Compact reticle inspection system capable of inspecting a reticle with high accuracy and method of inspecting the same

Even though a laser interferometer is affected by the changes in the environment, a reticle can be inspected with a high accuracy by synthesizing a reference image corrected appropriately to compare with an optical image. The reticle which a pattern is plotted in advance is irradiated with a light beam to obtain the optical image from the transmitted light to compare the optical image with the reference image synthesized by converting draft data used at plotting said pattern and to detect defects of the pattern. The reference image is corrected based on the deviation data obtained at the preceding pattern defect detection. The deviation data is the difference between a difference (an initial distance value) between a completion position data and an initiation position data measured before a laser interferometer is affected by the changes in the environment and a difference (a distance data) between the completion position data and the initiation position data measured after being affected by the changes in the environment, and is obtained using a scale is hardly affected by the changes in the environment.

BACKGROUND OF THE INVENTION 
 The present invention relates to a reticle inspection system and a method 
 of inspecting a reticle used for forming a predetermined pattern on a 
 semiconductor wafer. 
 Conventionally, in a manufacturing process of a Large Scale 
 Integration(LSI) circuit, a reticle is used for forming the predetermined 
 pattern on the semiconductor wafer constituted by a silicon or the like. 
 The pattern is formed on the semiconductor wafer by exposing the 
 semiconductor wafer to a light through the reticle and an optical lens. 
 Therefore, when the reticle has a pattern defect in itself, the defective 
 pattern is transferred on all the wafers manufactured by using the 
 reticle. As a results a large amount of defective LSI circuits are 
 manufactured. Accordingly, the pattern inspection of the reticle is very 
 important and essential for manufacturing LSI circuits. Moreover, since 
 the more fine pattern has come to be formed in recent years, a high 
 accuracy inspection of a defect detecting sensitivity of less than 0.2 
 .mu.m is required in a reticle inspection system. 
 In inspection methods of the reticle for manufacturing LSI circuits, there 
 are two kinds of the methods. One is a die-to-die inspection method for 
 comparing identical patterns formed at different positions on the same 
 reticle to each other while another is a die-to-database inspection method
 for comparing draft data used when plotting the reticle pattern with the 
 pattern on the actual reticle. Herein, "die" means a certain grouping of 
 pattern areas or the detection image thereof, which is defined as a unit 
 of a pattern comparison inspection. Further, "database" means a reference 
 image synthesized from the draft data with respect to an actual pattern 
 image detected by an optical system. 
 Conventionally, an ordinarily reticle inspection system comprises an X-Y 
 stage for setting a reticle, a laser interferometer for detecting a 
 position of the X-Y stage with a high accuracy, a laser-scanning 
 optical-device for scanning a laser beam in the direction of the Y-axis of
 the reticle, a transmitted-light detection section for detecting the 
 transmitted light, an optical image input section for receiving an optical
 image from the transmitted-light detection section, a data conversion 
 section for converting the draft data used where plotting the reticle to 
 synthesize the reference image, an image comparison section for comparing 
 the optical image with the reference image to detect a pattern defect, and
 a controller for controlling the entire system. 
 However, via a conventional method of inspecting a reticle using the 
 ordinarily reticle inspection system, it takes several hours to detent one
 sheet of the reticle. Therefore, error is inevitably caused to occur in 
 detection of the travel of the X-Y stage by the laser interferometer due 
 to changes of environment (temperature, humidity, atmospheric pressure) 
 during inspection. When the error is included in the detected results by 
 the laser interferometer, the X-Y stage cannot be made to travel correctly
 by a certain pitch. Consequently, a deviation is generated between the 
 optical image and the reference image, even though the reticle has 
 actually no defect in the pattern thereof. 
 In order to avoid this problem, it is considered that the entire reticle 
 inspection system is placed in an temperature control chamber to hold the 
 air flow constant as well as to hold the temperature and the humidity 
 constant. In addition, a wavelength compensator (a correcting means) is 
 provided foot detecting the change in a refractive index in the vicinity 
 of the optical path of the laser interferometer. Thereby, an effective 
 refractive index is calculated to correct the reference image in. real 
 time. Accordingly, the optical image is compared with the reference image 
 corrected in real time. In the wavelength compensator, a wavelength in an 
 actual environment is compared with that in vacuum by the use of a vacuum 
 tube with a certain distance to detect the change in a refractive index 
 and to correct it. However, this method requires a very large temperature 
 control chamber surrounding the entire system and the wavelength 
 compensator. The method therefore brings disadvantages in that the system 
 becomes large in size and very expensive. 
 SUMMARY OF THE INVENTION 
 Therefore, it is an object of the present invention to provide a reticle 
 inspection system and a method of inspecting a reticle in which the 
 reticle car be inspected with a high accuracy, by synthesizing the 
 reference image corrected appropriately to compare with the optical image,
 even in the case that the laser interferometer is subjected to the 
 influence of changes in the environment. 
 Other objects of the present invention will become clear as the description
 proceeds. 
 According to an aspect of the present invention, there is provided a method
 of inspecting a reticle comprising the steps of: irradiating a light bean 
 on a reticle having a pattern in advance to receive the transmitted light 
 and to form an optical image, while measuring the relative position of the
 X-Y table placing the reticle by a laser interferometer; comparing the 
 optical image with a reference image synthesized by converting draft data 
 used when forming the pattern; and detecting defects of the pattern. The 
 method further comprising the steps of: providing a scale to which changes
 in an environmental condition is less than that of the laser 
 interferometer and detecting the position of the X-Y table; obtaining 
 deviation data of measurement errors of the laser interferometer due to 
 the changes in the environmental condition using the scale; and 
 synthesizing the reference image which is corrected from the draft data by
 the amount of the deviation data. Therefore, a visual inspection of the 
 reticle can be performed accurately corresponding to the changes in the 
 environmental condition. 
 More particularly, the deviation data is calculated by: storing position 
 data of the laser interferometer and the scale in a light-beam irradiating
 initiation position to the reticle, and position data of the laser 
 interferometer and the scale in a light-beams irradiating completion 
 position; obtaining in advance an initial distance value of the difference
 between the position data of the laser interferometer in the light-beam 
 irradiating completion position and the position data of the laser 
 interferometer in the light-beam irradiating initiation position; moving 
 the X-Y table, placing the reticle of an inspection object so that the 
 position data detected by the scale matches with the stored position data 
 of the light-beam irradiating initiation position stored to store the 
 initiation position data of the laser interferometer at that time; 
 subsequently, initiating the light-beam irradiation to the reticle while 
 moving the X-Y table; storing the completion position data of the laser 
 interferometer at that time as well as stopping the light-beam 
 irradiating, at the time ill which the position data detected by the scale
 reaches a position matching with the position data of the light-beam 
 irradiating completion position; and calculating a distance data of the 
 difference between the completion position data of the laser 
 interferometer and the initiation position data of the laser 
 interferometer to obtain the difference between the distance data and the 
 initial distance value. 
 Moreover, the laser interferometer cancels vibration difference in phase 
 between a lens of the laser-scanning optical system irradiating the light 
 beam and the X-Y table. Thus, both the effects can be obtained using in 
 combination of the laser interferometer and the scale. 
 The detection of the defects in the pattern in a plurality of points of the
 reticle is performed sequentially, and when, in a reference image 
 synthesizing process, the correction is designed to be performed based on 
 the deviation data obtained at the preceding detection of the defect, the 
 synthesis of the reference image is efficiently corrected corresponding to
 the changes in the environment. 
 According to another aspect of the present invention, there is provided a 
 reticle inspection system comprising: an X-Y table for carrying a reticle 
 on which the pattern is plotted in advance; a light bean-scanning optical 
 system for irradiating on the reticle a light beam; a laser interferometer
 for measuring a relative position of the X-Y table, a scale to which an 
 influence of changes in the environment is less than that of the laser 
 interferometer, and for measuring the position of the X-Y table; a 
 transmitted-light detection section for obtaining an optical image based 
 on the, transmitted light of the light beam which is irradiated on the 
 reticle; a data conversion section for obtaining the deviation data of a 
 measurement error due to the changes in the environment of the laser 
 interferometer using the scale, to synthesize the reference image 
 corrected by the deviation data from draft data used when plotting the 
 pattern; and an image comparison section for comparing the optical image 
 with the reference image to detect defects of the pattern. 
 The deviation data is the difference between an initial distance value of 
 the difference between a completion position data and an initiation 
 position data measured before the laser interferometer is affected by the 
 changes in the environment and the distance data of the difference between
 the completion position data and the initiation position data measured 
 after the laser interferometer is affected by the changes in the 
 environment. 
 Moreover, the scale is a laser scale and the light beam is the laser beam.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
 Referring to FIG. 1, description is, at first, made about a conventional 
 reticle inspection system and a method using the conventional reticle 
 inspection system for a better understanding of the present invention. 
 FIG. 1 is a block diagram for showing a constitution of the conventional 
 reticle inspection system. The conventional reticle inspection system has 
 a structure similar to that of the ordinary one mentioned in the preamble 
 of the instant specification. However, in order to facilitate an 
 understanding of the problem in a reticle inspection method using the 
 conventional system, the constitution of the conventional reticle 
 inspection system is described again with reference numerals. 
 As illustrated in FIG. 1, the conventional reticle inspection system 
 comprises an X-Y stage 102 for setting a reticle 101, a laser 
 interferometer 104 for detecting a position of the X-Y stage with a high 
 accuracy, a laser scanning optical system 105 for scanning a laser beam in
 the direction of the Y-axis of the reticle 101, a transmitted light 
 detection section 107 for detecting the transmitted light, an optical 
 image input section 108 for receiving an optical image from the 
 transmitted light detection section 107, a data conversion section 109 for
 converting the draft data used when plotting the reticle to synthesize the
 reference image, an image comparison section 110 for comparing the optical
 image with the reference image to detect a pattern defect, and a 
 controller 111 for controlling the entire system. 
 Herein, description is made as regards a method of inspecting a reticle 
 using the conventional reticle inspection system by the above-mentioned 
 die-to-database inspection method. The defects on the entire reticle are 
 detected by dividing, at first, the reticle 101 into a plurality of 
 inspection regions slightly overlapping each other to inspect every each 
 inspection region sequentially, at last, to integrate the defects in each 
 region. The inspection in each inspection region is performed as follows. 
 First, the X-Y stage 102 places the reticle 101 at an inspection 
 initiation position or the associated inspection region. Second, the X-Y 
 stage 102 is fed in the direction of the X-axis, while monitoring with the
 laser interferometer 104, to scan the laser beam by the laser scanning 
 optical system 105 in the direction of the Y-axis every time it travels by
 a certain pitch. Then, the transmitted light is detected by the 
 transmitted light detection section 107 to receive the two-dimensional 
 image every one frame. The received optical image is transferred from the 
 optical image input section 108 to the image comparison section 110. In 
 the image comparison section 110, the received optical image is compared 
 with the reference image synthesized in the data conversion section 109 to
 detect dissimilarities (the defects). Moreover a "frame" is a unit 
 representing, the which may be simultaneously processed by the images; 
 comparison section 110 (refer to FIG. 3). 
 However, in the method of inspecting a reticle using the conventional 
 reticle inspection system, it takes several hours to detect one sheet of 
 the reticle 101. Therefore, error is inevitably caused to occur in 
 detection of the travel of the X-Y stage 102 by the laser interferometer 
 104 due to changes of environment (temperature, humidity, atmospheric 
 pressure) during inspection. When the error is included in the detected 
 results by the laser interferometer 104, the X-Y stage 102 cannot be made 
 to travel correctly by a certain pitch. Consequently, a deviation is 
 generated between the optical image and the reference image, even though 
 the reticle 101 has actually no defect in the pattern thereof. Namely, 
 even though, both images are approximately matched in the initial frame of
 the inspection region, as one increases the number of frames, the minute 
 errors of a traveling patch of the X-Y stage 102 are accumulated. Thereby 
 the amount of the deviation between the optical image and the reference 
 image is increased, so that it appears as a magnified deviation in the 
 inspection region inspected at the final process. 
 These errors are caused because the refractive index of the air in an 
 optical path of the laser interferometer 104 is varied due to changes in 
 the environment, and because the wavelength of the laser emitted from the 
 laser interferometer 104 is changed accordingly. For example, with the 
 laser interferometer made of Zygo Co., Ltd. having resolution of 1024, the
 factors of an effective distance card a refractive index with respect to 
 each change in the environment is as follows. 
 An effective distance=(a reading value of the interferometer.times.a 
 wavelength of the laser in a vacuum)/(1024.times.a refractive index) 
 A ratio of change in a refractive index for temperature: 1 [PPM/%] 
 A ratio of change in a refractive index for humidity: 0.01 [PPM/%] 
 A ratio of change in a refractive index for atmospheric pressure: 0.357 
 [PPM/%] 
 For example, in the case of measuring the distance of 100 .mu.m, the error 
 becomes 0.1 .mu.m when the refractive index is changed by 1 PPM. The 
 factor which especially influences upon a refractive index is changes of 
 atmospheric pressure. It can sometimes change by approximately 50 mmHg for
 a time period of several hours, resulting in an error in this case of up 
 to approximately 1.8 .mu.m. In the reticle inspection system in which the 
 accuracy of approximately 0.1 .mu.m in inspection resolution is required, 
 this error is not a negligible dimension and it appears as a large 
 deviation between the optical image and the reference image. 
 Referring now to FIGS. 2 through 6, description will proceed to a reticle 
 inspection system and a method of inspecting a reticle according to a 
 preferred embodiment of the present invention. 
 FIG. 2 is a block diagram for showing a constitution of a reticle 
 inspection system according to the embodiment. first, each component 
 constituting the reticle inspection system according to this embodiment 
 will be described. A reticle 1 is set on the upper surface of an X-Y stage
 2 and is movable in the direction of the X- and the Y-axis by a driving 
 mechanism (not shown). A laser interferometer 4 detects a relative 
 position in the direction of the x-axis of the X-Y stage accurately using 
 a laser. A laser scanning optical system (a light-beat scanning optical 
 system) 5 scans a laser beam (a light beam) through an objective lens 3 in
 the direction of the Y-axis. A transmitted light detection section 7 
 detects the transmitted light of the laser bean collected through a 
 collector lens 6 to obtain the optical image from this transmitted light. 
 The optical image is entered to an optical image input section 8 from the 
 transmitted light detection section 7. A data conversion section 9 
 converts the draft data used at plotting the reticle to synthesize the 
 reference image. An image comparison section 10 compares the optical image
 entered from the optical image input section 8 with the reference image 
 entered from the data conversion section 9 to detect a pattern defect. A 
 in controller 11 controls entire operations of this inspection system. A 
 respective position data is entered to a deviation detection section 13 
 from the lasers interferometer 4 and a laser scale 12 described below, and
 the deviation detection section 13 compares both data to obtain the 
 deviation. 
 The laser scale 12 is one for detecting a absolute position in the 
 direction of the X-axis of the X-Y stage 2 and is hardly affected by the 
 changes in the environment. Specifically, the laser scale 12 is one which 
 is formed in a manner of sandwiching a hologram grid prepared by means of 
 the laser light between two sheets of quartz plates, and is one 
 constituted by using the phenomenon that the phase of the diffracted light
 through the hologram grid is changed when moving the grid then irradiating
 the laser light. Such a device is hardly affected by changes in the 
 environment and is inexpensive since the a repeat accuracy is high and the
 optical path of the laser light can be shortened. 
 For example, the specification of the laser scale made of Sonny Precision 
 Technology Co., Ltd. is as follows; 
 Repeat accuracy: 0.02 .mu.m 
 A temperature coefficient: -0.7 PPM/deg. (mainly due to thermal expansion 
 of glass) 
 A humidity coefficient and a atmospheric pressure coefficient are 
 considerably small as compared with a temperature coefficient. 
 In a clean room in which such reticle inspection system is commonly set up,
 changes of an environment temperature are within the range of 
 approximately .+-.1 deg. With such degree of temperature change, the error
 in the laser scale becomes only approximately .+-.0.07 .mu.m for the 
 measurement of 100 mm at the maximum. Especially, with respect to changes 
 in a atmospheric pressure and a humidity, the error becomes negligible 
 small, whereby it shows superior resistance to the changes in the 
 environment as compared with the laser interferometer 4 or the like. 
 Next, referring to FIGS. 3 to 6, description proceeds to a reticle 
 inspection method according to this embodiment. 
 Now, FIG. 3 is a view illustrating the die-to-database inspection method 
 described above. FIG. 4 is a time chart for inspection in an inspection 
 region of the actual reticle 1. FIG. 5 is a flow chart of a preceding 
 process of the reticle inspection method according to this embodiment. 
 FIG. 6 is a flow chart of the reticle inspection according to this 
 embodiment. 
 In this embodiment, for example, the en-tire inspection region is divided 
 into a plurality of the inspection regions (a first to an eighth 
 inspection regions in this embodiment) so as to overlap each other 
 defining the direction of the x-axis as a longitudinal direction as shown 
 in FIG. 3, to inspect from the first inspection region at every inspection
 region in order, and at last integrating the defects of each inspection 
 region to inspect the entire defects of the reticle 1. 
 At this point, the details of the inspection in each inspection region is 
 described as shown below. Now, prior to initiate the inspection, the data 
 conversion section 9 takes the draft data of the reticle 1 of the 
 inspection object to make the data into the unfolded condition in advance 
 at each inspection region in order to synthesize the reference image. This
 permits synthesizing, in a short time, the reference image during 
 inspecting. 
 Moreover, as shown in FIG. 5, the initial value of the distance between the
 laser scanning initiation position and the laser scanning completion 
 position is measured. First, the X-Y stage 2 holding the reticle 1 is 
 moved up to the scanning initiation position (astep "a"), and a position 
 data SO1 (refer to FIG. 4) of the laser scale 12 and a position data Da of
 the laser interferometer 4 at that time are read by the 
 deviation-delection section 13 and stored (a step "b"). Subsequently, the 
 X-Y stage 2 is moved up to the scanning completion position (a step "c"), 
 and again a position data S45 (refer to FIG. 4) of the laser scale 12 and 
 a position data Db of the laser interferometer 4 are read by the 
 deviation-detection section 13 and stored (a step "d"). At this point of 
 time, the deviation-detection section 13 subtracts from the position data 
 Db of the laser interferometer 4 in the scanning completion position the 
 position data Da of the laser interferometer 4 of the scanning initiation 
 position and stores the value (Db-Da) as the initial value of the distance
 (a step "e"). By the ways since scanning of the laser beam by the 
 laser-scanning optical system 5 is executed in synchronism with a rising 
 edge of a pulse for moving a certain pitch from the laser interferometer 4
 to the laser-scanning optical system 5 described below, the position data 
 of the laser scale which becomes a timing for scanning initiation at the 
 inspection time is considered to be S01 and the position data of the laser
 scale which becomes a timing for a final scanning at the time inspection 
 is considered to be S45. 
 At this point of time, the actual reticle-inspection process shown in FIG. 
 4 and FIG. 6. is initiated. The X-Y stage 2 holding the reticle 1 is moved
 up to the scanning initiation position of the first inspection region (a 
 step "f" in FIG.6). Subsequently, the X-Y stage 2 initiates to travel at a
 constant speed (a step "g") in the X direction. The deviation-detection 
 section 13 detects the position of the X-Y stage 2, and when the position 
 data of the laser scale 12 matches with the scanning initiation position 
 S01 stored in the preceding process and it is judged that the X-Y stage 2 
 reaches the scanning initiation position (a step "h"), wherein a position 
 data D04 of the laser interferometer 4 is stored as the initiation 
 position data at the inspection time (a step "i"), Now, when the scanning 
 initiation position matches an inspection initiation position, the process
 traveling up to the scanning initiation position while monitoring the 
 laser interferometer 4 from the inspection initiation position can be 
 omitted. 
 Every time the laser interferometer 4 judges that the relative position 
 between the position of the objective lens 3 and the position of the 
 X-axis direction of the X-Y stage 2 is moved by a certain pitch (for 
 example, 1 .mu.m) (a step "j"), this laser interferometer, 4 sends to the 
 laser-scanning optical system 5 the pause providing the instruction to 
 move by a certain pitch (a step "k"). The laser-scanning optical system 5 
 scans the laser beam in the Y-axis direction every time the rising edge of
 this pulse (a step "m"). The laser beam scanned radiates the reticle 1 
 through the objective lens 3, and the transmitted light is collected 
 through the collector lens 6 to be detected at a transmitted-light 
 detection section 7. This scan is repeated at every certain pitch, and 
 when it is judged that the scan of one frame is completed (a step "n"), 
 the transmitted-light detection section 7 passes the two-dimensional 
 optical image of the associated frame (a step "p") through an optical 
 image input section 8 to the image comparison section 10 sequentially (a 
 step "q"). 
 The scan proceeds, and the deviation-detection section 13 detects the 
 position of the X-Y stage 2, and when the position data of the laser scale
 12 matches with the scanning completion position S45 stored in the 
 preceding process and it is judged that the X-Y stage 2 to reaches the 
 scanning completion position (a step "r"), a position data D92 of the 
 laser interferometer 4 is stored as the completion position data at the 
 time of this inspection (a step "s"), Subsequently, the initiation 
 position data D04 is subtracted from a completion position data (Dg2) of 
 the laser interferometer 4 to obtain the distance data (D92-DD4). Now, 
 since the position data are detected by the laser interferometer 4 and the
 laser scale 12 responsive to respective clock pulse and the position data 
 of the laser interferometer 4 is sent in synchronism with falling edge of 
 the clock pulse of the laser interferometer, the position data of the 
 laser interferometer 4 at the size of scanning initiation is considered to
 be D04 and the position data of the laser interferometer 4 at the time of 
 scanning completion is considered to be D92. 
 From this distance data (D92-D04), the distance initial value (Db-Da) is 
 subtracted to calculate a deviation data (D92-D04)-(Db-Da) ) (a step "t") 
 and send to the data conversion section 9. Although the position data of 
 the laser scale (for example, S01 and S45) is hardly affected by the 
 changes in the environment, and the sane position could substantially be 
 indicated at all time, it is considered that the reason why the distance 
 data of the laser interferometer 4 (D92-D04) differs from the distance 
 initial value (Db-Da) is as a consequence of it being affected by the 
 changes in the environment. Thus a deviation is caused in the detected 
 results of the laser interferometer 4. That is to say, the optical image 
 obtained would be expanded and contracted by this deviation data 
 ((D92-D04)-(Db-Da)). 
 On the other hand, the data conversion section 9 synthesizes the reference 
 image every one fraze in real time from the intermediate data unfolded in 
 the preceding process described above (it step "u") to send to the image 
 comparison section 10 (a step "v"). At this time, the synthesized 
 reference image is corrected accurately based in the deviation data sent 
 after being minimized. By the way, the deviation data used in this 
 correction is the deviation data obtained at inspection of the inspection 
 region preceding by one of the associated inspection region. For example, 
 the devlation data used for correcting when inspecting the third 
 inspection region is the data obtained when inspecting the second 
 inspection region. However, although there is no deviation data to be used
 for correcting with respect to the first inspection region, it is assumed 
 that a large error is not yet caused and is thus corrected. 
 Therefore, the image comparison section 10 compares the optical image 
 entered from the optional image input section 8 with the ready-to-correct 
 optical image entered from the data conversion section 9 at every frame to
 detect the defect (a step "w"). 
 As described above, a visual inspection of the reticle 1 is performed with 
 respect to one inspection region. This inspection process is performed 
 over the entire inspection region of one sheet of the reticle 1. If 
 inspection of the entire inspection region is not completed (a step "x"), 
 the X-Y stage 2 is moved to the inspection initiation position of the 
 subsequent inspection region (a step "y"), the steps "g" to "x" are 
 executed with respect to the subsequent inspection region. Moreover, when 
 the inspection of the entire inspection region is completed (a step "x"), 
 the inspected results of each inspection region are synthesized to obtain 
 the defects of the entire surface of the reticle 1 (a step "z") and to 
 output to a display means or the like (not shown) from the image 
 comparison section 10. 
 Thus, by correcting using the laser scale 12 being hardly affected by the 
 changes in the environment, the visual inspection of tile reticle can be 
 performed with a high accuracy. 
 Although the position-detection resolution and the repeat accuracy of the 
 laser scale 12 are substantially 0.01 .mu.m, and is less by one order of 
 magnitude as compared with the laser interferometer 4, the pitch of 
 scanning is substantially 0.1 .mu.m, so is no problem. 
 Moreover, although a method for detecting, the position of the x-axis by 
 only the laser scale 12 also can be thought without using the laser 
 interferometer 4, in general, the objective lens 3 is oscillated with a 
 different phase from one of the X-Y stage 2, whereby as far as this 
 oscillation is canceled by means of the laser interferometer 4, a defect 
 detection can not be performed with a high accuracy. Therefore, the use in
 combination of the laser interferometer 4 and the laser scale is required.
 In the interim, in order to obtain the optical irate of the reticle 1, as a
 substitute of using the combination of the laser-scanning optical system 5
 and the transmitted-light detection section 7, the combination of a 
 mercury lamp and CCD line sensor or the like also can be used. 
 As described above, according to the invention, by calculating, the 
 measurement error of the laser interferometer accompanying with the 
 charges in the environment using the scale which is hardly affected by the
 changes in the environment and synthesizing the reference image corrected 
 by the error thereof to compare with the optical image, the reticle can be
 inspected accurately. In particular, since the error of the laser 
 interferometer 4 is monitored always to be fed-back for synthesizing of 
 the reference image in inspection of the next time, it can be corresponded
 even in the case that the change in the environment goes on, thereby an 
 inspection performance with a high accuracy is able to be maintained. 
 Moreover, a very large temperature control chamber and the wavelength 
 compensator are not required, thereby the entire system is able to be made
 compact and to be reduced in price. Furthermore, it can be manufactured 
 simply and inexpensively by adding the scale to the conventional 
 constitution. 
 Moreover, the change in the environment is not detected by an indirect 
 measuring method such as the compensator, but the distance which the stage
 actually travels is detected by two kinds of methods of the laser 
 interferometer and the laser scale to obtain the deviation, whereby taking
 advantages of the both characteristics, the reference image appropriately 
 corrected can be obtained and inspected accurately. 
 While the present invention has thus far been described in conjunction with
 only a preferred embodiment thereof, it will now be readily possible for 
 one skilled in the art to put the present invention into effect in various
 other manners.