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In this research, the plasma etching mechanism which is applied to deposit AlF3 thin films has been discussed in detail. Different ratios of O2 gas were injected in the sputtering process and then the optical properties and microstructure of the thin films were examined. The best optical quality and smallest surface roughness was obtained when the AlF3 thin films were coated with O2:CF4 (12sccm:60sccm) at 30W sputtering power. To increase the deposition rate for industrial application, the sputtering power was increased to 200W with the best ratio of O2/CF4 gas. The results show that the deposition rate at 200W sputtering power was 7.43 times faster than that at 30W sputtering power and the extinction coefficients deposited at 200W are less than 6.8×10-4 at the wavelength range from 190nm to 700nm. To compare the deposition with only CF4 gas at 200W sputtering power, the extinction coefficient of the thin films improve from 4.4×10-3 to 6×10-4 at the wavelength of 193nm. In addition, the structure of the film deposited at 200W was amorphous-like with a surface roughness of 0.8nm.
©2008 Optical Society of America
1. Introduction
In order to achieve higher resolution in lithography, shorter exposure wavelength of light sources is necessary. AlF3 is one of the important low-index materials for UV and DUV coatings. The conventional thermal evaporation [1–4] and sputtering [5] methods normally use expensive AlFx as the starting materials, but they are subject to decomposition which could cause absorption. We have invented a new process to deposit AlF3 thin films by magnetron sputtering of an aluminum target with CF4 and CF4 mixed 5% O2 as the working gas [6–7]. In this process, stable CF4 gas was introduced and ionized to become CF+ 3 and F- ions or excited F* atoms. These particles stimulate the formation of AlF3 thin films on the substrates while at the same time causing the dissociation of carbon atoms from the CF+ 3 ions.
Then 5% O2 was added to react with the carbon atoms to formCO2 so the absorption and extinction coefficient decreased. However in this process, the absorption that was caused by the superfluous carbon became severer as the sputtering power increases. Besides, sputtering at high sputtering power requested more F- ions or excited F* atoms to react with larger number of Al particles to form AlF3. Although injecting more CF4 gas might be able to produce more F- ions or excited F* atoms, it would make the structure of films loose and cause larger surface roughness. Therefore AlF3 thin films with good quality can only be deposited at low sputtering power and this means that large quantity manufacturing of thin films is not realistic in the present-day optical industry. So the process above must be improved to overcome the disadvantages of sputtering AlF3 films at high sputtering power. In this research, we first tried to inject O2 gas at various flow rates during sputtering process and then found the best ratio of CF4/O2 gas to deposit the films at high sputtering power.
2. Experiment
2.1 Film preparation
Figure 1 shows a schematic representation of the pulsed DC magnetron sputtering system used in the experiment. The system consisted of a deposition chamber with two magnetron sputtering cathodes. An Al target was mounted on one cathode. The Al target was 15.24 cm (6 inch) in diameter and set about 8cm below the substrate. The sputtering cathode was surrounded with the chimney ring, and CF4 gas was directly injected over the surfaces of the target inside the chimney ring. A pulse generator that operated at a frequency of 20 KHz was located between the DC power supply and the sputtering cathode. The pulse generator helped to decrease the hard arcs and satisfied the demand to improve the film quality even fewer defects arose.
AlF3 thin films were coated on 29.0mm long, 27.3mm wide, and 1mm thick crystalline quartz substrates and polished silicon wafer (111) substrates 2.54cm (1 inch) in diameter and 1 mm thick. The substrates were cleaned in a UV photo cleaner for ten minutes prior to the deposition process. There were two main working wavelengths for the UV light of the UV photo cleaner, 185 nm and 254 nm, where organic dust could 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. Different ratios of CF4 to O2 gas were injected during sputtering of the aluminum targets. The total pressure was maintained between 1×10-3 to 3×10-3 torr and the sputtering power was 30W.
The best ratio of CF4 to O2 gas was chosen from the above experiments. The sputtering power was then changed from 30W to 200W. After coating, any further organic contaminants were removed by a 10 minute treatment in a UV photo cleaner. All of the experiments were carried out at room temperature.
2.2 Film characterization
The transmittance of thin films on crystalline quartz substrates was measured with a Hitachi U4100 spectrometer. The refractive indices and extinction coefficients of the AlF3 thin films were measured by a SOPRA GES5 Variable Angle Spectroscopy Ellipsometer and the analyses were carried out by a Forouhi-Bloomer model [8]. The cross-sectional morphology of the thin films was analyzed by scanning electron microscopy (SEM). The surface roughness was measured by atomic force microscopy (AFM)
3. Results and discussion
3.1 Transmittance
Fig. 2. Transmittance spectra of AlF3 thin films prepared with and without O2 gas at the same sputtering time and power.
Fig. 3. Transmittance spectra of AlF3 films prepared with 30W power and 60sccm CF4 mixed with various O2 gas flow rates at the same sputtering time.
Figure 2 shows the transmittance spectra of AlF3 thin films prepared with and without O2 gas at the same sputtering power (30W) and sputtering time (53 minutes). During the sputtering process, CF4 gas was ionized to become CF+ 3, F- ions or excited F* atoms. These CF+ 3 ions would sputter the Al target, and the F- ions and F* excited atoms would react with the Al or AlFX (X<3) to form AlF3 thin films on the substrate. However, the dissociation of carbon atoms from the CF4 gas during the sputtering process could contaminate the AlF3 films and Al target. This is the critical cause of a decrease in transmittance. When O2 gas was injected, it would react with the carbon atoms to form CO2. Moreover, the additional O2 gas encouraged the dissociation of CF4, CF+ 3, CF+ 2, and CF+ to form more F- ions and F* excited atoms[9–10]. This is why the transmittance became higher after the introduction of the O2 gas. Figure 3 shows the transmittance spectra of AlF3 thin films prepared with 30W sputtering power and 60sccm CF4 mixed with different O2 gas flow rates at the same sputtering time (53minutes). The transmittance increased as the O2 gas increased. This was because injecting more O2 gas led to a decrease in the carbon atoms and provided enough F* to react with Al to form AlF3 films. However when the O2 gas exceeded 12sccm, there was a negative inhomogeneous refractive index, due to the reduction in the bombarding energy. This reduction was caused by the excessive gas and the competition of oxygen atoms for chemisorption sites on the Al target, which made the surface layers of the target more oxidelike.
3.2 Film thickness, refractive index, and extinction coefficient
Fig. 4. Thickness of AlF3 thin films prepared with different O2 flow rates at the same sputtering power and time.
The thicknesses of AlF3 thin films prepared with 30W sputtering power and 60sccm CF4 mixed with different flow rates of O2 gas but at the same sputtering time (53minutes) are shown in Fig.4. At first, some carbon atoms were decompounded from the CF+ 3, CF+ 2, and CF+ ions to cover the Al target which decreased the sputtering rate. The introduction of O2 gas removed the carbon atoms from the Al surface so the thickness increased. When the amount of O2 gas exceeded 9sccm, the surface became oxide-like and the sputtering rate again decreased.
The refractive indices for various O2 gas flow rates are shown in Fig. 5. The refractive index decreased as the O2 gas flow rate increased. We conjecture that injecting more O2 gas reduced the sputtering energy which led to a decrease in the refractive index. In addition, injecting more O2 gas would also cause the Al particles to react more fully with the F* to produce more stable AlF3 films, so the refractive index decreased.
Figure 6 shows the extinction coefficient of AlF3 thin films prepared with different flow rates of O2 gas at 30W sputtering power. Obviously, the extinction coefficient decreased as the O2 gas flow rate increased. The O2 gas not only decreased the carbon atoms in the AlF3 films, but also supplied enough F* to react with Al to become AlF3.
Fig. 5. Refractive indices of AlF3 thin films prepared with different flow rates of O2 gas at 30 sputtering power.
Fig. 6. Extinction coefficient of AlF3 thin films prepared with different flow rates of O2 gas at 30W sputtering power.
3.3 Surface roughness
Table 1 shows the surface roughness of AlF3 thin films produced at different sputtering powers with 60sccm CF4 and different O2 flow rates. The root-mean-square (rms) surface roughness decreased as the O2 flow rates increased. The surface roughness was smallest when the O2 gas flow rate was increased to 12sccm. This was because of the presence of more electronegative atoms (e.g., O and F). These atoms captured the slow secondary electrons to become negative ions. These negative ions were accelerated by the electric field in the cathode dark space and they would bombard the surface of the thin films. There were more and more F- and F* as the O2 gas flow rate increased so the surface roughness became less and less. Although there were more F* excited atoms in the plasma at 15sccm O2, the excessive gas caused an even greater decrease in the sputtering energy and a slight increase in the surface roughness.
Table 1. Surface roughness of AlF3 thin films produced at the same sputtering powers with 60sccm CF4 and different O2 flow rates.
3.4 High sputtering power
Figure 7 shows the transmittance spectra of AlF3 films prepared with different gas flow rates and power and the deposition rates of curves (a) and (b) were 0.208nm/s and 0.028nm/s, respectively, which have been confirmed by SEM and elliposometer. Although curves (a) and (b) show that both films have very small absorption, the deposition rate in curve (a) is much higher than that in curve (b). The transmittance results (curves (a) an (c)) indicate that the deposition of AlF3 films without O2 gas at a high sputtering power will cause large absorption. Figures 8 and 9 show the extinction coefficients and refractive indices of AlF3 thin films prepared with: (a) 60sccm CF4; (b) 60sccm CF4 mixed with 12sccm O2 at both 200W sputtering power. The extinction coefficients of curve (b) are all smaller than 6.8×10-4 at the wavelength range from of 190nm to 700nm. The results indicate that the introduction of the appropriate O2 flow rate to CF4 gas can decrease the amount of absorption from carbon atoms and provide enough F-, and F* to react with Al to form AlF3 even at high sputtering power.
Fig. 7. Transmittance spectra of AlF3 films prepared with (a) 60sccm CF4 mixed with 12sccm O2 at 200W (b) 60sccm CF4 mixed with 12sccm O2 at 30W sputtering power (c) 60sccm CF4 at 200W.
Fig. 8. Extinction coefficient of AlF3 thin films prepared with (a) 60sccm CF4 (b) 60sccm CF4 mixed with 12sccm O2 at both 200W sputtering power.
The refractive indices in curve (a) of Fig. 9 are all larger than those of curve (b). This may be because the Al particles did not fully react with the F* to form AlF3. Instead, the Al particles became an unstable compound (AlFx, x<3).
Fig. 9. Refractive indices of AlF3 thin films prepared with (a) 60sccm CF4 (b) 60sccm CF4 mixed with 12sccm O2 at both 200W sputtering power.
Fig. 10. Cross-sectional morphology of AlF3 thin films coated with 60sccm CF4 mixed with 12sccm O2 at 200 sputtering power.
Figure 10 shows the cross-sectional morphology of AlF3 thin films coated with 60sccm CF4 mixed with 12sccm O2 at 200W sputtering power. The columnar structure is not as obvious in the AlF3 thin films as in other fluorides [11–12] and the surface is smooth with a surface roughness of 0.8nm. The amorphous-like structural phenomenon might have occurred because the deposition of AlF3 on the substrate was only a few molecules or a molecule by molecule at a time, instead of in a bunch or a cluster of molecules as with other type of materials. From the results described above, it can be seen that with this novel process, not only can we successfully deposit AlF3 films with good optical properties at high sputtering power, but the method is workable to be applied for manufacture in the real optical industry.
4. Summary
AlF3 thin films have been deposited by magnetron sputtering of Al target with different O2 gas flow rates. Three different mechanisms that influenced the optical properties of the AlF3 films are observed as the O2 gas was introduced. First, up to 13% (O2/(CF4+O2)), the deposition rate increased and the refractive index and extinction coefficient decreased. Second, between 13% and 20% (O2/(CF4+O2)), which made for the lowest extinction coefficient but the deposition rate began to decrease. Third, introducing O2 gas at 20%, O2/(CF4+O2), we obtained the lowest refractive index, the thinnest thickness of AlF3 films, and the highest extinction coefficient. The results of the surface roughness of AlF3 films show that injecting more O2 gas (up to16%) increased the number of F- ions to bombard the AlF3 films so the surface roughness decreased. The best ratio (16%) was chosen to sputter the Al target at high power, 200W. The extinction coefficients of the thin film were all smaller than 6.8×10-4 from 190nm to 700nm, and the structure was amorphous-like with a surface roughness of 0.8nm. The most important point is that the deposition rate at 200W was 7.43 times faster than that at 30W. In comparison to deposition with only CF4 gas at 200W sputtering power, the extinction coefficient of the thin film improved from 4.4×10-3 to 6×10-4 at the wavelength of 193nm. All of the results indicate that we have found a workable process suitable for the application of manufacture in the real-world optical industry.
Acknowledgements
The authors thank the National Science Council of Taiwan for financial support of this research under Contract NSC95-2221-E-008-159 and NSC95-2221-E-008-156.
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