JP3264263B2 - Overlay accuracy measuring device and measuring method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and a method for measuring overlay accuracy optically in a photolithography process.

[0002]

2. Description of the Related Art In a photolithography process at the time of manufacturing a semiconductor integrated circuit device, improvement in overlay accuracy has become more and more important with the miniaturization of pattern rules. In a conventional overlay accuracy measuring apparatus, a combination of a normal optical microscope and a CCD camera is used for an optical system for obtaining a mark waveform from a measurement mark.
FIG. 6 is a configuration diagram showing a conventional overlay accuracy measuring device. This optical system has a very large depth of focus, and cannot focus on, for example, only the bottom of the mark side wall.

[0003]

As described above, in the conventional overlay accuracy measuring device, a combination of a normal optical microscope and a CCD camera is used for an optical system.
It was not possible to focus only on the bottom of the mark side wall.

In particular, in the hole process, since the cross-sectional shape of the resist is strongly affected by the coma aberration of the exposure machine lens, the side wall of the resist 3 of the measurement mark is also asymmetric as shown in FIG.

[0005] Therefore, the peak position of the mark waveform is affected by the asymmetry of the side wall shape of the measurement mark, and a measurement error of several tens nm at the maximum occurs during overlay measurement.

An object of the present invention is to provide an overlay accuracy measuring apparatus and a measuring method capable of improving the overlay measurement accuracy.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a superposition accuracy measuring apparatus using a combination of an optical microscope using a confocal scanning method and a CCD camera as an optical system for obtaining a mark waveform of a measurement mark. The light reflected from the surface of the wafer passes through the objective lens and forms an image at the pinhole on the Nipkow disk. The light reflected from a position other than the focal point on the wafer is blocked, and the image is formed at the pinhole. After passing through an image lens, the formed light is re-imaged on the image sensor of the CCD camera, and is taken in as image data.

Further, according to the overlay accuracy measuring method of the present invention, an approximate focus position is obtained using the overlay accuracy measuring device, and then the peak intensity or peak position of a mark waveform obtained by a CCD camera is referred to. The focus position is moved, the true focus position is defined, and the overlay accuracy is measured.

According to the present invention, it is possible to focus only on an arbitrary position of the mark side wall, so that it is possible to prevent deterioration of measurement accuracy due to asymmetry of the measurement mark side wall shape.

[0010]

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an overlay accuracy measuring apparatus according to the present invention. As an optical system for obtaining a mark waveform, a combination of an optical microscope using a confocal scanning method and a CCD camera is used.

Illumination light emitted from a white or monochromatic illumination light source 21 such as a mercury lamp or an argon laser is
The dichroic mirror 24 is a scanning plate having pinholes arranged in a spiral shape.
Disk) 27, the wafer 2 via the objective lens 23
Guided on 2.

The wafer 22 is fixed to a wafer stage 28 which is movable in a direction parallel to and perpendicular to the wafer 22.

The light reflected on the surface of the wafer 22 passes through the objective lens 23 and forms an image on a pinhole on the Nipko disk 27. At this time, since the light reflected from a position other than the focal point on the wafer 22 is blocked, the overlay accuracy measuring apparatus according to this embodiment has a maximum of about 5 nm in the z-axis direction (perpendicular to the substrate). Is obtained. Actually, a plurality of pinholes are formed in the Nipko disk 27 so as to compensate for a decrease in light intensity when passing through the pinholes and to enable two-dimensional observation, and the Nipko disk 27 is rotating at high speed. .

The light imaged by the pinhole passes through the image lens 25 and then forms an image again on the CCD image sensor 26, and is captured as image data.

By using a combination of an optical microscope and a CCD camera using the above confocal scanning method,
Up to 5 nm in z-axis direction (perpendicular to substrate)
A degree of resolution is obtained. Therefore, for example, since it is possible to focus only on the bottom of the mark side wall, it is possible to prevent the measurement accuracy from being deteriorated due to the asymmetry of the measurement mark side wall shape.

Next, a description will be given of a first method for focusing on an arbitrary position of a measurement mark using the overlay accuracy measuring device shown in FIG.

FIG. 2 is a diagram showing the relationship between the cross-sectional shape of the measurement mark and the mark waveform, and FIG. 3 is a diagram showing an example of a change in peak intensity near the approximate focal position obtained by the laser. . As an example, a case where the focus is focused on the mark bottom 6 in FIG. 2 will be described.

First, the wafer stage 28 is moved up and down in the z-axis direction, and an approximate focus position is obtained using a laser beam or the like. Then, focusing on the vicinity of the focal position, the CCD
The wafer stage 28 is moved up and down again in the z-axis direction within a preset range with reference to the peak intensity obtained from the camera. Based on the relationship between the z-axis position of the wafer stage obtained at this time and the peak intensity, the true focus position (the bottom of the mark) is defined, and the overlay accuracy is measured.

For example, attention is paid to the peak intensities of the peaks 4 and 5 of the mark waveform 1 obtained from the measurement mark 2. FIG. 3 shows an example of a change in peak intensity near the approximate focal position obtained by the laser. Both peak intensities change from a linear region to a non-linear region at a certain point. When the focal position is on the side wall of the resist 3, changing the z-axis position does not significantly affect the peak intensity (linear region).

However, the focal position is set so that the focal position moves from the bottom of the resist 3 to the inside of the wafer.
When the axial position is changed, the resist 3 does not exist at the focal position, so that the peak intensity is rapidly attenuated (non-linear region).
However, the peak disappears from a certain position. Therefore, by defining the z-axis at the boundary between the linear region and the non-linear region and setting this as the true focal position, it is possible to accurately focus only on the bottom of the resist 3 (= the mark bottom 6).

Therefore, it is possible to obtain positional information on the bottom of the resist irrespective of the state of the side wall of the resist 3, and it is possible to suppress deterioration in measurement accuracy due to the asymmetry of the side wall shape of the resist.

The above-described method is for focusing only on the bottom of the mark. Similarly, the overlay inspection is performed based on the positional information on the top of the mark, or the overlay is inspected at an arbitrary distance from the bottom of the mark or the top of the mark. Obviously, it is also possible to measure the overlay accuracy based on the position information at the offset location.

As described above, according to this method, it is possible to focus on only an arbitrary position of the measurement mark, so that it is possible to suppress deterioration of measurement accuracy due to asymmetry of the side wall shape of the measurement mark.

Next, a second method for focusing on an arbitrary position of the measurement mark will be described. FIG. 4 is a diagram illustrating a relationship between a cross-sectional shape of a measurement mark and a mark waveform.
FIG. 5 is a diagram illustrating an example of a change in the peak position near the approximate focal position obtained by the laser.

In the first method described above, the true focus position is determined based on the peak intensity. In the second method, the true focus position is determined based on the peak position instead of the peak intensity. is there.

As in the first method, a combination of an optical microscope and a CCD camera using the confocal scanning method shown in FIG.

First, the wafer stage 28 is moved up and down in the z-axis direction, and an approximate focus position is obtained by using a laser beam or the like. Wafer stage 2
8 is moved up and down again in the z-axis direction within a preset range. The z of the wafer stage obtained at this time
Based on the relationship between the axial position and the peak position, a true focus position (a position near the bottom of the resist 53 and not affected by the footing shape) is defined, and the overlay accuracy is measured.

For example, attention is paid to the peak positions of the peak 54 and the peak 55 of the mark waveform 51 obtained from the measurement mark 52 in FIG. FIG. 5 shows an example of a change in the peak position near the approximate focal position obtained by the laser. Both peak positions change from a linear region to a non-linear region at a certain point. This is because the focal position is on the side wall of the resist 53 in the linear region, but is located near the bottom of the resist in the non-linear region.

When the focal position is on the side wall of the resist 53, the change in the peak position is relatively small even if the z-axis position is changed (linear region). However, if the focal position is
When the z-axis position is changed so as to be near the bottom of the resist 53, the peak position greatly changes due to the bottoming shape of the bottom of the resist 53 (non-linear region).

Here, z is defined at the boundary between the linear region and the nonlinear region.
By determining the axis and setting this as the true focal position, it is possible to accurately focus on a position near the bottom of the resist 53 (= the mark bottom 56) and not affected by the footing shape.

As described above, the second method uses the resist 53
It is possible to accurately obtain resist position information irrespective of the asymmetry of the side wall and the footing shape of the resist, thereby suppressing deterioration in measurement accuracy due to the asymmetry of the sidewall shape and the footing shape of the bottom due to these. be able to. Therefore, the second method is that the side wall of the measurement mark is particularly asymmetric,
This is effective when the bottom of the mark has a bottomed shape.

[0033]

As described above, according to the present invention, it is possible to focus on only an arbitrary position of the measurement mark, and thus it is possible to suppress the deterioration of the measurement accuracy due to the side wall shape asymmetry of the measurement mark. .

Further, according to the present invention, it is possible to accurately obtain resist position information irrespective of the asymmetry of the side wall of the measurement mark or the footing shape. Deterioration of measurement accuracy due to the skirting shape can be suppressed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an overlay accuracy measuring device according to the present invention.

FIG. 2 is a diagram illustrating a relationship between a cross-sectional shape of a measurement mark and a mark waveform.

FIG. 3 is a diagram illustrating an example of a change in peak intensity near an approximate focal position obtained by a laser.

FIG. 4 is a diagram showing a relationship between a cross-sectional shape of a measurement mark and a mark waveform.

FIG. 5 is a diagram illustrating an example of a change in a peak position near an approximate focal position obtained by a laser.

FIG. 6 is a configuration diagram showing a conventional overlay accuracy measuring device.

FIG. 7 is a diagram showing a relationship between a cross-sectional shape of a measurement mark and a mark waveform.

[Explanation of symbols]

 1, 51, 71 Mark waveform 2, 52, 72 Measurement mark 3, 53, 73 Resist 4, 5, 54, 55 Peak 6, 56 Mark bottom 21, 61 Illumination light source 22, 62 Wafer 23, 63 Objective lens 24, 64 Dichroic mirror 25,65 Image lens 26,66 CCD image sensor 27 Nipko disk 28 Wafer stage

Claims (5)

(57) [Claims] A superposition accuracy measuring apparatus using a combination of an optical microscope and a CCD camera using a confocal scanning method as an optical system for obtaining a mark waveform of a measurement mark, the apparatus being reflected on a surface of a wafer. The light passes through the objective lens, forms an image at a pinhole on the Nipko disk, blocks light reflected from a position other than the focal point on the wafer, and passes the image formed at the pinhole through the image lens. Later the CCD
An overlay accuracy measuring device characterized by re-imaging on an image sensor of a camera and taking in as image data. 2. An approximate focus position is obtained by using the overlay accuracy measuring apparatus according to claim 1, and then the focus position is moved while referring to a peak intensity of a mark waveform obtained by a CCD camera. A method for measuring overlay accuracy, wherein a focus position is defined and overlay accuracy is measured. 3. A linear region in which a change in peak intensity is relatively small even when changed in a direction perpendicular to the wafer, and a non-linear region in which peak intensity changes greatly when changed in a direction perpendicular to the wafer. 3. The method according to claim 2, wherein a boundary is defined as the true focal position. 4. An approximate focus position is obtained by using the overlay accuracy measuring device according to claim 1, and then the focus position is moved while referring to a peak position of a mark waveform obtained by a CCD camera to obtain a true focus position. A method for measuring overlay accuracy, wherein a focus position is defined and overlay accuracy is measured. 5. A linear region in which a change in peak position is relatively small even when changed in a direction perpendicular to the wafer, and a non-linear region in which a peak position changes greatly when changed in a direction perpendicular to the wafer. The method according to claim 4, wherein a boundary is defined as the true focal position.