JPS62262423A - Exposure device - Google Patents

Abstract

PURPOSE:To detect relative positions of a mask and a wafer with high precision and in a stable manner, by observing a mark on a mask as a latent image of resist formed by printing it on a resist layer on the top face of the wafer in a bright field of non-printing light. CONSTITUTION:A mark on a mask 9 is roughly aligned with a mark on a wafer 4 disposed on a wafer stage 2. An alignment mark exposing mask 20 is then placed within a flux of exposing light and a shutter 16 is driven so that only the alignment mark is exposed on the wafer. Further, the mark on the mask 9 formed on the resist of the wafer 4 is detected in a bright field by non-exposing light and, subsequently, a stepped mark on the wafer is detected in a dark field by non-exposing light. A positional error between these marks is read out therefrom. The positional error must be corrected by driving either the stage of the wafer or the stage of the mask, if the positional error exceeds an allowable value. If the error does not exceed the allowable value, however, the correction is not needed. Then, a pattern exposing mask 19 is placed within a flux of exposing light by a cylinder 21 and the shutter 16 is driven so that the circuit pattern on the mask 9 is exposed on the wafer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an exposure apparatus used in a photolithography process in the manufacturing process of semiconductor devices such as ICs and LSIs.

A specific example is an apparatus that uses a projection optical system to project and expose a pattern on a reticle onto a semiconductor substrate (wafer), and includes alignment optics for positional alignment between the pattern on the reticle and the pattern on the semiconductor substrate. The present invention relates to an exposure apparatus in which a system and the entire apparatus including a projection optical system are matched.

[Prior Art] The basic performance required of a mask aligner is resolution performance and alignment accuracy. If I had to mention one more thing, it would be processing capacity (throughput) because of its value as a production machine. As semiconductor devices become smaller and more highly integrated,
Higher resolution performance and alignment precision are required without limit. Mask aligners are broadly classified into contact method, proximity method, 1:1 mirror projection method, lens projection method, etc. depending on the exposure method. A reduction type lens projection method (so-called stepper) is becoming mainstream. The advantages of reduced lens projection in terms of resolution performance have already been introduced in literature, so we will omit them here, but on the other hand, when viewed as a system that takes alignment accuracy into consideration, this lens projection method has major drawbacks. There is.

Simply put, the projection lens has a certain wavelength (
This is due to the fact that imaging and aberration guarantees are only made for (usually the exposure wavelength).

Among the many alignment systems for lens projection that have been proposed or implemented to date, various methods have been adopted to overcome the above-mentioned obstacles in systems that use light of wavelengths other than those guaranteed by the lens. However, all of these methods include new error factors that occur during the process from alignment to exposure. Classifying these errors from the viewpoint of character, A1 errors due to the intervention of indirect references, B, errors due to movement of the mask (or reticle) or wafer (its feeding accuracy) between alignment and exposure, C. These include the time required from alignment to exposure, the temperature of each element, errors due to vibration, etc., D, the optical path difference between the alignment light and the exposure light between the mask and the sea urchin. In the system of JP-A No. 59-79527, the errors of B and C are the same as those of JP-A-55-135831 when an additional lens is used.
In the system of No. 8-145127, error factors such as A and D occur because the reticle and wafer are not observed simultaneously. All errors A to D occur in an off-axis type stepper. Although it is possible to reduce these errors, there is no doubt that stably maintaining the errors at a low level places a heavy burden on both the device manufacturer and the user.

The system proposed in Japanese Patent Application Laid-Open No. 58-25638 by the present applicant uses a He-CD laser [442 μm] with a wavelength very close to the exposure wavelength (g-line 436 μm) as an alignment light source, and uses a projection lens at these two wavelengths. This is an excellent system that avoids the occurrence of the error A-D by correcting the difference.

However, it is more difficult to move the 9 and the 7 Kamidai to the 2 katsu L-
+ Kobori lenses tend to use shorter wavelengths,
In this case, the advantages of the system cannot be utilized.

In addition, in order to use the resolution performance of the lens more effectively, it is necessary to reduce the standing wave effect that occurs in the resist, and for this purpose, anti-reflection treatment is applied to the surface of the sea urchin.
The number of processes that use absorbers in the resist or multilayer resists will increase in the future, but in such processes, it is necessary to take signals from the wafer during alignment at or near the exposure wavelength. is expected to become more difficult.

Regarding this problem, the present inventors have already solved the problem by suppressing the error derived from the alignment system mentioned above.
A patent application was filed in 1982-23 for a new exposure method and device based on alignment using non-burning light, in order to be compatible with new processes such as absorption resists and multilayer resists.
It is proposed as fi321.

The concept of the invention of Japanese Patent Application No. 59-236321 (hereinafter referred to as the prior invention) can be applied not only to conventional lens projection, but also to lens projection with shorter wavelengths, mirror projection, and even X-ray aligners. It is.

The key point of this prior invention is to detect partial changes in the resist layer resulting from exposure as signals on the mask side.

Resist causes a photochemical reaction when irradiated with light,
In an optical sense, it also causes a change in transmittance and refractive index, or in special cases, expansion or contraction causes a surface level difference with the non-irradiated area. Even with commonly used 0FPR or AZ resists, the results of selective exposure can be observed as a grayscale image under a white light microscope. In the following description, this image will be referred to as a latent image for convenience.

The latent image of the mask (resist latent image) printed on the resist by a regular exposure optical system has a relationship with the mask that does not include any errors at that point, and the resist latent image and the wafer mark (step difference) below it are ) reads the relationship between very close images on the same object, making extremely reliable detection unaffected by optical systems etc. possible.

FIG. 2 shows the basic alignment exposure procedure in this prior invention.

Errors that occur in this process are (a) detection error, (b) stage feeding error, and (c) error due to movement of the wafer or mask during the time from preliminary exposure to regular exposure. From the current track record of steppers, (
The feed amount of the stage in (b) is at most 2 to 3μ, (c)
The time is approximately 1 second, and it is possible to suppress these errors to extremely small values.

FIG. 3 shows an embodiment of this prior invention, and its operation will be explained.

The entire system is assembled on a surface plate 1 (the structure is not shown). A wafer stage 2 is provided on the surface plate 1, and allows a wafer holding plate 3 and a wafer 4 held thereto to be moved along a plane perpendicular to the optical axis of the projection lens 5. The position coordinates of the wafer mark 2 can be determined by a known method of shining light 7 from a laser interferometer onto an optical mirror 6 provided thereon, and the movement of the wafer mark 2 by a specified amount can be controlled. . Above the projection lens 5 is a reticle 9 held by a reticle holder 8, and when light is irradiated from the illumination optical system A above the reticle 9, the pattern taken into the reticle 9 is transferred to the wafer via the projection lens 5. 4. The configuration is maintained so as to be transferred to the surface.

The illumination optical system A includes first to third condenser lenses 11, 12, and 13 to uniformly irradiate the light emitted from the ultra-high pressure mercury lamp 10 onto the reticle 9, and first to third condenser lenses 11, 12, and 13 to bend the light beam. Second
mirrors 14 and 15. shutter 16
controls exposure.

Second. The third condenser lens 12.13 and the second mirror 15 are also conjugately connected to the reticle pattern surface 17 (iI
The reticle 9 is designed to have a descendant in the part B shown in the figure, and by masking this part, it is possible to illuminate only a specific part of the reticle 9.

On surface B, there is a pattern exposure mask 19 held in a frame 18.
Then, the cylinder 21 is switched and driven so that the alignment mark exposure mask 20 can be selectively inserted into the light beam.

An alignment optical system C is arranged with a part of it protruding between the projection lens 5 and the wafer 4.

The light emitted from the halogen lamp 22 is condensed by a condensing mirror 23 and a condensing lens 24, and a half prism 25
The light then passes through the objective lens 26 and reaches the movable mirror 27 . When the movable mirror 27 assumes a positional relationship of 45° with respect to the optical axis as shown by the broken line, the light collected by the objective lens 26 illuminates the wafer surface. The light reflected from the wafer surface passes through the movable mirror 27 and the objective lens 26, bends the optical axis upward at the half prism 25, passes through the relay lens 28, and forms a wafer image on the surface 30 of the imaging tube 29.

In the case of alignment, alignment of components in the three directions of X, Y, and θ is performed by simply aligning the reticle and the sea urchin in a plane, so two objective lenses are required.
In the case of the embodiment, it is assumed that there are two eyes.

The resist latent image is an alignment mark image of the mask formed on the photoresist of the wafer, so the system uses this resist latent image to align this latent image with the wafer alignment mark (wafer step). Positioning with less error becomes possible.

[Problems to be Solved by the Invention] Incidentally, the latent image can be detected based on contrast, color difference, etc. due to index differences between exposed and non-exposed areas, but it is difficult to detect scattered and diffracted light at the edges. In principle, it is very difficult to detect a latent image in dark field because the index difference is a factor. Therefore, it is advantageous to detect the latent image in bright field.

On the other hand, when detecting a wafer mark on which a photoresist is attached, it is difficult to obtain a stable output using bright field detection due to interference between the photoresist surface and the substrate. Therefore, dark field methods are generally advantageous for detecting marks on wafers.

In other words, when alignment is performed using a latent image, if the wafer mark is detected using the bright field observation system for detecting the latent image, the wafer mark detection output will be unstable. do.

An object of the present invention is to further improve the above-mentioned alignment method using latent images and to provide an exposure apparatus capable of more stable alignment.

[Means for Solving the Problems] The present invention enables stable alignment by coexisting two methods: a latent image in a bright field and a wafer mark in a dark field.

In one embodiment of the present invention, the above object is achieved by arranging an image sensor on the surface of the sea urchin and optically on the imaging surface, and switching between bright field and dark field using this as a reference.

[Example] An example is shown in FIG. Note that parts common or corresponding to those in FIG. 3 are given the same reference numerals.

In FIG. 1, an off-axis microscope C illuminates a wafer 4 with a light source 22. In FIG. The reflected light is imaged on the imaging tube 30 to detect the deviation between the wafer mark and the latent image. At this time, a diaphragm 50.51°52 is placed on the pupil plane of this optical system. The aperture 50 determines the NA of the detection optical system. Aperture 5
1 is for bright field illumination, and the center part is transparent and the outside part is opaque.

The aperture 52 is for dark field illumination, and the aperture 51
On the contrary, the center is opaque and the outside is transparent.

Among these apertures, a bright field aperture 51 and a dark field aperture 52
When detecting a latent image, the aperture 51 is used in a bright field, and when detecting a wafer mark, the aperture 51 or 52 is used to perform bright field or dark field detection.

With the above configuration, the detection of wafer marks in a bright field is the same as in the conventional example (FIG. 3). In the case of dark field detection, first the diaphragm 52 is attached to detect the wafer mark. At this time, the wafer mark is placed. Next, the aperture 52 is switched to the aperture 51, and a latent image is detected in a bright field. At this time as well, it is determined where on the imaging surface 30 the latent image corresponds, and the amount of deviation from the previously determined wafer position is detected. This makes it possible to detect the amount of deviation between the wafer and the mark.

Next, an example of the operation of the apparatus shown in FIG. 1 will be explained with reference to the flowchart shown in FIG.

In the exposure apparatus shown in Figure 1, first a mask (reticle) is
Mark 9 and wafer 4 placed on wafer stage 2
Align with the mark. This approximate alignment can be achieved by conventional methods, such as T
The TL method or the off-axis method in which the mark on the wafer is observed with an off-axis A/S (alignment scope) placed off the optical axis of the projection lens and aligned with a predetermined position reference can be applied. Next, the alignment mark exposure mask 20 is positioned within the exposure light beam, the shutter 16 is driven, and the alignment mark is set in the :XL area of the alignment tool h. Furthermore, the marks of the mask 9 formed on the resist of the wafer 4 by this preliminary exposure were observed in bright field using non-exposure light, and then the step marks on the wafer were observed in dark field using non-exposure light. Detect and read the position error of these marks. If this position error exceeds the allowable value, the stage on the wafer side or the mask side is driven to correct the position error, and if the position error is within the allowable value, then the cylinder 21 is used for pattern exposure. The mask 19 is positioned within the exposure light flux, and the shutter 16 is driven to expose the circuit pattern portion on the mask 9 (regular exposure).

Note that although the wafer was detected first and the latent image was detected here, this order does not limit the present invention in any way; conversely, the latent image was detected first and then the wafer mark was detected. However, there is no problem. The present embodiment is characterized in that it achieves both bright field detection and dark field detection using the imaging surface 30 as one reference. Processing of electrical signals from the image pickup tube can be easily performed by applying ordinary image processing techniques. In particular, since it is an off-axis microscope, white light can be used, and the monochromatic light effect caused by interference when using a laser is greatly reduced by the averaging effect, making it possible to perform stable measurements.

FIG. 4 is a flowchart showing the operation when the present invention is applied to a stepper. The flowchart in Figure 2 is to load an unexposed wafer (step 1), approximately align the wafer (step 2.3), perform the preliminary exposure for each shot, and detect errors between the mask latent image and the wafer mark. and the point at which regular exposure is performed (step 4~
11.16 loop), and during preliminary and regular exposure,
The difference is that the movable mirror 27 is retracted out of the exposure light beam (step 14.5). Note that after exposing only the alignment mark portion (step 4), pattern exposure mask 1 is
The point of switching between the mask 9 and the alignment mark mask 20 (step 13.15) is similar to the flowchart of FIG. 2, except that it is not shown in the flowchart.

FIG. 5 shows an example of the appearance of a mask (reticle) 9 used in a stepper. In the figure, 32 is an alignment mark, 33 is a circuit pattern section, and 34 is an alignment mark section.

FIG. 6 shows an example of the appearance of a wafer baked with the stepper. In the figure, 35 is an alignment mark (step), 36 is a scribe line, and 37 is an actual element portion.

The object of the present invention can also be achieved by having two off-axis microscopes and using a high-precision stage without changing the aperture of the off-axis microscope C.

In this case, one of the two microscopes is bright field and the other is bright field.
One is dark field. First, a latent image is detected in a bright field, and then the high-precision stage is driven a predetermined amount to detect a wafer mark using a dark field microscope. The image and wafer pattern can each be determined separately. On the other hand, the moving distance between the two microscopes is also accurately measured using a high-precision stage. As a result, the amount of deviation between the wafer and the reticle can be determined in the same manner as described above.

FIG. 7 shows an example using two such microscopes. In the figure, a dark field microscope C has a dark field aperture 52.
A bright field microscope C' is provided with a bright field aperture 51 (not shown).

In the embodiments shown in FIGS. 1 and 7, the off-axis microscope is not necessarily of a type that uses a movable mirror 27, but is of a fixed type without movable parts, such as that used in a normal stepper. It is clear that the present invention can be realized even if it is the same.

[Operations and Effects of the Invention] As described above, according to the present invention, a latent image of a resist formed by printing a mask mark on a resist layer on the upper surface of a wafer is observed in a bright field using non-printing light, and Since the wafer steps, which are marks on the wafer, are observed by selecting either the dark field or the bright field, the relative positions of the mask and the sea urchin can be detected with high precision and stability.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of a projection exposure apparatus according to an embodiment of the present invention, Fig. 2 is a flowchart explaining the basic process of the present invention, and Fig. 3 is a prior application apparatus applying latent image alignment. FIG. 4 is a flowchart of alignment exposure when the present invention is applied to a stepper; FIGS. 5A and B are diagrams showing an example of a reticle and alignment mark used in the present invention; FIG. 6 A and B are diagrams showing examples of a wafer and alignment marks used in the present invention, and FIG. 7 is a configuration diagram of a projection exposure apparatus according to another embodiment of the present invention. 2: stage, 4: wafer, 6: optical mirror, 9 reticle, 19: pattern exposure mask, 20: alignment mark exposure mask, 51: bright field diaphragm, 52: dark field diaphragm. Patent Applicant Canon Co., Ltd. Agent Patent Attorney Tatsuo Ito Agent Patent Attorney Tetsuya Ito Go to Figure 2 Figure 5 Figure 5 Date 6th Figure 6 Date

Claims (1)

[Claims] 1. An exposure device that aligns a mask pattern with a wafer pattern and prints the mask pattern onto a resist layer on the wafer surface, which is capable of selectively exposing different parts of the mask pattern. an exposure optical system, a stage capable of moving at least one of the wafer or the mask along a plane perpendicular to the optical axis of the exposure optical system, and a first alignment optical system capable of bright field observation of only the upper surface of the wafer with non-baking light. and a second alignment optical system that enables dark-field observation of only the top surface of the wafer using non-printing light. An exposure device characterized in that a latent image of a resist layer on the surface can be observed. 2. The first and second alignment optical systems are equipped with a bright-field aperture and a dark-field aperture, and one of these apertures is selectively arranged on the pupil plane of the optical system to achieve a bright-field image. and the same optical system capable of observing the upper surface of the wafer by switching to a dark field mode.
Exposure apparatus described in Section 2. 3. Claim 2, wherein the alignment optical system includes an illumination system and an observation system, and the bright field diaphragm and the dark field diaphragm are alternatively arranged on a pupil plane of the illumination system. Exposure apparatus described in Section 2.