JPH10282429A - Objective lens long in orerating distance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an objective lens having longer operating distance in comparison with a conventional objective lens and large numerical aperture capable of making the light beam of a near ultraviolet range to standard wavelength by composing a third group lens of plural single lenses and arranging at least plural achromatic combined surfaces in a second group lens. SOLUTION: The third group lens is composed of three or more single lenses and at least two or more achromatic combined surfaces are arranged in the second group lens. when the whole length of a lens system is defined as OAL, the focal distance of the third group lens is defined as (f III), the radius of the curvature of the first surface of a first group lens by counting it from an object point side is defined as Rf, that of the final surface of the first group lens is defined as R'f, that of the first surface of the third group lens by counting it from the object point side is defined as Rr, that of the final surface of the third group lens is defined as R'r and the synthetic thickness of the first group lens is defined as Σdf, the condition of an expression is satisfied by the objective lens having the long working distance. This objective lens having the long operating distance can be used as the high-magnification objective lens for a high-temperature microscope excellent in resolution impossible heretofore.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an objective lens used for an inspection device for a semiconductor element such as a reticle, a mask, a wafer, and the like. Also,
This objective lens can also be used for a microscope device using a fluorescent dye label, a high-temperature microscope device for observing phase inversion of metal, and the like.

[0002]

2. Description of the Related Art A long working distance objective lens according to the present invention is used for "ultra LSI device and defect inspection of reticles, masks, wafers and the like using a near ultraviolet laser beam as a light source. The main purpose is to use the laser beam scanner,
This is a dry objective lens having the above numerical aperture and a working distance longer than the focal length of the entire lens system. Conventionally, several objective lenses have been disclosed for such a purpose. Hereinafter, four typical examples will be described.

The objective lens disclosed in Japanese Patent Application Laid-Open No. 5-72482 is a dry-type high-magnification large-aperture objective lens that can be used with light in the far ultraviolet region near 250 mm. However, the working distance (WD) of this objective lens is of the order of 0.92 mm, and this value is only 1.1 of the overall lens system (OAL).
Since the value is less than 0%, there is a problem that the design of an inspection device (LSM) for an VLSI semiconductor device of 64M class or more is greatly restricted. The objective lens disclosed in Japanese Patent Application Laid-Open No. 5-196874
This is a dry-type, high-magnification, large-aperture objective lens that can be used with light in the near ultraviolet region near 51 mm. However, the working distance (WD) of this objective lens is also at a level of 1.57 mm,
Since the value is only less than 2.0% of the total length of the lens system (OAL), there is still a problem that the design of an inspection device (LSM) for an VLSI semiconductor device of 64M class or more is greatly restricted.

[0004] The objective lens disclosed in Japanese Patent Application Laid-Open No. 4-220615 is a dry system medium magnification medium aperture objective lens that can be used with light in the visible region. This objective has a relatively long working distance. However, since the numerical aperture stays at about 0.50 and is made of a glass material that absorbs ultraviolet rays, a method of improving the resolving power by shortening the wavelength used, which is often performed, is adopted. Have a defect that cannot be performed. Therefore, the objective lens of the present invention is not applicable to the inspection of a super LSI semiconductor device of 64M class or more, which is a main object, because of a remarkable lack of resolution. The objective lens disclosed in JP-A-4-220616 is a dry-type high-magnification large-aperture objective lens that can be used with light rays in the visible region. However, the working distance (WD) of this objective lens is also at a level of 2.48 mm, and the entire lens system (OA)
L) is less than 3.0%, and is made of a glass material that absorbs ultraviolet rays. Therefore, for the same reason as in the previous example, the resolution is insufficient and a short working distance is required. Therefore, there is a problem that the design of the LSM device is greatly restricted.

[0005] As a specific example of an inspection machine to which the long working distance objective lens of the present invention is applied, consider a photomask or reticle inspection machine. In recent years, a protective film called “pellicle” is generally applied to these photomasks and the like in order to prevent dust from being transferred to the surface of the mask when exposing the LSI pattern.
The pellicle is attached at a certain distance from the mask pattern surface because it is necessary to prevent an image from being formed on the LSI surface even if dust adheres to the pellicle. In the current standard,
The distance between the mask and the pellicle (referred to as the “stand-off” of the pellicle) is 6.3 mm, and it is unlikely that the stand-off will be reduced in the future. For this reason, an inspection machine for a mask or the like is required to inspect the pattern on the mask surface through the pellicle, and the long working distance objective lens of the present invention must secure a large working distance of about 7 mm. Strict conditions exist.

[0006] Each of the above-mentioned conventionally known objective lenses has a short working distance, a super-resolution (large aperture), and a near-ultraviolet light beam for the inspection of ultra LSI semiconductor devices of 64M class or more intended by the present invention. It is inadequate in terms of lens types with high transmittance and manufacturable tolerances. Photomask, reticle, wafer or L
When considering an inspection apparatus for inspecting a semiconductor element such as an SI, an objective lens having the following performance is required. That is, high N.P. A. (0.85 or more). The working distance must be long (about 7.0 mm). The wavelength range used is near-ultraviolet. (As an example of the wavelength to be used, 351 nm, 363.
8 nm, 325 nm of He-Cd laser, Nd: YA
A third harmonic of the G laser is possible. ) High transmittance in the above wavelength range. A tolerance that can be manufactured while satisfying the above conditions in the above wavelength region.

The above is the required performance determined from the required defect recognition resolution of 0.1 μm when targeting a highly accurate pattern such as a photomask. The above are the specifications required to inspect a photomask or reticle through a pellicle, as described above. Also,
The above is closely related to the S / N ratio of an image required when inspecting an object with a defect recognition resolution of 0.1 μm. Further, the above are self-evident conditions.

The long working distance objective lens of the present invention can also be used in a microscope apparatus using fluorescent dye labels, provided that the overall length of the lens is reduced to such an extent that it can be accommodated in a "45 mm parfocal length lens frame". Recently, in the cutting-edge field of molecular biology, for example, in the study of the mechanism of life development in sea urchin fertilized eggs at the molecular level, as with the study of the fluorescent dye single molecule, labeling with a fluorescent dye single molecule or several molecules There is a demand for the development of a technique for observing the behavior of a single active protein molecule and performing photometry. In order to capture such extremely weak light and obtain information with less noise, it is expected to supply a high-magnification, large aperture, long working distance dry system objective lens with extremely little autofluorescence. The specifications of high magnification and large aperture are necessary for the object to have a microstructure close to the resolution limit of the optical microscope or laser beam scanning microscope, and a long working distance is necessary for the manipulation of sample cells. This is because the drying system does not risk reduction in the S / N ratio due to the self-fluorescence of the immersion liquid.

[0009] The long working distance objective lens of the present invention has an extremely high transmittance for near-ultraviolet light that excites a fluorescent dye, and therefore has a feature that the amount of autofluorescence is extremely small. In addition, since it has a sufficiently long working distance of about 5 mm or more, there is an advantage that sample cell microsurgery by a manipulator in a microscope field of high magnification and high magnification becomes easy. On the other hand, a conventional fluorescent microscope objective lens on the market has a working distance of 5 mm or more like the long working distance objective lens of the present invention and has a magnification of 100 × class which almost completely realizes “self-fluorescence”. No dry objective lens has ever been available.

The long working distance objective lens of the present invention can also be used as an objective lens for a high resolution and high magnification high temperature microscope for observing phase transition of a metal in a high temperature furnace by utilizing its long working distance. Is possible. Conventional refraction objective lenses for high-temperature microscopes, which are currently commercially available, have a numerical aperture of only about 0.5 even at the highest magnification, and are designed for visible light. Since the resolution can reach only about 1/3 compared with the working distance objective lens, it has been unsatisfactory for the study of the metal phase transition under high temperature.

The present invention has been made in view of the above points, and has an objective lens having a longer working distance than a conventional objective lens and having a large numerical aperture having a standard wavelength of near-ultraviolet light. The purpose is to provide.

[0012]

According to the present invention, a first lens unit having a negative refractive power for an object point located at infinity and a third lens unit having a positive refractive power for an image point are provided. Between,
In the objective lens having the second group lens having a positive refractive power, the third group lens is constituted by three or more single lenses, and at least two or more achromatic junction surfaces are provided in the second group lens. , The overall length of the lens system is OAL, the focal length of the third lens unit is fIII, the radius of curvature of the first surface is Rf, counting from the object point side of the first lens unit, and the last surface of the first lens unit is Rf. , The radius of curvature of the first surface counted from the object point side of the third lens unit is Rr, the radius of curvature of the last surface of the third lens unit is Rr, and the radius of curvature of the first lens unit is R'f. When the total thickness is 肉 df, 0.2 ≦ fIII / OAL ≦ 0.4 (1) 1.4 ≦ R′f / Rf ≦ 3.4 (2) 2 0.3 ≦ Rr / R′r ≦ 4.7 (3) 0.4 ≦ Σdf / OAL ≦ 0.6 (4) A satisfactory long working distance objective. An objective lens satisfying the above conditional expressions (1) to (4), wherein the total focal length of the lens system is Fo, the absolute value of the focal length of the first lens unit is | fI |, and the second lens unit is When the focal length of the lens is fII, 1.6 ≦ fIII / | fI | ≦ 2.6 (5) 8.9 ≦ fII / Fo ≦ 15.0 (15.0) 6) a long working distance objective lens that satisfies each of the conditional expressions, and further an objective lens that satisfies the conditional expressions (1) to (4), wherein the total focal length of the lens system is Fo, Assuming that the absolute value of the focal length of the first lens unit is | fI | and the focal length of the second lens unit is fII, 0.05 ≦ fIII / | fI | ≦ 0.3 (7) 60 ≦ fII / Fo ≦ 29.0 (8) This is a long working distance objective lens that satisfies the conditional expressions (8).

[0013]

Embodiments of the present invention will be described below. Before giving a detailed description, first, for the sake of convenience, the concept regarding the elimination of chromatic aberration in the design of the long working distance objective lens of the present invention and the significance of the overall lens system (OAL) will be described.

The long working distance objective lens of the present invention is a laser scanning microscope (LMS) using near ultraviolet laser light.
The main purpose is to use it. The standard wavelength in the design is UV-Ar ⁺ (351 nm, 364 n
m), He-Cd (325 nm), YAG THG (3
Any one of various near-ultraviolet laser light sources (e.g., 54 nm) is selected and basically used, and it is necessary to secure the imaging performance within the diffraction limit for the standard single wavelength light within the entire scanning visual field. Further, these imaging performances must be manufactured so as to be surely realized over the entire lens material and the lens processing process, taking into account their manufacturing tolerances.

Therefore, it is necessary to adopt a lens type that increases the number of lenses, avoids a lens shape that is difficult to process, and that is easy to assemble and adjust. If the performance of the finished product can also be evaluated, the first step is to use single-wavelength light in the visible range and utilize the know-how of the conventional lens manufacturing process so that the performance of standard ultraviolet light can be estimated to some extent accurately. Care is important as a practical manufacturing procedure.

Considering the performance of the light source, the price, and the ease of handling, g-line (436 nm) of an ultra-high pressure mercury lamp is used as the visible single wavelength light by using an interference filter having a narrow half width at the same time. It is appropriate to do. In the embodiment of the present invention, the standard wavelength light is 3
The wavelength is set to 51 nm, the visible single-wavelength light for assistance is set to 436 nm, and the chromatic aberration is removed so that the best focus position coincides over the entire field of view for both wavelengths. In addition to the standard wavelength light, as well as the visible wavelength light assisted, the design is designed so that the imaging performance over the entire field of view is substantially within the diffraction limit. In this way, when a complete lens is manufactured by the conventional lens manufacturing process and the inspection technique using the visible wavelength light assisted, a fundamental portion of the imaging performance at the actual standard wavelength can be secured. After that, CCD for UV
Since it is only necessary to add a small adjustment operation of fine packing at a standard wavelength using a camera to this, it is extremely advantageous in manufacturing an inspection apparatus, and it is possible to surely manufacture the inspection apparatus.

Further, the fact that the “best focus” of the standard wavelength light and the assisting visible wavelength light over the entire field of view coincides with each other is as follows.
In a laser scanning microscope, it is possible to find an almost accurate focus position of the actual near-ultraviolet standard wavelength light by first visually focusing with the aid of visible wavelength light, so the use of the inspection device is extremely convenient. It becomes something.

However, the chromatic aberration of magnification between the standard wavelength light and the assisted visible wavelength light is not necessarily strictly removed because the two wavelengths are independently and separately used for focusing at different times. If an appropriate monochromatic light interference filter or the like is used in combination with the illumination light, the residual aberration of about ± 10% can sufficiently withstand practical use.

Next, in order to clarify the meaning of the conditional expression in each claim of the present invention, the significance of the overall lens system (OAL) will be described in advance. Due to the mechanical design of the LSM inspection apparatus or the microscope base, the objective lens facing the object to be inspected has naturally desired external dimensions. For example,
As for the “length”, a parfocal length of about 100 mm is used in an ordinary LSM inspection apparatus, and a value of 45.0 mm (ISO standard) to 90 mm is used in a microscope base.

Therefore, the overall length of the lens system (OAL) is naturally limited substantially to this range. Therefore, for example, when discussing the length of the working distance of the objective lens, it is more difficult to compare the working distance with the value of the working distance (WD / F _O ) normalized by the total focal length (F _O ) of the lens system, which is often seen in the related art. The method of comparison using a value (WD / OAL) normalized over the entire length of the lens system is more practical. This is because the total focal length (F _O ) of the lens system is basically different depending on the magnification of the objective lens, and it can be changed quite freely by the power distribution between the “element units of the lens system”. compared,
This is because the overall length of the lens system (OAL) is often limited to a certain practical range. Therefore, in the present invention, the normalization coefficient of each of the following conditional expressions is set to F _O or OAL, and in order to secure the clarity of its physical meaning,
They will be used as appropriate.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 to 6 are cross-sectional views showing the lens configurations of the first to sixth embodiments, respectively. That is, the lens system is divided into a first group lens I, a second group lens II, and a third group lens III, and configured under the following conditions. That is, the first lens unit I is configured to have a negative refractive power in total with respect to the object point at infinity, and plays a role of first converting a parallel light beam entering the entrance pupil into a divergent light beam. Give it.

The second lens group II has a total positive refractive power in order to convert the divergent light beam formed by the first lens unit I into a convergent light beam incident on the third lens unit III. The configuration is as follows. The second lens group II has at least two achromatic joining surfaces.

The third lens group III has a positive refractive power in total in order to converge the convergent light beam having a relatively small exit angle formed by the second lens group II on the final image plane with a large aperture angle. It is configured to have

Now, the total length of the entire lens system is OAL, the radius of curvature of the first surface facing the object surface at infinity is Rf, the radius of curvature of the last surface of the first lens unit I is R'f, and the first surface is The total thickness of the first lens unit I from the lens to the final surface is Σdf, the total focal length of the first lens unit I is fI, the total focal length of the second lens unit II is fII, and the object point side of the third lens unit III 1st counting
Assuming that the radius of curvature of the third surface is Rr, the radius of curvature of the final surface is R′r, and the total focal length of the third lens group III is fIII, the following four lens groups are arranged between the lens units configured as described above. It is important to configure so that the conditional expressions are simultaneously satisfied. 0.2 ≦ fIII / OAL ≦ 0.4 (1) 1.4 ≦ R′f / Rf ≦ 3.4 (2) 2.3 ≦ Rr / R′r ≦ 4.7 (3) 0.4 ≦ Σdf / OAL ≦ 0.6 (4) Hereinafter, the above-mentioned conditional expression, which is a fundamental part of the present invention, will be described in detail.

The first lens unit I is configured as a lens having a "thick meniscus shape" as a whole, in which the curvature radius Rf of the first surface is arranged so that its concave surface faces the object point at infinity. Accordingly, the radius of curvature of the first surface is Rf, and the radius of curvature of the last surface of the first lens unit I is R′f,
The ratio is taken into conditional expression (2), and at the same time, the total thickness Δd
By adopting f as in conditional expression (4), mainly the field curvature and astigmatic difference of the entire lens system are removed. The conditional expression (2) is a condition that defines the bending range of the first group lens I.

Further, in the first group lens I, there are at least two or more achromatizing surfaces and a plurality of air contacting surfaces. These are mainly composed of axial chromatic aberration, spherical aberration and coma. It plays a role in removing aperture aberration such as aberration. However, these means and effects are means and effects that should normally be taken on the lens design, and are not essential constituent elements of the present invention. This is clear from the various aberration curves corresponding to the various modes of the first group lens I of each embodiment.

Focusing on the working distance (WD / OAL) of the lens system, when the conditional expressions (1), (2) and (4) are satisfied,
The lower limits are closely related to each other. For example, now
Assuming that the value of Σdf / OAL in conditional expression (4) is invariable and the value of fIII / OAL in conditional expression (1) remains at a certain value, the value of R′f / Rf in conditional expression (2) becomes As approaching its upper limit of 3.4, the value of WD / OAL increases. Conversely, as the lower limit of 1.4 is approached, WD
The value of / OAL decreases. Similarly, when the value of R'f / Rf in the conditional expression (2) stays at a certain value, as the value of fIII / OAL in the conditional expression (1) approaches its upper limit 0.4, WD / OAL. Increases, and conversely, the lower limit of 0.
As the value approaches 2, the value of WD / OAL decreases.

As is well known in the design of an objective lens of a 35 mm camera, a so-called “retro-focus type lens system” in which a strong negative power is disposed in front of the objective lens system, eliminates the aperture aberration. Have difficulty.
Although the long working distance objective lens of the present invention basically belongs to the category of "retro focus type lens system",
This lens needs to remove various aberrations associated with an extremely large aperture of F number 1: 0.588 (numerical aperture 0.85). That is, the long working distance objective lens of the present invention must achieve a resolution of at least the design rule (0.35 μm) of the 64M class super LSI and a defect recognition resolution level of 0.1 μm from the purpose of use.

Therefore, according to the present invention, the light source laser,
Considering practical conditions such as the glass material to be used and the performance inspection technology, the working wavelength is limited to near-ultraviolet laser light, and the value of the working distance WD / F ₀ is first increased as much as possible. It is required as an absolute requirement that the aberrations of the imaging of the entire screen be sufficiently suppressed within the diffraction limit over the entire extremely large aperture until reaching the outermost peripheral portion. For this reason, the conditional expressions (1), (2) and (4) mainly satisfy the working distance W without causing a serious obstacle to the correction of the aperture aberration.
This is an essential condition when D / F ₀ can be increased to a level of 1.0 to 3.0.

The third group lens III is basically a "combined lens of three aplanatic-single lenses whose image point is the final point of the aplanatic point". However, in order to provide residual aperture aberration and astigmatism of the opposite sign which cancel out residual aperture aberration and astigmatism of the total system of the first lens group I and the second lens group II, each of the three single lenses is individually provided. Strict application of the Apranatech condition to each must be given an appropriate "deviation".

Conditional expression (3) defines the ratio between the radius of curvature Rr of the first surface and the radius of curvature R'r of the final surface, counted from the object side, where this "deviation" appears most significantly. It is determined. Therefore, the conditional expression (3), together with the conditional expression (1), is a condition for mainly controlling the amount of generation of astigmatism and the aperture aberration in the third lens group III and the sign thereof.

In the long working distance objective according to the present invention, the first lens group I and the "second lens group II include the third lens group".
The “lens group to which III has been added” has substantially self-correcting correction potentials for the aperture aberration and the axial chromatic aberration, respectively, and has a relationship that mutually cancels the annular aberration of the residual aberration. Further, in order to assist the understanding of the constituent features of claim 1 of the claims, the position of the entrance pupil and the restrictions on the glass material used will be additionally described.

Since the long working distance objective lens of the present invention is used for scanning a laser beam over an opaque object surface such as an VLSI semiconductor device placed at the image forming position,
Naturally, the exit pupil is designed to be located at infinity in the image space. That is, it is a so-called "field telecentric lens". Therefore, the entrance pupil coincides with the front focal position in the object space, and is designed to remove various aberrations of on-axis and off-axis rays incident on the entrance pupil.

Further, the long working distance objective lens of the present invention is used as a lens for a transmission scanning microscope and as a lens for a reflection scanning microscope. In particular, in the latter case, the light beam passes twice back and forth in the objective lens. Therefore, in order to obtain an effective information signal on the structure of the object, the value of the transmittance τ of the objective lens at one time must be set to the near ultraviolet standard. Since the wavelength of the light must exceed at least 70%, the long working distance objective lens of the present invention uses a special glass material such as i-line glass or artificial fluorite having an excellent value at the near ultraviolet standard wavelength. Must be designed heavily.

Next, the meaning of the upper and lower limits of each conditional expression assuming that the overall lens system length OAL is substantially constant will be described. In the conditional expression (1), if fIII / OAL> 0.4 for the third lens group III, the second group lens II must compensate for the lack of convex power of the lens system.
This causes a relative shortage of the concave power of the achromatic junction surface in the second lens group II, and further reduces the shortage.
The negative power surface inside must be strengthened and compensated for. However, as a result, insufficient correction of spherical aberration and coma aberration of on-axis and off-axis rays of the entire lens system occurs. , Will not fit within the diffraction limit. Also, fIII / O
When AL <0.2, the working distance WD / F ₀ becomes too small, and it is necessary to shorten the total focal length f I of the first lens unit I to recover it. However, shortening the focal length fI of the first lens unit I to counteract the shortening of the focal length fIII of the third lens unit III requires the first lens unit.
The negative power of the entire group lens I is unnaturally enhanced, and as a result, overcorrection of spherical aberration and coma aberration of on-axis and off-axis rays of the entire lens system increases,
It will not fit within the diffraction limit.

In the conditional expression (2), R'f / Rf>
In the case of 3.4, the negative total power of the first group lens I is excessively enhanced. Maintaining the self-sufficiency of the correcting power of the aperture aberration in the first lens unit I is an essential requirement for maintaining the aberration correction state of all the lens systems in a good condition. In order to correct the excessively corrected aperture aberrations, the positive power in the first lens unit I must also be excessively enhanced. This situation causes the first group lens I to increase the annular aberration of the aperture aberration, and also causes excessive correction of the axial residual chromatic aberration, so that it is impossible to suppress the aperture aberration of the entire lens system to the diffraction limit. It will cause it.

When R'f / Rf <1.4, contrary to the above, the negative total power in the first lens unit I is insufficient, and the first lens unit is balanced by aberration correction. lens
Even if the positive power in I is reduced, the first lens group I
Since the absolute value of the total negative power of the lens system itself is insufficient, the working distance WD / F _{O of the} entire lens system falls into a state of low vorticity. To avoid this, the value of the focal length fIII / OAL of the third lens group III must be excessively increased beyond its upper limit of 0.4. This causes the aperture aberration of the entire lens system to fall outside the diffraction limit for the reason already described.

In the conditional expression (3), R'f / Rf>
In the case of 4.7, when the amount of disturbance of this aplanatism in the third lens unit III is restored by the integrated system of the second lens unit II and the first lens unit I, on-axis spherical aberration of the entire lens system is obtained. Ring zone aberration increases, and asymmetry of off-axis aperture aberration occurs. In particular, the asymmetry of the "upper ray annular aberration" at the most peripheral angle of view exceeds the diffraction limit and becomes a level that cannot be ignored. Also, R'f / Rf
In the case of <2.3, the above-described defect appears in the opposite form, and in particular, the asymmetry of the off-axis aperture aberration “lower ray annular zone aberration” at the most peripheral angle of view limits the diffraction limit. Beyond, it will be a level that cannot be ignored.

In the conditional expression (4), the total length of the lens system Σ
As the value of df / OAL approaches 0.6, the pace occupied by the first lens unit I in the entire lens system becomes gradually longer, and the second lens unit II and the third lens unit II are correspondingly increased.
I's space becomes increasingly cramped. However, the third group lens III basically has a configuration in which three aplanatic lenses are arranged close to each other, and it is necessary to secure a required numerical aperture and working distance (WD / OAL). The minimum required space is naturally determined for the entire length of the lens. Therefore, after all, only the space of the second group lens II is gradually reduced.

On the other hand, in the second group lens II, at least two achromatic abutting surfaces are required in order to mainly remove chromatic aberration generated in the third group lens III which is a single lens combined lens. Reducing the overall length of the second lens group II eventually makes it increasingly difficult to increase the radius of curvature of the achromatic junction surface to the required amount. Of course, in this case, if the refractive index difference before and after the achromatic junction surface is made large, this difficulty can be avoided to some extent. However, for the purpose of using the long working distance objective lens of the present invention, the transmittance in the near ultraviolet region light is required. There is a restriction that a glass material having a low refractive index cannot be used, and the glass material that can be used for the concave lens is extremely limited. Therefore, it is practically impossible to make this refractive index difference sufficiently large.

Therefore, the total length of the lens system is Δdf / OAL
In the situation of> 0.6, the chromatic aberration of the on-axis and off-axis rays and the aperture aberration in the combined lens system of the second lens group II and the third lens group III are incompletely removed, and the correction thereof is not possible. Even if the first lens unit I is loaded, a large annular aberration is left as a whole lens system.

The total length of the lens system is Σdf / OAL <
In the situation of 0.4, the length of the first lens unit I becomes too small, and in order to obtain the initial working distance WD / F ₀ , the relative radius of curvature Rf / F _O of the first surface facing the object point is too small. Where large overcorrected aperture aberrations mainly occur with respect to oblique light beams. This aberration is
Even if the correction is made on the other air contact surface and the achromatic junction surface in the group lens I, a higher order asymmetric annular aberration that cannot be corrected as a whole lens system remains. For this reason, it becomes impossible to suppress various aberrations within the diffraction limit in the peripheral portion of the visual field.

Next, the conditional expression of claim 2 will be described. (1) to (4) of claim 1 described above.
Is a long working distance objective lens that satisfies the following conditional expression, and the focal length F _O / OAL of the entire lens system normalized by the total length OAL of the lens system is 0.044 mm class.
Further, it is essential to simultaneously satisfy the following conditional expressions in order to obtain a long working distance objective lens having excellent performance. 1.6 ≦ fIII / | fI | ≦ 2.6 ····· (5) 8.9 ≦ fII / F O ≦ 15.0 ······· (6)

That is, a long focal length F _{O of about} 2.6 mm
Has a working distance WD / F _O normalized by
To obtain a focal length F _O / objective lens of the long working distance and high magnification and large diameter having OAL of 44mm grade short entire lens system, the configuration requirements of the listed claims 1 (1) to (4) Satisfying, first-group lens with negative refractive power
The ratio fIII / | fI of the total focal length of the third lens group III having a positive refractive power to the absolute value | fI | of the total focal length of I
| Limit value to a relatively narrow range of 1.6 to 2.6 of [Condition (5)] Then simultaneously, the focal length F _O of the entire lens system
It is possible to limit the value of the focal length fII / F _O of the second lens unit II having the positive refractive power normalized by the above to a relatively narrow range of 8.9 to 15.0 [conditional expression (6)]. It is important.

The above-mentioned conditional expression (5) indicates that, in an objective lens having a value of F _O / OAL of 0.044 mm class, the focal length F _O / O is controlled while suppressing various aperture aberrations over the entire field of view within the diffraction limit.
This is a basic condition for increasing the working distance WD / OAL to its limit as compared with the AL, but in this case, by satisfying conditional expression (6) as an additional condition at the same time, the distance between each group in the entire lens system is increased. The power balance and the correction sharing for the various aperture aberrations can be made appropriate.

Hereinafter, the meaning of the upper and lower limits of each conditional expression will be described for an objective lens having a focal length F _O / OAL of 0.044 mm class, which is normalized by the total length OAL of the lens system. Before entering, the actual configuration restrictions imposed on each lens configuration group will be described first.

Regarding the third lens group III, first, fIII
It is necessary to satisfy the conditional expression (1) imposed on / OAL. On the condition that the conditional expression (4) imposed on Σdf and the second group lens II have at least two achromatic junctions A lens capable of securing a large image-side numerical aperture of about 0.85 or more in the remaining space for the third group lens III finally derived from the space for the configuration having the surface and the limit value of the OAL itself. There are constraints that must be met. On the other hand, for the first group lens I, it is necessary to simultaneously satisfy the conditional expression (2) imposed on R'f / Rf and the conditional expression (4) imposed on the thickness Σdf as well. the focal length of the first perforated diameter [psi _ED is the entire lens system of the lens facing the object in the infinite distance F _O and the entrance pupil [psi _ENT.P uniquely determined by the image-side numerical aperture NA (= 2F _O
There is a restriction that must be larger than NA).

As for the second lens group II, since this lens group is a lens having a positive focal length at the position where the light beam diameter is widened in the entire objective lens system, The glass material of the convex lens of the achromatic lens must be thick, and therefore, it is easy to lower the transmittance τ _UV of near-ultraviolet light of all lens systems, so that the fluorite with excellent transmittance τ _UV is used. There are restrictions that necessitate the use of artificial crystals. From the above actual configuration constraints imposed on the constituent groups of the long working distance objective lens of the present invention, the focal length fIII / F _O to obtain practically picking of the third lens group III is approximately 4.8
Stays in the range of 7.4. Further, the practically possible focal length | fI | / F _O of the first lens unit I is approximately 2.2 to 3.0.
It stays in the range of 9. Further, the focal length fII / F _O to obtain practically picking of the second lens group II remains in the range of approximately 8.9 to 15.0.

Now, in conditional expression (5), fIII / | f
When I |> 2.6, there are a case where fIII is excessively larger than | fI | and a case where | fI | is excessively smaller than fIII. In the former case, fII
Since I is long and the refractive index of the glass material is relatively low, the total lens length of the third lens group III becomes longer, and accordingly the total lens space of both the second lens group II and the third lens group III is reduced. As a result, the number of achromatic mating surfaces in the system cannot be sufficiently secured. Therefore, it becomes difficult to suppress the aperture aberration of all the lens systems within the diffraction limit. In the latter case, the absolute value | f of the focal length of the first lens unit I 1
I | becomes short, the Pepperl sum of the whole system becomes excessive with a negative value, and the overcorrected field curvature and the annular aberration of underline coma with respect to the principal ray increase, exceeding the diffraction limit at the outermost periphery of the field of view. Would.

In conditional expression (5), fIII / |
When fI | <1.6, the focal length fIII of the third lens unit III is equal to the absolute value | f of the focal length of the first lens unit I.
Case that is too small compared to I |
In some cases, the absolute value | fI | of the focal length of the third lens group III becomes excessively large compared to the focal length fIII of the third lens unit III. In the former case, the focal length fIII of the third lens group III is short,
Therefore, the working distance of the entire lens system is also shortened, so that the usability of the scanning microscope apparatus for inspection of the VLSI, which is the objective of the long working distance objective lens of the present invention, is seriously hindered. In the latter case, the absolute value | fI | of the focal length of the first lens unit I is long, and the Pebbal sum of the entire system remains at a relatively large positive value. Would. Further, the on-axis aperture aberration is also insufficiently corrected, and an aberration exceeding the diffraction limit remains in the periphery of the field of view.

If the lens satisfying the conditional expression (5) satisfies fII / F _O > 15.0 in the conditional expression (6), the positive refracting power of the second lens group II becomes extremely short. The third lens group III from the object side.
The positive refractive indices of the curvature radius Rr of the surface and the curvature radius R'f of the final surface counted from the object side of the first lens unit I must be strengthened and compensated. As a result, the value exceeds the lower limit of Rr / R'r according to the conditional expression (3) of 2.3 and the lower limit of R'f / Rf according to the conditional expression (2) of 1.4. For the reasons described in the description, it becomes impossible to suppress the aberration within the diffraction limit.

A lens satisfying the conditional expression (5) is a second lens group II normalized by F _O in the conditional expression (6).
When the focal length fII / F _O <8.9, conversely, the positive refractive power of the second group lens II is excessively strengthened, so that the first surface of the third group lens III Radius of curvature Rr of
In addition, the curvature radius R'f of the last surface of the third group lens III must be balanced by relaxing the positive refractive power. As a result, Rr /
R′f according to condition (2) and the upper limit of R′r of 4.7
Since the upper limit of / Rf exceeds 3.4, the aberration cannot be suppressed within the diffraction limit for the reason already described.

Next, claim 3 will be described. A objective lens satisfying the conditional expression according to claim 1 (1) to (4), the focal length F _O / O of the entire lens system is normalized OAL
When the objective lens has an AL of 0.095 mm class, it is indispensable to simultaneously satisfy the following conditional expressions in order to obtain a long working distance objective lens having excellent performance. 0.05 ≦ fIII / | fI | ≦ 0.3 (7) 9.60 ≦ fII / Fo ≦ 29.0 (8)

That is, a long focal length Fo of about 1.2 mm
Has a working distance WD / Fo normalized by
95mm class relatively long focal length Fo / OA
In order to obtain an objective lens having a long working distance, a medium magnification, and a large aperture having L, the lens unit satisfies the requirements of the conditional expressions (1) to (4) described in claim 1 and furthermore, fIII / | fI ｜
Is limited to the range of 0.05 to 0.3 [conditional expression (7)]
At the same time, it is important to limit the value of fII / Fo to the range of 9.6 to 29.0 [conditional expression (8)].

Conditional expression (7) represents the focal length Fo / fo of the entire lens system.
In an objective lens having an OAL of 0.095 mm class, Fo / O while suppressing various aperture aberrations over the entire field of view within the diffraction limit.
Working distance WD /
This is the basic condition for increasing the OAL to its limit. In this case, by simultaneously satisfying the conditional expression (8) as an additional condition, the power balance among the lens units in the entire lens system and the correction sharing for various aberrations in the aperture. Can be adjusted appropriately.

Hereinafter, the value of Fo / OAL is 0.095 mm
For the objective lens of the class, the meaning of the upper and lower limits of each conditional expression will be described. Before going into the main subject, first, the restrictions on the actual configuration imposed on each lens configuration group will be summarized and described. That is, the restrictions on the actual configuration of the third lens group III, the first lens group I, and the second lens group II are exactly the same as the restrictions of the related parts already described in the description of the conditional expression in claim 2. .

From these restrictions, the focal length fIII of the third lens group III normalized by the focal length of the entire lens system is used.
The value of / F _O remains practically in the approximate range of 2.1 to 4.1. The value of the focal length | fI | / F _O of the first lens unit I 1 normalized by the focal length of the entire lens system is practically almost 1
It remains in the range of 1.1 to 54.4. Also, the focal length fII / of the second lens group II normalized by the focal length of all lens systems
The value of F _O remains practically in the range of about 9.7 to 29.1. Now, in conditional expression (7), fIII / | fI |>
0.3 and fIII / | fI | <0.0
The troubles encountered when the value is 5 are exactly the same as the troubles of the related parts already described in the description of the conditional expression in claim 2.

In the condition (8), the lens satisfying the conditional expression (7) satisfies the focal length fII / F _O > 29.0 of the second lens group II normalized by the focal length of the entire lens system. The problem encountered when fII / F _O <9.6 is exactly the same as the problem in the related part already described in the description of the conditional expression in claim 2.

The description of the conditional expressions in each claim has been completed above, but the following conclusions can be easily derived from what has been described so far. That is, the focal length Fo / OAL of all the lens systems normalized by the total lens system length OAL is 0.044.
A long working distance intermediate magnification objective lens interposed between the mm class and the 0.095 mm class satisfies the constitutional requirements (1) to (4) of claim 1 and the respective conditions of claim 2 and claim 3. In the equations (5) to (8), the intermediate value of the lower limit corresponding value, that is, the upper limit of fIII / | fI | the values between 0.05 to 1.6, As for the value of the upper limit of FII / F _O, values between 15.0 to 29.0, for the lower limit from 8.9 to 9.6 ..), It is possible to reliably obtain an excellent long working distance objective having an imaging performance within the diffraction limit in the effective entire field of view. Therefore, an objective lens having such a specification is also included in the scope of the present invention.

The long working distance objective lens which satisfies the constitutional requirements of the present invention has a long working distance which cannot be achieved by a conventionally known objective lens which uses near-ultraviolet light at a standard wavelength, and has a long working distance. Has an imaging performance within the diffraction limit over the entire effective field of view when it is actually manufactured,
It is extremely useful when used in a laser beam scanning device for inspecting a test sample such as an I semiconductor element under a long working distance with a sufficient margin. Further, the long working distance objective lens of the present invention is a long working distance objective lens in which the generation of auto-fluorescence flare due to near-ultraviolet excitation light is extremely small, so that the entire lens length can be accommodated in the “45 mm parfocal length lens frame”. In recent studies in the field of molecular biology, it has become possible to provide a “very convenient means for various manipulations of test cell samples” that has not existed in recent studies in the field of molecular biology. Can be provided.

Further, since the long working distance objective lens of the present invention can be used as an objective lens for a high-temperature microscope having an unprecedented high-magnification resolution, it is more powerful for the study of phase transformation under high temperature of metal. Means can be provided.

[0062]

EXAMPLES Tables 1 to 6 show specific numerical examples from Example 1 to Example 6 and FIGS.
Is shown in the sectional view. FIG. 7 is an aberration curve diagram of each embodiment.
12 to FIG. However, the first to third embodiments are OAL
In the focal length of the entire lens system is normalized F _O / OAL correspond to 0.044mm grade lens, Examples 4 6
Corresponds to a 0.095 mm class lens with F _O / OAL.

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

[0066]

[Table 4]

[0067]

[Table 5]

[0068]

[Table 6]

Referring to the aberration curve diagrams shown in FIGS. 7 to 12 corresponding to each embodiment, all the lenses listed in the above embodiments have spherical aberration, coma aberration, and axial aberration for a large aperture of NA 0.85 class. It can be seen that "aperture aberrations" such as chromatic aberration of the above are sufficiently removed over the entire field of view. In the lenses of all these examples, the “Strehl of the diffraction image of the point image” reaches 0.90 or more for the standard wavelength light and 0.84 or more for the assisting wavelength light at any point in the entire field of view. Has almost the same performance as an “aberration-free lens”.

[0070]

As described above, according to the long working distance objective lens of the present invention, a long working distance which cannot be achieved by the conventionally known objective lens which uses near-ultraviolet light at a standard wavelength can be achieved. In addition, since the lens has an imaging performance within the diffraction limit over the entire effective field of view when the lens is actually manufactured, it can inspect a sample to be inspected, such as a super LSI semiconductor device, with a sufficiently long working distance. It is very effective when used in a laser beam scanning device for the purpose of performing. In addition, since the objective lens is a long working distance objective lens that generates very little auto-fluorescence flare due to near-ultraviolet excitation light, if the overall length of the lens is shortened to an extent that can be accommodated in a “45 mm parfocal length lens frame”, it is possible to reduce the recent molecular biology In research in the field, it has become possible to provide "extremely convenient means for various manipulations of test cell samples" that did not exist in the past, and to supply a fluorescence microscope device with more excellent features than conventional devices. it can. Furthermore, since it can be used as an objective lens for a high-temperature microscope having an unprecedented high-magnification resolution, it can provide a more powerful means for studying phase transformation of metals at high temperatures.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration of an objective lens according to a first embodiment.

FIG. 2 is a cross-sectional view illustrating a configuration of an objective lens according to a second embodiment.

FIG. 3 is a cross-sectional view illustrating a configuration of an objective lens according to a third embodiment.

FIG. 4 is a cross-sectional view illustrating a configuration of an objective lens according to a fourth embodiment.

FIG. 5 is a cross-sectional view illustrating a configuration of an objective lens according to a fifth embodiment.

FIG. 6 is a cross-sectional view illustrating a configuration of an objective lens according to a sixth embodiment.

FIG. 7 is an aberration curve diagram of the objective lens of Example 1.

FIG. 8 is an aberration curve diagram of the objective lens of Example 2.

FIG. 9 is an aberration curve diagram of the objective lens of Example 3.

FIG. 10 is an aberration curve diagram of the objective lens of Example 4.

FIG. 11 is an aberration curve diagram of the objective lens of Example 5.

FIG. 12 is an aberration curve diagram of the objective lens of the sixth embodiment.

[Explanation of symbols]

I curvature radius d _i axial distance of the i-th surface of the first lens group II second lens group III third lens r _i i-th surface

[Procedure amendment]

[Submission date] May 30, 1997

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0003

[Correction method] Change

[Correction contents]

The objective lens disclosed in Japanese Patent Application Laid-Open No. 5-72482 is a dry-type high-magnification large-aperture objective lens that can be used with light in the far ultraviolet region near 250 nm . However, the working distance (WD) of this objective lens is of the order of 0.92 mm, and this value is only less than 1.0% of the total length of the lens system (OAL).
There is a problem that greatly limits the design of the inspection device (LSM) for I semiconductor devices. The objective lens disclosed in Japanese Patent Application Laid-Open No. 5-196874
This is a dry-type, high-magnification, large-aperture objective lens that can be used with light in the near ultraviolet region near 51 mm. However, the working distance (WD) of this objective lens is also at a level of 1.57 mm,
Since the value is only less than 2.0% of the total length of the lens system (OAL), there is still a problem that the design of an inspection device (LSM) for an VLSI semiconductor device of 64M class or more is greatly restricted.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0014

[Correction method] Change

[Correction contents]

The long working distance objective lens of the present invention is a laser scanning microscope ( LS) using near-ultraviolet laser light.
M ). The standard wavelength in the design is UV-Ar ⁺ (351 nm, 364 n
m), He-Cd (325 nm), YAG THG (3
Any one of various near-ultraviolet laser light sources (e.g., 54 nm) is selected and basically used, and it is necessary to secure imaging performance within the diffraction limit within the entire scanning visual field for the standard single-wavelength light. Further, these imaging performances must be manufactured so as to be surely realized over the entire lens material and the lens processing process, taking into account their manufacturing tolerances.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0059

[Correction method] Change

[Correction contents]

The description of the conditional expressions in each claim is completed with the above, but the following conclusions can be easily derived from what has been described so far. That is, the focal length F ₀ / OAL of the entire lens system normalized by the total lens system length OAL is 0.04.
A long working distance intermediate magnification objective lens interposed between the 4 mm class and the 0.095 mm class satisfies the constitutional requirements (1) to (4) of claim 1 and the respective conditions of claim 2 and claim 3. In the equations (5) to (8), the intermediate value of the lower limit corresponding value, that is, the upper limit of fIII / | fI |, a value between 0.3 to 2.6, and a lower limit thereof 8.9 to 9 values between 0.05 to 1.6, As for the value of the upper limit of f II / _{F 0,} values between 15.0 to 29.0, for the lower limit. 6), it is possible to reliably obtain an excellent long working distance objective lens having an imaging performance within the diffraction limit in the effective entire field of view. Therefore, an objective lens having such a specification is also included in the scope of the present invention.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0069

[Correction method] Change

[Correction contents]

Referring to the aberration curve diagrams shown in FIGS. 7 to 12 corresponding to the respective embodiments, all the lenses listed in the above embodiments show spherical aberration, coma aberration, and on-axis aberration for a large aperture of NA 0.85 class. It can be seen that "aperture aberrations" such as chromatic aberration of the above are sufficiently removed over the entire field of view. In all of the lenses of these examples, the “Strehl intensity of the diffraction image of the point image” reaches 0.90 or more for the standard wavelength light and 0.84 or more for the reference wavelength light at any point in the entire field of view. Therefore, it has almost the same performance as the “aberration-free lens”.

[Procedure amendment 5]

[Document name to be amended] Drawing

[Correction target item name] Fig. 1

[Correction method] Change

[Correction contents]

FIG.

[Procedure amendment 6]

[Document name to be amended] Drawing

[Correction target item name] Figure 2

[Correction method] Change

[Correction contents]

FIG. 2

[Procedure amendment 7]

[Document name to be amended] Drawing

[Correction target item name] Figure 3

[Correction method] Change

[Correction contents]

FIG. 3

[Procedure amendment 8]

[Document name to be amended] Drawing

[Correction target item name] Fig. 4

[Correction method] Change

[Correction contents]

FIG. 4

[Procedure amendment 9]

[Document name to be amended] Drawing

[Correction target item name] Fig. 5

[Correction method] Change

[Correction contents]

FIG. 5

[Procedure amendment 10]

[Document name to be amended] Drawing

[Correction target item name] Fig. 6

[Correction method] Change

[Correction contents]

FIG. 6

Continuing from the front page (72) Inventor Osamu Maeda 3-5-3 Shinmachi, Setagaya-ku, Tokyo Showa Optronics Co., Ltd. (72) Inventor Shingo Murakami 5-7-1 Shiba 5-chome, Minato-ku, Tokyo NEC Corporation Inside

Claims (3)

[Claims] 1. A positive refractive power is provided between a first lens unit having a negative refractive power for an object point at infinity and a third lens unit having a positive refractive power for an image point. In the objective lens having the second group lens provided therein, the third group lens is constituted by three or more single lenses, and at least two or more achromatic junction surfaces are arranged in the second group lens. The overall length of the system is OAL, the focal length of the third lens unit is fIII, the radius of curvature of the first surface is Rf, counting from the object point side of the first lens unit, and the radius of curvature of the last surface of the first lens unit is R. 'F, the radius of curvature of the first surface counted from the object point side of the third lens unit is Rr, the radius of curvature of the last surface of the third lens unit is R'r, and the total thickness of the first lens unit is Σdf. Wherein a long working distance objective lens satisfies the following conditional expression. 0.2 ≦ fIII / OAL ≦ 0.4 (1) 1.4 ≦ R′f / Rf ≦ 3.4 (2) 2.3 ≦ Rr / R′r ≦ 4.7 (3) 0.4 ≤ ／ df / OAL ≤ 0.6 (4) 2. The objective lens according to claim 1, wherein
When the total focal length of the lens system is Fo, the absolute value of the focal length of the first lens unit is | fI |, and the focal length of the second lens unit is fII, the following conditional expression is satisfied. Long working distance objective lens. 1.6 ≦ fIII / | fI | ≦ 2.6 (5) 8.9 ≦ fII / Fo ≦ 15.0 (6) 3. The objective lens according to claim 1, wherein
When the total focal length of the lens system is Fo, the absolute value of the focal length of the first lens unit is | fI |, and the focal length of the second lens unit is fII, the following conditional expression is satisfied. Long working distance objective lens. 0.05 ≦ fIII / | fI | ≦ 0.3 (9) 9.60 ≦ fII / Fo ≦ 29.0 (8)