JPH0610696B2 - Auto focus method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an automatic focusing method for inspecting the appearance of a semiconductor device circuit pattern such as an LSI wafer.

[Background of the Invention]

Integrated circuits such as LSIs tend to be highly integrated and miniaturized. Therefore, formation of fine patterns in these circuits is
Even with the utmost care during the formation process, defects often occur in the pattern, and careful inspection is required. The initial inspection was performed by a large number of inspectors by visual inspection using a microscope, but eyes were easily tired, defects were often overlooked, and there was a problem in quality control. For this reason, automation of the inspection in the manufacturing process is desired, but focusing is important in order to automate the inspection process. A number of techniques relating to automatic focusing have been proposed mainly for focusing a camera.
No. 91409 is disclosed. FIG. 10 shows an automatic focusing method using the above technique. That is, in FIG. 10, the circuit pattern on the wafer 1 illuminated by the light source 15 is magnified by the objective lens 4 at a high magnification to form an image on the photoelectric converter 5, and the optical image is converted into an electric signal for defect determination. To do.
At the same time, the circuit pattern images at the same position are projected on the photoelectric converters 17a and 17b arranged at the same position before and after the plane conjugate with the photoelectric converter 5, and the output of the photoelectric converter is based on a predetermined evaluation function and the amount of image blurring. Are determined by the position detectors 25a and 25b, and the respective states of the front focus, the in-focus point, and the rear focus are determined based on these evaluation values. It seems that if the above technique is diverted, it can be used for pattern inspection of LSI wafers, etc., but in reality, various problems arise due to the difference between the inspection target and the subject.

The LSI pattern to be inspected is usually very fine, around 1 μm, and it is necessary to inspect at a high magnification. Moreover, miniaturization is progressing year by year, and there is a tendency to use the objective lens 4 having a high magnification. For example, the magnification of the objective lens is 5 × to 20 ×
, Or from 20 × to 40 ×, the magnification is getting higher,
In particular, recently, the use of 60 × and 100 × objective lenses has begun to be considered, and the depth of focus is thereby reduced, so improvement in focusing technology is desired. Conventional 5
With the × and 20 × objective lenses, the depth of focus was deep and the target wafer could be clearly captured over the entire surface.
However, the depth of focus is shallow for 60 × and 100 × objective lenses, so for wafers with steps as shown in FIG. 3 (a),
Focusing cannot be performed so that both the area A and the area B are in focus, and the conventional technique cannot be used. The performance required for using a high-magnification objective lens is desired not only in accuracy and stability, but also in selective focusing to match the desired position. Other than the automatic focusing method shown in FIG. 10, the air micro method is often used. However, this method also has good accuracy and stability, but cannot perform selective focusing.

[Object of the Invention]

An object of the present invention is to provide an automatic focusing method capable of automatically focusing on a fine pattern such as an LSI wafer.

[Outline of Invention]

In the present invention, a fringe pattern is projected on a specific point of the inspection target, and focusing is performed using the contrast of the fringe pattern.
The above contrast is taken into the optical system from a specific point,
Images are simultaneously picked up by the two detectors, and a position where the contrasts obtained from the two detectors are equal to each other is determined as a focus position. An image of the multilayer pattern is detected by another detector which forms an image at the above position and used for inspection. A plurality of sets of stripe patterns to be projected are prepared, or one stripe pattern is divided into a plurality of sets, and the detection light receiving units corresponding to one set of stripe patterns are grouped together to form a plurality of stripe patterns corresponding to a plurality of sets of stripe patterns. The output of the light receiving unit is handled independently, and the obtained multiple contrasts are combined for focusing. For example, if at least one of the obtained contrasts results in moving the stage downward, focusing is performed according to the result. That is, when the difference between the outputs of the light receiving parts of the photoelectric converters corresponding to the plurality of divided plane patterns is at least one and the rest are instructed to move the object in the same direction, the object is moved according to the above instruction. Focus by doing so. As a result, it is possible to always focus on the multilayer pattern, and particularly to focus on the uppermost layer. As still another means, the coordinates of a plurality of sets of divided stripe patterns projected on the wafer are calculated, the corresponding stripe pattern is prohibited in the inspection visual field, and only the stripe pattern projected in the inspection visual field is used. This ensures that the focus is always on the multilayer pattern.

In the automatic focusing method according to the present invention, a plurality of sets of flat patterns each including a light transmitting portion and a light shielding portion are projected onto a subject, and the projection pattern formed by the optical system on the predetermined focal plane is changed to the predetermined focal plane. Alternatively, a plurality of photoelectric converters arranged before and after the plane conjugate with it are used to detect an image, and a plurality of sets of projection patterns on the photoelectric exchanger are detected. However, the focusing is performed by moving the subject so that at least one is zero and the rest indicate subject movement in the same direction.

Example of Invention

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram showing an embodiment of an automatic focusing method according to the present invention, FIG. 2 is a diagram showing an example of a stripe pattern, and FIG. 3 is a receiver showing an example of a wafer and a stripe pattern projection position. (a) is a wafer cross-sectional view, (b) and (c) are examples of projected positions of stripe patterns, FIG. 4 is a view showing an example of the Z stage moving direction by the contrast obtained from the stripe patterns, and FIG. 5 is a stripe pattern. FIG. 6 is a diagram showing an example of contrast, (a) is a stripe pattern cross-sectional view, (b) and (c) are diagrams showing an output of an image sensor, FIG. 6 is a diagram showing an example of a normalized contrast curve, FIG. 7 is a diagram showing an example of a focusing method, FIG. 8 is a diagram showing a focusing method in which FIG. 4 and FIG. 7 are combined, and FIG. 9 is a configuration diagram embodying the above method. .

In FIG. 1, a circuit pattern on the wafer 1 irradiated by a light source 15 such as a mercury lamp is magnified by an objective lens 4 to a high magnification and formed on a photoelectric converter 5, and an optical image is converted into an electric signal. It is used to judge defects. A flat pattern 14 consisting of a light-transmitting portion and a light-shielding portion is inserted into a part of the illumination optical path, and a light source 15 is illuminated to project a bright and dark pattern on the wafer 1. The bright and dark pattern becomes a clearer pattern image than the circuit pattern on the wafer 1 corresponding to the portion irradiated with the illumination light and the portion not irradiated with the illumination light. The projection pattern on the circuit pattern is converted into photoelectric converters 17a, 1
Detected by 7b. The photoelectric converters 17a and 17b are arranged at equal distances in the front and rear on the optical axis of the photoelectric converter 5 for detecting a circuit pattern, and operate in synchronization.

A second example in which a linear image sensor is used as the photoelectric converter 5 that detects the projected light-dark pattern and the circuit pattern of the wafer 1 and a black and white striped pattern is used as the projected pattern 14
Shown in the figure. To avoid interfering effects such as the circuit patterns that are wired orthogonally to each other on the object to be inspected and become parallel to the stripe pattern, which causes contrast changes, tilt the parallel stripe pattern at a position different from the circuit pattern detection field of view by 45 °. The image is projected and detected by the linear image sensor for detecting the stripe pattern. The stripe pattern is configured such that light passes through the central portion so as not to overlap the visual field for detecting the circuit pattern. The inclination of the stripe pattern may be arbitrary, and the projection pattern 14 is arranged at a position that is least affected by the circuit pattern. Projection pattern 14 is pattern P ₁
And P ₂ . When the fringe patterns P ₁ and P ₂ projected in FIG. 1 are imaged through the half mirror 16 using the linear image sensors 17a and 17b, the wafer 1 is at the focus position, that is, the circuit pattern is the linear image sensors 17a and 1b.
When the image is formed at the midpoint on the optical axis of 7b, the image sensor 17
The outputs of a and 17b match. Because the two linear image sensors 17a and 17b image the same position on the wafer 1 at the same timing, the same circuit pattern and stripe pattern are imaged, and the light source 15 does not flicker. The above two image sensors 17a and 17b even if they exist
This is because they commonly contribute to Therefore, if the subtracter 19 calculates the difference between the outputs of the linear image sensors 17a and 17b via the amplifiers 18a and 18b, and the driver 22 of the Z moving mechanism 23 of the wafer 1 is controlled so that this difference becomes zero, the focus is focused. The alignment is possible, and the circuit pattern detecting linear image sensor 5 can always detect a clear image.

As shown in FIG. 3 (a), the target LSI wafer is generally formed with a multilayer pattern, but the step difference between the area where the pattern is formed and the area where the pattern is not formed is large and is close to several μm. As shown in FIG. 3 (b), stripe patterns P ₁ and P
_When both ₂ are projected in the area A, the circuit pattern can be focused, but as shown in FIG. 3 (c), the stripe patterns P ₁ and P ₂ are spread over the areas A and B. When projected, the areas A and B are focused on average, and the focus position is located between A and B. Therefore, defocus occurs around the area A, and complete inspection cannot be performed. In particular, the pattern formed in the area A needs to be inspected carefully, and the area B is not affected by the pattern.
Focusing on is essential. Therefore, the contrasts obtained from the stripe patterns P ₁ and P ₂ are calculated independently, and when the results differ, the stage is moved up and down by the Z moving mechanism 23 according to the movement shown in FIG. As a result, it is possible to focus on the area A regardless of the existence of the area B. FIG. 4 shows that at least one of the contrast differences obtained corresponding to a plurality of stripe patterns is zero, and the rest are moved such that the subject moves in the same direction. In FIG. 1, the average value circuit 24 and the arithmetic unit
Reference numeral 20 is a circuit for performing normalization for keeping the focusing accuracy uniform. In general, when an image of a material having a high reflectance and an image of a material having a low reflectance is taken, the output of the linear image sensor 17 is
As shown in the figure. FIG. 5 (a) shows the projection pattern,
(b) shows the output of the material with high reflectance, and (c) shows the output of the material with low reflectance. It can be seen that the material with higher reflectance has a larger contrast C _{A and a} larger average brightness μ _A. This can be expressed as C _A / C _B = μ _A / μ _B. Therefore, when the average value μ of the outputs of one of the linear image sensors is obtained by the average value circuit 24 and normalized by dividing the output of the subtractor 19 by the divider 20, as shown in FIG. It is possible to obtain a contrast curve that is not affected by the, and high-precision automatic focusing that is not affected by the type and material of the wafer. As shown in FIG. 1, the contrast calculation is actually performed only once at the end, the amplitude and phase of the output of the divider 20 are obtained by the contrast calculation circuit 21, and the Z movement mechanism of the wafer is controlled by this.

Further, a solving means different from the above means will be described with reference to FIG. In FIG. 1, the position detector 25 detects the coordinates of the wafer 1. Based on the coordinates detected by the position detector 25, stripe patterns P ₁ and P ₂ are projected as shown in FIG. 3 (b). In this case, the contrasts obtained from P ₁ and P ₂ are both valid, and the stage is moved up and down according to the movement shown in FIG. On the other hand, when the stripe pattern is projected as shown in FIG. 3 (c), the contrast obtained from P ₂ is valid, but the contrast obtained from P ₁ is invalid, and only P ₁ is used for focusing. . As a result, it is possible to focus on the area A regardless of the area B.

Although the pattern inside the chip has been described with reference to FIG. 3, this also holds true for the boundary area between the inside of the chip and the outside (scribe area) of the chip. The inside of the chip is about several μm higher than the outside of the chip, and according to the present invention, the inside of the chip can be accurately detected without being affected by the outside of the chip.

FIG. 8 shows a method of focusing by combining the methods shown in FIGS. 4 and 7 above. Based on the coordinates detected by the position detector 25, when the stripe patterns P ₁ and P ₂ are projected as shown in FIG. 3 (b), the contrasts obtained from P ₁ and P ₂ are both valid and The stage is moved up and down by the Z moving mechanism 23 according to the movement shown in FIG. On the other hand, when the stripe pattern is projected as shown in FIG. 3 (c), the contrast obtained from P ₂ is valid, but the contrast obtained from P ₁ is invalid, and only P ₁ is used for focusing. . A concrete implementation of the focusing method shown in FIG. 8 is shown in FIG. Contrast calculation circuit
The stripe pattern P ₁ contrast and the stripe pattern obtained by
Given P ₂ of contrast, and the fringe pattern P _1, P ₂ ON / OFF of the enable / disable and coordinate management as an input, 8
The judgment output as shown in the figure is obtained. However, the contrast of the stripe pattern is given as High when it is + and Low when it is 1. The structure shown in FIG. 9 is an extremely simple structure and can be easily realized. Further, the above configuration can be included in the contrast calculation circuit 21. Whether the stripe patterns P ₁ and P ₂ are valid or invalid can be determined by managing the stage coordinates by the position detector 25. The determination output shown in FIG. 9 is given to the driver 22 of the Z moving mechanism 23, and focusing is performed.

In the above embodiment, the case where the striped pattern is divided into two has been described, but the striped pattern can be divided into a larger number, and in this case, the above concept is maintained.

〔The invention's effect〕

As described above, the automatic focusing method according to the present invention projects a plurality of sets of flat patterns composed of a light-transmitting portion and a light-shielding portion onto a subject to form a projection pattern formed on the planned focal plane by the optical system. , A plurality of photoelectric converters arranged before and after the planned focal plane or a plane conjugate therewith, to detect a plurality of sets of projection patterns on the photoelectric converter, and to receive light from each of the light receiving units corresponding to the patterns. By moving the subject such that at least one of the partial outputs has a difference of zero and the rest indicate subject movement in the same direction,
Highly accurate pattern detection is possible for LSI wafers and the like, and since wafers made of various materials can be automatically focused with high accuracy regardless of the pattern step, it is possible to accurately detect the multilayer pattern of the LSI wafer. It is possible to automate the visual inspection.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an automatic focusing method according to the present invention, FIG. 2 is a diagram showing an example of a stripe pattern, and FIG. 3 is a diagram showing an example of a wafer and a stripe pattern projection position. (a) is a wafer cross-sectional view, (b) and (c) are examples showing projected positions of stripe patterns, FIG. 4 is a diagram showing an example of Z stage movement direction by contrast obtained from the stripe patterns, and FIG. 5 is stripes. FIG. 6 is a diagram showing an example of a pattern contrast, (a) is a cross-sectional view of a striped pattern, (b) and (c) are diagrams showing an output of an image sensor, and FIG. 6 is a diagram showing an example of a normalized contrast curve,
FIG. 7 is a diagram showing an example of a focusing method, FIG. 8 is a diagram showing a focusing method in which FIGS. 4 and 7 are combined, and FIG. 9 is a configuration diagram embodying the above method, FIG. 10 is a block diagram showing a conventional automatic focusing method. 1 ... Subject (LSI wafer) 4 ... Objective lens 5 ... Photoelectric converter (planned focal plane) 14 ... Plane pattern 17a, 17b ... Photoelectric converter 19 ... Subtractor 21 ... Contrast calculation circuit 23 ...... Z movement mechanism 25 …… Position detector

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI Technical display location H01L 21/027 7352-4M H01L 21/30 311 N

Claims (2)

[Claims] 1. A planned focal plane or a planned focal plane is defined as a projected pattern formed by projecting a plurality of sets of flat patterns consisting of a light-transmitting portion and a light-blocking portion onto a subject and forming a projected pattern on the subject by the optical system. Imaged with a plurality of photoelectric converters arranged before and after the surface conjugate with, and detecting a plurality of sets of projection patterns on the photoelectric converter, the difference in the light-receiving unit output of each light-receiving unit corresponding to the pattern An automatic focusing method in which focusing is performed by moving the subject so that at least one is zero and the rest indicate subject movement in the same direction. 2. The output of the effective photoelectric converter light receiving unit is obtained by switching the light receiving unit output of the photoelectric converter corresponding to a plurality of sets of plane patterns to valid or invalid according to the coordinates of the object. The automatic focusing method according to claim 1, characterized in that