Reflective / Refractive Objective Lens

Abstract

To provide an objective lens that performs aberration correction over a wide wavelength band including the deep ultraviolet region and has a long working distance. [Solution] Starting from the conjugate side at infinity, the optical system consists of a first group 210 composed of only one or more refractive optical elements, a second group 220 comprising multiple optical elements and including two reflective surfaces, and a third group 230 comprising at least one optical element, having an intermediate imaging point inside the optical system and satisfying the following condition. JPEG2026078417000028.jpg16170 However, f1: focal length of the first group, f3: focal length of the third group.

Description

Technical Field

[0001] The present invention relates to a refractive-reflective objective lens, and particularly to a refractive-reflective objective lens suitable for microscope observation and inspection applications in a wide wavelength band.

Background Art

[0002] In a wide range of fields such as materials, semiconductors, and biomedicine, various inspection devices using optical techniques are being utilized.

[0003] In such inspection devices, composite observation (inspection) may be performed by combining several illumination methods. As an inspection device of this type, for example, an inspection device that combines bright-field observation by epi-illumination transmitted through an objective lens device and dark-field observation by laser oblique-incidence illumination is known (for example, Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Non-Patent Documents

[0005] [Non-Patent Document 1] David R. Shafer et al., “Small catadioptric microscope optics“, Current Developments in Lens Design and Optical Engineering V, Proceedings of SPIE Vol. 5523 [Non-Patent Document 2] J. Webb et al., “Optical Design Forms for DUV&VUV Microlithographic Processes“, Optical Microlithography XIV, Proceedings of SPIE Vol. 4346 [Overview of the project] [Problems that the invention aims to solve]

[0006] Incidentally, there is a demand for using broad-wavelength light sources such as lamps for incident illumination and lasers for oblique incident illumination. To perform such inspections, an objective lens device is needed that can handle a broad wavelength range and has a long working distance that allows for oblique incident illumination. Many objective lenses have been proposed so far (Patent Documents 4-8, Non-Patent Documents 1-2), but no objective lens is known that can correct aberrations in a broad wavelength range including the deep ultraviolet region and achieve both a long working distance and a wide wavelength range.

[0007] One possible solution is to prepare multiple objective lens systems with different optical properties and switch between them as needed depending on the intended use. However, switching between multiple objective lenses presents challenges, such as increasing the complexity of the entire system and raising costs.

[0008] In view of these circumstances, the present invention aims to provide an objective lens that performs aberration correction over a wide wavelength band including the deep ultraviolet region and has a long working distance. [Means for solving the problem]

[0009] In order to solve the above problems, an aspect of the present invention is an infinity-corrected refractive-reflective objective lens, which includes, in order from the infinity conjugate side, a first group (10, 110, 210) composed of at least one optical element composed only of refractive optical elements, a second group (20, 120, 220) including two reflecting surfaces and composed of a plurality of optical elements, and a third group (30, 130, 230) composed of at least one optical element, has an intermediate imaging point inside the optical system, and satisfies the following conditional expression.

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[0010] According to this configuration, aberration correction over a wide wavelength band including the deep ultraviolet region is achieved for the objective lens.

[0011] In the above aspect, it is desirable that the first group satisfies the following conditions.

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[0012] According to the above aspect, in addition to chromatic aberration correction in a wide wavelength region, a high NA is achieved.

[0013] In the above aspect, it is desirable that the third group is composed of two reflecting surfaces (M3, M4, MM23, MM24, MM31, MM32) and refractive optical elements (L6 to L9, L26 to L28, L34 to L38).

[0014] According to this configuration, chromatic aberration correction in a wide wavelength region and a long working distance are compatible.

[0015] In the above aspect, all the refractive optical elements may be composed of one type of optical glass.

[0016] According to this configuration, in addition to chromatic aberration correction in a wide wavelength range, the change in optical performance due to environmental temperature changes is reduced.

Advantages of the Invention

[0017] According to the above aspects, the objective lens performs aberration correction over a wide wavelength band including the deep ultraviolet region and has a long working distance.

Brief Description of the Drawings

[0018] [Figure 1] Conceptual diagram of a refractive-reflective objective lens according to one embodiment of the present invention [Figure 2] Explanatory diagram of an optical system including one ideal lens [Figure 3] Explanatory diagram of an optical system including two ideal lenses [Figure 4] Configuration diagram of the refractive-reflective objective lens of Example 1 [Figure 5] Graph showing the axial chromatic aberration characteristics of the refractive-reflective objective lens of Example 1 [Figure 6] Configuration diagram of the refractive-reflective objective lens of Example 2 [Figure 7] Graph showing the axial chromatic aberration characteristics of the refractive-reflective objective lens of Example 2 [Figure 8] Configuration diagram of the refractive-reflective objective lens of Example 3 [Figure 9] Graph showing the axial chromatic aberration characteristics of the refractive-reflective objective lens of Example 3

Modes for Carrying Out the Invention

[0019] Hereinafter, embodiments of the refractive-reflective objective lens according to the present invention will be described with reference to FIGS. 1 to 9.

[0020] First, the principle of axial chromatic aberration correction of the refractive-reflective objective lens (hereinafter sometimes abbreviated as the objective lens) according to the present invention will be described with reference to FIG. 1.

[0021] As shown in Figure 1, consider a reflective / refracting objective lens 1 consisting of a first group 10 (front lens group), a second group 20 (intermediate group), and a third group 30 (rear lens group) arranged on a single optical axis. The first group 10 and the third group 30 are thin-walled lens systems made of the same glass material. The second group 20 is a lens system that does not depend on the wavelength λ.

[0022] Below, we examine the conditions for correcting axial chromatic aberration in this lens system using the ray matrix.

[0023] The ray matrix Rf of the first group of 10 is expressed using wavelength λ and power φ (refractive force).

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[0024] Furthermore, the ray matrix M of the second group 20 is defined as follows.

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[0025] At this time, the ray matrix A of the entire lens system is,

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[0026] Consider the image point corresponding to the object point at infinity in this lens system. The light rays incident on the lens system are

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[0027] Currently, since the second group 20 is assumed to be independent of wavelength λ, the only parameter that changes with wavelength λ is φ(λ). Therefore, in order to correct axial chromatic aberration, the coefficient of φ(λ) should be 0.

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[0028] Furthermore, the following equation holds true based on the properties of the ray matrix.

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[0029] In summary, the conditions for correcting axial chromatic aberration are:

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[0030] This equation indicates that the first group 10 and the third group 30 are in a conjugate configuration, and that the second group 20 has a predetermined multiplier determined by the power ratio of the first group 10 and the third group 30.

[0031] Furthermore, the fact that γ<0 in [Equation 12] indicates that the powers of the first group 10 and the third group 30 are opposite in sign. Expressing this specifically in a mathematical formula, if we let the focal length of the first group 10 be f1 and the focal length of the third group 30 be f3,

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[0032] Next, we will consider a specific power distribution using an ideal lens. Here, an ideal lens is a hypothetical optical element that does not produce aberrations, including chromatic aberration. It is important to note that a solution that satisfies the aforementioned axial chromatic aberration correction condition cannot be uniquely determined. Therefore, it is necessary to first determine the number of lens elements and then consider the solution by substituting specific numerical values.

[0033] Now, consider the case where a thin-walled lens, an ideal lens, and another thin-walled lens are placed with an air gap between them.

[0034] First, assuming the use of one ideal lens, the design solution shown in Figure 2 is obtained. As shown in Figure 2, this system consists of a positive thin-walled lens La, an ideal lens Lb, and a negative thin-walled lens Lc, in that order from the conjugate side at infinity. This can be interpreted as a system in which the field lens of an optical system called a Schupmann type is replaced with an ideal lens.

[0035] However, this system is unsuitable for achieving a long working distance because the image point is located inside the optical system.

[0036] Therefore, it is necessary to increase the number of ideal lenses to two to ensure design flexibility and achieve both axial chromatic aberration correction and a long working distance. The design solution shown in Figure 3 is obtained as such. This system consists of, in order from the conjugate side at infinity, a negative thin lens Ld, the first ideal lens Le, the second ideal lens Lf, and a positive thin lens Lg, and has an intermediate imaging point, with the final image point located outside the optical system.

[0037] In other words, with respect to the focal length f1 of the first group 10, the following equation is satisfied.

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[0038] Up to this point, we have considered power distribution using an ideal lens. While the ideal lens is, of course, a hypothetical element, if we limit ourselves to chromatic aberration, a system that does not produce it can be realized by using a reflective element. In other words, by replacing the ideal lens with a reflective optical element, a reflective-refractive objective lens that can be realistically manufactured can be obtained.

[0039] In actual design, optimization is necessary while considering specifications and imaging performance. Therefore, the actual design system does not necessarily have to strictly match the system spacing and focal length shown in Figure 3. However, the equation determined solely by the sign of the refractive power must be satisfied.

[0040] In summary, the objective lens 1 of this embodiment is an infinity-corrected reflective-refracting objective lens having, in order from the infinity conjugate side, a first group 10, a second group 20, and a third group 30. The first group 10 is composed only of a plurality of refractive optical elements. The second group 20 includes two reflective surfaces arranged opposite each other that realize the ideal lenses Le and Lf of the optical system shown in Figure 2. The third group 30 includes only refractive optical elements, or two reflective surfaces arranged opposite each other and a refractive optical element. The objective lens 1 has an intermediate imaging point inside the optical system and satisfies the following conditional equation (theoretical equation) with the focal length of the first group 10 being f1 and the focal length of the third group 30 being f3.

[0041]

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[0042] This objective lens 1 corrects aberrations over a wide wavelength range, including the deep ultraviolet region, and also has a long working distance.

[0043] Furthermore, group 10 satisfies the following conditions.

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[0044] This objective lens 1 achieves high numerical aperture (NA) in addition to chromatic aberration correction over a wide wavelength range and a long working distance.

[0045] It is preferable that all refractive optical elements used in the first group 10 and the third group 30 be made of a single type of glass material. This satisfies the premise of the aforementioned theory that the elements must be made of the same glass material, and broadband chromatic aberration correction is achieved. Furthermore, it becomes easier to match the coefficients of linear expansion of the optical material and the mechanical component material, resulting in a reflective refractive objective lens with less change in optical performance due to changes in ambient temperature. It is also preferable to include two reflective surfaces in the third group 30. This allows the image point to be located outside the optical system, achieving a long working distance.

[0046] Based on the considerations discussed so far, the significance of the present invention will be explained by comparing it again with prior art.

[0047] Patent Document 4 describes an objective lens using a refractive optical system that can handle a wavelength range of approximately ±5 nm in the deep ultraviolet region. However, with a refractive optical system like the one shown in Patent Document 4, it is difficult to correct chromatic aberration over a wide wavelength range from the deep ultraviolet to the visible region.

[0048] Patent documents 5-6 and non-patent document 1 describe reflective-refractive optical systems with chromatic aberration correction over a wide wavelength range. The objective lenses described in these documents are known to have evolved from an arrangement called the Schupmann type. However, as shown herein, although the Schupmann type is one particular solution that provides good correction of axial chromatic aberration, it is unsuitable for achieving long working distances. In fact, the patent documents only show systems with working distances of at most less than 1 mm. Furthermore, due to the large diameter of the optical elements near the image plane, it is unsuitable for oblique incidence illumination.

[0049] Patent Document 7 describes an optical system that partially applies the optical system of Patent Document 3 while increasing the number of reflective surfaces. However, the optical system described in Patent Document 7 does not achieve chromatic aberration correction over a wide wavelength range from the deep ultraviolet to the visible region. This is because it does not satisfy the theoretical formula disclosed in [Equation 12] of this specification.

[0050] Patent document 8 and non-patent document 2 describe a reflective refractive objective lens that has evolved from the so-called Schwarzschild type. They also point out that reflective refractive optical systems are superior to refractive optical systems in terms of chromatic aberration correction. However, this reflective refractive objective lens only corrects chromatic aberration in the wavelength range of about 10 nm and has not been able to achieve chromatic aberration correction over a wide wavelength range from the deep ultraviolet to the visible range.

[0051] The objective lens 1 of this embodiment is a new lens type that achieves chromatic aberration correction over a wide wavelength range from the deep ultraviolet to the visible region, while also achieving a long working distance, and is optically distinct from conventionally known lenses. Furthermore, the chromatic aberration correction theory and optical design method disclosed herein suggests the realization of more diverse and high-value-added reflective-refractive optical systems, and is of great technical significance.

[0052] (Example 1) The lens data for Example 1 is shown in Table 1. The corresponding values ​​for the conditional formula in Example 1 are shown in Table 2. The reflective / refracting objective lens shown in Example 1 has a wavelength λ = 266 nm to 900 nm, a focal length = -2.0 mm, and an NA = 0.85.

[0053] As shown in Figure 4, the reflective refractive objective lens of Example 1 is composed of, in order from the infinity conjugate side, a first group 110 having negative power, a second group 120 having positive power, and a third group 130 having positive power. The reflective refractive objective lens of Example 1 has an intermediate image point on the inside of the optical system (third group 130).

[0054] The first group 110 consists of five refractive optical elements L1 to L5. The second group 120 consists of two mirrors M1 (reflecting surface) and M2 (reflecting surface). The third group 130 consists of two mirrors M3 (reflecting surface) and M4 (reflecting surface) and four refractive optical elements L6 to L9.

[0055] The refractive optical elements L1 to L9 included in the reflective-refracting objective lens of Example 1 are all made of synthetic silica.

[0056] Figure 5 shows the axial chromatic aberration characteristics (wavelength-focus shift performance) of the reflective / refractive objective lens of Example 1. [Table 1] [Table 2]

[0057] (Example 2) The lens data for Example 2 is shown in Table 3. The corresponding values ​​for the conditional formula in Example 2 are shown in Table 4. The reflective / refractive objective lens shown in Example 2 has a wavelength λ = 193 nm to 1100 nm, a focal length of -4.0 mm, and an NA of 0.85.

[0058] As shown in Figure 6, the reflective refractive objective lens of Example 2 is composed of, in order from the infinity conjugate side, a first group 210 having negative power, a second group 220 having positive power, and a third group 230 having positive power. Similar to Example 1, the reflective refractive objective lens of Example 2 also has an intermediate imaging point inside the optical system.

[0059] The first group 210 consists of five refractive optical elements L21 to L25. The second group 220 consists of two reflective refractive optical elements (sometimes called manzine mirrors, manzian mirrors, or mangin mirrors) MM21 (reflective surface) and MM22 (reflective surface). The third group 230 consists of two reflective refractive optical elements MM23 (reflective surface) and MM24 (reflective surface) and three refractive optical elements L26 to L28. The image-plane side of the reflective refractive optical element MM24 has different radii of curvature at the center and the periphery, with the center being the reflective surface.

[0060] The refractive optical elements L21 to L28 in the reflecting-refracting objective lens of Example 2 are all made of fluorite (CaF2). Furthermore, the materials used in the reflecting-refracting optical elements MM21 to MM24 are all made of fluorite (CaF2).

[0061] Figure 7 shows the axial chromatic aberration characteristics (wavelength-focus shift performance) of the reflective / refractive objective lens of Example 2. [Table 3] [Table 4]

[0062] (Example 3) The lens data for Example 3 is shown in Table 5. The corresponding values ​​for the conditional formula in Example 3 are shown in Table 6. The reflecting refractive objective lens shown in Example 3 has a wavelength λ = 266 nm to 900 nm, a focal length = -10.0 mm, and an NA = 0.5. In the lens data in Table 5, lens surfaces 22 and 32 marked with an asterisk (*) are aspherical. The coefficient values ​​that give the aspherical shape of lens surfaces 22 and 32 marked with an asterisk (*) in the lens data are shown in Table 7.

[0063] The shape of the aspherical surface is such that, when y is the displacement in the direction perpendicular to the optical axis, z is the displacement in the direction of the optical axis from the intersection of the aspherical surface and the optical axis, K is the conic coefficient, and A4, A6, A8, A10, and A12 are the 4th, 6th, 8th, 10th, and 12th order aspherical coefficients, respectively, the coordinates of the aspherical surface are expressed by the following equation.

[0064]

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[0065] As shown in Figure 8, the reflective refractive objective lens of Example 3 is composed of, in order from the infinity conjugate side, a first group 310 having negative power, a second group 320 having positive power, and a third group 330 having positive power. Similar to Example 1, the reflective refractive objective lens of Example 3 also has an intermediate imaging point inside the optical system.

[0066] The first group 310 consists of three refractive optical elements L31 to L33. The second group 320 consists of two mirrors M31 (reflecting surface) and M32 (reflecting surface). The third group 330 consists of two reflective refractive optical elements MM31 (reflecting surface) and MM32 (reflecting surface) and five refractive optical elements L34 to L38. The image-side surface of the reflective refractive optical element MM32 has a reflective surface in the center and a refractive surface in the periphery.

[0067] The refractive optical elements L31 to L38 in the reflective-refractive objective lens of Example 3 are all made of synthetic quartz. Furthermore, the materials used in the reflective-refractive optical elements MM31 to MM32 are all made of synthetic quartz.

[0068] Figure 9 shows the axial chromatic aberration characteristics (wavelength-focus shift performance) of the reflective / refractive objective lens of Example 3. [Table 5] [Table 6] [Table 7]

[0069] Although the present invention has been described above in terms of preferred embodiments, as will be easily understood by those skilled in the art, the present invention is not limited to these embodiments and can be modified as appropriate without departing from the spirit of the invention. Furthermore, not all of the components shown in the above embodiments are necessarily essential, and they can be selected and omitted as appropriate without departing from the spirit of the invention. [Explanation of Symbols]

[0070] 1: Reflective / Refracting Objective Lens (Objective Lens) 10: 1st group 20: 2nd group 30: 3rd group 110: 1st group 120: 2nd group 130: 3rd group 210: 1st group 220: 2nd group 230: 3rd group 310: 1st group 320: 2nd group 330: 3rd group M3, M4: Mirror M31~M32: Mirror MM23, MM24: Reflective and refractive optical elements MM31, MM32: Reflective and refractive optical elements L6~L9: Refractive optical elements L21~L25: Refractive optical elements L34~L38: Refractive optical elements

Claims

1. An infinity-corrected reflective refractive objective lens, Starting from the infinity conjugate side, A first group consisting of only one or more refractive optical elements, A second group consisting of multiple optical elements, including two reflective surfaces, It consists of a third group comprising at least one optical element, A reflective-refracting objective lens characterized by having an intermediate image-forming point within the optical system and satisfying the following condition. [Number 18] However, f1: Focal length of the first group f3: Focal length of the third group

2. The first group satisfies the following conditional expression, characterized in that it is a reflective refractive objective lens according to claim 1. [Number 19]

3. The third group is characterized by including two reflective surfaces, as described in claim 2.

4. The reflective refractive objective lens according to any one of claims 1 to 3, characterized in that all refractive optical elements included in the reflective refractive objective lens are composed of one type of glass material.

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