JP2002107942A - Exposure method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily determine multilevel gradation in a narrow region, to improve the surface roughness and to reduce the manufacture cost of reticules and masks. SOLUTION: When a mask is used for exposure, the mask is relatively moved against a substrate 1 as shown in Fig. (a), (b), (c) and the substrate is exposed in each position. The movement range corresponds to the width of one gradation level and the mask is moved by 1/3 of the range at each time. Exposure is carried out as divided into three steps after each movement is stopper and the quantity of exposure light in each step is controlled to 1/3 of a specified quantity of exposure light. The obtained resist pattern by development after the exposure has the number of gradation levels increased three times that of the original as shown in Fig. (d).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure method for exposing a photosensitive material layer when manufacturing an article having a three-dimensional surface structure or a density distribution mask having a three-dimensional pattern of a light-shielding film. Things. This exposure method is used in the industrial field of forming fine irregularities and other three-dimensional shapes using photoengraving technology, for example, the field of manufacturing optical elements such as diffraction elements, the field of micromachining, the field of surface control technology, and the field of display for wall-mounted TVs. It is used in the field of utilizing the same microfabrication technology as that used in the semiconductor manufacturing process, such as the liquid crystal display field and the solar cell manufacturing field.

[0002]

2. Description of the Related Art A special surface shape represented by a spherical surface or an aspherical surface has been used for a refracting surface or a reflecting surface of an optical element. In recent years, a special surface shape has been required for an optical element such as a microlens in connection with a liquid crystal display element or a liquid crystal projector. Therefore, as a method of forming the refraction surface and the reflection surface without using molding or polishing, a layer of a photoresist (a typical example of a photosensitive material) is formed on the surface of an optical substrate, and the photoresist layer is exposed, Through development processing, a three-dimensional convex or concave surface shape is obtained as the surface shape of the photoresist. Thereafter, anisotropic etching is performed on the photoresist and the optical substrate, and the surface shape of the photoresist is engraved on the optical substrate. Attempts have been made to obtain desired three-dimensional refraction and reflection surface shapes on the surface of the optical substrate by copying and transferring.

As an exposure method, in the semiconductor manufacturing field,
“Stepper reduction exposure” and “aligner equal-size exposure” using a wiring pattern and a mask or reticle by a phase shift method are performed. In this method, a two-dimensional pattern such as a wiring pattern can be exposed, but a three-dimensional structure cannot be manufactured. In order to manufacture a three-dimensional structure, exposure of a binary structure in which a plurality of masks are sequentially exchanged for exposure is also known, but the exposure required four or more masks. In order to achieve optical performance (80% or more efficiency)
This is because at least four exposure steps are required.

[0004] As another method for producing a three-dimensional structure, a gradation distribution mask having gradation by changing the film thickness of a light-shielding film pattern in a stepwise manner, or a unit having an exposure region having an appropriate shape and size. A density distribution mask that is divided into cells and is set to have a value corresponding to the height of the corresponding position of the photosensitive material pattern to be obtained in the light transmission amount or the light shielding amount of the light shielding pattern in each unit cell. There has been proposed an exposure method using (see Japanese Patent Application Laid-Open No. 7-230159;
504515).

[0005]

When exposure is performed using a gradation distribution mask or a density distribution mask, it is necessary to provide a gradation difference in a narrow area. If an attempt is made to make such a step, a step may occur and the surface roughness may deteriorate, making it difficult to obtain desired performance. Also, the production cost of the reticle and the mask was high.
SUMMARY OF THE INVENTION It is an object of the present invention to facilitate setting of a large number of gradations in a narrow area, improve the surface roughness, and reduce the manufacturing cost of a reticle or a mask.

[0006]

According to the present invention, an intermediate gray scale is created between an adjacent density gray scale by slightly moving a mask relative to a photosensitive material during exposure. Exposure method. That is, the exposure method of the present invention is to form a three-dimensional photosensitive material pattern by irradiating and developing a photosensitive material through an exposure mask having a light shielding film pattern having two or more gradations of transmitted light, In the method of manufacturing an article with a three-dimensional structure by engraving the photosensitive material pattern on the base, the mask moves slightly in the exposure process relative to the photosensitive material in the exposure process. By doing so, the photosensitive material is exposed at a gradation of more steps than the gradation of the exposure mask.

The generation of intermediate gradations increases the number of gradations, reduces the level difference, improves the surface roughness, and makes it possible to obtain a desired structure and performance. In particular, it is possible to realize a gradation and a structure in a structure portion where a steep change in height occurs or an ultra-fine region where a concentration distribution mask cannot be manufactured. Further, it is not necessary to provide a large number of gradation differences in the exposure mask, and the cost of the mask can be reduced.

In the present invention, the mask is moved relatively to the photosensitive material. This movement may be performed by moving the mask while fixing the substrate on which the photosensitive material is applied. Then, the substrate coated with the photosensitive material may be moved. These two movements are referred to as “relative movement of the mask”.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Light irradiation can be performed during the relative movement of a mask. In this case, intermediate gradations are continuous. Further, the exposure step can be a step of performing multiple exposure by repeating relative movement of the mask and light irradiation at the time of stop. In this case, the control of the gradation is easy. In the case of performing multiple exposure by repeating the relative movement of the mask and the light irradiation at the time of stop, the relative movement range of the mask in one exposure step is set to be equal to or less than the distance corresponding to one gradation, and 1 /
By repeating the fine movement N times with a movement amount of N as one unit, exposure of N × n gradations can be realized with a mask of n gradations. Here, n and N are integers.

[0010] The relative movement direction of the mask may be only one-dimensional direction or two directions orthogonal to each other. Further, it can include a minute movement in the rotation direction. A preferred example of the mask used in this exposure method is one in which a light-shielding pattern having a two-dimensional light intensity distribution is formed on a transparent substrate, and the exposure region is divided without gaps by unit cells having an appropriate shape and size. This is a density distribution mask in which the light transmission amount or the light shielding amount of the light shielding pattern in each unit cell is set to a value corresponding to the height of the corresponding position of the photosensitive material pattern. In this case, one unit of the relative minute movement of the mask is set to an integral fraction of the unit cell size.

It is also preferable to perform exposure in a defocused state. Defocus means that the focus is out of the photosensitive material during exposure. By exposing in a defocused state, a three-dimensional pattern formed on the photosensitive material and, consequently, a surface shape of a target article can be made smoother. One example of a preferred exposure step is a reduced projection exposure step.

An example of an article having a three-dimensional structure formed by engraving a photosensitive material pattern on a base is an optical element such as a microlens array.
An article in which the surface of the substrate itself is processed. Another example of an article having a three-dimensional structure surface shape manufactured by engraving a photosensitive material pattern on a base is a density distribution sister mask having a light-shielding film pattern formed in the same size as the pattern of the target article. A two-dimensional pattern in which the light-shielding film pattern is formed on a transparent substrate by the light-shielding film,
At least a part of the two-dimensional pattern is a three-dimensional pattern having a film thickness distribution in which the film thickness of the light shielding film changes so that the amount of transmitted light changes. Here, the equal size refers to a dimension in a plane on which the light shielding film pattern is formed, and does not relate to a dimension in a height direction of the light shielding film pattern.

When such a density distribution sister mask is manufactured, the light-shielding film pattern itself has a thickness distribution in which the thickness of the light-shielding film changes so that the amount of transmitted light changes. The three-dimensional pattern of the surface shape can be formed on the photosensitive material layer also by the exposure method.

A stepper having a reduction optical system (lens reduction projection exposure machine) and a projection exposure machine can read a positioning pattern (register mark pattern) formed on the surface of an exposure target and perform mask positioning. It is not possible to read the register mark pattern formed on the surface opposite to the front surface and align the mask. Therefore, when manufacturing a desired surface shape on the front and back surfaces of a substrate using an exposure machine having a reduction optical system, a photolithography process for forming only the register mark patterns on the front surface and the back surface is required, and the manufacturing time is required. It is unavoidable that the length becomes very long, resulting in high cost. On the other hand, since the light-shielding film pattern of such a density distribution sister mask is formed to have the same size as the pattern of the target product to be formed, when forming the target product, a proximity exposure method, a contact exposure method, or the like is used. As a result, the light-shielding film pattern can be exposed by the equal-magnification exposure method. As a result, in the proximity exposure method or the contact exposure method, the light-shielding film pattern for the register mark pattern is exposed simultaneously with the light-shielding film pattern for the target surface shape. In addition, it is possible to read the alignment pattern on the back surface of the transparent substrate and align the mask on the front surface side. As a result, the number of steps for forming a desired surface shape on the front and back surfaces of the substrate can be reduced, and the manufacturing time can be shortened, so that the manufacturing cost of the target article can be reduced.

The use of such a density distribution sister mask makes it possible to transfer and manufacture optical elements to be formed on the front and back surfaces of a transparent substrate by different methods. For example, an optical element exposed to a stepper can be formed on one side of a transparent glass substrate, and an optical element exposed to a contact can be manufactured on the other side. By using such a method, for example, an optical product having optical surfaces on both sides can be manufactured. The method for manufacturing an optical surface includes a method which the present inventor has already applied for a patent (Japanese Patent Application Laid-Open No. 9-146259, Japanese Patent Application No. 2000-17).
No. 7847) can be applied.

When a three-dimensional structure is manufactured using such a density distribution sister mask, an optical element having a continuous surface such as a spherical surface, an aspherical surface, and a conical shape can be manufactured.
It is also possible to manufacture an optical element having a continuous surface and a discontinuous surface like a Fresnel shape. Further, it is possible to form a reflective optical surface by forming a reflective optical surface on such an optical element.

[0017]

(Example 1) Example of one-dimensional movement The density distribution shape (light-shielding film pattern) of the mask is shown in FIG.
As shown in the figure, there are three gradations of a level at which the transmittance is 0 (the highest density level), a level at which the transmittance is 100%, and an intermediate transmittance level. This light-shielding film pattern is a cylindrical pattern extending in the direction perpendicular to the plane of the drawing. When the positive resist is exposed and developed using this mask, the density distribution is transferred as it is, as shown in FIG. 1 is a substrate and 2 is a formed resist pattern.

Next, when exposing using this mask, as shown in FIGS. 1 (a), (b) and (c), it is moved relative to the substrate 1 and Exposure. The moving range was one gradation width, and the moving range was 1/3 of the range. Exposure was performed in three stages after each movement was stopped, and the amount of one exposure was １ ／ of the predetermined amount of exposure. After the exposure, the resist pattern obtained by development is shown in FIG.
As shown in (d), the number of gradations was tripled.

(Embodiment 2) Example of two-dimensional movement Using the mask used in Embodiment 1, as shown in Embodiment 1, as shown in FIGS. 1 (a), (b) and (c) After exposing at each position by relatively moving the substrate 1 one-dimensionally, the mask is returned to the original position, rotated by 90 °, and then moved in a direction orthogonal to the previous moving direction. The same exposure as shown in Example 1 was performed again. As a result, a resist pattern having a quadrangular pyramid (pyramid) structure could be manufactured. A cross-sectional view of the resist pattern along each moving direction is shown in FIG.

(Embodiment 3) Example of three-dimensional movement Using a density distribution mask shown in FIG. 3, exposure is performed while performing two-dimensional movement (X, Y, Θ) and small movement in the Z-axis direction. An example will be described. The density distribution mask illustrated in FIG. 3 shows one microlens portion of 18 μm × 18 μm in a reticle mask for a microlens array for a liquid crystal projector. The unit cell has a square shape like a grid. The unit cell does not necessarily have to be a square, but is desirably another polygonal shape according to a desired shape. The hatched portion is the portion where the Cr film remains.

For exposure, a stepper was used. FIG. 4 shows an outline of the stepper. Light from the light source lamp 30 is condensed by the condenser lens 31 and irradiates the exposure mask 32. The light transmitted through the mask 32 is incident on an image forming lens 33 having a reduced magnification, and is placed on a surface of a material 37 for an optical device or other articles placed on a stage 34 so that the mask 3
2, a reduced image of the transmittance distribution is formed. The stage 34 on which the material 37 is placed is a step motor 3
By the action of 5, 36, it is possible to displace in two directions perpendicular to each other in a plane perpendicular to the optical axis of the imaging lens 33, and to position the material 37 with respect to the optical axis of the imaging lens 33. It has become.

In this embodiment, in order to manufacture a micro lens array for a liquid crystal projector, a neoceram substrate is prepared, and a TGMR-950BE resist (a product of Tokyo Ohka Co., Ltd.) having a thickness of 8.56 μm is formed on the substrate. Was applied. Next, prebaking was performed on a hot plate at 100 ° C. for a baking time of 180 seconds.

This substrate was placed at the position of the stepper material 37 in FIG. 4 and exposed. The exposure forms a reduced image of the mask 32 by the imaging lens 33 on the surface of the photoresist layer of the material 37. The focal point of the image formation is set to a defocus state shifted from the photoresist layer surface as necessary. During this exposure, the mask 32 or the stage 34 is minutely moved. The exposure is performed densely over the entire surface of the material 37.

The minute movement amount at this time was set as follows as shown in FIG. Unit cell of density distribution mask (2
(μm × 2 μm) is exposed by a 1 / 2.5 stepper (the unit cell size after reduced exposure is 0.8 μm × 0.8 μm).
m), and the minute movement amount was 0.2 μm. When performing exposure with a 1/5 stepper (the unit cell size after reduced exposure is 0.4 μm × 0.4 μm), the minute movement amount is 0.1
μm. This is an amount corresponding to ４ of the unit cell size at the time of exposure, and this amount is used as a minute movement amount to shift the pixels.

The moving direction and the moving amount are as shown in FIG. The coordinates and exposure amount are as shown in Tables 1 and 2 below. Here, the exposure rate indicates a ratio of an exposure amount to a predetermined “required exposure amount (energy: mJ)”. That is, in this embodiment, 50% of the whole is exposed in the state of zero movement, and 5% of the whole is
Is exposed. In step n), the remaining 10% is exposed by using a separately prepared bottom punching pattern (cells having a photosensitive material remaining amount of zero (light shielding area zero)). Further, the defocus amount (DF) is DF
= 0, and all other steps were performed at DF = + 3 μm. A plus sign in the display of the defocus amount means that the focus is above the surface of the resist layer.

(Table 1) Stepper reduction ratio: 1 / 2.5 a) (X, Y) = (0, 0) 50% b) (X, Y) = (0.2, 0) 5% c) (X, Y) = (0.2, −0.2) 5% d) (X, Y) = (0, −0.2) 5% e) (X, Y) = (− 0.2, (-0.2) 5% f) (X, Y) = (-0.2, 0) 5% g) (X, Y) = (-0.2, 0.2) 5% h) (X, Y) Y) = (0,0.2) 5% li) (X, Y) = (0.2,0.2) 5% nu) Bottom-out pattern 10% (total exposure rate 100%)

(Table 2) Stepper reduction ratio: 1/5 b) (X, Y) = (0, 0) 50% b) (X, Y) = (0.1, 0) 5% c) (X , Y) = (0.1, −0.1) 5% d) (X, Y) = (0, −0.1) 5% e) (X, Y) = (− 0.1, −0) .1) 5% f) (X, Y) = (-0.1, 0) 5% G) (X, Y) = (-0.1, 0.1) 5% h) (X, Y) = (0, 0.1) 5% ri) (X, Y) = (0.1, 0.1) 5% nu) Bottom-out pattern 10% (total exposure rate 100%)

After exposure under these conditions, PEB (post-exposure bake) was performed at 100 ° C. for 180 seconds. Next, the photosensitive material was developed and rinsed. Thereafter, the resist was hardened by evacuating while irradiating ultraviolet rays for 180 seconds with an ultraviolet curing device. The ultraviolet curing device irradiates a wavelength that can cure the resist at a wavelength shorter than the wavelength used for exposing the resist. By this operation, the plasma resistance of the resist is improved, and the resist can be processed in the next step. At this time, the resist height was 7.5 μm. Due to the effects of micro-movement and defocus at the time of exposure, a resist pattern shape could be manufactured without any particular step.

Thereafter, the substrate is set in a TCP (inductively coupled plasma) dry etching apparatus, and the degree of vacuum is set as follows:
1.5 × 10 ⁻³ Torr, CHF ₃ : 5.0 sccm, C
F ₄ : 50.0 sccm, O ₂ : 15.0 sccm, substrate bias power: 600 W, upper electrode power: 1.25 kW,
Dry etching was performed at a substrate cooling temperature of -20 ° C. At this time, the etching was performed while changing the substrate bias power and the upper electrode power over time, and changing the selectivity to decrease with time. The average etching rate of the substrate was 0.52 μm / min,
The actual etching time required 11.5 minutes. The lens height after the etching was 5.33 μm.

In this embodiment, in particular, the structure of the four corner adjacent portions of each microlens having a steep rising angle, which has been difficult to manufacture with a microlens array having 18 μm × 18 μm microlenses for a liquid crystal projector, is manufactured as designed. I was able to. This part requires an ultra-fine unit cell arrangement (for example, a unit cell size of 1 μm × 1 μm or less) which is expensive with a normal concentration distribution mask.
According to the present invention, it is possible to fabricate the structural portions at the four corners adjacent to the microlens even with a mask having a unit cell arrangement that is not ultra-fine. In place of the exposure method of irradiating light after stopping the movement as shown in FIG. 5, the same effect is obtained by the continuous exposure method of irradiating light while moving.

(Embodiment 4) Example applied to the manufacture of a concentration distribution sister mask In each of the above embodiments, the article to be manufactured is a product such as an optical element. The present invention can also be applied to manufacture a density distribution sister mask used for manufacturing by exposure by an exposure method or proximity exposure method.

FIG. 6 is a process sectional view showing an embodiment in which the present invention is applied to the manufacture of a density distribution sister mask. FIG.
The manufacturing process will be described along. (A) Quartz substrate 21
A Cr film 23 was formed to a thickness of 0.2 μm on one surface, and a resist material layer 25 was applied thereon to a thickness of about 8 μm. OFPR-5 as a material for the resist material layer 25
000-800 (product of Tokyo Ohka Co., Ltd.) was used. Thereafter, when prebaked, the thickness of the resist material layer 25 was 7.5 μm. Quartz substrate 2 after pre-baking
1. The mask blanks 28 were made of the Cr film 23 and the resist material layer 25.

(B) The mask blanks 28 are made of a material 3 of a stepper shown in FIG.
7, exposure was performed under the exposure conditions (Table 2) described in Example 3 while moving the mask blanks 28 by minute amounts. The total irradiation amount is 390mW x 3.60 seconds,
The total exposure was about 1400 mJ. Under the above exposure conditions, a reduced image of the reticle 32 by the imaging lens 33 was formed on the resist material layer 25 of the mask blank 28. This exposure was performed over the entire surface of the mask blanks 28.

After exposure under these conditions, a development process including development and rinsing was performed to remove the exposed portion of the resist material layer 25 to form a resist material pattern 25a. The resist material pattern 25a has a “plateau-like cross-sectional structure”. The “plateau-like oblique angle” of the resist material pattern 25 a changes depending on the design of the reticle, the amount of defocus, the thickness of the resist material layer 25, and the sensitivity of the material of the resist material layer 25. This oblique angle has an important effect on the analog structure (light transmission amount distribution) of the optical density distribution of the density distribution mask.

In this step (B), the resist material pattern 25a was able to be manufactured without causing any special step in the surface shape because the defocusing was performed at the time of exposure. Further, due to the defocus effect, the light transmission amount distribution by the fine light shielding film pattern of the reticle 32 is further averaged, and the surface shape of the resist material pattern 25a becomes smoother.

Here, the surface shape of the resist material pattern 25a is further smoothed by defocusing at the time of exposure, but a certain degree of smoothing can be expected even with a focused just focus. further,
The smoothing effect is further enhanced by changing the amount of defocus during the exposure time while changing the amount of defocus under a preset condition, or by changing the amount of defocus from the side where the focus is largely shifted to the side where the focus is in focus. be able to.

Thereafter, while the temperature was gradually increased on a hot plate, post-baking was performed at 120 ° C. for 40 minutes so that the resist material pattern 25a was not deformed, and the resist material pattern 25a was cured. Here, a heat treatment is used as a means for curing the resist material pattern 25a, but an ultraviolet curing treatment (UV hardening treatment) may be used instead of the heat treatment.

(C) The mask blank 28 after the resist material pattern 25a is cured is set in an RIE (reactive ion etching) dry etching apparatus, and the degree of vacuum is set to 5.
0 × 10 ⁻³ (Torr), CF ₄ : 15.0 (scc)
m), Ar: 0.5 (sccm ), O 2: 10.0~15.
Under the condition of 0 (sccm), the resist material pattern 25a
And dry etching was performed on the Cr film 23.
At this time, the resist material pattern 25a and the Cr material were changed while changing the O ₂ introduction amount so that the selectivity changed over time.
The film 23 was etched. Etching time is 10
It took a minute.

In this manner, the resist material pattern 2
The surface shape of 5a was engraved on the Cr film 23 to form a Cr film pattern 23a. Thereby, the density distribution sister mask 2 having the same size as the target article to be manufactured is provided.
7 is completed. Since the Cr film pattern 23a was formed by engraving the surface shape of the resist material pattern 25a, the Cr film pattern 23a has a "plateau-like cross-sectional structure", and there is no particular step in the surface shape.

In the density distribution sister mask 27, C
The light transmission amount is 1 in the portion where the r film pattern 23a does not exist.
In the portion where the Cr film pattern 23a is present, the light transmission amount is large in the thin portion and the light transmission amount is small in the thick portion in accordance with the thickness of the Cr film pattern 23a. ing. That is, the optical density distribution of the density distribution mask 27 is realized by the thickness distribution of the Cr film pattern 23a. Since the film thickness of the Cr film pattern 23a changes continuously according to the surface shape of the target article, the optical density distribution of the density distribution mask 27 is analog. The density distribution mask 27 can be easily duplicated if the reticle 32 is manufactured, and a spare mask can be manufactured at low cost. The density distribution mask 27 manufactured in this manner is called a “sister mask” in the present invention.

[0041]

According to the present invention, the photosensitive material is moved in a plurality of gradations more than the gradation of the exposure mask by slightly moving the mask relative to the photosensitive material in the plane in the exposure step. Is exposed, so that a structure of N × n gradations can be obtained even with a mask of n gradations, and the surface roughness is improved. Further, ultra-fine gradation exposure has become possible. That is, it is possible to expose a fine line width and the number of gradations higher than the resolution at the time of manufacturing the mask. Since the exposure mask used in the exposure of the present invention does not need the gradation required for the product, the mask can be manufactured at low cost.

[Brief description of the drawings]

FIGS. 1A, 1B, and 1C show exposures according to the present invention using a mask having a cylindrical light-shielding film pattern.
It is a schematic sectional drawing which shows (c) and the formed resist pattern (d).

FIG. 2 is a schematic sectional view showing a mask (a) having a cylindrical light-shielding film pattern and a formed resist pattern (b).

FIG. 3 is a plan view showing one microlens portion of a density distribution mask for a microlens array for a liquid crystal projector.

FIG. 4 is a schematic perspective view showing a stepper.

FIG. 5 is a diagram illustrating a method of moving a substrate according to one embodiment.

FIG. 6 is a process sectional view showing one embodiment in which the present invention is applied to manufacture of a concentration distribution sister mask.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Resist pattern 21 Quartz substrate 23 Cr film 23a Cr film pattern 25 Resist material layer 25a Resist material pattern 27 Concentration distribution sister mask 28 Mask blank

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H095 BB02 BC09 2H097 AB01 JA02 LA15 5F046 AA11 AA25 BA04 CB17 CC01 CC02 CC05 DA02 DA14

Claims (12)

[Claims] 1. A photosensitive material having a three-dimensional structure is formed by irradiating a light-sensitive material with light through an exposure mask having an amount of light transmitted through a light-shielding film pattern having two or more gradations to form a photosensitive material pattern having a three-dimensional structure. In the exposure when producing an article having a three-dimensional structure surface by engraving a pattern on a base, in the exposure step, the mask is slightly moved in a plane relative to the photosensitive material in the exposure step. An exposing method for exposing the photosensitive material at a gradation of more steps than the gradation of the exposure mask. 2. The exposure method according to claim 1, wherein the light irradiation is performed during the relative movement of the mask. 3. The exposure method according to claim 1, wherein relative exposure of the mask and light irradiation at the time of stop are repeated to perform multiple exposure. 4. A relative movement range of the mask in one exposure step is set to be equal to or less than a distance corresponding to one gradation.
4. The exposure method according to claim 1, wherein N × n gradation exposure is realized with an n-gradation mask by repeating the fine movement N times with a movement amount of N as one unit. 5. Here, n and N are integers. 5. The exposure method according to claim 1, wherein a relative movement direction of the mask is only a one-dimensional direction. 6. The exposure method according to claim 1, wherein one exposure step includes a minute movement in two movement directions orthogonal to each other. 7. The exposure method according to claim 1, wherein one exposure step includes two minute movements in two movement directions and a rotation direction orthogonal to each other. 8. The mask has a light-shielding pattern having a two-dimensional light intensity distribution formed on a transparent substrate,
The exposure area is divided without gaps by unit cells of an appropriate shape and size, and the light transmission amount or light shielding amount of the light-shielding pattern in each unit cell depends on the height of the corresponding position of the photosensitive material pattern. 8. A density distribution mask which is set so as to have a predetermined value, wherein one unit of relative minute movement of the mask is set to an integer fraction of the unit cell size. Exposure method. 9. The exposure method according to claim 1, wherein the light irradiation is performed in a defocused state. 10. The exposure method according to claim 1, wherein the exposure step is a reduction projection exposure step. 11. The article having a three-dimensional structure surface shape produced by engraving the photosensitive material pattern on a base is an article having a processed surface of a substrate itself. Exposure method according to 1. 12. An article having a surface shape of a three-dimensional structure manufactured by engraving the photosensitive material pattern on an underlayer has a density distribution having a light-shielding film pattern formed in the same size as the pattern of the target article. A sister mask, in which the light-shielding film pattern is formed on the transparent substrate by the light-shielding film, and the thickness of the light-shielding film changes so that the amount of transmitted light changes in at least a part of the two-dimensional pattern. The exposure method according to any one of claims 1 to 10, wherein the exposure method is a three-dimensional pattern having a thickness distribution.