Method of and apparatus for bonding light-emitting element

A bonding apparatus has a probe for causing an LED chip to emit light before the LED chip is bonded on a board, an imaging system for recognizing the center of a light-emitting area of the LED chip and recognizing coordinates of a contour reference point of the LED chip with respect to the recognized center of the light-emitting area, and a light-emitting-element holding mechanism for positioning the LED chip in a bonding position on the board based on the recognized coordinates of the contour reference point. The center of the light-emitting area of the LED chip can be positioned highly accurately in a desired position on the board without being adversely affected by variations in the contour dimensions of the LED chip.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a method of and an apparatus for bonding a
 light-emitting element in a predetermined position on a board.
 2. Description of the Related Art
 Generally, a linear array of light-emitting elements such as laser diodes,
 light-emitting diodes, or the like is used as a light source for image
 reading and outputting (recording) applications. For example, as shown in
 FIG. 28 of the accompanying drawings, an LED array 1 comprises a plurality
 of LED chips (light-emitting elements) 3 mounted on a board 2 at equally
 spaced intervals and arranged in a linear pattern extending in one
 direction. The LED chips 3 are bonded on the board 2 by silver paste, with
 gold wires 4 extending from the respective LED chips 3.
 The LED array is required to have the LED chips 3 aligned highly accurately
 on the board 2 so that the central light-emitting regions of the LED chips
 3 will be spaced at equal distances. One known die bonder designed to meet
 such a requirement is disclosed in Japanese laid-open patent publication
 No. 6-216170, for example. In the disclosed die bonder, upper and lower
 solid-state imaging devices are moved to a position between a
 semiconductor device and a workpiece to which the semiconductor device is
 to be joined. The upper solid-state imaging device captures the image of a
 mark on the semiconductor device, whereas the lower solid-state imaging
 device captures the image of a mark on the workpiece. A processing
 controller calculates the relative positional relationship between the
 semiconductor device and the workpiece based on detected signals from the
 upper and lower solid-state imaging devices. The relative positional
 relationship between the semiconductor device and the workpiece is
 adjusted on the basis of the calculated data, and then the semiconductor
 device is bonded to the workpiece.
 On an LED chip, its central light-emitting area and the center of an
 alignment mark or a contour thereof are usually positionally misaligned
 with each other. Therefore, even if the LED chips are positioned
 relatively to the board by aligning the alignment marks of the LED chips
 with each other, a possible misalignment of the central light-emitting
 areas of the LED chips cannot effectively be avoided.
 When efforts are made to recognize the centers of the contours of the LED
 chips which are less misaligned with the central light-emitting areas
 thereof, since the contours of the LED chips tend to vary from chip to
 chip to a relatively large extent, the centers of the contours of the LED
 chips are liable to be recognized in error. For this reason, it is
 difficult to accurately position the central light-emitting regions of the
 LED chips, resulting in a failure to construct a highly accurate LED
 array.
 Japanese laid-open patent publication No. 7-43112 discloses a method of
 detecting a light-emitting spot of a light-emitting element and an
 apparatus for positioning such a light-emitting element. According to the
 disclosed arrangement, when a semiconductor laser chip is attracted by a
 suction nozzle and fed to a positioning location, an electric current is
 supplied to the semiconductor laser chip to enable the semiconductor laser
 chip to emit light, and a CCD camera positioned in confronting relation to
 the light-emitting area of the semiconductor laser chip captures an image
 of the semiconductor laser chip. The captured image is supplied from the
 CCD camera to a controller, which detects the position and orientation of
 the semiconductor laser chip based on the supplied image. Based on the
 detected position and orientation, the controller then controls the
 suction nozzle to correct the attitude of the semiconductor laser chip.
 The principles of the disclosed invention are, however, based on the
 configurations of semiconductor laser chips, and are not applicable to LED
 chips whose light-emitting areas are of a comparatively complex shape. In
 addition, whereas the light emission of a semiconductor laser chip can
 easily be detected while the semiconductor laser chip is being attracted
 because the attracted surface of the semiconductor laser chip is different
 from the light-emitting surface thereof, it would be difficult to detect
 the center of the light-emitting area of an LED chip as the attracted
 surface of the LED chip is oriented in the same direction as the
 light-emitting surface thereof. Furthermore, when a probe would be applied
 to an LED chip to enable the LED chip to emit light, the probe would
 shield the light-emitting area of the LED chip, with the result that the
 center of the light-emitting area would not be detected with high
 accuracy.
 Heretofore, the technique disclosed in Japanese laid-open patent
 publication No. 6-334022, for example, is known for bonding a plurality of
 LED chips on a board. According to the disclosed bonding process, an
 alignment mark on the board and alignment marks on the LED chips are read
 by individual cameras, and a biaxially movable stage, which supports the
 board and are movable along X- and Y-axes, is operated on the basis of
 positional information representing the read alignment marks, after which
 the LED chips are bonded on the board.
 In the disclosed bonding arrangement, the bonding accuracy of the LED chips
 depends largely on the accuracy with which the biaxially movable stage is
 positionally measured. The position of the biaxially movable stage is
 usually measured by encoders or linear scales mounted on slide guides.
 However, since pitching and yawing displacements of the upper surface of
 the biaxially movable stage, which serves as a workpiece support, cannot
 be fully measured, the bonding accuracy of the LED chips tends to be
 lowered.
 According to another bonding process, which is based on the above disclosed
 bonding process, the LED chips are simultaneously imaged by the camera,
 and then positionally corrected in order to equalize the distances between
 the alignment marks on the LED chips before the LED chips are bonded on
 the board.
 However, the other bonding process is disadvantageous in that if the LED
 chips are widely spaced apart, then they cannot be simultaneously
 recognized. Another problem is that since the LED chips themselves are
 tiny pieces, it is difficult to apply alignment marks to the LED chips. In
 addition, the bonding process is not versatile as it can be applied to
 transparent boards only.
 SUMMARY OF THE INVENTION
 It is therefore a general object of the present invention to provide a
 method of and an apparatus for bonding light-emitting elements on a board
 while easily positioning the centers of the light-emitting areas of the
 light-emitting elements highly accurately on the board without being
 unduly affected by variations of the centers of the light-emitting areas
 and contours of the light-emitting elements.
 A major object of the present invention is to provide a method of and an
 apparatus for bonding components by positioning the components highly
 accurately without being affected by spaced intervals between the
 components.
 The above and other objects, features, and advantages of the present
 invention will become more apparent from the following description when
 taken in conjunction with the accompanying drawings in which preferred
 embodiments of the present invention are shown by way of illustrative
 example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 shows in perspective a bonding apparatus 10 for carrying out a
 method of bonding a light-emitting element according to a first embodiment
 of the present invention. FIG. 2 shows the bonding apparatus 10 in side
 elevation.
 As shown in FIGS. 1 and 2, the bonding apparatus 10 has a probe 16 as a
 light-emitting means for enabling an LED chip 14 as a light-emitting
 element to emit light before the LED chip 14 is bonded on a board 12, an
 imaging means 18 for recognizing the center of a light-emitting area of
 the LED chip 14 and recognizing contour reference coordinates of the LED
 chip 14 with respect to the coordinates of the recognized center of the
 light-emitting area of the LED chip 14, and a light-emitting-element
 holding means 20 for positioning the LED chip 14 in a bonding position on
 the board 12.
 The bonding apparatus 10 has a mount base 22 with a displacement mechanism
 26 mounted on an upper surface 24 thereof. The displacement mechanism 26
 has a first movable stage 30 movable along a Y-axis of an orthogonal
 coordinate system by a first motor 28 and a second movable stage 34
 movable along an X-axis of the orthogonal coordinate system with respect
 to the first movable stage 30 by a second motor 32.
 The first movable stage 30 comprises a pair of guide rails 36a, 36b
 extending along the Y-axis and a ball screw 38 disposed between guide
 rails 36a, 36b and extending along the Y-axis. The first motor 28 has an
 output shaft coupled to an end of the ball screw 38. The ball screw 38 is
 threaded through a nut (not shown) fixed to a lower surface of a Y-axis
 movable table 40 that is movably supported on the guide rails 36a, 36b.
 The Y-axis movable table 40 is of an elongate shape along the X-axis. The
 Y-axis movable table 40 supports thereon a pair of guide rails 42a, 42b
 extending along the X-axis and a ball screw 44 disposed between guide
 rails 42a, 42b and extending along the X-axis. The guide rails 42a, 42b
 and the ball screw 44 belong to the second movable stage 34. The second
 motor 32 has an output shaft coupled to an end of the ball screw 44. The
 ball screw 44 is threaded through a nut (not shown) fixed to a lower
 surface of an X-axis movable table 46 that is movably supported on the
 guide rails 42a, 42b.
 The X-axis movable table 46 supports on an upper surface 48 thereof a chip
 carrier base 52 for carrying a plurality of LED chips 14, a .theta. stage
 54 for correcting the angular position of each of the LED chips 14, and a
 board suction base (board holding means) 56 for attracting and holding a
 board 12. The .theta. stage 54 has a turntable 58 which is rotatable about
 a vertical Z-axis by an actuator (not shown).
 A column 60 is vertically mounted on an end of the mount base 22. The
 column 60 supports thereon an actuating means 62 for moving the probe 16
 and the light-emitting-element holding means 20 back and forth along the
 Z-axis and the X-axis. The actuating means 62 has a frame 64 fixed to a
 vertical surface of the column 60, and a third motor 66 is fixed to an end
 of the frame 64 and has an output shaft coupled to a ball screw 68
 extending along the X-axis and threaded through an X-axis table 70. A
 vertical frame 72 is fixed to the X-axis table 70.
 A fourth motor 74 is fixed to an upper end of the frame 72 and has an
 output shaft coupled to a ball screw 76 extending along the Z-axis and
 threaded through a vertically movable base 78. The vertically movable base
 78 supports thereon a collet 80 of the light-emitting-element holding
 means 20 which is connected to a vacuum source (not shown). The probe 16
 is fixed to the vertically movable base 78, and a feeler 82 inclined with
 respect to the Z-axis is mounted on a lower distal end of the probe 16.
 The imaging means 18 has an arm 84 mounted on the column 60 supporting CCD
 cameras 86, 88 on its distal end, the CCD cameras 86, 88 being directed
 along the Z- and X-axes, respectively. A two-focus optical system 90 is
 positioned on the optical axes of the CCD cameras 86, 88. An image
 processor 100 for being supplied with images captured by the CCD cameras
 86, 88 and processing the supplied images to recognize the coordinates of
 a contour reference point L1 (described later on) is disposed on one side
 of the mount base 22.
 Operation of the bonding apparatus 10 will be described below with
 reference to flowcharts shown in FIGS. 3 through 5.
 The board 12 is set on the board suction base 56. The board 12 has been
 positioned with its edge along the X-axis being held in alignment with a
 station reference surface (not shown), and attracted to the board suction
 base 56 under a vacuum developed via suction holes (not shown) in the
 board suction base 56. A plurality of LED chips 14 in the form of a chip
 wafer 50 are placed on the chip carrier base 52.
 The displacement mechanism 26 is actuated to position the chip carrier base
 52 into alignment with a camera center of the imaging means 18, i.e., a
 chip removal position in step S1. In the displacement mechanism 26, the
 first motor 28 is energized to rotate the ball screw 38 about its own axis
 to move the Y-axis movable table 40 along the Y-axis, and the second motor
 32 is energized to rotate the ball screw 44 about its own axis to move the
 X-axis movable table 46 along the X-axis. Therefore, when the first and
 second motors 28, 32 are energized, the LED chips 14 on the chip carrier
 base 52 are brought into the component removal position.
 A certain LED chip 14 of the chip wafer 50 on the chip carrier base 52 is
 now imaged by the CCD camera 86, for example, of the imaging means 18 in
 step S2. An image signal representing the LED chip 14 which has been
 imaged by the CCD camera 86 is sent to the image processor 100, which
 processes the image signal to recognize a reference area of the LED chip
 14, e.g., an upper electrode center or a contour center, and calculate
 corrective quantities .DELTA.X, .DELTA.Y for the LED chip 16 in step S3.
 The corrective quantities .DELTA.X, .DELTA.Y calculated from the image
 signal are then compared with a preset reference value in step S4. If the
 corrective quantities .DELTA.X, .DELTA.Y are greater than the preset
 reference value, then the LED chip 14 is moved by the corrective
 quantities .DELTA.X, .DELTA.Y in step S5. Specifically, the LED chip 14 is
 moved by the corrective quantity .DELTA.X by the first motor 28 and by the
 corrective quantity .DELTA.Y by the second motor 32.
 If the corrective quantities .DELTA.X, .DELTA.Y are smaller than the preset
 reference value in step S4, then the collet 80 attracts and holds the LED
 chip 14 in step S6. Specifically, the collet 80 is positioned on the
 camera center of the imaging means 18 by the actuating means 62, and
 thereafter the fourth motor 74 is energized to lower the vertically
 movable base 78. The collet 80 on the vertically movable base 78 abuts
 against the LED chip 14 positioned as described above, and the
 non-illustrated vacuum source is actuated to cause the collet 80 to
 attract the LED chip 14. The fourth motor 74 is reversed to lift the
 vertically movable base 78 to elevate the LED chip 14 in unison with the
 collet 80 (see FIG. 6).
 The displacement mechanism 26 is actuated to move the .theta. stage 54 to
 the camera center of the imaging means 18 in step S7, after which the
 collet 80 is lowered in unison with the vertically movable base 78.
 Therefore, as shown in FIG. 7, the LED chip 14 attracted and held by the
 collet 80 is transferred onto the turntable 58 of the .theta. stage 54 in
 step S8. The collet 80 then releases the LED chip 14, and then moves
 upwardly with the vertically movable base 78. The CCD camera 86 of the
 imaging means 18 images the LED chip 14 on the turntable 58 in step S9.
 The captured image of the LED chip 14 is processed by the image processor
 100, which recognizes the contour edge of the LED chip 14 and calculates a
 corrective quantity .DELTA..theta. in step S10. The image processor 100
 compares the corrective quantity .DELTA..theta. with a preset reference
 value in step S11. If the corrective quantity .DELTA..theta. is greater
 than the reference value, then control goes to step S12 in which the
 turntable 58 is angularly corrected by the corrective quantity
 .DELTA..theta..
 After the angular correction on the .theta. stage 54 is finished, the third
 motor 66 of the actuating means 62 is energized to move the frame 72 along
 the X-axis to position the probe 16 in alignment with the camera center of
 the imaging means 18 in step S13. The fourth motor 74 is energized to
 lower the vertically movable base 78 until the feeler 82 on the distal end
 of the probe 16 contacts the LED chip 14 on the turntable 58 (see FIG. 8).
 Then, a current power supply (not shown) is turned on to energize the LED
 chip 14 to emit light in step S14, and the CCD camera 86 of the imaging
 means 18 images the center L0 of the light-emitting area of the LED chip
 14 in step S15 (see FIG. 9). An image signal generated by the CCD camera
 86 is sent to the image processor 100, which recognizes the coordinates of
 the center L0 of the light-emitting area of the LED chip 14. Then, the
 non-illustrated current power supply is turned off, after which the image
 processor 100 calculates the coordinates of a contour reference point L1
 (the coordinates relative to the center L0 of the light-emitting area)
 from contour reference lines S1, S2 of the LED chip 14 with respect to the
 recognized coordinates of the center L0 of the light-emitting area in step
 S16, as shown in FIG. 10.
 The fourth motor 74 of the actuating means 62 is energized to displace the
 vertically movable base 78 upwardly to disengage the probe 16 from the LED
 chip 14. Thereafter, the third motor 66 is energized to move the
 vertically movable base 78 together with the frame 72 along the X-axis for
 thereby moving the collet 80 into alignment with the camera center of the
 imaging means 18 in step S17. The fourth motor 74 is energized to lower
 the vertically movable base 78 for causing the collet 80 into abutment
 against the LED chip 14 on the turntable 58. The non-illustrated vacuum
 source is actuated to cause the collet 80 to attract the LED chip 14.
 As shown in FIG. 11, the collet 80 is lifted in unison with the vertically
 movable base 78 by the fourth motor 74 for thereby removing the attracted
 LED chip 14 from the turntable 58 in step S18. Control proceeds to step
 S19 in which the bonding position on the board 12 attracted and held by
 the board suction base 56 is brought into alignment with the camera center
 of the imaging means 18 by the displacement mechanism 26.
 Then, the collet 80 which has attracted the LED chip 14 is lowered by the
 fourth motor 74. The collet 80 stops its descending movement in a vertical
 position wherein the distance between the board 12 and the LED chip 14 is
 about 100 .mu.m in step S20 (see FIG. 12). Then, the CCD camera 88, for
 example, of the imaging means 18 images the LED chip 14 in step S21.
 The image processor 100 recognizes the contour reference lines S1, S2 and
 the contour reference point L1 of the LED chip 14, and calculates the
 center L0 of the light-emitting area of the LED chip 14 from the contour
 reference point L1 and also calculates the corrective quantities .DELTA.X,
 .DELTA.Y which represent a deviation from the bonding position on the
 board 12 in steps S22, S23. Then, control goes to step S24. If the
 corrective quantities .DELTA.X, .DELTA.Y are greater than a preset
 reference value in step S24, then the bonding position on the board 12 is
 corrected in step S25, and thereafter the LED chip 14 is bonded to silver
 paste on the board 12 in step S26. If the corrective quantities .DELTA.X,
 .DELTA.Y are smaller than the preset reference value in step S24, then
 control goes directly to step S26 in which the LED chip 14 is bonded to
 silver paste on the board 12.
 The processing in steps S2 through S18 is carried out on a next LED chip 14
 placed on the chip carrier base 52. In step S19, the board suction base 56
 is moved a constant pitch along the X-axis so that the distance to the
 previously placed LED chip 14 will be of a predetermined value, after
 which a new bonding position is set up on the board 12. The processing in
 step S20 and subsequent steps is carried out to position the next LED chip
 14 such that the distance between the centers L0 of the light-emitting
 areas of the previous and next LED chips 14 on the board 12 will be of a
 constant pitch, and then bond the next LED chip 14 (see FIG. 14).
 Similarly, a desired number of LEDs 14 are successively bonded on the board
 12 so that the centers L0 of their light-emitting areas are spaced at the
 constant pitch. Then, after the LED chips 14 are aligned with each other
 on the board 12, the silver paste on the board 12 is hardened with heat by
 an electric oven, for example.
 In the first embodiment, as described above, an LED chip 14 to be bonded on
 the board 12 is caused to emit light by the probe 16 to allow the center
 L0 of the light-emitting area of the LED chip 14 to be recognized. Then,
 the coordinates of the contour reference point L1 of the LED chip 14 with
 respect to the coordinates of the recognized center L0 of the
 light-emitting area of the LED chip 14 are recognized (calculated), after
 which the LED chip 14 is positioned in the bonding position on the board
 12 based on the coordinates of the contour reference point L1.
 Consequently, the center L0 of the light-emitting area of the LED chip 14
 can reliably and highly accurately be positioned in place without being
 adversely affected by variations in the contours and the centers of
 light-emitting areas of LED chips 14. As a result, it is possible to
 produce a highly accurate LED array which comprises a plurality of bonded
 LED chips 14 whose centers of light-emitting area have been spaced at a
 constant pitch. The LED array thus fabricated is capable of reading and
 writing images with increased accuracy.
 In the first embodiment, LED chips 14 are removed one by one from the chip
 wafer 50, and successively bonded on the board 12. However, an LED chip
 111 shown in FIG. 15 may also be employed in the first embodiment. The LED
 chip 111 shown in FIG. 15 is of a three-chip structure including a pair of
 collet-attractable chips 112a, 112b and a light-emitting chip 114
 positioned therebetween.
 The light-emitting chip 114 emits light, and the collet-attractable chips
 112a, 112b do no emit light, but are attracted by the collect 80. When the
 LED chip 111 is used, the light-emitting chip 114 can emit light while the
 collet-attractable chips 112a, 112b are being attracted by the collect 80
 on the .theta. stage 54. Therefore, it is possible to prevent the LED chip
 111 from being positionally displaced or deviated when the LED chip 111 is
 attracted by the collet 80.
 In the first embodiment, the LED chips 14, 111 are employed as
 light-emitting elements. However, the present invention is also applicable
 to the bonding of other minute chip arrays than LED arrays whose centers
 of light-emitting areas need to be positioned highly accurately. Though
 the LED chip 111 shown in FIG. 15 is of a three-chip structure, an LED
 chip to be bonded may be of a two-chip structure or a four- or more-chip
 structure.
 FIG. 16 shows in perspective a bonding apparatus 110 according to a second
 embodiment of the present invention.
 The bonding apparatus 110 has a probe 16 for enabling an LED chip 14 as a
 light-emitting element to emit light before the LED chip 14 is bonded on a
 board 12, an imaging means 18 for recognizing a light-emitting area 14a of
 the LED chip 14 while the LED chip 14 is emitting light, an image
 processor 112 for detecting the center of the light-emitting area 14a of
 the LED chip 14 from a captured image of the light-emitting area 14a, and
 a light-emitting-element holding means 20 for positioning the LED chip 14
 in a bonding position on the board 12. Those parts of the bonding
 apparatus 110 which are identical to those of the bonding apparatus 10
 according to the first embodiment are denoted by identical reference
 numerals, and will not be described in detail below.
 Operation of the bonding apparatus 110 will be described below with
 reference to flowcharts shown in FIGS. 17 through 19.
 Steps S1a through S12a shown in FIG. 17 are carried out in the same manner
 as steps S1 through S12 according to the first embodiment. Thereafter,
 control goes to step S13a shown in FIG. 18. In step S13a, the third motor
 66 of the actuating means 62 is energized to move the frame 72 along the
 X-axis to position the probe 16 in alignment with the camera center of the
 imaging means 18. The fourth motor 74 is energized to lower the vertically
 movable base 78 until the feeler 82 on the distal end of the probe 16
 contacts an upper electrode 114 in the light-emitting area 14a of the LED
 chip 14 on the turntable 58 (see FIG. 20).
 Then, a current power supply (not shown) is turned on to energize the LED
 chip 14 to emit light in step S14a, and the CCD camera 86 of the imaging
 means 18 captures a light-emission image of the LED chip 14 via an ND
 filter (not shown) in step S15a (see FIG. 20). An image signal generated
 by the CCD camera 86 is sent to the image processor 112.
 The fourth motor 74 is energized to displace the vertically movable base 78
 upwardly to disengage the feeler 82 from the LED chip 14. After the
 turntable 58 is rotated 180.degree. in step S16a, the probe 16 is lowered
 in unison with the vertically movable base 78, bringing the feeler 82 into
 contact with the LED chip 14 again to cause the LED chip 14 to emit light.
 The CCD camera 86 of the imaging means 18 captures another light-emission
 image of the LED chip 14 via the ND filter (not shown) in step S17a, and
 an image signal generated by the CCD camera 86 is sent to the image
 processor 112.
 The image processor 112 combines the two captured light-emission images of
 the LED chip 14 into a combined image 116 in step S18a (see FIG. 21).
 Specifically, each of the light-emission images captured by the CCD camera
 86 contains a shadow of the feeler 82 (see FIG. 20), and the shadow of the
 feeler 82 can be removed when the two captured light-emission images of
 the LED chip 14, which are angularly moved 180.degree. with respect to
 each other, are combined. Therefore, the combined image 116 includes a
 dark area corresponding to the upper electrode 114 of the LED chip 14 and
 a bright area representing a light-emitted surface 118 in step S18a.
 Then, the image processor 112 converts the combined image 116 into a binary
 image 119 as shown in FIG. 22. For producing the binary image 119, the
 image processor 112 uses, as a threshold, the brightness value of a
 certain number of pixels (substantially corresponding to the area of a PN
 junction of the LED chip 14) as counted from the highest-brightness pixel
 of the combined image 116. In this manner, a light-emitting area is
 determined, and the binary image 119 which is highly accurate can reliably
 be produced without being adversely affected by variations in the
 intensity of light emitted by various LED chips 14 which would otherwise
 poses problems if a predetermined brightness level is used as the
 threshold.
 Then, the image processor 112 calculates distributions of the numbers of
 pixels (total numbers of pixels) along the X- and Y-axes in the binary
 image 119, and also calculates an average value X0 of the numbers of
 pixels along the X-axis and an average value Y0 of the numbers of pixels
 along the Y-axis. The average values X0, Y0 represent the center of
 gravity of the area (X0, Y0) of the binary image 119. The image processor
 112 recognizes the center of gravity of the area as the center L0 of the
 light-emitting area of the LED chip 14 in step S19a.
 Then, the non-illustrated current power supply is turned off, after which
 the image processor 112 calculates the coordinates of a contour reference
 point L1 (the coordinates relative to the center L0 of the light-emitting
 area) from contour reference lines S1, S2 of the LED chip 14 with respect
 to the recognized coordinates of the center L0 of the light-emitting area
 in step S20a, as shown in FIG. 23.
 The fourth motor 74 of the actuating means 62 is energized to displace the
 vertically movable base 78 upwardly to disengage the probe 16 from the LED
 chip 14. Thereafter, steps S21a through S30a are carried out in the same
 manner as steps S17 through S26 according to the first embodiment.
 In the second embodiment, as described above, the LED chip 14 to be bonded
 on the board 12 is caused to emit light by the probe 16, and the
 light-emitting area 14a of the LED chip 14 is imaged by the imaging means
 18. At this time, the turntable 58 is turned 180.degree. to capture two
 images of the LED chip 14, which are then combined into the combined image
 116 from which the shadow of the feeler 82 has been removed. Then, the
 combined image 116 is converted into the binary image 119, and the center
 of gravity of the area (X0, Y0) of the binary image 119 is calculated as
 the center L0 of the light-emitting area of the LED chip 14.
 Consequently, the center L0 of the light-emitting area of the LED chip 14
 can reliably and highly accurately be detected without being adversely
 affected by variations in the contours and the centers of light-emitting
 areas of LED chips 14. As a result, it is possible to produce a highly
 accurate LED array which comprises a plurality of bonded LED chips 14
 whose centers of light-emitting area have been spaced at a constant pitch.
 The LED array thus fabricated is capable of reading and writing images
 with increased accuracy.
 A method of bonding a light-emitting element according to a third
 embodiment of the present invention will be described below. The method of
 bonding a light-emitting element according to the third embodiment is
 carried out according to the flowcharts shown in FIGS. 17 through 19,
 except that the process of recognizing the center of the light-emitting
 area of the LED chip 14 is carried out in a manner different from step
 S19a (FIG. 18). The process of recognizing the center of the
 light-emitting area of the LED chip 14 according to the third embodiment
 will be described below.
 As shown in FIG. 24, two light-emission images of the LED chip 14 as it
 emits light are imaged by the CCD camera 86 of the imaging means 18 and
 combined into a combined image 116a. In the combined image 116a, the
 pixels of the light-emitting area 14a are weighted depending on their
 brightness values, and distributions of the sums of brightness values
 along the X- and Y-axes are calculated. Then, an average value X1 of the
 brightness values along the X-axis and an average value Y1 of the
 brightness values along the Y-axis are calculated. The average values X1,
 Y1 represent the center of gravity of the area (X1, Y1) of the combined
 image 100a. The center of gravity of the area serves as the center L0 of
 the light-emitting area of the LED chip 14.
 In the third embodiment, therefore, the center L0 of the light-emitting
 area of the LED chip 14 can reliably and highly accurately be detected
 without being adversely affected by variations in the contours and the
 centers of light-emitting areas of LED chips 14, as with the second
 embodiment.
 FIG. 25 shows in front elevation a bonding apparatus 120 according to a
 fourth embodiment of the present invention. Those parts of the bonding
 apparatus 120 which are identical to those of the bonding apparatus 110
 according to the second embodiment are denoted by identical reference
 numerals, and will not be described in detail below.
 The bonding apparatus 120 has an imaging means 122 comprising two CCD
 cameras 124, 126 which are inclined at respective angles to the vertical
 direction. The CCD cameras 124, 126 serve to image the LED chip 14
 contacted by the probe 16 simultaneously. When the images captured by the
 CCD cameras 124, 126 are combined, the shadow of the feeler 82 of the
 probe 16 can be removed from the combined image.
 FIG. 26 shows in perspective a bonding apparatus 140 according to a fifth
 embodiment of the present invention, and FIG. 27 shows the bonding
 apparatus 140 in side elevation. Those parts of the bonding apparatus 140
 which are identical to those of the bonding apparatus 10 according to the
 first embodiment are denoted by identical reference numerals, and will not
 be described in detail below.
 The bonding apparatus 140 has a laser distance measuring mechanism 142 for
 directly measuring the distance of the board suction base 56 from the
 bonding position with laser beams. The X-axis movable table 46 supports on
 the upper surface 48 thereof a mirror block 144 for reflecting a laser
 beam L emitted from the laser distance measuring mechanism 142. The mirror
 block 144 is made of a material of small coefficient of thermal expansion,
 e.g., a glass material. The mirror block 144 has a first reflecting
 surface 146 extending along the X-axis and a second reflecting surface 148
 extending along the Y-axis. A mirror block 150 is fixed to the two-focus
 optical system 90. The mirror block 150 has a first reflecting surface 152
 extending along the X-axis and a second reflecting surface 154 extending
 along the Y-axis.
 The laser distance measuring mechanism 142 comprises a beam splitter 160 to
 which a laser beam L generated by a laser beam source (not shown) is
 applied, a first displaced position detecting means (e.g., a laser
 interferometer) 162 for dividing a laser beam L1 emitted from the beam
 splitter 160 along the X-axis into a reference beam L10 and a distance
 measurement beam L11, applying the reference beam L10, which is directed
 upwardly, via a mirror 166 to the first reflecting surface 152, which
 provides a reference position on the Y-axis, of the mirror block 150
 associated with the CCD cameras 86, 88, and applying the distance
 measurement beam L11, which is directed horizontally, to the first
 reflecting surface 146 of the mirror block 144 thereby to detect a
 position of the board suction base 56 displaced along the Y-axis with
 respect to the CCD cameras 86, 88, and a second displaced position
 detecting means (e.g., a laser interferometer) 164 for dividing a laser
 beam L2 emitted from the beam splitter 160 along the Y-axis into a
 reference beam L20 and a distance measurement beam L21, applying the
 reference beam L20, which is directed upwardly, via a mirror 168 to the
 second reflecting surface 154, which provides a reference position on the
 X-axis, of the mirror block 150 associated with the CCD cameras 86, 88,
 and applying the distance measurement beam L21, which is directed
 horizontally, to the second reflecting surface 148 of the mirror block 144
 thereby to detect a position of the board suction base 56 displaced along
 the X-axis with respect to the CCD cameras 86, 88.
 The first displaced position detecting means 162 causes the distance
 measurement beam L11 reflected by the first reflecting surface 146 of the
 mirror block 144 to interfere with the reference beam L10 reflected by the
 first reflecting surface 152 of the mirror block 150, producing an
 interference beam L12 which is applied to a first receiver 170. The second
 displaced position detecting means 164 causes the distance measurement
 beam L21 reflected by the second reflecting surface 154 of the mirror
 block 150 to interfere with the reference beam L20 reflected by the second
 reflecting surface 154 of the mirror block 150, producing an interference
 beam L22 which is applied to a second receiver 172.
 In the bonding apparatus 140, the laser distance measuring mechanism 142
 detects whether the bonding position on the board 12 is accurately
 established with respect to the imaging means 18 or not. In the laser
 distance measuring mechanism 142, specifically, the laser beam L emitted
 from the laser beam source is divided by the beam splitter 160 into the
 laser beam L1 along the X-axis and the laser beam L2 along the Y-axis, and
 these laser beams L1, L2 are applied to the respective first and second
 displaced position detecting means 162, 164. The first and second
 displaced position detecting means 162, 164 apply the upwardly directed
 reference beams L10, L20 via the respective mirrors 166, 168 to the first
 and second reflecting surfaces 152, 154 of the imaging means 18, which
 reflect the reference beams L10, L20 back to the respective first and
 second displaced position detecting means 162, 164. Furthermore, the
 horizontally directed distance measurement beams L11, L21 are applied
 respectively to the first and second reflecting surfaces 146, 148 of the
 mirror block 144, which reflect the distance measurement beams L11, L21
 back to the respective first and second displaced position detecting means
 162, 164.
 The reference beams L10, L20 and the distance measurement beams L11, L21
 interfere with each other, producing the interference beams L12, L22 that
 are emitted from the first and second displaced position detecting means
 162, 164 to the first and second receivers 170, 172, respectively.
 Consequently, the bonding position on the board 12 held in position near
 the mirror block 144 is biaxially detected.
 In the fifth embodiment, as described above, the laser beams L1, L2 emitted
 from the beam splitter 160 are divided by the first and second displaced
 position detecting means 162, 164 into the reference beams L10, L20 and
 the distance measurement beams L11, L21 that are reflected by the mirror
 blocks 150, 144. In this manner, relative positions with respect to the
 reference positions on the X- and Y-axes provided by the fixed CCD cameras
 86, 88 are detected for measuring the distance from the bonding position
 on the board 12 with the laser beams.
 Accordingly, the CCD cameras 86, 88 and the bonding position can be
 positioned accurately relatively to each other, so that the accuracy with
 which to position the bonding position can effectively be increased. The
 laser distance measuring mechanism 142 is versatile in applications as the
 board 12 does not need to be transparent.
 In the method of and the apparatus for bonding a light-emitting element
 according to the present invention, before the light-emitting element is
 bonded, it is caused to emit light for recognizing the center of the
 light-emitting area thereof, and then the light-emitting element is bonded
 while the contour reference point of the light-emitting element
 corresponding to the center of the light-emitting area is being observed.
 As a result, the center of the light-emitting area of each of
 light-emitting elements can reliably and highly accurately be positioned
 in place on the board without being adversely affected by variations in
 the contour dimensions of the light-emitting elements. It is thus possible
 to produce a light-emitting-element array whose centers of light-emitting
 areas are spaced at accurate intervals, through a simple operation and
 arrangement.
 According to the present invention, the light-emitting area of the
 light-emitting element is imaged while light is being emitted from the
 light-emitting area, and the center of gravity of the area of a binary
 image converted from the captured image is calculated and regarded as the
 center of the light-emitting area of the light-emitting element.
 Consequently, the center of the light-emitting area of each of
 light-emitting elements can reliably and highly accurately be positioned
 in place on the board without being adversely affected by variations in
 the contour dimensions of the light-emitting elements. It is thus possible
 to produce a light-emitting-element array whose centers of light-emitting
 areas are spaced at accurate intervals, through a simple process.
 The pixels of a captured image of the light-emitting area of the
 light-emitting element which is captured while light is being emitted from
 the light-emitting area are weighted depending on their brightness values,
 and the center of gravity of the area of a binary image converted from the
 captured image is calculated and regarded as the center of the
 light-emitting area of the light-emitting element, for thereby offering
 the same advantages as those described above.
 According to the present invention, furthermore, the distance of the board
 holding means for holding the board or the light-emitting-element holding
 means for holding the light-emitting element from the bonding position is
 directly measured by laser beams, so that the light-emitting element can
 be positioned highly accurately with respect to the light-emitting
 element. The accuracy of intervals or distances between light-emitting
 elements bonded on the board can thus effectively be increased.
 Although certain preferred embodiments of the present invention have been
 shown and described in detail, it should be understood that various
 changes and modifications may be made therein without departing from the
 scope of the appended claims.