Abstract:
The invention provides an optical surface defect inspection apparatus and an optical surface defect inspection method that reduces an influence from a dead zone of a sensor array and that reduces the influence from reduction of a detected light amount in a case of extending over light receiving elements, thereby enabling a defect inspection with high sensitivity. According to the invention, a subject is irradiated with an inspection light, an image is formed on the sensor array including the light receiving elements separated by the dead zone insensitive to light scattered by a surface of the subject and arranged in a plurality of lines, outputs from two adjacent light receiving elements are added, and a defect on the surface of the subject is inspected for based on the result of the addition.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an optical surface defect inspection apparatus and an optical surface defect inspection method, specifically to the optical surface defect inspection apparatus and the optical surface defect inspection method suitable for detecting a microdefect formed on a surface of a subject. 
       BACKGROUND ART 
       [0002]    Both a high-speed inspection applicable to 100% full inspection and a highly sensitive inspection are required for an optical surface defect inspection apparatus that inspects for a microdefect on a surface of a subject such as a magnetic disc, a glass or aluminum substrate used as a substrate thereof, and an IC wafer. It is especially required to inspect for a linear microdefect (scratch), which does a significant damage to a product. A highly sensitive defect detection generally employs a method of irradiating the surface with a microspot with a high intensity and scanning the surface therewith, thereby detecting a scattered light from the defect on the surface with high sensitivity. Moreover, for the high-speed inspection, the whole scanning must be completed in a short time by employing a rough scanning pitch, and the size of the irradiation spot in this case must be suitable to at least sufficiently cover the scanning pitch. However, there is a trade-off that a large spot size will result in a lower spot intensity and thus a lower detection sensitivity. 
         [0003]    One method of performing such a highly sensitive and high-speed surface defect inspection is disclosed in Japanese Patent LAID-Open 2001-174415. This technology includes a light transmitting system that emits a light beam having a width in a direction perpendicular to a main scanning direction and relatively scans a face plate, a light receiving system that includes a sensor array having n (n is an integer larger than one) light receiving elements arranged in perpendicular directions and receiving lights reflected by the face plate and forms an image of scanned position on the face plate on the n light receiving elements, a stripe pattern filter that reverses relation of transmission and shielding of adjacent light receiving elements substantially on the right side and the left side from the center of a light receiving surface of the light receiving elements, and a detection circuit that generates a detection signal corresponding to a difference in the received light amount between the adjacent light receiving elements as a signal for detecting a defect, wherein data acquired from the n light receiving elements is processed to detect a microdefect. The technology of Japanese Patent LAID-Open 2001-174415 is intended for defects from which the detection signal is available with respect to an area of approximately one light receiving element at the smallest. 
       SUMMARY OF THE INVENTION 
       [0004]    However, demands for the highly sensitive detection are growing still severer in these days. There is a dead zone between light receiving elements in a sensor array in which a plurality of light receiving elements are arranged in series (actually about 100 of the width of a single light receiving element). There is a problem that the dead zone reduces the detected light amount of the scattered light, thereby reducing the detection sensitivity. The reduction of the detection sensitivity greatly affects the size of the defect that can be detected at the width of a single light receiving element. There is another problem that, when the scattered light from the defect extends over two light receiving elements, the detected light amount is distributed to both elements, thereby reducing the detected light amount. 
         [0005]    The present invention was made in the light of the above problems, and aims to provide the optical surface defect inspection apparatus and the optical surface defect inspection method that reduces the influence from the dead zone of the sensor array and reduces the influence from reduction of the detected light amount in the case of extending over the light receiving elements, thereby enabling the defect inspection with high sensitivity. 
         [0006]    To achieve the above objectives, the present invention has at least the following features. 
         [0007]    The apparatus according to the invention includes: an irradiation means irradiating a subject with an inspection light; a sensor array including light receiving elements separated by dead zones insensitive to light scattered by a surface of the subject and arranged in a line; a scattering optical means light focusing the scattered light onto the sensor array; a plurality of addition means adding outputs from two adjacent the light receiving elements; and a processing unit inspecting for a defect on the surface of the subject based on outputs from the addition means. 
         [0008]    The method according to the invention includes: an irradiation step of irradiating a subject with an inspection light; a step of forming an image on a sensor array including light receiving elements separated by dead zones insensitive to light scattered by a surface of the subject, the receiving elements being arranged in a line; a plurality of addition steps of adding outputs from two adjacent light receiving elements; and a processing step of inspecting for a defect on the surface of the subject based on the result of the addition step. 
         [0009]    The present invention can provide an optical surface defect inspection apparatus and an optical surface defect inspection method that reduces the influence from the dead zone of the sensor array and reduces the influence from reduction of the detected light amount in the case of extending over the light receiving elements, thereby enabling the defect inspection with high sensitivity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows an embodiment of an optical surface detect inspection apparatus according to the invention; 
           [0011]      FIG. 2  is an illustration of a mechanism and operation of scanning a whole surface of a subject by spiral scanning; 
           [0012]      FIG. 3A  shows a configuration of a sensor array according to the embodiment and a relation between the sensor array and the subject; 
           [0013]      FIG. 3B  shows the configuration of the sensor array according to the embodiment and a configuration of a preprocessing unit of the sensor array; 
           [0014]      FIG. 4A  shows a configuration of a conventional sensor array and a relation between the conv. sensor array and the subject; 
           [0015]      FIG. 4B  shows the configuration of the conventional sensor array and a configuration of a preprocessing unit of the conventional sensor array; and 
           [0016]      FIG. 5  shows a general configuration of the preprocessing unit according to the embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]      FIG. 1  shows an embodiment of an optical surface defect inspection apparatus (hereinafter, referred to merely as “inspection apparatus”)  100 . The inspection apparatus  100  includes an inspection optical system  1  that irradiates a surface of a subject  2  in a form of a disc such as a magnetic disc, an IC wafer, or the like being a workpiece with an inspection light, a frame  9  that supports the inspection optical system  1  on the apparatus, and a scanning unit  10  that scans the subject  2  so as to scan a whole surface of the subject  2 . The inspection apparatus  100  also includes a preprocessing unit  50  that processes an output from the inspection optical system  1 , and a data processor  11  that controls the scanning unit  10  and includes a processing unit  12  that inputs the output from the preprocessing unit  50  and processes the data. 
         [0018]    A mechanism and operation of scanning the whole surface of the subject by spirally scanning the doughnut-shaped subject  2  as shown in  FIG. 2  is explained below. 
         [0019]    A work table  3  is, as shown in  FIG. 1 , supported by a linear movement table  5  and a  8  rotation table  6 . The linear movement table  5  linearly moves in a direction R, and the A rotation table  6  is provided on the linear movement table  5 . The θ rotation table  6  is provided with an encoder  6   a  that generates a signal indicative of a rotation angle, and the linear movement table  5  is provided with an encoder  5   a  that generates a signal indicative of a movement position in the direction R. The signal from each encoder  5   a,    6   a  is transmitted to a data processor  11  (interface  14 ) as a scanning position signal. 
         [0020]    Denoted by  2   a  is a sensor detecting that the subject  2  is placed on the work table  3 . Denoted by  3   a  is a guide pin for setting the subject  2  such that the center of the doughnut-shaped subject  2  coincides with the center of rotation of the θ rotation table  6 . Denoted by  8  is a θ-direction drive circuit that drives the A rotation table  6 , and the rotating direction, the rotating speed, the stopping position and the like of the work table  3  are controlled through the drive circuit. Denoted by  7  is an R-direction drive circuit that linearly moves the linear movement table  5  in the direction R. These drive circuits are controlled in accordance with a control signal from the data processor  11 . 
         [0021]    By controlling such a mechanism with a constant-speed spiral scanning program  13   b  stored in a storage unit  13 , the subject  2  is spirally scanned. Specifically, the subject  2  is placed such that the center of the subject  2  coincides with the center of rotation of the θ rotation table  6 , and an inspection light  21  is set at an inner edge of the doughnut. Subsequently, while rotating the work table  3  at a constant speed by the θ rotation table  6 , the work table  3  is moved in the radial (R) direction of the subject  2 , for example in the left-to-right direction in  FIG. 1 , by the linear movement table  5 . This allows for scanning, i.e. inspecting, the whole surface of the subject  2  with the inspection light  21 . 
         [0022]    The scanning is not limited to the spiral shape but may be performed in a rectangular shape, or the scanning may be performed by moving the inspection optical system. 
         [0023]    Measured data of the scattered light at each measurement point when the whole surface is scanned is digitally converted by the preprocessing unit  50  and transferred to the data processor  11 , and each measurement point (scanning) position specified by each encoder  5   a,    6   a  and the measured value at the point are stored in a measurement result storing area  13   c  of the storage unit  13 . A defect analysis program  13   a  stored in the storage unit  13  analyzes the data from each measurement point of which position is identified, whereby the defect such as a scratch S or a foreign substance can be inspected for and the result can be displayed on a display device  15 . In  FIG. 1 , denoted by  16  is a bus. 
         [0024]    A configuration of the inspection optical system  1  being a feature of the embodiment of the present invention is explained below with reference to  FIG. 2 . The inspection optical system  1  includes a laser unit (light source)  20  that irradiates the surface of the subject  2  with the laser light  21  and a scattering optical system  30  that forms an image on a light receiving surface of a sensor array  40  with scattered light  31  from among the light reflected by the defect S on the subject  2 . An irradiation point in this embodiment is a position offset from the center of the doughnut-shaped subject  2 , and the whole surface is scanned by moving the subject  2  in the direction R. 
         [0025]    The scattering optical system  30  includes an objective lens  32 , a mask  34  that blocks a regular reflected light  26  from among the whole reflected light, and an imaging lens that focuses the scattered light  31 , which its regular reflected light  26  has been cut, onto the sensor array  40 . A horizontal resolution in an array direction of the light receiving element, i.e. in the horizontal direction with respect to the thickness direction, can be defined by the size of the light receiving element in the array direction/detection magnification of the scattering optical system. For example, assuming here the size of the light receiving element in the array direction as 500 μm and the detection magnification of the scattering optical system as  100 , the horizontal resolution is 5 μm. The width of the dead zone is, based on 10% of the width of the light receiving element, 0.5 μm. 
         [0026]      FIGS. 3A and 3B  show a configuration of the sensor array  40  according to the embodiment (hereinafter, “the present sensor array”) and a relation between the present sensor array  40  and the subject  2  ( FIG. 3A ) as well as a configuration of the preprocessing unit  50  of the present sensor array  40  ( FIG. 3B ). On the other hand,  FIGS. 4A and 4B  show a configuration of a conventional sensor array (hereinafter, “conv. sensor array”)  70  and a relation between the conv. sensor array  70  and the subject  2  ( FIG. 4A ) as well as a configuration of a preprocessing unit  80  of the conv. sensor array  70  ( FIG. 4B ). 
         [0027]    The present sensor array  40  and the conv. sensor array  70  include a plural number (n) of light receiving elements  40   1  to  40   n ,  70   1  to  70   n , respectively, and both are arranged in parallel with the subject  2  in the direction R. The conv. sensor array  70  is, as shown in  FIG. 4B , provided with a dead zone  71  that is an insulator separating light receiving elements so as to be perpendicular to the array direction of the sensor array  70 . 
         [0028]    On the other hand, as shown in  FIG. 3B , the present sensor array  40  is provided with a dead zone  41  obliquely to the array direction of the sensor array  40 , making the structure of the light receiving element rhombic. In this embodiment, the oblique angle θ is 30 degrees. When the oblique angle θ is smaller than 25 degrees, it approaches the conv. sensor array leading to decrease of an effect to be described later, and when it is larger than 45 degrees, the scattered light  31  always extends over the dead zone  41  or even extends over two dead zone  41  in some locations, which is not desirable. Accordingly, the oblique angle θ is preferably in a range of 25 to 45 degrees. 
         [0029]    As a result, as shown in  FIGS. 3B and 4B , in a case of a defect Sb causing the scattered light  31  extending over the dead zone  41 ,  71 , a detection region Rb indicated by a shadow mark is divided into regions of light receiving elements  40   m  and  40   m+1  (m≧1, integer, the same hereinafter) or the light receiving elements  70   m  and  70   m+1 , respectively, leading to reduction of detection sensitivity. 
         [0030]    Especially in a case of a defect Sc with the size equal to or smaller than the width of the dead zone  71 , a detection region Rc indicated by a shadow mark is buried in the dead zone  71  of the conv. sensor array  70  at the worst, and an output signal cannot be obtained. For example, if the dead zone is 5 μm, a defect of 0.5 μm or less may not be detected. 
         [0031]    On the other hand, with the present sensor array  40 , because the dead zone  41  is provided obliquely, the detection region Rc on either side of the dead zone  41  has a region in which the output can be obtained in at least one of the light receiving elements  40   m  and  40   m+1 . Accordingly, although the detection sensitivity of the present sensor array  40  may be reduced by the amount of the crossing dead zone, the output signal can be obtained. 
         [0032]    To compensate for the reduction in the detection sensitivity due to the dead zone  41 , as shown in  FIG. 3B , the embodiment is provided with adders  52  ( 52   1  to  52   n−1 : see  FIG. 5 ) adding outputs from two adjacent light receiving elements. As a result, the reduction of the detection sensitivity can be suppressed to the amount of the crossing dead zone  41 . The influence by the reduction of the light receiving amount can also be reduced. 
         [0033]    The effect of providing the adders  52  can also benefit the case of using the conv. sensor array  70 . When using the conv. sensor array  70 , not much effect is brought about in a case where the detection region Rc is buried in the dead zone  71  at the worst, but the effect similar to that of the present sensor array  40  is brought about in a case where the detection region Rb extends over two light receiving elements  70 . 
         [0034]    A prior art preprocessing unit  80  shown in  FIG. 4B  inputs, for example, an output from an average value calculation circuit  82   m+1  for the outputs from the light receiving elements  70   m  and  70   m+2  on both sides of the central light receiving element  70   m+1  and an output from the central light receiving element  74   m+1 , into a differential amplifier  83   m+1  to generate an output signal. The prior art processing circuit  80  detects difference in light receiving positions depending on the defect type such as an unevenness and therefore it is suitable for detecting the size and the type of the defect, but may not be very effective in inhibiting the reduction of the detection sensitivity. For example, in the aforementioned case of the defect Sb, taking a comprehensible example in which the outputs from the light receiving elements  70   m  and  70   m+1  are equal to Ta and the outputs from the light receiving elements  70   m−1 ,  70   m+1  are zero, the outputs from the average value calculation circuit  82   m+1  and the differential amplifier  83   m+1  are both Ta/2, resulting in the reduction of the detection sensitivity. 
         [0035]    However, by inputting the output from the adder  52  according to the embodiment to the prior art preprocessing unit  80  as the output from the present sensor array  40  or the conv. sensor array  70 , the effect of the prior art is also available. 
         [0036]      FIG. 5  shows a general configuration of the preprocessing unit  50  according to the embodiment. The output from each light receiving element  40   1  to  40   n  is input to each current/voltage converter  51  ( 51   1  to  51   n ) that converts the output current to a voltage. The outputs from two adjacent current/voltage converters  51 , such as the current/voltage converter  51   1  and  51   2 , are input to the adder  52   1  described above. The output from the adder  52   1  is, to reduce noise, passed through a high frequency cut filter  53   1  and low frequency cut filter  54   1 , converted to a digital signal by an A/D converter  55   1 , and introduced into the data processor  11  shown in  FIG. 1  via the interface  14 . 
         [0037]    The adder may be a summing amplifier. The signal from each of the light receiving elements  40   1  to  40   n  may be amplified and then A/D converted, and the subsequent post-processing may be performed by the processing unit  12 . Furthermore, both the A/D conversion and the processing corresponding to the preprocessing unit  80  may be performed by the processing unit  12 . 
         [0038]    As already described with reference to  FIG. 1 , with the aid of the defect analysis program  13   a  stored in the storage unit  13 , the data of each measurement point of which position is identified can be analyzed, an inspection of the defect S such as the scratch or the foreign substance can be performed, and the result can be displayed on the display device  15 . 
         [0039]    According to the embodiment described above, it is possible to provide the optical surface defect inspection apparatus or the optical surface defect inspection method that reduces the influence from the dead zone of the sensor array and enables the defect inspection with high sensitivity.