JPH0799732B2 - Alignment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is applied to an apparatus for projecting and transferring a pattern on a reticle onto a wafer coated with a thin film such as a resist via an imaging optical system. And a method for aligning a wafer.

[Prior Art] The two basic performances of this type of exposure apparatus are resolution and overlay accuracy. The resolution is very simple to handle. This is because there are few parameters that determine the resolving power, and in a device called a stepper, the resolving power of the optical system can be easily inferred if only the wavelength used and the numerical aperture (NA) of the projection lens are known. In addition, even in the case of X-ray exposure, there are only limited parameters such as penumbra blurring due to the size of the light source.

Since the realization of one transistor of the memory cell, it has been the progress of lithography, that is, the fine line width printing technology and the progress of the process technology such as etching, which has played a role in the high integration of semiconductors. Regarding the resolving power, the optical system has been steadily improving, as can be understood by tracing the history of stepper lenses. The optical method breaks the wall of 1 μm, and lenses for the submicron era are being announced one after another.

On the other hand, in the process as well, new ideas have been realized with the idea of three-dimensional IC in combination with the low step and high step such as the trench digging method. Advances in resolution on the exposure equipment side and advances in the process side will find the greatest point of contact in the stage of superimposing patterns in each process. In that sense, it can be said that overlay accuracy is becoming more and more important in exposure equipment.

It is difficult to display overlay accuracy with simple parameters that deal with resolution. This shows the variety of wafer processes, but on the other hand, it can be said that it is due to the variety of configurations of the alignment system for overlaying. The reason why the wafer process factor is made more complicated is that the problem is not limited to one wafer substrate, and it is necessary to discuss the photoresist including the photoresist coated on the wafer. One of the clear directions of current semiconductors is the flow toward the three-dimensional construction of ICs. Among them, it is unavoidable that the wafer surface has a high level difference, but this high level difference has a definite adverse effect on the state of photoresist application. Wafers are becoming larger and larger, from 6 inches to 8 inches and even 10 inches. When photoresist is applied to a large-diameter wafer by the spin method, it is obvious that the resist application situation is different between the central part and the peripheral part, and the difference may be more remarkable as the step difference on the wafer surface is larger. it is obvious. In fact, it is known that the alignment state changes under the influence of resist coating, and conversely, research has been conducted on how to perform uniform coating.

Another point to note in photoresists is the trend toward multilayering in the submicron era. Measures for improving the resolution, such as the multi-layer resist process and CEL, are inevitably adopted in some steps, and countermeasures against them are also necessary. It can be said that the exposure apparatus is forced to deal with these new wafer processes in the stage of overlaying.

On the other hand, the variety of alignment systems is a proof of the flexibility and difficulty of system configuration.
At present, no single alignment system has been proposed and implemented, and each system has its own advantages and disadvantages. For example
58-25638 “Exposure device” is one example. This system uses a telecentric optical system for both the reticle and wafer as the projection optical system, and uses TTL on Axis.
This is one of the excellent configuration examples that realized the idea. Although the projection lens has been corrected for aberrations with respect to the g-line (436 nm), the same performance is also exhibited at the He-Cd laser wavelength (442 nm). In one embodiment disclosed in this patent application, a laser beam scanning method using a He-Cd laser is adopted as an alignment signal detection method, and as a result, TT
It is possible to immediately start the exposure operation with L on Axis, that is, in the aligned state. The TTL on Axis system has the least systematic error factor in the sense that there is only one alignment signal detection error as an exposure device, and it is close to the ideal system.
The only drawback of this system is that it is vulnerable to processes such as multilayer resists that absorb wavelengths near the exposure wavelength.

On the other hand, on the other hand, a wavelength other than the exposure wavelength, specifically e
Many system configuration examples using longer wavelengths such as line (546 nm) and He-Ne laser (633 nm) have been proposed.
This system has the advantage of being robust against absorption-type processes such as multilayer resists, since wavelengths longer than the exposure wavelength are used. However, the image height for alignment is usually fixed with respect to the projection lens due to various color aberrations of the projection lens, and an error factor of moving the wafer to the exposure position after detecting the alignment is introduced. Become. Therefore, the alignment system with light other than the exposure wavelength is inevitably a TTL off Axis system.

However, in recent years, the requirement for overlay accuracy has become more and more strict, and even the detection error of the alignment signal, which is the error factor in the ideal system as shown in Japanese Patent Laid-Open No. 58-25638, can be put into a problem area. ing. The present invention is characterized by improving the alignment accuracy by analyzing the detection error component of the alignment signal based on the conventional example and removing the error factor.

According to the analysis of the detection error component of the alignment signal by the inventors of the present application, the error component is mainly due to the photoresist coating problem.
It was found that most of them were caused by the problem of the step structure of the AA mark) on the surface of the wafer substrate.

There are various causes of error due to the photoresist, but it is considered that the largest of them is the following two factors.

The first is the effect of interference between the light reflected from the surface of the resist and the light that passes through the resist and strikes the wafer substrate and returns. In particular, as described above, the photoresist is not always uniformly applied within the wafer, and the applied state is often different between the center and the periphery. The wafer substrate itself also has a problem of uniformity in the wafer such as etching and sputtering. Therefore, the structure of the AA mark of each shot in the wafer is different depending on the place and place when considering the resist coating, and therefore the interference effect is also different. It is considered that the effect of this interference is the largest in causing the alignment error due to the influence of the resist coating.

The second factor is multiple reflection. The resist has a character as an optical waveguide. Therefore, a part of the light reflected by the wafer substrate is reflected by the interface between the resist and air, and returns to the wafer to be re-reflected. This effect is more remarkable as the reflectance of the substrate is higher, and this multiple reflected light eventually causes interference and deteriorates the alignment accuracy.

The cause of the alignment error is the problem of the step structure of the AA mark. The step structure of this AA mark becomes non-uniform due to etching, sputtering and the like. For example, in the AA mark step structure after aluminum sputtering, the aluminum film thickness in the vicinity of the edge may become non-uniform, and the part that is originally a horizontal plane may have an inclination. Therefore, the light reflected by the wafer substrate is directly taken in as a false signal, which becomes a factor that deteriorates the alignment accuracy.

In addition to this, there is a problem on the surface of the wafer substrate as a factor of alignment error. That is, if the surface of the wafer substrate is a rough surface, scattered light from the rough surface is affected by the resist and causes interference, which causes an alignment error.

[Problems to be Solved by the Invention] Therefore, according to the present invention,
An object of the present invention is to provide a positioning method that reduces the error component of the alignment signal due to the AA mark step structure and the wafer substrate surface state and realizes higher alignment detection accuracy.

[Means and Actions for Solving Problems] In order to solve the above object, the present invention relates to an exposure apparatus for projecting and exposing a pattern drawn on a reticle onto a wafer coated with a resist via a projection optical system. In the alignment method for aligning the reticle and the wafer, the step of arranging the reference plate on which the resist is not applied at the wafer position, the image of the mark on the reticle and the projection optical system are moved by the first optical system. The first detection step of detecting the positional relationship between the reticle and the substrate by detecting the image of the mark on the reference plate through the mark, and the mark on the wafer that is not affected by the resist as compared with the first optical system. Based on the detection result of the first and second detection stages, and the second detection stage that detects the image of the mark on the reference plate at the same position as the first detection stage by the second optical system that detects hand , The relative relationship between the image position of the mark on the reticle detected by the first optical system and the image position of the mark on the wafer detected by the second optical system, in which the reticle and the wafer are in a predetermined positional relationship. Is stored as an offset and the relative relationship between the reticle mark image position detected by the reticle imaging optical system and the wafer mark image position detected by the wafer imaging optical system when aligning the reticle and the wafer. It is characterized by means for correcting the offset and detecting the positional deviation between the reticle and the wafer.

According to this, when the detection result detected individually has a relative relationship, the offset indicating whether the reticle and the wafer have a predetermined positional relationship is accurately detected in advance,
At the time of actual alignment, the offset is corrected from the relative relationship of the individually detected results to accurately detect the positional deviation between the reticle and the wafer. Moreover, this offset does not have to be taken again even if the reticle is replaced.

Embodiments Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an example in which the alignment apparatus according to one embodiment of the present invention is applied to a stepper, that is, a reduction projection exposure apparatus.

In the figure, the reduction lens 3 serves to project and expose the pattern 2 on the reticle 1 onto the wafer 4. The reduction lens 3 is provided with the unevenness of the wafer to be printed,
Usually, the wafer side is usually telecentric so that the distortion and the magnification do not change due to focus fluctuations caused by measurement drive errors of the autofocus system arranged between the lens 3 and the wafer 4. is there. It should be noted that in FIG. 1, the five locations on the wafer correspond to the pattern 2 on the reticle 1.

The greatest feature of the present invention is the wafer mark imaging optical system arranged below the projection lens 3. In FIG. 1 the AA mark 6 for the part 5 of the wafer to be printed is
An image is formed on the mark 6 by a wafer mark image forming optical system 21 arranged in a predetermined direction and at an angle. This is the most important part of the present invention. And
This image is detected as an electric signal by the wafer mark image detecting element 14. Reference numerals 12 and 13 are a mirror and a lens for guiding the light from the mark 6 to the wafer mark imaging optical system 21.

As described above, the largest cause of the detection error of the alignment signal is the interference between the reflected light from the resist surface and the reflected light from the wafer substrate. There are several methods to eliminate this effect, but the most fundamental solution is to prevent the surface reflected light from entering the wafer mark imaging optical system. When observing the resist coating state with an SEM or an interference microscope, the inclination of the surface of the resist coated on it, even on a wafer with a very large step structure, is up to around 5 °, which is even steeper. It turned out that there is no such slope. Due to the problem of step coverage, it is usual to apply a resist having a thickness larger than that for a large step, and as a result, the value is set to about 5 °.

Therefore, the present invention is characterized in that the following conditions are added to the UHEHA mark imaging optical system 21 as shown in FIG. That is, the slope angle of the resist is 5
゜, and the divergence angle of illumination light 17 is A, surface reflected light 18
Is an angle B = 10 °, and the multi-reflected light 19 is about 25 ° if the light is reflected twice by the substrate. In the case of three or more reflections, the reflection intensity is generally sufficiently small for practical use and can be ignored.
Therefore, the angle E at which the image of the AA mark 6 is captured is E ≧ C + A (1).

In this way, the reflected light and the multiple reflected light of the illumination light on the resist surface do not enter the wafer mark imaging optical system 21.

Furthermore, the AA mark can be imaged, the AA mark shape can be recognized by an image processing system (not shown), false signals due to the AA mark step structure and the wafer surface shape can be eliminated, and the true AA mark position can be detected. it can.

The TTL method that has been proposed for the stepper has hitherto illuminated the illumination light through the projection lens and received the reflected light through the projection lens. On the other hand, the present invention is a method of receiving light outside the projection lens, and further, by providing a regulation value of E ≧ C + A, reflection on the resist surface and multiple reflection are removed, and a mark image is further formed to obtain image data. It succeeded in extracting only the AA mark by performing image processing by using. The fact that only the AA mark itself can be separated and taken out is directly linked to the improvement of alignment accuracy.

Although the wafer mark image forming optical system has been described above, the reticle mark image forming optical system, as shown in FIG. 1, first uses the beam of the light source 9 through the mirror 8, the beam splitter 11, etc. to form the reticle mark on the reticle 1. Illuminate 15 And
The mark image formed by the image forming optical system 7 is received by the detecting element 10 to obtain image data. The beam transmitted through the reticle 1 illuminates the above-mentioned wafer mark 6 via the projection system.

Here, it is necessary to previously obtain the relative positions of the two images obtained by the reticle mark imaging optical system and the wafer mark imaging optical system.

As this method, a reference wafer having a vertical edge not coated with resist is used, and the amount of deviation between the reticle and the wafer is set to zero based on the image data of the reticle mark and the wafer mark imaged by the reticle mark imaging optical system. Fit in. At this time, the relative position in the screen of both the wafer mark imaged by the wafer mark imaging optical system and the reticle mark imaged by the reticle mark imaging optical system is stored as an offset. Then, at the time of actual alignment, the offset value is subtracted from the relative positional deviation amount between the reticle mark image obtained by the reticle mark imaging optical system and the wafer mark image obtained by the wafer mark imaging optical system. , By aligning, it became possible to perform highly accurate alignment.

In the above embodiment, it is desirable that the light source 9 in FIG. 1 has the same wavelength as the exposure wavelength, but when a wavelength other than the exposure wavelength is used, the correction optical system 16 is used to correct the aberration of the illumination light projected on the wafer. It is also possible to do so. If necessary, the correction optical system can be retracted at the time of exposure. The light source 9 is preferably a laser, but may be a high-luminance light source, and may be a mercury lamp spectral line.

FIG. 3 shows an example in which signals are detected at a plurality of wavelengths. In addition to FIG. 1, a light source 9'of another wavelength, a light receiver 14 'and a beam splitter 11 are arranged. The correction optical system 16 moves in and out according to the wavelength used. Of course, it is possible to use three or more wavelengths in the same manner. By doing so, it is possible to always obtain a good AA mark image regardless of the process conditions of the wafer.

FIG. 4 shows an example in which light is received by the image guide 17. The detection optical system can be easily mounted at low cost.

Although the wafer mark image forming optical system 21 is shown as a pair in FIG. 1, the mark can be observed in the direction of the arrow in FIG. 5 if necessary due to the relationship between the mark and the direction of diffraction light spread. It is also possible to have two or more pairs. Further, the arrangement direction of the wafer horizontal plane of the wafer mark imaging optical system is a direction orthogonal to the AA mark edge.

Further, the wafer mark image forming optical system and the reticle mark image forming optical system are not limited to those in the above-mentioned embodiments, and may be any one as long as they form an image of the mark for obtaining image data.

EFFECTS OF THE INVENTION As described above, according to the present invention, in the method of individually detecting the positions of the reticle and the wafer with different optical systems and aligning the reticle and the wafer, the detection result detected individually. When the actual alignment is performed, the offset indicating how the reticle and the wafer have a predetermined positional relationship with each other is accurately detected in advance.
By correcting the offset from the relative relationship of the individually detected results, it is possible to accurately detect the positional deviation between the reticle and the wafer. Moreover, this offset has an effect that it is not necessary to retake it even if the reticle is exchanged.

[Brief description of drawings]

FIG. 1 is a schematic view of a main part of a stepper to which an alignment apparatus according to an embodiment of the present invention is applied, and FIG. 2 is an explanatory view showing a state of reflection on a wafer surface by illumination light on the wafer. 3 to 5 are views for explaining another embodiment of the present invention. 1: reticle, 3: projection lens, 4: wafer, 6: wafer step, 7: reticle mark image forming optical system, 8: reflection mirror, 9: light source, 10, 14, 14 ': mark image detection element, 21 : Wafer mark imaging optical system.

Claims (2)

[Claims] 1. An alignment method for aligning a reticle and a wafer in an exposure apparatus, which projects and exposes a pattern drawn on a reticle onto a wafer coated with a resist via a projection optical system, Disposing a reference plate on which a resist is not applied at the position of the wafer; a first optical system displays an image of the mark on the reticle and an image of the mark on the reference plate via the projection optical system. A first detection step of detecting the positional relationship between the reticle and the substrate; and a second optical step of detecting a mark on the wafer without being affected by the resist as compared with the first optical system. A second detection step of detecting an image of a mark on the reference plate at the same position as the first detection step by a system; and based on the detection results of the first and second detection steps, the reticle Image position of the mark on the reticle detected by the first optical system and the image of the mark on the wafer detected by the second optical system when the lens and the wafer are in a predetermined positional relationship. Storing the relative relationship with the position as an offset; when aligning the reticle and the wafer, the mark image position of the reticle detected by the reticle imaging optical system and the wafer imaging optical system are detected. The step of correcting the offset from the relative relationship with the mark image position of the wafer, and detecting the positional deviation between the reticle and the wafer. 2. The alignment method according to claim 1, wherein the second optical system detects a mark on the wafer without passing through the projection optical system.