JPH0542102B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a scanning electron microscope with a beam locking function.

[Prior Art] Recently, crystal analysis of the surface of a bulk sample has been widely performed by observing electron channeling patterns using a scanning electron microscope. The electron channeling pattern uses an electron beam with good parallelism, and while the irradiation position of the electron beam on the sample is fixed at one point, the incident angle θ and the azimuth angle φ of the electron beam are adjusted.
By scanning and performing so-called beam locking, detection signals such as reflected electrons and secondary electrons obtained in conjunction with this scanning are supplied to a display means such as a cathode ray tube that is scanned in synchronization with this scanning. You can get it.

Figures 1a and 1b are ray diagrams for obtaining a normal scanning image and an electron channeling pattern in a scanning electron microscope, respectively. In both figures, 1 is an electron gun and 2 is a focusing lens. , 3 is an objective lens, and 4 is a sample. 6 is the X direction and Y direction
A first stage deflector is composed of a pair of deflection coils for each direction, and 7 is a second stage deflector composed of a pair of deflection coils, and EB indicates an electron beam. As is clear from these figures, in the scanning image observation mode, a crossover image 5 of the electron gun 1 is formed by the focusing lens 2, and this image 5 is projected onto the sample surface S by the objective lens 3. This projected point is scanned on the sample surface S by the deflectors 6 and 7. On the other hand, in the electron channeling pattern observation mode, the crossover image of the electron gun passes through the focusing lens 2 to the front focal plane 8 of the objective lens 3.
The electron beam is excited so that a parallel electron beam is formed, and the sample 4 is irradiated with a parallel electron beam. Therefore, when the electron beam EB is deflected by the first stage deflector 6, the crossover image 5' moves on the focal plane 8 of the objective lens 3, so that the electron beam EB incident on the sample 4 is The incident angle θ and the azimuth angle φ can be changed while the point is fixed at one point on the optical axis C. However, in such a conventional device, due to the influence of spherical aberration of the objective lens and the object plane of the objective lens not coinciding with the principal deflection surface of the deflector 6,
When the electron beam was rocked, the irradiation position of the electron beam on the sample surface actually moved by several tens of micrometers, making it impossible to analyze minute portions of the sample. In order to solve these shortcomings,
For example, it is conceivable to flow a correction current in accordance with the deflection angle of the electron beam through the objective lens, but if the correction current is passed directly through the objective lens with large inductance, the objective lens will not be able to fully respond to high-speed angular scanning. Spherical aberration cannot be corrected sufficiently.
Another method is to place an auxiliary lens with a small inductance near the objective lens, and change the excitation intensity of this auxiliary lens according to the deflection angle of the electron beam, so that the point of incidence of the electron beam on the sample is fixed. Although it is possible to do so, it is not only complicated to correct the axis misalignment between the auxiliary lens and the objective lens, but also
Providing an auxiliary lens also increases the manufacturing cost of the device.

[Object of the Invention] The present invention has been made to solve these conventional drawbacks, and has a relatively simple structure with good operability that can prevent fluctuations in the irradiation position of the electron beam due to beam locking. The aim is to provide a device that is inexpensive to manufacture.

[Structure of the Invention] The present invention is a scanning electron microscope that enables observation by controlling a deflector to switch between a scanning image and an electron channeling pattern by electron beam rocking. In a scanning electron microscope configured to scan an incident angle θ and an azimuth angle φ with respect to a sample, an electron channeling pattern observation mode is performed based on a signal representing the maximum value α of the incident angle θ in the electron beam locking state. The present invention is characterized in that it includes means for aligning the position of the circle of least confusion formed when the electron beam is tilted to the maximum with the position of the sample surface.

[Operation of the Invention] Hereinafter, the basic idea of the present invention will be explained based on FIGS. 2 and 3, in which the same components as in FIG. 1 are given the same numbers.

As shown in FIG. 2, in the scanning image observation mode, the objective lens 3 is excited so as to form the crossover image 5 on the sample surface S as shown by the optical path L1. Therefore, if the excitation of the objective lens 3 remains unchanged when the mode shifts to the electron channeling pattern observation mode, the main deflection surface 9 of the first deflector 6 is located below the crossover image 5. Therefore, when rocking is performed by the first deflector 6, the electron beam should be rocked by the sample surface S around the point 10 below Δf as shown in the optical path L2. The incident point of the electron beam changes. Furthermore, the second
In the figure, the spherical aberration of the objective lens 3 is not taken into consideration, but since spherical aberration actually exists, the fluctuation of the incident point due to rocking becomes even larger. That is, as shown in FIG. 3, paraxial rays L2 and L2' emitted from the intersection of the main deflection surface 9 of the first deflector 6 and the optical axis C are focused on the point 10 below the sample surface S. However, due to the spherical aberration of the objective lens 3, the rays with higher-order trajectories L3 and L3' are located at points above the sample surface S.
The light is focused on Cm, and a point (circle of least confusion) where the irradiation position moves the least during locking is created at this point Cm. Therefore, if the position of the sample surface S can be aligned with this point Cm, fluctuations due to rocking of the electron beam irradiation position can be minimized.

Now, the point where the minimum disturbance circle is formed with the sample surface S
The distance to Cm is ΔF, Cs is the spherical aberration coefficient of the objective lens 3, α is the rocking angle that is the maximum value of θ, and Z ₀ and Z ₀ ' are the distance between the crossover image 5, the objective lens 3, and the first deflector. 6 and the distance between objective lens 3,
If Zi and Zi' are the distances between the objective lens 3 and the sample surface S, and the distance between the objective lens 3 and the image 10 of the principal deflection surface 9, and d is the diameter of the circle of least confusion, then 1/Z ₀ +1Zi= 1/Z ₀ ′+1/Zi′ Zi′−Zi=Δf On the other hand, ΔF+Δf=3/4・Cs・α ² d=1/2・Cs・α ³ , so ΔF=3/4Csα ² −Zi ² ( Z ₀ −Z ₀ ′)／Z ₀ Z ₀ ′＋Z
₀ ′Zi−Z ₀ Zi……(1). When the electron beam probe current is constant, (1)
The second term in the equation is a constant, and since the change in the spherical aberration coefficient Cs when the excitation intensity of the objective lens 3 is changed is negligibly small, ΔF can be considered to depend only on the rocking angle α in practical terms. Therefore, when switching the observation mode from the scanning image mode to the electronic channeling pattern mode, the distance between the focal point of the objective lens 3 and the sample surface S is changed by an amount corresponding to ΔF given by equation (1) above. By doing so, it is possible to analyze a minute area while minimizing the movement of the electron beam irradiation point due to rocking.

[Example] The present invention is based on the above-mentioned idea, and the following:
Embodiments of the present invention will be described in detail based on the drawings.

FIG. 4 is for showing one embodiment of the present invention, and in FIG. 4, the same components as in FIG. 1 are given the same numbers.

In the figure, 11 is a scanning signal generation circuit, and each scanning signal from this circuit 11 is sent to a variable gain amplifier 12.
and 13 to the first and second deflectors 6 and 7, and the cathode ray tube 1
4 and is supplied to the deflection coil D. 15 is a switch circuit for controlling the supply of the scanning signal to the second deflector 7 when the observation mode is switched between the scanning image observation mode and the electronic channeling pattern observation mode. 16 is a locking angle designation signal generation circuit that generates a signal to designate the locking angle of the electron beam when observing an electron channeling pattern;
The signal from 6 is supplied to a variable gain amplifier 12. 17 and 18 are excitation power sources for the focusing and objective lenses 2 and 3, respectively. Objective lens excitation power supply 1
8 is controlled based on a control signal from an excitation control circuit 19. The excitation control circuit 19 includes a first circuit 19a that generates an excitation designation signal for observing a scanned image, a second circuit 19b that generates an excitation designation signal for observing an electronic channeling pattern, and an output signal from the circuit 19. It consists of a switch circuit 19c for switching between the output signals of these two circuits 19a and 19b. A signal representing the locking angle α from the locking angle designation signal generation circuit 16 is supplied to the circuit 19b.
b is a signal for exciting the objective lens 3 based on the specified signal from the circuit 16 so as to form the circle of least confusion Cm at a position distant from the position specified by the circuit 19a by ΔF in equation (1). Occur. Further, the switch circuit 15 and the switch 19c are designed to work together. 20 is electron beam EB sample 4
This detector 2 is a detector for detecting reflected electrons etc. generated from the sample 4 by irradiating the
The output signal from 0 is supplied to the cathode ray tube 14 via an amplifier 21 as a luminance signal.

In such a configuration, when observing the scanning line, the switch circuits 15 and 19c
are connected as shown by dotted lines in FIG. 4, and the excitation power source 17 is switched so that the image of the electron gun 1 is formed by the focusing lens 2 at point 5 in FIG. 1a. Therefore, the objective lens 3 is excited so as to form the crossover image 5 on the sample surface S. or,
The amplification factors of the variable amplifiers 12 and 13 are switched to a predetermined value for scanning image observation by a control means not shown. Therefore, if a sawtooth scanning signal is generated from the scanning signal generation circuit 11, the electron beam scans the sample surface two-dimensionally, and the detection signal obtained from the detector 20 along with this scanning is transmitted to the cathode ray tube. 14, a normal scanned image is displayed on the cathode ray tube 14. Next, when an electronic channeling pattern is to be observed, the switch circuit 19c is connected as shown by the solid line in FIG. 4, and the switch circuit 15 is switched to a non-conductive state. Furthermore, the focusing lens 2 is excited so that a crossover image of the electron gun 1 is formed on the front focal plane 8 of the objective lens, as shown in FIG. 1b. Furthermore, since the excitation power source 18 of the objective lens is excited based on the excitation designation signal from the second excitation designation signal generation circuit 19, the circle of least confusion Cm of the objective lens 3 is smaller than the case shown in FIG. 1 by the above-mentioned ΔF. It is excited so that it comes to a far position.
Therefore, when a scanning signal is supplied from the scanning signal generation circuit 1, this scanning signal is supplied to the first deflector 6, and as a result, the crossover image 5' of the electron gun 1 is
Since the electron beam scans the front focal plane 8 of the objective lens, the incident angle θ and the azimuth angle φ are scanned while the incident point on the sample 4 is fixed to the electron beam. Therefore, detector 2
An electron channeling pattern is displayed on the cathode ray tube 14 based on the output signal from 0, but the focal length of the objective lens 3 is made longer by an amount corresponding to ΔF than in the scanning image observation mode. Therefore, the point Cm where the circle of least confusion is formed also moves by ΔF and exactly coincides with the position of the sample surface S. Therefore, movement of the electron beam incident point on the sample surface S can be suppressed to a minimum.

Note that the above-described embodiment is only one embodiment of the present invention, and the present invention may be implemented in many other forms.

For example, in the above-mentioned embodiment, the focal length of the objective lens was changed, but when changing the observation mode, the sample could be automatically moved by ΔF in the optical axis direction. Also good.

[Effects] The scanning electron microscope of the present invention is capable of detecting the electron beam formed when the electron beam is tilted to the maximum in the electron channeling pattern observation mode based on a signal representing the maximum value α of the incident angle θ in the electron beam locking state. Since it is equipped with a means to match the position of the circle of least confusion with the position of the sample surface, it can lock the electron beam without moving the incident point of the electron beam and create an electron channeling pattern in a minute area. Since there is no need to install an auxiliary lens, there is no need to align the axes of the auxiliary lens and the objective lens, improving operability. Furthermore, since no auxiliary lens is provided, it is possible to provide a device with a relatively simple structure and low manufacturing cost.

[Brief explanation of the drawing]

FIGS. 1a and 1b are ray diagrams of the conventional apparatus in scanning image observation mode and electronic channeling pattern observation mode, respectively; FIGS. 2 and 3 are diagrams for explaining the basic idea of the present invention; FIG. 4 is a diagram showing an embodiment of the present invention. 1: Electron gun, 2: Focusing lens, 3: Objective lens, 4: Sample, 5: Crossover image of the electron gun in scanning image observation mode, 5': Crossover image of the electron gun in electron channeling pattern observation mode , 6, 7: Deflector, 11: Scanning signal generation circuit, 12, 13: Variable gain amplifier, 14:
Cathode ray tube, 15, 19c: Switch circuit, 16:
Rocking angle designation signal generation circuit, 17, 18:
Excitation power supply, 19: Excitation control circuit, 19a, 19
b: Excitation designation signal generation circuit, 20: Detector, C:
Optical axis, Cm: point where the circle of least confusion exists, S: sample surface.

Claims (1)

[Scope of Claims] 1. A scanning electron microscope capable of observing a scanned image and an electron channeling pattern by electron beam rocking by controlling a deflector, wherein the scanning electron microscope is capable of observing an electron channeling pattern by controlling a deflector, and in the electron beam rocking, an electron channeling pattern of an electron beam is In a scanning electron microscope configured to scan the incident angle θ and the azimuth angle φ, the electron beam is scanned in the electron channeling pattern observation mode based on a signal representing the maximum value α of the incident angle θ in the electron beam locking state. 1. A scanning electron microscope characterized by comprising means for aligning the position of a circle of least confusion formed when the is tilted to the maximum with the position of a sample surface. 2. Based on the signal representing the maximum value α of the incident angle θ in the electron beam rocking state, the position of the circle of least confusion formed when the electron beam is tilted to the maximum in the electron channeling pattern observation mode is determined on the sample. 2. A scanning electron microscope according to claim 1, wherein the means for matching the position of the surface comprises means for changing the focal length of the objective lens. 3 Based on the signal representing the maximum value α of the incident angle θ in the electron beam rocking state, the position of the circle of least confusion formed when the electron beam is tilted to the maximum in the electron channeling pattern observation mode is determined on the sample. 2. A scanning electron microscope according to claim 1, wherein the means for matching the position of the surface comprises means for mechanically moving the sample along the optical axis.