JPH07183215A - Projection exposure method and device therefor - Google Patents

Abstract

PURPOSE:To ensure both of high resolving power and sufficient focal depth to a pattern, e.g. a contact hole, etc., by a method wherein the changing step of image-forming position for a substrate as well as the changing step of exposure energy in respective image-forming positions are included in the projection exposure step. CONSTITUTION:A mask 7 is illuminated with a beam of light from light sources 1-4 so as to transfer a pattern on the mask 7 to a substrate through the intermediary of a projection optical system 8. At this time, a changing step of image- forming position for a substrate as well as the changing step of exposure energy in respective image-forming positions are included in the projection exposure step. For example, the changing step of the image-forming positions is composed of the changing step of the beam from the light sources 1-4. Furthermore, the changing step of exposure energy wherein the beam used for pattern exposure is pulse-oscillated excimer laser beam is composed of the narrowing step of the oscillation wavelength and the changing step of pulse numbers corresponding to the narrowed wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reduction projection exposure apparatus used for fine processing of solid state elements such as semiconductor element elements, magnetic bubble elements and superconducting elements.

[0002]

2. Description of the Related Art Conventionally, a fine pattern in a solid-state element such as an LSI has been formed mainly by using a fine projection exposure method. In the above method, the mask pattern is transferred by forming an image on a resist-coated substrate using a projection lens. Since the limiting resolution in the reduced projection exposure method is proportional to the exposure wavelength and inversely proportional to the numerical aperture of the projection lens, improvement of the resolution has been promoted by shortening the wavelength of the exposure light and increasing the numerical aperture of the projection lens. . On the other hand, the depth of focus of the projection lens is proportional to the exposure wavelength and inversely proportional to the numerical aperture of the projection lens. For this reason, the depth of focus has been drastically reduced by improving the resolution. That is,
In the above-mentioned conventional method, it is difficult to make the pattern finer and secure a sufficient depth of focus at the same time, and the depth of focus becomes extremely shallow especially when aiming for high resolution. Note that the projection exposure method is discussed in, for example, semiconductor lithography technology, Fichiro Ichiro, Industrial Graphics, Chapter 4, pages 87 to 93.

[0003]

With the high integration of LSIs, the miniaturization and three-dimensionalization of elements have progressed at the same time, and the need to form fine patterns on a substrate having a large uneven step structure on the surface has become even stronger. It was However, as described above, in the current reduction projection exposure method, if the resolution limit is raised, the depth of focus becomes shallow. For this reason, it has become difficult to keep the surface of the substrate within the depth of focus over the entire exposure region and resolve the fine pattern. In particular, in forming a contact hole, the depth of focus is originally small, and in general, it is often performed after a considerable surface step is formed on the element. Therefore, the shortage of the depth of focus is serious.

An object of the present invention is to secure high resolution for a general fine pattern on a flat surface by using an optical system having a short exposure wavelength and a large numerical aperture, and in particular, the above contact holes and the like. It is an object of the present invention to provide a projection exposure apparatus capable of ensuring both a high resolution and a sufficient depth of focus for a pattern.

[0005]

SUMMARY OF THE INVENTION In the reduction projection exposure apparatus, the above-mentioned object is to provide a means for varying the wavelength dependence of the light intensity of the light involved in the exposure, particularly the wavelength bandwidth, and the above-mentioned wavelength bandwidth. This is achieved by providing projection exposure means capable of continuously forming an image of the mask pattern within a wide distance that is constant in the optical axis direction.

[0006]

According to the study by the present inventor, the depth of focus of an isolated pattern such as a hole pattern having a small proportion of light-transmitting portions is effectively increased by continuously forming the image in the optical axis direction. To do. On the other hand, when exposure is performed using light having different wavelengths, the mask pattern is imaged at different positions depending on the wavelength due to chromatic aberration of the projection lens. Therefore, the mask pattern can be continuously imaged in the optical axis direction by performing exposure using light in a relatively wide wavelength band.
Therefore, by widening the wavelength band for each exposure, it is possible to effectively increase the depth of focus of an isolated pattern such as a hole pattern having a small proportion of transparent portions.

However, when the wavelength band is widened in a pattern having a relatively large light-transmitting portion such as a line-and-space pattern, the light intensity contrast is lowered due to chromatic aberration, as is well known. For this reason, it is necessary to narrow the wavelength band width for patterns having a large proportion of these light transmitting portions to sufficiently reduce chromatic aberration.

In the exposure apparatus according to the present invention, since the wavelength band width of each exposure can be changed by the means for varying the wavelength dependence of the light intensity of the light involved in the exposure, the proportion of the light transmitting portion such as the hole pattern is small. For an isolated pattern, a relatively wide wavelength band is set for exposure, while for a pattern with a relatively large light-transmitting portion such as a line-and-space pattern, a light with a sufficiently narrow band is used. Can be used for exposure. This makes it possible to form a fine pattern on a flat surface using a large numerical aperture and short wavelength exposure light, while forming a hole pattern with a sufficient depth of focus even on a substrate having a large step on the surface. Becomes

Next, it is shown that the depth of focus of the hole pattern is increased by continuously imaging the pattern in the optical axis direction.

In an orthogonal coordinate system having the Z axis as the optical axis, Z
The light intensity function of the hole pattern that forms an image at = 0 is i (x,
y, z). FIG. 3A shows an example of calculating i (x, y, z). However, the calculation example is 0.3 exposed by a projection lens with a numerical aperture of 0.4 by a KrF excimer laser beam.
This is for a μm square contact hole. Now
Assuming that the pattern is uniformly imaged at + ∞ from Z = −∞ on the same optical axis, the light intensity distribution function I (x,
y, z) is

[0011]

[Equation 1]

Therefore, it does not depend on the position z in the optical axis direction. By the way, as can be seen from FIG. 3A, z <−3 μm, 3 μ
It can be regarded that i (xyz) ~ for m <z. Therefore,

[0013]

[Equation 2]

Discretization for numerical calculation,

[0015]

[Equation 3]

However, ΔZ = 0.25 μm FIG. 3B shows I (x, y, z) obtained by the above equation. However, the scale of the light intensity (vertical direction) is changed from that shown in FIG. As can be seen by comparing FIG. 3A and FIG. 3B, the image quality does not change even if images are continuously formed, and in principle, the depth of focus can be increased to an unlimited extent.

[0017]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a reduction projection exposure apparatus according to an embodiment of the present invention. This apparatus is required for a reflecting mirror 1, a wavelength band setting apparatus 2, an excimer laser resonator 3, a mirror 5, an illumination optical system 6, a reticle 7, a projection lens 8, a substrate stage 9, a control computer 10 and other reduction projection exposure apparatus. It is composed of various elements.

The wavelength band setting device 2 has a plurality of etalons having different Q values, and any one of them can be inserted into the laser optical path. Therefore, the exposure wavelength band can be arbitrarily set by selecting an etalon having a Q value corresponding to a desired wavelength band within the allowable range and inserting it into the laser optical path. The designation of the exposure wavelength band is made by an input to the control computer 10 of the exposure apparatus, and the wavelength band setting apparatus 2 automatically selects and inserts the etalon based on this data.

Although the wavelength band setting device 2 is installed outside the laser resonator 3 in FIG. 1, it is not limited to this method in practice. That is, it is possible to adopt a method of inserting the wavelength band setting device 2 into the laser resonator 3 or integrating the both. Also, for example, a grating may be used instead of the etalon having a different Q value. in this case,
The relatively narrow bandwidth is due to the etalon with high Q
Further, it is preferable that the relatively wide bandwidth is set by the grating.

Next, by using this apparatus, the pattern on the reticle 7 was projected and exposed onto the substrate coated with the resist fixed on the substrate sturgi 9, and then developed to form a resist pattern. K is used as the excitation gas for the excimer laser resonator 3.
rF was used. The wavelength band width of the laser when the band was not controlled by the wavelength band setting device 2 was about 0.5 nm.

First, the exposure wavelength band width was set to about 0.003 nm by the wavelength band setting device 2. For the projection lens used, the chromatic aberration due to this bandwidth is very small,
It is at a level that can be practically ignored. The surface arrangement of the substrate was variously set in the optical axis direction to perform exposure, and the range of the optical axis direction in which various fine patterns were resolved, and the depth of focus for the pattern were investigated. At the same time, using a substrate having a flat surface and a substrate having various uneven steps on the surface, the uniformity of pattern resolution over the entire exposed region was examined. As a result, hole patterns with diameters of 0.3 μm and 0.5 μm were resolved at focal depths of 1 μm and 2 μm, respectively. Also, 0.3μ
Line and space patterns of m and 0.5 μm resolved at depths of focus of about 1.5 μm and 2.5 μm, respectively. Even when the substrate surface is extremely flat, it was difficult to resolve a hole pattern having a diameter of 0.3 μm over the entire exposed area due to the influence of image plane distortion of the projection lens. In addition, as for the other patterns described above, it became difficult to resolve the entire surface as the unevenness of the substrate surface increased.

Next, the wavelength band setting device 2 was used to change the exposure wavelength band width to about 0.1 μm, and an attempt was made to form a similar pattern. According to the setting data of the projection lens used, the pattern is continuously imaged over about 10 μm in the optical axis direction with respect to the above band width due to the chromatic aberration of the lens. As a result, regarding the line and space pattern,
No resolution was possible at all, while the depths of focus of the 0.3 μm and 0.5 μm diameter hole patterns were increased to about 8 μm, respectively. This makes it possible to form the hole pattern on the entire surface of the exposed region even when the substrate surface has a large unevenness.

(Embodiment 2) FIG. 2 is a block diagram of another embodiment of the present invention. In this device, a wavelength scanning device 12 is provided instead of the wavelength band setting device in the first embodiment.

The wavelength scanning device 12 is composed of an etalon and a mechanism for controlling the angle of the etalon with respect to the laser beam.
The wavelength band of the laser light is narrowed to about 0.003 nm, and the central wavelength of the band is changed by changing the angle. Therefore, the image forming position of the mask pattern can be moved in the optical axis direction by changing the angle of the etalon during exposure and scanning the exposure wavelength. In addition, the control computer can synchronize the pulse oscillation of the excimer laser and the timing of the etalon angle setting as specified in advance, so that exposure can be performed using any exposure amount for each wavelength within the above scan range. Can be done.

As means for changing the central wavelength, in addition to changing the angle of the etalon with respect to the laser beam, the distance between the two flat plates constituting the etalon is changed, or the two flat plates are hermetically sealed. Known means such as changing the pressure or type of gas may be used.

A pattern is actually formed using this apparatus,
The same effect as the first embodiment was confirmed.

Next, another embodiment of the present invention will be described.

According to a study by the present inventor, the depth of focus of a fine pattern is effectively increased by superimposing images whose image forming positions continuously change in the optical axis direction on the same optical axis.

In the above method, the mask pattern is projected and exposed onto the substrate by using light in a relatively wide band, and the mask pattern is continuously connected in the optical axis direction within the range of chromatic aberration corresponding to the wavelength band. It is realized by making an image.

This method is not preferable because it causes a decrease in image contrast in a dense pattern having a large proportion of light transmitting portions, but has a remarkable effect in an isolated hole pattern having a small proportion of light transmitting portions.

In order to show the effect of the present invention, first, the light intensity distribution of images when the images whose image forming positions continuously change in the optical axis direction are superimposed will be discussed.

In a Cartesian coordinate system with the Z axis as the optical axis direction, the number of three-dimensional light intensity distribution planes of an image having the plane Z = 0 as the image plane is i (x, y, z). The light intensity distribution function I (x, y, z) of the composite image when this image is continuously formed from positions Z = -L to Z = L on the same optical axis is

[0034]

[Equation 4]

It is represented as follows. Where w (l) is Z
Is the relative intensity ratio of the image formed at = 1 to the image formed at Z = 0. For simplicity, assuming that this relative intensity ratio is all 1 and scattering the integral,

[0036]

[Equation 5]

However,

[0038]

[Equation 6]

To obtain In FIG. 4, N = 8, Δl = 0.25
The calculation results of each term on the right side and the left side of the equation when μm is shown. However, the scale of the light intensity is appropriately adjusted. Note that the numerical aperture was calculated by using a KrF excimer laser beam.
This is for a contact hole of 0.3 μm square exposed using the projection lens of No. 4. I (x, y, z) and i
From the comparison of (x, y, z), it can be seen that the range in the optical axis direction where a good light intensity distribution can be obtained by the superposition, that is, the depth of focus is increased by about 70%.

The depth of focus generally increases as the number of N species increases, but there is a concern that the image may be deteriorated. Therefore, attention is paid to the light intensity distribution at Z = 0 when the value of N is increased. By the way, Z <-8 · Δl, 8
-For Δl <z, it can be regarded as i (x, y, z) ~ 0, so the above light intensity distribution is

[0041]

[Equation 7]

That is, even if the value of N is 8 or more, the obtained light intensity distribution is almost the same as that when N is 8. This is a phenomenon peculiar to an isolated pattern in which the light intensity distribution approaches 0 as the defocus increases and a small proportion of the light transmitting portion is represented by a contact hole or the like. Therefore, for the above pattern, by increasing N and enlarging the range of continuous image formation, it is possible in principle to increase only the depth of focus without degrading the image quality.

By carrying out projection exposure using exposures having a finite wavelength band and a projection lens which is not image-point achromatic with respect to the wavelength band, the same mask pattern is continuously formed in the optical axis direction. Can be imaged on. That is, since the image forming positions of the respective wavelengths in the band differ due to chromatic aberration, the image forming surface can be continuously distributed in the optical axis direction. At this time, when the image forming surface is shifted in the optical axis direction due to the change of the wavelength within the band, the fluctuations of the image forming magnification, the field curvature, and the image distortion must be kept within a certain allowable range.

Further, since the light intensity generally changes within the band depending on the wavelength, the light intensity of the image differs depending on the image forming position.
By the way, considering a narrow wavelength range, the relationship between the wavelength and the image forming position can be generally expressed by a linear expression. Therefore, w (l) in the expression is defined as the light intensity wavelength dependence (wavelength spectrum) of light involved in exposure. Can be considered equivalent. Needless to say, it is desirable that w (l) can be regarded as 1 in order to obtain the uniformity of the image in the optical axis direction. Therefore, it is preferable that the wavelength spectrum of the light involved in the exposure is a rectangular type or a trapezoidal type having a constant light intensity only within the band.

(Example 3) A mask pattern was transferred to a resist film coated on a substrate by using a KrF excimer laser beam narrowed to a wavelength band of 0.1 nm, and then developed to form a resist pattern. . According to the design data of the projection lens, as shown in FIG. 5, the mask pattern is continuously imaged for about 10 μm in the optical axis direction corresponding to the wavelength band.

Within the range including the above-mentioned image forming position, the surface position of the substrate coated with the resist was variously set in the optical axis direction for exposure, and the range in which the pattern was resolved was examined.
The pattern used for evaluation is a 0.3 μm square hole pattern. It was found that the image formation, the range in the optical axis direction of the substrate surface position where the pattern is resolved, that is, the effective depth of focus is about 8 μm. On the other hand, in the conventional method in which light is exposed using light whose band is narrowed enough to ignore chromatic aberration, the depth of focus of the pattern was only about 1 μm. In an actual LSI, there are uneven steps on the surface. A minimum depth of focus of 2 μm is required in order to resolve the above pattern on the entire surface of the exposure area regardless of the unevenness. Therefore, in the conventional method, it was difficult to apply the above pattern to an actual LSI, but by using this method, this is possible.

The projection lens of the projection exposure apparatus used in this example was a monochromatic lens made of a single material of synthetic quartz. For this reason, a slight change in the imaging magnification was observed depending on the exposure wavelength, that is, the image forming position, and when the substrate surface position was different in the optical axis direction, pattern displacement occurred in the peripheral portion of the exposure area. This is not the effect intended by the present invention and should be suppressed as much as possible. Therefore,
The projection lens is designed so that the image plane can be shifted in the optical axis direction while keeping fluctuations in magnification, image plane distortion, field curvature, etc. within a certain allowable range when the exposure wavelength is changed. You should use the configured one. Further, the light source used for exposure and the wavelength band width of each exposure are not limited to those shown in this embodiment, and can be used. However, the above wavelength band width is such that the distance on the optical axis at which the mask pattern is continuously imaged corresponding to the band width is 0.5 × (λ / NA ² ) (where λ is the light that contributes to exposure). The central wavelength and NA are preferably set to be equal to or larger than the numerical aperture of the projection lens used for exposure. This is because if the distance is less than 0.5 × (λ / NA ² ), the effect of increasing the depth of focus can hardly be expected.

(Embodiment 4) Next, another embodiment of the present invention will be described.

The KrF excimer laser light is narrowed to a wavelength band of 0.0025 nm by using an etalon, and the etalon angle is changed so that the central wavelength of the KrF excimer laser light is 0.0025 n from 248.3 nm to 248.35 nm.
It was shifted by m and exposed for 94 pulses. At this time, in order to set the wavelength dependence of the light intensity of the light contributing to exposure as shown in FIG. 6, the number of pulses for each shifted wavelength was changed as shown in FIG. The projection lens using X, the evaluated pattern, and the evaluation method are the same as in the first embodiment.

According to this embodiment, a hole pattern of 0.3 μm square can be resolved with a depth of focus of about 4 μm. Therefore, the following pattern forming method is possible.

(1) The reduction projection exposure includes the steps of reducing projection exposure onto a resist film through a mask pattern having a desired shape, and developing the resist film.
In a pattern forming method which is performed with light having a central wavelength λ using a projection lens having a numerical aperture NA, the image forming position on the optical axis of the mask pattern is at least -0.5 with the image forming position with respect to the central wavelength λ as the center. (Λ / NA ² ) to 0.5
A pattern forming method characterized by being continuously distributed over a range of (λ / NA ² ).

(2) The image forming position on the optical axis of the mask pattern is determined by the chromatic aberration of the projection lens corresponding to the wavelength band width of light involved in the reduction projection exposure.
The pattern forming method as described in (1) above, wherein the pattern forming method is defined so as to be continuously distributed within the range.

(3) By scanning the wavelength of light involved in the reduction projection exposure during the exposure, chromatic aberration of the projection lens causes the image forming position of the mask pattern on the optical axis over the range. By simultaneously moving, the scanning speed of the wavelength is adjusted to control the moving speed of the imaging position as specified in advance, or the exposure amount is changed in synchronization with the scanning of the wavelength as specified in advance. A method of forming a pattern, which comprises exposing the film to light.

(4) The pattern forming method as described in (2) or (3) above, wherein the wavelength dependence of the light intensity of the light involved in the reduction projection exposure is trapezoidal or bimodal. .

(5) The pattern forming method as described in (1) above, wherein the light used for the reduction projection exposure is an excimer laser light.

(6) In the projection lens, the variation of the image-forming magnification of the mask pattern, the image plane of the image-forming surface of the mask pattern, with respect to each wavelength within the wavelength bandwidth or the range of the wavelength scan. The pattern forming method according to (2) or (3), wherein the curvature and the image plane distortion are within a predetermined allowable range, and the image planes are different in the optical axis direction.

[0057]

As described above, in the reduction projection exposure apparatus according to the present invention, in the reduction projection exposure apparatus for reducing and exposing the mask pattern onto the substrate, the wavelength dependence of the light intensity of the light involved in the exposure is changed. By providing a means to
Since the image forming positions of the pattern on the mask by the projection lens can be arbitrarily distributed by using the chromatic aberration of the lens, the depth of focus can be increased according to the exposure pattern. Therefore, it is possible to cope with the shortage of the focal depth due to the shortening of the wavelength of each exposure, the increase of the numerical aperture of the projection lens, and the increase in the unevenness of the surface of the substrate due to the three-dimensional element structure. . Therefore, by using the reduction projection exposure apparatus according to the present invention, the reduction projection exposure method can be applied to the LSI.
It is possible to eliminate a major obstacle when applied to the formation of a fine pattern in a solid-state device such as the above, and it is possible to expand the scope of application of the reduction projection exposure method to the manufacture of a finer solid-state device.

Further, by using the pattern forming method according to the present invention, it is possible to increase the depth of focus of a pattern having an isolated light transmitting portion represented by a contact hole or the like in the reduced projection exposure method. It is possible to deal with the lack of depth of focus due to the increase in the numerical aperture of the projection lens, the increase in the numerical aperture of the projection lens, the increase in the unevenness of the substrate surface due to the three-dimensional element structure, the inclination of the substrate, and the curvature of field of the projection lens Is.

Therefore, by appropriately using the pattern forming method according to the present invention in manufacturing a solid-state element such as an LSI, a large obstacle in applying the reduced projection exposure method to the pattern can be eliminated, and the reduced projection exposure method can be used. The scope of application can be extended to the manufacture of finer solid-state devices.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a configuration diagram showing another embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the effect of the present invention.

FIG. 4 is a principle diagram showing an operation of the present invention.

FIG. 5 is a characteristic diagram showing the principle in one embodiment of the present invention.

FIG. 6 is a characteristic diagram showing conditions of an example of the present invention.

FIG. 7 is a diagram for setting conditions of an embodiment of the present invention.

[Explanation of symbols]

1 ... Reflective mirror, 2 ... Wavelength band setting device, 3 ... Excimer laser resonator, 4 ... Output mirror, 5 ... Mirror, 6 ... Illumination optical system,
7 ... Reticle, 8 ... Projection lens, 9 ... Substrate stage, 1
0 ... Control computer, 12 ... Wavelength scanning device.

Claims (4)

[Claims] 1. A projection exposure method for illuminating a mask with light from a light source and transferring a pattern on the mask onto a substrate via a projection optical system, wherein an image forming position is formed on the substrate during exposure. And a step of varying the exposure energy at each imaging position in the step of varying. 2. The projection exposure method according to claim 1, wherein the step of changing the image forming position comprises a step of changing the wavelength of light from the light source. 3. The step of varying the exposure energy is a pulse oscillation excimer laser used for pattern exposure, the step of narrowing the oscillation wavelength, and the number of pulses corresponding to the narrowed wavelength. The projection exposure method according to claim 1, further comprising the step of varying 4. A light source is an excimer laser, means for narrowing pulse-oscillated light, and means for synchronizing a change in the narrowed central wavelength and the number of pulses of the light as specified in advance. , A projection exposure apparatus comprising means for transferring synchronized light onto a substrate through a reduction lens.