Yilin Hong

Fabrication of varied-line-spacing grating by elastic medium

We demonstrate a method of fabricating varied-line-spacing grating by employing elastic medium with variable vertical section. The advantages of this technique include continuous variation of line spacing, high fabrication efficiency, low cost, etc. It is very difficult or impractical to produce this variation of line spacing by traditional interference lithography. This kind of grating can be used in an optical position sensor to improve the precision of the sensor.

Alignment method combining interference lithography with anisotropic wet etch technique for fabrication of high aspect ratio silicon gratings.

A method was developed for aligning interference fringes generated in interference lithography to the vertical {111} planes of <110> oriented silicon wafers. The alignment error is 0.036°. This high precision method makes it possible to combine interference lithography with anisotropic wet etch technique for the fabrication of high aspect ratio silicon gratings with extremely smooth sidewalls over a large sample area. With this alignment method, 320 nm and 2 μm period silicon gratings have been successfully fabricated. The highest aspect ratio is up to 100. The sample area is about 50 mm × 60 mm. The roughness (root mean square) of the sidewall is about 0.267 nm.

Soft X-ray holographic grating beam splitter including a double frequency grating for interferometer pre-alignment

Grating beam splitters have been fabricated for soft X-ray Mach- Zehnder interferometer using holographic interference lithography. The grating beam splitter consists of two gratings, one works at X-ray laser wavelength of 13.9 nm with the spatial frequency of 1000 lines/mm as the operation grating, the other works at visible wavelength of 632.8 nm for pre-aligning the X-ray interferometer with the spatial frequency of 22 lines/mm as the pre-alignment grating. The two gratings lie vertically on the same substrate. The main feature of the beam splitter is the use of low-spatial- frequency beat grating of a holographic double frequency grating as the pre-alignment grating of the X-ray interferometer. The grating line parallelism between the two gratings can be judged by observing the diffraction patterns of the pre-alignment grating directly.

Vacuum-ultraviolet blazed silicon grating anisotropically etched by native-oxide mask

We describe a simple and convenient method of controlling the profile of a blazed grating that consists of using a patterned native-oxide layer of off-cut silicon (111) wafers as the mask of anisotropic etching to maximize the smooth blaze facets of the desired blaze angle and to minimize the deficiencies of the groove apexes. With the blazed-grating profile well controlled by this technique, a 1200 g/mm blazed grating was fabricated that had a blaze angle of 5.0 degrees and smooth blaze facets of about 0.2 nm rms. It was measured to have blaze efficiency in the vacuum-ultraviolet wavelength region.

VUV and soft x-ray diffraction grating fabrication by holographic ion beam etching

Holographic and ion beam etching technique has become routine means for fabricating vacuum ultraviolet and soft x-ray diffraction gratings. A novel technique has been successfully, in which oxygen reactive ions etching was used to achieve resist ashing of the grating, to fabricate diffraction gratings with holographic ion beam etching. The new technique was used to fabricate a spherical blazed grating, 1200g/mm and 130 nm blazed wavelength, and some laminar gratings for monochromators in the beamline of National Synchrotron Radiation Laboratory. The results show that the new technique can considerably lower the stringent requirements of holographic exposure and development, and makes it controllable to make smooth grooves with desirable depth and duty cycle. A gold transmission grating is one of the critical elements in the soft x-ray spectrometer for plasma diagnostics. With holographic-ion beam etching technique, a number of self-supporting transmission gratings have been fabricated for inertial confinement fusion (ICF) diagnosis.

Fabrication of self-supporting transmission gratings for plasma diagnostics

With holographic-ion beam etching technique, a number of self-supporting transmission gratings have been fabricated for inertial confinement fusion diagnosis. In addition to the general process, a practical method for monitoring the evolution of the grating structures exposed in photoresist during development of the resist. The real-time monitor technique developed here is relatively simple in comparison with a He-Ne laser and detector, which needs delicate control.

Reactive ion beam etching of large-aperture multilayer diffraction gratings by radio frequency ion beam source

The major etching processes of a large-aperture multilayer diffraction grating, including the uniformity of the ion beam current along the major axis and the on-line measurement of the diffraction intensity distribution are described. A large-aperture ion beam etcher with radio frequency linear source has been developed to fabricate large-aperture diffractive optical elements. The length with ±5.1% uniformity of the ion beam current distribution along the major axis is 30 cm. A series of multilayer diffraction gratings were etched successfully by using this etcher with CHF3 chemistry. Multilayer diffraction gratings on a 80 mm×150 mm BK7 substrate etched for laser systems are shown. The grating exhibits an averaged diffraction efficiency about 96% at TE polarization of 1 064 nm light viewed at Littrow angle.

Fabrication of the beam splitters for soft X-ray laser application

The soft X-ray interferometry is completed by the Mach-Zehnder interferometer using a soft X-ray laser, and it is also an important method to measure the electron densities of a laser-produced plasma near the critical surface. It is apparently demonstrated in this paper that the incident angle of each optical element in the soft X-ray Mach-Zehnder interferometer should be near normal incidence based on the polarized characteristics of the soft X-ray multilayers, and the product of reflectivity and transmission of the beam splitter should be taken as a standard of design according to the structure of the soft X-ray Mach-Zehnder interferometer. The beam splitters used in the soft X-ray interferometry at 13.9 nm are fabricated using the ion beam sputtering. The figure error of the beam splitter has reached the nanometer magnitude, in which the product of reflectivity and transmission of the beam splitter is more than 1.6%.

Design and fabrication of condenser zone plates for a soft x‐ray microscopy beamline in Hefei

A synchrotron radiation soft x‐ray beamline for scanning microscopy mainly, is being constructed in Hefei. It contains a linear monochromator consisting of a diaphragm of 7 μm diameter and one of three condenser zone plates (CZP). The three CZPs are for three overlapping wavelength ranges 1.97–2.78 nm (CZP23), 2.74–3.87 nm (CZP32), and 3.84–5.44 nm (CZP45), respectively. The processing steps for the condenser zone plate fabrication are summarized, especially the holographic exposure with a special optical system as a key step. Parameters of a prototype of CZP45 fabricated in Hefei are the following: D=2.9 mm, n=826, R1=0.05 mm, and ΔRn≊0.9 μm.

Finely control groove-depth variations of large-area diffraction gratings

We proposed a technique for conducting on-the-fly fine adjustment of etch depths with sub-nanometer precision during the course of ion beam etching (IBE). Simulations were performed to evaluate the etch-depth control precision. The simulation prediction shows that the precision of fine control of etch depths is at the level of 0.1nm. The preliminary experiment was conducted. The early result and the simulation prediction are in agreement with each other, which indicates that this approach is feasible for finely controlling groove-depth variations of large-area diffraction gratings.