JPH10288733A - Automatic focusing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automatic focusing device capable of easily focusing an object. SOLUTION: The automatic focusing device is provided with a slit plate 3 with plural focus detecting slits formed, an illuminating optical system ILS for projecting the image of the slit on the object, a focusing optical system AF for forming the reflected image of the image and a photoelectric converter 17 for detecting the reflected image. And also, the device is provided with a calculating part (focus signal selecting part etc.) for calculating plural focal positions based on the focus signals as for plural slit images.

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 an optical system such as a microscope or a semiconductor circuit inspection apparatus on an object. In particular, the present invention relates to an autofocus apparatus suitable for an apparatus for observing and inspecting a circuit formation surface (on the way and in a completed state) of a semiconductor device or a liquid crystal device.

[0002]

2. Description of the Related Art The prior art will be described by taking a semiconductor device inspection apparatus as an example. FIG. 3 is a diagram schematically showing a configuration of an optical system (knife edge method) and a control system of a conventional semiconductor inspection apparatus. This inspection apparatus is an apparatus that forms an enlarged image of a semiconductor circuit layer and a photoresist layer formed on the upper surface of a wafer 7 as an object, and inspects the shapes and dimensions of those layers. The wafer 7 is fixed on a wafer stage 8 via a holder such as a vacuum chuck or an electrostatic chuck. The wafer stage 8 is scanned in the optical axis direction and in a vertical plane (Z direction, XY plane).

[0003] The inspection apparatus shown in FIG. 3 includes an observation optical system OBS disposed immediately above the wafer 7, an illumination optical system ILS and an autofocus optical system AFS disposed beside the optical system OBS. .

In the illumination optical system ILS, a light source 1, a condenser lens 2, an AF slit plate 3, a projection lens 4, and a half mirror 5 are sequentially arranged on the optical axis. The illumination light IL emitted from the light source 1 is condensed through the condenser lens 2 and then enters the AF slit plate 3. A slit for autofocus is formed in a peripheral portion of the AF slit plate 3. The central portion of the AF slit plate 3 is transparent. The illumination light emitted from the AF slit plate 3 is given an appropriate illumination magnification by the projection lens 4. A first half mirror 5 is placed at a point where the optical axes of the illumination optical system ILS and the observation optical system OBS intersect. The illumination light IL exiting the projection lens 4 strikes the half mirror 5 and is reflected downward, and then the first objective lens 6
The wafer 7 is illuminated via.

[0005] In the observation optical system OBS, a first objective lens 6, a first half mirror 5, a second half mirror 9, a second objective lens 10, and an imaging lens 1
One and two-dimensional photoelectric conversion means (CCD camera) 12 are arranged. Among them, the first objective lens 6 is shared by both the autofocus optical system AFS and the illumination optical system ILS.

The illumination light incident on the wafer 7 is reflected on the upper surface of the wafer 7. The reflected light RL is directed upward, passes through the first objective lens 6 and the first half mirror 5, and reaches the second half mirror 9. The second half mirror 9 receives the reflected light RL
Are passed upwards and partially reflected laterally. The transmitted light OL is used for observation or measurement, passes through the second objective lens 10 and the imaging lens 11, and forms an enlarged image on the photoelectric conversion surface of the CCD camera 12.

The light FL reflected by the second half mirror 9 is
Enter the autofocus optical system AFS. In the autofocus optical system AFS, a relay lens 13, a mirror 26, a knife edge 22, an imaging lens 15, a cylindrical lens 16, and an AF photoelectric conversion unit 17 are arranged in this order from the half mirror 9. The knife edge 22 is disposed substantially at the pupil position of the autofocus optical system AFS, and its tip coincides with the optical axis. Photoelectric conversion means 17
Is an autofocus optical system A including the first objective lens 6
In the FS, it is located at a position conjugate with the pupil provided with the wafer 7 and the knife edge 22. Cylindrical lens 1
6 compresses the autofocus light FL in the width direction and forms an image on the photoelectric conversion means 17. The photoelectric conversion means 17 in this device is a linear CCD sensor having a width of about 10 μm and a length of 20 mm. As a result, a portion of the image of the slit for AF projected on the wafer 7 that is not shielded by the knife edge 22 is imaged on the AF photoelectric conversion unit 17.

A signal relating to the AF slit detected by the AF photoelectric conversion means 17 is sent to a focus position detecting section 18 in a control section of the inspection apparatus, where the signal is processed (details will be described later), and a normal focus position is detected. . The signal relating to the focus position is sent to the focus actuator drive unit 21, which sends the signal to the Z stage of the wafer stage 8.
The actuator for driving the table is driven to move the wafer 7 up and down, and the upper surface of the wafer 7 is positioned at a regular focus position.

FIG. 4 shows an image profile of an AF slit on the AF photoelectric conversion means in the apparatus shown in FIG.
The horizontal axis represents the longitudinal position of the photoelectric conversion means, and the vertical axis represents the output of the photoelectric conversion means at the position. In the figure, the one-dot chain line CL in the vertical direction is a line that bisects the area of the profile at the time of focusing, and has already been given at the time of adjustment of the apparatus. The focus position detection unit 18
Upon receiving a signal representing the profile of the photoelectric conversion means 17, the areas a and b of the profile on the left and right of the line CL are calculated. And from the difference between the two areas, the amount of defocus,
That is, a dimension for moving the wafer up and down to the focus position is calculated.

FIG. 4A shows a focus state in which the profile areas a and b on the left and right sides of the line CL are equal.
The profile has a curve that sharply drops on the right side where the knife edge exists. FIG. 4B shows a defocused state in which the area a 'of the left profile of the line CL is larger than the area b' of the right profile (a '>b'). This is because when defocusing occurs, light passing through the knife edge side is blocked by the knife edge, while light passing through the anti-knife edge side passes through to the photoelectric conversion means. is there.

[0011]

In the above conventional example, there is one AF slit for detecting the focus position. And it was common to place this AF slit outside the field of view of the observation optical system. Also, various patterns are formed on the wafer surface of the observation target by a semiconductor process or a photoresist, and there are irregularities exceeding the depth of focus of the observation optical system. For this reason, the autofocus device may be focused at a different focus from the visual field of the measurement / observation target.

Further, even when an AF slit is placed in the measurement visual field, other regular patterns are arranged in a region near the edge of the pattern where pattern regularity is lost, such as a wafer edge. In some cases, the focus was unexpectedly changed with a different focus. In such a case, the focus can be adjusted by giving a focusing offset to the apparatus, but since the current level of the focusing position changes from time to time, it was not possible to perform processing by giving a universal offset.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an autofocus apparatus which can easily obtain a focus on an object.

[0014]

In order to solve the above-mentioned problems, an auto-focusing device according to the present invention is an auto-focusing device for focusing an optical system on an object; a plurality of sensors for detecting a focus state. Mechanism and
A focus actuator that adjusts a focus position by moving an object or an optical system in an optical axis direction; a calculation unit that calculates a plurality of focus positions based on focus signals from the plurality of sensor mechanisms; And a selector for selecting one of the focus positions.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one embodiment of the present invention,
A plurality of focus detection slits provided in a field of view of an objective lens of the optical system, an illumination optical system that projects an image of the focus detection slit onto an object, and a reflection image of the image. A focus optical system that forms an image; and a photoelectric converter that detects the reflected image. The object is a circuit surface of a semiconductor device, and the selection unit selects a position closest to or farthest from the objective lens of the optical system (particularly a position closest to the objective lens) among the plurality of focus positions. be able to. When the highest position of the test object (the surface of the photoresist in the example of the semiconductor wafer) is the most stable surface in the test object, such as a semiconductor wafer subjected to photoresist coating, exposure, and development. Thus, even when the AF slit is projected on the edge of the test object or outside the observation field of view, it is possible to realize a focus position that is hardly affected by differences in circuit patterns. Also, when an offset is given to the autofocus, the current in-focus position is somewhat clear, and a value estimated as a step between the observation target and the top surface may be given as the offset. Focusing can be achieved quickly.

Hereinafter, description will be made with reference to the drawings. FIG.
FIG. 1 is a diagram schematically showing a configuration of an optical system (pupil division prism type) and a control system of a semiconductor device inspection apparatus according to one embodiment of the present invention. This inspection apparatus is an apparatus that forms an enlarged image of a semiconductor circuit layer and a photoresist layer formed on the upper surface of a wafer 7 as an object, and inspects the shapes and dimensions of those layers. The wafer 7 is fixed on a wafer stage 8 via a holder such as a vacuum chuck or an electrostatic chuck. The wafer stage 8 is scanned in the optical axis direction and in a vertical plane (Z direction, XY plane).

The inspection apparatus shown in FIG. 1 includes an observation optical system OBS disposed immediately above the wafer 7, an illumination optical system ILS and an autofocus optical system AFS disposed beside the optical system OBS. .

In the illumination optical system ILS, a light source 1, a condenser lens 2, an AF slit plate 3, a projection lens 4, and a half mirror 5 are sequentially arranged on the optical axis. The light source 1 is a lamp such as a halogen lamp. Light source 1
The illumination light IL emitted from the lens is condensed through the condenser lens 2 and then enters the AF slit plate 3. A
A slit for auto focus (the size on the wafer 7 and the magnification of the objective lens are 1
In the case of 00 ×, a plurality of 10 μm × 100 μm, generally, a length of 1/2 to 1 of the visual field and a width of 1/20 to 1/10 of the visual field are formed. The central portion of the AF slit plate 3 is transparent. The illumination light emitted from the AF slit plate 3 is given an appropriate illumination magnification by the projection lens 4. A first half mirror 5 is placed at a point where the optical axes of the illumination optical system ILS and the observation optical system OBS intersect. The illumination light IL exiting the projection lens 4 strikes the half mirror 5 and is reflected downward, and then illuminates the wafer 7 via the first objective lens 6.

In the observation optical system OBS, a first objective lens 6, a first half mirror 5, a second half mirror 9, a second objective lens 10, and an imaging lens 1 are arranged in this order from the side closer to the wafer 7.
One and two-dimensional photoelectric conversion means (CCD camera) 12 are arranged. Among them, the first objective lens 6 is shared with the autofocus optical system AFS and the illumination optical system ILS.

The illumination light incident on the wafer 7 is reflected on the upper surface of the wafer 7. The reflected light RL is directed upward, passes through the first objective lens 6 and the first half mirror 5, and reaches the second half mirror 9. The second half mirror 9 receives the reflected light RL
Are passed upwards and partially reflected laterally. The transmitted light OL is used for observation or measurement, passes through the second objective lens 10 and the imaging lens 11, and forms an enlarged image on the photoelectric conversion surface of the CCD camera 12.

The light FL reflected by the second half mirror 9 is
Enter the autofocus optical system AFS. In the autofocus optical system AFS, a relay lens 13, a pupil splitting prism 14, an imaging lens 15, a cylindrical lens 16, and an AF photoelectric conversion unit 17 are arranged in order from the half mirror 9. The pupil splitting prism 14 is arranged substantially at the pupil position of the autofocus optical system AFS, and its dividing line (center ridge) coincides with the optical axis. In the autofocus optical system AFS including the first objective lens 6, the photoelectric conversion unit 17 is located at a position conjugate with the wafer 7 and the pupil where the pupil division prism division line is provided. The cylindrical lens 16 compresses the autofocus light FL in the width direction and forms an image on the photoelectric conversion unit 17. The photoelectric conversion means 17 in this device is a linear CCD sensor having a width of about 10 μm and a length of 20 mm. Eventually, the AF slit image projected on the wafer 7 is split into two by the pupil splitting prism 14 and formed on the AF photoelectric conversion means 17.

Signals relating to a plurality of AF slits detected by the AF photoelectric conversion means 17 are sent to focus position detecting sections 18 and 19 via a focus slit dividing section 23 in a control section of the inspection apparatus, and are processed ( The focus position is detected (described later in detail). The signal related to the focus position is sent to the focus signal selection unit 20, and the signal closest to the objective lens 6 (highest one) is selected. The signal of the selected focus position is sent to the focus actuator driving unit 21, and the unit 21
The actuator for driving the Z table of the wafer stage 8 is driven to move the wafer 7 up and down, and the upper surface of the wafer 7 is positioned at the highest focus position.

FIG. 2 shows an image profile of an AF slit on the AF photoelectric conversion means in the apparatus shown in FIG.
The horizontal axis represents the longitudinal position of the photoelectric conversion means, and the vertical axis represents the output of the photoelectric conversion means at the position. In the figure, a dashed-dotted line CL in the vertical direction is a line that bisects the area of the profile at the time of focusing. In FIG. 2, in each stage (FIGS. 2A, 2B, and 2C), four peak-like profiles 31, 31 ', 3
2, 32 'are shown. Profile 3
Reference numerals 1 and 31 ′ denote reflected light from an image of one AF slit on the wafer, which is split into two by the pupil splitting prism 14. 32 and 32 'are the other (second) AF
Slits are similarly divided.

FIG. 2A is a slit image at the time of balance focusing, and is divided into left and right profiles 31,
The intervals between 31 'and 32, 32' are equal in the left and right profiles 31, 32, and are L, which is a standard value at the time of focusing. FIG.
(B) is a slit image at the time of defocusing, and is divided into left and right profiles 31, 31 'and 32, 32'.
Is equal in the left and right profiles 31 and 32, but is L 'different from the standard value L at the time of focusing. FIG.
(C) is a slit image at the time of unbalance focus, and is divided into left and right profiles 31, 31 'and 3
The intervals between 2 and 32 'are different for the left and right profiles 31 and 32, and are L "and L', respectively.
There are steps at the two wafer surfaces where the F slits form an image, and the focus of each AF slit is focused on the wafer surface at a different step. That is, when there are steps on two wafer surfaces where two AF slits form an image, the divided left and right profiles 31 are used.
The distance between 31 'and 32, 32' is the left and right profile 3
It is a figure showing that L ″ and L ′ are different for Nos. 1 and 32, respectively.

The photoelectric conversion means signal shown in FIG. 2 is first sent to the focus slit dividing section 23 and divided into a pair of profiles for each AF slit. The mechanism of this division may be to count the number of slit images,
It may be divided independently at a position on the photoelectric conversion means 17. When there are two photoelectric conversion units, the focus slit dividing unit is unnecessary. Next, the signal of the pair profile of each AF slit is output to two focus position detection units 1.
8 and 19 respectively. Here, the intervals L, L ', L "between the pairs are detected, and the corresponding focus position signals are sent to the focus signal selector 20.

In the case of an unbalanced focus as shown in FIG. 2C, the focus signal selector 20 compares the above L 'and L "with the focus position comparing means. Is longer, the longer L ″, that is, the focus position closer to the objective lens 6 is selected by the focus position selector. This focus position is applied to the focus actuator drive unit 21,
The Z stage of the wafer stage 8 is moved to focus on the highest slit imaging portion.

In the above embodiment, the case where the number of AF slits is two has been described, but the number of AF slits may be two or more. The arrangement of the AF slit may be in any of a tangential direction, a normal direction, and an inclined direction of the field circle. Further, the focus position comparing means can be configured using circuits such as a magnitude comparator and an analog comparator, and can also be configured by software on a computer. The focus position selecting means may be constituted by a circuit such as a data selector and an analog switch or may be realized by software.

[0028]

As is apparent from the above description, according to the present invention, it is possible to provide an autofocus apparatus which can easily obtain a focus on an object. In particular, in the embodiments described in this specification, AF is set as a focus detection position at a position close to the objective lens among the obtained focus positions, so that a semiconductor wafer that has been subjected to photoresist coating, exposure, and development is used. In the case where the highest position of the test object (the surface of the photoresist) is the most stable surface in the test object, it is possible to realize a focus position that is hardly affected by the pattern.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of an optical system (pupil division prism type) and a control system of a semiconductor inspection apparatus according to one embodiment of the present invention.

FIG. 2 is a view showing A on photoelectric conversion means for AF in the apparatus of FIG. 1;
3 shows an image profile of an F slit. The horizontal axis represents the longitudinal position of the photoelectric conversion means, and the vertical axis represents the output of the photoelectric conversion means at the position.

FIG. 3 is a diagram schematically showing a configuration of an optical system (knife edge method) and a control system of a conventional semiconductor inspection apparatus.

FIG. 4 is a diagram showing A on the photoelectric conversion means for AF in the apparatus shown in FIG. 3;
3 shows an image profile of an F slit. The horizontal axis represents the longitudinal position of the photoelectric conversion means, and the vertical axis represents the output of the photoelectric conversion means at the position.

[Explanation of symbols]

Reference Signs List 1 light source 2 condenser lens 3 AF slit 4 projection lens 5 first half mirror 6 first objective lens 7 wafer 8 wafer stage 9 second half mirror 10 second objective lens 11 imaging lens 12 two-dimensional photoelectric conversion means 13 relay lens Reference Signs List 14 pupil division prism 15 imaging lens 16 cylindrical lens 17 photoelectric conversion means for AF 18 first focus position detection means 19 second focus position detection means 20 focus signal selection means 21 focus / actuator drive means 22 knife edge 23 focus slit division means 26 mirror

Claims (3)

[Claims] An auto-focus device for focusing an optical system on an object; a plurality of sensor mechanisms for detecting a focus state; and adjusting a focus position by moving the object or the optical system in an optical axis direction. A focus actuator that calculates a plurality of focus positions based on focus signals from the plurality of sensor mechanisms; and a selection unit that selects one of the calculated plurality of focus positions. An autofocus device characterized by: 2. A sensor system comprising: a plurality of focus detection slits provided in a field of view of an objective lens of the optical system; an illumination optical system for projecting an image of the focus detection slit onto an object; The autofocus device according to claim 1, further comprising: a focus optical system that forms a reflection image of the image; and a photoelectric converter that detects the reflection image. 3. The method according to claim 2, wherein the object is a circuit surface of a semiconductor device, and the selector selects a position closest to or farthest from the objective lens of the optical system among the plurality of focus positions. The autofocus device according to claim 1 or 2, wherein