JPH03246407A - Optimum focus setting method in electron beam length measuring instrument - Google Patents

Abstract

PURPOSE:To perform measurement with an optimum focus at all of a small number of focusing points by performing the focusing at plural points on the sample and finding the bend of the sample from their coordinates on the sample and the current value of a focus correcting coil. CONSTITUTION:A focusing position (x1, y1) which is registered in a ROM 17 is irradiated first with an electron beam 12 to carry on the focusing, and the current value OI1 of the optimum focus correcting coil 11 is inputted to a CPU 1 from a lens control circuit 4. Similarly, values OIn of the coil 11 are obtained for focusing points (xn, yn) as many as (n) focusing points. Then coefficients a1-a10 with which the relation shown by equation I is obtained between the currents OI of the coil 11 and coordinates (x,y). Then an XY stage 14 is moved so as to irradiate a measurement pattern position which is registered 17 with the electron beam 12. The current coordinates (Xp1, Yp1) are used to calculate a current OIp1 from equation I and sets it in a control circuit 4. Consequently, the focus of the electron beam 12 in the pattern measurement is optimized and the measurement pattern measuring using excellent electron beam 12 is available.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electron beam length measuring device, and more particularly to a focus correction method for an electron beam length measuring device that can measure patterns at high speed and with high accuracy.

[Conventional technology]

In conventional devices, as described in Japanese Patent Application Laid-Open No. 58-48989, various techniques have been described regarding automatic focusing techniques for each measurement point, but most of them involve changing the electron lens current. The sample is scanned with an electron beam, and the electron lens current value is set so that the detection signal at that time is optimal, and the process takes several seconds.

[Problem to be solved by the invention]

In a length measurement device that uses an electron beam, when focusing the electron beam on the sample, the current flowing through the focus correction coil is changed and the secondary electron signal at that time is captured to obtain the optimal secondary electron signal. As.

Since the focus correction coil is determined, it takes several seconds to focus at one point on the sample.

If there are many measurement points on the sample, if you want to make good measurements, you can focus on each measurement point, but it takes time to focus on the measurement point x (focusing time at - point). However, there was a problem with throughput.

Furthermore, if focusing is performed at each measurement point in consideration of throughput, the throughput will be improved, but there is a problem that the measurement points between the measurement points where focusing may be out of focus. Ta.

An object of the present invention is to take into consideration throughput and to enable measurement to be carried out at optimal focus at all measurement points with a small number of focusing points.

[Means to solve the problem]

In order to achieve the above purpose, before measuring the pattern to be measured, focusing is performed at multiple arbitrary points on the sample, and when the coordinates on the sample at that time and the optimum focus are obtained, focus correction is performed. When measuring the pattern to be measured by determining the curvature in the height direction of the sample from the coil current value, the amount of curvature is determined from the coordinates on the sample and the corresponding current is applied to the focus correction coil, allowing for high throughput. Moreover, the optimum focus can be obtained over all measurement points.

[Effect]

Before measurement, focus is performed at multiple points on the sample, and a relational expression between the coordinates on the sample and the focus position is determined. During measurement, the focus position is calculated from the measurement point coordinates using the relational expression, and the hardware By setting , it is possible to scan the electron beam at the optimum focus and obtain good measurement results without performing focusing at each measurement point.

〔Example〕

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram of the main parts of an electron beam length measuring apparatus according to an embodiment of the present invention.

In the figure, the CPUI controls the entire electron beam length measurement apparatus, and the control is executed by the program and registration data stored in the ROM 17.

When measuring the pattern on the sample 13, the CP
A command to move to the measurement pattern coordinates input to the UI is issued. Stage control circuit 6 moves XY stage 14 under the supervision of laser optical system 7. Further, an electron beam 12 emitted from the electron gun 8 passes through an aperture 9 and is deflected by a deflection coil 10. At this time, the deflection control circuit 3 generates a deflection current according to the amount of deflection specified to the deflection control circuit 3 from the CPUI. The deflected electron beam 12 is condensed by a focus correction coil 11 so that it is focused on a sample 13. The current flowing through this focus correction coil can be controlled by the CPU via the I/F 2 and the lens control circuit 4.

When the electron beam is scanned over the sample in this manner, secondary electrons 16 are generated from the sample. This secondary electron 16 is detected by the detector 1
5 and is taken into the CPUI through the signal processing circuit 5 and I/F 2.

The CPUI calculates the dimensions of the pattern from the captured secondary electronic signal.

When performing focusing in such an electron beam length measuring device, the electron beam 12 is scanned over the sample, and while changing the focus correction coil current, a secondary electron signal is captured and the secondary electron signal optimal for length measurement is detected. Determine the focus correction coil current so that an electronic signal can be obtained.

This focusing method may be a manual method in which an operator observes the SEM image and determines the focus, or an automatic method in which the CPU controls the device and determines the focus.

An optimal focus setting method for an electron beam length measurement apparatus having the above-mentioned focusing function will be explained according to the flowchart shown in FIG.

First, in step S1, the XY stage 14 is moved so that the focusing position 1j (x(1), y(1)) registered in advance in the ROM 17 is irradiated with the electron beam.
In step S2, focusing is performed at the position after movement. The optimum focus correction coil value (1) at that time is taken from the lens control circuit to the CPUI through I/F2. In step S3, it is determined whether focusing has been completed for the n number of focusing points registered in the ROM 17. If not, the process moves to the next focusing point (x(2), y(2)), and similarly the optimum focus correction coil Obtain the value, OI (2). In this way, the focusing points (X(1), y(1)), (X(2)).

y(2)), ... (x(n), y(n)), the optimal focus correction coil value ○I (1), OI (2),
...obtain ○I(n).

From the relationship obtained in this way, between the focus correction coil current ○ and the coordinates (x, y), ○I=az+azx+aay+aax2+agxy+a
ey"+a7x'+agx2y+aexy2+atoy
's...(1) Coefficients a1 to alo that obtain the relationship are determined in step S4.
Determine using the least squares method.

Next, in step S5, the XY stage 14 is moved so that the registered measurement pattern position is irradiated with the electron beam. Coordinates at this time (Xp(1).

In step S6, the current 0Ip(1) for the focus correction coil is calculated by applying the coordinate value to the equation (1) using Yp(1)), and is set in the lens control circuit 4 through the I/F 2. As a result, the focus of the electron beam during pattern measurement in step S7 is optimized, making it possible to perform pattern measurement using the electron beam in good quality.

In step 8, it is determined whether all registered measurement points have been measured, and if not, steps 85 to S7 are repeated for the first measurement pattern, and when all points have been measured. The process ends.

According to this embodiment, measurement can be performed at the optimum focus of the electron beam at each measurement point without performing focusing at each measurement point.

〔Effect of the invention〕

According to the present invention, by focusing only at several points on the sample,
When measuring the pattern to be measured on the sample, the measurement can be performed with the optimum focus, so the time required for measurement can be significantly reduced compared to performing focusing on the entire measurement pattern on the sample.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the main part of the control system of an electron beam length measuring apparatus which is an embodiment of the present invention, and FIG. 2 is a flowchart of an optimal focus setting method. 1... CPU, 2... I/F, 3... Deflection control circuit, 4... Lens control circuit, 5... Signal processing circuit,
6... Stage control circuit, 7... Laser optical system, 8
... Electron gun, 9... Aperture, 10... Deflection coil, 11... Focus correction coil, 12... Electron beam, 13... Sample, 14... XY stage, 15...
-Secondary electron detector, 16...Secondary electron. Diagram

Claims (1)

[Claims] 1. An electron gun that emits an electron beam, and means for moving the focal position of the electron beam when the electron beam emitted from the electron gun is irradiated onto a sample to be measured; An electron beam that has the function of capturing a secondary electron signal generated when an electron beam is irradiated onto a sample, and determining the optimal focal position for measurement based on the secondary electron signal and the result of movement of the focal position of the electron beam. In a length measuring device, before measuring the pattern to be measured on the sample to be measured, the optimal focal position at multiple arbitrary positions on the sample is determined, the curvature of the sample is determined from the relationship, and the curvature of the sample is determined from the relationship between the positions and the sample position when measuring the pattern to be measured. An optimal focus setting method for an electron beam length measuring device, characterized in that the method includes a function of determining the optimal focal point position for measurement based on the curvature.