Nano-Scale Imaging With Tabletop Soft X-Ray Lasers:
Sub-38 nm Resolution
C. S. Menoni1,2, G. Vaschenko1,2, F. Brizuela1,2, C. Brewer1,2, Y. Wang1,2,
M. A. Larotonda1,2, B. M. Luther 1,2, M. C. Marconi1,2, and J. J. Rocca1,2,
W. Chao1,3, J. A. Liddle1,3, Y. Liu1,3, E. H. Anderson1,3, and D. T. Attwood1,3,4 A. V. Vinogradov5, I. A. Artioukov5, Y. P. Pershyn6 and V. V.
Kondratenko6
1

NSF ERC for Extreme Ultraviolet Science and Technology
Department of Electrical and Computer Engineering, Colorado State University, Fort Collins, CO 80523
3
Center for X-ray Optics, Lawrence Berkeley National Laboratory,
4
University of California, Berkeley, CA 94720
5
P. N. Lebedev Physical Institute, Russia
6
National Technical University “KhPI”, Ukraine
2

Summary. We report the demonstration of soft x-ray microscopes with resolution
down to sub-38 nm using tabletop soft x-ray laser illumination. One of the compact microscopes combines the 46.9 nm wavelength output from a capillary discharge Ne-like Ar laser with a reflective condenser and a free standing zone plate
objective. High quality images were acquired with this microscope in transmission
mode and reflection mode. The latter includes images of an integrated circuit pattern containing polysilicon features on silicon. The spatial resolution for this microscope is between 120-150 nm. Increased resolution was demonstrated in another microscope using 13.2/13.9 nm wavelength illumination from Ni-like Cd/Ag
transient soft x-ray lasers in combination with diffractive zone plate optics for
both the condenser and objective. Using an objective zone plate with a 50 nm
outer zone width, this optical system achieved a record sub-38 nm spatial resolution. These results demonstrate the feasibility of using compact high repetition
rate soft-x-ray laser sources in nanometer-scale microscopy.

1. Introduction
The development of practical imaging tools with nanometer-scale spatial
resolution are of importance for many applications ranging from material
science and microelectronics to biology. A direct pathway to realize mi-

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croscopy with tens of nanometers resolving power is to use soft x-ray
(SXR) light. The best spatial resolution for photon-based microscopes to
date, 15 nm, was obtained using soft x-ray illumination from a synchrotron
source [1]. Aside from the experiments conducted at synchrotron facilities, a variety of imaging experiments have been carried out using both coherent and incoherent short wavelength sources. Early imaging work with
soft x-ray lasers demonstrated submicrometer resolution in reflection mode
with a SXR recombination laser at λ=18.2 nm. [2] A resolution of 75 nm
was reported using the λ=4.48 nm output from a SXR laser pumped by the
very large fusion-driver NOVA which was limited to firing several shots a
day. [3] In the last several years smaller-scale short wavelength sources
including high order harmonics [4], and incoherent laser-plasma-based
sources [5,6], have been used for submicron resolution imaging. Of these
experiments, the best performance in terms of spatial resolution, reported
as sub-100 nm, was obtained with a laser-created incoherent plasma source
emitting at λ = 3.37 nm [6].
In this paper we discuss the implementation and demonstration of soft
x-ray microscopy with compact high repetition rate laser sources. We present imaging results in transmission and reflection modes obtained with a
microscope that combines the output from a λ = 46.9 nm capillary discharge pumped Ne-like Ar laser with a reflective condenser and objective
zone plate. This instrument is capable of acquiring high quality images
with a large field of view in exposure times of ~10 - 20 seconds and with a
spatial resolution between 120-150 nm. Another transmission mode microscope was developed using a similar geometry but with Fresnel zone
plate (FZP) optics for both the condenser and the objective in combination
with shorter 13.2/13.9 nm wavelength illumination from laser-pumped Nilike Cd or Ag collisionally excited soft x-ray lasers. This compactable-top
photon-based imaging system is shown to have a spatial resolution better
than 38 nm.

2. λ=46.9 nm imaging system based on a table-top capillary
discharge laser
The tabletop soft x-ray microscope implemented using a capillary discharge Ne-like Ar laser as the illumination source is configured to operate
either in transmission or in the more versatile reflection mode. In transmission mode, illustrated in Fig. 1a, the laser output is collected and focused onto a sample using a Sc/Si multilayer coated Schwarzschild con-

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denser. A free standing FZP objective then forms the image onto a backilluminated CCD detector.

Fig. 1. (a) Schematic diagram of the 46.9 nm soft x-ray laser microscope in its
transmission mode implementation. (b) Photograph of the inside of the vacuum
chamber that houses the microscope.

The Sc/Si multilayer coated Schwarzschild condenser contains a primary convex mirror of 10.8 mm in diameter and a 50 mm diameter secondary concave mirror. [7] Together these two mirrors produce a hollow
cone of λ = 46.9 nm light that is focused onto the sample, which is positioned at ~ 5 cm from the output of the condenser. The Schwarzschild
condenser has a numerical aperture (NA) of 0.18 and a throughput of ~
1%, due to the less than optimum reflectivity of the coatings at λ = 46.9
nm
The FZP objective was fabricated to be “freestanding,” as the use of any
substrate material would significantly attenuate the λ = 46.9 nm light. The
zone plate, manufactured onto a thin nickel foil attached to a silicon frame,
contained pseudo-random bridges connecting the different zones to provide structural stability. [8] The FZP objective has a diameter of 0.5 mm,
an outer zone width of 200 nm, a NA of 0.12 and a focal distance of 2.13
mm. To achieve high magnification (~ 750×) the FZP objective to CCD
distance was chosen to be 1.6 m, which required the working distance of
the objective be very close to its focal distance.
For these experiments, the capillary discharge laser was equipped with
an Al2O3 capillary 18 cm long, resulting in an average power ~ 0.1 mW.
[9,10] This choice of capillary length provided a good compromise between output power and degree of coherence of the source [11]. Although
a single laser shot produced discernable images, most images were acquired by accumulating several shots to improve the signal-to-noise ratio.
Nevertheless, since the laser can be operated at a repetition rate of several

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Hz, the exposure time was only several seconds. The images were collected with a back-illuminated CCD camera with a 1024×1024 array of
24×24 μm2 pixels.

Fig. 2. a) Soft x-ray image of the free standing zone plate objective showing the
different zones connected with pseudo-random bridges. The outermost zones of
width 200 nm and the corresponding intensity cross-section are shown in (b) and
(c) respectively.

The test sample used in the transmission imaging experiment was a
freestanding FZP similar to the one used as the objective. Fig. 2a is an image of the central portion of this test pattern and was acquired in 20 seconds at a magnification of 250×. The resolution of the imaging system
was obtained from the high magnification images of the 200 nm outer edge
of the FZP sample, as shown in Fig. 2b (×750 magnification, 10 sec. exposure). From this image the intensity modulation or lineout along the outer
edge portion of the FZP was obtained (Fig. 2c). An average of 100 lineout
traces shows the intensity modulation is ~ 94 %. This intensity modulation
is significantly higher than the 26.5% set by the Rayleigh resolution criterion, suggesting that the resolution of the instrument is significantly better
than 200 nm. To estimate the spatial resolution of the imaging system,
simulations were performed using the SPLAT program [12]. From the
simulations a 120 - 150 nm resolution was estimated. This program assumes the use of incoherent light illumination, which is justifiable for this
imaging system when using a short length capillary to ensure low coherence. [11].

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Fig. 3. Soft x-ray image of a silicon wafer patterned with 100 nm polysilicon
lines (top right) and 250 nm half-period lines with equal lines and spaces (bottom
right). This image was obtained with the 200 nm outer zone objective using a 20
second exposure (20 laser shots) and a magnification of 750×.

To acquire images in reflection mode the object was rotated by 45º with
respect to the incoming beam, and the FZP objective and CCD were rotated by 90º. In this configuration, the depth of focus of the objective FZP
limited the area of the image in focus at the CCD to 3×30 μm2. This was
overcome by digitally compensating the images. We imaged a test pattern
consisting of polysilicon lines on a silicon substrate. This test pattern was
originally produced for optimization of the lithography process for chip
manufacturing. Fig. 3 shows an image of this sample obtained with a 20
second exposure. The condenser was scanned during acquisition to ensure
complete illumination of the sample and to reduce coherence effects on the
image. In the upper right corner of Fig. 3, 100 nm polysilicon lines separated by 800 nm spaces are clearly discernible, and in the lower right region, 250 nm lines with 250 nm separations are well resolved. A more
compact “desk-top” size version of this microscope is currently under development. It will utilize the desk-top size Ne-like Ar capillary discharge
laser developed at CSU in combination with an objective FZP with a 120
nm outer-most zone to obtain sub 100 nm resolution images.

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3. Imaging at 13 nm with a laser-pumped tabletop laser
We have also demonstrated the operation of a microscope at wavelengths
near 13.5 nm. The shorter wavelength of the illumination coupled with
aberration free zone plate optics allowed a record spatial resolution to be
obtained for a tabletop photon-based full-field microscope system. The illumination is provided by either a 13.2 nm Ni-like Cd or a 13.9 nm Nilike Ag laser-pumped transient collisional laser generating highly monochromatic light (Δλ/λ < 1 × 10-4) with microwatt average power. [13,14]
The condenser and objective FZPs were fabricated by electron beam lithography in a ~ 120 nm thick nickel film supported by a 100 nm Si3N4
layer that is ~ 40 % transparent to the incident 13 nm light. The condenser
FZP has a diameter of 5 mm and consists of 12,500 zones of decreasing
width down to 100 nm. It has a numerical aperture NA = 0.07 and a focal
distance of 38 mm for the wavelength of 13.2 nm. Two different objective
zone plates were used. One has a diameter of 0.2 mm and contains 625
zones with an outer zone width of Δr = 80 nm. The second one has a 0.1
mm diameter with Δr = 50 nm. Microscope magnifications of 290 –
1750× were obtained by selecting the objective working distance very
close to its focal length and by selecting the distance between the objective
zone plate and the CCD camera in the 0.335 to 0.635 m range.
The spatial resolution of the microscope was determined by imaging a
series of dense grating test patterns of period ranging from 38 to 310 nm.
The sample also contains a set of 64 radial spokes decreasing in width
down to ~ 40 nm as they converge towards the center of a circular pattern.
Figure 4 shows an image of the radial spokes pattern obtained with the objective zone plate of Δr = 80 nm, using λ = 13.9 nm illumination and an
exposure time of 20 sec (100 laser pulses at 5 Hz). The central part of the
image contains 60 nm half-period lines that are clearly visible. The acquisition of images with a relatively large field of view (~ 12 × 20 µm2) is facilitated by the high brightness of the laser, which allows the condenser to
efficiently collect the laser emission and focus it onto the test pattern.
The spatial resolution of the microscope was experimentally determined
from the images of grating patterns with different periods, all of them with
nominal 1:1 line/space ratio. Figure 5 shows a soft x-ray image of gratings with 50 nm dense lines obtained with the FZP of Δr = 50 nm and λ =
13.2 nm illumination. The intensity lineout corresponding to the 38 nm
grating image is also shown in Fig. 5. The ~ 70 % intensity modulation
clearly demonstrates that the spatial resolution of the microscope is better
than 38 nm.

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Fig. 4. Image of a pattern consisting of radial spokes of width increasing outwards
obtained at λ = 13.9 nm with the Δr= 80 nm FZP and a 6 sec. exposure.

Fig. 5. Soft x-ray image of 50 nm dense lines (left) obtained at λ=13.2 nm with
the Δr = 50 nm FZP. The image of the 38 nm dense lines and its lineout are
shown on the right. The ~ 70% intensity modulation shows that the spatial resolution of the system is better than 38 nm.

4. Summary
We have demonstrated high resolution full field imaging at soft x-ray
wavelengths by combining the output from high repetition rate tabletop lasers at λ = 46.9, 13.9 and 13.2 nm with diffractive FZP optics. The λ =
46.9 nm microscope renders images in transmission and in the challenging
reflection mode of operation. Its spatial resolution has been determined to

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be 120-150 nm. The best spatial resolution, 38 nm, was realized with an
all zone plate imaging system at λ = 13.2 nm using an objective FZP with
an outer zone width of Δr = 50 nm. The spatial resolution of these microscopes could be readily improved by using objective zone plates with
smaller outer zone widths and increased numerical apertures.

5. Acknowledgments
This work was supported by the Engineering Research Centers Program of
the National Science Foundation under NSF Award Number EEC0310717.

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