JP2807936B2 - Fine pattern projection exposure equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a projection exposure apparatus for forming a fine pattern such as an LSI on a wafer (substrate) using a mask and a projection lens.

[0002]

2. Description of the Related Art Conventionally, a projection exposure apparatus for forming a fine pattern such as an LSI has been required to have a high resolution. Therefore, a projection lens of a recent projection exposure apparatus has a resolution close to a theoretical limit determined by the wavelength of light.
Nevertheless, in order to respond to recent miniaturization of LSI patterns, higher resolution is required. In order to meet this demand, in recent years, one
By providing a phase difference close to 80 degrees, a phase shift method for reducing the light intensity at the light shielding portion to zero has been proposed, and it has been shown that the resolution is improved.

However, in the phase shift method, a pattern having a high degree of miniaturization can be obtained in a pattern such as an L & S pattern (line-and-space pattern) in which a phase difference of 180 degrees can be easily provided in the adjacent light transmitting portion. On the other hand, with a random pattern, it is difficult to satisfy this condition, and the effect is reduced. That is, the effect of improving the resolution differs depending on the type of the pattern. For this reason, there are disadvantages such as an effective shifter arrangement method for random patterns, technical difficulties such as shifter production, inspection, and correction, and a significant increase in reticle production costs.

On the other hand, the assignee of the present application has proposed a phase shift method by irradiating a light incident on a reticle in a fine pattern projection exposure apparatus at an angle corresponding to a numerical aperture of a projection optical system from an optical axis. A method for realizing equivalent resolution has been proposed (Japanese Patent Application No. 3-135317). Unlike the phase shift method, this method has a great advantage over the phase shift method because the effect of improving resolution does not depend on the type of pattern and the conventional mask can be used as it is. This method will be briefly described with reference to FIG.

FIG. 6A shows the above-mentioned oblique incidence projection method (Japanese Patent Application No. 3-135317) proposed by the same applicant.
FIG. 6B shows a conventional projection method. In the conventional projection method, as shown in FIG. 6B, the incident light I (wave number k ₀ )
Is perpendicularly incident on the surface of the reticle 31 and generates diffracted light (wave number k ₁ ) on both sides of the optical axis. The reticle 31 under the aperture stop (aperture) 32, a maximum wave number that can pass through the projection system is k _1, and the thereby diffracted by pattern of large optical i.e. shorter period than 2 [pi / k ₁ wave number is blocked I will. Here, if the diffraction angle is α,

K ₁ = k ₀ · sin α

Therefore, the minimum resolution dimension is 2π / (k ₀
(Sin α). On the other hand, in the oblique incidence projection method shown in FIG. 6A, when the 0th-order light obtained by the straight traveling of the illumination light I (wave number k ₀ ) has an inclination such that it passes through the outermost periphery of the projection system aperture stop 32, As shown in the figure, the diffracted light having the diffraction angle 2α and the zero-order light give the highest resolution. The wave number of this diffracted light is

K ₁ ′ = 2k ₀ · sin (2α) = 2k ₁

Therefore, the minimum resolution dimension is 2π / (2k
₀ · sin α), that is, half the size of the conventional exposure method can be resolved. Where si
Since nα is the numerical aperture of the projection lens: NA, this oblique incidence illumination system raises the resolution by passing light with a larger diffraction angle by tilting the irradiation light with respect to the optical axis by an angle corresponding to NA. Can be done.

As a specific method of realizing the oblique incidence illumination, a prism, a grating, or the like is disclosed in "Mask illumination optical system and projection exposure apparatus and method using the same" in Japanese Patent Application No. 3-142772 filed by the same applicant. Is provided in a part of the illumination optical system. Before describing this method, the configuration of an optical system of a conventional typical fine pattern projection exposure apparatus, that is, a stepper will be described with reference to FIG.

In FIG. 7, a light source lamp 1 is placed at a first focal point of an elliptical reflecting mirror 2, and a light beam is once collected near a second focal point 3 of the elliptical reflecting mirror 2 via a cold mirror 12. The luminous flux is converted into a parallel luminous flux by the input lens 4 which shares the focal point substantially with the second focal point 3. The light beam that has passed through the filter 10 enters the optical integrator 6 where the light flux is collected at the center by the cone lens 5 to make the light intensity distribution uniform, and the irradiation intensity distribution on the reticle surface is made uniform. The emitted light uniformly illuminates the reticle 9 by the output lens 7, the cold mirror 13, and the condenser lens group 8. An aperture stop 11 is placed on the emission side of the optical integrator 6, and determines the emission size. Reticle 9
Is passed through the projection optical system 14, and the image of the fine pattern on the reticle 9 is projected and exposed on the resist on the wafer 15. A stop 1 for determining the numerical aperture is included in the projection optical system 14.
There are six.

In such a projection exposure apparatus, it is necessary to use the theory of partially coherent light in order to accurately handle the imaging characteristics of the projection optical system. The amplitude transmittance of the mask is A
(X) (one-dimensional representation for simplicity, but in reality x, y
), And its Fourier transform is A (k) (however, in the text, the symbol 表 わ す representing the Fourier transform is omitted, and simply A (k), and the following J (k; k ′), J
_The same applies to ₀ (k _s ), K (k) and the like. ), Image plane intensity I
(X) is given by the following equation.

[0013]

(Equation 1)

Where A ^* (x) is the complex conjugate of A, J
(K; k ') is called a mutual transfer coefficient and is given by the following equation.

[0015]

(Equation 2)

Here, J ₀ (k _s ) is a light source expressed in a pupil space, and K (k) is a pupil function (Born, Wolff “Principles of Optics I, II, III” (translated by Kusagawa and Yokota)) See Tokai University Press). The optical transfer function (OTF) can be roughly evaluated by J (k; 0) using equation (2) (completely coincident at the limit of incoherent light).

In the case of coherent light, the amplitude depends on the principle of superposition, whereas in the case of incoherent light, the frequency dependence of each value can be clearly defined according to the principle of superposition. Then
As shown in the equations (1) and (2), it is represented by a function of two frequencies (k, k '), and represents a superposition of interference of waves of frequencies k, k'. As can be seen from equation (1), since the frequency component in the image plane intensity also depends on the frequency distribution A (k) of the mask pattern, it is difficult to objectively evaluate the performance of the illumination system and the projection optical system using this equation. is there.

Therefore, the frequency dependence of the contrast (M
Regarding the evaluation of TF), the mask pattern, that is, the type of the mask is A ₀ , A _{1 and} the pattern is limited to the L & S pattern represented by the equation (3), and A ₀ = 1/2, A ₁
= 1/4,

[0019]

(Equation 3)

This is convenient. At this time, the image plane intensity I
(X) is (the symbol 表 す representing Fourier series is omitted)

[0021]

(Equation 4)

## EQU1 ## Contrast MTF = (I _max −
Focusing on the basic period, I _min ) / (I _max + I _min )

[0023]

(Equation 5)

Is given by (5) The denominator of the equation gives the mean intensity _{(I max + I min) /} 2. By using the function representing the filter configuration as the pupil function K (k) in the equation (2), the calculation for optimizing the oblique incidence illumination system and the filter configuration can be easily performed. Fig. 8 (a)
Is an example to which the above-mentioned Japanese Patent Application No. 3-135317 is applied. As shown in FIG. When an annular light source is used and the radius of the pupil circle is 1, the width w of the annular light source is 0.1, 0.2,
The contrast of equation (5) is calculated for 0.3 and 0.4. In FIG. 8, the horizontal axis represents the spatial frequency normalized by the wave number of the illumination light, and the vertical axis represents the contrast: MTF.

FIG. 8 shows a conventional light source having a coherence factor σ = 0.5, that is, FIG.
As shown in (b), the case of the light source 20 having a size that is half the size of the entrance pupil circle 19 and whose center is the optical axis is also shown (indicated by a reference symbol B in the figure). As is clear from FIG. 8, it is understood that the use of the annular light source increases the contrast in the high wavenumber band and can greatly improve the resolution.

[0026]

On the other hand, whether or not a pattern having a certain wave number can be actually formed is determined by the resist material and the MTF threshold that can be determined by the resist process.
For example, if the MTF is 0.5 or more, there is a resist process that can form a pattern, while the MTF is 0.7
There is also a resist process in which a pattern cannot be formed without the above.

In general, the larger the MTF threshold, the more stable the resist process with a larger dimensional margin. From such a viewpoint, M by the annular light source in FIG.
Looking at the TF curve, as the wave number increases and the MTF gradually decreases, a resist process capable of forming a pattern with a small MTF is required to achieve high resolution, and a large MTF is required. When this method is applied to a resist process to be performed, a large improvement in resolution cannot be expected. As described above, the conventional annular light source has a disadvantage that the process margin of pattern formation is low because the MTF gradually changes depending on the wave number.

The present invention has been made in view of the above points, and has been made in order to solve the above-described problems. An object of the present invention is to provide an optical system which does not reduce MTF in a high wavenumber region and provides a sharp MTF curve. It is an object of the present invention to provide a projection exposure apparatus capable of increasing a process margin of pattern formation by configuring a system.

[0029]

In order to achieve the above-mentioned object, a projection exposure apparatus according to the present invention comprises means for giving a light beam irradiating a mask an inclination with respect to an optical axis at an angle corresponding to the numerical aperture of a projection lens. When viewed in a plane perpendicular to the optical axis, it is illuminated by four partial annular light sources from four directions symmetrical to the optical axis.
The shape of the partial toroidal light source is a reduction magnification m of the projection optical system,
The numerical aperture of the projection lens is projected onto a plane containing NA and the optical axis
The angle between the optical axis of the luminous flux center line and the optical axis is α ',
The divergence angle is β, and the normalized incident angle of the partial annular light source is R (= m
Sin α ′ / NA), and the width of the partial annular light source is set to 2σ _D (σ
_D = m · sinβ / NA), the aperture on the circumference of the partial annular light source
When the inclination angle is 2θ, R + σ _D = 1 and 2σ _D is 0.3
Below, 2θ is set to satisfy the condition of 60 ° or less .

[0030]

Therefore, according to the present invention, by arranging four partial annular light sources at symmetrical positions with respect to the optical axis,
The resolution of orthogonal L & S patterns can be improved, and the MTF curve can be steeper with respect to the spatial frequency.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to embodiments.
The oblique incidence illumination according to the present invention uses four partial annular light sources and specifies the direction in which the light is illuminated, so that the effect of improving the resolution also has directionality. For ease of explanation, here, an X axis and a Y axis orthogonal to the optical axis as the origin are set on a plane perpendicular to the optical axis, and are arranged symmetrically in four directions of X, -X, Y and -Y. Further, four partial annular light sources having the same shape are set.

In such a system, the normalized incident angle R of the partial annular light source and 2σ _D representing the width of the partial annular light source are defined as follows. R = m · sin α ′ / NA σ _D = m · sin β / NA where m is the reduction magnification, NA is the numerical aperture of the projection lens, and β
Is the spread angle of the reticle illumination light beam, and α 'is the angle formed by the light axis of the light beam center line projected on a plane including the optical axis. further,
The open angle on the circumference of the partial annular light source is defined as 2θ.

FIG. 1 shows an embodiment of the present invention, in which a typical arrangement of partial annular light sources 21 is represented by an entrance pupil space. When the size of the entrance pupil circle 22 is represented by a circle having a radius of 1 standardized by the NA of the projection lens, R, σ _D , and θ are as illustrated. In the present invention, R + σ _D = 1 and 2σ _D is 0.
Under the condition of 3 or less and 2θ of 60 ° or less, a high effect of the present invention can be obtained.

In the off-axis illumination system, the DC component of the spatial frequency is generally large and lowers the contrast. To address this, Japanese Patent Application No. 3-15740 filed by the same applicant
It is effective to apply a transmittance adjusting filter to the opening of the projection lens as proposed in No. 1. This filter has a predetermined transmittance only in the peripheral portion where the 0th-order light passes through the opening of the projection lens, and when viewed in the entrance pupil space, a filter having a predetermined transmittance is provided at a position just overlapping with the light source. The highest effect can be obtained when they are arranged so that they are arranged.

However, in the partial annular light source according to the present invention, a remarkable difference in effect is obtained between the case where the partial annular filter having a predetermined transmittance is used at the position just overlapping the light source and the case where the annular filter is used. Was not seen, so the latter filter arrangement was applied. The filter transmittance was set to 0.35 which gives the highest MTF as described later.

The pattern for evaluating the effect of the present invention is as follows:
In the following description, an L & S pattern parallel to the X-axis and the Y-axis and an L & S pattern having an inclination of 45 ° with respect to the X-axis and the Y-axis are represented. L (‖): L & S pattern parallel to X-axis or Y-axis L (#): L & S pattern having an inclination of 45 ° with respect to X-axis or Y-axis Since there are four symmetrical directions, the L & S pattern parallel to the X-axis and the L & S pattern parallel to the Y-axis have the same resolution.
All L & S patterns having an inclination of 45 ° with respect to the axis have the same resolution.

Hereinafter, some embodiments will be described with reference to the drawings using these parameters. FIGS. 2, 3, 4 and 5 show an embodiment according to the present invention, and show the contrast calculated based on the above equation (5) under the conditions of the arrangement and size of each light source. The conditions and evaluation target patterns are as follows.

FIG. 2: R = 0.95, 2σ _D = 0.1 and 2θ is 20
L (‖) in the case of °, 40 °, 60 °, 90 ° Fig. 3: R and σ under the condition of 2θ = 20 ° and R + σ _D = 1
L (‖) when _D is changed FIG. 4: R = 0.95, 2σ _D = 0.1, θ is 20 °,
L (#) at 40 ° and 60 ° FIG. 5: Under the condition of 2θ = 20 ° and R + σ _D = 1, R and σ
L (#) when _D is changed

As is apparent from FIG. 2, the application of the present invention makes the MTF curve longer and steeper than the conventional method and the annular light source. It is understood that the use of a resist process having an MTF of 0.7 or less can improve resolution and provide a large process margin. Also, by reducing the open angle 2θ on the circumference of the partial annular light source,
It is also clear that the effect of the present invention increases. Note that the MTF curve according to the conventional method is indicated by reference numeral B.

FIG. 3 shows a case in which the width of the partial annular light source is changed. The effect of the present invention is increased when the width is reduced. Conversely, if the width is increased, the effect of the present invention is reduced, but 2σ _D is 0.
If 3 or less, the effect of the present invention as compared to the circular light sources of the corresponding 2 [sigma] _D of FIG. 8 is obtained.

In FIGS. 2 and 3, the normalized spatial frequency is 0.5
The MTF is about 0.7 from 1.5 to 1.5
In equation (5), the transmittance is set to 0.35 for the k = 0 component passing through the filter, and the others are set to transmittance 1, and the value of each mutual transfer coefficient is expressed as J (k; 0) = 0.35 J (−k; 0) = 0.35 J (0; 0) = 0.35 × 0.35 J (k; k) = 1 J (−k; k) = 1 J (k; −k) = 0

By substituting A ₀ = 1 and A ₁ = に in equation (5), the MTF becomes about 0.7. Conversely, the value of the transmittance 0.35 is set as t, and dM / dt is obtained from the expression (5).
When t is determined to be = 0, the transmittance t = (route 2) /4=0.35 of the filter providing the highest resolution, and then, MTF = (route 2) /2≒0.7 is obtained. .

FIGS. 4 and 5 show the results of evaluating L (#) under the conditions corresponding to FIGS. 2 and 3. In any case, when the MTF is around 0.7 or less, the MT is higher than that of the annular light source.
Although F calms down, it shows higher resolution than the conventional method. This result shows the worst value of the present optical system.
The resolution of the L & S pattern parallel to the inclination other than 45 ° with respect to the axis and the Y axis exists between the MTF curve of the present result and the MTF curves of FIGS. 2 and 3 corresponding to the conditions.

In general, the LSI pattern is mostly composed of orthogonal line patterns or rectangular patterns, and a diagonal pattern is used in order to partially make the space work highly efficient. In this case, since the requirements for dimensional accuracy and resolution for the oblique pattern are generally lower than those for the orthogonal pattern, there is no problem in actual LSI pattern formation.

In the present invention, four congruent partial annular light sources are
Although it is used by arranging it at a position symmetrical with respect to the optical axis, there are the following two methods as concrete means for realizing the partial annular light source, and both provide the same effect. A method of obtaining a partial annular light source by using a lens system or an optical fiber according to the method proposed in Japanese Patent Application No. 3-148133 for an annular light source. A method in which an aperture having an opening corresponding to a partial annular light source is mounted immediately after a secondary light source surface in a conventional light source having a coherent factor σ = 1.

[0046]

As described above, the present invention improves the resolution of orthogonal L & S patterns and makes the MTF curve steeper with respect to the spatial frequency, as compared with a projection exposure apparatus using an annular light source. Therefore, a projection exposure apparatus (lithography) having a high process margin such as dimensional accuracy and reproducibility can be provided. By using such a projection exposure apparatus according to the present invention, it is possible to miniaturize the pattern in a general LSI pattern formation that requires high resolution with orthogonal patterns, and it is possible to greatly improve the degree of integration and device performance. Can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an arrangement of a representative partial annular light source in an entrance pupil space according to an embodiment of the present invention.

FIG. 2 is a diagram showing a contrast calculated based on the equation (5) according to the arrangement of the light source and the size of the light source in the embodiment according to the present invention in comparison with the conventional method.

FIG. 3 is a diagram showing a contrast calculated based on Expression (5) according to another embodiment of the present invention based on the conditions of the arrangement and size of the light source in comparison with the conventional method.

FIG. 4 is a diagram showing contrast calculated based on Expression (5) according to another embodiment of the present invention based on the conditions of arrangement and size of a light source in comparison with a conventional method.

FIG. 5 is a diagram showing a contrast calculated based on equation (5) according to another embodiment of the present invention based on the conditions of the arrangement and size of the light source in comparison with the conventional method.

FIG. 6A is an explanatory diagram showing reticle irradiation by an oblique incidence exposure method proposed by the same applicant, and FIG. 6B is an explanatory diagram of reticle irradiation by a conventional method for comparison with this.

FIG. 7 is a schematic view showing an example of a conventional projection exposure apparatus.

FIG. 8: Japanese Patent Application No. 3-1353 proposed by the same applicant.
It is a characteristic view of MTF which shows an example to which the thing of No. 17 is applied.

9A is a diagram showing an example of a light source arrangement in an entrance pupil space of the above-mentioned Japanese Patent Application No. 3-135317, and FIG. It is.

[Explanation of symbols]

 Reference Signs List 1 light source lamp 4 input lens 5 cone lens 6 optical integrator 7 output lens 8 condenser lens group 9 reticle 14 projection optical system 15 wafer 16 stop 21 partial annular light source 22 entrance pupil space

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuhiko Komatsu 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Emi Tamechika 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Yoshiaki Mimura 1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-61-91662 (JP, A)

Claims (1)

(57) [Claims] 1. A projection exposure apparatus for projecting and exposing a pattern on an object surface mask onto a wafer via a projection optical system, wherein a light beam illuminating the mask has an angle with respect to an optical axis corresponding to a numerical aperture of a projection lens. It has means for giving an inclination, and when viewed in a plane perpendicular to the optical axis, it is illuminated by four partial annular light sources from four directions symmetrical to the optical axis, and the shape of the partial annular light source is System
m, the numerical aperture of the projection lens is projected onto a plane containing the NA and the optical axis
The angle between the center of the light beam and the optical axis is α ', the reticle illumination light
The divergence angle of the bundle is β, and the normalized incident angle of the partial annular light source is R
(= M · sin α ′ / NA), the radiation of the partial annular light source is 2σ
_D (σ _D = m · sin β / NA), circumference of partial annular light source
Assuming that the upper opening angle is 2θ, R + σ _D = 1, 2σ _D is 0.3 or less, and 2θ is 60 ° or less
A fine pattern projection exposure apparatus characterized by satisfying the following conditions: