Objective lens

Abstract

To establish a configuration of a compact objective lens that is less likely to be damaged, is achromatized in an oscillation wavelength of a laser, has high NA, and can take a wide viewing angle equally to that of a normal microscope.SOLUTION: An objective lens 1 is composed of nine lenses of a first lens to a ninth lens arranged in order from an enlargement side. The first lens L1 is formed of a meniscus lens having negative refractive power. The second lens L2 is formed of a biconcave lens. The third lens L3 is formed of a meniscus lens having positive refractive power. The fourth lens L4 is formed of a biconvex lens. The fifth lens L5 is formed of a meniscus lens having negative refractive power. The sixth lens L6 is formed of a biconvex lens. The seventh lens L7 is formed of a lens with an arbitrary shape having positive refractive power. The eighth lens L8 is formed of a meniscus lens having positive refractive power. The ninth lens L9 is formed of a meniscus lens having positive refractive power. The objective lens 1 does not include a cemented lens, has a numerical aperture larger than 0.75, and has a half viewing angle of 2.5° or more. A thickness of the ninth lens L9 is 1.5-3 times a focal length of the objective lens 1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an objective lens, and more particularly to a small objective lens suitable for focusing a laser light source or for observation under illumination by a laser light source. [Background technology]

[0002] Objective lenses with aberration correction for various laser wavelengths are widely used for imaging applications in laser processing equipment, various inspection devices, etc. Examples of laser wavelengths include harmonics of YAG lasers such as 266 nm, 355 nm, and 532 nm, excimer lasers with 193.4 nm and 248 nm or solid-state laser wavelengths similar to these, and laser diode light sources with 405 nm.

[0003] It is desirable for this type of objective lens to have a large numerical aperture (NA), a wide field of view, and at the same time, be small and low cost. A detailed explanation will be given below.

[0004] First, it is desirable that the NA be 0.75 or greater. This is because the spot diameter of the lens is determined by the NA, and the larger the NA, the higher the resolving power that can be achieved.

[0005] For observation purposes, it is desirable to have a field of view equivalent to that of a typical microscope objective. Here, the half field of view angle ω is defined by the following formula, where D is the field diameter and f is the focal length of the objective. ω=arctan(D÷2÷f) Alternatively, it can be defined as follows using the image height y: ω=arctan(y÷f)

[0006] For example, consider a 50x objective with a common field number of 20. In this case, if the focal length of the tube lens is f = 200 mm, then the focal length of the objective is f = 4 mm. The field number is the field diameter measured in mm on the image plane side of the tube lens, so the diameter of the objective's sample plane is D = 20 mm ÷ 50 = 0.4 mm. In this case, the field half angle of the objective is ω = arctan(0.4 mm ÷ 2 ÷ 4 mm) = 2.86°.

[0007] Furthermore, it is desirable for this type of objective lens to be about the same size as commercially available microscopes. Specifically, it is desirable for the overall length to be 100 mm or less, and for the diameter, including the lens barrel, to be 40 mm or less. This is because an inexpensive commercially available piezo stage can be used to drive the lens.

[0008] In terms of cost, it is desirable that this type of objective lens not include aspherical surfaces or diffractive optical elements. In other words, it is desirable that each element that makes up the objective lens be made up of conventional, ordinary spherical lenses. This is because special optical elements such as aspherical surfaces and diffractive optical elements not only significantly increase costs, but also introduce error factors that are not present in spherical lenses.

[0009] Naturally, it is desirable to minimize the number of lenses while still achieving achromatism within the wavelength range of the laser. This is due not only to cost considerations, but also to the fact that reducing the number of lenses eliminates the adverse effects of flare and ghosting that occur on the lens surfaces.

[0010] Therefore, even for special objective lenses specialized for focusing a single-wavelength laser light source or for observation under illumination by a laser light source, they must have a larger NA, a wider field of view, and be low cost while achieving a size comparable to that of commercially available objective lenses. Furthermore, for practical use, they must be achromatic within the spectral range of the laser light source (several pm to several hundred pm), and it is desirable to keep the number of lenses as small as possible.

[0011] Here, as conventional objective lenses, those disclosed in Non-Patent Document 1 and Patent Documents 1 to 10 are known. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 6-242381 [Patent Document 2] Japanese Patent Application Publication No. 11-30754 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-182116 [Patent Document 4] US Patent Application Publication No. 2004 / 0070846 [Patent Document 5] US Patent Application Publication No. 2006 / 0087725 [Patent Document 6] Japanese Patent Application Laid-Open No. 2000-155267 [Patent Document 7] Japanese Patent Application Laid-Open No. 2004-212920 [Patent Document 8] Japanese Patent Application Laid-Open No. 2004-118072 [Patent Document 9] Japanese Patent Application Laid-Open No. 2010-55006 [Patent Document 10] U.S. Patent No. 6,952,256 [Non-patent literature]

[0013] [Non-Patent Document 1] J. Webb et al., "Optical Design Forms for DUV&VUV Microlithographic Processes", Optical Microlithography XIV, Proceedings of SPIE Vol. 4346 (2001) Summary of the Invention [Problem to be solved by the invention]

[0014] However, no conventional objective lens is known that satisfies all of the above requirements. The problems with conventional objective lenses will be explained below.

[0015] Non-Patent Document 1 describes specific examples of objective lenses suitable for various ultraviolet lasers. However, none of these are suitable for the above-mentioned application of the present invention. For example, Figure 1 of Non-Patent Document 1 shows an example of an objective lens design with an NA of 0.6 and a wavelength of 248.4 nm. However, the NA is small at 0.6 and the track length is extremely long at 315 mm.

[0016] Patent Document 1 discloses an example of an objective lens that can accommodate various laser wavelengths by using a common single lens and varying the spacing between them. While the example disclosed therein is a compact objective lens, as noted in paragraphs

[0006] and

[0013] of Patent Document 1, its configuration is only effective when the field of view is narrow. Calculating the specific field of view, for example, f = 2.5 mm and image height y = 0.05 mm, the half-field angle is ω = arctan(0.05 ÷ 2.5) = 1.14°, which is less than half the half-field angle of a typical microscope objective lens. Furthermore, as shown in the table in paragraph

[0011] of Patent Document 1, the NA cannot be very large except in the design reference state. Therefore, it is not suitable for the application of the present invention described above.

[0017] Patent Document 2 discloses a method for observing various depths of an object while maintaining aberrations constant by dividing the objective lens into a lens closest to the sample and a group of other lenses and moving the lens closest to the sample along the optical axis. According to paragraph

[0014] of Patent Document 2, the focal length of the objective lens is f = 8 mm and the NA is 0.8. However, as will be described later, this configuration does not provide a wide field of view. According to the aberration diagrams (Figures 2 and 3) of the examples in Patent Document 2, the half-angle of the field of view is at most ω = 1.076°. This means that the field of view is less than half that of a conventional microscope, making it unsuitable for the application of the present invention described above.

[0018] Patent Documents 3 and 4 disclose examples using aspherical or diffractive optical elements. However, for the applications envisioned in the present invention, an objective lens consisting entirely of spherical surfaces is optimal in order to construct the objective lens at the lowest possible cost. Therefore, the examples disclosed in these documents are not suitable for the applications of the present invention described above.

[0019] Patent Document 5 discloses various examples of refractive and catadioptric objective lenses at a wavelength of 157 nm. These include immersion lenses with NAs of 1.3, 1.1, and 0.9, but their fields of view are extremely narrow (see paragraph

[0031] ) and their achromatic ranges are also extremely narrow (see paragraph

[0023] ), making them unsuitable for the applications of the present invention described above.

[0020] Patent Documents 6, 7, 8, 9, and 10 also exemplify objective lenses for various ultraviolet laser light sources, but the size of these objective lenses is significantly larger than commercially available microscope objectives. As a result, none of these objective lenses can be mounted on commercially available piezo stages, and they are also extremely expensive. Therefore, they are not suitable for the application of the present invention described above.

[0021] As we have seen so far, there have been no inexpensive, high-performance objective lenses for laser processing equipment or various inspection devices. As a result, equipment has been configured using commercially available objective lenses that were not originally designed for ultraviolet lasers. As a result, there have been problems such as insufficient processing precision, inability to obtain high-quality images, and damage to the objective lenses due to strong laser light sources.

[0022] In view of the above background, the present invention aims to establish the configuration of a small objective lens that is resistant to damage, achromatic within the laser oscillation wavelength, has a high NA, and has a wide field of view equivalent to that of a conventional microscope. [Means for solving the problem]

[0023] In order to solve the above problems, one embodiment of the present invention is an objective lens having nine lenses, including no cemented lenses, a numerical aperture of greater than 0.75, and a half angle of view of 2.5° or more, the objective lens including, arranged in order from the magnification side, a first lens (L1) made of a meniscus lens having negative refractive power with a convex surface facing the magnification side, a second lens (L2) made of a biconcave lens, a third lens (L3) made of a meniscus lens having positive refractive power with a concave surface facing the magnification side, a fourth lens (L4) made of a biconvex lens, and a negative lens having a convex surface facing the magnification side. the fifth lens (L5) consisting of a meniscus lens having a positive refractive power, the sixth lens (L6) consisting of a biconvex lens, the seventh lens (L7) consisting of a lens of any shape having a positive refractive power, the eighth lens (L8) consisting of a meniscus lens having a positive refractive power with its convex surface facing the magnification side, and the ninth lens (L9) consisting of a meniscus lens having a positive refractive power with its convex surface facing the magnification side, and the thickness of the ninth lens closest to the sample surface is 1.5 to 3 times the focal length of the objective lens.

[0024] According to this aspect, a small objective lens is constructed that is resistant to damage, achromatic within the laser oscillation wavelength, has a high NA, specifically 0.75 or more, and can provide a wide viewing angle equivalent to that of a normal microscope.

[0025] In the above-mentioned aspect, it is preferable that the thickness of the ninth lens is 2.13 to 2.15 times the focal length of the objective lens. Furthermore, in the above-mentioned aspect, it is preferable that the dispersion of the fifth lens is greater than the dispersion of the sixth lens. This is because the combination of the fifth lens, which is a concave lens, and the sixth lens, which is a convex lens, can reduce overall chromatic aberration. Furthermore, in the above-mentioned aspect, it is preferable that the third lens, the fourth lens, and the sixth lens to the ninth lens, which have positive refractive power, are made of CaF2 (calcium fluoride, fluorite), and at least one of the first lens, the second lens, and the fifth lens, which have negative refractive power, is made of quartz. This is also because constructing the concave lens from a high-dispersion material and the convex lens from a low-dispersion material helps to reduce overall chromatic aberration. [Effects of the Invention]

[0026] According to the above aspects, it is possible to establish a compact objective lens configuration that is resistant to damage, achromatic within the laser oscillation wavelength, has a high NA, and has a wide field of view comparable to that of a conventional microscope. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a diagram illustrating an optical path of an objective lens according to an embodiment of the present invention; [Figure 2] Illustration of aberration caused by refraction through a thick parallel plate [Figure 3] Longitudinal aberration diagram of the objective lens of Example 1 [Figure 4] Lateral aberration diagram of the objective lens of Example 1 [Figure 5] Optical path diagram of the objective lens of Example 2 [Figure 6] Longitudinal aberration diagram of the objective lens of Example 2 [Figure 7] Lateral aberration diagram of the objective lens of Example 2 [Figure 8] Optical path diagram of the objective lens of Example 3 [Figure 9] Longitudinal aberration diagram of the objective lens of Example 3 [Figure 10] Lateral aberration diagram of the objective lens of Example 3 [Figure 11] Optical path diagram of the objective lens of Example 4 [Figure 12] Longitudinal aberration diagram of the objective lens of Example 4 [Figure 13] Lateral aberration diagram of the objective lens of Example 4 [Figure 14] Optical path diagram of the objective lens of Example 5 [Figure 15] Longitudinal aberration diagram of the objective lens of Example 5 [Figure 16] Lateral aberration diagram of the objective lens of Example 5 DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0029] FIG. 1 is a diagram of optical paths for an objective lens 1 according to an embodiment. As shown in FIG. 1, the objective lens 1 is made up of nine lenses, first lens L1 to ninth lens L9, arranged in order from the magnification side. The first lens L1 is a meniscus lens with negative refractive power and a convex surface facing the magnification side. The second lens L2 is a biconcave lens. The third lens L3 is a meniscus lens with positive refractive power and a concave surface facing the magnification side. The fourth lens L4 is a biconvex lens. The fifth lens L5 is a meniscus lens with negative refractive power and a convex surface facing the magnification side. The sixth lens L6 is a biconvex lens. The seventh lens L7 is a lens of any shape with positive refractive power. In the illustrated example, the seventh lens L7 is a meniscus lens with a convex surface facing the magnification side. The eighth lens L8 is a meniscus lens with positive refractive power and a convex surface facing the magnification side. The ninth lens L9 is a meniscus lens having a positive refractive power and a convex surface facing the magnification side.

[0030] With this configuration, this embodiment achieves the goals of achieving a high NA (high numerical aperture), a wide field of view, and achromatism across the laser wavelength range using a small number of lenses, without increasing the overall length or outer diameter through an optimal lens arrangement. Furthermore, the objective lens 1 does not have a cemented lens in which lenses are cemented together with resin, so the objective lens 1 is less likely to be damaged even when the laser light source is strong.

[0031] First, a wide field of view can be achieved by the action of a negative lens group consisting of a meniscus lens (first lens L1) with negative refractive power and a biconcave lens (second lens L2) with a convex surface facing the magnification side from the most magnification side.

[0032] The field of view of the objective lens 1 is determined by the field curvature of the objective lens 1 itself, and the amount of this curvature is determined by the Petzval sum. The Petzval sum is defined as the sum of the powers of each lens in the optical system divided by the refractive index, so the Petzval sum of an objective lens 1 that has a positive refractive power overall will necessarily be a positive value. Therefore, in order to bring the Petzval sum of the objective lens 1 close to zero, a group with a strong negative refractive power is required.

[0033] The two lenses constituting the first lens L1 and the second lens L2 have strong negative refractive powers, thereby reducing the overall Petzval sum. Furthermore, the shapes of these two lenses, characteristic of the present invention, are close to the shapes that minimize spherical aberration for on-axis marginal rays, thereby minimizing the occurrence of spherical aberration. In other words, if lenses were constructed with shapes other than these, overcorrected spherical aberration would occur, making it impossible to maintain good overall spherical aberration. Conventional objective lenses that do not have this configuration (e.g., Patent Documents 1 and 2, and Figure 1 in Non-Patent Document 1) cannot reduce the Petzval sum, and therefore have large field curvatures, making it impossible to achieve a wide field angle.

[0034] The NA can be increased by the action of the next two lenses: a meniscus lens (third lens L3) with positive refractive power and a concave surface facing the magnification side, and a biconvex lens (fourth lens L4). These two lenses are characterized by having nearly minimal spherical aberration for a diverging light beam. Therefore, these two lenses can widen the width of the diverging light beam while suppressing newly occurring aberrations. Conventional objective lenses that do not have this configuration (for example, Figure 1 in Non-Patent Document 1) cannot correct spherical aberration for a large NA, and therefore cannot achieve a large NA.

[0035] Furthermore, the action of the following meniscus lens (fifth lens L5) with negative refractive power and a convex surface facing the magnification side and the biconvex lens (sixth lens L6) corrects spherical aberration, further increasing the NA. These two lenses have a so-called split doublet shape, which actively corrects spherical aberration. Conventional objective lenses that do not have this configuration (for example, Figure 1 in Non-Patent Document 1) are unable to sufficiently correct spherical aberration and are therefore unable to achieve a large NA.

[0036] Furthermore, the glass materials constituting these two lenses (fifth lens L5 and sixth lens L6) have different dispersions, which allows for better correction of chromatic aberration. For example, in the wavelength range from visible to 355 nm, ordinary optical glass can be selected, with flint glass being used for the concave lens and crown glass being used for the convex lens. Furthermore, in the wavelength range shorter than 355 nm, synthetic quartz can be used for the concave lens and CaF2 for the convex lens.

[0037] Finally, we will explain the functions of the seventh lens element L7, the eighth lens element L8, and the ninth lens element L9. These three lenses allow the axial light beam, which has been expanded once, to be focused with a high NA without causing significant aberrations.

[0038] Here, we will add a bit more about the effect of another feature of the present invention, namely, "the thickness of the ninth lens L9, which is closest to the sample surface, being 1.5 to 3 times the focal length of the entire lens." A thick lens placed on the sample surface side, i.e., on the reduction or focusing side, is useful for correcting spherical aberration as well as chromatic aberration.

[0039] Let us consider theoretically a state in which a parallel plate with a thickness of t and a refractive index of n is inserted into a convergent beam with a convergence angle of U0, as shown in Figure 2, by comparing a thick lens to a parallel plate.

[0040] According to the law of refraction and plane geometry, the image point shift Δx caused by a parallel plate is expressed by the following equation:

number

[0041] If we perform a Taylor expansion of this using NA=sinU0 and take the quadratic terms, we obtain the following approximate equation.

number

[0042] The first term in equation (2) represents the image point movement due to the parallel plate. Since the denominator of the first term includes the refractive index n, it represents axial chromatic aberration when considering the change in refractive index for each wavelength. Considering two refractive indices n1>n2 within the wavelength range used, the difference Δx c Considering this, the following formula is obtained:

number

[0043] For two refractive indices n1>n2, Δx c Since is positive, this indicates over-corrected axial chromatic aberration. Since a normal lens is under-corrected, this cancels out the over-corrected axial chromatic aberration caused by the thick parallel plate, indicating that the axial chromatic aberration can be reduced overall.

[0044] The second term in equation (2) is a positive value that increases as the NA increases. In other words, the second term indicates the amount of over-corrected spherical aberration. Because a normal lens is under-corrected, this cancels out the over-corrected spherical aberration caused by the thick parallel plate, thereby reducing the overall spherical aberration. Furthermore, a thick meniscus lens has the effect of reducing the Petzval sum.

[0045] These effects are more advantageous the thicker the parallel plate corresponding to the meniscus lens. However, if the parallel plate is too thick, it shortens the working distance of the objective lens 1, which is disadvantageous in practical use. Therefore, there is naturally an optimal range for the thickness of the parallel plate. Here, the thickness of the ninth lens is preferably at least in the range of 1.5 to 3 times the focal length of the entire lens. This is because if the thickness of the ninth lens is below the lower limit of 1.5 times the focal length, spherical aberration and chromatic aberration will be undercorrected. On the other hand, if the thickness of the ninth lens is above the upper limit of 3 times, spherical aberration will be overcorrected and the working distance will be adversely affected. Furthermore, as a result of further investigation, it was found that the thickness of the ninth lens L9, which is closest to the sample surface, is more preferably about 2.14 times (2.13 to 2.15 times, preferably 2.135 to 2.14 times) the focal length of the entire lens.

[0046] For the above reasons, in order to achieve a small size and high performance with a small number of lenses and consisting of only spherical surfaces, the configuration shown in Figure 1 is essential. This will be explained using a cited example that does not meet these conditions.

[0047] For example, in Figure 1 of Non-Patent Document 1, there are no "meniscus lenses and biconcave lenses with negative refractive power and convex surfaces facing the magnification side" corresponding to the first lens L1 and second lens L2 located closest to the magnification side. Also, there are no "meniscus lenses and biconvex lenses with negative refractive power and convex surfaces facing the magnification side" corresponding to the fifth lens L5 and sixth lens L6. As a result, the NA of this cited example is small at 0.65, and the overall length is very long.

[0048] In Patent Document 2, there are no "meniscus lenses and biconcave lenses with negative refractive power and convex surfaces facing the magnification side" that correspond to the first lens L1 and second lens L2 arranged on the most magnification side, and therefore the field curvature cannot be corrected, resulting in a narrow field angle.

[0049] As mentioned above, Patent Documents 3 and 4 disclose objective lenses with configurations different from those of the present invention, which use aspherical or diffractive optical elements. However, the configurations of these conventional objective lenses are contrary to the objective of the present invention, which is to configure the objective lens 1 at the lowest possible cost.

[0050] As mentioned above, Patent Document 5 also discloses various examples of refractive objective lenses and catadioptric objective lenses at a wavelength of 157 nm. These conventional objective lenses differ from the configuration of the present invention, and as a result, their field of view is extremely narrow (see paragraph

[0031] ) and their achromatic range is also extremely narrow (see paragraph

[0023] ), making them unsuitable for the above-mentioned applications of the present invention.

[0051] In Patent Document 6, the shapes of the "meniscus lens and biconcave lens with negative refractive power and convex surface facing the magnification side" corresponding to the first lens L1 and second lens L2 arranged on the most magnification side are different. Also, there is no meniscus lens (fifth lens L5) or biconvex lens (sixth lens L6) with negative refractive power and convex surface facing the magnification side. Furthermore, the thickness of the convex meniscus lens on the most reduction side is thin, about 1x the focal length. As a result, more lenses are required to correct spherical aberration than in the configuration disclosed in the present specification, making it impossible to shorten the overall length. Also, chromatic aberration is not corrected.

[0052] In Patent Document 7, as in Patent Document 6, the thickness of the lens on the reduction side is nearly equal to the focal length, so the lens thickness is not actively used to correct chromatic aberration, and spherical aberration and chromatic aberration are corrected by combining multiple concave and convex lenses. In this configuration, the combination of concave and convex lenses worsens the Petzval sum, making it difficult to correct field curvature and preventing a wide field of view. The same is true in Patent Documents 8 and 9, where these conventional objective lenses require a large number of lenses and a long lens length to achieve both a wide field of view and a high NA.

[0053] In Patent Document 10, a combination of a meniscus lens (L5) with negative refractive power and a biconvex lens (L6) with a convex surface facing the magnification side is used, but there is no thick meniscus lens on the reduction side. This makes it difficult to correct spherical aberration and Petzval sum, so one of these must be sacrificed. As a result, a long focal length and overall length are required to ensure a wide field of view.

[0054] The objective lens 1 of the present invention effectively corrects Petzval sum, spherical aberration, and chromatic aberration through an optimal lens arrangement, thereby achieving a high NA, a wide field of view, and achromatism across the laser wavelength range with a small number of lenses. Therefore, the objective lens 1 of the present invention has the advantage of being able to configure a system with high imaging performance when combined with an inexpensive stage that is optimal for inspection equipment and laser processing equipment. [Example]

[0055] The lens data for Example 1 of the objective lens 1 are as shown in Table 1. In Example 1, the objective lens 1 has a focal length of f=4 mm, a wavelength of 193.4±0.004 nm, an NA of 0.85, and a field diameter of D=0.45 mm. The field half angle ω is ω=arctan(0.45÷2÷4)=3.21°. The thickness of the ninth lens L9, which is closest to the sample surface, is 8.544 mm, and (thickness of the lens closest to the sample surface)÷(focal length of the entire lens system)=2.136. The fifth lens L5 is made of quartz, and the sixth lens L6 is made of CaF2. Therefore, the fifth lens L5 is made of a material whose dispersion is larger than that of the sixth lens L6. In addition, all lenses with positive refractive power (the third lens L3, the fourth lens L4, and the sixth lenses L6 to L9) are made of CaF2. At least one of the lenses having negative refractive power (first lens L1, second lens L2, and fifth lens L5) is made of quartz.

[0056] [Table 1]

[0057] The longitudinal aberration diagram of the objective lens 1 of Example 1 is as shown in FIG. The lateral aberration diagram is shown in Figure 4. [Example]

[0058] FIG. 5 is an optical path diagram of the objective lens 1 of Example 2. The lens data of the objective lens 1 of Example 2 are as shown in Table 2. In Example 2, the objective lens 1 has a focal length of f=4 mm, a wavelength of 266±0.015 nm, an NA of 0.85, and a field of view of φ0.45 mm. The field half angle ω is ω=arctan(0.45÷2÷4)=3.21°. The thickness of the ninth lens L9, which is closest to the sample surface, is 8.56 mm, and (thickness of the lens closest to the sample surface)÷(focal length of the entire lens system)=2.14. The fifth lens L5 is made of quartz, and the sixth lens L6 is made of CaF2. Therefore, the dispersion of the fifth lens L5 is larger than that of the sixth lens L6. In addition, all lenses with positive refractive power (the third lens L3, the fourth lens L4, and the sixth lenses L6 to L9) are made of CaF2. At least one of the lenses having negative refractive power (first lens L1, second lens L2, and fifth lens L5) is made of quartz.

[0059] [Table 2]

[0060] The longitudinal aberration diagram of the objective lens 1 of Example 2 is as shown in FIG. The lateral aberration diagram is shown in Figure 7. [Example]

[0061] FIG. 8 is an optical path diagram of the objective lens 1 of Example 3. The lens data of the objective lens 1 of Example 3 are as shown in Table 3. In Example 3, the objective lens 1 has a focal length of f=4 mm, a wavelength of 355±0.05 nm, an NA of 0.85, and a field of view of φ0.45 mm. The field half angle ω is ω=arctan(0.45÷2÷4)=3.21°. The thickness of the ninth lens L9, which is closest to the sample surface, is 8.54 mm, and (thickness of the lens closest to the sample surface)÷(focal length of the entire lens system)=2.135. The fifth lens L5 is made of quartz, and the sixth lens L6 is made of CaF2. Therefore, the dispersion of the fifth lens L5 is larger than that of the sixth lens L6. In addition, all lenses with positive refractive power (the third lens L3, the fourth lens L4, and the sixth lenses L6 to L9) are made of CaF2. At least one of the lenses having negative refractive power (first lens L1, second lens L2, and fifth lens L5) is made of quartz.

[0062] [Table 3]

[0063] The longitudinal aberration diagram of the objective lens 1 of Example 3 is as shown in FIG. The lateral aberration diagram is shown in FIG. [Example]

[0064] FIG. 11 is an optical path diagram of the objective lens 1 of Example 4. The lens data for Example 4 of the objective lens 1 are shown in Table 4. In Example 4, the objective lens 1 has a focal length of f=4 mm, a wavelength of 266±0.015 nm, an NA of 0.85, and a field of view of φ0.45 mm. The field of view half angle ω is ω=arctan(0.45÷2÷4)=3.21°. The thickness of the ninth lens L9, which is closest to the sample surface, is 6 mm, and (thickness of the lens closest to the sample surface)÷(focal length of the entire lens system)=1.5. This value is the lower limit of the conditional expression relating to the thickness of the ninth lens L9. If the value of the conditional expression falls below this value, spherical aberration and chromatic aberration will be insufficiently corrected, and the objective lens 1 will not be able to exhibit the desired performance. The fifth lens L5 is made of quartz, and the sixth lens L6 is made of CaF2. Therefore, the fifth lens L5 is made of a material whose dispersion is greater than that of the sixth lens L6. All of the lenses having positive refractive power (third lens L3, fourth lens L4, and sixth lens L6 to ninth lens L9) are made of CaF2. At least one of the lenses having negative refractive power (first lens L1, second lens L2, and fifth lens L5) is made of quartz.

[0065] [Table 4]

[0066] A longitudinal aberration diagram of the objective lens 1 of Example 4 is as shown in FIG. 12, and a lateral aberration diagram of the objective lens 1 of Example 4 is as shown in FIG. [Example]

[0067] Figure 14 is an optical path diagram of the objective lens 1 of Example 5. The lens data for Example 5 of the objective lens 1 are shown in Table 5. In Example 5, the objective lens 1 has a focal length of f = 4 mm, a wavelength of 266 ± 0.015 nm, an NA of 0.85, and a field of view of φ of 0.45 mm. The field of view half angle ω is ω = arctan(0.45 ÷ 2 ÷ 4) = 3.21°. The thickness of the ninth lens L9, which is closest to the sample surface, is 12 mm, and (thickness of the lens closest to the sample surface) ÷ (focal length of the entire lens system) = 3. This value is the upper limit of the conditional expression relating to the thickness of the ninth lens L9. If the value of the conditional expression exceeds this value, spherical aberration will be overcorrected, and at the same time, the working distance will be adversely affected, and the objective lens 1 will not be able to exhibit the desired performance. The fifth lens L5 is made of quartz, and the sixth lens L6 is made of CaF2. Therefore, the fifth lens L5 is made of a material with a larger dispersion than the sixth lens L6. All of the lenses having positive refractive power (third lens L3, fourth lens L4, and sixth lens L6 to ninth lens L9) are made of CaF2. At least one of the lenses having negative refractive power (first lens L1, second lens L2, and fifth lens L5) is made of quartz.

[0068] [Table 5]

[0069] A longitudinal aberration diagram of the objective lens 1 of Example 5 is as shown in FIG. 15, and a lateral aberration diagram of the objective lens 1 of Example 5 is as shown in FIG.

[0070] Although the description of the specific embodiment has been completed above, the present invention is not limited to the above embodiment and examples, and can be widely modified and implemented. [Explanation of symbols]

[0071] 1: Objective lens L1: First lens L2: Second lens L3: Third lens L4: Fourth lens L5: Fifth lens L6: 6th lens L7: Seventh lens L8: 8th lens L9: 9th lens

Claims

1. An objective lens having nine lenses, no cemented lenses included, a numerical aperture greater than 0.75, and a half angle of view ω of 2.5° or more, arranged in order from the magnification side. a first lens element consisting of a meniscus lens element having negative refractive power and a convex surface facing the magnification side; a second lens consisting of a biconcave lens; a third lens element consisting of a meniscus lens element having positive refractive power and a concave surface facing the magnification side; a fourth lens consisting of a biconvex lens; a fifth lens element consisting of a meniscus lens element having negative refractive power and a convex surface facing the magnification side; a sixth lens consisting of a biconvex lens; a seventh lens element consisting of a lens element of any shape having positive refractive power; an eighth lens element consisting of a meniscus lens element having a positive refractive power and a convex surface facing the magnification side; and The ninth lens is a meniscus lens with positive refractive power and a convex surface facing the magnification side. The thickness of the ninth lens closest to the sample surface is 1.5 to 3 times the focal length of the objective lens.

2. 2. The objective lens according to claim 1, wherein the thickness of the ninth lens is 2.13 to 2.15 times the focal length of the objective lens.

3. 3. The objective lens according to claim 1, wherein the dispersion of the fifth lens is greater than the dispersion of the sixth lens.

4. The third lens, the fourth lens, and the sixth to ninth lenses each having a positive refractive power are made of CaF ２ 4. The objective lens according to claim 1, wherein at least one of the first lens, the second lens and the fifth lens, each having a negative refractive power, is made of quartz.

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