JPH06250378A - Exposure method and photomask used in the same - Google Patents

Abstract

PURPOSE:To expose a bright isolated pattern such as a contact hole with sufficient depth of focus. CONSTITUTION:The surface 21 of an opening in a photomask is made convex or concave within the height of the edge of the light shielding film 12 of the photomask and the photomask is illuminated with an illuminating optical system whose numerical aperture NA almost satisfies the relation of NA= W.n.(n-1)/(2¦R¦) [where (n) is the refractive index of the transparent substrate 11 of the photomask, W is the width of the opening on the substrate 11 and R is the radius of curvature of the convex or concave surface 21 of the opening].

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure method for printing a pattern of a photomask as an original plate on a photosensitive substrate when, for example, manufacturing a semiconductor device or a liquid crystal display device by using a photolithography technique, and an exposure method therefor. The present invention relates to a photomask used in an exposure method.

[0002]

2. Description of the Related Art For example, when a semiconductor device or a liquid crystal display device is manufactured by using a photolithography technique, a pattern of a photomask or reticle (hereinafter referred to as "photomask") is exposed through a projection optical system. 2. Description of the Related Art A projection exposure apparatus that exposes a material coated substrate (wafer, glass plate, or the like) is used. As a photomask for such a projection exposure apparatus, conventionally, a photomask in which a pattern is formed on a transparent substrate with a light shielding film has been generally used.

FIG. 5A shows a photomask made of a conventional light-shielding film. In FIG. 5A, a light-shielding film 12 such as a chromium film is formed on a transparent substrate 11 such as a glass substrate. The opening 12a formed in 12 corresponds to the bright pattern. Recently, as the pattern of semiconductor elements and the like becomes finer, the projection exposure apparatus is required to have improved resolution. The projection optical system, the illumination optical system, and the photomask have been improved in order to enhance the resolution, but a so-called phase shift mask has been attracting attention as a photomask for enhancing the resolution.
Various variations have been proposed for the phase shift mask. Now, assuming that the pattern to be transferred on the photosensitive substrate is an isolated bright pattern, an example of a phase shift mask for such an isolated pattern is shown in FIGS. 5B and 5C.
Shown in.

FIG. 5B shows a phase shift mask called “auxiliary pattern type”. In FIG. 5B, an opening 12b is formed around the opening 12a of the light shielding film 12 on the transparent substrate 11. The phase member 13 is formed on the opening 12b. On the other hand, FIG. 5C shows a photomask called “edge-enhanced type” or the like. In FIG. 5C, the edge portion of the opening 12 a of the light shielding film 12 on the transparent substrate 11 is covered. The phase member 14 having an opening is formed in the.

In both of the phase shift masks of FIGS. 5B and 5C, the phase member 13 or 14 covers a part of the transmissive portion formed of the opening 12b or 12a, and the phase member 13 is formed.
Alternatively, the phase of the light passing through 14 is changed by π as compared with the light passing through the opening without passing through the phase member. That is, when the refractive index of the phase members 13 and 14 is n, the thickness is d, the wavelength of the exposure light is λ, and k is an integer, the thickness d is set to satisfy the following equation. There is. (N-1) d = (k + 1/2)? It has been confirmed that the resolution of the pattern is improved by using such a conventional phase shift mask.

[0006]

Generally, when the pattern to be transferred is miniaturized, the numerical aperture of the projection optical system increases accordingly, and as the numerical aperture increases,
The depth of focus, which is the range in which good image formation is performed, becomes shallow.
This is the same even when the phase shift mask is used, and there is a disadvantage that the depth of focus is not always sufficient even if the resolution is increased by using the phase shift mask.
Further, as the exposure apparatus, a contact type or proximity type exposure apparatus that does not use a projection optical system is also used. However, in such an exposure apparatus, especially when a pattern to be transferred is miniaturized, the transfer of the pattern may occur. It will be difficult.

However, it has been found that for a periodic pattern such as a line-and-space pattern having a predetermined pitch, the so-called modified light source method can improve the resolution and secure the depth of focus. However, there is currently no effective method for ensuring the depth of focus for isolated patterns such as contact holes. In view of the above point, the present invention has an object to provide an exposure method capable of exposing a bright isolated pattern such as a contact hole with a sufficient depth of focus. Furthermore, the present invention aims to provide a photomask used in such an exposure method.

[0008]

In the exposure method according to the present invention, as shown in FIG. 3, for example, a photomask having a light shielding member (12) arranged on a transparent substrate (11) so as to surround a predetermined transparent portion. In the method of illuminating the substrate with the illumination light from the illumination optical system to expose the image of the pattern of the predetermined transmission portion on the photomask on the substrate, the surface (21) of the transmission portion on the photomask is shielded It is formed into a convex or concave curved surface within a range not exceeding the edge portion of the member (12), the refractive index of the transparent substrate (11) is n, the width of its transparent portion on the transparent substrate (11) is W, When the radius of curvature of the curved surface (21) of the portion is R, the numerical aperture NA of the illumination optical system is
The photomask is illuminated while the following relations are almost satisfied.

NA = Wn (n-1) / (2│R│) Further, the first photomask according to the present invention is shown in FIG.
As shown in (A), a light shielding member (12) is arranged on a transparent substrate (11) so as to surround a predetermined transparent portion, and the surface (21) of the transparent portion is provided with an edge portion of the light shielding member (12). It is formed in a convex or concave curved surface shape within a range not exceeding.

In the second photomask of the present invention, a light blocking member (12) is arranged on a transparent substrate (11) so as to surround a predetermined transparent portion, and the light blocking member (1) is provided on the surface of the transparent portion.
The phase member having a convex curved surface is formed within a range not exceeding the edge portion of 2).

[0011]

The structure of the photomask used in the exposure method of the present invention is as shown in FIG. Figure 1 (A)
Then, the light shielding film 12 of chromium or the like is arranged on the transparent substrate 11 (lower side in FIG. 1A). Shading film 12 in the center
There is no light and it is a light transmission part. The surface 21 of the transparent substrate 11 in the light transmitting portion is a convex curved surface. Further, in FIG. 1B, the surface 22 of the transparent substrate 11 in the light transmitting portion has a concave curved surface in the same configuration.

Now, the principle of exposure when the photomask having such a structure is used will be described with reference to FIG. Figure 2
When the photomask is illuminated with illumination light having a predetermined numerical aperture, it is a geometrical optical representation of how the light travels. For simplicity, the transparent substrate 11 is omitted in the drawing. FIG. 2A shows the case of a conventional normal photomask, and in FIG. 2A, the light shielding film 12 is shown.
The light passing through the opening 12a of the light travels straight. Therefore, FIG.
Considering the envelope of all the light fluxes shown in (A), the light flux is the thinnest light flux at the opening 12a of the light-shielding film 12 and becomes thicker as it goes away from it. When this is imaged on the photosensitive substrate side via the projection optical system, the light-shielding film 12
A sharp image is formed when the photosensitive substrate is present at the conjugate position of the opening 12a of the optical system through the projection optical system, but when the photosensitive substrate is displaced from the conjugate position in the optical axis direction, a blurred image is formed. .

Next, the photomask of FIG. 2B corresponding to FIG. 1A is equivalent to having a convex lens 23 in the opening 12a of the light shielding film 12. In this case, of the light that illuminates the photomask from above, the mutually parallel light components are focused on the position separated by the focal length F of the convex lens 23,
The entire luminous flux behaves as shown in FIG. Here, considering the envelope of the total light flux, it can be seen that the thickness of the light flux is kept substantially constant from the position of the light shielding film 12 to the position of the focal length F downward. Assuming that the magnification of the projection optical system is M, the portion where the thickness of this light flux is constant is in the range of F · M ² on the photosensitive substrate side, and it can be expected that the depth of focus increases by this amount.

Next, the photomask of FIG. 2C corresponding to FIG. 1B is equivalent to having a concave lens 24 in the opening 12a of the light shielding film 12. In the case of FIG. 2C, as compared with the case of FIG. 2B, a portion where the thickness of the light flux is constant can be formed as a virtual image on the upper side of the photomask. A portion where the thickness of the light flux as the virtual image is constant is shown by a broken line in FIG. Further, for simplicity, a part of the incident light beam is omitted in FIG. When this state is viewed from the projection optical side, it is equivalent to that the thickness of the light beam is constant within the range of the focal length F in FIG. 2C, and therefore, the same as in the case of FIG. 2B. The depth of focus increases for the reason.

The photomask according to the present invention can be optimized with respect to the illumination system. Consider this with reference to FIG. Assuming that the surface 21 of the opening of the photomask is a convex curved surface, the conditions for forming a constant light beam on the lower side of the photomask are the focus position P formed by the light ray component incident perpendicularly to the photomask, The distance h between the focus position Q formed by the light ray component incident on the photomask at the largest incident angle
Is half the width W of the opening. That is, the following equation is established.

H = W / 2 (1) Further, when the numerical aperture of the illumination optical system is NA, the following equation is established, where θ is the maximum incident angle of the illumination light. NA = sin θ (2) Then, assuming that the refractive index of the transparent substrate 11 is n and the incident angle of the maximum incident angle of the illumination light to the light shielding film 12 is θ ′, the following formula is established from Snell's law.

Sin θ = n · sin θ ′ (3) Further, when the radius of curvature of the surface 21 which is a convex curved surface is R and the focal length due to the convex curved surface is f, the following equation holds from the geometrical optics relational expression. 1 / f = (n−1) / R (4) h = f · tan θ ′ (5) In the expressions (1) to (5), the angle θ and the angle θ
′ Is sufficiently small, that is, θ≈sin θ≈tan θ,
The NA of the illumination optical system is calculated as follows.

NA≈W · n · (n−1) / (2R) (6) Since the approximation process is further performed using geometrical optics, the maximum value of NA is about ± 20%. You should think about it. Further, the expression (6) can be expressed as follows. R≈W · n · (n−1) / (2 · NA) (7) This formula (7) is a formula that gives the radius of curvature of the opening of the photomask when the NA of the illumination optical system is given. Has become. Further, so far, FIG. 3 assumes a convex curved surface in the opening, but the same applies if a concave curved surface is assumed. Therefore, (6)
The radius of curvature R in equation (7) must be interpreted as an absolute value.

The above description has been made on the basis of geometrical optics, but in order to carry out a more detailed analysis, it is necessary to consider the diffraction of light at the opening of the photomask by means of wave optics. However, since the phenomenon of diffraction spreads with a predetermined angular distribution around the traveling direction of light predicted by geometrical optics, the effects described above apply qualitatively as they are even when considered by wave optics. .

In the photomask of FIG. 1A, for example, the surface 21 has a convex curved surface, but the transparent substrate 1
It is clear that even if the surface member of No. 1 is coated with the convex phase member having a curved surface, the same operational effect can be obtained.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1A and 1B show a part of the photomask of this embodiment. In the photomask of FIG. 1A, a light shielding film 12 such as a chromium film is provided on a transparent substrate 11 made of a glass substrate or the like. Is provided, and a light pattern including the opening 12 a is formed in a part of the light shielding film 12. The shape of the surface 21 of the transparent substrate 11 in the opening 12a is a convex curved surface. On the other hand, in the photomask of FIG. 1B, the surface 22 of the transparent substrate 11 in the opening 12a is formed.
Has a concave curved surface.

The operation of this embodiment will be described with reference to FIGS. 2 (B) and 2 (C). 2B and 2C show a convex curved surface and a concave curved surface as a convex lens 23 and a concave lens 24, respectively. In this case, due to the lens effect of the opening, a region having a substantially constant luminous flux width can be formed in the front (FIG. 2C) or the rear (FIG. 2B) direction of the opening. The effect is to increase the depth of focus through.

Although FIG. 1 and FIG. 2 show a one-dimensional cross-sectional shape, in reality, a two-dimensional pattern must be considered. First, in the case of a one-dimensional isolated line pattern, FIG.
In the state of (B), translational symmetry may be given in the direction perpendicular to the plane of FIG. Therefore, FIG. 1 (A) or (B)
The surface 21 or 22 has a cylindrical surface shape.

Next, as shown in FIG.
In (A) and (B), in the case of an isolated pattern that can be regarded as being isolated in both the left-right direction within the plane of FIG. 1 and the direction perpendicular to the plane of FIG.
The surface shape of No. 2 is a toric surface shape having a radius of curvature in both directions. Next, the shape of the curved surface will be described with reference to FIG. As shown in FIG. 3, the in-plane radius of curvature of the surface 21 of the transparent substrate 11 in the opening 12a in the direction parallel to the paper surface of FIG. 3 is R, and the width of the opening 12a in the direction parallel to the paper surface of FIG. Is W, the relationship between the opening width W and the radius of curvature R is as shown in equation (6).
And are directly proportional. Therefore, it can be said that a smaller radius of curvature R is required when the width W becomes smaller. This relationship is shown in Figure 3.
Also holds for the opening width W ′ and the radius of curvature R ′ in the direction perpendicular to the plane of FIG. In particular, when the opening widths in both directions are equal to each other, the shapes of the surfaces 21 and 22 in FIGS. 1A and 1B are preferably spherical surfaces.

Next, the optimum radius of curvature R is obtained using the above equation (7). The numerical aperture NA ₀ of the projection optical system of the reduction projection exposure apparatus using the photomask of FIG. 3 is set to 0.5.
0, coherence factor σ of 0.5, magnification M
Is 1/5, the numerical aperture NA of the illumination optical system (same as NA in the expression (7)] is as follows: NA = NA ₀ × M × σ = 0.50 × (1/5 ) × 0.5 (8) If the pattern width is 0.3 μm on the photosensitive substrate side,
The width W on the photomask is as follows.

W = 0.3 × (1 / M) = 0.3 × 5 = 1.5 [μm] (9) Further, the refractive index n of the transparent substrate 11 is set as follows. n = 1.5 (10) Then, by substituting the equations (8), (9), and (10) into the equation (7), the optimum radius of curvature R is as follows.

R = 11.25 [μm] (11) Further, the distance at which the light flux width is constant is (1 / f) = (1.5-1) / 11. 25 Therefore, the focal length f is 22.5 μm. When converted into the amount on the photosensitive substrate side, it becomes 22.5 × (1/5) ² = 0.9, and the depth of focus increases by about 0.9 μm.

In this embodiment, the surface of the transparent substrate 11 in FIGS. 1A and 1B has a curved shape, but the surface of the transparent substrate 11 is actually etched. The desired shape may be obtained, or a transparent material having a desired shape may be applied to the opening to achieve the same effect. However, what is important here is that the opening 12a, which is a light transmitting portion, should not be completely covered and the edge portion of the light shielding film 12 should not be covered with a transparent material having a curvature.

FIG. 4 shows such a case. In FIG. 4, the opening 12a of the light shielding film 12 is covered with the transparent substance 15 having a convex radius of curvature including the edge portion of the light shielding film 12. . In this case, there arises a problem that the original pattern width W and the pattern width W ′ observed from the projection optical system side are different. This means that when viewed from the projection optical system side, the pattern width itself is viewed through the transparent substance 15 having a lens effect, so that a virtual image of the pattern can be seen and the pattern width W apparently changes. This is because it will end up. Therefore, in applying the present invention, it is important to form a curved surface inside the opening 12a which is the light transmitting portion of the photomask.

However, as shown in FIG. 1B, the opening 1
If the surface 22 of 2a is concave, the surface 22A
As shown by, it may be widened beyond the edge of the light shielding film 12. In the case of a concave surface, the opening 12a seen from the projection optical system
This is because the width of does not change. Also, in the above embodiment,
Although it has been described on the assumption that projection exposure is performed, in particular, when the curved surface of the opening is convex, a portion with a constant light flux width is formed below the photomask. The present invention is directly applied to the exposure method using the photomask.

The present invention is not limited to the above-described embodiments, and it goes without saying that various configurations can be adopted without departing from the gist of the present invention.

[0032]

According to the exposure method of the present invention, of the patterns on the photomask, for a bright pattern isolated on a dark background, the surface of the transparent substrate of the bright pattern (transmission part) is a convex surface or Since the surface is concave, a region having a substantially constant luminous flux width is formed below or above the photomask. Therefore, if this pattern is imaged on the substrate side via the projection optical system, there is an advantage that the depth of focus increases. Further, according to the present invention, the numerical aperture NA of the illumination optical system, the pattern width W, the radius of curvature R of the pattern opening, the refractive index n of the transparent substrate, etc. are optimized.

Further, as a side effect of the present invention, FIG.
As seen in (B) and (C), depending on whether the curved surface of the pattern opening is convex or concave, the region where the light flux width is constant changes to the lower or upper part of the photomask. This means that if there is a step structure on the substrate due to the process, the depth of focus range can be changed for each pattern on the photomask side in accordance with the step structure. As described above, according to the present invention, not only the depth of focus for one isolated pattern can be increased, but also the total depth of focus can be increased in accordance with the step structure on the substrate.

Further, the above-mentioned exposure method can be carried out by using the photomask of the present invention.

[Brief description of drawings]

FIG. 1A is an end view along a cross section showing a photomask having a convex opening surface according to an embodiment of the present invention, and FIG. 1B is a photomask having a concave opening surface according to the embodiment. It is an end view which follows the cross section shown.

FIG. 2 is an explanatory diagram for explaining an effect of the present invention in comparison with a conventional technique.

FIG. 3 is a diagram for explaining optimization of a photomask and an illumination optical system according to the present invention.

FIG. 4 is an end view along a cross section showing a photomask having a transparent material deposited to an inappropriate extent.

5A is an end view along a cross section showing a conventional ordinary photomask, and FIGS. 5B and 5C are end views along a cross section showing a conventional phase shift mask, respectively.

[Explanation of symbols]

 11 transparent substrate 12 light shielding film 12a opening

Claims (3)

[Claims] 1. A pattern of the predetermined transmission part on the photomask, which is obtained by illuminating a photomask having a light-shielding member arranged on a transparent substrate so as to surround the predetermined transmission part with illumination light from an illumination optical system. In the method of exposing the image on a substrate, the surface of the transparent portion on the photomask is formed into a convex or concave curved surface within a range not exceeding the edge portion of the light shielding member, and the refractive index of the transparent substrate is n, the width of the transparent portion on the transparent substrate is W, and the radius of curvature of the curved surface of the transparent portion is R.
Then, the photomask is illuminated in a state where the numerical aperture NA of the illumination optical system substantially satisfies the relationship of NA = Wn (n-1) / (2 | R |). Exposure method. 2. A light shielding member is arranged on a transparent substrate so as to surround a predetermined transparent portion, and the surface of the transparent portion is formed into a convex or concave curved surface within a range not exceeding an edge portion of the light shielding member. A photomask characterized by. 3. A light shielding member is arranged on a transparent substrate so as to surround a predetermined transparent portion, and a convex curved surface phase member is formed on a surface of the transparent portion within a range not exceeding an edge portion of the light shielding member. A photo mask characterized by that.