Abstract:
A device and method for inspecting a test piece with a laser beam in which the laser beam is divided into plural beams, and each of the plural beams has an identification marker, such as a particular polarity or intensity. Each of the marked beams, scans a different portion of the test piece to reduce the time needed to inspect the test piece.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inspection method and an inspection apparatus associated with inspecting the external views of the reticles for large scale integrated circuits (LSI) fabrication or the patterns of LSIs themselves. 
     2. Description of the Related Art 
     Conventionally, it is a general practice that reticles for LSI fabrication or the patterns of LSIs themselves are inspected by providing two optical systems using metallurgical microscopes, simultaneously observing identical portions of the patterns under test, and obtaining a difference between these portions to detect a defect. 
     As the LSI patterns have decreased in size, the above-mentioned conventional method can no longer cope with the recent LSI patterns due to the limitation in resolving power. To overcome this problem, a method as disclosed in U.S. Pat. No. 5,572,598 was proposed. In this method, a laser beam having good convergence characteristics is used as the light source. This laser is collected into a microscopic spot. The surface of a test piece, such as a pattern for LSI fabrication, is scanned with this laser beam. An image of the observed surface of the test piece is constructed based on the variation in light quantity of the laser beam transmitted through or reflected from the test piece. 
     The above-mentioned method, however, uses a technique in which the test piece is scanned with the laser spot in a two-dimensional manner to obtain an observed image. Therefore, as compared with the conventional technique in which an observed image is obtained in a batch by using a camera or an equivalent detector in a single-dimensional or two-dimensional manner, the time for observed image detection increases remarkably. 
     Moreover, as the resolving power for inspection continues to increase, there is also a drastic increase in data processing requirements. It is therefore strongly desired to shorten the detection time for the observed image of test pieces. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to solve the above-mentioned problems by providing an inspection method and an inspection apparatus that shortens the time for inspecting the reticles for LSI fabrication semiconductor chips, or the patterns of LSIs themselves (hereinafter referred to as “test pieces”) and that have higher precision than the prior art. 
     In carrying out the invention and according to one aspect thereof, a method of inspecting a high-precision pattern by scanning a surface of a test piece with a laser beam and using at least one of a light beam reflected from the surface of the test piece and a light beam transmitted through the surface of the test piece includes the steps of: branching the laser beam into a plurality of laser beams in order to scan the surface of the test piece with the laser beam as the plurality of branched scan laser beams simultaneously; assigning an identification marker to each of the plurality of branched scan laser beams; and identifying each of the plurality of branched scan laser beams by the identification marker to provide an image of the surface of the test piece corresponding to each of the identified branched scan laser beams. 
     The laser beam branching step may include a step for splitting the laser beam into two, a step for tilting the optical axis of one of the split laser beams, and a step for synthesizing the two split laser beams. The identification marker may either be a different polarized state assigned to each branched scan laser beam or a variation in light intensity assigned to each branched scan laser beam in a time division manner. 
     Preferably, the laser beam has an ultraviolet wavelength. 
     In carrying out the invention and according to another aspect thereof, an apparatus for inspecting a high-precision pattern by scanning a surface of a test piece with a laser beam and using at least one of a light beam reflected from the surface of the test piece and a light beam transmitted through the surface of the test piece may include: a scanning means for scanning the surface of the test piece with the laser beam; a laser beam branching means for branching the laser beam into a plurality of laser beams in order to scan the surface of the test piece with the laser beam as the plurality of branched scan laser beams simultaneously; an identification marker assigning means for assigning an identification marker to each of the plurality of branched scan laser beams; a radiating means for radiating the plurality of branched scan laser beams assigned with the identification markers onto the surface of the test piece; an image signal detecting means for detecting at least one of the light reflected from the surface of the test piece and the light transmitted through the surface of the test piece; a system control having an image processing unit for identifying each of the plurality branched scan laser beams by the identification markers and detecting a defect by obtaining an image of the surface of the test piece by using a detect signal obtained from the image signal detecting means, an image display section for displaying an desired image, and an input section for inputting data from outside; and an XY stage for holding the test piece to drive the same in X-axis and Y-axis directions. 
     The laser beam branching means may include a splitting means for splitting the laser beam into two, an optical axis changing means for tilting the optical axis of one of the two split laser beams, and a synthesizing means for synthesizing the two split laser beams. 
     The laser beam branching means may be a plurality of unit laser beam branching means provided in at least one of parallel and series arrangements, the unit laser beam branching means including one splitting means for splitting the laser beam into two, one optical axis changing means for tilting the optical axis of one of the two split laser beams, and one synthesizing means for synthesizing the two split laser beams. 
     The optical axis changing means may include a wedge-shaped glass plate. 
     The identification marker to be assigned by the identification marker assigning means may be a different polarized state assigned to each of the plurality of branched scan laser beams or a variation in a light intensity assigned to each of the plurality of branched scan laser beams in a time division manner. 
     The identification marker assigning means for assigning the variation in light intensity that provides the identification marker may have an ultrasonic modulating means for performing analog modulation on each of the plurality of branched scan laser beams to change a light intensity thereof and a modulation signal generating means for outputting a modulation signal to the ultrasonic modulating means in a predetermined time division manner. 
     Further, preferably, the laser beam may have an ultraviolet wavelength. 
     The laser beam radiated from the laser light source is branched into a plurality of laser beams by the laser beam branching means, so that the test piece surface can be scanned with the plurality of laser beams for scanning predetermined ranges of the surface. By combining, in at least one of parallel and series arrangements, a plurality of unit laser beam splitting means for splitting the laser beam into two, tilting the optical axis of one of the split laser beams, and synthesizing the split laser beams, a desired number of branched scan laser beams including an odd number thereof can be generated. 
     By assigning an identification marker to each of the branched scan laser beams, the branched scan laser beams can be identified in the detect signal obtained from the image signal detecting means for detecting at least one of the light beams reflected from and transmitted through the test piece surface. Consequently, an image of the wide test piece surface can be obtained in a short time for defect detection. 
     By the XY stage for holding the test piece and driving the same in the X-axis and Y-axis directions relative to the laser beam radiation position, the test piece can be scanned in the X-axis direction. When the test piece has been scanned in the X-axis direction once, the test piece is step-fed in the Y-axis direction to be scanned in the direction opposite to the X-axis direction. This operation is repeated to scan all over the subject area of the test piece in a scan width in which the plurality of branched scan laser beams are arranged side by side. 
     A first advantage of the present invention is that the inspection method in which the ultrasonic deflector and the technique of splitting the laser beam into two are combined can expand the image signal detection per unit time from conventional 500 points to 1000 points. Further splitting of the laser beam can further increase the number of points per unit time. Consequently, the time for inspecting defects of reticles for LSI fabrication, for example, can be shortened to enhance productivity. This also significantly reduces the cost of LSI itself. 
     A second advantage of the present invention is that the UV light having wavelength of 363.8 nm is used for the light source, so that, as compared with the prior-art resolution of about 0.3 μm for defect detection, a defect size as small as 0.1 μm can be realized, enabling the defect detection of higher precision patterns than the prior art. This provides an extremely effective inspection technique for the recent LSI fabrication reticles for example that are getting more microscopic in feature. 
     A third advantage of the present invention is that, compared with the prior-art technique in which the same laser light source is used for both illumination and autofocusing, optical axis adjustment and the like can be made easily and, at the same time, the accuracy of autofocus detection can be enhanced because He—Ne laser (wavelength 632.8 nm) is used for the autofocusing light source independently of the light source for illumination. 
     A fourth advantage of the present invention is that highly precise alignment of test piece can be performed by using the stage having 3 degrees, of freedom, i.e. x, y and θ directions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general block diagram illustrating an apparatus for inspecting high-precision patterns; 
     FIG. 2 is a detailed block diagram illustrating an optical system in the inspection apparatus practiced as the first embodiment of the invention; 
     FIG. 3 is a timing chart of the deflection scan in the first embodiment of the invention; 
     FIGS.  4 ( a ) and  4 ( b ) are diagrams illustrating a state of scan laser beam scanning on the test piece in the first embodiment of the invention; 
     FIG. 5 is a block diagram illustrating operations of a circuit for generating the deflection scan signal in the first embodiment of the invention; 
     FIGS.  6 ( a ) to  6 ( c ) are schematic diagrams illustrating operations of the XY stage, in which 
     FIG.  6 ( a ) shows the perspective drawing illustrating the XY stage, 
     FIG.  6 ( b ) shows the operation of the first embodiment of the present invention and 
     FIG.  6 ( c ) shows the operation of a prior-art example; 
     FIG. 7 is a detailed block diagram illustrating an optical system of an apparatus for inspecting high-precision patterns, practiced as the second embodiment of the invention; 
     FIG. 8 is a timing chart of the deflection scan of the second embodiment; 
     FIGS.  9 ( a ) and  9 ( b ) are diagrams illustrating a state of scan laser beam scanning on the test piece in the second embodiment of the invention, in which 
     FIG.  9 ( a ) is a schematic diagram of the scan state and FIG.  9 ( b ) is a graph showing a relationship between the Y-axis deflection scan signal and the elapsed time; 
     FIG. 10 is a schematic diagram illustrating in detail the relationship between scan areas, the modulation signals, and the Y-axis deflection scan signal of the second embodiment; 
     FIG. 11 is a general block diagram illustrating an optical system for quartering the laser beam of an apparatus for inspecting high-precision patterns, practiced as the third embodiment of the invention; 
     FIG. 12 is a schematic diagram illustrating the operation of the XY stage practiced as the third embodiment of the invention; 
     FIG. 13 is a timing chart of the deflection scan in the third embodiment of the present invention; 
     FIG. 14 is a schematic diagram illustrating a scan state of the laser beam in the third embodiment; and 
     FIG. 15 is a general block diagram illustrating an optical system for splitting the laser beam into eight in the apparatus for inspecting high-precision patterns, practiced as a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following describes in detail a first embodiment of the present invention with reference to FIGS. 1 and 2. The high-precision pattern inspection apparatus according to the present invention is an inspection apparatus for locating defects, for example, errors of shape, size, registration and lack of pattern and so on, of the test pieces. 
     As shown in FIG. 1, the high-precision pattern inspection apparatus according to the present invention is composed of the optical system  110  including the laser light source  111  providing illumination light, the deflection scan means  120  and  125  for deflectively scanning the laser beam  119  output from the laser light source, the halving optical system  130  that is a laser beam branching means for splitting the laser beam into two, and other optical units, the XY stage  181  for holding thereon the test piece  182  to be inspected, and the system controller  190  having the display  192  and the data input section  193 . 
     The XY stage  181  comprises an X table (not shown) that is automatically fed in the X-axis direction and a Y table that is step-fed in the Y-axis direction. Each table is driven by a mechanism (not shown) that converts the rotary movement of an AC servo motor into the linear movement by a ball screw. 
     The following describes the optical system  110  with reference to FIG. 2 in detail. This optical system  110  is composed of a first transmission system for deflecting the laser beam and transmitting the resultant laser beam to the halving optical system  130 , the halving optical system  130 , a second transmission system for transmitting the laser beam coming from the halving optical system to the test piece  182 , the reflected beam detector  150 , the transmitted beam detecting section  160 , and the autofocus section  170 . 
     The first transmission system is composed of the laser light source  111  for radiating the laser beam  119  for illumination, the attenuator  112  for adjusting the output intensity of the laser beam  119 , the spatial filter  113 , the deflection scan means  120  and  125  (including the beam expander  122 , the half-wave plate  123 , and the ultrasonic deflectors  121  and  126 ) for deflectively scanning the test piece with the laser beam  119  at high speeds, the cylindrical lens  114  for condensing the laser beam from the deflection scan means  120  and  125  in the X-axis direction, the beam splitter  115  for changing the progression direction of the laser beam  119  and to transmit part thereof, the power monitor  116  for detecting the laser beam transmitted through the beam splitter  115  and monitoring the output intensity of the laser beam  119  at the output side of the attenuator  112 , the relay lens  117  on the side of the reflected light from the beam splitter  115 , and the quarter-wave plate  118 . The first transmission system thus constituted transmits the laser beam reflected by the beam splitter  115  to the halving optical system  130 . 
     The halving optical system  130  for splitting the laser beam into two laser beams of P polarization and S polarization is composed of the polarizer  131  for splitting the incident laser beam  128  into two laser beams of P polarization and S polarization, the mirror  132   a  for changing the direction of one split laser beam  139 , the wedge plate  133  composed of a wedge-shaped glass plate for altering the optical axis of the laser beam  139 , the mirror  132   b  for changing the direction of the laser beam  139 , the mirrors  132   c ,  132   d ,  132   e , and  132   f  for changing the progression direction of the other split laser beam  138  and adjusting the transmission distance, and the polarizer  134  for synthesizing the split laser beams  138  and  139  into combined laser beams  129 . The halving system  130  thus constituted transmits the synthesized laser beam  129  to the second transmission system. 
     The second transmission system is composed of the half mirror  141  for transmitting the laser beam  129  therethrough, the galvanomirror  142  and the mirror  143  for changing the direction of the laser beam to the X-axis direction instead of stage scan and for obtaining the image in a certain range, the telescope  144 , the dichroic mirror  145  for changing the direction of the laser beam by reflecting the same, and the objective lens  146 . The laser beam  149  is collected on the pattern surface of the test piece through the objective lens  146 . 
     In the reflected beam detector  150 , which is one of the image signal detecting means, the laser beam reflected from the test piece  182  returned in the optical path of the original laser beam enters the polarizer  152  after being reflected by the half mirror  141 . 
     The reflected beam detector  150  is composed of the polarizer  152  for transmitting the P-polarized component of the reflected laser beam therethrough and reflecting the remaining component in the direction of the polarizer  155 , the condenser lens  153  for condensing the P-polarized component transmitted through the polarizer  152  onto the detection surface of the reflected beam detector  154 , the polarizer  155  for separating the S-polarized component from the light reflected from the polarizer  152 , and the condenser lens  156  for condensing the separated S-polarized component onto the detection surface of the reflected beam detector  157 . 
     The transmitted beam detecting section  160 , which is another of the image signal detection means, is composed of the collector lens  161  for collecting the laser beam transmitted through the test piece  182 , the polarizer  162  for transmitting the P-polarized component therethrough and reflecting the remaining portion in the direction of the polarizer  165 , the condenser lens  163  for condensing the light transmitted through the polarizer  162  onto the detection surface of the transmitted beam detector  164 , the transmitted beam detector  164 , the polarizer  165  for separating the S-polarized light from the reflected light of the polarizer  162 , the condenser lens  166  for condensing the separated S-polarized component onto the detection surface of the transmitted beam detector  167 , and the transmitted beam detector  167 . 
     The autofocus section  170  is composed of the autofocus light source  171  that uses a linear polarized laser beam such as He—Ne laser (wavelength 632.8 nm), the beam expander  172  for expanding the laser beam radiated from the autofocus light source  171 , the beam splitter  173  for transmitting the laser beam therethrough radiated from the autofocus light source  171  and reflecting to the split detector  177  the reflected light from the test piece  182  and returned along the same optical path, the quarter-wave plate  178  for converting the linearly polarized light into the circularly polarized light, the compensating lens  174  for making the autofocus laser beam to focus on the same plane as the laser beam  149  after transmission through the objective lens  146 , the mirror  175  for radiating along with the laser beam  129  transmitted through the dichroic mirror  145  onto the test piece  182  and reflecting the light reflected from the test piece  182  to the beam splitter  173 , the collective lens  176  for collecting the light reflected from the beam splitter  173 , the knife edge  179 , and the split detector  177 . 
     With further reference to FIG. 1, the image processing unit  191  is composed of the image data storage  191   b  for receiving the detect signals detected by the detectors from the light reflected from and transmitted through the test piece  182 , generating images from the received detect signals, and sequentially storing the generated images, and storing image data of already inspected reference test pieces for use in the inspection based on the comparison between test pieces, the database storage  191   a  for storing corresponding image data to be obtained from the database when performing inspection by comparison between a test piece and a test piece database, the defect information storage  191   c  for storing the information about defects found by inspection, and the image processing controller  191   d  for collectively controlling the digital operations of the components of the image processing unit and interfacing with external devices. 
     The display  192  displays images, defect information, and so on, thereby informing the operator of the contents of detected defects through a display device such as CRT. The display is also used for editing the inputs from the data input section  193  to be described later. 
     The data input section  193  is used as man-machine interface for inputting data into the above-mentioned database and determining the contents of the display on the display  192  for example. Normally, a keyboard is used for the data input section. 
     The following describes the operation of the first embodiment of the present invention. The laser beam to be used preferably has a shorter ultraviolet wavelength from the viewpoint that the laser beam has a good focus, or the spot size on the test piece  182  must be as small as possible. However, in the wavelength band below 360 nm, the available lens materials are extremely limited, making it extremely difficult to fabricate a lens having a small aberration and, at the same time, difficult to fabricate the means for deflecting the laser beam. Consequently, a good result was obtained from the UV light of an Ar laser having a wavelength of 363.8 nm in the present embodiment. 
     The laser beam  119  radiated from the laser light source for illumination is deflected by the deflection scan means  120  and  125  in the Y direction and the deflected laser beam is split by the halving optical system  130  into P polarization and S polarization, which are radiated onto the test piece  182  held on the XY stage. 
     With reference now to FIG. 3, a drive signal as indicated by the Y-axis deflection scan signal  307  is applied to the ultrasonic deflectors  121  and  126  of the deflection scan means  120  and  125 . In synchronization with a clock pulse CLK, when the X stage comes to a predetermined position, the first-time Y scan start signal  301  is generated, from the first-time Y scan start signal  301 , the first-time in-scan signal  305  is generated, and the drive signal indicated by the Y-axis deflection scan signal  307  is generated. When the first-time in-scan signal  305  is ON, the pattern surfaces in the scan area  1 - 1  and the scan area  1 - 2  of the test piece  182  are deflectively scanned with each of the two branched laser beams and the resultant image data is sampled. 
     When the first-time in-scan signal  305  goes OFF at the time the scan of the scan area  1  comes to an end, the Y-axis deflection scan signal  307  quickly returns to the 0 level  308  of the Y-axis deflection scan signal and is kept in the wait state until the second-time in-scan signal  306  is turned ON by the second-time Y scan start signal  302 . The wait time is set as short as possible. 
     With reference to FIG.  4 ( a ), the laser beam spot on the pattern surface of the test piece  182  moves as follows. Each of the two branched laser beams is initially at the positions  411  and  421  and moves in the direction of arrow A by the Y-axis deflection scan signal  307 . The Y-axis scan signal  307  increases in steps (not linearly) at each spot position as shown in FIG.  4 ( b ). 
     The first-time in-scan signal  305  goes OFF when the end positions  414  and  424  of the Y-axis scan of the area  1  are deflectively scanned with each laser beam. When the first-time in-scan signal  305  goes OFF, the Y-axis deflection scan signal  307  quickly returns to the 0 level  308  of the Y-axis deflection scan signal, so that each laser beam also returns to the radiation start position. When the XY stage moves in the X-axis direction, each laser is positioned at the second deflection scan start positions  415  and  425  and kept in the wait state until the second-time in-scan signal  306  is turned ON by the second-time Y scan start signal  302 . 
     When the second-time Y in-scan signal  306  goes ON, the Y-axis deflection scan signal  307  is generated in the same manner as the first-time Y scan and the generated signal is applied to the deflector. Subsequently, the above-mentioned operation is repeatedly performed by the predetermined number of times on all areas of scan area  1 . 
     With reference to FIG. 5, the Y-axis deflection scan signal  307  to be inputted in the deflection scan means  120  and  125  is, as shown in FIG. 5, generated from the outputs of the counter  332  for counting the number of times the clock pulse CLK  331  is inputted, the memory  333  for performing the arithmetic operation to output an accumulated value every time the clock is counted by the predetermined number, the D/A converter  334  for converting the digital output of the memory  333  into an analog value, and the amplifier  335 . 
     Referring to FIG.  6 ( b ), the XY stage  181  carries the test piece  382  and moves in one direction along X-axis and, when the test piece end portion  387  reaches the scan laser beam radiation position  340 , stops moving, upon which the X-axis feed for the first-time scan area  310  shown in FIG.  6 ( b ) comes to an end. 
     The position  340  is a fixed position. The laser beam is branched into two at this position, which are scanned in Y direction to provide the P-polarized scan laser beam  341  and the S-polarized scan laser beam  342  for example. It should be noted that, for convenience of description, an end  341   a  of the laser beam  341  and an end  342   b  of the laser beam  342  are shown with a space in between; actually, however, the scan width of each laser beam is set such that both the laser beams  341  and  342  overlap with each other. 
     Next, when the XY stage  181  has been fed two steps in Y direction (the direction indicated by arrow A), the XY stage  181  is fed in X-axis direction, which is perpendicular to the Y direction, upon which the second X-axis feed of the scan area  320  starts. When the test piece end  387  subject to inspection has reached the scan laser beam radiation position  340 , the XY stages stops. 
     Although not shown in FIG.  6 ( b ), if there are multiple scan areas such as a third scan area and a fourth scan area, the X-axis scan and the Y-axis step feed are repeated such that all the scan areas on the test piece are scanned with the scan laser beams  341  and  342 . 
     Meanwhile, the output intensity of the laser beam  119  is monitored by the power monitor  116 . Using a detect signal of this power monitor  116 , the attenuator  112  is controlled by a laser power controller (not shown) to maintain the output intensity of the laser beam  119  at a constant level. 
     The above-mentioned operations are performed in cooperation with the operations of other components under the control of the system controller  190 . 
     Referring to FIG.  6 ( c ), in the prior-art example, the two split laser beams as used in the present embodiment are not used. Therefore, the first-time scan area is half the first-time scan area of the present embodiment in scan width, resulting in the same X-axis direction as the present embodiment but a single-step feed in Y direction. Consequently, as compared with the present invention, the time for inspecting the test piece is approximately doubled. In other words, the present invention can reduce the inspection time to a half of the prior-art example, providing a significant advantage. 
     The following describes in detail the operation of the optical system  110  with reference to FIG.  2 . The laser beam  119  radiated from the laser light source  111  using UV-Ar laser is adjusted in output intensity by the attenuator  112  and the adjusted laser beam is transmitted through the spatial filter  113  to be deflectively scanned in Y-axis direction on the inspection images (as shown in FIG.  6 ( b )) by the deflection scan means composed of a pair of the ultrasonic deflector  121  and the ultrasonic deflector  126 . The laser beam outputted from the ultrasonic deflector  126  is focused in the X-axis direction through the cylindrical lens  114 . In the Y-axis direction, the laser is focused, as with the cylindrical lens  114 , to the same focus position by the condensing action of the ultrasonic deflector  126  itself. Then, the laser is transmitted through the relay lens  117  and the quarter-wave plate  118  via the beam splitter  115  to be branched by the halving optical system  130 . 
     The laser beam transmitted through the beam splitter  115  is received by the power monitor  116 , by which variations in the laser beam intensity are monitored. 
     The laser beam entering the halving optical system  130  is put in the circularly polarized state by the quarter-wave plate  118  to be branched into two laser beams of P polarization and S polarization. One of the branched laser beams, or the laser beam  139  is altered in the tilt of the optical axis thereof through the mirror  132   a  to be entered in the polarizer  134  via the mirror  132   b.    
     The other laser beam  138  is entered in the polarizer  134  through the transmission distance adjusting mirrors  132   c,    132   d,    132   e,  and  132   f.  The polarizer  134  synthesizes the laser beams  138  and  139  branched by the polarizer  131 . The resultant synthesized laser beam consists of P polarization and S polarization. 
     These laser beams have a slightly different light axis direction from each other. This difference in the direction of the axes is adjusted by adjusting the tilt of the wedge plate  133  such that these laser beams are separated from each other by a width equivalent to the scan area width shown in FIG.  6 ( b ). In the state in which the laser beams are focused on the surface of the test piece, a positional change results from the tilt of the wedge plate  133  is about 0.05 μm on the test piece surface, an extremely precise adjustment. Thus, in spite of having only one system as a laser beam scanning means in the Y-axis direction, two areas on the test piece  182  can be simultaneously scanned and the parallel processing can be performed. 
     The laser beam  129  outputted from the halving optical system  130  passes through the half mirror  141  and is reflected from the galvanomirror  142  and the mirror  143  to enter the telescope  144 . The galvanomirror  142  changes the direction of the laser beam to the X-axis direction instead of stage scan to obtain an image of a certain area. In inspecting the patterns of reticles, as described later, the XY stage  181  mounting reticles is moved to obtain an image. 
     The laser beam outputted from the telescope  144  is reflected by the dichroic mirror  145  to be mixed with the autofocusing laser beam transmitted through the dichroic mirror  145 , which will be described later. The resultant laser beam enters the objective lens  146 . 
     The telescope  144  can alter the size of the scan spot and scan area of the laser beam on the test piece  182 , so that the minimum value of detectable defects can be selected. It is also possible to design the lens system of the telescope  144  such that the length of the scan range is increased in proportion to the scan spot size. The laser beam coming from the telescope  144  is focused onto the surface of the test piece  182  through the objective lens  146 . 
     The laser beam  119  radiated from the laser light source  111  onto the test piece  182  is partially reflected from the pattern surface of the test piece  182  to return along the original laser beam optical path and is further reflected from the half mirror  141  to go in the direction of the polarizer  152  of the reflected beam detector  152 . This reflected laser beam enters the polarizer  152  and the P polarization transmitted through the polarizer  152  is condensed by the condenser lens  153  to be received by the reflected beam detector  154 . On the other hand, as for the beam reflected from the polarizer  152 , the S polarization transmitted through the polarizer  155  enters the condenser lens  156  to be received by the reflected beam detector  157 . 
     The laser beam transmitted through the test piece  182  is collected by the collector lens  161  and P polarization is transmitted through the polarizer  162  and condensed by the condenser lens  163  to be received by the transmitted beam detector  164 . On the other hand, as for the laser beam reflected from the polarizer  162 , the S polarization is reflected by the polarizer  165  and enters the condenser lens  166  to be received by the transmitted beam detector  167 . 
     The laser beam radiated from the autofocusing light source  171  that uses He-Ne laser (linear polarized laser of wavelength 632.8 nm) in the autofocus section  170  passes through beam expander  172  to obtain the desired beam size, beam splitter  173 , quarter wave plate  178 , compensating lens  174 , mirror  175  to alter the tilt of light path, and dichroic mirror  145 , so that the autofocusing laser beam is focused onto the surface of the test piece  182  through objective lens  146  and the laser beam is reflected by the surface of the test piece  182 . 
     The reflected beam returns in the direction opposite to the direction of incidence to the test piece  182 , transmits through the dichroic mirror  145 , reflected by the mirror  175  to change direction, reflected again by the beam splitter  173  by the action of the quarter-wave plate  178 , collected by the collective lens  176 , and enters the split detector  177 . 
     In front of the split detector  177  is a light cutting plate, that is to say, knife edge  179  is located so as to cut off half of the light. 
     The knife edge  179  is aligned so that equal quantities of light enter both detectors of split detector  177  only when the test piece  182  is at such position that the reflected He-Ne laser beam is focused on the surface of the split detector  177 . When the height of the surface of the test piece  182  changes, the focus position of the reflected laser beam changes and light quantity entering each detector of the split detector  177  becomes unbalanced because part of reflected light is cut off by the knife edge  179 . By taking out the difference between detectors as a detect signal and altering the up-down position of the objective lens  146  through servo mechanism (not shown) that drives the objective lens  146 , feed-back position control is performed so that the focus of the laser beam from the laser light source  111  for illumination and the focus of the laser beam from the laser light source  171  for autofocusing are matched to each other on the test piece surface. 
     The present embodiment has a laser beam for autofocusing that has a wavelength band different from that of the laser light source for pattern visual checking for the following reason. In the inspection of high-precision patterns to which the present invention is to be applied, it is required to focus the laser spot size on the surface of the test piece  182  as small as the order of the wavelength. To do so, the numerical aparture (NA) of the objective lens  146  must be made fairly large. The objective lens having a high NA is inevitably shallow in focal depth. In the present embodiment, the required accuracy of autofocusing is 0.05 μm. Key to achieving such a high accuracy is to clearly detect the signal difference in the split detector  177 , which is the of autofocusing detector. Since only a semiconductor device is available for the practical split detector  177 , it is not advantageous to use UV light because of the low UV sensitivity of reception by the semiconductor device. Therefore, the present embodiment uses the He—Ne laser in a wavelength band wherein the semiconductor split detector  177  has greater sensitivity. 
     Next, the following describes the operation of the XY stage  181  during inspection with reference to FIG.  6 . The test piece  182  is set on the XY stage  181  with 3-degrees, that is to say, X, Y and θ directions, of freedom. In this embodiment, XY stage  181  has a stacked structure of the X stage  186 , the Y stage  187  and the θ stage  188  in this order and moves in the X direction when the inspection images are obtained. 
     The test piece  182  is set precisely and held on the stage  181  by a transfer mechanism (not shown). Next, the θ stage  188  is operated and an alignment is performed. By aligning with the θ stage, a more precise operation is maintained than by aligning with only an XY interpolation operation. 
     The operation of the XY stage  181  during inspection is as illustrated below. First of all, the X stage  186  is automatically fed with a constant velocity and, until the image obtaining points pass through the inspection area  383 ,  384 ,  385 , the Y stage  187  maintains register and does not perform the interpolating operation. The image obtaining points pass through the inspection area  385  and when the X stage  186  stops, the Y stage  187  is stepfed and the X stage is automatically fed in the opposite direction with a constant velocity. 
     While the X stage  186  is fed with the constant velocity, the UV-Ar laser beams repeatedly scan on the inspection area  383 ,  384 ,  385  in the Y-axis direction by the ultrasonic deflector  121 ,  126  and the reflected light and transmitted light are detected by the above-mentioned reflected beam detector  150  or transmitted beam detecting section  160 . 
     Each table is driven by a mechanism (not shown) that converts the rotary movement of an AC servo motor into linear movement by a ball screw. 
     In closing, the following describes the signal processing method. The image processing unit  191  receives the detect signals from the detectors of the beams transmitted through and reflected from the test piece  182 , generates image data from the received detect data, and stores the generated image data in the image data storage  191   b.  Normally in the case of inspecting defects of shapes, size and so on, transmitted light is only used and in the case of inspecting for foreign particles, the reflected light is only used. 
     In the case where a plurality of the same circuit patterns are formed on the same test piece and the piece-to-piece inspection method is adopted, when a comparison is made between the image data taken out by the detector and the image data of a test piece having the same features already inspected and stored in the image data storage  191   b  and, if a mismatch is found, the position of the detected defect, defect image data, and reference data are stored as defect information into the defect information storage  191   c.    
     On the other hand, for test piece-to-database inspection, a comparison is made between the image data of the piece inspected and the corresponding image information stored in the database storage  191   b  to detect a defect. The detected defect is stored as defect information. 
     The display  192  shows the image, defect information, and so on to inform the operator of the contents of the detected defect. 
     The following describes a second embodiment of the present invention with reference to drawings. In FIG. 7, reference numeral  711  denotes a laser light source, reference numeral  712  an attenuator, reference numeral  713  a spatial filter, reference numeral  714  a cylindrical lens, reference numeral  715  a beam splitter, reference numeral  716  a power monitor, reference numeral  717  a relay lens, reference numeral  718  a quarter-wave plate, reference numeral  719  a laser beam, reference numeral  721  an ultrasonic deflector, reference numeral  722  a beam expander, reference numeral  723  a half-wave plate, reference numerals  728  and  729  laser beams, reference numeral  730  a halving optical system, reference numeral  731  a beam splitter, reference numerals  732   a,    732   b,    732   c,    732   d,    732   e,  and  732   f  mirrors, reference numeral  734  a prism, reference numerals  735  and  736  ultrasonic modulators, reference numerals  738  and  739  laser beams, reference numeral  741  a half mirror, reference numeral  742  a galvanomirror, reference numeral  743  a mirror, reference numeral  744  a telescope, reference numeral  745  a dichroic mirror, reference numeral  746  an objective lens, reference numeral  749  a laser beam, reference numeral  750  a reflected beam detector, reference numeral  753  a condenser lens, reference numeral  754  a reflected beam detector, reference numeral  760  a transmitted beam detecting section, reference numeral  761  a collector lens, reference numeral  763  a condenser lens, reference numeral  764  a transmitted beam detector, reference numeral  770  an autofocus section, reference numeral  771  a light source, reference numeral  772  a beam expander, reference numeral  773  a beam splitter, reference numeral  774  a compensating lens, reference numeral  775  a mirror, reference numeral  776  a collective lens, reference numeral  777  a split detector, reference numeral  778  a quarter-wave plate, and reference numeral  782  a test piece. 
     Referring to FIG. 7, the second embodiment differs from the first embodiment in the halving optical system  730 . Namely, the polarizer  131  of the halving optical system  130  in the first embodiment shown in FIG. 2 is replaced by the beam splitter  731  and the polarizer  134  is replaced by the prism  734  that synthesizes the branched laser beams  738  and  739  into laser beam  729 . Further, the ultrasonic modulators  735  and  736  are arranged in the optical paths of the branched laser beams  738  and  739 . 
     Next, the reflected beam detector  750  does not require the polarizers  152  and  155  shown in FIG.  2  and is composed of a pair of the condenser lens  753  and the reflected beam detector  754 . The transmitted beam detecting section  760  does not require the polarizers  162  and  165  is composed of only the collector lens  761 , the condenser lens  763 , and the transmitted beam detector  764 . The other components are the same as those of FIG.  2  and the description of these components will be omitted. 
     The following describes the operation of the second embodiment mainly in the differences from the first embodiment. Referring to FIG. 7, the laser beam radiated from the laser light source  711  changes its direction through the beam splitter  715  to enter the halving optical system  730 . 
     The laser beam entered in the halving optical system  730  is split by the beam splitter  731  into two laser beams  738  and  739 . 
     The laser beam  738  and the laser beam  739  are analog-modulated by the ultrasonic modulator  735  and the ultrasonic modulator  736  respectively to be changed in the intensity of light. This analog modulation is performed in a time division manner, which will be described later. 
     FIG. 8 is a timing chart of the deflection scan of the second embodiment. In the figure, reference numeral  801  denotes a first-time Y scan start signal, reference numeral  802  a second-time Y scan start signal, reference numeral  805  a first-time in-scan signal, reference numeral  806  a second-time in-scan signal, reference numeral  807  a Y-axis deflection scan signal, reference numeral  808  the 0 level of Y-axis deflection scan signal, reference numeral  811  a first beam modulation signal (a), and reference numeral  812  a second beam modulation signal (b). 
     Corresponding to two branched scan areas of the first-time scan area  1 , the ultrasonic modulator  735  outputs the first beam modulation signal  811  to the first beam scan area using the timing of a  1 — 1  modulation signal a and the ultrasonic modulator  736  outputs the second beam modulation signal  812  to the second beam scan area using the timing of a  1 - 2  modulation signal b to perform a modulating operation. 
     The laser beam  739  of FIG. 7 is altered in the tilt of its optical axis by the wedge plate  733 . As with the first embodiment, this tilt is adjusted in units of the scan area width of the test piece  382  shown in FIG.  6 ( b ). 
     Both the analog-modulated laser beams are synthesized by the synthesizing prism  734  to be radiated onto the test piece  782  as described with reference to the first embodiment. 
     The following describes the operation of the scan laser beam on the surface of the test piece  782  with reference to FIGS. 8,  9 , and  10 . 
     FIG. 9 is a diagram illustrates a state of scan laser beam scanning on the test piece in the second embodiment of the invention. In the figure, FIG.  9 ( a ) is a schematic diagram of the scan state and FIG.  9 ( b ) is a graph showing a relationship between the Y-axis deflection scan signal and the elapsed time. Reference numeral  810  denotes a scan area  1 , reference numeral  811  a first beam scan area  1 - 1 , reference numeral  812  a second beam scan area  1 - 2 , reference numeral  911  a first beam spot start position, reference numerals  912 ,  913 ,  914 ,  915 ,  916 , and  917  spot positions, reference numeral  918  first-time end position, reference numeral  919  a second-time start position, reference numeral  921  a second beam spot start position, reference numerals  922 ,  923 ,  924 ,  925 ,  926 , and  927  spot positions, reference numeral  928  a first-time end position, and reference numeral  929  a second-time start position. 
     The two branched laser beams are initially at the spot positions  911  and  921  and then moved in the direction of arrow A by the Y-axis deflection scan signal  807 . Meanwhile, the two branched laser beams are modulated alternately as described above, so that the strong laser beam and the weak laser beam appear alternately on the scan areas  1 - 1  and  1 - 2  of the pattern surface of the test piece  782 . 
     In the figure, thick-line circles  911 ,  913 ,  915 ,  917 , and  919  denote strong laser beams and thin-line circles  912 ,  914 ,  916 , and  918  denote laser beams of which intensity is nearly zero. 
     For the convenience of description, the laser beam on the side of scan area  1 - 1  is P polarization, while the laser beam on the side of scan area  1 - 2  is S polarization. When the laser beam of P polarization corresponding to the scan area  1 - 1  is at one of the positions  911 ,  913 ,  915 , and  917 , or the first-time-axis in-scan signal  805  of FIG. 8 is ON and the  1 - 1  modulation signal a of the first beam modulation signal  811  is ON, the reflected beam detected by the reflected beam detector  750  of FIG. 7 provides the reflected beam detect signal corresponding to the scan area  1 - 1  of the test piece  782 . 
     At this moment, the reflected beam of the S polarization laser beam is also detected from the scan area  1 - 2  by the reflected beam detector  750 . Since this laser beam is of intensity nearly equal to zero corresponding to the thin-line circles  921 ,  923 ,  927  and so on, the reflected beam detect signal of this laser beam is distinguished from the strong reflected beam detect signal coming from the scan area  1 - 1 . 
     When the S-polarization laser beam corresponding to the scan area  1 - 2  is at one of positions  922 ,  924 ,  926 , and  928 , or the first-time-axis in-scan signal  807  of FIG. 8 is ON the  1 - 2  modulation signal b of the second beam modulation signal  812  is ON, the reflected beam detected by the reflected beam detector  750  provides the reflected beam detect signal corresponding to the scan area  1 - 2  of the test piece  782 . At the same time, the reflected beam of the P-polarization laser beam is also detected from the scan area  1 - 1  by the reflected beam detector  750 . Since this laser beam has an intensity nearly equal to zero corresponding to thin-line circles  912 ,  914 ,  916 ,  918  and so on, this reflected beam is distinguished from the strong reflected beam detect signal coming from the scan area  1 - 2 . As described above, the second embodiment uses the reflected beam detecting means that synchronizes with the modulation signal and does not require the separating means for separating P polarization from S polarization in the reflected beam detector  29  in the first embodiment, enabling the simultaneous detection of the branched laser beams at two positions regardless of the laser beam polarized state. 
     Also, the second embodiment is applicable to an optical system that branches the laser beam into two by use of polarization, in which the detection of better S/N ratio is possible. 
     FIG. 10 is a diagram illustrating in detail the relationship between the modulation signals a and b, the scan areas  1 - 1  and  1 - 2 , and the Y-axis deflection scan signal  907 . 
     The beam transmitted through the test piece  782  is detected by the transmitted beam detector  764  through the collector lens  761  and the condenser lens  763  of the transmitted beam detecting section  760 . The method in which the transmitted beam is detected is the same as the method in which the reflected beam detect signal is detected and therefore the description will be omitted. 
     The other operations are the same as those described with reference to the first embodiment and therefore the description will be omitted. 
     The following describes a third embodiment of the invention with reference to FIG.  11 . FIG. 11 is a block diagram illustrating an optical system for quartering the laser beam of an apparatus for visually inspecting high-precision patterns, practiced as the third embodiment of the invention. In the figure, reference numeral  1120  denotes a first halving optical system, reference numeral  1121  a beam splitter, reference numeral  1122  a mirror, reference numeral  1123  a wedge plate, reference numerals  1127 ,  1128 , and  1129  laser beams, reference numeral  1130  a second halving optical system, reference numeral  1133  a wedge plate, reference numeral  1135  an ultrasonic modulator, reference numeral  1140  a synthesizing optical system, reference numeral  1141  a mirror, reference numeral  1142  a synthesizing prism, and reference numeral  1149  a laser beam. 
     The third embodiment comprises the first halving optical system  1120  composed of the beam splitter  1121 , the mirror  1122 , and the wedge plate  1123 , two sets of the second halving optical systems the same as that described with reference to the second embodiment, and the synthesizing optical system  1140  for synthesizing two branched laser beams composed of the mirror  1141  and the synthesizing prism  1142 . 
     The following describes the operation of the third embodiment. Referring to FIG. 11, the laser beam  1129  is branched into two by the first halving optical system  1120 . The branched laser beams  1127  and  1128  enter the different second halving optical systems  1130  to be further branched into two. Namely, the laser beam  1129  is eventually branched into four, the four branched laser beams are processed separately, and the processed laser beams are synthesized by the synthesizing prism  1142  into the laser beam  1149  to be radiated onto the test piece  1282  (FIG.  12 ). 
     The following describes the states of the branched laser beams. The laser beam  1129  is branched into two by the first halving optical system  1120 . The optical path of one laser beam  1127  is tilted by the wedge plate  1123  toward the optical path of the other laser beam  1128  to change the position between both the laser beams, which enter the different second halving optical systems  1130 . The amount of the positional change between the two laser beams is adjusted to a separated width such that the laser beam  1127  can scan the scan area  1 - 1  of the test piece  1282  and the other laser beam  1128  can scan the scan area  1 - 3  of the test piece  1282 . 
     The tilted laser beam  1127  is further branched into two by the second halving optical system  1130 . At this moment, one of the branched laser beams is separated from the other by the area width equivalent to the scan area  1 - 1  by the wedge plate  1133 . The position between the two laser beams is determined by adjusting the tilt such that the scan area  1 - 2  can be scanned. Consequently, the laser beam  1127  is branched into two, one of them scanning the scan area  1 - 1  and other scanning the scan area  1 - 2 . 
     As for the other laser beam  1128  coming from the first halving optical system, the mutual position is adjusted by the wedge plate  1133  like the laser beam  1127  and is radiated to the scan area  1 - 3  and the scan area  1 - 4 . Namely, the four branched laser beams are radiated to the four divided scan areas  1 - 1 ,  1 - 2 ,  1 - 3 , and  1 - 4  of the scan area  1  respectively. 
     The four branched laser beams are synthesized by the prism  1134  of the second halving optical system  1130  and the synthesizing optical system  1140  into the laser beam  1149  to be radiated onto the test piece  1282 . A total of four optical paths in the halving optical systems are analog-modulated by the ultrasonic moudulators  1135  in a time division manner using different timing for light intensity changes. 
     FIG. 12 is a schematic diagram illustrating the operation of the XY stage practiced as the third embodiment of the invention. In the figure, reference numeral  1210  denotes a first-time scan area, reference numeral  1220  a second-time scan area, reference numeral  1227   a  a first laser beam, reference numeral  1227   b  a second laser beam, reference numeral  1228   a  a third laser beam, reference numeral  1228   b  a fourth laser beam, reference numeral  1282  a test piece, reference numeral  1283  a first inspection area, reference numeral  1284  a second inspection area, and reference numeral  1285  a third inspection area. 
     The scan area  1 - 1  is scanned with the laser beam  1227   a,  one of the laser beams resulting from branching the laser beam  1127 , and the scan area  1 - 2  is scanned with the other laser beam  1227   b.  The scan area  1 - 3  is scanned with the laser beam  1128   a,  one of the laser beams resulting from branching the laser beam  1128  and the scan area  1 - 4  is scanned with the other laser beam  1128   b.    
     It will be apparent that the same effect as above can be obtained if the correspondence between the branched laser beams  1127  and  1128  and the scan areas is reverse to the above-mentioned relationship. 
     As described above, the laser beam is split into four to be radiated onto the surface of the test piece  1282 . These laser beams are analog-modulated as described above, which is illustrated in FIGS. 13 and 14. 
     FIG. 13 is a timing chart of the deflection scan in the third embodiment of the present invention. In the figure, reference numeral  1301  denotes a first-time Y scan start signal, reference numeral  1302  a second-time Y scan start signal, reference numeral  1305  a first-time in-scan signal, reference numeral  1306  a second-time in-scan signal, reference numeral  1307  a Y-axis deflection scan signal, reference numeral  1308  the 0 level of the Y-axis deflection scan signal, reference numeral  1311  a first beam modulation signal, reference numeral  1312  a second beam modulation signal, reference numeral  1313  a third beam modulation signal, and reference numeral  1314  a fourth beam modulation signal. 
     FIG. 14 is a schematic diagram illustrating a scan state of the laser beam in the third embodiment. In the figure, reference numeral  1210  denotes a scan area  1 , reference numeral  1401  a first beam spot start position, reference numerals  1402 ,  1403 ,  1404 ,  1407 ,  1408 , and  1409  weak spot positions, reference numerals  1405  and  1406  strong spot positions, reference numeral  1410  a first-time end position, reference numeral  1411  a second-time start position, reference numeral  1421  a second beam spot start position, reference numerals  1423 ,  1424 ,  1425 ,  1426 ,  1428 , and  1429  weak spot positions, reference numerals  1422  and  1427  strong spot positions, reference numeral  1430  a first-time end position, and reference numeral  1431  a second-time start position. 
     Referring to FIG. 13, the laser beam to be radiated onto the scan area  1 - 1  is analog-modulated using the timing of a  1 - 1  modulation signal a, which is the first beam modulation signal  1311 , the laser beam to be radiated onto the scan area  1 - 2  is analog-modulated using the timing of a  1 - 2  modulation signal b, which is the second beam modulation signal  1312 , the laser beam to be radiated onto the scan area  1 - 3  is analog-modulated using the timing of a  1 - 3  modulation signal c, which is the third beam modulation signal  1313 , and the laser beam to be radiated onto the scan area  1 - 4  is analog-modulated using the timing of a  1 - 4  modulation signal d, which is the fourth beam modulation signal  1314 . 
     FIG. 14 shows a state in which the spots of the above-mentioned modulated laser beams operate on the test piece  1282 . Details of branching the laser beam into four of FIG. 14 will be omitted from the following description because the two branched laser beams of FIG. 9 are simply converted to the four branched laser beams. 
     As shown in FIG. 14, in the scan area  1 - 1 , the strong laser beam as indicated by thick-line circles  1401 ,  1404 ,  1406 ,  1410 ,  1411  and so on equivalent to the “ON” timing of the modulation signal a is radiated on the surface of the test piece  1282  and during “OPP” timing the weak laser beam of which intensity becomes nearly zero is radiated on the surface of the test piece  1282 . In the scan area  1 - 2 , the strong laser beam is radiated at the thick-line circles  1422 ,  1427  and so on equivalent to the timing of the modulation signal b and, when “OFF” , the weak laser beam of which intensity becomes nearly zero is radiated. 
     Likewise, in the scan area  1 - 3 , the strong laser beam is radiated in the timing of the modulation signal c. In the scan area  1 - 4 , the strong laser beam is radiated in the timing of the modulation signal d. In another timing, the weak laser beam of which intensity becomes nearly zero is radiated. 
     Identification of the signals of the beams reflected from the test piece  1282  is made in the similar manner in which the identification is made in the second embodiment, except that the modulation timing for the branched laser beams is quartered instead of halved. 
     Briefly described, the reflected beams are detected by use of the modulation signals a, b, c, and d and the first-time in-scan signal  1305  in a time division manner as shown in FIG.  13 . The reflected beam detect signal in the timing of the modulation signal a in the first-time in-scan signal  1305  of FIG. 13 is identified and detected as the reflected beam detect signal from the scan area  1 - 1  of FIG. 12, the reflected beam detect signal in the timing equivalent to the modulation signal b is identified by detected as the reflected beam detect signal from the scan area  1 - 2 , the reflected beam detect signal in the timing of the modulation signal c is identified and detected as the reflected beam detect signal from the scan area  1 - 3 , and the reflected beam detect signal in the timing of the modulation signal d is identified as the reflected beam detect signal from the scan area  1 - 4 . 
     As for the scan area  2 , the reflected beam detect signals are likely identified and detected by use of the second-time in-scan signal  1306  and the modulation signals a, b, c, and d. Although not shown, if there are scan areas  3 ,  4  and so on, the reflected beam detect signals are identified and detected in the same manner as above. Identification of the transmitted beam signals is made in the same manner, so that the description thereof will be omitted. 
     FIG. 15 is a general block diagram illustrating an optical system for splitting the laser beam into eight in the apparatus for visually inspecting high-precision patterns, practiced as a fourth embodiment of the invention. In the figure, reference numeral  1520  denotes a first halving optical system, reference numeral  1521  a beam splitter, reference numeral  1522  a mirror, reference numeral  1523  a wedge plate, reference numeral  1529  a laser beam, reference numeral  1530  a second halving optical system, reference numeral  1540  a synthesizing optical system, and reference numeral  1549  a laser beam. 
     As shown in FIG. 15, it will be apparent that the laser beam  1529  may further be branched into eight, 10, 12, and so on. 
     It will also be apparent that the laser beam may be branched into odd numbers. For example, to branch the laser beam into five, the laser beam of the mirror  1522  of FIG. 15 may only be synthesized with another laser beam by skipping the next first halving optical system  1520  and the halving optical system  1530 . 
     It will be further apparent that the technique of the second embodiment and the technique of the first embodiment using polarization may be used in combination. The present invention includes such a variation. 
     For the autofocusing method, the knife edge method using the split detector  177  in front of which the knife edge  179  is positioned is described. It will be apparent that, instead of the knife edge method, an astigmatic method using a collective lens having astigmatism and a quartering detector may be used.