All conventional x-ray dispersive elements including multilayers, crystals and diffraction gratings provide multi-order diffraction spectra, which bring problems in spectroscopy application especially in the region of from VUV to soft x-ray region. The emergence of the so-called soft x-ray single-order diffraction grating (SXSDG) changed such a situation and may bring promised big benefits in lots of fields such as laser plasma diagnostics, x-ray astronomy, synchrotron radiation beam monochromatic; laser generated high harmonics and etc.
In this presentation, the authors provid that the introduction of the concept of the so-called soft x-ray single-order diffraction grating, the evolution and development of such a new emergened dispersive elements, the state of the art and its application, the perspect and ets. A possible worldwide collaboration in future is also suggested.
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.
A Schwarzschild microscope at 18.2 nm for ultra-fast laser plasma diagnostics has been developed.
Based on the third-order aberration the microscope is designed for numerical aperture of 0.1 and
magnification of 10. Spatial resolution of the objective can achieve 1250 lp/mm within the field of ±1
mm. Mo/Si multilayer films with peak throughout at 18.2 nm is designed and deposited by magnetron
sputtering, and the measured reflectivity of optical elements is 45%. The 600 lp/inch copper grid
backlit by laser produced plasma is imaging via Schwarzschild microscope on CCD. The spatial
resolution is measured as 3 μm approximately in the field of 1.2 mm.
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.
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.
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.
We introduce a combined e-beam and x-ray lithographic method to fabricate microzone plates (MZP) on free-standing silicon nitride films. An automatic design program is developed to draw the complex layout of MZP with very smooth boundaries. A gold MZP master mask with a minimum ring width of 250 nm is fabricated by e-beam lithography. The master mask is replicated using x-ray lithography (XRL) and nickel electroplating to obtain the final MZP. The combined lithographic technique produces a MZP with a pattern aspect ratio of 4.4:1.