One of the reasons why the current optics used in 193 nm lithography do not have sufficiently high reflectivity and have poor optical timing is the lack of a low-refractive-index material with low absorption, low scattering, low residual stress, amorphous structure, and good aging resistance. In this study, we investigated the effect of the AlF3 content in MgF2-based low-refractive-index materials on the microstructure and optical properties of Mg1-xAlxF nanocomposite coatings prepared by thermal evaporation. Based on these findings, this study successfully solved the problems of large absorption, easy crystallization tendency, and poor aging performance of thermally evaporated AlF3 films. A novel low-refractive-index nanocomposite material with improved absorption reduction, scattering suppression, and aging resistance was obtained, which can be applied to the study of reflective optics for 193 nm lithography.
La1-xAlxF3 nanocomposites with different AlF3 doping ratios were synthesized using a dual-source electron beam co-evaporation technique to achieve amorphous La1-xAlxF3 coatings with low loss and tensile stress. We analyzed the evolution laws of optical constants, microstructures, film stress, and film loss of La1-xAlxF3 nanocomposites with the change of element content. When x ≥ 0.30, high-refractive-index nanocomposites La1-xAlxF3 demonstrated reduced absorption, integrated amorphous structure, and lower tensile stress. The nanocomposite exhibited superior performance with an excellent overall structure, as well as reduced tensile stress. Additionally, this material was employed to create a high-performance reflective film with a high reflectivity.
HfO2 prepared by ion beam sputtering (IBS) is widely used as a high-refractive index material for making low-loss laser films. The challenge of a high-performance HfO2 film is to simultaneously obtain an amorphous morphology with low scattering and a stoichiometric structure with low absorption. Furthermore, nanometer-sized voids are commonly present in IBS films due to excessive oxygen and argon adsorption during deposition, which is the primary barrier to achieve a smooth surface and low optical loss of HfO2. Thin amorphous SiO2 layers were added periodically into HfO2 coatings using the IBS process to synthesize amorphous HfO2 / SiO2 nanolaminate-based composites. The resulting composites exhibited excellent comprehensive performance with a dense amorphous microstructure and a void-free smooth surface. High-temperature annealing was performed to ensure superior stoichiometry and lower absorption. However, the crystalline states and microstructure of some composites evolved during the gradual annealing. We present a detailed study of the crystallization, surface topography, and absorption evolution in HfO2 / SiO2 nanolaminates as a function of HfO2 sublayer thickness and thermal annealing temperature. Moreover, the interplay between crystallization, surface topography, and absorption is elucidated. The HfO2 / SiO2 nanolaminate with 19 thin layers maintained a dense amorphous structure with low absorption after annealing. Finally, a 1064-nm HfO2 / SiO2 nanolaminate-SiO2 high-performance reflector was prepared and achieved lower absorption with a smooth surface after annealing, which demonstrated the great potential of the HfO2 / SiO2 nanolaminates for considerably improving optical loss.
HfO2 film prepared by ion beam sputtering (IBS) has greater potential in low loss laser films. However, it is prone to form sub-stoichiometric and polycrystalline structure resulting in high absorption and scattering loss. Besides, the adsorption of excess oxygen atoms will produce oxygen inclusions in the surface of HfO2 during fabrication, leading to the formation of surface hole defects, thereby increasing the surface roughness. Annealing is an important method to achieve better stoichiometry, but it tends to induce crystallization or recrystallization of the film. In this paper, thin amorphous SiO2 layers were periodically added into HfO2 coatings to synthesize HfO2/SiO2 nanolaminate layers, which can effectively suppress crystallization during HfO2 growth, increase the crystallization threshold temperature during thermal annealing, eliminate hole defects on the film surface and minimize the absorption and scattering loss. We present a detailed study of the crystallization, surface topography and absorption evolution in HfO2/SiO2 nanolaminate layers as a function of sub-layer thickness and thermal annealing temperature. It was found that thinner HfO2 layers show higher threshold temperature of crystallization, so it can maintain almost zero absorption and amorphous state under high temperature annealing.
Third harmonic separator has proven to be a critical component in high-power laser systems, which can be used for beam splitting and optical path change of 355 nm laser. It is desirable for the coating to have high spectral efficiency and laser induced damage threshold. Through precise fabrication, the spectrum efficiency can easily meet the requirements with high reflection at 355nm and high transmittance at 532nm and 1064nm. However, the issue is more complicated and quite challenging for obtaining high LIDT, because high refractive index materials tend to have high absorption at ultraviolet, which contribute to a decreased LIDT together with the electric field inside the film. It is significant to investigate the interplay between absorption, electric field intensity and LIDT. In this paper, materials of HfO2 and SiO2 are used to design and fabricate the two third harmonic separator with similar film structure, high spectral efficiency and different electric field distribution by electron beam evaporation. Spectral measurements results show that the prepared samples all have similar spectra with high spectral efficiency. Damage threshold and weak absorption test results show that high intra-film electric field distribution results in higher weak absorption and lower damage threshold. Our research has important reference significance for the design and preparation of high damage threshold third harmonic separator.
We have designed and experimentally demonstrated a periodic multilayer structure of SiO2 and Cr thin interlayers to achieve an ultra-broadband perfect absorber based on optical admittance matching method. The successive Nano-Cr-film make significant contribution to improving the absorption intensity of the structure. Measurements reveal high absorption over 85%, when averaged over the range 0.4–7.2 μm. Remarkably, it is the most broadband planar absorber film without involving lithography in fabrication. Incident angle and polarization dependence of the absorption spectra are also considered. The manufactured absorber also has potential applications for thermal shielding, detecting, imaging, photovoltaics (PVs), sensing, etc.
Low absorption, low scattering and high-density HfO2 coatings lead to an important improvement of
high power laser systems. In order to suppress the crystallization of HfO2 coatings fabricated with ion
assisted deposition (IAD), we used double electron-beam (EB) coevaporation with ion beam assisting
method to fabricate HfO2-SiO2 mixtures from two independent material sources. Crystallization
following the different mixture ratios was investigated. Several prototypes were designed, featuring
different HfO2-SiO2 ratios with similar physical thickness (500nm). The samples were deposited on
fused silica and silicon substrates. X-ray diffraction patterns show that the degree of crystallization
gradually fades away with increasing SiO2 contents and when SiO2 component reach 18%, the mixture
film becomes almost amorphous. To decrease the high absorption originating from IAD method,
thermal annealing in air at progressive temperatures was subsequently performed. It was found that
post-annealing treatment at 600°C could eliminate the absorption at 1064nm. However, high
temperature annealing would induce the crystallization of these initial amorphous coatings. In order to
suppress the crystallization further to obtain amorphous structure even after 600°C annealing, the 25%
SiO2 sample was fabricated and it successfully obtain low roughness and low absorption equivalent to
the bare substrate. Finally, a 1064 nm HR coating using SiO2 and mixed film of 25% SiO2
concentration was prepared and annealed to prove its practical application in low loss and high LIDT
optical elements.
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