# **Fabrication of free-standing zone plates for soft x-ray microscopy**

Anne Sakdinawat, EE290B class project

## UCB/UCSF Bioengineering Graduate Group

> Imaging radiation sensitive samples with the soft x-ray microscope
> requires a high-efficiency, high-resolution Fresnel zone plate.
> Through a combination of removing the plating base (which makes the
> zone plate free-standing) and increasing the zone thickness, high
> efficiency nickel zone plates were designed and optimized for 30 nm
> resolution imaging of biological samples at λ =2.4 nm wavelength.
> Nickel free-standing zone plates having a 45 μm diameter, 35 nm outer
> zone width, and a 200 nm thickness were fabricated using electron beam
> lithography. Initial imaging tests performed with soft x-rays (λ =2.4
> nm or 516.6 eV) at the XM-1 microscope (Beamline 6.1.2, Advanced Light
> Source, Berkeley, CA) confirmed the accurate placements of the zones
> and a resolution capability of 30 nm for 1:1 and 1:2 spaced lines.
> Efficiency calculations for the fabricated zone plate were made based
> on the zone plate thickness and gap to zone ratio. Efficiency
> measurements were designed and were planned for the future.

**I. Introduction**

#### A. X-ray microscopy and the 'water window'

Soft x-ray microscopes using synchrotron radiation are being used to
study biological specimens due to their unique capability of being able
to image non-stained, hydrated biological samples up to 10 μm thick.

The primary optical components used to focus the x-rays in these
microscopes are Fresnel zone plates, which are circular diffraction
gratings like the ones depicted in figure 1. The imaging characteristics
of the Fresnel zone plates (condenser zone plate and micro zone plate)
determine the overall system resolution and performance.

![](media/image1.png){width="3.0833333333333335in" height="1.48125in"}

Figure 1: Schematic of a typical full-field transmission soft x-ray
microscope^1^ There are two zone plates used in this configuration: the
condenser zone plate and the micro zone plate. The condenser zone plate
focuses the incoming light onto the sample, and the micro zone plate
focuses the light coming out of the sample onto the CCD.

For biological imaging, the energy range commonly used is between the
carbon k-edge (λ = 4.36 nm or 284.4 eV) and the oxygen k-edge (λ = 2.28
nm or 543.8 eV), and is known as the "water window". The water window is
shown in figure 2. In this range, the differences in photoattenuation
cross sections of carbon and oxygen allow for an order of magnitude
dynamic range in natural contrast between cellular components/molecules
and water. Due to the imaging of thick samples and the need for greatest
contrast between carbon and oxygen, an energy bordering the oxygen
k-edge is used. This energy is around 516.6 eV or 2.4 nm. However,
depending on the sample thickness and other properties of the sample,
other energies within the water window or beyond the water window may be
used.

![](media/image2.wmf){width="3.125in" height="1.8680555555555556in"}

Figure 2: Photoattenuation cross section v. energy for carbon and water.

#### B. Fresnel zone plate efficiency

While zone plate resolution has been improved greatly in the past years,
to allow for practical imaging of radiation sensitive samples,
efficiency must also improve.^2,3^ The absolute efficiency of a zone
plate can be dependent upon many different factors. The two that will be
addressed here are: overall transmission and first order diffraction
efficiency. All calculations will be based upon the off-Bragg zone
plate. Absolute diffraction efficiency is defined as the product of the
overall transmission and the first order diffraction efficiency.

Transmission loss of a zone plate (equation 1) is due to the presence of
the plating base and silicon nitride membrane.

$I = I_{0}e^{- (\mu_{\text{Au}}t_{\text{Au}} + \mu_{\text{Cr}}t_{\text{Cr}} + \mu_{\text{SN}}t_{\text{SN}})}$
(1)

where μ is the photoabsorption coefficient for a particle material
multiplied by the density of the material, and t is the thickness of the
material.

For gold, μ~Au~ = 2.35x10^5^ cm^-1^, t~Au~ = 12 nm

For chromium, μ~Cr~ = 2.44 x10^4^ cm^-1^, t~Cr~ = 5 nm

For silicon nitride, μ~SN~ = 4.39x10^4^ cm^-1^, t~SN~ = 100 nm

Using equation 1, the total transmission through a typical plating base
consisting of 5 nm chromium, 12 nm gold, and 100nm silicon nitride is ≈
48% for 2.4 nm x-rays. The transmission will be less if lower wavelength
light is used and greater if higher wavelength light is used.

The first order diffraction efficiency of an off-Bragg zone plate is
dependent upon the zone thickness and other zone optical properties,
such as phase shift. Assuming a first order diffraction efficiency of
approximately 10% or less, the absolute diffraction efficiency of the
zone plate would then be only around 4.8% or less.

However, if the plating base and silicon nitride membrane were removed,
and replaced with a minimally obstructing interleaving support
structure, the total transmission through the structure would be much
greater than 48%, and the overall efficiency would thereby improve.

**II. Experimental Procedures**

#### A. Free-standing zone plates (FSZP)

Free-standing zone plates are zone plates that lack the plating base and
silicon nitride membrane. During certain fabrication methods of zone
plates, the plating base is necessary for structural support and in some
cases, the electroplating of various metals. Without the plating base,
though, a structure that will hold the zones in place must be formed.
This structure can take the form of a "spoke-like" pattern or have
"bridges" that connect concentric zones. Free-standing zone plates with
outermost zone widths as small as 50 nm have been fabricated^4^.

#### B. Design

Aiming to resolve 30 nm features in a biological sample at λ = 2.4 nm,
the FSZP design (figure 3) had 10% randomly placed bridges and the
following parameters:

Outer zone width: 35 nm

Number of zones: 320 zones

Numerical aperture: 0.0344

Diameter: 45 μm

Focal length: 650 μm

Depth of focus: 1 μm

Zone thickness: 200 nm

![](media/image3.png)

Figure 3: A slice of the data set used to generate the pattern of the
zone plate. The dark areas represent those areas that were exposed by
the electron beam. The bridges were randomly placed between each zone at
a density of 10%.

Absolute diffraction efficiency calculations (figure 4) were made for
different cases of the free-standing zone plate and for the regular zone
plate^5,6^. It can be seen that the free-standing zone plate design with
10% bridges theoretically improves the efficiency by approximately 2
times the typical zone plate with 10% bridges.

![](media/image4.wmf){width="3.298611111111111in"
height="2.259027777777778in"}

Figure 4: Absolute diffraction efficiency v. zone plate thickness curves
for various types of zone plates. The FSZP with bridges efficiency
approaches that of the FSZP without bridges (an ideal case) as the % of
bridges decreases. By comparing the efficiency of the regular zone plate
(ZP) with 10% bridges to the FSZP with 10% bridges, the design aimed to
double the efficiency

#### C. Resist properties

![](media/image5.wmf){width="3.9166666666666665in"
height="2.502083333333333in"}To ensure the proper thickness of resist
(30% AZPN 114), spin curve measurements were taken around the thickness
of interest. From these data points, it was determined that an
appropriate spin speed to use was 2000 rpm.

Figure 5: A spin curve of AZPN 114 used to determine that a 2000 rpm
spin speed was necessary to achieve around 200 nm thickness.

#### D. Dose tests

Dose tests were performed by exposing a test zone plate wafer with a 7 x
7 array of zone plates using a large dose range from 2500 μC/cm^2^ To
3781 μC/cm^2^. Scanning electron microscope (SEM) images of the
resulting zone plate structures revealed that a dose of around 3500
μC/cm^2^ or higher was necessary to achieve the correct zone widths. The
real zone plate wafer was exposed with a 5 x 5 array of zone plates
using a dose range of 3000 μC/cm^2^ to 3810 μC/cm^2^.

#### E. Fabrication Methods

A 100 μm thick silicon wafer with an array of 70 μm x 70 μm x 100 nm
thick square silicon nitride windows (spaced a quarter inch apart) was
used. The plating base, consisting of 5 nm chromium and 12 nm gold, was
evaporated on the front side of the wafer. 30% AZPN 114 was spun on the
wafer at 2000 rpm for 45 seconds and the wafer was baked for 15 minutes
at 170˚ C. HSQ was spun on the wafer at 4000 rpm for 45 seconds and was
baked for 5 minutes at 170˚ C. This should result in a \~200 nm thick
layer of AZPN 114 and a \~30 nm thick layer of HSQ. The HSQ was then
exposed with the electron beam system in the desired FSZP pattern (as
seen in figure 3). The exposure took approximately 40 minutes. Then, the
HSQ was developed in LDD-26W. Using the patterned HSQ as a mask, the
wafer was cryogenically dry etched resulting in the removal of AZPN 114
in 7, 15 second increments. Each step after the fifth step was monitored
such that the wafer was inspected for success in etching but not
over-etched.

The wafer was then nickel electroplated for 14 minutes to achieve a
thickness of \~200 nm. The wafer was rotated 180˚ every 2 minutes to
achieve a more uniform plating distribution. Wedges, measured throughout
the ends of the process showed a plating thickness of \~198 nm at the
top wedge and \~180 nm at the center wedge after 14 minutes. Then, the
HSQ was stripped with a 40 second HF dip, and the AZPN 114 was stripped
using a cryogenic dry etch in 7, 15 second increments.

The silicon nitride window and plating base were then etched from the
backside using SF~6~ for 5 minutes, resulting in the free-standing zone
plate.

**III. Results and Discussion**

#### A. SEM images

SEM images of the zone plate were taken. The images revealed that the
zone plate was still under-dosed, in that the zone to space ratio, as
seen in the following figure, deviates from the desired 1:1 gap to zone
ratio to an approximately 1:2 gap to zone ratio.

![](media/image6.png){width="2.875in" height="2.15625in"}

Figure 6: The backside of the FSZP.

![](media/image7.png){width="2.875in" height="2.15625in"}

Figure 7: The outermost zones of the FSZP.

#### B. Resolution Tests

Imaging experiments to ensure the proper placements of the zones and
achievement of the desired resolution for the free-standing zone plate
were performed at the Advanced Light Source on the x-ray microscope
'XM-1' at beamline 6.1.2. Images of a gold test pattern containing 30 nm
1:1 lines were resolved at an average modulation of 30 %. 30 nm 1:2
lines (line: space) were resolved at an average modulation of 56 %.
Average line profiles and images are given in figures 8 and 9.

![](media/image8.png){width="2.0in" height="2.0in"}

![](media/image9.wmf){width="3.0006944444444446in"
height="1.9041666666666666in"}

Figure 8: The image of the 30 nm 1:1 lines and the corresponding
averaged line profile.

![](media/image10.png){width="2.0in" height="2.0in"}

![](media/image11.wmf){width="3.25in" height="1.9166666666666667in"}

Figure 9: The image of the 30 nm 1:2 lines and the

corresponding averaged line profile.

#### C. Efficiency Tests

Efficiency tests were designed but unable to be set up or performed due
to time constraints. The tests were designed to take place at the ALS at
beamline 6.3.2. In order to measure first order diffraction efficiency,
the ratio of the transmitted light through the zone plate and order
sorting aperture to the transmitted light without the zone plate (but
with a pinhole having a similar aperture size as the zone plate
diameter) will be measured.

Absolute diffraction efficiency estimates of the fabricated zone plate
were made based upon the gap to zone ratio shown in the SEM images.
Figure 10 shows that the deviation from the 1:1 gap to zone ratio may
cause this free-standing zone plate to have an efficiency greater than
that of a regular zone plate with bridges, but less than the optimal
efficiency of a free-standing zone plate with bridges.

![](media/image12.wmf){width="3.2493055555555554in"
height="2.3493055555555555in"}

Figure 10: Efficiency v. zone thickness estimate of the fabricated zone
plate compared to the efficiencies of the FSZP with 10% bridges and the
ZP with 10% bridges.

### IV. Conclusion

A free-standing zone plate with a 10% bridge support structure was
fabricated using electron beam lithography and an HSQ-AZPN114 bilayer
resist. The resulting zone plate had a zone thickness of 200 nm but had
a gap to zone ratio of approximately 1:2 rather than 1:1. This was
probably due to insufficient dose. Testing of zone plate imaging
properties was performed on the XM-1 microscope at the ALS and resulted
in a resolution of 30 nm. Efficiency calculations were performed for
different cases of the FSZP and ZP, and efficiency measurements were
planned for the future.

^1^D.Attwood, *Soft X-rays and extreme ultraviolet radiation: Principles
and Applications*, Cambridge University Press, Cambridge, United
Kingdom, 1999 pp. 337-394.

^2^M. Peuker, AIP Conference Proceedings, **507**, 682 (2000).

^3^M. Peuker, Applied Physics Letters, 78, 2208 (2001).

^4^ S. Tennant, S. Spector, A. Stein, C.Jacobsen, AIP Conference
Proceedings, 507, 601(2000).

^5^J. Maser, "Theoretical description of the diffraction properties of
zone plate with small outermost zone width" in *X-Ray Microscopy IV,*
edited by

V.V.Aristov and A.I.Erko, Bogorodskii Pechantnik Publishing Company,
Chernogolovka, Moscow region, 142432, Russia, 1992 pp. 523-530.

^6^H.W. Schnopper, Appl Optics **16**, 1088 (1977).