JPS62181432A - Automatic aligning apparatus - Google Patents

Abstract

PURPOSE:To observe a wafer directly and to make it possible to perform detection without considering the deterioration in transmittance, by inserting observing optical systems between the wafer and an exposing optical system, and keeping a magnification and focus unchanged. CONSTITUTION:Observing optical systems 41, 5 and 42 are inserted between wafer 2 and an exposing optical system 40. The magnification and the focus of a projected image are kept unchanged eve if the systems are inserted. The wafer can be directly observed without using the exposing optical system 40. This is an advantage. When the numerical aperture NA of the exposing optical system s small, high alignment accuracy is required. In this case, how the wafer can be observed with good resolution becomes a problem. The system, which directly observes the wafer, can detect light with a numerical aperture NA larger than the numerical aperture NA of the exposing optical system. Therefore, said system is more advantageous in comparison with a method using the exposing optical system. Since the wafer can be directly detected without considering the deterioration due to the transmittance of the exposing optical system 40, this method is advantageous when the transmittance is poor.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic alignment device for a pattern printing (exposure) device on a semiconductor.

Semiconductor chips are manufactured by layering complex circuit patterns on Si or other semiconductor substrates. One of the problems in miniaturization is how fine a pattern can be printed on a semiconductor substrate, and the other is how accurately to align circuit patterns that are superimposed in multiple layers. The question is whether it can be done. Generally, a severe value of 115 to 1/10 of the minimum line width of the circuit pattern is required for alignment accuracy. Therefore, in reality, alignment accuracy is often a constraint in chip production. It is against this background that in recent years, devices having a function of automatically performing this alignment work, a so-called auto-alignment function, have appeared. Auto alignment frees humans from the simple repetitive task of positioning.
Speed up and homogenize work.

This resulted in higher precision.

The problems when designing a device for [auto alignment] are:
The problem is how well the signal to be processed can be detected. Electrical signal processing is not as flexible as it is for humans to visually judge and perform alignment work. Therefore, in order to detect the signal with a good S/N ratio (2), special measures are required for printing or observation optical system. It has a simple structure in which a thin film of chromium-chromium oxide is deposited and patterned. However, the other object, the wafer, undergoes many layers of circuit baking, impurity diffusion, and other treatments. In addition, a thin film of photoresist is always coated on the wafer for baking, and the absolute value and unevenness of the film thickness of the photoresist also affect the wafer signal. This has a large impact on detection.

Various methods have been known in the past to deal with wafers having such diverse properties as objects and to detect signals with a good S/N ratio. A representative method is a scanning method using a laser beam (Japanese Patent Laid-Open No. 52-132851)
). In this application, a laser is used as a high-brightness light source to image a laser spot or slit on the object plane,
Scan it. Furthermore, this application is characterized in that the signal to be photoelectrically detected is only scattered light in order to optically improve the S/N ratio. If this method is directly applied to an alignment scope that observes a wafer through a reticle, the coherence of laser light becomes a problem.

In particular, in the signal from the reticle, the light that is directly scattered from the reticle and the light that is scattered and then returned via the wafer interfere with each other, causing fluctuations in time and causing instability in measurement. A method using polarized light to eliminate this interference phenomenon is shown in Japanese Patent Laid-Open No. 56-24504. In this application, an element equivalent to a λ/4 plate is placed in the optical system between the reticle and the wafer. It is characterized by polarizing the light scattered directly from the reticle and the light that is not directly scattered.The fact that an element equivalent to a λ/4 plate must be included in the optical system is a major design constraint. For this reason, conventionally, burn-in has been prevented by changing part of the lens during alignment and placing a λ/4 plate on the alignment lens, or by applying a special coating to the mirror of the optical system. , this condition was satisfied.

The present invention aims to eliminate the conventional printing restriction of arranging something equivalent to a λ/4 plate in a space through which a light beam passes, and to detect signals from a reticle and a wafer using a new method. In the present invention, the relative automatic alignment of the reticle and wafer is further performed based on the detection signal.

Furthermore, the present invention is not affected by the chromatic aberration of the exposure optical system, and aims to solve the problem of color restriction of the exposure optical system, that is, the problem of image sticking due to the difference between the wavelength and the alignment wavelength.

A further object of the present invention is to efficiently extract reticle and wafer signals by adopting a method that matches the properties of the exposure optical system. For this reason, the present invention is characterized by using a polarizing beam splitter corresponding to the alignment wavelength between the reticle and the wafer.

Details of the invention are provided in the Examples below.

FIG. 1 shows an example in which an embodiment related to the present invention is applied to a step-and-repeat type exposure apparatus, a so-called stepper. In the figure, l is a reticle or mask, 2 is a wafer, and 3 is a stage on which the wafer is placed. Reference numeral 4 indicates an exposure optical system that exposes a reticle (mask) image onto a wafer at reduced or equal magnification, 4-1 indicates a front group of the optical system, and 4-2 indicates a rear group. In this example, 4-1 is used as part of the exposure optical system.
A beam splitter 5 is arranged between and 4-2.

When the wavelength for alignment and the wavelength for exposure are different, the beam splitter 5 acts as a polarizing beam splitter for the alignment light and as a mere plane parallel plate for the exposure light.

When the wavelengths of the alignment light and the exposure light can be considered to be the same, priority is given to functioning as a beam splitter. It is convenient for the beam splitter 5 to be placed at the pupil position of the exposure optical system 4, for example. FIG. 1 shows a case where a beam splitter 5 is placed at the pupil position.

The laser light emitted from the laser source 14 passes through the condenser lens 13 and enters the rotating polygon mirror 12 . The beam scanned as the rotating polygon mirror 12 rotates passes through the relay lens 10 and then enters the beam splitter 5. The beam splitter 5 splits the light into two, but when using a polarized laser, the polarization direction of the laser must be determined in advance, or a λ/4 plate or a An element for adjusting the polarization state such as a λ/2 plate may be included. A beam splitter 11 is provided between 12 and 10 to guide the light to the photoelectric detection system, but if a polarizing beam splitter is used as 11, the polarization direction of the light entering the relay lens 10 will be naturally determined. In that case, it is convenient to adjust the polarization state using a λ/4 plate as 9. Although it is assumed that the beam splitter 5 is placed at the pupil position of the lens, the reflection point of the rotating polygon mirror and the pupil position are in a common relation with each other. The beam reflected by the splitter 5 passes through the optical system 4-2, forms an image on the wafer 2, and scans the alignment mark portion on the wafer 2 as the mirror 12 rotates.

Aberrations are corrected in the relay lens 10 so that the laser spot light is imaged on the upper surface of the wafer 2 via the optical system 4-2. The light reflected by the wafer 2 is reflected again by the polarizing beam splitter and returns to the relay lens 10.
into the photoelectric detection system. 15 is a spatial frequency filter for performing dark field detection, 16 is a condenser lens, and 17 is a photodetector.

The dark field filter 15 is located at a position common to the reflection point of the polygon. Even if the laser spot light scans the wafer 2'', the reflection point of the mirror 12 is a spatially fixed point, and therefore, even on the spatial frequency filter 15, the position of the light that is specularly reflected by the wafer 2 surface and returns remains unchanged. It is. The light projected from the mirror 12 is indicated by diagonal lines in the figure.

The photoelectric detection system detects the output of only scattered light.

That is, out of the light reflected by the wafer 2, the unscattered light reenters the projected light beam within the shaded area and is eventually blocked by the spatial frequency filter I5. On the other hand, since the scattered light at the pattern edge of the alignment mark does not follow regular reflection, it passes through the optical path of the part other than the shaded part. At the position of the spatial frequency filter I5, therefore, the scattered light and the non-scattered light are spatially separated. Due to the effect of the filter 15, only the scattered light is transmitted and enters the photodetector 17.

On the other hand, the light transmitted through the polarizing beam splitter 5 is used to detect alignment mark information on the reticle.

This principle will be explained. The light transmitted through the splitter 5 enters the λ/4 plate 6. The action of 6 serves to change the polarization direction of the light reflected by mirror 8 and returned. The light whose polarization direction has been changed is reflected by the polarizing beam splitter 5, enters the front group 4-1 of the exposure optical system, and reaches the alignment mark portion on the lower surface of the reticle l.

In view of the principle of laser spot scanning, the light from the laser beam 14 must form a spot on the lower surface of the reticle 1. The lens 7 and mirror 8 play this role. Therefore, when the beam splitter 5 is located at the pupil position of the optical system,
Lens 7 needs to be a telecentric lens.

In order to form a spot on the lower surface of the reticle 1, the position of the mirror 8 may be adjusted in the optical axis direction of the lens 7. The light reflected or scattered on the lower surface of the reticle l is first reflected by the splitter 5, passes through the λ/4 plate 6, and then passes through the mirror 8.
, and passes through the λ/4 plate 6 again. At this time, the polarization direction is rotated, so that the polarization direction now passes through the splitter 5 and is guided from the relay lens system 10 to the photodetector 17.

Generally, when a configuration is adopted in which a polarizing beam splitter is placed at the position of the pupil, the position of the mirror 8 is near the focal position of the lens 7. The system from 7 to 8 constitutes a kind of cat's eye optical system. This cat's eye optical system connects a laser spot to the lower surface of the reticle through the exposure optical system 4-1. In other words, it has become possible to simultaneously image the laser spot on both the lower surface of the reticle and the upper surface of the wafer. The exposure optical system 4 uses a specific wavelength for image burn-in, such as G-line (
436 nm), if He-Ne light of 633 nm is selected as the wavelength of the laser source 14, a large chromatic aberration will generally occur in both values. If reticle 1 and wafer 2 are focused using g-line, 633 nm
In this case, the reticle image becomes significantly defocused on the wafer. However, in this embodiment, by using an auxiliary optical system of 7 or less, it is possible to form a 633 nm spot on both the reticle and the wafer with the g-line in focus.

Signals from the reticle are similarly captured by scattered light. This situation is clear from the behavior of the light flux shown by diagonal lines on the mask side in FIG. Scattered light signals obtained when the two surfaces of the wafer and the lower surface of the mask are scanned as the rotating polygon mirror 12 rotates are detected by the first detector 17 and amplified 23.
18. After passing through the pulse shaping circuit 19, the signal processing system 20
to go into. Based on signals obtained from the processing system 20, the wafer drive device 21 adjusts the relative position of the mask l and the wafer 2. The same effect can be obtained by driving the mask 1 instead of the wafer.

In that case, the driving accuracy on the mask side becomes looser by the magnification of the exposure optical system.

What should be noted here is that since the mask and wafer are detected independently, there is no interference of light between the mask and wafer. Therefore, the obtained signal is extremely stable, and it is possible to perform highly accurate measurements with good repeatability. This stability is a feature of this example and also applies to all of the examples below.

As an alignment mark, the one shown in FIG. 2 is conventionally known. In the figure, reference numeral 25 indicates a mark on the reticle, a dotted line 26 indicates a mark on the wafer, and a dashed line 27 indicates a laser scanning line. The images of the reticle and wafer observed through the relay lens 10 are thus observed so that they overlap. The scattered light is detected when the scanning line hits the mark and is converted into a train of electrical pulse waves. By measuring the mutual time interval between pulses, the relative position of the reticle and wafer can be detected.

FIG. 3 shows another embodiment of FIG. 1. The difference from FIG. 1 is that the state of the laser beam incident on the rotating polygon mirror 12 has been changed, and a relay lens 31 has been newly placed.
The difference is in the configuration of the photoelectric detection system. The system in Figure 3 is lens 3.
1, the wafer through 10.4-2, and 31. 10.
7. 7. Although the reticle is scanned through 4-1, it is desirable to correct aberrations so that both have a total f-θ characteristic, that is, the scanning spot moves at a constant speed on the object plane. What is shown in FIG. 3 is the imaging relationship of the scanning spot, and the shaded area is the effective diameter of the incident laser beam. This relationship is the same as in FIG.

Another characteristic in Fig. 3 is the beam splitter 11.
This is the photoelectric detection system after being divided. In the figure, 32 is a pupil imaging lens, 33 is a polarizing beam splitter, 15 is a spatial frequency filter, 16 is a condenser lens, and 17 is a first filter.
・It is a detector. The reason for using the No. 33 polarizing beam splitter was to focus on the fact that the polarization directions of the two lights reflected by the wafer and reticle and integrated again by the polarizing beam splitter are orthogonal to each other.

The polarization state of the light incident on the polarizing beam splitter 5 must have both P and S components so that the light is split into the wafer side and the reticle side. When using a polarized laser, it is sufficient to set the polarization direction of the laser beam to a predetermined direction in advance, but it is also possible to place a λ/4 plate at position 9 to create P and S circumferential components, for example. It is possible. In that case, it is necessary to insert another λ/4 plate at the 9' position. In any case, the light reintegrated by the splitter 5 contains two pieces of information in the form of polarization plane separation: the wafer in one polarization direction and the reticle in the polarization direction perpendicular to the wafer. The photoelectric detection system shown in FIG. 3 and subsequent parts separate this signal again, take it out electrically, and perform signal processing. At this time, the signal from the reticle 1 may be detected in a bright field because the target objects are glass and chromium (or chromium oxide), which have considerably different reflectances. In order to obtain a bright field, a transparent glass 34 may be provided in place of the spatial frequency filter 15, or the filter 15 may be removed. Wafer signals must be captured in the dark field due to the S/N ratio, but the amount of light that can be detected in the dark field is nearly an order of magnitude smaller than in the bright field. Therefore, if it is possible to capture the reticle signal in bright field, the ratio of the P component and the S component that first enters the polarizing beam splitter can be controlled by, for example, changing the polarization direction of the laser, so that more of the reticle signal can be captured on the bonded wafer side. It is also possible to distribute the light of Even if the bright field output of the wafer changes due to resist thickness, etc., the bright field output of the reticle remains stable because it is separated by polarization.
/N ratio can be obtained well.

Further, in FIG. 3, numeral 35 represents an erector, numeral 36 represents an eyepiece lens, and represents an optical system for observing the reticle and wafer.

The observation optical system is necessary for the operation of the device, but the first
Since the diagrams in FIG. 1 and FIG. 11 become complicated, they are omitted and shown only in FIG. 3.

FIG. 4 shows a wafer 2 and an exposure optical system 4 according to an embodiment of the present invention.
This is an example of an optical system in which an observation optical system 41.5.4.2 is inserted between 0 and 0. The optical systems 41 to 42 are insertion optical systems with a magnification of 1, and maintain the magnification and focus of the projected image unchanged even after insertion. In this case as well, it is preferable to utilize the polarizing beam splitter 5. λ/4 plate6. The role of the correction lens 451 and reflection mirror 8 is the same as in FIGS. 1 and 3.

This embodiment of the present invention has the advantage that the wafer can be observed directly without using the exposure optical system 40.

Even when the numerical aperture NA of the exposure optical system is small, high alignment accuracy may be required.

In this case, the problem is how well the wafer can be observed with good resolution.A reticle has clear edges and is easy to observe, whereas a wafer has a complex structure.

In this embodiment, the numerical aperture NA of the observation optical system can be set regardless of the numerical aperture NA of the exposure optical system, so the scanning spot diameter can be made smaller than when scanning through the exposure optical system. Reducing the scanning spot diameter corresponds to increasing the numerical aperture NA of the scanning spot. Since the numerical aperture NA of the exposure optical system as a whole is fixed, if the numerical aperture NA of the scanning spot is increased, there is no margin for extracting scattered light, and the photoelectric detection output to be detected decreases.

In FIG. 4, this corresponds to the fact that the proportion of the shaded portion of the light flux passing through the entire system increases, and the portion for detecting scattered light decreases. Therefore, in a system that directly observes a wafer, the numerical aperture N is larger than the numerical aperture NA of the exposure optical system.
Since the light can be detected using A, it is advantageous compared to a method that uses an exposure optical system.

A system such as that shown in FIG. 4 is also advantageous when the exposure optical system 40 has poor transmittance. This is because the wafer, which is a difficult target to detect, can be directly detected without considering deterioration due to the transmittance of the exposure optical system. On the other hand, the signal from the reticle is once reflected by the mirror 8 and then reflected by the polarizing beam splitter 5 due to the effect of the λ/4 plate 6. Thereafter, the light passes through the exposure optical system 40, is reflected and scattered by the reticle 1 (not shown), and returns to the original path. In FIG. 4, the pupil imaging lens 44.
Although a spatial frequency filter 15 and the like are arranged and the configuration is similar to that shown in FIG. 1, of course a configuration like that shown in FIG. 3 is also possible.

Since the scattered light from the reticle has a good S/N ratio, a sufficiently good signal can be obtained even with the method shown in FIG.

Another modification is to arrange a polarizing beam splitter on the reticle and exposure optical system side, but since the system shown in FIG. 4 can be applied as is, illustration is omitted.

Further, the present invention is not limited to the laser scanning method, but can be easily applied to other autoalignment methods, such as methods using a TV or an image sensor.

According to the present invention, it is possible to detect burn-in using light by focusing the reticle and wafer, which are common to each other, with alignment light while keeping them as they are. Furthermore, the alignment wavelength can also be arbitrarily selected.

Furthermore, the present invention eliminates interference between the reticle and the wafer, making it possible to extract signals from both independently.

This enabled stable measurements and improved measurement accuracy.

Another advantage of the present invention is that the wafer can be directly observed and detected with a high numerical aperture NA. That is, it increases the detection resolution and contributes to improving the auto-alignment accuracy.

Yet another effect of the present invention is that it makes it possible to arrange the exposure optical system in accordance with its transmittance. Therefore, even when the transmittance of the exposure optical system is low with respect to the alignment wavelength, the present invention can sufficiently cope with the problem.

[Brief explanation of drawings]

Fig. 1 is a layout diagram when an automatic alignment device according to an embodiment of the present invention is applied to an exposure device, Fig. 2 is a diagram showing an alignment markter, and Fig. 3 is a diagram showing a second embodiment in which multiple Side Port of Detector FIG. 4 is a diagram showing an embodiment of the present invention, showing a side port in which a detection optical system is placed between an exposure optical system and a wafer. In the figure, 1 is the reticle, 2 is the wafer, 3 is the wafer mounting table, and 4 is the reticle.
, 40 is a projection optical system, 5 is a polarizing beam splitter, 6 is a λ/4 plate, 7 is a correction lens, 8 is a mirror, IO is a lens, 12 is a rotating polygon mirror, 14 is a laser, 15 is a spatial frequency filter, 17 20 is a photodetector, 20 is a signal processing circuit, and 25. 26 is Ord Alignment I Mark, 3
3 is a polarizing beam splitter. No./Meal

Claims (1)

[Claims] an exposure means for exposing the pattern on the first substrate to the same size or reduced size on the second substrate; a first reading means for reading the first alignment mark portion on the lower surface of the first substrate; a second reading means for reading a second alignment mark portion in a region facing the exposure means on the upper surface of the second substrate; 1. An automatic alignment device for an exposure apparatus, comprising means for relatively aligning a first and a second substrate.