JPS649606B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical device having an observation optical system and a photoelectric detection optical system, such as a so-called aligner used in the manufacturing process of semiconductor devices.

The process of manufacturing semiconductor devices such as transistors and integrated circuits involves printing fine patterns and boarding lead wires, and it is necessary to precisely align the wafer or pellet that is the printing substrate in a predetermined position. Since the fine structure of this substrate and the like has reached a precision on the order of microns, the alignment precision must also be on the order of microns or submicrons. However, this type of work is quite cumbersome even for skilled workers and requires a long period of experience to become proficient. Furthermore, with such severe precision, it is inevitable to work while looking through a microscope, and considering that wafers are processed at a rate of several wafers per minute, there is a risk of worker fatigue and variations in precision due to fatigue. It becomes a big problem.

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

Photoelectric detection means are commonly used in autoalignment. For example, on a board on which part of the actual element pattern that makes up the circuit has already been formed,
Consider the case where a circuit is created by layering and printing another pattern. At this time, in the auto-alignment method, alignment is performed by guiding an alignment mark provided in advance on a substrate to be printed and an alignment mark formed on a mask having an overlapping pattern into a predetermined relationship. Since the alignment marks and the patterns constituting the actual device have a predetermined relationship, if the alignment marks are brought into a predetermined relationship, the actual device patterns on the substrate and the mask are properly aligned. 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 make adjustments.

FIG. 1 shows an example of a conventionally known optical system used in such an autoaligner. In the figure, 1 is a laser light source, 2 is a condenser lens, 3 is a rotating polygon mirror, and 4
is a relay lens, 5 is a beam splitter for guiding light to an optical system for visual observation of 22 or less, 6 is a field lens, 7 is a beam splitter for guiding light to a photoelectric detection system for 14 or later, and 8 is a beam splitter for guiding light to a photoelectric detection system for 14 or later. Relay lens; 9 is a beam splitter for guiding light from the visual observation illumination optical system consisting of 19 to 21; 1;
0 is a pupil of an objective lens 11, 12 is a mask, and 13 is a wafer, which is an optical system for laser beam scanning. Note that although Figure 1 assumes the contact printing method or proximity method as the method for transferring the mask image onto the wafer, in the case of the projection imaging method using lenses or mirrors, formation of particles between the mask and the wafer may occur. An imaging optical system is arranged. In the case of the projection imaging method, if the imaging is telecentric, the wafer 13 in FIG. 1 can be considered as the image of the wafer. The imaging relationship of the scanning spot in the scanning optical system shown in FIG. 1 is as follows. In the figure, 34 is the scanning location, but 3
When the spot is traced backward from 4 toward the rotating polygon mirror, the spot is once imaged in the vicinity of the field lens 32 via the objective lens 11 and the relay lens 8.
After being reflected again by the rotating polygon mirror via the relay lens 4, the image is formed at the position 30. i.e. 30, 32, 3
4 are in a conjugate relationship with each other. Therefore, the laser spot diameter at position 30 is φ,
If the imaging magnification from 30 to 34 is β, then 34
The scanning spot diameter at this point 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 of the optical system shown in FIG. 1 is also important. The pupil of the objective lens 11 is 1 in the figure.
0, a point 33 on the optical axis which is the center point of 10 and a reflection point 31 of the rotating polygon mirror 3 are conjugate with each other. That is, the arrangement shown in FIG. 1 can be said to be equivalent to placing a rotating polygon mirror exactly at the position of the pupil 10 from the viewpoint of the incidence of the laser beam on the objective lens.

To understand the optical system in Figure 1, objective lens 1
You must understand the function of number 1. FIG. 2 is a diagram showing the principle of the operation of the objective lens 11. The objective lens 11 has a telecentric arrangement. A telecentric objective lens is one in which the position of the pupil, which determines the size of the light flux passing through the objective lens, matches the focal position of the objective lens. The ray that passes through the center of the pupil is called the chief ray because it becomes the center line of the luminous flux. 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 edge-scattered light 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 scattered light component are spatially separated within the pupil. Second
The figure shows this separation. The specularly reflected light flux that enters the pupil, passes through the objective lens, and then passes over the pupil is indicated by diagonal lines in the figure, and 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 to 0.7.
It is desirable that it be within the nearby range.

Returning to FIG. 1 again, consider the photoelectric detection optical system that separates from the beam splitter 7 and reaches the photodetector 18. 14 in the figure is 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. A position 16 is where an image of the pupil 10 is formed by the lens 14 that forms an image of the pupil.
A light shielding plate 16 is provided here that allows only scattered light to pass through and blocks non-scattered light. Scattered light passing through the light shielding plate 16 is collected by a condenser lens 17 onto a photodetector 18. Therefore, the pupil 10, the light shielding plate 16, and the photodetector 18 are in a conjugate relationship with each other. The blocking plate can be easily created by patterning a transparent glass substrate using a substance such as metal or ink.
This photoelectric detection system, which detects scattered light, therefore produces an output only when the scanning spot approaches the edge of the pattern. If the pattern is an alignment mark shown on the mask and the wafer, the relative positional deviation between the mask and the wafer 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, 19 is a light source, and 20 is a condenser lens, which serves to form an image of the light source on the pupil of the objective lens to perform so-called Koehler illumination. 2
Reference numeral 1 denotes a filter that transmits wavelengths in a non-photosensitive region of a photoresist coated on a wafer 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 of 22 or less, 22 is an erector that rotates the image in the normal direction, 23 is a filter that attenuates the photoelectric detection light, specifically, in the example of FIG. 1, the scanning laser beam, and 24 is an eyepiece.

FIG. 3 shows an example in which the optical system shown in FIG. 1 is three-dimensionally constructed. Since it is necessary to control all two-dimensional degrees of freedom, alignment of the mask and wafer is usually performed by observing a plurality of locations on the mask and wafer.

The example shown in FIG. 3 is an arrangement example in which accurate detection is performed at two locations, and the roof mirror 25 plays the role of dividing the scanning laser beam into two optical paths. The ridgeline of the reflection roof 25 is set to intersect with the optical axis, and the ridgeline or the vicinity of the ridgeline is formed by the objective lens 11 and the relay lens 8. The ridgeline of the roof, which is the primary imaging plane of the mask, acts as a dividing line of the field of view when observing through the eyepiece. The numbers in FIG. 3 correspond to the numbers in the principle diagram of FIG. 1, but unnumbered parts represent mirrors or prisms used to bend the optical path. Also, the pupil of the objective lens 1
0 is shown as a diaphragm for convenience of explanation, but the effective diameter of 10 is necessarily determined from the effective diameter of the objective lens, etc., so there is no need to provide a mechanical diaphragm. 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. The problem with the wire connection method is that since an imaging optical system exists between the mask and the wafer, the light is attenuated and the light reflected from the wafer becomes weak. In the case of a visual system, the reflection from the back of the mask becomes flare light that spreads over the entire surface.
This worsens the contrast of the wafer image.

A known visual system method for preventing such a decrease in contrast is a ``mask pattern printing apparatus'' disclosed in Japanese Patent Application Laid-Open No. 49-28363 filed by the present applicant. This method places a quarter-wave plate in the imaging optical system, and determines the polarization direction of the light that passes through the mask, hits the wafer, is reflected, and returns to the mask, and the polarization direction of the incident light. This is a method that orthogonally intersects the Therefore, the flare component due to reflection from the back surface of the mask and the reflected light from the wafer can be easily separated using polarization means. In this case, it is convenient to use linearly polarized light as the incident light.

The present invention aims to efficiently combine a conventionally known method using polarized light for visual viewing systems with an optical system for auto-alignment that has been newly developed in recent years. For this purpose, the present invention is characterized by utilizing a polarizing beam splitter to control the polarization directions of the photoelectric detection system and the visual system.

FIG. 4 shows a preferred embodiment of the invention. The configuration is such that the system shown in FIG. 1 is placed directly on the imaging optical system 40. A 1/4 wavelength plate 41 is disposed in an imaging optical system that transfers a mask image onto a wafer. The optical system mounted on the mask differs from that in FIG. 1 in that a dichroic beam splitter 50 is placed in place of the beam splitter 5. A polarizing beam splitter 51 is arranged in place of the beam splitter 7, and a polarizing plate 52 is included in the illumination optical system. The conjugate relationship between the object and the pupil is the same as that in FIG. 1, so the explanation will be omitted.

What is important in the configuration of the system shown in Figure 4 is the quarter-wave plate 41.
and the polarizing plate 52 and polarizing beam splitter 51 combined therewith.

Further, in order to increase the extraction efficiency of photoelectric detection light, it is desirable to use a linearly polarized laser light source 1.

Next, the action of the system will be explained. In the figure, the polarizing beam splitter 51 transmits the polarized light in the plane of the paper (this is referred to as P polarized light) and reflects the polarized light in the direction perpendicular to the plane of the paper (this is referred to as S polarized light). . First, considering laser light, the laser light is set so that the polarization direction of the emitted beam is within the plane of the paper, that is, it becomes P-polarized light. The laser beam is reflected by the rotating polygon mirror 3 and then enters the beam splitter 50. The laser enters the beam splitter 50 as P polarized light, but 50
shall have sufficient transmittance for P-polarized light at the laser wavelength. After passing through 50, the light proceeds to a polarizing beam splitter 51, but since it still enters 51 as P-polarized light, it is transmitted and scanned over the mask 12 via the objective lens. The light further passes through the imaging optical system 40 from the mask, is reflected by the wafer 13, and is reflected again at 40.
The light passes through the 1/4 wavelength plate 41 twice and returns to the objective lens, but at this time, the polarization direction is rotated by 90 degrees and the P-polarized light becomes S-polarized light. Therefore, the laser beam for photoelectric detection is reflected by the polarizing beam splitter 51,
A system leading to a photoelectric detector 18 is provided. During this time, the attenuation factor of the laser beam can be almost eliminated.

On the other hand, a visual optical system is also important as well as a photoelectric detection system. This is because the visual viewing optical system plays an important role not only in observing patterns, but also in performing essential tasks during auto-alignment such as initial mask setting. For this reason, the present invention is characterized in that the polarization direction of the laser, which is the photoelectric detection light, and the polarization direction of the illumination light for visual observation are made perpendicular to each other at the time of incidence on the mask.

In order to make the polarization direction of the illumination light of the visual observation system orthogonal to the laser, in the system shown in Fig. 4, the polarizing plate 52 is used.
This means that it is set to allow S-polarized light to pass through. The light that reaches the mask as S-polarized light turns into flare light during observation, weakening the contrast of the entire system.The light reflected from the back of the mask remains S-polarized, so it is reflected by the polarizing beam splitter 51 and is sent to the photoelectric detection system side. It ends up. This light is filtered by 15
is prevented from reaching the photodetector 18 by the.

The visual illumination light other than the reflected light from the back surface of the mask reaches the wafer and is reflected back, but at that time only 1/4
The S-polarized light is changed to P-polarized light by the action of the wave plate 41. Therefore, after passing through the objective in the reverse direction, since it is P-polarized light, it passes through the polarization beam splitter 51 and enters the dichroic beam splitter 50. The visual observation light is reflected here and enters an optical system of 22 or less. Therefore, the characteristic of 50 is that it transmits P-polarized light of laser wavelength and reflects P-polarized light for visual observation. Taking He-Ne laser (light source 632.8 nm) as a laser and E-line (wavelength 546.1 nm) as a typical visual observation light, the characteristics for P-polarized light are required to change significantly between these wavelengths. . This kind of characteristic is realized by a so-called dichroic film, which is a multi-layered dielectric film. However, the characteristics of 50 do not need to be so strict, and may be such that sufficient light can reach the photoelectric detection system and the visual system.

FIG. 5 again shows the three-dimensional layout of the portion above the mask of the optical system. When the x, y, and z axes are set in the directions shown in the figure, the polarization direction of the light emitted from laser 1 is set in the z-axis direction, whereas the transmitted polarization direction of the polarizing plate in the illumination system is set in the y-axis direction. be done. Further, if the beam splitter 9 is made to have characteristics such that it reflects the photoelectric detection light and acts as a half mirror for the visual light, the loss of light can be reduced.

As described above, the present invention makes it possible to use both a visual system and a photoelectric detection system by skillfully combining polarized light, and without losing any of the merits of conventional optical systems. A detection system can be added. Furthermore, various configurations can be considered in addition to the example shown in FIG. 5, and the application of orthogonality of polarization as intended by the present invention can be applied to any of these cases. If the transmission and reflection characteristics of P-polarized light and S-polarized light are a problem, the characteristics of the system can be improved by appropriately selecting the characteristics of the dichroic film at the position 50.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a conventional optical device suitable for applying the present invention, FIG. 2 is a diagram showing the action of the objective lens in FIG. 1, and FIG. 3 is a three-dimensional arrangement of the device in FIG. 1. FIG. 4 is a diagram showing an embodiment of the optical device of the present invention, and FIG. 5 is a diagram showing an example of the three-dimensional arrangement of the device shown in FIG. 4. In the figure, 1 is a laser, 2... condensing lens, 3... rotating mirror, 4, 8... relay lens, 6... field lens, 5, 7, 9... beam splinter, 1
1... Objective lens, 12... Mask, 13... Wafer, 14, 17... Condensing lens, 16... Stopper, 18... Photo detector, 19...
...Visual light source, 22...Erector, 24...Eyepiece, 25...Field division prism, 40...
Imaging optical system, 41...1/4 wavelength plate, 50...Dichroic beam splinter, 51...Polarizing beam splinter, 52...Polarizing plate.

Claims (1)

[Claims] 1. A light source means for illuminating an object with observation light and detection light having different wavelengths, an observation means for observing the object using the observation light, a detection means for detecting the object using the detection light, and a means for detecting the object from the object. a 1/4 wavelength plate provided in the optical path of the light, and an optical system that receives the light from the 1/4 wavelength plate; The beams are incident on the quarter-wave plate in a state perpendicular to each other, and the object is illuminated through the quarter-wave plate, and a polarizing beam splitter and a dichroic beam splitter included in the optical system, The observation light from the 1/4 wavelength plate is guided to the observation means, and the 1/4 wavelength plate
An optical device characterized in that the detection light from the wavelength plate is guided to the detection means.