Objective Fluoride film is used in infrared bands because of its excellent optical properties, including a large bandgap that results in increased transparency. Particularly, yttrium fluoride (YF₃) has a broad transmission range, from ultraviolet (UV) to infrared (IR). Different deposition methods are used to obtain different optical properties for YF₃thin films. Electron beam deposition is the most popular technique owing to its high productivity. However, this film is porous and has a low packing density. Therefore, it absorbs much IR energy in the water absorption bands and its optical properties are likely to be unstable. Alterenatively, sputtering creates films with a high packing density. However, YF₃ thin films are easily contaminated with oxygen and crumbled. Unfortunately, to our knowledge, little work has been done to address the issue. Therefore, investigating how the deposition affects the material’s composition, structure, and optical properties is important.

Methods The sputtering material was YF₃ ceramic target (99.9% purity, 100-mm diameter, 3-mm thickness). All YF₃ thin films were deposited on germanium wafers at room temperature by radio frequency magnetron sputtering at different deposition power in a vacuum chamber. Sputtering power of 150, 200, 250, 300, and 350 W were selected to deposit YF₃ films. However, the thin films easily crumbled when sputtering power was more than 300 W. Therefore, three power parameters of 150, 200, and 250 W were selected. The vacuum chamber was evacuated to a 5×10 ^-5 Pa base pressure with turbomolecular and mechanical pumps. Presputtering was performed for approximately 10 min with a shutter covering the substrate before film deposition. The target-substrate distance is 15 cm. Under 1.0 Pa at room temperature, the deposited power changed step by 50 W from 150 W to 250 W. To maintain the film thickness at 1000 nm, deposition time lasted for 5, 4, and 3 h at 150, 200, and 250 W, respectively. The sputtering deposition conditions of the YF₃ films are listed in Table 1.

The crystalline structure of the films was identified using glancing incident X-ray diffraction (GIXRD, Philips X’Pert-Pro) with Cu Kα source (40 kV, 40 mA). The incidence angle was 1.5° and samples were scanned in a 2θ range of 10°--90° with a scan step size of 0.05°. The sample compositions were examined using X-ray photoelectron spectroscopy using a monochromatized Al Kα X-ray source with a step size of 0.8 eV. In addition, these coating samples’ surface morphology and spectrum transmittance were analyzed using atomic force microscopy and Fourier transform tnfrared spectrometer.

Results and Discussions Fig. 1 shows the XRD patterns of as-deposited YF₃ films grown on Ge wafers at 150, 200, and 250 W, and YF₃ ceramic target, respectively. Different deposition power contributes to forming the orthorhombic YF₃ crystal structure with different preferential orientations, and no evident peak belonging to cubic yttrium oxide appears in the samples. Fig. 2 shows the XPS survey spectra for the surface of the YF₃ thin films using different deposition power in the 0--1400 eV range, no peaks from other elements appear in the scan except yttrium (Y), fluorine (F), oxygen (O), and carbon (C). Besides Y and F elements, all samples contained O. When deposited by 150 W, the ratio of atomic number fraction between F and Y elements is the smallest (~2.03), and the oxygen atomic number fraction is 9.59%, which is the highest. The atomic concentrations of Y and F are close to the theoretical values for the stoichiometry of the YF₃ at 200 W and 250 W. Simultaneously, the films have an oxygen atomic number fraction of 5.74% and 5.19% from Table 2. The refractive index of YF₃ thin film was fitted from 2000 nm to 8000 nm using OptiChar software; the refractive index parameter increases with power. To ensure the accuracy of refractive index fitting, the refractive index parameters obtained through fitting were substituted into OptiChar for transmittance parameters. The black line in Fig. 7 represents experimental transmittance, whereas the red dot line represents fitted transmittance, indicating that the refractive index is accurate. Film RMS and TIS values are listed in Table 3.

Conclusions Some work has been done with more than 200 ℃, the thickness of the thin films was less than 300 nm, and oxygen atomic number fraction was more than 15%. In this work, we successfully deposited 1000-nm-thick YF3 films through radio frequency magnetron sputtering on germanium substrates at room temperature by adjusting the deposition process. The research shows that YF₃ thin films deposited using 200 W had less than 6% oxygen atomic number fraction, low absorption, and a refractive index are higher than 1.6 from 2 μm to 8 μm.