JPH02295042A - Dynamic focus circuit - Google Patents

Abstract

PURPOSE:To enable focusing on the whole surface of a sample even when the distance (WD) between the poll piece of an objective lens and the sample is varied, as the sample is inclined, by using multiplication machines, an adder, a dynamic focus coil and a variable resistor. CONSTITUTION:A WD value is input to ROM 1, 2 and a cosine value cosalpha corresponding to the WD value is output from ROM1 and a sine value sinalpha corresponding to the WD value is output from ROM2, and a multiplication machine 3 multiplies the cosalpha by a horizontal scanning signal H, and a multiplication machine 4 multiplies the sinalpha by a vertical scanning signal V. The output of the multiplication machines 3, 4 is output to an adder 5, and (Vsinalpha-Hcosalpha) is output and input to a multiplication machine 6, and amplitude is controlled according to the angle theta of inclination of a sample, and A(Vsinalpha-Hcosalpha) is output and is subjected to voltage to current conversion and supplied to a dynamic focus coil 8. In this case, a variable resistor 9 has its value varied according to magnification, and when the resistance value is R, then the value of an exciting current supplied to the dynamic focus coil 8 is A(Vsinalpha-Hcosalpha)/R. Focusing is thus enabled on the whole surface of the sample.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a dynamic focus circuit for a scanning electron microscope (hereinafter referred to as SEM), and particularly relates to a dynamic focus circuit for a scanning electron microscope (hereinafter referred to as SEM). Distance between samples (
Hereinafter, this distance will be referred to as WD (WORKING DISTAN).
The present invention relates to a dynamic focus circuit that can perform focusing over the entire surface of a sample even when changing the NCE (referred to as NCE).

[Prior Art] In SEM, as shown in FIG. 2, a sample is sometimes observed while being tilted at a certain angle θ (θ≠0). In FIG. 2, the sample 26 is placed at an angle θ rather than perpendicular to the optical axis.

The electron beam 21 is scanned in the horizontal and vertical directions by a first scanning coil 22 and a second scanning coil 23, and is focused on the entire surface of the sample 26 by a dynamic focus coil 24 and an objective lens coil 25. A predetermined range of the surface of 26 is two-dimensionally scanned. At this time, the dynamic focus coil 24 includes a first
A signal obtained by amplitude modulating the scanning signal supplied to the second scanning coils 22 and 23 in accordance with the tilt angle θ of the sample 26 is supplied. This makes it possible to obtain an image that is in focus over the entire surface even if the sample 26 is tilted.

[Problems to be Solved by the Invention] However, in order for the conventional dynamic focus circuit to function effectively, the tilting direction of the sample and the scanning direction of the electron beam must match, and if the WD is changed, In this case, it was necessary to align the scanning direction of the electron beam with the tilt direction of the sample each time, and the operation was troublesome. In other words, when observing a sample with a SEM, the WD1, that is, the distance between the objective lens pole piece and the sample, may be changed depending on various conditions such as the size of the sample.
When changing the value, the excitation current supplied to the objective lens coil must be changed in order to focus. However, when the excitation current of the objective lens coil is changed, the scanning direction of the electron beam changes, and as a result, the scanning direction of the electron beam no longer matches the tilt direction of the sample, and as a result, dynamic focus no longer functions.

Therefore, conventionally, each time the WD is changed, it is necessary to perform so-called raster rotation to match the scanning direction of the electron beam with the tilt direction of the sample, resulting in poor operability.

The present invention solves the above-mentioned problems, and aims to provide a dynamic focus circuit that can obtain a focused SEM image over the entire surface of a sample even if the WD is changed while the sample is tilted. This is the purpose.

[Means for Solving the Problems] In order to achieve the above object, the dynamic focus circuit of the present invention includes a first circuit that modulates the amplitude of the horizontal scanning signal according to the working distance, and a first circuit that modulates the amplitude of the horizontal scanning signal according to the working distance. The second modulates the working distance according to the working distance.
a third circuit that combines the output of the first circuit and the output of the second circuit, and a fourth circuit that modulates the amplitude of the output of the third circuit in accordance with the tilt angle of the sample. and a fifth circuit that supplies the output of the fourth circuit to a dynamic focus coil.

[Operations and Effects of the Invention] According to the present invention, a signal is generated in which the amplitude of the horizontal and vertical scanning signals is adjusted in accordance with the WD value, and the signals are synthesized and supplied to the dynamic focus coil. It is possible to automatically obtain a SEM image that is in focus on the entire surface of the sample.

Furthermore, since it is possible to change the current supplied to the dynamic focus coil according to the magnification, an optimal SEM image can be obtained even if the magnification is changed.

[Example] Examples will be described below with reference to the drawings.

FIG. 1 shows one of the dynamic focus circuits according to the present invention.
1 is a diagram showing the configuration of an embodiment, and in the diagram, 1 and 2 are ROM1
3.4 is a multiplier, 5 is an adder, 6 is a multiplier, 7 is a drive circuit, 8 is a dynamic focus coil, 9 is a variable resistor, and 10 controls all the operations of the dynamic focus circuit according to the present invention. It is a control device.

In the configuration shown in FIG. 1, the WD value is input to ROM1 and ROM2, and the cosine value corresponding to the WD value is output from ROM1.
The ROM 2 outputs each sine value corresponding to the WD value. In order to make the WD value correspond to the sine value and cosine value in this way, for example, the midpoint of the WD range is set as O'', and the maximum value of WD is +180° and the minimum value is -
It should be 180°. Furthermore, the WD value can be determined from the value of the excitation current supplied to the objective lens. That is, W
If autofocus is performed when D is changed, the excitation current of the objective lens coil for the relevant WD is automatically determined, but since the excitation current corresponds to the WD value, the control device 10 The WD value can be known, and the obtained WD value is A/D converted and stored in the ROM.
1 and 2. COSα and S1nα thus obtained are input to multipliers 3 and 4, respectively. Multiplier 3 multiplies cos α by horizontal scanning signal H, and multiplier 4 multiplies sin α by vertical scanning signal V. Note that these multipliers 3 and 4 are multiplication type D
/A converter. The outputs of multipliers 3 and 4 are input to adder 5, and (Vslnα−Hcosα)
is output. The output is input to the multiplier 6, and the amplitude is controlled according to the inclination angle θ of the sample, and A (Vslnα−
H cos α) is output. The output is the drive circuit 7
The signal is input to the dynamic focus coil 8, undergoes voltage-to-current conversion, and is supplied to the dynamic focus coil 8. The value of the variable resistor 9 is changed depending on the magnification, and the resistance value is set to R.
Then, the value of the excitation current supplied to the dynamic focus coil 8 is A(Vslnα-Hcosa)/R. This is a process performed in response to the fact that when the magnification changes, the focus state also changes. Note that the resistance value of the variable resistance value 9 may be adjusted manually by the operator while observing the SEM image, and since the control device 10 can know the magnification, the control device 10 may adjust the resistance value automatically. It's also good.

Also, regarding the adjustment of the amplitude according to the inclination angle θ of the sample,
This may be done manually by an operator while observing the SEM image, or the inclination angle may be measured using a known method and the control device 10 may automatically perform this based on the result.

Note that the amplitudes of the horizontal scanning signal and vertical scanning signal input to the multipliers 3 and 4 are set to values that allow optimum operation based on the acceleration voltage, WD value, and magnification value of the electron beam. Needless to say.

As is clear from the above description, in the present invention, S
Since the EM image is not rotated, the SEM image obtained is rotated as the WD value is changed, but it is possible to obtain a SEM image in which the entire surface of the sample is in focus.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible.

For example, as a method for detecting the WD value, it is also possible to link a potentiometer to a mechanism that moves the sample in the optical axis direction, and have the control device take in and detect the value of the potentiometer. Alternatively, it is also possible to detect by interlocking a rotary encoder with a mechanism that moves the sample in the optical axis direction, and by having the control device count the number of pulses output from the rotary encoder. In particular, according to these methods, the excitation current to be supplied to the objective lens coil can be determined from the detected WD value.

This is because, as described above, the excitation current supplied to the objective lens coil corresponds to the WD value.

Further, in the above embodiment, the WD value is converted into a cosine value and a sine value, but in short, the amplitudes of the horizontal scanning signal and the vertical scanning signal may be set to values corresponding to the WD value.

[Brief explanation of the drawing]

FIG. 1 shows one of the dynamic focus circuits according to the present invention.
FIG. 2 is a diagram showing the configuration of the embodiment. FIG. 2 is a diagram showing an outline of the optical system of the SEM. 1, 2... ROM1 3.4... Multiplier, 5...
Adder, 6... Multiplier, 7... Drive circuit, 8...
Dynamic focus coil, 9...variable resistor,
10...Control device. Applicant: JEOL Ltd.

Claims (1)

[Claims] (1) a first circuit that modulates the amplitude of the horizontal scanning signal according to the working distance; a second circuit that modulates the amplitude of the vertical scanning signal according to the working distance;
a third circuit that combines the output of the first circuit and the output of the second circuit; a fourth circuit that modulates the amplitude of the output of the third circuit according to the tilt angle of the sample; A fifth circuit that supplies the output of the fourth circuit to the dynamic focus coil.
A dynamic focus circuit characterized by comprising a circuit.