JP4192290B2 - Charged particle beam projection optical system and adjustment method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a charged particle beam projection optical system and an adjustment method thereof, and more particularly to a charged particle beam projection optical system for observing and inspecting an object surface using a charged particle beam such as an electron beam or an ion beam, and It relates to the adjustment method.
[0002]
[Prior art]
Conventionally, a charged particle beam microscope using an electron beam (electron beam) or the like is often used for observing and inspecting semiconductor elements and the like that have been miniaturized and highly integrated. Among charged particle beam microscopes, there is a so-called mapping electron microscope in addition to a scanning electron microscope (SEM). In recent years, development of a charged particle beam mapping projection optical system for this mapping electron microscope has been actively performed. Has been done.
The configuration of the charged particle beam mapping optical system will be briefly described below. First, a primary electron beam (irradiation electron beam) emitted from an electron gun passes through a primary optical system (irradiation optical system) and enters an electromagnetic prism called an e-cross bee (E × B). To do. The primary electron beam after passing through the e-cross bee becomes an electron beam whose cross-sectional shape is linear or rectangular, passes through the cathode lens (objective optical system), and passes through the object surface of the sample. Use epi-illumination. When the object surface is irradiated with the primary electron beam, a reflected electron beam having a relatively high energy reflected from the object surface and a low energy secondary electron beam emitted from the object surface are generated. Of these electron beams, a secondary electron beam is usually used for imaging. The secondary electron beam (observation electron beam) passes through the cathode lens and enters the e-cross bee. The secondary electron beam that has passed through the e-Cross beam passes through the secondary optical system (imaging optical system) and enters the electron beam detector. Based on the information of the secondary electron beam incident on the electron beam detector, the object surface is observed and inspected.
[0003]
[Problems to be solved by the invention]
In the charged particle beam projection optical system, it is necessary to accurately adjust the charged particle beam projection optical system in advance in order to accurately observe and inspect the object surface of the sample. Specifically, in order to make the illumination visual field of the primary optical system coincide with the observation visual field of the secondary optical system, voltage adjustment between the primary optical system and the secondary optical system, and electromagnetic field adjustment of e-Cross Bee To adjust the optical axis and correct aberrations.
However, the conventional charged particle beam projection optical system cannot adjust the primary optical system, the secondary optical system, and the e-cross bee, respectively. I spent a lot of work. That is, while the primary electron beam is emitted from the electron gun, the illumination visual field of the primary optical system and the observation visual field of the secondary optical system are matched.
Accordingly, it is an object of the present invention to provide a charged particle beam projection optical system capable of quick and accurate adjustment regardless of the level of skill of an operator, and a method for adjusting the objective optical system and the imaging optical system thereof. To do.
[0004]
[Means for Solving the Problems]
The present invention has been made to solve the above-described problems. That is, when the reference numerals in the attached drawings are added in parentheses, the present invention is directed to charged particle beams for irradiation emitted from an irradiation source (15) ( S) is incident on the optical path switching means (6) through the irradiation optical system, and the charged charged particle beam (S) that has passed through the optical path switching means (6) passes through the object optical system (30) through the objective optical system (5). The observation charged particle beam (K) emitted from the object plane (30) is incident on the optical path switching means (6) through the objective optical system (5) and irradiated by the optical path switching means (6). The observation charged particle beam (K) is guided in a direction different from the direction reaching the radiation source (15), and the observation charged particle beam (K) after passing through the optical path switching means (6) is passed through the imaging optical system. In the charged particle beam projection optical system to be incident on the detection means (14), the object plane (3 To be placed in a position of) a charged particle beam image projection optical system, characterized in that adjustment-ray source that emits adjustment charged particle beam (T) a (1) is provided.
[0005]
In the present invention, the charged charged particle beam (S) emitted from the irradiation source (15) is incident on the optical path switching means (6) through the irradiation optical system, and passes through the optical path switching means (6). The charged particle beam (S) is incident on the object plane (30) via the objective optical system (5), and the observation charged particle beam (K) emitted from the object plane (30) is passed through the objective optical system (5). The charged particle beam for observation (K) is guided in a direction different from the direction reaching the irradiation source (15) by the optical path switching means (6), and the optical path switching means (6). In the adjustment method of the charged particle beam projection optical system in which the observation charged particle beam (K) after passing through the beam is incident on the detection means (14) via the imaging optical system, it is adjusted to the position of the object plane (30). A step of disposing an adjustment source (1) for emitting charged particle beams (T), and an objective optical system (5) A step of adjusting the optical axis or aberration of the objective optical system (5) by applying a voltage to the optical system, and an optical axis of the imaging optical system by applying a voltage to the imaging optical system in addition to the objective optical system (5). A method for adjusting a charged particle beam projection optical system, comprising the steps of:
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of a charged particle beam mapping projection optical system according to the present invention. The outer appearance of the charged particle beam projection optical system mainly includes a primary column 2, a secondary column 3, and a chamber 4. They are provided with an evacuation system (not shown). The interior of the charged particle beam projection optical system is evacuated by evacuation by the turbo pump of the evacuation system. An X stage 31 that can be moved in the X direction by the X stage driving unit 35 and a Y stage 32 that can be moved in the Y direction by a Y stage driving unit (not shown) are installed in the chamber 4. On the X stage 31, the cold cathode 1 (adjustment source), the sample 30, the X moving mirror 33, and the Y moving mirror (not shown) are placed.
Here, the cold cathode 1 is a so-called self-luminous radiation source that emits an electron beam having a low initial energy (approximately 0.5 to 2 eV). This initial energy value is close to the initial energy value of the secondary electron beam K emitted from the object plane of the sample 30 described above. Examples of the cold cathode 1 include a MOS tunnel cold cathode, a Poly-Si / i-Si / n-Si cathode, a silicon field emitter, and the like.
Further, the cold cathode 1 can freely form self-luminous patterns such as dots, line and space patterns, cross marks, L-shaped marks, etc., by processing using a lithography method.
[0007]
As shown in FIG. 1, the primary electron beam S emitted from the electron gun 15 installed inside the primary column 2 passes through the primary optical system and enters the e-cross bee 6. Here, the primary optical system includes a field stop FS1, irradiation lenses 17, 18, and 19, aligners 23 and 24, a scan aligner 25, an aperture 26, and the like. The irradiation lenses 17, 18, and 19 are electronic lenses, and for example, circular lenses, quadrupole lenses, octupole lenses, or the like are used.
After the optical path of the primary electron beam S is deflected by the e-cross bee 6, the primary electron beam S reaches the aperture stop AS and forms an image of a crossover of the electron gun 15 at this position. After passing through the first aligner 9, the primary electron beam S that has passed through the aperture stop AS is subjected to the lens action by the cathode lens 5 to Koehler illuminate the sample 30.
[0008]
When the sample 30 is irradiated with the primary electron beam S, the sample 30 generates a secondary electron beam K and a reflected electron beam having a distribution corresponding to the surface shape, material distribution, potential change, and the like. Of these, the secondary electron beam K mainly serves as an observation electron beam. As described above, the initial energy of the secondary electron beam K is low, about 0.5 to 2 eV.
The secondary electron beam K emitted from the sample 30 passes through the cathode lens 5, the first aligner 9, the aperture stop AS, the e-cross bee 6, and the secondary optical system in this order, and then enters the electron beam detector 14. To do. Here, the secondary optical system includes an imaging lens front group 7, an imaging lens rear group 8, stigmeters 12 and 13, a second aligner 10, a third aligner 11, a field stop FS2, and the like. Further, the field stop FS2 has a positional relationship conjugate with the object plane of the sample 30 with respect to the cathode lens 5 and the imaging lens front group 7. Further, the imaging lens front group 7 and the imaging lens rear group 8 of the secondary optical system are electronic lenses, and for example, a circular lens, a quadrupole lens, an octupole lens, or the like is used.
[0009]
The secondary electron beam K incident on the detection surface of the electron beam detector 14 forms an enlarged image of the sample 30 by the secondary optical system. Here, the electron beam detector 14 is converted to an MCP (Micro Channel Plate) for amplifying the electrons, a fluorescent plate for converting the electrons into light, and the outside of the secondary column 3 kept in a vacuum state. And a vacuum window for emitting light.
The light emitted from the electron beam detector 14, that is, the optical image of the sample 30 is transmitted through the relay lens 40 and is incident on the image sensor 41 such as a CCD. The light incident on the image sensor 41 is converted into a photoelectric signal and transmitted to the control unit 42. Further, the photoelectric signal transmitted to the control unit 42 is converted into an image signal and transmitted to the CPU 43. This image signal is transmitted to the display 44, and the image of the sample 30 is displayed on the display 44.
[0010]
The CPU 43 sends the control signal to the first voltage control unit 45, the second voltage control unit 46, and an electromagnetic field control unit (not shown). Here, the first voltage control unit 45 performs voltage control of the primary optical system, the second voltage control unit 46 performs voltage control of the cathode lens 5, the first aligner 9, and the secondary optical system, and the electromagnetic field control unit. Performs electromagnetic field control of the e-cross bee 6.
Further, the CPU 43 transmits the control signal to the X stage driving unit 35 and the Y stage driving unit, and receives stage position information from the X interferometer 34 and the Y interferometer (not shown). Observation and inspection can be performed sequentially.
[0011]
Next, referring to FIG. 2, the configuration and operation of the e-cross bee 6 will be described. As shown in FIG. 5A, the primary electron beam S emitted from the electron gun 15 is converged by the lens action of the primary optical system, and is incident on the e-cross bee 6. The trajectory (optical path) is bent by the deflection action of the cross bee 6.
This is because, as shown in FIG. 5B, when an electron of charge q (primary electron beam S) travels in a + Z direction at a velocity v in an electric field E and a magnetic field B orthogonal to each other, This is because the resultant force of the force F _E (= qE) due to the electric field acting in the direction and the force F _B (= qvB) due to the magnetic field is received. Thereby, the trajectory of the primary electron beam S is bent in the XZ plane.
[0012]
On the other hand, the secondary electron beam K generated from the sample 30 irradiated with the primary electron beam S is subjected to the lens action by the cathode lens 5 and passes through the aperture stop AS arranged at the focal position of the cathode lens 5. After entering the e-cross bee 6, the e-cross bee 6 goes straight ahead.
This is due to the following reason. As shown in FIG. 2C, when an electron of charge q (secondary electron beam K) travels in a −Z direction at a velocity v in an electric field E and a magnetic field B orthogonal to each other, in the −X direction. A resultant force of a force F _E caused by a working electric field and a force F _B caused by a magnetic field acting in the + X direction is received. At this time, the absolute values of the force F _E by the electric field and the force F _B by the magnetic field are set to be equal (E = vB), that is, to satisfy the Wien condition. Accordingly, the force F _E caused by the electric field and the force F _B caused by the magnetic field cancel each other, and the apparent force received by the secondary electron beam K becomes zero, and the secondary electron beam K passes through the e-cross bee 6. You will go straight.
As described above, the e-cross bee 6 has a function as a so-called electromagnetic prism that selects an optical path of an electron beam passing therethrough.
[0013]
Next, with reference to FIG. 3, the adjustment of the charged particle beam mapping projection optical system according to an embodiment of the present invention using the cold cathode 1 on which the dot pattern is formed will be described. As an overall rough adjustment procedure, first, the optical axis of the secondary optical system is adjusted using the spot pattern of the cold cathode 1, and then the electromagnetic field adjustment of the e-cross bee 6 and the optical axis of the primary optical system are adjusted. And do.
A detailed adjustment procedure will be described. First, as shown in FIG. 3, the cold cathode 1 is disposed below the cathode lens 5. Next, a state in which a voltage is applied to the cathode lens 5 (power on) is set, and a state in which no voltage is applied to other lens systems (power off) is set. The adjustment electron beam T emitted from the cold cathode 1 enters the cathode lens 5. At the cathode lens 5, the adjustment electron beam T receives a force due to the electric field of the cathode lens 5. The adjustment electron beam T that has passed through the cathode lens 5 passes through the first aligner 9, the aperture stop AS, the e-cross bee 6, and the secondary optical system in the same manner as the secondary electron beam K described above. The light enters the electron beam detector 14.
[0014]
As with the secondary electron beam K, information on the adjustment electron beam T incident on the electron beam detector 14 is transmitted in the order of the relay lens 40, the image sensor 41, the control unit 42, and the CPU 43 and then to the display 44. Then, an image of a point pattern is displayed on the display 44.
As described above, since no voltage is applied to the lens system other than the cathode lens 5, the force that the adjustment electron beam T receives before reaching the electron beam detector 14 depends on the electric field of the cathode lens 5. It is only power. In this state, when the voltage of the cathode lens 5 is changed (wobbled) in an alternating manner, the image of the point pattern on the detection surface of the electron beam detector 14 is defocused. At this time, if the point pattern is not on the optical axis of the cathode lens 5, the image of the point pattern on the display 44 moves in a plane orthogonal to the optical axis with this defocusing. The X stage 31 and the Y stage 32 are adjusted and moved so that the dot pattern image on the display 44 does not move regardless of defocusing. In this way, the position of the point pattern at which the image of the point pattern on the display 44 stops moving is the position on the optical axis of the cathode lens 5. Thus, the optical axis of the cathode lens 5 was adjusted.
[0015]
Next, in addition to the cathode lens 5, a voltage is applied to the imaging lens front group 7. At that time, the voltage condition is determined so that an image of the spot pattern of the cold cathode 1 is formed on the electron beam detector 14, and the voltage is changed in an alternating manner as in the optical axis adjustment of the cathode lens 5. However, the voltage of the first aligner 9 is adjusted so that the dot pattern image on the display 44 does not move. Thereby, the optical axis of the imaging lens front group 7 can be made to coincide with the optical axis of the cathode lens 5 adjusted previously.
Next, in addition to the cathode lens 5 and the imaging lens front group 7, a voltage is applied to the imaging lens rear group 8. At that time, the voltage condition is determined so that the image of the point pattern is formed on the electron beam detector 14, and the image of the point pattern on the display 44 does not move while changing the voltage in an alternating manner. The voltage of the second aligner 10 is adjusted. Thereby, the optical axis of the imaging lens rear group 8 can be made to coincide with the optical axis of the sword lens 5 and the imaging lens front group 7 adjusted in advance.
[0016]
Finally, the voltage of the third aligner 11 is adjusted to move the point pattern image to the center of the electron beam detector 14 so that the center of the electron beam detector 14 and the optical axis coincide. Thus, the optical axes of the cathode lens 5 and the secondary optical system were adjusted.
The adjusting electron beam T can be accelerated by providing a potential difference between the cold cathode 1 and the first electrode located on the object plane side of the cathode lens 5 by the acceleration power supply 47.
As described above, since the optical axes of the cathode lens 5 and the secondary optical system are adjusted individually, the primary optical system and the e-cross bee 6 are adjusted as the next step. At this time, the Wien condition for the secondary optical system of the e-cross bee 6 is determined so that the dot pattern image on the display 44 does not move even when the e-cross bee 6 is turned on / off. .
As described above, in this embodiment, it is possible to quickly and accurately match the illumination visual field of the primary optical system and the observation visual field of the secondary optical system to obtain a good image of the charged particle beam optical system.
[0017]
In this embodiment, a point pattern is formed on the cold cathode 1 and the optical axis of the secondary optical system is adjusted. Similarly, a point image on the electron beam detector 14 is obtained using the point pattern. Various aberrations can be analyzed by detecting an intensity distribution and an image when defocused.
Further, if a line and space pattern is used instead of the point pattern for the pattern formed on the cold cathode 1, the spherical aberration of the secondary optical system can be corrected. Further, if a cross mark or L-shaped mark is used, the distortion of the secondary optical system can be corrected.
In the present embodiment, the cold cathode 1 is used as the adjustment source, but an adjustment electron gun may be used instead of the cold cathode 1. The light emission shape of the adjustment radiation source on the object plane may be at least one of a point shape, a line shape, a cross shape, or an L shape.
[0018]
In this embodiment, the trajectory of the primary electron beam S is bent by the e-cross bee 6 and the secondary electron beam K is caused to advance straight. On the contrary, the primary electron beam S is caused to advance straight. It is also possible to bend the trajectory of the secondary electron beam K.
In this embodiment, the charged particle beam projection optical system using an electron beam is shown. However, a charged particle beam map projection optical system using an ion beam may be used instead of the electron beam.
The charged particle beam mapping optical system of the present embodiment is a so-called surface-to-surface charged particle beam mapping optical system that illuminates the object surface with an electron beam from a radiation source and forms an image on the image surface. The present invention can be easily applied to a semiconductor exposure apparatus or the like, not as a single apparatus for an observation apparatus and an inspection apparatus.
[0019]
【The invention's effect】
As described above, in the present invention, a self-emitting adjustment source is provided on the object surface, and the objective optical system and the imaging optical system can be adjusted independently, so that a charged particle beam map that can be adjusted quickly and accurately. A projection optical system and an adjustment method thereof can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a charged particle beam mapping optical system according to an embodiment of the present invention.
2A is a schematic diagram of E-Cross Bee of a charged particle beam projection optical system, FIG. 2B is a schematic diagram showing an electric field and a magnetic field acting on a primary electron beam, and FIG. 2C is a secondary electron. It is the schematic which shows the electric field and magnetic field which act on a beam.
FIG. 3 is a schematic view showing a charged particle beam mapping projection optical system during adjustment according to an embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Cold cathode 2 ... Primary column 3 ... Secondary column 4 ... Chamber 5 ... Cathode lens 6 ... E cross bee 7 ... Imaging lens front group 8 ... Imaging lens rear group 9 ... First aligner 10 ... First 2 aligner 11 ... third aligner 12, 13 ... stigmeter 14 ... electron beam detector 15 ... electron gun 17, 18, 19 ... irradiation lenses 23, 24 ... aligner 25 ... scanning aligner 26 ... aperture 30 ... sample 31 ... X Stage 32 ... Y stage 33 ... X moving mirror 34 ... X interferometer 35 ... X stage drive unit 40 ... relay lens 41 ... imaging element 42 ... control unit 43 ... CPU 44 ... display 45 ... first voltage control unit 46 ... second Voltage control unit 47 ... acceleration power sources FS1, FS2 ... field stop AS ... aperture stop T ... adjusting electron beam S ... primary electron beam K ... secondary electron beam

Claims (7)

An irradiation charged particle beam emitted from an irradiation source is incident on an optical path switching unit through an irradiation optical system, and the irradiation charged particle beam that has passed through the optical path switching unit is incident on an object surface through an objective optical system. The observation charged particle beam emitted from the object plane is incident on the optical path switching means via the objective optical system, and the observation charge is applied in a direction different from the direction reaching the irradiation source by the optical path switching means. In the charged particle beam mapping projection optical system for guiding the particle beam and causing the observation charged particle beam after passing through the optical path switching unit to enter the detection unit via the imaging optical system,
An charged particle beam mapping projection optical system comprising an adjustment source that emits an adjustment charged particle beam so as to be disposed at a position on the object plane. 2. The charged particle beam projection according to claim 1, wherein a light emission shape on the object plane of the adjustment radiation source has at least one of a point shape, a line shape, a cross shape, and an L shape. Optical system. 3. The charged particle beam mapping optical system according to claim 1, wherein the adjusting charged particle beam is an electron beam. 4. The charged particle beam projection optical system according to claim 1, wherein an initial energy of the adjustment charged particle beam is equal to an initial energy of the observation charged particle beam. The charged particle beam mapping optical system according to claim 1, wherein the adjustment radiation source is formed of a cold cathode. The potential difference for accelerating the charged particle beam for adjustment is provided between the adjustment source and the object-side surface of the objective optical system. The charged particle beam mapping optical system described. An irradiation charged particle beam emitted from an irradiation source is incident on an optical path switching unit through an irradiation optical system, and the irradiation charged particle beam that has passed through the optical path switching unit is incident on an object surface through an objective optical system. The observation charged particle beam emitted from the object plane is incident on the optical path switching means via the objective optical system, and the observation charge is applied in a direction different from the direction reaching the irradiation source by the optical path switching means. In a method for adjusting a charged particle beam projection optical system that guides a particle beam and causes the observation charged particle beam after passing through the optical path switching unit to enter a detection unit via an imaging optical system,
Arranging an adjusting radiation source for emitting an adjusting charged particle beam at a position of the object plane;
Adjusting the optical axis or aberration of the objective optical system by applying a voltage only to the objective optical system;
A charged particle beam mapping comprising the step of applying a voltage to the imaging optical system in addition to the objective optical system to adjust the optical axis of the imaging optical system or to evaluate or adjust the aberration. Adjustment method of projection optical system.

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