JPH0529190A - Projection exposing method for fine pattern - Google Patents

Abstract

PURPOSE:To impart resolution equivalent to that of a phase shift masking method for a linear pattern to an imaging system via a projection lens without using a phase shift film for a mask pattern by inclining an optical axis of an irradiating light to irradiate a mask, and setting the value of the inclination corresponding to various basic amounts of the lens. CONSTITUTION:When a light I having a spatially coherent wave number K0 irradiates at an oblique angle alpha to an optical axis Z for a normal mask M, i.e., in a direction directed from a center of a masklike (M) optical axis Z toward an edge of an aperture A, a wave I1 diffracted by the mask M and transmitted becomes a wave having an angle 2alpha' to an incident light and hence becomes a wave having a wave number K1. Thus, an imaging can be performed with maximum resolution of a projection lens. Further, resolution equivalent to that of a phase shift masking method can be performed for an arbitrary pattern. Accordingly, integration and reliability can be improved for the formation of a fine pattern for an LSI, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fine pattern forming method for forming a fine pattern such as an LSI on a substrate using a projection lens, a so-called projection exposure method.

[0002]

2. Description of the Related Art Conventionally, a projection exposure apparatus for forming a fine pattern such as an LSI is required to have high resolution. Therefore, the projection lens of the recent projection exposure apparatus has a resolution close to the theoretical limit determined by the wavelength of light.
Nevertheless, in order to cope with the recent miniaturization of LSI patterns, higher resolution is required. There is a phase shift mask method as a method for answering this. In this phase shift mask method, a phase shift film is added at intervals of each transmission pattern so that a phase difference of π appears in the transmitted light between adjacent patterns on the mask placed on the object plane of the projection lens. To do. The improvement of the resolution will be described with a simple example.

FIG. 17 is an explanatory diagram of the mask resolution limit by the phase shift mask method, and FIG. 18 is an explanatory diagram of the mask resolution limit by the normal irradiation method. Consider a mask in which many line figures having the same width are arranged in parallel at intervals equal to the line width. The mask M shown in FIG. 18 is a cross-sectional view as seen from a plane perpendicular to the line direction, and the repeating period length of the line figure of the mask M is d. In the phase shift mask MP shown in FIG. 17, the phase shift film S is provided at every interval of the transmissive portions so that the phase difference π is generated between the lights passing through the adjacent transmissive portions. That is, since the amplitude becomes negative at every interval of the pattern, the cycle becomes 2d and the DC component becomes zero. Therefore, the irradiation light I ₀ having the wave number k ₀ that is vertically incident on the phase shift mask MP is diffracted by the mask to generate a wave I ₁ having an inclination in the optical axis z. The wave number of this wave is k ₁ = k ₀ s, where α ′ is the diffraction angle.
Although in (α '), since the length of the basic repeating cycle of the pattern is 2d, k ₁ = 2π / 2d. The electric field amplitude u of the transmitted light has a component in the ± x direction and is represented by the following equation.

[0004]

[Equation 1]

However, x is a coordinate in the repeating direction of the pattern. It is considered that when the wave of Expression (1) goes to the projection lens, it passes around the aperture A (entrance pupil). That is, finer patterns than this are not resolved by the projection lens. The light intensity at the image plane is 2 which is the absolute value of the electric field amplitude.
Since it is proportional to the power of

| U | ² = (1/2) | u ₀ | ² (1 + cos2k ₁ x) (2)

Thus, the pattern having the basic period d and the wave number k ₁ is reproduced. On the other hand, when the normal mask M without the phase shift mask is vertically irradiated with the light I ₀ having the wave number k ₀ , FIG.
2k ₁ = 2k ₀ sin (α ′) ≈k ₀ si
A wave I ₁ ′ having a wave number of n (2α ′) is generated, but this wave does not reach the image plane because it is blocked by the aperture A. That is, the normal mask vertical irradiation does not resolve the pattern.
From the above, it can be seen that the phase shift mask method improves the resolution.

[0008]

As described above, the phase shift mask method is effective for adjacent linear patterns, but is not necessarily effective for adjacent patterns of different sizes or isolated patterns. In addition, the process of adding the phase shift film in the mask manufacturing causes a decrease in yield and a high mask manufacturing cost. The ring light source aperture method is another method for answering the high resolution required for the miniaturization. This method is disclosed in Japanese Patent Application No. 59-21 of the same applicant.
This is a method of using a ring aperture as a light source aperture as described in "Projection Exposure Apparatus" of No. 1269. In such a method, the grounds for its operation are experimental facts, and it is stated that in the ring light source aperture, the higher the resolution is, the more the outer light source is used. However, nothing is said about the sizing of the ring source aperture to obtain the highest resolution of the projection lens.

The present invention has been made in view of the above points, and an object thereof is a phase shift mask method for a linear pattern without using a phase shift film for a mask pattern having an arbitrary shape and arrangement. Another object of the present invention is to provide a projection exposure method which gives a resolution equivalent to that of (3) to an imaging system using a projection lens.

[0010]

In order to achieve the above object, the projection exposure method of the present invention provides an irradiation light for irradiating a mask with an inclination with respect to an optical axis, and the value of the inclination is used as a basic value of a projection lens. It is set according to various quantities.

[0011]

The operation of the present invention will be described first with the linear pattern (wave number 2
The description will be made for k ₁ ). Here, the mask M in FIG. 1 shows a cross section viewed from a plane perpendicular to the line direction, and the repeating period length of the line figure is d. As shown in FIG. 1, the wave number k ₀ is spatially coherent with respect to the ordinary mask M at an inclination angle α ′, that is, in the direction from the center of the optical axis z on the mask M to the helicopter of the aperture A. Of the light I, the wave I diffracted by the mask M and transmitted therethrough
_{Since 1} becomes a wave having an angle 2α 'with respect to the incident light, it becomes a wave having a wave number k ₁ . A wave that goes straight in the incident direction and passes through the helicopter of aperture A is considered to be a wave having a wave number k ₁ . Alternatively, assuming that the wave traveling straight in the incident direction is ₁ , the wave number of the diffracted wave is 2k ₁ . Therefore, the electric field amplitude u of the transmitted light is

[0012]

[Equation 2]

[0013] The light intensity at the image plane is

[0014] ^{| u | 2 = (| u} 0 | 2/2) (1 + cos2k 1 x) ...... (4)

And is in agreement with the equation (2). That is,
It is understood that when the normal mask M is irradiated from the oblique direction, the same action as that of the phase shift mask occurs. In addition, this is 0, π, 0,
It can be understood from the fact that a phase difference of π, ... Can be given. As described above, the fact that the phase shift method and the oblique irradiation method are equivalent in the case of the highest resolution was first discovered by the present inventors, and the present invention is based on this.

Next, the operation of the present invention when the periodicity of the mask pattern is bidirectional will be described. Most of the integrated circuit patterns are composed of vertical and horizontal patterns, and in this case, if the oblique irradiation is performed from the x direction and the y direction, the same effect as the linear pattern is expected. The effect of the irradiation in the two directions or the irradiation in the four directions with improved symmetry will be described with reference to FIG.

When incoherent irradiation is used from a plurality of directions, it is the same as when irradiation is performed independently of each other.
No extra interference occurs. In FIG. 2 (a), the horizontal axis represents the inclination amount βx in the x direction (wavenumber βx), and the vertical axis represents the inclination amount βy in the y direction.
(Wave number βy) is taken, and points AP1 to AP4 indicating the amount of inclination of the light source are placed at four symmetrical points on the circle A1 centered on the origin. The radius R of A1 is equal to the numerical aperture NA1 when the exit pupil of the projection lens is viewed from the center of the optical axis of the image plane. Here, the irradiation angle corresponding to the numerical aperture of the projection (optical) lens indicates α'in the following equation with respect to the reduction ratio 1 / m.

[0018]     sin α '= (1 / m) sin α ...... (5)

Where sin α is the numerical aperture of the projection lens. For example, when the reduction ratio is 1/5 and the numerical aperture of the projection lens is 0.5, sin α ′ = (1/5) × 0.5,
α'≈5.7 °. Since it is the essence of the present invention that obliquely irradiates with an inclination equal to this value, there is a point at which irradiation light is shown on A1. Circles A1 'and A1 "indicate ranges in which high-order components corresponding to wave numbers k ₁ and 2k ₁ are taken into the projection lens when the mask pattern is decomposed into spatial harmonics. A1' is an optical axis It shows the position of the best resolution circle that would be obtained if the irradiation was performed in parallel. A1 ″ represents the range in which the resolved wave number component obtained by the present invention is taken in, and its center Px is of the parallel irradiation. It becomes a value.

The optical transfer function OTF of FIG. 2 (a) is shown in FIG. 2 (b).
As shown in ( ₁₎ , the maximum wave number in the x-axis and y-axis directions is twice the value of k ₁ in the case of parallel irradiation with the optical axis, that is, 2 k ₁ . In the intermediate direction between the x-direction and the y-direction, the route of parallel irradiation of the optical axis is doubled, which is suitable for the pattern to be resolved. Next, if the mask pattern is required to be symmetrical about the optical axis, the irradiation light will have a numerical aperture N with respect to the optical axis.
Irradiation light having an inclination corresponding to A1 may be irradiated isotropically (axisymmetrically). At that time, as shown in FIG. 3 (a), a fine annular shape is formed, and the optical transfer function OTF is

F (k) = (1 / π) cos ⁻¹ (k / 2k ₁ ) ... (6)

It becomes This is shown in FIG. 3 (b). Compared with a normal irradiation method that includes a component parallel to the optical axis to a certain inclination, the characteristics are slightly reduced at high frequencies, but the value is quite large up to the maximum frequency, which is superior to the normal irradiation method. If the numerical aperture of the irradiation light does not correspond exactly to the numerical aperture of the projection lens system, the use of an annular irradiation system does not make much difference from the case of normal partial coherent irradiation, which has been described so far. Understood from the calculation of.

Next, a method for further improving the image forming characteristics and improving the contrast will be described. In the OTF shown in FIG. 3 (b), the component at k = 0 is sharply large, which means that the direct current component of the spatial frequency component of the pattern is large, which lowers the contrast. To remove this DC component, it is sufficient to remove the DC component of the tilt irradiation when passing around the projection lens aperture. However, since the higher order component of the pattern also passes through this peripheral part, it is necessary to set the transmittance of the peripheral part in consideration of the resolution and the contrast in correspondence with the oblique irradiation. If such a projection lens aperture is used, further excellent imaging characteristics can be obtained. Here, the projection lens numerical aperture refers to a portion of an aperture stop attached at a position where an image of a light source is formed in the projection lens system. O when the amplitude transmittance is set to 50% only on the arc line around the projection lens aperture
The TF is shown in FIG.

In the case of the 4-point oblique irradiation method shown in FIG.
To remove the sharp DC component of the OTF in (b), a projection lens aperture as shown in FIG. 5 is used. Where AP
Reference numerals 1 ', AP2', AP3 'and AP4' are portions corresponding to oblique irradiation points, and the amplitude transmittance is set to "1/2". The amplitude transmittance in the square area indicated by AP5 is set to "1", and the four half-moon shaped areas indicated by AP6 are set so as not to transmit light, that is, the transmittance is set to "0". By using such a projection lens aperture, the effect of improving the contrast can be obtained.

Supplementing the OTF used in the above description, in the case of a partially coherent light source, the mutual transmission coefficient has to be used, but this is a function of two wavenumbers and depends on the type of pattern. To do. Therefore, the OTF is defined by a mutual transmission coefficient that is made into a variable by excluding the pattern dependence and used for the explanation. A list of the relationship between the arrangement of the light sources and the transmittance of the projection lens aperture is shown in FIG. Here, FIG. 6 (a) shows the case of one point oblique irradiation, and FIG. 6 (b) shows the case of 2 points.
FIG. 6 (c) shows the case of point oblique irradiation, FIG. 6 (c) shows the case of four-point oblique irradiation shown in FIG. 5, and FIG. 6 (d) shows the case of using an annular irradiation system. Further, FIGS. 6A1 to 6D1 respectively show the transmittances of the projection lens apertures corresponding to the arrangement of the respective light sources in FIGS. 6A to 6D. That is, FIG. 6 (a1) shows a substantially circular area AP7.
The projection lens aperture has a transmittance of 1 and a transmittance of the arcuate region AP8 corresponding to the oblique irradiation point is 1/2. Further, FIG. 6 (b1) shows that the transmittances of the portions AP1 'and AP2' corresponding to the oblique irradiation points are "1/2", the transmittance of the substantially rectangular area AP9 is "1", and three half-moon shaped areas. AP
6 shows the projection lens aperture with the transmittance of 6 being “0”.
(c1) corresponds to FIG. 5 above, and FIG. 6 (d1) shows a circular area AP1.
A projection lens aperture in which the transmittance of 0 is "1" and the transmittance of the annular region AP11 is "1/2" is shown.

Although the above description is for clarifying the basic principle, an outline of an optical system of an actual reduction projection exposure apparatus will be additionally described here. FIG. 7 shows a basic optical system for forming an image on the wafer surface 15 by irradiating the mask pattern (reticle) 11 with the illumination light from the point light source slightly converged by using a condenser lens (not shown in the drawing). The configuration is shown. The projection lens system is classified into a first lens system 12 and a second lens system 13 in terms of function. Although each lens is represented by one lens in FIG. 7, it is actually composed of a plurality of lenses for optimizing aberrations and the like, but in terms of function, each lens may be represented by one lens. An aperture stop 14 is provided between the first lens system 12 and the second lens system 13, and an image of a point light source for illumination is normally formed at the center thereof. The image (virtual image) of the aperture stop 14 viewed through the first lens system 12 is called an entrance pupil. Mask pattern 1
It can be seen that the light diffracted by 1 (from p, p ') can pass through the projection lens system within the range of the angle at which the entrance pupil 16 is seen, and can contribute to the image formation on the wafer surface 15. However, the exit pupil is upward ∞.

Aperture A of FIG. 1 for illustrating the basic principle
Is actually meant to be the entrance pupil described above. Further, since a telecentric optical system in which the principal ray passing through the center of the aperture stop 14 is perpendicular to the wafer surface 15 is usually used in the image forming portion, as shown in the figure for the focal length f ₂ of the second lens system 13. The aperture stop 14 and the wafer surface 15 are set to come to the position. When the reduction ratio is ⅕, the mask pattern (reticle) 11 is set at the position of the focal length f ₁ of the first lens system 12, and f ₁ / f ₂ ≈5.

FIG. 7 shows a point p on the optical axis of the mask pattern 11.
And the diffracted light from the point p ′ outside the optical axis is Q on the wafer surface 15,
The state of forming an image on Q'is shown. The respective light rays are parallel light rays between the first lens system 12 and the second lens system 13. Focusing on the aperture stop portion 14, the diffracted light having the largest diffraction angle passes around the aperture stop portion 14 without depending on the positions of p and p ′ on the mask pattern 11. That is, the aperture stop 14 corresponds to the Fourier transform surface of the mask pattern 11. Further, in order to adjust the transmittance of the projection lens aperture to improve the imaging characteristics, a filter is provided at the aperture stop 14 to adjust the light transmittance, and the entire surface of the mask pattern 11 is adjusted. It is understood that the imaging characteristics (contrast) are improved over.

[0029]

EXAMPLES Examples of the present invention are shown below by exemplifying specific numerical values. FIG. 8 shows an example of a mask pattern.
The rectangular portion shown in the pattern 3 to 3 is a light transmitting portion, and other areas do not transmit light. The whole area of 3 μm × 3 μm is x
It is assumed to be arranged infinitely in the direction and y direction. The width a of each pattern is 0.3 μm, and the distance b between patterns is 0.3 μm.

In FIG. 9, the range of tilt irradiation is NAs = 0.3.
7 to 0.40 and the numerical aperture NA1 of the projection lens is 0.4
It is a numerical example of the present invention when 0, the wavelength λ of light is 0.365 μm, and the reduction ratio is 1/1. FIG. 9A is a contour line of the light intensity distribution in the 3 μm × 3 μm region on the image plane. The maximum value max and the minimum value min are displayed, and these are max = 3.830900e-02 and min = 8.079000e, respectively.
-04. The contour line is drawn by a thick line when the 0-max value is divided into 10. Distance from the coordinate origin to the center of the display frame to the x-direction edge of the frame, alx and y-direction distance
aly is expressed in μm. 9 (b) is the same as that of FIG. 9 (a).
It is the figure which looked at the light intensity distribution along the marked line of the x direction, and patterns 1-3 correspond to the patterns shown by numerals 1-3 of FIG. 8, respectively. The numerical values shown as (xa, ya) and (xb, yb) represent the coordinate values at both ends of the line segment marked with an X in FIG. 9 (a) in μm unit, and (xa, ya) = -1.500
542,0.00674, (xb, yb) = 1.493813, 0.026966.

As can be seen from FIG. 9, the light intensity decreases in the order of pattern 3, pattern 2, and pattern 1. Although it depends on the resist system whether or not the pattern 1 is resolved after development when the resist photosensitive film coated on the substrate is exposed by this light intensity distribution, it is presumed that the situation is close to the limit. FIG. 10 shows the light intensity distribution of the image when the pattern 1 and the pattern 3 are left as they are and the pattern 2 is added with a phase shift film for shifting the phase by π in the mask of FIG.
The image forming conditions are the same as those in FIG. 9 except that the irradiation condition is NAs = 0.0 to 0.2. As can be seen from FIG. 10 (a), pattern 1 and pattern 3 are connected,
Not resolved. This is because pattern 1 and pattern 3 are in phase. The phase shift mask method is an example in which the 0-π phase arrangement is difficult depending on the pattern figure.

FIG. 11 shows the light intensity distribution when the mask of FIG. 8 without phase shift is imaged under the same optical conditions as in FIG. Patterns 1, 2 and 3 are not resolved.
FIG. 12 shows a light imaged by the oblique irradiation method of the present invention using a mask in which the patterns 1, 2, and 3 do not transmit light in the mask of FIG. The intensity distribution is shown. The imaging conditions are the same as in the case of FIG. FIG. 13 shows an image when the same mask as that in FIG. 12 is imaged under a normal condition, that is, the same condition as in FIG. As can be seen by comparing FIGS. 12 and 13, it is clear that the present invention is effective. Although the example of the phase shift mask is not shown for the mask in this case, it is quite difficult to design this phase shift arrangement.

FIG. 14 shows a light intensity distribution when an image is formed by the four-point oblique irradiation method of the present invention using the mask of FIG. Here, a square area is set in AP1 of FIG. 2 (a) (βx = 0.35 to 0.4, βy = 0.55 to 0.05),
Similar areas were set for AP2, AP3, and AP4. As is clear from FIG. 14, the present invention is more effective than the conventional method. FIG. 15 shows an image formed by the four-point oblique irradiation method of the present invention using an inversion mask so that the patterns 1, 2, and 3 do not transmit light in the mask of FIG. 2 shows the light intensity distribution of. As can be seen from FIG. 15, it is clear that the present invention is more effective than the conventional method.

Next, a numerical example in which the transmittance of the aperture of the projection lens is adjusted to improve the contrast will be described. FIG. 16 shows that the same reversal mask as in FIG. 12 is used and the transmittance of the portion around NA1 = 0.37 to 0.40 of the aperture of the projection lens is set to 50% under the same condition of the axisymmetric tilt irradiation. , NA1 = 0.0 to 0.37 is a light intensity distribution diagram when the transmittance is 100%. It can be seen that the contrast is improved about twice and the image forming property is significantly improved as compared with FIG.

Although the calculation results using the i-line of the Hg lamp as the light source have been concretely shown above, the same effect can be obtained when the g-line of the Hg lamp or various excimer lasers are used as the light source. Is clear. Further, since the present invention is a method in which the periphery of the projection lens is slightly emphasized, it is expected that the influence of the aberration of the lens or the influence on the depth of focus is more affected than the conventional method. Using an aperture stop that is slightly lower than the maximum numerical aperture of the lens,
If the present invention is applied with a slightly reduced numerical aperture, the projection exposure with a sufficiently improved frequency band, a sufficient depth of focus and a small aberration becomes possible.

[0036]

As described above, according to the present invention, the tilt range of the irradiation light used in the conventional apparatus is set according to the numerical aperture of the projection lens, so that the projection lens forms an image with the highest resolution. be able to. Moreover, there is an advantage that a resolution equivalent to that of the phase shift mask method by the conventional apparatus can be achieved for an arbitrary pattern without using the phase shift mask. As described above, the use of the present invention has an effect that the degree of integration and the reliability can be improved in forming a fine pattern such as an LSI.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of resolution of a 4-point or 2-point oblique irradiation method according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram of resolution of an axially symmetric oblique irradiation method according to another embodiment of the present invention.

FIG. 4 is a diagram showing an OTF when the amplitude transmittance on an arc line around the projection lens aperture according to the present invention is 50%.

FIG. 5 is a diagram showing a projection lens aperture for improving contrast in the 4-point oblique irradiation method according to the present invention.

FIG. 6 is a diagram showing the relationship between the arrangement of light sources and the transmittance of the projection lens aperture according to the present invention, wherein (a) to (d) are diagrams showing the arrangement of light sources, and (a1) to (d1). FIG. 4 is a diagram showing projection lens apertures corresponding to the respective light sources of (a) to (d).

FIG. 7 is a schematic view of an optical system for forming an image on a wafer surface by irradiating a mask pattern with light emitted from a point light source according to the present invention through a condenser lens.

FIG. 8 is a diagram showing an example of a mask pattern used in this embodiment.

FIG. 9 is a light intensity distribution diagram of an image by the tilt irradiation method of the present invention.

FIG. 10 is a light intensity distribution diagram of an image by a conventional phase shift mask.

FIG. 11 is a light intensity distribution chart of an image obtained by a normal irradiation method.

FIG. 12 is a light intensity distribution diagram of an image according to the invention of a reversal mask.

FIG. 13 is a light intensity distribution diagram of an image of a reversal mask by a normal method.

FIG. 14 is a light intensity distribution diagram of an image by irradiation light by the 4-point oblique irradiation method of the present invention.

FIG. 15 is a light intensity distribution chart of an image of the inversion mask by the four-point oblique irradiation light of the present invention.

FIG. 16 is a light intensity distribution chart of an image when the transmittance of the periphery of the projection lens of the inversion mask according to the present invention is 50%.

FIG. 17 is an explanatory diagram of a phase shift mask method according to a conventional example.

FIG. 18 is an explanatory diagram of a normal irradiation method.

[Explanation of symbols]

Inclination angle with respect to the optical axis-frequency M normal mask z optical axis α with wavenumber of 'the irradiated light I ₁ k of A aperture I wavenumber k _{0 0} sin .alpha

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Emi Tamekichi             1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation (72) Inventor Katsuyuki Harada             1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation

Claims (5)

[Claims] 1. A fine pattern projection exposure method, wherein an inclination of an angle corresponding to the numerical aperture of a projection lens is given to an optical axis with respect to a light beam irradiating an object surface mask on which a pattern is drawn. 2. A fine pattern projection exposure method according to claim 1, wherein a plurality of incoherent irradiation lights satisfying the above requirements are superimposed. 3. The fine pattern projection exposure method according to claim 2, wherein when viewed in a plane perpendicular to the optical axis, oblique irradiation is performed from four directions that are equiangular or two directions that are orthogonal to each other. . 4. The fine pattern projection exposure method according to claim 2, wherein tilt irradiation is performed at an angle that is axisymmetric with respect to the optical axis and that corresponds to the numerical aperture of the projection lens. 5. The projection lens aperture according to claim 1, wherein the projection lens aperture has an adjusted transmittance around the aperture when the range of inclined irradiation is made to correspond to the aperture. Fine pattern projection exposure method.