PROCEEDINGS OF SPIE · SPIE 10691, Advances in Optical Thin Films VI, 106910C (5 June 2018); doi: ... They conclude that "The experimental results reported in this article have shown

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Stable, durable, low-absorbing, low-scattering MgF<sub>2</sub> filmswithout heat or added fluorine

Ronald R. Willey, Reza Shakoury

Ronald R. Willey, Reza Shakoury, "Stable, durable, low-absorbing, low-scattering MgF<sub>2</sub> films without heat or added fluorine," Proc.SPIE 10691, Advances in Optical Thin Films VI, 106910C (5 June 2018); doi:10.1117/12.2309812

Event: SPIE Optical Systems Design, 2018, Frankfurt, Germany

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Stable, durable, low-absorbing, low-scattering MgF2 films without heat or added fluorine

RONALD R. WILLEYa,* and R. SHAKOURYb

aWilley Optical, Consultants, 13039 Cedar Street, Charlevoix, Michigan 49720, USA bDepartment of Physics, Faculty of Science, Imam Khomeini International University, Qazvin, Iran

ABSTRACT

The goal achieved by this work has been to produce stable, durable, low-absorbing, low-scattering magnesium fluoride (MgF2) films without additional heat or fluorine. This was done with IAD in a chamber that could do production on a commercial basis. Stability with respect to humidity and temperature shifts and durability with respect to abrasion resistance depend on the density of the films. Absorption depends on the stoichiometry of the films. Scattering partially depends on stress cracking due to mismatch of the coefficients of thermal expansions of the substrates and the coatings. In recent decades, MgF2 has become a preferred material in the DUV to wavelengths as short as 180 nm. It has been shown that MgF2 often suffers from a fluorine deficiency when deposited by energetic processes. It has been demonstrated that much of the resulting absorption can be eliminated by ultraviolet annealing which provides enough energy to give mobility to fluorine atoms to reunite with nearby magnesium atoms. This implies that the fluorine deficiency is due to dislocations of the fluorine atoms, but that they are not all lost from the matrix of the film and pumped away by the vacuum process. It is believed that energetic ions from sputtering or IAD sources cause the dissociation of the F atoms from the Mg if their energy level exceeds some threshold. Present results provided "fully" dense films for hardness, low temperature/humidity shifts, and low absorption and scattering by using IAD at 160 eV or less and no added fluorine.

Keywords: magnesium fluoride, ion-assisted-deposition, no process heat, no fluorine, fluorine deficiency, low energy, low absorption, low scattering

1. INTRODUCTION

Historically, magnesium fluoride (MgF2) has been a preferred low index material for antireflection (AR) coatings and other more complex designs in the visible and near infrared spectrum. In recent decades, MgF2 has been a preferred material in the deep ultraviolet (DUV) to wavelengths as short as 180 nm. It has been known for over half a century that MgF2 films, when deposited on substrates at room temperature, can be easily removed by mild abrasion; however, it can be highly abrasion resistant when deposited at 250-300°C. Dumas, et al. [1] found the mass densities ranged from 0.77 to 0.97 for films deposited without heat versus those deposited at 300°C. These findings were further supported by infrared spectra in the 3 and 6 µm region, showing atmospheric water absorption bands in the porous films deposited at low temperatures and not in those deposited at 300°C. Similar results had been reported by Ritter[2] and Guenther[3]. Dumas, et al. [1] also showed a slight fluorine deficiency and high tensile residual stresses, due to the contribution of the thermal stress arising primarily from the mismatch of thermal expansion coefficients between film and substrate, as also mentioned by Ristau, et al [4]. The Dumas group suggested that a "way to obtain dense and low-stress MgF2 thin films would be to deposit the material at low substrate temperature under ion assistance." That is one of the objectives of the work reported here.

The effects of fluorine deficiency have been the subject of many papers such as references [4-12]. Quesnel, et al.[5] showed that color centers (F-centers) were a primary cause of absorption in the ultraviolet (UV) and deep UV (DUV). These color centers are one or more electrons where the fluorine atoms should have been in the MgF2 molecule. They

* [email protected] phone 1-231-237-9392

Advances in Optical Thin Films VI, edited by Michel Lequime, H. Angus Macleod, Detlev Ristau, Proc. of SPIE Vol. 10691, 106910C · © 2018 SPIE

CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2309812

Proc. of SPIE Vol. 10691 106910C-1


showed that these defects could be annealed by UV light from a mercury lamp, and the absorption virtually eliminated in a few hours or less. The implication of this is that the fluorine atoms (F, not to be confused with F-centers) were not missing from the film matrix, but only dislocated. They suggest that "This fluorine could come from F atoms positioned in interstitial sites and/or at grain boundaries."

Dumas, et al. [6] later went on to discuss the role of magnesium atoms in forming colloids that contributed to

absorption and the probable role of oxygen and/or hydroxyls in producing MgO which cause more absorption in the DUV near 300 nm. They state that "The absorption setup of MgF2 deposited by argon IBAD is thus complicated and appears to result from a combined effect of fluorine deficiency and oxygen incorporation into the material deposited."

This absorption reduction is another major goal of the present work. The work of the Dumas group is focused in the

UV, whereas the present report is concerned only with the visible spectral region where the longer wavelength "tails" of their absorption bands near 300 nm still have an influence.

Alvisi, et al. [7] report similar results to the Dumas group in their work to find favorable laser induced damage

thresholds (LIDT). They state "However, the ion beam energy should be lower than 250 eV to avoid the decomposition of MgF2 resulting in high absorbing films." They conclude that "The experimental results reported in this article have shown that assisting ion beams of low current (0.2 A) obtained at low anode voltages (VA=100 V) allow getting films of good packing density, of low k values and therefore of high laser damage fluence." Liu, et al. [8] studied LIDT via the deposition of MgF2 from molybdenum boats without ion assisted deposition (IAD) and concluded that the films are best at 300 °C and at a deposition rate of 0.05 nm/s.

Jaing, et al. [9] performed IAD with argon at 300 eV and obtained absorbing films. This is consistent with and

contrary to the work of the Alvisi group wherein lower anode voltages were needed to reduce absorption. The Jaing group found smaller grain structure with IAD, implying that the IAD somewhat disrupted the columnar crystal growth.

Bischoff, et al. [10] reported on the IAD of MgF2, AlF3, and LaF3 using the Leybold LION ion beam source, but the

anode voltage was not reported. They used both metal boats and electron beam evaporation and found both acceptable. However, molybdenum boats were found to have a problem in that "the molybdenum reacts with the layer material inside the heated boat to form MoFx" which evaporates and contributes to absorption in the films. They also discuss the UV annealing to remove absorption which takes 3-10 hours. The Bischoff group makes the statement that "To compensate the loss of the fluorine by sputtering, it is inevitable to use fluorine as a reactive gas for the ion assistance process." This might be correct in the case of sputtering, but it is one of the goals of the present work to achieve films with low absorption at least in the visible spectrum without any additional fluorine.

Wilbrandt, et al. [11] discuss the details of optical monitoring when the in situ optical properties of the films (fluorine

deficiency) are quite different than the ex situ values after UV annealing. They found the colloid or embedded metal cluster model to best suit their needs.

Sun, et al. [12] showed the effects of water vapor and oxygen on MgF2 films deposited by e-beam evaporation

without IAD at various temperatures. The oxygen could reduce the color centers to produce MgO and reduce the absorption from the color centers, but the MgO added to the absorption. At 300°C, the oxygen effects tend to disappear.

Mertin, et al. [13] deposited MgF2 by reactive pulsed DC magnetron sputtering from a metallic magnesium target

with a mixture of argon, oxygen, and carbon tetrafluoride (CF4). The CF4 provides the needed fluorine and the oxygen removes the carbon as CO2 in the reactions. By careful tuning of the process, residual carbon and oxygen can be reduced to negligible levels. This process appears to have the potential to produce large area coatings of high quality.

Preliminary work has been done by Hennessy, et al. [14] to produce films of MgF2 by atomic layer deposition (ALD)

at 100°C. This shows promise as a conformal coating and possibly impervious layers. The index and absorption reported seem to equal that of conventional physical vapor deposition (PVD) at 300°C.

It is apparent from all of the foregoing that to produce dense, low absorbing, low stress MgF2 films, more energy is

needed than that provided by normal evaporation (<0.2 eV) onto substrates at ambient temperature. It is also apparent



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that sputtering and some conditions of IAD cause dissociation of the magnesium from the fluorine, which causes absorption. The large difference in the coefficients of thermal expansion (CTE) between the layers and the substrates will typically cause high stress, when the process is done at high temperature. This stress often causes cracking, which in turn causes scattering losses (which can be as great as absorption losses).

It is hypothesized here that absorption could be avoided by applying IAD at lower ion energies than commonly used,

to obviate the need for additional fluorine in the process. It is thought that such lower energy (eV) ions would not break the bonds between the Mg and the F atoms, and thereby avoid the loss or dislocation of the fluorine.

It is evident that coatings deposited with some kind of energetic ion bombardment can show higher refractive indices

due to increased film density than non-IAD evaporated samples. Cuomo, Rossnagel, and Kaufman[15] illustrated the effects of ions on film growth in charts such as Fig. 1. The ion-to-atom-arrival rate (IAAR) is the rate of arrival of ions at the depositing surface to the atoms of deposition material arriving at that same surface area. This is only a relative measurement since the film is also bombarded by energetic neutrals which are not readily measured. In areas of Fig. 1 where the ion energy (eV) is low and the IAAR is low, there is no effect. In areas where the ion energy (eV) is too high and the IAAR is too high, sputtering of the depositing materials and possibly even the substrate will occur. The point of maximum densification before some sputtering occurs of the depositing film will be near the 100% curve in the figure, and that is one of the goals of this work. However, this may be compromised and reduced in some cases to reduce compressive stress in the coating.

The IAAR in the present work was a key parameter to optimize. It was varied to find its optimum by the change of

the ion source drive current (Anode Drive Amps in Table 1) which provides a proportionate variation of the ion arrival rate at the substrate. The atom arrival rate at the substrate of depositing material was held to a given value by a constant evaporation rate of the MgF2; therefore, the IAAR is simply proportional to the Anode Drive Amps.

Fig. 1. Chart after the form of Cuomo, Rossnagel, and Kaufman[15] showing the effects on film growth as a function of ion

energy and ion/atom arrival ratio. Momentum transfer from the accelerated gas ions or atoms to the atoms forming the layer is the effective factor in

increasing in packing density according to Macleod and Targrove[16]. They used Ne of atomic mass (amu) 20.18, Ar of 39.95 amu, and Xe of 131.29 amu for their experiments in order to compare the effects of momentum in IAD. The momentum transfer efficiency can be increased by appropriate choice of the working gas. Momentum transfer is maximum when masses of colliding particles are equal. Therefore it seems that argon would be better than nitrogen



(14.01 amu) to get good momentum transfer. However, results with argon gas were found in our earlier work to be not as good as those with nitrogen which provided the least absorption[17]. This is thought to be because the lower amu of the nitrogen atom causes less dissociation of the magnesium from the fluorine. Nitrogen is also much less expensive than neon. Oxygen (16.0 amu) would also have a lower mass than argon, but is not considered here because it would produce MgO which is undesirable due to its increased solubility and ultraviolet absorption.

The ion energy as seen in Fig. 1 is proportional to the ion source drive voltage, as displayed in Figs. 5 to 9. Figure 3

in Sec. 3 below shows more detail on this relationship. With this class of ion sources, the drive voltage is a function of the gas flow through the ion source and the chamber pumping speed. However, with the Veeco Mark-II (MK-II) ion source used here, the voltage is set in the controller, and the controller automatically adjusts the gas flow to provide that set voltage.

The goals of this work have been to produce hard, low absorbing, and low scattering MgF2 films on glass (or plastic)

at low temperatures using IAD in a typical "box coater" as is commonly used for production in the optical coating industry. The effects of the anode voltage and current of a MK-II ion/plasma source, when performing IAD with nitrogen gas on magnesium fluoride films, have been quantified with respect to refractive index, hardness, absorptance, and scattering.

2. EXPERIMENTAL DETAILS

All of the MgF2 single layers were deposited on BK7 glass substrates. The evaporation source was an E-gun at 10 KV, 5 KW, and 270 degree bent-beam (EV M-5, Ferrofluidics, GmbH, Germany). The vacuum system consisted of a turbomolecular pump (F-400/3500, KYKY, China) that was backed with a rotary pump (RVP-24, KYKY, China). The diameter of the chamber is 90 cm and its height is 110 cm. The distance from the source to the substrate was 60 cm.

The pumping speed for the system for nitrogen was approximately 2500 liters/second. The base pressure of the

system was evacuated to less than 1×10-5 Torr. Substrates could be heated by quartz heaters to reach 300°C and the temperature could be held constant during deposition by a control system, but this heat was not used on the IAD runs. The E-gun includes sweep pattern capability. Magnesium fluoride granules were placed in an alumina liner in the water cooled copper crucible of the E-gun. The ion source was used at ambient temperature for all IAD depositions of the MgF2 films. During the course of the deposition, the chamber temperature increased from ambient up to 80° C due to thermal energy from the E-gun and the ion source. The deposition rates were 5Å/s for all of the samples. In order to investigate the influence of the ion deposition parameters on the optical properties, the MgF2 films were prepared with different ion drive voltage, drive current, and neutralizer current as seen in Table 1. Nitrogen gas was used through the ion source.

Table 1. Parameters used for the experiments done for the DOE.

D.O.E ANODE ANODE NITROGEN CHAMBER NEUTRAL. TEST DRIVE DRIVE GAS FLOW PRESSURE FILAMENT

NUMBER VOLTS AMPS SCCM mTORR mAMPS 1 92 0.80 7.8 0.038 181 2 80 1.51 12.3 0.053 352 3 90 2.20 14.3 0.060 441 4 120 0.50 5.7 0.037 91 5 120 1.50 10.2 0.047 332 6 120 2.50 14.8 0.053 499 7 148 0.80 6.7 0.035 160 A 160 0.80 6.2 0.042 164 B 180 0.81 7.3 0.039 150 C 160 1.20 9.1 0.048 234

250°C 0 0.00 0.0 0.005 0 The transmittance and scattering of samples was measured by a Cary 6000 spectrometer with an integrating sphere

attachment. In order to perform useful measurements on scattering samples, it was necessary to collect a high proportion



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of the scattered radiation. The integrating sphere was the collector of scattered radiation as shown in Fig. 2. The optical constants (refractive indices n and k) were extracted from transmittance data using the index fitting procedures in the FilmStar optical thin film evaluation and design software [18].

Fig. 2. Integrating Sphere configurations used in these scattering measurements

3. RESULTS

The independent deposition parameters were the drive voltage (ion beam energy), drive current (ion beam flux density), neutralizer current percentage, and deposition rate (which was not varied). Nitrogen flow (in SCCM) and working pressure depend on drive voltage, drive current, and neutralizer current percentage, and are controlled by the MK-II system.

Zhurin[19] has shown that the cathode (neutralizer) current has a significant effect on the voltage (eV) distribution of

the ions in the plasma. Figure 3 is adapted from Zhurin’s work and shows the effects of doubling the cathode output of an End-Hall source such as the MK-II with the drive volts at 90V. Under the minimum neutralization current condition to avoid sparking on the substrates, which had been our common practice, there seems to be a low voltage secondary peak. When more cathode emission current is available, this low voltage peak tends to disappear, and the chamber pressure to produce a given ion voltage is also reduced. This reduced voltage was investigated further by Willey, et al.[20] and has been applied in this work by using ~20% neutralizer current instead of the usual ~10%.

Fig. 3. Energy analysis for End-Hall source with argon and Vd = 90V, discharge current Id = 5A and cathode current Iem = 5A

and 10A .



Design Of Experiments (DOE) methodology was used to plan the tests to be performed and to process and plot the resulting data. The software used was DOE PRO XL 2010[21]. The data in Table 1 are not all those generated by the experiments, but just for a brief representation of the work.

Ten (10) representative deposition runs of MgF2 using IAD, and one at 250°C without IAD with parameters as shown

in Table 1, were performed and measured for four different results: refractive indices n and k, scattering, and hardness, as shown in Table 2.

Table 2. Results at 550 nm of the experiments done in Table 1.

D.O.E. INDEX INDEX PERCENT HARDNESS

ROW n-VALUE

k×10-3

550 nm SCATTER SCORE

NUMBER ±0.005 ±0.005 ±0.01 - 1 1.3463 0.11 0.00 4 2 1.3136 0.60 0.05 8 3 1.3681 3.18 0.18 8 4 1.3485 0.00 0.04 7 5 1.3806 2.48 0.22 5 6 1.4037 2.62 0.35 7 7 1.3843 0.30 0.10 1 A 1.3919 0.18 0.10 1 B 1.3757 3.81 0.22 1 C 1.3786 0.64 0.26 4

250°C 1.3750 0.06 0.33 1 The hardness was measured on a scale of 1 to 10 as shown in Table 3, where 1 is the hardest and represents surviving

50 strokes of the common eraser rub test.

Table 3. Quantitative scale used in the evaluation of the hardness of the coatings. SCORE NUMBER

RESULTS DESCRIPTION

1 No scratches with 50 strokes severe eraser Mil test (2 lbs) 2 No scratches with 40 strokes severe eraser Mil test (2 lbs) 3 No scratches with 20 strokes severe eraser Mil test (2 lbs) 4 No scratches with 5 strokes severe eraser Mil test (2 lbs) 5 No scratches with 40 strokes Moderate eraser Mil test (1 lb) 6 No scratches with 20 strokes Moderate eraser Mil test (1 lb) 7 No scratches with 5 strokes Moderate eraser Mil test (1 lb) 8 Scratched with dry cotton wipe 10 strokes 9 Scratched with dry cotton wipe 5 strokes 10 Coating wipes out with acetone + cotton wipe (peel off)

Figure 4 compares three example spectra as processed using FilmStar software to determine n and k. The measured

spectra are transferred from the spectrophotometer in a comma separated variable (CSV) file to a "Spectrum" file in FilmStar. Those transmittance values versus wavelength are set as design targets. An estimated thickness is entered into FilmStar and a dispersion formula is selected. In these cases, simplified Cauchey fitting equations with absorption were used, $QUADK, which are: n = A + B/wavelength2 and k = C + D/wavelength2. FilmStar then optimizes these A, B, C, and D coefficients and the thickness to give the best fit to the target spectra. These fitting results are plotted as smooth curves in Fig. 4, whereas the measured curves show the noise in the measured data, particularly in the long wavelength end of the B curves.

It can be seen that the measured and fit curves diverge in the C case at the short wavelength end. This is because the

$QUADK function is not a perfect model for the real k-dispersion over the spectral region from 400 to 435 nm. Tikhonravov, et al.[22] reported a better model, but such was not required for the purposes of these experiments.



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Fig. 4. Example spectra of IAD MgF2: (A) (from Table 1) at 160V and 0.8A having 583.9±2 nm thickness, (B) at 180V and 0.81A having 421.9±2 nm thickness, and (C) at 160V and 1.2A having 455.1±2 nm thickness. Over-plotted are the measured spectra and the best curve fit spectra using the $QUADK function from FilmStar. Case C is the only one where the measured and fit diverge noticeably in the 400-435 nm range.

Figures 5 through 8 show plots of the best fits of the measured data from Table 2 to give results and predictions for k-values, n-

values, % scattering, and hardness using DOE software. The n-value of 1.40347 in Row number 6 in Table 2 is thought to be outside of the error estimates due to that particular sample

being thinner than the others by approximately a factor of two. This adds uncertainty to the n and k-values. Dodge[23] showed that the bulk value of MgF2 at 560 nm was 1.37829 nO and 1.39013 nE.

Fig. 5. Index of refraction n-value at 550 nm fit to the measured results of the DOE versus ion source drive voltage and drive

current. The “*” is the point for experiment “A” where the actual value was 1.3919, which is well above the 1.36 minimum goal.



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Fig. 7. Percent scattering value around 550 nm fit to the measured results of the DOE versus ion source drive voltage and drive current. The “*” is the point for experiment “A” where the actual value was 0.11%, just above the 0.10% maximum goal.



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The non-shaded areas in Figs. 5 through 8 satisfy the goals of: a) <1.0×10-3 k (absorption at 550 nm), b) n>1.36, c) scattering <0.1%, and d) hardness <2. When the “bird’s-eye views” of each of these four figures are superimposed on top of each other, Fig. 9 is produced. The area with a large “X” in the lower center satisfies the n, k, and scattering goals, but does not quite meet the hardness goal. The area with a large “Y” in the lower left satisfies the n, hardness, and scattering goals, but does not quite meet the k goal. These curves are all based on the statistical fit to the actual measured data. However, the best of the actual measured data was found to be “Row A” in Tables 1 and 2 at the position shown by the asterisk (*) on each of Figs. 5 through 8, which actually satisfies each of the four goals.

Fig. 8. Hardness score of the measured results of the DOE versus ion source drive voltage and drive current. The “*” is the point

for experiment “A” where the actual value was equal to 1 (the best). As seen in the introduction, it is the conclusion of many authors in the past that absorption in MgF2 can be caused by

dissociation of the magnesium atoms from the fluorine atoms due to excess energy of the ions used in the IAD. Whereas, from our experience, TiO2 seems to absorb at over 200 eV and SiO2 at over 600 eV, MgF2 seems to have problems at drive voltages even as great as 200 eV. The results of these current experiments are consistent with this conjecture, since test runs A and C in Table 1 at 160 drive volts have low absorption but run B at 180 drive volts has much higher absorption. The authors think that even lower ion voltages can produce absorption-free MgF2 films without adding fluorine to the process.

4. CONCLUSIONS

It has been found that satisfactory magnesium fluoride coatings which have an index of refraction of over 1.39 at 550

nm (and thereby have higher density) without significant absorption, scattering, and humidity shift and which pass the common 50-stroke eraser-rub hardness/abrasion test can be deposited on glass (and probably polymer) substrates with ion assisted deposition and without process heat or additional fluorine in the process. The scattering appears to be primarily due to cracking from the mismatch of the CTE of the substrate to the MgF2. The suitable parameters to use with the Veeco Mark-II ion source were found using Design Of Experiments Methodology in a chamber with a pumping speed of approximately 2500 liters/second for nitrogen.



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point for experiment “A” where the actual values satisfy all of the goals. See text for further details.

ACKNOWLEDGEMENTS

The authors would like to thank H. Haydari for the technical contributions and F. Ansari for spectral measurements. REFERENCES

[1] L. Dumas, E. Quesnel, J.-Y. Robic, and Y. Pauleau, "Characterization of magnesium fluoride thin films deposited

by direct electron beam evaporation," J. Vac. Sci. Technol. A, 18(2), 465-469 (2000). [2] E. Ritter, "Optical film materials and their applications," Appl. Opt. 15, 2318-2327 (1976). [3] K. H. Guenther, “Growth structures in a thick vapor deposited MgF2 multiple layer coating," Appl. Opt. 26, 188-190

(1987). [4] D. Ristau, S. Günster, S. Bosch, A. Duparré, E. Masetti, J. Ferré-Borrull, G. Kiriakidis, F. Peirò, E. Quesnel, and A.

Tikhonravov, “Ultraviolet optical and microstructural properties of MgF2 and LaF3 coatings deposited by ion-beam sputtering and boat and electron-beam evaporation,” Appl. Opt. 41(16), 3196–3204 (2002).



[5] E. Quesnel, L. Dumas, D. Jacob, and F. Peiro, “Optical and microstructural properties of MgF2 UV coatings grown by ion beam sputtering process,” J. Vac. Sci. Technol. A 18(6), 2869–2876 (2000).

[6] 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(1), 102-105 (2002).

[7] 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), 714-720 (2002).

[8] Ming-Chung Liu, Cheng-Chung Lee, Masaaki Kaneko, Kazuhide Nakahira, and Yuuichi Takano, "Microstructure of magnesium fluoride films deposited by boat evaporation at 193 nm," Appl. Opt. 45(28), 7319-7324 (2006).

[9] Jaing, C.-C., Shiao, M.-H., Lee, C.-C., Lu, C.-J., Liu, M.-C., Lee, C.-H., Chen, H.-C., "Effects of ion assist and substrate temperature on the optical properties and microstructure of MgF2 films produced by e-beam evaporation," Proc. SPIE 5870, 1-6 (2005).

[10] M. Bischoff, M. Sode, D. Gäbler, H. Bernitzki, C. Zaczek, N. Kaiser, and A. Tünnermann, “Metal fluoride coatings prepared by ion-assisted deposition,” Proc. SPIE 7101, 71010L-1 - 71010L-10 (2008).

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PROCEEDINGS OF SPIE · SPIE 10691, Advances in Optical Thin Films VI, 106910C (5 June 2018); doi: ... They conclude that "The experimental results reported in this article have shown