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Aluminum oxyfluoride films were deposited by a combined HIPIMS/CFUBMS deposition technique with Al targets at room temperature. In order to obtain better optical and mechanical properties, films were investigated under different duty cycles and CF4 gas flow rates. The optical properties in deep ultraviolet range, microstructure, surface roughness, and crystalline structure of aluminum oxyfluoride films have been studied. The aluminum oxyfluoride films deposited with 45/255 duty cycle and 0.8 sccm CF4 had the lowest extinction coefficient (8 × 10−4) and the highest refractive index (1.72) at 193 nm. Additionally, aluminum oxyfluoride films revealed a dense and amorphous structure with 0.155 nm surface roughness.
© 2016 Optical Society of America
1. Introduction
Shorter wavelength light sources in the DUV and VUV regions have been implemented for the semiconductor industries, laser material processing industries and biological research over the past few decades [1–4]. With regard to selection of materials, fluoride and oxide materials are the two common choices in the industries. Fluoride materials are better candidates for the DUV and VUV wavelengths because of their large energy band gap [5–8]. However, the use of fluorides is accompanied by several drawbacks, for example inferior adhesion, over-sensitivity to air and light irradiation, the porous or multicrystalline structure as well as the lower refractive index. On the other hand, oxide materials such as HfO2, ZrO2, Nb2O5, Ta2O5 and Al2O3 possess high refractive index and better mechanical properties. Nevertheless, they show strong absorption and are therefore not suitable for interference coating design purposes in the VUV and DUV spectral region. Both materials have their own weaknesses.
To overcome their shortcomings, a technique [9] was proposed to deposit fluorine doped oxide films by DC pulse magnetron sputtering from aluminum targets at room temperature. Its refractive index was around 1.69 and the extinction coefficient was on the order of 10−4 at 193 nm and it also possessed excellent mechanical properties. During the process of improvement, other relative problems appeared. In order to grow stoichiometric compound films in reactive sputtering, deposition was usually from poisoned target and it was highly unstable. It usually exhibited hysteresis effect, and the deposition rate was significantly lower than those obtained from a metallic target. Besides, the deposition energy was tailored by around 300-400 voltage between the cathode and anode, which was too small to enhance the packing density.
Under such circumstances, a high power impulse magnetron sputtering (HIPIMS) system and a closed-field unbalanced magnetron sputtering (CFUBMS) system were considered to be applied. HIPIMS is a newly developed sputtering technique [10, 11] in which the pulse of power applied to the target has a low duty cycle (on/off time ratio) of less than 10% and a frequency of less than 10 kHz. This leads to pulse power densities of several kW cm−2 on the target. This sputtering technique generates ultra-dense plasmas with a higher concentration of ionized sputtered atoms. These unique features make the deposited coatings dense and smooth, even on complex-shaped substrates and make possible optical coatings with a high refractive index. In a closed-field unbalanced magnetron sputtering system [12], adjacent magnetrons are made with magnets of opposite polarity. The closed-field unbalanced magnetron sputtering creates a magnetic confinement that extends the electron mean free path leading to the high ion current density. By properly tilting the sputtering gun, the high ion current density and the high reaction rate can just happen in the volume around the rotating substrate at room temperature. As a result, the process does not require a separate ion or plasma source for reaction, which makes the process simpler, less expensive and more stable.
In this research, a closed-field unbalanced magnetron sputtering system with high power impulse sources was designed and fabricated. In order to stabilize the sputtering process and improve the deposition rate, Al2O3 films were deposited in transition mode from Al targets at room temperature. Then, various flow rates of CF4 gas were added to deposited dense aluminum oxyfluoride films for DUV coating.
2. Experiments
2.1 Film deposition
Aluminum oxide and aluminum oxyfluoride films were deposited by a closed field magnetron sputtering system with RF and high power impulse sources. Films were deposited on 0.5 mm thick square fused silica substrates with side length 1 inch and square polished silicon wafer (100) substrates with side length 1 inch and surface roughness around 0.13 nm. From previous study [13], increasing the deposition temperature would increase the absorption so thin films were deposited at room temperature. The home-made closed field magnetron sputtering system was schematically illustrated in Fig. 1. The system consisted of a deposition chamber and two quartz crystal microbalances with three co-sputtering magnetron cathodes. The cathodes were all set at a lean angle to improve the thickness uniformity. Al targets (99.99% purity) were mounted on each cathode. The Al target was 101.6 mm (4 inch) in diameter and located approximately 8 cm below the substrate. Base vacuum, smaller than 6 × 10−6 torr, was attained prior to reactive sputtering experiments. The DC sputtering power kept at 300 W and different RF power was utilized to prepare stoichiometric Al2O3 films. During sputtering, a high power impulse source with a constant pulsed on-time period of 45 μs was applied. In order to increase the deposition energy and refractive index, repetition frequency of the pulse was varied from 1 to 20 kHz by changing the off-time periods of the high power impulse source.
The working gas was Ar and the reactive gases were O2 and CF4. The total pressure was maintained around 2.1 × 10−3 Torr. The oxide films were formed by the sputtering action of Ar and O2 and then CF4 gas would react with the oxide films to become aluminum oxyfluoride films. During the sputtering procedure, O2 was not just the reactive gas. It could also react with C atoms to form CO, which would reduce the contamination of films. Meanwhile, it created more fluorine atoms as well. The equations for the reactions were as follows [7, 14, 15]:
2.2 Film characterization
The transmittance of thin films on fused silica substrates was measured with a Hitachi U3900 spectrometer. Its guaranteed wavelength range was 190 to 850 nm and photo metric accuracy and reproducibility were ± 0.3% and ± 0.1%, respectively. From the transmittance spectral analysis, the refractive index, extinction coefficient and physical thickness for a weakly absorbing thin film could be determined by the envelope method [16]. The cross-sectional and surface morphology of the thin films were analyzed by scanning electron microscopy (SEM). The surface roughness was measured with an atomic force microscope (AFM).The composition and depth profiles of the films were examined by X-ray photoelectron spectroscopy (XPS).
3. Results and discussion
Figure 2 shows the transmittance spectra of Al2O3 films deposited with Ar 30 sccm and O2 6 sccm from Al targets at room temperature. The on/off ratio of HIPIMS remained at 45/255 with an average power of 300 W and a fixed deposition time of 13 minutes. From the spectral analysis, the physical thickness was determined by the envelope method. The thicknesses of all films were about 314 nm. In order to stabilize the plasma and decrease the arcing for optical coating, Al2O3 films were deposited in the transition mode. However, the reactive gas (O2) was too less to fully react the sputtered Al atoms so the transmittance was very low in the VIS and UV region. As increasing the RF power, the closed-field unbalanced magnetron sputtering created a magnetic confinement that extended the electron mean free path leading to high ion current densities. Thus, the high reaction just happened in the volume around the rotating substrate at room temperature. Making a comparison between the highest and lowest transmittance curves of the Al2O3 thin films, the transmittance increased more than 70% around 250 nm as the RF power was larger than 60 W. Al2O3 thin films deposited with RF 80 W could reach the highest transmittance (88.5%) at 227 nm, accompanying a high deposition rate (24.2 nm/min).
Figure 3(a) shows the transmittance spectra of Al2O3 thin films deposited with an average DC power of 300 W, RF power 80 W, and various HIPIMS on/off ratio at room temperature. The deposition rates of Al2O3 thin films were 23.8, 24.2, and 19.9 nm/min for 45/5, 45/255, and 45/555 on/off ratio, respectively. Figure 3(b) shows the dispersion of the refractive index of Al2O3 films depicted in Fig. 3(a). The working gas (Ar) was 30 sccm and reactive gas (O2) was 6 sccm. The extinction coefficients of Al2O3 films were all smaller than 5 × 10−4 from the wavelength range 300 nm to 700 nm. In the beginning, the refractive index of Al2O3 films increased with increasing pulse off time. When the pulse off time was larger than 255 us, the refractive index of Al2O3 films didn’t keep increasing. The refractive index of Al2O3 films prepared with 45/255 pulse on/off ratio at 550 nm was 1.666, which was larger than those refractive indexes of Al2O3 films deposited by various techniques depicted in reference 17 [17].
Fig. 3 (a) Transmittance spectra and (b) refractive index of Al2O3 thin films prepared with different HIPIMS on/off time.
Figure 4(a) shows the transmittance spectra of aluminum oxyfluoride films coated with an average DC power of 300 W, RF power 80 W and HIPIMS 45/255 on/off ratio at room temperature. In order to increase the transmittance of Al2O3 films in DUV range, aluminum oxyfluoride films were deposited with Ar 30 sccm, O2 6 sccm and various CF4 flow rates. The thickness of Al2O3 films was about 314 nm. In order to compare the transmittance in the DUV range (especially around 193 nm), aluminum oxyfluoride films (deposited with 0.6 and 0.8 sccm) were deposited with the similar physical thickness, about 100 nm. The transmittance increased as increasing the CF4 gas in DUV range. There was almost no absorption when introducing 0.8 sccm CF4 gas. The comparison of the highest and lowest transmittance curves showed that the highest transmittance around 200 nm increased from 77% (without CF4 gas) to 90%. The improvement of transmittance could be attributed to the formation of the ionized F- ions and excited F* atoms from the CF4 plasma [14, 15]. They reacted with Al2O3 films to form aluminum oxyfluoride films which possessed larger energy band gap. This phenomenon would also be discussed and proved below.
Fig. 4 (a) Transmittance spectra of aluminum oxyfluoride films prepared with different CF4 gas flow rates and (b) refractive index and extinction coefficient of aluminum oxyfluoride films prepared with 0.8 sccm CF4.
The dispersion of the refractive index and extinction coefficient of aluminum oxyfluoride films prepared with Ar 30 sccm, O2 6 sccm and 0.8 sccm CF4 gas is shown in Fig. 4(b). The refractive index at 193 nm was 1.72, higher than that (1.69) deposited by pulse DC magnetron sputtering in our previous research [9]. It was also higher than GdF3 (1.70) and LaF3 (1.70) films deposited by ion-beam sputtering [5, 18]. The higher refractive index would be a consequence of the ultra-dense plasma with a higher concentration of ionized sputtered atoms generated by the HIPIMS technique. The extinction coefficients of films were all below 8 × 10−-4 for the wavelength range from 193 nm to 300 nm. The small extinction coefficients were due to the generation of the aluminum oxyfluoride films which possessed a large energy band gap rather than the intermixture of aluminum oxide and aluminum fluoride, which would also be proved at XPS results in Fig. 6(b).
Figure 5 shows the evolution of refractive index and extinction coefficient of aluminum oxyfluoride films described in Fig. 4(a) at 248 nm wavelength. The refractive index and extinction coefficient of aluminum oxyfluoride films obviously decreased when increasing the CF4 flow rates. Increasing more CF4 flow rates could generate more fluorine atoms so the refractive index and extinction coefficient showed a clear trend toward those of AlF3 films.
Fig. 5 The evolution of refractive index and extinction coefficient of aluminum oxyfluoride films deposited with various CF4 flow rates at 248 nm.
Figure 6(a) shows XPS spectra of as-deposited aluminum oxyfluoride films over the full energy range of 0-1100 eV. Four relevant peaks were identified and they were C, F, O, and Al atoms. After etching the film by an Ar sputter etching process with 30 seconds, there was only a trace amount of carbon atoms. This result explained most of the carbon element came from the atmosphere and just existed on the surface of films. The respective atomic concentrations of Al, O, and F were 41.4%, 46.1% and 12.5%. The trend of lower refractive index (1.72 at 193 nm) for the sample compared to pure Al2O3 films (1.87 at 200 nm) [19, 20] could be attributed to the 12.5% F atomic concentrations. Figure 6(b) shows the XPS spectra of the Al2p for the aluminum oxyfluoride films. Energy calibration was performed by adopting the C 1s core level at 284.8 eV as a reference peak. The peak positioned around 75.6 eV could not simply be assigned as an aluminum oxide peak. The initial peak position lay between the Al2O3 peak (EB = 74.4 eV) and the AlF3 peak (EB = 76.9 eV) [21, 22]. This result suggested that the chemical composition of the films was in the form of AlOxFy. Meanwhile, the chemical composition of AlOxFy films could also explain the reason why the extinction coefficient was so small at 193 nm. The very small extinction coefficient was consistent with the result shown in Fig. 4(b).
Fig. 6 (a) XPS spectra over the entire energy range (0-1100 eV) and 6.(b) high-resolution XPS spectra of Al2p for the aluminum oxyfluoride films.
Figure 7 shows the AFM surface images and cross-sectional morphology of Al2O3 ((a) and (c)) and AlOxFy ((b) and (d)) with the similar thickness around 151 nm. The cross-sectional morphology of films showed amorphous structures with no columnar or porous structures in the films. The surface of the films was very smooth; the surface roughness of Al2O3 and AlOxFy films were 0.172 nm and 0.155 nm, respectively.
Fig. 7 AFM surface images and cross-sectional morphology of Al2O3 ((a) and (c)) and AlOxFy ((b) and (d).
5. Summary
Aluminum oxyfluoride films were coated by a combined HIPIMS/CFUBMS deposition technique with Al targets at room temperature. Different RF power and CF4 gas flow rates were applied to get better optical properties. Increasing the RF power and the CF4 gas could decrease the extinction coefficient in the UV range. Aluminum oxyfluoride films deposited with DC 300 W, RF 80 W and 0.8 sccm CF4 had the best optical properties. The extinction coefficient was smaller than 8 × 10−4 from 193 nm to 700 nm and the refractive index was 1.72 at 193 nm. The AFM surface images and cross-sectional morphology of AlOxFy films showed a dense and amorphous structure with 0.155 nm surface roughness. X-ray photoelectron spectroscopy indicated that the Al, O, and F atomic concentrations of aluminum oxyfluoride films (AlOxFy) were 41.4%, 46.1%, and 12.5%, respectively. All of the results indicated that we have found a cost effective and mass production process which was suitable for the application of manufacture in the real-world industry.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology of Taiwan for their financial support of this work under Contract No. MOST 104-2221-E-492 −025 -MY2.
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