JP2566567B2 - Pattern forming method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a pattern forming method using a projection exposure method in the production of, for example, semiconductor elements, magnetic bubble elements, superconducting elements, and the like. A pattern forming method that is particularly effective for forming a fine pattern by an exposure method will be described.

[Conventional technology]

A projection exposure method is widely used for forming various fine patterns such as those in the surroundings such as semiconductor devices and magnetic bubble memory devices. The projection exposure method, especially the reduced shadow exposure method is useful for forming a fine pattern. In the projection exposure method, the resolution is improved by increasing the numerical aperture of the lens and shortening the exposure wavelength. However, in the conventional projection exposure method, the depth of focus of the exposure optical system strongly depends on the numerical aperture of the projection lens and the exposure wavelength. The depth of focus of the projection lens is inversely proportional to the square of its numerical aperture, and is proportional to the exposure wavelength. Therefore, if the numerical aperture is increased or the wavelength is shortened to increase the resolution, the depth of focus becomes shallower accordingly. turn into. For this reason, it is becoming increasingly difficult to cope with obstacles caused by image plane distortion of the projection lens and uneven steps on the substrate surface. The obstacle caused by the step due to the relatively fine pattern has been dealt with by the smoothing by the well-known multi-layer resist method, but even with this method, the step caused by the large area pattern is completely flattened. This is not possible, and it is inevitable that defective imaging will occur above or below the step.

The multilayer resist method is described in JP-A-51-107775.

[Problems to be solved by the invention]

With the recent high integration of semiconductor integrated circuits, the miniaturization of patterns and the unevenness of irregularities on the substrate surface are remarkably increased, and it is required to cope with them. When using the projection exposure method for pattern formation, in order to cope with an increase in uneven steps,
A deeper depth of focus is required for the exposure optical system. However, in order to improve the resolution, it is necessary to increase the numerical aperture of the projection lens, so the depth of focus is conversely shallow. Further, since the image plane is not a perfect plane due to the image plane distortion of the projection lens, it is becoming difficult to secure the depth of focus corresponding to the uneven surface step over the entire exposure area.

In the above-mentioned conventional technique, it is not possible to completely flatten the unevenness caused by the large area pattern, and even if perfect flattening is achieved, the image plane of the mask pattern is formed over the entire exposure region due to the image plane distortion of the lens. It was not possible to match the substrate surface, and it was difficult to address the above problems.

An object of the present invention is to provide a pattern forming method capable of forming a fine pattern with sufficiently high accuracy even if a step exists on the surface. Another object of the present invention is to prevent a substantial decrease in the depth of focus of the optical system even when the numerical aperture of the lens is large and the exposure wavelength is short, and to form an image defect over the entire exposure region regardless of the presence or absence of a step. It is to form a good fine pattern with no defects.

[Means for solving problems]

The above object is achieved by forming a reversible bleaching film on a photoresist, and then performing multiple exposure by changing the position of the image plane in the optical axis direction. Here, the reversible light selective transmission film (bleaching film) is opaque to the exposure light when not exposed, but becomes transparent by the irradiation of the exposure light and is opaque when the irradiation of the exposure light is stopped. A film having a property of returning to a film.

[Action]

When forming a fine pattern, the light intensity distribution of a so-called defocused image at a position distant from the image forming position has a gentle mountain shape with a wide skirt, and its peak intensity is the peak intensity of the image at the image forming position. Compared to.

The reversible bleaching film blocks light below a certain amount,
When it exceeds a certain threshold, it has the property of becoming transparent and transmitting light. Further, since it has reversibility, when the light irradiation is stopped, it returns to the initial opaque state and is reset. Therefore, if the image formation position is changed, multiple exposure is performed, and the amount of light is appropriately selected, the defocused image is shielded by the bleaching film, the photoresist is not exposed, and the image is in focus.
Only the so-called best focus image passes through the bleaching film to expose the photoresist. Due to the reversibility of the bleaching film, the history of exposure due to the defocused image does not remain, and its influence can be removed. Even if there is a step on the surface and the focus position differs depending on the location, multiple exposure is performed by changing the image formation position, so the image of the best focus is formed in either exposure, and only that image is selectively Since the photoresist is exposed, it is possible to solve the problems of defocusing due to the substrate step and defocusing due to the lens surface distortion.

〔Example〕

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

As shown in FIG. 1 (a), a photoresist was applied on a substrate 1 having a step on the surface to form a photoresist layer 2. Thereafter, as shown in FIG. 1B, a light selective transmission film (bleaching film) 3 was formed on the photoresist layer 2. The light selective transmission film 3 was formed by spin coating a solution of 4-dimethylamino-4'-nitroazobenzene dissolved in an aqueous solution of polyvinyl alcohol. The film thickness is about 0.
Although the thickness is 5 μm, the thickness is not limited to this as long as the bleaching effect can be obtained. Then, the pattern was exposed by using a projection exposure apparatus (not shown), and then developed to form a resist pattern 4 as shown in FIG. 1 (c). This exposure was performed as follows. First, the stage on which the substrate was fixed was moved so that the image plane was on the convex surface 1'of the substrate 1, and the first exposure was performed at that position. Next, substrate 1
The stage was moved in the direction of the optical axis so that the image plane was located on the concave surface of 1 ″, and the second exposure of the above pattern was performed. Hitachi RA101H is used as the projection exposure apparatus.
An L-type reduction projection exposure device was used. The exposure wavelength is 436 nm.

By this method, even if there is a step of 2 μm or more, a fine pattern of 0.7 μm both above and below the step can be formed without deterioration of shape. On the other hand, in the conventional method, when the step exceeds about 1.5 μm, the resolution is poor.

It should be noted that in the above embodiment, TS is used as a photoresist.
MR8800 (trade name of Tokyo Ohka Kogyo Co., Ltd.) was used, but not only this, MP1400 (trade name of Shipley company), OFPR5000
(Tokyo Ohka Kogyo Co., Ltd. product name), AZ1300SFD (Hochst company product name), Kodak809 (Kodak product name) PMMA, PGMA, PMIPK
A variety of positive resists such as RD2000N, polyvinylphenol negative resists such as RUI1000N (Hitachi Chemical Co., Ltd. product name), cyclized rubber negative resists such as CBR (Nippon Synthetic Rubber Co., Ltd. product name), etc. A resist can be used. Further, a three-layer resist structure and a two-layer resist structure are also effective. Although 4-dimethylamino-4'-nitroazobenzene was used as the dye of the light selective transmission film in this example, it is not limited to this material, and 3-methylamino-4-nitroazobenzene, 4-nitroazobenzene, 4-nitroazobenzene, Any reversible bleaching material such as an azobenzone derivative such as dimethylaminoazobenzene or 3-methyl-4-dimethylamino-4′-nitroazobenzene or a spiropyran derivative may be used. Further, although the dye was dissolved in water in this example, it is not limited to water, and a mixture of propanol and water, a mixture of propanol, methylcyclohexane and toluene, a mixture of methylcyclopentane and methylcyclohexane, and the like can also be used.
When these solvents erode the photoresist, for example, polysiloxane in the case of propanol and perfluoropolyether in the case of other solvents, an intermediate film that is insoluble in the above solvent and does not erode the photoresist is used as the photoresist and the light. By providing it between the selectively permeable membranes, the problem of erosion can be completely eliminated. Also, the interval between the first exposure and the second exposure was about 0.5 seconds or more,
This is a value determined by the time required for the light transmittance of the dye to return to the initial state, and depends on the material. The temperature may be raised to shorten the time.

In this example, after exposure, development was performed to form a pattern. This is because an aqueous solution of tetramethylammonium salt was used as the developer and a water-soluble film was used as the light selective transmission film, so that the light selective transmission film could be automatically removed during development. When the light selective transmission film cannot be removed with a developing solution, it is necessary to develop after removing the light selective transmission film. Although the numerical aperture of the projection lens is set to 0.38 in this embodiment, a remarkable effect is recognized even if the numerical aperture is changed.

In this embodiment, the exposure wavelength is 436 nm, but any wavelength that causes bleaching in the light selective transmission film can be used without being limited to this value. For example, when a light selective transmission film containing 4-dimethylaminoazobenzene as a dye is used, the exposure wavelength can be 405 nm.

Example 2 As shown in FIG. 2 (a), a photoresist was applied on a substrate 11 having steps to form a photoresist layer 12.
Thereafter, as shown in FIG. 2 (b), polysiloxane was applied to form an intermediate layer 13. After that, a light selective transmission film 14 was formed as shown in FIG. 4-Nitroazobenzene was used as the dye of the light selective transmission film, propanol was used as the solvent, and the light selective transmission film 14 was formed by spin coating.
After that, projection exposure was performed using a XeCI excimer pulse laser. The exposure wavelength is 308 nm. This exposure was performed as follows. First, the main plane 15 of the substrate surface is set 10 μm below the image plane of the projection optical system (in a direction away from the projection optical system) to perform exposure, and then the stage with the substrate fixed is moved about 1 μm along the optical axis. Each of them was moved upward, and exposure was performed each time. This operation was continued until the principal plane of the substrate surface was 10 μm above the image plane.

Then, the light selective transmission film 14 was removed using propanol, the intermediate layer 13 was subsequently removed using xylene, and then development was performed to form a resist pattern 16 as shown in FIG. 2 (d).

In this embodiment, the substrate step is 3 μm at the maximum, the image plane distortion of the projection optical lens is about 2 μm at the maximum, and the tilt of the substrate in the exposure plane is about 1 μm at the maximum. The difference in position on the substrate surface was about 6 μm. Regardless of this difference in position, a sharp, fine, and highly accurate pattern could be obtained over the entire exposed surface. For example 0.45
Dimensional accuracy of 0.1 μm for line and space patterns of μm
Could be formed with. On the other hand, according to the conventional method, if the substrate step exceeds about 2 μm, the resolution is poor at the step above or below, and it is not possible to form a pattern. A poor resolution occurred in the department. When this method is used, the light selective transmission film is a CEL film (Contras
Since it also functions as a t Enhancement Layer), the pattern shape becomes steeper than the conventional method. For example 0.5
The cross-sectional inclination angle of the μm pattern has been improved from 82 ° of the conventional method to 87 °.

Since this method causes reversible bleaching, the sensitivity of the photoresist and the bleaching characteristics can be matched by setting the exposure amount and the number of exposures, and the efficiency as a CEL film was also high. Although the defocused pattern was also exposed, a good pattern could be formed without being adversely affected by the reversibility of the bleaching characteristics of the light selective transmission film.

In this example, the substrate was moved up to 21 μm in the optical axis direction. If a large amount of movement is used, a good pattern can be formed without knowing the level difference of the substrate, the image plane distortion of the lens, the amount of tilt of the substrate, etc., but it takes time to expose. Therefore, exposure is performed by adjusting the stage position so that the highest position on the substrate surface is about 1 μm above the image plane, and then the substrate is moved about 1 μm along the optical axis until it is near the lowest position on the substrate surface. It was moved upward and exposed each time. In this case, the sum of the movement amounts of the substrates was about 5 μm, and accordingly, the time required for exposure was reduced to about 1/4 as compared with the above case. It should be noted that although the position movement amount of the substrate is set to 1 μm each, the present invention is not limited to this, and the same effect can be obtained if it is within the depth of focus of the projection optical system. Further, the position may be swept continuously instead of stepwise. However, if the value is larger than the depth of focus, poor resolution occurs at a part of the step.
For example, when the position movement step is set to 2 μm, resolution failure occurs at the substrate step 3 μm.

In this embodiment, the relative position between the image plane and the substrate surface is changed by moving the position of the stage on which the substrate is placed. Not limited to this method, the reticle on which the mask pattern exists is moved in the optical axis direction, a substance having a refractive index different from that of air is inserted into the exposure optical system, and the atmospheric pressure of a part including the whole or a part of the exposure optical system , Using a multifocal lens, superimposing light from a plurality of optical systems with different set image planes, and exposing with a plurality of different or continuous wavelengths of light using the same optical system. May be used to change the relative position of the imaging surface and the substrate surface.

In this example, 4-nitroazobenzene was used as the light selective transmission film, but the present invention is not limited to this, and azobenzene-based derivatives such as azobenzene and spiropyran-based derivatives were also effective.

Example 3. A well-known three-layer resist structure was formed by forming an organic film as a lower layer film on a substrate having a steep step of 10 μm, forming an inorganic film on the lower film, and applying a photoresist thereon. . Thereafter, as in Example 2, an intermediate film and a light selective transmission film were formed, exposed to light, sequentially removed from the light selective transmission film and the intermediate film, and then developed to form an upper resist pattern. Then, anisotropic etching was performed to form a three-layer resist pattern.

By this method, it was possible to form a pattern of 0.5 μm both above and below the step including the step portion of the substrate. on the other hand,
In the conventional method, the pattern could be formed only above or below the step.

As the organic film for the lower layer film, RG390OB (trade name of Hitachi Chemical Co., Ltd.) spin-coated and heat-treated at about 200 ° C. was used, but not limited to this. Any film that can be used as a lower layer film in a three-layer resist method such as a film may be used. Although the inorganic film is an intermediate film using SOG (S pin o n G lass ), used is not limited to this as long as the material functions as an intermediate layer of CVD SiO _2, sputtering SiN, TiOx such as a three-layer resist method be able to. Further, not only the three-layer resist method but also the two-layer resist method has the same effect.

Example 4. 0.35 μ was formed on the flat substrate by the same method as in Example 2.
A line% space pattern of m was formed. However, the stage feed was about 0.5 μm each.

In the present embodiment, the pattern could be formed over the entire exposed area, but due to image surface distortion, the conventional method could only form the pattern in about 70% of the area. Further, since the focus latitude is expanded by the amount of moving the position of the substrate by using this method, it is possible to secure the focus latitude as much as possible.

Example 5 This method was applied to the step of forming fine auxiliary holes for conduction and the wiring step of a device having a step of about 2 μm. With this application, it is possible to completely prevent conduction failure, wiring disconnection and short circuit,
Yield improved from about 60% to about 80%.

〔The invention's effect〕

As is clear from the above description, according to the present invention, the effective depth of focus in the projection exposure method can be increased, so that it is possible to cope with the high numerical aperture of the projection lens, the image plane distortion, and the increase in the unevenness of the surface of the substrate. It is possible to The amount of increase in the focus latitude changes depending on how much the relative position between the image plane and the substrate surface is changed for exposure, but the focus latitude can be increased by increasing the amount of change. Therefore, the pattern can be formed even when there is a substrate step difference of 10 μm.

[Brief description of drawings]

1 and 2 are process diagrams showing different embodiments of the present invention. 1 ... Substrate, 2 ... Resist layer, 3 ... Light selective transmission film,
4 ... resist pattern, 11 ... substrate, 12 ... photoresist layer, 13 ... intermediate layer, 14 ... light selective transmission film, 15 ...
Main surface of the substrate, 16 …… resist pattern.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor, Akio Taniguchi, 1-280, Higashi Koigakubo, Kokubunji City, Hitachi Research Laboratory (72) Inventor, Toshiharu Matsuzawa, 1-280, Higashi Koigakubo, Kokubunji Ltd., Central Research Laboratory, Hitachi, Ltd.

Claims (5)

(57) [Claims] 1. A step of projecting and exposing a resist film through a mask pattern having a desired shape, and a step of developing the resist film to form a resist pattern, wherein the projection exposure is performed in the optical axis direction. An optically reversible light-selective transmission film laminated on the resist film is formed so that the image planes of the mask patterns are formed at a plurality of positions relative to the resist film. And the plurality of desired regions of the resist film are selected by the projection exposure in which the imaging planes are formed at the desired regions or at positions near the desired regions in the optical axis direction. Pattern forming method characterized in that the pattern is exposed to light. 2. The pattern forming method according to claim 1, wherein the resist film is formed on a surface of a substrate having a step. 3. The pattern forming method according to claim 2, wherein the projection exposure is performed such that the image forming planes are formed at the top and bottom of the step or in the vicinity thereof, respectively. 4. The pattern forming method according to claim 1, wherein the light selective transmission film contains an azobenzene-based derivative or a spiropyran-based derivative. 5. The relative position of the image plane with respect to the resist film in the optical axis direction is controlled by moving the resist film in the optical axis direction. The pattern forming method according to any one of items 1 to 4.