We introduce an innovative technique for the deposition of fluorine doped oxide (F:Al2O3) films by DC pulse magnetron sputtering from aluminum targets at room temperature. There was almost no change in transmittance even after the film was exposed to air for two weeks. Its refractive index was around 1.69 and the extinction coefficient was smaller than 1.9 × 10−4 at 193 nm. An AlF3/F:Al2O3 antireflection coating was deposited on both sides of a quartz substrate. A high transmittance of 99.32% was attained at the 193 nm wavelength. The cross-sectional morphology showed that the surface of the multilayer films was smooth and there were no columnar or porous structures.
© 2011 OSA
The development of high-quality optical components for applications in the deep ultraviolet (DUV) and vacuum UV (VUV) spectral regions has garnered considerable interest over the past few decades. Their major applications are in the laser material processing industries , semiconductor industries [2,3], and biological research . Moreover, the spatial light modulator (SLM) consists of an array of micromirrors with an individual mirror size of 16 μm × 16μm, which forms an addressable and reflective surface. Now this technology benefits from DUV and VUV optical coatings, for example high-reflection DUV (HRDUV) coatings on individual mirror. This technology in microelectro mechanical systems (MEMS) and optoelectronics has evolved to develop a class of integrated microsystems with brand new application domains and great potential for development in the near future [5,6].
The coating materials applied in the DUV regions must possess high energy band gap to avoid light to be absorbed. They can be part of fluoride and oxide and the better choices are fluoride films . Magnesium fluoride (MgF2) and aluminum fluoride (AlF3) are usually used as low-index materials in the DUV region. Lanthanum fluoride (LaF3) and gadolinium fluoride (GdF3) are potential high-index materials in the DUV region.
Generally speaking, coating fluoride thin films with resistive heating deposition can get better optical properties films. Nevertheless, the low deposition energy is accompanied by several drawbacks, for example inferior adhesion, sensitivity to exposure to air and light irradiation, and their porous or multicrystalline structure. Therefore, Toshiya Yoshida used IBS with injected fluorine gas to obtain fluoride films characterized by a high packing density, smooth surfaces and very small extinction coefficients on the order of 10−4 in the DUV range . Yusuke Taki used magnetron sputtering with the injected reactive gas SF6 to deposit bulky-structure fluoride films. Their extinction coefficients reached on the order of 10−4 in the DUV range . In both of these improved deposition processes, the sputtering targets are always expensive fluoride compounds. The high refractive index materials are usually the rare LaF3 and GaF3. In addition, it is dangerous to use fluorine gas to control the stoichiometry, because such gas is detrimental to both human health and experimental instruments. Sulfur atoms on the other hand may dissociate from the SF6 gas to contaminate the fluoride films .
Compared to these improved deposition processes, in this study, fluorine doped oxide films were deposited as the high index material. Efforts are made to solve the problems of environmental instability and inferior adhesion of fluoride films applied by DC pulse magnetron sputtering at room temperature. A cheaper metal (Al) was chosen as the target material instead of the rare metals (La and Gd). In order to reduce the optical absorption in the short wavelength, stable CF4 reactive gas was introduced rather than F2 for the deposition of F doped oxide films. In addition, the AlF3 deposition process which we have discussed before is also improved [10,11]. During sputtering, the working and reactive gas (CF4) cause a polymerization effect on the surface of the Al target. This phenomenon affects the stability of the sputtering and causes large optical absorption in DUV range. Hence we attempt to decrease the amount of CF4 gas and properly added the O2 gas to decrease the arcing effect on the surface of Al target. Finally a 193 nm antireflective (AR) coating is added to prove the process is a promising method for applications in DUV optics.
2. Theoretical basis for the deposition process
Figure 1 shows a schematic diagram of the new deposition process for fluorine doped oxide and fluoride films. For the deposition of fluorine doped oxide films, the working gas is Ar and reactive gases are O2 and CF4. The oxide films are formed by the sputtering action of Ar and O2 and then CF4 gas will react with the oxide films to become fluorine doped oxide films. For the deposition of fluoride films, CF4 is both the working gas and a reactive gas. CF ions will be ionized from CF4 gas and then they will bombard the metal target. The sputtered metal particles will react with fluorine atoms to form fluoride films on the substrate. During the sputtering procedure, the O2 is not just the reactive gas. It can also react with C atoms to become CO, which can reduce contamination. It can also create more fluorine atoms. We believe the reactions in our deposition process are similar to those of Ref.  as follows.
Fluorine doped Al2O3 and AlF3 films were coated onto crystalline quartz substrates 2.54 cm (1 inch) in diameter and 0.5 mm thick. The substrates were cleaned in a UV photo cleaner for five minutes prior to the deposition process. The UV light of the UV photo cleaner has two main working wavelengths, 185 nm and 254 nm, at which organic dust can be dissociated into CO, CO2 and H2O gases. The deposition chamber was pumped down to a base pressure of less than 8 × 10−6 torr by a cryopump. Fluorine doped Al2O3 and AlF3 films were deposited by pulsed DC magnetron sputtering of Al targets (with a purity of 99.99%) at room temperature. The sputtering powers were 1kW and 100W, respectively. When depositing fluorine doped Al2O3 films, the injected gases were Ar, CF4, and O2. On the other hand, CF4 and O2 were introduced for the deposition of AlF3 films. The pressures for the deposition of F:Al2O3 and AlF3 were around 1.1 × 10−3 torr and 3.5 × 10−4 torr, respectively. The transmittance of thin films on crystalline quartz substrates was measured with a Hitachi U4100 spectrometer. From the spectral analysis, the refractive index, extinction coefficient and physical thickness were determined by the envelope method. The cross-sectional and surface morphologies of the thin films were analyzed by scanning electron microscopy (SEM).
4. Results and discussion
Figure 2 shows the transmittance spectra of fluorine doped Al2O3 thin films coated with 1 kW DC power at room temperature. The films were prepared with different ratios of CF4/O2 gas but the same Ar (60 sccm) and O2 (14.4 sccm) flow rate and the pressure for the deposition of F:Al2O3 was around 1.1 × 10−3 torr. In the plasma, CF4 would be ionized to CF3 + and F- ions, or excited to become excited F* atoms. The sputtered aluminum particles would react with the F- ions and the F* and O* excited atoms to form fluorine doped Al2O3 films on the substrates. The transmittance in the DUV range increased as the CF4/O2 gas ratio increased. There was almost no absorption when the ratio of CF4/O2 gas was 0.243. The comparison of the highest and lowest transmittance curves showed that the transmittance of the improved fluorine doped Al2O3 thin films increased more than 20% around 193 nm. This phenomenon was believed to be the result of AlF3 molecules which enhanced the energy bandgap of the Al2O3 films.Figure 3 shows the aging effect of F:Al2O3 films prepared with a CF4/O2 ratio of 0.243. Even after two weeks of exposure to air, there was no change in transmittance from 190 to 700 nm. This is due to the dense structure which avoids the water and air penetrating in the film [5,6]. The results show that the new deposition can deposit the F:Al2O3 films with excellent mechanical properties.
Figure 4 shows the transmittance spectra of AlF3 thin films coated with 100 W DC power with CF4 15 sccm and O2 9 sccm at room temperature. During sputtering, the excess working and reactive gas (CF4) would not only deposit the AlF3 films but also yield fluorinated amorphous carbon (a-C:F) films. Films with good electrical insulation capacity and a low dielectric constant were built up on the substrate and the conductive targets . The former would cause absorption and the latter would generate arcs on the target. Hence, the amount of CF4 gas used was decreased and the proper amount of O2 gas added to generate more fluorine atoms. From the transmittance results, there was almost no optical absorption at short wavelengths.
The dispersion of the refractive index and extinction coefficient of F:Al2O3 for 0.243 CF4/O2 gas ratios is shown in Fig. 5 . The refractive index at 193 nm was around 1.69 when the CF4/O2 ratio was 0.243. Since AlF3 has a low refractive index, it can be conjectured that the reduction of the refractive index could be due to the doping of AlF3 in the Al2O3 films. The extinction coefficients of films prepared with a CF4/O2 ratio of 0.243 were all below 1.9 × 10−4 for the wavelength range from 190 nm to 194 nm. This result was due to the generation of F- ions or excited F* atoms reacting with the Al2O3 to produce F:Al2O3 films which possessed a large band gap.The refractive index of AlF3 films in Fig. 4 can be determined by the envelope method and the result is depicted in Fig. 6 . The polymerization effect is suppressed so the refractive index is around 1.46 at 193 nm and the extinction coefficient is almost zero.
Figure 7 shows the total thickness, as indicated by the image of SEM, and the transmittance spectrum deposited on both sides of the quartz substrate of the AlF3/F:Al2O3 antireflection (AR) coating. This is known as a V-coat because of the shape of the characteristic. The refractive indexes of substrate, low-index (AlF3) and high-index (F:Al2O3) layers are 1.66, 1.46 and 1.69, respectively at 193 nm. The design structure at 193nm is quartz substrate / AlF3(40.7 nm) / F:Al2O3(31.5 nm) / AlF3 (28.5 nm) / Air. A high transmittance of 99.32% is attained at wavelengths of 193 nm. Figure 8 shows the cross-sectional morphologies of the AlF3/F:Al2O3 AR coating. The surface of the films is smooth and the film is amorphous in structure, with no columnar or porous structures in it.
In this study, fluorine doped Al2O3 and AlF3 films were coated by DC pulse magnetron sputtering of aluminum targets with purities of 99.99% at room temperature. During the deposition of F:Al2O3 films, different ratios of CF4/O2 gas were applied to obtain better optical properties while still retaining the original good mechanical properties. The transmittance spectra showed there was almost no optical loss from 190 nm to 700 nm when the ratio was 0.243. There was no change in transmittance even after two weeks of exposure to air. The extinction coefficient was smaller than 1.9 × 10−4 when the wavelength range was from 190 nm to 194 nm. On the other hand, the refractive index was around 1.46 and the extinction coefficient was almost zero at 193nm when AlF3 films were deposited with lower amounts of CF4. High transmittance of 99.32% was attained at wavelengths of 193 nm. In addition, the cross-sectional morphologies showed that the surface was smooth and the film structure was amorphous, with no columnar or porous structures. All of the results indicated that we have developed an easy and cost effective process to be applied in DUV optics.
The authors would like to thank the National Science Council of Taiwan for their financial support of this work under Contract No. NSC 96-2221-E-008-065-MY3.
References and links
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