JPS60241009A - Long working distance high magnification objective lens - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a microscope objective, particularly a high magnification objective with a large working distance.

(Background of the Invention) Recent microscopes are used for a wide variety of purposes, and objective lenses are required to have high N and A, a long working distance, and high resolution. In particular, in the case of IC-related specimens, it is absolutely necessary to keep the specimens small and free from scratches, and objective lenses with short working distances, especially high-magnification objectives, are extremely useful for this purpose. It had to be used carefully, and its operability was extremely poor. For example, in the case of a 100x objective lens, the working distance is 0.3 to 0.4 mm.
This is normal, and when a working distance of 1 to 2 mm is required, the actual situation is to use a 20 to 40x objective lens and a high magnification eyepiece to obtain a total magnification of about 1000x. there were. However, in this case, even though the overall magnification is high, the resolving power itself is determined by the N and A of the objective lens, so the resolving power is insufficient and only a poor quality image is obtained.

In addition, when observing the bottom of the concave part of the test specimen, an objective lens with a working distance longer than the depth of the four parts is of course required, and when the depth is 1 to 2 mm, an objective lens with a working distance of about 20 to 40 times Observation could only be carried out using a 200-degree objective lens. Furthermore, when applying some kind of environmental change to the specimen from the outside, such as shading an electric or magnetic field, cooling or heating, a long working distance is essential for attaching various devices for these purposes. It is.

However, high magnification means that the focal length is small, and if! Usually, as the focal length decreases, the working distance also decreases.

In order to increase the working distance, it is necessary to significantly increase the focal length of the front group of the objective lens base, but this makes it difficult to correct spherical aberration and chromatic aberration. In particular, the amount of residual secondary spectra becomes large, and high-order spherical aberrations for light rays on the short wavelength side become significant, making it extremely difficult to properly correct them.

(Objective of the Invention) An object of the present invention is to provide an objective lens that has a long working distance and excellent imaging performance despite its high magnification.

(Summary of the Invention) The objective lens according to the present invention has, in order from the object side, a positive meniscus lens component with a concave surface facing the object side and a plurality of cemented positive lens components, as shown in the examples shown in FIGS. A second lens group G2 with positive refractive power has a positive lens component with a surface of stronger curvature facing the object side, and a concave surface on the image side. The 117th
The refractive index of the positive meniscus lens component closest to the object in the first group G1 is N1, the radius of curvature of the concave surface on the object side is P6, the focal length of the second lens group G2 and the third lens group G3 is fz, respectively. h, the center thickness of the second lens group G2 and the third lens group G3 is 12 and 13, respectively, the air distance between the second lens group G2 and the third lens group G3 is D, and the composite focal point of the entire system is When the distance is F, the following conditions are satisfied.

2.5F<llc/(N,-1) l<5. OF
(1) 2.0 < l fz/ f+ l < 4.
0 (2) 5, OF<t2+t, +D <IO,OF
(3) These conditional expressions will be explained below.

The conditions of equation (1) relate to correction of spherical aberration, coma aberration, and field curvature. If the lower limit of this condition is exceeded,
The convergence effect at the frontmost lens surface of the objective lens becomes weaker, and the beam diverges significantly, worsening spherical aberration and making it difficult to correct it well.

In particular, higher-order spherical aberration and coma aberration worsen as the working distance increases, making it difficult to sufficiently correct them in subsequent lens groups. Furthermore, if this condition of 1++g is exceeded, the Petzval sum becomes large, making it difficult to correct the curvature of field and making it difficult to obtain a flat image surface.

The condition of equation (2) is that the second lens group G2 and the third lens group G
This relates to the balance of refractive power with 3.

When the lower limit is exceeded, the positive refractive power of the second lens group G2 becomes relatively strong, and the overall length of the lens system tends to become short. For this reason, the center thicknesses of the second lens group G2 and the third lens group G3 and the air gap between both groups are reduced.

When the total length is increased due to the divergence effect at the rear of the lens system, the Petzval sum increases, field curvature remains, and flatness deteriorates. Moreover, when the upper limit is exceeded, the negative refractive power of the third lens group G3 becomes relatively too strong. 3rd lens group G,
For example, longitudinal chromatic aberration is reversely erased, and lateral chromatic aberration is slightly undercorrected, but as these correction effects become stronger, longitudinal chromatic aberration becomes undercorrected. If this is canceled by weakening the effect of reverse achromatic aberration in the fourth lens group G4, the correction effect of the lateral chromatic aberration of the fourth lens group G4 will be weakened, and an even larger amount of aberration will be generated in addition to the amount remaining up to the third lens group G3. It will remain.

Conditional expression (3) is based on the second lens group G2 and the third lens group G3.
It is related to the thickness of each lens and the distance between both groups, and the balance between the object-image distance, that is, the total length of the lens system, and the objective lens length (parfocal distance) to maintain parfocality with other objective lenses. This is for the purpose of achieving this goal. If the lower limit of this condition is exceeded, the light beam exiting the third lens group G3 will be subjected to a strong diverging effect in the fourth lens group G4 before it is sufficiently narrowed down, resulting in an increase in the overall length of the lens system. . For this reason, when the entire lens system is proportionally reduced, the working distance also becomes shorter, resulting in poor operability. Furthermore, if the refractive power of the second lens group G2 is strengthened to reduce the total length of the lens system, it becomes difficult to maintain the flatness of the image plane, as in the case where the lower limit of condition (2) is exceeded. On the other hand, if the first limit of this condition is exceeded, the light beam incident on the fourth lens group G4 is too narrowed down, so that the total length of the lens system must be maintained at a predetermined length even with the strong diverging action of the fourth lens group G4. becomes difficult. For this reason, by proportionally enlarging the entire length of the lens system, the working distance can be increased, but the length of the objective lens barrel becomes too large compared to other objective lenses, making it difficult to maintain parfocality. .

In the configuration of the present invention as described above, axial chromatic aberration is over-corrected by the convergent lens group G and the second lens group G2, but is corrected by the divergent third lens group G3 and the fourth lens group G. , and chromatic aberration of magnification is preferably corrected by each lens group. For this purpose, in the present invention, the third lens group G3 is configured by laminating a positive lens and a biconcave negative lens in order from the object side, and the fourth lens group G4
is constructed by laminating a biconcave negative lens and a biconvex positive lens in order from the object side, and the number of positive lenses and negative lenses in the third lens group G is set to Vp3. When Vn3 and the vertical numbers of the positive and negative lenses in the fourth lens group G4 are νp4+Vn4, 15 <
(Vnz Vp3) + (Vn4Vp4) <60
It is desirable to satisfy condition (4).

If the lower limit of formula (4) is exceeded, the third lens group It is necessary to increase the curvature of the bonding surfaces in G3 and the fourth lens group G4, and as a result, the higher-order spherical aberration for short wavelength light becomes insufficiently corrected. Also,
If the amount of axial chromatic aberration in the first lens group G and the second lens group G2 is reduced, it becomes difficult to correct the chromatic aberration of magnification and it becomes difficult to maintain good performance as a whole. On the other hand, beyond this eye, the curvature of the bonded surfaces in the third lens group G3 and the fourth lens group G4 becomes weaker, resulting in overcorrection of higher-order spherical aberration for short wavelength light. Furthermore, the fourth
The lens group G4 can also be composed of a biconcave negative lens component made by laminating a positive meniscus lens with a concave surface facing the object side and a biconcave negative lens; in this case as well, the above ( 4) It is desirable to satisfy the second condition.

Further, when the surface layer refractive power of the object side lens surface of the second lens group G2 is Φ2, and the surface rudder power of the image side lens surface of the third lens group G3 is Φ3, 2<lΦ3/Φ21<5 ( 5) It is also desirable to satisfy condition 1. Here, Φ2 and Φ3 are the third lens group G, where the radius of curvature of the object-side lens surface of the second lens group G2 is R2, and the refractive index is N2.

When the radius of curvature of the image-side lens surface of is R3 and the refractive index is (■), they are defined as 2-(N2-1)/R2 Φ3 = N-N:l)/L, respectively.

When the lower limit of equation (5) is exceeded, the convergence effect of the object-side lens surface of the second lens group G2 becomes stronger, and in order to make the total length of the lens system a predetermined length, the second lens group G2 and the third It is necessary to reduce the center thickness of lens group G3 and the air gap between both groups to reduce the distance between it and the subsequent diverging lens group. For this reason, the Pessochard sum increases, making it difficult to maintain the flatness of the image plane, and furthermore, the meridional image plane becomes over-corrected, making it difficult to perform good correction using the third lens group G3 or the fourth lens group G4. . Moreover, if the upper limit of condition (5) is exceeded, the meridional image plane will be undercorrected too much, and the flatness of the image plane will deteriorate. Furthermore, the second
The refractive power of the object side lens surface of the lens group G2 becomes weaker, and the refractive power of the second lens group G2 becomes weaker, and the refractive power of the second lens group G2 becomes weaker.
Similar to the case where the negative refractive power of the lens group G3 becomes strong and exceeds the upper limit of the condition of equation (2) above, large chromatic aberration of magnification remains.

(Example) Examples of the present invention will be described below. FIG. 1 is a lens configuration diagram of a first embodiment of the present invention. The lens group G includes, in order from the object side, positive meniscus lenses 1. Similarly, a positive meniscus lens 1.2 with its concave surface facing the object side, a biconvex positive lens 3, and three laminated positive lenses I and 4°LS+l-6 each consisting of a negative meniscus lens and a positive lens laminated together. The second lens group G2 consists of a double-convex positive lens 1.7 and negative lenses 1 and 8 bonded to this and having a surface with a stronger curvature facing the object side, and the third lens group G3 has a positive lens 1.7 facing the object side. The fourth lens group G consists of a positive lens L9 with a strong curvature side facing, and a biconcave negative lens LIO bonded to this, and a biconcave negative lens Ll + a biconvex positive lens I bonded to this. , 1□.

The second embodiment according to the present invention has almost the same lens configuration as the first embodiment shown in FIG. 1, so a diagram of the lens configuration is omitted. Both the first and second embodiments have a magnification of 100 times, N and A of 0.85, and a working distance that is 1.1 times or more the focal length of the entire system.

The third embodiment according to the present invention, as shown in the lens configuration diagram in FIG. L4 and the air distance is large, and the air distance between the second lens group G2 and the third lens group G3 is large, and the air distance between the third lens group G3 and the fourth lens group G4 is small. It is characteristic that The fourth embodiment is based on the second
The lens configuration diagram is omitted because it has almost the same lens configuration as the third embodiment shown in the figure. Third example and fourth example
In both examples, the magnification is 100 times, N and A are 0.80, and the working distance is 1.4 times or more the focal length of the entire system.

The table below shows the specifications of each example. However, in each table, the numbers on the left side represent the order from the object side, β is the magnification, N, A are the numerical aperture, Wisdom, D are the working distance M, and P is the Penzval sum. shall be expressed. Furthermore, the refractive index n
and Atsube number ν are the d-line (λ−
587.6 nm).

3 z 4 1st ji J! ! i 3rd implementation (Masu 5 17 -- 6 The various aberration diagrams for the above-mentioned first, second, third, and fourth examples are shown in FIG. 3, FIG. 4, FIG. 5, and FIG. 6, respectively. Each aberration diagram shows the d-line (λ-5
The spherical aberration, astigmatism, coma aberration, and distortion for 87.6 nm) are shown, and in the spherical aberration diagram,
C line (λ-656.3 nm), F line (λ-486.1 nm)
nm) and 8-line (λ-435, 8 nm) are also shown.

From the various aberration diagrams, it is clear that all the examples have a large working distance and excellent imaging performance even though they have a high magnification of 100 times. In particular, it can be seen that high-order spherical aberrations for short wavelength light, which tend to occur in objective lenses with high magnification and long working distances, are extremely well corrected.

(Effects of the Invention) As described above, according to the present invention, an objective lens having a long working distance and excellent imaging performance despite its high magnification can be achieved. Therefore, it is possible to fully inspect the bottom of the four parts of the object to be inspected, and it can also be used for various inspections of semiconductor devices, improving operability, and is suitable for semiconductor devices that have recently become more highly integrated. This greatly contributes to the inspection of

[Brief explanation of the drawing]

FIG. 1 is a lens configuration diagram of the first embodiment according to the present invention, and the second
The figure is a lens configuration diagram of the second embodiment of the present invention, and the third, fourth, fifth, and sixth figures are the first, second, and third lens configuration diagrams, respectively.
and FIG. 7 is a diagram of various aberrations of the fourth example. [Explanation of symbols of main parts] G1...1121st group G2...2nd lens group G1...3rd lens group G4...4th lens group Applicant: Nippon Kogaku Kogyo Co., Ltd. Agent Takao Watanabe 9

Claims (1)

[Scope of Claims] A 1121st group of positive refractive power that includes, in order from the object side, a positive meniscus lens component with a concave surface facing the object side and a plurality of cemented positive lens components and converts a light beam from the object into a convergent light beam; A second lens group with positive refractive power having a positive lens component with a surface of stronger curvature facing the object side, a third lens group with negative refractive power having a negative cemented lens component with a concave surface facing the image side, and a third lens group with negative refractive power having a negative cemented lens component with a concave surface facing the image side; Consisting of a fourth lens group with a negative refractive power having a negative cemented lens component facing a concave surface, the refractive index of the positive meniscus lens component closest to the object in the 1121st group is N, and the curvature of the concave surface on the object side The radius is R, the focal length of the second lens group and the third lens group are respectively h, and the center thickness of the second lens group and the third lens group is t2. t3, an objective lens that satisfies the following conditions, where D is the air distance between the second lens group and the third lens group, and F is the combined focal length of the entire system. 2.5 F<IRI/(N,-1)l<5.
OF (1) 2.0 < l f2/f31 <4.0
(2) 5, OF<t2+t, -1-D <IO,OF
(3)