A variety of high NA and very broad spectral band catadioptric microscope objectives is described which only use one glass type.

1. Small catadioptric microscope optics
David R. Shafera
, Yung-Ho Chuangb
, J. Joseph Armstrongb
a
David Shafer Optical Design, 56 Drake Lane, Fairfield CT, USA 06430
b
KLA-Tencor, 160 Rio Robles, San Jose, CA, USA 95134
ABSTRACT
New catadioptric objectives have been developed with large bandwidths in the DUV, high numerical apertures (NAs),
and small size. This can be achieved using a single glass material. Designs are specifically optimized for loose alignment
tolerances and ease of manufacturability. Applications include broad band DUV imaging and high resolution immersion
imaging for industrial microscopy and life sciences.
Keywords: Catadioptric, DUV, broadband, microscope, objective
1. INTRODUCTION
There are many microscopy applications that can benefit from broad band DUV imaging. Some applications further
require immersion, cover glass correction, or operation over a wide temperature range. To be suitable for general use,
microscope objectives need to be small enough to fit into a standard microscope and have a reasonable cost. Currently
available microscope optics do not satisfy these requirements. We present a new catadioptric approach with the
capability to address these requirements. A 0.9 NA self corrected objective with a bandwidth from 266-800nm and field
size of 0.15 mm is possible using only seven fused silica elements. Variations on this design with up to eleven elements
have been optimized with many applications in mind including broad band DUV imaging and high resolution immersion
imaging for semiconductor inspection. NA’s of 1.2 are possible for broad band DUV or 193nm water immersion. The
designs are also ideally suited for biological applications in DUV fluorescence imaging and spectroscopy, UV resonance
Raman spectroscopy, multi-photon absorption imaging, and Optical Coherence Tomography. In addition, the design is
insensitive to variations in the glass index. This makes it suited for applications where optical performance must be
maintained over a very wide temperature range such as in aerospace are possible. The index tolerance for many of these
designs in fused silica is 500 ppm providing good broad band performance over an extreme temperature range. With
design modifications and lens material selections, broad band optics from the IR to DUV is possible. This also allows
broad band microscope objectives to be made with reduced cost using inexpensive materials and replication
technologies. In this report we present a prototype objective that is specifically optimized for loose alignment tolerances
and ease of manufacturability. Mechanical assemblies for DUV optics include nitrogen purging to prevent
contamination.
2. CURRENT OBJECTIVE APPROACHES
There are three different approaches to microscope objective design; dioptric (refractive), catoptric (reflective), and
catadioptric (reflective + refractive). Of the currently available objectives utilizing these approaches, there are a limited
number that are capable of good performance with high resolution below 400nm.
2.1 Dioptric
Dioptric or refractive microscope objectives are the standard for visible microscopy. The complexity and cost for this
style of objective increases depending of the amount of color correction. Refractive objectives for use in the DUV are
limited by the availability of optical materials. Unfortunately high NA refractive objectives can not be produced with
significant bandwidths that include wavelengths below 350nm because of the lack of glass types with high transmission
and the desired dispersion. Even if some glasses with low transmission are considered, they can not significantly
increase the bandwidth. Objectives have been designed using only fused silica and calcium fluoride material which have
high transmission below 350nm. However, due to the similarities in their dispersions, the bandwidths are limited by

2. chromatic aberration. Some single line objectives have been made using fused silica for 248nm and 266nm laser light
sources. However, these objectives have a very high cost making them impractical for many applications. In addition,
most high NA DUV refractive objectives have limited field sizes.
2.2 Catoptric
If an objective with a broad bandwidth is desired, a catoptric or all reflective solution seems to be an attractive option.
With this type of objective there is no issue with bandwidth or chromatic aberrations. Bandwidth is limited only by the
mirror coatings. However, designs that are axially symmetric, like the Schwarzschild two mirror design, have limited
NA, large central obscuration causing poor performance in the mid spatial frequencies, and limited field size even when
large diameter mirrors are used. There are many varieties of off axis mirror systems, however these have limited NA’s,
tend to be large, and offer challenging optical alignment.
2.3 Catadioptric
A typical form for a catadioptric objective is shown in Fig. 1. It has three primary lens groups: the catadioptric group, the
field lens group, and the focusing lens group. In this design the field lens group is located partially within the
catadioptric group.
Fig. 1 A typical catadioptric objective with three primary lens groups: Focusing group,
field lens group, and catadioptric group. In this design the field lens group is substantially
within the catadioptric group.
The catadioptric group is typically composed of two second surface mirrors or Mangin elements. Light from the object
enters the catadioptric group through an aperture in the mirror coating of the first Mangin element. The light then reflects
from the mirror coatings on second Mangin element and then the first Mangin element before passing through an
aperture or hole in the second Mangin element, forming an internal image near the field lens group. The location of each
Mangin element in relation to the object or internal image and its diameter determines the amount of obscuration. In
practice the obscuration can be limited to around 2% of the area so it has no significant impact on the performance. Light
from the internal field then passes though the field lens before entering the focusing lens group. The focusing lens group
is designed to correct for the residual aberrations of the catadioptric group and the field lens group. These objectives can
be self corrected and designed for an infinite conjugate. The currently available catadioptric objectives have diameters of
greater 80mm and lengths of greater than 60mm1, 2
. This is driven by the desire for large working distances, large
numerical apertures, and large field sizes. Optical manufacturing is also a factor in the size for catadioptric designs.
Highly accurate surfaces are required for the Mangin elements. Any surface figure error from the polishing of the
reflective surface is especially important because it is the equivalent of six refractive surfaces. Many high accuracy
optical polishing methods are not applicable to small sized, high curvature optics.
Focusing lens group
Catadioptric group
Field lens group

3. 3. NEW SMALL CATADIOPTRIC APPROACH
Previous catadioptric objective designs were primarily targeted for semiconductor applications such as inspection and
lithography. An objective for general microscopy doesn’t require the large field size or long working distances common
to these applications. These requirements, along with improvements in the design form, allow for a small catadioptric
objective to be developed.
There are several standard parameters that will constrain the designs. Some of these include the focal length, total system
length, working distance, NA, field size, telecentricity in object space, and infinite conjugates. In addition, for an
objective to be practical for general microscopy volume manufacturability and reduced cost are also required. These
designs focus on reducing the alignment tolerances for the optical elements. This simplifies the typically time consuming
process of decenter adjustment and makes the design more robust to withstand mechanical shock. Additional design
goals help improve the final system performance. This includes low angles of incidence for good broad band coating
performance. All these requirements can be achieved with the new small catadioptric objective.
This style of catadioptric objective is based on the Schupmann medial telescope. The Schupmann telescope is a two lens
design that corrects for first order axial color using a single material. However, because the Schupmann telescope forms
a virtual image between the two lenses, a mirror arrangement is used instead to create a real image. Second order axial
color is then corrected with the addition of a field lens as suggested by Offner3
. This theory assumes an infinitely thin
field lens with zero dispersion with the field lens located at the internal paraxial image point.
In reality, the field lens has a finite thickness and dispersion which contributes its own aberration. One approach to
reduce the impact of the field lens dispersion is to achromatize the field lens by making it a doublet. This can effectively
correct for higher order chromatic aberrations. In the UV-DUV this can be accomplished using fused silica and calcium
fluoride materials for the field lens. However, this requires high curvature surfaces and the tolerances on the element
thickness, decenter, and index are quite tight. Another approach is look for a more general solution to achromatizing a
three lens system that also includes the thickness and dispersion of the field lens. A more general solution can be derived
and is used in these new designs. It is possible to use a finite thickness, single material field lens to correct for the axial
chromatic and first order lateral chromatic correction beyond what can be achieved by using the Schupmann and Offner
conditions. This improved correction is achieved by shifting the field lens away from the internal field. Figs 2a and 2b
show the transverse axial and lateral color aberrations for a shifted field lens compared to a field lens at the paraxial
focus.
Fig. 2a This plot shows the paraxial transverse axial color Fig. 2b This plot shows the paraxial transverse lateral color
for the Schupmann/Offner approach and the extended design for the Schupmann/Offner approach and the extended design
obtained from a general solution to the achromatic three lens obtained from a general solution to the achromatic three lens
problem. problem.
Transverse lateral color single material design
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
260 320 380 440 500 560 620 680 740 800
Wavelength (nm)
Distancefromaxis(m)
Schupmann + Offner
Extended design
Diffraction limited radius
Transverse axial color for single material design
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
260 320 380 440 500 560 620 680 740 800
Wavelength (nm)
Distancefromaxis(m)
Schupmann + Offner
Extended design
Diffraction limited radius

4. The Schupmann and Offner conditions provide 2nd
order correction and shifting the field lens increases the correction
capability to third order. Dotted lines are shown as a reference and indicate the increasing spot size with wavelength. The
distance between the dotted lines is equivalent to the diffraction limited spot diameter. The total transverse aberration is
equal to the sum of both the axial and lateral aberrations. The position of the field lens is chosen to minimize the total
aberration. For a larger fields, lateral color becomes the dominant aberration and more of the budget must be allocated to
lateral color correction.
The single material catadioptric design also has the advantage of being very insensitive to the absolute index of the glass.
This allows the design to maintain performance over a wide temperature range. This design has a refractive index
tolerance of 0.005. This is very large compared to the temperature sensitivity of the refractive index of fused silica at
approximately 0.00001 per deg C.
Many small catadioptric design forms with between 7 and 11 elements have been developed. The requirements for the
NA, field size, and bandwidth, and acceptable tolerances will drive the design complexity. One of the primary
constraints is the distance from the object to the last surface. This is set to be less than 45mm, the distance between the
object and the mounting flange on a standard microscope. An example design with 9 elements is shown in Fig. 3. The
diameter of the largest catadioptric element is 25mm. The polychromatic Strehl of this design is greater than 0.87 over
the field.
One of the important advantages of this design is the loose decenter tolerances. This makes the alignment of the
objective during manufacturing easier. A chart showing the decenter sensitivity of the objective is shown in Fig. 4. The
element with the largest decenter sensitivity is the Mangin element nearest the object. This sensitivity is driven by the
radius of curvature requirement for the surface nearest the object. The objective must fit on a standard microscope
objective turret. In practice many turrets accept a larger radius of curvature so this tolerance can be substantially
loosened.
If a somewhat larger obscuration is acceptable, the design can be simplified to seven elements while maintaining the
loose tolerances and good performance of the previous nine element design. An example of this design is shown in Fig.
5. The reduction in the required number of lenses comes from placing the second field lens on the opposite side of the
internal field compared to the 9 element design. This allows for better correction of the chromatic variation of
monochromatic aberrations, especially spherical and coma. The small catadioptric objective design is very flexible. This
design has extra degrees of freedom that can produce many solutions. These extra degrees of freedom are used to reduce
the manufacturing tolerances and limit the angles of incidence at each lens surface. This will produce the best objective
with the least cost.
Fig 3. An example 9 element design corrected from 266-
800nm. The diameter of the largest catadioptric element is
25mm. This design has an NA of 0.9 at the object with a
field size of 0.13mm. The obscuration is 0.8%.
Fig 4. This chart shows the wavefront error at
313nm from an element decenter of 10 microns with
no compensation. The objective exhibits a loose
decenter sensitivity.
Wavefront error from 10 micron decenter
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
9 8 7 6 5 4 3 2 1
Lens
RMSwavefronterrorat313nm
0 micron decenter
10 micron decenter

5. When using polychromatic Strehl as a metric with a
broad band system it is important to consider the
definition. The monochromatic Strehl is a ratio of the
intensity at the peak of the diffraction pattern of an
aberrated point image to that at the peak of an
aberration free image as formed by the same optical
system. The polychromatic Strehl is determined by the
incoherent addition of the peak intensities for each
wavelength being considered. One additional constraint
is the energy at each wavelength must be equal or
weighted appropriately. However the spot size is
smaller for the short wavelengths and the peak intensity
is proportional to the inverse square of the wavelength.
For example lets examine a system using wavelengths at
400nm and 800nm. The peak intensity at 400nm will be
4 times as high as the peak intensity at 800nm with
equal wavelength weighting. In this case the 400nm
wavelength will have 4 times the influence on the
polychromatic Strehl compared to the 800nm
wavelength. For this reason, the polychromatic Strehl may not be the best metric for many applications. An alternative
would be to specify the monochromatic aberrations, the axial color, and the lateral color separately. These metrics are
also more applicable for comparison to testing data gathered in the manufacturing process.
4. APPLICATIONS
Potential applications for this type of small catadioptric objective include semiconductor inspection and review, life
sciences, and other general microscopy applications. Objectives for semiconductor inspection have a variety of
challenging requirements including a large field size and very high Strehl ratio. The bandwidth these designs can support
is generally limited by higher order lateral color. These requirements generally add significant cost to the objectives.
Objective versions for semiconductor review don’t require a large field size, so their designs are more flexible and can
support larger bandwidths. Objective versions for biology have a wide variety of requirements that are driven by the
many detection and imaging techniques. The table in Fig. 6 summarizes some of these techniques and why they may be
suitable for the small catadioptric objective. Many standard techniques such as high resolution microscopy and
fluorescence detection may directly benefit by extending them into the DUV. Some of relatively new techniques
including UV resonance Raman spectroscopy, three photon imaging, and Optical Coherence Tomography may
substantially benefit from the broad band capabilities of the objective.
Many biological applications have additional requirements for cover glass correction and immersion imaging. Small
catadioptric objective designs have been investigates to address these requirements. The objective in Fig. 7 is an
objective corrected from 266-436nm over a 0.15mm field for a cover glass with a standard 0.17mm thickness. The major
challenge for broad band correction with a cover glass is allowing for variations in the cover glass thickness. By
adjusting air gaps it is possible to allow for thickness variations of ±10 microns. In principle this air gap adjustment can
be made with one mechanical motion of an adjusting ring on the objective housing. This design is also corrected over a
much larger field than the 10-20 micron field of a typical 0.9 NA objective. Designs with smaller field sizes would allow
for improved performance. The objective in Fig. 8 is an immersion objective corrected from 266-800nm over a 0.10mm
field with 1.2NA. Additional elements are added on one side of the internal field and between the Mangin element and
the object.
Fig 5. An example 7 element design corrected from 266-
800nm. The diameter of the largest catadioptric element is
25mm. This design has an NA of 0.9 at the object with a field
size of 0.13mm. The obscuration is 1.5% of the area.

6. DUV optics High NA
optics
Broad band
optics
Comment/Application advantage
High-resolution
microscopy
    DUV provides high resolution, but may kill living
samples
 Can generate UV/DUV artificial color maps.
 Can be used for sample review and classification.
Fluorescence imaging     Extend the fluorescence imaging to UV/DUV
 May not need dyes
UV absorption
spectroscopy
   Identify structural groups, for example carbonyl
group
 Extend the absorption spectroscopy to UV/DUV
Fluorescence time
domain analysis
  Analysis techniques include FIMDA, FCS, FxCS
Fluorescence
spectroscopy
   Extend to UV/DUV range
UV resonance Raman
spectroscopy
    Can analyze spectrally, temporally, and spatially
 In 220-270nm range, biology samples have little
fluorescence; good for analyzing phenylalanine,
tyrosine, tryptophan
Three photon
absorption
    Does not need DUV laser
Optical coherence
tomography
   Need UBB, not DUV
5. RESULTS
Prototype objectives using a similar design form to Fig. 3 are being developed. They are optimized for a field size of
0.13 mm and support a bandwidth of 266-800nm. Even though the design can support a broad range of wavelengths, the
wavelengths supported by the prototype objective are limited by the available optical coatings. The first prototype
objective has coatings with optimized transmission from 266-436nm. Later prototypes will test different coatings that
should extend the upper wavelength to 550nm.
Fig 7. An example 9 element design corrected from 266-
436nm over a 0.15mm field for use with a standard 0.17mm
cover glass.
Fig 8. An example 11 element design corrected
from 266-800nm with 1.2NA. Additional elements are
added on one side of the internal field and between the
Mangin element and the object.
Fig 6. This table summarizes some of the biological detection and imaging techniques that may be suitable for the small
catadioptric objective.

7. Fig 9. Picture of the first prototype objective using a
similar design to that presented in Fig. 3. The objective is
shown in the center position on a standard 6 position
microscope turret between two large diameter darkfield
objectives.
Fig 10. Images using the first prototype objective and
varifocal optics. Images are of 0.5 micron line and spaces at
266nm, 313nm, 365nm, and 436nm.
One of the primary goals for the prototype objectives is to demonstrate manufacturing feasibility. The first objective has
demonstrated monochromatic wavefront performance of 0.9 Strehl at 405nm. The chromatic aberrations have been
measured at close to the design value. Further development and testing of the prototypes is ongoing including wavefront
measurements at other wavelengths and transmission measurements. A picture of this objective is shown in Fig 9.
In addition to the objective, a varifocal imaging system was manufactured that can continuously adjust from 50 to 300x
magnification and is corrected over the 266-800nm bandwidth of the objective. Fig. 10 shows four images taken using
this objective and varifocal imaging system at wavelengths of 266nm, 313nm, 365nm, and 436nm. The images are of 0.5
micron lines and spaces taken at 300x magnification.
This prototype objective addresses other practical issues related to DUV operation and general microscopy.
Transmission degradation from DUV photo-contamination is a critical problem that must be considered for such an
objective. Many precautions are taken including using stainless steel or nickel coated aluminum for many critical parts.
The objective is also capable of being purged with nitrogen. The nitrogen flows across the external surface near the
object as it exits the objective to prevent contamination. The external surface is also protected from direct contact with
the object by using a thin metal cover plate.
REFERENCES
1. D. Shafer, Y. Chuang, B. Tsai, U.S. Patent No. 5,717,518. Washington, DC: U.S. Patent and Trademark Office.
(1998).
2. J. Webb, T. Tienvieri, U.S. Patent No. 6,560,039. Washington, DC: U.S. Patent and Trademark Office. (2003)
3. A. Offner, “Field Lenses and Secondary Axial Aberration,” Appl Opt. 8. 1735-1736 (1969).
266nm 313nm
365nm 436nm