Abstract:
A surface comparison apparatus for measuring surface characteristics of a specimen through which visible ray can not be transmitted comprises: a light source generating light having a predetermined wavelength; an optical device changing the light into collimated light; an irradiating unit for irradiating the collimated rays on both surfaces of a specimen which needs to be measured after dividing them into two paths and making the lights, which are reflected on both surfaces, be focused on opposite direction of the incident light after passing through the paths and be interfered with each other; and a display means for displaying the interfered lights which are made by interfering the reflected lights, and thereby, a parallelism or surface characteristics for both surfaces of the specimen can be measured simultaneously through an interference pattern which is obtained by dividing the light into two paths, irradiating the light on both surfaces of the specimen, and reflecting to be interfered with each other, and an interferometer can be constructed and aligned in a simple way.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a plane surface measurement apparatus, and particularly, to an optical interferometric surface comparison apparatus for comparing and measuring relative surface characteristics of a thin window type non-transparent plate through which visible light cannot be transmitted.  
           [0003]    2. Description of the Background Art  
           [0004]    A thin plate type infrared window such as a Germanium window, which is used in a FLIR (forward looking infrared) equipment mounted on the top or nose or bottom of a helicopter or UAV (unmanned aerial vehicle), or an Infrared filter plate, which has, a function of selecting the special wavelength in equipment for measuring the modulation transfer function (MTF) of an infrared optical system, etc., is installed in the optical path through which the infrared rays are transmitted. Therefore, surface characteristics, that is, non-parallelism and surface shape errors of the two face planes of the window distort the wave front of the transmitted infrared rays and affect the performance of optical devices or measurement devices incorporating such windows.  
           [0005]    Additionally, measurement of the surface characteristics of both faces of planar opaque specimens such as disc platters and semiconductor wafers is often desirable when flatness, parallelism, etc are critical.  
           [0006]    On the other hand, the surface shape of a transparent plane window (i.e. one which is able to transmit visible light) can be conveniently measured by using an optical interferometer employing a visible wavelength laser. In this case, the wavefront representing relative surface characteristics of the plane window as well as the surface shape of each plane face can be measured and then, the influences of the plane window on the performance of the optical device can be grasped precisely.  
           [0007]    However, in the case of the infrared window or a metal window through which visible light can not be transmitted as well as other opaque planar objects such as disc platters and semiconductor wafers, the surface characteristics must be measured using another method instead of the conventional one employing the visible laser optical interferometer.  
           [0008]    [0008]FIG. 1 is a diagram showing a conventional optical interferometer surface measurement apparatus.  
           [0009]    As shown therein, the conventional optical interferometer apparatus for measuring the surface characteristics of a specimen such as an infrared window, a metal window, etc., includes a light source  1  generating a light beam of a predetermined wavelength; an optical device  2  changing the light beam generated from the light source I into a collimated light beam  5 ; a beam splitter (semi-reflective mirror)  3  splitting the light beam  5  into two beams, a reference light beam  5   a  and a test light beam  5   b  which respectively irradiate a specimen  4  of which the surface characteristics are to be measured; and a reference mirror  6 , whereby, the reflected reference light beam  5   a ′ reflected from the reference mirror  6  and the beam splitter  3  interferes with the reflected test light beam  5   b ′ reflected from-the surface of the specimen  4  and transmitted back through the beam splitter  3 ; and an imaging device  7  imaging the interference fringe pattern made by interference of the two reflected light beams  5   a ′,  5   b′.    
           [0010]    The imaging device  7  includes a semi-reflective mirror (beam splitter)  7   a  located between the light source  1  and the optical device  2 , and a camera device  7   b.    
           [0011]    The conventional surface measurement apparatus having the above-described structure measures the surface characteristics of the specimen  4  by an interferometric technique. However, from this only one of the surfaces of the specimen  4  can be measured, and therefore, overall analysis of the wavefront transmitted by both surfaces of the specimen  4  cannot be accomplished simultaneously and the optical performance of the window cannot be measured precisely.  
           [0012]    If an interferometer using an infrared laser, which penetrates, through the infrared window is used, the above problems can be solved. This is the same as using the visible laser interferometer for measuring the transparent window. However, in the case of the infrared laser interferometer, the light is invisible, and therefore, optical alignment of the measuring system is difficult and a different laser should be used according to the infrared media, which have respective transparent bands of wavelength.  
         SUMMARY OF THE INVENTION  
         [0013]    Therefore, an object of this present invention is to provide a surface comparison apparatus which is able to measure the parallelism and relative surface shape of both planar faces of a thin plate specimen simultaneously.  
           [0014]    To achieve the above object of the present invention, as embodied and broadly described herein, there is provided a surface comparison apparatus comprising: a light source for generating light having a predetermined wavelength; an optical device for changing, the light generated from the light source into a collimated light beam; a beam splitter for splitting the collimated light beam into two light beams, a first test light beam and a second test light beam; a plurality of mirrors for directing the first test light beam and the second test light beam to irradiate the respective surfaces of a specimen and for directing first and second reflected light beams from the specimen back to the beam splitter and optical device to form an interference fringe pattern; and a display device for displaying the interference fringe pattern made by interference of the two reflected light beams.  
           [0015]    Also, there is provided a surface comparison method comprising: a step of irradiating parallel collimated light beams onto both surfaces of a specimen after dividing it into two paths; a step of causing the reflected light beams reflected from the both surfaces of the specimen to interfere with each other; and a step of measuring surface shape characteristics of the surfaces relative to the each other.  
           [0016]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.  
         [0018]    In the drawings:  
         [0019]    [0019]FIG. 1 is a schematic diagram showing an optical interferometric conventional surface measurement apparatus;  
         [0020]    [0020]FIG. 2 is a schematic diagram showing an optical interferometric surface comparison apparatus according to the present invention;  
         [0021]    [0021]FIG. 3 is a schematic diagram showing a case that a pentagonal prism is used in the optical interferometric surface comparison apparatus according to the present invention;,  
         [0022]    [0022]FIG. 4 is a schematic diagram showing a case that the paths of parallel collimated light beams form a triangle shape in the optical interferometric surface comparison apparatus according to the present invention;  
         [0023]    [0023]FIG. 5 is a diagram showing an interference pattern when the optical interferometric surface comparison apparatus according to the present invention is aligned in a state that there is no specimen;  
         [0024]    [0024]FIG. 6A is a diagram showing an interference pattern for one surface of a specimen measured by the conventional optical interferometric surface measurement apparatus, and FIG. 6B is a diagram showing a surface profile of the one surface of the specimen obtained from the interference pattern in FIG. 6A;  
         [0025]    [0025]FIG. 7A is a diagram showing, an interference pattern for another surface of the specimen measured by the conventional optical interferometric surface measurement apparatus; and FIG. 7B is a view showing a surface profile of the other surface of the specimen obtained from the interference pattern in FIG. 7A; and  
         [0026]    [0026]FIG. 8A is a view showing an interference pattern of the light beams  15   a ′ and  15   b ′ reflected from both surfaces of a specimen measured by the optical interferometric surface comparison apparatus according to the present invention, and FIG. 8B is a view showing a surface profile of the specimen obtained from the interference pattern in FIG. 8A. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0028]    [0028]FIG. 2 is a diagram showing an optical interferometric surface comparison apparatus according to the present invention.  
         [0029]    As shown therein, the optical interferometric surface comparison apparatus according to the first embodiment of the present invention includes: a light source  11  generating light having a predetermined wavelength; an optical device  12  changing the light into a collimated light beam  15 ; an irradiating system dividing the collimated light beam  15  into two light beams  15   a ,  15   b  and conducting the respective beams along respective separate paths (P 1  and P 2 ) to be made incident normally upon both respective surfaces  16   a  and  16   b  of a specimen  16  to be examined, and for conducting the reflected light beams  15   a ′,  15   b ′, which are respectively reflected off the surfaces  16   a ,  16   b , in respectively opposite directions to the incident light beams  15   a ,  15   b  and combining the reflected beams  15   a ′,  15   b ′; and an imaging device  17  imaging and displaying the interference pattern to be observed.  
         [0030]    The present invention adopts a visible laser source and optical path of a Twyman-Green interferometer. Herein, the Twyman-Green interferometer (referring to http://cord.org/cm/leot/course10_Mod06/Module10-6.htm) is known as useful for measuring defects of the components of an optical device such as a lens, prism, and plane window. The irradiating system includes a beam splitter  13  such as a semireflective mirror which is capable of dividing the collimated light beam  15  into two beams,  15   a  and  15   b  which propagate along the separate respective paths (P 1  and P 2 ), and reflectors arranged on the respective paths (P 1  and P 2 ) to change i.e., bend the propagating direction of the light beams  15   a  and  15   b.    
         [0031]    As shown in FIG. 2, the paths (P 1  and P 2 ) construct a quadrangle such as a rectangle (in an exemplary embodiment) or parallelogram. The beam splitter  13  is located at a first apex of the quadrangle formed by the paths P 1  and P 2  as a dividing portion in which the beam splitter  13  divides the collimated light beam  15  into the two light beams  15   a  and  15   b  and the quadrangle has first, second, third, and fourth sides, and second, third, and fourth apexes in a counter-clockwise direction from the first apex, referring to FIG. 2.  
         [0032]    The first path P 1  is bent at the first apex and the second apex, and reaches one surface  16   a  of the specimen  16  located along the second side of the rectangle so at to be normally incident thereon. In addition, the second path P 2  passes through the first apex, is bent at the fourth and third apexes, and reaches the other surface  16   b  of the specimen  16  so at to be normally incident thereon. The specimen  16  may be located along any of the first, third, or fourth sides of the rectangle according to circumstances.  
         [0033]    The first path P 1  is formed by the beam splitter  13  located on the first apex and the reflector M 1  located on the second apex, and the second path P 2  is formed by the beam splitter  13  located on the first apex, by a pair of reflectors M 3  and M 4  arranged as an erecting corner reflector and located on the fourth apex and by a reflector M 2  located on the third apex.  
         [0034]    [0034]FIG. 3 is a schematic diagram showing another embodiment of the optical interferometric surface comparison apparatus according to the present invention wherein a pentagonal prism  18  serving as an erecting reflector is used in instead of the pair of mirrors M 3 , M 4 , in the embodiment shown in FIG. 2, and FIG. 4 is a schematic diagram showing yet another embodiment of the optical interferometric surface comparison apparatus according to the present invention wherein the paths P 1 , P 2  of the parallel light beams  15   a ,  15   b  form a triangle.  
         [0035]    In addition, in case of the surface comparison apparatus shown in FIG. 2, the paths P 1 , P 2  of the collimated light beams  1   5   a ,  1   5   b  form a square. However, as shown in FIG. 4, the paths (P 1  and P 2 ) of the light beams  15   a ,  15   b  in the surface comparison apparatus according to the present invention may form a triangle. In this case, the beam splitter  13  is located in a first apex of the triangle formed by the paths P 1  and P 2 , as a dividing portion in which the beam splitter  13  divides the light beam generated by the light source  11  the into the two light beams  15   a  and  15   b  and the quadrangle have first, second, and third sides and second, and third apexes in counterclockwise direction from the first apex referring to FIG. 3.  
         [0036]    The incident light beam  15   a  propagated along the first path P 1  is bent at the first apex, bent again at the reflector M 1  located on the second apex, and reaches and is normally incident upon one surface  16   a  of the specimen located along the second side. In addition, the second incident light beam  15   b  propagated along the path P 2  passes through the first apex, is bent at the reflector M 2  located on the third apex, and reaches and is normally incident upon the other surface  16   b  of the specimen  16 .  
         [0037]    Also, the first and second paths (P 1  and P 2 ) are formed to be precisely aligned normally on a point where the specimen is located after the parallel light beams  15   a ,  15   b  have passed along the respective paths (P 1  and P 2 ).  
         [0038]    The incident light beams  15   a ,  15   b  are reflected from the respect surfaces of the specimen, and the reflected light beams  15   a ′,  15   b ′ passes along the respective paths P 1 , P 2  and are combined at the beam splitter  13 . The combined light beam  15 ′ progresses toward the optical device  12 , and incident on a semi-reflective mirror  17   a  of the imaging device  17  described hereafter.  
         [0039]    The imaging device  17  is located between the light source  11  and the optical device  12 , and includes a semi-reflective mirror  17   a  for reflecting i.e. bending the interfered parallel light beams  15   a ′,  15   b ′ induced through the optical device  12 , and a camera device  17   b  for photographing the interference fringe pattern produced by the interfered light beams  15 ,  15 ′ reflected by the semi-reflective mirror  17   a.    
         [0040]    The surface comparison apparatus according to the present invention having the above construction, is able to measure the relative parallelism of the two surfaces of a specimen by comparing the wavefronts of visible rays as if they were transmitted through a nontransparent window. That is, if it is assumed that an index of refraction of the infrared window medium is n(λ′) and the index of refraction is distributed evenly, and if the measured relative shape error is W(r,ψ) for a window aperture, the wavefront function of light W′ after the light is transmitted through the window can be represented by the following equation 1.  
           W ′( r ,ψ)= n (λ′) W ( r ,ψ)/λ′  (1)  
         [0041]    Herein, λ represents the wavelength of the infrared rays.  
         [0042]    [0042]FIG. 5 is a diagram showing an interference pattern when the surface comparison apparatus according to the present invention is aligned.  
         [0043]    As shown in FIG. 2, the surface comparison apparatus is aligned such that the parallel light beam  15  of the interferometer is divided at the beam splitter  13 , and the reflected light beam  15   a ′ proceeding in the clockwise direction and the reflected light beam  15   b ′ proceeding in the counter clockwise direction meet at the beam splitter  13 , and are converged by the optical device  12 , and bent at the semi-reflective mirror  17   a.    
         [0044]    In addition, circular images of the light beams may not be agree with each other due to the structure of the optical system. Therefore, in optical axis alignment of the optical system, the reflectors are adjusted so that the circular images are coincided on a screen placed in the position of the specimen, a pin hole is put on the position of the screen, and fine controlling of the optical axis alignment is performed while viewing the image received by the camera on a monitor so that the two images of the pin hole formed by the light beams proceeding in the two directions are coincided. The alignment is identified while moving the position of the pin hole, and after that, when the alignment is completed, the pin hole is removed and the interference pattern is identified.  
         [0045]    The optical alignment is adjusted so that the number of the interference pattern fringes is less than 1 throughout the entire screen, and the interference pattern obtained after the optical alignment is shown in FIG. 5. After that, the specimen  16  to be measured is put in the screen position of the surface comparison apparatus, and the paths of the light beams  15   a ,  15   b  are controlled so that the beams irradiated onto the surfaces  16   a ,  16   b  of the specimen  16  can be incident normally onto the respective surfaces  16   a ,  16   b  of the specimen  16   a ,  16   b , i.e. perpendicularly.  
         [0046]    [0046]FIG. 6A shows an interference pattern for one surface of the specimen measured by the conventional surface measurement apparatus, and FIG. 6B shows the surface profile of the one surface of the specimen obtained from the interference pattern shown in FIG. 6A.  
         [0047]    [0047]FIG. 7A shows an interference pattern for the other surface of the specimen measured by the conventional optical interferometric surface measurement apparatus, and FIG. 7B shows the surface profile of the other surface of the specimen obtained from the interference pattern shown in FIG. 7A.  
         [0048]    [0048]FIG. 8A shows an interference pattern of the light beams  15   a ′,  15   b ′ respectively reflected from the both surfaces  16   a ,  16   b  of the specimen measured by the optical interferometric surface comparison apparatus according to the present invention, and FIG. 8B shows the surface profile of the specimen  16  obtained from the interference pattern shown in FIG. 8A.  
         [0049]    On the other hand, the result of measuring the surfaces of a specimen using the conventional surface measurement apparatus and the result of measuring the surfaces of a specimen using the surface comparison apparatus according to the present invention will be compared as follows.  
         [0050]    The object the relative surface shape of which is to be measured is an infrared filter used in equipment for measuring the modulation transfer function (MTF) of an infrared optical system. The interference pattern of one surface of the specimen measured by the conventional surface measurement apparatus or method is shown in FIG. 6A, and the surface profile of the one surface obtained from the interference pattern by using an interference pattern analyzing program is shown in FIG. 6B.  
         [0051]    The surface shape error is the result of dividing the wavefront error by 2. In Zernike coefficients of the measured wavefront error, the defocus is 0.83λ, the astigmatism is 5.43λ, the coma is 4.01λ, and the spherical aberration is 1.98λ. Herein, λ is the wavelength of the He—Ne laser, which is the light source, used in the surface comparison apparatus, and the wavelength is 0.631μm.  
         [0052]    The interference pattern of the other surface of the specimen measured by the conventional surface measurement apparatus or method, and the surface profile of the other surface obtained by using the interference pattern analyzing program are shown in FIG. 7A and FIG. 7B, respectively. The surface shape error is the result of dividing the wavefront error by 2 the same as above. In Zernike coefficients of the measured light wave surface, the defocus is 3.13λ, the astigmatism is 3.7λ, the coma is 9.27λ, and the spherical aberration is 13.13λ.  
         [0053]    On the other hand, the interference pattern measured by the surface comparison apparatus and method according to the present invention, and the wavefront error of the window obtained on the basis of the interference pattern are shown in FIG. 8A and FIG. 8B, respectively.  
         [0054]    As a result of analyzing the interference pattern, in Zernike coefficients of the measured wavefront error, the tilt is 1.98λ, the defocus is −0.68λ, the astigmatism is 0.69λ, the coma is 1.83λ, and the spherical aberration is −1.68λ.  
         [0055]    Herein, the tilt indicates the parallelism between both surfaces, and the other coefficients indicate relative shape errors between the respective surfaces of the specimen.  
         [0056]    As described above, in the case of the conventional surface measurement apparatus and method, if the surfaces of the specimen are measured alternately, the parallelism between the two surfaces cannot be measured and it is difficult to coincide the positions, which are to be measured, and thereby many errors are liable to be generated in the measuring process.  
         [0057]    However, according to the surface comparison apparatus and method of the present invention, the interference pattern obtained and the shape of the light wave front obtained from the interference pattern have relative values for the both surfaces, and therefore, the parallelism of the both surfaces of the specimen and the surface characteristics can be measured in a simple way.  
         [0058]    In the surface comparison apparatus and method according to the present invention, the collimated light beam is split into two beams, and the light beams are irradiated onto and reflected from the both respective surfaces of the specimen, and interfered with each other to obtain the interference pattern. And the parallelism and the relative surface characteristics for the both surfaces of the specimen can be measured simply from the interference pattern, and the interferometer can be constructed and aligned in a simple way.  
         [0059]    In particular, the optical interferometric surface comparison apparatus in accordance with the present invention compares one surface of a specimen referring to the other surface without using another reference surface, and easily measures relative surface profile errors between both surfaces which would distort the wavefront of the transmitted infrared rays and affect the performance of optical devices or measurement devices incorporating infrared windows.  
         [0060]    Meanwhile, the optical interferometric surface comparison apparatus in accordance with the present invention, can be easily adapted for comparing the surfaces of non-planar specimens such as lenses, etc.  
         [0061]    As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the appended claims.