Patent Publication Number: US-2023137226-A1

Title: Inspection apparatus and focal position adjustment method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-177889, filed Oct. 29, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an inspection apparatus and a focal position adjustment method for inspecting a pattern formed on a sample. 
     BACKGROUND 
     In a manufacturing process of a semiconductor device, a circuit pattern is transferred onto a semiconductor substrate by means of reduction exposure with an exposure apparatus (also referred to as a “stepper” or a “scanner”). In such an exposure apparatus, a mask (also referred to as a “reticle”) on which an original pattern (hereinafter also simply referred to as a “pattern”) is formed is used. 
     One of the causes of reduced yields in manufacturing of semiconductor devices is defects in the mask pattern. 
     In most-advanced devices, it is required, for example, to form a pattern with a line width of several nanometers. With the miniaturization of the pattern, defects in the mask pattern are also miniaturized. Accordingly, a mask inspection apparatus with enhanced precision for detection of minute defects has been demanded. 
     In a mask inspection apparatus, a mask is placed on a stage. The stage moves, and thereby light with which the stage is irradiated via an optical system scans the mask. Light that has transmitted through or been reflected from the mask is caused by a lens to form an image on a sensor. Thereby, an optical image is acquired. 
     To capture an optical image with a fine pattern, development has been made to increase the magnification and the numerical aperture (NA) of the optical system of the inspection apparatus. This causes the optical system to have a shallow depth of focus for the mask. With the shallow depth of focus, a slight change in the distance between the optical system and the mask causes defocusing of light. This results in blurring of the pattern image in the optical image, adversely affecting the optical image acquisition and the defect detection processing. An inspection apparatus with a real-time autofocus mechanism for suppressing defocusing is known. 
     For example, Patent Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2010-217317) discloses a method in which focal position information of an inspection region is subjected to computation processing using a polynomial approximation, and focal position control of an adjacent inspection region is performed based on results of the computation. Enhancing the speed of movement of the stage during optical scanning for enhancing the inspection speed may cause a delay in autofocus tracking, possibly causing defocusing in the presence of an abrupt step height. 
     The present invention has been made in view of the above-described circumstances. That is, the present invention aims to provide an inspection apparatus and a focal position adjustment method capable of decreasing an autofocus tracking delay. 
     SUMMARY 
     According to a first aspect, an inspection apparatus includes: a stage on which a sample is placed; an illumination optical system configured to irradiate the sample with light used for optical scanning of the sample; an imaging optical system including a sensor that detects a focal position, and configured to cause the light with which the sample is irradiated to form an image on the sensor; a detection circuit configured to detect a focal position signal of the light received by the sensor; a setting circuit configured to set a first focus offset value of a first region based on a result obtained by shifting, in an advancement direction of the stage, coordinate data of first focal position data generated based on a result obtained by optically scanning the first region, the first region being included in a plurality of stripe-shaped regions into which the sample is virtually divided in a direction orthogonal to the advancement direction of the stage; and a control circuit configured to control a height position of the stage based on the focal position signal and the first focus offset value. 
     According to a second aspect, a focal position adjustment method includes: optically scanning, in a first direction, a first region included in a plurality of stripe-shaped regions into which a sample is virtually divided in a direction orthogonal to an advancement direction of a stage on which the sample is placed; generating first focal position data based on a result obtained by optically scanning the first region; setting a first focus offset value of the first region based on a result obtained by shifting coordinate data of the first focal position data in the advancement direction of the stage; and optically scanning the first region by controlling a height position of the stage based on the first focus offset value and a focal position detection result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram showing an overall configuration of an inspection apparatus according to an embodiment. 
         FIG.  2    is a block diagram of an illumination optical system  140  and an imaging optical system  150  of the inspection apparatus according to the embodiment. 
         FIG.  3    is a diagram showing an example of an inspection region of a mask in an inspection apparatus according to the embodiment. 
         FIG.  4    is a flowchart of an inspection process in an inspection apparatus according to the embodiment. 
         FIG.  5    is a flowchart showing a flow of optical image acquisition in an inspection apparatus according to the embodiment. 
         FIG.  6    is a diagram showing a concrete example of approximation data of a Z coordinate obtained by means of a moving average and a polynomial approximation. 
         FIG.  7    is a diagram showing an example of pre-reading of focal position data in scanning of a stripe in a FWD direction in the inspection apparatus according to the embodiment. 
         FIG.  8    is a diagram showing an example of pre-reading of focal position data in scanning of a stripe in a BWD direction in the inspection apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments will be described with reference to the drawings. Each embodiment illustrates an apparatus and a method for embodying the technical idea of the invention. The drawings are either schematic or conceptual, and the dimensions, ratios, etc. of each drawing are not necessarily identical to the actual ones. The technical idea of the present invention is not designated by the shape, structure, arrangement, etc. of the components. 
     Hereinafter, a mask inspection apparatus will be described as an example of a sample inspection apparatus. In the present embodiment, a case will be described where an inspection apparatus captures an optical image; however, the configuration is not limited thereto. The inspection apparatus may capture an electron-beam image as an inspection image using, for example, a scanning electron microscope (SEM). 
     Moreover, in the present embodiment, a case will be described where a sample to be inspected is a mask used in photolithography, etc.; however, the configuration is not limited thereto. It may be any sample with a pattern provided on the surface, such as a template used in nanoimprint lithography (NIL), a wafer (semiconductor substrate), etc. 
     Furthermore, in the present embodiment, a die-to-database (D-DB) mode, in which an inspection image is compared with a reference image that is based on design data, will be described as a mask defect inspection mode in the inspection apparatus; however, the configuration is not limited thereto. The defect inspection mode may be a die-to-die (D-D) mode, in which an inspection image is compared with images of a plurality of regions with an identical pattern formed on the mask. 
     Furthermore, the focal position adjustment method described in the embodiment is not limited to an inspection apparatus, and may be applied to, for example, other apparatuses such as a charged particle beam irradiation apparatus used for producing a mask. 
     1. Overall Configuration of Inspection Apparatus 
     First, an example of an overall configuration of an inspection apparatus  1  will be described with reference to  FIG.  1   .  FIG.  1    is a diagram showing an overall configuration of the inspection apparatus  1 . The example of  FIG.  1    shows a configuration in which an optical image is acquired using light (hereinafter also referred to as “reflected light”) reflected from a mask  2 ; however, the configuration of the inspection apparatus  1  is not limited thereto. The inspection apparatus  1  may be configured to acquire an optical image using light that has transmitted through the mask  2  (hereinafter also referred to as “transmitted light”). Alternatively, the inspection apparatus  1  may be configured to acquire both an optical image using reflected light and an optical image using transmitted light. 
     As shown in  FIG.  1   , the inspection apparatus  1  includes an image acquisition mechanism  10  and a control mechanism  20 . The inspection apparatus  1  according to the present embodiment includes an autofocus mechanism. 
     The image acquisition mechanism  10  includes a stage  110 , an XY drive unit  120 , a Z drive unit  130 , an illumination optical system  140 , an imaging optical system  150 , a sensor circuit  160 , a focal position detection circuit  170 , a laser length-measuring system  180 , and an autoloader  190 . 
     The stage  110  is movable in an X direction that is parallel to a surface of the stage  110 , a Y direction that is parallel to the surface of the stage  110  and that intersects the X direction, and a Z direction that is perpendicular to the surface of the stage  110 . The mask  2  is placed on the stage  110 . 
     The XY drive unit  120  includes a drive mechanism for allowing the stage  110  to move in an XY plane configured of the X and Y directions. More specifically, the XY drive unit  120  includes an X-axis motor  121  that drives the stage  110  in the X direction and a Y-axis motor  122  that drives the stage  110  in the Y direction. A stepping motor, for example, may be used as each of the X-axis motor  121  and the Y-axis motor  122 . The XY drive unit  120  may include, for example, a rotation axis motor that rotates the stage  110  around a rotation axis on the XY plane, with the Z direction being the rotation axis. 
     An inspection region of the mask  2  is virtually divided into, for example, a plurality of stripe-shaped portions as viewed in the Y direction. Hereinafter, each of the portions into which the inspection region is divided will be referred to as a “stripe”. The XY drive unit  120  controls the operation of the stage  110  in such a manner that each of the divisional stripes are optically scanned continuously. 
     The Z drive unit  130  includes a drive mechanism for moving the stage  110  in the Z direction. 
     More specifically, the Z drive unit  130  includes, for example, a plurality of Z-axis actuators  131  which drive the stage  110  in the Z direction. For the Z-axis actuators  131 , actuators that use, for example, piezoelectric elements can be used. 
     The illumination optical system  140  includes a light source, and irradiates the mask  2  with illumination light emitted from the light source. Details of the illumination optical system will be described later. 
     The imaging optical system  150  causes reflected light from the mask  2  to form an image on a sensor. The imaging optical system  150  includes an optical image capturing sensor and a focal position measuring sensor. Details of the imaging optical system  150  will be described later. 
     The sensor circuit  160  subjects an electric signal received from the optical image capturing sensor in the imaging optical system  150  to an analogue-to-digital (A/D) conversion. The sensor circuit  160  transmits optical image data that is based on a digital signal obtained by the conversion to a comparison circuit  211  of the control mechanism  20 . The optical image is based on a pattern of the mask  2 . For the optical image, brightness (luminance) of each of the pixels into which the image capturing region is divided on the XY plane is expressed as a tone value. If, for example, the tone value is represented as 8-bit data, the pixel value of each pixel is represented as a tone value from 0 to 255. 
     The focal position detection circuit  170  detects a focal position (a deviation amount of a focal point) based on an electric signal (a focal position signal) received from the focal position measuring sensor in the imaging optical system  150 . The focal position detection circuit  170  generates a focal position adjustment signal based on the detected focal position. The focal position adjustment signal is a signal for bringing the focal position of the illumination light into focus on a pattern surface of the mask  2 . The focal position adjustment signal contains information on a correction amount of a Z coordinate of the stage  110 . The focal position detection circuit  170  transmits the focal position adjustment signal to a Z drive unit control circuit  207 . The focal position detection circuit  170  is included in the autofocus mechanism. 
     The laser length-measuring system  180  measures a position (also referred to as a “stage position”) of the stage  110  in the X and Y directions. The laser length-measuring system  180  transmits information on the measured stage position in the XY plane to a position circuit  212  of the control mechanism  20 . 
     A plurality of masks  2  are set on the autoloader  190 . The autoloader  190  loads a mask  2  to be inspected onto the stage  110 . The autoloader  190  unloads, from the stage  110 , a mask  2  for which an optical image capturing has ended. 
     The control mechanism  20  includes a control calculator  200 , a storage unit  201 , a display unit  202 , an input unit  203 , a communication unit  204 , an autoloader control circuit  205 , an XY drive unit control circuit  206 , the Z drive unit control circuit  207 , an FD generation circuit  208 , a development circuit  209 , a reference circuit  210 , the comparison circuit  211 , and the position circuit  212 . These are coupled to each other via, for example, a bus line. 
     The autoloader control circuit  205 , the XY drive unit control circuit  206 , the Z drive unit control circuit  207 , the development circuit  209 , the reference circuit  210 , the comparison circuit  211 , and the position circuit  212  may be configured of programs executed by the control calculator  200 , configured of hardware or firmware included in the control calculator  200 , or configured of individual circuits controlled by the control calculator  200 . Hereinafter, a case will be described where the functions of these circuits are implemented based on programs executed by the control calculator  200 . 
     The control calculator  200  controls the entirety of the inspection apparatus  1 . More specifically, the control calculator  200  controls the image acquisition mechanism  10 , the storage unit  201 , the display unit  202 , the input unit  203 , the communication unit  204 , the autoloader control circuit  205 , the XY drive unit control circuit  206 , the Z drive unit control circuit  207 , the development circuit  209 , the reference circuit  210 , the comparison circuit  211 , and the position circuit  212 . The control calculator  200  acquires an optical image by controlling the image acquisition mechanism  10 . Also, the control calculator  200  generates a reference image by controlling the control mechanism  20 . The control calculator  200  compares the optical image with the reference image, and detects a defect, etc. in the pattern. The control calculator  200  includes, for example, an unillustrated central processing unit (CPU). The CPU executes, for example, an inspection program  223  in the storage unit  201 . The control calculator  200  may be a CPU device such as a microprocessor, or a computer apparatus such as a personal computer. Also, at least some of the functions of the control calculator  200  may be served by other integrated circuits such as an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or a graphics processing unit (GPU). 
     The storage unit  201  stores information relating to inspection. More specifically, the storage unit  201  stores design data  220 , inspection condition data  221 , inspection data  222 , an inspection program  223 , etc. The storage unit  201  may include, as an external storage, various storage apparatuses such as a magnetic disk storage (a hard disk drive, HDD) or a solid-state drive (SSD). 
     The design data  220  is design data of the mask  2 . 
     The inspection condition data  221  include parameters such as image capturing conditions and inspection conditions. 
     The inspection data  222  contains data relating to the reference image, the optical image, and the inspection results. Also, the inspection data  222  contains focal position data FD. The focal position data FD is data based on X, Y, and Z coordinates of the mask  2  (stage  110 ) during optical scanning of the mask  2 . 
     The inspection program  223  is a program for executing a pattern inspection. The storage unit  201  stores the inspection program  223  as a non-transitory storage medium. The inspection program  223  is stored in, for example, a read-only memory (ROM). 
     The display unit  202  is a display apparatus such as a cathode ray tube (CRT) display, a liquid crystal display, or an organic electro luminescence (EL) display. The display unit  202  may include a speech output apparatus. 
     The input unit  203  is an input apparatus such as a keyboard, a mouse, a touch panel, or a button switch. 
     The communication unit  204  is an apparatus for connection to a network to perform data transmission and reception to and from an external apparatus. For such communications, various communication standards may be used. The communication unit  204  is configured, for example, to receive design data from an external apparatus, and transmits results of inspection to the external apparatus. 
     The autoloader control circuit  205  controls the operation of the autoloader  190 . The autoloader control circuit  205  operates the autoloader  190  and loads the mask  2  to be inspected onto the stage  110 . Also, the autoloader control circuit  205  operates the autoloader  190  and unloads the mask  2  from the stage 
     The XY drive unit control circuit  206  controls the XY drive unit  120 . For example, the XY drive unit control circuit  206  acquires results of position measurement (X and Y coordinates) of the stage  110  on the XY plane measured by the laser length-measuring system  180  via the position circuit  212 , and controls the XY drive unit  120  based on the acquired results. 
     The Z drive unit control circuit  207  controls the Z drive unit  130 . The Z drive unit control circuit  207  of the present embodiment determines the Z coordinate based on a focus offset value and the focal position adjustment signal (autofocus control signal). The focus offset value is a setting value (X, Y, and Z coordinates) of the position of the stage  110  used for scanning. The Z drive unit control circuit  207  is included in the autofocus mechanism. 
     The Z drive unit control circuit  207  includes an offset setting circuit  230 . The offset setting circuit  230  sets the focus offset value based on the focal position data FD read from the storage unit  201 . In the present embodiment, the offset setting circuit  230  sets the focus offset value using the focal position data FD at the time of scanning of a given stripe of the mask  2  in an identical direction. At this time, the offset setting circuit  230  sets the focus offset value by, for example, pre-reading information on the Z coordinate of the focal position data FD with respect to the movement direction of the stage  110 . Details of the setting of the focus offset value will be described later. 
     The FD generation circuit  208  generates the focal position data FD based on the results of the scanning of the stripe. More specifically, the FD generation circuit  208  calculates approximation data by means of a moving average using, for example, data of continuous Z coordinates (height positions of the stage  110 ) during scanning of a single stripe. By associating, for example, the approximation data of the Z coordinate and the data of the X and Y coordinates, the FD generation circuit  208  generates the focal position data FD. The focal position data FD may further contain information on the stripe number, etc. The data of the X, Y, and Z coordinates may be results of measurements of the position of the stage  110  acquired from the position circuit  212 , or may be information on control of the position of the stage  110  performed by the XY drive unit  120  and the Z drive unit  130 . The FD generation circuit  208  transmits the focal position data FD to the storage unit  201 . 
     The development circuit  209  develops (converts) the design data  220  into, for example, binary or multivalued image data. More specifically, the development circuit  209  develops, for example, the design data  220  stored in the storage unit  201  into data of each pattern (figure), and interprets a figure code indicating the figure shape of the figure data, figure dimensions, etc. The development circuit  209  develops the design data  220  into a binary or multivalued image (hereinafter referred to as an “development image”) as a pattern arranged in a unit square of a grid of a predetermined quantization size. The development circuit  209  computes an occupancy indicating a rate at which the figure occupies pixel of the development image in each pixel. The computed figure occupancy in each pixel is the pixel value of the development image. If, for example, the pixel value of the development image is represented in an 8-bit tone value, the pixel value of each pixel is represented as a tone value from 0 to 255. The development circuit  209  transmits the generated development image to the reference circuit  210 . 
     The reference circuit  210  generates a reference image using the development image. The reference circuit  210  transmits the generated reference image to the comparison circuit  211 . 
     The comparison circuit  211  compares the optical image received from the sensor circuit  160  with the reference image received from the reference circuit  210  using an appropriate algorithm. The comparison circuit  211  determines, if the difference in the tone values of the optical image and the reference image exceeds a preset threshold value, that there is a defect at a corresponding coordinate position (X and Y coordinates) of the mask  2 . The comparison circuit  211  stores the inspection data in the storage unit  201 . The comparison circuit  211  may be configured to display the inspection data on the display unit  202 , or output the inspection data to the outside via the communication unit  204 . 
     The position circuit  212  generates position data of the stage  110  on the XY plane based on data received from the laser length-measuring system  180 . 
     2. Configuration of Illumination Optical System and Imaging Optical System 
     Next, an example of a configuration of the illumination optical system  140  and the imaging optical system  150  will be described with reference to  FIG.  2   .  FIG.  2    is a block diagram of the illumination optical system  140  and the imaging optical system  150 . 
     A configuration of the illumination optical system  140  will be described. 
     As shown in  FIG.  2   , the illumination optical system  140  includes light sources  301  and  302 , a dichroic mirror  303 , a lens  304 , a slit  305 , a lens  306 , a half mirror  307 , and an objective lens  308 . 
     The light sources  301  and  302  emit light with which the mask  2  is to be irradiated. The wavelengths of light emitted from the light sources  301  and  302  are different each other. The light may be emitted by either one of the light sources  301  and  302 , or both of them at the same time. The light source  301  is used for capturing an optical image and detecting a focal position. Depending on the relationship between the wavelength of light from the light source  301  and the layout of the mask pattern, the precision in detection of the focal position may deteriorate by the effect of diffracted light of light reflected from the mask  2 . In such a case, the light source  302  is used for detecting the focal position. A case has been described where two light sources are provided; however, only a single light source (e.g., the light source  301 ) may be provided, and in such a case, the dichroic mirror  303  need not be provided. 
     The dichroic mirror  303  is used as an optical isolation means for the light sources  301  and  302 . The dichroic mirror  303  of the present embodiment allows light from the light source  301  to transmit therethrough, and reflects light from the light source  302 . The dichroic mirror  303  may be configured to reflect the light from the light source  301 , and to allow the light from the light source  302  to transmit therethrough. 
     The light that has transmitted through or been reflected from the dichroic mirror  303  propagates along an optical axis L 1 , and is made incident on the lens  304 . 
     Part of the light that has passed through the lens  304  passes through the slit  305 . The light that has passed through the slit  305  is used for detecting the focal position. The light that has passed through the lens  304  and the light that has passed through the slit  305  are made incident on the lens  306 . 
     The light refracted by the lens  306  is made incident on the half mirror  307 . Part of the light that is made incident on the half mirror  307  is reflected, and is made incident on the objective lens  308 . 
     The objective lens  308  concentrates the light onto the pattern surface of the mask  2 . At this time, the light that has passed through the slit  305  is concentrated on the outside of the optical image capturing region of the mask  2 . The objective lens  308  may have a structure in which a plurality of lenses are arranged inside an optical column. 
     The stage  110  moves in the Z direction in such a manner that the light that has passed through the objective lens  308  is focused at a position on the pattern surface of the mask  2 . In the description that follows, a direction away from the objective lens  308  and a direction approaching the objective lens  308  as viewed in the Z direction are respectively referred to as a “+Z direction” and a “−Z direction”. 
     Next, a configuration of the imaging optical system  150  will be described. The imaging optical system  150  includes the objective lens  308 , half mirrors  307 ,  314 , and  316 , lenses  309 ,  311  to  313 , and  315 , a mirror  310 , slits  317  and  318 , a photodiode array  320 , and sensors  321  to  323 . The lenses  313  and  315 , the half mirrors  314  and  316 , the slits  317  and  318 , and the sensors  321  to  323  are included in the autofocus mechanism. 
     Part of the light that has been reflected from the mask  2  and passed through the objective lens  308  passes through the half mirror  307  and is made incident on the lens  309 . The light that has refracted at the lens  309  becomes approximately parallel to the optical axis. 
     Part of the light that has passed through the lens  309  passes through the lenses  311  and  312 . The lenses  311  and  312  cause the light to form an image on the photodiode array  320 . 
     The photodiode array  320  is an optical image capturing sensor. The photodiode array  320  generates an electric signal by photoelectrically converting the light that has formed an image. The photodiode array  320  transmits an electric signal to the sensor circuit  160 . More specifically, the photodiode array  320  includes an unillustrated image sensor. Examples of the image sensor that may be used include a line sensor in which a plurality of charge-coupled device (CCD) cameras functioning as image capturing devices are aligned in a line. Examples of the line sensor include a time delay integration (TDI) sensor. The TDI sensor captures, for example, images of the pattern of the mask  2  placed on the stage  110 , which is moving continuously. 
     Part of the light that has passed through the lens  309  is reflected from the mirror  310  and is made incident on the lens  313 . The light that has been reflected from the mirror  310  includes light that has passed through the slit  305  and been reflected from the mask  2 . The mirror  310  may be a half mirror. 
     The light refracted by the lens  313  is made incident on the half mirror  314 . 
     The light that has been reflected from the half mirror  314  is made incident on the sensor  321 . 
     The sensor  321  is used for measuring the optical intensity distribution of the pupil of the illumination optical system  140 , namely, the pupil of the objective lens  308 , or a pupil at a position that is conjugate thereto. Based on the received light, the sensor  321  generates an electric signal, and transmits the generated electric signal to the focal position detection circuit  170 . For the sensor  321 , a CCD image sensor, which is a two-dimensional sensor, is used. If, for example, the effects of the diffracted light can be confirmed from the light intensity measured by the sensor  321 , the light source  302  is used for detecting the focal position. 
     The light that has transmitted through the half mirror  314  passes through the lens  315 , and is made incident on the half mirror  316 . 
     The light that has transmitted through the half mirror  316  irradiates the slit  317 . The light that has passed through the slit  317  is made incident on the sensor  322 . The light that has been reflected from the half mirror  316  irradiates the slit  318 . The light that has passed through the slit  318  is made incident on the sensor  323 . The slits  317  and  318  are arranged to allow light that has passed through the slit  305  and been reflected from the mask  2  to pass therethrough. At this time, the slits  317  and  318  are arranged so as to be positioned at the front-side conjugate position and the rear-side conjugate position with respect to the mask  2 . 
     The slit  317  may be arranged on the front side and the slit  318  may be arranged on the rear side, or vice versa. It is preferable that the widths of the slits  317  and  318  correspond to half a spread light flux based on the numerical aperture (NA) of the objective lens  308 , namely, half the pupil diameter of the objective lens  308 . 
     The sensors  322  and  323  are sensors for detecting the focal position. Based on the received light, the sensors  322  and  323  generate an electric signal (a focal position signal), and transmit the generated electric signal to the focal position detection circuit  170 . For the sensors  322  and  323 , a photodiode or a photoelectron amplifier tube, for example, is used. The focal position detection circuit  170  calculates a focal position (a deviation amount from the focal position) by comparing the light amount at the front-side conjugate position and the light amount at the rear-side conjugate position. The position where the light amount at the front-side conjugate position and the light amount at the rear-side conjugate position are equal is the optimum focal position. 
     In the imaging optical system  150 , the light from the light source  301  and the light from the light source  302  may be isolated, and the focal position may be detected from each of them. In this case, a dichroic mirror is provided, for example, between the half mirror  314  and the lens  315 . The dichroic mirror allows, for example, the light from the light source  301  to transmit therethrough, and reflects the light from the light source  302 . Also, two focal point detection systems each configured of the lens  315 , the half mirror  316 , the slits  317  and  318 , and the sensors  322  and  323  may be provided in such a manner that the focal position is detected from each of the light that has transmitted through and the light that has been reflected from the dichroic mirror. 
     3. Inspection Region of Mask 
     Next, an example of an inspection region of the mask  2  will be described with reference to  FIG.  3   .  FIG.  3    is a diagram showing a surface of the mask  2 . Hereinafter, a case will be described where the mask  2  is scanned in the X direction, and a plurality of stripes are aligned in the Y direction. A direction going toward the left side in  FIG.  3    and a direction going toward the right side in  FIG.  3    as viewed in the X direction will be respectively referred to as a “−X direction” and a “+X direction”. Also, a direction going toward the bottom side in  FIG.  3    and a direction going toward the top side in  FIG.  3    as viewed in the Y direction will be respectively referred to as a “−Y direction” and a “+Y direction”. 
     As shown in  FIG.  3   , the mask  2  includes an inspection region  400 . In the inspection region  400 , an unillustrated pattern is provided. The inspection region  400  is, for example, a transmission region of the mask  2 , and an outer periphery of the inspection region  400  is a light-shielding region. In a reduction projection exposure apparatus, a pattern is exposed by light that has transmitted through the transmission region. Part of the light-shielding region may be included in the inspection region  400 . 
     The inspection region  400  is virtually divided into a plurality of stripes SP as viewed in the Y direction. The stripe SP has a rectangular shape of a preset width. End portions of adjacent stripes SP in the Y direction may overlap one another. In the example of  FIG.  3   , the inspection region  400  is divided into n stripes SP1 to SPn (where n is a natural number equal to or greater than one). The stripes SP1 to SPn are arranged in this order toward the +Y direction. The XY drive unit  120  moves the stage  110  in such a manner that the stripes SP1 to SPn are scanned continuously. 
     More specifically, in the stripe SP1, the region irradiated with the light moves in the +X direction. Hereinafter, moving in the +X direction may also be referred to as moving in a “FWD direction”. At this time, the stage  110  moves in the −X direction. Hereinafter, moving in the −X direction may also be referred to as moving in a “BWD direction”. After scanning of the stripe SP1 ends, scanning of the stripe SP2 is executed. In the scanning of the stripe SP2, the irradiated region moves in the −X direction (BWD direction). At this time, the stage  110  moves in the +X direction (FWD direction). Subsequently, in the scanning of the stripe SP3, the irradiated region moves in the FWD direction. At this time, the stage  110  moves in the −X direction (BWD direction). Subsequently, in the scanning of the stripe SP4, the irradiated region moves in the BWD direction. At this time, the stage  110  moves in the +X direction (FWD direction). In this manner, scanning is executed until scanning of the stripe SPn ends, by allowing the movement direction of the irradiated region to alternate between the FWD direction and the BWD direction. That is, if the number of the stripes SP is odd, scanning is executed in the FWD direction, and if the number of the stripes SP is even, scanning is executed in the BWD direction. 
     4. Overall Flow of Inspection Process 
     Next, an example of an overall flow of an inspection process will be described with reference to  FIG.  4   .  FIG.  4    is a flowchart of an inspection process. 
     As shown in  FIG.  4   , the control calculator  200  controls the image acquisition mechanism  10  to execute calibration (step S 1 ). Through the calibration, a pixel value (tone value) of an optical image acquired by the sensor circuit  160  is adjusted. 
     Subsequently, the control calculator  200  executes scanning, and acquires an inspection image (optical image) of the mask  2  (step S 2 ). The acquired optical image is transmitted to the comparison circuit  211 . 
     The development circuit  209  executes development processing of design data  220  (step S 3 ). More specifically, the development circuit  209  reads the design data  220  stored in the storage unit  201 . The development circuit  209  develops (converts) the design data  220  into, for example, 8-bit image data (a development image). The development circuit  209  transmits the generated development image to the reference circuit  210 . 
     The reference circuit  210  generates a reference image (step S 4 ). The reference circuit  210  transmits the generated reference image to the comparison circuit  211 . The acquisition of the inspection image and the generation of the reference image may be in reverse order, or may be performed at the same time. 
     The comparison circuit  211  performs comparison processing (step S 5 ). More specifically, the comparison circuit  211  executes alignment between the optical image and the reference image, and performs positioning of a pattern in the optical image and a pattern in the reference image. Subsequently, the comparison circuit  211  compares the optical image with the reference image. 
     The comparison circuit  211  calculates a difference in pixel value (tone value) of each pixel, and determines that the pixel is defective if the difference in tone value is equal to or greater than a preset threshold value. 
     The control calculator  200  outputs a comparison result (inspection data) (step S 6 ). The control calculator  200  saves the inspection results in the storage unit  201 . The control calculator  200  may display the inspection results on the display unit  202 , or may output them to an external apparatus (e.g., a review apparatus, etc.) via the communication unit  204 . 
     5. Focal Position Adjustment 
     Next, focal position adjustment will be described. In the present embodiment, upon scanning, focal position adjustment (height position adjustment of the stage  110 ) is performed based on a focus offset value and an autofocus control signal. The focus offset value is set based on focal position data FD obtained by scanning a given stripe SP in an identical direction. More specifically, upon scanning of the stripe SPn, for example, the focus offset value is set based on focal position data FD (n−2) of a stripe SP (n−2) subjected to second-to-last scanning in the same direction. Accordingly, in the present embodiment, prior to scanning of the stripes SP1 and SP2, dummy scanning is executed in a direction corresponding to the scanning advancement direction of each stripe. In dummy scanning, the focal position data FD is acquired, and an optical image is not acquired. If, for example, the variable n is a numerical value less than n=1, which corresponds to the stripe SP1, namely, if n=−1 or n=0 is set, dummy scanning is executed. 
     5. 1. Flow of Optical Image Acquisition 
     First, an example of a flow of optical image acquisition will be described with reference to  FIG.  5   , with a focus on scanning.  FIG.  5    is a flowchart showing a flow of optical image acquisition. 
     As shown in  FIG.  5   , the control calculator  200  sets variable n=−1 (step S 101 ). Subsequently, the control calculator  200  executes dummy scanning in the FWD direction (step S 102 ). The dummy scanning in the FWD direction corresponds to the stripe SP1. For the dummy scanning in the FWD direction, for example, the stripe SP1 is used. At this time, an optical image of the stripe SP1 is not acquired. 
     Subsequently, the FD generation circuit  208  generates approximation data of the Z coordinate (height position of the stage  110 ) by means of a moving average based on the results of the scanning. The FD generation circuit  208  generates focal position data FD (−1) based on the approximation data of the Z coordinate (step S 103 ). The focal position data FD (−1) is stored in the storage unit  201 . 
     Subsequently, the control calculator  200  increments the variable n, to satisfy n=0 (step S 104 ). Subsequently, the control calculator  200  executes dummy scanning in the BWD direction (step S 105 ). The dummy scanning in the BWD direction corresponds to the stripe SP2. Accordingly, for the dummy scanning in the FWD direction, for example, the stripe SP2 is used. 
     Subsequently, the FD generation circuit  208  generates approximation data of the Z coordinate by means of a moving average based on the results of the scanning. The FD generation circuit  208  generates focal position data FD0 based on the approximation data of the Z coordinate (step S 106 ). The focal position data FD0 is stored in the storage unit  201 . 
     Subsequently, the control calculator  200  increments the variable n, to satisfy n=n+1 (step S 107 ). 
     Subsequently, the offset setting circuit  230  reads focal position data FD (n−2) from the storage unit  201 . The offset setting circuit  230  sets a focus offset value for the scanning of the stripe SPn based on the focal position data FD (n−2) (step S 108 ). At this time, the offset setting circuit  230  pre-reads the focal position data FD. In other words, the offset setting circuit  230  shifts the data of the X coordinate of the focal position data FD in the advancement direction of the stage  110 . More specifically, the stripe SP1 is scanned in, for example, the FWD direction. That is, the stage  110  moves in the −X direction. In such a case, the offset setting circuit  230  sets the focus offset value based on results obtained by shifting the coordinate data of the focal position data FD (−1), more specifically, the value of the X coordinate in the −X direction. The stripe SP2 is scanned in, for example, the BWD direction. That is, the stage  110  moves in the +X direction. In such a case, the offset setting circuit  230  sets the focus offset value based on results obtained by shifting the value of the X coordinate of the focal position data FD0 in the +X direction. The shift amount is determined based on a scanning speed and an autofocus tracking speed. Similarly, in the stripe SP3 and the stripes that follow (n=3 or greater), for example, the offset setting circuit  230  sets the focus offset value based on results obtained by shifting the focal position data FD of the stripe SP subjected to second-to-last scanning. 
     In the case of setting of the focus offset value, focal position data FD to which pre-reading is to be applied and focal position data FD to which pre-reading is not to be applied may coexist. For example, the offset setting circuit  230  may pre-read the focal position data FD (−1) and FD0 acquired by dummy scanning, and may not pre-read the focal position data FDn (n&gt;1) acquired by scanning other than the dummy scanning. 
     In the present embodiment, a case has been described where the offset setting circuit  230  reads the focal position data FD (n−2) from the storage unit  201 ; however, the configuration is not limited thereto, as long as the scanning advancement direction is the same. If, for example, the stripe SP (n−4) is determined to be more similar to the pattern of the stripe SPn than the stripe SP (n−2) by referring to the design data  220 , the offset setting circuit  230  may read the focal position data FD (n−4) from the storage unit  201 . 
     Subsequently, the control calculator  200  executes scanning of the stripe SPn (step S 109 ). At this time, the Z drive unit control circuit  207  controls a height position (Z coordinate) of the stage  110  based on the focus offset value and the focal position adjustment signal of the focal position detection circuit  170 . Optical image data acquired by the scanning is transmitted to the comparison circuit  211 . 
     Subsequently, the FD generation circuit  208  generates approximation data of the Z coordinate by means of a moving average based on the results of the scanning. The FD generation circuit  208  generates focal position data FDn based on approximation data of the Z coordinate (step S 110 ). The focal position data FDn is stored in the storage unit  201 . 
     The control calculator  200  confirms whether or not the stripe SPn subjected to the scanning is the final stripe SP (step S 111 ). If the stripe SPn subjected to the scanning is not the final stripe SP (step S 111  No), the control calculator  200  proceeds to step S 107 , and increments the variable n. On the other hand, if the stripe SPn subjected to the scanning is the final stripe SP (step S 111 _Yes), the control calculator  200  ends acquisition of the optical image. 
     5. 2. Concrete Example of Approximation Data of Z Coordinate 
     Next, a concrete example of approximation data of a Z coordinate will be described with reference to  FIG.  6   .  FIG.  6    is a diagram showing a concrete example of approximation data of a Z coordinate. In the example of  FIG.  6   , X and Z coordinates of the stage  110  in a single stripe SP are shown. 
     The graph at the top of  FIG.  6    shows coordinates of the stage  110  during scanning. The overall rise of the Z coordinate toward the +X direction shows the effects of distortion of the mask  2 . In the present example, an abrupt step height exists at coordinates X1 and X2. 
     The graph at the bottom shows two items of approximation data of the coordinates shown in the graph at the top, obtained by two types of computing. The approximation data shown by the solid line indicates the case where the FD generation circuit  208  of the present embodiment calculates approximation data by means of a moving average. The dashed line indicates the case where, as a comparative example, approximation data is calculated by means of a polynomial approximation. With the use of a polynomial approximation as in the comparative example, a step height between X1 and X2 may not be reflected if the order of the polynomial is small. On the other hand, with a moving average, it is possible to reflect a step height in the approximation data. In a moving average, the number of items of data used for calculating an average value of Z coordinates with respect to each X coordinate can be freely set. Such setting may be based on, for example, the noise of the Z coordinate. 
     5. 3. Concrete Example of Pre-Reading of Focal Position Data 
     Next, a concrete example of pre-reading of focal position data FD will be described with reference to  FIGS.  7  and  8   .  FIG.  7    is a diagram showing an example of pre-reading of focal position data FD (−1) in scanning of the stripe SP1 in the FWD direction.  FIG.  8    is a diagram showing an example of pre-reading of focal position data FD0 in scanning of the stripe SP2 in the BWD direction. 
     First, a case will be described where the stripe SP1 is scanned in the FWD direction. 
     The top of  FIG.  7    shows part of a cross section of the mask  2  to be scanned. The graph at the middle shows a relationship between the Z coordinate and the X coordinate of the stage  110  based on dummy scanning of the stripe SP1. The graph at the bottom shows a relationship between the Z coordinate and the X coordinate of the stage  110  based on scanning of the stripe SP1. 
     A projection  500  (e.g., a pattern) with a height T1, for example, exists between the X coordinates P1 and P2 of the mask  2 . Such a portion is subjected to dummy scanning in the FWD direction. That is, the stage  110  is moved in the −X direction. The focus offset value at this time is constantly Z0, regardless of the X coordinate. In the focal position data FD (−1), an autofocus tracking delay occurs in the +X direction, with respect to the actual X coordinate of the projection  500 . More specifically, the Z coordinate of the surface of the mask  2  with respect to the X coordinate P1 fluctuates (rises) by +T1. On the other hand, in the focal position data FD (−1), as the X coordinate shifts from P1 to P1+d, the Z coordinate shifts from Z0 to Z0+T1. That is, an autofocus tracking delay of a magnitude |d| occurs in the +X direction. Similarly, the Z coordinate of the surface of the mask  2  with respect to the X coordinate P2 fluctuates (drops) by −T1. On the other hand, in the focal position data FD (−1), as the X coordinate shifts from P2 to P2+d, the Z coordinate shifts from Z0+T1 to Z0. That is, an autofocus tracking delay of a magnitude |d| occurs in the +X direction. Based on the above results, the focal position data FD (−1) is generated. Accordingly, the focal position data FD (−1) contains coordinate data in which the autofocus tracking delay has occurred. 
     The offset setting circuit  230  sets, prior to scanning the stripe SP1, a focus offset value used for scanning the stripe SP1, using the focal position data FD (−1) read from the storage unit  201 . At this time, the offset setting circuit  230  pre-reads the focal position data FD (−1). More specifically, for scanning in the FWD direction, the offset setting circuit  230  causes the X-coordinate data to shift in the −X direction. Assuming, for example, that the magnitude of the shift amount is |s|, the offset setting circuit  230  sets data obtained by subtracting the X coordinate of the focal position data FD (−1) by |s| as the focus offset value. The shift amount |s| is a value equal to or smaller than the tracking delay amount |d|. 
     Thereby, in scanning of the stripe SP1, fluctuation of the Z coordinate can be started based on the focus offset value prior to fluctuation of the Z coordinate based on the autofocus control signal. For example, fluctuation of the Z coordinate starting from the coordinate P1 in dummy scanning starts from the coordinate P1−s in scanning. Accordingly, the autofocus tracking delay is decreased by the shift amount |s|. 
     Next, a case will be described where the stripe SP2 is scanned in the BWD direction. 
     The top of  FIG.  8    shows part of a cross section of the mask  2  to be scanned. The graph at the middle shows a relationship between the Z coordinate and the X coordinate of the stage  110  based on dummy scanning of the stripe SP2. The graph at the bottom shows a relationship between the Z coordinate and the X coordinate of the stage  110  based on scanning of the stripe SP2. 
     In the example of  FIG.  8   , dummy scanning in the BWD direction is executed. That is, the stage  110  is moved in the +X direction. The focus offset value at this time is constantly Z0, regardless of the X coordinate. In the focal position data FD0, an autofocus tracking delay occurs in the −X direction, with respect to the actual X coordinate of the projection  500 . More specifically, the Z coordinate of the surface of the mask  2  with respect to the X coordinate P2 fluctuates (rises) by +T1. On the other hand, in the focal position data FD0, as the X coordinate shifts from P2 to P2−d, the Z coordinate shifts from Z0 to Z0+T1. That is, an autofocus tracking delay of a magnitude |d| occurs in the −X direction. Similarly, the Z coordinate of the surface of the mask  2  with respect to the X coordinate P1 fluctuates (drops) by −T1. On the other hand, in the focal position data FD0, as the X coordinate shifts from P1 to P1−d, the Z coordinate shifts from Z0+T1 to Z0. That is, an autofocus tracking delay of a magnitude |d| occurs in the −X direction. Based on the above results, the focal position data FD0 is generated. Accordingly, the focal position data FD0 contains coordinate data in which the autofocus tracking delay has occurred. 
     Prior to scanning the stripe SP2, the offset setting circuit  230  sets the focus offset value used for scanning the stripe SP2 using the focal position data FD0 read from the storage unit  201 . At this time, the offset setting circuit  230  pre-reads the focal position data FD0. More specifically, for scanning in the BWD direction, the offset setting circuit  230  shifts the X-coordinate data in the +X direction. The offset setting circuit  230  sets data obtained by adding |s| to the X-coordinate value of the focal position data FD0 as a focus offset value. 
     As a result, fluctuation of the Z coordinate, starting from the coordinate P2 in dummy scanning, starts from the coordinate P2+s in scanning. Accordingly, the tracking delay is decreased by the shift amount |s|. 
     6. Advantageous Effects of Present Embodiment 
     According to the configuration of the present embodiment, an inspection apparatus includes an FD generation circuit which calculates approximation data of the Z coordinate by means of a moving average based on results of scanning of a stripe SP and generates focal position data FD using the approximation data, and an offset setting circuit which sets a focus offset value by pre-reading (shifting) the focal position data in the advancement direction of the stage  110 . By calculating approximation data by means of a moving average, the inspection apparatus is capable of calculating approximation data in which the effects of noise are reduced and information on an abrupt step height is reflected. Also, by pre-reading the focal position data and setting the focus offset value, the inspection apparatus is capable of starting moving of the height position of the stage  110  based on the focus offset value prior to moving of the height position of the stage  110  based on the autofocus control signal. It is thereby possible to decrease the tracking delay of the autofocus control signal. This decreases defocusing in an optical image, thus facilitating image capturing of a fine pattern and enhancing the precision of detecting pattern defects. 
     7. Others 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the new embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.