Improved Magnesium Fluoride Process by Ion-Assisted Deposition

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1 Improved Magnesium Fluoride Process by Ion-Assisted Deposition R.R. Willey, Willey Optical, Charlevoix, MI; and K. Patel and R. Kaneriya, Astro Optics Pvt. Ltd., Mumbai, India ABSTRACT When Ion Assisted Deposition (IAD) started to appear on the scene, there was great hope that could be done with IAD on room temperature substrates and also allow robust AR coating on plastic substrates. The use of IAD for on plastics is almost mandatory to get reasonable adhesion and hardness. A review of the history of attempts to use IAD with at low temperatures has shown limited success. has been and will continue to be an attractive and well used low index material. It has appeared that there is still an opportunity for study and improvement of deposition using IAD with respect to absorption, stress, and scattering. The goal of this work has been to produce, on a chamber-full production scale, dense and non-absorbing films on glass and plastic at temperatures below 100 C which are equal to or better than those deposited at C without IAD. The effort of this study has been to investigate the realm of IAD variables from ev and ion-atom-arrival rates (IAAR) well below the resputtering rates with argon or nitrogen gas, to the extent possible with a broad beam ion source of the End-Hall type. INTRODUCTION Magnesium fluoride (, index near 500 nm) has been used successfully for well over half a century. It has been the most common optical coating on glass as a quarter wave optical thickness (QWOT) layer antireflection coating. It can be hard and relatively insoluble. It transmits well from about 120 nm in the vacuum UV out to about 7µm the mid-infrared. Olsen and McBride [1] showed that a 2.75 mm thick single crystal of was clear from at least 200 nm out to about 6 µm and then started to increase in absorption toward longer wavelengths. At 10 µm, the transmittance fell to about 2%. A thin layer can be used as a top protective layer on coatings for the 8 to 12 µm region, although it has too much absorption for a thick layer in that region. There is currently much interest in as a material in vacuum ultraviolet for lithographic optics in the semiconductor industry. The hardness, durability, and density of without IAD is a strong function of the substrate temperature during deposition. At room temperature, the films can usually be wiped off, have high humidity shifts, have an index of about 1.32 in vacuum and a packing density of about 0.82 according to an extensive study by Ritter [2]. With a 300 C deposition temperature, Ritter shows that the packing density is about 0.98 and the index nearly The 300 C films will pass an eraser rub test and have a low humidity shift. is often evaporated from molybdenum or tantalum resistance boat sources where it melts to form a liquid and evaporates. The use of an E-gun with this and other fluorides is common, but can have some problems. Granular from an E-gun can easily cause explosive scattering of the material if the E-bean applies too much power density to the material. It is thought that the rush of the fluoride vapors from evaporated granules below the surface of the granule pile blows the solid granules above like dust. It is therefore necessary to use great care and slowly bring up the power on the E-gun. Also, fusing the top layer all over during pre-melt helps avoid spatter. It is also found that the material in the E-gun often becomes almost black. The black condition has also been seen with a boat when the rate control is set for low rates such as 0.2 nm/sec. It is our opinion that a boat is the best approach unless there are extenuating circumstances forcing the use of an E-gun. Without IAD, magnesium fluoride films tend to have a columnar structure as shown by Guenther [3], scattering, and high tensile mechanical stress as shown by Ennos [4]. HISTORY McNeil et al. [5] reported early IAD work with a gridded Kaufman ion source using oxygen gas on SiO 2 and TiO 2 films where they showed the effects of ion energy (ev) and ion current on the hardness and absorption of the films. They did not work with, but their results are generally applicable to this work. The report of Kennemore and Gibson [6] points to the likelihood that energies lower than the ev that they had used may be optimal for. Absorption inducing damage is thought by Tsou and Ho [7] and others to be preferential sputtering of the fluorine from the films. Figure 1 (in a form after Figure 18a in Cuomo, Rossnagel, and Kaufman [8]) shows that the conditions reported by Kennemore and Gibson might have been too far toward the resputtering realm. They reported 0.5 nm/sec., 150 ev, and 33 µa/cm 2. On this figure, the region of low ion energy and low IAAR are 2010 Society of Vacuum Coaters 505/ rd Annual Technical Conference Proceedings, Orlando, FL April 17 22, 2010 ISSN

2 found to have no effect, while the region of too high values for these variables will sputter away all of the film. The region desired for this work is where the values are high enough to densify the film but not to sputter it. concern for those working in the visible spectrum. However, the MgO is more soluble than the and is therefore not as environmentally stable and satisfactory. The MgO also raises the index while decreasing the UV transmittance. Targrove and Macleod [12] verified that momentum transfer is the dominant densifying mechanism by using Ne, Ar, and Xe for IAD. Note also that the atomic weight of oxygen is only 16, so it has less than half the momentum of argon when used for IAD. However, O 2 + (32) has been shown by McNeil et al. [5] to be the dominant species over O + by about 3:1, so that the most common bombarding O 2 + ion is more nearly like Ar (40). Allen et al. [13] obtained good results with dual ion beam sputtering (DIBS) of by adding a background gas of Freon (CF 4 ) at about 1x10-4 torr, which they describe as refluorinating the sputter target while the fluorine was being preferentially sputtered. They also found the least absorption was obtained when nitrogen was used as the sputtering gas, which they attribute to its lower atomic weight (14) as compared to Ar (40). Figure 1: Ion/atom arrival ratio versus ev for Kennemore and Gibson reports. Martin and Netterfield [9] reported on the use of IAD at 700eV with Ar, O 2, and H 2 O at ambient temperature. They reported high absorptance at 550 nm with Ar IAD, but none with O 2, and H 2 O. The implication here is that the IAD preferentially sputtered away the fluorine, but that oxygen filled the fluorine vacancies to make MgO where needed. Gibson and Kennemore [10] subsequently did more work where they used Freon 116 (C 2 F 4 ) to replace some of the lost fluorine. Their results were less successful than hoped for. They also did some work at 80 ev with Ar and with Freon. In Figure. 1, the small dots on the 80 ev line show the IAAR for some of their tests. They did conclude that low bombarding energies may prove a useful and less absorbing combination. They also describe how the MgO content causes the films to dissolve in a salt water bath after 48 hours. They review the availability of oxygen in the chamber from water vapor, and describe baking out the chamber before deposition to reduce H 2 O and oxygen so that the films have less opportunity to acquire oxygen. Martin et al. [11] did an extensive study with argon and oxygen IAD. They showed increased absorption in the VUV from nm for IAD films as compared to conventional evaporation at 440 C. Their IAD was done at ev. They showed that the additional absorption in the UV between 120 and 180 nm was probably due to MgO. This might be of no Kolbe et al. [14] used fluorine gas as the reactive species for several metal fluorides with good results. However, they did mention the corrosive effects of the fluorine on the ion source. Their work seems to be consistent with the concepts that low energy IAD is important, and that oxygen in the process will create a deposit which has some metal oxide and/or oxyfluoride that causes higher index and absorption at the short wavelength end of the spectrum. Robic et al. [15], in their Figure 11, show the decrease in extinction coefficient with decreasing anode voltage from 170eV to 60eV for the IAD of YF 3 using a Mark II ion source. This seems to be further evidence for the case of using lower ion energies for IAD with metal fluoride coatings. Single IBS bombards the target to be sputtered with high energy atoms (>1000eV). The pressures can generally be lower than with IAD. However, there is significant risk of high energy atoms reaching the substrate and disassociating the fluorine from the magnesium. Yoshida et al. [16] report on such work where fluorine gas is used to replenish the lost fluorine atoms in the metal fluoride coating and to avoid oxygen and water vapor which would cause MgO and absorption. Tsou and Ho [7] used an Advanced Plasma Source (APS) for IAD of with argon. They reported that absorption in the visible region occurred with IAD at discharge voltages exceeding 75 V, and that absorption was found to increase with increasing discharge current and voltage of the plasma. This would imply moving upward and to the right in Figure 1 where resputtering and preferential sputtering of fluorine would occur. 314

3 Baldwin et al. [17] report on work at 36eV to 72eV ( anode drive volts) argon IAD which supports the hypothesis that low energy IAD is key without absorption. They obtained robust films on plastic at low temperatures. Ristau et al. [18] compared the results of IBS with both boat and E-beam Physical Vapor Deposition (PVD) for. They found the index at 248 nm to be for the IBS films and higher by about.03 for the PVD films. It is conjectured that this is due to MgO and other contaminants in the PVD films. The IBS film showed an index of about at 500 nm. Coating stresses were compressive for IBS and tensile for PVD. Work at ev by Alvisi et al. [19] produced reasonable laser damage results, but none as good as non-iad at C. The evidence seems to point to IAD with energies between ev ( drive volts) should produce the desired results at low temperatures, but even there, it will be necessary to keep the IAAR below the resputtering rates to avoid absorbing films. Dumas et al. [20] used argon IAD and E-beam evaporated to compare UV absorption in the nm range with films deposited with no IAD. They modeled the effects of lost fluorine with oxygen and hydroxyl ion replacement. They used normalized momentum per the work of Targrove and Macleod [12] as the most salient variable of the process. It was shown that the stoichiometry dropped from over 1.95 F/Mg at low momentum to 1.7 above a certain threshold momentum. This again seems to confirm that the differential sputtering of fluorine is the major cause of absorption in IAD films of. EXPERIMENTS For ease of production of films for the visible spectrum in a typical 0.5 to 1.0 meter box coater, it is desirable to have a process that does not require fluorine replacement. Hard coatings without MgO are needed for environmental durability. A low process temperature (<100 C) is needed for the ability to coat plastic (and other temperature sensitive objects) and for reduced process time. It was the goal of this work to find the process parameters which meet these requirements using the available broad beam IAD source. The equipment setup used for this work was a Leybold L560 box coater with a Leybold Turbovac 1000 turbomolecular pump of 1000 liter/sec capacity. The starting vacuum was 1x10-5 torr. The ion source was a Kaufmann Robinson, Inc. (KRI ) model EH400 (end-hall). The distance from the KRI to the substrates was approximately 43 cm. The was evaporated from a tantalum boat at rates of nm/ sec. Argon gas was supplied at 4-8 sccm in the argon-only experiments which gave x10-4 torr deposition pressure. For nitrogen-only experiments, gas at 6-15 sccm gave x10-4 torr. An issue with most broad beam IAD sources is that large gas flows are needed to obtain low discharge voltages (50-100V). This increases the chamber pressure to the point that most of the ions and energetic atoms are nearly thermalized to the energy of the gas in the chamber before they reach the substrate, and therefore they have little energetic effect. At these higher pressures, the gas atoms are also competing with the depositing atoms at the surface which tends to cause the films to be porous and weak. It appears that, even at the lowest pressures and ion currents, very few IAD atoms reach the substrates without having multiple collisions with other vapor molecules/atoms. The mean free path (MFP) in centimeters is 5x10-3 divided by the pressure in torr. Therefore, at 1x10-4 torr, the MFP would be 50 cm., or approximately the source to substrate distance here. The percentage of ions which have not had a collision while travelling a distance X is: This means that, at a pressure of 1x10-4 torr, only 37% of the ions have reached the substrate 50 cm distant without a collision, and only 0.67% at a pressure of 5x10-4 torr. This implies that the ion flux reaching the substrate has had many collisions and lost much of its energy and momentum. The actual ev of the ions reaching the substrate is probably an order of magnitude below the drive voltage of the ion source, and therefore the observed effects are only relative with respect to what is shown in Figure 1. However, the concern of this work is with what control parameters produce the desired results. Therefore, knowledge of the actual details at the surface are of interest but not critical to the goals of this work. It is seen, from the history of the foregoing investigations, that: 1) the ion energies (ev) may need to be kept below some threshold, 2) the IAAR (momentum per depositing atom) need to be adequate but not too much, and 3) oxygen needs to be avoided in the deposited films. The ion energies depend on the gas flow and drive current of the KRI, and to some extent the deposition rate of the. The momentum imparted to the coating depends on the IAAR, the atomic mass of the IAD gas atoms, and the ion energy. The gas flow (pressure), drive current, and deposition rate are the independent variables of this process that need to be investigated with respect to the ability to get the desired densification and low absorption. As seen in Figure 1, the ultimate parameters of importance are IAAR and the energy of the arriving ions; these are derived from the results of controlling flow, current, and rate. 315

4 Nitrogen, because of its lower AMU of 14, is the most promising for the IAD gas. All but one case cited in the history above used argon with an AMU of 40, and/or oxygen with and effective AMU [5] more nearly 32. The momentum for oxygen (O 2+ ) is 90% that of Ar, and nitrogen is 60% that of Ar. Nitrogen is an inexpensive alternative to Ar and much less costly than neon which has an AMU of 20. Reports [13] indicate that nitrogen does not seem to become incorporated into the the way that oxygen does, even though it is not inert like Ar and Ne. Therefore, it is has been desirable to investigate the relative merits of Ar and N 2 IAD over the low ev energy ranges. DESIGN OF EXPERIMENTS The methodology for Design of Experiments (DOE), as detailed in Schmidt and Launsby [21], and DOEKISS Software [22] were used here for data processing. The choice of experimental parameters for flow rate, deposition rate, and drive current have been found by DOE methodology for both Ar and N 2. The key performance parameters measured were the film durability and absorption. Drive voltage and thereby drive power were recorded. Drive current and deposition rate allow the computation of a relative IAAR. Peak process temperature and process speed were also measured. Table 1 shows the data and results for the argon IAD tests, where the first four columns contain the independent variables, and rest are measured results and derived data. Table 2 shows the data and results for the nitrogen IAD tests. The preliminary results with the argon DOE did not seem promising; therefore, the emphasis was shifted to the nitrogen DOE. After the normal 15 DOE experiments for nitrogen, the results were used to guide additional tests to search for a hard and absorption free solution. RESULTS Normally the results, such as absorption and hardness, would be plotted versus the independent variables of rate, sccm, and drive current as shown in Figure 2. The checkered area here shows the region where less than 1% absorption can be expected when the deposition rate is 0.5 nm/second. These plots could also be made for other rates between 2 and 8 of the DOE. Historical findings make it more beneficial to plot the results in the derived variables of log-drive-voltage (Vd) (where the peak of ev is approximately 60% of Vd)) versus log-ion/atom arrival rate to be compared with Figure 1. The absorption (which is continuous with the variables) is plotted in Figures 3 and 4 on a contour plot versus Vd and IAAR. Since the hardness is not a continuous variable, but more of a pass/fail of the 50 strokes eraser-rub test, pass is indicated by an X and fail by an O where they occur in Figures 3 and 4. Figure 2: DOE plot of absorption at a deposition rate of 0.5 nm/s versus sccm and drive amps. Figure 3 shows the results of the absorption at 400 nm (rest of visible has less absorption) from the argon experiments with the hardness marks. It shows that the absorption is the least at low values of Vd and IAAR and is the most at high values. The shape of the curves are consistent with shape in Figure 1. This combination predicts that the best results which combine low absorption and high hardness, based on the experiments executed, would not be expected to have zero absorption, but would be likely to lie near the checkered area. It appears that the same process and levels of IAD which harden (densify) the films also sputter the fluorine and cause absorption. This still should provide a usable process for practical work since a QWOT for a visible antireflection coating would be ¼ as thick as those of these tests, and will have approximately ¼ the absorption. Figure 4 is similar to Figure 3 but for the nitrogen experiments. The region expected to yield less than 2% absorption and 40 or more strokes of hardness is upper middle part of the figure. The parameters which might provide a usable process are approximately: 0.3 nm/s, 10 sccm of nitrogen, 1.36 drive amps. These should result in about: 150 Vd (2.17 log Vd), 0.43 relative IAAR (-0.36 log IAAR), and a pressure of 3.3x10-4 torr. 316

5 Table 1: Argon IAD experimental details and results. Table 2: Nitrogen IAD experimental details and results. 317

REFERENCES 1. A.L. Olsen and W.R. McBride: Transmittance of Single- Crystal Magnesium Fluoride and IRTRAN-1 in the 0.2- to 15-µ Range, J. Opt. Soc. Am. 53, 1003-1005 (1963). 2. E.

4. A.E. Ennos: Stresses Developed in Optical Coatings, Appl. Opt. 5, 51-61 (1966). Figure 3: Contour plot of % Absorption versus drive voltage and ion/atom arrival rate for argon IAD. 5. J.R.

6 REFERENCES 1. A.L. Olsen and W.R. McBride: Transmittance of Single- Crystal Magnesium Fluoride and IRTRAN-1 in the 0.2- to 15-µ Range, J. Opt. Soc. Am. 53, (1963). 2. E. Ritter: Optical film materials and their applications, Appl. Opt. 15, (1976). 3. K.H. Guenther: Growth structures in a thick vapor deposited multiple layer coating, Appl. Opt. 26, (1987). 4. A.E. Ennos: Stresses Developed in Optical Coatings, Appl. Opt. 5, (1966). Figure 3: Contour plot of % Absorption versus drive voltage and ion/atom arrival rate for argon IAD. 5. J.R. McNeil, A.C. Barron, S.R. Wilson, and W.C. Herrmann, Jr.: Ion-assited deposition of optical thin films: low energy vs high energy bombardment, Appl. Opt. 23, (1984). 6. C.M. Kennemore and U.J. Gibson: Ion beam processing for coating onto ambient temperature substrates, Appl. Opt. 23, (1984). 7. Y. Tsou and F.C. Ho, Optical properties of hafnia and coevaporated hafnia:magnesium fluoride thin films, Appl. Opt. 35, (1996). 8. J.J. Cuomo, S.M. Rossnagel, and H.R. Kaufman, Handbook of Ion Beam Processing Technology, Noyes Publications, New Jersey, P.J. Martin and R.P. Netterfield: Ion-assisted deposition of magnesium fluoride films on substrates at ambient temperatures, Appl. Opt. 24, (1985). Figure 4: Contour plot of %Absorption versus drive voltage and ion/atom arrival rate for nitrogen IAD. CONCLUSIONS It does not appear that there are fully abrasion resistant and absorption-free solutions for the IAD of with argon or nitrogen at room temperature. It appears that IAD conditions which provide sufficiently densified films to be hard enough will also selectively sputter some fluorine and cause some absorption. However, it does appear that there are practical room temperature solutions which will be hard enough and will have low enough absorption to be tolerable in most applications. 10. U.J. Gibson and C.M. Kennemore III: Ambient temperature deposition of MgF2 with noble and fluorocarbon ion assistance, SPIE 678, (1986). 11. P.J. Martin, W.G. Sainty, R.P. Netterfield, D.R. McKenzie, D.J.H. Cockayne, S.H. Sie, O.R. Wood, and H.G. Craighead, Influence of ion assistance on the optical properties of, Appl. Opt. 26, (1987). 12. J.D. Targrove and H.A. Macleod, Verification of momentum transfer as the dominant densifying mechanism in ion-assisted deposition, Appl. Opt. 27, (1988). 13. T.H. Allen, J.P. Lehan, and L.C. McIntyre, Jr., Ion beam sputtered metal fluorides, SPIE 1323, (1990). 318

7 14. J. Kolbe, H. Schink, and D. Ristau, Optical Losses of Fluoride Coatings for UV/VUV Applications Deposited by Reactive IAD and IBS Processes, Proc. Soc. Vac. Coaters 36, (1993). 15. J.Y. Robic, V, Muffato, P. Chaton, M. Ida, and M. Berger, Optical and structural properties of YF3 thin films prepared by ion assisted deposition or ion beam sputtering techniques, SPIE 2253, (1994). 16. T. Yoshida, K. Nishimoto, K. Sekine, and K. Etoh, Fluoride antireflection coatings for deep ultraviolet optics deposited by ion-beam sputtering, Appl. Opt. 35, (1996). 17. D.A. Baldwin, J.B. Ramsey and J.E. Miles: Optical Films: Ion-Beam-Assisted Deposition of Magnesium Fluoride in a Conventional Electron Beam Evaporator and the Resulting Film Properties, 40th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, pp , D. Ristau, W. Arens, S. Bosch, A. Duparré, E. Masetti, D. Jacob, G. Kiriakidis, F. Peiró, E. Quesnel, and A. Tikhonravov: UV-Optical and Microstructural Properties of -Coatings Deposited by IBS and PVD Processes, SPIE 3738, (1999). 19. M. Alvisi, F. De Tomasi, A. Della Patria, M. Di Giulio, E. Masetti, M.R. Perrone, M.L. Protopapa, and A. Tepore, Ion assistance effects on electron beam deposited films, J. Vac. Sci. Technol. A 20(3), (2002). 20. L. Dumas, E. Quesnel, F. Pierre, and F. Bertin, Optical properties of magnesium fluoride thin films produced by argon ion-beam assisted deposition, J. Vac. Sci. Technol. A. 20, (2002). 21. S.R. Schmidt and R.G. Launsby, Understanding Industrial Designed Experiments, (Air Academy Press, Colorado Springs, 1994 Edition). 22. DOEKISS Software, Six Sigma Products Group, 1650 Telstar Drive, Suite 110, Colorado Springs, CO 80920, USA (1997). 319

PROCEEDINGS OF SPIE. Stable, durable, low-absorbing, lowscattering. without heat or added fluorine

PROCEEDINGS OF SPIE. Stable, durable, low-absorbing, lowscattering. without heat or added fluorine PROCEEDINGS OF SPIE SPIEDigitalLibrary.org/conference-proceedings-of-spie Stable, durable, low-absorbing, lowscattering MgF2 films without heat or added fluorine Ronald R. Willey, Reza Shakoury