U.S. patent application number 09/897398 was filed with the patent office on 2002-03-07 for microscope objectives lens.
This patent application is currently assigned to Nikon Corporation. Invention is credited to Ono, Kenji.
Application Number | 20020027707 09/897398 |
Document ID | / |
Family ID | 18701485 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020027707 |
Kind Code |
A1 |
Ono, Kenji |
March 7, 2002 |
Microscope objectives lens
Abstract
A microscope objective lens of the present having, in the order
from the object side, a first lens group, a second lens group and a
third lens group. The first lens group comprises a meniscus lens
with its concave surface facing the object side and a plurality of
cemented lenses, and has a positive refractive power on the whole.
The second lens group comprises a plurality of cemented lenses and
has a positive refractive power on the whole. The third lens group
comprises a plurality of cemented lenses and having a negative
refractive power on the whole, and fulfills the following
conditions (1) to (3): 2.5<.linevert split.f1/f.linevert
split.<5 (1) 30<.linevert split.f2/f.linevert split.<70
(2) 15<.linevert split.f3/f.linevert split.<30 (3) where f1
is the focal length of the first lens group, f2 Is the focal length
of the second lens group, f3 is the focal length of the third lens
group, and f is the focal length of the whole microscope objective
lens system.
Inventors: |
Ono, Kenji; (Kawasaki-shi,
JP) |
Correspondence
Address: |
MILES & STOCKBRIDGE PC
1751 PINNACLE DRIVE
SUITE 500
MCLEAN
VA
22102-3833
US
|
Assignee: |
Nikon Corporation
|
Family ID: |
18701485 |
Appl. No.: |
09/897398 |
Filed: |
July 3, 2001 |
Current U.S.
Class: |
359/368 ;
359/656 |
Current CPC
Class: |
G02B 21/02 20130101;
G02B 21/16 20130101 |
Class at
Publication: |
359/368 ;
359/656 |
International
Class: |
G02B 021/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2000 |
JP |
2000-204179 |
Claims
What is claimed is:
1. A microscope objective lens comprising, in order from the object
side, a first lens group, a second lens group and a third lens
group; said first lens group comprising a meniscus lens with its
concave surface facing the object side and a plurality of cemented
lenses, and having a positive refractive power on the whole: said
second lens group comprising a plurality of cemented lenses and
having a positive refractive power on the whole: and said third
lens group comprising a plurality of cemented lenses and having a
negative refractive power on the whole; the lens fulfilling the
following conditions (1) to (3):2.5<.linevert
split.f1/f.linevert split.<5 (1)30<.linevert
split.f2/f.linevert split.<70 (2)15<.linevert
split.f3/f.linevert split.<30 (3)where; f1 is the focal length
of the first lens group; f2 is the focal length of the second lens
group; f3 is the focal length of the third lens group; and f is the
focal length of the whole microscope objective lens system.
2. The microscope objective lens according to claim 1, wherein said
second lens group has at least three groups of cemented lenses each
having at least one cementing surface.
3. The microscope objective lens according to claim 1, wherein said
third lens group has at least one group of cemented lens having at
least one cementing surface.
4. The microscope objective lens according to claim 1, wherein said
first lens group comprises at least one convex meniscus lens, at
least one group of cemented lens having at least one cementing
surface, and at least one single lens.
5. The microscope objective lens according to claim 1. wherein the
lenses of said lens groups each comprise a glass material
comprising fluorite, quartz, barium fluoride or strontium
fluoride.
6. The microscope objective lens according to claim 1, wherein, in
the cemented lenses the microscope objective lens has, lens
components are cemented with a cement comprising fluoropolymers.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an objective lens having a long
working distance and a magnification of about 50, and more
particularly to an objective lens used in optical systems such as
microscopes which utilize ultraviolet light.
[0003] 2. Related Background Art
[0004] Objective lenses for an ultraviolet region of around 250 nm
wavelength include, e.g., a lens disclosed in Japanese Patent
Application Laid-Open No. 7-20385. This lens Is an objective lens
constituted of i) a first lens group which has cemented lens groups
each composed of a convex lens and a concave lens, and ii) a second
lens group which comprises two or more cemented lens groups, at
least one of which includes a triple cemented lens. Correction of
aberrations has been made simultaneously for the visible region and
the near-ultraviolet region.
[0005] An ultraviolet objective lens disclosed in Japanese Patent
Application Laid-Open No. 3-188407 also has a high NA (numerical
aperture) and a high magnification, where color correction has been
made over the range of from the visible region to the ultraviolet
region and a part of lenses can be set floating In accordance with
service wavelength. Herein, "floating" is meant to move lens groups
so as to make aberrations small In accordance with service
wavelength while keeping the length of the whole optical system at
a constant value.
[0006] In these conventional cases, the lenses can be used under
ultraviolet light when used for the inspection of wafers,
inspection of pits of, e.g., digital video disks (DVD) and
inspections of, e.g., hard disk drives (HDD). However, when used in
a microscope incorporated with an automatic focus detector which
emits near-infrared light onto an object surface to be observed, to
detect focal position on the object surface on the basis of the
light which casts back therefrom, it is difficult for the object
surface to be focused because the near-infrared light or
near-ultraviolet light is not at the same focal position with the
ultraviolet light.
[0007] The ultraviolet objective lens disclosed in Japanese Patent
Application Laid-Open No. 3-188407 also has a disadvantage of poor
operability because it is a lens some part of which is set floating
in accordance with service wavelength and also it has a very short
working distance (WD).
[0008] In the case when objective lenses are used in the
near-ultraviolet region, some materials constituting an optical
system may cause an abrupt fall of transmittance or may cause
solarization. Accordingly, there is a limitation on the materials
to be used. In particular, where the wavelength is 300 nm or
shorter, mediums usable as glass materials are necessarily limited
to quartz and fluorite. These optical materials have a small
difference in Abbe constants. but the chromatic aberration can
sufficiently be corrected by using triplets In a large number as
cemented lenses. However, any cement is not available which has a
sufficient transmittance at around a wavelength .lambda. of 250 nm
and has a good operability and a superior cementing force. Thus,
there has been a problem that those making use of highly precise
triple cemented lenses In a large number have had a difficulty in
manufacture.
[0009] Moreover, the light used for observation and that for
automatic detection of focal points are different in wavelength
from each other. Hence, because of the above limitations on optical
materials to be used. it is difficult to materialize an objective
lens In which image planes in respect of the light having
wavelength used for observation and the light having wavelength for
automatic detection of focal points have been brought into
substantial agreement and at the same time a high numerical
aperture and a high magnification have been ensured and also the
longitudinal chromatic aberration of two wavelengths has been
corrected. There has also been such a problem.
SUMMARY OF THE INVENTION
[0010] The present invention was made taking account of the above
problems. Accordingly, an object of the present invention Is to
provide a microscope objective lens which is durable against its
use in the ultraviolet region, near-ultraviolet region and
near-infrared region and in which materials having ensured a
sufficient transmittance are used and at the same time the
longitudinal chromatic aberration and various aberrations have well
been corrected, also having a relatively long working distance.
[0011] To achieve the above object, the present invention provides
a microscope objective lens comprising, in order from the object
side, a first lens group, a second lens group and a thrid lens
group;
[0012] said first lens group comprising a meniscus lens with its
concave surface facing the object side and a plurality of cemented
lenses, and having a positive refractive power on the whole;
[0013] said second lens group comprising a plurality of cemented
lenses and having a positive refractive power on the whole; and
[0014] said third lens group comprising a plurality of cemented
lenses and having a negative refractive power on the whole;
[0015] the lens fulfilling the following conditions (1) to (3):
2.5<.linevert split.f1/f.linevert split.<5 (1)
30<.linevert split.f2/f.linevert split.<70 (2)
15<.linevert split.f3/f.linevert split.<30 (3)
[0016] where;
[0017] f1 is the focal length of the first lens group;
[0018] f2 is the focal length of the second lens group;
[0019] f3 is the focal length of the third lens group; and
[0020] f is the focal length of the whole microscope objective lens
system.
[0021] According to the present invention, a microscope objective
lens can be provided which Is durable against its use in the
ultraviolet region, near-ultraviolet region and near-infrared
region and in which materials having ensured a sufficient
transmittance are used and at the same time the longitudinal
chromatic aberration and various aberrations have well been
corrected, also having a relatively long working distance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 illustrates the lens construction of a microscope
objective lens according to a first embodiment of the present
invention.
[0023] FIG. 2 presents diagrams showing various aberrations of the
microscope objective lens according to the first embodiment.
[0024] FIG. 3 illustrates the lens construction of a microscope
objective lens according to a second embodiment of the present
invention.
[0025] FIG. 4 presents diagrams showing various aberration of the
microscope objective lens according to the second embodiment.
[0026] FIG. 5 illustrates the lens construction of an imaging
lens.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] The microscope objective lens of the present invention has,
in the order from the object side, a first lens group, a second
lens group and a third lens group. The first lens group comprises a
meniscus lens with its concave surface facing the object side and a
plurality of cemented lenses, and has a positive refractive power
on the whole, the second lens group comprises a plurality of
cemented lenses and has a positive refractive power on the whole,
and the third lens group comprises a plurality of cemented lenses
and has a negative refractive power on the whole, and fulfills the
following conditions (1) to (3):
2.5<.linevert split.fl/f.linevert split.<5 (1)
30<.linevert split.f2/f.linevert split.<70 (2)
15<.linevert split.f3/f.linevert split.<30 (3)
[0028] where f1 is the focal length of the first lens group, f2 is
the focal length of the second lens group, f3 is the focal length
of the third lens group, and f is the focal length of the whole
microscope objective lens system.
[0029] The condition (1) is a condition for ensuring the necessary
working distance and also bringing residual secondary spectra of
the longitudinal chromatic aberration into a correctable range. If
the value is outside the upper limit of the condition (1), the lens
can have a longer working distance but may have an undesirable
longitudinal chromatic aberration In respect of residual secondary
spectra. If on the other hand the value is outside the lower limit
of the condition (1), the first lens group may undesirably have too
strong a power to balance with the second lens group in the
correction of spherical aberration.
[0030] The condition (2) is a condition which defines a proper
range of the focal length of the second lens group. If the value is
outside the upper limit of the condition (2), the second lens group
may have so weak a refractive power as to have an undesirable
spherical aberration In respect of the light in the range of color
correction width of .+-.3 nm with respect to the reference
wavelength .lambda. of 248 nm. If it is attempted to change the
cementing surface of each lens so as to avoid this, the lens may
have undesirable long-wavelength-side spherical aberration and
short-wavelength-side comatic aberration, making it impossible to
effect any sufficient correction of aberrations. If on the other
hand the value is outside the lower limit of the condition (2), the
second lens group may have too strong a refractive power. This may
lower the height of light rays to make them unable to enter the
second lens group G3, making It impossible to effect any good
correction of aberrations.
[0031] The condition (3) Is a condition which defines the focal
length of the third lens group having a negative refractive power.
If the value is outside the upper limit of the condition (3), the
third lens group may have so weak a refractive power as to make the
spherical aberration undesirable in respect of the light having the
reference wavelength .lambda. of 248 nm and make the image plane
curve. if it is attempted to change the refractive power of each
lens so as to avoid this, the image plane may more greatly curve,
making it impossible to effect any sufficient correction of
aberrations. If on the other hand the value is outside the lower
limit of the condition (3), the third lens group may have too
strong a refractive power, so that the third lens group may have
too weak a refractive power to effect any sufficient correction of
longitudinal chromatic aberration, spherical aberration and comatic
aberration. The lower limit value and upper limit value of the
condition (3) may more preferably be 20 and 25, respectively,
because the present invention can be made much more effective.
[0032] In a preferred embodiment of the present invention, it is
preferable to use the cemented lens in plurality. In such a case,
in order to achieve a state having neither longitudinal chromatic
aberration nor chromatic aberration of magnification, the cemented
lenses in the first lens group G1 and second lens group G2 may
preferably be achromatic lenses, and the cemented lenses In the
third lens group G3 be chromatic lenses. Accordingly, the lenses
may preferably be so formed that the Abbe constant of positive
lenses is larger than the Abbe constant of negative lenses, of the
cemented lenses in the first lens group G1 and second lens group
G2, and the Abbe constant of positive lenses Is smaller than the
Abbe constant of negative lenses, of the cemented lenses In the
third lens group G3.
[0033] In a preferred embodiment of the present invention, as glass
materials which constitute respective lens components, it is also
preferable to use not only fluorite and quartz but also strontium
fluoride and barium fluoride. It is well known to constitute an
optical system by using at least two types of glass materials in
order to correct chromatic aberration. Especially in the case of
objective lenses which transmit ultraviolet light and near-infrared
light, it is preferable to use only materials having a sufficient
transmittance over the wavelength of from around 200 nm to around
800 nm, such as fluorite, quartz, strontium fluoride and barium
fluoride.
[0034] In a preferred embodiment of the present invention, it is
also preferable to use as a cement for cementing each lens
component a fluoroplastic(s). Cements commonly used under visible
light tend to absorb ultraviolet light to cause deterioration of
optical transmission properties as a result of irradiation for a
long time. In particular, epoxy-type cements may change in color
into yellow or brown as a result of irradiation by ultraviolet
light having a wavelength .lambda. of 365 nm or shorter. Hence, It
is not preferable to use them in objective lenses. Also,
additive(s) such as stabilizing agent, decomposer or the like
contained in the cement may absorb the .lambda. 365 nm or shorter
ultraviolet light to be decomposed in the cement and colored, so
that the cement layer(s) in the interiors of lenses may become
cloudy to damage the original optical performance.
[0035] A fluoroplastic has superior resistance to ultraviolet rays,
moisture resistance, weatherability and chemical resistance.
Accordingly, a fluoroplastic used as a cement according to the
present invention is required to comprise carbon and fluorine only
as its constituent elements, but has no other limitation. The
fluoroplastic has no particular limitation in its structure and may
have a ring, linear or branched structure, as well as a
cross-linked structure. The use of such substance as above may
provide the optical system which can be used In a stable state. In
the present invention, as examples of the substance having a
fluorine resin, the cement may be comprised of any of
fluoropolymers such as CYTOP (trade name of Asahi chemical Glass
Company (Japan)) or Teflon AF (trade name of Dupon-Mitsui Company
(Japan)), and fluororubbers.
[0036] Embodiments of the microscope objective lens according to
the present invention are described below with reference to the
accompanying drawings. All embodiments are designed in an infinity
system. Also, when used actually as objective lenses of
microscopes, for example, an imaging lens constructed as shown in
FIG. 5 is provided on the image side. Specific values of this
imaging lens are described later.
First Embodiment
[0037] FIG. 1 illustrates the lens construction of a microscope
objective lens according to First Embodiment. This microscope
objective lens comprises, in order from the object side, a first
lens group G1, a second lens group G2 and a third lens group G3.
The first lens group G1 comprises a meniscus lens with its concave
surface facing the object side and a plurality of cemented lenses,
and has a positive refractive power on the whole, the second lens
group G2 comprises a plurality of cemented lenses and has a
positive refractive power on the whole and the third lens group G3
comprises a plurality of cemented lenses and has a negative
refractive power on the whole.
[0038] Specific values of the present Embodiment are listed In
Table 2. In the whole specific values, f is the focal length of the
whole system at the time of infinity with respect to light rays of
248 nm wavelength, and is the specific value itself without using
the above imaging lens. Also, NA represents the numerical aperture
on the object side; .beta., the magnitication; and W.D., a value
corresponding to the working distance, i.e,, the distance between
the object plane and the vertex of front lens surface. In the lens
data, the left-end numerical values represent the order of lenses
counted in order of incidence of light rays; R, the curvature
radius of lens surface; d, the distance between lens surfaces; and
Material, the name of a material. The refractive index of each
material to each wavelength is also shown in Table 1. In Table 1,
the columns corresponding to n248 and n486 provide refractive
indexes to light of 248 nm wavelength and 486 nm wavelength.
respectively. Also, .nu. represents the Abbe constant of each lens
on the basis of light of 248 nm wavelength .lambda., and is
expressed by the following equation.
v=(n248-1)/(n248-n486)
[0039]
1 TABLE 1 Material n248 n486 .nu. Quartz 1.508569 1.463166 11.201
Fluorite 1.468016 1.437019 15.098 SrF.sub.2 1.474122 1.441252
14.424 BaF.sub.2 1.519923 1.478546 12.565
[0040] In specific values in all Embodiments given below, the same
letter symbols as those in the present Embodiment are used. Also,
"mm" is commonly used as the units of length of focal length,
curvature radius, surface distance and others. However, the units
are by no means limited thereto because optical systems can provide
equivalent optical performances even when magnified or reduced
proportionally.
[0041]
2TABLE 2 (Total specific values) f = 4.02 NA = 0.55 .beta. =
50.times. W.D. = 7.485 (Lens data) No. R d Material 1 0.00 7.485 2
-24.4298 1.30 Quartz L1 3 -127.5017 4.10 Fluorite L2 4 -9.2053 0.10
5 -46.0400 3.10 Fluorite L3 6 -20.8995 0.15 7 47.9329 1.30 Quartz
L4 8 15.6025 4.50 Fluorite L5 9 -24.7360 0.15 10 50.5372 4.90
Fluorite L6 11 -15.4830 1.35 BaF.sub.2 L7 12 17.6252 4.55 Fluorite
L8 13 -28.3871 0.10 14 64.3340 4.50 Fluorite L9 15 -13.7278 1.30
Quartz L10 16 9.8383 5.25 Fluorite L11 17 -47.1234 1.10 18 -14.8275
1.30 Quartz L12 19 7.7813 4.50 Fluorite L13 20 -26.2162 1.50 21
-13.8401 1.30 Quartz L14 22 10.8989 4.50 SrF.sub.2 L15 23 -11.2815
1.30 Quartz L16 24 -45.5926 0.11 25 9.2155 3.20 Fluorite L17 26
-301.0727 1.35 Quartz L18 27 8.3953 11.50 28 -38.8726 1.90 Fluorite
L19 29 11.1236 3.20 Quartz L20 30 -11.8642 0.30 31 8.1297 3.10
Quartz L21 32 -25.3738 1.30 Fluorite L22 33 6.0366 2.50 34 -5.6201
1.30 Fluorite L23 35 4.5665 2.00 Quartz L24 36 28.1701
[0042] Condition-corresponding values
[0043] fl/f=3.57
[0044] f2/f=60.5
[0045] f3/f=20.47
[0046] FIG. 2 presents diagrams showing various aberrations of the
microscope objective lens according to the present Embodiment. In
the aberration diagrams, solid lines each indicate aberration at
248 nm, broken lines 251 nm, chain lines 245 nm, and chain
double-dashed lines 486 nm. In aberration diagrams in all
Embodiments given below, the same indications as those in the
present Embodiment are used. Also, the aberration diagrams in all
Embodiments are drawn from images formed using the imaging lens
mentioned above.
[0047] As can be seen from the aberration diagrams, various
aberrations have well been corrected at wavelengths of 248.+-.3 nm
and 486 nm.
Second Embodiment
[0048] FIG. 3 illustrates the lens construction of a microscope
objective lens according to Second Embodiment. This microscope
objective lens comprises, in order from the object side, a first
lens group G1, a second lens group G2 and a third lens group G3.
The first lens group G1 comprises a meniscus lens with its concave
surface facing the object side and a plurality of cemented lenses,
and has a positive refractive power on the whole. The second lens
group G2 comprises a plurality of cemented lenses and has a
positive refractive power on the whole and the third lens group G3
comprises a plurality of cemented lenses and has a negative
refractive power on the whole.
[0049] Specific values of the present Embodiment are listed in
Table 3.
[0050]
3TABLE 3 (Total specific values) f = 4.02 NA = 0.55 .beta. =
50.times. W.D. = 7.485 (Lens data) No. R d Material 1 0.00 7.485 2
-26.5625 1.30 Quartz L1 3 -127.5017 4.10 Fluorite L2 4 -9.2130 0.10
5 -44.8702 3.10 Fluorite L3 6 -20.3411 0.15 7 56.5862 1.30 Quartz
L4 8 15.0958 5.70 Fluorite L5 9 -25.5130 0.15 10 46.0073 4.90
Fluorite L6 11 -16.2574 1.35 BaF.sub.2 L7 12 17.5122 4.90 Fluorite
L8 13 -28.8393 0.10 14 65.6196 4.50 Fluorite L9 15 -14.5208 1.30
Quartz L10 16 9.8793 6.20 Fluorite L11 17 -17.4204 1.10 18 -15.1415
1.30 Quartz L12 19 7.6954 4.80 Fluorite L13 20 -27.1941 1.50 21
-14.0609 1.30 Quartz L14 22 11.1782 4.50 SrF.sub.2 L15 23 -11.7254
1.30 Quartz L16 24 -49.6395 0.11 25 9.1794 3.20 Fluorite L17 26
-648.1708 1.35 Quartz L18 27 8.329 11.50 28 -41.5761 1.90 Fluorite
L19 29 11.0686 3.20 Quartz L20 30 -11.741 0.30 31 8.196 3.10 Quartz
L21 32 -24.7625 1.30 Fluorite L22 33 6.0579 2.50 34 -5.596 1.30
Fluorite L23 35 4.5599 2.00 Quartz L24 36 27.9908
[0051] Condition-corresponding values
[0052] f1/f=3.66
[0053] f2/f=46.94
[0054] f3/f=23.01
[0055] FIG. 4 presents diagrams showing various aberrations of the
microscope objective lens according to the present Embodiment. As
can be seen from the aberration diagrams, various aberrations have
well been corrected at wavelengths of 248.+-.3 nm and 486 nm.
[0056] FIG. 5 illustrates the lens construction of the Imaging lens
mentioned previously. Specific values of this imaging lens are
listed in Table 4.
4TABLE 4 (Total specific values) f = 200 (Lens data) No. R d
Material 1 -30.630 2.00 Quartz 2 2406.000 5.00 Fluorite 3 -39.100
1.00 4 -417.400 5.00 Quartz 5 -51.920
[0057] Incidentally, shown in the aberration diagrams of the above
Embodiments are those obtained at 248 .+-.3 nm and 486 nm. Without
limitations thereto, in the present invention the various
aberrations have well been corrected at wavelengths of around 800
nm, too.
* * * * *