JPH10311911A - Diffractive optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a diffraction optical element (BOE) in which a diffraction efficiency is prevented from being lowered by dully forming ridgelines of fine difference in level parts of a cyclic step structure to reduce undesired lights. SOLUTION: Ridgelines 11 of difference in level parts having a height h=λ/(n-1) for every one cycle are sharply formed, however, ridgelines 12 of fine difference in level parts having a height d-h/2M being in one cycle are dully formed to be dully formed with respect to conventional ridgelines 15. Provided λ is a wavelength to be used, n is the refractive index of glass and M is the number of sheets of masks to be used in a lithographic process. Then, the phase function of the wave front passed through the BOE approaches the phase function 25 of an ideal wave front as compared with the phase function 24 of the wave front passed through the BOE by dulling ridgelines of the fine difference in level parts of the step structure. Besides, a part in which the phase of the wave front 19 of one part of a wave front 17 is delayed by 2 πand a part in which the phase of the wave front 20 of one part of a wave front 18 is advanced by 2 π are combined at discontinuity parts 21. Thus, it is required that phase functions for every one cycle are to be given sharply so that phases of different wave fronts are easy to connect.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diffractive optical element, a method for manufacturing the diffractive optical element, an optical device having the diffractive optical element, and a method for manufacturing an element using an exposure apparatus having the diffractive optical element. is there.

[0002]

2. Description of the Related Art FIG.
(A) and (b) show the BOE (B
There is a shape-phase type diffractive optical element called an “inary optical element”. The process from the conventional refraction lens to the BOE will be described using the phase function shown in FIG. In the figure, the horizontal axis r represents the position in the radial direction from the optical axis, and the vertical axis φ represents the phase given to the wavefront of the light beam transmitted through each optical element. If the refractive index distribution in the optical element is uniform, the phase function given to the light beam transmitted through the optical element may be considered to be proportional to the inverted shape of the optical element.

FIG. 10 (a) shows an example of a phase function given to a transmitted wavefront by a refracting concave lens. FIG.
In order for the optical element to have the same effect as the lens providing the phase distribution as shown in FIG. 9A, for each radius rm where the phase difference from the phase at r = 0 becomes 2mπ (m: integer), What is necessary is just to provide a discontinuous part with a step of 2π. That is, if the optical element is configured to give a phase function as shown in FIG. 10B, it is possible to combine wavefronts having a phase difference of 2π at discontinuous portions. A diffractive optical element having such a periodic structure can achieve a diffraction efficiency of 100% for light of a design wavelength.

However, as the pitch of the periodic structure becomes smaller, it becomes very difficult to form a continuous shape within one period for providing a phase function as shown in FIG. In order to solve this difficulty, a BOE was created in which a continuous shape within one cycle was approximated in a stepwise manner. B
The OE is an element that gives the curved part of the phase function in FIG. 10B as a phase function that is approximated in a stepwise manner by a straight line parallel to the r-axis and a straight line parallel to the φ-axis. The phase difference for each step in one cycle is 2π / ^2M . Here, M is the number of masks used in the lithography process. FIG. 10 (c)
Shows the phase function of a four-stage BOE created with M = 2 sheets.

FIG. 11 is an enlarged sectional view of a part of a periodic structure of a conventional BOE. 1 to 5 are wavefronts of light transmitted through the BOE, and 6 is a partial cross-sectional shape of the BOE. Such a shape is created, for example, by a lithography process. The BOE in FIG. 11 is an 8-stage BOE using three masks in a lithography process.

In the BOE of FIG. 11, a step having a height h corresponding to a phase difference of 2π is provided for each radius rm to form one cycle. Further, within this one cycle, the height of one step d = h / 2
A step-like step of ^M (corresponding to a phase difference of 2π / 2 ^M ) is provided.
For an element that plays the role of a convex lens with a focal length f, the radius rm is

[0007]

[Outside 1] Given by Here, λ is the wavelength to be used, n is the refractive index of glass, m is a positive integer, and M is the number of masks used in the lithography process.

[0008] In a step portion having a height h, wavefronts whose phases are shifted by 2πm are coupled to each other. For example, wavefronts 2 and 3 are BOE
Is the same wavefront before passing through
The wavefront 2 and the wavefront 4 are coupled. When the combined wavefront is sufficiently far from the BOE, it behaves equivalently to a continuous wavefront, such as 5.

[0009]

However, the conventional BOE has a problem that scattered light is generated due to an edge portion of a minute step in one cycle and becomes unnecessary light, thereby lowering diffraction efficiency. .

An object of the present invention is to provide a diffractive optical element that reduces unnecessary light and prevents a reduction in diffraction efficiency.

[0011]

To achieve the above object, a diffractive optical element according to the first aspect of the present invention is characterized in that a ridgeline of a minute step portion of a periodic stair structure is dull.

A method of manufacturing a diffractive optical element according to a second aspect of the present invention is characterized in that the method includes a step of forming a periodic step structure having a dull ridge.

The method of manufacturing a diffractive optical element according to the third aspect of the present invention is characterized by comprising a step of forming a periodic step structure and a step of dulling a ridgeline of a minute step portion of the step structure.

The method of manufacturing a diffractive optical element according to the fourth aspect of the present invention includes a step of forming a mold having a periodic step structure having a dull ridge, and a step of forming a diffractive optical element using the mold. It is characterized by:

An optical apparatus according to a fifth aspect of the present invention includes the diffractive optical element according to the first aspect of the present invention.

The device manufacturing method according to the sixth aspect of the present invention is directed to the first aspect of the present invention.
The invention is characterized in that the element is manufactured by an exposure apparatus having the diffractive optical element.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

(First Embodiment) A BOE according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1A is a cross-sectional view of the BOE of Example 1 in which a periodic staircase structure is formed concentrically and cut along a plane including an optical axis, showing only a part of the periodic staircase structure. I have. FIG. 1 (b) is the same as FIG.
It is an enlarged view of 13 parts in the inside. FIG.
It is the figure which showed the phase function of the wavefront which passed one periodic structure 10 of BOE of the shape of (a). FIG.
It is an enlarged view of 22 part in (c). FIG. 1E is an explanatory diagram of a discontinuous portion of the phase.

In the BOE of this embodiment, as shown in FIG. 1A, the ridgeline 11 of the step portion having a height of h = λ / (n−1) for each period is formed sharply. As shown in FIG. 1B, the ridgeline 1 of the minute step portion of d = h / 2 ^M in one cycle
2 is formed to be blunt with respect to the conventional ridge line 15. At this time, the average inclination angle of the ridgeline 12 is set to be equal to or less than the inclination angle of the continuous shape 14 before being approximated by the staircase structure. Here, the tilt angle is an angle formed by a plane perpendicular to the optical axis of the BOE. The average inclination angle is a curve (curved surface) 16 in which the ridge line 12 changes continuously as in the present embodiment. It is obtained by dividing by the width Δr.

The phase of the wavefront of light passing through one periodic structure 10 of the BOE of this embodiment is as shown in FIG.
As shown in FIG. 1D, the BOE of the present embodiment is formed by dulling the ridge line of the minute step portion of the staircase structure as in the present embodiment.
The phase function 23 of the wavefront that has passed through is closer to the phase function 25 of the ideal wavefront than the phase function 24 of the wavefront that has passed through the conventional BOE, and scattered light (unnecessary light) at the ridgeline of the minute step portion And diffraction efficiency is improved.

On the other hand, it is necessary to form the ridge line 11 of the step portion for each period sharply. This will be described with reference to FIG. In FIG. 1E, reference numerals 17 and 18 represent the phases of a certain wavefront and the next wavefront. A portion of the wavefront 17 where the phase of the wavefront 19 is delayed by 2π and a portion of the wavefront 18 where the phase of the wavefront 20 is advanced by 2π are combined at a discontinuous portion 21 of the phase. Therefore, in order to easily combine phases of different wavefronts, it is necessary to provide a sharp phase function for each period, and a shape requires a sharp ridge at a discontinuous portion of the phase for each period.

Next, a method of manufacturing the BOE of this embodiment will be described with reference to FIG. In FIG. 2, a mask used for pattern exposure is indicated by a bold line, a resist is indicated by a hatched portion, and a substrate which finally becomes a BOE is indicated by a thin line.

FIGS. 2A to 2D show a normal BOE manufacturing process in which a ridgeline of a minute step in one cycle is sharp.
First, in FIG. 2A, a resist-coated SiO
Exposure is performed on a glass substrate such as ₂ using a mask having the narrowest line width in the subsequent steps, and development is performed. Next, FIG.
In (b), the pattern is etched into a glass substrate by highly anisotropic etching. In FIG. 2C, exposure and development are performed using a mask having the second smallest line width, and etching is performed in FIG. 2D. Thus, the staircase structure within one cycle forms a four-stage BOE. Although only four steps are shown here, the same steps are repeated while increasing the number of masks, so that B, such as eight, sixteen, and thirty steps, can be obtained.
An OE can be formed.

In the BOE of this embodiment, after the normal BOE manufacturing process as described above, a process for dulling the ridgeline of the minute step portion in one cycle is further performed. First, FIG.
In step (2), exposure and development are performed so that the resist remains at discontinuous portions where a phase difference is given by 2π (step portions at each period).
Next, in FIG. 2 (f), etching is performed under conditions of low anisotropy by increasing the pressure of the etcher or lowering the bias voltage, and a portion without a resist, that is, a minute step portion in one cycle of BOE is formed. Blunt the ridge. Finally, in FIG. 2G, the BOE of this embodiment is formed by removing the resist.

If, after the step of FIG. 2 (f), a step larger than an allowable value is formed between the part covered with the resist and the exposed part, before the step of FIG. 2 (f), FIG.
The substrate may be processed into a shape that allows for the amount of etching in (f), so that unnecessary steps are not generated by the etching in FIG.

(Embodiment 2) FIG. 3 is for explaining a second embodiment of the present invention. FIG. 3A is a cross-sectional view of the BOE of Example 2 in which a periodic step structure is formed concentrically and cut along a plane including the optical axis, and shows only a part of the periodic step structure. I have. FIG.
It is an enlarged view of 33 part in (a). FIG. 3 (c)
It is the figure which showed the phase function of the wavefront which passed one periodic structure 30 of BOE of the shape of FIG. FIG. 3 (d)
It is an enlarged view of 36 parts in FIG.3 (c).

In FIG. 3A, reference numeral 30 denotes B of the present embodiment.
The OE has a one-period structure, 31 is a step portion for each period having a phase difference of 2π, and 32 is a ridgeline of a minute step portion in one period. In FIG. 3B, reference numeral 34 denotes a continuous shape before approximation by the staircase structure, and reference numeral 35 denotes a conventional ridgeline before the ridgeline 32 is dulled. In FIG. 3D, reference numeral 37 denotes B of the present embodiment.
The phase function of OE, 38 is the phase function of conventional BOE, 39
Is the ideal phase function.

In this embodiment, only the ridgeline 32 of the convex portion in one cycle is blunted. The average inclination angle of the ridge line 32 is shown in FIG.
As shown in (b), the inclination angle of the continuous shape 34 before being approximated by the staircase structure is equal to or smaller than the inclination angle.

The phase imparted to the wavefront of light passing through one periodic structure 30 of the BOE of this embodiment is as shown in FIG. As shown in FIG. 3D, the phase function 37 of the BOE according to the present embodiment is changed from the phase function 3 of the conventional BOE by dulling the ridge line of the minute step portion of the staircase structure as in the present embodiment.
8, it approaches the ideal phase function 39,
Scattered light (unnecessary light) on the ridgeline of the minute step is reduced,
Diffraction efficiency is improved.

Next, a method of manufacturing the BOE of this embodiment will be described with reference to FIG.

In FIG. 4, after a normal BOE having a sharp ridge line of a minute step in one cycle is obtained on a glass substrate such as SiO _{2 in the same} manner as in the first embodiment, as shown in FIG. A thin layer of resist is applied on the top to form a resist layer having a general substrate shape. Next, in FIG. 4B, until the ridgeline of the minute step portion in one cycle is exposed from the resist layer,
The resist is etched back by O ₂ RIE. Subsequently, FIG.
In (c), a gas having a mixing ratio of CF ₄ and CHF ₃ of 1: 1 is used.
The exposed ridge lines are etched using the resist as a mask. Finally, in FIG. 4D, the BOE of this embodiment can be formed by removing the resist.

(Embodiment 3) FIG. 5 is for explaining a third embodiment of the present invention. FIG. 5A is a cross-sectional view of the BOE of Example 3 in which the periodic step structure is formed concentrically and cut along a plane including the optical axis, and shows only a part of the periodic step structure. I have. FIG.
It is an enlarged view of 53 part in (a). FIG. 5 (c)
It is the figure which showed the phase function of the wavefront which passed one periodic structure 50 of BOE of the shape of FIG. 5 (a). FIG. 5 (d)
It is an enlarged view of 56 part in FIG.5 (c).

In FIG. 5A, reference numeral 50 denotes B of this embodiment.
The structure of one period of the OE, 51 is a step portion for each period having a phase difference of 2π, and 52 is a ridge line of a minute step portion in one period. In FIG. 5B, reference numeral 54 denotes a continuous shape before approximation by the staircase structure, and reference numeral 55 denotes a conventional ridgeline before the ridgeline 52 is dulled. In FIG. 5D, reference numeral 57 denotes B of the present embodiment.
OE phase function, 58 is a conventional BOE phase function, 59
Is the ideal phase function.

In this embodiment, only the ridge line 52 of the concave portion in one cycle is blunted. The average inclination angle of the ridge line 52 is shown in FIG.
As shown in (b), the inclination angle of the continuous shape 34 before being approximated by the staircase structure is equal to or smaller than the inclination angle.

The phase imparted to the wavefront of light passing through one periodic structure 50 of the BOE of this embodiment is as shown in FIG. As shown in FIG. 5 (d), the phase function 57 of the BOE of this embodiment is changed to the phase function 5 of the conventional BOE as shown in FIG.
8, the phase function 59 approaches the ideal phase function.
Scattered light (unnecessary light) on the ridgeline of the minute step is reduced,
Diffraction efficiency is improved.

The BOE of this embodiment can be formed by depositing a fluorine-based release layer on a SiO ₂ mold manufactured in the same manner as in the second embodiment and pressing the mold.

(Embodiment 4) A fourth embodiment of the present invention will be described with reference to FIG. In this embodiment, the BOE of the present invention is mounted on an exposure apparatus.

6, reference numeral 71 denotes a light source; 72, a reticle; 73, a reticle stage; 74, a projection optical system;
Is a refracting lens constituting a part of the projection optical system, 76 is a wafer, 77 is a wafer stage, and 78 is the BOE of the present invention. In the exposure apparatus of this embodiment, the wafer 76 is positioned at a desired position by the wafer stage 77, and the height of the wafer 76 is adjusted to a focus position by focus detection means (not shown). Thereafter, a shutter (not shown) is opened, the reticle 72 is illuminated by the illumination light from the light source 71, and the circuit pattern on the reticle 72 is projected onto the wafer 76 by the projection optical system 74.

Incidentally, an ArF excimer laser or KrF
When an excimer laser is used as a light source of an exposure apparatus, light absorption is relatively large in an optical system, and therefore, there is a problem that the use efficiency of exposure light is reduced. Since the thickness of the BOE is smaller than that of a normal lens, the BOE has a relatively small light absorption amount, and can be said to be an optical element suitable for an exposure apparatus. In addition, since the BOE of the present invention can reduce scattered light and prevent a decrease in diffraction efficiency, an optical system having a high transmittance can be realized by applying the BOE to an optical system of an exposure apparatus. Can be improved.

(Embodiment 5) Next, an embodiment of a method of manufacturing a semiconductor device using the exposure apparatus of FIG. 6 will be described.

FIG. 7 shows a flow of manufacturing semiconductor devices (semiconductor chips such as ICs and LSIs, liquid crystal panels and CCDs).
In step 1 (circuit design), the circuit of the semiconductor element is designed. In step 2 (mask production), a mask (reticle 72) on which the designed circuit pattern is formed is produced. On the other hand, in step 3 (wafer manufacture), a wafer (wafer 76) is manufactured using a material such as silicon. Step 4
The (wafer process) is called a pre-process, and actual circuits are formed on the wafer by lithography using the prepared mask and wafer. The next step 5 (assembly) is called a post-process, and is a process of forming chips using the wafer created in step 4, and includes processes such as an assembly process (dicing and bonding) and a packaging process (chip encapsulation). Including. In step 6 (inspection), inspections such as an operation confirmation test and a durability test of the semiconductor device created in step 5 are performed. Through these steps, a semiconductor device is completed and shipped (step 7).

FIG. 8 shows a detailed flow of the wafer process. In step 11 (oxidation), the wafer (wafer W)
Oxidizes the surface of the Step 12 (CVD) forms an insulating film on the surface of the wafer. Step 13 (electrode formation) forms electrodes on the wafer by vapor deposition. In step 14 (ion implantation), ions are implanted into the wafer. In step 15 (resist processing), a resist (sensitive material) is applied to the wafer. In step 16 (exposure), the wafer is exposed to an image of the circuit pattern of the mask (reticle 72) by the exposure apparatus. Step 17 (development) develops the exposed wafer. In step 18 (etching), portions other than the developed resist are removed. In step 19 (resist stripping), unnecessary resist after etching is removed. By repeating these steps, a circuit pattern is formed on the wafer.

By using the manufacturing method of this embodiment, it is possible to manufacture a highly integrated semiconductor device which has been difficult in the past.

[0044]

As described above, according to the diffractive optical element of the present invention, unnecessary light can be reduced and a decrease in diffraction efficiency can be prevented.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a BOE according to a first embodiment.

FIG. 2 is a view for explaining a BOE manufacturing method according to the first embodiment;

FIG. 3 is a diagram illustrating a BOE according to a second embodiment.

FIG. 4 is a diagram for explaining a BOE manufacturing method according to a second embodiment.

FIG. 5 is a diagram illustrating a BOE according to a third embodiment.

FIG. 6 is a schematic configuration diagram of an exposure apparatus having a BOE of the present invention.

FIG. 7 is a view showing a manufacturing process of the semiconductor element.

FIG. 8 is a diagram showing details of a wafer process during the step of FIG. 7;

FIG. 9 is a front view and a cross-sectional view of the BOE.

FIG. 10 is a diagram for explaining a process from a conventional refractive lens to a BOE.

FIG. 11 is an enlarged cross-sectional view of a partial periodic structure of a conventional BOE.

[Explanation of symbols]

 Reference Signs List 10 Periodic structure of one period 11 Step part for one period 12 Ridge of minute step part in one period 14 Continuous shape before approximation 15 Conventional ridgeline 16 Ridge of the present invention 23 Phase function of ridgeline 24 Ridgeline 15 Phase function of 25 ideal phase function

Claims (14)

[Claims] 1. A diffractive optical element, wherein a ridgeline of a minute step portion of a periodic staircase structure is dull. 2. The staircase structure has a shape approximating a continuous shape, and an absolute value of an average inclination angle at a portion where the ridge line is blunt is equal to or less than an absolute value of an inclination angle of the continuous shape. A diffractive optical element, characterized in that: 3. The diffractive optical element according to claim 2, wherein the inclination angle is an angle formed with a plane perpendicular to the optical axis. 4. The diffractive optical element according to claim 1, wherein the ridge lines are a ridge line of a convex portion and a ridge line of a concave portion of the minute step portion. 5. The diffractive optical element according to claim 1, wherein the ridge is a ridge of a convex portion of the minute step. 6. The diffractive optical element according to claim 1, wherein the ridge is a ridge of a concave portion of the minute step. 7. The diffractive optical element according to claim 1, wherein the ridge is curvedly dull. 8. The diffractive optical element according to claim 1, wherein the ridge is linearly dull. 9. A method for manufacturing a diffractive optical element, comprising a step of forming a periodic step structure having a dull ridge. 10. A step of forming a periodic stair structure;
A method for manufacturing a diffractive optical element, comprising a step of dulling a ridgeline of a minute step portion of the stair structure. 11. A method for manufacturing a diffractive optical element, comprising: a step of forming a mold having a periodic step structure with a dull ridge line; and a step of forming a diffractive optical element using the mold. . 12. An optical apparatus comprising the diffractive optical element according to claim 1. 13. An optical apparatus according to claim 12, wherein said optical apparatus is an exposure apparatus for manufacturing an element. 14. An element manufacturing method, wherein an element is manufactured using the exposure apparatus according to claim 13.