The present invention relates to an electron beam apparatus such as a scanning electron microscope and a transmission electron microscope, and particularly to an electron beam apparatus suitable for observation of an electron channeling pattern.
As an application of a scanning electron microscope (SEM) or a transmission electron microscope (TEM), there is a selected area electron channeling pattern (SACP) method that can obtain a microscopic crystal structure of an observation portion. In the SACP method, an electron channeling pattern (ECP) image can be obtained by fixing an electron beam at one point on a sample and performing angular scanning that changes an incident angle of the electron beam. A crystal structure and crystal strain of the sample can be analyzed from crystal orientation distribution in the sample appearing in the ECP image.
When the angular scanning is performed to obtain the ECP image, the electron beam is deflected by a deflector, and the electron beam deflected to the outside of an optical axis is swung back by an objective lens to change the incident angle of the electron beam onto the sample. When the electron beam deviates significantly out of the axis and passes through the objective lens, the electron beam is not fixed at one point due to spherical aberration of the objective lens, an irradiation area expands, and a clear ECP image cannot be obtained. For this reason, an electron optical system that observes the ECP image by applying the SACP method is provided with a corrector that corrects the spherical aberration.
Examples of citation list include PTLs 1 and 2 which disclose an electron beam apparatus for obtaining an ECP image. PTL 1 aligns an axis of a dynamic focus lens which corrects the spherical aberration with an axis of an objective lens in order to correct spherical aberration with high accuracy, and discloses that a phase of a correction current supplied to the dynamic focus lens is controlled for this axis alignment. Further, PTL 2 discloses that even when a deflection frequency increases, in order to follow a deflection signal to correct spherical aberration, a lens unit is provided between a deflector and an objective lens and a focal length thereof is adjusted, or an electrostatic lens is provided in the objective lens and a lens action of the electrostatic lens is adjusted to correct the spherical aberration.
PTL 1: JP-A-S61-4145
PTL 2: JP-A-H04-32143
As a related art of PTL 2, a configuration is disclosed in which a spherical aberration correction coil which corrects spherical aberration is provided in a gap portion of a yoke (magnetic pole) of an objective lens. In this configuration, due to an influence of a magnetic path generated by an objective lens coil, the spherical aberration correction coil apparently acts as a cored coil, and inductance thereof increases. Therefore, there is a problem that when a deflection frequency increases, a current waveform of the spherical aberration correction coil cannot follow a deflection signal due to the inductance of the spherical aberration correction coil, it is difficult to reliably respond to lens intensity, and fast angular scanning cannot be performed.
The invention provides, in a configuration in which a spherical aberration correction coil is provided in a gap portion of a yoke (magnetic path) of an objective lens, an electron beam apparatus capable of following a deflection signal to correct spherical aberration and obtaining a clear ECP image even when a deflection frequency increases.
According to an embodiment of the invention, there is provided an electron beam apparatus performing angular scanning that changes an incident angle of an electron beam incident at a predetermined incident position on a sample, and having a plurality of scanning modes having different scanning speeds of the angular scanning, the electron beam apparatus including: a scanning coil that deflects the electron beam, an objective lens that swings back the electron beam deflected to the outside of an optical axis by the scanning coil, a correction coil disposed in a gap portion of a magnetic pole of the objective lens, and a main control unit that controls an electron optical system including the scanning coil, the objective lens, and the correction coil. The main control unit sets predetermined phase change amounts with respect to control of the scanning coil in control of the correction coil, and the predetermined phase change amounts differ depending on the plurality of scanning modes.
By setting a phase change amount suitable for a scanning mode, accurate spherical aberration correction can be performed even at different scanning speeds.
Other problems and novel features will be apparent from a description of the description and accompanying drawings.
The electronic optical system is stored in a lens body 10, optical elements constituting the electron optical system are controlled by control units 11 to 15 that control the optical elements respectively, and a main control unit 16 controls the control units 11 to 15. The main control unit 16 controls each of the control units 11 to 15 to irradiate the sample 7 with the electron beam 5 under a desired optical condition, and detects a signal electron emitted from the sample 7 by a detector (not shown) to obtain an ECP image.
Control of the correction coil 3 performed by the main control unit 16 will be described. In
Here, as shown in
ΔZi=A·CS·θ02 (Equation 1)
Here, CS is spherical aberration coefficient of the objective lens 6, and A is a proportionality constant.
In addition, when a current Icor is passed through the correction coil 3 disposed in the gap portion of the magnetic pole of the objective lens 6, a focus change amount ΔZc generated in the objective lens 6 is expressed by Equation 2.
ΔZc=B·Icor (Equation 2)
Here, B is a proportionality constant.
Therefore, if ΔZc is controlled such that the sum of ΔZc and ΔZi becomes zero, the spherical aberration of the objective lens 6 is corrected. In an ideal system, the correction current Icor flowing in the correction coil 3 to correct the spherical aberration is represented by (Equation 3), in which IX represents the current amount of the X component flowing in the scanning coil 4, and IY represents the current amount of the Y component.
I
cor
=C·C
S·(IX2+IY2) (Equation 3)
Here, C is a proportionality constant.
The scanning coil 4 and the correction coil 3 are synchronously controlled.
I
cor
=C·C
S·{(IX−a)2+(IY−b)2} (Equation 4)
Here, a, b are phase change amounts. That is, by setting the phase change amounts a, b according to a scanning speed and a frequency or a cycle of the scanning coil signal 102 in the example of
Further, PTL 1 shows that when an axis of the objective lens 6 deviates from an axis of the correction coil 3, a correction can be performed by a function of the same format as Equation 4. Normally, an electron beam apparatus has a plurality of scanning modes having different scanning speeds. Therefore, taking one of the scanning modes as a reference scanning mode, phase change amounts a′, b′ that align the axis of the objective lens 6 with the axis of the correction coil 3 are obtained, and the phase change amounts are corrected according to the scanning mode. At this time, the function of the current value Icor of the correction coil 3 is expressed by Equation 5.
I
cor
=C·C
s·[{IX−(a′+αi)}2+{IY−(b′+βi)}2] (Equation 5)
Here, αi, βi are phase correction amounts from the reference scanning mode in a scanning mode i. (a′+αi) and (b′+βi) respectively correspond to the phase change amounts a, b in Equation 4 in each scanning mode.
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/JP2018/021335 | 6/4/2018 | WO | 00 |