Patent Publication Number: US-2022236049-A1

Title: Measuring apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a measuring apparatus having a holding unit for holding a measurand, i.e., an object to be measured, and a measuring unit for measuring a height of an upper surface of the measurand held by the holding unit or a thickness of the measurand. 
     Description of the Related Art 
     Semiconductor wafers with devices formed on their face side have their reverse side ground by a grinding apparatus so that they are thinned down to a predetermined thickness. Then, each of the semiconductor wafers is divided into individual device chips having the respective devices by a processing apparatus such as a cutting apparatus or a laser processing apparatus. For processing such a semiconductor wafer as a workpiece on a laser processing apparatus, it is necessary to position a focused spot of a pulsed laser beam having a wavelength transmittable through the workpiece at a predetermined depth in the workpiece from the upper surface of the workpiece. For thus positioning the focused spot of the pulsed laser beam, a height of the upper surface of the workpiece is measured by a measuring apparatus (see, for example, Japanese Patent Laid-Open No. 2008-170366 and Japanese Patent Laid-Open No. 2011-122894). 
     The measuring apparatus has a light source for emitting light in a predetermined wavelength band. Light emitted from the light source is guided through an optical fiber to a head that includes a lens having an axial chromatic aberration. The light in the predetermined wavelength band is diffracted by the lens due to the axial chromatic aberration into light rays that are converged at different positions on an optical axis depending on the wavelengths of the light rays. Of the light rays applied from the head to the measurand, the light ray focused on the upper surface of the measurand and having a predetermined wavelength is reflected by the measurand and mainly introduced through the lens into an optical fiber. The reflected light ray is applied through the optical fiber to a spectroscope or the like, which specifies the wavelength of the light ray. 
     Since the lens has predetermined focal lengths for the respective wavelengths, a distance from the head to the upper surface of the measurand can be determined on the basis of the specified wavelength of the reflected light ray. Accordingly, the measuring apparatus can measure the height of the upper surface of the measurand and the thickness of the measurand (see, for example, PCT Patent Publication No. WO 2015/199054). The measuring apparatus disclosed in PCT Patent Publication No. WO 2015/199054 has a light source, a spectroscope, a head, and a 1×2 fiber optic coupler. The light source is connected to a first port positioned on one side of a branching member of the coupler, and the spectroscope is connected to a second port positioned on the same one side. 
     Moreover, the head is connected to a third port positioned on the other side of the branching member of the coupler. With this configuration, as the head can only measure one point on the measurand in one measuring session, the measuring process is inefficient. In view of the difficulty, there has been proposed a technology in which an optical fiber including a 2×2 fiber optic coupler is used to connect a light source, a spectroscope, and two heads for measuring two points on the measurand. However, the proposed technology is problematic in that since reflected light rays received by the respective heads are simultaneously detected by a light detector via the spectroscope, the light detector is unable to establish a correspondence between the reflected light rays from the heads and detected light signals. 
     To solve the above problem, there has been proposed a measuring apparatus that measures different wavelength bands with respective heads by providing filters that pass respective light rays in the different wavelength bands for the respective heads (see, for example, Japanese Patent Laid-Open No. 2014-178287). However, as the number of divided wavelength bands increases due to an increase in the number of heads, the measurement wavelength band per head, i.e., the measurement range, is reduced. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above drawback. It is an object of the present invention to provide a measuring apparatus for measuring the height of the upper surface of a measurand, etc. without reducing a measurement wavelength band per head and at two or more points thereon using two or more heads. 
     In accordance with an aspect of the present invention, there is provided a measuring apparatus including a holding unit for holding a measurand and a measuring unit for measuring a height of an upper surface of the measurand held by the holding unit or a thickness of the measurand held by the holding unit. The measuring unit includes a light source for emitting light in a predetermined wavelength band, a first optical fiber for transmitting the light emitted from the light source, a branching member for branching the light transmitted by the first optical fiber to at least two measuring optical fibers, a head unit having a plurality of heads including respective beam condensers for converging the light branched by the branching member onto the measurand, a shutter device for shifting the timings of application of the light from the heads to the measurand, a second optical fiber branched from the branching member, for transmitting returning light reflected from the measurand and transmitted through the measuring optical fibers, a spectroscopic unit having a light detector for detecting the returning light transmitted through the second optical fiber, and a controller for controlling operation of the shutter device to control the timings of application of the light from the heads to the measurand and controlling the light detector to detect the returning light from the heads individually. 
     Preferably, the light detector includes a two-dimensional sensor. 
     Preferably, the measuring apparatus further includes an additional branching member disposed between the first optical fiber and the branching member, a second branching member for branching light from the additional branching member to at least two different measuring optical fibers, a second head unit having a plurality of second heads including respective beam condensers for converging the light branched by the second branching member onto the measurand, a second shutter device for shifting timings of application of the light from the second heads to the measurand, a fourth optical fiber branched from the second branching member differently from a third optical fiber that interconnects the additional branching member and the second branching member, for transmitting the returning light reflected by the measurand and transmitted through the measuring optical fibers, and a second spectroscopic unit having a second light detector for detecting the returning light transmitted through the fourth optical fiber. The controller controls operation of the second shutter device to control the timings of application of the light from the second heads to the measurand and controlling the second light detector to detect the returning light from the second heads individually. 
     Preferably, the measuring apparatus further includes an additional branching member disposed between the first optical fiber and the branching member, a second branching member for branching light from the additional branching member to at least two different measuring optical fibers, a second head unit having a plurality of second heads including respective beam condensers for converging the light branched by the second branching member onto the measurand, a second shutter device controllable in operation by the controller, for shifting timings of application of the light from the second heads to the measurand, and a fourth optical fiber branched from the second branching member with respect to a third optical fiber that interconnects the additional branching member and the second branching member, for transmitting the returning light reflected by the measurand and transmitted through the measuring optical fibers. The returning light transmitted through the second optical fiber and the fourth optical fiber is detected by the light detector, the light detector has a two-dimensional sensor, and the two-dimensional sensor detects the returning light transmitted through the second optical fiber and the returning light transmitted through the fourth optical fiber at different positions, thereby separating the returning light transmitted through the second optical fiber and the returning light transmitted through the fourth optical fiber from each other. 
     In accordance with another aspect of the present invention, there is provided a measuring apparatus including a holding unit for holding a measurand and a measuring unit for measuring a height of an upper surface of the measurand held by the holding unit or a thickness of the measurand held by the holding unit. The measuring unit includes a light source for emitting light in a predetermined wavelength band, a first optical fiber for transmitting the light emitted from the light source, a light-source-side branching member for branching the light transmitted by the first optical fiber to at least two rays of light, a plurality of couplers for transmitting rays of light branched from the light-source-side branching member to respective measuring optical fibers, a head unit having a plurality of heads including respective beam condensers for converging the rays of light transmitted through the measuring optical fibers onto the measurand, a plurality of spectroscopic-unit-side optical fibers branched from the couplers, for transmitting returning light reflected from the measurand and transmitted through the measuring optical fibers, and a plurality of spectroscopic units associated respectively with the spectroscopic-unit-side optical fibers and having respective light detectors for detecting the returning light transmitted through the spectroscopic-unit-side optical fibers, respectively. 
     Preferably, each of the couplers includes an optical circulator. 
     The measuring apparatus according to the aspect of the present invention includes the shutter device that shifts the timings of application of the light from the heads to the measurand. The measuring apparatus also includes the controller that controls operation of the shutter device. The shutter device operates to allow the light detector to detect individually the light applied from the heads to the measurand and reflected from the measurand. The height of an upper surface of the measurand, etc. can thus be measured without reducing a measurement wavelength band per head and at multiple points, i.e., two or more points, on the upper surface of the measurand using the two or more heads. 
     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 some preferred embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a measuring apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view of a spectroscopic unit of the measuring apparatus; 
         FIG. 3  is a diagram illustrating detected light signals by way of example; 
         FIG. 4  is a schematic view illustrating a modified shutter device; 
         FIG. 5  is a schematic view of a measuring apparatus according to a second embodiment of the present invention; 
         FIG. 6  is a schematic view of a measuring apparatus according to a third embodiment of the present invention; 
         FIG. 7  is a schematic view of a measuring apparatus according to a fourth embodiment of the present invention; and 
         FIG. 8  is a schematic view of a measuring apparatus according to a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinbelow with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  schematically illustrates a measuring apparatus  2  according to a first embodiment of the present invention. In  FIG. 1 , some of components of the measuring apparatus  2  are illustrated in functional block form. Z-axis directions indicated in  FIG. 1  represent vertical directions, and X-axis directions and Y-axis directions, not illustrated, extend perpendicularly to the Z-axis directions and represent horizontal directions. The measuring apparatus  2  is incorporated in a processing apparatus, such as a grinding apparatus or a polishing apparatus, where it is necessary to measure a height of an upper surface  11   a  of a measurand  11  or a thickness  11   c  of the measurand  11 , i.e., an object to be measured, for example. However, the measuring apparatus  2  may not necessarily be required to be ancillary to the processing apparatus but may be independent of the processing apparatus. 
     The measuring apparatus  2  according to the present embodiment has a chuck table  4 , i.e., a holding unit. The chuck table  4  has a lower portion coupled to a rotary actuator, not illustrated, such as an electric motor for rotating the chuck table  4  about its own central axis in a predetermined direction. The chuck table  4  has a disk-shaped frame body having a fluid channel, not illustrated, defined in a lower portion thereof for applying therethrough a negative pressure generated by a suction source, not illustrated, such as an ejector to a disk-shaped porous plate, not illustrated. 
     The porous plate, not illustrated, is securely fitted in a disk-shaped recess defined in an upper portion of the frame body. The upper surface of the porous plate and the upper surface of the frame body lie substantially flush with each other, jointly making up a substantially flat holding surface  4   a.  The holding surface  4   a  holds under suction thereon the measurand  11 , i.e., a workpiece, to be processed by the processing apparatus. The measurand  11  includes a silicon wafer, for example, and has a lower surface  11   b  held under suction on the holding surface  4   a  with a dicing tape, not illustrated, of resin interposed therebetween. The measurand has an upper surface  11   a  opposite the lower surface  11   b.  When the measurand is held on the holding surface  4   a,  the upper surface  11   a  is exposed upwardly. The upper surface  11   a  is irradiated with measuring light  13  from each of heads  18   a  and  18   b  of a measuring unit  6  to be described in detail later. 
     The measuring unit  6  has a light source  8 . According to the present embodiment, the light source  8  includes a stimulating light source, not illustrated. The stimulating light source includes, for example, a laser diode (LD) for emitting stimulating light that includes blue light having a wavelength λ=450 nm, for example. The stimulating light is applied through an optical system including a lens, a prism, etc. to a phosphor. The phosphor includes a YAG:Ce phosphor, e.g., (Y,Gd) 3 Al 5 O 12 :Ce, for example. The phosphor absorbs the simulating light and emits light in a predetermined wavelength band (for example, 500 nm to 700 nm) that includes light in green, yellow, red, and so on. The light emitted from the phosphor travels through a condensing lens, not illustrated, of the light source  8  and is converged onto an end of a first optical fiber  12  that acts as a first port of a 2×2 fiber optic coupler  10 , from which the light is transmitted through the first optical fiber  12  to a branching member  14 . 
     The fiber optic coupler  10  includes two optical fibers having respective portions fused and stretched, for example. Two measuring optical fibers  16   a  and  16   b  that act as third and fourth ports, respectively, of the fiber optic coupler  10  are connected to a side of the branching member  14  that is opposite the first optical fiber  12 . The light in the predetermined wavelength band from the light source  8  is branched from the branching member  14  into the measuring optical fibers  16   a  and  16   b  at a ratio of 50 to 50, for example. 
     The measuring optical fiber  16   a  is connected to the head  18   a.  The head  18   a  includes a beam condenser  20   a  therein. According to the present embodiment, the beam condenser  20   a  includes a diffractive optical element and a condensing lens. Because of an axial chromatic aberration of the beam condenser  20   a,  a vertical position of a converged spot of the light that has passed through the beam condenser  20   a  varies depending on its wavelength. Specifically, the longer the wavelength of the light is, the closer the converged spot of the light is positioned to the beam condenser  20   a,  and the shorter the wavelength of the light is, the farther the converged spot of the light is positioned from the beam condenser  20   a.  Note that the beam condenser  20   a  may have a chromatic aberration lens instead of the diffractive optical element and the condensing lens. According to such an alternative, the longer the wavelength of the light is, the farther the converged spot of the light is positioned from the beam condenser  20   a,  and the shorter the wavelength of the light is, the closer the converged spot of the light is positioned to the beam condenser  20   a.    
     Part of the measuring light  13  applied from the beam condenser  20   a  to the measurand  11  is reflected by the upper surface  11   a  thereof and travels as returning light  15  back through the beam condenser  20   a  to the measuring optical fiber  16   a.  However, while the wavelength component that has been converged on the upper surface  11   a  mainly passes into the measuring optical fiber  16   a,  those wavelength components that have not been converged on the upper surface  11   a  are essentially blocked by the measuring optical fiber  16   a.  The head  18   a  and the measuring optical fiber  16   a  thus provide a confocal optical system. Incidentally, insofar as the head  18   a  provides a confocal optical system, the head  18   a  is not limited to the configuration illustrated in  FIG. 1  but may be of other configurations. For example, an additional condensing lens may be positioned between the beam condenser  20   a  and the measuring optical fiber  16   a.    
     The measuring optical fiber  16   b  is connected to the head  18   b.  Note that the head  18   a  and the head  18   b  jointly make up a first head unit  18 . The head  18   b  also includes a beam condenser  20   b,  and the head  18   b  and the measuring optical fiber  16   b  provide a confocal optical system. Part of the measuring light  13  applied from the beam condenser  20   b  to the measurand  11  is also reflected by the upper surface  11   a  thereof and travels as returning light  15  back through the beam condenser  20   b  to the measuring optical fiber  16   b.    
     The returning light  15  reflected by the upper surface  11   a  and passing through the measuring optical fiber  16   a  or  16   b  is transmitted via the branching member  14  to a second optical fiber  22  that acts as a second port of the 2×2 fiber optic coupler  10 . A spectroscopic unit  24  is connected to an end of the second optical fiber  22 . The spectroscopic unit  24  has a collimator lens  26  (see  FIG. 2 ) that turns the returning light  15  into substantially parallel-ray light. 
       FIG. 2  schematically illustrates the spectroscopic unit  24 . As illustrated in  FIG. 2 , the returning light  15  that has passed through the collimator lens  26  is reflected by a diffraction grating  28  and detected by a light detector  32  via a condensing lens  30 . The light detector  32  includes a complementary metal-oxide-semiconductor (CMOS) or a charge-coupled-device (CCD) image sensor, i.e., a two-dimensional sensor, having a two-dimensional matrix of light detecting elements. 
     The light detector  32  includes a rectangular light detecting area  32   a  having two sides that are substantially parallel to a first direction A 1  and two sides that are substantially parallel to a second direction A 2  extending perpendicularly to the first direction A 1 . The returning light  15  is detected by the light detector  32  at different positions in the second direction A 2  on the light detecting area  32   a  depending on the wavelength thereof. A detected light signal generated by the light detecting area  32   a  is input to a controller  34  that controls the measuring apparatus  2  (see  FIGS. 1 and 2 ). The controller  34  specifies a position of the light detecting element where light intensity in the second direction A 2  is maximum, thereby specifying a peak wavelength of the returning light  15 . Since the second direction A 2  and the wavelength (λ) are associated with each other, the second direction A 2  is denoted by A 2 (λ) in  FIG. 2 . 
     The controller  34  includes, for example, a computer including a processor such as a central processing unit (CPU), a main storage device such as a dynamic random access memory (DRAM), and an auxiliary storage device such as a memory or a hard disk drive. The controller  34  has its functions performed by operating the processor, etc. according to software or programs stored in the auxiliary storage device. The controller  34  controls operation of a shutter device  36  in addition to the light source  8  and the spectroscopic unit  24 . The shutter device  36  has a shutter  36   a  disposed between the head  18   a  and the holding surface  4   a  and a shutter  36   b  disposed between the head  18   b  and the holding surface  4   b.    
     According to the present embodiment, each of the shutters  36   a  and  36   b  includes a focal plane shutter that is slidable along the X-axis directions. However, each of the shutters  36   a  and  36   b  may be slidable along predetermined directions other than the X-axis directions in an X-Y plane that is defined by the X-axis directions and the Y-axis directions. Note that each of the shutters  36   a  and  36   b  may include a mechanical shutter such as a lens shutter or a shutter incorporating a transmissive liquid crystal that selectively transmits and blocks light when electrically driven. 
     The controller  34  controls the shutters  36   a  and  36   b  such that when one of the shutters  36   a  and  36   b  is open, the other of the shutters  36   a  and  36   b  is closed, thereby shifting timings of the application of the measuring light  13  from the heads  18   a  and  18   b  to the upper surface  11   a  of the measurand  11 . The returning light  15  from the heads  18   a  and  18   b  is thus detected by the light detector  32  individually, i.e., at different timings. 
       FIG. 3  illustrates detected light times by way of example. In  FIG. 3 , a horizontal axis represents the wavelength λ(nm) and a vertical axis represents intensity of the detected light signals. A broken-line curve indicates a detected light signal having a peak wavelength λ A  representing returning light  15  from the head  18   a  that is detected by the light detector  32  at time t 1 . A solid-line curve indicates a detected light signal having a peak wavelength λ B  representing returning light  15  from the head  18   b  that is detected by the light detector  32  at time t 2  different from the time t 1 . 
     An associated relation between the peak wavelengths of returning light  15  and distances from the heads  18   a  and  18   b  to the converged spots is pre-registered in the controller  34 . Therefore, the distances from the heads  18   a  and  18   b  to the upper surface  11   a  are determined depending on the peak wavelengths of the returning light  15 . According to the present embodiment, the returning light  15  is detected at different timings depending on the heads  18   a  and  18   b.  Consequently, the height of the upper surface  11   a,  etc. can be measured without reducing a measurement wavelength band per head  18   a  or  18   b  and at two points using the two heads  18   a  and  18   b.  Note that the height of the upper surface  11   a  means a relative height with respect to a predetermined reference height. The predetermined reference height refers to the height of the holding surface  4   a,  for example. The thickness  11   c  (see  FIG. 1 ) of the measurand  11  can be measured when the controller  34  calculates the difference between the height of the holding surface  4   a  and the height of the upper surface  11   a.    
     Incidentally, the controller  34  stores peak wavelengths of the returning light  15  that correspond to respective measurement timings. For example, while alternately opening and closing the shutters  36   a  and  36   b,  the controller  34  stores peak wavelengths of the returning light  15  that correspond to respective timings of the opening of the shutters  36   a  and  36   b.  The height of the upper surface  11   a  is measured on the basis of the stored peak wavelengths. According to an example, the heads  18   a  and  18   b  are positioned in an array on a radius established on the upper surface  11   a  of the measurand  11  held on the holding surface  4   a.  While the shutters  36   a  and  36   b  are being alternately opened and closed, the chuck table  4  is rotated about its own central axis by the rotary actuator. In this manner, the height of the upper surface  11   a  can be measured in a concentric area on the upper surface  11   a.    
     A display monitor, not illustrated, may be electrically connected to the controller  34 . The display monitor displays an outline of the upper surface  11   a  and pieces of information representing the heights of measured points on the upper surface  11   a.  Providing the display monitor displays color-coded pieces of information representing the heights of measured points, the operator can visually recognize surface irregularities of the upper surface  11   a  by seeing the displayed color-coded pieces of information on the display monitor. 
     (First Modification) 
     For the purpose of multi-point measurement, three or more measuring optical fibers may be branched from the branching member  14 . For example, three measuring optical fibers are branched from the branching member  14  and connected to respective heads. In this case, the controller  34  controls the shutter device  36  to apply measuring light  13  selectively from the three heads successively one at a time to the upper surface  11   a  of the measurand  11 . 
     (Second Modification) 
     A second modification will be described below with reference to  FIG. 4 .  FIG. 4  illustrates a modified shutter device  36  according to the second modification. According to the second modification, the shutter device  36  includes a single polishing unit  40 . The polishing unit  40  has a spindle housing  42  in the form of a substantially hollow cylinder whose axis extends along the Z-axis directions. An X-axis moving mechanism, not illustrated, for moving the polishing unit  40  along the X-axis directions is coupled to the spindle housing  42 . 
     A cylindrical spindle  44  has a portion rotatably housed in the spindle housing  42 . A rotary actuator, not illustrated, such as an electric motor, is connected to an upper end of the spindle  44  in the spindle housing  42 . The spindle  44  has a lower end coupled to a disk-shaped polishing tool  46 . The polishing tool  46  is smaller in diameter than the holding surface  4   a.  For example, the diameter of the polishing tool  46  ranges from 1/10 to approximately ½ of the diameter of the holding surface  4   a.  The polishing tool  46  has a disk-shaped mount  46   a  made of metal. The mount  46   a  has a lower surface to which a polishing pad  46   b  having substantially the same diameter as the mount  46   a  is fixed. 
     The X-axis moving mechanism moves the polishing unit  40  along the X-axis directions selectively to a first position between the upper surface  11   a  and the head  18   a  and a second position between the upper surface  11   a  and the head  18   b,  thereby selectively blocking the measuring light  13  from the heads  18   a  and  18   b.  Therefore, the light detector  32  can detect the returning light  15  at different timings depending on the heads  18   a  and  18   b,  as with the first embodiment. In addition, with the heads  18   a  and  18   b  positioned in an array on a radius established on the upper surface  11   a,  while the polishing unit  40  is selectively blocking the measuring light  13  from the heads  18   a  and  18   b,  the chuck table  4  is rotated about its own central axis by the rotary actuator. In this manner, the height of the upper surface  11   a  can be measured in a concentric area on the upper surface  11   a.    
     (Third Modification) 
     According to a third modification, the light source  8 , the head  18   a,  and the spectroscopic unit  24  may be interconnected by a first three-port optical circulator, not illustrated, and the light source  8 , the head  18   b,  and the spectroscopic unit  24  may be interconnected by a second three-port optical circulator, not illustrated. In this case, the first and second three-port optical circulators correspond to the branching member  14 . An optical fiber interconnecting a first port of the first three-port optical circulator and the light source  8  and an optical fiber interconnecting a first port of the second three-port optical circulator and the light source  8  correspond to the first optical fiber  12 . Moreover, an optical fiber interconnecting a third port of the first three-port optical circulator and the spectroscopic unit  24  and an optical fiber interconnecting a third port of the second three-port optical circulator and the spectroscopic unit  24  correspond to the second optical fiber  22 . 
     (Fourth Modification) 
     According to a fourth modification, a first port of a 1×2 fiber optic coupler, not illustrated, may be connected to the light source  8 , a first port of a first three-port optical circulator, not illustrated, may be connected to a second port of the 1×2 fiber optic coupler, and a first port of a second three-port optical circulator, not illustrated, may be connected to a third port of the 1×2 fiber optic coupler. The head  18   a  may be connected to a second port of the first three-port optical circulator, and the spectroscopic unit  24  may be connected to a third port of the first three-port optical circulator. Furthermore, the head  18   b  may be connected to a second port of the second three-port optical circulator, and the spectroscopic unit  24  may be connected to a third port of the second three-port optical circulator. 
     In this case, the first port of the 1×2 fiber optic coupler corresponds to one end of the first optical fiber  12 , and the third port of the first three-port optical circulator and the third port of the second three-port optical circulator correspond to one end of the second optical fiber  22 . According to the third and fourth modifications, the shutter device  36  and the polishing unit  40  are used to shift the timings of the application of the measuring light  13  from the heads  18   a  and  18   b  to the upper surface  11   a,  so that the returning light  15  from the heads  18   a  and  18   b  can be detected by the light detector  32  at different timings. 
     Second Embodiment 
     A second embodiment of the present invention will be described below with reference to  FIG. 5 .  FIG. 5  schematically illustrates a measuring apparatus  50  according to the second embodiment. The measuring apparatus  50  is mainly different from the measuring apparatus  2  according to the first embodiment in that it has a plurality of spectroscopic units  24 , i.e., a first spectroscopic unit  24   a  and a second spectroscopic unit  24   b.  A 1×2 fiber optic coupler having an additional branching member  52   a  is connected between the light source  8  and the branching member  14 . A first port of the 1×2 fiber optic coupler corresponds to one end of the first optical fiber  12  that transmits light from the light source  8 . 
     The 1×2 fiber optic coupler has a second port and a third port that are connected to a side of the additional branching member  52   a  that is opposite the first optical fiber  12 . The second port is connected to the branching member  14 , and the third port is connected to a second branching member  52   b.  Therefore, light emitted from the light source  8  is branched from the additional branching member  52   a  to the branching member  14  and is also branched from the additional branching member  52   a  to the second branching member  52   b  via a third optical fiber  54 . 
     As with the first embodiment, two measuring optical fibers  16   a  and  16   b  are connected to the branching member  14 . The head  18   a  is connected to the measuring optical fiber  16   a,  and the head  18   b  is connected to the measuring optical fiber  16   b.  The shutter device  36  is disposed between the heads  18   a  and  18   b  and the holding surface  4   a.  Returning light  15  from the heads  18   a  and  18   b  is detected by the light detector  32  of the first spectroscopic unit  24   a,  which is of the same structure as the spectroscopic unit  24 , via the branching member  14 . 
     Similarly, two measuring optical fibers  56   a  and  56   b  are connected to the second branching member  52   b.  A head, i.e., a second head,  58   a  having a beam condenser  60   a  similar to the beam condenser  20   a  is connected to the measuring optical fiber  56   a.  Furthermore, a head, i.e., a second head,  58   b  having a beam condenser  60   b  similar to the beam condenser  20   b  is connected to the measuring optical fiber  56   b.  Incidentally, in the present invention, the head  58   a  and the head  58   b  jointly make up a second head unit  58 . 
     The heads  58   a  and  58   b  have the same function as the heads  18   a  and  18   b.  Furthermore, a shutter  62   a  is disposed between the head  58   a  and the holding surface  4   a,  and a shutter  62   b  is disposed between the head  58   b  and the holding surface  4   a.  As with the shutter device  36 , the shutters  62   a  and  62   b,  which jointly make up a second shutter device  62 , are controlled by the controller  34 . The shutters  36   a,    36   b,    62   a,  and  62   b  are slidable along the Y-axis directions, though they may be slidable along predetermined directions other than the Y-axis directions in the X-Y plane. 
     The second shutter device  62  shifts the timings of the application of the measuring light  13  from the heads  58   a  and  58   b  to the upper surface  11   a  of the measurand  11 . The returning light  15  reflected by the upper surface  11   a  is transmitted successively through the heads  58   a  and  58   b,  the measuring optical fibers  56   a  and  56   b,  and the second branching member  52   b  to a fourth optical fiber  64 . The fourth optical fiber  64  is branched from the second branching member  52   b  differently from the third optical fiber  54 . The returning light  15  is transmitted via the fourth optical fiber  64  to the second spectroscopic unit  24   b.    
     As with the first spectroscopic unit  24   a  (see  FIG. 2 ), the second spectroscopic unit  24   b  has a collimator lens  26 , a diffraction grating  28 , a condensing lens  30 , and a light detector, i.e., a second light detector,  32 . According to the second embodiment, the first spectroscopic unit  24   a  detects the returning light  15  from the heads  18   a  and  18   b  individually at different timings and the second spectroscopic unit  24   b  detects the returning light  15  from the heads  58   a  and  58   b  individually at different timings. 
     Consequently, the height of the upper surface  11   a,  etc. can be measured without reducing a measurement wavelength band per head and at four points using the four heads  18   a,    18   b,    58   a,  and  58   b.  Incidentally, according to the second embodiment, the four heads  18   a,    18   b,    58   a,  and  58   b  may be arrayed on a radius established on the upper surface  11   a.  In addition, the first through fourth modifications described above may be applied to the second embodiment. 
     Third Embodiment 
     A third embodiment of the present invention will be described below with reference to  FIG. 6 .  FIG. 6  schematically illustrates a measuring apparatus  70  according to the third embodiment. The measuring apparatus  70  is mainly different from the measuring apparatus  50  according to the second embodiment in that it has a single spectroscopic unit  24 . The returning light  15  from the first head unit  18  is transmitted through the second optical fiber  22  to the spectroscopic unit  24 , and the returning light  15  from the second head unit  58  is transmitted through the fourth optical fiber  64  to the spectroscopic unit  24 . 
     However, because the positions where light from the second optical fiber  22  and light from the fourth optical fiber  64  fall on the collimator lens  26  are different from each other, the returning light  15  from the first head unit  18  and the returning light  15  from the second head unit  58  are detected by the light detector  32  at different positions in the first direction A 1  on the light detecting area  32   a.  In  FIG. 6 , the returning light  15  from the first head unit  18  is detected at a position B 1  on the light detecting area  32   a,  and the returning light  15  from the second head unit  58  is detected at a position B 2 , different from the position B 1 , on the light detecting area  32   a.  Furthermore, the returning light  15  is detected by the light detector  32  at different positions in the second direction A 2  on the light detecting area  32   a  depending on the wavelength thereof. 
     In this manner, the returning light  15  is detected on the light detecting area  32   a  separately with respect to the head units. According to the third embodiment, the number of spectroscopic units may be reduced compared with the second embodiment. The height of the upper surface  11   a,  etc. can be measured without reducing a measurement wavelength band per head and at four points using the four heads  18   a,    18   b,    58   a,  and  58   b.  Incidentally, according to the third embodiment, the four heads  18   a,    18   b,    58   a,  and  58   b  may be arrayed on a radius established on the upper surface  11   a.  In addition, the first through fourth modifications described above may be applied to the third embodiment. 
     Fourth Embodiment 
     A fourth embodiment of the present invention will be described below with reference to  FIG. 7 .  FIG. 7  schematically illustrates a measuring apparatus  72  according to the fourth embodiment. The measuring apparatus  72  is mainly different from the measuring apparatus  50  according to the second embodiment illustrated in  FIG. 5  in that it does not have the shutter device  36 , the second shutter device  62 , and the first and second heads  58   a  and  58   b.  Light emitted from the light source  8  is transmitted through the first optical fiber  12  to a light-source-side branching member  74 , which branches the light to at least two head-side branching members, i.e., combiners,  76   a  and  76   b.    
     Each of the light-source-side branching member  74  and the head-side branching members  76   a  and  76   b  includes a 1×2 fiber optic coupler. The head  18   a  is connected to the head-side branching member  76   a  through the measuring optical fiber  16   a.  Similarly, the head  18   b  is connected to the head-side branching member  76   b  through the measuring optical fiber  16   b.  The head-side branching members  76   a  and  76   b  transmit the light branched by the light-source-side branching member  74  to the corresponding measuring optical fibers  16   a  and  16   b,  respectively. 
     According to the present embodiment, no shutter device  36  is disposed between the heads  18   a  and  18   b,  i.e., the head unit, and the holding surface  4   a.  Therefore, the light transmitted through the measuring optical fibers  16   a  and  16   b  is applied to the measurand  11  held on the holding surface  4   a  without being blocked by the shutters  36   a  and  36   b,  etc. The returning light  15  reflected by the upper surface  11   a  of the measurand  11  is transmitted through the measuring optical fiber  16   a  and a spectroscopic-unit-side optical fiber  78   a  branched from the head-side branching member  76   a  to the first spectroscopic unit  24   a.    
     Likewise, the returning light  15  reflected by the upper surface  11   a  of the measurand  11  is transmitted through the measuring optical fiber  16   b  and a spectroscopic-unit-side optical fiber  78   b  branched from the head-side branching member  76   b  to the second spectroscopic unit  24   b.  In this fashion, the light applied from the heads  18   a  and  18   b  is reflected by the upper surface  11   a  and detected individually by the light detectors  32  of the first and second spectroscopic units  24   a  and  24   b.  Consequently, the height of the upper surface  11   a,  etc. can be measured without reducing a measurement wavelength band per head  18   a  or  18   b  and at two points using the two heads  18   a  and  18   b.  Note that providing a measuring apparatus includes a plurality of sets including three or more sets of head-side branching members, measuring optical fibers, heads, spectroscopic-unit-side optical fibers, and spectroscopic units, the measuring apparatus can measure the height of the upper surface  11   a,  etc. at a plurality of points including three or more points. 
     Fifth Embodiment 
     A fifth embodiment of the present invention will be described below with reference to  FIG. 8 .  FIG. 8  schematically illustrates a measuring apparatus  80  according to the fifth embodiment. The measuring apparatus  80  is different from the measuring apparatus  72  illustrated in  FIG. 7  in that the head-side branching members  76   a  and  76   b  illustrated in  FIG. 7  are replaced with three-ports optical circulators, i.e., combiners,  86   a  and  86   b,  respectively. The optical circulator  86   a  has a first port connected to the light source  8  through the light-source-side branching member  74  and the first optical fiber  12 . Furthermore, the optical circulator  86   a  has a second port connected to the measuring optical fiber  16   a  and a third port connected to the spectroscopic-unit-side optical fiber  78   a.    
     Similarly, the optical circulator  86   b  has a first port connected to the light source  8  through the light-source-side branching member  74  and the first optical fiber  12 . Furthermore, the optical circulator  86   b  has a second port connected to the measuring optical fiber  16   b  and a third port connected to the spectroscopic-unit-side optical fiber  78   b.  According to the present embodiment, since a loss of the amount of the returning light  15  that is detected by the first and second spectroscopic units  24   a  and  24   b  is smaller than if the head-side branching members  76   a  and  76   b  are used, the light from the light source  8  can be used more effectively. 
     The structural details, processes, and other features according to the above embodiments and modifications may be changed or modified appropriately without departing from the scope of the present invention. For example, the above embodiments and modifications remain effective and functional even if the head  18   a  is of an interference optical system. Specifically, the head includes a condensing lens free of axial chromatic aberrations and provides a first optical path along which the measuring light  13  travels through the condensing lens and a second optical path along which the measuring light  13  travels around the condensing lens (see, for example, Japanese Patent Laid-Open No. 2011-122894). When interference light produced from returning light  15  that has traveled along the first optical path through the condensing lens and measuring light  13  that has traveled along the second optical path is detected by the light detector  32 , the distance from the head to the upper surface  11   a  can be measured on the basis of the peak wavelength of the interference light and the difference between the lengths of the first and second optical paths. 
     Incidentally, according to another modification, a fiber optic combiner, not illustrated, having a beam splitter and a plurality of sets of lenses and optical fibers may be used instead of a fiber optic coupler, e.g., the fiber optic coupler  10  (see  FIG. 1 ) including the branching member  14  and the plural optical fibers. 
     The present invention is not limited to the details of the above described preferred embodiments. 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.