# Patent

## Title
Method and apparatus for pattern inspection

## Abstract

Because a mirror electron imaging type inspection apparatus for obtaining an inspection object image with mirror electrons has been difficult to optimize inspection conditions, since the image forming principles of the apparatus are different from those of conventional SEM type inspection apparatuses. In order to solve the above conventional problem, the present invention has made it possible for the user to examine such conditions as inspection speed, inspection sensitivity, etc. intuitively by displaying the relationship among the values of inspection speed S, inspection object digital signal image pixel size D, inspection object image size L, and image signal acquisition cycle P with use of a time delay integration method as a graph on an operation screen. The user can thus determine a set of values of a pixel size, an inspection image width, and a TDI sensor operation cycle easily with reference to the displayed graph.

## Background
CLAIM OF PRIORITY
The present application claims priority from Japanese application JP 2006-027861 filed on Feb. 6, 2006, the content of which is hereby incorporated by reference into this application.
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for inspecting an electrical defect of a microstructure circuit formed on a semiconductor wafer.
BACKGROUND OF THE INVENTION
As a method for detecting a defect of a circuit pattern formed on a wafer by comparing images in a manufacturing process of a semiconductor device, for example, a pattern comparing inspection method is disclosed in JP-A No. 258703/1993. The method uses an SEM in which, a point focused electron beam is scanned. The SEM type inspection apparatus is higher in resolution than optical inspection systems and it has a feature for enabling such an electrical defect as a connecting failure to be detected. However, because the SEM type inspection apparatus scans an electron beam on a specimen surface two-dimensionally to obtain an image, the scanning time is long. This disadvantage will become a substantial obstacle for reducing the inspection time.
As an electron beam inspection method that has successfully reduced the inspection time, for example, the JP-A No. 249393/1995 discloses a projection type inspection apparatus, which radiates a rectangular electron beam onto semiconductor wafer and forms an image of buck scattering and secondary electrons with use of electron lenses. The projection type inspection apparatus can radiate an electron beam with a larger current than that of the SEM type at a time, thereby obtaining a plurality of images collectively. The projection type is thus expected to form images more quickly than those of the SEM type, that is, the scanning electron type.
On the other hand, a secondary electron mapping type inspection apparatus cannot obtain a sufficient resolution due to the aberration of the objective lens, thereby it is difficult to obtain a required defect detection sensitivity. The JP-A No. 108864/1999 points out such disadvantages of the apparatus. The JP-A No. 108864/1999 discloses a mirror electron imaging type wafer inspection apparatus that uses electrons pulled back before colliding with a specimen due to a negative electric field formed just above the wafer (hereunder, to be referred to as mirror electrons or mirror reflecting electrons) as imaging electrons.
Here, the mirror electron imaging type wafer inspection apparatus will be described. The mirror electron imaging type wafer inspection apparatus obtains an image to be used for inspection with use of a mirror electron microscope. An inspection image is obtained by radiating an electron beam onto a specimen and forms an image of the reflecting electron beam. At this time, a negative potential is applied onto the surface of the specimen in advance so that the radiated electron beam is reflected on a specific equipotential surface in the vicinity of the specimen surface without reaching the specimen surface. The electrons reflected on an equipotential surface in the vicinity of the specimen surface such way are referred to as “mirror electrons”. Because the equipotential surface of the specimen surface is influenced by the information of an unevenness and a potential change of the specimen surface itself, the image to be obtained is also influenced by the information of the unevenness and the potential change of the specimen surface when the mirror electrons are imaged. Consequently, shape and electrical defects of the specimen surface can be detected by comparing such a mirror electron image with a reference image, respectively.
SUMMARY OF THE INVENTION
As described above, the mirror electron imaging type wafer inspection apparatus differs completely from any of the conventional SEM type inspection apparatuses. Consequently, to put the mirror electron imaging type wafer inspection apparatus for practical use, it is needed to think out a method for setting inspection conditions optimized for the apparatus. Under such circumstances, it is an object of the present invention to realize an inspection condition setting method optimized for the object mirror electron imaging type wafer inspection apparatus and make it easier to operate the apparatus.
Upon thinking out a method for setting such inspection conditions optimized for the mirror electron imaging type wafer inspection apparatus, the present inventor has examined the following circumstances.
In a semiconductor device manufacturing line, a mirror electron imaging type wafer inspection apparatus is often used for defect inspection in all or some specific portions of every wafer flowing on the manufacturing line. Thus, it should be carefully determined between which semiconductor processes an inspection process is to be inserted allocated for the inspection process by giving consideration to the productivity of the semiconductor manufacturing line. In other words, if accurate inspection process is designed to improve the yield, the productivity is lowered in proportion to an increase of the inspection time. On the other hand, if the inspection time is reduced, both the inspection accuracy and the productivity are lowered. As such, for various manufacturing lines, each of which has different defect generating rate, defect generating process, and productivity, there is an optimum inspection time for each manufacturing line to achieve the best productivity. The optimum inspection time for each manufacturing line is varied among those manufacturing lines. This is why the conditions for the inspection speed should be set flexibly for the mirror electron imaging type wafer inspection apparatus so as to make inspections most efficiently by giving consideration to the circumstances specific to each of various semiconductor device manufacturing lines.
The inspection speed of the mirror electron imaging type wafer inspection apparatus means an area of a wafer that can be inspected per unit time. FIG. 2 shows the arrangement of pixels for composing an inspection object image. In FIG. 2, each cell means a pixel 201. The call is usually a square of which the length of this side is represented by D. An inspection image of the mirror electron imaging type wafer inspection apparatus is obtained with use of a time delay integration data acquisition method (TDI method). In the TDI method, integration is made by sending image signals in the vertical direction of the image synchronously with the movement of the wafer (as shown with a white arrow in FIG. 2). The cycle in which one signals of pixel are sent in the vertical direction is defined as P. And the length in a direction (horizontal direction in FIG. 2) normal to the movement of the wafer in the image region is defined as L. The image data of length L×width D area (gray region shown in FIG. 2) is sent in a cycle P to an image processing apparatus. Consequently, the inspection speed S can be described by using D, P, and L in the following expression:

S=D×L×P. 

To operate the apparatus at an optimized speed, therefore, the user is requested to satisfy the relationship among D, L, and P shown above and adjust the D, L, and P values so as not to degrade the inspection sensitivity. Particularly, in the case of the mirror electron imaging type wafer inspection apparatus, the pixel size optimized for inspection is changed depending on the magnification of the imaging optical system for mirror reflecting electrons. This change depends on the characteristics such as the material, structure, etc. of the specimen. And such characteristics are never generated in any of SEM and secondary electron projection type electron optical systems; the characteristics are specific to the mirror optical systems. The user of the apparatus, therefore, is requested to adjust the D, L, and P values by giving consideration to the magnification of the optical system.
Conventionally, the apparatus manager and the apparatus developer have set such D, P, and L values by trial and error, by giving consideration to the characteristics of the mirror electron imaging type wafer inspection apparatus and the inspection object. Furthermore, the user's interest is usually just the inspection speed. The user would thus feel very troublesome when requested to set such conditions and will feel that the apparatus is inconvenient when operating the apparatus.
In order to solve the above described conventional problems, therefore, the present invention has enabled such S, D, L, and P values to be displayed on an operation screen so that the user can examine such conditions as inspection speed, inspection sensitivity, etc. intuitively. Furthermore, the present invention has provided a process newly for converting user determined conditions to conditions for operating an electron optical system, a time delay integration type imaging device, and a wafer moving stage respectively so that the user can make inspections in accordance with the circumstances of various semiconductor manufacturing lines without understanding the details of the inspection apparatus.
According to the present invention, therefore, it is possible to set such conditions as optimized pixel size, irradiated area size, etc. to easily realize an inspection speed capable of preventing an semiconductor device manufacturing line from delay so that the user can inspect defects of each semiconductor pattern efficiently under optimized conditions. Because such inspection conditions can be set easily such way, the total inspection time from condition setting to end of inspection can be reduced. And because the apparatus can be operated easily, the apparatus will also have advantages in sales policy.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram for describing a basic configuration of a mirror electron imaging type wafer inspection apparatus;
FIG. 2 is a diagram for describing how to obtain an inspection image with a TDI image acquisition method;
FIG. 3 is a diagram for describing an example of an inspection condition setting screen;
FIG. 4 is a diagram for describing another example of an inspection condition setting screen;
FIG. 5 is a diagram for describing still another example of an inspection condition setting screen;
FIG. 6 is a diagram for describing principles of defect detection with a mirror electron image;
FIG. 7 is a diagram for describing an example of a defect expansion effect in an inspection object image with a mirror electron imaging method;
FIG. 8 is a diagram for describing an application example of the present invention to a mirror electron imaging type wafer inspection apparatus;
FIG. 9 is a diagram for describing an example of an inspection condition setting screen; and
FIG. 10 is a diagram for describing an application example of the present invention to a mirror electron imaging type wafer inspection apparatus.



DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Hereunder, a description will be made in detail for a configuration of a mirror electron imaging type wafer inspection apparatus in a preferred embodiment of the present invention with reference to the accompanying drawings.
First Embodiment
FIG. 1 shows an example of a hardware configuration of the mirror electron imaging type wafer inspection apparatus in the preferred embodiment of the present invention. In FIG. 1, vacuum pumps, their controller, pipes for evacuating systems, etc. are omitted.
At first, main elements of the electron optical system of the apparatus will be described. An electron beam 100a emitted from an electron gun 101 is focused by a condenser lens 102 and deflected by an ExB deflector 103 to form a cross-over 100b, then radiated onto a specimen wafer 104 as an approximate parallel flux. In FIG. 1, although only one condenser lens 102 is used, a plurality of lenses may be combined into a lens system to optimize the optical conditions. The electron gun 101 is usually a Zr/0/W type Schottky electron source. Such voltages and currents as an extracting voltage applied to the electron gun 101, an accelerating voltage to extracted electrons, a heating current of an electron source filament, etc. required for operating the apparatus are supplied and controlled by an electron gun controller 105.
The ExB deflector 103 is disposed in the vicinity of an imaging plane 100d of an imaging electron beam 100c. At this time, an aberration occurs in the radiated electron beam 100a due to the ExB deflector 103. If this aberration must be corrected, another ExB deflector 106 for correcting the aberration is disposed between an radiation system condenser lens 102 and the ExB deflector 103. The electron beam 100a deflected by the ExB deflector 103 so as to go along an axis perpendicular to the wafer 104 is formed by an objective lens 107 as a planar electron beam entered in a direction perpendicular to the surface of the specimen wafer 104. On the focal plane of the objective lens 107 is formed a finer cross-over by the radiation system condenser lens 102. Thus the electron beam can be radiated onto the specimen wafer 104 just in parallel. A region of the specimen wafer 104, in which the electron beam 100a is radiated, is an area as large as, for example, 2500 μm2, 10000 μm2, or the like.
The specimen wafer 104 mounted on a wafer stage 108 receives a negative voltage almost equal to or slightly higher larger absolute value than the accelerating voltage of the electron beam. This negative potential works on the radiated electron beam 100a so that it slows down just before reaching the wafer 104 and pulled back upward to become as reflecting mirror electrons, thereby it does not collide the wafer 104 in most cases. The voltage applied to the wafer 104 is supplied and controlled by a wafer voltage controller 109. In order to make the radiated electrons reflected in the vicinity of the wafer 104, a difference from the accelerating voltage of the radiated electron beam 100a is required to be adjusted accurately and the wafer voltage controller 109 and the electron gun controller 105 are controlled so that they are interlocked with each other.
Mirror electrons flying from the wafer side includes information related to an electrical defect of an object circuit pattern formed on the wafer 104. Thus its image is formed with use of an electron imaging optical system to be fetched in the apparatus as an image for determining whether there is a defect in the pattern or not. The mirror electrons are focused by the objective lens 107. And the ExB deflector 103 is controlled so as not to deflect an electron beam advancing from below, so that the mirror electrons go up perpendicularly as are, then magnified and projected by an intermediate lens 110 and a projection lens 111 at an image detection part 112. In FIG. 1, only one projection lens 111 is used, but a plurality of lenses may be composed into a projection system to provide a higher magnification and correct distortions of images. The image detection part 112 converts an image to an electrical signal and sends a distribution of the local charging potential of the surface of the wafer 104, that is, a defect image to an image processing part 112. The electron optical system is controlled by an electron optical system controller 113.
Next, the image detection part 112 will be described. A fluorescent plate 112a, an optical image detector 112b, and an optical image transmission system 112c are used for optical coupling to convert a mirror electron image to an optical image and detect the image. In this embodiment, an optical fiber bundle is used as the optical image transmission system 112c. The optical fiber bundle consists of the same number of thin optical fibers as the number of pixels and it can transmit optical images efficiently. In case where a fluorescent image is used with a sufficient light, the optical transmittance may be set lower. In such cases, instead of the optical fiber bundle, an optical lens is used and an optical image on the fluorescent plate 112a is formed by the optical lens on a light detecting surface of the optical image detector 112b. Furthermore, an amplifier is inserted in the optical image transmission system to transmit an optical image with a sufficient light. The optical image detector 112b converts an optical image formed on the light receiving plane to an electrical image signal and outputs the signal. As the optical image detector 112b, an TDI sensor is used. The TDI sensor uses a time delay integration (TDI) type CCD.
The image processing part 116 is composed of an image memory 116a and a defect determination part 116b. The image memory 116a inputs electron optical condition, image data, and stage position data from the electron optical system controller 113, the image detection part 112, and the stage controller 115 respectively and stores the image data by relating it to the coordinate system used on the specimen wafer. The defect determination part 116b uses image data related to the coordinates on the wafer and compares it with a preset value or with an adjacent pattern image or an image of the same pattern position in an adjacent die, or the like with use of various defect determination methods so as to determine a defect. The defect coordinates and an intensity of its corresponding pixel signal are transferred to and stored in the inspection apparatus controller 117. The user sets any one of those defect determination methods or the inspection apparatus controller 117 selects a method corresponding to the object wafer type in advance.
The inspection apparatus controller 117 inputs/outputs conditions for operating each part of the apparatus. The inspection apparatus controller 117 inputs beforehand various preset conditions such as electron beam accelerating voltage, current conditions for electron optical devices, wafer stage moving speed, timing for acquisition an image signal from an image detection element. The inspection apparatus controller 117 controls the controller of each element as an interface with the user. The inspection apparatus controller 117 may be composed of a plurality of computing devices connected to each another through a communication line and having a specific function. The apparatus further includes user interface device 118 with a monitor.
In the mirror electron imaging type wafer inspection apparatus, the electron beam hardly collides with the object wafer. Thus the specimen wafer may not be charged sufficiently in some cases. To detect an electrical defect, however, the wafer must be charged sufficiently to cause a difference from that of normal parts. The present invention has solved this problem by providing pre-charging devices 119a and 119b. Those devices are controlled by a pre-charging controller 120. The pre-charging controller 120 controls a charging potential generated on the wafer by the pre-charging devices 119a and 119b with the wafer voltage controller 109 and the electron gun controller 105 not to disturb the status of the electron beam that is reflected in the vicinity of the wafer surface.
FIG. 3 is a first embodiment of a screen on which the user operates the inspection apparatus. This screen 301 is an “inspection condition setting screen” on which the user select an inspection speed and an inspection sensitivity or part of the screen. The screen 301 belongs to the user interface device with a monitor 118. A graph 302 displayed on the screen 301 has a horizontal axis that indicates a pixel size D and a vertical axis that indicates an inspection speed S. In the graph 302, the inspection speed S means an area on a wafer surface to be inspected per unit time and it is represented by an inspection area (cm2/h) per hour. The user may select this unit to make it easier to understand. For example, the unit may be the number of wafers to be processed per hour or an inverse number to define a processing time of one wafer and a time required for a unit area inspection (e.g., h/cm2).
In the graph 302, a range from 0 cm2/h to 600 cm2/h is shown. The pixel size D is represented by a value corresponding to an actual size on the object wafer. It is within a range from 0 nm to 250 nm. The graph 302 also has a plurality of characteristic straight lines 303. These straight lines are used for different cycle P values of the TDI sensor respectively. The graph 302 uses values of 100 to 700 kHz selected as P values. The width L of an inspection image is displayed on an inspection image width display part 304. In this example, it is set as 60 μm. A plurality of conditions are displayed as a pull-down menu for this value when the user clicks the selection arrow 305. The user can select and change any of the conditions. If the user selects another value, a newly calculated straight line is displayed as shown in FIG. 4. FIG. 4 shows an example in which 120 μm is selected as an L value. The L value has its upper limit, which depends on the aberration of the objective lens. If the value goes over 200 μm, the distortion and the resolution degradation in marginal area of the field of view advance significantly. Consequently, the upper limit of the field of view usable as an inspection image is about 200 μm×200 μm.
The user can search an inspection speed and a pixel size by moving a white arrow pointer with a mouse on the graph 302. The values of the inspection speed, pixel size, TDI cycle calculated from the position of the pointer on the graph 302 are displayed in a display field 307 at the bottom of the screen. The user can select conditions with reference to such concrete values. In the graph 302, not only the values on straight lines, but also values between straight lines are calculated from the pointer position and displayed.
When conditions are determined on the graph 302, the user clicks the mouse button (not shown) or press a specific key on the keyboard (not shown) to fix the conditions. Those conditions are sent to the controller of the inspection apparatus when the user clicks the [ENTER] button 308 on the screen, then those conditions are converted to detailed operation conditions of the apparatus. Each of those conditions has its upper limit, which depends on the specifications of the apparatus. For example, if P=700 kHz is the upper limit in the specifications of the TDI camera and a condition is set in a region over 700 kHz, the condition is ignored.
The user can determine values of a set of the pixel size, inspection image width, and TDI sensor operation cycle with reference to the drawing.
Using the condition setting method in this embodiment makes it possible for the user to set an inspection speed of the mirror electron imaging type wafer inspection apparatus without trial and error.
Second Embodiment
In the first embodiment, the user determines conditions for operating the inspection apparatus with reference to mainly the values of inspection speed and pixel size and the relationship between the defect detection sensitivity and the pixel size is not clear. In this second embodiment, therefore, the horizontal axis of the graph displayed on the inspection speed S setting screen is used for defect detection sensitivity, thereby the user comes to know the relationship between the defect detection sensitivity and the pixel size intuitively. Instead of the horizontal axis, the vertical axis may also be used for the defect detection sensitivity.
FIG. 5 shows a schematic diagram of the inspection speed S setting screen displayed on the user interface device with a monitor 118 of the mirror electron imaging type wafer inspection apparatus. The user operation screen shown in FIG. 5 is displayed on the monitor of the mirror electron imaging type wafer inspection apparatus or mirror electron imaging type specimen inspection apparatus. Unlike that shown in FIG. 3, the horizontal axis of the graph 501 is used for the defect detection sensitivity. Explanations of the same items such as the pointer, the characteristic curve, etc. as those shown in FIG. 3 will be omitted to simplify the description.
The relationship between the pixel size and the defect detection sensitivity is based on the magnification function of defect images specific to the mirror electron imaging type wafer inspection apparatus. An inspection object image of the mirror electron imaging type wafer inspection apparatus is obtained by imaging a distortion of an equipotential surface caused by existence of a defect. FIG. 6 is a diagram for describing principles of such mirror electron imaging. FIG. 6A shows a view of an equipotential surface 603 and a view of a trajectory 604 of radiated electrons which are reflected from the equipotential surface 603 when a protruded defect 601 such as a foreign particle and a recessed defect 602 such as a scratch are detected on the object wafer surface. FIGS. 6B and 6C are diagrams for showing a distortion of the equipotential surface 606 and a trajectory 607 of radiated electrons when connecting failure 605a and 605b exist as defects in vias 605 used for connecting to the lower layer wiring embedded in an oxide film respectively. In FIG. 6B, the electrically open via 605a is negatively charged. In FIG. 6C, the open via 605b is positively charged. Due to any of the unevenness of the shape of the wafer surface and the electrical difference on the wafer surface, the area of the distortion of the equipotential surface is larger than the actual size of the defect. In addition, when the equipotential surface is positioned higher, the area of the distortion is larger while the distortion level is low. Consequently, a larger image is obtained when compared with the actual defect size by adjusting the mirror electron imaging optical system. This means that defects can be detected even when defect images are obtained with pixels larger than the actual defect size.
FIG. 7 is an example of an inspection image obtained with use of a mirror electron imaging method. FIG. 7A shows a schematic diagram of a circuit pattern. This pattern consists of 200 nm×200 nm square vias 702 embedded in an oxide film 701 and composed like a matrix patterns arranged in 5 rows×5 columns at pitches of 800 nm. Each normal via is continued to a wiring 703 in the lower layer. FIG. 7B is an inspection image, that is, a mirror electron image obtained with use of a mirror electron imaging method. The size of the via patterns in this mirror electron image, except for the center one, is as large as 600 nm, which is about 3 times the actual one. This magnification is made due to a distortion of the equipotential surface caused by a difference of voltage between the via voltage and the voltage of its peripheral insulation film. In the mirror electron image shown in FIG. 7B, the center via 704 has a disconnect defect and its voltage differs from that of other normal ones by about +1.5V. The size of the mirror electron image of this defect via 704 is about 1200 nm, which is about double that of a normal via and magnified up to 6 times that of the actual via pattern.
It can thus be concluded from those data that the size of a mirror electron image is magnified to from 3 times to 6 times the actual size due to the via voltage. According to this result, in this embodiment, it is expected to be able to detect defects of patterns up to 1⁄3 of the pixel size and the horizontal axis of the graph 501 for the inspection speed is displayed for the detection sensitivity, which is 1⁄3 of the pixel. The value indicated by the horizontal axis of the graph 501 shown in FIG. 5 is converted from the value indicated by the horizontal axis of the graph 501 shown in FIG. 3 on the basis of the above detection sensitivity information. The calculation for converting a value of the horizontal axis or vertical axis such way is executed by a computing device built in the inspection apparatus controller 117 or user interface device with a monitor 118. In the same way, the relationship between the detection sensitivity and the pixel is stored in the memory means built in the inspection apparatus controller 117 or in the user interface device with a monitor 118. As the memory means, for example, any of a memory, a hard disk, etc. may be used.
Next, a description will be made for how to transmit such conditions as user specified inspection speed, etc. to the electron optical system and the wafer stage.
FIG. 8 shows a schematic diagram of a hardware configuration of a mirror electron imaging type wafer inspection apparatus in this embodiment. In FIG. 8, the same reference numbers will be used for the components of the same functions and operations as those shown in FIG. 1. In the mirror electron imaging type wafer inspection apparatus shown in FIG. 8, the inspection apparatus controller 117 is provided with a conversion part 801. Conditions for inspection operations inputted by the user through the user interface device 118 are sent to the inspection apparatus controller 117. In this embodiment, the inspection apparatus controller 117 is provided with a condition conversion part 801. Conditions inputted through the user interface device with a monitor 118 are values of the pixel size D, inspection speed S, TDI camera image acquisition cycle P, and size of the field of view L. The moving speed Vs of the wafer stage 108 is calculated on the basis of those values. The Vs is determined by the following relational expression according to the TDI camera image acquisition cycle P and the pixel size D.

Vs=P×D 

This Vs value is sent to the stage controller 115. The stage controller 115 controls a stage driving mechanism by monitoring the stage position information received from the position detector 114 so as to keep the speed Vs while the stage is moving. The TDI camera image acquisition cycle P is sent to the condition conversion part 801 as is. The condition conversion part 801 controls the TDI camera image acquisition cycle so that the image acquisition is synchronized with the stage movement. The pixel size D, as well as a preset pixel size Dp on the TDI camera light detecting surface are used to calculate a magnification Dp/D of the imaging electron optical system. The magnification Dp/D of the imaging electron optical system is sent to the electron optical system controller 113 and used to control the voltage and the electromagnet current applied to the objective lens 107, the intermediate lens 110, and the projection lens 111 respectively. The voltage and current conditions of each electron optical element of the imaging optical system with respect to the magnification of the imaging optical system are stored as a numerical table beforehand in the electron optical system controller 113 or condition conversion part 801. The voltage and current conditions are determined with reference to those values in the table. If a magnification value that is not stored in the table is referred to, current and voltage values are determined by interpolating the values for the nearest magnification value.
The condition conversion part 801 or electron optical system controller 113 stores a numerical table that records conditions of both the condenser lens 102 and the objective lens 107 with respect to the size of the field of view L. And upon a user's determination for the size of the field of view L, the voltage and current values of the radiated electron optical system are referred to from the numerical table for controlling. With such a configuration, the user selected inspection conditions are reflected correctly in the inspection apparatus.
As described above, therefore, in this embodiment, the user can set such inspection conditions as inspection speed and defect detection sensitivity as parameters. Consequently, the user can operate the apparatus more easily.
Third Embodiment
In the second embodiment, a description is made for a user operation screen on which the defect magnification is set as 3 times. This third embodiment enables the user to change the defect magnification.
FIG. 9 shows a user operation screen in this third embodiment. The user operation screen shown in FIG. 9 is displayed on the monitor of the mirror electron imaging type wafer inspection apparatus or mirror electron imaging type specimen inspection apparatus. In this third embodiment, the description for the same components as those shown in FIG. 5 will be omitted. In FIG. 9, there is only a difference from that shown in FIG. 5; a defect magnification selection field 901 is provided. The user can select a magnification from a plurality of defect magnification values by clicking the arrow in the defect magnification selection field 901. The value of the horizontal axis of the graph 902 is corrected by the selected defect magnification, thereby the displayed characteristic curve is also corrected.
The user can change the defect magnification according to the objective lens focal condition, the height of the equipotential surface for reflecting mirror electrons, etc. in the mirror electron imaging. A mirror electron image is always formed due to a distortion of the equipotential surface even when the defect type is an uneven surface or a voltage variation caused by an electrical defect. Consequently, the user can estimate a defect image magnification in advance by using the height of the equipotential surface for causing mirror reflection of electrons and focal conditions of the objective lens as parameters.
For such an estimate, the user is just requested to obtain a mirror electron image with respect to a different focal point of the objective lens and a different voltage potential value of the wafer and measure a magnification according to the actual defect size by using an Si wafer of which size is already known and having a protruded or recessed shape defect that is already processed. If the defect type is not such a shape defect, but it is a potential variation, a voltage that causes the equipotential surface to be distorted as much as a distortion generated by a protruded or recessed shape may be calculated by computer simulation and adjusted precisely with the relationship between the electron optical condition and the magnification in the shape defect. Because such calculation of a level of a distortion of an equipotential surface with respect to a voltage is simple calculation of an electrical field, it is so easy. Instead of using a standard specimen as described above, it is also possible to analyze a trajectory of electrons by computer simulation and change the condition of the objective lens, thereby calculating an image to be obtained, then obtaining a relationship with a magnification.
In this embodiment, the inspection apparatus is provided with an inspection condition evaluation device 1001 for holding a table that stores a condition of the objective lens, a negative voltage value to be applied to each wafer to change its equipotential surface used for mirror reflection, and a defect magnification obtained as described above. FIG. 10 shows a schematic diagram of a system provided newly with the inspection condition evaluation device 1001.
When an inspection is made with a defect magnification using this inspection apparatus, the equipotential surface for reflecting mirror electrons must be kept constant. Thus it is important to keep the wafer surface potential constant. This is why pre-charging devices 119a and 119b are used. Those charging devices are controlled by a pre-charging controller 120. For example, assume now that a wafer is passed under the pre-charging device, then just under the objective lens and moved just under the pre-charging device 119b. In such a case, the wafer surface potential is set to a prescribed potential by the pre-charging device 119a. This potential makes it possible to obtain a desired defect magnification. The potential is given from the numerical table of the inspection condition evaluation device 1001 and controlled by the pre-charging controller 120. As the pre-charging device, for example, such an electron beam radiation device as a flood gun may be used. After the wafer passes just under the objective lens, the disturbance of the equipotential surface potential, caused by slight charging of the wafer when in observation of mirror electrons, must be eliminated so to as return the potential to a required level with use of the pre-charging device 119b again.
According to this third embodiment, therefore, it is possible to optimize the such conditions as the inspection time including the user specified defect magnification, thereby the object semiconductor manufacturing line can be managed efficiently.
Although a description has been made for the preferred embodiments of the present invention, the present invention also includes a combination of the first to the third embodiment described above.

## Claims

1. A wafer defect inspection method, comprising the steps of:
radiating an electron beam on a certain area of a wafer;
reflecting radiated electrons by applying a negative voltage to said wafer just before said radiated electrons reach a surface of said wafer;
forming an image of the reflected electrons;
moving said wafer at a certain speed with respect to said radiated electron beam;
obtaining a digital image with a time delay integration data acquisition synchronously with a moving speed of said wafer; and
extracting a defect of said wafer using an obtained image and displaying a position and an image of said defect,
wherein said method further includes a step of displaying on an operation screen a relationship among the values of S, D, L, and P when L is defined as a length in a direction perpendicular to a moving direction of said wafer of which image is being obtained in a range of an inspection object image in the region on which said electron beam is radiated, S is defined as an area of said wafer to be inspected per unit time, D is defined as a size on said wafer, corresponding to a unit pixel of said obtained digital image, and P is defined as an image signal acquisition frequency in said time delay integration formula, as well as a step of adjusting an electron optical system and a wafer moving speed on the basis of user determined S, D, L, and P values,
wherein a graph for S=D×L×P is displayed with respect to a plurality of L values and P values respectively by assuming the horizontal axis as D and the vertical axis as S, and
wherein said horizontal axis of said graph is displayed as a defect sensitivity obtained by multiplying said D value by a constant less than 1.

2. The method according to claim 1, wherein the L value is 200 μm or under.

3. The method according to claim 1, wherein the D value is 250 nm or under.

4. The method according to claim 1, wherein a condition for operating an electron lens is determined to realize user determined values of D and L with reference to a numerical table that holds said condition for operating said electron lens corresponding to each of said D and L values beforehand.

5. The method according to claim 1, wherein said constant is 1⁄3 or under.

6. The method according to claim 1, wherein an Si wafer marked with an uneven pattern is used when said user selects said constant, thereby measuring beforehand a potential of said Si wafer and a focal point of an objective lens, as well as a ratio between the size of said marked pattern and the size of a mirror electron image so that said measurement results are set in said numerical table, then a wafer surface potential with respect to said constant is related to an objective lens focal condition according to said results in said numerical table.

7. A defect inspection apparatus, comprising:
means for radiating an electron beam on a certain area of a wafer;
means for reflecting radiated electrons by applying a negative voltage to said wafer just before said electrons reach a surface of said wafer;
means for forming an image of the reflected electrons;
means for moving said wafer at a certain speed with respect to said radiated electron beam;
means for obtaining a digital image with a time delay integration formula synchronously with a moving speed of said wafer; and
means for extracting a defect of said wafer using an obtained image and displaying a position and an image of said defect,
wherein said method further includes means for displaying a relationship among values of S, D, L, and P when L is defined as a length in a direction perpendicular to a moving direction of said wafer of which image is being obtained in a range of an inspection image in the region on which said electron beam is radiated, S is defined as an area of said wafer to be inspected per unit time, D is defined as a size on said wafer, corresponding to a unit pixel of said obtained digital image, and P is defined as an image signal acquisition frequency in said time delay integration formula, as well as means for adjusting an electron optical system and a wafer moving speed on the basis of user determined values of S, D, L, and P,
wherein said apparatus further includes means for displaying an graph for S=D×L×P with respect to a plurality of values of L and P respectively by assuming the horizontal axis as D and the vertical axis as S, and
wherein said apparatus further includes means for displaying the horizontal axis of said graph as a defect sensitivity obtained by multiplying a D value by a constant less than 1.

8. The apparatus according to claim 7, wherein the L value is selected within 200 microns or under.

9. The method according to claim 7, wherein the D value is displayed as 250 nm or under.

10. The apparatus according to claim 7, wherein said apparatus further includes means for holding a numerical table in which a condition for operating an electron lens corresponding to each of D and L values is set beforehand and means for determining a condition for operating said electron lens to realize user determined values of D and L with reference to said numerical table.