Charge-up prevention method and charge-up prevention device

Abstract

PURPOSE: To provide a charge-up prevention method and a charge-up prevention device that prevent charge-up of an EUV mask in a device that irradiates an EUV mask with an electron beam, detects the electrons generated, and acquires an image.CONSTITUTION: A charge-up prevention method includes an electron gun, an objective lens, a deflection system, a stage, detection means for detecting secondary electrons or reflected electrons to generate an image, providing a band-shaped groove around an area where a pattern is formed on the surface of an EUV mask to electrically isolate it, electrically connecting the internal pattern of a black border that reduces the effects of flare from an EUV light source to the surrounding area, and providing a claw electrode that externally controls the potential of the internal pattern or a conductive connecting portion provided on the substrate of the EUV mask that externally controls the potential of the internal pattern, and the claw electrode or the conductive connecting portion prevents charge-up caused by the black border in the area where the pattern is formed on the surface of the EUV mask.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a charge-up prevention method and a charge-up prevention device.

Background Art

[0002] Semiconductor devices have been shrinking every year according to Moore's law, and in the most advanced devices, the minimum feature size has become 20 nm or less. In order to achieve a small feature size, an exposure technique capable of forming a smaller pattern is required.

[0003] Conventionally, a laser beam with a wavelength of 193 nm has been used for exposure. However, since the optical resolution has already been greatly exceeded, in recent years, an exposure technique using EUV light with a wavelength of 13.5 nm has been actively promoted. As a result, in the logic 7-nanometer generation, TSMC and others have successfully achieved practical use for the first time in the world.

[0004] The evolution of exposure technology means the reduction of the feature size of the photomask pattern and the increase in the number of patterns. To perform semiconductor exposure, a quartz plate of an insulator with a pattern drawn on it, called a photomask, is used. A fine pattern based on circuit design data is drawn on this quartz plate. A master plate called a template for nanoimprint is also made of quartz.

[0005] In order to examine whether the patterns formed on these masks are of the correct dimensions, electron microscope technology is essential because optical technology has insufficient resolution. In order to obtain a high-definition electron beam image, it is necessary to irradiate electrons with a small beam spot size on the sample surface and detect the signal electrons. At that time, it is essential that the sample surface is not charged and does not adversely affect the irradiated electron beam and signal electrons.

Disclosure of the Invention

Problems to be Solved by the Invention

[0006] Conventionally, the retarding method has been used for the purpose of inexpensively reducing the aberration of an electron beam optical system. In this method, once the energy of an electron beam emitted from an electron gun is accelerated to a high energy of several tens of kV, the electron beam is then focused using an electron optical system, and the energy of the electrons is decelerated to 1 kV or less by the reverse bias voltage applied to the sample, and the sample is irradiated. It is characterized in that the resolution can be kept high despite the low energy during irradiation of the sample.

[0007] The higher the energy of the electron beam, the shorter the wavelength, and the beam size can be reduced, improving the image resolution. However, when directly scanning a semiconductor device or the like, which is the sample, with a high-energy electron beam to obtain and observe an image, it has been found that the high-energy electrons cause a semi-permanent effect on the semiconductor that is the sample and damage the device. For the purpose of preventing this, in a CDSEM for semiconductors, the retarding method is used while reducing the beam size and lowering the energy of the electron beam during irradiation.

[0008] In the retarding method, it is necessary that the measured point potential on the sample surface is at a desired potential. However, in the case of a photomask made of a dielectric material such as quartz or a template for nanoimprinting, unlike the case of a semiconductor wafer which is usually a conductor, the potential is different between the voltage applied to the electrode provided below the sample used for applying the voltage and the surface of the photomask. Therefore, it is extremely difficult to always adjust to the target surface potential. Further, when the sample moves due to the movement of the XY stage, the capacitance constituting the measurement system changes accordingly, and there is a problem that the retarding voltage substantially applied to the sample surface changes.

[0009] Furthermore, in an apparatus that forms an image by accelerating secondary electrons generated on the sample surface to a predetermined energy, such as a multi-beam inspection apparatus, there has been a problem that the retarding voltage becomes indefinite, causing a defect in image formation.

Means for Solving the Problem

[0010] The present invention is characterized in that, in a sample in which a pattern is formed on an insulating substrate such as a photomask as described above in the prior art, a desired retardation voltage can be stably applied to the surface, and a high-resolution and stable image can be obtained.

[0011] In order to solve the above problems, the present invention provides an electron beam inspection apparatus that applies a retardation voltage to a sample to slow down a high-speed electron beam and irradiate it to generate a high-resolution image. The apparatus includes an electron gun that generates an electron beam with a predetermined high acceleration voltage, an objective lens that narrows down the electron beam with the predetermined high acceleration voltage generated by the electron gun and irradiates the sample, a deflection system that two-dimensionally scans the electron beam narrowed down by the objective lens on the surface of the sample, a retardation voltage application device that applies a predetermined retardation voltage to the sample, a detection device provided facing the sample at the tip of the objective lens that detects the potential or capacitance of the surface of the sample, and a correction device that corrects the first retardation voltage so as to keep the potential of the surface of the sample constant based on the potential or capacitance of the surface of the sample detected by the detection device.

[0012] At this time, a sample surface potential control device is provided to control the second potential on the surface of the sample by applying a voltage from the outside. Based on the potential or capacitance detected by the detector, the sample surface potential control device is instructed to control the second potential, or the correction means is instructed to control the first retardation voltage, or both are controlled.

[0013] Also, the measurement point corresponding coordinates or the surface of the sample are divided in the vertical and horizontal directions and associated with the partial regions of each intersection point. Based on the potential or capacitance measured in real time, the first retardation voltage or the second potential or both are corrected according to the movement of the sample.

[0014] Further, the surface of the sample is divided in the vertical and horizontal directions and associated with partial regions at each intersection, and a table in which the previously measured potential or capacitance is registered is provided. Based on the table, the first retardation voltage or the second potential or both are corrected in accordance with the movement of the sample.

[0015] Further, a groove is provided in a band shape around the region where the pattern is formed on the surface of the sample to electrically separate the internal pattern and the external pattern of the black border, and a claw electrode is provided to electrically connect the internal pattern and the external pattern so that the potential of the pattern can be controlled from the outside by the claw electrode.

[0016] Further, instead of the claw electrode, a conductive connecting portion is formed on the substrate.

[0017] Further, the sample is configured to form a pattern on an insulating substrate.

Advantages of the Invention

[0018] In the present invention, in a sample in which a pattern is formed on an insulating substrate such as a photomask, a desired retardation voltage can be stably applied to the surface, and a high-resolution and stable image can be obtained.

[0019] Further, the potential or capacitance of the surface of the sample is measured in real time, the retardation voltage or the potential of the surface of the sample is controlled, and a stable and high-resolution image can always be obtained even when the sample moves.

[0020] Further, by registering the potential or capacitance of the surface of the sample in a table in advance by dividing the sample vertically and horizontally, the retardation voltage and the potential of the surface of the sample are automatically corrected without following the movement of the sample, and a stable and high-resolution image can always be obtained.

Example 1

[0021] FIG. 1 shows a configuration diagram of one embodiment of the present invention.

[0022] In FIG. 1, the electron gun 1 is a known device that generates electrons, and generates electrons from, for example, TFE, LaB6, a field emitter, a photoelectron gun, or the like. After being adjusted through a focusing lens and an aperture (not shown) so as to have an acceleration voltage of about several hundred to several tens of kV and a current of, for example, about several pA to several hundred nA, it is narrowed down from the nm order to the order of several hundred nm by the objective lens 5 described later, and then irradiated onto the surface of the sample 9. A bias voltage (retarding voltage 8) is applied to the substrate (mask) of the sample 9 to optimize the irradiation energy or the energy of the electrons generated on the surface of the sample 9 (about several hundred V to 1 kV).

[0023] The blanking device 2 turns the electrons generated by the electron gun 1 on and off at high speed.

[0024] The blanking aperture 3 is a diaphragm that blocks the electrons deflected by the blanking device 2.

[0025] The objective aperture 4 limits the electrons incident on the objective lens 5.

[0026] The objective lens 5 is for narrowing the electron beam and irradiating the sample 9.

[0027] The deflection device 6 deflects the electron beam and two-dimensionally scans the narrowed electron beam onto the sample 9, and is a two-stage deflection system (electrostatic, electromagnetic).

[0028] The surface potential and capacitance detection device 7 is a disk-shaped plate with a hole in the center installed at the tip of the objective lens 5, and is an electrode for measuring the potential of the surface of the sample 9 or measuring the capacitance (capacitance) (measuring the potential or capacitance with a device not shown). The shape of the electrode does not have to be circular and can be other shapes. It does not necessarily have to be at the center, and a hole is not necessary either. The electrode for capacitance measurement can also use the metal itself that constitutes the objective lens.

[0029] The potential and capacitance detection signal 71 is a signal (surface potential signal or capacitance detection signal) detected by the surface potential and capacitance detection device 7, and based on this signal, a personal computer (not shown) is used to detect the potential or capacitance of the surface of the sample 9 in real time. In the case of the surface potential signal, it is a signal for measuring the potential of the surface of the sample 9 (or the retardation upper electrode 81), generating a minute potential corresponding to the potential of the surface of the sample 9 (retardation upper electrode 81), amplifying this, and measuring (detecting) the potential, which is a known method. In the case of the capacitance detection signal, it is a known method for supplying a minute high-frequency voltage, detecting the current flowing at that time, and calculating the capacitance. In addition, surface potential and capacitance measurement means known in the art can be used.

[0030] The retardation voltage 8 is a retardation voltage applied to the sample 9, and is a deceleration voltage (in this example, a deceleration voltage that is 500 V to 1 KV lower than 5 KV to 35 KV) for decelerating a predetermined high-voltage electron beam emitted from the electron gun 1, for example, from 5 KV to 35 KV to about 500 V to 1 KV.

[0031] The retardation upper electrode 81 is an electrode for extracting the potential of the upper surface of the sample 9 to the outside.

[0032] The retardation lower electrode 82 is an electrode for extracting the potential of the lower surface of the sample 9 to the outside. Usually, the retardation voltage 8 is applied to this retardation lower electrode 82.

[0033] The insulator 10 electrically insulates the retardation lower electrode 82.

[0034] Sample 9 is a sample such as a mask. Here, it is fixed on the insulator 10, and a retarding voltage 8 is applied. An electron beam decelerated only by the retarding voltage 8 will irradiate the sample 9. After being focused into a minute spot at a high voltage in the objective lens 5, it is decelerated by the retarding voltage 8 and irradiates the sample 9 with a low-energy electron beam to reduce damage, contamination, etc., and a high-resolution image can be generated.

[0035] The XYZ stage 11 is a stage that fixes the sample 9 and automatically moves it to arbitrary coordinates (X, Y), and further (Z). The movement can be precisely measured and moved in real time in the X, Y, and further in the Z direction as required by a laser interferometer (not shown). More specifically, there is an insulator on the stage, and further, there is a photomask holding mechanism made of a conductor called a mask pallet thereon, which can also serve as a lower electrode. On the other hand, a claw electrode is separately arranged on the pallet, and a mechanism is provided to apply a voltage from there to the upper part of the photomask.

[0036] The vacuum chamber 12 is a container that houses the XYZ stage 11 on which the sample 9 is fixed, etc., in a vacuum.

[0037] The vacuum pump 13 is an oil-free vacuum exhaust system that evacuates the vacuum chamber 12, such as a dry pump or a TMP.

[0038] The vibration isolation device 14 is for vibration isolation so that vibration is not transmitted from the outside to the vacuum chamber 12, etc.

[0039] The electron detection device 21 is a known detector that detects secondary electrons emitted when the sample 9 is irradiated with an electron beam and two-dimensionally scanned.

[0040] Next, the operation of the configuration in FIG. 1 will be briefly described. (1) Automatically transfer and fix a sample 9 such as a mask (a mask fixed to a holder) to the XYZ stage 11 by a robot. (2) The XYZ stage 11 is moved to the indicated position while measuring the position of the photomask in real time with a laser interferometer according to the design data of the pattern provided on the photomask, so that a predetermined position of the sample 9 is irradiated with an electron beam. At the same time, the surface potential / capacitance detection device 7 measures the potential (or capacitance) of the surface of the sample 9 (or the retardation upper electrode 81) in real time, and corrects it (corrects one or both of the retardation voltage 8 and the potential of the retardation upper electrode 81) so as to become a reference value (or the value of the reference point), and corrects it so that the potential of the surface of the sample 9 always remains constant even when the sample 9 is moved. As a result, even when the sample 9 is moved, the surface of the sample 9 always has the same potential, and while irradiating the sample 9 with a low-acceleration electron beam decelerated by the retardation voltage 8 and scanning it, it becomes possible to always obtain a stable and high-resolution secondary electron image or the like. This will be described in detail sequentially below.

[0041] FIG. 2 shows an example of the sample (mask) of the present invention. FIG. 2 shows a cross-sectional structure of a typical photomask. FIG. 2(a) shows a structural example of an EUV mask, and FIG. 2(b) shows a structural example of a DUV mask.

[0042] In FIG. 2(a), generally, as shown in the figure, an EUV mask is provided with a conductive reflective film 32 such as MoSi on an ultra-low thermal expansion quartz which is an insulating substrate 31, and a Ta-based absorption layer on which a pattern 33 is formed is provided thereon. Therefore, the region where the pattern is formed is characterized by having overall electrical conductivity.

[0043] In FIG. 2(b), in the case of the illustrated DUV mask, since there may be no conductive film under the layer of the pattern 34, the pattern 34 may often be in an island floating state where it is not electrically connected anywhere. Such a mask will change the surface potential due to charging when irradiated with an electron beam, and it will be impossible to obtain a stable and high-resolution secondary electron image.

[0044] FIG. 3 shows an example of the black border of the present invention. FIG. 3(a) shows a side view, and FIG. 3(b) shows a top view.

[0045] In FIG. 3, the photomask 35 is an example of the sample 9.

[0046] The bias electrode 36 is an electrode for applying a potential to the back surface of the photomask 35.

[0047] The black border 37 is provided to reduce the influence of flare generated by the EUV exposure apparatus, and as a result, has a structure in which the surrounded area is electrically isolated from the surroundings.

[0048] The claw electrode 38 is an electrode that is electrically connected to apply the bias voltage 1 to the surface of the photomask 35. The shape of the electrode is arbitrary, but it can also be a needle-shaped electrode in order to break through the insulator on the surface.

[0049] The bias voltage 1 is the voltage applied to the claw electrode 38 (the surface of the photomask 35).

[0050] The bias voltage 2 is the voltage applied to the back surface of the photomask 35.

[0051] Next, the configuration will be described. (1) In FIG. 3, as described above, the EUV mask of FIG. 2(a) is formed by providing about 50 layers of a reflective film material such as MoSi on a quartz substrate and then forming an absorption layer for forming the pattern 33. If left as it is, if a voltage is applied from the outside to the peripheral portion, the entire surface of the photomask can be set to the target potential. (2) However, since the EUV exposure apparatus uses a large number of mirrors with a short wavelength of 13.5 nm and the light source is not necessarily high-performance, a problem phenomenon called flare occurs, and light scatters in directions other than the target direction, resulting in a deterioration of the light source quality. Therefore, EUV light also reaches adjacent locations that should not be exposed originally, causing problems. For the purpose of preventing this, an electrically insulating boundary region called the black border 37 is provided as shown in the figure so as to surround the region where the main pattern of the photomask is written.

[0052] The boundary called the black border 37 is formed by engraving the MoSi conductor forming the reflective layer down to the quartz glass substrate. Therefore, the area surrounded by the black border 37 is electrically separated from the peripheral area and becomes in an insulator state.

[0053] When it becomes in an insulating state, even if a voltage is applied from the periphery of the mask surface, the area surrounded by the insulator will not reach the desired potential. Also, when observing by irradiating an electron beam such as SEM, charges gradually accumulate at the observation location and the image deteriorates. To avoid this, in the present invention, as shown in FIG. 3, a special claw electrode 38 for energizing the area surrounded by the black border 37 is provided. Thereby, conduction occurs between the peripheral part and the inside of the mask, charges can be released, and the same surface potential can be achieved. The claw electrode 38 used at this time is as thin and flat a structure as possible, and the surface is coated with an insulator having a high dielectric breakdown voltage such as Teflon (registered trademark) so that discharge does not occur between it and the objective lens 5 (the distance between them is about several millimeters (for example, 3 mm)). There may be provided not only one claw electrode 38 but also a plurality of them at the peripheral part of the photomask. Also, since there may be a plurality of insulating regions partitioned by the black border, the voltages applied to the plurality of provided claw electrodes 38 may be independently controllable. Furthermore, it is desirable to be able to apply independent voltages inside and outside the black border.

[0054] FIG. 4 shows an example of the black border of the present invention.

[0055] In FIG. 4, the black border 41 is formed by engraving the MoSi conductor forming the reflective layer down to the quartz glass substrate as described above.

[0056] The peripheral area 42 is the peripheral area electrically separated by the black border 41.

[0057] The measurement target area 43 is the measurement target area separated by the black border 41.

[0058] The connecting portion 44 is a conductive portion formed to electrically connect the measurement target region 43 and the peripheral region 42 electrically separated by the black border 41.

[0059] Next, FIG. 4 will be described in detail. (1) In FIG. 4, the black border itself has a very large size compared to the actually formed pattern size. Therefore, a pattern (connecting portion 44) is formed in which a part of the border is left so that the performance of the black border 41 is not limited and the peripheral and inner regions are connected somewhere. In this way, since electrical conduction occurs between the mask peripheral part and the inside partitioned by the black border 41, the potentials of the two regions can be made the same. Also, it becomes possible to release the charges generated by electron beam irradiation, and the image is not disturbed by the charge-up phenomenon. Applying a voltage by the claw electrode is sufficient if it is performed on the region of the outer periphery of the mask. (2) On the other hand, there may be a case where it is not always possible to form the above pattern (connecting portion 44). In that case, wiring can also be formed with a conductive material such as Ta or W having the property of absorbing EUV light using an FIB or an electron beam mask correction apparatus used for mask correction on the black border 41 to establish conduction. A plurality of conduction points can be taken. Since wiring can be formed three-dimensionally with a mask correction apparatus, it is easy to wire across the black border 41. As a result, the regions partitioned by the black border 41 are connected to the outer periphery, and conduction can be taken by contacting an electrode from the outside to the outer periphery. The entire mask surface becomes the desired potential, and charging of the EUV mask can be prevented.

[0060] FIG. 5 shows a schematic explanatory diagram of the surface potential of the present invention.

[0061] In FIG. 5, C1 represents the capacitance between the lower surface of the objective lens 5 and the surface of the mask (retarding upper electrode 81).

[0062] C2 represents the capacitance between the surface of the mask (retarding upper electrode 81) and the bias electrode (retarding lower electrode 82).

[0063] E represents the retarding voltage 8.

[0064] Next, the operation will be described. (1) Fig. 5 schematically shows the principle of determining the potential generated in the internal region partitioned by a black border and insulated within the electron beam inspection apparatus, or on the surface of a DUV photomask in a floating state. Since the photomask does not necessarily have a conductive reflective layer like an EUV mask, the entire surface is not always conductive. In that case, when an external voltage is applied from the substrate lower electrode to the floating region, the surface potential is determined capacitively. Inside the electron beam inspection apparatus chamber, there is an objective lens 5 made of metal facing the photomask surface (the surface of sample 9). Since the photomask surface and the objective lens 5 are in close proximity, it is equivalent to the presence of two metal electrodes and a large capacitance C1 is formed. On the other hand, since a dielectric such as quartz exists between the photomask surface and the back surface of the photomask, a large capacitance C2 is formed between the pattern on the photomask surface and the photomask lower electrode 82.

[0065] Therefore, the potential of the photomask surface in the middle is approximately determined by the ratio of the magnitudes of the two capacitances. (2) When the photomask is moved by the XYZ stage 11 during measurement, the distance from the objective lens 5 and the distance from peripheral members change, so the capacitance C1 of the aforementioned measurement system changes. As this changes, the bias voltage (retarding voltage 8) applied to the substrate is distributed. Therefore, when the same voltage is applied, the surface potential changes as the XYZ stage 11 moves. That is, if measurement is performed while applying a constant retarding voltage, the bias potential generated on the substrate surface varies depending on the location, resulting in completely different measurements. In the case of a measurement device sensitive to the surface potential, at worst, an image may not be formed, or the position where the image is formed may fluctuate, making the measurement unstable. (3) Therefore, by correcting the potential of the bias electrode (retarding voltage 8) or the potential of the mask surface (first potential) so that the potential on the mask surface (the potential distributed by capacitance C1 and capacitance C2) always remains constant even when the XYZ stage 11 moves, and automatically controlling it so that the potential on the mask surface becomes constant, the potential on the mask surface will remain constant even when the XYZ stage is moved, and it becomes possible to obtain a stable and high-resolution secondary electron image.

[0066] FIG. 6 shows an example of a map table of capacitance and potential of the mask of the present invention.

[0067] FIG. 6(a) shows an example of a capacitance map table, and FIG. 6(b) shows an example of a surface potential map table.

[0068] FIG. 6(a) is obtained by dividing the XYZ stage 11 (or the mask) into i parts in the X direction and j parts in the Y direction, and measuring and registering the capacitance C1(i,j) at the position (i,j) of the intersection.

[0069] FIG. 6(b) is obtained by dividing the XYZ stage 11 (or the mask) into i parts in the X direction and j parts in the Y direction, and measuring and registering the potential V(i,j) at the position (i,j) of the intersection.

[0070] Here, the capacitance C1 and the potential V are measured as follows. (1) Sample surface potential V: There are various methods for knowing the sample surface potential V. The surface potential can be known using various commercially available surface potentiometers based on different principles and forms. The surface potentiometer can be arranged at the tip of the objective lens 5 to measure the potential at the measurement location of the mask. Automatically adjust the voltage applied to the substrate (the retarding voltage 8 in FIG. 1 or the potential of the retarding upper electrode 81) during movement or stop of the XYZ stage 11 so that the measured potential becomes a constant value. (2) Similar results can be obtained by measuring the capacitance generated between the objective lens and the sample surface. The capacitance is measured at a specific coordinate on the XY coordinates. The sample 9 is moved by the XYZ stage 11 to measure the capacitance. Since the capacitance difference represents the change in surface potential, the voltage applied to the sample 9 by the change in capacitance (the retarding voltage 8 in Fig. 1 or the potential of the retarding upper electrode 81) is changed and adjusted so that the surface potential substantially reaches the desired value. (3) The surface potential V can also be known by examining the energy of the secondary electrons actually generated by irradiating the electron beam. Specifically, a mesh grid or the like is provided on the surface of the secondary electron detector, a predetermined voltage is applied, and the energy of the secondary electrons can be examined by examining the amount of secondary electrons detected passing through the mesh. By changing the grid voltage, the energy distribution can be understood, and as a result, the change in the sample surface potential V that forms the element of the energy shift of the secondary electrons can be detected. The surface potential can be maintained at the desired value by automatically adjusting the bias voltage applied to the substrate (the retarding voltage 8 in Fig. 1 or the potential of the retarding upper electrode 81) so that the change in the measured value disappears. (4) The surface potential change or capacitance change accompanying the movement of the XYZ stage 11 obtained as described above is a quantity specific to a particular device and is constant unless the inside of the device is changed.

[0071] Therefore, once the data of the surface potential change or capacitance change accompanying the movement of the XYZ stage is acquired and registered, the correction amount can be estimated by inputting the position coordinates thereafter. Thus, the data of the surface potential or capacitance change corresponding to such measurement point coordinates XY is stored in the storage device as a map table as shown in (b) and (a) of Fig. 6, and at the actual measurement, the voltage applied to the mask (sample 9) is automatically adjusted based on the map table, so that the surface potential can be kept constant at the desired value.

[0072] Fig. 7 shows the mask surface potential setting flowchart of the present invention.

[0073] In FIG. 7, S1 moves the XY stage. Here, the coordinates of the moving position are set as (Xi, Yj).

[0074] S2 measures the surface potential or capacitance. This is to measure the surface potential Vij of the mask facing the objective lens 5 or the capacitance Cij between the surface of the mask facing the objective lens 5 with the XY stage moved to the coordinates (i, j).

[0075] S3 calculates the correction value. This calculates the correction value based on the surface potential Vij or capacitance Cij measured by S2 so as to match any of the reference value, the potential (capacitance) at a specific location, and an arbitrary specified value.

[0076] S4 sets the applied voltage. This sets the potential of the retardation lower electrode 82 or the retardation upper electrode 81 in FIG. 1 to a predetermined value and corrects it so that the surface potential or capacitance always remains constant.

[0077] S5 determines whether the measurement is completed. If YES, the process ends. If NO, the steps after S1 are repeated.

[0078] By the above method, each time the XYZ stage 11 is moved, the surface potential or capacitance is measured and corrected (correcting the potential of the retardation lower electrode 82 or the retardation upper electrode 81 in FIG. 1) so that the surface potential of the mask (sample 9) can be kept constant, and a stable and high-resolution secondary electron image can be obtained. When correcting in real time, the surface potential and capacitance at an arbitrary measurement point can be measured and the retardation voltage value to be applied can be corrected.

[0079] FIG. 8 shows the mask surface potential setting flowchart (part 2) of the present invention. FIG. 8 reads and corrects the surface potential and capacitance at the mask coordinates from the surface potential map table in FIG. 6(b) or the capacitance map table in FIG. 6(a) measured and registered in advance, instead of measuring the surface potential and capacitance in real time as in FIG. 7.

[0080] In FIG. 8, S11 moves the XY stage. Here, the movement position is the coordinates (Xi, Yj).

[0081] S12 reads out the correction value. As described above, this reads out the surface potential or capacitance at the coordinates of the mask from the surface potential map table in FIG. 6(b) or the capacitance map table in FIG. 6(a) registered in advance. When there are no coordinates matching the table, the values (potential or capacitance) of a plurality of coordinates in the vicinity are read out.

[0082] S13 calculates the correction value. Based on the surface potential Vij or capacitance Cij read out by S12, this calculates the correction value so as to match any of a reference value, the potential (capacitance) at a specific location, and an arbitrary specified value.

[0083] S14 sets the applied voltage. This sets the potential of the retardation lower electrode 82 or the retardation upper electrode 81 in FIG. 1 to a predetermined value and corrects it so that the surface potential or capacitance always remains constant.

[0084] S15 determines whether the measurement is complete. If YES, the process ends. If NO, the steps after S11 are repeated.

[0085] By the above method, every time the XYZ stage 11 moves, the surface potential or capacitance is read out from the pre-registered table, and correction is performed so that it always remains constant (correcting the potential of the retardation lower electrode 82 or the retardation upper electrode 81 in FIG. 1), thereby enabling the surface potential of the mask (sample 9) to be kept constant and making it possible to obtain a stable and high-resolution secondary electron image. Note that since the Z-axis height affects the capacitance value, it is desirable to perform automatic control so that the height is a desired constant height (the distance between the sample surface and the objective lens).

[0086] FIG. 9 shows a configuration diagram of another embodiment of the present invention. FIG. 9 shows an external view of a multi-electron beam inspection apparatus. The multi-electron beam inspection apparatus is a device capable of realizing ultra-high-speed two-dimensional image acquisition by irradiating a sample 9 with a plurality of electron beams in a planar manner at once, re-accelerating secondary electrons generated in the sample 9, and forming an image. In this apparatus, it is very important to uniformly accelerate the secondary electrons generated on the surface of the sample 9. If the surface potential varies, the operation of the beam splitter 541 will be hindered and the image will not be formed at the desired position. Therefore, the control of the surface potential is very important.

[0087] Generally, since inspection apparatuses measure at high speed and in large quantities, the XYZ stage 11 may move continuously as in the case of normal SEMs and review apparatuses, or may move step by step. Even in such cases, the present invention can be utilized. Specifically, it is as follows. (1) In the case of step-by-step movement, a correction value may be held as a map (see FIG. 6) for each step. On the other hand, when the stage 11 moves continuously, the following method is adopted. For example, as shown in FIG. 6, the area of the photomask is divided into about 100 equal parts, and the surface potential or capacitance of the sample 9 at the center coordinates of each area is measured. This is stored as a map (see FIG. 6). Since the surface potential or capacitance value has the property of changing smoothly as the photomask moves, the correction value at coordinates that cannot be directly measured can be estimated by interpolation. (2) For example, using the measurement point coordinates output from a laser interferometer and the data of the map, interpolation calculations such as the least squares method, spline, Lagrange interpolation, etc., or two-dimensional weighting operations such as a Gaussian filter are performed on the intermediate coordinates not in the table of FIG. 6 to estimate the surface potential or capacitance at the measurement point coordinates. The correction value at any coordinate can be estimated from this value. When the stage is moved continuously, the XYZ coordinates of the measurement points are taken in and correction is performed in real time. (3) On the one hand, the overall substantially approved retardation potential of the measurement target area is determined by the aforementioned principle. However, when considering local capacitance changes, the area of the pattern contributing to capacitance formation directly below the objective lens 55 is calculated using the pattern design data (CAD data such as GDI) written on the photomask, and the surface potential can be estimated by calculating the capacitance. By calculating the correction amount from the capacitance estimated in this way and performing control (controlling the potential of the upper retardation electrode 81 or the lower retardation electrode 82), the potential of the measurement point on the surface of the sample 9 can be set to a desired value.

Brief Description of the Drawings

[0088]

Figure 1

Figure 2

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Explanation of Reference Numerals

[0089] 1: Electron gun 2: Blanking device 3: Blanking aperture 4: Objective aperture 5: Objective lens 6: Deflection device 7: Surface potential and capacitance detection device 71: Potential and capacitance detection signal 8: Retarding voltage 81: Retarding upper electrode 82: Retarding lower electrode 83: First potential 9: Sample 10: Insulator 11: XYZ stage 12: Vacuum chamber 13: Vacuum pump 14: Vibration isolator 21: Electron projection transfer 31: Insulating substrate 32: Conductive reflective layer 33: Absorbing layer (pattern) 34: Pattern 35: Photomask 36: Bias electrode 37: Black border 38: Claw electrode 41: Black border 42: Peripheral region 43: Measurement target region 44: Connection part

Claims

1. 1. A method for preventing charge-up of an EUV mask in an apparatus for acquiring an image by detecting electrons generated by irradiating an EUV mask with an electron beam, comprising: an electron gun that generates an electron beam; an objective lens for narrowing the electron beam generated by the electron gun and irradiating it onto an EUV mask; a deflection system that two-dimensionally scans the electron beam narrowed by the objective lens onto the surface of the EUV mask; a stage for mounting the EUV mask and moving it to a predetermined position; a detection means for irradiating a narrow electron beam onto a surface of the EUV mask and detecting emitted secondary electrons or reflected electrons to generate an image; a groove is provided in a strip shape around an area where a pattern is formed on the surface of the EUV mask to electrically isolate the area; an internal pattern of a black border that reduces the effect of flare from an EUV light source is electrically connected to the peripheral area; a claw electrode is provided for externally controlling the electric potential of the internal pattern, or a conductive connection part is provided on a substrate of the EUV mask for externally controlling the electric potential of the internal pattern; The claw electrode or the conductive connecting portion prevents charge-up caused by the black border in the patterned area on the surface of the EUV mask. A method for preventing charge-up, comprising:

2. 2. The method for preventing charge-up according to claim 1, wherein the claw electrode or the conductive connecting portion connects the inner region to the periphery while leaving the black border to an extent that the performance of the black border is not restricted.

3. 3. The method for preventing charge-up according to claim 1, wherein the conductive connecting portion is made of a conductive material selected from the group consisting of Ta and W, which have a property of absorbing EUV light.

4. 4. The method for preventing charge-up according to claim 1, wherein the claw electrode or the conductive connecting portion connects the inner region and the periphery across the black border.

5. An apparatus for preventing charge-up of an EUV mask in an apparatus for acquiring an image by detecting electrons generated by irradiating an EUV mask with an electron beam, comprising: an electron gun that generates an electron beam; an objective lens for narrowing the electron beam generated by the electron gun and irradiating it onto an EUV mask; a deflection system that two-dimensionally scans the electron beam narrowed by the objective lens onto the surface of the EUV mask; a stage for mounting the EUV mask and moving it to a predetermined position; a detection means for irradiating a narrow electron beam onto a surface of the EUV mask and detecting emitted secondary electrons or reflected electrons to generate an image; a groove is provided in a strip shape around an area where a pattern is formed on the surface of the EUV mask to electrically separate the area; a black border is provided to electrically connect the pattern inside the black border and the peripheral area, the black border reducing the effect of flare from the EUV light source; a claw electrode is provided to externally control the electric potential of the internal pattern, or a conductive connection part is provided on a substrate of the EUV mask to externally control the electric potential of the internal pattern; The claw electrode or the conductive connecting portion prevents charge-up caused by the black border in the patterned area on the surface of the EUV mask. A charge-up prevention device characterized by the above.

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