Patent Publication Number: US-2021170524-A1

Title: Laser beam adjustment system and laser processing apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a laser beam adjustment system and a laser processing apparatus. 
     Description of the Related Art 
     Among laser processing apparatuses that process a workpiece by irradiating the workpiece with a laser beam, there are apparatus differences that the beam diameter of a laser beam emitted from a laser oscillator varies from laser oscillator to laser oscillator. Therefore, a beam expander is used to adjust the beam diameter of the laser beam to a preset diameter and also to adjust the laser beam to a parallel beam of light, which is hereinafter referred to simply as “a parallel light.” 
     A beam expander adjusts a laser beam to a parallel light, and also adjusts the beam diameter of the laser beam to a predetermined size. Use of the beam expander can make the diameter of a laser beam emitted from a laser oscillator substantially uniform among apparatuses. The apparatus differences among laser processing apparatuses can be reduced accordingly. 
     In a beam expander, a first concave lens, a convex lens and a second concave lens are arranged side by side in this order from a laser oscillator as disclosed in Japanese Patent Laid-open No. 1996-015625. Focal points of the individual lenses are located on the same optical axis. 
     SUMMARY OF THE INVENTION 
     The beam diameter of a laser beam in a laser processing apparatus is measured from a reacted area on a photodetector (power meter) by irradiating the photodetector with the laser beam as disclosed in Japanese Patent Laid-open No. 1996-015625. 
     An adjustment of a laser beam by a beam expander is performed before processing as will be described hereinafter. First, a worker performs an adjustment to convert a laser beam to a parallel light or to a collimated beam by moving a lens. Next, the worker measures the beam diameter, and performs an adjustment to change the beam diameter of the laser beam to a predetermined size (beam diameter adjustment) by moving another lens. When this beam diameter adjustment is performed, the parallelism is broken so that an adjustment to a parallel light and a beam diameter adjustment are performed again. As described above, the worker repeats an adjustment to a parallel light and a beam diameter adjustment to obtain a desired parallelism and beam diameter for a laser beam. It therefore takes labor and time for the adjustment of a laser beam by a beam expander. 
     If the laser beam becomes no longer the parallel light or varies in beam diameter during processing, the worker is hard to notice such a change because the adjustment to a parallel light and the beam diameter adjustment, which uses the photodetector, are performed before the processing. 
     The present invention therefore has as an object thereof the provision of a laser beam adjustment system that can facilitate an adjustment to a parallel light and a beam diameter adjustment for a laser beam and, if the laser beam varies in beam diameter during laser processing, allows a worker to notice such a change. 
     In accordance with an aspect of the present invention, there is provided a laser beam adjustment system for adjusting a laser beam emitted from a laser oscillator to a parallel light. The laser beam adjustment system includes a beam adjustment unit having a plurality of lenses arranged in an optical path of the laser beam, a first mirror that reflects the laser beam, which has passed through the beam adjustment unit, to change the optical path thereof, a second mirror that reflects the laser beam, the optical path of which has been changed by the first mirror, to change the optical path thereof, a first camera configured to capture an image of a first light having passed through the first mirror, a second camera configured to capture an image of a second light having passed through the second mirror, and a control unit. The control unit includes a first calculation section configured to calculate a first beam diameter of the first light from pixels in a region having a higher brightness than a preset first brightness in the image captured by the first camera, a second calculation section configured to calculate a second beam diameter of the second light from pixels in a region having a higher brightness than a preset second brightness in the image captured by the second camera, and a lens adjustment section configured to move one of the plurality of lenses of the beam adjustment unit in a direction parallel to the optical path of the laser beam so that the first beam diameter calculated by the first calculation section and the second beam diameter calculated by the second calculation section match each other. 
     Preferably, the lens adjustment section may be configured to move another one of the plurality of lenses of the beam adjustment unit in the direction parallel to the optical path of the laser beam so that the first beam diameter or second beam diameter calculated by the first calculation section or second calculation section falls within a predetermined range set beforehand. 
     In accordance with another aspect of the present invention, there is provided a laser processing apparatus including a chuck table configured to hold a workpiece, a laser processing unit configured to process the workpiece, which is held on the chuck table, by a laser beam irradiation, a processing feed mechanism that carries out processing feed of the chuck table in an X-axis direction relative to the laser processing unit, an indexing feed mechanism that carries out indexing feed of the chuck table in a Y-axis direction, which intersects the X-axis direction at right angles, relative to the laser processing unit, and a notification unit that performs a notification to a worker. The laser processing unit includes a laser oscillator that oscillates a laser, a condenser that focuses a laser beam emitted from the laser oscillator, and a laser beam adjustment system arranged between the laser oscillator and the condenser. The laser beam adjustment system includes a beam adjustment unit having a plurality of lenses arranged in an optical path of the laser beam, a first mirror that reflects the laser beam, which has passed through the beam adjustment unit, to change the optical path thereof, a second mirror that reflects the laser beam, the optical path of which has been changed by the first mirror, to change the optical path thereof, a first camera configured to capture an image of a first light having passed through the first mirror, a second camera configured to capture an image of a second light having passed through the second mirror, and a control unit. The control unit includes a first calculation section configured to calculate a first beam diameter of the first light from pixels in a region having a higher brightness than a preset first brightness in the image captured by the first camera, a second calculation section configured to calculate a second beam diameter of the second light from pixels in a region having a higher brightness than a preset second brightness in the image captured by the second camera, and a lens adjustment section configured to move one of the plurality of lenses of the beam adjustment unit in a direction parallel to the optical path of the laser beam so that the first beam diameter calculated by the first calculation section and the second beam diameter calculated by the second calculation section match each other. The notification unit is configured, if the beam diameter calculated by the first calculation section or second calculation section falls outside a predetermined range during laser processing, to notify the worker accordingly. 
     According to the laser beam adjustment system, for the adjustment of the laser beam to the parallel light, the lens adjustment section moves one of the lenses of the beam adjustment unit so that the first beam diameter of the first light and the second beam diameter of the second light match each other. According to the laser beam adjustment system, the laser beam can therefore be adjusted to the parallel light without substantial involvement of work by a worker. Therefore, a load on the worker can be reduced, and the laser beam can be easily adjusted to the parallel light. 
     Preferably, the lens adjustment section may move another lens of the beam adjustment unit so that the beam diameter of the laser beam has a value in the predetermined range set beforehand. The beam diameter of the laser beam can also be easily set at an appropriate value without substantial involvement of work by the worker. 
     In other words, the present invention can adjust, without substantial involvement of work by a worker, a laser beam so that it has a high parallelism and an appropriate beam diameter. Concerning the adjustment of the laser beam, it is therefore possible to significantly reduce the worker&#39;s labor and to successfully suppress human errors. As a consequence, it is possible to suppress effects of apparatus differences among laser oscillators. Substantially the same processing results can hence be obtained in a plurality of laser processing apparatuses. 
     If the beam diameter of the laser beam falls outside the predetermined range set beforehand during laser processing by the laser processing apparatus, the notification unit notifies the worker of that accordingly. Even if the beam diameter varies during the laser processing, the worker can therefore readily notice such a variation. As a consequence, it is possible to suppress a failure in processing a workpiece. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings illustrating a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating the configuration of a laser processing apparatus; 
         FIG. 2  is a schematic diagram illustrating the configuration of a laser processing unit; 
         FIG. 3  is a perspective view illustrating the configuration of the laser processing unit; 
         FIG. 4  is a flow chart illustrating adjustment operations of a laser beam; 
         FIG. 5  is a schematic view illustrating an example of an image captured by a first camera (or a second camera); and 
         FIG. 6  is a graph illustrating an example of a relation between pixels, which are located side by side on a straight line that passes through a center of a beam region, and brightnesses thereof with respect to each of the captured image illustrated in  FIG. 5  and a similar image captured by the second camera (or the first camera). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A laser processing apparatus  10  illustrated in  FIG. 1  is useful in subjecting a wafer  1  to laser processing. The laser processing apparatus  10  includes a parallelepiped bed  11 , an upright wall portion  13  disposed upright on an end portion of the bed  11 , a notification unit  50  that performs a notification to a worker, and a control unit  51  that controls individual members of the laser processing apparatus  10 . 
     On an upper surface of the bed  11 , a chuck table moving mechanism  14  is disposed to move a chuck table  43 . The chuck table moving mechanism  14  carries out processing feed and indexing feed of the chuck table  43  in an X-axis direction and a Y-axis direction, respectively. The chuck table moving mechanism  14  includes a chuck table assembly  40  having the chuck table  43 , an indexing feed mechanism  20  that moves the chuck table  43  in an indexing feed direction relative to a laser processing unit (laser beam irradiation unit)  12 , and a processing feed mechanism  30  that moves the chuck table  43  in a processing feed direction relative to the laser processing unit  12 . 
     The indexing feed mechanism  20  includes a pair of guide rails  23  extending in the Y-axis direction, a Y-axis table  24  mounted on the guide rails  23 , a ball screw  25  extending in parallel to the guide rails  23 , and a drive motor  26  that rotates the ball screw  25 . 
     The paired guide rails  23  are arranged in parallel to the Y-axis direction on the upper surface of the bed  11 . The Y-axis table  24  is arranged on the paired guide rails  23  slidably along these guide rails  23 . On the Y-axis table  24 , the processing feed mechanism  30  and the chuck table assembly  40  are mounted. 
     The ball screw  25  is maintained in threaded engagement with nut portions (not illustrated) arranged on a side of a lower surface of the Y-axis table  24 . The drive motor  26  is connected to an end portion of the ball screw  25 , and rotationally drives the ball screw  25 . Accompanied by the rotational drive of the ball screw  25 , the Y-axis table  24 , the processing feed mechanism  30  and the chuck table assembly  40  move in an indexing feed direction (in the Y-axis direction that intersects the X-axis direction at right angles) along the guide rails  23 . 
     The processing feed mechanism  30  includes a pair of guide rails  31  extending in the X-axis direction, an X-axis table  32  mounted on the guide rails  31 , a ball screw  33  extending in parallel to the guide rails  31 , and a drive motor  35  that rotates the ball screw  33 . The paired guide rails  31  are arranged in parallel to the X-axis direction on an upper surface of the Y-axis table  24 . The X-axis table  32  is disposed on the paired guide rails  31  slidably along these guide rails  31 . On the X-axis table  32 , the chuck table assembly  40  and a power meter  80  are mounted. 
     The ball screw  33  is maintained in threaded engagement with nut portions (not illustrated) arranged on a side of a lower surface of the X-axis table  32 . The drive motor  35  is connected to an end portion of the ball screw  33 , and rotationally drives the ball screw  33 . Accompanied by the rotational drive of the ball screw  33 , the X-axis table  32  and the chuck table assembly  40  move in a processing feed direction (in the X-axis direction) along the guide rails  31 . 
     The chuck table assembly  40  is used to hold the wafer  1 . As illustrated in  FIG. 1 , the wafer  1  that is an example of a workpiece is held as a wafer unit W, which includes a ring frame F, an adhesive tape S, and the wafer  1 , on the chuck table assembly  40 . 
     The chuck table assembly  40  has the chuck table  43  that holds the wafer  1 , clamps  45  arranged around the chuck table  43 , and a θ table  47  that supports the chuck table  43  thereon. The θ table  47  is arranged rotatably in an XY-plane on an upper surface of the X-axis table  32 . The chuck table  43  is a member for holding the wafer  1  under suction. The chuck table  43  is formed in a disc shape, and is arranged on the θ table  47 . 
     On an upper surface of the chuck table  43 , a holding surface is formed including a porous ceramic material. This holding surface is in communication with a suction source (not illustrated). Around the chuck table  43 , the clamps  45  are arranged as many as four. Each clamp includes a supporting arm. The four clamps  45  are activated by an air actuator (not illustrated), whereby the ring frame F around the wafer  1  held on the chuck table  43  is held and fixed in four directions. 
     The upright wall portion  13  of the laser processing apparatus  10  is disposed upright behind the chuck table moving mechanism  14 . On a front surface of the upright wall portion  13 , the laser processing unit  12  is arranged to process the wafer  1 , which is held on the chuck table  43 , by a laser beam irradiation. 
     The laser processing unit  12  includes a processing head  18  from which the laser beam is applied to the wafer  1 , and an arm portion  17  that supports the processing head  18 . The arm portion  17  protrudes from the upright wall portion  13  in a direction toward the chuck table moving mechanism  14 . The processing head  18  is supported on a distal end of the arm portion  17  so that the processing head  18  opposes the chuck table  43  or the power meter  80  in the chuck table assembly  40  in the chuck table moving mechanism  14 . 
     In the arm portion  17  and the processing head  18 , an optical system of the laser processing unit  12  is arranged. As illustrated in  FIG. 2 , the laser processing unit  12  includes, in the arm portion  17 , a laser oscillator  61  that emits a laser beam B, a beam expander  62  that adjusts the laser beam B, and a beam measurement system  63  for measuring the parallelism and beam diameter of the laser beam B. 
     On the other hand, the laser processing unit  12  has, in the processing head  18 , a reflection mirror  65  that reflects the laser beam B, and a condenser (condenser lens)  66  that focuses and outputs the laser beam B. The laser oscillator  61  is, for example, a solid-state laser beam source. The laser oscillator  61  emits the laser beam B in a −Y direction in the arm portion  17 . 
     The beam expander  62  corresponds to an example of a beam adjustment unit having a plurality of lenses. The beam expander  62  is used to adjust the laser beam B emitted from the laser oscillator  61 . 
     The laser beam B adjusted by the beam expander  62  passes through the beam measurement system  63 , and enters the reflection mirror  65  in the processing head  18 . The laser beam B is reflected in a −Z direction by the reflection mirror  65 , and is guided to the condenser  66 . The condenser  66  focuses the laser beam B to be applied in the −Z direction toward an outside of the processing head  18 . 
     When processing the wafer  1  illustrated in  FIG. 1 , the wafer  1  on the chuck table  43  is irradiated with the laser beam B that has been focused by the condenser  66 . When adjusting the laser beam B, on the other hand, the laser beam B is applied to the power meter  80  as illustrated in  FIG. 2 . 
     The notification unit  50  is, for example, a touch panel including a speaker, and a variety of information such as conditions for processing by the laser processing apparatus  10  is presented by an image and a voice message. The notification unit  50  is also used to set various information such as processing conditions. As appreciated from the foregoing, the notification unit  50  functions not only as input means for inputting information but also as presentation means for presenting the information so inputted. 
     A description will next be made about the laser beam adjustment system of the laser processing apparatus  10 . The laser beam adjustment system adjusts the laser beam B emitted from the laser oscillator  61  to a parallel light, and also adjusts the beam diameter of the laser beam B. 
     The laser beam adjustment system of the laser processing apparatus  10  includes the optical system of the laser processing unit  12  built in the above-described arm portion  17  and processing head  18 , and is arranged between the laser oscillator  61  and the condenser  66 . The laser beam adjustment system also includes the control unit  51  illustrated in  FIG. 2 . 
     As illustrated in  FIG. 2 , the beam expander  62  includes, in an optical path B 1  of the laser beam B, a first concave lens  71 , a convex lens  72 , and a second concave lens  73 . The first concave lens  71 , the convex lens  72 , and the second concave lens  73  have focal points located on the optical path B 1  of the laser beam B. 
     The first concave lens  71  is fixed in the beam expander  62 . On the other hand, the convex lens  72  and the second concave lens  73  are configured to be movable in a direction parallel to the optical path B 1  of the laser beam B. The beam expander  62  therefore includes a convex lens moving mechanism  74  and a second concave lens moving mechanism  75 . The convex lens moving mechanism  74  moves the convex lens  72  in the direction parallel to the optical path B 1  of the laser beam B. The second concave lens moving mechanism  75  moves the second concave lens  73  in the direction parallel to the optical path B 1  of the laser beam B. 
     If the second concave lens  73  is moved, a distance L 2  between the first concave lens  71  and the second concave lens  73  changes. As a consequence, it is possible to adjust the parallelism of the laser beam B outputted from the beam expander  62 . A distance between the convex lens  72  and the second concave lens  73  after the adjustment of the parallelism will be assumed to be L 3 . 
     The term “parallelism of the laser beam B” means the degree of uniformity of the beam diameter (width) of the laser beam B along the optical path B 1 . Being high in parallelism means that the laser beam B is a parallel light or a collimated beam, in other words, the beam diameter of the laser beam B is substantially uniform along the optical path B 1 . Being low in parallelism, in contrast, means that the beam diameter of the laser beam B spreads out (or narrows) along the optical path B 1 . 
     If the convex lens  72  is moved, a distance L 1  between the first concave lens  71  and the convex lens  72  are changed. As a consequence, the size of the beam diameter of the laser beam B can be adjusted. When moving the convex lens  72 , it is desired to maintain the distance L 3  after the adjustment of the parallelism. A position SP 0  on the optical path B 1  as illustrated in  FIG. 2  indicates the mutually registered positions of the focal points of the convex lens  72  and the second concave lens  73 . 
     The beam measurement system  63  located in a subsequent stage of the beam expander  62  includes a first mirror  91  and a second mirror  92  that reflect the laser beam B, a first camera  93  arranged on a back side of the first mirror  91 , and a second camera  94  arranged on a back side of the second mirror  92 . 
     The first mirror  91  reflects the laser beam B having passed through the beam expander  62  to change the direction of the optical path B 1  of the laser beam B. The second mirror  92  further reflects the laser beam B the optical path of which has been changed by the first mirror  91 , so that the optical path B 1  is changed further. The laser beam B reflected by the second error  92  enters the reflection mirror  65 . 
     These first mirror  91  and the second mirror  92  reflect the applied laser beam B substantially in its entirety, but allow the laser beam B to pass at a very low rate (0.05% to 0.1%). 
     Then, the first camera  93  arranged on the back side of the first mirror  91  captures an image of a first light P 1  that is a light having passed through the first mirror  91 . On the other hand, the second camera  94  arranged on the back side of the second mirror  92  captures an image of a second light P 2  that is a light having passed through the second mirror  92 . 
     As illustrated in  FIG. 3 , the laser beam oscillator  61 , the beam expander  62 , and the first mirror  91 , the second mirror  92 , the first camera  93  and the second camera  94  of the beam measurement system  63  are arranged in the arm portion  17  so that the laser beam B is reflected in the XY plane by the first mirror  91  and the second mirror  92 . Further, the reflection mirror  65  and the condenser  66  are arranged in the processing head  18  so that they are located side by side along a Z-axis direction. 
     The power meter  80  is arranged downstream of the second mirror  92 , the reflection mirror  65 , and the condenser  66  in the optical path B 1  of the laser beam B. The power meter  80  is exposed to the laser beam B focused by the condenser  66 . As a consequence, the power meter  80  measures the amount of energy (illuminance) of the applied laser beam B. 
     The control unit  51  controls the individual elements of the laser processing apparatus  10  to perform processing on the wafer  1 . The control unit  51  also controls the optical system of the laser processing unit  12  and the power meter  80  illustrated in  FIG. 2  to perform the adjustment of the laser beam B. 
     As illustrated in  FIG. 2 , the control unit  51  includes, as elements, a lens adjustment section  52 , a first calculation section  53 , and a second calculation section  54 . Adjustment operations of the laser beam B under control by the control unit  51  will hereinafter be described along with functions of the elements of the control unit  51 . 
       FIG. 4  is a flow chart illustrating the adjustment operations of the laser beam B by the control unit  51 . As illustrated in this figure, the control unit  51  first sets the positions of the convex lens  72  and the second concave lens  73  of the laser beam adjustment system at predetermined initial positions (initialization: S 1 ). The control unit  51  also controls the chuck table moving mechanism  14  to arrange the power meter  80  right below the condenser  66  in the processing head  18 . 
     Subsequently, the control unit  51  controls the laser oscillator  61  to emit the laser beam B. The power meter  80  is irradiated with the laser beam B emitted from the laser oscillator  61  via the reflection mirror  65  and the condenser  66 . 
     Next, the control unit  51  performs beam diameter acquisition processing (S 2 ). Described specifically, the control unit  51  controls the first camera  93  to capture an image of the first light P 1  having passed through the first mirror  91 , and also controls the second camera  94  to capture an image of the second light P 2  having passed through the second mirror  92 . 
     The first calculation section (the first beam diameter calculation section)  53  of the control unit  51  then calculates the beam diameter of the laser beam B (the first light P 1 ) from pixels in a region having a higher brightness than a preset first brightness in the image captured by the first camera  93 . Similarly, the second calculation section (the second beam diameter calculation section)  54  of the control unit  51  calculates the beam diameter of the laser beam B (the second light P 2 ) from pixels in a region having a higher brightness than a preset second brightness in the image captured by the second camera  94 . 
       FIG. 5  illustrates an example of an image captured by the first camera  93  (or the second camera  94 ). As illustrated in this figure, the captured image is a multi-gradation image containing, for example, a plurality of 5.5 by 5.5 μm square pixels. In the captured image, a color close to white is presented near a center pixel O corresponding to a central part of high intensity (brightness) in the first light P 1  (or the second light P 2 ). As the distance from this center pixel O increases, the pixels of the captured image are presented in a color closer to black. 
     The first calculation section  53  and the second calculation section  54  calculate, based on such captured images, the beam diameters of the first light P 1  and the second light P 2  of the laser beam B, respectively. 
       FIG. 6  illustrates, with respect to each of the captured images based on the first light P 1  and the second light P 2 , an example of a brightness curve presenting a relation between pixels, which are located side by side on a straight line that passes through the center pixel O, and brightnesses thereof. 
     In the examples illustrated in  FIG. 6 , a brightness curve G 1  corresponding to the first light P 1  is presented by a broken line. On the other hand, a brightness curve G 1  corresponding to the second light P 2  is presented by a solid line. Concerning each of these brightness curves G 1  and G 2 , the brightness of the center pixel O presented in  FIG. 5  has a maximum value, and the brightness of each pixel decreases as its distance from the center pixel O increases. Further, the maximum value (100%; corresponding to white color) of the brightness on the brightness curve G 1  is greater than the maximum value (approx. 70%; corresponding to gray color) of the brightness on the brightness curve G 2 . 
     As illustrated in  FIG. 6 , the first calculation section  53  then calculates a first beam diameter R 1 , which is the beam diameter of the first light P 1 , as a width W 1  between pixels that have a brightness 1/e 2  (13.5%) times the value of the peak intensity of the brightness curve G 1  corresponding to the first light P 1 . 
     In other words, the first calculation section  53  determines first border pixels K 1  (at two locations) as the pixels having the brightness 1/e 2  (13.5%) times the value of the peak intensity. The first calculation section  53  next determines that the pixels on an inner side than the first border pixels K 1  are the pixels in the region having the brightness higher than the preset first brightness. The first calculation section  53  then calculates the first beam diameter R 1 , which is the beam diameter of the first light P 1 , as the width W 1  between the two first border pixels K 1 . 
     Similarly, the second calculation section  54  calculates, as illustrated in  FIG. 6 , a second beam diameter R 2 , which is the beam diameter of the second light P 2 , as a width W 2  between pixels that have a brightness 1/e 2  times the value of the peak intensity of the brightness curve G 2  corresponding to the second light P 2 . 
     In other words, the second calculation section  54  determines second border pixels K 2  (at two locations) as the pixels having the brightness 1/e 2  (13.5%) times the value of the peak intensity of the brightness curve G 2  corresponding to the second light P 2 . The second calculation section  54  next determines that the pixels on an inner side than the second border pixels K 2  are the pixels in the region having the brightness higher than the preset second brightness. The second calculation section  54  then calculates the second beam diameter R 2 , which is the beam diameter of the second light P 2 , as the width W 2  between the two second border pixels K 2 . 
     The lens adjustment section  52  of the control unit  51  next moves the second concave lens  73  of the beam expander  62  in the direction parallel to the optical path B 1  of the laser beam B so that the first beam diameter R 1  of the first light P 1  as calculated by the first calculation section  53  and the second beam diameter R 2  of the second light P 2  as calculated by the second calculation section  54  match each other. 
     For example, the control unit  51  calculates a difference between the first beam diameter R 1  and the second beam diameter R 2  (S 3  in  FIG. 4 ). The control unit  51  then determines whether the diameter difference thus calculated falls within a preset tolerance. Described specifically, if the diameter difference thus calculated is determined not to fall within the tolerance, the control unit  51 , based on the determination result, makes an adjustment to the position of the second concave lens  73  (see  FIG. 2 ) in the beam expander  62  (S 4 ). 
     In other words, any diameter difference within the preset tolerance means that the laser beam B has a substantially similar beam diameter at both the first mirror  91  and the second mirror  92  located apart from each other in the direction of the optical path B 1  of the laser beam B. 
     Accordingly, in this case, the control unit  51  therefore determines that the laser beam B is a parallel light having high parallelism, and hence determines that a position adjustment is unnecessary (“Yes” in S 4 ). In contrast, any diameter difference greater than the tolerance means that the laser beam B does not have a substantially similar beam diameter at both the first mirror  91  and the second mirror  92 . 
     Accordingly, in this case, the control unit  51  therefore determines that the laser beam B is not a parallel light, and hence determines that a position adjustment is necessary (“No” in S 4 ). In this case, the lens adjustment section  52  of the control unit  51  controls the second concave lens moving mechanism  75  to move the second concave lens  73  in the direction parallel to the optical path B 1  (S 6 ). 
     Described specifically, if the second beam diameter R 2  is greater than the first beam diameter R 1  by more than the tolerance, the control unit  51  determines that the laser beam B has spread out. In this case, the lens adjustment section  52  moves the second concave lens  73  illustrated in  FIG. 2  in the −Y direction to make greater the distance L 2  between the first concave lens  71  and the second concave lens  73 . As a consequence, the laser beam B can be suppressed from spreading out. 
     If the first beam diameter R 1  is greater than the second beam diameter R 2  by more than the tolerance, in contrast, the control unit  51  determines that the laser beam B has narrowed. Accordingly, in this case, the lens adjustment section  52  moves the second concave lens  73  in a +Y direction to make smaller the distance L 2 . As a consequence, the laser beam B can be suppressed from narrowing. 
     In the manner described above, the diameter difference between the first beam diameter R 1  and the second beam diameter R 2  is reduced to the preset tolerance or smaller, and the processing operations of S 2  to S 4  are repeated until the control unit  51  determines that the laser beam B is a parallel light. 
     After the laser beam B has become the parallel light, the control unit  51  determines whether the beam diameter (the first beam diameter R 1  or the second beam diameter R 2 ) is in a predetermined range set beforehand, for example, in a range of 1.63 mm±50 μm (S 5 ). 
     If the beam diameter is in the predetermined range (“Yes” in S 5 ), the control unit  51  ends the processing. If the beam diameter is not in the predetermined range (“No” in S 5 ), in contrast, the lens adjustment section  52  of the control unit  51  controls the convex lens moving mechanism  74  so that the beam diameter has a value in the predetermined range, thereby moving the convex lens  72  in the direction parallel to the optical path B 1  of the laser beam B (S 7 ). 
     Described specifically, if the beam diameter is greater than the predetermined range, the lens adjustment section  52  controls the convex lens moving mechanism  74  to make smaller the distance L 1  between the first concave lens  71  and the convex lens  72 . As a consequence, the beam diameter of the laser beam B can be made smaller. 
     If the beam diameter is smaller than the predetermined range, in contrast, the lens adjustment section  52  controls the convex lens moving mechanism  74  to make greater the distance L 1  between the first concave lens  71  and the convex lens  72 . As a consequence, the beam diameter of the laser beam B can be made greater. 
     Subsequently, the control unit  51  returns the processing to S 2 , and the processing operations of S 2  to S 7  are repeated until the laser beam B becomes a parallel light and its beam diameter increases to a value in the predetermined range. When the laser beam B has become the parallel light and its beam diameter has increased to the value in the predetermined range, the control unit  51  ends the adjustment operations of the laser beam B. Using the notification unit  50 , the control unit  51  then notifies the worker of that accordingly. 
     After an initiation of laser processing by the worker, the control unit  51  performs the processing operation presented as S 3  in  FIG. 4  as needed, whereby the difference between the first beam diameter R 1  and the second beam diameter R 2  and/or the first beam diameter R 1  or the second beam diameter R 2  (one of the beam diameters) is acquired. 
     If the diameter difference between the first beam diameter R 1  and the second beam diameter R 2  falls outside the preset tolerance or if one of the beam diameters falls outside the predetermined range set beforehand during the laser processing, the control unit  51  controls the notification unit  50  to notify the worker of that accordingly. 
     As has been described above, to adjust the laser beam B to the parallel light by the laser beam adjustment system according to this embodiment, the beam diameter acquisition processing ( FIG. 4 ; S 2 ) is performed by the control unit  51 , and the control unit  51  then calculate the difference between the first beam diameter R 1  of the first light P 1  and the second beam diameter R 2  of the second light P 2 , that is, the diameter difference (S 3 ). In order to make this diameter difference fall within the preset tolerance, the lens adjustment section  52  next controls the second concave lens moving mechanism  75  to move the second concave lens  73  in the direction parallel to the optical path B 1  (S 4 , S 6 ). These processing operations are then repeated until the above-described diameter difference falls within the preset tolerance. 
     According to this embodiment, the laser beam B can therefore be adjusted to the parallel light without substantial involvement of work by the worker. Therefore, a load on the worker can be reduced, and the laser beam B can be easily adjusted to the parallel light. 
     In this embodiment, the lens adjustment section  52  further controls the convex lens moving mechanism  74  to move the convex lens  72  in the direction parallel to the optical path B 1  so that the beam diameter (the first beam diameter R 1  or the second beam diameter R 2 ) of the laser beam B has a value in the predetermined range set before hand (S 7 ). The processing operations of S 2  to S 7  are then repeated until the beam diameter has the value in the predetermined range. In this embodiment, the beam diameter of the laser beam B can also be easily set at an appropriate value without substantial involvement of work by the worker. 
     As has been described above, it is possible in this embodiment to adjust, without substantial involvement of work by the worker, the laser beam B so that it has a high parallelism and an appropriate beam diameter. In this embodiment, it is therefore possible to significantly reduce the worker&#39;s labor and to successfully suppress human errors, both, with respect to the adjustment of the laser beam B. 
     As a consequence, it is possible to suppress effects of the apparatus differences (for example, differences in beam diameter) among laser oscillators. Substantially the same processing results can hence be obtained in a plurality of laser processing apparatuses. 
     In this embodiment, the control unit  51  acquires the difference between the first beam diameter R 1  and the second beam diameter R 2  and the beam diameter (the first beam diameter R 1  or the second beam diameter R 2 ) as needed during the laser processing. If the diameter difference falls outside the preset tolerance or the beam diameter falls outside the predetermined range set beforehand, the notification unit  50  notifies the worker of that accordingly. 
     Even if the laser beam B no longer becomes the parallel light or the beam diameter varies during the laser processing, the worker can therefore notice such a change in this embodiment. As a consequence, it is possible to suppress a failure in processing the wafer  1 . 
     In this embodiment, as illustrated in  FIG. 2 , the second camera  94  is arranged on the back side of the second mirror  92  to capture the image of the second light P 2  that is the light having passed through the second mirror  92 . As an alternative, the second camera  94  may be configured to be arranged on the back side of the reflection mirror  65  in the processing head  18  and to capture an image of a light passed through the reflection mirror  65 . 
     In addition to the first camera  93  and the second camera  94  illustrated in  FIG. 2 , a third camera may also be arranged on a back side of the reflection mirror  65  to capture an image of a light passed through the reflection mirror  65 . In this case, the control unit  51  may further include a third calculation section to calculate the beam diameter of the laser beam B from pixels in a range brighter than a preset third brightness in the image captured by the third camera. The lens adjustment section  52  may then move the second concave lens  73  in the direction parallel to the optical path B 1  of the laser beam B in the beam expander  62  so that the beam diameter calculated by the first calculation section  53 , the beam diameter calculated by the second calculation section  54 , and the beam diameter calculated by the third calculation section match one another. 
     In this embodiment, the images captured by the first camera  93  and the second camera  94  are multi-gradation images. As an alternative, these captured images may be binarized images. 
     In the example illustrated in  FIG. 6 , the maximum value of the brightness on the brightness curve G 1 , which corresponds to the first light P 1  having passed through the first mirror  91 , is higher than the maximum value of the brightness on the brightness curve G 2 , which corresponds to the second light P 2  having passed through the second mirror  92 . In this regard, the brightness of the second light P 2  may become higher than that of the first light P 1 . It is to be noted that the brightness of a light passed through a mirror has a different value depending on the type of the mirror. 
     The laser processing unit  12  according to the embodiment may be a processing unit for subjecting the wafer  1  to ablation processing, or a processing unit for performing stealth dicing processing to form modified layers inside the wafer  1 . 
     The present invention is not limited to the details of the above-described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.