Abstract:
A method for measuring a height of a bump formed on a work of substrate, the method comprising: irradiating X-rays having a predetermined wavelength and an incident intensity toward a first work of substrate which is the same as the above-mentioned work of substrate, but no bump is formed thereon, and detecting a first X-ray transmitted intensity at a position on which the bump is to be formed; irradiating X-rays having the same wavelength and incident intensity toward a material constituting the bumps and detecting a linear absorption coefficient of the X-rays; memorizing the first X-ray transmitted intensity and the linear absorption coefficient as known data; and irradiating X-rays having the same wavelength and the same incident intensity toward a second work of substrate which is the same as the above-mentioned work of substrate, but bump is formed thereon, and detecting a second X-ray transmitted intensity at a position on which the bump is formed; and determining the height of the bump from the second X-ray transmitted intensity on the basis of the known data.

Description:
DETECTING BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method and an apparatus for advantageously measuring heights of bumps, such as connecting terminals provided on a semiconductor package, such as BGA (ball grid array) package, or flip-chip connection bumps by which a semiconductor chip is mounted on a substrate.  
           [0003]    2. Description of the Related art  
           [0004]    A surface-mount type semiconductor device, such as a BGA substrate, is commonly provided on a surface of the substrate with semi-spherical bumps formed as external connecting terminals. These bumps can be formed by adhering solder balls to lands provided on the surface of the substrate. However, in the production of such a semiconductor device, when the dimension in the radial direction of the bumps is small and when the bumps are densely arranged, the bumps are generally formed as follows. That is to say, a solder paste is supplied onto the lands by a printing method to reflow the solder paste so as to form the bumps on the substrate. In this case, however, if the amount of solder paste to be supplied onto the lands fluctuates, the height or size of the bumps will not uniform.  
           [0005]    Therefore, in a semiconductor device in which bumps are formed on a surface of the substrate as external connecting terminals, the height of the bumps must be measured by a suitable measuring device after the bumps are formed on the substrate. In a measuring device known in the prior art, an optical method is generally used to detect the height of the bumps.  
           [0006]    [0006]FIG. 6 schematically shows a method for optically detecting the height of the bumps  12  formed on a substrate  10 . The height of the bumps  12  is measured by a suitable measuring device after the bumps are formed on the substrate. One of the methods for optically detecting the height of the bumps is a method in which a surface of protective film  14 , such as solder resist film, covering the surface of the substrate  10  is defined as a reference surface and a distance from this reference surface to the top of the bumps  12  is then optically detected. Otherwise, also known is a method in which the shape of the top of bumps is flattened by a coining process and then the flattened top portion of the bump is optically detected to measure the height of bumps.  
           [0007]    However, in the prior art method for optically detecting the height of the bumps, the optical reflection rate is significantly different case by case and greatly depends on a gloss of the bumps and, therefore, the accuracy of the measured results may fluctuate due to a fluctuation of the state of the bumps. In the case that the height of bumps are measured on the basis of a surface of the protective film  14 , as the reference surface, the measuring accuracy is lowered since the height of bumps from the referenced surface fluctuates because the thickness of the solder resist as the protective film fluctuates. In addition, if there is any warp on the surface, the optical focus is deviated and therefore an accurate measurements cannot be expected.  
           [0008]    In addition, since the number of terminals has been gradually increased in recent semiconductor devices having fine connecting terminals, a high accuracy in the measuring results of the height of bumps has been more and more required. As mentioned above, the deviations of the height of bumps are caused by the fact that the amount of solder paste which should be supplied to the individual lands fluctuates or the amount of flux contained in the solder paste fluctuates. In the case of bumps having a height of several hundred μm, a deviation in the amount of solder paste is not a significant problem. However, if the bumps have a lower height, such as the heights of bumps being several tens of μm, the height and size of the bumps are greatly affected by even small deviations in the amount of solder paste.  
           [0009]    Also, in the case of the size of bumps being small, if a surface which might easily deviate, such as the surface of the protective film of the solder resist or the like, is used as the reference surface, the accuracy will be reduced and, also, even a small warp in the substrate causes reduced accuracy.  
         SUMMARY OF THE INVENTION  
         [0010]    An object of the present invention is to provide a method and an apparatus for accurately measuring the height of bumps formed on a semiconductor package or substrate.  
           [0011]    Another object of the present invention is to provide a method and an apparatus for measuring the height of bumps in which the above-mentioned drawbacks in the prior art can be overcome.  
           [0012]    According to the present invention, there is provided a method for measuring a height of a bump formed on a work of substrate, said method comprising: irradiating X-rays having a predetermined wavelength and incident intensity toward a first work of substrate which is the same as the above-mentioned work of substrate, but no bump is formed thereon, and detecting a first X-ray transmitted intensity at a position on which the bump is to be formed; irradiating X-rays having the same wavelength and incident intensity toward a material constituting the bumps and detecting a linear absorption coefficient of the X-rays; memorizing the first X-ray transmitted intensity and the linear absorption coefficient as known data; and irradiating X-rays having the same wavelength and the same incident intensity toward a second work of substrate which is the same as the above-mentioned work of substrate, but a bump is formed thereon, and detecting a second X-ray transmitted intensity at a position on which the bump is formed; and determining the height of the bump from the second X-ray transmitted intensity on the basis of the known data.  
           [0013]    According to another aspect of the present invention, there is provided a method for measuring heights of first and second bumps formed on first and second surfaces, respectively, of a work of substrate in which materials of the first and second bumps are different from each other, a planar arrangement of the first bumps is the same as that of the second bumps, said method comprising: irradiating at least two kinds of X-rays having different wavelengths toward the materials constituting the first and second bumps, respectively, and detecting first and second linear absorption coefficients of the X-ray; memorizing the first and second linear absorption coefficients of the X-rays as known data; and irradiating the two kinds of X-ray toward the first and second works respectively, and detecting the first and second X-ray transmitted intensities at a position on which the first and second bumps are formed; and determining the heights of the first and second bumps from the first and second X-ray transmitted intensities on the basis of the known data.  
           [0014]    According to still another aspect of the present invention, there is provided an apparatus for detecting a height of bumps, said apparatus comprising: a stage on which a work of substrate having a plurality of bumps formed thereon is disposed; a X-ray generating unit for generating X-rays incident to the work; and a detecting unit for detecting an intensity of X-rays transmitted through the work at a position where the bump is located.  
           [0015]    The detecting unit may comprise a CCD camera for detecting the intensity of X-rays transmitted through the work, and an analyzing unit for analyzing an image data output from the CCD camera to detect the intensity of X-rays transmitted through the bump.  
           [0016]    Otherwise, the detecting unit may comprise an image intensifier for detecting the intensity of X-rays transmitted through the work, and an analyzing unit for analyzing an image data output from the image intensifier to detect the intensity of X-rays transmitted through the bump.  
           [0017]    Alternatively, the detecting unit may comprise a photomultiplier tube for detecting the intensity of X-ray transmitted through the work, and an analyzing unit for analyzing an image data output from the photomultiplier tube to detect the intensity of X-rays transmitted through the bump. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    FIGS.  1 ( a ) and  1 ( b ) are schematic illustrations of a method for measuring the height of the bumps using X-rays according to the present invention;  
         [0019]    [0019]FIG. 2 is a schematic illustration of a method for measuring the height of the bumps provided on the respective surfaces of a substrate according to the present invention;  
         [0020]    [0020]FIG. 3 is a schematic illustration of an embodiment of a bump height measuring apparatus according to the present invention;  
         [0021]    [0021]FIG. 4 is a schematic illustration of another embodiment of a bump measuring apparatus according to the present invention;  
         [0022]    [0022]FIG. 5 is a schematic illustration of still another embodiment of a bump height measuring apparatus according to the present invention; and  
         [0023]    [0023]FIG. 6 is a schematic illustration of a method for measuring the height of the bumps known in the prior art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Referring now to the drawings, wherein FIGS.  1 ( a ) and  1 ( b ) show a method for measuring the height of the bumps provided on a semiconductor substrate using X-rays according to the present invention. In the bump detecting method according to this invention, X-rays are irradiated onto the work and the intensity of X-rays which transmit through the work is measured to determine the heights of the bumps.  
         [0025]    In the measuring method of FIGS.  1 ( a ) and  1 ( b ), the measurement results for one sample, wherein a bump  12  is not formed on the land  16  of the substrate  10 , and the measurement results for another sample, wherein a bump  12  is formed on the land  16  of the substrate  10 , are compared to each other to determine the height of bump  12 .  
         [0026]    As shown in FIG. 1( a ), in the case of no bump being formed, if the intensity of incident X-ray is I 0  and the intensity of transmitted X-ray is I 1 , the following relationship between I 0  and I 1  is given. In the following representation, μ 1  is Planck&#39;s linear absorption coefficient. 
           I   1   =I   0 exp(−μ 1   x )  (1) 
         [0027]    On the other hand, as shown in FIG. 1( b ), in the case of bump  12  being formed on the land  16 , if the intensity of incident X-rays is I 0  and the intensity of transmitted X-rays is I, the following relationship between I 0  and I is given. In the following representation, μ B  is a linear absorption coefficient of the material constituting the bumps and h is the height of bump. 
           I={I   0 exp(−μ 1   x )}·exp(−μ B   h )= I   1 ·exp(−μ B   h )  (2) 
         [0028]    The relationship between I and I 1  can be deformed as follows. 
           h=−{l   n ( I/I   1 )}/μ B   (3) 
         [0029]    According to this representation, if the intensity I 1 , of transmitted X-rays in the state that no bumps  12  are formed on the lands  16  is measured beforehand and, on the other hand, if the linear absorption coefficient μ B  of the material constituting the bumps  12  is measured beforehand, the height (h) of bumps can be detected by only measuring the intensity of transmitted X-rays in the state that the bumps  12  are formed on the lands  16 .  
         [0030]    The thickness of the lands  16  formed on the substrate  10  is relatively thin, about  18  μm, the deviation of the thickness is relatively small, and in general the linear absorption coefficient is relatively small. Therefore, the lands  16  do not result in a great influence to the measuring results. Therefore, it is sufficient to measure the intensity (I 1 ) of transmitted X-rays with respect to the samples of substrate having no bumps to use the measured results as a reference value.  
         [0031]    The intensity I 1  of transmitted X-rays with respect to the samples of substrate having no bumps  12  formed on the lands  16  reflects the intensity of transmitted X-rays through the land  16  as well as the intensity of transmitted X-rays through the base body of the substrate  10 . Therefore, the measuring method of this embodiment can be used for detecting the height of bumps  12 , either for a substrate having no inner conductive layer other than the lands  16 , as well as a substrate having such an inner conductive layer. This is because the height of bumps  12  can be determined in such a manner that the intensity of transmitted X-ray with respect to the substrate having no bumps  12  is defined as a reference value and then the difference is determined.  
         [0032]    The wavelength of the X-rays which transmit through the work can be optionally selected. Therefore, depending on the bumps to be measured, the absorption spectrum of X-ray is measured beforehand and the wavelength of X-ray which is the most favorable for measuring can be determined. For example, if the bumps  12  are formed of solder, the absorption spectrum of X-ray with respect to solder is measured beforehand. Thus, it is possible to measure using X-rays having such a wavelength of large linear absorption coefficient. Since the absorption spectrum may be different depending on the products even if the substrate itself is the same kind, it is effective to measure the absorption spectrum of X-rays to use the optimum wavelength.  
         [0033]    [0033]FIG. 2 shows an example wherein the bumps  12   a  and  12   b  are provided on the respective surfaces of a substrate  10 . As a product of the semiconductor device, there are many kind of such a substrate having the respective surfaces of a substrate  10  on which the bumps  12   a  and  12   b  are formed. If the materials of the bumps  12   a  and  12   b  formed on the respective surfaces of the substrate are different from each other, such as, if the bumps  12   a  on one of the surfaces are formed of tin-lead and the bumps  12   b  on the other surface are formed of tin-silver, the respective heights h a  and h b  of the bumps  12   a  and  12   b  can be detected.  
         [0034]    In order to detect the respective heights h a  and h b  of the bumps  12   a  and  12   b  formed on the respective surfaces of the substrate, two or more types of X-rays having different wavelength can be used to measure the intensity of the transmitted X-ray.  
         [0035]    That is to say, if the intensities of X-rays, the wavelength thereof being λ 1  and λ 2 , are I 1  and I 2  respectively; the linear absorption coefficients of the material constituting the bumps  12   a  and  12   b  respectively for X-rays, the wavelength thereof being λ 1  and λ 2 , are μ a1 , μ a2 , μ b1  and μ b2 , I 1  and I 2  are given by the following representations. 
           I   1   =I   0 exp(−μ a1 h a )·exp(−μ b1 h b )  (4) 
           I   2   =I   0 exp(−μ a2 h a )·exp(−μ b2 h b )  (5) 
         [0036]    In this representations, μ a1 , μ a2 , μ b1 , and μ b2  are given beforehand by measurements and I 1  and I 2  can be given as the results of measurements. Therefore, the heights h a  and h b  of the bumps  12   a  and  12   b  can thus be obtained from the above-mentioned representations (4) and (5).  
         [0037]    In order to measure the height of the bumps by detecting the intensity of the transmitted X-rays, it is necessary to measure the respective intensity of the transmitted X-rays for the respective bumps. In this connection, FIGS.  3  to  5  show some examples of measuring apparatus for measuring the intensity of the transmitted X-ray. The embodiment shown in FIG. 3 uses an X-ray CCD camera, the embodiment shown in FIG. 4 uses X-ray image intensifier, and the embodiment shown in FIG. 5 is a measuring apparatus using a photomultiplier tube.  
         [0038]    The bump detecting apparatus shown in FIG. 3 comprises a X-ray generating device for generating X-rays which should be incident to the work  30  and a CCD camera  22  for detecting X-ray transmitted through the work  30 . The image data detected by the X-ray CCD camera  22  is converted to a digital value by an AD converter  24  and detected by a computer  26  as an intensity of transmitted X-ray. The apparatus further includes a stage  28  on which works are set and a stage guide  29  for moving the stage  28  in the direction parallel to the optical axis of the CCD camera to regulate the focus position of the work  30 .  
         [0039]    AS mentioned above, as far as both the linear absorption coefficient (μ b ) of the material constituting the bumps of the work  30  and the intensity (I 1 ) of transmitted X-rays of the substrate having no bumps are available in advance, the height of the bumps can be determined by detecting the intensity of transmitted X-rays transmitted through the work  30 . In the computer  26 , the respective intensities (I) of X-ray transmitted through the respective bumps of the work  30  and the linear absorption coefficient (μ b ) and the intensities (I 1 ) of transmitted X-rays are analyzed to determined the height (h B ) of bumps. From these detected results, whether the bumps have a predetermined dimension of height or not can be determined to discriminate allowable and non-allowable products. In this embodiment, the CCD camera  22  and the AD converter  24  constitute detecting means and the computer  26  constitutes an analyzing means.  
         [0040]    In the embodiment as shown in FIG. 3, a standard sample  32  can be provided for correcting the intensity of X-rays. The standard sample  32  is disposed on the stage  28  and the X-ray transmitted intensity with respect to the standard sample  32  is detected by the X-ray CCD camera  22 , thereby the intensity of the incident X-rays can be regulated. Otherwise, the results of measurements can be revised in accordance with the intensity of the incident X-rays.  
         [0041]    In the embodiment of FIG. 4, X-rays are incident toward the work  30  from the X-ray generating device  20  and the X-rays transmitted through the work  30  are incident on the X-ray image intensifier  34 . The X-ray image intensifier  34  converts the weak X-ray image into a visible image and a digital CCD camera  36  is arranged at the optical output side. Thus, the X-ray transmitted image of the work  30  can be output to the computer  26  as a digital image. In the computer, the X-ray permeability of the respective bumps are analyzed from the digital image of the works  30  and the heights of the respective bumps can thus be detected on the basis of these data.  
         [0042]    Since the X-ray image intensifier  34  has a high image quality, even though a large number of fine bumps are highly densely arranged on the work  30 , the bumps can be accurately detected. Also, as the X-ray image intensifier  34  can detect a relatively wide image range, if the work itself is small in size, as a semiconductor device, it is possible to measure at all the bumps once. In this embodiment, a standard sample can be used for correcting the intensity of the incident X-rays. In this embodiment, the X-ray image intensifier  34 , the digital CCD camera  36  and the computer  26  constitute a detecting device.  
         [0043]    [0043]FIG. 5 shows a further embodiment of a measuring apparatus for detecting the transmitted X-ray intensity, in which X-rays are incident toward the work  30  from the X-ray generating device  20 , the X-rays transmitted through the work  30  are detected by a photomultiplier tube  38  and thus the intensity of X-rays transmitted through the work can be measured. The photomultiplier tube  38  serves to amplify the photoelectron energized by the X-rays and the data counted by a photon counter  40  is input into the computer  26 . A high voltage power  39  supplies power to actuate the photomultiplier tube  38 .  
         [0044]    In the measuring method using the photomultiplier tube  38 , the count number of the respective bumps of the work are individually measured to detect the intensity of transmitted X-rays. Therefore, the work  30  must be moved for each bump with respect to the photomultiplier tube  38 . In this embodiment, a stage  28  is supported on a movable stage  42  and the position of the movable stage  42  is controlled by the computer  26 .  
         [0045]    The output (count number) from the photomultiplier tube  38  is memorized by the computer for each bump of the work  30  and thus the intensity of transmitted X-rays can be measured for all of the bumps of the work  30 . The process for detecting the height of the individual bumps from the intensity of transmitted X-ray is the same as that of the previous embodiments. Although the detecting method using the photomultiplier tube  38  is complicated as the detecting process must be performed for each bump, a more precise measurement can be expected. In this embodiment, the photomultiplier tube  38 , the photon counter  40  and the computer  26  constitute a detecting device.  
         [0046]    As mentioned above, according to the present invention, since the height of the bumps is measured using X-rays transmitted through the work, the following advantages can be obtained. That is to say, a precise measurement can be effected regardless of the nature of the object, such as the gloss of the bumps. Also, as the height of the bumps from the land can be measured by using transmitted X-rays, it is no longer necessary to measure the height of bumps using a protective film, such as a solder resist, as in the prior art. Therefore, there are no fluctuations in the measurement results due to deviations in the thickness of protective film. This is particularly advantageous when a semiconductor element is mounted on a substrate by flip-chip connection using bumps densely arranged on the substrate.  
         [0047]    In addition, according to the measuring method of this invention, any warps of the substrate do not affect the measured heights of bumps.  
         [0048]    It should be understood by those skilled in the art that the foregoing description relates to only some of the preferred embodiments of the disclosed invention, and that various changes and modifications may be made to the invention without departing the sprit and scope thereof.