JP2004247321A - Scanning electron microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning electron microscope that produces a scanning image of high spacial resolution in low acceleration voltage region. <P>SOLUTION: An acceleration cylinder 9 is placed in the electron beam path of an objective lens 8. The post stage acceleration voltage 10 for a primary electron beam is then applied. A deceleration electric field for the primary electron beam is formed between the acceleration cylinder 9 and the specimen 12 by applying a superimposed voltage 13 to a specimen 12. The secondary signal 23 of secondary electrons or reflected electrons or the like generated from the specimen 12 are attracted into the acceleration cylinder 9 by the electric field (the deceleration electric field ) in front of the specimen and detected by a secondary electron detector placed upper part than the acceleration cylinder 9. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a scanning electron microscope that obtains a two-dimensional scanning image representing the shape or composition of a sample surface by scanning an electron beam on the surface of a sample to be inspected, and particularly to a scanning electron microscope having a high resolution in a low acceleration voltage region. It relates to a scanning electron microscope suitable for obtaining.

  A scanning electron microscope accelerates electrons emitted from a heating type or field emission type electron source, converts the electrons into a thin electron beam (primary electron beam) with an electrostatic or magnetic lens, and uses the primary electron beam with a scanning deflector. Scans the sample to be observed, and detects secondary signals such as secondary electrons or reflected electrons secondary from the sample by primary electron beam irradiation, and scans the detected signal intensity in synchronization with the primary electron beam scanning A two-dimensional scanned image is obtained by using the brightness modulation input of the CRT. In a general scanning electron microscope, electrons emitted from an electron source are accelerated between an electron source to which a negative potential is applied and an anode at a ground potential, and an electron beam is scanned on a test sample at a ground potential.

  Scanning electron microscopes have been used for the process of semiconductor device fabrication or for inspection after completion (for example, dimension measurement by an electron beam or inspection of electrical operation). A high resolution of 10 nm or less at a low accelerating voltage has been required.

  A factor that hinders high resolution in the low acceleration voltage region is blurring of the electron beam due to chromatic aberration due to variation in energy of the electron beam emitted from the electron source. In a scanning electron microscope with a low accelerating voltage, a field emission type electron source having a small variation in energy of an emitted electron beam is mainly used in order to reduce the blur caused by the chromatic aberration. However, even with a field-emission electron source, the spatial resolution at 500 volts is limited to 10 to 15 nm, and cannot meet user requirements.

As a solution to this problem, the acceleration of the primary electron beam between the electron source and the anode at the ground potential is set to a voltage value higher than the final acceleration voltage, and the inspection with the objective lens at the ground potential and the negative potential applied. There is a method of setting the final low acceleration voltage by decelerating the primary electrons between samples (see: I Triple E, 9th Annual Symposium on Electron Ion and Laser Technology Proceeding, pp. 176-186, IEEE 9th
Annual Symposium on Electron, Ion and Laser Technology).

  The effect of this method has already been confirmed in experiments, but since a high voltage is applied to the sample, it is difficult for secondary electrons to be drawn into the mirror body by the deceleration electric field, and it is difficult to detect. Since a sample stage is required, there are few examples adopted in commercially available devices.

  The present invention provides a breakthrough for the aforementioned problems. In the present invention, there is provided means for detecting a secondary signal such as a secondary electron or a reflected electron attracted into an opening of the objective lens by an electric field applied between the objective lens and the sample after passing through the objective lens. In order to solve the problem of secondary signal detection, and to provide a post-acceleration means in the objective lens passage, the negative potential applied to the sample is reduced to a value that can be used practically. is there.

  That is, the present invention provides an electron source, a scanning deflector for scanning a primary electron beam generated from the electron source on a sample, an objective lens for converging the primary electron beam, and a secondary electron beam generated from the sample by irradiation of the primary electron beam. In a scanning electron microscope that includes a secondary signal detector that detects a secondary signal and obtains a two-dimensional scanning image of a sample, an accelerating cylinder arranged in an electron beam path of an objective lens, and a post-stage of a primary electron beam on the accelerating cylinder A means for applying an accelerating voltage and a means for forming a deceleration electric field for the primary electron beam between the accelerating cylinder and the sample are provided, and the secondary signal detector is arranged at a position closer to the electron source than the accelerating cylinder. I do.

  According to the present invention, it is possible to solve the problem that the detection of secondary electrons or reflected electrons is difficult, and the problem of handling caused by the application of a high voltage to a sample. Thus, a scanning electron microscope with reduced chromatic aberration can be realized.

  The secondary signal detector includes a conductive reflecting plate having an opening through which the primary electron beam passes, a suction unit for sucking the secondary electrons generated by the reflecting plate, and a detecting unit for detecting the sucked secondary electrons. Can be included. The attraction means can be formed by an electric field and a magnetic field orthogonal thereto, so that the deflection of the primary electron beam by the electric field can be canceled by the magnetic field. The secondary signal detection method of this system is applicable even when no accelerating cylinder is provided or when the accelerating cylinder is set to 0 V (ground potential).

  The secondary signal detector may be constituted by a multi-channel plate having an opening for passing the primary electron beam, or a photodetector for detecting light emission of the phosphor and the phosphor having the opening for passing the primary electron beam. May be configured.

  The location of the secondary signal detector may be between the accelerating cylinder and the scanning deflector, or between the scanning deflector and the electron source, or both. When two secondary signal detectors are provided, a scanning image can be formed using either one of the detection signals, or a scanning image is formed by calculating the detection signals of the two detectors. You can also. Which method is used to form a scan image may be automatically selected according to the scan image magnification or a given observation condition. The method using these two secondary signal detectors is also applicable when no accelerating cylinder is provided or when the accelerating cylinder is set to 0 V (ground potential).

  The scanning deflector for the primary electron beam, by combining electrostatic deflection and magnetic field deflection, gives the desired deflection for the primary electron beam but gives the deflection for the secondary signal sucked from the sample side. Can not be. This deflection method combining electrostatic deflection and magnetic field deflection can be applied even when no accelerating cylinder is provided.

  While maintaining a constant ratio between the post-acceleration voltage applied to the accelerating cylinder and the electron gun voltage applied to the electron source, and the ratio between the voltage applied to the sample and the electron gun voltage applied to the electron source, the post-acceleration voltage and the sample applied voltage are kept constant. When controlled, the crossover point of the secondary signal generated from the sample can be maintained at a fixed position.

  Eliminating distortion of the scanned image due to the action of the electrostatic lens formed by the deceleration electric field by matching the lens center created by the magnetic field of the objective lens with the lens center of the electrostatic lens formed between the acceleration cylinder and the sample Can be.

  When the upper magnetic pole of the objective lens is electrically insulated from the rest of the objective lens, and an acceleration cylinder is used as the upper magnetic pole by applying a post-acceleration voltage thereto, the center of the magnetic lens and the electrostatic lens can be matched. It will be easier.

  According to the present invention, the acceleration voltage of the primary electron beam when passing through the inside of the objective lens can be made higher than the final acceleration voltage, and the blur of the beam due to the aberration generated in the objective lens can be reduced. Further, by providing a reflector in the electron beam path, it is possible to efficiently detect a secondary signal such as a secondary electron or a reflected electron, which has been difficult so far.

  FIG. 1 is a schematic view of an embodiment of a scanning electron microscope according to the present invention. When an extraction voltage 3 is applied between the field emission cathode 1 and the extraction electrode 2, emitted electrons 4 are emitted. The emitted electrons 4 are further accelerated (or decelerated) between the extraction electrode 2 and the anode 5 at the ground potential. The energy (acceleration voltage) of the electron beam passing through the anode 5 coincides with the electron gun acceleration voltage 6. In the present invention, the primary electron beam 7 that has passed through the anode 5 is further accelerated by an acceleration cylinder 9 provided through the objective lens 8. The energy of the electron beam when passing through the objective lens 8 is the sum of the electron gun acceleration voltage 6 and the post-stage acceleration voltage 10 applied to the acceleration cylinder 9. The post-accelerated primary electron beam 11 is decelerated by the negative superimposed voltage 13 applied to the sample 12 to a desired acceleration voltage. The actual acceleration voltage in this method is the difference between the electron gun acceleration voltage 6 and the superimposed voltage 13 irrespective of the post-stage acceleration voltage 10.

  The primary electron beam 7 that has passed through the anode 5 undergoes scanning deflection by the condenser lens 14, the upper scanning deflector 15, and the lower scanning deflector 16, and then is further accelerated by an acceleration cylinder 9 provided in the path of the objective lens 8. Get 10 acceleration. The primary electron beam 11 accelerated in the latter stage is narrowed down on the sample 12 by the objective lens 8. The primary electron beam 11 that has passed through the objective lens 8 is decelerated by the deceleration electric field 17 created between the objective lens 8 and the sample 12, and reaches the sample 12.

  According to this configuration, the acceleration voltage of the primary electrons when passing through the objective lens 8 is higher than the final acceleration voltage. As a result, compared with the case where the primary electron beam of the final acceleration voltage is passed through the objective lens 8, the chromatic aberration at the objective lens is reduced, and a finer electron beam (high resolution) can be obtained. The divergence angle of the primary electron beam of the objective lens 8 is determined by a stop 18 placed below the condenser lens 14. Centering of the aperture 18 is performed by an adjustment knob 19. Although the mechanical adjustment is performed in the figure, an electrostatic or magnetic field deflector may be provided before and after the stop 18 to deflect the electron beam to make adjustment.

The electron beam narrowed down by the objective lens 8 is scanned on the sample 12 by the upper scanning deflector 15 and the lower scanning deflector 16. At this time, the deflection direction and intensity of the upper scanning deflector 15 and the lower scanning deflector 16 are set. Is adjusted so that the scanned electron beam always passes through the center of the objective lens 8. The sample 12 is fixed on a sample holder 20 to which the superimposed voltage 13 is applied. The sample holder 20 is placed on a sample stage 22 via an insulating table 21 so that the horizontal position can be adjusted.

  When the primary electron beam 11 irradiates the sample 12, secondary electrons 23 are generated. The decelerating electric field 17 created between the objective lens 8 and the sample 12 acts as an accelerating electric field for the secondary electrons 23, so that it is attracted into the passage of the objective lens 8 and undergoes lens action by the magnetic field of the objective lens 8. Going up. The secondary electrons 23 that have passed through the objective lens 8 are attracted by the transverse electric field of the suction electrode 24 placed between the objective lens 8 and the lower scanning deflector 16, and after passing through the mesh of the suction electrode 24, 10 kV ( (Positive potential) is accelerated by the scintillator 25 to which the scintillator 25 is applied, causing the scintillator 25 to emit light. The emitted light is guided to a photomultiplier tube 27 by a light guide 26 and is converted into an electric signal. The output of the photomultiplier tube 27 is further amplified and becomes the luminance modulation input of a cathode ray tube (not shown).

  The feature of this configuration is that the accelerating voltage of the electron beam when passing through the condenser lens 14, the diaphragm 18, and the objective lens 8 is higher than the final energy, and especially when passing through the objective lens 8 which controls chromatic aberration. Further, the latter stage acceleration is added. In a typical example, the electron gun acceleration voltage is 1000 volts, the post-stage acceleration voltage is 1000 volts, the negative superimposed voltage on the sample 12 is 500 volts, and the actual acceleration voltage is 500 volts. When passing through the objective lens 8, the chromatic aberration is reduced to about 50% because the voltage is 2000 volts. When the acceleration voltage is set to 500 volts, the beam diameter (resolution) which was 15 nm is improved to 7 nm. .

  In the above-described embodiment, the secondary electrons 23 are taken out of the electron path by the suction electrode 24 and detected. In this method, as the superimposed voltage 13 increases, the energy of the secondary electrons 23 increases. Therefore, it is necessary to increase the voltage applied to the suction electrode 24 accordingly. As a result, there arises a problem that the primary electron beam (the primary electron beam 7 passing through the anode 5) is also deflected.

The embodiment using the reflector shown in FIG. 2 solves the above-mentioned problem and enables highly efficient detection. In this embodiment, a reflection plate 29 having a central hole 28 is provided in the electron beam path. The surface of the reflection plate is coated with a material such as gold, silver, and platinum that easily generates secondary electrons by electron irradiation. The primary electron beam 7 that has passed through the anode 5 passes through the central hole 28 of the reflection plate 29 and then enters the acceleration cylinder 9. The diameter of the central hole 28 is set so that the electron beam deflected by the scanning deflectors 15 and 16 does not collide with the reflecting plate 29. The secondary electrons 23 generated in the sample 12 and accelerated by the superimposed voltage 13 pass through the accelerating cylinder 9 while diverging due to the lens action of the objective lens 8 and collide with the back surface of the reflection plate 29. Although the trajectory is different from that of the secondary electrons, the reflected electrons generated in the sample 12 also collide with the back surface of the reflector 29.

  Secondary electrons 30 formed on the back surface of the reflecting plate 29 are attracted by the electric field of the attracting electrode 24, and are converted into electric signals through the scintillator 25, the light guide 26, and the photoelectron amplifier 27 as in FIG. The feature of this method is that even if the superimposed voltage 13 applied to the sample is high and the acceleration of the secondary electrons 23 is high, what is detected is the secondary electrons 30 made by the reflector 29 that has not been accelerated. Therefore, the voltage applied to the suction electrode 24 may be low. Therefore, the influence of the electric field generated by the suction electrode 24 on the primary electron beam 7 passing through the anode 5 can be reduced. Here, the scintillator 25 is used to detect the attracted secondary electrons, but an electron detection amplifier such as a channel plate or a multi-channel plate may be used.

  FIG. 3 shows an example in which a magnetic field B is applied orthogonal to an electric field E that attracts the secondary electrons 30 formed by the reflector 29. With this structure, it is possible to correct the deflection of the primary electron beam due to the attraction electric field E described above. That is, the deflection of the primary electron beam 7 that has passed through the anode 5 is corrected by the deflection by the magnetic field B. Here, 31 'and 31 "are electric field deflection electrodes for generating an attraction electric field, and 31" is meshed so that the secondary electrons 30 can pass therethrough. Reference numerals 32 'and 32 "denote orthogonal magnetic field deflection coils (the coils 32' and 32" for generating the magnetic field B are shown symbolically in the figure). The magnetic field B generated by the orthogonal magnetic field deflection coils 32 'and 32 "is orthogonal to the electric field E, and the intensity of the magnetic field B is adjusted so as to cancel the deflection of the accelerated electron beam 7 due to the electric field E. In the example, the orthogonal magnetic field deflecting coils 32 are one set. However, if the orthogonal magnetic field deflecting coils are two sets arranged at an angle, the degree of orthogonality with the electric field can be adjusted by adjusting the current and the like flowing through each set of coils. Even if the direction of the electric field is adjusted by using two sets of electric field deflection electrodes instead of two sets of the orthogonal magnetic field deflection coils 32, the degree of orthogonality between the electric field and the magnetic field can be strictly adjusted. It goes without saying that it is possible.

  The secondary signal detection method using the reflector 29 shown in FIGS. 2 and 3 works effectively even when the acceleration cylinder 9 is not provided or when the acceleration cylinder 9 is grounded.

FIG. 4 shows an embodiment in which the secondary signal detector is provided above the upper scanning deflector 15. In the figure, a secondary signal detector is provided between the upper scanning deflector 15 and the diaphragm 18. As in FIG. 2, a central hole 28 is provided in the reflecting plate 29. However, since the primary electron beam has not been scanned and deflected yet, the size of the central hole 28 limits the minimum primary electron beam aperture angle. The diameter may be the same as the diameter of the stop 18 to be adjusted. In the embodiment shown in the figure, a reflector 29 having a central hole 28 having a diameter of 0.1 mm is provided below the diaphragm 18. The diaphragm 18 and the reflection plate 29 can be shared.

  When the reflecting plate 29 is provided below the scanning deflector, the diameter of the central hole 28 is set to a size that does not cause the deflected electron beam to collide. Comparing the size of the central hole 28 with a typical example, a size of 3 to 4 mm is required when installed below, but may be 0.1 mm or less when installed above. As described above, when the reflector is provided above the scanning deflector, the central hole of the reflector can be made sufficiently small, so that the efficiency of capturing the secondary electrons by the reflector is improved.

  In the embodiment of FIG. 4, the sample 12 is placed in the pole gap of the objective lens 8. This arrangement reduces the chromatic aberration coefficient of the objective lens 8 and has a shape pursuing higher resolution. The sample stage 22 is also provided in the objective lens 8.

FIG. 5 shows an embodiment in which secondary signal detectors are provided at both positions above and below the scanning deflector. An upper detector 33 is provided on the upper scanning deflector 15, and a lower detector 34 is provided between the lower scanning deflector 16 and the acceleration cylinder 9. As shown in FIG. 3 and FIG. 4, the upper detector 33 and the lower detector 34 include reflectors 29a and 29b, electric field deflection electrodes 31a and 31b, orthogonal magnetic field deflection coils 32a and 32b, scintillators 25a and 25b, and light guides, respectively. 26a, 26b and photomultiplier tubes 27a, 27b.

In this embodiment, the upper detector 33 can detect secondary electrons or reflected electrons that have passed through the central hole 28b of the reflector 29b of the lower detector 34. Since the secondary signal detected by the upper detector 33 contains many secondary electrons and reflected electrons emitted from the sample 12 in the vertical direction, an image having a contrast different from that of the lower detector 34 is obtained. For example, in the inspection of a contact hole in a manufacturing process of a semiconductor element, an image in which a portion of the contact hole is emphasized from the periphery is obtained by using the lower detector 34, and a fine image of the bottom of the contact hole is obtained by using the upper detector 33. Is obtained. It is also possible to create a contrast that emphasizes the characteristics of the sample by calculating the signals of the detectors 33 and 34.

  Which of the upper and lower detectors produces the scan image can be selected by the operator, but may be automatically selected under predetermined conditions. For example, when the observation magnification is 2000 times or less, the lower detector 34 is selected, and when the observation magnification is higher, the upper detector 33 is selected. Alternatively, the selection may be made according to the sample to be observed. In this case, a procedure such as inputting the type of the sample to be observed to the apparatus is performed. For example, when the observation of the contact hole of the semiconductor element is input, the upper detector 33 that emphasizes the inside of the hole is automatically selected, and when the resist on the surface is observed, the lower detector 34 is selected.

  In the embodiment shown in FIG. 4 or FIG. 5, even if the accelerating cylinder 9 is removed or the accelerating cylinder 9 is grounded, the effect is large and sufficiently practical.

FIG. 6 shows an embodiment in which a secondary signal is detected using a multi-channel plate detector. The multi-channel plate 35 has a disk shape and has a central hole 28 through which a primary electron beam passes. A mesh 37 is provided below the multi-channel plate 35 and is grounded. In such a configuration, the primary electron beam 7 that has passed through the anode 5 passes through the central hole 28 of the microchannel plate, and is then converged by the objective lens and irradiated onto the sample. The secondary electrons 23 generated in the sample pass through the mesh 37 and enter the channel plate 35. The secondary electrons 23 incident on the channel plate 35 are accelerated and amplified by an amplification voltage 38 applied to both ends of the channel plate 35. The amplified electrons 39 are further accelerated by the anode voltage 40 and captured by the anode 41. After the captured secondary electron signal is amplified by the amplifier 42, it is converted into an optical signal 44 by the optical conversion circuit 43. The conversion into the optical signal 44 is because the amplifier 42 is floating with the amplified voltage 38 of the channel plate main body 35. The light signal 44 is converted again into an electric signal by the electric conversion circuit 45 of the ground potential, and is used as a luminance modulation signal of the scanned image.

  Here, it is also possible to obtain information on the emission direction of the secondary electrons 23 by dividing the anode 41 into two or four parts. In this case, it is needless to say that the amplifiers 42, the optical conversion circuits 43, and the electric conversion circuits 45 are required in a number corresponding to the division, and signal processing for calculating the divided signals is performed.

FIG. 7 shows an embodiment in which a secondary signal is detected using a single crystal scintillator. In FIG. 7, a single crystal scintillator 46 is, for example, a cylinder-shaped YAG single crystal that is cut obliquely, and an opening 47 for passing a primary electron beam is provided on the cut surface. Alternatively, a conductive thin film 48 such as carbon is coated, and the conductive thin film 48 is grounded. Light emitted by irradiating the scintillator 46 with the secondary electrons 23 generated from the sample 12 is reflected at an oblique portion, guided to the photomultiplier tube 27 by a light guide formed by a cylindrical portion, and detected and amplified. . In the present embodiment, both the light emitting portion of the scintillator 46 and the light guide are described as being formed of a single crystal of YAG. However, only the light emitting portion for detecting secondary electrons is formed of a single crystal of YAG or a fluorescent material, and the light guide is formed. You may make it comprise a transparent body, such as glass and resin.

  A control method for efficiently detecting the secondary signal will be described with reference to FIG. Since the secondary signal (for example, secondary electrons) 23 passes through the magnetic field of the objective lens 8, it undergoes a lens action, and a crossover 49 of secondary electrons is created. If the secondary electrons are focused on the opening 47 of the scintillator 46 by the lens action, most of the secondary electrons pass through the opening 47 and cannot be detected. Therefore, the focus is adjusted so as to focus on the front and rear of the reflector, thereby increasing the detection efficiency. In the embodiment, the post-stage acceleration voltage and the superimposed voltage applied to the sample are controlled so that the focal position of the secondary electrons does not change when the acceleration voltage (substantial acceleration voltage) is changed.

The focal length of the magnetic lens is a function of a variable IN / V1 / 2, where I is the current flowing through the lens coil, N is the number of turns of the coil, and V is the electron acceleration voltage when passing through the lens magnetic field. The acceleration voltage when the primary electrons pass through the lens magnetic field is (Vo + Vb) when Vo is the electron gun acceleration voltage and Vb is the post-acceleration voltage applied to the acceleration cylinder. Since the sample position (focal length) is constant, I / N / (Vo + Vb) 1/2 always takes a constant value (= a). The accelerating voltage when the secondary electrons pass through the lens magnetic field is given by:
In (Vr + Vb), the variable IN / V1 / 2 is expressed by the following equation.

I · N / V1 / 2 = a (Vo + Vb) 1/2 / (Vr + Vb) 1/2
= A ｛1+ (Vb / Vo)｝ 1/2 / ｛(Vr / Vo)
+ (Vb / Vo)｝ 1/2
From this equation, if the Vr / Vo and Vb / Vo ratios are controlled to be constant, the focal position of the secondary electrons will be constant. That is, if the Vr / Vo and Vb / Vo ratios are fixed and the post-acceleration voltage Vb and the superimposed voltage Vr of the sample are controlled, the focal position of the secondary electrons can be kept constant without depending on the acceleration voltage (substantial acceleration voltage). Can control.

  FIG. 8 shows an example in which a control electrode for controlling the electric field applied to the sample surface is added. A control electrode 36 is provided between the objective lens 8 and the sample 12, and a control voltage 50 is applied to the control electrode 36. The control electrode 36 has a hole through which an electron beam passes. The control electrode 36 controls the electric field intensity applied to the surface of the sample 12 between the acceleration cylinder 9 and the sample 12. This configuration is effective when it is inconvenient if a strong electric field is applied to the sample. For example, there is a case where there is a problem of element damage due to a strong electric field of a wafer on which a semiconductor integrated circuit is formed.

It is also effective for the problem of electrical non-contact with the sample holder 20 when the periphery of the wafer is covered with an oxide film. More specifically, when the side surface and the back surface of the sample (wafer) are covered with an insulator, electrical connection for lettering cannot be performed. Also, the sample (wafer) 12 is in an electric field created between the sample holder 20 and the objective lens 8, and when there is no control electrode, the superimposed voltage 13 applied to the sample holder 20 and the potential of the objective lens 8 at the ground potential This is because normal observation cannot be performed because only an intermediate potential is applied.

  Further, by setting the potential of the control electrode 36 to the same potential as the potential of the sample holder 20 to which the superimposed voltage 13 is applied or a positive potential of several tens of volts from the sample holder 20, breakage of the element and the floating of the wafer from the potential of the sample holder 20 are achieved. Can be prevented. In this case, the control electrode 36 is made large enough to always cover the sample (wafer).

  FIG. 9 is a diagram showing an example in which a control electrode is added.

  A control electrode 60 having an opening 59 through which a primary electron beam passes is provided above the sample (wafer) 12, and the same voltage as the superimposed voltage 13 applied to the sample holder 20 is applied to the control electrode 60. When the control electrode 60 having the same potential as the sample holder 20 is placed on the sample (wafer) 12, the wafer is surrounded by the metal having the same potential, and the wafer has the same potential as the potential of the surrounding metal. Strictly speaking, the intrusion of the electric field from the opening 59 through which the primary electron beam 7 passed through the anode 5 causes an error with the potential of the metal. This error is roughly the ratio of the area of the sample (wafer) 12 to the area of the opening 59. For example, if the wafer is 8 inches and the diameter of the opening 59 is 10 mm, the area ratio is 1/400 and the potential error is 1%, which is a sufficiently small value.

  With the above configuration, the same potential as that of the metal surrounding the wafer can be applied to the wafer.

  This makes it possible to apply a voltage for lettering even when the front and back surfaces of the wafer are covered with an insulating film and cannot be electrically connected to a sample stage or the like. .

  In this embodiment, at least a portion of the sample holder 20 located under the sample (wafer) 12 is formed of a conductor for applying the superimposed voltage 13, so that the wafer has the same potential as described above. It will be surrounded by metal. The sample holder itself may be a conductor, or a conductor may be inserted into the sample holder.

  FIG. 10 shows another example in which a control electrode is added.

A control electrode 60 is provided between the sample (wafer) 12 and the objective lens 8, and the same voltage as the superimposed voltage 13 applied to the sample holder 20 is applied to the control electrode 60. As a result, the sample (wafer) 12 is surrounded by the sample holder 20 and the control electrode 60 to which the same voltage is applied. Even if the sample (wafer) 12 is covered with the insulating film as described above, 13 can be applied to the sample (wafer).

The opening 59 of the control electrode 60 is usually circular, but may be other than circular. The size of the opening 59 does not disturb the visual field to be observed. In this embodiment, the size of the opening 59 is 4 mm in diameter. Since the distance between the control electrode 60 and the sample (wafer) 12 is 1 mm, there is a visual field with a diameter of 4 mm. Further, since the deceleration electric field reaches the wafer through the opening 59, the secondary electrons can be efficiently lifted onto the objective lens 8. When the aperture diameter is reduced, the deceleration electric field does not reach the sample (wafer) 12. However, when the wafer is tilted or the sample has irregularities, such a condition is better. The visual field displacement can be reduced.

A stage 22 is provided for observing an arbitrary position of the sample (wafer) 12. In this case, when a position largely deviated from the center point of the sample (wafer) 12 is to be observed, the sample (wafer) 12 needs to be largely moved. At this time, the sample (wafer)
When the electrode 12 comes off the control electrode 60, the potential of the sample (wafer) 12 changes, and it becomes impossible to apply a constant lettering voltage.

  In order to cope with this situation, in this embodiment, the control electrode is formed along the movement trajectory of the sample (wafer) 12. With this configuration, a constant lettering voltage can be applied even if the position of the sample (wafer) 12 is changed by the stage 22, and further, element destruction due to an electric field generated between the objective lens 8 and the sample (wafer) 12 can be prevented.

  In this embodiment, it is desirable to dispose a control electrode having a size larger than the movement range of the wafer. Specifically, the diameter of the control electrode for observing the entire surface of the 8-inch wafer is 400 mm in diameter. With this configuration, the voltage applied to the wafer can be kept constant regardless of how the wafer is moved.

  In this embodiment, the control electrode is a plate-like electrode. However, the control electrode may be formed in a mesh shape, a shape in which a large number of holes or slits are formed, and the evacuation property can be improved. In this case, it is desirable that the hole diameter and the slit width are smaller than the distance between the wafer and the control electrode.

In FIG. 10, the primary electron 63c passes through the opening 59 of the control electrode 60, and the sample (wafer)
When irradiating 12, secondary electrons 62 are generated. The generated secondary electrons 62 are inversely accelerated by the deceleration electric field with respect to the primary electrons 63c, and are guided above the objective lens 8. At this time, since the lens acts as a lens due to the magnetic field of the objective lens 8, it is guided onto the objective lens 8 while making a focal point as shown in the figure.

  The guided secondary electrons 62 collide with the reflecting plate 29 and generate secondary electrons 30. The secondary electrons 30 are deflected by the electric field created by the negative electrode deflecting electrode 31 'and the positive electrode deflecting electrode 31 "placed opposite to each other. The deflecting electrode 31" is made of a mesh. Therefore, the secondary electrons 30 pass through the mesh and are detected by the scintillator 25. Reference numerals 32 "and 32 'denote deflecting coils, which generate a magnetic field orthogonal to the electric field generated by the deflecting electrodes 31' and 31" and cancel the deflecting action of the primary electron beam 63b by the electric field generated by the deflecting electrodes 31 'and 31 ". ing.

  Although not shown, it is also possible to reduce contamination caused by scanning the sample with the primary electron beam 63c by cooling the control electrode 64.

  FIG. 11 shows still another example in which a control electrode is added.

Components such as a field emission cathode 1, an extraction electrode 2, an anode 5, a condenser lens 14, an objective lens 8, a sample 12, a sample holder 20, an insulating table 21, and a sample stage 22 are contained in a vacuum housing 66. The illustration of the vacuum exhaust system is omitted.

  Here, in a state where a negative superimposed voltage is applied to the sample 12, it is necessary to avoid a sample exchange operation by the sample exchange mechanism 67 and to keep the inside of the vacuum housing 66 at atmospheric pressure. In other words, the superimposed voltage 13 may be applied only when the electron beam is scanned on the sample 12.

  Therefore, in this embodiment, the first condition in which the switch 68 is closed and the accelerating voltage 6 is applied, which is a preparatory operation for mounting and replacing the sample, and the valve provided between the field emission cathode 1 and the sample 12 When both the second condition in which both the valve 69 and the valve 70 are open and the third condition in which the valve 71 through which the sample exchange mechanism 67 passes to place the sample 12 on the sample stage 22 are closed are satisfied. Only in this case, control is performed in which the switch 72 is closed and the superimposed voltage 13 is applied to the sample 12.

  Further, the sample holder 20 and the sample stage 22 are electrically connected via a discharge resistor 73, and when the switch 72 is opened, the charges charged in the sample 12 are transferred to the sample holder 20, the discharge resistor 73, and the sample stage 22. (The sample stage 22 is grounded.) The discharge is quickly performed under a constant time constant, and the potential of the sample 12 is reduced. The discharge resistor may be built in the power supply of the superimposed voltage 13.

Under the condition that the vacuum around the field emission cathode 1 is equal to or lower than the set value, the acceleration voltage can be applied from the anode 5 and the valve 69 is provided only when the vacuum of the vacuum housing 66 is equal to or higher than the set value. , 70 are released.

In this embodiment, the superimposed voltage 13 is described as being applied when all of the above three conditions are satisfied. However, when one or two of these conditions are satisfied, The switch 72 may be closed.

  FIG. 12 shows still another example in which a control electrode is added, which is applied to a scanning electron microscope having a sample stage 22 capable of tilting a sample. In this embodiment, the control electrode 76 is attached so as to cover the upper surface of the sample 75. Also, depending on the viewpoint, it can be said that they are arranged along the shape of the objective lens. The shape of the objective lens is formed so as not to hinder the movement of the sample 12. In the case of an apparatus having a tilting device as shown in FIG. 12, the objective lens has a sharp shape toward the sample 12. By forming the control electrodes along the objective lens formed under such conditions, it is possible to arrange the control electrodes without hindering the movement of the sample.

  In this case, the sample (wafer) 12 is surrounded by the sample holder 20 and the control electrode 76 regardless of the position and the inclination angle of the sample (wafer) 12. According to this configuration, no electric field is generated on the surface of the sample (wafer) 12. 20a shows a state where the sample (wafer) 12 is inclined. Reference numeral 74 denotes a tilting mechanism incorporated in the sample stage 22. In this embodiment, the diameter of the opening 65 of the control electrode 76 may be smaller than the distance between the opening 65 and the sample 12 so that the electric field created between the control electrode 76 and the sample (wafer) 12 does not change when tilted. desirable. The efficiency of detecting secondary electrons can be improved by setting the voltage applied to the control electrode 76 to several tens of volts more positive than the voltage applied to the sample 12.

  At this time, for the purpose of applying a voltage for lettering to the sample, a desired potential is applied to the sample in consideration of the effect of a composite electric field based on the voltage applied to the sample and the voltage applied to the control electrode. It is desirable to set the voltage applied to each of the sample and the control electrode so that the voltage is applied.

  It is also possible to increase the aperture diameter and apply an electric field to the sample 12 for attracting secondary electrons. In this case, the observation position shifts due to the tilt, but the tilt angle and the amount of the shift are measured in advance to deflect the electron beam. Alternatively, this shift can be eliminated by performing a correction such as moving the sample stage 22 horizontally. The control electrode 76 in this embodiment is made of a non-magnetic material so as not to affect the characteristics of the objective lens 8.

  In this embodiment, the control electrode is arranged so as to cover the inside of the sample chamber, but it is not always necessary to arrange it in this way. That is, it is sufficient that the sample is formed at least along the moving range of the sample. Even with such a configuration, the sample is surrounded by the sample holder and the control electrode. Although the sample holder has been described as a conductor in the present invention, for example, the conductor may be arranged above or below the sample holder. Further, in the case of the above-described embodiment, by forming the conductor larger than the sample, the sample (wafer) is substantially surrounded by the control electrode and the conductor, and it becomes possible to apply a constant lettering voltage. I have.

  FIG. 13 shows still another example in which a control electrode is added. In this example, the control electrode is not grounded between the objective lens 8 and the sample 12, but is opposed to the sample 12 in the objective lens 8 including the excitation coil 78, the upper magnetic path 77, and the lower magnetic path 79. The lower magnetic path 79 at the position is electrically insulated from the upper magnetic path 77, and the superimposed voltage 13 is applied thereto. The potential applied to the lower magnetic path 79 can be made more positive than the sample 12 to efficiently guide the secondary electrons onto the objective lens 8.

  FIG. 14 is a diagram illustrating an electron beam scanning deflector combining an electric field and a magnetic field. When a secondary electron detector is provided on the scanning deflector, secondary electrons generated in the sample are deflected by the scanning deflector when passing through the scanning deflector. For this reason, the deflection of the secondary electrons also increases at a low magnification when the scanning deflection angle of the electron beam becomes large, and the electron beam may collide with the inner wall of the electron beam path and become undetectable. The present embodiment has solved this problem. The scanning deflector comprises eight-pole electrostatic deflectors 51a to 51h and magnetic deflectors 52a to 52d.

  Now, considering the deflection in the x-axis direction, a positive electric potential is applied to the electrodes 51h, 51a, 51b and a negative electric potential is applied to 51d, 51e, 51f in the eight-electrode electrostatic deflector to generate a deflection electric field Ex. Here, as shown in FIG. 14, a potential of magnitude Vx is applied to the electrodes 51a and 51e, and a potential of 1/2 1/2 thereof is applied to the electrodes 51h, 51b, 51d and 51f on both sides thereof. Apply. This is a well-known method for producing a uniform electric field. At the same time as the electric field, a current Ix is supplied to the coils 52a and 52c of the magnetic field deflector 52 to generate a magnetic field Bx in a direction orthogonal to the electric field Ex as shown. The electric field Ex and the magnetic field Bx cancel the deflection of the secondary electrons coming from below, and act to reinforce the primary electrons from above.

  The deflection θ (S) for the secondary electrons coming from below is the difference between the deflection θ (B) due to the magnetic field and the deflection θ (E) due to the electric field as shown in the following equation.

θ (S) = θ (B) −θ (E)
= L / 8.Ex / Vr- (e / 2m) 1/2 BxL / Vr1 / 2
Here, L is the working distance of the electric and magnetic fields, e and m are the charge and mass of the electrons, respectively, and Vr is the acceleration voltage when the secondary electrons pass through the scanning deflector. If the ratio of Ex to Bx is given by the following equation, secondary electrons coming from below will not be deflected.

Bx / Ex = (2m / e) 1/2 / 8Vr1 / 2
On the other hand, regarding the deflection of the primary electrons, the electric field deflection is added to the magnetic field deflection, and the following equation is obtained. Where Vo is the electron gun acceleration voltage.

θ (o) = θ (B) + θ (E)
= (E / 2m) 1 / 2BxL / Vo1 / 2 + L / 8.Ex / Vo1 / 2
Therefore, the deflection angle θ (o) under the condition that the secondary electrons are not deflected is as follows.

θ (o) = (e / 2m) 1 / 2BxL {1+ (Vr / Vo) 1/2} / Vo1 / 2
So far, the deflection in the x-axis direction has been described. The deflection in the y-axis direction is performed in the same manner. That is, the potential of the electrode 51c is set to Vy, the potential of the electrodes 51b and 51d is set to Vy / 21/2, the potential of the electrode 51g is set to -Vy, and the potentials of the electrodes 51f and 51h are set to -Vy / 21/2. An axial deflection electric field Ey is generated. At the same time, a current Iy flows through the coils 52b and 52d of the magnetic field deflector to generate a magnetic field By orthogonal to the electric field Ey. As described above, the electric field Ey and the magnetic field By are sized to cancel the deflection of the secondary electrons coming from below and reinforce the primary electrons coming from above.

  In practice, deflection is performed by combining deflection in the x-axis direction and deflection in the y-axis direction. Accordingly, as shown in FIG. 14, the potential of each deflection electrode is the sum of the deflection potential in the x-axis direction and the deflection potential in the y-axis direction. In an actual apparatus, this deflector has two stages of an upper scanning deflector and a lower scanning deflector so that deflected primary electrons pass through the center of the objective lens. Then, by changing the potentials Vx, Vy of the deflecting electrodes and the deflecting coil currents Ix, Iy with time while maintaining the above relationship, the desired primary electron beam can be scanned on the sample.

  Next, the relationship between the lens center of the magnetic field type objective lens and the lens center of the electrostatic lens formed between the acceleration cylinder and the sample will be described. FIG. 15A is a diagram illustrating a problem when the center CB of the magnetic field type objective lens 8 and the center CE of the electrostatic lens formed between the acceleration cylinder 9 and the sample 12 do not match. In this case, the primary electron beam 11 accelerated in the latter stage is deflected so as to pass through the center CB of the magnetic lens, but passes through with a distance d from the center CE of the electrostatic lens. When the shift amount d increases, spherical aberration is added to the lens function of the electrostatic lens, and the scanned image is distorted.

  FIG. 15B shows an example in which the lens center CB of the magnetic field type objective lens 8 and the lens center CE of the electrostatic lens formed between the acceleration cylinder 9 and the sample 12 are matched. In this embodiment, the upper magnetic pole 53 of the objective lens 8 is projected so as to face the sample 12, and a magnetic field is created between the sample 12 on which the electrostatic lens is formed and the accelerating cylinder 9 so that the centers of both lenses coincide. It was made. As a result, since the primary electron beam 11 accelerated in the subsequent stage is not affected by the lens action of the electrostatic lens, a scan image without distortion can be obtained.

FIG. 16 shows a structure of the objective lens 8 that more effectively realizes the coincidence of the centers of the magnetic lens and the electrostatic lens. In the embodiments described so far, the acceleration cylinder 9 is inserted inside the electron beam path of the objective lens 8. In this case, if the axial centers of the electrostatic lens formed by the accelerating cylinder and the magnetic lens formed by the objective lens deviate, the resolution is reduced. Therefore, it is necessary to precisely match the mechanical centers of the two. In this embodiment, attention is paid to this point, and the upper magnetic pole 53 of the objective lens 8 is protruded to the end level of the lower magnetic pole 54 to face the sample 12. Further, the upper magnetic pole 53 is electrically insulated from the rest of the objective lens by the insulating plate 55, and the post-stage acceleration voltage 10 is applied thereto.

  According to the present embodiment, since the upper magnetic pole that determines the lens center of the objective lens 8 and the latter-stage acceleration electrode are also used, the above-described displacement between the electrostatic lens and the magnetic field lens does not occur. In addition, since the upper magnetic pole 53 of the magnetic lens directly faces the sample 12 and the post-acceleration voltage is applied thereto, not only the axial center but also the positions of the lens centers of the electrostatic lens and the magnetic lens coincide. Can be done.

FIG. 1 is a schematic diagram of one embodiment of the present invention. FIG. 4 is an explanatory diagram of an embodiment using a reflector for detecting a secondary signal. FIG. 4 is an explanatory diagram of an embodiment in which a reflector is used for detecting a secondary signal, and the deflection of primary electrons is prevented by using an electric field and a magnetic field that are orthogonal to the attraction of secondary electrons. FIG. 2 is a schematic diagram of an embodiment in which a secondary signal detector is placed above a scanning deflector. FIG. 4 is a schematic diagram of an embodiment in which secondary signal detectors are provided at two positions above and below a scanning deflector. FIG. 4 is an explanatory diagram of an embodiment using a channel plate for detecting a secondary signal. FIG. 4 is an explanatory diagram of an embodiment using a scintillator for detecting a secondary signal. FIG. 4 is a schematic diagram of an embodiment in which the electric field intensity applied to the sample is controlled. The figure which shows an example of the Example provided with the control electrode on the sample. The figure which shows an example of the charged particle microscope provided with the control electrode. The figure which shows another example of the charged particle microscope provided with the control electrode. The figure which shows an example of the charged particle microscope in which the control electrode was formed along the sample chamber inner surface. The figure which shows the example which used a part of objective lens magnetic path as a control electrode. FIG. 4 is an explanatory diagram of an embodiment in which a scanning deflector is a combination of an electric field and a magnetic field. FIG. 5 is an explanatory diagram of an embodiment in which the lens centers of a magnetic field lens and an electrostatic lens are matched. FIG. 4 is an explanatory view of an embodiment in which the magnetic pole of the magnetic lens and the acceleration cylinder are shared.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 ... Field emission cathode, 2 ... Extraction electrode, 3 ... Extraction voltage, 4 ... Emission electron, 5 ... Anode, 6 ... Electron gun accelerating voltage, 7 ... Primary electron beam passing through anode 5, 8 ... Objective lens, 9 ... Accelerating cylinder,
10: Post-acceleration voltage, 11: Primary electron beam accelerated in the post-stage, 12: Sample, 13: Superimposed voltage, 14: Condenser lens, 15: Upper scanning deflector, 16: Lower scanning deflector, 17: Deceleration electric field, 18 ... Aperture, 19 ... Adjustment knob, 20 ... Sample holder, 21 ... Insulating table, 22 ... Sample stage, 23 ... Secondary electron, 24 ... Suction electrode, 25 ... Scintillator, 26 ... Light guide,
27: Photoelectron amplification tube, 28: Central hole, 29: Reflector, 30: Secondary electron made of reflector, 31: Electric field deflection electrode, 32: Orthogonal magnetic field deflection coil, 33: Upper detector, 34: Lower Detector,
35 channel body, 36 control electrode, 37 mesh, 39 amplified electrons, 40 anode voltage, 41 anode, 42 amplifier, 43 optical conversion circuit, 44 optical signal, 45 electrical conversion Circuit, 46 single crystal scintillator, 47 opening, 48 conductive coating, 49 secondary electron crossover, 50 control voltage, 51a to 51h electrostatic deflection electrode, 52a to 52d magnetic field deflection coil, 53 ... upper magnetic pole, 54 ... lower magnetic pole, 55 ... insulating plate.

Claims (6)

  In a charged particle microscope that irradiates a charged particle beam emitted from a charged particle source onto a sample and obtains a sample image based on a secondary signal generated from the sample, a conductive material for applying a negative potential under the sample is used. A charged particle microscope, wherein a body is arranged and an electrode for applying the same potential as the negative potential or a potential on the positive side from the potential is provided on the sample.   The charged particle microscope according to claim 1, wherein the electrode is formed along a movement trajectory of the sample.   The charged particle microscope according to claim 1, wherein the electrode is formed so as to cover a moving area of the sample.   4. The charged particle microscope according to claim 1, wherein the electrode is formed along a shape of an objective lens arranged under the charged particle source. 5.   The charged particle microscope according to claim 1, wherein the electrode is formed along an inner surface of a sample chamber surrounding the sample. The charged particle microscope according to claim 1, wherein the conductor located under the sample has a size equal to or larger than the sample.

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