Abstract:
The present invention provides a method and apparatus for inspecting a surface of a substrate. The apparatus includes: a rotatable stage on which a substrate to be inspected is placed; an inspection optical system having an illumination light source for emitting light to a substrate placed on the stage and a detector for detecting light from the substrate which is irradiated with the light from the illumination light source; an A/D converter for amplifying and A/D converting signals output from the detector in the inspection optical system; and a defect detector for detecting defects in a surface of the substrate by processing signals output from the detector and converted by the A/D converter and classifying the defected defects. The defect detector extracts micro defects in the surface of the substrate by processing the signals output from the detector, and detects linear defects existing discretely in a linear region.

Description:
BACKGROUND 
     The present invention relates to a method and apparatus for detecting a defect in a surface of a substrate and, more particularly, to a substrate surface inspecting method and apparatus suitable for detecting a linear defect in a surface of a magnetic disk substrate. 
     For an apparatus for inspecting a surface of a substrate for a magnetic disk, there is a need to classify detected defects in order to contribute to sophistication in process management and improvement in process. Generally, a detection optical system in an apparatus for inspecting a surface of a substrate for a magnetic disk includes plural detectors. In addition to classification of micro defects by detection signals from the plural detectors, classification of the defects on the basis of a feature to a distribution shape of the defects in a magnetic disk face is requested. Specifically, although continuous micro defects can be recognized as linear defects, defects distributed discretely and linearly (hereinbelow, written as a linear scratch defect) have to be discriminated from a collection of the other micro defects. 
     For example, Japanese Patent Application Laid-Open Publication No. 2000-180376 (patent document 1) discloses an apparatus for detecting a defect in a surface of a magnetic disk, which emits a laser beam to a magnetic disk as a sample to be inspected, receiving reflection light and scattering light from the surface of the magnetic disk by plural detectors, and classifying micro defects in accordance with light reception conditions of light receivers of the detectors. By determining continuity in a plane of the detected micro defects, the length of the defect is detected and, the defects are classified to a linear defect and a lump defect. 
     Japanese Patent Application Laid-Open Publication No. 2009-180590 (patent document 2) describes that, using an apparatus for detecting a defect in a surface of a magnetic disk similar to that described in the patent document 1, the cycles of linear defects are detected to detect a wrinkled defect. 
     Further, Japanese Patent Application Laid-Open Publication No. 2006-352173 (patent document 3) describes a technique of classifying defects in accordance with a state of a distribution of defects obtained by testing the surface of a semiconductor wafer. 
     Defects linearly distributed include continuously linear defects and defects which are discretely distributed in a linear region. In the defect classifying method described in the patent document 1, although continuously linear defects can be detected as linear defects, detection of defects which are discretely distributed in a linear region (continuous scratch defect) so as to be discriminated from random micro defects is not considered. In the invention disclosed in the patent document 2, since increase/decrease in reflection light generated by mild roughness in the surface of a magnetic disk is detected, micro defects cannot be detected. Further, in the defect classifying method described in the patent document 3, only the defect position information is used. Consequently, it is difficult to select and process a defect point having a specific feature from defects obtained from the plural detectors. 
     SUMMARY 
     The present invention is to address the problems of the above-described related arts and provide a method and apparatus for inspecting a surface of a substrate, capable of classifying defects (linear scratch defects) which are discretely and linearly distributed in the surface of a substrate so as to be discriminated from micro defects which are distributed at random. 
     To achieve the object, according to an aspect of the present invention, there is provided a substrate surface inspecting apparatus including: a rotatable stage on which a substrate to be inspected is placed; an inspection optical system having one or more illumination light sources for emitting light to a substrate placed on the stage and one or more detectors for detecting reflection/scattering light from the substrate which is irradiated with the light from the illumination light sources; an A/D converter for amplifying and A/D converting signals output from the one or more detectors in the inspection optical system; and a defect detector for detecting defects in a surface of the substrate by processing signals output from the one or more detectors and converted by the A/D converter and classifying the defected defects. The defect detector extracts micro defects in the surface of the substrate by processing the signals output from the one or more detectors, and detects linear defects existing discretely in a linear region from the extracted micro defects. 
     To achieve the object, according to another aspect of the present invention, there is provided a method for inspecting a surface of a substrate, including the steps of: emitting one or more illumination beams from one or more illumination light sources to a substrate placed on a rotatable stage while rotating the stage; detecting reflection/scattering light from the substrate which is irradiated with the light from the one or more illumination light sources by one or more detectors; amplifying and A/D converting signals output from the one or more detectors which have detected the reflection/scattering light from the substrate; detecting defects in a surface of the substrate by processing the A/D converted signals output from the one or more detectors; and classifying the defected defects. In the step of detecting the defect, micro defects in the surface of the substrate are extracted by processing the signals output from the one or more detectors and, in a step of classifying the detected defects, linear defects existing discretely in a linear region are detected from the extracted micro defects. 
     According to the aspects of the present invention, defects which occur linearly and discretely in a surface of a substrate of a magnetic disk can be detected as linear defects so as to be distinguished from defects which occur at random. 
     By grasping the shapes of the detected defects which occur linearly and discretely in the surface of a magnetic disk and the occurrence positions in the substrate of the magnetic disk, places as causes of defects in a manufacturing process can be narrowed down. 
     These features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a general schematic configuration of a surface inspecting apparatus as an embodiment of the present invention; 
         FIG. 2  is a flowchart of processes for detecting and classifying a defect in the embodiment of the invention; 
         FIG. 3  is a flowchart of processes for extracting linear defects existing discretely in a linear region from micro defects in the embodiment of the invention; 
         FIGS. 4A and 4B  are diagrams for explaining the principle of the Hough transform; 
         FIGS. 5A and 5B  are diagrams showing an example of linear defects discretely existing in a linear region, which are extracted from detected micro defects; 
         FIGS. 6A and 6B  are diagrams showing an example of linear defects discretely existing in a curved region, which are extracted from defected micro defects; 
         FIG. 7  is a flowchart of a modification of the processes for detecting and classifying defects shown in  FIG. 2 ; and 
         FIG. 8  is a front view of a display screen for displaying a result of linear defects extracted in the embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. 
       FIG. 1  shows a schematic configuration of an apparatus  1000  for inspecting a surface of a magnetic disk according to an embodiment. The apparatus  1000  for inspecting a surface of a magnetic disk has two illumination detection optical systems (hereinbelow, called optical systems). A first optical system  100  has one illuminating unit and three detecting units. The illuminating unit is a first illuminating unit  110  for emitting a laser beam having a first wavelength to the surface of a magnetic disk as a sample  1  from a high-angle direction. The three detecting units are a high-angle detecting unit  120 , an intermediate-angle detecting unit  130 , and a low-angle detecting unit  140 . 
     The high-angle detecting unit  120  has a lens  121  condensing reflection/scattering light including regular-reflection light which travels in a high-angle direction out of directions indicated by broken lines, emitted from the first illuminating unit  110  and reflected/scattered by the surface of the sample  1 , a wavelength selection filter  122  which transmits light having the same wavelength as that of the laser beam emitted from the first illuminating unit  110  in the light condensed by the condenser lens  121  and blocks light having different wavelengths, a high-angle detector  123  for detecting the light which passed through the wavelength selection filter  122 , a mirror  124  for reflecting the regular reflection light from the sample  1  in the light condensed by the condenser lens  121 , a wavelength selection filter  125  which transmits light having the same wavelength as that of the laser beam emitted from the first illuminating unit  110  in the regular reflection light from the sample  1  reflected by the mirror  124  and blocks light having different wavelengths, and a regular-reflection light detector  126  for detecting the light which passed through the wavelength selection filter  125 . 
     The intermediate-angle detecting unit  130  has a condenser lens  131  for condensing scattering light which travels in an intermediate-angle direction in the light emitted from the first illuminating unit  110  and reflected/scattered by the surface of the sample  1 , and an intermediate-angle detector  132  for detecting the light condensed by the condenser lens  131 . 
     The low-angle detecting unit  140  has a condenser lens  141  for condensing scattering light which travels in a low-angle direction in the light emitted from the first illuminating unit  110  and reflected/scattered by the surface of the sample  1 , and a low-angle detector  142  for detecting the light condensed by the condenser lens  141 . 
     A second optical system  200  has one illuminating unit and one detecting unit. The illuminating unit of the second optical system  200  is a second illuminating unit existing in a plane different from that of the first optical system  100  and emitting a laser beam having a second wavelength to the surface of the magnetic disk as the sample  1  from an oblique direction. The detecting unit of the second optical system  200  has a detector  222  for detecting light passed through a pinhole device  221  in regular-reflection light which travels in a direction indicated by the solid line in light condensed/diffused by the roughness of the surface of the sample  1  in the laser beam emitted from the second illuminating unit  210 . Signals output from the detectors  123 ,  222 ,  126 ,  132 , and  142  are amplified and A/D converted by A/D converters  151 ,  152 ,  153 ,  154 , and  155 , respectively, and the resultant digital signals are supplied to a processor  160 . 
     The processor  160  has: a defect detector  161  for receiving the A/D converted signals output from the detectors  123 ,  222 ,  126 ,  132 , and  142  and detecting a micro defect; a defect candidate point selector  162  for receiving a signal from the defect detector  161 , determining the kind of the micro defect and selecting a defect candidate point; an arithmetic processor  163  for receiving a signal from the defect candidate point selector  162  and performing arithmetic process on the signal; and a linear defect kind determining unit  164  for receiving a result of the process performed by the arithmetic processor  163  and determining the kind of a linear defect. 
     To the processor  160 , an input/output unit  170  having a display screen  171  is connected. The processor  160  and the input/output unit  170  are connected to an overall controller  180 . The overall controller  180  controls a stage unit  190  having a stage on which the sample  1  is placed and which rotates the sample  1  and is movable at least in one direction in a plane in which the sample  1  rotates. 
     With the above-described configuration, under control of the overall controller  180 , the sample  1  on the stage unit  190  is rotated around the normal to the surface of the sample  1  as a rotation center, and starting to move at constant speed in a direction orthogonal to the normal. 
     In this state, a laser beam having a first wavelength is emitted from the first illuminating unit  110  in the first optical system  100  to the surface of the sample  1  which is on the rotating stage of the stage unit  190 . Also from the second illuminating unit  210  in the second optical system  200 , a laser beam having a second wavelength is emitted to a region irradiated with the first laser beam in the surface of the sample  1  which is on the rotating stage of the stage unit  190 . 
     The optical path of a regular-reflection-light component in the light condensed by the condenser lens  121  is bent by the mirror  124  toward the detector  126 . Light obtained by removing the regular-reflection light in the light condensed by the condenser lens  121  (scattering light around the optical axis of the regular-reflection light) enters the wavelength selection filter  122 . The light of the component having the same wavelength as that of the first laser beam passes through the wavelength selection filter  122  and light of the other wavelength components is blocked by the wavelength selection filter  122 . The light from which the regular-reflection light is eliminated and passed through the wavelength selection filter  122  is incident on the detector  123  and detected. 
     On the other hand, only light components having the same wavelength as that of the first laser beam in the regular-reflection-light component whose optical path is bent by the mirror  124  toward the detector  126  pass through the wavelength selection filter  125 , and the light of the other wavelength components are blocked by the wavelength selection filter  125 . The regular-reflection light which passed through the wavelength selection filter  125  is incident on the detector  126 . That is, the laser beam emitted from the second illuminating unit  210  and reflected/scattered from the sample and condensed by the condenser lens  121  is blocked by the wavelength selection filters  122  and  125  and is not detected by any of the detectors  123  and  126 . 
     The second optical system  200  is disposed at an angle different from the angle of the first optical system  100  in plan view in which the magnetic disk as the sample  1  is seen in a circular shape so that the influences of the regular-reflection light components of the systems are ignorable. 
     The sample  1  is moved straight while being rotated and an inspection is conducted from the outer rim toward the inner rim of the sample  1  to inspect the entire surface of the sample  1 . The sample  1  is put upside down by using a substrate turning mechanism (not shown) so that the rear face which is not inspected yet faces upward. By conducting an inspection similar to that on the surface, both faces of the sample can be inspected. 
     In the embodiment, the wavelength selection filters  122  and  125  are used as optical elements in the optical path in the first optical system  100 . Instead of them, an optical mask filter (including a pinhole device) or a polarization filter may be used singularly or together with a wavelength selection filter to make light of a specific component pass. 
     Next, the flow of processes performed to detect a defect in the surface of the sample  1  by using the inspection apparatus shown in  FIG. 1  will be described with reference to  FIG. 2 . 
     First, the sample  1  is continuously moved in one direction while being rotated by the stage unit  190 . In this state, a first laser beam emitted from the first illuminating unit  110  in the first optical system  100  and a second laser beam emitted from the second illuminating unit  210  in the second optical system  200  are illuminating an inspection portion on the surface of the sample  1 . 
     The reflection/scattering light generated by the sample  1  by the irradiation of the first and the second laser beams is detected by the high-angle detecting unit  120 , the intermediate-angle detecting unit  130  and the low-angle detecting unit  140  in the first optical system  100 , and the detecting unit in the second optical system  200 . The detection signals output from the detectors  123 ,  222 ,  126 ,  132 , and  142  are amplified and A/D converted by the A/D converters  151 ,  152 ,  153 ,  154 , and  155 , respectively, and the A/D converted signals are supplied to the processor  160  (S 201 ). 
     The defect detector  161  extracts a defect candidate from the detection signals supplied from the detectors to the processor  160  (S 202 ). A feature amount of the extracted defect candidate is calculated (S 203 ), and defects which are continuous on the surface of the sample are extracted on the basis of the calculated feature amounts (S 204 ). Whether the extracted defects are linear or not is determined (S 205 ). When it is determined that the defects are linear, the defects are determined as continuous linear defects (S 206 ). When it is determined that the defects are not linear but are spread, the defects are determined as planar defects (S 207 ). Subsequently, a check is made to see whether or not the defect which is determined as a discontinuous defect in S 204  is detected by the detector  132  of the intermediate-angle detecting unit  130  and the detector  142  of the low-angle detecting unit  140  (S 208 ). When it is detected, the defect is determined as a foreign matter (S 209 ). When it is not detected, the defect is determined as a micro defect (S 210 ). 
     The flow of processes of extracting linear defects which are discretely distributed in a linear small region from defects determined as micro defects in S 210  will now be described with reference to  FIG. 3 . 
     First, a signal of a defect determined as a micro defect in S 210  is sent together with information of the feature amount which is calculated in S 203  to the defect candidate point selector  162  (S 301 ). The detected micro defects are distributed on the disk plane by using position information, and kinds of defects which are components of one of linear defects are selected (S 302 ). The number of kinds of defects to be selected is not limited to one but plural kinds may be selected. In case of selecting the classified defect kinds, a micro defect group forming linear defects may be selected by directly designating a light reception condition of the detectors. 
     The information of the selected defect kind or the micro defect group is sent to the arithmetic processor  163 . Using the data of the position of each of the micro defects, defect candidates existing linearly are extracted from them by, for example, a Hough transform process (S 303 ). 
     As shown in  FIG. 4A , the Hough transform determines a straight line on which feature points (the positions of defects in the embodiment) exist the most. For example, angle θ to the x axis formed by a normal extending from the point θ to a straight line connecting two points P 1  and P 2  and length ρ of the normal are sequentially calculated for each of straight lines connecting two points, and are plotted to a two-dimensional space as shown in  FIG. 4B . A straight line expressed by a set of the length ρ and the angle θ determined by a point P(θm, ρm) as a crossing point is determined as the straight line on which feature points exist most. 
     By the Hough transform, even when distributions of points which construct a line are discontinuous, the line on which the points exist can be detected. The Hough transform is effective to extract linear defect components from defect points which exist discretely. 
     In the Hough transform process, a line on which at least a predetermined number of defect points exist is detected. In consideration of positional errors in detecting defect candidates, as shown in  FIGS. 5A and 5B , defect points existing within a distance “w” of a margin of a detected straight line are regarded as linear defect points, and the total number of linear defect points and defect density are calculated including the regarded defect points. 
     Using a line end at which the defect density is equal to or less than a predetermined value as a defect end point, the length and position of the linear defect candidates are determined. From the determined linear defect candidates, defects are determined on the basis of spatial feature amounts such as length, position, and density. 
     The process is not limited to detection of defects in a straight line shape. By defining a circle or a circular arc, or a figure indicated by an arbitrary function using the generalized Hough transform, for example, as shown in  FIGS. 6A and 6B , micro defects in a curved-line shape along the defined shape and the center position of the curvature of the curved line can be detected. 
     By performing the process as described above, even linear defects formed by a group of discrete points can be detected so as to be discriminated from other random defects. 
     To shorten the process time, a detection shape condition may be limited in advance in accordance with a process condition. In the case where the correspondence between a geometric shape of a detected defect and a defect generating process is determined empirically or logically, it may be formulated and a determination may be made by using the formula. 
     The data processed by the arithmetic processor  163  is sent to the linear defect kind determining unit  164 , and the kind of the linear defect is determined on the basis of the physical shape of the detected linear defect (S 304 ). For example, when a line to be detected is a circular or a circular arc, the center point of the circle or the circular arc and the radius are also calculated as geometric feature amounts. 
     Although the discrete defect points are classified to a foreign matter or a micro defect depending on the presence or absence of a detection signal of the scattering light detection system in S 208 , they may not be classified in S 208 . In  5303 , whether defects (random defects) which are not determined as candidates existing in a linear shape may be determined as foreign matters or not on the basis of the condition of S 208 . 
     The defects determined as continuous linear defects in S 206  may be supplied together with the micro defect in S 210  to the defect candidate point selector in S 301 . In such a manner, the defects as linear defects generated by the same source and having the features classified to the continuous part and the discrete part can be detected as a single defect. The flow of the modification is shown in  FIG. 7 . 
     The flow of  FIG. 7  will be described. 
     First, the sample  1  is continuously moved in one direction while being rotated by the stage unit  190 . In this state, a first laser beam emitted from the first illuminating unit  110  in the first optical system and a second laser beam emitted from the second illuminating unit  210  in the second optical system  200  are illuminating an inspection portion on the surface of the sample  1 . The reflection/scattering light generated by the sample  1  by the irradiation of the first and the second laser beams is detected by the high-angle detecting unit  120 , the intermediate-angle detecting unit  130  and the low-angle detecting unit  140  in the first optical system  100 , and the detecting unit in the second optical system  200 . The detection signals output from the detectors  123 ,  222 ,  126 ,  132 , and  142  are amplified and A/D converted by the A/D converters  151 ,  152 ,  153 ,  154 , and  155 , respectively, and the A/D converted signals are input to the processor  160  (S 701 ). 
     The defect detector extracts a defect candidate from the detection signals input from the detectors to the processor  160  (S 702 ). A feature amount of the extracted defect candidate is calculated (S 703 ), and defects which are extracted on the basis of the calculated feature amounts are determined whether continuous defects or not (S 704 ). Then, the defects determined as continuous defects (YES in S 704 ) are determined whether linear defects or not (S 705 ). In the case of YES, the defects are determined as continuous linear defects (S 706 ). In the case of NO, the defects are determined as planar defects (S 707 ). On the other hand, a defect which is determined as a discontinuous defect (NO) in S 704  is classified as a micro defect (S 708 ), and the information of the micro defect is supplied to the defect candidate point selector  162  (S 709 ). A defect candidate point is selected from the information of the micro defect which is supplied to the defect candidate point selector  162  and the defects determined as continuous linear defects in S 706  (S 710 ). The linear defects are extracted from the information of the defect candidate points (S 711 ), and the kind of the extracted linear defects is determined (S 712 ). 
     By selecting the defect candidate points from both of the micro defects as discontinuous defects and the continuous linear defects, defects as linear defects generated by the same source and separated to the continuous part and the discrete part can be detected as a single defect. On the other hand, the defects which are not extracted as linear defects in S 711  are determined as nonlinear (random) defects. The input signals are checked to see whether or not there are detection signals from the scattering light detection systems  132  and  142  on the nonlinear defects (S 713 ). In the case where there are detection signals from the scattering light detection systems  132  and  142 , the micro defects are determined as foreign matters (S 714 ). On the other hand, in the case where there are no detection signals from the scattering light detection systems  132  and  142 , the defect is determined as a small flaw in the surface of the substrate (S 715 ). 
     Generally, in a magnetic disk polishing process, while making a lower board having a top face to which a donut-shaped grinder is attached rotate, an upper board having an under face to which a grinder is attached is made rotate and revolve, the top and under faces of a magnetic disk substrate sandwiched by the grinders are polished by the grinders. The configuration of such a polishing apparatus is described in, for example, Japanese Patent Application Laid-Open Publication No. H04-013553. Since the magnetic disk substrate is polished in such a manner, the size and the center position of an arc of linear defects caused in the polishing process are limited to a certain degree. Consequently, whether the center point of the arc of the defects is the center of the magnetic disk or not is determined. In the case of NO, a defect source process can be estimated depending on whether the center point of the arc of the defects is in the plane of the magnetic disk or on the outside. In the case where the center point of the arc coincides or almost coincides with the center of the magnetic disk, the defect can be estimated as a defect caused by a head touch. Obviously, defect which occurs in the process such as a defect which occurs in handling the substrate can be estimated. 
       FIG. 8  illustrates an example of the screen  171  of the input/output unit  170  for outputting a result of detection of discrete linear defects detected in the embodiment. 
     The screen  171  outputting an inspection result includes a defect map display region  810  showing the positions on the magnetic disk, of detected linear defects in a map form, and a data display region  820  displaying data related to the detected linear defects. In the defect map display region  810 , a linear defect A  812 , a linear defect B  813 , and a linear defect C  814  are displayed by kinds of the linear defects detected in an outline  811  of the magnetic disk. The data display region  820  includes a region  821  displaying disk information such as lot No. and disk No. of a magnetic disk to be inspected, a region  822  displaying date and time of an inspection, and a region  823  displaying information on a linear defect such as length of each linear defect, defect density, defect width (width W in  FIG. 4 ), the number of micro defects, and average size of micro defects. 
     In the embodiment, micro defects which are discretely detected are extracted from defect signals obtained from detection signals of reflection/scattering light from the surface of a magnetic disk. From the extracted discrete defects, linear defects (in straight line and curved line) existing in linear regions can be detected. Therefore, linear defects having relatively low density, which occur in a magnetic disk manufacturing process can be detected at higher sensitivity. 
     Although the present invention achieved by the inventors herein has been concretely described above on the basis of the embodiments, obviously, the present invention is not limited to the foregoing embodiments but can be variously modified without departing from the gist. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.