JPS63177420A - Pattern forming method - Google Patents

Abstract

PURPOSE:To form a fine pattern with sufficiently high accuracy even if a step exists on its surface by projecting to expose a plurality of positions on the relatively different optical axes of a mask pattern focused surface and a resist film, and photosensing the resist only by exposing at near relative positions. CONSTITUTION:A plurality of positions on the different optical axes of a mask pattern focused surface and a resist film are projected to be exposed, and a resist is selectively photosensed only by the exposure at the near relative positions of the focused surface and the resist film face. For example, a photoresist layer 2 is formed on a substrate 1 having a step on its surface, and a light transmissible selective film 3 which has properties to be opaque to an exposed light at the time of unexposure and transparent upon irradiation of the exposed light and returns to an opaque film upon interruption of the exposed light is formed. The pattern is exposed by a projecting exposure device, and then developed to form a resist pattern 4. In this exposure, first exposure is conducted by bringing the focused surface to the protruded surface 1' of the substrate 1, and second exposure is conducted by then bringing the focused surface to the recess surface 1'' of the substrate 1.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a pattern forming method using a projection exposure method in the production of, for example, semiconductor devices, magnetic bubble devices, superconducting devices, etc. The present invention provides a pattern forming method that is particularly effective for forming fine patterns by exposure.

[Conventional technology]

As is well known, projection exposure methods are widely used to form various fine patterns included in semiconductor devices, magnetic bubble memory devices, and the like. Projection exposure methods, particularly reduction projection exposure methods, are useful for forming fine patterns. In the projection exposure method, resolution is improved by increasing the numerical aperture of the lens and shortening the exposure wavelength.

However, in conventional projection exposure methods, 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 a projection lens is inversely proportional to the square of its numerical aperture and proportional to the exposure wavelength, so if you increase the numerical aperture or shorten the wavelength to increase resolution,
As a result, the depth of focus becomes shallow. For this reason, it is becoming increasingly difficult to deal with obstacles caused by image plane distortion of the projection lens and irregularities on the surface of the substrate.

Problems caused by steps caused by relatively fine patterns have been dealt with by smoothing using a well-known multilayer resist method. However, even if this method is used, it is not possible to completely flatten the level difference caused by the large-area pattern, and it is inevitable that imaging defects will occur above or below the level difference.

Regarding the multilayer resist method, please refer to Japanese Patent Application Laid-Open No. 51-1077.
It is described in issue 75 etc.

[Problem that the invention seeks to solve]

As semiconductor integrated circuits have become more highly integrated in recent years, patterns have become finer and the unevenness of the substrate surface has increased significantly, and measures to cope with these problems are required. When a projection exposure method is used to form a pattern, the exposure optical system needs to have a deeper depth of focus in order to cope with an increase in uneven steps. However, in order to improve the resolution, it is necessary to increase the numerical aperture of the projection lens, so the depth of focus becomes shallower.

Furthermore, because the image plane is not completely flat due to image plane distortion of the projection lens, it has become difficult to ensure a depth of focus over the entire exposure area in response to the unevenness of the surface.

With the above-mentioned conventional technology, it is not possible to completely flatten the unevenness caused by a large-area pattern, and even if complete flattening is achieved, the image plane of the mask pattern cannot be completely flattened over the entire exposure area due to the image plane distortion of the lens. It was difficult to deal with the above problem because it could not be made to match the surface of the substrate.

With the above-mentioned conventional technology, it is not possible to completely flatten the unevenness caused by a large-area pattern, and even if complete flattening is achieved, the imaging plane of the mask pan is not completely flattened over the entire exposure area due to the image plane distortion of the lens. It was difficult to deal with the above problem because it could not be made to match the surface of the substrate.

An object of the present invention is to provide a pattern forming method that can form fine patterns with sufficiently high precision even if there are steps on the surface. Another object of the present invention is to prevent the substantial depth of focus of the optical system from decreasing even when the numerical aperture of the lens is large and the exposure wavelength is short, and to prevent image formation defects over the entire exposed area regardless of the presence or absence of steps. The goal is to form a fine pattern with no blemishes.

Means for Solving Problem C] The above object is achieved by forming a reversible bleaching film on a photoresist, and then performing multiple exposures by changing the position of the imaging plane. Here, a reversible bleaching film is opaque to exposure light when unexposed, but has the property of becoming transparent as it is irradiated with exposure light.
It also refers to a film that has the property of returning to its opaque state when irradiation with exposure light is stopped.

[Effect]

When forming a fine pattern, the light intensity distribution of a so-called defocused image at a position far from the imaging position becomes a gentle mountain shape with a wide base, and the peak intensity of the light intensity is the same as that of the image at the imaging position. It decreases compared to the peak intensity of .

A reversible bleaching film has the property of blocking light below a certain amount of light, and becoming transparent and transmitting light when a certain threshold is exceeded. Furthermore, since it has reversibility, when light irradiation is stopped, it returns to its initial opaque state and is reset. Therefore, if multiple exposure is performed by changing the imaging position and the light intensity is appropriately selected, the defocused image will be blocked by the bleaching film, and the foresist will not be exposed to light. The photoresist is exposed to light through the bleaching film. Due to the reversibility of the bleaching film, no history of exposure due to a defocused image remains, and its influence can be removed. Even if the focus position differs depending on the location, multiple exposures are performed by changing the imaging position, so there is an image with the best focus in one of the exposures, and only that image selectively exposes the photoresist, so the substrate Problems with out-of-focus caused by steps and out-of-focus caused by field distortion of the lens can be resolved.

〔Example〕

Example 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 onto a substrate 1 having a step on its surface to form a photoresist layer 2. As shown in FIG. Thereafter, a light selective transmission film 3 was formed on the photoresist layer 2 as shown in FIG. 1(b). The selectively transmitting light membrane 3 was formed by spin coating a solution of 4-dimethylamino-4'-nitroazobenzene dissolved in an aqueous solution of polyvinyl alcohol. Although the film thickness was set to about 0.5 μm, it is not limited to this film thickness as long as a bleaching effect can be obtained.

Thereafter, a pattern was exposed using a projection exposure apparatus (not shown), and then development was performed to form a resist pattern 4 as shown in FIG. 1(c). This exposure was carried out as follows. First, the stage on which the substrate was fixed was moved so that the imaging plane was located on the convex surface 1' of the substrate 1, and the first exposure was performed at that location. Next, the stage was moved in the optical axis direction so that the image plane was positioned above the concave surface 1' of the substrate 1, and a second exposure of the above pattern was performed. The exposures were carried out at intervals of 0.5 seconds or more after the first exposure.

The projection exposure apparatus mentioned above is Hitachi RAIOIHL.
A mold reduction projection exposure system was used. Exposure wavelength is 436nm
It is.

By this method, even when there was a step difference of 2 μm or more, a fine pattern of 0.7 μm both above and below the step could be formed without deterioration in shape. On the other hand, in the conventional method, resolution failure occurred when the step exceeded about 1.5 μm.

In addition, in the above embodiment, TS was used as the photoresist.
MR8800 (trade name of Tokyo Ohka Kogyo Co., Ltd.) was used, but not only this, MP1400 (trade name of Shipley Co., Ltd.), 0FPR5000 (trade name of Tokyo Ohka Kogyo Co., Ltd.), AZ 1300 S F D (product of Hochst Co., Ltd.) were used. name), Kodak809 (Kodak company product name)
Positive resists such as PMMA, PGMA, PMIPK, RD20ON, RUlooON (Hitachi Chemical Co., Ltd.)
Various resists can be used, such as a polyvinylphenol negative resist such as Co., Ltd. (trade name), and a cyclized rubber negative resist such as CBR (trade name, Nippon Synthetic Rubber Co., Ltd.). Also, a three-layer resist structure and a two-layer resist structure are similarly effective. In addition, in the examples, 4-dimethylamino-4'-nitroazobenzene was used as the dye for the light selectively transmitting film, but the material is not limited to this material. Aminoazobenzene, 3-methyl-4-
Any reversible bleaching material such as an azobenzene derivative such as dimethylamino-4'-nitroazobenzene or a spiropyran derivative may be used. Furthermore, although this dye was dissolved in water in this example, it is not limited to this solvent; a mixture of propatool and water, a mixture of propatool, methylcyclohexane and toluene, a mixture of methylcyclopentane and methylcyclohexane, etc. can also be used. can. If this solvent corrodes the photoresist, e.g. polysiloxane for propatool, perfluoropolyether for other solvents, etc., which are not soluble in the above solvent,
Moreover, by providing an intermediate film that does not corrode the photoresist between the photoresist and the selectively transmitting light film, the problem of corrosion can be completely eliminated. In addition, the interval between the first and second exposures was set to approximately 0.5 seconds or more, but this value is determined by the time required for the light transmittance of the dye to return to its initial state, and varies depending on the material. . To shorten the time, just raise the temperature.

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 membrane was used as the selectively transmitting light membrane, so that the selectively transmitting light membrane could be automatically removed during development. If the selectively transmitting light film cannot be removed with a developer,
It is necessary to perform development after removing the selectively transmitting light film. In this example, the numerical aperture of the projection lens was set to 0.38, but significant effects were observed even when the numerical aperture was changed.

Although the exposure wavelength was set to 436 nm in this embodiment, it is not limited to this value and can be used as long as the wavelength causes bleaching of the selectively transmitting light film. For example, when using a light selective transmission film containing 4-dimethylaminoazobenzene as a dye, the exposure wavelength can be set to 4Q5 nm.

Example 2 As shown in FIG. 2(a), a photoresist layer 12 was formed by applying a photoresist onto a substrate 11 having a step. Thereafter, as shown in FIG. 2(b), polysiloxane was applied to form the intermediate layer 13. Thereafter, a light selective transmission film 14 was formed as shown in FIG. 2(c). The selectively transmitting light film 14 was formed by spin coating, using 4-nitroazobenzene as the dye and propatool as the solvent. Thereafter, projection exposure was performed using a XeCl excimer pulse laser. The exposure wavelength was 308 nm.

This exposure was carried out as follows.

First, exposure is performed with the main plane 15 of the substrate surface set 10 μm below the imaging plane of the projection optical system (in the direction away from the projection optical system), and then the stage on which the substrate is fixed is moved approximately 1 μm along the optical axis. The light was moved upward one by one, and each exposure was performed. This operation is performed so that the main plane of the substrate surface is 10μ from the image plane.
Continue until you reach m above.

Thereafter, the selectively transmitting light film 14 was removed using a propatool, and then the intermediate layer 13 was removed using xylene, and then development was performed to form a resist pattern 16 as shown in FIG. 2(d). .

In this example, the substrate height difference is at most 3 μm, the image plane distortion of the projection optical lens is at most about 2 μm, and the substrate inclination within the exposure plane is at most about 1 μm. The difference in surface position was about 6 μm. Regardless of this positional difference, a steep, fine, and highly accurate pattern could be obtained over the entire exposed surface. For example 0.45
Dimensional accuracy of μm line & space pattern ±0.1μ
It was possible to form the m. On the other hand, in the conventional method, the substrate level difference is approximately 2 μm.
Beyond this, poor resolution occurs because the step is below the ground, and not only is it impossible to form a pattern, but also poor resolution occurs in a part of the exposed peripheral area due to image plane distortion even on a flat/flat surface. . In addition, when this method is used, the light selective transmission film is a CEL film (
Contrast Enhancement Layer
), the pattern shape is steeper than that of the conventional method. For example, the cross-sectional inclination angle of a 0.5 μm pattern was improved from 82° in 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, so the efficiency as a CEL film was also high. Although a defocused pattern was also exposed, due to the reversibility of bleaching of the selectively transmitting light film, a good pattern could be formed without any adverse effects.

In this example, the substrate was moved by a maximum of 21 μm.

If the amount of movement is long, differences in the board level, field distortion of the lens,
Although it is possible to form a good pattern even if the amount of tilt of the substrate is not accurately known, it takes time for exposure. Therefore, the highest point on the substrate surface is approximately 1 μm higher than the imaging plane.
The stage position was adjusted so as to be above m, and exposure was performed, and then the substrate was moved upward by about 1 μm along the optical axis until it was near the lowest point on the substrate surface, and exposure was performed each time. In this case, the total amount of movement of the substrate is approximately 5 μ
m, and accordingly, the time required for exposure was reduced to about 1/4 compared to the above case. Although the amount of positional movement of the substrate is set to be 1 μm at a time, the present invention is not limited to this, and the same effect can be obtained if the distance is within the depth of focus of the projection optical system. Further, the position may be swept continuously instead of in a stepwise manner. However, if the value is larger than the depth of focus, poor resolution will occur in a part of the step. For example, when the step of positional movement was 2 μm, poor resolution occurred at a portion where the substrate step was 3 μm.

In this embodiment, the relative position between the imaging plane and the substrate surface was changed by moving the position of the stage on which the substrate was placed. This method is not limited to this method, but includes moving the reticle on which the mask pattern is present in the optical axis direction, inserting a substance with a refractive index different from air into the exposure optical system, and atmospheric pressure in the part containing all or part of the exposure optical system. various methods, such as varying the wavelength, using a multifocal lens, superimposing light from multiple optical systems with different set imaging planes, and exposing the same optical system to light of multiple different or continuous wavelengths. may be used to change the relative position of the imaging plane 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 derivatives such as azobenzene, spiropyran derivatives, and the like were similarly effective.

Example 3 An organic film was formed as a lower layer film on a substrate having a steep step of 10 μm, an inorganic film was formed on top of the organic film, and a photoresist was applied on top of the organic film to form a well-known three-layer resist structure. . Thereafter, in the same manner as in Example 2, an intermediate film and a selectively transmitting light film were formed, exposed, and after sequentially removing the selectively transmitting light film and the intermediate film, development was performed to form an upper resist pattern. Thereafter, 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 substrate step. on the other hand,
With conventional methods, patterns could only be formed above or below the step.

Note that the organic film for the lower layer used was RB3900B (trade name, Hitachi Chemical Co., Ltd.) spin-coated and heat-treated at about 200°C; however, it is not limited to this, and may also be a heat-treated ordinary resist or polyimide. Any material may be used as long as it can be used as a lower layer film in a three-layer resist method, such as a film.

SOG (Spin on
Although a material such as CVD 5in2, sputtered SiN, or TiOx can be used as long as it functions as an intermediate film in a three-layer resist method. Furthermore, not only the three-layer resist method but also the two-layer resist method was similarly effective.

Example 4 0.35° was applied on a flat substrate using the same method as in Example 2.
A μm line and space pattern was formed. However, the stage was moved in increments of approximately 0.5 μm.

Although this method was able to form a pattern over the entire exposed area, the conventional method was only able to form a pattern in about 70% of the area due to image plane distortion. Furthermore, when one method is used, the focus latitude can be adjusted by the amount by which the position of the substrate is moved, so any amount of focus latitude can be secured.

Example 5 This method was applied to the process of forming fine conductive holes and the wiring process of a device having a step of about 2 μm.

By applying this method, it was possible to completely prevent conduction defects, disconnections and short circuits in the wiring, and the yield increased from about 60% to about 80%.

〔Effect of the invention〕

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

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 are process diagrams showing different embodiments of the present invention, respectively. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Resist layer, 3...Selective light transmission film, 4...Resist pattern, 11...Substrate, 1
2... Photoresist layer, 13... Intermediate layer, 14.
・・Light selective transmission film, 15 ・・Major plane of the substrate surface, 16
...Resist pattern.

Claims (1)

[Scope of Claims] 1. A pattern forming method for forming a resist pattern including a step of projecting exposure onto a resist film through a mask pattern having a desired shape and a step of developing the resist film, the method comprising: is performed at a plurality of positions on the optical axis that are relatively different between the imaging plane of the mask pattern and the resist film, and selectively resists only the exposure where the imaging plane and the resist film surface are close in relative position. 1. A pattern forming method comprising exposing to light. 2. The method according to claim 1, wherein the selective exposure is performed by forming a light transmission selective film on the resist film and exposing the resist film to light through the film. Pattern formation method. 3. The light transmission selective film has optical reversibility such that it is opaque to exposed light when unexposed, becomes transparent as it is irradiated with exposure light, and returns to its opaque state when the exposure light is blocked. 3. The pattern forming method according to claim 2, wherein the pattern forming method is a film comprising: 4. The pattern forming method according to claim 2, wherein the light transmission selective film is a film containing an azobenzene derivative. 5. The pattern forming method according to claim 2, wherein the light transmission selective film is a film containing a spiropyran derivative. 6. The pattern forming method according to claim 1, wherein the image forming plane is set near a convex surface and a concave surface of a substrate step. 7. There are two or more image forming planes, one of which is set at the same position as the highest position on the top surface of the substrate (closest to the projection exposure optical system), or at a position closer to the projection optical system, and one is set at the same position as the lowest position on the top surface of the substrate or at a position farther from the projection optical system than that, and 3
2. The pattern forming method according to claim 1, wherein in the case of more than one spot, the remaining imaging plane is set between the two. 8. If there are two or more imaging planes, one of which is set at a position farther from the projection optical system than the highest position on the substrate surface by the depth of focus of the optical system, and if there are three or more, the remaining 2. The pattern forming method according to claim 1, wherein the imaging plane is set between the two. 9. The exposure is performed while the position of the imaging plane is swept from at least the highest position on the upper surface of the substrate to the lowest position (furthest from the optical system). The pattern forming method described in section. 10. The pattern forming method according to claim 1, wherein the exposure is performed multiple times.