JP2980479B2 - Photomask and exposure method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photomask and an exposure method. Especially semiconductor devices, liquid crystal devices, dielectrics,
The present invention is applied to a projection exposure apparatus used for manufacturing ferroelectric elements, magnetic elements, and superconductor elements, and relates to improvement in resolution and depth of focus of a resist pattern formed on a substrate.

[0002]

2. Description of the Related Art In recent years, as semiconductor devices have become more highly integrated, a circuit configuration with a finer pattern has been required, and there has been a remarkable demand for further miniaturization of lithography technology. Therefore, studies have been made using a projection type exposure apparatus called a stepper to meet the demand. The focus of conventional studies has been on shortening the wavelength of the light source used in the exposure apparatus and, accordingly, on improving peripheral technologies such as optical systems and resists. The g-line (wavelength: 436 nm), which is a bright line of a high-pressure mercury lamp, has been focused on. ), I-line (wavelength: 365 nm), and further to the Deep UV region (wavelength: 248 nm) using a KrF excimer laser. However, with the rapid integration of semiconductor devices in particular, the minimum line width requirement of the circuit is 0.3 μm or less and 0.2 μm or less, which is approaching the value equal to or less than the wavelength of the light source.

[0003] The critical resolution (R) and depth of focus (DOF) of a projection type exposure apparatus are generally obtained by the following Rayleigh equation. R = k ₁ λ / NA (1) k ₁ : constant determined by process, λ: wavelength of light source (nm), NA: numerical aperture of projection lens DOF = k ₂ λ / (NA) ² (2) k ₂ : Therefore, in order to improve the resolution without shortening the wavelength of the light source from equation (1), it is necessary to increase the numerical aperture (NA) of the projection lens. However, increasing the NA increases the other important characteristic in lithography, as is apparent from the equation (2), which leads to a decrease in the depth of focus, and it is necessary to optimize the NA in order to achieve both characteristics well. Further, increasing the NA involves difficulties in manufacturing technology, and the maximum NA that can be obtained at present is 0.
It is about 5. Further, with a generally used quartz lens material, it is difficult to correct chromatic aberration in a shorter wavelength range, and the absorption increases, so that lens distortion due to heat generation becomes a problem.

Therefore, in recent years, the wavelength of the light source and the N
Three methods for improving resolution and depth of focus without improving A have been proposed. First method (Japanese Patent Laid-Open No. 57-620)
52) is compared with a lens having the same NA by forming a transparent film that is 180 ° out of phase with a light transmitting part adjacent to at least one of the light transmitting parts on both sides of the opaque part of the periodic mask pattern. Resolution can be increased.
The second method (JP-A-62-67547) is isolated (single)
As a means for improving the resolution of the light transmitting portion, an attempt is made to provide a light transmitting portion having a resolution equal to or lower than the resolution limit where the phase of the transmitted light is inverted by 180 ° on both sides of a single light transmitting portion. The third method (Japanese Patent Laid-Open No. 4-101148) discloses a method in which at least one of the pupil plane of the illumination optical system or its conjugate plane or the pupil plane of the projection optical system is used.
It is proposed to provide a light-shielding plate having at least one set of light-transmitting parts determined based on the Fourier transform pattern obtained from the fine pattern of the mask at several places, and to improve the resolution and the depth of focus as compared with lenses of the same NA Have been.

[0005]

It is important to shorten the wavelength of the light source to improve the resolution and the like. However, the construction of an optical system free from aberration (with a light source having a narrow band) associated with the shortening of the wavelength has been proposed.
In addition, it is necessary to establish a high-performance exposure system as a whole, such as the construction of a peripheral process centered on a resist that has a transmittance suitable for the exposure wavelength range. Optical systems are approaching their limits. In addition, as a matter of course, investment in these exposure apparatuses and exposure processes also increases, and it is difficult to reduce costs.

On the other hand, a method of improving resolution, depth of focus, and the like by processing a mask or installing a light shielding plate in an optical system of an exposure apparatus can use the same light source wavelength and lens as before, and By utilizing the peripheral processes such as resist, it can be developed without a significant increase in cost, and is advantageous, but has the following problems. In the first method, although improvements are observed in both characteristics, the effect cannot be obtained unless a regular repetitive pattern is obtained. In addition, a pattern not present in the mask pattern may be transferred due to phase inversion at the shifter end. is there. Furthermore, when laying out an actual circuit, it is necessary to arrange shifters so that a good effect can be obtained in a complicated circuit pattern, and this work becomes extremely complicated, and depending on the circuit configuration, the shifter layout may be required. Inconsistency and it is difficult to arrange shifters.
This is a major obstacle for practical use. In addition, even when the shifter is formed at any position above or below the opaque pattern in the production of the mask, a superposition process which is not included in the conventional mask production process is required, which requires high precision and increases the number of processes. In contrast to the conventional Cr film, the inspection and correction of defects are performed on the transparent film, unlike the conventional Cr film, so that a practical method for inspection and correction has not been obtained.

The second method is useful for improving the resolution of a single portion. However, the second method requires a light transmitting portion (referred to as an auxiliary pattern) in which the phase of the transmitted light formed on both sides is inverted by 180 °, so that the pattern layout is also required. Is large, and depending on the size of the auxiliary pattern, the pattern may be transferred, which is a problem. The third method is advantageous in that the characteristics can be improved simply by arranging a light-shielding plate in the illumination system using a conventional ordinary mask without forming a shifter or the like, but the applicable pattern is limited to a regular repetitive pattern. In addition, the pattern has a layout direction dependence in which a good effect can be obtained only in a limited layout direction, and the effect is not effective for all patterns and layout conditions, which is a practical problem. .

In view of the above problems, the present invention can improve the depth of focus of exposure light and the resolution of a fine pattern without improving the wavelength of a projection type exposure apparatus and the NA of a lens. An object of the present invention is to provide a photomask and an exposure method using the same.

According to the present invention, a light-shielding film having one or a plurality of openings on a transparent substrate, a transparent film formed on the entire surface including the light-shielding film, and a semi-transparent film formed thereon After the transparent film passes through the transparent film from the transparent substrate through the opening of the light-shielding film, the light is reflected by the semi-transparent film and travels back and forth in the transparent film and light that is not reflected by the semi-transparent film. Provided is a photomask having a film thickness capable of making the phase difference of 180 °. That is, in a conventional photomask using diffraction,
Although the resolution of the pattern has an eye field, in the present invention,
Use a photomask with even finer openings
Technology that can increase the resolution of one pattern
Can be

The transparent substrate can be formed of, for example, glass, quartz, fused quartz, synthetic quartz, fluorite, or the like.
The light-shielding film is for shielding the light incident from the substrate side other than the opening and for reflecting the light incident from the opening, passing through the transparent film, reflecting on the translucent film, and again passing through the transparent film. Therefore, a film having a predetermined thickness such as Cr, Ta, Mo, W, and Ti can be used. This film thickness is a size necessary for light shielding, and is usually 0.05 to 0.25 μm.
It is.

The transparent film is a light that passes through this film without being reflected by the lower translucent film, and a light that is reflected once by the lower translucent film and is reflected by the upper light shielding film and passes through this film. It is preferable to set the film thickness so as to interfere with the phase difference of 180 °. This film thickness is substantially equal to the formula I

[0012]

(Equation 1) (Where t is the film thickness, m is a positive odd number, and λ is light (exposure light)
, N is the refractive index of the transparent film, and θ is the angle of incidence of light on the transparent film). The light is more effective when the light is obliquely incident on the transparent film surface of the opening. Considering the resolution pattern size of 0.3 μm or less at the wavelength of the i-line, the incident angle θ is usually 2 μm.
Although it is set to １１11 °, it is also effective at a direct incidence of 0 ° to 2 °.

In the present invention, when a light-shielding film having a plurality of openings is used, it is preferable to form another translucent film before forming the light-shielding film on the transparent substrate. The semi-transparent film (upper) is for reflecting light that enters from the opening of the light-shielding film, passes through the transparent film, is reflected translucently (below), and again passes through the transparent film. , Ta,
Mo, Al or the like having a predetermined thickness can be used. This film thickness is usually 0.03 to 0.12 μm.

The photomask of the present invention can be used, for example, by disposing it in a projection exposure apparatus as follows. That is, the light on the light source side of the projection type exposure apparatus is sequentially exposed to the photosensitive material on the substrate through the light shielding plate, the condenser lens, the photomask according to claim 1 and the projection lens.

The light-shielding plate receives light from the light source side, blocks light toward the center of the condenser lens and transmits light toward the periphery, and transmits light mainly received by the condenser lens to the periphery. it can be incident obliquely to the click. When the light-shielding plate is expressed by a coherent factor σ used in a projection type exposure apparatus, the central region where a light-shielding portion is normally formed is 0.1 mm.
The radius is 2 to 0.5, and the peripheral region forming the light transmitting portion has an inner diameter of 0.2 to 0.5 and an outer diameter of ≧ 0.3. Furthermore,
The light-shielding plate is provided on the pupil plane of the condenser lens so that light from the light source
Even if it has at least one set of openings that only transmit
Good.

[0016]

In the case of a single pattern, if the pattern becomes fine and the light intensity difference (contrast) of the transfer mask pattern on the wafer surface decreases, the pattern according to the mask cannot be resolved as a matter of course. This point, as mentioned earlier,
Even if a shifter is formed in the transparent portion adjacent to the mask and the phase of the exposure light is inverted by 180 °, or a method in which a light-shielding plate is provided in the light source system cannot be improved. Therefore, improvement of the resolution for a single pattern by the mask of the present invention will be described.

FIG. 4A is a sectional view of a conventional photomask. 1 is a glass substrate, 2 is a Cr film as a light shielding film, and the exposure light is diffracted by a light transmitting portion 3 as shown in FIG.
Wafer 5 by a projection lens with three light beams of 0-order light and ± 1st-order light
It is imaged on the upper side (the first-order diffracted light is shown by a broken line in the figure). In addition, first-order and higher-order diffracted lights also occur, but they hardly contribute to imaging. FIG. 4C shows a case where the pattern becomes a finer pattern. ± 1 diffracted by the light transmitting part 3
The next light cannot pass through the projection lens (vignetting on the pupil of the projection optical system), and this fine pattern cannot be resolved. Therefore, the coherence factor (σ) of the exposure optical system is adjusted to reduce the coherency of the exposure light, or the oblique incidence component of the exposure light entering the mask is reduced by disposing an appropriate light shielding plate in the illumination optical system. Fig. 4 (d)
As shown in (2), the resolution of a fine pattern that cannot be resolved by normal direct incidence is attempted by two-beam image formation using the 0-order light and one of the first-order diffracted lights (the other first-order diffracted light is vignetting on the pupil). However, when the phase of the exposure light is inverted by 180 ° in the adjacent light transmitting portions as in the regular repeating pattern shown in FIG. 4E, the 0th-order light and the + 1st-order diffracted light (for example,
(Referred to as + 1st-order diffracted light). Each of them interferes with each other to effectively reduce the diffraction angle determined by the pitch of the fine pattern, thereby obtaining the effect of oblique incidence. However, this effect cannot be achieved with a single pattern because there is no adjacent pattern. .

FIG. 1A is a sectional view of a mask according to the present invention. In order to improve the resolution of a fine pattern even in a single pattern, in addition to the usual photomask structure shown in FIG. 4, another layer is formed on the Cr film and on the entire surface of the glass substrate.
A transparent film 6 is formed, and a translucent film 7 is further provided thereon. For this reason, as shown in FIG. 1B, the 0th-order light and the + 1st-order light diffracted by the single light transmitting portion 3 are each a Cr film and a translucent film within a transparent region formed on the glass substrate. Can be regarded as a pseudo-repeated pattern by multiple reflections between. Further, if the transparent region is formed with a film thickness such that the phase is inverted by 180 ° in one reciprocation, the interfering light reduces the effective diffraction angle similarly to the description of the repetitive pattern in FIG. And a good fine pattern resolution effect can be obtained even in a single pattern.

Next, the contents will be described with reference to the light intensity distribution on the wafer surface. FIG. 2 shows a case where an oblique incidence effect is applied to a normal photomask, and FIG. FIG. 2B is a schematic view of the amplitude distribution on the wafer surface, and FIG. 2C is a conceptual diagram of the light intensity distribution on the wafer surface. As described above, only oblique incidence results in a light hardness distribution that extends to the light-shielding portion, and a fine pattern cannot be resolved. On the other hand, FIG. 3 shows a system utilizing multiple reflection according to the present invention ( shown in the same configuration as FIG. 2 ) . The phase of the adjacent light beam is inverted by 180 °, and the transmittance is reduced at every reflection, so that the amplitude distribution conceptually becomes as shown in FIG. 3B. The light intensity distribution shown in FIG. 3C in which the spread is significantly suppressed as compared with the mask is obtained, and good resolution of the ultrafine pattern can be achieved. (However, although the light intensity is slightly reduced, there is a problem in contrast.) No). In FIG. 3B, a is the light reflected once, b
Is the light that has never been reflected, and c is the amplitude of the light that has been reflected twice.
Each distribution is shown. In FIG. 3C, d is
In the case where there is no translucent film (conventional), e indicates the case where a translucent film is provided.
3 shows an intensity distribution of the present invention (the present invention).

FIG. 5 shows a case of a regular repeating pattern.
(A). In the case of a repetitive pattern, see FIG.
As shown in (e), even in the ordinary photomask, the phase of the light from the adjacent light transmitting part 3 can be improved by inverting the phase by 180 ° by increasing the oblique incident component. In the method of forming the film 6 and the translucent layers 7 and 8 and using multiple reflections, the phase is inverted by 180 ° by one round trip reflection, so that the multiple reflection light and the phase of the input light or the first-order diffraction light are adjacently transmitted. Are matched, and the multiple reflection lights of the same phase are superimposed (a schematic diagram is shown in FIG. 5B), and FIG.
The resolution can be improved in the same manner as described above. Therefore, even when a repeated pattern and a single pattern coexist, the mask of this method is useful for improving the resolution.
However, in the case of a repetitive pattern, it is desirable that the translucent film 8 be formed in advance also below Cr of the light shielding film as shown in FIG.

As shown schematically in FIG. 4 (b), the depth of focus (DOF) is 0. +-.
An image of three light beams of the next light is formed, and an optical path difference occurs between the 0th light and the ± 1st light, so that the depth of focus is reduced. On the other hand, for example, as schematically shown in FIG. 4D, in the method using the oblique incident component including the present invention, the two-beam image formation of the zero-order light and one of the first-order diffracted lights (the other one). The first-order diffracted light is vignetting on the pupil)
Thus, a good depth of focus can be obtained with almost no optical path difference between the two light beams.

Although the above description has focused on the characteristics of the exposure light due to the oblique incidence component, the effect of improving the characteristics of the mask of the present invention is not limited to the oblique incidence exposure light. As described above, even in an optical system in which direct incidence is dominant, good resolution and an improved depth of focus can be obtained by utilizing multiple reflection of ± 1st-order diffracted light diffracted at a predetermined angle by an aperture pattern.

[0023]

Embodiments of the present invention will be described below. Embodiment 1 FIG. 1A shows a sectional structure of a mask having a single pattern as a first embodiment of the present invention. 1 is a glass substrate, 2 is a light-shielding film of Cr, and the production is 2 C on 1 glass substrate.
After the formation of the r film, the Cr film is subjected to EB exposure and etching into a predetermined shape to obtain 3 light transmitting portions. Thereafter, a transparent film 6 is formed on the entire surface of the glass substrate. In this embodiment, SOG (spin-on-glass) is used as a material for forming the transparent film, and the transparent film is formed at 570 nm.
Formed to a thickness of Further, i-line (wavelength (λ): 365 nm) was used as the exposure light, and the refractive index (n) for the exposure light was 1.45. Finally, a translucent film 7 was formed on the transparent film with Cr having a thickness of 50 nm in this embodiment. In addition, as a material of the transparent film 6, SiO ₂ or the like formed by sputtering is also effective. The thickness of the translucent film is optimized by improving characteristics and appropriate light intensity (exposure amount).
It is desirable to be about 0 nm or less. Depending on the resolution pattern size, the translucent film 8 is also formed above the light shielding film 6 as shown in the example of the repetitive pattern in FIG. The thickness (t) of the transparent film 6 needs to be set so that the phase of the multiple reflected light changes 180 ° in one reciprocation with respect to the light traveling straight without being reflected, as shown in the schematic diagram of FIG. If the optical path length on one side at this time is L,

[0024]

(Equation 2) In this example, the design value was set to 9λ / 2, and a design value of L = about 575 nm was obtained. This L is an optical path based on an oblique incident component as shown in FIG. 1B and the like, and an accurate appropriate film thickness can be obtained from the optical path L and the incident angle. In this example, an incident angle θ of about 8.5 ° was obtained based on the pattern size on the mask and the exposure wavelength in consideration of application to a pattern of ≦ 0.3 μm. Here, the film thickness (t)
Is determined by t = Lcos θ (4), and t = about 570 nm was obtained.

In this embodiment, the mask of the present invention manufactured by the above steps is applied to a projection type exposure apparatus in which the coherent factor (σ) of the exposure optical system is σ = 0.6. The excellent contrast characteristics as shown were achieved, a good resolution pattern was obtained, and the depth of focus was improved. Further, the σ value of the exposure optical system is desirably about 0.3 or more.

Embodiment 2 As a second embodiment of the present invention, an example of application to a regular repeating pattern will be described. FIG. 5A is a cross-sectional view of the mask.
Although the method of manufacturing the mask and the method for exposure are almost the same as those of the first embodiment, in the case of a repetitive pattern, the translucent film 8 is necessary even before the formation of the transparent film 6, and A Cr film of about 50 nm is formed similarly to the translucent film. Further, in order to improve the controllability and reproducibility of the thickness of the lower Cr film 8, it is effective to form an etching stop layer 9 made of a material such as ITO as shown in FIG.

Embodiment 3 As a third embodiment of the present invention, as shown in FIG. 6, a light shielding plate 10 is arranged almost on the pupil plane of the illumination optical system to increase the oblique incidence component of the exposure light to the mask. An exposure method in which the mask of the present invention is applied to a projection type exposure apparatus having an exposure optical system. As the light-shielding area, an optical system that circularly shields the area a corresponding to a radius of 2/3 from the center with respect to the opening area of a = 0.6 and the area near a = 0.4 in both x and y directions from the center. An optical system provided with two sets of apertures having a radius of about 0.2 was used. By increasing the oblique incidence component, the contrast was further improved, and the resolution and the depth of focus were improved.

The effect of the present invention is independent of the exposure wavelength.
In the above embodiment, an example in which the i-line (365 nm) is used as the exposure wavelength has been mainly described. However, the present invention is not limited to this wavelength and the g-line (436n) is used.
m), KrF excimer laser light (248 nm), ArF excimer laser light (193 nm), and the like can provide similar results.

[0029]

According to the photomask and the exposure method of the present invention, a significant improvement in pattern resolution and depth of focus can be obtained as compared with the conventional photomask. In comparison with the improvement method using a phase shift mask that controls the temperature and the improvement method using a two-beam image by disposing a light-shielding plate etc. in the exposure optical system, especially for a single pattern, both resolution and depth of focus are excellent. The improvement effect has been achieved. In addition, the mask manufacturing process is relatively simple, is close to the conventional photomask manufacturing process, and can simplify the process greatly without the complexity of the shifter arrangement as compared with the phase shift mask. Is as high as a conventional photomask, and is extremely useful in practice.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a photomask manufactured in an embodiment of the present invention.

FIG. 2 is an explanatory diagram of image formation using a conventional photomask.

FIG. 3 is an explanatory diagram of image formation using a photomask manufactured in an embodiment of the present invention.

FIG. 4 is an explanatory diagram of a conventional photomask.

FIG. 5 is an explanatory diagram of a photomask manufactured in an example of the present invention.

FIG. 6 is an explanatory diagram of an exposure method performed in an embodiment of the present invention.

FIG. 7 is an explanatory diagram of multiple reflection in a transparent film of a photomask manufactured in an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Glass substrate (transparent substrate) 2 Light shielding film 3 Opening (light transmission part) 4 Projection lens 5 Wafer 6 Transparent film 7,8 Translucent film 9 Etching stop film 10 Light shielding plate 11 Condenser lens 12 Photo mask

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-101148 (JP, A) JP-A-6-161092 (JP, A) JP-A-2-248949 (JP, A) JP-A-62-162 67547 (JP, A) JP-A-4-40455 (JP, A) JP-A-4-80756 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G03F 1/08-1 / 16 H01L 21/027

Claims (3)

(57) [Claims] 1. A light-shielding film having one or a plurality of openings on a transparent substrate, a transparent film formed on the entire surface including the light-shielding film, and a translucent film formed on at least an upper surface of the transparent film. After the transparent film passes through the transparent film from the transparent substrate through the opening of the light-shielding film, the light reflected by the translucent film and reciprocated in the transparent film once and the light not reflected by the translucent film. A photomask having a film thickness capable of setting a phase difference to 180 °. 2. The photomask according to claim 1, wherein the opening of the light shielding film is a single hole, dot, space, line, or irregular pattern. The 3. A light source side of the projection exposure apparatus, the light shielding plate in order, a condenser lens, one of photomasks of claim 1 or 2, and the photosensitive material on the substrate through a projection lens exposure An exposure method, wherein the light shielding plate transmits light from a light source to a center of a condenser lens.
It has a configuration to block light in the part and transmit light in the peripheral part
Or from the light source on the pupil plane of the condenser lens.
At least one set of apertures that transmit only part of the light
Exposure method is characterized in that it is incident light received in the peripheral portion of the main condenser lens through the light shielding plate obliquely to the photomask.