Patent Grant
7355174
The present invention relates to a charged-particle beam emitting device using a charged-particle beam such as an electron beam or ion beam. More particularly, it relates to a charged-particle beam emitting device and its optical-axis adjusting method which are preferable for acquiring a high-resolution image by suppressing a degradation in the image resolution even when the charged-particle beam is tilted on a sample.
In a charged-particle beam emitting device the representative of which is a scanning electron microscope, a narrowly converged charged-particle beam is scanned on a sample, thereby acquiring desired information (e.g., sample image) from the sample. In the charged-particle beam emitting device like this, implementation of high resolution has been in progress year by year. Simultaneously, in recent years, it has become necessary to tilt the charged-particle beam with respect to the sample so as to acquire tilted image of the sample.
In order to irradiate the sample with the charged-particle beam in the state of being tilted, there exists a method of utilizing the swing-back effect of the charged-particle beam in the off-axis of an objective lens. For example, in JP-U-55-48610 and JP-A-2-33843, the following method has been disclosed: The charged-particle beam is caused to enter the off-axis of the objective lens, thereby utilizing the converging effect (swing-back effect) of the objective lens. Also, in JP-A-2000-348658, the following technology has been disclosed: There is provided a two-stage deflecting unit for deflecting the charged-particle beam in mutually opposite directions within a converging magnetic field of the objective lens. This allows correction of transverse chromatic aberration which occurs when the charged-particle beam is tilted in the off-axis of the objective lens. Also, in JP-A-2001-15055, the following technology has been disclosed: A deflecting unit for causing the charged-particle beam to pass through the off-axis of the objective lens is provided on the electron-source closer side than the objective lens. Then, the chromatic aberration (transverse chromatic aberration) which occurs in the off-axis of the objective lens is corrected using a Wien filter which is provided on the electron-source closer side than the objective lens. This allows a reduction in the image-resolution degradation at the time when the charged-particle beam is tilted. Moreover, in WO 01/33603, the following technology has been disclosed: The Wien filter, which generates orthogonal electrostatic and electromagnetic fields in arbitrary two-dimensional directions orthogonal to the optical axis, is located on the optical axis on the electron-source closer side than the objective lens. This allows correction of the transverse chromatic aberration in an arbitrary direction.
In any one of the above-described conventional technologies, the charged-particle beam is tilted with respect to the sample by utilizing the swing-back effect of the beam in the off-axis of the objective lens. Meanwhile, in order to mutually cancel out aberrations which occur in the off-axes of a plurality of converging lenses including the objective lens, the following operation has been required: Namely, adjustment of an astigmatic corrector and adjustment of the optical axis are repeated, thereby driving the optical axis so that an image acquired turns out to become the sharpest one. This operation, however, requires significant amount of skill and experience. Accordingly, an axis adjusting method or axis adjusting function which is simpler and easier has been desired.
It is an object of the present invention to provide a charged-particle beam emitting device which allows a high-resolution image to be easily acquired at a high-angle beam tilt by using the axis adjusting method and axis adjusting function which are simple and necessitate no skilled person.
In order to accomplish the above-described object, there is provided a unit for changing control quantities for all of optical components (such as a correction lens) simultaneously and by predetermined quantities. Here, these optical components make contributions to correction of the aberration which occurs when the charged-particle beam is tilted in the off-axis of the objective lens.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.
Referring to
The condition that the transverse chromatic aberration is equal to 0 means that the position of a trajectory for beam center on the sample does not change depending on a difference in the energy. In
M×(P1-P1′)+(P2-P2)=0 (1)
turns out to become the condition under which the transverse chromatic aberrations will be cancelled with each other (i.e., aberration correcting condition).
As the operation of finding out the condition in the expression (1), in substitution for changing the energy of the beam, changing the excitation current of the objective lens 7 is also preferable. Letting the excitation current of the objective lens 7 be Iobj, winding number of a excitation coil be Nobj, and the acceleration voltage (the beam energy) be Vi, the lens effect (strength of the lens: Ex) on the primary charged-particle beam is represented by
Ex=(Iobj×Nobj)/√{square root over (Vi.)} (2)
Now, assuming that the beam energy has been changed from Vi to Vi+ΔV, the lens strength (Ex) will change as follows:
Ex→Ex+ΔEx. (3)
Here, from the relation in the expression (2), ΔEx is represented by the following expression:
ΔEx=−0.5×Ex×(ΔV/Vi). (4)
As an operation of creating the same change ΔEx as the one in the expression (4) by changing the excitation current from Iobj to Iobj+ΔIobj, it is preferable to set the change quantity ΔIobj in the excitation current as being
ΔIobj=−0.5×Iobj×(ΔV/Vi). (5)
Similarly, when letting the excitation current of the condenser lens 6 be Ic, setting of
ΔIc=−0.5×Ic×(ΔV/Vi) (6)
makes it possible to create, in the condenser lens 6, the same excitation change as the one created when the beam energy has been changed from Vi to Vi+ΔV.
Accordingly, assume that the excitation currents Iobj and Ic are simultaneously changed by the values (ΔIobj, ΔIc) indicated by the expression (5) and the expression (6). This makes it possible to create the same state as the one created when the beam energy has been changed by the quantity ΔV. This fact shows the following findings: Namely, changing Iobj and Ic at the change rates (ΔI/I) determined by the energy of the beam passing through the respective lenses is equivalent to changing the beam energy. When applying an acceleration electric-field or deceleration electric-field to the objective-lens portion, there occurs a change in the average energy (Vi) of the charged particles passing through the objective-lens portion. As a result, the current change rate in the objective lens and the current change rate in the condenser lens become different values from each other. In whatever case, the lens currents Iobj and Ic are simultaneously changed by the quantities ΔIobj and ΔIc at the current change rates determined in correspondence with the beam energy passing through the respective lenses. Then, the optical axis is adjusted so that a change in the beam position at this time (i.e., displacement of the field-of-view) will become its minimum. This operation allows implementation of the condition under which the transverse chromatic aberrations will be cancelled with each other as the combined effect of the objective lens 7 and the condenser lens 6 (i.e., aberration correcting condition). This operation turns out to become an exceedingly easier adjustment as compared with the adjustments in the conventional technologies. This is because this operation is the adjustment of minimizing the displacement of the image, and because, in the conventional technologies, changes in the picture quality and image resolution are directly judged which are dependent on human's sensory capabilities and qualities.
Letting aligner control value (complex-number representation, j: imaginary-number unit) for the optical-axis adjustment be WAL=XAL+j ·YAL, the relation between the aligner control value (WAL) and the field-of-view's displacement (ΔW) at the time of changing the lens currents is represented by the following expression: Here, control values XAL and YAL mean current values caused to flow through the aligner, and more practically, mean numerical values set to a DAC (Digital-to-Analogue Converter) for setting the current values.
ΔW=A×(C+D×WAL) (7)
Here, A denotes a coefficient determined by the change quantities in the excitation currents, C denotes an initial axis-shift quantity, and D denotes a coefficient dependent on action condition of the electron-optics system and position of the aligner. Excluding A, both C and D are given by complex numbers.
According to the present invention, it becomes possible to easily make the axis adjustment in the case where the beam tilt is implemented by utilizing the converging effect of the objective lens. In addition thereto, it also becomes possible to implement the high-accuracy automatization of the axis adjusting operation.
Hereinafter, the explanation will be given below concerning embodiments of the present invention.
The primary electron beam 4 is scanned on the sample 10 in a two-dimensional manner by a scanning coil 9 which is controlled by a scanning-coil control power-supply 24. At this time, by the irradiation with the primary electron beam 4, a secondary signal 12, such as secondary electrons, is generated from the sample 10. Then, after traveling onto the upper portion of the objective lens 7, the secondary signal 12 is separated from the primary electrons by a device 11 which produces orthogonal-electrostatic-and-electromagnetic-fields for separating the secondary signal 12. The secondary signal 12 separated is detected by a secondary-signal detector 13. Moreover, the secondary signal 12 detected by the secondary-signal detector 13, after being amplified by a signal amplifier 14, is transferred to an image memory 25. Furthermore, the secondary signal 12 is displayed as a sample image by an image display device 26.
A two-stage deflecting coil 51 is located at the same position as that of the scanning coil 9. The deflecting coil 51 allows position of the primary electron beam 4 entering the objective lens 7 to be controlled in a two-dimensional manner by a tilt control power-supply 31 so that object point of the objective lens 7 becomes the deflection fulcrum. An astigmatism correction coil 53, which is located in the vicinity of the converging lens 6, is controlled in conjunction with beam tilt conditions by an astigmatic correction power-supply 33. A two-stage deflecting coil 52 is located between the converging lens 6 and the diaphragm plate 8. The deflecting coil 52 allows position of the primary electron beam 4 entering the converging lens 6 to be controlled in a two-dimensional manner by an aberration control power-supply 32 so that object point of the converging lens 6 becomes the deflection fulcrum. In addition to a primary-electron-beam position control signal for permitting the object point of the objective lens 7 to become the deflection fulcrum, a control signal for permitting the irradiation position with the primary electron beam on the sample to be controlled in a two-dimensional manner can also be caused to flow along the deflecting coil 51. This control signal makes it possible to correct shift in the irradiation position in conjunction with the beam tilt conditions. The deflecting coil 51 also carries out the function as the above-described aligner.
A sample stage 15 is capable of displacing the sample 10 in at least two directions (X direction and Y direction) within a plane perpendicular to the primary electron beam. An input device 42 makes it possible to specify image fetching conditions (such as scanning velocity and acceleration voltage), the beam tilt conditions (such as tilt direction and tilt angle), output of images, saving of the images into a storage device 41, and the like.
Concerning an embodiment for correcting the transverse chromatic aberration which occurs at the time of the beam tilt by the scanning electron microscope having the configuration illustrated in
In correspondence with the set condition for the beam tilt angle, the deflecting coil 52 deflects the primary beam 4 so that the object point of the converging lens 6 becomes the deflection fulcrum, thereby causing the primary beam 4 to enter the off-axis of the converging lens 6. Next, the primary beam 4, which has entered the off-axis of the converging lens 6, is swung back by the lens effect of the converging lens 6, thereby attaining to a point P1. At the convergence point of the converging lens 6, the deflecting coil 51 is located. The deflecting coil 51 causes the primary beam 4 to enter the off-axis of the objective lens 7. Next, the primary beam 4, which has entered the off-axis of the objective lens 7, is swung back by the lens effect of the objective lens 7, thereby entering the upper surface of the sample 10 in a state of being tilted. The control quantities for the deflecting coils 51 and 52 are set in correspondence with the beam tilt angle and in accordance with a predetermined relationship therebetween. Ideally, the traverse aberrations (i.e., chromatic aberration and coma aberration) of the objective lens 7 are cancelled out by the off-axis aberrations of the converging lens 6. Under actual circumstances, however, the canceling between the off-axis aberrations of the objective lens 7 and the off-axis aberrations of the converging lens 6 cannot completely be achieved because of factors such as slight amount of axis shift and control error. Accordingly, in the present embodiment, the processing proceeds to the next axis-adjusting stage.
At the axis adjusting stage, same-phase and simultaneous variations (i.e., periodic changes in time) whose amplitudes are ΔIc and ΔIobj respectively are provided to current of the converging lens 6 and current of the objective lens 7. Incidentally, the amplitudes ΔIc and ΔIobj are controlled such that the following expression will be satisfied:
(ΔIc/Ic)=(ΔIobj/Iobj) (8)
If field-of-view of the SEM image is displaced in synchronization with the variations in these lens currents, it means that the correcting condition for correcting the off-axis aberrations has become irrelevant. Consequently, the deflecting coil 51 is adjusted so that the field-of-view displacement of the SEM image will become its minimum, thereby adjusting the primary-beam incidence position into the objective lens 7. When the field-of-view displacement has become its minimum in this operation, the off-axis aberrations of the converging lens 6 and the off-axis aberrations of the objective lens 7 are cancelled with each other. This permits a high-resolution SEM image to be acquired in the state where the beam is tilted with respect to the sample. Incidentally, like the case where a voltage is applied to the sample, if the primary beam passing through the objective-lens area and the primary beam passing through the converging-lens area differ from each other in their energies,
(ΔIc/Ic)=k·(ΔIobj/Iobj) (9)
is employed in substitution for the expression (8). Here, k denotes a coefficient dependent on the difference between the beam energies in the objective-lens area and the converging-lens area. The coefficient k can be determined in advance by calculation or experiment.
In the present embodiment, referring to
In correspondence with the beam tilt angle relative to the sample, the control conditions for the deflecting coils 52 and 51 illustrated in
(i) S1
In this processing, based on the expression (9), the current change quantities (ΔIc, ΔIobj) of the converging lens 6 and the objective lens 7 are calculated.
(ii) S2 to S4
From two pieces of images acquired by changing the lens currents, field-of-view shift quantity (W1) is detected.
(iii) S5
A change quantity ΔA1 determined in advance is added to the aligner (i.e., the deflecting coil 51), thereby changing the aligner control value.
(iv) S6 to S7
The processings at S2 to S4 are repeated, thereby detecting field-of-view shift quantity (W2) between two pieces of images with respect to the aligner whose control value has been changed.
(v) S8
Optimum control value for the aligner is calculated from the field-of-view shift quantities W1 and W2. This calculation can be performed based on the expression (7). Namely, from the field-of-view shift quantity W1 at the time of WAL=A0, the following relation:
W1=A×(C+D×A0) (10)
can be acquired. Next, from the field-of-view shift quantity W2 at the time of WAL=A0+ΔA1, the following relation:
W2=A×(C+D×(A0ΔA1)) (11)
can be acquired.
From the expressions (10) and (11), the unknown quantities C and D can be solved as follows:
C=(1/A)·[W1−(A0/ΔA1)(W2−W1)] (12)
D=(1/A)·(W2−W1)/ΔA1. (13)
The optimum control value for the aligner, which is the condition under which ΔW=0 is implemented in the expression (7), is acquired by
WAL=−C/D. (14)
Accordingly, the optimum control value for the aligner (i.e., the correcting condition for correcting the off-axis aberrations) can be acquired from the expressions (12) and (13) as
WAL=−[W1−(A0/ΔA1)(W2−W1)]/[(W2−W1)/ΔA1] (15).
Consequently, even if the unknown quantity A is contained in the expressions (12) and (13), the optimum control value for the aligner can be calculated.
(vi) S9
The aligner control value WAL calculated by the expression (15) is set to the aligner.
In the Wien filter 70, an electric field and a magnetic field are generated which are orthogonal to each other. Moreover, magnitudes of these electric and magnetic fields are set as follows: Namely, these fields exert deflecting effects in opposite directions on the electrons which have energy Vi and are passing through these fields, so that these deflecting effects will be cancelled with each other. The magnitudes of these electric and magnetic fields satisfying this condition can be easily set by making an adjustment for allowing one and the same field-of-view to be acquired before and after the operation of the Wien filter 70. At this time, letting the field-of-view shift quantities caused to occur by the electric field and the magnetic field be rE and rB, respectively, the Wien condition for preventing the primary beam 4 from being deflected is represented by the following expression:
rE=rB. (16)
Meanwhile, the deflection quantities (rE, rB) caused to occur by the electric field and the magnetic field are represented by the following expressions, letting the acceleration voltage be Vi, voltage for inducing the electric field be VE, and excitation current for inducing the magnetic field be IB:
rE=KE·VE/Vi (17)
rB=KB·IB/Vi1/2. (18)
Here, KE and KB denote coefficients dependent on factors such as configuration of electrodes and the coils and layout of the Wien filter. In this way, in the electron beam which has passed through the Wien filter, the electrons having the average energy (Vi) are not deflected by the difference in the dependence of the deflection quantities (rE, rB) upon the acceleration voltage Vi. A variation ΔV in the energy, however, causes an energy dispersion (i.e., chromatic aberration) Δrc to occur on the sample. This energy dispersion is represented by the following expression:
Δrc=0.5×rB×(ΔV/Vi)=0.5×rE×(ΔV/Vi). (19)
In order to detect magnitude of the energy dispersion Δrc as the displacement of the image, a change ΔIB in the excitation current IB is determined which is equivalent to changing the average energy of the beam from Vi to Vi+ΔV. As a result of this, the following expression is acquired from basically the same idea as the one in the case of the converging lens:
ΔIB=−0.5×IB×(ΔV/Vi). (20)
Namely, when ΔIobj calculated by the expression (5) and the change ΔIBin the excitation current IB of the Wien filter calculated by the expression (20) have been simultaneously provided, if there occurs none of the displacement of the image, it means that the chromatic aberration has been corrected. Meanwhile, changing the voltage VE of the Wien filter also makes it possible to implement the same effect. A voltage change ΔVE which is equivalent to the change ΔV in the beam energy becomes equal to
ΔVE=−VE×(ΔV/Vi). (21)
Accordingly, the excitation current Iobj of the objective lens and the operating condition (the current or voltage) on the Wien filter are simultaneously changed. This makes it possible to determine the magnitude of the Wien filter 70 or the condition for the deflecting coils 61 and 62 so that the displacement of the field-of-view will become its minimum. Also, executing processing steps S11 to S19 in
(i) S11
In this processing, based on the expression (20), the current change quantities (ΔIobj, ΔIB) of the objective lens and the Wien filter are calculated.
(ii) S12 to S14
From two pieces of images acquired by simultaneously changing the objective-lens current and the Wien-filter current, field-of-view shift quantity (W1) is detected.
(iii) S15
A change quantity ΔA1 determined in advance is added to the aligner, thereby changing the aligner control value.
(iv) S16 to S17
The processings at S12 to S14 are repeated, thereby detecting field-of-view shift quantity (W2) between two pieces of images with respect to the aligner whose control value has been changed.
(v) S18
Optimum control value for the aligner is calculated from the field-of-view shift quantities W1 and W2. This calculation can be performed based on the expression (7). Namely, from the field-of-view shift quantity W1 at the time of WAL=A0 , the following relation:
W1A×(C+D×A0) (22)
can be acquired. Next, from the field-of-view shift quantity W2 at the time of WAL=A0+ΔA1, the following relation:
W2=A×(C+D×(A0+ΔA1)) (23)
can be acquired. From the expressions (22) and (23), the unknown quantities C and D can be solved as follows:
C=(1/A)·[W1−(A0/ΔA1)(W2−W1)] (24)
D=(1/A)·(W2-W1)/ΔA1. (25)
The optimum control value for the aligner, which is the condition under which ΔW=0 is implemented in the expression (7), is acquired by
WAL=−C/D. (26)
Accordingly, the optimum control value for the aligner (i.e., the correcting condition for correcting the off-axis aberrations) can be acquired from the expressions (24) and (25) as
WAL=−[W1−(A0/ΔA1)(W2−W1)]/[(W2−W1)/ΔA1] (27).
Consequently, even if the unknown quantity A is contained in the expressions (24) and (25), the optimum control value for the aligner can be calculated.
(vi) S19
The aligner control value WAL calculated by the expression (27) is set to the aligner.
Incidentally, at S11, the same result can be acquired using ΔVE in the expression (21) instead of using the current change quantity ΔIB of the Wien filter.
In an embodiment illustrated in
In the case of the present embodiment, the entire chromatic aberration which will occur in accompaniment with the beam tilt occurs within the objective lens 7. As a consequence, an optical-axis condition under which the displacement of the sample image disappears in association with the change in the excitation current of the objective lens 7 turns out to become the condition for preventing the chromatic aberration in accompaniment with the beam tilt from occurring. Incidentally, a coil for creating the optical-axis condition like this is not limited to the deflecting coil 61b. For example, combination of the deflecting coils 61b and 62b, or some combination other than this one also allows implementation of the optical-axis condition like this. In whatever case, the adjusting-operation of minimizing the field-of-view's displacement while watching the field-of-view's displacement allows implementation of the chromatic-aberration correcting condition. This makes it possible to make the adjustment easily. Furthermore, executing processing steps S21 to S29 in
(i) S21
In this processing, the current change quantity (ΔIobj) is calculated in correspondence with the set value for the objective-lens current.
(ii) S22 to S24
From images acquired by changing the objective-lens current, field-of-view shift quantity (W1) is detected.
(iii) S25
A change quantity ΔA1 determined in advance is added to the aligner (e.g., the deflecting coil 61b), thereby changing the aligner control value.
(iv) S26 to S27
The processings at S22 to S24 are repeated, thereby detecting field-of-view shift quantity (W2) with respect to the aligner whose control value has been changed.
(v) S28
Optimum control value for the aligner is calculated from the field-of-view shift quantities W1 and W2. This calculation can be performed based on the expression (7). Namely, from the field-of-view shift quantity W1 at the time of WAL=A0, the following relation:
W1=A×(C+D×A0) (28)
can be acquired. Next, from the field-of-view shift quantity W2 at the time of WAL=A0+ΔA1, the following relation:
W2A×(C+D×(A0+ΔA1)) (29)
can be acquired.
From the expressions (28) and (29), the unknown quantities C and D can be solved as follows:
C=(1/A)·[W1−(A0/ΔA1)(W2−W1)] (30)
D=(1/A)·(W2−W1)/ΔA1. (31)
The optimum control value for the aligner, which is the condition under which ΔW=0 is implemented in the expression (7), is acquired by
WAL=−C/D. (32)
Accordingly, the optimum control value for the aligner (i.e., the correcting condition for correcting the off-axis aberrations) can be acquired from the expressions (30) and (31) as
WAL=−[W1−(A0/ΔA1)(W2−W1)]/[(W2−W1)/ΔA1] (33).
Consequently, even if the unknown quantity A is contained in the expressions (30) and (31), the optimum control value for the aligner can be calculated.
(vi) S29
The aligner control value WAL calculated by the expression (33) is set to the aligner.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-189442 | Jun 2004 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 6603128 | Maehara et al. | Aug 2003 | B2 |
| 6614026 | Adamec | Sep 2003 | B1 |
| 20050236570 | Morokuma et al. | Oct 2005 | A1 |
| 20060060781 | Watanabe et al. | Mar 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| 55-048610 | Mar 1980 | JP |
| 02-033843 | Feb 1990 | JP |
| 2003-348658 | Dec 2000 | JP |
| 2001-015055 | Jan 2001 | JP |
| 2001-283759 | Dec 2001 | JP |
| 2002-352758 | Dec 2002 | JP |
| 2003-022771 | Jan 2003 | JP |
| 2004-127930 | Apr 2004 | JP |
| WO 0133603 | May 2001 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20050285036 A1 | Dec 2005 | US |
