Abstract:
A method of measuring solder bumps formed on a substrate mounting a semiconductor element thereon includes mounting a work to be measured on a work position mechanism, and scanning the work by an optical micro head to measure errors of a mount posture of the work. Each stage is controlled to correct the errors, and thereafter, the apex positions of the bumps are scanned and measured. The measurement results are collected by a personal computer, and the measurement results together with control data of each axis are sent to a main personal computer and displayed on its screen. An error of an apex position of each bump from a regression plane is calculated, and if the error is smaller than a reference value, the work is judged to be satisfactory.

Description:
This application is a division of Ser. No. 08/759,949, filing date Dec. 3, 1996 now U.S. Pat. No. 5,906,309. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for measuring solder bumps formed on a semiconductor module such as LSI, on TAB (Tape Automated Bonding), and the like. 
     2. Description of the Related Art 
     FIGS. 1A to  1 C illustrate the structure of a semiconductor module  1  having a number of solder bumps. FIG. 1A is a diagram showing the overall structure of the semiconductor module. FIG. 1B is an enlarged view of a bump area A of the semiconductor module and illustrates a relative position of solder bumps. FIG. 1C is an enlarged view of a bump  10  (corresponding to β in FIG.  1 A). 
     A number of solder bumps  10  are formed on the bottom of the semiconductor module  1 . Each bump is used for connection to a wiring board on which the semiconductor module  1  is mounted. The semiconductor module  1  is of a square shape having a side length of, for example, 10 mm. Solder bumps  10  are formed on the surface  8  at a pitch of, for example, 450 μm, totalling in number to 23×23. 
     Each bump  10  is generally spherical as shown in the enlarged view of FIG.  1 C. 
     There is no apparatus for automatically measuring a height of a number of spherical bumps to date. Therefore, height is measured visually by using a focus-of-depth microscope or the like. 
     Works (to be measured) such as semiconductor modules formed with a number of bumps are positioned at a later process on a wiring board, and bumps are heated and melted in a heating furnace to be connected to the wiring board. 
     In order to reliably connect the bumps, it is necessary to correctly measure a height of each bump at its apex. 
     The size of each bump is required to have predetermined values so that adjacent bumps and wiring connections are prevented from being electrically shorted. 
     However, it is very difficult to measure heights of a number of bumps correctly and in a short time. 
     One of the inventors of this invention has proposed techniques of measuring a height of a work by using an optical beam, as disclosed in JP-A-2-80905. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus capable of measuring bumps on a work correctly at high speed. 
     The measuring method of this invention comprises the steps of: mounting a work to be measured on a table, the work having solder bump rows; detecting a reference bump from the solder bumps by using a laser optical micro head; scanning the solder bump rows with the optical micro head and detecting a shift amount of a mount posture of the work; correcting the shift amount of the mount posture of the work by scanning the table; and measuring height positions of apexes of all the solder bumps with the optical micro head. 
     The method of the invention further comprises the steps of calculating a regression plane formed by the apexes of the solder bumps and calculating a relative apex position of each solder bump relative to the calculated regression plane. The method of the invention further comprises the steps of calculating a standard deviation of relative heights and judging from the standard deviation and the apex height of each bump whether the work is defective or not. 
     A measuring apparatus of this invention comprises: a table for placing thereon a work to be measured, the work having solder bump rows; moving means for moving the table in a two-dimensional plane in two perpendicular axis directions; rotating means for rotating the table about a vertical axis; inclining means for inclining the table; a laser optical micro head mounted on the table to be movable in the vertical axis direction; a personal computer for recording data measured by the optical micro head in the form of digital data; controlling means for controlling the table and the optical micro head; a personal computer for calculating a height position of an apex of each solder bump in accordance with the data measured by the optical micro head; a display screen for displaying the calculated results; and a printer for printing out the calculated results. 
     According to the present invention, a work is measured by scanning a light spot by the optical micro head. Therefore, the drive speed can be changed depending upon a necessary measurement precision and the size of the work, so that high speed, high precision, and versatility can be realized for various applications. 
     As above, the invention provides a method and apparatus for measuring heights of all of a number of bumps formed on a work such as an LSI and judging whether the work is good or not. The invention is applicable to works having highly dense bumps expected in the future. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a semiconductor module used as a work to be measured, and FIGS. 1B and 1C are enlarged views showing bumps and a bump on the semiconductor module. 
     FIG. 2 is a perspective view of a measuring apparatus of this invention. 
     FIG. 3 is a block diagram showing the structure of the measuring apparatus of the invention. 
     FIG. 4 is a perspective view showing the structure of an optical micro head. 
     FIG. 5 is a plan view illustrating scanning of an optical micro head. 
     FIG. 6 is a diagram showing an example of measurement results. 
     FIGS. 7A and 7B are diagrams illustrating a method of measuring the apex of a solder bump on a substrate surface. 
     FIG. 8 is a diagram illustrating the effects of swell of a substrate. 
     FIG. 9 is a plan view illustrating a mount of a work on a work table. 
     FIG. 10 is a plan view illustrating a method of detecting a bump apex position. 
     FIG. 11 is a diagram illustrating measurement results with substrate mount errors. 
     FIG. 12A is a diagram illustrating measurement results with substrate mount errors, and FIG. 12B is a diagram illustrating rotation correction of mount errors. 
     FIG. 13 is a diagram illustrating measurement results with substrate mount errors. 
     FIG. 14 is a diagram illustrating a regression plane of apexes of solder bumps. 
     FIG. 15 is a graph showing a standard deviation of measurement results. 
     FIG. 16 is a diagram illustrating a display screen of a monitor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a diagram showing an apparatus used with a solder bump measuring method of this invention. 
     This measuring apparatus generally indicated by reference numeral  100  has an operation stage  105  on which a work is placed and measured, a control unit  120 , an operation panel  130 , a printer  140  for outputting measurement results, a monitor TV for monitoring a measurement area of the operation stage  105 , and the like. 
     FIG. 3 is a block diagram showing the control system of the measuring apparatus  100  shown in FIG. 2. A table frame on which a work  1  is placed is mounted on an anti-vibration base  210 . The table frame has a θ stage  212  for controlling an angle around the work vertical axis (Z axis), an X stage  214  for controlling a motion in a guide axis (X axis) in the plane perpendicular to the vertical axis, a Y stage  214  for controlling a motion in a guide axis (Y axis) perpendicular to the X axis, and a β inclination stage  218  and an α inclination stage  220  respectively for controlling inclination of the surface of a work positioning mechanism (work table)  230 . 
     Each control axis is controlled by outputs from an axis controller/driver apparatus  240 . 
     An optical microsensor  250  is mounted on a Z axis driver mechanism  270  and is used for controlling the optical microsensor in the Z axis relative to the work table  230 . This optical microsensor  250  can be retracted away from a measuring position of the work table  230  by a retracting mechanism  272  so that the work  1  can be easily placed on or dismounted from the operation stage. A motion amount in the Z axis is measured by a digital micrometer  274 . 
     An optical camera  252  is mounted at the side of the optical microsensor  250 , and the state of the measuring area can be monitored by a CRT  150 . 
     The optical microsensor  250  is controlled by a controller  260 , and the measured data is A/D converted and supplied via a digital input/output interface  264  to a computing apparatus, for example, a personal computer  266 . 
     The measurement results are supplied via a digital input/output interface  280  to a computing apparatus, for example, a master personal computer  110 . The measurement results are displayed on the personal computer  110  and printed out from a printer  140 . 
     The measurement results of solder bumps are judged in accordance with coordinate data of each bump position and correction amount data of each control axis, by using an operation switch  284 . 
     FIG. 4 shows the structure of the optical micro head  250  which has a semiconductor laser  252  and a light receiving element  255 . A laser beam radiated from the semiconductor laser  252  passes through a lens  253  and applied to a bump  10  as a laser beam LB. The laser beam reflected at the surface of the bump  10  passes through a lens  254  and is received by the light receiving element  255 . The light receiving element  255  measures the height position of the bump surface in accordance with a position where the reflected light is received, by using the principle of triangulation, and also detects the amount of reflected light. 
     The X axis of the table is aligned with the optical axis of the laser beam LB. 
     FIG. 5 shows scan paths along which the bump  10  is scanned with the laser beam LB. The surface area including the apex of the bump  10  is measured by three paths. Three paths are used for obtaining a correct value while considering the position displacement of each bump. Three paths are only illustrative and a plurality of paths may be set depending upon the measurement conditions. 
     FIG. 6 shows an example of the measurement results wherein the abscissa represents the X axis and the ordinate represents a detected level. A first curve C 1  indicates a level of reflected and received light, and a second curve C 2  indicates a change in height of the bump. 
     A reference value TL of the level of the reflected and received light C 1  is preset. An X axis position where the received light becomes larger than the reference value TL and an X axis position where the received light becomes smaller than the reference value are detected. The value of the height signal C 2  at the middle coordinate position X 10  is used as the height of the apex of the bump. 
     FIGS. 7A and 7B illustrate a relationship between the surface  8  of the substrate  2  and the height position of the bump  10 . 
     The laser beam LB scans the surface  8  of the substrate  2  and the bump  10  to detect the height positions of the surface  8  and bump  10 . 
     The substrate is not always absolutely flat, since it may have a swell or the like. Therefore, as shown in FIG. 8, a regression plane P 1  of the surface  8  of the substrate  8  and a regression plane P 2  of the apexes of bumps are calculated. 
     An absolute height H 1  from the regression plane P 1  and a relative height H 2  from the regression plane P 2  are then obtained. 
     There is not always a constant relationship between the size and shape of the substrate of a work to be measured and the position of each solder bump on the substrate, because a work precision of the substrate has a limit. Therefore, after the work is placed on the table, the mount posture of the work is aligned before measurement. 
     The regression plane means a virtual plane which minimizes the distances to bumps. This is the same concept as a regression line. It is more effective from the viewpoint of process to evaluate the bump height from the distance to the regression plane than using the absolute bump height. 
     FIG. 9 shows a positioning device for positioning an LSI carrier as a work on the work table  230  which moves in X and Y directions. 
     A work  1  is of a square shape. A right angle block  232  conformal to the work  1  is mounted on the table  230 . The block  232  has reference stoppers  233 . The work  1  is pushed against the stoppers  233  by a pushing pin  234  which moves toward the stoppers  233 . 
     The table  230  has a suction device  235  which uses a negative pressure to suck the bottom of the work  1 . 
     There are some errors of the outer dimension of the LSI substrate and bump positions because of substrate scribe errors, shrinkage of substrate material, or the like. 
     For this reason, a process of detecting the position of a first bump  10 - 1  of the work  1  is executed as illustrated in FIG.  10 . 
     The controller knows in advance the position, as designed, of the first bump  10 - 1  relative to the substrate of the work  1 . Therefore, first, the coordinates of the X axis of the table are aligned with the design coordinates of the first bump  10 - 1 , and a first scan S- 1  is performed for measurement while moving the table in the Y direction. 
     With this scan S- 1 , a curve indicating a change in the amount of reflected and received light of the first bump  10 - 1 , such as the curve C 1  shown in FIG. 6, can be obtained. The center position of the light amount of this curve is obtained, this position being assumed as the center (origin) of the first bump  10 - 1 . If the received light amount does not exceed the reference value, the X axis coordinates are shifted by a distance D 1  to execute a second scan S- 2  to detect the temporary origin and determine the Y axis coordinates. 
     Next, after the Y axis coordinates are fixed, a scan S- 3  is performed in the X axis direction to determine the position of the first bump  10 - 1  from the center of the received light amount. If the received light amount does not exceed the reference value at the X axis coordinates, the Y axis coordinates are shifted by a distance D 2  to execute a scan S- 4  to perform similar operations as above. 
     The coordinate values obtained in the above manner are used as a temporary origin K 1 . 
     A first row of bumps including the temporary origin K 1  is scanned (S- 10 ), and the apex positions of bumps are obtained which show a received light amount in excess of the reference value. An average value of shift amounts between the X axis coordinates of apex positions and the designed X axis coordinates is calculated (refer to FIG.  11 ). 
     Next, a similar scan (S- 11 ) is performed for the bump row remotest from the above bump row to thereby calculate an average value of shift amounts in the X axis coordinates. 
     In accordance with shift amounts, a correction value of a distance from the end of the work substrate to the first bump in the X axis direction is determined to correct the X axis coordinates of the origin. 
     Similar scans are executed in the Y axis direction to calculate the shift amounts of the Y axis coordinates and correct the Y axis coordinates of the origin. 
     If the work is mounted being rotated about the center axis and a scan is performed by shifting the scan position by a designed value, then the peak values of the received light amount on the scan line change in a predetermined direction and a shift occurs between the peak position and the design position (refer to FIG.  12 A). 
     In accordance with this change, a correction value for the work rotation angle θ is calculated to rotate the θ stage and correct its position (refer to FIG.  12 B). 
     While a scan for the θ correction is executed in the X axis direction, the heights at positions  8 - 1 ,  8 - 2 ,  8 - 3 , . . . on the work substrate are (Refer to FIG.  13 ). In accordance with the change in the heights, a regression line L- 1  is calculated and a correction value a for an inclination angle from the reference line (horizontal line) in the X axis direction is calculated to correct the a inclination stage  220 . 
     Similarly, a correction value β for an inclination angle in the Y axis direction is calculated to correct the β inclination stage  218 . 
     After the above alignment processes are completed, all bumps are scanned to measure the apex heights as already described with FIG.  5 . 
     With precise alignment, a measuring laser beam can be scanned highly precisely at high speed. Therefore, the bump heights can be measured highly precisely at high speed. 
     FIG. 14 shows a regression plane PL- 1  formed by the measured apex positions of bumps  10 . A shift of each bump apex position is calculated from this regression plane PL- 1  and the judgement results are displayed on the display  112 . 
     FIG. 15 is a graph showing the measurement results displayed on the screen. 
     The abscissa represents a difference between each bump apex and the regression plane, and the ordinate represents the number of bumps. 
     This work has 5200 bumps, the standard deviation is 0.5 μm, and 3δ is +1.95. 
     If a target value is set to +/−2 μm, the work is judged good from the standard deviation, and is transported to the next process. The work may be judged good to transport it to the next process, if all bumps are in a range of predetermined reference values. 
     FIG. 16 shows examples of various displays displayed on the screen  112  of the measuring apparatus. 
     An operation screen  300  has a display area  302  for a present operation mode and the like, an area  304  for displaying a relative height occurrence frequency distribution graph, an area  306  for displaying judgement results, and an area  308  for displaying the detailed results for each LSI as a work. 
     In the area  302 , the following items are displayed. 
     (a) Present operation mode (alignment, apex measurement, base measurement, judgement calculation, second time individual measurement). 
     (b) Present inspection position (row, column) with graphics and values. 
     (c) Coordinates of defective bump of inspected LSI with graphics and values. 
     For the display of the position of a defective bump, for example, the position of a defective bump  10 -N of the work  1  is displayed with graphics. 
     The relative height occurrence frequency distribution graph  304  is shown in FIG.  15 . The judgement results are displayed in the area  306  by OK or NG. For the detailed results for each LSI in the area  308 , the alignment results are displayed by values of X, Y, Z, θ, α, and β, and also a bump average height, standard deviation of relative heights, received light average amount, and base shrinkage ratios in X and Y directions are displayed. 
     A position shift graph screen  310  shows a distribution of a shift amount of an apex position from a design value for each bump. 
     A height three-dimensional distribution graph screen  320  shows a 3D graph of relative heights of respective bumps and a two-dimensional distribution graph in different colors. 
     A defective LSI information graph screen  330  shows a judgement mode of a defective LSI after full automatic operation (in units of magazine). 
     An inspection condition setting screen  340  is used for setting inspection parameters such as judgement threshold values. 
     With this invention, for example, an operation speed of 32 mm/sec, optical micro head frequency of 16 kHz, measurement resolution in the horizontal direction of 2 to 3 μm, and measurement resolution in the height direction (Z direction) of 0.05 μm, can be realized.