JPH0942925A - Alignment optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To match a reference pattern with a target pattern with a high degree of accuracy, by providing a spatial filter having a specific number of apertures which are line-symmetric with respect to a plurality of line segments that pass through the center of the pupil of an objective lens and that are parallel with a regular arrangement direction of the pattern and, orthogonal to each other. SOLUTION: A spatial filter 11 has more than four apertures which are line-symmetric with respect to two line segments that pass through the center of the pupil of an objective lens 12 in parallel with a regular arrangement direction of a pattern and that cross together, orthogonal to one another. Light emitted from a fiber bundle 16 is projected onto the filter 11 through an illuminating lens 17 and a half-mirror 18, and passes through one point of one of the apertures so as to serve as a secondary light source. Then, it is turned into a parallel ray beam which is irradiated onto a sample 13 by an angle ϕwith respect to an optical axis Lc by means of a lens 12. A zero-degree diffraction light beam among the diffraction light beams passes, as a normal reflection light beam, through the aperture of the filter 11 by means of the lens 12, and is led to an image plane by a focusing lens 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for precisely positioning and aligning an object such as a semiconductor wafer having a regular pattern formed on its surface, and more particularly to an optical system thereof.

[0002]

2. Description of the Related Art As a device of this type, Japanese Patent Publication No. 3-27
No. 043 proposes an automatic precision alignment system for a dicing machine. In addition, actual fairness 5
No. 37476 discloses an automatic aligner for a prober. The positioning method of semiconductor wafers common to these conventional devices, that is, the alignment method, is first compared by aiming at a break point between chips called "street" by an optical system capable of observing a wide range at a relatively low magnification. After performing a rough alignment, the optical system that observes a narrow range at a relatively high magnification is then used to perform a precise alignment by targeting a characteristic pattern, such as a wiring cross section or a wire bonding pad. Is. Further, the shapes of the streets and the bonding pads are determined by a template matching method with a standard pattern stored in the image memory in advance.

In recent semiconductors, the number of inspection items in the manufacturing process has increased as the size and functionality of chips have increased.
Inspection devices such as probers are also becoming larger and more expensive. Therefore, semiconductor manufacturers are trying to speed up the inspection and improve the cost performance of the inspection device by providing one inspection device with a plurality of inspection functions. As an example, in the case of the prober, its optical system can be used not only for positioning the semiconductor wafer as described above, but also for inspecting needle traces remaining on the bonding pad at the end of the probe test, for example. Such complex inspection devices are also beginning to be manufactured.

[0004]

However, if one optical system (particularly a high-magnification one) is commonly used for inspection of alignment and needle marks, the following problems will occur. FIG. 5 is a diagram showing an example of a wiring pattern of a semiconductor chip. A plurality of wirings run on the upper portion of this semiconductor chip, and the wiring to the bonding pad 1 is drawn from a part thereof. . For pattern matching, characteristic portions such as the corner portion 2 of the bonding pad 1 and the wiring cross portion 3 are used. As shown in the figure, the surface of the metal part is roughened to stabilize the bonding.
There are many fine agglomerates. These agglomerates are not preferable because they act as background noise for the measured value of the degree of matching when performing pattern matching. Further, in the inspection of needle traces, an image with the highest possible resolution is required.

Therefore, as an optical system used for alignment of semiconductor wafers, the accuracy is ensured no matter what kind of semiconductor chip wiring pattern shape is measured or when edge sagging occurs. A very special condition is required that the isolated points such as the above-mentioned agglomerates must be erased without damage. On the other hand, as an optical system used at the time of inspecting a needle trace, it is desired that even an ordinary observation system can supply a clear image to the extent of reaching the resolution limit. Therefore, when the semiconductor wafer is aligned and inspected using the same optical system, the optical system is required to satisfy all the above conditions.

In view of the above-mentioned problems of the prior art, the present invention provides performance required for alignment and inspection of an object such as a semiconductor wafer having a regular pattern formed on its surface. It is an object to provide an optical system equipped with everything.

[0007]

In order to achieve the above object, the alignment optical system according to the present invention provides a precise alignment of an object in which a regular pattern is repeatedly formed in the X and Y axis directions of standard coordinates. In an observation optical system for acquiring an image for performing the above, the center of the pupil in the pupil of the objective lens is located in or near a plane that is in a positional relationship of Fourier transform with the sample surface with respect to the objective lens. As a result, it is line-symmetric with respect to any two line segments that are parallel to the regular array direction of the pattern and intersect each other at right angles.
A shielding plate (spatial filter) having at least one opening is arranged. Further, the optical system of the present invention is used in a state in which a constant defocus amount is given when matching the pre-stored standard pattern with the observed pattern, and is used for other inspections. When acquiring an image, it is characterized in that it is used in a focused state.

That is, the alignment optical system of the present invention is
It is configured by arranging a spatial filter 11 having four or more openings as shown in FIG. 2 in or near a plane having a positional relationship of Fourier transform with the sample surface with respect to the objective lens. Further, the aperture of the spatial filter 11 is in the pupil of the objective lens, and passes through the center O of the pupil and is parallel to the regular arrangement direction of the pattern of the semiconductor wafer (not shown), and is perpendicular to each other. It is provided so as to be line-symmetric with respect to both the parts A and B.
In FIG. 2, x is Y in the reference coordinate system of the filter 11.
The distance between the diameter in the axial direction (line segment A) and the center of the opening, and y is the distance between the diameter in the X-axis direction (line segment B) and the center of the opening,
r ₀ is the shortest distance between the line segment A and the circumference of the opening, and r _n is the line segment A
And the longest distance between the circumference of the opening and the circumference of the opening, respectively.

An image obtained by the optical system of the present invention equipped with such a spatial filter 11 has a defocused isolated point which appears as a grain agglomerate when the objective lens is slightly defocused from the focal point. Disappears,
The wiring patterns and edges of the semiconductor wafer provided in the X and Y axis directions in FIG. 2 can be clearly observed.

Therefore, in the optical system of the present invention, in pattern matching of a semiconductor wafer, by capturing a slightly defocused image, a characteristic pattern shape is accurately captured, and a background noise is generated. An image with isolated points removed can be obtained. Further, during inspection of needle marks and the like, by observing in a focused state, it is possible to obtain a delicate image having a resolution limit or higher.

The principle of the present invention will be described in detail below with reference to FIG. 3 and in comparison with the conventional optical system of this type shown in FIG. 3 and 4, epi-illumination is adopted in the optical system for actually aligning the objects, but here, for convenience of explanation, the configuration of the optical system adopting the transillumination type is shown. Therefore, the illustrated spatial filter 11 and the spatial filter 11 ′, the objective lens 12 and the objective lens 12 ′ are exactly the same, and in the case of the epi-illumination type, they should be commonly used because the illumination light is reciprocated. is there.

The alignment optical system of the present invention, as shown in FIG. 3, has a light source (not shown) side (left side in the figure) in order.
The light source side spatial filter 11, the light source side objective lens 12, the light receiving side objective lens 12 ′, the light receiving side spatial filter 11 ′, and the imaging lens 14 are arranged and configured. Reference numeral 13 indicates the sample to be observed, and 15 indicates the image plane. As the light source, a light source such as a fiber bundle that projects a relatively uniform surface ray onto the spatial filter 11 is adopted. The spatial filters 11 and 11 'are filters having the configuration shown in FIG. Further, f in the figure is the objective lens 1.
The focal length of 2,12 ', f _i denotes respectively the focal length of the imaging lens 14.

In the optical system of the present invention, first, the spherical wave emitted from one point of the aperture of the light source side spatial filter 11 is made into a parallel light flux by the light source side objective lens 12, and the sample 13 is made at an angle φ with respect to the optical axis L _C. Irradiate. And this light
Diffracted by the sample 13, first, the 0th-order diffracted light (shown by the solid line in the figure) enters the light-receiving side objective lens 12 ′ while maintaining the angle φ with the optical axis L _C, and the light-receiving side objective lens 1
The light is condensed on the aperture of the light-receiving side spatial filter 11 ′ arranged in a Fourier transform relationship with the sample 13 based on 2 ′. The 0th-order diffracted light that has passed through the aperture of the light-receiving side spatial filter 12 ′ is subjected to inverse Fourier transform by the imaging lens 14 and is imaged on the image plane 15 by interfering with the + 1st-order diffracted light described later. At this time, the distance between the light-receiving side spatial filter 11 ′ and the imaging lens 14 may be arbitrary, but in order to perform ideal inverse Fourier transform, the imaging lens 14
It is preferable to set the same distance as the focal length f _{i of} . On the other hand, the + 1st order diffracted light (indicated by the broken line in the figure) by the sample 13 is sin θ + sin φ = λ / P _S (1) (where λ, depending on the spatial frequency component of the pitch P _S in the sample 13). Is the objective lens 1 on the light-receiving side while maintaining the angle θ that satisfies the relationship of the + 1st order diffracted light by the sample 13).
2 ', enters the image plane 15 through the light-receiving side spatial filter 11' and the imaging lens 14 similarly to the 0th-order diffracted light,
An image of the sample 13 is formed by the interference action with the 0th-order diffracted light.

On the other hand, FIG. 4 is a sectional view taken along the optical axis showing the structure of a conventional optical system of this type. Spatial filter 2
The light emitted from one point of the aperture of No. 1 is diffracted by the spatial frequency component of the pitch P _S in the sample 13 as in the optical system shown in FIG. 3, but the 0th-order light is on the optical axis L _C. Go straight along and reach the image plane 15. Further, ± 13th-order diffracted light is generated in the sample 13, and these lights satisfy the relationship of sin θ ′ = ± λ / P _S (2) (where, ± λ is the wavelength of ± 1st-order diffracted light) angle ± theta with respect to the optical axis L _C 'respectively held by the light-receiving-side objective lens 12' is incident to, similarly to the optical system shown in FIG. 3,
An image is formed on the image plane 15 by interfering with the 0th-order diffracted light.

Now, assuming that the objective lens 12 'on the light-receiving side or the sample 13 moves in the direction along the optical axis L _C by a distance Δf, the phase change Δl between the 0th order diffracted light and the ± 1st order diffracted light is Δl. = Δf · sin ² θ ′ / 2 (3), and the image on the image plane 15 is blurred.

On the other hand, in the optical system shown in FIG. 3, for the spatial frequency P _S satisfying sin θ = sin φ = λ / 2P _S (4), the light-receiving side objective lens 12 ' Alternatively, even if the sample 13 moves in the direction along the optical axis L _C , the phase relationship between the 0th-order diffracted light and the 1st-order diffracted light on the image plane 15 is kept constant, and the image formed on the image plane 15 is There is no blur. Further, the above formula (4) can also be written as r ₁ / f = r ₂ / f = λ / 2P _S ... (5). Therefore, the spatial frequency P _S satisfying the above equation (4) is determined by the position and size of the openings provided in the spatial filters 11 and 11 ′, and P _S = fλ / 2r (6) (However, r ₀ ≤r ≤r _n ) (r ₀ and r _n are shown in FIG.
As shown in).

As described above, when the alignment optical system according to the present invention is used, the depth of focus can be remarkably increased with respect to the spatial frequency components within a certain range. However, it is known that this effect is effective only for the spatial frequency in the X or Y axis direction of the standard coordinate and has no effect in the direction of 45 ° (SPIE Vol.167).
4 Optical / Laser Microlithography V (1992) 741). Therefore, when the wiring pattern of the semiconductor wafer as shown in FIG. 5 is observed using the optical system of the present invention, the image of the pattern having an edge in the X-axis or Y-axis direction is less likely to be blurred due to defocus, and the isolated point is As before, the image will be blurred and unobservable.

Further, as an advantage of the optical system of the present invention, the resolution can be improved. That is, in the optical system shown in FIG. 3 (hereinafter referred to as the first optical system) and the optical system shown in FIG. 4 (hereinafter referred to as the second optical system), the resolution is 0th order diffracted light. Is determined by the angle formed by the diffracted light of ± 1st order and the numerical aperture of the objective lens and the cutoff frequency of the spatial filter are equal in the first optical system and the second optical system, the angle (φ + θ) shown in FIG. ) Is about 1.5 times the angle θ ′ shown in FIG. 4, and it can be seen that the resolution of the first optical system is higher.

[0019]

Embodiments of the present invention will be described below.

FIG. 1 is a sectional view taken along the optical axis showing the configuration of the alignment optical system according to this embodiment. The optical system of the present embodiment is of a reflection type. First, the light emitted from the fiber bundle 16 whose tip is configured as a surface light source for illumination light is reflected by the half mirror 18 via the illumination lens 17, It is projected on the spatial filter 11 arranged at the focal position of the objective lens 12. This spatial filter 1
The configuration of No. 1 is as described above with reference to FIG. The light flux that has passed through one point of the aperture provided in the spatial filter 11 becomes a secondary light source, and is converted into parallel light so as to be Keller illumination as shown by a thick solid line in the figure, and then the objective lens 12 The sample 13 at an angle φ with respect to the optical axis L _C
It is irradiated toward. 0 out of diffracted light from sample 13
The secondary diffracted light (thick solid line in the figure) passes through the aperture of the spatial filter 11 again via the imaging lens 12 as specular reflection light, and reaches the image plane 15 by the imaging lens 14. still,
In this embodiment, the distance between the spatial filter 11 and the imaging lens 14 is set to be the same as the focal length f _i of the imaging lens 14, so that the Fourier transform relationship is accurately maintained, The light flux reaching the image plane 15 is parallel light.

Here, the diffracted light with the spatial frequency P _{S in the} X-axis direction contained in the sample 13 is different from that expressed by the above equation (1) in the case of the optical system of this embodiment. That is, as shown by the broken line in FIG. 1, it occurs in the direction of the angle θ satisfying −sin θ + sin φ = λ / P _S (7). this is,
This is because the −1st-order diffracted light is shown, and the + 1st-order or higher-order diffracted light cannot be captured by the aperture of the spatial filter 11, and is omitted. Thus, in the optical system of the present embodiment, 0 as with the next optical system and diffracted light and -1 order diffracted light is described with reference to FIG. 3, the interference fringes of the pitch P _S interfere in the image plane 15 Will be formed. Further, for P _{S where} θ = φ, there is no phase difference between the 0th-order diffracted light and the −1st-order diffracted light due to defocusing. As described above, the range of the spatial frequency P _S involved in the image formed on the image 15 is determined.

As described above, the difference between the optical system of this embodiment and the optical system shown in FIG.
Since only the difference between the first order and the first order and the other things are exactly the same, the operation and effect are also exactly the same as those described based on FIG.

Further, the alignment optical system of the present invention is not limited to a semiconductor wafer, but can be applied as long as it has a regular pattern on the surface (for example, LCD substrate, color filter substrate, etc.). .

[0024]

As described above, according to the alignment optical system of the present invention, when aligning a semiconductor wafer or the like having a regular pattern in the X and Y axis directions of standard coordinates, it is possible to perform pattern matching of the substrate. With respect to the evaluation value of the matching degree, only minute isolated points that become background noise are erased in an out-of-focus state, and an image in which a repeating pattern in the X and Y axis directions or an edge in the X and Y axis directions is captured is The sharpness can be maintained. Therefore, the reference pattern in the memory and the observed target pattern can be matched with high accuracy without performing special image processing, and the accuracy of alignment can be improved.

Further, since the optical system of the present invention can obtain a resolution higher than that of an observation optical system having a similar numerical aperture in a normal focusing state, other than pattern matching,
For example, it can exhibit higher performance than before even for delicate inspection such as needle mark inspection after a probe test, and is extremely useful as an observation optical system for a composite inspection apparatus for semiconductor wafers.

[Brief description of drawings]

FIG. 1 is an embodiment showing a configuration of an alignment optical system according to the present invention.

FIG. 2 is a front view showing the configuration of a spatial filter provided in the optical system of the present invention.

FIG. 3 is a diagram for explaining the principle of the present invention.

FIG. 4 is a diagram for explaining a configuration of an optical system for aligning a conventional semiconductor wafer or the like.

FIG. 5 is a diagram showing an example of a wiring pattern of a semiconductor chip.

[Explanation of symbols]

1 Bonding pad 2 Corner part of bonding pad 3 Crossing part of wiring 11, 11 ', 21, 21' Spatial filter 12, 12 'Objective lens 13 Sample 14 Imaging lens 15 Image plane 16 Fiber bundle 17 Illumination lens 18 Half mirror L _C optical axis

Continuation of the front page (51) Int.Cl. ⁶ Identification code Reference number within the agency FI technical display location // H01L 21/66 G06F 15/62 405C

Claims (2)

[Claims] 1. An observation optical system for acquiring an image for aligning an object, in which a regular pattern is repeatedly formed in the X and Y axis directions of standard coordinates, in an observation optical system with reference to an objective lens and a Fourier plane. In any of two line segments in the pupil of the objective lens, passing through the center of the pupil, parallel to the regular arrangement direction of the pattern, and perpendicular to each other in the plane having the positional relationship of conversion. An alignment optical system in which a shield plate having four or more openings that are line-symmetric with respect to each other is arranged. 2. When performing matching between a standard pattern stored in advance and an observed pattern, the standard pattern is used with a given defocus amount, and an image for other inspection is acquired. In some cases, the alignment optical system is used in a focused state.