JPH10189416A - Pattern forming method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to transfer micro patterns lower than a resolution limit by using a pulsed light source for a projecting light source which projects a masking pattern drawn on a masking substrate on a wafer substrate via a projecting optical system and for a light source for a demodulating step which represents the masking pattern. SOLUTION: A light radiated from a light source 2 which uses a second harmonic pulse light of a first titanium sapphire laser for exposure is converted into a beam having a requested even light strength of section area by a beam expander 4 and irradiates a mask substrate 5. A pattern 6 drawn on the mask substrate 5 is projected on a wafer 8 through a projecting lens 7 having a magnification of 1/5. Interference fringes 34 are formed on the surface of the wafer 8 by using a light from a light source 3 which uses a second harmonic pulse light from a second titanium sapphire laser. A micropattern is formed on a resist 35 by synchronizing both in a synchronous circuit 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a pattern forming method and apparatus which improve the resolution of a transfer pattern formed on a sample substrate of a pattern forming apparatus.

[0002]

2. Description of the Related Art A method of transferring a pattern drawn on a mask onto a sample substrate, which is called a lithography technique, is widely used for forming a pattern of a semiconductor integrated circuit or a liquid crystal element. In order to perform this pattern transfer, a reduced-projection type projection exposure apparatus for reducing and transferring a pattern on a mask is generally used.

As the pattern miniaturization of semiconductor integrated circuits and the like progresses, projection exposure apparatuses are required to have fine pattern transfer performance with higher resolution than ever before. In general, the higher the numerical aperture (NA) of the projection lens or the shorter the wavelength of light used for exposure, the higher the resolution. However, the projection lens is also required to have a large exposure area at the same time, and there is practically a limit in improving the NA. In addition, the shortening of the wavelength of light used for exposure is approaching its limit due to restrictions on the light source, the material of the projection lens, and the resist material that can be used.

[0004] Attempts have been made to transfer a fine pattern exceeding the conventional resolution limit using the current projection exposure apparatus. Japanese Patent Publication No. Sho 62-50811 discloses a technique for significantly improving the resolving power, especially for an aperture pattern having periodicity, by introducing a device for giving a phase difference of light used for exposure to a mask itself. ing.
Japanese Patent Application Laid-Open No. 62-67514 discloses a method of improving the resolution by newly arranging an auxiliary phase difference pattern even for a substantially isolated pattern. Further, Japanese Patent Application Laid-Open Nos. 4-101148 and 4-2 disclose a method for improving the resolving power by increasing illumination light in a specific direction to an illumination optical system of a projection exposure apparatus.
No. 67515.

[0005]

Problems to be Solved by the Invention Japanese Patent Publication No. Sho 62-50
No. 811 and Japanese Patent Application Laid-Open No. 62-67514 disclose a mask technique, in which a transparent phase member for changing the phase of illumination light by 180 degrees is provided at every other opening in the light-shielded area of the mask. This realizes an improvement in resolution. Further, Japanese Patent Application Laid-Open No. Hei 4-267515 discloses a method of giving a resolution exceeding the conventional resolution performance by employing a so-called oblique incidence illumination method in which a mask is obliquely irradiated.

In general, the resolution performance of a projection lens of a projection exposure apparatus is determined by the wavelength λ of light for exposing a pattern and the numerical aperture NA of the projection lens, and the dimension R of the resolution limit is usually given by the following equation (1).

[0007]

(Equation 1)

In practice, the value of the constant k1 is about 0.6 to 0.8, and depends on the pattern exposure and development method. As for the resolution performance of the projection lens, the contrast of the projected image approaches the maximum value 1 for a relatively large pattern, and the contrast of the projected image decreases to the minimum value 0 as the pattern becomes finer, that is, as the spatial frequency increases. Approach. Then, a fine pattern having a high frequency exceeding λ / 2NA cannot be transferred.

Under the above restrictions, the above-mentioned Japanese Patent Publication No. Sho 62-5
The methods disclosed in JP 0811 and JP-A-62-67514 improve the projected image contrast at a high spatial frequency. Further, Japanese Unexamined Patent Publication No.
The method disclosed in JP-A-48-48 and JP-A-4-267515 is intended to slightly lower the projected image contrast in a low spatial frequency region and to improve the projected image contrast in a high spatial frequency region.

[0010] In any of the methods, the resolution performance is improved in the high spatial frequency region as compared with the conventional method. However, as described above, since the limit of the spatial frequency of a transferable pattern is λ / 2NA, it is impossible to transfer a fine pattern having a higher frequency than this.

An object of the present invention is to provide a pattern forming method and apparatus capable of transferring a fine pattern exceeding a conventional resolution limit.

[0012]

According to the present invention, there is provided a pattern forming method for forming a pattern by projecting a mask pattern drawn on a mask substrate onto a wafer substrate via a projection optical system. A modulating step of shifting a position of the Fourier transform pattern by a predetermined amount on a surface on which a Fourier transform pattern of the mask pattern is formed to change a spatial frequency; and, on the wafer substrate, a modulation amount in the modulating step. In a pattern forming method having a demodulation step of reproducing the mask pattern by shifting a spatial frequency in association therewith, a pulse light source is used for both the projection light source and the light source for the demodulation step.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a conventional projection exposure method, each of the 0th, ± 1st, ± 2nd,... Diffracted lights generated in a mask pattern passes through a projection exposure lens and forms an image on a wafer. Was. On the other hand, in the present invention, a pattern having a different spatial frequency, that is, diffracted light constituting a moiré fringe passes through the projection lens by superimposing the first lattice fringe on the mask pattern, thereby transmitting the pattern information onto the wafer.
Then, pattern transfer is performed by demodulation on the wafer. As a method of forming a pattern having a different spatial frequency, a mask pattern may be illuminated obliquely, and the position where the Fourier transform pattern is formed on the Fourier transform plane may be shifted.

Here, a coordinate system will be described to simplify the description. Now, assuming that the absolute value of the magnification of the projection lens is m and the numerical aperture on the wafer side is NA, the numerical aperture on the mask side of the projection lens is expressed as mNA. In an ordinary projection exposure apparatus, the value of m is widely adopted as 0.2. Here, the unit of the length on the mask side is λm / NA, and the unit of the length on the wafer side is λ / NA. By adopting a dimensionless unit, it is not necessary to consider the magnification of the lens, and it can be handled as an imaging optical system with an apparent magnification of 1. Therefore, also in this case, the above-mentioned non-dimensional coordinates are considered.

Now, a spatial frequency ν represented by a function of only x
Is arranged, and the spatial frequency of the first lattice pattern superimposed on the mask pattern is τ. At this time, ν + τ, ν-τ
(Moire fringes) having the sum and difference spatial frequencies. The coherent cutoff frequency of the projection lens (λ / N
A) When a suitable first lattice fringe is superimposed on a mask pattern having a larger frequency ν, the difference frequency can be made equal to or lower than the cutoff frequency (FIG. 5). Since this passes through the optical system of the projection lens, the mask pattern can be projected and transferred by demodulation. This principle is described, for example, in page 111 of Teruji Kose, "Fourier Imaging Theory" (Kyoritsu Shuppan), or pages 853 to 8 of Applied Physics Vol. 37, No. 9 (1968).
It is described on page 59.

In the present invention, as a mask pattern modulating means, a transmission type phase grating (first lattice fringe) having a large number of linear irregularities arranged on the surface of a transparent substrate is used. The same effect can be obtained by obliquely illuminating the mask pattern. Also, the lattice is
The diffraction grating is not necessarily a transmission type phase grating, and may be a diffraction grating in which a large number of linear light-shielding lines are arranged on the surface of the transparent substrate.

Assuming that the spatial frequency of the first lattice fringe is τ, a fringe having a spatial frequency of ν−τ is obtained as described above, and passes through the projection lens. Now, also on the surface of the wafer, a fringe having a spatial frequency of the sum of (ν−τ) + τ = ν and (ν−τ) −τ = ν−2τ is formed by the second lattice fringe having a spatial frequency of τ. can get. Of these, the pattern is transferred by extracting only the stripes having the sum spatial frequency (ν).

As a method of overlaying the second lattice pattern on the surface of the wafer, a material whose transmittance or refractive index changes in response to light for a very short time is applied to the surface of the wafer. Light is irradiated from two directions on the demodulation thin film to form two-beam interference fringes in the thin film. This interference fringe acts as a second interference fringe for light for exposure for pattern transfer.

The pattern transfer is performed according to the above principle. When the second grating is, for example, a light-dark type stripe, in principle, the higher the contrast between light and dark, the greater the demodulation effect. To get high contrast,
It is desirable that the intensity of light for forming the second lattice fringes be high. However, when the exposure is constantly performed at a high intensity, the energy stored in the demodulation material increases, and the demodulation material is thermally damaged, which is a problem.

For this reason, in the present invention, pulse excitation for exposing only a short time is performed as a light source, although the intensity is strong. When the second lattice fringe is formed by pulse excitation, the fringe formed in the demodulation material disappears in a short time and returns to the original transparent state. This is because the first grid pattern can be mechanically scanned because the upper and lower grid patterns are synchronously scanned, but the lower second grid pattern is optically scanned with interference fringes. Is a characteristic desired from the measurement principle that it is necessary to return to the original transparent state as soon as the light beam stops scanning.

Further, if the demodulation material is constantly irradiated with the pattern exposure light, it is exposed even after the lattice fringes disappear, so that the demodulation effect is reduced. Since the second lattice fringe is the clearest fringe immediately after the fringe is formed, pattern exposure only immediately after the fringe formation is a method for maximizing the demodulation effect. Therefore, it is the best exposure method that the light source of the pattern exposure is also pulsed, synchronized with the pulse light for forming the second grid pattern, and the pattern is exposed only immediately after the second grid pattern is formed.

As the pulse light source, a continuous light source may be pulsed by a mechanical shutter, or a titanium sapphire
Laser or YAG laser, excimer
A laser that oscillates from the beginning, such as a laser, may be used. In particular, a titanium sapphire laser, a laser using an opto-parametric phenomenon, or the like can continuously set the oscillation wavelength to a desired value, and is particularly suitable for implementing the present invention.

The optical characteristics of the second grating for demodulation are desirable in the case of a light-dark type demodulation grating because the larger the absorbance of the dark part, the greater the demodulation effect. Specifically, the absorbance is desirably 1 or more. In a phase shift type demodulation grating, it is preferable that the phase difference changes greatly as the demodulation effect increases. Specifically, it is desirable that the phase difference is λ / 2 or more.

[0024]

Embodiments of the present invention will be described below. These embodiments are merely examples of the present application, and the present invention is not limited to these embodiments.

(Embodiment 1) First, an outline of a configuration example of a projection exposure apparatus for realizing the pattern forming method of the present invention will be described with reference to FIG. As light (wavelength: 365 nm) used for exposure, second harmonic pulse light of the first titanium sapphire laser was used. The light emitted from the light source 2 is converted by a beam expander 4 into a beam having a cross section of a desired uniform light intensity, and illuminates the mask substrate 5. In the following, the mask substrate 5
Is referred to as a reticle 5.

The pattern 6 drawn on the reticle 5 is projected onto a wafer 8 via a projection lens 7 having a magnification of 1/5. The reticle 5 is placed on the stage 10 controlled by the reticle position control means 9, and its center and the optical axis of the projection lens 7 are accurately aligned.

The wafer 8 is placed on a Z stage 11 which is movable in the direction of the optical axis of the projection lens 7, that is, in the z direction. The Z stage 11 and the XY stage 12 each drive means 1 in accordance with a control command from the main control system 13.
Since it is driven by 4 and 15, it can be moved to a desired exposure position with respect to the base 16 of the projection exposure apparatus.
The position is accurately monitored by a laser length measuring device 18 as a position of a mirror 17 fixed to the Z stage 11. The surface position of the wafer 8 is measured by a normal focus position measuring means of the projection exposure apparatus. By driving the Z stage 11 according to the measurement result, the surface of the wafer 8 can always be made to coincide with the image plane of the projection lens 7.

In this embodiment, a transmissive lattice-shaped optical element 19 acting as a first lattice fringe is arranged very close to the reticle 5. Here, a phase grating was adopted, and the pitch of the phase grating 19 was 1.0 μm. As shown in FIG. 2, the phase grating 19 is provided with linear irregularities 20 on the surface of a transparent substrate, and inverts the phase of illumination light transmitted through the concave and convex portions.

The spatial frequency of the unevenness 20 of the phase grating 19 is τ
Then, the spectrum becomes as shown in FIG.
On the entrance pupil 21 of the projection lens, the spectra 22 and 2
It has the distribution shown in FIG. Further, the phase grating 19 is mounted on a stage 24 and can be scanned in a plane by a driving unit 25.

On the other hand, the reticle 5 is provided with a pattern 6 similar to the conventional one, and the spatial frequency v
Exceeds the cut-off frequency of the projection lens, the distribution shown in spectra 26, 27 and 28 as shown in FIG.
Here, when the reticle 5 and the first lattice fringe 22 are superimposed, Fourier spectra 29 and 30 of a composite pattern as shown in FIG. 5 are obtained, and the pupil 21 of the projection lens is obtained.
Will pass through the inside of

The second harmonic (wavelength: 490 nm) from the second titanium-sapphire laser was used as a light source 3 for forming demodulation fringes. On the surface of the wafer 8, the light from the light source 3 is caused to interfere with two beams in the demodulating thin film 36 using the beam splitter 31 and the mirrors 32 and 33, and the pitch is 0.2 μm.
Is formed. This pitch is equal to the pitch of the first grid pattern (1.0 μm) and the magnification (1/1 /
5).

A photoresist 35 is applied on the wafer 8, and a material whose photochemical reaction is reversible is applied on the surface thereof to a thickness of 0.3 μm to form a demodulation thin film 36. In this example, a solution of acridine orange and polyvinylpyrrolidone was spin-coated on the resist and dried by heating to produce a demodulation thin film 36.

The demodulation thin film 36 has a time response of less than millisecond to light having a wavelength of 460 nm and reacts reversibly. That is, only when the pulse light of 460 nm is irradiated, the complex refractive index changes, and the refractive index changes instantaneously at 365 nm.
4 and returns to the original refractive index within a short time with a lifetime of less than milliseconds.

The first titanium-sapphire laser 2 and the second titanium-sapphire laser 3 are combined by the synchronizing circuit 1 into the second titanium-sapphire laser 3 of 460 nm.
Immediately after forming the lattice fringe 34 for demodulation, the lattice fringe 34 is converted to the first titanium sapphire laser 2 for pattern exposure.
Is set to pass. Therefore, when the first titanium-sapphire laser 2 passes through the demodulation layer 36, the lattice fringes 34 having a sufficient contrast are formed, so that the demodulation is efficiently performed.

This second lattice fringe 34 has the spatial frequency τ
Works as a lattice. The second plaid 34 is the mirror 32
Is driven by the mirror driving means 37, and moves within the demodulation thin film 36 by continuously changing the phase of light. By performing pattern exposure while synchronously scanning the first lattice and the second lattice fringe 34, the photoresist 3
In No. 5, a fine pattern exceeding the conventional resolution limit was exposed.
Thereafter, the demodulation thin film 36 was removed, the resist 35 was developed, and the pattern shape was observed with a scanning electron microscope. As a result, a fine pattern having a good shape was formed.

When the phase shift amount of the demodulation grating was measured, it was found to be λ / 2. Similar effects were obtained by using a yag laser as the pulse laser.

(Embodiment 2) In the second embodiment, the second harmonic of the titanium sapphire laser (wavelength:
436 nm) to form a second interference fringe in the thin film for demodulation using the fourth harmonic of a yag laser (wavelength: 266 n).
m), using a polymethyl methacrylate thin film in which 2-acenatofutone is dispersed in the demodulation material,
The pattern formation was performed with the same device configuration as in Example 1 except that the pitch of the lattice fringes was 0.5 μm and the pitch of the second lattice fringes was 0.1 μm. The demodulation thin film 36 has a wavelength of 2
When the light of 66 nm is absorbed, the light has a transient absorption at the exposure wavelength of 436 nm, has a time response of less than millisecond, and undergoes a reversible coloring / fading reaction. A transitional light-and-dark lattice pattern is formed. Also in this embodiment, the yag laser for producing the second grating and the titanium sapphire laser for exposure are controlled synchronously, and when the titanium sapphire laser for exposure passes through the demodulation layer, sufficient contrast is obtained. Are formed, the demodulation action is performed efficiently, and a good pattern transfer can be performed.

When the absorbance of the demodulation grating was measured, it was found to be 1.2.

According to the present invention described above, since the pulse light source is used for forming the demodulated lattice fringes, an ideal lattice fringe having high contrast can be formed without thermally damaging the demodulated material. Therefore, the demodulation operation is performed efficiently, and good pattern transfer becomes possible.

Since both the exposure light source and the light source for forming the demodulation lattice fringes are synchronized by using a pulse light source, it is possible to make the exposure pulse for pattern formation incident on the demodulation material layer in a time region where the contrast of the demodulation lattice fringes is high. it can. Therefore, the demodulation operation is performed efficiently, and good pattern transfer becomes possible.

[0041]

According to the present invention, a fine pattern exceeding the conventional resolution limit can be transferred using an optical projection exposure apparatus. That is, finer processing than before can be realized without introducing a higher-performance projection lens, so that high integration of a semiconductor integrated circuit or the like is facilitated.

Further, individual effects will be described.

[Brief description of the drawings]

FIG. 1 is a block diagram of a projection exposure apparatus of the present invention.

FIG. 2 is a cross-sectional view showing a phase grating, which is a first grating fringe, and a reticle.

FIG. 3 is an explanatory diagram showing an example of a Fourier transform pattern of a phase grating.

FIG. 4 is an explanatory diagram showing an example of a Fourier transform pattern of a reticle pattern.

FIG. 5 is an explanatory diagram showing an example of a Fourier transform pattern of a pattern obtained by overlapping a reticle pattern and a phase grating.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Synchronous circuit, 2 ... Pulse light source for exposure, 3 ... Pulse light source for demodulation, 4 ... Beam expander, 5 ... Reticle, 6 ... Pattern, 7 ... Projection lens, 8 ... Wafer, 9 ...
Reticle position control means, 10 reticle stage, 11 Z stage, 12 XY stage, 13 system main control system, 14 Z stage drive means, 15 X
Y stage driving means, 16: Projection exposure apparatus base, 17
Mirror, 18 Laser length measuring device, 19 Lattice, 24 Stage, 25 Stage driving means, 31 Beam splitter, 32 and 33 Mirror, 34 Grid check, 35
Resist, 36: demodulation thin film, 37: mirror driving means.

Claims (8)

[Claims] A modulation step of shifting a position of the Fourier transform pattern by a predetermined amount on a surface of the mask pattern on which a Fourier transform pattern is formed, to change a spatial frequency; A demodulation step of reproducing the mask pattern on a wafer substrate by shifting, wherein a pulse light source is used for both the projection light source and the light source for the demodulation step. 2. The method according to claim 1, wherein when a thin film is formed on the wafer substrate and an excited state is formed in the thin film by the pulse light source for the demodulation step, the change in absorbance of the thin film with respect to the wavelength of the projection light source is 1. The pattern forming method described above. 3. The method according to claim 1, wherein a thin film is formed on the wafer substrate, and when the excited state is formed in the thin film by the pulse light source for the demodulation process, the refractive index of the thin film with respect to the wavelength of the projection light source changes. The optical path difference in the thin film is 0.
A pattern forming method having five or more wavelengths. 4. The pattern forming method according to claim 1, wherein an excitation state is formed by the pulse light source for the demodulation step, and the projection pulse light is irradiated within the life of the excitation state. 5. A modulating means for shifting a position of the Fourier transform pattern by a predetermined amount on a surface of the mask pattern on which a Fourier transform pattern is formed to change a spatial frequency, and a modulating amount of the modulating means on a wafer substrate. A pattern forming apparatus having demodulating means for reproducing the mask pattern by shifting a spatial frequency in association therewith, wherein a pulse light source is used for both the projection light source and the light source for the demodulating means. 6. The method according to claim 5, wherein when a thin film is formed on the wafer substrate and an excited state is formed in the thin film by the pulse light source for the demodulation means, the change in absorbance of the thin film with respect to the wavelength of the projection light source is one. The pattern forming apparatus described above. 7. The method according to claim 5, wherein when a thin film is formed on the wafer substrate and an excited state is formed in the thin film by the pulse light source for the demodulating means, a change in the refractive index of the thin film with respect to the wavelength of the projection light source. The optical path difference in the thin film is 0.
A pattern forming apparatus having five or more wavelengths. 8. A pattern forming apparatus according to claim 5, wherein an excitation state is formed by the pulse light source for the demodulating means, and the projection pulse light is irradiated within the life of the excitation state.