JP2008090051A - Microscope objective lens - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a microscope objective lens which can realize chromatic aberration compensation in a wide wavelength breadth ranging from a deep ultra violet region to a visible region. <P>SOLUTION: Microscope objective lens carries out color compensation at a wavelength of 2λ<SB>1</SB>which is about twice a wavelength λ<SB>1</SB>in a UV region. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a microscope objective lens used in an optical system such as a microscope used in the ultraviolet region, particularly in the deep ultraviolet region.

  Conventionally, as shown in Patent Document 1, for example, an optical system using a diffractive optical element (Differential Optical Element, abbreviated as DOE) is known.

Specifically, as shown in the patent document 1, the first group of positive refractive power as a whole including a plano-convex lens whose object side is a plane or a meniscus lens having a concave surface facing the object side in order from the object side. A second group including at least one diffractive optical element, satisfying the following condition (1), and at least one diffractive optical element satisfying the conditions (2) and (3): It is characterized by satisfying at least one.
(1) 0.5 <| R | / t <5
(2) D1 / D> 0.8
(3) (h × f) / (L × I)> 0.07
Where R is the radius of curvature of the image-side surface of the meniscus lens, t is the thickness of the meniscus lens, D1 is the marginal beam diameter on the surface of the diffractive optical element, and D is the maximum margin in the microscope objective lens. The light beam diameter, h is the principal ray height on the surface of the diffractive optical element, f is the focal length of the entire microscope objective lens, L is the focal distance of the microscope objective lens, and I is the maximum image on the specimen surface. Is high.

In the configuration satisfying the above conditions (1) and (2), the plano-convex lens having a flat object side or the meniscus lens having a concave surface facing the object side is disposed closest to the object side. The refractive lenses constituting the first group and the second group are both single lenses. The refractive lenses constituting the first group and the second group are the same glass material.
Japanese Patent No. 3312061

  By the way, in the deep ultraviolet region (Deep Ultra Violet, abbreviated as DUV) such as a wavelength of 248 nm, the glass material of the objective lens that can be used is limited, and it is difficult to correct chromatic aberration over a wide wavelength range.

  In an optical system of such a wavelength range, a fluorite lens and a quartz lens are generally used, but even if the microscope objective lens described in Patent Document 1 described above is used, chromatic aberration correction from the DUV region to the visible region is not possible. difficult.

  Accordingly, an object of the present invention is to provide a microscope objective lens that can realize chromatic aberration correction in a wide wavelength range from the deep ultraviolet region (DUV Ultra Violet, abbreviated as DUV) to the visible region. .

  In order to solve the above-mentioned problems, the microscope objective lens according to the first aspect of the present invention is characterized in that color correction is performed with respect to a wavelength of 2λ, which is approximately twice the wavelength λ in the ultraviolet region.

  The microscope objective lens according to the second aspect of the present invention is characterized in that color correction is performed with respect to a wavelength of 488 nm which is almost twice as large as an ultraviolet wavelength λ = 248 ± 5 nm.

  The microscope objective lens of the invention of claim 3 performs color correction using a second-order diffracted wave for the wavelength λ = 248 nm in the ultraviolet region and using a first-order diffracted wave for the nearly doubled λ = 488 nm. It is characterized by

  As described above, the present invention can realize chromatic aberration correction over a wide wavelength range from the deep ultraviolet region (DUV Ultra Violet, abbreviated as DUV) to the visible region.

  Embodiments of a microscope objective lens according to the present invention will be described below.

  The microscope objective lens according to the present invention includes, in order from the object side, a lens group G4 having a positive refractive power, a lens group G3 having a diffractive optical element (referred to as DOE for short), and a thick meniscus. A lens group G2 having a shape and a lens group G1 having a negative refractive power are provided. Here, the object means the near conjugate point side, and the image means the far conjugate point side.

  In such a microscope objective lens, it is possible to realize flatness of the image plane and a long back focus (working distance) WD by adopting a retrofocus type arrangement in which the negative lens group G1 is placed on the parallel light beam side. it can. Here, WD≈2EFL. EFL (focal length) is an abbreviation for Effective Focal Length.

  Further, in the cemented lens in the optical system of the microscope objective lens, chromatic aberration is removed by using a relatively low dispersion fluorite as a positive lens and, conversely, a high dispersion quartz as a negative lens.

  However, in the lens group G1 having negative refractive power, it is desirable that the negative lens of the cemented lens is made of fluorite, and the lens group G2 on the contrary is made of quartz as a positive lens. This configuration has an effect of expanding chromatic aberration, but is useful for removing astigmatism. The negative lens group G1 has a concave surface facing the image side, or is formed of two lenses and has a concave surface facing each other. In the positive lens group G4, the most object side lens preferably has a meniscus shape with a concave surface facing the object side, but can also be a plano-convex lens in which one surface is flat and the other surface is convex.

  In L4 and L13 positive G4 in Example 1 and Example 2, which will be described later, the direction of the cemented lens may be either.

  However, fluorite is used for the positive lens and quartz is used for the negative lens. Further, the condition 0.6 <γ <0.8 with respect to the position of the diffractive optical element DOE is not only the effect of the diffractive optical element DOE on the axial chromatic aberration but also the off-axis chromatic aberration by separating the DOE surface from the diaphragm to some extent. This is also because the influence of the diffractive optical element is sufficiently exerted.

  Here, γ = hDOE / hMAX is defined. Here, hMAX is the maximum beam height (radius of the light beam) taken by the peripheral light, and hDOE is the height of the peripheral light beam (radius of the beam) on the surface of the diffractive optical element DOE.

  In the microscope objective lens of the present invention, color correction is performed for the wavelength λ = 248 ± 5 nm in the deep ultraviolet region and the wavelength of 488 nm which is almost doubled. The second-order diffracted wave of the diffractive optical element DOE is used for the wavelength λ = 248 nm in the deep ultraviolet region, and the first-order diffracted wave of the diffractive optical element DOE is used for the wavelength λ = 488 nm. The color correction can be performed without reducing the diffraction efficiency η.

  The diffractive optical element DOE is determined by optimizing the phase function of the following equation (1).

ここで、ψは光路差関数、ｈは径方向の距離、aは係数である。
位相関数を回折型光学素子の形状に変換すると、回転対称な曲面をもった輪帯の回折格子になる。この輪帯の周期性から透過光の関数をフーリエ級数に展開でき、その級数の係数から各回折次数の回折効率が（４）式で求まる。
文献: 1) D.A.Buralli,G.M.Morris and J.R.Rogers:“Optical performance of holographic kinoforms,”Appl.Opt.,28(1989)976-983
２) 回折光学素子入門［光設計研究グループ監修］増補改定版（オプトロニクス社,2006）

Here, ψ is an optical path difference function, h is a radial distance, and a is a coefficient.
When the phase function is converted into the shape of the diffractive optical element, an annular diffraction grating having a rotationally symmetric curved surface is obtained. The function of the transmitted light can be expanded into a Fourier series from the periodicity of the annular zone, and the diffraction efficiency of each diffraction order can be obtained by the equation (4) from the coefficient of the series.
References: 1) DABuralli, GMMorris and JRRogers: “Optical performance of holographic kinoforms,” Appl. Opt., 28 (1989) 976-983
2) Introduction to diffractive optical elements [Supervised by Optical Design Research Group] Supplement revised edition (Opttronics, 2006)

ここで、Pは最適化に用いた回折光の次数、ｍは任意の回折光の次数、λ_０は最適化波長、λは他波長、ｎ（λ）は波長λでの屈折率である。回折型光学素子の曲面形状は半導体製造のリソグラフィー技術によりバイナリー近似的にも製作される。そのバイナリー近似の程度はリソグラフィーでの露光回数、つまりマスクの枚数により決まる。バイナリー近似された回折光学素子の回折効率は、階段の数をＮとすると（6）式で表される。

Here, P is the order of the diffracted light used for optimization, m is the order of the arbitrary diffracted light, λ ₀ is the optimized wavelength, λ is the other wavelength, and n (λ) is the refractive index at the wavelength λ. The curved surface shape of the diffractive optical element is also produced in a binary approximate manner by lithography technology for semiconductor manufacturing. The degree of the binary approximation is determined by the number of exposures in lithography, that is, the number of masks. The diffraction efficiency of the binary approximated diffractive optical element is expressed by equation (6) where N is the number of steps.

回折型光学素子のリソグラフィー製造においてＭ枚のマスクを使用した場合、素子の階段の数（量子化数）Ｎは、Ｎ＝Ｍ^２になる。尚、短波長の波長で回折光学素子の形状を最適化すると、（６）式で計算される周辺波長の回折効率の低下を抑えることが出来る。

When M masks are used in the lithographic production of a diffractive optical element, the number of steps (quantization number) N of the element is N = M ² . When the shape of the diffractive optical element is optimized at a short wavelength, it is possible to suppress a decrease in diffraction efficiency of the peripheral wavelength calculated by the equation (6).

  The data of the glass material of the objective lens is as follows.

＜第１実施例＞
図１は顕微鏡対物レンズの例を示したものである。この第１実施例の顕微鏡用対物レンズは、焦点距離ｆ＝４ｍｍ、ＮＡ０．５５ 深紫外（Ｄｅｅｐ Ｕｌｔｒａｖｉｏｌｅｔ）波長２４８nmと４８８ｎｍに対応する光学系を有する。

<First embodiment>
FIG. 1 shows an example of a microscope objective lens. The objective lens for a microscope according to the first embodiment has an optical system corresponding to a focal length f = 4 mm and NA 0.55 deep ultraviolet wavelengths 248 nm and 488 nm.

  This microscope objective lens includes, in order from the object side, a lens group G4 having a positive refractive power, a lens group G3 having a diffractive optical element, a thick meniscus lens group G2, and a lens group having a negative refractive power. G1 is provided. Here, the object means the near conjugate point side, and the image means the far conjugate point side.

  The lens group G1 includes a positive lens L1 made of quartz and a cemented lens La. The cemented lens La has a negative lens L2 made of fluorite and a positive lens L3 made of quartz joined to the negative lens L2.

  The lens group G2 includes a positive lens L4 made of fluorite and a negative lens L5 made of quartz bonded to the positive lens L4.

  Further, the lens group G3 includes a quartz diffractive optical element DOE (Differential Optical Element), cemented lenses Lb and Lc, and a diaphragm surface AS positioned between the cemented lenses Lb and Lc. This cemented lens Lb has a quartz negative lens L7 having concave surfaces on both sides, and fluorite positive lenses L6 and L8 cemented on both sides of the negative lens L7. Further, the cemented lens Lc includes a quartz negative lens L10 having both surfaces formed in a concave shape, and fluorite positive lenses L9 and L11 cemented on both sides of the negative lens L10.

  The lens group G4 includes a cemented lens Ld and a positive lens L14 made of quartz. The cemented lens Ld includes a positive lens L12 made of fluorite, and a negative lens L13 made of quartz joined to the positive lens L12.

  In such a microscope objective lens, it is possible to realize flatness of the image plane and a long back focus (working distance) WD by adopting a retrofocus type arrangement in which the negative lens group G1 is placed on the parallel light beam side. it can. Here, WD≈2EFL. EFL (focal length) is an abbreviation for Effective Focal Length.

  The aberration diagram of the optical system of the microscope objective lens is drawn as an aberration diagram for an image formed on the object side when parallel light is incident from the image side according to the custom.

  Further, in the cemented lens in the optical system of the microscope objective lens, chromatic aberration is removed by using a relatively low dispersion fluorite as a positive lens and, conversely, a high dispersion quartz as a negative lens.

  In the lens group G1 having negative refractive power, the negative lens L2 of the cemented lens La is made of fluorite, and the lens group (junction lens) G2 is made of the positive lens L3 made of quartz. This configuration has an effect of expanding chromatic aberration, but is useful for removing astigmatism.

  The negative lens group G1 includes a negative lens L1 having a concave surface facing the image side, and two cemented lenses La. The concave surface of the negative lens L2 and the concave surface of the positive lens L1 are the cemented lens La. Face to face.

  In the positive lens group G4, the most object side lens preferably has a meniscus shape with the concave surface facing the object side as described above, but one surface is flat and the other surface is convex as shown in FIG. The plano-convex positive lens L14 is used.

  However, fluorite is used for the positive lens and quartz is used for the negative lens. Further, the condition 0.6 <γ <0.8 with respect to the position of the diffractive optical element DOE is not only the effect of the diffractive optical element DOE on axial chromatic aberration by separating the analytical optical element DOE surface to some extent from the stop This is because the influence of the diffractive optical element is sufficiently exerted on the off-axis chromatic aberration.

  Here, γ = hDOE / hMAX is defined. Here, hMAX is the maximum beam height (radius of the light beam) taken by the peripheral light, and hDOE is the height of the peripheral light beam (radius of the beam) on the surface of the diffractive optical element DOE.

  In the microscope objective lens of the present embodiment, color correction is performed for the wavelength λ = 248 ± 5 nm in the deep ultraviolet region and the wavelength of 488 nm which is almost twice as large. Then, color correction is performed without decreasing the diffraction efficiency η by using a second-order diffracted wave for the wavelength λ = 248 nm in the deep ultraviolet region and using a first-order diffracted wave for the wavelength λ = 488 nm. be able to. Note that, by optimizing the shape of the diffractive optical element DOE with a short wavelength, it is possible to suppress a decrease in the efficiency of the peripheral wavelength.

  Table 2 shows the radius of curvature, the surface interval, and the glass material of an optical member such as a lens in the optical system of the microscope objective lens.

表３は、深紫外（ＤＵＶ）波長２４８ｎｍ、４８８ｎｍに対する光学性能を示す。

Table 3 shows the optical performance for deep ultraviolet (DUV) wavelengths 248 nm and 488 nm.

また、表４は解析型光学素子ＤＯＥの係数を示したものである。

Table 4 shows the coefficients of the analytical optical element DOE.

このような第１実施例における顕微鏡対物レンズの収差図は図２〜図４に示した縦収差図と図５，図６に示した横収差図のようになる。この縦収差図は、図２の球面収差図，図３の非点収差図，図４の歪曲収差図である。また、横収差図は、図５の軸上の横収差図、図６のｈ´＝０．２における横収差図となる。

The aberration diagrams of the microscope objective lens in the first embodiment are as shown in the longitudinal aberration diagrams shown in FIGS. 2 to 4 and the lateral aberration diagrams shown in FIGS. This longitudinal aberration diagram is the spherical aberration diagram of FIG. 2, the astigmatism diagram of FIG. 3, and the distortion diagram of FIG. Further, the lateral aberration diagram is the lateral aberration diagram on the axis in FIG. 5 and the lateral aberration diagram at h ′ = 0.2 in FIG. 6.

As can be seen from the spherical aberration diagram of FIG. 2, spherical aberrations at deep ultraviolet (DUV) wavelengths 243, 253, and 248 nm are small and corrected in a preferable state. Also, the spherical aberration at 488 nm is within a small range. In addition, as can be seen from the astigmatism diagram of FIG. 3, astigmatism such as ΔS (sagittal image plane aberration) and ΔM (meridional image plane aberration) at a deep ultraviolet (DUV) wavelength of 248 nm is small and corrected in a preferable state. ing. Further, as can be seen from the distortion diagram of FIG. 4, the distortion aberration DST (DISTORTION) at a deep ultraviolet (DUV) wavelength of 248 nm is also corrected in a preferable state. As can be seen from the on-axis lateral aberration diagram of FIG. 5, the on-axis lateral aberrations at deep ultraviolet (DUV) wavelengths 243, 253, and 248 nm are small and corrected in a preferable state. Further, as can be seen from the lateral aberration diagram on the axis in FIG. 6, the lateral aberration at h ′ = 0.2 is corrected in a preferable state because the lateral aberrations on the deep ultraviolet (DUV) wavelengths 243, 253, and 248 nm are small. Has been.
<Second embodiment>
FIG. 7 shows another example of the microscope objective lens. This microscope objective lens has an optical system corresponding to a focal length f = 4 mm and an NA 0.55 deep ultraviolet (DUV) wavelength of 488 nm.

  This microscope objective lens includes, in order from the object side, a lens group G4 having a positive refractive power, a lens group G3 having a diffractive optical element, a thick meniscus lens group G2, and a lens group having a negative refractive power. G1 is provided. Here, the object means the near conjugate point side, and the image means the far conjugate point side.

  The lens group G1 includes a positive lens L1 made of quartz and a cemented lens La. The cemented lens La has a negative lens L2 made of fluorite and a positive lens L3 made of quartz joined to the negative lens L2.

  The lens group G2 includes a positive lens L4 made of fluorite and a negative lens L5 made of quartz bonded to the positive lens L4.

  Further, the lens group G3 includes a quartz diffractive optical element DOE (Differential Optical Element), cemented lenses Lb and Lc, and a diaphragm surface AS positioned between the cemented lenses Lb and Lc. This cemented lens Lb has a quartz negative lens L7 having concave surfaces on both sides, and fluorite positive lenses L6 and L8 cemented on both sides of the negative lens L7. Further, the cemented lens Lc includes a quartz negative lens L10 having both surfaces formed in a concave shape, and fluorite positive lenses L9 and L11 cemented on both sides of the negative lens L10.

  The lens group G4 includes a cemented lens Ld and a positive lens L14 made of quartz. The cemented lens Ld has a positive lens L2 made of quartz and a negative lens L13 made of fluorite joined to the positive lens L2.

  In such a microscope objective lens, it is possible to realize flatness of the image plane and a long back focus (working distance) WD by adopting a retrofocus type arrangement in which the negative lens group G1 is placed on the parallel light beam side. it can. Here, WD≈2EFL. EFL (focal length) is an abbreviation for Effective Focal Length.

  The aberration diagram of the optical system of the microscope objective lens is drawn as an aberration diagram for an image formed on the object side when parallel light is incident from the image side according to the custom.

  Further, in the cemented lens in the optical system of the microscope objective lens, chromatic aberration is removed by using a relatively low dispersion fluorite as a positive lens and, conversely, a high dispersion quartz as a negative lens.

  In the lens group G1 having negative refractive power, the negative lens L2 of the cemented lens La is made of fluorite, and the lens group (junction lens) G2 is made of the positive lens L3 made of quartz. This configuration has an effect of expanding chromatic aberration, but is useful for removing astigmatism.

  The negative lens group G1 includes a negative lens L1 having a concave surface facing the image side, and two cemented lenses La. The concave surface of the negative lens L2 and the concave surface of the positive lens L1 are the cemented lens La. Face to face.

  In the positive lens group G4, the most object side lens preferably has a meniscus shape with the concave surface facing the object side as described above. However, as shown in FIG. 1, one surface is convex and the other surface is concave. The positive lens 12 is a plano-convex positive lens L13, L14 having one surface flat and the other surface convex.

  However, fluorite is used for the positive lens and quartz is used for the negative lens. Further, the condition 0.6 <γ <0.8 with respect to the position of the analytical optical element DOE is not limited to the effect of the analytical optical element DOE on axial chromatic aberration by separating the analytical optical element DOE surface to some extent from the stop. This is because the influence of the diffractive optical element is sufficiently exerted on the off-axis chromatic aberration.

  Here, γ = hDOE / hMAX is defined. Here, hMAX is the maximum beam height (radius of the light beam) taken by the peripheral light beam, and hDOE is the peripheral beam height (radius of the light beam) on the analytical optical element DOE surface.

  In the microscope objective lens of the present embodiment, color correction is performed for the wavelength λ = 248 ± 5 nm in the deep ultraviolet region and the wavelength of 488 nm which is almost twice as large. Then, color correction is performed without decreasing the diffraction efficiency η by using a second-order diffracted wave for the wavelength λ = 248 nm in the deep ultraviolet region and using a first-order diffracted wave for the wavelength λ = 488 nm. be able to. Note that by optimizing the shape of the analytical optical element DOE at a short wavelength, it is possible to suppress a decrease in the efficiency of the peripheral wavelength.

  The curvature radius, surface space, and glass material of an optical member such as a lens in the optical system of the microscope objective lens are shown.

表６は、深紫外（ＤＵＶ）波長２４８ｎｍ、４８８ｎｍに対する光学性能を示す。

顕微鏡対物レンズの深紫外（ＤＵＶ）波長２４８ｎｍ、４８８ｎｍに対する光学性能を示す。

Table 6 shows the optical performance for deep ultraviolet (DUV) wavelengths 248 nm and 488 nm.

The optical performance with respect to deep ultraviolet (DUV) wavelength 248nm and 488nm of a microscope objective lens is shown.

このような第２実施例における顕微鏡対物レンズの収差図は図８〜図１０に示した縦収差図と図１１，図１２に示した横収差図のようになる。この縦収差図は、図８の球面収差図，図９の非点収差図，図１０の歪曲収差図である。また、横収差図は、図１１の軸上の横収差図、図１２のｈ´＝０．２における横収差図となる。

The aberration diagrams of the microscope objective lens in the second embodiment are as shown in the longitudinal aberration diagrams shown in FIGS. 8 to 10 and the lateral aberration diagrams shown in FIGS. This longitudinal aberration diagram is the spherical aberration diagram of FIG. 8, the astigmatism diagram of FIG. 9, and the distortion diagram of FIG. Further, the lateral aberration diagram is the lateral aberration diagram on the axis in FIG. 11 and the lateral aberration diagram at h ′ = 0.2 in FIG. 12.

As can be seen from the spherical aberration diagram of FIG. 8, spherical aberrations at deep ultraviolet (DUV) wavelengths 243, 253, and 248 nm are small and corrected in a preferable state. Also, the spherical aberration at 488 nm is within a small range. As can be seen from the astigmatism diagram of FIG. 9, astigmatism such as ΔS (aberration on sagittal image surface) and ΔM (aberration on meridional image surface) at a deep ultraviolet (DUV) wavelength of 248 nm is also corrected in a preferable state. ing. Furthermore, as can be seen from the distortion diagram of FIG. 10, the distortion aberration DST (DISTORTION) at a deep ultraviolet (DUV) wavelength of 248 nm is also corrected in a preferable state. As can be seen from the on-axis lateral aberration diagram of FIG. 11, the on-axis lateral aberrations at deep ultraviolet (DUV) wavelengths 243, 253, and 248 nm are small and corrected in a preferable state. In addition, as can be seen from the on-axis lateral aberration diagram of FIG. 12, the lateral aberration at h ′ = 0.2 is corrected in a preferable state with small on-axis lateral aberrations of deep ultraviolet (DUV) wavelengths 243, 253, and 248 nm. Has been.
(Other 1)
The present invention is a microscope objective lens that is color-corrected for the wavelength λ = 248 ± 5 nm in the deep ultraviolet region and the wavelength of 488 nm, which is almost twice as described above, but second-order diffraction is performed for λ = 248 nm. By using a wave and using a first-order diffracted wave for λ = 488 nm, color correction can be performed without reducing the diffraction efficiency η. The diffraction efficiency is calculated by the above formulas (1) and (2). According to this formula, by optimizing the shape of the analytical optical element DOE with a short wavelength, it is possible to suppress a decrease in the efficiency of the peripheral wavelength. The diffraction efficiency of a binary approximated diffractive optical element is expressed by equation (6).

（６）において、ｍは回折次数、Ｎは回折型光学素子ＤＯＥの階段の数、λ_０は最適化波長、ｐは最適化の次数を表す。回折型光学素子のリソグラフィー製造においてＭ枚のマスクを使用した場合、素子の階段の数（量子化数）Ｎは、Ｎ＝２^Ｍになる（４マスクを使用する場合は１６）。

In (6), m is the diffraction order, N is the number of steps of the diffractive optical element DOE, λ ₀ is the optimization wavelength, and p is the optimization order. When M masks are used in the lithographic production of a diffractive optical element, the number of steps (quantization number) N of the element is N = 2 ^M (16 when using 4 masks).

Moreover, the phase function method used when calculating | requiring diffraction efficiency is represented by the formula of (1)-(3) mentioned above.
That is, the above formulas (1) to (3) are as shown in Equation 5.

ここで、ψは光路差関数、ｈは径方向の距離、aは係数、光学系は光軸に対し回転対称である。

Here, ψ is an optical path difference function, h is a radial distance, a is a coefficient, and the optical system is rotationally symmetric with respect to the optical axis.

  Using such formulas, 16 levels considering the difference (dispersion) in the refractive index depending on the wavelength of the base when optimized with the second-order diffracted light of 248 nm and the first-order diffracted light of 488 nm (the number of steps, ie, quantization, described later) The diffraction efficiency (quartz) of the diffractive optical element DOE of several N) is shown by the graph shown in FIG.

As can be seen from FIG. 13, in the vicinity of 248 nm, the efficiency is better when optimized with the second order diffracted light of 248 nm. The efficiency of secondary light around 248 nm is about 0.95, and the efficiency of primary light around 488 nm is about 0.97. An overall graph of the diffraction efficiency of a 16 level diffractive optical element DOE optimized with 248 nm second order diffracted light is shown in FIG. In this optimization, dispersion of the substrate of the diffractive optical element DOE is also taken into consideration.
(Other 2)
A method for manufacturing the diffractive optical element DOE shown in FIGS. 1 and 7 will be described using an example in which two masks are used.

  As a primary operation for manufacturing the diffractive optical element DOE, as shown in FIG. 15A, a photoresist is applied on the substrate B to form a photoresist layer Pr on the substrate B. The photoresist layer Pr is exposed with light indicated by an arrow A1 through the next mask (M = 1), and the pattern of the primary mask (M = 1) is baked on the photoresist layer Pr.

  Next, the portion of the photoresist layer Pr where the pattern of the primary mask (M = 1) is baked is removed as shown in FIG. 15B, and the photoresist layer removal portions B1 and B2 and the photoresist are removed. A portion of the layer Pr is provided on the substrate B.

Then, the photoresist layer removing portions B1 and B2 of the substrate B in FIG. 15B are etched to a desired depth as shown in FIG. 15C, thereby forming the lower step surfaces Lb1 and Lb2 on the substrate B. After that, the photoresist layer Pr remaining on the substrate surface in FIG. 15C is removed by cleaning, and lower step surfaces Lb1 and Lb2 and upper step surfaces Ub1 and Ub2 are provided as shown in FIG. The formed substrate B is formed. At this time, the depth d ₁ of the lower step surfaces Lb1 and Lb2 is d ₁ = λ / 2 (n−1).

  Next, as a secondary operation, a photoresist is applied on the stepped surfaces Ub1, Ub2, Lb1Lb2 of the substrate B formed in a rectangular shape as shown in FIG. 15E, and the stepped surfaces Ub1, Ub2, Lb1Lb2 of the substrate B are applied. After the photoresist layer Pr is formed, the photoresist layer Pr is exposed to light indicated by an arrow A1 through a primary mask (M = 1), and a pattern of the secondary mask (M = 2) is formed on the photoresist layer Pr. Bake.

  Next, the portion of the photoresist layer Pr where the pattern of the primary mask (M = 2) is baked is removed as shown in FIG. 15 (f), and the photoresist layer removal portions B3, B4, B5, B6 and the photoresist layer Pr are provided on the substrate B.

  Then, the photoresist layer removing portions B3 to B6 of the substrate B in FIG. 15 (f) are etched to a desired depth as shown in FIG. 15 (g), whereby the lower step surfaces Ub12, Ub22, Lb1 , Lb12, Lb2, and LB22, the photoresist layer Pr remaining on the substrate surface in FIG. 15G is removed by cleaning, and the lower step surfaces Ub12, Ub22, A substrate B provided with Lb1, Lb12, Lb2, LB22 and upper step surfaces Ub1, Ub2 is formed. In this way, a large number of step surfaces can be further provided on the rectangular substrate B. Note that a step of half the depth of the primary work is formed by such a secondary work. When two masks at this time are used (quantization number 4), the step between each step face, That is, the depth dz is dz = pλ / 4 (n−1) where p is the optimized diffraction order and λ is the wavelength. Further, when four masks are used in this procedure (quantization number 16), the step between the step surfaces, dz, is dz = pλ / 16 (n−1).

  Through such operations, a diffraction grating can be provided on the surface of the diffractive optical element DOE of the objective lens shown in FIGS.

  Further, by repeating the above-described operation, a BOE (Binary Optical Element) as shown in FIG. 16 can be formed as one of the diffractive optical elements DOE. The BOE of FIG. 16 is formed in a Fresnel lens shape from small ring lens portions Ls1 and Ls2 that are provided in a ring band shape at the peripheral edge and have a sawtooth cross section and a central convex lens portion Lc. The diffraction grating portions dg1 and dg2 formed as described above are formed on the surfaces of the small ring lens portions Ls1 and Ls2, and the diffraction grating portion dg3 formed as described above also on the surface of the convex lens portion Lc. Is formed. Moreover, the height d of the diffraction grating portions dg1 and dg2 of the small ring lens portions Ls1 and Ls2 is d = pλ / (n−1).

  The repeated formation of the above-described diffraction grating can be regarded as the quantization number of the optical element. If the relationship between the diffraction efficiency and the quantization number when optimized with the first-order diffracted light is graphed, FIG. As shown in FIG. In FIG. 17, the horizontal axis represents the number of steps of the diffraction grating, and the vertical axis represents the diffraction efficiency. As can be seen from FIG. 17, when the quantization number is 8, the diffraction efficiency is 0.95, and when the quantization number 16 is 0.99.

  By the way, when secondary light is used as described above, d = 2λ / (n−1) from d = pλ / (n−1). In addition, if the wavelength λ is determined, the refractive index n is also determined, so that the depth d is determined. In the embodiment described above, the shape is optimized and calculated using a short wavelength of 248 nm (secondary). In this case, if the refractive index n is almost constant, the depth d is not so different between the second order of 248 nm and the first order of 488 nm, that is, good diffraction efficiency is obtained at both wavelengths.

  The diffraction grating part dg3 is determined by how much the depth determined by the diffraction grating parts dg1 and dg2 is quantized (how many times the above operation is repeated). That is, when the work is performed twice, since four steps are made, dz = d / 4. Moreover, in the case of 3 times, it becomes d / 8, and in the case of 4 times, it becomes d / 16. As described above, the diffraction efficiency when optimized with the first-order diffracted light is 0.95 when the quantization number is 8, and 0.99 when the quantization number is 16. However, when optimized with high-order p diffracted light, the quantization efficiency N corresponds to a diffraction efficiency equivalent to 1 / p of the diffraction efficiency when optimized with primary diffracted light. That is, the diffraction efficiency is the quantization number (N / p). Accordingly, the diffraction efficiency at a wavelength of 248 nm, a diffraction order of 2nd order, and a quantization number of 16 corresponds to a quantization number of 8 and is 0.95. At 498 nm (2 × 248 nm), if the refractive index of the substrate is the same as 248 nm, it corresponds to the efficiency at a wavelength of 498 nm, the first diffraction order, and the quantization number 16, and the diffraction efficiency is 0.99. Further, the wavelength of 488 nm is close to the wavelength of 498 nm, and the diffraction efficiency becomes 0.97 even when the refractive index of the substrate is taken into consideration.

  Further, the shape of the diffraction grating is determined by the phase function φ (h) of the above-described equation (1). For example, when a two-level BOE is created, h whose phase function changes by 2π / 2 is the width of the mask. In the case of 4 levels, in addition to the above, a mask created with a width of h determined by 2π / 4 is used in the second step.

  Further, in FIG. 16, the radius of the convex lens portion Lc is h where the phase function is changed by 2π, and the depth d is equivalent to a change of 2π of the phase function. Similarly, the width of the small ring lens portion Ls2 corresponds to h in which the phase function changes by 2π.

As described above, the microscope objective lens according to the embodiment of the present invention is color-corrected with respect to the wavelength of 2λ ₁ which is almost twice the wavelength λ _{1 in} the ultraviolet region.

  According to this configuration, it is possible to realize chromatic aberration correction over a wide wavelength range from the deep ultraviolet region (Deep Ultra Violet, abbreviated as DUV) such as a wavelength of 248 nm to the visible region.

  In addition, the microscope objective lens according to the embodiment of the present invention is color-corrected with respect to a wavelength of 488 nm which is almost twice as large as the wavelength λ = 248 ± 5 nm in the ultraviolet region.

  According to this configuration, it is possible to realize chromatic aberration correction over a wide wavelength range from the deep ultraviolet region (Deep Ultra Violet, abbreviated as DUV) such as a wavelength of 248 nm to the visible region.

  Furthermore, the microscope objective lens according to the embodiment of the present invention uses the second-order diffracted wave for the wavelength λ = 248 nm in the ultraviolet region, and uses the first-order diffracted wave for the nearly doubled λ = 488 nm. Color correction is designed.

  According to this configuration, it is possible to realize chromatic aberration correction over a wide wavelength range from the deep ultraviolet region (Deep Ultra Violet, abbreviated as DUV) such as a wavelength of 248 nm to the visible region.

It is explanatory drawing which shows 1st Example of the optical system of the microscope objective lens based on this invention. It is explanatory drawing of the spherical aberration of the microscope objective lens of FIG. It is explanatory drawing of the astigmatism of the microscope objective lens of FIG. It is explanatory drawing of the distortion aberration of the microscope objective lens of FIG. It is explanatory drawing of the lateral aberration of the microscope objective lens of FIG. It is explanatory drawing of the lateral aberration of the microscope objective lens of FIG. It is explanatory drawing which shows 2nd Example of the optical system of the microscope objective lens based on this invention. It is explanatory drawing of the spherical aberration of the microscope objective lens of FIG. It is explanatory drawing of the astigmatism of the microscope objective lens of FIG. It is explanatory drawing of the distortion aberration of the microscope objective lens of FIG. It is explanatory drawing of the lateral aberration of the microscope objective lens of FIG. It is explanatory drawing of the lateral aberration of the microscope objective lens of FIG. It is explanatory drawing of the difference in diffraction efficiency when a diffractive optical element is optimized by 248 nm and 488 nm. FIG. 14 is an overall explanatory diagram when optimizing with the second order diffracted light of 248 nm of FIG. (A)-(h) is explanatory drawing which shows the manufacturing method of the diffractive optical element of FIG. 1, FIG. It is explanatory drawing which shows an example of the diffractive optical element manufactured by the manufacturing method of FIG. It is explanatory drawing which shows the relationship between the quantization number of the diffractive optical element manufactured by the manufacturing method of FIG. 15, and diffraction efficiency.

Explanation of symbols

G1 ... Negative lens group G2 ... Lens group G3 ... Lens group G4 ... Positive lens group L1-L14 ... Lens

Claims (3)

  A microscope objective lens that is color-corrected for a wavelength of 2λ that is approximately twice the wavelength λ in the ultraviolet region.   A microscope objective lens which is color-corrected for a wavelength of 488 nm which is almost twice as large as a wavelength λ = 248 ± 5 nm in the ultraviolet region.   A microscope objective lens, wherein color correction is performed using a second-order diffracted wave for a wavelength λ = 248 nm in the ultraviolet region, and using a first-order diffracted wave for nearly doubled λ = 488 nm.

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