JPH10301026A - Optical system having diffractive optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct deterioration in image forming performance due to deformation of a diffraction optical element and to keep excellently optical performance by providing the diffraction optical element provided with a diffraction grating having a periodical structure converting an incident wave front to a prescribed wave front on a substrate surface and a correcting optical element correcting a change in an optical characteristic after the diffraction optical element is deformed when the diffraction optical element is arranged in an optical system. SOLUTION: The diffraction grating 101 is formed on a transparent substrate 104. The correcting optical element 102 corrects the change in the optical characteristic due to own weight of the diffraction optical element 101. The correcting optical element 102 is constituted by forming an aspherical surface 103 rotation symmetrically for an optical axis 105 on the transparent substrate 106. An effect of own weight deformation is prevented by properly setting the aspherical surface amount of the correcting optical element 102. Various aberrations occurring when the diffraction optical element 101 having positive power (diffractive force) causes the deformation such as the own weight deformation and lens barrel pressure deformation, etc., are corrected by the correcting optical element 102 having the aspherical surface arranged in the vicinity of it, and the optical performance is kept excellently.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system having a diffractive optical element, for example, using a binary diffractive optical element as the diffractive optical element, and using the binary diffractive optical element (hereinafter also referred to as "diffractive optical element"). Is suitable for various optical systems in which the optical performance is maintained well by compensating for the deterioration of the optical performance due to its own weight deformation and lens barrel deformation when the lens is disposed in the optical system by using an aspherical surface. It is something.

[0002]

2. Description of the Related Art In recent years, various optical systems using a diffractive optical element utilizing a light diffraction phenomenon have been proposed. As a diffractive optical element, for example, a Fresnel zone plate, a diffraction grating, a hologram and the like are known.

[0003] A diffractive optical element is used as an optical element for converting an incident wavefront into a predetermined wavefront. This diffractive optical element has features not found in refraction lenses. For example, it has features such as having a dispersion value opposite to that of a refraction lens, and having a compact optical system because it has substantially no thickness.

In general, when a diffractive optical element is formed into a binary shape, a semiconductor element manufacturing technique can be applied to its manufacture, and a fine pitch can be realized relatively easily. For this reason, research on a binary diffractive optical element in which a blazed shape is approximated by a step shape has been actively pursued recently.

FIG. 8 is an explanatory diagram of a binary diffractive optical element. The binary diffractive optical element shown in FIG.
The portion giving an optical path difference of an integral multiple of the wavelength to the shape of the plano-convex refraction lens 201 shown in FIG. 8 is removed, and a diffractive optical element (Fresnel lens) 202 having a sectional shape as shown in FIG. Further, the diffractive optical element 204 is manufactured by quantizing the shape with a thickness of a fraction of the wavelength and approximating the shape with a step-like shape structure as shown in FIG.

Here, reference numerals 203 and 205 in the figure denote transparent substrates, and the diffractive optical element 2 having a fine shape on its surface.
02, 204 are formed.

FIG. 9 is an explanatory view of a conventional method for manufacturing a binary diffractive optical element having a four-stage structure. In the figure, 300 is a transparent glass substrate (refractive index: n), 301 is a resist,
02 denotes a mask used for the first exposure, and 303 denotes exposure light. Here, it is assumed that the resist 301 is a positive type.

First, in process A, a pattern of a mask 302 is transferred onto a resist 301 by exposure light 303. In process B, development of the resist 301 is performed, and in process C, etching of the glass substrate 300 is performed. And in process D,
By removing unnecessary resist on the substrate 300, a binary diffractive optical element having a two-stage structure is completed.

Here, the etching depth d1 is determined by d1 = λ / 2 (n−1), where λ is the wavelength when a binary diffractive optical element is used.

Next, a resist 304 is again applied to the glass substrate 300 on which the binary diffractive optical element having the two-stage structure is formed, and a second exposure using the mask 305 is performed in the process E. The pattern on the mask 305 has a half pitch of the pattern of the mask 302, and the exposure in the process F is performed by accurately aligning the edge of the light shielding portion with the edge of the two-stage binary structure and performing exposure. After the processing, a resist pattern as shown is formed.

Next, a second etching is performed in process G, and unnecessary resist is removed in process H, thereby completing a four-stage binary diffractive optical element. Here, the etching depth d2 in the second etching is determined by d2 = λ / 4 (n−1).

Although the description here has been made for a four-stage structure, it is well known that a binary diffractive optical element having an eight-stage or sixteen-stage structure can be manufactured by repeating the above process. .

In the above-described method, the number of steps that can be manufactured is limited to 2 ⁿ (n: natural number).
By freely selecting the number of masks to be used and the pattern line width, a binary diffractive optical element having an arbitrary number of steps can be manufactured.

Although the diffraction efficiency is reduced to some extent by approximating the shape in a stepwise manner, it is approximately 95% in approximation of eight steps.
%, A diffraction efficiency of about 99% is obtained by approximation of 16 steps, and it can be used without any problem in practical use.

[0015]

When the diffractive optical element is used as a part of the optical system, the above-mentioned various advantages can be obtained. However, the substrate shape of such a binary diffractive optical element often uses a parallel plane plate for ease of manufacture, and the thickness of the substrate is generally thin. When used in an optical system, the substrate is arranged near the pupil. In many cases, the effective diameter of the diffractive optical element becomes extremely large due to a request such as an increase in NA. As described above, when the effective diameter of the diffractive optical element is large and the thickness of the substrate is small, there is a concern that the imaging performance may be deteriorated due to its own weight deformation.

According to the present invention, when a diffractive optical element is used as a part of an optical system, deterioration of the imaging performance due to deformation of the diffractive optical element can be corrected, and the optical performance can be maintained satisfactorily. And an optical system having the diffractive optical element.

[0017]

An optical system having a diffractive optical element according to the present invention comprises: (1-1) a diffraction grating having a periodic structure for converting an incident wavefront into a predetermined wavefront is provided on a substrate surface. It is characterized by having a diffractive optical element and a correcting optical element for correcting a change in optical characteristics after deformation of the diffractive optical element when the diffractive optical element is arranged in an optical system.

(1-2) A diffractive optical element in which a diffraction grating having a periodic structure for converting an incident wavefront into a predetermined wavefront is provided on a substrate surface and the diffractive optical element is caused by its own weight deformation or deformation of a lens barrel. A correction optical element for correcting a change in optical characteristics.

[0019]

FIG. 1 is a sectional view of a main part of a first embodiment of the present invention. In the figure, reference numeral 101 denotes a diffractive optical element, which is formed on a transparent substrate 104. A correction optical element 102 corrects a change in optical characteristics due to the weight of the diffractive optical element 101. The correction optical element 102 is a transparent substrate 1
The configuration is such that the aspheric surface 103 is formed on the optical axis 105 in a rotationally symmetric manner with respect to the optical axis 105.

The feature of this embodiment is that it occurs when the diffractive optical element 101 having positive power (refractive power) (hereinafter also referred to as "diffraction substrate") undergoes deformation such as its own weight deformation or lens barrel deformation. The objective is to correct various aberrations by the correction optical element 102 having an aspheric surface disposed in the vicinity thereof, and to maintain good optical performance.

Next, the features of the optical system having the diffractive optical element of the present embodiment will be described. First, the amount of deformation of the substrate under its own weight is determined using a simple model. Deformation when a uniform load is applied to a thin disk in a direction perpendicular to the plane can be calculated using a theoretically obtained formula. Here, the amount of deformation is determined for a simple case of simple peripheral support (no sliding constraint).

FIG. 2A shows a thin circular plate 401 (parallel flat plate) when there is no deformation as a substrate. here,
a is the distance from the optical axis 105, that is, the radius (m
m) and t is its thickness (mm). FIG. 2
(B) shows the state of the thin circular plate 404 after deformation.
w indicates the amount of deformation (mm) in the thickness t direction.

From the above figures, it is known that the deformation amount w at a distance h from the optical axis 105 can be obtained by the following equation.

[0024]

(Equation 1) Here, E: Young's modulus [N / mm ² ], ν: Poisson's ratio [Dimensionless amount], p: Load per unit area [N / mm]
² ], a: radius of the disc [mm], t: thickness [mm],
h: Radial coordinate [mm] If the discussion is limited to the own weight deformation, the load p per unit area is given by p = 9.81ρt [N / mm ² ] using a density of ρ [kg / mm ³ ]. (2) given by

Next, in equation (1), for example, the physical property value of fused quartz E = 7.31 × 10 ⁴ [N / mm ² ] ν = 0.170 ρ = 2.22 × 10 ⁻⁶ [kg / mm ^3] } When the equation (3) is substituted, the deformation amount W is expressed as a fourth-order function of the radius h from the equation (1) as follows.

W = α1 + α2 · h ² + α3 · h ⁴ [mm] ‥‥‥ Formula (4) Here, the radius a of the disk 401 is a = 75 [mm], and the thickness t is t = 1 [mm]. When the coefficient of Expression (4) is obtained as
It looks like this:

Α1 = 7.58e−3, α2 = −1.65e−6, α3 = 5.42e−11 Equation (5) In this embodiment, the shape of the substrate 401 changes due to its own weight deformation. Only the influence of the phase distribution function as the diffractive optical element is ignored. The reason is that the amount of self-weight deformation in the direction perpendicular to gravity is usually at a level that can be ignored compared to the amount of change in the direction of gravity. For example, as shown in FIG. 3B, the diffractive optical element 502 after its own weight deformation has substantially the same pitch as the pitch of the phase distribution function of the diffractive optical element 101 before its own weight deformation in FIG. For example, the pitch is p). That is, this corresponds to the fact that the position of the boundary of the annular zone in the radial direction r does not change before and after the deformation.

In the present embodiment, the weight deformation other than the diffractive optical element 101 is not considered. Appears as. Here, as shown in FIG. 3, both surfaces of the diffractive optical element 101 are s,
It is represented by s + 1, and the diffractive optical element 101 is assumed to be formed on the s plane (s = 1 in this embodiment). Further FIG.
As shown in the figure, the surface distance between the substrate 104 of the diffractive optical element 101 and the correction optical element 102 is represented by ds + 1.

The substrate shapes of the first surface and the second surface of the substrate 104 are represented by the following aspherical general formula.

[0030]

(Equation 2) Here, c is the curvature of the surface, x is the coordinates in the optical axis direction (thickness direction t), and k, A, and B are aspherical coefficients, respectively.

Here, assuming that k (cone constant) =-1, the surface shape x after its own weight deformation can be expressed as follows.

[0032]

(Equation 3) Compared to equation (4),

[0033]

(Equation 4) Becomes Further, the variation Δds of the surface distance ds + 1 can be considered in direct connection with the maximum displacement (α1) due to the own weight deformation.

In this way, the change amount Δds of the surface deformation amount W and the surface distance ds + 1 of the diffractive optical element due to its own weight deformation is obtained. A correction optical element 102 having an aspherical surface is arranged beforehand or after its own weight deformation in the vicinity of the diffractive optical element, and the aspherical amount is appropriately set so as to correct the imaging performance change due to the surface deformation and the change in the surface interval. By doing so, the effect of own weight deformation is prevented.

Next, specific numerical examples of the present embodiment will be shown. FIG. 4 is a schematic diagram of an optical system according to the present embodiment. In the figure, reference numeral 601 denotes a diffractive optical element;
Denotes a correction optical element, and 603 denotes an optical axis. Reference numeral 604 denotes an aspherical surface of the correction optical element 602. Also 60
Reference numeral 5 denotes an on-axis light beam.

First, various data of the optical system in a state where the diffractive optical element 601 does not have its own weight deformation is shown in (Numerical Example 1).
The imaging performance is shown in FIG. This figure shows the spherical aberration. In this case, the substrate of the diffractive optical element 601 is a plane-parallel plate, and no consideration is given to deformation due to its own weight. It is also assumed that the aspheric surface of the correction optical member 602 is not introduced. Note that the coefficients of the phase distribution function in the numerical examples are defined by the following equations.

F (h) = a1 · h ² + a2 · h ⁴ + a3
· H ⁶ + a 4 · h ⁸ + ‥‥ g (h) = 2π / λ · f (h) where f (h) is an optical path length function, g (h) is a phase distribution function, and a1, a2, a3 a4, ‥‥: coefficients of the phase polynomial lambda: wavelength (numerical example 1) object distance = _{infinity, λ = 248nm i r i d} i 1 0 1.0 n = quartz 2 0 5.0 3 0 10.0 n = quartz 4 0 diffractive optical Coefficient of phase distribution function of element i a1 a2 a3 a4 1 -0.00333 0.359917e-7 -0.74132e-12 0.140112e-16 Next, the diffractive optical element 601 has its own weight deformation. Consider a state in which the correction is not performed by the aspherical surface provided in the correction optical element 602. Various data of the optical system in this case are shown in (Numerical Example 2), and the imaging performance is shown in FIG. That is, the diffractive optical element 601 and the surface distance ds + 1 are deformed and changed according to the equations (4) and (8). The aspheric surface of the correction optical element 602 is not introduced. At this time, as can be seen from FIG. 5B, the spherical aberration is deteriorated due to the own weight deformation of the diffractive optical element.

[0038] (Numerical Example 2) object distance = infinite _{i r i d i 1 -302396.4251 1.0} n = quartz 2 -302396.4251 4.992416 3 0 10.0 n = coefficient of the phase distribution function of quartz 4 0 diffractive optical element i a1 a2 a3 a4 1 -0.00333 0.359917e-7 -0.74132e-12 0.140112e-16 Aspherical surface coefficient i KA 1 -1.0 0.54246e-10 2 -1.0 0.54246e-10 Further, the influence of the self-weight deformation of the diffractive optical element 601 is considered. hand,
One surface 60 of the correction optical element 602 disposed in the vicinity thereof
FIG. 5 shows various data of the optical system when the imaging performance is corrected by applying an aspheric surface to No. 4 and a spherical aberration diagram in FIG.
It is shown in (C). As is apparent from FIG.
It can be seen that the effect of the self-weight deformation of No. 1 has been sufficiently corrected.

[0039] (Numerical Example 3) object distance = infinite _{i r i d i 1 -302396.4251 1.0} n = quartz 2 -302396.4251 4.992416 3 0 10.0 n = coefficient of the phase distribution function of quartz 4 0 diffractive optical element i a1 a2 a3 a4 1 -0.00333 0.359917e-7 -0.74132e-12 0.140112e-16 Aspherical surface coefficient i KA BCD 1 -1.0 0.54246e-10 2 -1.0 0.54246e-10 3 -1.0 -0.157455e-9 0.601459e -13 -0.13130e-16 0.102233e-20 FIG. 6 is a schematic view of a main part of an optical system having a diffractive optical element according to a second embodiment of the present invention. FIG. 6A is a diagram in which the back surface of the correcting optical element 102 in FIG.
(B) is a schematic diagram in the case where a plurality of correction optical elements are arranged.

In the drawing, reference numerals 801 and 810 denote diffractive optical elements in a state of being deformed by their own weights, and reference numerals 802 and 811 denote correction optical elements. 805 is an optical axis, 803, 806,
Reference numeral 807 denotes an aspherical portion, and reference numerals 804, 807, and 809 denote substrate portions of the correction optical element.

Incidentally, both surfaces of the substrate 804 may be aspherical. Further, the aspherical surface is not limited to be rotationally symmetric with respect to the optical axis, and may have any aspherical shape as long as the shape corrects the deformation of the diffractive optical element. Also,
Even if the configuration as in the above embodiment is applied to a part of a certain optical system, the same effect as described above can be obtained. For example, in the vicinity where a diffractive optical element is arranged in an imaging optical system,
By arranging a correcting optical element for canceling the influence of the weight deformation of the diffractive optical element, the same effect of correcting the imaging performance degradation as in the above embodiment can be obtained.

In this embodiment, the diffractive optical element having a positive refractive power is described, but the optical system may have a negative refractive power.

In the above embodiment, the "deformation of its own weight" is mentioned as the deformation of the diffractive optical element. However, the present invention is similarly applied to the correction of the deformation of the substrate of the diffractive optical element (for example, the deformation of the lens barrel). can do.

FIG. 7 is a schematic view of a principal part of a third embodiment when an optical system having a diffractive optical element according to the present invention is applied to a projection exposure apparatus for manufacturing a semiconductor element. In the figure, a circuit pattern provided on a reticle R illuminated with exposure light from an illumination system ER is projected on a wafer W surface by a projection optical system TL. Here, the projection optical system TL has an optical system BO having a diffractive optical element and a correction optical element. Then, semiconductor devices are manufactured on the wafer W through a known development process.

[0045]

According to the present invention, by setting each element as described above, when a diffractive optical element is used as a part of an optical system, deterioration of the imaging performance due to deformation of the diffractive optical element is prevented. It is possible to achieve an optical system having a diffractive optical element that has been corrected so as to maintain good optical performance.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a main part of a first embodiment of the present invention.

FIG. 2 is an explanatory view of the deformation of the substrate by its own weight.

FIG. 3 is an explanatory view of the weight change of the diffractive optical element.

FIG. 4 is an explanatory view of the weight change of the diffractive optical element.

FIG. 5 is an explanatory diagram of an aberration with respect to a substrate's own weight deformation.

FIG. 6 is a schematic diagram of a main part of a second embodiment of the present invention.

FIG. 7 is a schematic diagram of a main part of a third embodiment of the present invention.

FIG. 8 is an explanatory diagram of binary optics according to the present invention.

FIG. 9 is an explanatory diagram of a method for producing binary optics according to the present invention.

[Explanation of symbols]

101,601,801,810 Diffractive optical element 102,602,802,811 Correction optical element 103,604,803,806,807 Aspheric surface 104,401,404,804,807,809
substrate

Claims (3)

[Claims] 1. A diffractive optical element having a diffraction grating having a periodic structure for converting an incident wavefront into a predetermined wavefront provided on a substrate surface, and a diffractive optical element when the diffractive optical element is arranged in an optical system. An optical system having a diffractive optical element, comprising: a correction optical element for correcting a change in optical characteristics after deformation. 2. A diffractive optical element having a diffraction grating having a periodic structure for converting an incident wavefront into a predetermined wavefront on a substrate surface, and the optical characteristics of the diffractive optical element due to its own weight deformation or deformation of a lens barrel holder. An optical system having a diffractive optical element, comprising: a correcting optical element for correcting a change. 3. The optical system according to claim 1, wherein the correcting optical element has an aspherical surface.