JPS638402B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photoelectric detection device, and more particularly to a photoelectric detection device that photoelectrically detects light from two objects placed in an optical back-and-forth relationship.

Although the present invention can be applied to various fields, it is particularly suitable for application to recently developed auto-alignment devices for IC printing machines. The auto-alignment device will be described below as a specific example.

The process of manufacturing semiconductor devices such as transistors and integrated circuits involves printing fine patterns and bonding lead wires, and it is necessary to precisely align wafers and pellets, which are the substrates for printing, in predetermined positions. . Since the fine structure of this substrate and the like has reached a precision on the order of microns, the precision of alignment must also be on the order of microns or submicrons. however,
This type of work is quite complicated even for experienced workers, and requires a long period of experience to become proficient.
Furthermore, with this level of precision, it is inevitable to work while looking through a microscope, and considering that wafers are being processed at a rate of several wafers per minute, there is a risk of worker fatigue and variations in accuracy due to fatigue. is also a big problem.

Traditionally, the alignment of the mask and wafer was manually adjusted by the operator while looking through a microscope, but recently this type of alignment has been developed to solve the problems of skill and fatigue mentioned above. Attempts have been made to do the work automatically. This is a device called an auto aligner.

An auto aligner, that is, an auto alignment device, is used, for example, when creating a circuit by overlaying and printing another pattern on a substrate on which a part of the actual element pattern constituting the circuit has already been formed.
The substrate and the mask are aligned by guiding an alignment mark previously provided on the substrate to be printed and an alignment mark formed on the mask having an overlapping pattern into a predetermined relationship. The alignment marks and the patterns constituting the actual device have a predetermined relationship, so if the alignment marks are brought into a predetermined relationship, the actual device pattern on the substrate and the actual device pattern on the mask will be properly overlapped. It turns out.
The positional relationship of the alignment marks is detected photoelectrically, and if this deviates from a predetermined relationship, the drive mechanism is operated in accordance with the value of the photoelectric output to perform adjustment.

FIG. 1 shows an example of the optical system of a conventional photoelectric detection device used in such an autoaligner. In the figure, 1 is a laser light source, 2 is a condensing lens, 3 is a rotating polygon mirror, 4 is a relay lens, 5 is a beam splitter for guiding light to an optical system for visual observation of 22 or less, and 6 is a field lens. , 7 is a beam splitter for guiding light to the photoelectric detection system after 14, 8 is a relay lens, 9 is a beam splitter for guiding light from the visual observation illumination optical system consisting of 19 to 21, and 10 is a beam splitter for guiding light from the visual observation illumination optical system consisting of 19 to 21. The pupil of the objective lens 11, 12 a mask,
13 is a wafer. In addition, although FIG. 1 assumes the contact printing method or the broximity method as the method for transferring the mask image to the wafer, in the case of the projection imaging method using lenses or mirrors, the mask image is transferred to the wafer. An imaging optical system is arranged at. In the case of the projection imaging method, if the imaging is telecentric, the wafer 13 in FIG. 1 can be considered as an image of the wafer.

The imaging relationship of the laser beam scanned by the optical system shown in FIG. 1 is as follows. In the figure, reference numeral 34 is the scanning point of the laser beam, and when the laser beam is traced backward from 34 toward the rotating polygon mirror 3, the laser beam passes through the objective lens 11 and the relay lens 8, and once reaches the vicinity of the field lens 6. Imaged at 32,
After being reflected again by the rotating polygon mirror 3 via the relay lens 4, an image is formed at the position 30. That is, 30,3
2 and 34 are in a conjugate relationship with each other. Therefore, the spot diameter of the laser beam at position 30 is φ, and the imaging magnification from 30 to 34 is β.
Then, the spot diameter of the laser beam scanned by 34 is represented by βφ.

In addition to the conjugate relationship of the scanning beam on the actual object plane as explained above, the image formation relationship of the pupil is also important in the optical system of FIG. The pupil of the objective lens 11 is indicated by 10 in the figure, and a point 33 on the optical axis, which is the center point of the pupil 10, and a reflection point 31 of the rotating polygon mirror 3 are conjugate with each other. That is, the optical system shown in FIG. 1 can be said to be equivalent to the case where the rotating polygon mirror 3 is placed exactly at the position of the pupil 10, from the viewpoint of the incidence of the laser beam on the objective lens 11.

To understand the optical system in Figure 1, objective lens 1
You must understand movement number 1. FIG. 2 shows a principle diagram showing the operation of the objective lens 11 used in the optical system of FIG. 1. That is, in the optical system shown in FIG. 1, the objective lens 11 is arranged in a telecentric manner. A telecentric objective lens is one whose focal point coincides with the position of the pupil, which determines the size of the light flux passing through the objective lens. The ray that passes through the center of the pupil is called the chief ray because it becomes the center of the luminous flux. In this case, the center of the pupil is the focal point of the objective lens, so after passing through the objective lens, the chief ray becomes parallel to the optical axis of the objective lens and enters the object plane perpendicularly. When a vertically incident ray hits a part of an object that undergoes specular reflection, it is reflected and follows the original path, and after passing through the objective lens returns to the original center of the pupil.

On the other hand, if there is a pattern in the area where the light hits, the light will be scattered at the boundary lines that make up the pattern. If we collectively refer to the boundary line as an edge, the light scattered at the edge does not follow the original optical path, unlike in the case of specular reflection.
Therefore, when the scattered light is captured again by the objective lens and passes through the pupil, it no longer passes through the center of the pupil. This shows that when the reflected light passing through the objective lens is observed on the pupil, the specular reflection component and the scattering component are spatially separated within the pupil. FIG. 2 shows this separation. In the figure, the specularly reflected light flux that enters the pupil, passes through the objective lens, and returns to the pupil is indicated by diagonal lines, and the scattered light that can be captured by the objective lens is indicated by dots. In order to effectively capture scattered light, the effective diameter of the light beam in the specular reflection area is normally set to be sufficiently small relative to the pupil diameter, and the ratio of the diameters is usually 0.1.
It is desirable that it be within the range of ~0.7.

Returning to FIG. 1 again, consider the photoelectric detection optical system that separates from the beam splitter 7 and reaches the photodetector 18. In the figure, 14 is the objective lens 1
A lens 15 that forms an image of the pupil 10 of the lens 1 is a filter that allows light for photoelectric detection to pass through and substantially blocks light of other wavelengths, for example, in a wavelength range used in a viewing optical system. 16 is a spatial filter (shading plate) for passing scattered light and blocking specularly reflected light.
The lens 14 is placed at a position where an image of the pupil 10 is formed. The scattered light that has passed through the spatial filter 16 is collected by a condenser lens 17 onto a photodetector 18. Therefore, pupil 1
0, spatial filter 16, photo detector 1
8 are in a conjugate relationship with each other. The spatial filter 16 can be easily created by patterning a light shielding part on a transparent glass substrate using a substance such as metal or ink.

Therefore, this photoelectric detection system for detecting scattered light will produce an output only when the scanning beam approaches the edge of the pattern. If the patterns are alignment marks provided on the mask 12 and the wafer 13, the relative positional deviation between the mask 12 and the wafer 13 can be detected from the output signal. According to the detected amount of deviation, a drive system (not shown) corrects the relative relationship between the two, and auto-alignment is performed.

In addition to this in Figure 1, 19 to 2 are provided.
1 is a visual illumination system, and 22 to 24 are observation systems.
In the illumination system, a condenser lens 20 is formed on the pupil 10 of the objective lens 11 of the light source 19, and serves to perform so-called Koehler illumination. Reference numeral 21 denotes a filter that transmits wavelengths in the non-photosensitive region of the photoresist coated on the wafer 13 to be aligned. The filter 21 may need to have a low transmittance for light in the photoelectric detection wavelength range so as not to transmit excess light to the photoelectric detection system, but it may be selected appropriately in consideration of the filter 15.

In the observation system, 22 is an erector that rotates the image forward;
23 is a filter that attenuates the intensity of a laser beam for photoelectric detection, and 24 is an eyepiece lens.

FIG. 3 shows an example of a three-dimensional construction of the optical system shown in FIG. The alignment of the mask 12 and the wafer 13 is usually performed by observing a plurality of locations on the mask 12 and the wafer 13 because it is necessary to suppress all two-dimensional degrees of freedom. The example in FIG. 3 is an arrangement array in which two locations are observed and detected, and the roof mirror 25 plays the role of dividing the scanning laser beam into two optical paths. The ridgeline of the roof of the roof mirror 25 is set to intersect with the optical axis.
Furthermore, the ridgeline or the vicinity of the ridgeline serves as a primary imaging plane of the mask 13 formed by the objective lens 11 and the relay lens 8. That is, the ridgeline of the roof of the roof-shaped mirror 25 functions as a dividing line of the visual field when observing through the eyepiece lens 24. The numbers in FIG. 3 correspond to those in FIG. 1, with unnumbered parts representing mirrors or prisms used to bend the optical path. Further, although the pupil 10 of the objective lens 11 is shown as an aperture for convenience of explanation in FIG. 1, the effective diameter of the pupil 10 is necessarily determined from the effective diameter of the objective lens 11, etc.
There is no particular need to place a mechanical aperture. Therefore, in FIG. 3, placing special parts at the position of the pupil 10 is omitted.

The system shown in FIG. 1 can be applied as is to an IC pattern printing method using a projection imaging optical system using lenses or mirrors. In particular, if the projection imaging optical system is telecentric both to the mask side and to the wafer side, the system shown in FIG. 1 can be applied as is. In this case, the operation of the system can be easily understood by considering the wafer 13 in FIG. 1 as an image of the wafer 13 created by the projection imaging optical system, rather than the wafer itself.

Problems in the IC pattern printing method using a projection imaging optical system become apparent when the projection imaging optical system is outside the telecentric condition. FIG. 4 shows an optical path of a projection imaging optical system in which the wafer 13 side is telecentric and the mask 12 side is not telecentric. In the figure, 40 is a projection imaging optical system using lenses or mirrors. The principal ray of the imaging light beam on the mask 12 side is not telecentric and is therefore inclined to the optical axis; on the wafer 13 side, the principal ray is parallel to the optical axis because it is telecentric.
Therefore, the light is incident perpendicularly to the wafer 13. Here, consider a case where a point 41 on the mask 12 is observed using the objective lens 11. The dotted line in FIG. 4 shows the optical path within the projection imaging optical system 40 when the scanning beam is incident perpendicularly to the mask 12 as in FIG. 1. As is clear from the figure, the light that has been specularly reflected in a straight line by the mask 12 returns to the objective lens 11 perpendicularly, but the light that has specularly reacted at the conjugate point 42 on the wafer 13 and returns is no longer perpendicular to the objective lens 11. I won't go back. In this case, the distribution of specularly reflected light and scattered light on the pupil plane 10 is not as simple as shown in FIG.
The distribution of light (specularly reflected light and scattered light) from the mask 12 and the distribution of light (specularly reflected light and scattered light) from the wafer 13 are shifted from each other and overlap with each other, as shown in FIG. 4b. Therefore, in order to apply the system shown in FIG. 1 as is, it is necessary to devise the shape of the light shielding part of the spatial filter 16. However, it is not easy to completely block the specularly reflected light as shown in Figure 4b with a light shielding part of a specific shape without impairing the passing efficiency of scattered light, so the system in Figure 1 can be used as is. If so, there is a possibility that a part of the light specularly reflected on the object surface (mask 12 or wafer 13) may enter the photodetector 18 as noise. Generally, the intensity of specularly reflected light is stronger than that of scattered light, so even if only a portion of it is incident, it will cause false detection.

On the other hand, the present applicant, in Japanese Patent Application Laid-Open No. 49-28363,
In order to separate the desired light reflected from the wafer from the backside reflection of the mask, which is directly reflected by the mask and becomes a flare component during observation, polarized light is installed in the projection imaging optical system that projects the mask image onto the wafer. We propose that by arranging a means (1/4 wavelength plate), the polarization direction of the light that passes through the mask, is reflected by the wafer, and returns to the mask is orthogonal to the polarization direction of the light that enters the mask. are doing.
The fact that polarized light can be used to separate light from each of the mask and wafer is particularly advantageous for a photoelectric detection system that shares an optical path with a visual observation system. This is because if a polarized laser is used as the light source of the photoelectric detection system, it is possible to minimize the loss of light in the optical system.

The present invention was devised in view of the drawbacks of conventional photoelectric detection systems as described above, and its purpose is to polarize light from two objects placed in an optical back-and-forth relationship, such as a mask and a wafer. By separating the scattered light and reflected light using separate spatial filters, even if the reflected light from each of the two objects is shifted, the scattered light from each of the two objects can be separated. An object of the present invention is to provide a photoelectric detection device that can efficiently and accurately detect light.

Hereinafter, the present invention will be explained in detail based on embodiments shown in the drawings. In the following embodiments, the same components as those described above are denoted by the same reference numerals as in FIGS. 1 to 4, and the explanation will be omitted. do.

FIG. 5 shows an embodiment of the present invention.
The system shown in FIG. 5 is a system in which the system shown in FIG. lens 17,
The difference is that the photodetector 18) is divided into a plurality of parts, and that a quarter wavelength plate 50 is disposed within the imaging optical system 40. Furthermore, the imaging optical system 4
Of course, the aberrations of the laser beam 0 are corrected with respect to the wavelength of the laser beam for photoelectric detection with the 1/4 wavelength plate 50 inserted.

The operation of the system shown in FIG. 5 will be explained. In order to efficiently perform photoelectric detection, the laser beam from the laser light source 1 is assumed to be linearly polarized light, and is passed through the objective lens 11 to the mask 12 while maintaining the linearly polarized state.
and the wafer 13 at the same time. mask 1
As described above, the polarization directions of the light that directly returns from the wafer 13 and the light that travels to the wafer 13 and returns are orthogonal to each other due to the action of the quarter-wave plate 50. mask 1
In order to align the mask 12 and the wafer 13, the position signals of both the mask 12 and the wafer 13 must be photoelectrically detected. The light that directly returns from the mask 12 has only a signal related to the mask pattern, but the light that has reached the wafer 13 has both a wafer signal and a mask signal.
Therefore, although it is possible to detect the positions of both the mask 12 and the wafer 13 by capturing the light from the wafer 13, the reflectance of the wafer 13 is generally low due to the influence of the photosensitive layer provided on its surface. For the mask signal, it is preferable to detect the light directly returning from the mask 12 in terms of detection accuracy. As explained in FIG. 4, the imaging optical system 40 is on the mask side,
If the beam is not telecentric to both sides of the wafer, the light from the mask 12 and the wafer 13
However, in this embodiment, the two are separated by the polarizing beam splitter 51 due to the difference in linearly polarized components (P component and S component). The position of each spatial filter 16 after passing through 51 (pupil 10
The distribution of light observed on the imaging plane (image formation plane) is such that the specularly reflected light and scattered light are separated as shown in Figure 2b, avoiding the overlapping and misaligned state as shown in Figure 4b. can do. In order to increase the efficiency of extracting the scattered light, it is desirable to make the boundary between the specularly reflected light and the scattered light smaller and to reduce the area of the blocking part of the spatial filter 16. It's a big loss to do so. The polarizing beam splitter 51 functions to remove the overlap by reseparating the overlapping mask light and wafer light in a polarized separated state. The improvement in the scattered light extraction efficiency due to the cancellation of the overlap is remarkable. In this embodiment, a spatial filter 16 is inserted to filter the mask light and wafer light, which are separated for the above purposes, in order to block the specularly reflected light and transmit the scattered light. The scattered light from the wafer 13 is detected by each photodetector 18. The mask signal is simultaneously outputted from both photodetectors 18, and the wafer signal is outputted to the photodetector 18 on the side where the reflected light from the wafer 13 reaches. 52 is an adder for adding the outputs from the two photodetectors 18.

As described above, according to the present invention, in a device that detects light scattered by two objects placed in an optical back-and-forth relationship, the non-scattered light that is specularly reflected by each object is shifted. Even if
Scattered light from each object can be detected efficiently and accurately. Therefore, if the present invention is used to align a mask and a wafer, the accuracy of the alignment can be improved.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an example of the optical arrangement of a conventional photoelectric detection device, Figure 2 is a diagram explaining the action of the objective lens in Figure 1, and Figure 3 is an example of the three-dimensional arrangement of the optical system in Figure 1. FIG. 4 is a diagram showing the optical path when there is a deviation from telecentricity in the projection imaging optical system, and FIG. 5 is a diagram showing the optical arrangement of an embodiment of the photoelectric detection device of the present invention. be. 1... Laser light source, 2... Condensing lens, 3...
...Rotating polygon mirror, 4, 8...Relay lens, 6...
Field lens, 5, 7, 9...beam splitter, 11...objective lens, 12...mask,
13...Wafer, 14,17...Condenser lens,
16... Spatial filter (shading plate), 18... Photo detector, 19... Light source, 22... Erector, 24... Eyepiece, 25... Roof mirror, 40... Projection imaging optical system, 50 ...1/4 wavelength plate, 51...Polarizing beam splitter.

Claims (1)

[Claims] 1 first row second arranged in optical anteroposterior relationship
When an object is illuminated from the first object side, the linearly polarized component of the light reflected or scattered by the first object is different from the linearly polarized component of the light reflected or scattered by the second object on the first object side. an optical system, a beam splitter for separating the first and second object beams guided from the first object side based on differences in linear polarization components, and a separate filter for the separated first and second object beams. By having a plurality of spatial filters for filtering, the scattered light within the first object beam and the second
A photoelectric detection device characterized by photoelectrically detecting scattered light within object light.