JPH07147223A - Pattern forming method - Google Patents

Abstract

PURPOSE:To realize the focal depth of a large transfer pattern, and excellent resolution, by arranging a multiple amplitude imagery filter or a filter having synthetic amplitude transmittance of said filter and a high frequency emphasis filter, on the pupil surface of a projection lens, and constituting an effective light source in an illumination optical system having almost ring belt type intensity distribution. CONSTITUTION:Ring belt illumination 7 is applied to a secondary light source surface. A multiple amplitude imagery filter 8 or a filter 9 having synthetic amplitude transmittance of the filter 8 and a high frequency emphasis filter or an approximate phase filter 10 of them is applied to the pupil surface. When the multiple imagery filter is applied to slant illumination, the amplitude distribution of an image formed by the slant illumination is generated in a plurality of part in the optical axis direction. The image formed on each imagery surface has a comparatively large focus, but the focal length is further extended by multiple amplitude imagery. When the high frequency emphasis filter is used, the image contrast to a fine pattern of high spatial frequency is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pattern forming method used for forming a fine pattern on various solid-state devices.

[0002]

2. Description of the Related Art In order to improve the degree of integration and operation speed of solid-state elements such as LSI, circuit patterns are becoming finer. At present, a reduction projection exposure method, which is excellent in mass productivity and resolution performance, is widely used for forming these patterns.

FIG. 2 is a schematic diagram of a reduction projection exposure apparatus. The light emitted from the effective light source on the secondary light source surface 1 illuminates the mask 3 via the illumination optical system 2. The pattern on the mask 3 is projected onto the substrate 5 via the projection lens 4. Since the resolution limit of this method is proportional to the exposure wavelength and inversely proportional to the numerical aperture (NA) of the projection lens, the resolution limit has been improved by increasing the NA and shortening the wavelength. However, after the 64-Mbit DRAM, the circuit size becomes smaller than the wavelength of light, and it has become difficult to improve the resolution by increasing the NA and shortening the wavelength in the related art.

Further, since the depth of focus of this method is proportional to the exposure wavelength and inversely proportional to the square of NA, the depth of focus is remarkably decreasing as the resolution limit is improved. On the other hand, L
Along with the high integration of SI, the three-dimensional structure of elements and the multi-layer wiring are being advanced. Therefore, large unevenness is generated on the surface of the LSI substrate, which is the projected surface of the mask pattern,
The surface exceeds the depth of focus. As a result, it has become difficult to form a fine pattern on the entire surface of the LSI chip.

Therefore, various methods for improving the resolution and the depth of focus of the projection exposure method have been proposed. The inventors of the present invention have disclosed in Japanese Unexamined Patent Publication No. 4-348017 that the projection lens 4
The amplitude transmittance distribution t (r) of the pupil 6 is approximately t (r) = cos (2πβr ² −θ / 2) (where r is the radial coordinate of the pupil and β and θ are appropriate constants). A method (Super-FLEX method) of providing a spatial filter (hereinafter, referred to as a multi-amplitude imaging filter or a Super-FLEX filter) is proposed. By using this method, images of the same mask can be simultaneously formed at two locations in the optical axis direction, and the amplitudes of the two can be combined while controlling the phase relationship of each. This improves depth of focus and resolution. Furthermore, in the application, when a filter is applied to a periodic pattern, the image contrast is lowered, and therefore, a method of superposing a high frequency enhancement filter having a large transmittance on the outer edge of the pupil was proposed. This is because the pupil plane is the Fourier transform plane of the object plane, and in principle the outside (peripheral part)
Since higher-order diffracted light (high spatial frequency component of the mask pattern) passes through, the higher-order diffracted light is emphasized by increasing the transmittance of the outer edge portion of the pupil to improve the image contrast.

On the other hand, the resolution and the depth of focus are improved by making the shape of the effective light source into a ring (ring) shape. This method has long been known as an annular illumination method in the field of microscopes and the like. However, in this case as well, the image contrast is lowered, so that a sufficient resolution improving effect cannot be obtained. Therefore, the inventors have proposed to use the same high-frequency emphasis filter also for the oblique incidence illumination method such as the annular illumination method. For the combination of the oblique incidence illumination and the high frequency enhancement filter, see, for example, Japanese Journal of Applied Physics, Volume 31, Part 1, 41.
Pages 26-4130 (1992) (Japanese Journal of
Applied Physics, Vol. 31, Part. 1, pp. 4126-4130 (19
92)).

[0007]

Among the various image improving methods described above, the pupil filtering (Super-FLEX) method is an LSI method.
It is particularly suitable for a fine isolated pattern such as a contact hole, and its resolution is improved by 20% and the depth of focus is tripled or more. However, with respect to the periodic pattern, the depth of focus is increased by about 1.5 times, but the resolution is 1
It decreased by 0%, and it cannot be said that a sufficient effect can be obtained.
On the other hand, in the combination of the ring illumination method and the high frequency emphasis filter,
Although the resolution is improved by about 20%, the increase in the depth of focus is still only about 1.5 times.

For this reason, the wavelength of the exposure apparatus is shortened to 193 nm of the ArF excimer laser, and the NA is set to 0.
When the resolution is improved by increasing it to about 6 to 0.7, there is a problem that the depth of focus becomes insufficient by any of the methods. Therefore, a method capable of further increasing the depth of focus is desired.

Further, since the high-frequency emphasis filter needs to use an absorption filter or a partial reflection mirror to partially transmit light, there is a new problem that heat and stray light generated by the filter must be removed. It was

An object of the present invention is to provide a pattern forming method and projection method capable of obtaining a transfer pattern faithful to a mask pattern even with a complicated LSI circuit pattern, with a large depth of focus, which is more than twice that of the conventional one, and excellent resolution. An object is to provide an exposure apparatus.

Another object of the present invention is to provide a pattern forming method and a projection exposure apparatus which can improve the resolution of a periodic pattern without being adversely affected by heat or stray light even if a pupil filter is used. .

[0012]

The first object is to irradiate a mask with light emitted from a light source through an illumination optical system and project and expose a pattern on the mask onto a substrate through a projection lens. A multi-amplitude imaging filter, or a filter having a composite amplitude transmittance of this and a high-frequency emphasis filter, is provided on the pupil plane of the projection lens, and the effective light source in the illumination optical system has a substantially annular intensity distribution. It is achieved by

The second object is achieved by substituting a multi-focus filter or a filter having a composite amplitude transmittance with these approximate phase filters.

[0014]

As a result of the study by the inventor, the multi-amplitude imaging (Supe
It has been clarified that the r-FLEX) filter, the annular illumination method, and the high-frequency emphasis filter work independently. Therefore, the present invention intends to obtain the advantages of each by using them together.

That is, according to the present invention, in order to obtain a larger depth of focus than the conventional method, as shown in FIG. 1, the annular illumination 7 is applied to the secondary light source surface and the multiple amplitude imaging filter 8 is applied to the pupil surface. , Or a filter 9 having a combined amplitude transmittance of a high-frequency emphasis filter and this, or an approximate phase filter 10 thereof.

In general, an image formed by oblique illumination has a relatively large depth of focus because it is difficult to receive defocus aberration. Next, when the multi-amplitude imaging filter is applied to the oblique illumination, the amplitude distribution of the image formed by the oblique illumination occurs at a plurality of points in the optical axis direction. As described above, the image formed on each image plane has a relatively large depth of focus, but the depth of focus is further expanded by performing multi-amplitude image formation. This results in a very large depth of focus. Further, the use of the high frequency enhancement filter improves the image contrast for a fine pattern having a large spatial frequency.

Next, the fact that the multi-amplitude imaging filter, the annular illumination method, and the high-frequency emphasis filter work independently of each other will be described using some mathematical expressions.

The amplitude distribution U of the projection image emitted from an arbitrary point s on the secondary light source surface and formed by an optical system having an arbitrary pupil function p is as a function of the position x in the image plane and the defocus z. You can write

[0019]

[Equation 1] U (x, z, s) = exp (iφ) ∫a (f−s) · p (| f |, z) · exp (2πix · f) df (Equation 1) where a ( f) is the Fourier spectrum of the mask pattern, p
(r, z) is the pupil function, f is the spatial frequency standardized by λ / NA, and r is the radial direction coordinate of the pupil plane standardized by the maximum aperture radius. The defocus z has a relationship of D = 2 · z · λ / NA ² with the actual defocus amount D on the optical axis. exp (iφ)
Is a term representing the phase of light, and φ = 2πD / λ = 4πz / NA
² , expressed as

Next, with respect to U (x, z, s), the image plane is moved by + β in the optical axis direction to shift the phase by + Δφ, and conversely, it is moved by -β to shift the phase by -Δφ. Considering the amplitude distribution U ′ (x, z, s) of the image by the multiple amplitude imaging that synthesizes the shifted images,

[0021]

U ′ (x, z, s) = U (x, z−β, s) exp (iΔφ) + U (x, z + β, s) exp (−iΔφ) (Equation 2) Substituting 1) into (Equation 2)

[0022]

U ′ (x, z, s) = exp (iφ) ∫a (f−s) · p ′ (| f |, z) · exp (2πix · f) df p ′ (| f |, z) = cos (2πβr ² −θ / 2) · p ′ (| f |, z) (Equation 3). However, θ = 2Δφ-8πβ / NA ² , which corresponds to a net phase difference obtained by subtracting the phase change due to the movement of the distance between the image planes from the phase difference between the two images.

From the above, the light is emitted from an arbitrary point s on the light source,
The multiple imaging amplitude image for the projection image formed by the optical system having the arbitrary pupil function p is the original pupil function (pupil transmittance distribution).
By cos (2πβr ² −θ / 2). This means that when the oblique illumination, the high-frequency emphasis filter, and the multi-amplitude imaging filter are used together, each works independently.

When the illumination distribution is σ (s), the light intensity distribution is given by I (x, z) = ∫σ (s) | U '(x, z, s) | ² ds.

The effects of the present invention depend on parameters such as the amplitude transmittance of the multi-amplitude imaging filter and the high frequency enhancement filter and the ring radius, and it is desirable to optimize these.
As a result of the inventor's study, it was found that when the conditions were optimized, a depth of focus more than twice that of the conventional case could be obtained.

3 and 4, a lattice-like (L / S) pattern obtained by optimizing the conditions of the multi-amplitude imaging filter and the annular illumination.
An example of the pattern dimension dependency of the pattern focal depth will be shown. 3 and 4 show β = for annular illumination of σ = 0.4 to 0.6, respectively.
0.325, θ = 0 degree, and β = 0.55, θ = 140
The result is obtained by using a multi-amplitude imaging filter of degree.

In each drawing, (a) is the amplitude transmittance distribution in the radial direction of the pupil, and (b) is the pattern size dependency of the depth of focus (however, the pattern size and defocus are λ / NA and λ / N, respectively).
A ² standardized). For comparison, the results of the conventional method (normal illumination, no filter) are also shown. In either case, it can be seen that an extremely deep focal depth which is more than twice that of the conventional method (dotted line in the figure) can be obtained for the L / S pattern having a dimension of about 0.6 · λ / NA.

Next, as a result of studying a case where a high-frequency emphasis filter is further stacked on the multi-amplitude imaging filter, FIG.
(A) With the filter having the amplitude transmittance distribution shown by the solid line, good results as shown in FIG. 5 (b) were obtained. The filter indicated by the solid line in FIG. 5 (a) is a multi-amplitude imaging filter of FIG. 3 overlaid with a high-frequency emphasis filter in which the amplitude transmittance inside the outer diameter of the annular illumination conjugate region on the pupil is reduced to 50%. It is a synthesis filter. As a result, the resolution is improved by about 10%.

The amplitude transmittance distribution of the synthesis filter shown by the solid line in FIG. 5 (a) can be further approximated by a perfect phase filter as shown in FIG. 6 (a). The filter of FIG. 6A is obtained by inverting the phase of a region of 89% or more of the pupil radius, and FIG. 6B shows the pattern dimension / defocus dependency of the image contrast when this is used. It can be seen that almost the same effect can be obtained also by the approximation filter. Since the filter of (b) is a perfect phase filter, it is not necessary to remove heat and stray light generated by the filter, and it is not adversely affected by heat and stray light.

[0030]

EXAMPLE A ring stop was inserted in the secondary light source surface of an i-line reduction projection exposure apparatus having a numerical aperture of 0.5, and a mask was illuminated by a ring light source with σ = 0.4 to 0.6. In addition, a pupil filter was inserted at the pupil position of the projection lens. The pupil filter has a radius of 80 on a quartz substrate polished on both sides to be optically flat.
A light-shielding plate (not shown) having an opening of mm is provided, a phase shifter film is vapor-deposited on the inner zone of the ring-shaped region having an inner diameter of 71 mm and an outer diameter of 80 mm, and an antireflection film (not shown) on both surfaces thereof.
Is applied.

Next, using a reduction projection exposure apparatus, a novolac-based positive resist film (film thickness 1 was applied on a Si substrate.
μm), masks containing L / S patterns of various arrangements and dimensions were transferred under various defocus conditions.

According to this embodiment, the entire exposed area is 0.4.
The μmL / S pattern could be formed with a dimensional accuracy of ± 10% over a defocus range of about 3 μm.
The cross-sectional shape of the resist pattern was also good. Further, it was possible to transfer the patterns having various arrangements and sizes to the resist pattern as designed.

The wavelength, the numerical aperture, the pupil filter size, the resist process used, the mask pattern size, etc. of the exposure apparatus are not limited to those shown in this embodiment.

[0034]

According to the present invention, in the exposure method, the light emitted from the light source is applied to the mask through the illumination optical system, and the pattern on the mask is projected and exposed onto the substrate through the projection lens. A multi-amplitude imaging filter, or a filter having a combined amplitude transmittance of this and a high-frequency emphasis filter, or an approximate phase filter of these is provided on the pupil plane of, and an effective light source in the illumination optical system is provided with a substantially annular intensity distribution. Shall have. As a result, a transfer pattern faithful to the mask pattern can be obtained with a large depth of focus and excellent resolution. This makes it possible to form an LSI having a circuit pattern size of 0.15 to 0.3 μm by using optical lithography.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram showing a configuration of a conventional method.

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

FIG. 4 is a characteristic diagram showing characteristics of the present invention.

FIG. 5 is a characteristic diagram showing characteristics of the present invention.

FIG. 6 is a characteristic diagram showing characteristics of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Effective light source, 2 ... Illumination optical system, 3 ... Mask, 4 ... Projection lens, 5 ... Substrate, 6 ... Pupil, 7 ... Ring illumination, 8 ... Multi-amplitude imaging filter, 9 ... Multi-amplitude imaging filter A filter having a composite amplitude transmittance of a high-frequency emphasis filter, 10 ...
Approximate phase filter.

Claims (4)

[Claims] 1. A pattern is formed on a substrate by irradiating a mask with light emitted from a light source through an illumination optical system and projecting and exposing the pattern on the mask onto a substrate through a projection lens. In the method, a multi-amplitude imaging filter, or a filter having a composite amplitude transmittance of a multi-amplitude imaging filter and a high-frequency emphasis filter is provided on the pupil plane of the projection lens, and the effective light source in the illumination optical system has a substantially annular shape. A pattern forming method having an intensity distribution. 2. The multi-amplitude imaging filter or the filter having the composite amplitude transmittance according to claim 1, wherein the phase of light transmitted through an annular region centered on the optical axis and other regions is substantially the same. A pattern forming method approximated by an inversion phase filter. 3. The pattern forming method according to claim 2, wherein the region where the phase of the transmitted light is inverted on the pupil plane is a region outside 85% to 95% of the pupil radius. 4. The illuminance distribution of the effective light source according to claim 1, wherein 95% or more of the total amount of light is from an inner diameter of 0.35 when the radius of the region conjugate with the pupil on the secondary light source surface is 1. 0.4
5. A method of forming a pattern within an annular zone having an outer diameter of 0.55 to 0.65.