JP2002040322A - Autofocusing device and mark observing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately perform autofocusing. SOLUTION: This autofocusing device is provided with a slit part 3 having an aperture, an irradiation means 1 irradiating the slit part 3 with light, 1st projection means 4, 5 and 6 projecting the image of the slit part 3 to an object surface 21, 2nd projection means 11, 12 and 13 projecting a luminous flux reflected on the surface 21 to an observed image projection surface 8, a light splitting means 31 splitting the luminous flux reflected on the surface 21 into two, a photoelectric conversion means 34 receiving two luminous fluxes split by the splitting means 31, and control means 35 and 36 focusing the surface 21 on the projection surface 8 based on the signal intensity profile of the two luminous fluxes based on an output signal from the conversion means 34. The control means 35 and 36 analyze the signal intensity profile of the two luminous fluxes by plural threshold levels.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus apparatus for automatically focusing on a test substrate and a mark observing apparatus for observing marks on the test substrate such as a semiconductor wafer. In particular, the present invention is suitably applied to a microscope for observing marks on a test substrate, an alignment system mounted on an exposure apparatus, and the like.

[0002]

2. Description of the Related Art As a conventional autofocus device, there is one disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 6-214150. According to this device, the target surface is irradiated with the index light transmitted through the index slit, the light beam reflected from the test surface is separated into two light beams by the pupil division prism, and the distance between the two light beams is measured. As a result, information on the in-focus position was obtained.

[0003]

In the above prior art,
Based on the light intensity profiles of the two light beams, the edge portion of each light beam is obtained, and the distance between the edges is obtained to obtain information on the in-focus position.

However, when an edge portion is determined from a light intensity profile, if a pattern having a low reflectance is applied to an irradiation position corresponding to the edge portion on the surface to be inspected, the edge portion cannot be detected accurately. There is.
In such a case, there is a problem that the autofocus cannot be performed accurately.

[0005] It is an object of the present invention to provide an auto-focusing device that can accurately perform auto-focusing even in the case described above.

[0006]

Means for Solving the Problems To solve the above problems,
The autofocus device according to the present invention includes a slit section 3 having an opening, an irradiation unit 1 for irradiating the slit section 3 with light,
First projection means 4, 5, 6 for projecting the image of the slit section 3 on the object plane 21 and second projection means 11, 12, 13 for projecting the light beam reflected on the object plane 21 on the observation image projection plane 8;
A light splitting unit 31 that splits the light beam reflected by the object surface 21 into two, a photoelectric conversion unit 34 that receives the two light beams split by the light splitting unit 31, and an output signal from the photoelectric conversion unit 34. Control means 35 and 36 for focusing the object plane 21 on the observation image projection plane 8 based on the signal intensity profiles of the two light fluxes.
Has a configuration in which the signal intensity profiles of the two light beams are analyzed by a plurality of threshold levels.

Further, the control means 35, 36 may select an edge of the signal intensity profile of the two light beams by the analysis, and perform a focusing operation based on the selected distance between the edges.

The mark observation apparatus of the present invention comprises a slit section 3 having an opening, an irradiating means 1 for irradiating the slit section 3 with light, and an image of the slit section 3 on a mark 2 on an object plane 21.
First projecting means 4, 5, 6 for projecting to 0, and object plane 2
The second beam splitter 11, 12, and 13 project the light beam reflected by 1 on the observation image projection surface 8, the light beam splitter 31 that splits the light beam reflected by the object surface 21 into two, and the light beam splitter 31 Photoelectric conversion means 34 for receiving the two light beams
And 2 based on the output signal from the photoelectric conversion means 34.
Object plane 21 based on the signal intensity profiles of the two luminous fluxes
Control means 35, 36 for focusing on the observation image projection surface 8;
The image projected on the observation image projection plane 8 is captured, and the mark 2
It has an image processing means 9 for detecting the position of 0, and the control means 35 and 36 are configured to analyze the signal intensity profiles of the two light beams by a plurality of threshold levels.

Further, the control means 35 and 36 may be configured to select an edge of the signal intensity profile of the two light beams by the analysis and perform a focusing operation based on the selected distance between the edges.

[0010]

FIG. 1 is a diagram showing a configuration of a mark observation device provided with an autofocus device according to an embodiment of the present invention. In FIG. 1, an illuminating light beam of a broadband wavelength emitted from a light source 1 is condensed by a condenser lens 2 and uniformly illuminates a field stop 3. The field stop 3 has an opening (slit) S1 as shown in FIG. The luminous flux emitted from the field stop 3 is converted into an illumination relay lens 4
And the beam is split by the beam splitter 5. Further, the light is condensed by the first objective lens 6 and vertically falls on the wafer 21. Here, the field stop 3
And the wafer 21 are at conjugate positions, so that the image of the slit S1 of the field stop 3 is formed on the wafer 21 via the illumination relay lens 4 and the first objective lens 6.

The image of the slit S1 illuminates the mark 20 on the wafer 21. Here, the reflected light of the image of the slit S1 irradiated onto the wafer 21 is defined as L1. At this time,
The light beam L1 reflected from the surface of the wafer 21 is collimated by the first objective lens 6, transmitted through the beam splitter 5, and collected again by the second objective lens. The light beam transmitted and branched by the beam splitter 14 is the first light beam.
An image of a wafer mark is formed on the imaging surface of the CCD 8 serving as an imaging device by the relay lens 12 and the second relay lens 13. The output signal from the CCD 8 is processed by the image processing unit 9, and the position of the wafer mark is detected, and observation by a television monitor is performed.

On the other hand, the light beam reflected and branched by the beam splitter 14 is collimated by the AF first relay lens 30 and then forms an image of the light source on the pupil division mirror 31. The light beam L1 is split into two light beams by the pupil splitting mirror, and is condensed again by the AF second relay lens 32.
Then, the light flux L <b> 1 is imaged in two directions on the AF sensor 34 in the measurement direction via the cylindrical lens 33. The AF sensor 34 is a line sensor, and the measurement direction is the direction of arrow A in the figure. This measurement direction is shown in FIG.
(A) corresponds to the direction of arrow A '.

FIGS. 2A, 2B and 2C show the AF sensor 3
4 illustrates an image-forming state of a two-divided light beam L1 obtained on FIG. FIG. 2B shows an image-forming state when the wafer 21 is in focus with respect to the CCD 8, and the centers of the image-forming positions of the two divided light beams are P1 and P2. On the other hand, when the wafer 21 is below the in-focus state, as shown in FIG. 2A, the centers of the image forming positions of the two light fluxes are shifted in a direction closer to each other than P1 and P2. State. On the other hand, when the wafer 21 is located above the in-focus position, the state shown in FIG. 2C is shifted in a direction away from P1 and P2. Therefore, by moving the stage 22 up and down in the vertical direction, the images of the two light beams L2L and L2R move closer to or away from each other in the measurement direction. Then, a detection signal from the AF sensor 34 including information on the image forming position of the two light beams is processed by the signal processing unit 35 to calculate a distance between the two light beams. Advantages of such an AF detection method include an effect of averaging the reflectance unevenness of the wafer by increasing the length of the slit S1 in the non-measurement direction, and an effect of reducing the influence of the wafer reflectance unevenness by pupil division.

Further, the image processing unit 35 compares the calculated distance between the two light beams with a previously stored distance between the two light beams in the focused state, calculates the difference between the two light beams, and obtains the difference as focus position information. Output to the stage control unit 36. The stage control unit 36 moves the stage up and down based on the input information, and focuses the wafer mark image on the wafer 21 with respect to the CCD 8.

Next, a signal processing section and a processing method of the present embodiment will be described.

FIG. 3 is a view showing the observation field including the portion illuminated by the slit S1 and the observation mark, and shows a state in which a pattern outside the observation field is applied to the edge of the slit S1. . In addition, the intensity profile in the measurement direction of the slit image is also described. As described above, in order to have an effect of averaging the reflectance unevenness, a slit region is widely secured outside the observation visual field. Therefore, the pattern around the observation mark easily enters the slit area. In FIG. 3, since the pattern outside the observation visual field that is darker than the other portions is applied to the edge of the slit S1, the edge portion of the intensity profile has a gentle slope.

In such a state, the intensity profile based on the detection signal from the AF sensor 34 is as shown in FIG. In FIG. 4, the intensity profiles L and R are
2 shows two intensity profiles obtained as a result of being divided into two and forming an image on the AF sensor 34.

In the present embodiment, two slice levels are provided and an intensity profile is analyzed. In FIG. 4, SL1 and SL2 each represent a slice level. The intersection points between the slice level SL1 and the edge portions of the two intensity profiles L and R are denoted by L11, L21, and L21, respectively.
Let R11 and R21 be the intersections between the slice level SL2 and the edges of the two intensity profiles L and R, respectively L12 and R21.
L22, R12, and R22.

First, the distance between edges of two intensity profiles is obtained. In FIG. 4, the horizontal axis is the X axis, and the intersection L
X coordinates of 11, L21, R11, R21, L12, L22, R12, and R22 are respectively represented by l11, l21, r11, r21, l12, l22,
r12 and r22.

Intersection points L11 and R11 with the slice level SL1
Is the distance D11 between the intersection L21 and R21 with the slice level SL1.
Is the distance D12 between the intersection L12 and R12 with the slice level SL2.
Is the distance D22 between the intersection L22 and R22 with the slice level SL2.
Is D22 = r22-l22.

Next, the difference between the distances between edges at each slice level is determined.

Edge distance D at slice level SL1
Assuming that the difference between the edge distance D12 at the slice level SL2 and the edge distance D12 at the slice level SL2 is ΔD2, the difference between the edge distance D21 at the slice level SL2 and the edge distance D22 at the slice level SL2 is ΔD2. Then, ΔD2 = | D21−D22 | ΔD1 is the difference between the distances between edges corresponding to edge E1, and ΔD2 is the difference between the distances between edges corresponding to edge E2.

The distance D1 between edges obtained as described above
1, D21, D12, and D22 should ideally have substantially equal values.

In FIG. 4, the slopes α1 and α2 (generated by the pattern outside the observation field) of the two intensity profiles obtained as a result of being divided into two and formed on the AF sensor have the same shape. Must have. However, actually, the two do not have the same shape due to the aberration of the optical system of the autofocus.

In such a case, if the slice level is high, the slopes α1 and α2 of the edge E2 are affected by the slice level. Therefore, if the distance between edges is calculated based on the slice level, an error is included. If the slice level is sufficiently low with respect to the maximum value of the intensity profile, the above error may be avoided in some cases. It is not enough as a countermeasure. Therefore, in the present embodiment, a plurality of slice levels are provided,
The focus operation is performed by detecting only the edge information on the side not affected by the pattern outside the observation visual field.

.DELTA.D1 and .DELTA.D which are differences between the edge distances.
When comparing No. 2, the value at the edge affected by the pattern outside the observation visual field becomes larger. That is, in the above example, the value of ΔD2 is larger. Therefore, a focusing operation is performed based on edge information based on the smaller value. In this case, the focusing operation is performed based on the information of the edge E1 based on ΔD1.

It is also possible to provide two or more slice levels, obtain the distance between edges at each slice level, and determine the edge to be used for the focusing operation from the variation in the distance between edges. It is considered that there is little variation in the distance between edges that are not affected by the pattern outside the observation visual field.

As described above, in the present embodiment, by providing the signal processing unit capable of analyzing the intensity profile at a plurality of slice levels at a high speed, it is possible to determine an edge on the side outside the field of view that is not affected by the pattern. . Therefore, the detection accuracy for the focal position information is improved. Therefore, when this apparatus is applied to autofocus of a mark observation position on a test substrate such as a semiconductor wafer, a clear wafer mark image can be captured, and highly accurate mark dimension measurement and mark position detection can be performed. It becomes possible.

[0029]

As described above, according to the first to fourth aspects of the present invention, the focus control is performed by analyzing the signal intensity profile using a plurality of threshold levels. The focus control can be performed accurately even when a pattern having a low reflectance on the object surface is applied to the edge of the image.

According to the second and fourth aspects of the present invention,
Select the edges of the signal intensity profile of the two beams,
Since the focusing operation is performed based on the selected distance between edges, even if a pattern with a low reflectance on the object surface is applied to the edge of the image of the slit portion projected on the object surface, the pattern is affected by the pattern. It is possible to select a non-existing edge, so that focusing control can be performed accurately.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of a mark observation device provided with an autofocus device according to an embodiment of the present invention.

FIG. 2 is a diagram showing an image forming state of a two-divided light beam L1 obtained on an AF sensor 34 of the autofocus device according to the embodiment of the present invention.

FIG. 3 is a diagram showing an observation field including a portion illuminated by a slit S1 and an observation mark in the autofocus apparatus according to the embodiment of the present invention.

FIG. 4 is a diagram showing an intensity profile based on a detection signal from an AF sensor in the autofocus device according to the embodiment of the present invention.

[Explanation of symbols]

1: light source, 2: condenser lens, 3: field stop, 4:
Illumination relay lens, 5: half prism, 6: first objective lens, 7: second objective lens, 8: CCD, 9: image processing unit, 10: illumination aperture stop, 11: imaging aperture stop, 1
2: 1st relay lens, 13: 2nd relay lens, 1
4: half prism, 20: mark, 21: wafer, 2
2: stage, 30: AF first relay lens, 31: pupil division mirror, 32: AF second relay lens, 33: cylindrical lens, 34: AF sensor, 35: signal processing unit, 36: stage control unit.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01L 21/30 526A

Claims (4)

[Claims] A slit having an opening; an irradiating means for irradiating the slit with light; a first projecting means for projecting an image of the slit on an object plane; and a light beam reflected by the object plane. A second projection unit that projects the light beam reflected on the object surface into two, a light splitting unit that splits the light beam reflected by the object surface, a photoelectric conversion unit that receives the two light beams split by the light splitting unit, Control means for focusing the object plane on the observation image projection plane based on a signal intensity profile of the two light fluxes based on an output signal from a photoelectric conversion means, wherein the control means comprises: An autofocus apparatus for analyzing the signal intensity profile of the image signal based on a plurality of threshold levels. 2. The auto-focusing device according to claim 1, wherein said control means selects an edge of a signal intensity profile of said two light beams by said analysis, and selects an edge based on the selected distance between edges. An auto-focusing device for performing a focusing operation. 3. A slit section having an opening, an irradiating means for irradiating the slit section with light, and a first section for projecting an image of the slit section onto a mark on an object plane.
Projection means for projecting a light beam reflected on the object surface onto an observation image projection surface, a light dividing device for dividing the light beam reflected on the object surface into two, and divided by the light dividing device. Photoelectric conversion means for receiving the two light fluxes, and control means for focusing the object plane on the observation image projection plane based on a signal intensity profile of the two light fluxes based on an output signal from the photoelectric conversion means. Image processing means for capturing an image projected on the observation image projection surface and detecting the position of the mark, wherein the control means sets the signal intensity profile of the two light fluxes by a plurality of threshold levels. A mark observation device characterized by analyzing. 4. The mark observing device according to claim 3, wherein the control means selects an edge of the signal intensity profile of the two light beams by the analysis, and selects an edge based on the selected distance between edges. A mark observing device for performing a focusing operation.