Patent Publication Number: US-2021178512-A1

Title: Detecting apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a detecting apparatus including a detecting mechanism for detecting the inside of a workpiece held on a chuck table. 
     Description of the Related Art 
     Wafers with a plurality of devices such as integrated circuits (ICs), large-scale integration (LSI) circuits, etc. formed in respective areas that are demarcated on a face side thereof by a plurality of intersecting projected dicing lines are divided by a dicing apparatus or a laser processing apparatus into individual device chips which will be used in electric appliances such as mobile phones, personal computers, and so on. 
     For dividing a wafer with devices, which should be free of dirt, such as micro-electric-mechanical systems (MEMS), charge-coupled devices (CCD), complementary metal oxide semiconductors (CMOS), or the like formed on its face side into individual device chips, the face side of the wafer is held on a protective tape, projected dicing lines established on the face side of the wafer are detected from a reverse side of the wafer by an infrared camera, and the wafer is diced or laser-processed from the reverse side thereof. 
     SUMMARY OF THE INVENTION 
     However, if a reverse side of a wafer is overlaid with a metal film, then since infrared rays cannot be transmitted through the metal film, the projected dicing lines established on the face side of the wafer cannot be detected from the reverse side of the wafer by an infrared camera. 
     It is therefore an object of the present invention to provide a detecting apparatus that is capable of detecting projected dicing lines on a face side of a workpiece from a reverse side thereof even in a case where the projected dicing lines on the face side cannot be detected from the reverse side by an infrared camera. 
     In accordance with an aspect of the present invention, there is provided a detecting apparatus including a chuck table having a holding surface defined by X- and Y-axis coordinates for holding a workpiece thereon and a detecting mechanism for detecting an inside of the workpiece held on the chuck table. The detecting mechanism includes a laser oscillator for oscillating pulsed laser in a wide-band range of wavelengths, wavelength delaying means for outputting each pulse of a pulsed laser beam emitted from the laser oscillator with time differences imparted to respective wavelengths as a pulsed laser beam, ring-shaped generating means for generating a ring-shaped pulsed laser beam from the pulsed laser beam with the time differences imparted to the respective wavelengths and diffracting the ring-shaped pulsed laser beam into ring-shaped laser beams ranging from small to large ring-shaped laser beams at the respective wavelengths, a beam splitter for branching the ring-shaped pulsed laser beam diffracted into the ring-shaped laser beams ranging from the small to large ring-shaped laser beams, a scanning scanner for scanning the ring-shaped pulsed laser beam branched into a first direction by the beam splitter in X-axis coordinate directions, an indexing scanner for indexing the ring-shaped pulsed laser beam in Y-axis coordinate directions, an fθ lens for applying the ring-shaped pulsed laser beam diffracted into the ring-shaped laser beams ranging from the small to large ring-shaped laser beams to an upper surface, defined by X- and Y-axis coordinates, of the workpiece held on the chuck table, a laser beam applying device for applying a detecting laser beam branched into a second direction by the beam splitter, a half-silvered mirror disposed between the laser beam applying device and the beam splitter, a returning mirror disposed such that the half-silvered mirror and the returning mirror are disposed one on each side of the beam splitter, for returning the detecting laser beam that has passed through the half-silvered mirror to the half-silvered mirror, a photodetector for detecting a laser beam reflected by the half-silvered mirror, and image generating means for generating an image from intensity of the laser beam detected by the photodetector and the X- and Y-axis coordinates of the upper surface of the workpiece to which the ring-shaped laser beams are applied. When the ring-shaped laser beams ranging from the small to large ring-shaped laser beams are applied to the upper surface of the workpiece held on the chuck table, the ring-shaped laser beams generate ultrasonic waves in the workpiece, and an interference wave of the ultrasonic waves is converged at a position defined by a Z-axis coordinate in the workpiece to produce vibrations, and the detecting laser beam is applied to the upper surface of the workpiece at a position aligned with the position where the vibrations are produced, and reflected by the upper surface of the workpiece as a first return laser beam modulated by the vibrations, and an interference laser beam produced from the first return laser beam and a second return laser beam generated from the detecting laser beam returned by the returning mirror is guided by the half-silvered mirror to the photodetector, and the image generating means generates an image representing a state near the position where the interference wave is converged. 
     Preferably, the time differences of the pulsed laser beam diffracted into the ring-shaped laser beams ranging from the small to large ring-shaped laser beams at the respective wavelengths by the ring-shaped generating means are adjusted by the wavelength delaying means thereby to adjust the position defined by the Z-axis coordinate at which the interference wave of the ultrasonic waves is converged in the workpiece. 
     Preferably, the position defined by the Z-axis coordinate at which the interference wave of the ultrasonic waves is converged in the workpiece is adjusted by delaying, with the wavelength delaying means, time t calculated according to: 
       ( H 1 −H 2)/ V=t    
     where H 1  represents a distance from the position at which the interference wave of the ultrasonic waves is converged in the workpiece to the large ring-shaped laser beam on the upper surface of the workpiece, H 2  represents a distance from the position at which the interference wave of the ultrasonic waves is converged in the workpiece to one of the ring-shaped laser beams that is positioned adjacent to the large ring-shaped laser beam, and V represents a speed of the ultrasonic waves propagated in the workpiece. Preferably, the ring generating means is an axicon lens assembly including a pair of axicon lenses and a diffraction grating or a diffractive optical element. 
     According to the present invention, even though no infrared rays are transmittable through a workpiece because of a metal film on a reverse side thereof and a state of a face side of the workpiece cannot be detected from the reverse side by an infrared camera, projected dicing lines, for example, on the face side can nevertheless be detected from the reverse side by the detecting mechanism. 
     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 showing a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a laser processing apparatus incorporating therein a detecting apparatus according to an embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating structural details of a detecting mechanism of the detecting apparatus illustrated in  FIG. 1 ; and 
         FIG. 3  is a conceptual view illustrating the manner in which ultrasonic waves are generated on the basis of a plurality of ring-shaped laser beams applied to a wafer to detect the state of the wafer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A detecting apparatus according to an embodiment of the present invention will be described in detail below with reference to the accompanying drawings.  FIG. 1  illustrates in perspective a laser processing apparatus  1  incorporating therein the detecting apparatus according to the present embodiment. 
     As illustrated in  FIG. 1 , the laser processing apparatus, denoted by  1 , includes a holding unit  20  for holding a workpiece, a moving mechanism  30  for moving the holding unit  20 , a laser beam applying unit  40  for applying a laser beam to the workpiece held by the holding unit  20 , a display unit  50 , and a detecting mechanism  60  that cooperates with the holding unit  20  in making up the detecting apparatus according to the present embodiment. 
     The holding unit  20  includes a rectangular X-axis movable plate  21  mounted on an apparatus base  2  and movable in X-axis directions indicated by the arrow X in the figure, a rectangular Y-axis movable plate  22  mounted on the X-axis movable plate  21  and movable in Y-axis directions indicated by the arrow Y in the figure, which are perpendicular to the X-axis directions, a hollow cylindrical support post  23  fixedly mounted on an upper surface of the Y-axis movable plate  22 , and a rectangular cover plate  26  fixedly mounted on an upper end of the support post  23 . The cover plate  26  has an oblong hole defined therein with a circular chuck table  25  disposed therein. The chuck table  25  is rotatable about a central axis thereof by rotary actuator means, not illustrated. The chuck table  25  has an upper surface acting as a holding surface  25   a  defined in a horizontal plane by an X-axis parallel to the X-axis directions and a Y-axis parallel to the Y-axis directions. The holding surface  25   a  is made of a porous material and is hence permeable to air, and is connected to a suction means, not illustrated, through a fluid channel defined in and extending through the support post  23 . 
     A plurality of clamps  27  are disposed on the chuck table  25  for securing an annular frame F that supports the workpiece through a protective tape T. As illustrated in  FIG. 1 , the workpiece according to the present embodiment is a wafer  10 , for example, including a silicon substrate that has a plurality of devices  12  formed on a face side thereof in respective areas demarcated by a grid of projected dicing lines  14 . The wafer  10  is held by the annular frame F with the face side  10   a  inverted and affixed to the protective tape T for protection of the devices  12  and a reverse side  10   b  facing upwardly. The reverse side  10   b  is overlaid with a metal film that prevents infrared rays from being transmitted therethrough, so that the face side  10   a  cannot be detected by infrared rays applied to the wafer  10  from the reverse side  10   b.    
     The moving mechanism  30  is disposed on the apparatus base  2 , and includes an X-axis feeding mechanism  31  for processing-feeding the holding unit  20  along the X-axis directions and a Y-axis feeding mechanism  32  for indexing-feeding the holding unit  20  along the Y-axis directions. The X-axis feeding mechanism  31  converts rotary motion from a stepping motor  33  into linear motion via the ball screw  34  and transmits the linear motion to the X-axis movable plate  21  to move the X-axis movable plate  21  in one of the X-axis directions and the other along a pair of guide rails  2   a  on the apparatus base  2 . The Y-axis feeding mechanism  32  converts rotary motion from a stepping motor  35  into linear motion via the ball screw  36  and transmits the linear motion to the Y-axis movable plate  22  to move the Y-axis movable plate  22  in one of the Y-axis directions and the other along a pair of guide rails  21   a  on the X-axis movable plate  21 . Although not illustrated, position detecting means are disposed respectively on the X-axis feeding mechanism  31 , the Y-axis feeding mechanism  32 , and the chuck table  25  for detecting X- and Y-axis coordinates and angular position of the chuck table  25 . Positional information representing the X- and Y-axis coordinates and angular position of the chuck table  25  is sent from the position detecting means to a control unit, not illustrated. On the basis of the supplied positional information, the control unit issues instruction signals for actuating the X-axis feeding mechanism  31 , the Y-axis feeding mechanism  32 , and the rotary actuator means combined with the chuck table  25  to position the chuck table  25  in a desired position over the apparatus base  2 . 
     As illustrated in  FIG. 1 , an upstanding frame body  4  is mounted on the apparatus base  2  alongside of the moving mechanism  30 . The frame body  4  includes a vertical wall  4   a  disposed on the apparatus base  2  and a horizontal beam  4   b  extending from an upper end of the vertical wall  4   a.  The horizontal beam  4   b  houses therein an optical system, not illustrated, including the laser beam applying unit  40 . The optical system also includes a beam condenser  42  disposed on a lower surface of a distal end of the horizontal beam  4   b.    
     The horizontal beam  4   b  also houses therein the detecting mechanism  60  for detecting the inside of the wafer  10  held by the holding unit  20 . The holding unit  20  and the detecting mechanism  60  jointly make up the detecting apparatus according to the present embodiment as described above. 
       FIG. 2  illustrates in block diagram an optical system of the detecting mechanism  60 . As illustrated in  FIG. 2 , the detecting mechanism  60  includes a laser oscillator  61  for oscillating pulsed laser in a wide wavelength range from 400 to 800 nm, for example, wavelength delaying means  62  for outputting each pulse of a pulsed laser beam PL 0  emitted from the laser oscillator  61  with time differences imparted to respective wavelengths as a pulsed laser beam PL 1 , ring-shaped generating means  64  for generating a ring-shaped pulsed laser beam from the pulsed laser beam PL 1  and diffracting the ring-shaped pulsed laser beam into ring-shaped laser beams ranging from small to large ring-shaped laser beams at the respective wavelengths, and generating and outputting a pulsed laser beam PL 2 , a beam splitter  65  having a function to branch a laser beam passing therethrough into appropriate directions, an indexing scanner  67  constructed as a galvanometer scanner, for example, for indexing the pulsed laser beam PL 2  branched into a first direction D 1  by the beam splitter  65  in Y-axis coordinate directions over the chuck table  25  of the holding unit  20 , a scanning scanner  68  constructed as a resonant scanner, for example, for scanning the pulsed laser beam PL 2  in X-axis coordinate directions over the chuck table  25 , and a detecting beam condenser  69  including an fθ lens  691  for converging the pulsed laser beam PL 2  diffracted into the ring-shaped laser beams ranging from the small to large ring-shaped laser beams onto the reverse side  10   b,  facing upwardly, of the wafer  10  held on the chuck table  25  at positions defined by X-axis and Y-axis coordinates thereon. 
     The pulsed laser beam PLO emitted from the laser oscillator  61  is guided through an optical fiber  620  to the wavelength delaying means  62 . The wavelength delaying means  62  may be realized by an optical fiber that causes wavelength dispersion, for example. More specifically, the optical fiber, not illustrated, of the wavelength delaying means  62  may include therein diffraction gratings having different reflecting positions at respective wavelengths, e.g., providing shorter reflection distances for laser beams having longer wavelengths and longer reflection distances for laser beams having shorter wavelengths. The wavelength delaying means  62  can thus generate the pulsed laser beam PL 1  that is emitted from an optical fiber  621  connected to an output side of the wavelength delaying means  62  such that each pulse of the pulsed laser beam PL 1  is output with predetermined respective time differences imparted to a sequence of longer wavelengths, e.g., a red laser beam PL 1   a , a yellow laser beam PL 1   b , a green laser beam PL 1   c , and a blue laser beam PL 1   d  are output with predetermined respective time differences. 
     The pulsed laser beam PL 1  that has been given the time differences imparted to the respective wavelengths by the wavelength delaying means  62  is converted by a collimation lens  63  to a parallel beam, which is then introduced into the ring-shaped generating means  64 . The ring-shaped generating means  64  is arranged as an axicon lens assembly including a pair of axicon lenses  641  and  642  and a doughnut-shaped diffraction grating  643  that is radially symmetrical, for example. When the pulsed laser beam PL 1  travels through the axicon lenses  641  and  642 , the pulsed laser beam PL 1  is converted to a ring-shaped laser beam. When the ring-shaped laser beam travels through the diffraction grating  643 , the ring-shaped laser beam is diffracted into ring-shaped laser beams ranging from small to large ring-shaped laser beams at the respective wavelengths, thereby generating the pulsed laser beam PL 2 . The sizes of the ring-shaped laser beams of the pulsed laser beam PL 2  can be adjusted by adjusting the distance between the axicon lenses  641  and  642 . According to the present embodiment, the axicon lens assembly is used as means for diffracting the pulsed laser beam PL 1  into ring-shaped laser beams ranging from small to large ring-shaped laser beams at the respective wavelengths. According to the present invention, however, the means for diffracting the pulsed laser beam PL 1  is not limited to the axicon lens assembly, but may be a diffractive optical element (DOE), for example. 
     The pulsed laser beam PL 2  that has been diffracted into ring-shaped laser beams ranging from small to large ring-shaped laser beams at the respective wavelengths is introduced into the beam splitter  65 . The pulsed laser beam PL 2  introduced into the beam splitter  65  passes through a reflecting surface  65   a  therein and is guided to travel in the first direction Dl to a reflecting mirror  66 , which changes the optical path of the pulsed laser beam PL 2  to guide the pulsed laser beam PL 2  to the indexing scanner  67  that indexes the pulsed laser beam PL 2  in the Y-axis coordinate directions. The indexing scanner  67  has its reflecting surface  67   a  controlled by the control unit, not illustrated, to control the position on the chuck table  25  to which the pulsed laser beam PL 2  is applied, accurately in indexing feed directions, i.e., the Y-axis directions, perpendicular to the sheet of  FIG. 2 . The pulsed laser beam PL 2  reflected by the reflecting surface  67   a  of the indexing scanner  67  is guided to the scanning scanner  68 . The scanning scanner  68  has its reflecting surface  68   a  controlled by the control unit, not illustrated, to control the position on the chuck table  25  to which the pulsed laser beam PL 2  is applied, accurately in scanning directions, i.e., the X-axis directions, parallel to the sheet of  FIG. 2 . The pulsed laser beam PL 2  that has been controlled by the indexing scanner  67  and the scanning scanner  68  in terms of the directions along which the pulsed laser beam PL 2  travels toward the chuck table  25  is guided to the fθ lens  691  and converged thereby onto predetermined positions defined by X- and Y-axis coordinates on the upper surface, i.e., the reverse side  10   b,  of the wafer  10  on the chuck table  25 . 
     The detecting mechanism  60  according to the present embodiment further includes a laser beam applying device  71  for applying a detecting laser beam LB 0 , the laser beam applying device  71  being disposed in a second direction D 2  along which a laser beam is branched by the beam splitter  65 , a half-silvered mirror  72  disposed between the laser beam applying device  71  and the beam splitter  65 , a returning mirror  73  disposed on a side of the beam splitter  65  remote from the half-silvered mirror  72  such that the half-silvered mirror  72  and the returning mirror  73  are disposed one on each side of the beam splitter  65 , a photodetector  74  for detecting a laser beam returning via the beam splitter  65  to the half-silvered mirror  72  and reflected by the half-silvered mirror  72 , and image generating means, i.e., an analyzer,  75  for generating an image from the intensity of the laser beam detected by the photodetector  74  and information of the positions defined by X- and Y-axis coordinates on the chuck table  25  to which the pulsed laser beam PL 2  is applied. 
     The laser beam applying device  71  is constructed as a laser diode (LD), for example. The detecting laser beam LB 0  emitted from the laser beam applying device  71  passes through the half-silvered mirror  72  into the beam splitter  65 , which divides the detecting laser beam LB 0  into a branched laser beam LB 1  reflected by the reflecting surface  65   a  and traveling in the first direction D 1  and a branched laser beam LB 2  transmitted through the reflecting surface  65   a.  The branched laser beam LB 1  reflected by the reflecting surface  65   a  travels in the center of the pulsed laser beam PL 2 , and is indexed and scanned by the indexing scanner  67  and the scanning scanner  68  to travel through the fθ lens  691  to the predetermined positions defined by X- and Y-axis coordinates on the reverse side  10   b  of the wafer  10  on the chuck table  25 . 
     The branched laser beam LB 1  applied to the reverse side  10   b  of the wafer  10  is reflected thereby as a first return laser beam LB 1 ′, is reflected by the scanning scanner  68 , the indexing scanner  67 , and the reflecting mirror  66 , is then reflected by the reflecting surface  65   a  of the beam splitter  65  to travel in the second direction D 2 , and is applied to the half-silvered mirror  72 . The branched laser beam LB 2  transmitted through the reflecting surface  65   a  of the beam splitter  65  is reflected by the returning mirror  73  and transmitted through the reflecting surface  65   a  as a second return laser beam LB 2 ′. At this time, the second return laser beam LB 2 ′ travels along the same optical path as the first return laser beam LB 1 ′. The first return laser beam LB 1 ′ that is modulated by vibrations of the reverse side  10   b  of the wafer  10  and the second return laser beam LB 2 ′ functioning as a reference laser beam not affected by the wafer  10  jointly make up an interference laser beam, which is reflected by the half-silvered mirror  72  to travel to the photodetector  74 . The image generating means  75  generates an image on the basis the intensity of the laser beam detected by the photodetector  74 , i.e., the interference laser beam that combines the first return laser beam LB 1 ′ and second return laser beam LB 2 ′, and the X- and Y-coordinates representing the positions on the reverse side  10   b  of the wafer  10  to which the detecting laser beam LB 1  is applied, and outputs the generated image to the display unit  50 . 
     The detecting mechanism  60  is generally configured as described above. Functions and operation of the detecting mechanism  60  will be described below also with reference to  FIG. 3 . 
     As illustrated in  FIG. 1 , the wafer  10  prepared as the workpiece is held under suction on the chuck table  25  of the holding unit  20  and secured in position by the clamps  27 . With the wafer  10  secured to the chuck table  25 , the moving mechanism  30  is actuated to move the chuck table  25  to position a predetermined detection area of the wafer  10  directly below the detecting beam condenser  69  that includes the fθ lens  691 . Then, the laser oscillator  61  is energized to oscillate pulsed laser and to generate and output, through the wavelength delaying means  62  and the ring-shaped generating means  64 , the pulsed laser beam PL 2  having time differences imparted to respective wavelengths and diffracted into a plurality of ring-shaped laser beams ranging from small to large ring-shaped laser beams at the respective wavelengths. The pulsed laser beam PL 2  is transmitted through and branched by the beam splitter  65  to travel in the first direction D 1 , and then applied to positions defined by X- and Y-axis coordinates in the detection area on the reverse side  10   b  of the wafer  10  held on the chuck table  25  through the indexing scanner  67  and the scanning scanner  68  that are controlled by the control unit, not illustrated, and also through the fθ lens  691 . 
     According to the present embodiment, as illustrated in  FIG. 3 , the pulsed laser beam PL 2  includes a ring-shaped laser beam PL 2   a  produced from a red laser beam PL 1   a , a ring-shaped laser beam PL 2   b  produced from a yellow laser beam PL 1   b , a ring-shaped laser beam PL 2   c  produced from a green laser beam PL 1   c , and a ring-shaped laser beam PL 2   d  produced from a blue laser beam PL 1   d , arranged successively in the order of descending diameters. The ring-shaped laser beams PL 2   a,  PL 2   b,  PL 2   c , and PL 2   d  are applied concentrically around a center C on the reverse side  10   b  of the wafer  10 . The ring-shaped laser beam PL 2   a  that is of the largest diameter reaches the reverse side  10   b  of the wafer  10  at the earliest timing, and then the ring-shaped laser beams PL 2   b,  PL 2   c , PL 2   d  that are successively smaller in diameter reach the reverse side  10   b  of the wafer  10  with respective time differences t 1 , t 2 , and t 3 . According to the present embodiment, the pulsed laser beam PL 2  is diffracted into four wavelength ranges for illustrative purposes. Actually, however, the pulsed laser beam PL 2  is diffracted into a range of 10 to 20 wavelength ranges. 
     According to the present embodiment, the reverse side  10   b  of the wafer  10  is overlaid with the metal film, and hence the pulsed laser beam PL 2  including the ring-shaped laser beams PL 2   a  through PL 2   d  are not transmitted through the wafer  10 . However, when the ring-shaped laser beams PL 2   a  through PL 2   d  reach the reverse side  10   b,  they generate ultrasonic waves that are propagated through the wafer  10  from the points where the ring-shaped laser beams PL 2   a  through PL 2   d  reach the reverse side  10   b.  By appropriately establishing the time differences t 1  through t 3  with which the ring-shaped laser beams PL 2   a  through PL 2   d  reach the reverse side  10   b  with respect to the ring-shaped laser beam PL 2   a,  it is possible to converge an interference wave of the ultrasonic waves at a position P defined by a desired Z-axis coordinate in the thicknesswise directions of the wafer  10  on the center of the ring-shaped laser beams PL 2   a  through PL 2   d  applied to the reverse side  10   b.  According to the present embodiment, the position P is established in the vicinity of the face side  10   a  of the wafer  10  in order to detect a state near the face side  10   a.    
     The time differences t 1  through t 3  are appropriately established as follows: The diameters of the ring-shaped laser beams PL 2   a  through PL 2   d  applied to the reverse side  10   b  of the wafer  10  are of values established by the diffraction grating  643  of the ring-shaped generating means  64 . For example, the diameters of the ring-shaped laser beams PL 2   a  through PL 2   d  are established respectively as a 1  through a 4  as illustrated in  FIG. 3 . If the Z-axis coordinate, i.e., the depth, of the position P where the operator wishes to converge the interference wave of the ultrasonic waves generated by the ring-shaped laser beams PL 2   a  through PL 2   d,  from the center C of the ring-shaped laser beams PL 2   a  through PL 2   d  in the thicknesswise directions of the wafer  10  is represented by Pz, then distances Hl through H 4  from the points where the ring-shaped laser beams PL 2   a  through PL 2   d  reach the reverse side  10   b  of the wafer  10  to the position P are calculated according to the following equations: 
         H 1=( a 1 2   +Pz   2 ) 1/2    
         H 2=( a 2 2   +pz   2 ) 1/2    
         H 3=( a 3 2   +pz   2 ) 1/2    
         H 4=( a 4 2   +pz   2 ) 1/2    
     When the ring-shaped laser beams PL 2   a  through PL 2   d  reach the reverse side  10   b  of the wafer  10  with the time differences t 1  through t 3  and generate ultrasonic waves that are propagated through the wafer  10 , the time differences t 1  through t 3  should be established to satisfy the equations illustrated below in order to converge the interference wave of the ultrasonic waves at the position P. In the equations, V indicates the speed (m/s) at which the ultrasonic waves are propagated through the wafer  10 , the speed depending on the material of the wafer  10 . 
       ( H 1 −H 2)/ V=t 1 
       ( H 2 −H 3)/ V=t 2 
       ( H 3 −H 4)/ V=t 3 
     The time differences t 1  through t 3  can be adjusted by the wavelength delaying means  62 . Specifically, the positions of the diffraction gratings, not illustrated, disposed correspondingly to the respective wavelengths of the pulsed laser beam PL 0  in the optical fiber of the wavelength delaying means  62  may be changed to provide the above time differences t 1  through t 3 . 
     When the ring-shaped laser beams PL 2   a  through PL 2   d  are applied with the time differences t 1  through t 3  satisfying the above conditions to the reverse side  10   b  of the wafer  10 , the ring-shaped laser beams PL 2   a  through PL 2   d  generate ultrasonic waves that are propagated through the wafer  10 . The interference wave of the ultrasonic waves are converged at the position P, producing intensive vibrations in the wafer  10 . Part of the vibrations is reflected in the vicinity of the position P and propagated through the wafer  10  to a point on the upper surface of the wafer  10  aligned with the position P, i.e., the center C of the right laser beams PL 2   a  through PL 2   d  on the reverse side  10   b  of the wafer  10 , vibrating the reverse side  10   b.  The vibrations of the reverse side  10   b  depend on the state in the vicinity of the position P where the ultrasonic waves are converged. 
     According to the present embodiment, the laser beam applying device  71  applies the detecting laser beam LB 0  to the beam splitter  65 , which divides the detecting laser beam LB 0  into the branched laser beam LB 1  that is applied to the reverse side  10   b  at the center C of the ring-shaped laser beams PL 2   a  through PL 2   d.  When the branched laser beam LB 1  reaches the center C and is reflected from the reverse side  10   b,  it is reflected as the first return laser beam LB 1 ′ that is modulated by the vibrations of the reverse side  10   b.  The reflected and modulated first return laser beam LB 1 ′ is reflected by the scanning scanner  68 , the indexing scanner  67 , the reflecting mirror  66 , and the reflecting surface  65   a  of the beam splitter  65 , and reaches the half-silvered mirror  72 . At the same time, the branched laser beam LB 2  of the detecting laser beam LB 0  emitted from the laser beam applying device  71  and transmitted through the beam splitter  65  is reflected by the returning mirror  73  as the second return laser beam LB 2 ′, which is combined with the first return laser beam LB 1 ′ by the reflecting surface  65   a  of the beam splitter  65  and reaches the half-silvered mirror  72 . The first return laser beam LB 1 ′ that is reflected by the half-silvered mirror  72  and the second return laser beam LB 2 ′ functioning as the reference laser beam not affected by the wafer  10  jointly produce the interference laser beam whose intensity is detected by the photodetector  74 . The detected intensity and the X- and Y-axis coordinates of the center C on the reverse side  10   b  of the wafer  10  are transmitted to the image generating means  75 . 
     According to the present embodiment, the detecting mechanism  60  includes the scanning scanner  68  and the indexing scanner  76 . The scanning scanner  68  and the indexing scanner  76  are actuated to apply the ring-shaped laser beams PL 2   a  through PL 2   d  and the branched laser beam LB 1  successively to positions in the entire predetermined detection area defined by X- and Y-axis coordinates on the wafer  10 , and the photodetector  74  detects the intensities of the pulsed laser beam PL 2  and the branched laser beam LB 1  each time the ring-shaped laser beams PL 2   a  through PL 2   d  and the branched laser beam LB 1  are applied to one of the positions and transmits the detected intensities, together with information of the pulsed laser beam PL 2  and the position to which the branched laser beam LB 1  is applied, i.e., the positional information of the X- and Y-axis coordinates of the center C of the ring-shaped laser beams PL 2   a  through PL 2   d,  to the image generating means  75 . 
     The image generating means  75  generates an image of the face side  10   a  at the position where the branched laser beam LB 1  of the detecting laser beam LB 0  is applied to the detection area, on the basis of changes in the intensity of the first return laser beam LB 1 ′ which are clearly grasped from the interference wave generated from the first return laser beam LB 1 ′ and the second return laser beam LB 2 ′. For example, in a case where there are devices  12  near the position P where the ultrasonic waves are converged in the wafer  10 , the converged ultrasonic waves are reflected by the devices  12  and vibrations thereof reach the center C of the reverse side  10   b,  so that the photodetector  74  detects a strong interference wave. On the other hand, in a case where the position P is located near projected dicing lines  14 , the ultrasonic waves are essentially not reflected and no vibrations are applied to the reverse side  10   b  of the wafer  10 , so that the photodetector  74  detects essentially no interference wave. 
     Image information including the positional information of the devices  12  and the projected dicing lines  14  on the face side  10   a  in the detection area is displayed on the display unit  50  as illustrated in  FIG. 2 . The image information in the detection area displayed on the display unit  50  is stored, together with the information of the corresponding X- and Y-axis coordinates, in the control unit, not illustrated. The moving mechanism  30  is actuated to move the chuck table  25  to position different areas of the wafer  10  successively in the detection area detected by the detecting mechanism  60 , and the devices  12  and the projected dicing lines  14  on the face side  10   a  are detected and stored according to the sequence described above. When the state of the face side  10   a  of the wafer  10  has been detected, the chuck table  25  is positioned directly below the beam condenser  42  of the laser beam applying unit  40 , and the laser beam applying unit  40  is energized to process the wafer  10  using the positional information. 
     According to the present embodiment, even though no infrared rays are transmittable through the wafer  10  because of the metal film on the reverse side  10   b  thereof and the projected dicing lines  14  on the face side  10   a  cannot be detected from the reverse side  10   b  by an infrared camera, the projected dicing lines  14  on the face side  10   a  can nevertheless be detected from the reverse side  10   b  by the detecting mechanism  60 . 
     The present invention is not limited to the embodiment described above, but covers changes and modifications therein. For example, while the pulsed laser beam PL 0  is diffracted into four ring-shaped laser beams PL 2   a  through PL 2   d  with time differences imparted to respective four wavelengths. However, the present invention is not limited to such details, but the pulsed laser beam PL 0  may be diffracted into a plurality of ring-shaped laser beams with no limitations posed on the number of ring-shaped laser beams. 
     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.