A novel design of X-ray spectroscopic photon sieves (SPS) was realized to eliminate the higher diffraction orders. SPS gratings consist of randomly distributed circular holes, forming an approximately sinusoidal transmission function. Due to the intensive absorption of soft X-rays in any known material, these gold nanoholes are free-standing without supporting membrane. For applications in soft X-ray region, a hybrid lithographic method was used to manufacture spectroscopic photon sieves (SPS) of 1000 lines/mm in high throughput. In the fabrication process, an electron beam was focused to write patterns on the membrane substrate to achieve a master mask. Using this mask XRL and gold electroplating were performed to efficiently replicate SPS structures. After that, UVL was used to define the supporting coarse frame. In the replication process of XRL, the deviation of circle patterns caused by overheating problem in exposure has been resolved by inserting appropriate filters in X-ray beam path. The spectrum of X-ray source for exposure can be restricted in the 1.0- 2.0 keV energy band. Therefore, less heat are produced in exposure due to less absorption of higher energy X-rays in resist. After the SPS has been finished, the diffraction pattern was achieved at the soft X-ray beam line on Beijing Synchrotron Radiation Facility. The calibration results show that higher-order diffraction orders were efficiently suppressed along the axis of symmetry.
We report the nanofabrication and characterization of x-ray transmission gratings with a high aspect ratio and a feature size of down to 65 nm. Two nanofabrication methods, the combination of electron beam and optical lithography and the combination of electron beam, x-ray, and optical lithography, are presented in detail. In the former approach, the proximity effect of electron beam lithography based on a thin membrane of low-z material was investigated, and the x-ray transmission gratings with a line density of up to 6666 lines/mm were demonstrated. In the latter approach, which is suitable for low volume production, we investigated the x-ray mask pattern correction during the electron beam lithography process and the diffraction effect between the mask and wafer during the x-ray lithography process, and we demonstrated the precise control ability of line width and vertical side-wall profile. A large number of x-ray transmission gratings with a line density of 5000 lines/mm and Au absorber thickness of up to 580 nm were fabricated. The optical characterization results of the fabricated x-ray transmission gratings were given, suggesting that these two reliable approaches also promote the development of x-ray diffractive optical elements.
We review our recent progress on the fabrication of x-ray diffractive optical elements (DOEs) by combining complementary advantages of electron beam, x-ray, and proximity optical lithography. First, an electron beam lithography tool with an accelerating voltage of 100 kV is used to expose initial x-ray mask based on SiC membrane with a low aspect ratio. Second, x-ray lithography is used to replicate x-ray DOEs and amplify the aspect ratio up to 14:1. Third, proximity optical lithography is used to fabricate a large-scale gold mesh as the supporting structures. We demonstrate that this method can achieve high aspect ratio metal nanometer structures without the need of a complicated multilayer resist process. A large number of x-ray DOEs have been fabricated with feature sizes down to 100 nm for the purpose of laser plasma fusion applications. Among them, the ninth-order diffraction peak on the positive side of the zeroth order can be observed for both 3333 and 5000 lines/mm x-ray gold transmission gratings.
Transmission gratings with a period of 100 nm for extreme ultraviolet interference lithography are
fabricated with 2 groups of 50 nm thick Cr bars on a 100 nm thick Si3N4 film. The fabrication process
starts with depositing Si3N4 on both sides of (100) Si wafers by LPCVD, followed by electron beam
lithography of ZEP520A resist, evaporation of Cr and resist lift-off. A 120 nm thick stop layer of Au is
then evaporated onto the surrounding area to eliminate unwanted transmission. Finally, a pair of Si3N4
windows are opened on the back side by dry etching, and the Si under the grating pattern is removed by
KOH anisotropic wet etching. Diffraction measurement shows an acceptable first order efficiency of
the gratings at the wavelength of 13.4 nm. Using the fabricated gratings at the interference lithography
beam line of Shanghai Synchrotron Radiation Facility, economic and efficient fabrication of gratings
with a doubled pitch, namely 50 nm period gratings, can be expected.
We present a novel diffractive optical element, the quantum dot array diffraction grating (QDADG), used in soft x-ray spectroscopy. Because of its sinusoidal transmission it effectively suppress higher order diffractions, which can improve the precision and SNR of soft x-ray spectroscopy in laser plasma diagnosis. There are, however, many difficulties in the fabrication of a soft x-ray spectroscopy QDADG because of its small dimensions and complex pattern. We propose a hybrid lithography to fabricate a QDADG, including electron-beam lithography and x-ray lithography. The diffraction property of the QDADG is also proved to be consistent with a theoretical prediction using experiments.
In this paper, we calculated the numeric results of diffraction field in space of X-ray (λ=4.5nm) Fresnel zone plate based
on angular spectrum method, analyzed the axial and radial distribution patterns of X-ray Fresnel zone plate. The Full
Width at Half-Maximum (FWHM), Depth Of Focus (DOF) and Strehl efficiency of focus spot were studied. Discussed
the relationships between FPZ's design parameter and focus spot properties. At the condition of λ=4.5nm and the
outmost width Wn=50nm, the size of focus spot is proportional to the outmost width, and increase slowly with the
increase of number of zones and focus length; the DOF of focus spot increase at first; when the focus length increased to
40µm, the DOF incline to a constant; the focus spot's Strehl efficiency increased slowly with the increase of number of
zones and focus length.
X-ray transmission gratings (TG) have attracted much interest because of its wide use in x-ray
telescope, synchrotron radiation facilities, and target diagnostic in inertial confinement fusion, etc. In
this work, a 200 nm period master TG to diffract x-ray in the energy range 0.1-8keV has been
successfully fabricated by electron beam lithography followed by gold electroplating. In fabrication
processes, 500 nm resist was exposed by focused electron beam on polyimide free-standing-membrane
coated with a Cr/Au plating base. According to numerical simulation, the proximity effect due to
electron back-scattering from the substrate can be sharply reduced because of the thin polyimide
free-standing membrane substrates. PMMA resist was chosen due to its high resolution and good
performance in subsequent processes. After delicate dose test and shape modification of the proximity
effect caused by electron front-scattering, resist grating bars with 95 nm width and 200 nm period were
achieved. Subsequently, resist patterns were transferred to gold layer by electroplating. In future work,
with this master mask of TG, thousands of TG to diffract x-ray can be sufficiently replicated using
A novel diffractive optical element (DOE), quantu-dot-array diffraction grating(QDADG), used in soft X-ray
spectroscopy has been fabricated for the first time. The QDADG, which consists of a large number of quantum dots
distributed on a substrate as sinusoidal function, has many advantages in theory over conventional transmission grating
(TG) in soft X-ray spectroscopy, such as doubtless diffraction efficiency, no higher-order diffraction and no
subordination diffraction maximum, and so on. So, it can be predicted theoretically to improve the precision and Signal
Noise Ratio of soft X-ray spectroscopy in laser plasma diagnosis. But, there are many difficulties in the fabrication of
soft X-ray spectroscopy QDADG because of its much small dimension and complex pattern. In this paper, a combined
lithography was proposed to fabricate QDADG including electron beam lithograph (EBL) and proximity X-ray
lithograph(XRL). The diffraction property of QDADG has also been proved to be consistent with theoretical prediction
from test experiment. In the process of fabrication, because of the thin film substrate of soft X-ray QDADG, the
backscattering of incidence electrons can be effectively restrained in the electron beam lithograph, which can cause
much higher resolution. Without proximity effect correction, QDADG with 250nm minimal unit has been successfully
fabricated. In order to further increase the spectroscopy resolution and dispersion power of QDADG, it is necessary to
carry out proximity effect correction in electron beam lithograph.
Grating patterns with approximately 150 nm period were achieved by X-ray lithography with a
single exposure through a 300 nm period grating mask, which was manufactured by e-beam
lithography. BPM simulation of the X-ray propagation through the mask structure, which acts here
in a way much like that of a wave guide with many layers, was carried out. Considering also the
light propagation in the uniform space between the mask and the wafer, preferable parameters of
the optical setup, such as the exposure dose, the distance between the mask and the wafer, and
resist thickness, are suggested and their process windows are discussed. The dependence of the
resulted pattern profiles on the mask design is analyzed and an optimized design of the mask
grating is presented for this process. By carefully choosing the process parameters, the doubling of
grating resolution by X-ray lithography can be expected under precise control.
Hydrogen silsesquioxane (HSQ) is a kind of inorganic negative-tone resist for electron beam lithography with high pattern resolution of about 5 nm. It is a kind of promising resist used in fabrication of nanostructures such as transmission grating (TG), dots array, and chiral structures. But the poor sensitivity limits the extensive application of HSQ. And the property of HSQ in electron beam lithography is also studied little before. In this paper, from the viewpoint of chemical structure the property of HSQ in electron beam lithography has been proposed and experiments have also been presented with the variety of the exposure dose and development conditions. It is proved by experiments not only the sensitivity and contrast of HSQ but also the influence of proximity effect can be modulated by changing the baking temperature and concentration of developer with the same exposure conditions. 100 nm lines at 200 nm pitch grating patterns with excellent vertical side-wall and line-edge roughness have been achieved in more than 450 nm thickness HSQ layer by increasing the concentration of developer and reducing the baking temperature in combination with optimization of exposure conditions.
The combination of electron beam lithography (EBL) and x-ray lithography (XRL) has been developed to successfully fabricate x-ray transmittive diffractive optical elements (DOE) such as Fresnel zone plates (FZP) and transmittive gratings (TG). In fabrication processes, the master masks of FZP and TG were patterned with high resolution on free standing membranes by EBL and followed by electroplating. Subsequently, the final gold FZP and TG with vertical cross section were efficiently and economically replicated by XRL and electroplating. By using this combined method, FZP based on silicon nitride (SiNx) free standing membrane was achieved with 150 nm width of outermost ring and 6.7 high aspect ratio, due to a novel sandwich resist structure. A series of TG master masks (2000 g/mm, 3333 g/mm, and 5000 g/mm) were fabricated by EBL. Furthermore, final gold TGs with 2000 g/mm and 3333 g/mm were replicated by XRL. The spectrum of 2000 g/mm TG has shown its perfect performance in x-ray spectroscopy.