JP4283839B2 - Astigmatism adjustment method using electron beam apparatus - Google Patents

Description

  The present invention relates to an astigmatism adjustment method using an electron beam apparatus for observing and inspecting a sample such as a wafer or a mask on which a pattern having a minimum line width of 0.1 μm or less is formed with high throughput and high reliability. .

  An apparatus and a method for observing and evaluating a sample by irradiating the sample with an electron beam and detecting secondary electrons, reflected electrons, or backscattered electrons emitted from the sample are known (for example, see Patent Document 1).

  In such a sample surface observation evaluation apparatus, astigmatism adjustment is indispensable for performing observation at a high magnification. This is because after the electron beam passes through a circular aperture or the like, the image is blurred as a result of being deformed into an ellipse in any rotation direction and the major axis direction deviating from the spot. In order to correct such blurring of the image, the electron beam may be formed in a spot shape by applying an electric field or a magnetic field by a lens such as an octupole to twelve pole to narrow the major axis direction of the electron beam. For example, Patent Document 2 discloses a method of adjusting astigmatism using a magnetic field.

Specifically, as shown in (A) in FIG. 8, when the electron beam is deformed into an elliptical shape, the cross-sectional shape on the sample surface is elongated deformed in the direction of the azimuth angle theta, in FIG. 8 ( As shown in B), by assigning an optimum voltage to the pair of opposing electrodes R ₁ and R ₂ positioned in the direction of the azimuth angle θ, the cross-sectional shape of the electron beam can be adjusted to be a spot shape. . Therefore, for example, when the major axis direction θ of the electron beam can be set to 0 °, 45 °, 90 °, 135 °, 180 °, 225 °, 270 °, and 315 ° as shown in FIG. By arranging the electrodes facing each other on the line forming the angles and optimizing the voltage applied to the corresponding pair of electrodes on the line along the cross-sectional shape of the electron beam to be adjusted, the electrode is deformed into an elliptical shape, for example. The electron beam can be changed to a beam having a circular cross section or a spot shape.

  Conventionally, in order to optimally set the azimuth angle θ and the voltage V applied to the electrode, this pattern is sharpened in all azimuth directions while looking at a radial or annular pattern existing in the test pattern. The applied voltage R was adjusted. For example, Patent Document 3 discloses an SEM that performs astigmatism correction using a circular pattern.

However, the conventional algorithm for automatically adjusting astigmatism is complicated and difficult to understand. This is because the azimuth angle at which the pattern is blurred due to astigmatism cannot be extracted using the autofocus function. In addition, when the fine adjustment of the various wafer, it is necessary to observe the test pattern previously, when the wafer to be inspected Target as the test pattern does not exist, the astigmatism adjustment for each wafer There is also the problem that it is impossible.
Japanese Patent Laid-Open No. 2001-22986 Japanese Patent Laid-Open No. 10-247466 Japanese Patent Laid-Open No. 10-247466

  The present invention has been proposed in order to solve the above-described problems, and an object of the present invention is to provide an electronic device capable of adjusting astigmatism for each wafer without preparing a test pattern for adjustment in advance. An astigmatism adjustment method using a beam device is provided.

In order to solve the above problems, the invention of claim 1
In an electron beam apparatus including an electron optical system having an astigmatism adjusting unit including a multipole element for adjusting astigmatism of an electron beam, a reflected electron emitted from the sample is irradiated with the electron beam. A method for adjusting astigmatism of an electron beam by detecting secondary electrons including backscattered electrons,
(A) acquiring an image of a first pattern including lines formed in the vertical direction and the horizontal direction on the sample;
(B) adjusting the electron optical system to obtain a first auxiliary voltage when the vertical line sharpness in the image is maximized;
(C) applying the first auxiliary voltage to the multipole to adjust the cross-sectional shape of the electron beam;
(D) When the electron optical system is adjusted with the first auxiliary voltage fixed when the vertical line looks the clearest, and the sharpness of the horizontal line in the image is maximized Obtaining a second auxiliary voltage of:
(E) Applying the second auxiliary voltage to the multipole so as to minimize the difference between the vertical line sharpness and the horizontal line sharpness in the cross-sectional shape of the electron beam. Adjusting the process to
(F) sequentially performing the steps (a) to (e) on a pattern having a line having a smaller line width than the line of the first pattern;
A method comprising:
I will provide a.

The invention of claim 2 is characterized in that the multipole element includes a plurality of pairs of electrodes or coils facing each other with the optical axis of the electron beam as a center.
According to a third aspect of the present invention, the electrode includes a first auxiliary electrode for adjusting a focus value for the vertical line, and a second auxiliary electrode for adjusting a focus value for the horizontal line. It is characterized by including.

According to a fourth aspect of the present invention, the automatic focusing is performed using the autofocus function of the optical system.
The invention according to claim 5 is characterized in that the sample in the middle of the process is evaluated using the method according to any one of claims 1 to 4.

  FIG. 1 is a diagram schematically showing a configuration of a mapping projection type electron beam apparatus which is an embodiment of an electron beam apparatus according to the present invention. In the figure, an electron beam apparatus 100 includes an electron beam emitting unit 101, a primary optical system 102, a secondary optical system 103, a secondary electron detection unit 104, and an astigmatism adjustment unit 105. The electron beam emitting unit 101 includes an electron gun 1, a Wehnelt electrode 2, and an anode 3, and an electron beam emitted from the electron gun 1 is accelerated by the anode 3 and enters the primary optical system 102.

  The primary optical system 102 includes an electrostatic lens 4, a square aperture 5, a multi-stage quadrupole lens 6, an E × B separator 7 and an objective lens 8, and the traveling direction of the electron beam by the E × B separator 7. And the primary electron beam is made to travel perpendicular to the wafer W on the XY-θ stage S, and the electron beam is formed in a desired cross-sectional shape by the objective lens 8 and irradiated onto the wafer W. The XY-θ stage S is movable so as to be movable with respect to two orthogonal directions X and Y, and is supported so as to be able to rotate with respect to one of them, whereby the surface of the wafer W is scanned with an electron beam. Is done.

  Secondary electrons including reflected electrons and backscattered electrons emitted from the wafer W by irradiation of the electron beam pass through the secondary optical system 103 and enter the secondary electron detection unit 104. The secondary optical system 103 includes an objective lens 8, an E × B separator 7, a first stage condenser lens 9, and a second stage condenser lens 10. The secondary electron detection unit 104 includes a fluorescent plate 11, a TDI 12, an MCP 13, a detector 14, and an image processing unit 15. The fluorescent plate 11 converts incident secondary electrons into an optical signal, which is converted into an electrical signal by the TDI 12 and transmitted to the detector 14. The detector 14 generates an electrical signal corresponding to the intensity of the received secondary electrons and sends it to the image processing unit 15, and the image processing unit 15 performs A / D conversion on the received electrical signal to form a digital image signal. . Such an operation is performed throughout the scanning period of the wafer W, and as a result, the image processing unit 15 can output an image of the wafer W.

  The astigmatism adjustment unit 105 includes an astigmatism controller 16 and an astigmatism adjuster 17, and an output from the image processing unit 15 is given to the astigmatism controller 16. The astigmatism adjuster 17 is a multipole including a plurality of pairs of, for example, two or more pairs of electrodes or coils facing each other around the optical axis in a plane perpendicular to the optical axis of the secondary optical system 103. Astigmatism adjustment accuracy increases as the number of electrode or coil pairs increases.

  On the other hand, FIG. 2 is a diagram schematically showing a configuration of a scanning electron beam apparatus which is another embodiment of the electron beam apparatus according to the present invention. In the figure, an electron beam apparatus 200 includes an electron beam emitting unit 201, an electron optical system 202, a secondary electron detecting unit 203, and an astigmatism adjusting unit 204. The electron beam emitting unit 201 includes an electron source 21 and a Wehnelt electrode 22, and an electron beam emitted from the electron source 21 passes through the Wehnelt electrode 22 and enters the electron optical system 202.

  The electron optical system 202 includes a square aperture 23, a plurality of quadrupole lenses 24, and a scanning coil 25, and changes the traveling direction of the electron beam from the electron source 21 by adjusting the voltage applied to the scanning coil 25. The electron beam is incident on the wafer W on the XY-θ stage S. The XY-θ stage S is movable in two orthogonal directions and supported so as to be able to rotate with respect to one of the two directions, whereby the surface of the wafer W is scanned with an electron beam.

  Secondary electrons including reflected electrons and backscattered electrons emitted from the wafer W by the irradiation of the electron beam are incident on the secondary electron detector 203. The secondary electron detector 203 receives the secondary electrons emitted from the wafer W and converts them into electrical signals corresponding to the strength of the secondary electrons, and processes the electrical signals output from the detectors 26. And an image processing unit 27 for imaging. The detector 26 generates an electrical signal corresponding to the intensity of the received secondary electrons and sends it to the image processing unit 27. The image processing unit 27 performs A / D conversion on the received electrical signal to form a digital image signal. . Such an operation is performed throughout the scanning period of the wafer W. As a result, the image processing unit 27 can output an image of the wafer W.

  The astigmatism adjustment unit 204 includes an astigmatism controller 28 and an astigmatism adjuster 29, and an output from the image processing unit 27 is given to the astigmatism controller 28. The astigmatism adjuster 29 is a multipole including a plurality of pairs, for example, two or more pairs of electrodes or coils facing each other around the optical axis in a plane perpendicular to the optical axis of the electron optical system 202.

  In order to perform astigmatism adjustment in the electron beam apparatus shown in FIGS. 1 and 2, the present invention is capable of simultaneously observing lines running vertically and horizontally among wiring patterns formed on the wafer W, for example, Utilize a pattern having lines and spaces running in two orthogonal directions. An example of such a pattern is a box pattern having a line and space width of 180 nm as shown in FIG. Therefore, an electron beam is irradiated to such a pattern, and a digital image of the pattern is obtained from the image processing units 15 and 27. As a result, as shown in FIG. 3B, when the digital image indicating that the cross-sectional shape of the electron beam is an ellipse 31 having a major axis in the X-axis direction is output from the image processing units 15 and 27, Astigmatism adjustment of an electron beam is performed by applying a correction voltage of an appropriate magnitude to a pair of electrodes or coils on the X axis of the astigmatism adjusters 17 and 29. As a result, the cross-sectional shape of the electron beam can be adjusted to a circle 33 via an ellipse 32 having a longer diameter than the ellipse 31. In FIG. 3, the width d represents the sharpness of the pattern in the Y-axis direction.

  Similarly, when the digital image indicating that the cross-sectional shape of the electron beam is an ellipse having a major axis in the Y-axis direction perpendicular to the X-axis is output from the image processing units 15 and 27, the astigmatism adjuster. By applying an appropriate correction voltage to a pair of electrodes or coils on the Y axis, the cross-sectional shape can be made a circle.

However, actually, the cross-sectional shape of the electron beam is not deformed only in one axial direction, that is, the X-axis direction or the Y-axis direction. For example, as shown in FIG. Generally, it is deformed into an inclined ellipse 41. Therefore, in the electron beam apparatus shown in FIGS. 1 and 2, first, the cross section of the ellipse 41 is observed using the astigmatism adjusters 17 and 29 while observing the images output from the image processing units 15 and 27. The astigmatism adjustment in the X-axis direction is performed on the shaped electron beam, and the cross-sectional shape of the electron beam is adjusted to be an ellipse 42 having a major axis in the Y-axis direction, whereby the line resolution in the Y-axis direction is adjusted. Raise. Next, the astigmatism adjustment in the Y-axis direction is performed on the electron beam having an elliptical cross-sectional shape, so that the cross-sectional shape of the electron beam is a circle 43. As a result, an ideal round beam can be obtained as the electron beam, and astigmatism adjustment for the electron beam is completed. 4, the width d _X in the X-axis direction represents the sharpness of the Y-axis direction of the line, the width d _Y in the Y-axis direction represents the sharpness of the X-axis direction line.

  In the above description, it has been described that the astigmatism adjustment in the Y-axis direction is performed after the astigmatism adjustment in the X-axis direction, but conversely, the astigmatism adjustment in the Y-axis direction is performed. Astigmatism adjustment in the X-axis direction may be performed so that the cross-sectional shape of the electron beam is circular 43.

  Therefore, in order to adjust astigmatism in the above-described process with respect to the electron beam having the cross-sectional shape of the ellipse 41 shown in FIG. In the following description, when the electron beam shape is corrected in the X-axis direction, the vertical line of the captured pattern is cleared, and when the correction in the Y-axis direction is performed, the horizontal pattern is cleared. Please keep in mind.

(1) An electron beam is irradiated onto a wafer having a pattern having a relatively large line and space width (for example, a line width of 180 nm), and a still image thereof is acquired with a predetermined resolution.
(2) A box pattern having lines that run vertically or horizontally in an acquired still image (for example, a box pattern having a line and a space of a certain width as shown in FIG. 3A). Move to.

(3) Using the autofocus function of the electron beam apparatus, the focus value Fv when the vertical line looks the clearest is obtained for the box pattern. Specifically, finding a focus value Fv1 when vertical lines waving correction voltage V _X to correct astigmatism in the longitudinal direction of the multipole astigmatism regulator 28 looks the most clear.

(4) Next, the focus value Fh when the horizontal line looks the clearest is obtained. Specifically, with the correction voltage V _X1 that is the vertical correction voltage at Fv1 fixed, the correction voltage V _Y that corrects the horizontal astigmatism of the multipole element of the astigmatism adjuster 28 is changed. Then, the focus value Fh1 when the horizontal line looks the clearest is obtained.

  If the irradiation beam shape is circular, Fv1 and Fh1 should match, but since the line width of the observation target is relatively large, for example, 180 nm, the beam shape is considered to be distorted in this order. It is done. Therefore, as the next step, a pattern image having a line with a smaller width is acquired, and the astigmatism of the electron beam is corrected based on the acquired image.

(5) An electron beam is irradiated to a wafer having a pattern in which the width of the line and space is smaller than (1) (for example, the line width is 150 nm), and a still image thereof is acquired with a predetermined resolution.
(6) Move to a box pattern having lines running vertically and horizontally in the acquired still image.

(7) Using the autofocus function of the electron beam apparatus, the focus value Fv when the vertical line looks the clearest is obtained for the box pattern. As described above, if the irradiation beam shape is circular, Fv1 and Fh1 should be the same, so the focus value to be obtained is considered to be between Fv1 and Fh1. Therefore, here shaking correction voltage _{V X} the average value Fo = (Fv1 + Fh1) / 2 of Fv1 and Fh1 as the initial value, determine the longitudinal best focus value Fv2.

(8) Next, horizontal adjustment is performed. With the correction voltage V _X2 , which is the correction voltage in the X-axis direction at the time of Fv2, fixed, the correction voltage V _Y for correcting the astigmatism in the horizontal direction among the multipole elements of the astigmatism adjuster 28 is shaken to change the horizontal direction The focus value Fh2 when the line looks the clearest is obtained.

  Thereafter, the process (5) to (8) is repeated for a box pattern having a smaller line width (for example, 130 nm) to correct the astigmatism of the electron beam. This process is repeated in consideration of the line width of the actual inspection object.

  In the present embodiment, a multipole of the astigmatism adjuster 28, that is, a quadrupole is used as the correction electrode. However, in the case of octupole and twelve, the X axis and the Y axis are orthogonal to each other. Since correction is possible not only in two axes but also in a multi-axis direction, the electron beam shape may be corrected to a sufficiently circular shape with only one box pattern.

In general, when the astigmatism adjustment is insufficient, the difference ΔF (= Fv1−Fh1) has a certain value. However, as the astigmatism adjustment proceeds, the difference ΔF decreases, and the difference between the X-axis direction and the Y-axis direction. The resolution of the pattern goes up. FIG. 5 shows the correlation between the magnitudes of the correction voltages V _X and V _Y and the autofocus evaluation value Fh or Fv. It can be seen from the figure that the autofocus evaluation value becomes maximum at a certain correction voltage value.

  FIG. 6 shows the transition of Fv and Fh when the astigmatism adjustment is performed in order using the box pattern with the line and space widths of 180 nm, 150 nm, and 130 nm by the above steps using the autofocus function. Shown in In the figure, “after 180 L & S adjustment” indicates a state after the astigmatism adjustment is performed for a box pattern having a line and space width of 180 nm, and “after 150 L & S adjustment” and “after 130 L & S adjustment”. "Has the same meaning. As shown in FIG. 6, every time astigmatism adjustment is performed using a box pattern with a small line width, the difference ΔF between Fv and Fh decreases, and after astigmatism adjustment with a box pattern with a line width of 130 nm. The difference ΔF converged to zero, and good resolution was obtained in both XY directions.

FIG. 7 shows V _X and V _Y in each box pattern when the adjustment in FIG. 6 is performed. In the figure, α = tan ⁻¹ (V _Y / V _X ), and R is the intensity. In FIG. 7, the units of V _X and V _Y are volts, and the unit of α is degrees. It only took about 20 seconds to complete the astigmatism adjustment using the above three different line width box patterns.

  As understood from the above description, the present invention obtains the optimum correction voltage for astigmatism adjustment using the autofocus evaluation value of the image acquired from the pattern formed on the sample. Astigmatism adjustment can be completed quickly with a simpler algorithm than before.

It is a figure which shows schematically the structure of one Embodiment of the electron beam apparatus which concerns on this invention. It is a figure which shows schematically the structure of other embodiment of the electron beam apparatus which concerns on this invention. (A) shows an example of a box pattern, and (B) is a conceptual diagram showing how the cross-sectional shape of an electron beam that spreads in one direction changes with the correction voltage during astigmatism adjustment. It is a figure which shows the cross-sectional shape of a general electron beam. It is a graph which shows the correlation of the correction voltage value and autofocus evaluation value in the electron beam apparatus shown in FIG.1 and FIG.2. It is a graph which shows the relationship between the line width of a pattern, and a focus value at the time of astigmatism adjustment performed using the electron beam apparatus shown in FIG.1 and FIG.2. It is a graph which shows the correction voltage value and azimuth which were used at the time of astigmatism adjustment in FIG. (A) And (B) is a conceptual diagram explaining astigmatism adjustment. It is a conceptual diagram which shows how the cross-sectional shape of an electron beam changes according to an azimuth and a correction voltage.

DESCRIPTION OF SYMBOLS 100, 200: Electron beam apparatus 101, 201: Electron beam emission part 102: Primary optical system 103: Secondary optical system 104, 203: Secondary electron detection part 105, 204: Astigmatism adjustment part 14, 26: Detector 15, 27: Image processing unit 16, 28: Astigmatism controller 17, 29: Astigmatism adjuster 202: Electro-optical system

Claims (5)

In an electron beam apparatus including an electron optical system having an astigmatism adjusting unit including a multipole element for adjusting astigmatism of an electron beam, a reflected electron emitted from the sample is irradiated with the electron beam. A method for adjusting astigmatism of an electron beam by detecting secondary electrons including backscattered electrons,
(A) acquiring an image of a first pattern including lines formed in the vertical direction and the horizontal direction on the sample;
(B) adjusting the electron optical system to obtain a first auxiliary voltage when the vertical line sharpness in the image is maximized;
(C) applying the first auxiliary voltage to the multipole to adjust the cross-sectional shape of the electron beam;
(D) When the electron optical system is adjusted with the first auxiliary voltage fixed when the vertical line looks the clearest, and the sharpness of the horizontal line in the image is maximized Obtaining a second auxiliary voltage of:
(E) Applying the second auxiliary voltage to the multipole so as to minimize the difference between the vertical line sharpness and the horizontal line sharpness in the cross-sectional shape of the electron beam. Adjusting the process to
(F) sequentially performing the steps (a) to (e) on a pattern having a line having a smaller line width than the line of the first pattern;
A method comprising the steps of: The method according to claim 1, wherein the multipole comprises a plurality of pairs of electrodes or coils facing each other about the optical axis of the electron beam. 3. The method according to claim 2, wherein the electrode has a first auxiliary electrode for adjusting a focus value for the vertical line and a second for adjusting a focus value for the horizontal line. And a supplementary electrode. The method according to claim 1, wherein the method is automatically performed using an autofocus function of the optical system. The method according to claim 1, wherein the sample is evaluated during the process using the method according to claim 1.

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