JPH0654751B2 - Focus detection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a focus detection method, and particularly in the manufacture of integrated circuits such as IC and LSI, a pattern on a mask or reticle surface is projected onto a wafer surface by a projection optical system. The present invention relates to a focus detection method suitable for performing projection exposure with high precision focusing.

(Prior Art) In recent years, a reduction type projection exposure apparatus (so-called stepper) has been used as an IC or an LSI as an apparatus for exposing and transferring a fine circuit pattern.
Have been widely used in the production site of semiconductor devices such as. This reduction type projection exposure apparatus reduces an image of a circuit pattern drawn on a reticle (mask) by a projection optical system to expose a layer of photoresist (photosensitive agent) on a semiconductor wafer.

Recently, further miniaturization of the printing pattern in the projection exposure apparatus has been required.

As a conventionally known relationship, the line width of the printing pattern is W,
When the focus search is d, the exposure wavelength is λ, and the numerical aperture of the projection optical system is NA, W = k ₁ λ / NA d = k ₂ λ / (NA) ² (where k ₁ and k ₂ are constants ) Is established.

For this reason, if the wavelength is shortened or the numerical aperture is increased in order to reduce the line width of the printing pattern, the depth of focus becomes smaller in proportion to it or in proportion to the square. Then, for example, focus position fluctuation due to the following causes becomes a big problem.

That is, when the projection optical system is a refraction system, the refractive index of the glass material changes due to changes in the ambient temperature, the atmospheric pressure of the air, etc., and the focal position fluctuates.

In recent years, various projection exposure apparatuses using an excimer laser as a light source have been proposed. Among them, in the case of an excimer laser whose spectrum width is narrowed, the oscillation wavelength changes with time, which results in a focal point. The position is changing.

In order to solve such a problem, the focus position may be detected by the exposure light immediately before the exposure and the focus position may be adjusted each time, but in the recent resist process, the multilayer resist has a property of absorbing the exposure light. Therefore, it is difficult to effectively detect a signal associated with focus detection.

Therefore, it is very difficult to detect the focus position with high accuracy and obtain a printing pattern image with high resolution.

(Problems to be Solved by the Invention) In the present invention, when a pattern on a reticle surface is projected onto a wafer surface through a projection optical system and exposed, environmental conditions such as ambient temperature and atmospheric pressure change, and An object of the present invention is to provide a focus detection method that can always position a wafer at a focal position with high accuracy even if the oscillation wavelength of a light source fluctuates slightly and the focal position fluctuates, and that a high resolution can be easily obtained.

(Means for Solving Problems) In the focus detection method of the present invention, a pattern formed on a first object is projected on a second object placed on a movable stage and coated with a photosensitive layer. An irradiation step of irradiating a pattern formed on the first object with light in the focus detection method for detecting the focus state of the projection optical system; A step of moving in a direction orthogonal to the axial direction; and a step of sequentially performing the irradiation step and the moving step,
Exposing and transferring images of the patterns having different in-focus states to different positions on the photoconductor layer; observing the undeveloped pattern image with an observation system; And a step of determining the focal position based on the above.

(Embodiment) FIG. 1 is an overall schematic view of a focus detection method according to an embodiment of the present invention. In the figure, the light emitted from the illumination optical system 1 irradiates the reticle 2 as the first object, and the pattern thereof is transferred onto the wafer 4 as the second object by the projection optical system 3. The wafer 4 is coated with a resist as a photoconductor and is fixed on a movable stage 5 arranged on a surface plate 16.

The movable stage 5 is driven by a drive system (not shown) to move X, Y and Z.
Direction (optical axis direction of the projection optical system 3), and the amount of movement is monitored by a laser measuring instrument for each of the X, Y and Z directions. In FIG. 1, the laser measuring device 8 for the X direction is shown, but the laser measuring device for the Y direction and the Z direction is not shown. The height of the optical axis of the laser measuring device 8 is the same as the surface of the wafer 4 in order to eliminate Abbe's error.

Reference numeral 6 denotes an integrating exposure meter. A signal indicating the exposure amount from the integrating exposure meter is sequentially input to the controller 100, and when it is confirmed that a predetermined exposure amount has been reached, the shutter of the optical system 1 is controlled by the controller 100. The signal is closed and the exposure is stopped.

When the focus detection pattern is transferred to the resist, if the exposure amount that illuminates the pattern fluctuates, the pattern state changes according to the change in the exposure amount even if the pattern is transferred in the in-focus state. Alternatively, it prevents the defocus amount from being not accurately detected.

Reference numeral 7 denotes a masking mechanism in the illumination optical system 1, which is removed during normal exposure. Then, a pattern 9 for focus detection on the surface of the reticle 2 is formed in an area on the surface of the wafer 5.
Is projected, and masking is applied when an image of the pattern 9 is formed on one area on the resist surface of the wafer 4 so that only the vicinity of the focus detection pattern 9 on the reticle 2 surface is illuminated.

The masking at this time may be arranged directly above or below the reticle 2.

An observation system 10 is for observing an undeveloped pattern image which is a latent image of the focus detection pattern 9 formed on the resist surface on the reticle 4 surface. Reference numeral 11 denotes a light source, which emits a light flux having a wavelength that is not sensitive to the resist. The light source 11 is imaged by the lens 12 near the pupil position of the objective lens 13. The light flux from the light source 11 illuminates the surface of the wafer 4 through the lens 12, the half mirror 15 and the objective lens 13. Then, the light flux from one area on the surface of the illuminated wafer 4 is passed through the objective lens 13 and the half mirror 15 to T
The light is guided to the V camera 14 for observation.

In the present embodiment, when the pattern image of the focus detection pattern 9 provided on the reticle 2 is formed by projection exposure on the surface of the wafer 4, first, the stage drive means 110 is used to form the stage 5
Is moved to position the pattern image forming position on the surface of the wafer 5 directly below the projection optical system 3.

Then, by controlling the masking mechanism 7 by the controller 100, masking is provided in the optical path of the illumination light, and the pattern 9 is selectively illuminated by the illumination optical system 1, and the projection optical system 3 patterns 9 on the resist surface of the wafer 4. To form. Then, the wafer stage 5 is moved again by the stage driving means 110 so that the undeveloped pattern image is positioned directly below the observation system 10. The image formation state of the undeveloped pattern image at this time is observed using the TV camera 14.

The undeveloped pattern image captured by the TV camera 14 is processed by a computer in the controller 100 to determine the shift amount δ from the focus position from the contrast of the pattern image or the size compared with the original pattern. At this time, the conversion formula of the contrast of the predetermined pattern image or the size-deviation amount δ is stored in advance in the computer, and the shift amount δ of the focus position is calculated from the contrast calculated from the image of the undeveloped pattern image. Looking for.

Then, for example, the wafer 4 is moved in the vertical direction (z direction) in the direction for correcting the deviation amount δ to match the focus position, and then the masking is removed to perform projection exposure of the actual element pattern on the reticle 2 surface. It has started.

The flow chart of the above sequence is shown in FIG. 2 for reference.

In this embodiment, the wafer stage driving means 110
By moving the wafer 4 in the vertical direction by a predetermined step, the stage 5 is further moved leftward and rightward, and the pattern 9 is exposed and transferred onto the surface of the wafer 4, so that the pattern 9 is moved to a different position on the surface of the wafer 4. A plurality of pattern images in different focused states are formed. When the undeveloped pattern image is observed with a TV camera, as shown in FIG. 4, since the focus states are different (exposure is performed at different positions in the optical axis direction), pattern images having different blur states are formed. Thus, different image signals are obtained.

From the above results, the relationship between the contrast of the image signal and the position of the wafer surface in the optical axis direction is as shown in FIG.
The position in the optical axis direction on the wafer surface where the contrast of the pattern image is maximum can be obtained as the best focus position.

In particular, if a pulse laser such as an excimer laser whose light emission time is very short is used as a light source, pattern images can be formed by continuous feeding at each exposure position of each pattern image without stopping the stage 5 each time. Through-but can be improved.

In the present embodiment, the undeveloped pattern image of the pattern 9 is used to perform the alignment between the reticle 2 and the wafer 4, the magnification detection of the projection optical system 3 and the like simultaneously with the focus detection through the observation optical system 10. Is also good.

As the focus position adjusting method, in addition to the vertical movement of the wafer 4, for example, the vertical movement of the reticle 2, the movement of the projection optical system 3 and some of its members, and the change of the environment in the projection optical system 3 are performed. You may go in.

Alternatively, a wavelength tunable laser such as a narrow band excimer laser may be used as a light source to adjust the angle of a mirror, an ethylon or the like, which is a part of the narrow band element, to change the wavelength to perform focus alignment. .

In the embodiment described above, the pattern image of the focus detection pattern 9 is formed on the resist on the wafer 4. The so-called latent image forming method is used.

The following embodiment is a method completely different from the latent image forming method, and instead of forming a latent image on a resist, an optical recording medium such as a magneto-optical recording medium and photochromic which can be freely written and erased is used.

FIG. 3 is a partial constitutional view showing another embodiment of the present invention, and shows a portion including the wafer stage 5 of the apparatus shown in FIG. 1 and an observation system 10 '. In the figure, 20 is a magneto-optical recording medium, 21 is a magnetic field generating means for applying a bias magnetic field perpendicular to the recording surface of the medium 21, and is composed of a permanent magnet or the like.
The observation system 10 'is different from the observation system 10 shown in FIG. 1 and is composed of a polarization microscope. Needless to say, other reference numerals indicate the same members as those shown in FIG.
The description is omitted here.

The wafer stage 5 shown in FIG. 3 also has a high precision X,
It can be moved in the Y and Z directions. Further, the Z axis can be rotated about the rotation axis.

Also in this embodiment, the pattern image of the focus detection pattern is formed on the magneto-optical recording medium 20. At this time, the magneto-optical recording medium 20 is arranged so that the recording surface of the recording medium 20 and the resist surface on the wafer 4 are substantially flush with each other.

The portion of the magneto-optical recording medium 20 on which the pattern image is formed is
Due to the photothermal effect, the magnetization direction of the perpendicular magnetization image forming the medium is reversed. Therefore, the recording pattern corresponding to the focus state of the pattern image is recorded on the magneto-optical recording medium 20.

On the other hand, the recording state of the pattern of the magneto-optical recording medium 20 on which the pattern is recorded is observed by the observation optical system 10 'including a polarization microscope. That is, the polarization direction of the linearly polarized light that is incident on the portion where the magnetization reversal occurs according to the pattern and is reflected and returned, and the polarization direction of the linearly polarized light that is incident on the portion that has not been magnetized reversal and is reflected and returned. Since it is different, it is possible to know the state of the recording pattern formed on the recording medium, that is, the blurring degree and size of the pattern, by selectively extracting the reflected light from the pattern via an analyzer etc. and observing it. Is.

Since the sequence for performing the above observation is almost the same as that of the embodiment shown in FIG. 1, its explanation is omitted here.

After the focus detection is completed, by applying a magnetic field of a predetermined intensity from the direction perpendicular to the recording surface of the magneto-optical recording medium, the part where the magnetization reversal occurs can be returned to its magnetization state, and the next measurement can be performed. Can be used.

By using the writable and erasable optical recording medium in this way, unlike the latent image forming method of forming an image on the resist, it is possible to obtain the special effect that the same recording medium is always used for focus detection. Is.

As the writable and erasable optical recording medium, various optical recording media such as the above-mentioned photochromic material, amorphous material which causes a phase transition between crystalline and non-crystalline, in addition to the magneto-optical recording medium 20. Can be used.

When the photochromic material is used as the recording medium, 20 in FIG.
It is preferable that 1 is the cooling means. The lower the temperature of the photochromic material is, the faster the coloring and decoloring speeds are. Therefore, the focus detecting pattern image forming speed or the used pattern image erasing speed can be increased or decreased by the action of the cooling means 21. Can be controlled.

(Effect of the Invention) According to the present invention, a focusing pattern on the reticle surface is formed on the wafer surface by the projection optical system, and the undeveloped pattern image at this time is observed by the observation optical system to determine the focus position. By performing detection and focusing, it is possible to achieve a focus detection method that can always position the wafer at the focus position with high accuracy without being affected by variations in the focus position of the projection optical system.

[Brief description of drawings]

FIG. 1 is a schematic view of an embodiment of the present invention, and FIG. 2 is an explanatory view of a flow chart of an embodiment when focusing is performed in the present invention. FIG. 3 is an explanatory view of essential parts of an embodiment of the present invention. FIG. 4 is an explanatory view showing the relationship between the pattern on the wafer surface and the image signal according to the present invention, and FIG. 5 is an explanatory view showing the relationship between the position of the wafer surface in the optical axis direction and the contrast of the pattern image. . In the figure, 1 is an illumination optical system, 2 is a reticle, 3 is a projection optical system, 4 is a wafer, 5 is a movable stage, 6 is an integrating exposure meter,
7 is a masking mechanism, 8 is a laser length measuring device, 9 is a pattern for focus detection, 10 is an observation system, 11 is a light source, 13 is an objective lens, and 14 is a TV camera.

Claims (3)

[Claims] 1. A pattern formed on a first object,
A focus detection method for detecting a focus state of a projection optical system which is mounted on a movable stage and projects onto a second object coated with a photosensitive layer, comprising irradiating a pattern formed on the first object with light. An irradiation step of: moving the movable stage stepwise in an optical axis direction of the projection optical system and in a direction orthogonal to the optical axis direction; and by performing the irradiation step and the moving step sequentially,
Exposing and transferring images of the patterns having different in-focus states to different positions on the photoconductor layer; observing the undeveloped pattern image with an observation system; And a step of determining a focus position based on the focus detection method. 2. The focus detecting method according to claim 1, wherein the observing step includes a step of measuring a contrast of the observed pattern image. 3. The focus detecting method according to claim 1, wherein the observing step includes a step of measuring a size of the observed pattern image.