JPH06163350A - Projection exposure method and device thereof - Google Patents

Abstract

PURPOSE:To improve resolution and focus depth simultaneously and to improve contrast even when a line width of a repetitive pattern is changed by reducing a rate of zero-order light directed to a pupil by dividing illumination source and by directing just a part of each divided light source to a pupil. CONSTITUTION:Light projected from a water source lamp 1 which is an exposure light source and converged by an oval mirror 2 is reflected by a mirror 3 and injected to an annular lens 4. Light passed through an illumination aperture diaphragm 5 is reflected by a mirror 6 and illuminates a reticule 8 by a condenser lens 7. A pupil surface of an illumination system is made to be illuminated as shown is (b). Four circles B which partially overlap with a pupil A show a projection image a pupil surface of each divided light source. When an illumination system is improved in this way, only a part of light (zero order light) which does not receive diffraction by a mask is injected and a rate of zero order light is reduced to primary light. Therefore, contrast of a projection image is improved.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to a projection exposure method and apparatus for use in photolithography.

[0002]

2. Description of the Related Art In recent years, the development of semiconductor technology has been remarkable.
As for the pattern of a semiconductor device, a pattern having a size of 0.5 μm is being formed. Such miniaturization is rooted in a breakthrough in elemental technology called lithography technology. Lithography technology is broadly divided into the steps of resist application, exposure and development. The miniaturization of the pattern has been achieved by improving the resist material, exposure apparatus and exposure method, development method and the like. In particular,
The refinement of the pattern has been remarkably promoted by the improvement of the exposure apparatus and the exposure method.

A method of increasing the numerical aperture (NA) of the projection lens has been used as a method of miniaturizing the pattern. However, since increasing the NA simultaneously causes a decrease in the depth of focus, there is a limit in achieving resolution improvement with increasing the NA. Therefore, there is a demand for a method of improving the resolution without changing the numerical aperture of the projection lens. Therefore, a method of forming a finer resist pattern by improving the illumination system of the projection exposure apparatus has been proposed.

In an ordinary projection exposure apparatus, the aperture stop placed at the exit of the fly's eye lens is circular as shown in FIG. 6 (a). In the figure, A is a pupil and B is a secondary light source image. The exit of the fly-eye lens corresponds to the pupil plane of the illumination optical system, and is a position optically conjugate with the pupil plane of the projection optical system. This aperture stop is called a secondary light source, and an image (secondary light source image) obtained by projecting the secondary light source on the pupil plane is called an equivalent light source. Since the secondary light source plane and the pupil plane are at conjugate positions, the illumination system is characterized by an equivalent light source. Therefore, in the following discussion, the illumination system will be described using the secondary light source image projected on the pupil plane. By normalizing the radius of the pupil as 1, the image formed by the secondary light source on the pupil surface becomes the same circle as the aperture stop. The radius σ of this circle is defined as the degree of coherence of the projection optical system.

On the other hand, as a method of using the improved illumination system (hereinafter referred to as modified illumination), for example, Japanese Patent Laid-Open No. 4-101.
For example, there are JP-A No. 148, JP-A-4-180612 and the like. In the method proposed here, an aperture stop having a shape different from a circle is provided at a position conjugate with the pupil plane of the illumination system, and a higher resolution is realized by illuminating the mask in an oblique direction. This principle has long been widely known as dark field illumination in microscopes. As an example of applying this principle to a projection exposure apparatus, the proceedings of the 32nd Joint Lecture on Applied Physics by Horiuchi et al., P. 294 (1985), "Pursuit of optical exposure resolution limit using annular light source diaphragm". In this example, high resolution is obtained by arranging the ring-shaped aperture shown in FIG. 6B at a position optically conjugate with the pupil plane of the illumination system. On the other hand, JP-A-4-180
In the method described in Japanese Patent No. 612, attention is paid to the fact that the LSI is mainly composed of horizontal and vertical patterns, and it is arranged in the illumination system in order to more efficiently resolve the resolution of the vertical and horizontal patterns. The aperture (diaphragm) has a shape as shown in FIG. The illumination light is divided into four on the 45-degree diagonal line of the mask by this opening and enters the mask surface. Such an improvement of the illumination light source can increase the incident angle of the diffracted light on the mask to the pupil. It has been confirmed that this four-division illumination can improve the resolution and depth of focus of horizontal and horizontal patterns. In this four-division illumination method, since only one side (1/2) of the diffracted light at the mask is used, the resolution is improved as compared with the normal method, but the contrast of the projected image is lowered as compared with the normal method. There is a problem.
In order to prevent this, a method has been proposed in which the four-division illumination method is used in combination with a halftone type phase shift mask.

[0006]

The illumination method shown in FIG. 6 (c) in which the illumination light source is divided into four areas on a diagonal line of 45 degrees (Japanese Patent Laid-Open No. 4-180612) has a resolution as described above. Although it is improved, since only half of the diffracted light is used, the contrast of the projected image is lowered. The decrease in contrast becomes more remarkable as the opening ratio of the mask pattern increases. This is because as the aperture ratio increases, the ratio of direct light to diffracted light increases.

FIGS. 7A and 7B show the relationship between the opening ratio a of the mask pattern and the contrast of the projected image. Figure 7
The mask pattern of (a) is a simple line / spacing pattern, and the line width of the opening is shown as a when normalized by the repeating cycle. The contrast values shown in FIG. 7B are for the case where only the lowest-order diffracted light (first-order diffracted light) is incident on the pupil, and the second-order and higher-order diffracted light is not incident on the pupil (during transfer of the fine mask pattern). Is the calculation result of.

Even when the line and the space are equal (a = 0.5), the contrast is reduced in the four-division illumination compared with the normal method, and the contrast is about 0.9. Four
In the split illumination method, the aperture ratio sharply decreases as the aperture ratio increases, and when the line-spacing ratio exceeds 1: 2 (a> 0.67), the contrast becomes 0.7 or less. On the other hand, in the conventional method, the contrast can be maintained at 0.8 or more until the line / spacing ratio becomes 1: 4 (a <0.8).

As is apparent from the above, the four-division illumination method has a problem that the contrast of the projected image is lowered in exchange for a high resolution, and the reduction of the contrast is particularly remarkable when the mask aperture ratio is large. I understand. In the LSI manufacturing process, the line-to-spacing ratio is often not 1: 1 and the fact that the contrast changes greatly with the change in the line-to-spacing ratio of the mask pattern is a serious problem in practical use.

In order to solve such a problem, a method of using a halftone type phase shift mask for the four-division illumination method has been proposed. The halftone type phase shift mask is a light-shielding pattern (light transmittance: 0
%) Is a kind of phase shift mask using a phase shifter pattern that has a constant light transmittance and inverts the phase of transmitted light.

By using this mask, it is possible to reduce the 0th order light (direct light) from the mask. The halftone type phase shift mask and the contrast of the projected image when the mask is used are shown in FIGS. Figure 8
The mask pattern of (a) is the same as the mask pattern shown in FIG. However, in the mask of FIG. 7A, the light transmittance of the light-shielding portion was 0%, but in the halftone phase shift mask of FIG. 8A, the transmittance of the exposure light was t.
It is a light-shielding pattern in which the phase of transmitted light is inverted by (%). As is clear from FIG. 8B, as the light transmittance is increased, the mask having a line-to-space ratio of 1: 1 (a = 0.
The contrast of 5) increases.

However, as the aperture ratio increases, the tendency for the contrast to sharply decrease does not change, and the contrast also decreases in the portion where the aperture ratio is small. The halftone phase shift mask has a line-to-space ratio of 1: 1 (a = 0.
Although the contrast of 5) is improved, there is a problem that it is not effective when the line / spacing ratio of the mask pattern changes. Such problems also exist in the same manner when a shifter light shielding type phase shift mask is used.

Therefore, it is an object of the present invention to provide a projection exposure method and apparatus capable of improving the resolution and the depth of focus and improving the contrast.

[0014]

The present invention is characterized in that an illumination light source is divided, and only a part of each divided light source is incident on the pupil.

[0015]

According to the present invention, the proportion of the 0th-order light incident on the pupil can be reduced, and the resolution can be improved even when the line width of the repetitive pattern is changed or when the mask pattern includes a diagonal component. And the depth of focus can be improved at the same time, and the contrast can be improved.

[0016]

Embodiments of the present invention will be described in detail with reference to FIG. The light emitted from the mercury lamp 1 as an exposure light source and converged by the elliptical mirror 2 is reflected by the mirror 3 and enters the fly's eye lens 4. Here, the exit surface of the fly-eye lens 4 is the pupil surface of the illumination optical system. An illumination system aperture stop 5 is provided on or near the pupil plane of the illumination optical system. The light passing through the illumination system aperture stop 5 is reflected by the mirror 6 and illuminates the reticle 8 by the condenser lens 7. The light diffracted by the pattern on the reticle 8 is projected onto the wafer 10 on the wafer stage by the projection lens 9.
Image on top. A projection system aperture stop 11 is provided on the pupil plane of the projection optical system in the projection lens 9.

The projection exposure apparatus shown in FIG. 1 (a) is basically the same as the optical system of the projection exposure apparatus used conventionally. The difference from the conventional apparatus is the shape of the illumination system aperture stop 5. In the conventional projection exposure apparatus, the illumination system aperture stop 5 has a circular shape, and the projection image on the pupil plane of the stop has a circular shape as shown in FIG. The radius σ with the radius of the pupil as a unit is the coherence degree of the projection exposure apparatus, and the coherence degree in a normal projection exposure apparatus is a value of about 0.5 to 0.6.

On the other hand, in the four-division illumination method, the illumination system aperture stop 5 is modified as shown in FIG. 6 (c) and the mask surface is illuminated by the divided light sources. Japanese Patent Application Laid-Open No. 4-18 has disclosed the principle of improving resolution and depth of focus by a method of dividing illumination light in a direction rotated by 45 degrees from a horizontal direction and a vertical direction of a fine pattern requiring resolution of a mask pattern.
It is described in detail in Japanese Patent Publication No. 0612. As described above, the problem with this four-division illumination method is that since only half of the 1st-order diffracted light is used, the proportion of direct light (0th-order light) increases and the contrast of the projected image decreases. is there. In order to improve the contrast, it is necessary to reduce the proportion of 0th-order light. In this invention, the illumination system is optimized for the purpose of reducing the proportion of 0th-order light.

The illumination system has a shape such that the pupil plane is illuminated as shown in FIG. 1 (b). FIG. 1B shows a pupil plane, and a circle having a radius of 1 shown by A represents the projection aperture stop 11, that is, the pupil. Only the light that enters the inside of this circle contributes to image formation. The four small circles B partially overlapping the pupil A represent the projected images of the divided light sources on the pupil plane.

The difference from the four-division illumination method shown in FIG. 6C is that a part of the illumination light is outside the pupil (outside the projection system aperture stop 11).
Is designed to illuminate. Of the equivalent light sources represented by the four small circles, the part that enters the pupil plane (the part where the pupil and the projected image overlap) is an important part. The radius of each of the four divided light sources is σ r, and the center of the light source is deviated from the center of the pupil by σ 1 both in the horizontal and vertical directions.

When the illumination system is improved in this way, only a part of the light (0th order light) which is not diffracted by the mask is incident on the pupil. On the other hand, one of the lights diffracted by the horizontal and vertical patterns on the mask surface is almost completely incident on the pupil until it crosses the pupil. In this method, the proportion of the 0th-order light is reduced with respect to the 1st-order light, as compared with the conventional split illumination method in which all the direct light (0th-order light) of the split illumination light source is incident on the pupil. Therefore, the contrast of the projected image is improved by using the present invention.

Next, the illumination light source of the present invention will be described in more detail. First, let us consider the proportion of each divided light source (equivalent light source) incident on the pupil plane.
Let k be the proportion of light that directly enters the pupil plane among the respective divided light sources. As shown in FIG. 2A, the radius σ r of the circle in which each of the divided light sources is projected on the pupil plane (the pupil radius is standardized to 1), and the area Sc (S of the secondary light source image on the pupil plane)
c = π · σ r ² ), where Sp is the area of the secondary light source image that is directly incident on the pupil plane, and k = Sp / Sc.

The radius σr in the pupil plane of each divided light source is smaller than the radius of the pupil (σr << 1),
When the pitch of the mask pattern is sufficiently small and only the direct light and the diffracted light of the lowest order (first order light) contribute to the image formation, the contrast of the projected image on the wafer according to the change of the line / spacing ratio a of the mask pattern The calculated value of is shown in FIG. When all the light is incident on the pupil plane (k = 1), FIG.
The contrast is the same as that of the four-division illumination, and the contrast is rapidly reduced in the region where the aperture ratio a is large.

On the other hand, when 1/2 of the light is incident on the pupil surface (k = 0.5), the conventional method (when a circular aperture stop is used)
Is equal to the contrast of. Even when the latest resist is used, the contrast of the projected image must be 0.6 or more in order to form a pattern on the resist having a film thickness of 1 μm. Furthermore, the contrast shown here is a contrast when no focus shift occurs. When defocus occurs, the contrast decreases. Therefore, the contrast on the focal plane is preferably 0.8 or more.

When the line-spacing ratio is 1: 2 to 2: 1, a contrast of 0.8 or more is obtained in the range of k <0.4 <k <0.8. The value of k is particularly preferably in the range 0.5 <k <0.6. At this time, the contrast of the projected image in the above range is 0.9 or more.

The center of the circle in which the light source is projected on the pupil plane is the edge of the pupil.
, The value of k is 0.5 to 0.45 (0 <σr <
0.5). Therefore, the projected image of the light source on the pupil plane
Set so that the center is slightly inside the edge of the pupil
Thus, almost ideal imaging characteristics can be obtained. Next, each
About the radius σr of the projected image on the pupil plane of the divided illumination source
Consider. If the radius σr is too large, only direct light
In addition, part of the diffracted light always goes out of the pupil. this
When the ratio of diffracted light decreases, the contrast is low.
Will be defeated. The radius σr is used as a parameter in Fig. 3.
, The spatial frequency ν of the transfer pattern and the projection onto the wafer
The relationship of image contrast is shown. One transfer pattern
The line and spacing pattern was 1. In the figure, νc is νc = NA / λ. However, NA is the numerical aperture of the projection lens, λ
Is the exposure wavelength. Cutoff frequency with increasing radius σr
Increase, but the contrast in the low frequency region gradually increases
descend. Low spatial frequency when radius σ r is smaller than 0.3
There is no decrease in contrast, but the radius σr is 0.4.
If it is too large, the contrast at low spatial frequencies drops sharply
To do. Therefore, the radius σr must be 0.35 or less
Is desirable. Therefore, each divided equivalent light source
Area that illuminates the pupil plane (area that directly enters the pupil and outside the pupil
The sum of the illuminated areas is approximately 1/8 of the total area of the pupil plane (≈0.
35 ²) The following is a high contrast.
It is necessary for

Further, in order to obtain a sufficient amount of light on the wafer, the radius σr needs to have a large value to some extent. It is desirable that the radius σ r is at least 0.1 or more in order to adjust each of the divided light sources to be equal to each other. Further, the center position of each divided illumination light source will be considered here. Figure 4
As shown in (a), the distance between the center of the divided illumination light source and the center of the pupil is σ2 (σ2 = σ1 · √2), and the radius σr of the divided illumination light source and the proportion k of the light directly incident on the pupil are The relationship is shown in FIG. As already shown, the radius σr is preferably in the range of 0.1 to 0.35, and the value of the pupil incidence ratio k is 0.4 to 0.8 (particularly preferably 0.5 to 0.5).
It is desirable to be in the range of 0.6). Under these conditions, the optimum value of the center σ2 of the equivalent light source is the portion surrounded by the diagonal lines in FIG. 4 (b).

FIG. 5 shows the image forming characteristics of the normal illumination method (σ = 0.5) and the four-division illumination method (σ1 = 0.5, σr = 0.25) and the illumination method of the present invention (σ1 = 0). .7, σ r = 0.
25) is shown for comparison. The pattern width shown in the upper part of FIG.
5 nm) Transfer pattern size when a stepper is used.

When the resolution limit is set to 0.7 for contrast, 0.38 μm for the normal method and 0.25 μ for the four-division illumination method.
m, in the present invention, it is 0.24 μm, which shows that the resolution is improved. Furthermore, it can be seen that the contrast in the low frequency region is improved by the illumination method of the present invention, while the contrast in the low frequency region is only about 0.9 in the four-division illumination method. In addition, this method significantly improves the depth of focus for patterns near the resolution limit. The depth of focus is determined by the wavefront aberration associated with the path difference between the direct light (0th order light) and the diffracted light. In this method, direct light always passes through the edge of the projection lens. Further, the first-order light of the diffracted light from the reticle pattern near the resolution limit passes through the end of the lens opposite to the direct light. For this reason, the aberrations of the direct light and the first-order diffracted light approach 0 near the resolution limit, and a large depth of focus can be obtained.

In order to actually confirm the effectiveness of the illumination method of the present invention, the aperture stop of the i-line stepper with a numerical aperture of 0.5 is set to 4 mm.
We performed a transfer experiment on a wafer by changing to a light shield plate with two circular openings. The aperture stop used is σ1 = 0.7, σr
= 0.25. In a test pattern transfer experiment on a substrate in which 1 μm thick photoresist was coated on bare silicon, a 0.3 μm line / spacing pattern was 1.2 μm.
It could be formed with a focal depth of m.

This stepper has a normal circular aperture stop (σ =
0.6), the resolution limit is 0.35 μm
(At this time, the depth of focus is 0.6 μm), and the illumination method of the present invention can realize high resolution and improvement in depth of focus. Further, the wiring pattern of the dynamic RAM of the rule of 0.3 μm could be formed on the substrate having the step. The method of the present invention is particularly effective for manufacturing a semiconductor device such as a memory device or a solid-state image pickup device having a large number of repetitive periodic patterns.

In the above-described embodiment, each of the divided illumination lights illuminates the pupil plane in a circular shape. However, the shape of the light source is not limited to this example, and may be any shape as long as the shapes of all the divided light sources are equal to each other and the positional relationship with the pupil plane is symmetric to each other. . In the present invention, the area of each of the divided light sources that illuminates the pupil plane is ⅛ or less of the entire pupil plane, so that the shape of each light source does not significantly contribute to the imaging characteristics.

Further, in the above embodiments, the optimization of the divided illumination is realized by changing the shape of the illumination system aperture stop at the exit of the fly's eye lens. However, the effect of the present invention can be obtained by adopting an illumination method equivalent to this in the pupil plane of the projection system. That is, the equivalent light source shape on the pupil plane may be four light sources rotated by 45 degrees with respect to the mask, and a part of each equivalent light source may go out of the pupil. For example, the diffraction grating pattern is arranged at a position conjugate with the reticle surface of the illumination optical system, and only the first-order component of the diffracted light generated by this is selected by the aperture stop placed on the pupil plane of the illumination system to select the reticle surface. It is also possible to adopt a configuration of an illumination system for illuminating. Alternatively, in the above-mentioned embodiment, the light from one illumination lamp is divided by using the aperture stop, but the same effect can be obtained by disposing four independent illumination light sources. It is also possible to configure four secondary light sources by using an optical fiber from one light source. Furthermore, in a projection exposure apparatus using a laser light source (eg, krypton fluoride or an excimer laser of argon fluoride), a laser beam as illumination light is time-integrated on the pupil plane to illuminate as shown in FIG. 1 (b). It can also be realized by scanning as described above.

[0034]

As described above, according to the present invention, it is possible to improve the resolution and the depth of focus for various patterns while maintaining the high contrast of the projected image.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a projection exposure apparatus that is an embodiment of the present invention.

FIG. 2 is a diagram showing image forming characteristics of a projection exposure apparatus that is an embodiment of the present invention.

FIG. 3 is a diagram showing imaging characteristics of a projection exposure apparatus that is an embodiment of the present invention.

FIG. 4 is a diagram showing imaging characteristics of a projection exposure apparatus that is an embodiment of the present invention.

FIG. 5 is a diagram showing image forming characteristics of a projection exposure apparatus according to an embodiment of the present invention in comparison with a conventional exposure apparatus.

FIG. 6 is a diagram showing a conventional equivalent light source (effective light source on a pupil plane).

FIG. 7 is a diagram showing a problem of the conventional split illumination method.

FIG. 8 is a diagram showing a problem when a conventional split illumination method and a halftone type phase shift mask are combined.

[Explanation of symbols]

 1 Lamp 2 Elliptical Mirror 3 Mirror 4 Fly's Eye Lens 5 Illumination System Aperture Stop 6 Mirror 7 Condenser Lens 8 Reticle 9 Projection Lens 10 Wafer 11 Projection System Aperture Stop A Pupil B Projected Image of Light Source

Claims (5)

[Claims] 1. A projection exposure method for projecting a mask pattern onto a substrate, wherein a secondary light source image formed on a pupil plane is divided into four in a diagonal direction of the mask, and a part of each of the divided illumination light. A projection exposure method characterized in that only the light enters the pupil. 2. The projection exposure method according to claim 1, wherein a ratio k of light that directly enters the pupil in each of the divided secondary light source images is 0.4 <k <0.8. 3. The projection exposure method according to claim 1, wherein the area of illumination of the pupil plane by each of the divided secondary light source images is ⅛ or less of the total area of the pupil. 4. A method for manufacturing a semiconductor device, which comprises forming a circuit pattern by the projection exposure method according to claim 1. 5. A projection exposure apparatus for projecting a mask pattern onto a substrate, wherein a secondary light source image formed on a pupil plane is divided into four in a diagonal direction of the mask, and a part of each of the divided illumination light. A projection exposure apparatus, which is equipped with an illumination system in which only the light enters the pupil.