JPH0319923B2 - - Google Patents

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to an electron beam length measuring device that measures the distance between two points on a sample to be measured by scanning an electron beam.
In particular, it relates to the improvement of measurement errors due to the influence of stray magnetic fields from commercial frequency power sources, etc.

[Prior Art] Conventional devices of this type have already been commercialized as applied devices for scanning electron microscopes, and two types of methods described below are generally used.

FIG. 1 is a diagram showing an example of a general distance measurement method using two-dimensional scanning. In this method, like a scanning electron microscope, a finely focused electron beam 1 is scanned two-dimensionally on the surface of a sample 2, and signals such as secondary electrons generated from the sample 2 are used to scan the CRT.
A sample image 3 is displayed on the screen, and a position-variable cursor mark 4 is displayed that overlaps with this sample image 3 to indicate two points to be measured, and the interval D between the cursor marks 4 is
and the known magnification of the sample image to determine the actual size L of the sample 2 between the two points indicated by the cursor mark 4.
Measure.

FIG. 2 is a diagram showing an example of a distance measurement method using linear scanning. The electron beam is scanned in a straight line passing between two points to be measured on the sample 2 to be measured, and the distance L between the two points on the sample 2 to be measured is measured based on the signal waveform of secondary electrons etc. generated from the sample 2. It is something.
For example, when the sample to be measured 2 as shown in FIG. 2a is scanned with the electron beam 1, the waveform of the secondary electron signal generated from the sample 2 becomes as shown in FIG. 2b. in this case,
Since the scanning width of the electron beam 1 is known, the dimension L of the sample to be measured 2 can be measured from the distance D between, for example, two peak values of the secondary electron signal waveform. This method is
It is possible to automatically detect the peak point of the secondary electron signal waveform or the point where the slope is maximum, etc.
It has the advantage of automating measurement operations.

[Problems to be Solved by the Invention] One of the causes of measurement errors in dimension measurement using such an electron beam is the influence of stray magnetic fields.

The target of the present invention, the electron beam length measurement device, is mainly used for semiconductor products in the manufacturing process, and the measurement accuracy and resolution of the measurement distance must be 1 μm or less. Since this type of sample to be measured is an insulator, the accelerating voltage of the electron beam must normally be around 1 kV as a condition to prevent the electron beam irradiated onto the sample from accumulating on the sample surface. When a conventional scanning electron microscope uses an electron beam with an acceleration voltage of this level, if the stray magnetic field around the device changes by a few milligrams, the electron beam irradiation position on the sample surface changes from a few hundredths of a micrometer to a few hundredths of a micrometer. It is known that it varies by several tenths of a μm. Naturally, a similar phenomenon occurs in electron beam length measurement devices that have the same configuration as this scanning electron microscope, and this value ranges from a few percent to several tens of percent of the measured value, and changes in stray magnetic fields of several milligrams or more. If there is, accurate measurements will not be possible.

By the way, among the stray magnetic fields that actually exist around devices, those that exceed several milligrams are mostly the earth's magnetic field and AC magnetic fields at commercial power frequency generated from commercial AC frequency power supply wires or power equipment using them. I understand. Of these, since the earth's magnetic field is constant, the irradiation position of the electron beam that irradiates the two points on the sample to be measured changes by the same amount, and therefore does not affect the distance measurement between the two points. I won't give it. However, when an alternating magnetic field exists, the magnetic field strength at the measurement target is different, and the electron beam passes through these two points with a certain time difference, so the electron beam irradiation position at the two measurement target points is different. Only the amount changes. FIG. 3 simply shows this situation. In Figure 3,
When the floating AC magnetic field acts in the direction of arrow A while the electron beam 1 is scanning, and its phase φ changes as shown in the figure b, the electron beam 1 receives a force in the direction perpendicular to the scanning direction, and the floating AC magnetic field If there is no magnetic field, the distance between the peaks of the sample image or secondary electron signal waveform displayed on the cathode ray tube after passing through points a' and b' at the time when it would have to pass through points a and b is , wider or narrower than the actual width, which appears as a measurement error.

Therefore, when displaying a sample image by two-dimensional scanning, if the phases of the floating AC magnetic fields at the starting points of each horizontal scanning line are not the same, for example, the third
The image of sample 2 shown in the figure apparently changes in width and position with each horizontal scan, so
As shown in Figure a, the sides of the scanning end become wavy. Furthermore, even if the horizontal scanning period is synchronized with the commercial power frequency to align the phase of the floating AC magnetic field at the starting point of each horizontal scan, as is done with scanning electron microscopes, the sample image may not be accurate at first glance. However, as shown in Figure 4b,
It is displayed with a width different from the actual width L,
This is a cause of measurement error.

SUMMARY OF THE INVENTION An object of the present invention is to provide an electron beam length measuring device that can significantly reduce measurement errors caused by stray alternating current magnetic fields, particularly stray alternating current magnetic fields at commercial power frequency.

[Means for Solving the Problems] In order to achieve the above object, the present invention scans a narrowly focused electron beam on a sample to be measured, and detects the target based on the secondary electron signal generated from the sample to be measured. In an electron beam length measuring device that measures the distance between two points on a measurement sample, there is a means for determining the positions of two points on an image of the sample to be measured in the scanning range of the electron beam, and a position determined by the position determination means on the image. An electron beam length measuring device is provided with a means for adjusting the scanning width of the sample so that the phase of the commercial frequency AC power source is equal when the electron beam passes through the two points, and measuring the distance between the two points. This is a proposal.

[Function] The present invention is based on the premise that in an electron beam length measuring device, the actual dimension between two points to be measured is the most important concern, and that the shape of the sample to be measured itself may appear to be somewhat distorted. The electron beam is scanned so that the phase of the floating AC magnetic field is the same when the electron beam passes through two points to be measured.

Specifically, since most of the stray AC magnetic field is generated directly or indirectly due to the influence of the commercial frequency power source, it is important that the phase of the commercial frequency power source is the same when the electron beam passes through the two points being measured. Scan the electron beam so that

When scanning in this manner, the influence of the stray alternating current magnetic field becomes the same at least at two points on the measurement target, so measurement errors are significantly reduced.

[Embodiment] Next, an embodiment of the present invention will be described with reference to FIGS. 5 and 6.

FIG. 5 is a block diagram showing the configuration of an embodiment of the electron beam length measuring device according to the present invention. In the figure, 1 is an electron beam, 2 is a sample to be measured, 5 is a first horizontal scanning signal generator for display, and 6 is a second horizontal scanning signal generator for length measurement.
a horizontal scanning signal generator; 7 a changeover switch having a two-point structure for switching between display and length measurement; 8 a vertical scanning signal generator; 9 a horizontal deflection amplifier;
is a vertical deflection amplifier, 11 is a deflection means consisting of a deflection coil, etc., 12 is a secondary electron detector, 13 is an image amplifier, 14 is a CRT, 15 is a comparator, 16
is a comparator, 17 is a horizontal cursor mark moving means, 18 is a vertical cursor mark moving means, 19 is a control circuit, and 20 and 21 are deflectors for deflecting the electron beam of the CRT 14 in synchronization with electron beam scanning. 22 is a length measuring circuit, and 23 is a display.

In this embodiment, in addition to the first horizontal scanning signal generator 5 for displaying the sample image, a second horizontal scanning signal generator 5 for length measurement is used.
A horizontal scanning signal generator 6 is provided. When displaying the sample image 3 shown in FIG. 6 and aligning the cursor with two points to be measured, the changeover switch 7 is switched to the A side. In this state, the outputs of the first horizontal scanning signal generator 5 and vertical scanning signal generator 8 for displaying the sample image 3 are transmitted through the horizontal deflection amplifier 9 and the vertical deflection amplifier 10, respectively.
The deflection means 11 is supplied with the deflection means 11 . The electron beam 1 is two-dimensionally scanned over the sample 2 by the deflection means 11,
A secondary electron signal is generated from the surface of the sample 2. This secondary electron signal is detected by the secondary electron detector 12, amplified by the image amplifier, and applied to the CRT 14 as a brightness signal. Deflection amplifiers 20 and 21
Since the deflector deflects the electron beam of the CRT 14 in synchronization with the electron beam scanning, a sample image 3 as shown in FIG. 6a is displayed on the screen of the CRT 14. Although not shown here, the sample 2 can be mechanically moved by a sample moving table, and while observing the sample image, the part to be measured is moved within the image, and the part to be measured is moved to the center in the vertical direction. It is possible to move it to This mechanical moving stage is known from scanning electron microscopes and the like. On the other hand, the output of the first horizontal scanning signal generator 5 is compared with the output signals of the horizontal cursor mark moving means 17 and the vertical cursor mark moving means 18 in comparators 15 and 16, respectively. Comparator 1
5 and 16 output a cursor signal at the matched timing when the levels of both signals match as a result of this comparison. This cursor signal is added to the luminance signal of the sample 2 in the image amplifier 13. Therefore, on the CRT14 screen, the 6th
As shown in Figure a, two cursor marks 4 are displayed in a superimposed manner along with the sample image 3. At this time,
If the cursor mark moving means 17 and 18 are adjusted so that the two cursor marks 4 match the two points of the measurement target on the sample image 3, the difference between the output signals of the cursor mark moving means 17 and 18 will be Since this corresponds to the distance between two points on the sample where the mark 4 is aligned, if this is corrected according to the magnification of the sample image, the actual distance between the two points can be calculated.

What has been described so far is the conventional length measurement method. The sample image 3 displayed at this time may be distorted due to the influence of the stray AC magnetic field.
It is unclear whether the measurements are accurate or not.

Therefore, in this embodiment, after aligning the two cursor marks 4 with the measurement end of the sample 2 as described above, the changeover switch 7 is switched to the B side.
The output signal of the second signal generator 6 for length measurement causes the electron beam 1 to linearly scan the vertical center of the image. As shown in FIG. 6b, the second signal generator 6 generates a sawtooth voltage waveform that starts at a certain phase of the commercial frequency AC voltage of FIG. 6c and ends after, for example, one and a half cycles of this AC voltage. Generates a scanning signal. The amplitude and starting point voltage of this scanning signal are
The voltage at points P ₁ and P ₂ of the same phase of the commercial frequency AC voltage is the cursor mark moving means 17 and 1.
It is controlled by a control circuit 19 so as to match the output voltage of 8. The horizontal scanning signal is set at the second point for length measurement so that the phases of the commercial frequency components of the commercial frequency AC power supply, that is, the floating magnetic field, are almost the same when the electron beam 1 passes through the two points where the cursor mark 4 was previously set. It is output from the horizontal scanning signal generator 6. The secondary electron signal generated from the sample 2 by such scanning is detected by the secondary electron detector 12, then amplified by the image amplifier 13, and then sent to the length measurement circuit 2.
2 is output. As described above, the length measuring circuit 22 calculates the distance between two points using, for example, the peak value of the secondary electronic signal waveform, and displays the distance on the display 23.

In this way, the points P ₁ and P ₂ of the same phase of the commercial frequency AC voltage coincide with the two points where the cursor mark 4 was set, so the influence of the stray AC magnetic field appears exactly the same, and when subtracted, the stray AC Distance measurement without errors due to magnetic fields is realized.

Note that, as mentioned above, the displayed sample image 3 is distorted, so strictly speaking, the setting position of the first cursor mark may not match the two measurement points on the sample. The phase of the stray alternating current magnetic field when the electron beam 1 passes through the points is not strictly the same. However, even if the deviation is about 1/10 cycle, the difference in magnetic field strength is about 8% of the peak value, and the measurement error is improved by more than an order of magnitude compared to the conventional method.

Considering the rate of change of the alternating current magnetic field, even if some deviation occurs, the influence of changes in the magnetic field is small near the peak or bottom of the alternating current waveform. Therefore, as shown in Fig. 6b and c, for example, one cycle from peak to peak of the AC waveform and approximately 1/4 cycle each of the left and right slopes for confirmation, totaling about 1 and a half cycles. It is desirable to display the information.

Further, in this embodiment, a method is adopted in which the electron beam 1 is first scanned two-dimensionally to display the sample image 3, and the cursor mark 4 is aligned with this, but linear scanning may be performed from the beginning. That is, first, a straight line scan is performed without considering the phase of the commercial frequency power source, and the positions containing errors at the two points at which the distance is to be measured are determined from the secondary electronic signals at that time, and the position of the commercial frequency power source at these two points is determined. Linear scanning may be performed so that the phases are the same.

[Effects of the Invention] According to the present invention, length measurement errors due to stray AC magnetic fields caused by at least commercial power supply frequency components can be significantly reduced, and means for shielding stray AC magnetic fields can be simplified.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an example of a general distance measurement method using two-dimensional scanning, Figure 2 is a diagram showing an example of a distance measurement method using linear scanning, and Figures 3 and 4 are diagrams showing an example of a distance measurement method using linear scanning. 5 is a block diagram showing the configuration of an embodiment of the electron beam length measuring device according to the present invention, and FIG. 6 is a diagram showing the relationship between the displayed image, scanning waveform, and floating AC magnetic field waveform in the embodiment of FIG. FIG. DESCRIPTION OF SYMBOLS 1... Electron beam, 2... Sample, 3... Sample image, 4... Cursor mark, 5... First horizontal scanning signal generator, 6...
2nd horizontal scanning signal generator, 7... changeover switch,
8... Vertical scanning signal generator, 9, 10... Deflection amplifier, 11... Electron beam deflection means, 12... Secondary electron detector, 13... Image amplifier, 14... CRT, 15, 1
6... Comparator, 17, 18... Cursor mark moving means, 19... Control circuit, 20, 21... Deflection amplifier,
22...Length measurement circuit, 23...Display device.

Claims (1)

[Claims] 1. An electron beam that scans a narrowly converged electron beam over a sample to be measured and measures the distance between two points on the sample based on a secondary electron signal generated from the sample. In the line length measuring device, means for determining the positions of two points on the image of the sample to be measured in the scanning range of the electron beam, and when the electron beam passes through the two points determined by the position determination means on the image. an electron beam length measuring device, comprising: means for measuring the distance between the two points by adjusting the scanning width of the sample so that the phases of the commercial frequency alternating current power sources are equal;