Patent Application
20080283744
1. Field of the Invention
The present invention relates to a scanning charged particle beam device which irradiates an object with charged particles, and particularly relates to a technique which corrects astigmatism, off-axis aberrations, out-of-focus state and the like.
2. Description of the Related Art
Examples of scanning charged particle beam devices are scanning electron microscopes (SEM), scanning transmission electron microscopes (STEM) and ion beam machining devices. Devices using them include scanning tunnel microscopes.
The scanning electron microscopes scan samples with electron beams as probes, and detect secondary electrons generated from the samples due to the scanning. The scanning transmission electron microscopes scan samples with electron beams as probes, and detect electrons transmitted through the samples due to the scanning. The ion beam machining devices narrow down ion beams to irradiate desired positions on sample surfaces and can machine the samples. Further, the ion beam machining devices scan the sample surfaces with ion beams and detect the amount of secondary electrons generated at the time of ion collision so that shapes of the surface can be observed microscopically.
For example, focused ion beam (FIB) devices have a scanning ion microscope (SIM) function. In scanning tunnel microscopes, a sample and a probe are made to be extremely close to each other, and both of them are scanned comparatively two-dimensionally, an electric current flowing in the probe is detected so that an image is displayed based on the detected electric current.
In the charged particle beam devices, astigmatism occurs due to machining error and assembly error of optical components. For this reason, an astigmatism corrector which corrects the astigmatism is provided. In order to improve yield of secondary electrons generated from a sample, an electrostatic lens is provided in a vicinity of an objective lens, and a voltage is applied to it. Further, this applied voltage is slightly changed so that a focus of a charged particles beam can be corrected at high speed.
Japanese Patent Application Laid-Open No. 5-114378 (Patent Document 1) discloses a method for obtaining an optimum focus automatically using a focus correcting function. An exciting current of the objective lens is changed into a sawtooth shape, and an optimum exciting current is derived based on an integration value of image signals obtained at respective steps.
In the charged particle beam devices having such an optical system, the axes of the objective lens and the electrostatic lens occasionally do not align with each other. As a result, when the exciting current of the objective lens or the applied voltage of the electrostatic lens are changed in a state where the astigmatism has been corrected by the astigmatism corrector, astigmatism occurs due to off-axis. Therefore, as described in Japanese Patent Application Laid-Open No. 8-306331 (Patent Document 2), for example, a technique which changes the exciting current of the astigmatism corrector according to each changed contents of optical conditions so as to prevent the occurrence of the astigmatism is known.
As described in Japanese Patent Application Laid-Open No. 2002-83563 (Patent Document 3), in addition to the above optical system, a reflecting plate made of metal is occasionally provided between a sample and a detector. As a result, secondary electrons generated from the surface of the sample are separated from reflected electrons, and they are captured by independent detectors. Shape information about the sample surface can be acquired based on the secondary electrons, and cubic information about sample unevenness can be acquired based on the reflected electrons. In this example, the applied voltage of the electrostatic lens controls the separation of the secondary electrons and the reflected electrons, and accelerates a charged particle beam passing through the objective lens so as to reduce chroma aberration generated on the objective lens.
In the astigmatism correcting unit of Japanese Patent Application Laid-Open No. 8-306331, the astigmatism is corrected by correction coefficients obtained in advance for the respective applied voltages of the electrostatic lens, so that an observation image without astigmatism can be obtained. An inductance of the astigmatism corrector, however, has time constant. For this reason, response time of the astigmatism corrector limits an effect of a high-speed focus correction of the electrostatic lens. Optical components such as a deflector, an objective lens and an electrostatic lens are provided on the side of the sample with respect to the astigmatism corrector. For this reason, chroma aberration and off-axis aberration which occur in the respective components cannot be eliminated. Therefore, resolution of the observation image has a problem.
In order to solve this problem, a means that an aberration correcting device is provided on a side closer to a sample with respect to the objective lens can be considered. In the optical system of Japanese Patent Application Laid-Open No. 2002-83563, since the electrostatic lens, the reflecting plate, the detector for detecting reflected electrons and the like are present near the objective lens, it is structurally difficult to provide the aberration corrector. When the aberration corrector is provided on the side closer to the sample with respect to the objective lens, a problem that a working distance becomes long arises.
It is an object of the present invention to provide a charged particle beam device which is capable of correcting astigmatism, off-axis aberration and out-of-focus state simultaneously at high speed.
According to the charged particle beam device of the present invention, an electrostatic lens which generates an electric field on a trajectory of a charged particle beam is provided between an objective lens and a sample. The electrostatic lens is divided into a plurality of electrodes, and a voltage can be applied to the electrodes independently. When the voltage is adjusted, at least one of the astigmatism, the off-axis aberration and the out-of-focus state of the objective lens is corrected.
According to the present invention, the astigmatism, the off-axis aberration and the out-of-focus state can be corrected simultaneously at high speed.
The present invention is described with reference to
A voltage can be applied to the electrostatic lens (114) by control of the control section (106). When a voltage is applied to the electrostatic lens (114), an electric field is generated on a trajectory of an electron beam (102). Adjustment of the voltage applied to the electrostatic lens (114) provides a focus correcting function and an aberration correcting function.
In this example, the electrostatic lens (114) is constituted as a magnetic pole of the objective lens. That is to say, the electrostatic lens (114) composes a magnetic path of the objective lens, and a voltage can be applied thereto.
The electron beam (102) from the electron source (101) is extracted by the extraction electrode (103), and is converged by the condenser lens (104). Off-axis aberration of the electron beam (102) is corrected by the alignment lens (108), and the electron beam (102) is scanned two-dimensionally by the deflecting lens (110), is focused by the objective lens (112), and is emitted to a sample (116). A secondary electron (107) generated from the sample (116) and a reflected electron (115) from the sample (116) are separated by the reflecting plate (113). The secondary electron (107) whose energy is comparatively low and does not have a directional property is detected by the secondary electron detector (105). The reflected electron (115) whose energy is comparatively high and has a directional property is detected by the reflected electron detector (111).
Scintillator is used for electron detectors of the secondary electron detector (105) and the reflected electron detector (111). When a positive voltage is applied to the scintillator, the electron is attracted to the detector and is simultaneously accelerated, and collides with the scintillator. Light emission from the scintillator is amplified by a photoelectron multiplier, and the emitted light is transmitted to the control section (106) as a signal of the detector. In the control section (106), the signal of the detector is synchronized with a deflecting signal of the deflecting lens (110) so that a scanned image is displayed on the display (109).
When a positive voltage is applied to the electrostatic lens (114), an electric field is generated on a trajectory of the electron beam (102), and the secondary electron (107) generated from the sample (116) and the reflected electron (115) from the sample (116) are separated from each other. When a positive voltage is applied to the electrostatic lens (114), a positive electric field is generated, and a force, which pulls up the secondary electron (107) and the reflected electron (115) vertically, exerts. As a result, the secondary electron (107) passes through a hole on a center of the reflecting plate (113), and is attracted to the secondary electron detector (105) provided above it. The reflected electron (115) collides with the reflecting plate (113), and turns into a secondary electron (not shown) generated from the surface of the reflecting plate (113) so as to be attracted to the reflected electron detector (111).
The correction of the aberration in the scanning electronic microscope apparatus of this example is described. When a primary electron beam (102) shifts from a center axis line of the objective lens (112), an off-axis aberration occurs on the scanned image. Therefore, the alignment lens (108) is excited, the shift of the primary electron beam (102) is corrected so that the occurrence of the off-axis aberration is prevented.
When the positive voltage is applied to the electrostatic lens (114), the primary electron beam (102) is accelerated. As a result, the chroma aberration which occurs at the time when the primary electron beam (102) passes through the objective lens (112) is reduced. By adjusting the positive voltage applied to the electrostatic lens (114), the off-axis aberration, the astigmatism and the out-of-focus state are corrected as described below.
In this example, since the electrostatic lens (114) is used, inductance is not present unlike a magnetic field lens using a coil. For this reason, an element, which can operates at high speed, is used for controlling the voltage applied to the electrostatic lens, so that the off-axis aberration, the astigmatism and the out-of-focus state can be corrected at high speed.
In the scanning electronic microscope apparatus of this example, a memory device and an input device, not shown, are provided. Functions which show relationships among a value of the voltage applied to the electrostatic lens (114), the optical condition, and the off-axis aberration, the astigmatism and the out-of-focus state, are stored in the memory device. In order to obtain these functions, the operator changes the value of the voltage applied to the electrostatic lens (114) with respect to each image pick-up condition via the input device, so as to measure the off-axis aberration, the astigmatism and the out-of-focus state. In such a manner, the functions or the graphs of the off-axis aberration, the astigmatism and the out-of-focus state with respect to the value of the applied voltage are obtained.
When such functions or graphs are obtained, the operator uses the functions or the graphs at the time of observing the sample. When the image pickup condition is input, the control section (106) searches the memory device for the image pickup condition and reads the function corresponding to the image pickup condition. The value of the voltage applied to the electrostatic lens (114), by which the off-axis aberration, the astigmatism and the out-of-focus state become at a minimum, is read based on the function read. The correcting amount of the applied voltage is adjusted based on the data read.
The control section (106) can detect the off-axis aberration, the astigmatism and the out-of-focus state automatically from the image of the sample. The aberration and the out-of-focus state may be detected by using a well-known image process.
A method for correcting the aberrations of the primary electron beam (102) according to the present invention is described with reference to
V
1
=v
1
+V
st1
V
2
=v
2
+V
st2
V
3
=v
3
−V
st3
V
4
=v
4
−V
st4
V
5
=v
5
+V
st5
V
6
=v
6
+V
st6
V
7
=v
7
−V
st7
V
8
=v
8
−V
st8 Formula (1)
v1 to v8 of the first term on the right side of the formula (1) are the initial voltages, and Vst1 to Vst8 of the second term are the correcting amounts. As shown in the formula (1), the voltages V3, V4, V7, V8, applied in the direction along the long axis of the ellipse of the section of the primary electron beam (102) reduce, and the voltages V1, V2, V5, V6 applied in the direction along the short axis of the ellipse increase. As a result, the section of the primary electron beam (102) deforms so as to be compressed in the direction along the long axis and to be pulled in the direction along the short axis. In such a manner, the astigmatism of the primary electron beam (102a) is eliminated, and the primary electron beam (102a) passes through the center of the electrostatic lens (114) and has a circular section.
Even after the voltages are adjusted, the center position of the primary electron beam (102a) does not change. That is to say, the center position of the primary electron beam (102a) passes through the center of the electrostatic lens (114). In order to obtain this state, the correcting amounts of the three pairs of the electrodes whose paired electrodes are arranged on both sides of the long axis of the ellipse of the section of the primary electron beam (102) should be equal. For example, the first electrode (201) and the sixth electrode (206) are symmetrical with respect to the long axis of the ellipse. Therefore, the correcting amount Vst1 of the voltage applied to the first electrode (201) and the correcting amount Vst6 of the voltage applied to the sixth electrode (206) are equal to each other. Similarly, the second electrode (202) and the fifth electrode (205) are symmetrical with respect to the long axis of the ellipse. Therefore, the correcting amount Vst2 of the voltage applied to the second electrode (202) and the correcting amount Vst5 of the voltage applied to the fifth electrode (205) are equal to each other. Similarly, the third correcting amount Vst3 and the fourth correcting amount Vst4 are equal to each other, and the seventh correcting amount Vst7 and the eighth correcting amount Vst8 are equal to each other.
V
1
=v
1
+kV
ax1
V
2
=v
2
+V
ax2
V
3
=v
3
+V
ax3
V
4
=v
4
+kV
ax4
V
5
=v
5
−k
Vax5
V
6
=v
6
−V
ax6
V
7
=v
7
−V
ax7
V
8
=v
8
−k
Vax8 Formula (2)
v1 to v8 of the first term on the right side of the formula (2) are the initial voltages, and Vax1 to Vax8 of the second term are correcting amounts. k is a constant which is larger than 0 and smaller than 1. In this example, since the primary electron beam (102a) is moved along the x axial direction, the correcting amount of the voltages applied to the electrodes arranged above the x axis is equal to the correcting amount of the voltages applied to the electrodes arranged below the x axis. That is to say, the correcting amounts of the three pairs of the electrodes whose paired electrodes arranged on both sides of the x axis should be equal to each other. For example, the first electrode (201) and the fourth electrode (204) are symmetrical with respect to the x axis. Therefore, the correcting amount Vst1 of the voltage applied to the first electrode (201) is equal to the correcting amount Vst4 of the voltage applied to the fourth electrode (204). Similarly, the eighth electrode (202) and the fifth electrode (205) are symmetrical with respect to the x axis. Therefore, the correcting amount Vst8 of the voltage applied to the eighth electrode (202) is equal to the correcting amount Vst5 of the voltage applied to the fourth electrode (205).
Similarly, the second correcting amount Vst2 is equal to the third correcting amount Vst3, and the sixth correcting amount Vst6 is equal to the seventh correcting amount Vst7. If the correcting amounts of the voltages applied to the electrodes symmetrical with respect to the x axis are different from each other, the primary electron beam (102a) shifts to the y direction.
In this example, the voltages V1, V2, V3, V4, V5, V6, V7 and V8 applied to the electrodes (201) to (208), respectively, are set to the following values.
V
1
=v
1
+Vf
V
2
=v
2
+Vf
V
3
=v
3
+Vf
V
4
=v
4
+Vf
V
5
=v
5
+Vf
V
6
=v
6
+Vf
V
7
=v
7
+Vf
V
8
=v
8
+Vf Formula (3)
v1 to v8 of the first term on the right side of the formula (3) are the initial voltages, and Vf of the second term is the correcting amount. In order to enlarge the section of the primary electron beam (102a), the correcting amount Vf is set to a negative value, and in order to reduce the section of the primary electron beam (102a), the correcting amount Vf is set to a positive value.
As shown in the formulas (1) to (3), the correcting amount varies depending on which of the off-axis aberration, the astigmatism and the out-of-focus state is corrected. For example, the correcting amount by which the off-axis aberration becomes zero is not always equal to the correcting amount by which the astigmatism becomes zero. In general, the correcting amount, which simultaneously makes the off-axis aberration, the astigmatism and the out-of-focus state zero, does not exist. However, actually these aberrations simultaneously occur. In order to simultaneously correct the astigmatism, the off-axis aberration and the out-of-focus state, their correcting amounts may be added up. The voltages V1, V2, V3, V4, V5, V6, V7 and V8 applied to the electrodes (201) to (208), respectively, are set to the following values.
V
1
=v
1
+ΔV
1
V
2
=v
2
+ΔV
2
V
3
=v
3
+ΔV
3
V
4
=v
4
+ΔV
4
V
5
=v
5
+ΔV
5
V
6
=v
6
+ΔV
6
V
7
=v
7
+ΔV
7
V
8
=v
8
+ΔV
8 Formula (4)
ΔV1 to ΔV8 are the correcting amounts of the electrodes, by which the total sum of the off-axis aberration, the astigmatism and the out-of-focus state becomes minimum. The correcting amounts ΔV1 to ΔV8 may be the total sum of the correcting amount of the off-axis aberration, the correcting amount of the astigmatism and the correcting amount of the out-of-focus state. In the example of
ΔV1=+Vst1+kVax1+Vf
ΔV2=+Vst2+Vax2+Vf
ΔV3=−Vst3+Vax3+Vf
ΔV4=−Vst4+kVax4+Vf
ΔV5=+Vst5−kVax5+Vf
ΔV6=+Vst6−Vax6+Vf
ΔV7=−Vst7−Vax7+Vf
ΔV8=−Vst8−kVax8+Vf Formula (5)
In this example, the correcting amounts of the voltages are expressed by a primary expression. Therefore, as shown in the formula (5), when a plurality of aberrations is corrected, the correcting amount is the sum of their respective correcting amounts. However, when the plurality of aberrations is corrected, the aberrations can be occasionally reduced by a high-order aberration correction. According to the present invention, when the plurality of aberrations is corrected, the high-order aberration correction may be made.
When the image pickup condition is changed, the initial voltages and the correcting amounts change. The correcting amounts change also according to the initial voltages. Therefore, the correcting amounts are also functions with respect to the image pickup condition and the initial voltages. The image pickup condition includes optical conditions such as voltages, magnifications and scanning conditions of the condenser lens (104), the deflecting lens (110), the objective lens (112) and the like.
A procedure of the aberration correcting method according to the present invention is described with reference to
The functions stored in the memory device are read, and the correcting amounts of the voltages applied to the electrodes are determined so that the astigmatism amount, the off-axis aberration amount and the focal accuracy each become optimum at step S407. That is to say, the correcting amounts of the voltages applied to the electrodes in the case where the astigmatism amount becomes minimum, the correcting amounts of the voltage applied to the electrodes in the case where the off-axis aberration amount becomes minimum, and the correcting amounts of the voltages applied to the electrodes in the case where the predetermined out-of-focus state becomes minimum are determined. As shown in the formula (5), the correcting amounts of the voltages applied to the electrodes of the electrostatic lens (114) in the case where the astigmatism amount, the off-axis aberration amount and the out-of-focus state simultaneously become minimum are determined. An image is acquired and is displayed on the display (109) at step S408.
When the functions are derived at steps S401 to S405, the image at the time of changing the correcting amounts of the voltages may be displayed on the display (109).
The aberration and the focus are corrected by adjusting the voltage of the electrostatic lens, but in this case, the methods in Japanese Patent Application Laid-Open No. 5-114378 and Japanese Patent Application Laid-Open No. 8-306331 can be used. Japanese Patent Application Laid-Open No. 5-114378 refers to the invention relating to the automatic focus correction by means of an exciting current of the objective lens, but the similar sequence can be applied to the voltage of the electrostatic lens.
The procedure of another example of the aberration correcting method according to the present invention is described with reference to
A method in which the operator makes various adjustments manually is described with reference to
One example of the display screen of the display (109) in the scanning electronic microscope apparatus of the present invention is described with reference to
The review window is described first. The review window includes an information portion (501), an ADR (AutoDefect Review) progress and ADR throughput portion (502), an image display portion (503), a userclass histogram portion (504) and a beam parameter portion (506). The information portion (501) shows a review condition. The ADR progress and ADR throughput portion (502) shows a progress, an operating speed and a residual time of a defect review operation. The image display portion (503) shows a low magnification image for defective alignment and respective images obtained by the optical system of Japanese Patent Application Laid-Open No. 2002-83563. The userclass histogram portion (504) shows the amounts according to defect classification. The beam parameter portion (506) is a function display window at steps S411 and S412 of
The astigmatism function, the off-axis aberration function, the focal accuracy function can be displayed on the beam parameter portion (506) individually or simultaneously. The display/non-display is selected by clicking respective buttons (505) in the beam parameter portion (506) via a mouse of a PC (not shown) used for operator. The border of the clicked button is highlighted or is colored so that the operator is informed of the selected button. The selected function is displayed on the function window (507), and an optimum value is displayed by a dotted line in the window. The function displayed on the function window (507) is obtained at step S406.
The electronic microscope operating screen is described below. The electronic microscope operating screen includes a detector selecting portion (508), an image magnification display portion (509), an electron source (101) state display portion (510), an image display portion (511), and an optical condition operating portion (515). On the detector selecting portion (508), the secondary electron detector (105) provided to the optical system in Japanese Patent Application Laid-Open No. 2002-83563 or the right and left reflected electron detectors (111) is selected for an image to be displayed. The image display portion (511) shows the image of the detector selected on the detector selecting portion (508). The optical condition operating portion (515) provides a user interface for manually adjusting the optical condition by the operator.
Alphabets which represent their respective detectors are displayed on the detector selecting portion (508). S is the secondary electron detector (105), L is the reflected electron detector (111) on the left side, and R is the reflected electron detector (111) on the right side. The selection of the detector to be used for displaying on the image display portion (511) and the displaying of the selected button are similar to the buttons (505) on the beam parameter window.
Tags showing groups' names in which the optical conditions to be operated are classified are provided to the optical condition operating portion (515). When the items in the respective tags are operated, the optical conditions can be adjusted manually.
As one example, a case where the voltages of the electrostatic lens (Booster tag) is described. The booster tag is provided with selection buttons (512) of the adjusting contents, and they are a button (Axis Align.) for adjusting the off-axis aberration, a button (Stigma) for adjusting the astigmatism and a button (Focus) for adjusting the focus. A process for displaying the selected button is similar to the detector selecting portion (508). When the operator selects the adjusting content, parameter correcting bars (516) on a lower portion are associated with control values of the applied voltage to the electrostatic lens (114). When in this state, an increase/decrease button X/Y (513) is clicked, or adjustment bars (514) are pulled so as to be moved right and left, biaxial manual adjustment can be carried out. When the Auto Align. button (517) is pushed down, automatic adjustment sequence is carried out arbitrarily by the operator so that the optimum optical conditions can be obtained.
One example of the display screen on the display (109) is described, but the arrangement of the window and the method for displaying selected items using the buttons and bars in the above description do not restrict another method. For example, the arrangement of the window can be changed arbitrarily by the operator. Instead that the border of the button is highlighted, the selected item can be displayed by use of a pull-down menu and a radio button.
The present invention can be utilized in scanning microscopes, electron beam drawing devices, ion beam devices and similar devices using the scanning charged particle beams, which observe and machine living things, materials, semiconductor integrated circuits and the like.
The examples of the present invention are described above, but the present invention is not limited to the above examples, and a person skilled in the art easily understands that various modifications may be made without departing from the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-014909 | Jan 2007 | JP | national |
