panels, which have been widely used in electronic products, has become more complex. An LCD panel usually comprises an upper glass substrate, a lower glass substrate and intermediate layers sandwiched between the glass layers. The intermediate layers may include a color filter layer, indium tin oxide (“ITO”) layers, alignment films and a liquid crystal cell filled with a liquid crystal. The thickness or the cell gap of the liquid crystal cell is an important factor to control because the properties such as display color, response speed and orientation stability of a liquid crystal cell depend upon the cell gap. Accordingly, in order to use a liquid crystal cell, it is important to measure the cell gap. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to a system and method for measuring the thickness of an object that obviate one or more problems resulting from the limitations and disadvantages of the prior art. 
   In accordance with an embodiment of the present invention, there is provided an optical measuring system that comprises a first confocal microscope for providing a first light beam in a first direction converging at a first focal plane, and a second confocal microscope for providing a second light beam in a second direction substantially opposed to the first direction converging at a second focal plane. 
   Also in accordance with the present invention, there is provided an optical measuring system that comprises a first confocal microscope including a first objective lens for providing a first light beam in a first direction converging at a first focal plane of the first objective lens, a second confocal microscope including a second objective lens for providing a second light beam in a second direction substantially opposed to the first direction converging at a second focal plane of the second objective lens, and a device for adjusting the position of one of the first focal plane and the second focal plane along an axis defined by the first and second directions. 
   Further in accordance with the present invention, there is provided a method for thickness measurement that comprises providing a first confocal microscope, emitting a first light beam from the first confocal microscope in a first direction, focusing the first beam at a first focal plane, providing a second confocal microscope, emitting a second light beam from the second confocal microscope in a second direction substantially opposed to the first direction, focusing the second beam at a second focal plane, and adjusting the relative position of the first and second microscopes by overlapping the first and second focal planes. 
   Still in accordance with the present invention, there is provided a method for thickness measurement that comprises providing a first confocal microscope including a first objective lens, emitting a first light beam from the first confocal microscope in a first direction, focusing the first beam at a first focal plane of the first objective lens, providing a second confocal microscope including a second objective lens, emitting a second light beam from the second confocal microscope in a second direction substantially opposed to the first direction, focusing the second beam at a second focal plane of the second objective lens, and providing an object including at least one layer. 
   Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
   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, as claimed. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
     In the drawings: 
       FIG. 1A  is a schematic diagram of a confocal microscope suitable for use in the present invention; 
       FIG. 1B  is a schematic diagram illustrating an out-of-focus situation with the confocal microscope shown in  FIG. 1A ; 
       FIG. 1C  is a plot diagram illustrating the relationship between the displacement and relative intensity of a light beam received at a detector of the confocal microscope shown in  FIG. 1A ; 
       FIG. 2A  is a schematic diagram of an optical measuring system in accordance with one embodiment of the present invention; 
       FIG. 2B  is a schematic diagram illustrating a method for operating the optical measuring system in accordance with one embodiment of the present invention; 
       FIG. 3  is a flow diagram illustrating a method for thickness measurement in accordance with one embodiment of the present invention; 
       FIG. 4  is a schematic diagram of an interferometer suitable for use in the present invention; 
       FIG. 5  is a schematic diagram of an optical measuring system in accordance with another embodiment of the present invention; 
       FIG. 6A  is a schematic diagram of an object including multiple transparent layers; and 
       FIG. 6B  is a flow diagram illustrating a method for measuring the thickness of the object shown in  FIG. 6A  in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIG. 1A  is a schematic diagram of a confocal microscope  10  suitable for use in the present invention. Referring to  FIG. 1A , the confocal microscope  10  includes a light source  11 , a first lens  12 - 1 , a second lens  12 - 2 , a beam splitter  13 , a first objective lens  14 - 1 , a second objective lens  14 - 2  and a detector  15  with a pinhole  15 - 1 . The light source  11 , first lens  12 - 1  and second lens  12 - 2  provide a laser beam toward an object  16 . The laser beam, passing through the first lens  12 - 1 , second lens  12 - 2 , beam splitter  13 , is focused by the first objective lens  14 - 1  on a focal plane  17 - 1  associated with the first objective lens  14 - 1 , and then reflected therefrom. The object  16 , which is disposed at the focal plane  17 - 1 , is in-focus with the first objective lens  14 - 1 . The reflected light beam, which coincides with the incident laser beam, is recollected by the first objective lens  14 - 1  and reflected by the beam splitter  13  toward the detector  15  through the second objective lens  14 - 2 . Since the pinhole  15 - 1  is located at a focal plane associated with the second objective lens  14 - 2 , the reflected light beam from the focal plane  17 - 1  is focused at the pinhole  15 - 1  and entirely pass to the detector  15 . 
     FIG. 1B  is a schematic diagram illustrating an out-of-focus situation with the confocal microscope  10  shown in  FIG. 1A . Referring to  FIG. 1B , an incident light beam  17  is focused by the first objective lens  14 - 1  on the focal plane  17 - 1 . However, since the object  16  is disposed away from the focal plane  17 - 1 , only a portion of the reflected light beam (illustrated in dotted lines) is received by the detector  15 . Specifically, a light beam from below the focal plane  17 - 1  comes to a focus before reaching the pinhole  15 - 1 , and then expands out so that most of the light beam is physically blocked from reaching the detector  15  by the pinhole  15 - 1 . In the same way, a light beam from above the focal plane  17 - 1  is focused behind the pinhole  15 - 1 , so that most of light beam also hits the edges of the pinhole  15 - 1  and is not detected. 
     FIG. 1C  is a plot diagram illustrating the relationship between the displacement and relative intensity of a light beam received at the detector  15  of the confocal microscope  10  shown in  FIG. 1A . Referring to  FIG. 1C , for an in-focus situation where an object is located at a focal plane, the relative intensity ratio, i.e., the intensity of a received light beam to that of an incident light beam, is approximately 1, which may be used as a measurement threshold for the confocal microscope  10 . For an out-of-focus situation where an object is located away from a focal plane, the relative intensity ratio decreases as the displacement increases. 
     FIG. 2A  is a schematic diagram of an optical measuring system  20  in accordance with one embodiment of the present invention. The optical measuring system  20  includes a first confocal microscope  30  and a second confocal microscope  40 . The first confocal microscope  30 , having a similar structure to the confocal microscope  10  illustrated in  FIG. 1A , includes a light source  31 , a first lens  32 - 1 , a second lens  32 - 2 , a beam splitter  33 , a first objective lens  34 - 1 , a second objective lens  34 - 2  and a detector  35  with a pinhole  35 - 1 . The light source  31 , first lens  32 - 1  and second lens  32 - 2  provide a first light beam in a first direction  37 . The first light beam passes through the beam splitter  33  and is focused by the first objective lens  34 - 1  at a focal plane of the first objective lens  34 - 1 . A reflected light beam is collected by the first objective lens  34 - 1  and reflected by the beam splitter  33  toward the detector  35  through the second objective lens  34 - 2  and pinhole  35 - 1 . 
   In the same manner, the second confocal microscope  40 , having a similar structure to the confocal microscope  10  illustrated in  FIG. 1A , includes a light source  41 , a first lens  42 - 1 , a second lens  42 - 2 , a beam splitter  43 , a first objective lens  44 - 1 , a second objective lens  44 - 2  and a detector  45  with a pinhole  45 - 1 . The light source  41 , first lens  42 - 1  and second lens  42 - 2  provide a second light beam in a second direction  47  substantially opposed to the first direction  37 . The second light beam passes through the beam splitter  43  and is focused by the first objective lens  44 - 1  at a focal plane of the second objective lens  44 - 1 . A reflected light beam is collected by the first objective lens  44 - 1  and reflected by the beam splitter  43  toward the detector  45  through the second objective lens  44 - 2  and pinhole  45 - 1 . The second confocal microscope  40  further includes a sliding device  46  for moving the first objective lens  44 - 1  with respect to the light source  41  along an axis defined by the first direction  37  and second direction  47 . In one embodiment according to the present invention, the first objective lens  44 - 1  is loaded on the sliding device  46  to move along the axis. Skilled persons in the art will understand that other device capable of adjusting the focus point of the first objective lens  44 - 1  along the axis may be used to replace the sliding device  46 . Furthermore, the first confocal microscope  30  may include a device similar to the sliding device  46  for moving the first objective lens  34 - 1  along the axis. 
   An object  50 , for example, a liquid crystal cell, including a liquid crystal layer  53  sandwiched by transparent layers  51  and  52  such as glass substrates, is disposed between the first confocal microscope  30  and second confocal microscope  40 . To measure the thickness of the object  50 , or the cell gap of the liquid crystal cell, the relative position of the first confocal microscope  30  and the second confocal microscope  40  is reset.  FIG. 2B  is a schematic diagram illustrating a method for operating the optical measuring system  20  in accordance with one embodiment of the present invention. Referring to  FIG. 2B , before positioning the object  50  between the first confocal microscope  30  and second confocal microscope  40 , the second confocal microscope  40  is moved along the axis with respect to the first confocal microscope  30  till the focal plane of the first objective lens  34 - 1  overlaps the focal plane of the first objective lens  44 - 1  at a first position plane  48 . The position of the sliding device  46  is then recorded. 
   Next, referring again to  FIG. 2A , the object  50  is positioned between the first confocal microscope  30  and second confocal microscope  40 . The object  50  is moved along the axis till a first interface  531  between the liquid crystal layer  53  and one transparent layer  51  overlaps the focal plane of the first objective lens  34 - 1 . Next, the sliding device  46 , on which the first objective lens  44 - 1  is loaded, is moved along the axis toward the light source  41  till a second interface  532  between the liquid crystal layer  53  and the other transparent layer  52  overlaps the focal plane of the first objective lens  44 - 1 . The new position of the sliding device  46  is then recorded. The thickness of the object  50  is determined by the recorded positions of the sliding device  46 . 
     FIG. 3  is a flow diagram illustrating a method for thickness measurement in accordance with one embodiment of the present invention. Referring to  FIG. 3 , at step  51 , a first confocal microscope is provided for providing a first light beam in a first direction along an axis converging at a first focal plane. At step  52 , a second confocal microscope is provided for providing a second light beam along the axis converging at a second focal plane. The second light beam travels in a second direction substantially opposed to the first direction. Next, at step  53 , the relative position of the first confocal microscope and the second confocal microscope is reset by moving the second confocal microscope along the axis till the first focal plane overlaps the second focal plane. When overlapped, a maximum relative intensity ratio is detected. The position of an objective lens of the second confocal microscope associated with the second focal plane is then recorded. At step  54 , an object including a layer further including a first side and a second side is positioned between the first confocal microscope and the second confocal microscope. Next, at step  55 , the object is moved along the axis till the first side overlaps the first focal plane. At step  56 , the position of the second focal plane is adjusted by moving the objective lens associated with the second focal plane till the second side overlaps the second focal plane. The new position of the objective lens of the second confocal microscope is then recorded. The layer thickness of the object is determined from the recorded positions of the objective lens associated with the second focal plane. 
     FIG. 4  is a schematic diagram of an interferometer  60  suitable for use in the present invention. An interferometer works on the principle that two optical waves that coincide with the same phase will amplify each other while two optical waves that have opposite phases will cancel each other out. Referring to  FIG. 4 , the interferometer  60 , for example, a Michelson interferometer, includes a detector  61 , reflecting mirrors  62 - 1  and  62 - 2 , and a beam splitter  63 , which is usually a semitransparent mirror. There are two optical paths from a light source toward the detector  61 . One reflects off the beam splitter  63 , travels to one reflecting mirror  62 - 2  and then reflects back, goes through the beam splitter  63  to the detector  61 . The other one travels through the beam splitter  63  to the other reflecting mirror  62 - 1 , reflects back to the beam splitter  63 , then reflects therefrom into the detector  61 . If these two optical paths differ by a whole number (including 0) of wavelengths, there is constructive interference and a strong signal at the detector  61 . If they differ by a whole number and a half wavelengths, there is destructive interference and a weak signal at the detector  61 . 
     FIG. 5  is a schematic diagram of an optical measuring system  70  in accordance with another embodiment of the present invention. Referring to  FIG. 5 , the optical measuring system  70  has a similar structure to the optical measuring device  20  illustrated in  FIG. 2A  except that a first confocal microscope  80  includes an interferometer  89  and a second confocal microscope  90  includes an interferometer  99 . The interferometer  89 , which has a similar structure to the interferometer  60  illustrated in  FIG. 4 , is disposed between the first objective lens  34 - 1  and the focal plane of the first objective lens  34 - 1 . In the same manner, the interferometer  99 , which has a similar structure to the interferometer  60  illustrated in  FIG. 4 , is disposed between the first objective lens  44 - 1  and the focal plane of the first objective lens  44 - 1 . With the interferometers  89  and  99 , the sensitivity and resolution of the first confocal microscope  80  and second confocal microscope  90  may be increased. In one embodiment according to the present invention, the measurement threshold of the optical measuring system  70 , also referring to  FIG. 1C , is approximately 1, and the maximum slope of the plot A is greater than or equal to that of an envelop B. The maximum slope of the envelop B may occur at a section corresponding to the ratio ranging between 0.4 to 0.6. 
     FIG. 6A  is a schematic diagram of an object  100  including multiple transparent layers. Referring to  FIG. 6A , the object  100  includes a liquid crystal layer  101 , a pair of alignment layers  102 - 1  and  102 - 2  sandwiching the liquid crystal layer  101 , a pair of indium tin oxide (“ITO”) layers  103 - 1  and  103 - 2  sandwiching the alignment layers  102 - 1  and  102 - 2 , and a pair of glass substrates  104 - 1  and  104 - 2  sandwiching the ITO layers  103 - 1  and  103 - 2 . 
     FIG. 6B  is a flow diagram illustrating a method for measuring the thickness of the object  100  shown in  FIG. 6A  in accordance with one embodiment of the present invention. Referring to  FIG. 6B , at step  111 , a first confocal microscope is provided for providing a first light beam in a first direction along an axis converging at a first focal plane. At step  112 , a second confocal microscope is provided for providing a second light beam along the axis converging at a second focal plane. The second light beam travels in a second direction substantially opposed to the first direction. Next, at step  113 , the relative position of the first confocal microscope and the second confocal microscope is reset by moving the second confocal microscope along the axis till the first focal plane overlaps the second focal plane. The position of an objective lens of the second confocal microscope associated with the second focal plane is then recorded. At step  114 , an object including multiple layers transparent to the first light beam and second light beam is positioned between the first confocal microscope and the second confocal microscope. Next, at step  115 , the object is moved along the axis till a first interface of the multiple layers overlaps the first focal plane. The first interface includes a first side of the object. At step  116 , the position of the second focal plane is adjusted by moving the objective lens associated with the second focal plane till a second interface corresponding to the first interface overlaps the second focal plane. The second interface includes a second side of the object. The new position of the objective lens of the second confocal microscope is then recorded. The thickness of a layer, for example, the liquid crystal layer, of the object is determined from the recorded positions of the objective lens associated with the second focal plane at step  117 . Next, a step  118 , it is determined whether to continue the measurement. If confirmative, the steps  113  to  117  are repeated for measuring the thickness of one of the remaining layers. 
   It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 
   Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.