Ion beam-assisted deposition of MgF2 and YbF3 ®lms M. Kennedya,*, D. Ristaua, H.S. Niederwaldb aLaser Zentrum Hannover e.V., Hollerithallee 8, 30419Hannover, Germany bCarl Zeiss, Optical Technology Centre, 73446Oberkochen, Germany Received 12 November 1997; accepted 24 April 1998 Abstract For ion-assisted deposition of thin MgF2 and YbF3 ®lms a gridless end-Hall ion source has been used. The effects of ion energy, ion current and different working gases on the optical and mechanical properties of the single layers, deposited at ambient temperatures, have been investigated. For both materials, IAD with xenon proved to be superior to argon and argon±oxygen mixture, respectively. Compared to hot conventional ®lms, the refractive indices of single layers deposited with optimized parameters could be increased. No signi®cant absorption was observed in the vis spectral range for these layers, in the UV spectral range low absorption for IAD-coatings was achieved. With respect to the mechanical properties, the optimized IAD-MgF2-®lms were comparable to conventional coatings, and the durability of YbF3-®lms could be clearly enhanced by the applied IAD-process. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Optical coatings; Fluorides; Ion bombardment; Optical properties 1. Introduction Ion-assisted deposition is known as a deposition techni- que with a positive effect on the optical properties of thin ®lms [1]. Extensive research activities especially on oxide materials are documented in the open literature [2±4]. For the majority of oxide materials commonly used in optical thin ®lm technology, it is possible to deposit ®lms with high densities, high refractive indices, low extinction coef®cients and good mechanical properties even at ambient tempera- tures. As a consequence of the increased packing densities of IAD-®lms, the vacuum-to-air and temperature shifts are reduced in these coatings. On the basis of these progresses, IAD-processes for oxide materials are implemented in special commercial applications in laser technology and coating of substrates sensitive to high temperatures. With the exception of MgF2, [3,5±7] as the most promi- nent ¯uoride material, only a limited number of studies are published on ¯uoride-materials. In contrast to oxide materi- als, ¯uoride materials are more susceptible to stoichiometric degradation caused by extensive ion bombardment. The major problem is the preferential sputtering of the ¯uoride atoms resulting in a stoichiometric de®ciency, which can not be completely eliminated by reactive gases in the deposition process [5, 8±10]. In the beginning of IAD- research work, most ion sources were of the Kaufman- type [1], which is limited by the speci®cally high ion ener- gies. In recent years, modern ion source designs with reduced ion energy and improved ion current densities were employed increasingly [3,4,8,11]. In principle, these ion sources provide reduced preferential sputtering and, therefore, offer advantages for the deposition of ¯uoride materials. Besides MgF2, YbF3 is an interesting ¯uoride material, which has been investigated for the production of high power laser coatings in the FIR-spectral range [12]. In the argon range, only limited data about this ®lm material is available today, the most comprehensive survey can be found in [13] and references therein. The major targets of the present investigation are concen- trated on the ion assisted deposition of MgF2 and YbF3-®lms at ambient temperatures with high refractive indices, low absorption in the vis, reduced inhomogeneities and superior mechanical properties. 2. Experimental Coatings were deposited in a cryo-pumped standard Leybold A700 boxcoater (base-pressure below 1023 Pa) at Zeiss optical technology centre in Oberkochen. MgF2 and YbF3 (Merck, Patinal quality) were evaporated from molyb- denum-boats, typical evaporation rates were 0.7±1 nm/s, ®lm-thickness was approximately 250 nm. The deposition rate and the ®lm thickness were controlled by a quartz crys- tal in situ. For ion-assisted deposition, an end-Hall ion Thin Solid Films 333 (1998) 191±195 0040-6090/98/$ - see front matter q 1998 Elsevier Science S.A. All rights reserved. PII S0040-6090(98)00847-5 * Corresponding author. Fax: 149 511 2788100; e-mail:
[email protected]. source (MarkII, Commonwealth Scienti®c Corp.) was employed. As working gases argon with and without oxygen back®ll as well as xenon were applied, typical process-pres- sures were 1:4±2:8 £ 1022 Pa. According to other investiga- tions in the operating parameters of this broad-beam ion- source, the median kinetic ion energy can be approximated by 60% of the anode voltage and the generated ion current can be expressed as a fraction of 20% of the anode current [7]. The ion-bombardment caused no signi®cant tempera- ture increase during the process (DT , 308C). The ®lms were deposited on Quartz and crown glass (BK7, Schott) substrates, the latter were used for the eraser testing. For the determination of the optical properties, transmit- tance and re¯ectance measurements were performed with a Lambda9 (Perkin±Elmer) spectrometer. The refractive indices and the extinction coef®cients were calculated by means of a least square ®t to the transmittance and re¯ection data with a thin ®lm software [14]. This programme also enables the calculation of ®lm inhomogeneities by subdivid- ing the solid single layer into several sublayers with inde- pendent dispersion behaviour. The relative error of the complete algorithm for the determination of the refractive index is mainly dependent on the difference between the refractive indices of the substrate and the thin ®lms, the degree of inhomogeneity and the absorption of the sample coating, as well as the error of the spectral measurements. For the coating materials considered in this study, the rela- tive error may be estimated below 3% for the refractive index. With respect to the extinction coef®cient, the relative error of the data reduction algorithm is markedly increased for values below 1023. The coatings were also tested with respect to their mechanical properties, by employing an eraser test accord- ing to MIL-13508-C. For an approach towards a quantitative evaluation of the induced degradation by the eraser, the transmittance of the samples was measured before and after the test. The reduction of the transmittance can be attributed to an increased scattering caused by surface defects induced by the eraser. This decrease in transmittance is integrated over a distinct range (MgF2: 350±490 nm, YbF3: 360±500 nm) to obtain a representative value for the deterioration. 3. Results and discussion 3.1. Magnesium ¯uoride In order to achieve high quality ®lms with a packing density close to unity, conventionally evaporated MgF2- ®lms have to be deposited at high substrate temperatures in the range from 3008C [15] to 4508C [16]. For deposition at room temperature, the packing densities p, of conven- tional MgF2-®lms are limited to values below p < 0:72 [17]. In this study, conventional reference samples were produced at 2708C for comparison with coatings produced with adapted IAD-processes at ambient temperatures. Three IAD-series with different working gases: argon, argon with oxygen back®ll, and xenon, were investigated. The oxygen back®ll was chosen to compensate for the stoichiometric effects of the preferential sputtering in the growing ¯uoride layers. By this technique, local stoichiometric de®ciencies of ¯uorine atoms may be saturated by oxygen forming a fraction of MgO in the layers, which impair the optical properties. On one hand, this effect may result in reduced extinction coef®cients of the layers, on the other hand, a high fraction of MgO in the MgF2-®lm is undesirable, since MgO is susceptible to degradation [18]. To minimize the effects of preferential sputtering, extremely low anode voltages, i.e. very low median ion energies were used. Anode voltage was varied in a range between 40 and 80 V, the anode current from 2 to 5 A and the evaporation rate was selected in an interval from 0.7±1.0 nm/s. In Fig. 1 the refractive indices of all MgF2-®lms are depicted in comparison to the ®lm deposited conventionally. The data are ranked with respect to the degree of ion assistance, which is expressed in terms of the ion energy dose per deposited thin ®lm molecule (EPM: energy per molecule). Taking into account the equivalence between the ion beam properties and the operation parameters this quantity can be calculated by the product of the anode voltage, the anode current and the reciprocal of the evaporation rate (propor- tionality of the anode voltage/current with the ion energy/ density). In the following ®gures the EPM-values are cali- brated to the total electric power dissipated in the ion-source during the process. For an estimation of the actual energy dose per admolecule, the constants of proportionality for the ion energy respective to the ion current density have to be considered. Typical ion current densities were in the range of 1±2.5 mA/cm2 with mean ion energies of 30±70 eV. Inhomogeneities could not be observed for all MgF2-®lms produced with different deposition parameters. For the series with argon ions, the data indicate a trend towards higher refractive indices for increasing EPM-values, M. Kennedy et al. / Thin Solid Films 333 (1998) 191±195192 Fig. 1. Refractive index (n at 500 nm) of the three MgF2-deposition-series in dependence on the IAD-parameters; stated are the EPM-values of the different working gas series. The refractive index of the conventional refer- ence sample is depicted with an EPM-value of zero. which may be attributed to an increased densi®cation of the layer structure. Layers produced at only 60 V anode voltage exhibit exceptionally low refractive indices. Obviously, the behaviour of the refractive indices as a function of the EPM- values is also dependent on the average ion energy of the beam. Further studies are necessary to clarify whether a threshold energy exists for this effect. Taking into account the error budget of the data reduction algorithm for the refractive indices of the layers, the effect of an additional oxygen back®ll to the IAD-process with argon can not be veri®ed unambiguously. A possible expla- nation for the observed tendency towards reduced refractive indices with increasing ion assistance could be an excessive oxygen partial pressure during the deposition process. The highest refractive indices were achieved with xenon- IAD: this effect can be explained by the greater momentum transfer of the heavier xenon ions to the admolecules of the growing layer. The observation is in agreement with the results of Targove and Macleod [19], who report a similar tendency for ion assisted deposition of LaF3 and state that momentum transfer is the dominant densifying mechanism in IAD-processes. In contradiction to the trend towards higher refractive indices with argon ions, highest EPM- values result in lower refractive indices for xenon ions. The observed reduction in the refractive indices could be related to the speci®c operational conditions of the ion source at the higher anode current of 3 A, which generates a high background pressure of xenon during the deposition process. RBS measurements on the samples, which reveal higher xenon contents in these layers (0.9 at.%), support this hypothesis. For a survey on extinction coef®cients of the MgF2-®lms, k-values are compared in Fig. 2 at a wavelength of 300 nm, which was selected for an enhancement of contrast: in the vis all xenon-IAD and the argon-IAD (respectively, Ar/O2 2) samples with lower ion assistance showed no absorption detectable by the data reduction algorithm. The ®lms produced with argon-IAD in conjunction with oxygen back- ®ll also exhibit low extinction coef®cients when deposited at modest EPM-values. The increasing absorption with the EPM-values can be explained by the preferential sputtering of the ¯uoride atoms [9,10], which is reduced when working with xenon ions compared with the other working gas combinations employed in these experiments. The extinc- tion coef®cient is most susceptible to small deviations in the stoichiometry; however, depth-resolved XPS measurements of the investigated low-energy-IAD coatings showed no signi®cant ¯uorine de®ciencies within the experimental error budget. The results of the eraser test, which are considered as representatives for the mechanical properties of the MgF2- ®lms, are summarized in Fig. 3 in terms of their integrated reduction of transmittance. In this series two samples failed by means of partial removal of the coating. In comparison to the conventional reference coating, most of the single layers produced by IAD indicate a lower mechanical stability. An unexpected effect can be observed for the argon-IAD ®lms: in contradiction to the increased ®lm-densities of the layers deposited at higher EPM-values, the abrasive resistance shows a tendency towards lower values. This effect could not be clari®ed within the framework of this preliminary investigation. As a possible explanation, the dependence of the mechanical properties on the stoichiometry of the MgF2-®lms may be considered. In the argon-IAD series with oxygen back®ll an improved stability with increasing EPM-value can be observed, meanwhile the best results were found for IAD with xenon ions: The eraser test reveals a durability of the IAD-coatings deposited at ambient temperatures which is comparable to the mechanical proper- ties of the conventionally evaporated ®lm at 2708C. 3.2. Ytterbium ¯uoride Ytterbium ¯uoride has been investigated as a replacement material for the radioactive ThF4, which is used predomi- nantly for coatings in the FIR spectral region [13]. In that study IAD with argon was employed to improve the thin ®lm density and to minimize water incorporation as well as M. Kennedy et al. / Thin Solid Films 333 (1998) 191±195 193 Fig. 2. Extinction coef®cients (k at 300 nm) of all MgF2-®lms calculated from the spectral data. Also stated are the EPM-values for the different working gas combinations. Fig. 3. Integrated (350±490 nm) reduction in transmittance of MgF2-®lms after the eraser tests for different working gas combinations and EPM- values. the corresponding absorption at 10.6 mm. Best results were achieved with maximum EPM-values of the deposition system, which was equipped with an rf-ion source (RIM4). In the present investigation, YbF3-®lms deposited by IAD with argon and xenon ions are compared to reference samples conventionally evaporated at 1508C substrate temperature. The refractive indices at 500 nm and the extinction coef®cients at 300 nm of YbF3-®lms deposited with different process parameters are depicted in Figs. 4 and 5, respectively. The conventional YbF3-®lms and the IAD- ®lms deposited with low ion assistance revealed a negative inhomogeneity, i.e. the refractive index of the ®lm decreases with the growing ®lm-thickness from the substrate to the surface of the layer. For calculation of the inhomogeneity, the ®lm was separated into ®ve sublayers of identical thick- ness (approx. 50 nm) with independent dispersion. The refractive indices plotted in Figs. 4 and 5 indicate the mean value of the respective sublayers, and the error bars give the mean standard deviation. For the single layers deposited with argon ion assistance, an improvement of the ®lm density and the refractive indices with increasing EPM-values was observed (in the diagram from the left to the right). The simultaneous increase of the extinction coef- ®cient in the UV spectral region may be attributed to prefer- ential sputtering of the ¯uoride atoms in the growing layer. This effect is not signi®cant for application in the visible spectral range, were the coatings exhibit negligible absorp- tion. With respect to the inhomogeneity of the ®lms, an unexpected effect can be observed: As the conventional process, IAD employing low ion assistance result in a nega- tive inhomogeneity. Coatings deposited with medium EPM- values reveal a tendency towards a higher standard varia- tions in the refractive indices, which may be interpreted as an increased grade of inhomogeneity. Maximum ion assis- tance results in a reduced inhomogeneity of the produced YbF3-layers. The investigations on ion assistance with xenon are in agreement with the experimental series on M. Kennedy et al. / Thin Solid Films 333 (1998) 191±195194 Fig. 4. Refractive index (at 500 nm) and extinction coef®cient (at 300 nm) of YbF3-®lms produced with argon-IAD, stated are the EPM-values. Stan- dard evaporation rate was 1 nm/s. The error bars give information about the inhomogeneity of the ®lms. Fig. 5. Refractive index (at 500 nm) and extinction coef®cient (at 300 nm) of YbF3-®lms produced with xenon-IAD, stated are the EPM-values. The error bars give information about the inhomogeneity of the ®lms. Fig. 6. Normalized (to the starting value) refractive index in dependence of the ®lm thickness. Compared to the conventional ®lm is the average of the argon-IAD-series and the average of the xenon-IAD-series with high ion assistance plus the argon-IAD ®lm with maximum ion bombardment. Fig. 7. Integrated (360±500 nm) transmittance reduction of YbF3-®lms after rub eraser test, the reduction in transmittance is due to increased scattering. Also stated are the EPM-values for the different working gas combinations. argon ion assistance: similar tendencies can be observed for the behaviour of the refractive indices and the extinction coef®cients as a function of the applied EPM-values. Fig. 6 demonstrates the positive effect of high EPM-values for assisted deposition with xenon ions. The corresponding coatings exhibit an adequate homogeneity, a high packing density and highest refractive indices compared to the conventional evaporation process or IAD employing argon. The results of the eraser tests (Fig. 7) also prove the superior mechanical properties of YbF3-®lms produced by IAD-processes, especially by assistance with xenon ions. 4. Conclusions The results of the present experimental investigation indi- cate major advantages of ion-assisted deposition at ambient substrate temperature for MgF2- and YbF3-®lms. On the basis of optimized deposition parameters, single layers can be produced by IAD with comparable or even improved optical and mechanical properties with respect to conven- tional coatings deposited at substrate temperatures of 2708C. For IAD-MgF2-®lms further fundamental studies on the microstructure and the stoichiometry are necessary for a clari®cation of unexpected dependency of the refractive indices and the mechanical properties ®lm properties on the selected IAD-parameters. MgF2-®lms deposited under assistance with xenon ions exhibit highest refractive indices, lowest absorption and a higher mechanical durability compared with ®lms produced by argon-IAD. Deposition processes involving high EPM-values cause considerable optical absorption for MgF2-®lms in the UV-vis spectral range, and therefore, have to be carefully optimized with respect to the particular application. The xenon-IAD MgF2- ®lms of this investigation, which were deposited with moderate EPM-values, showed markedly low absorption values in the UV-vis spectral range. The experimental results on YbF3-®lms produced by IAD indicate a high potentiality of this material for coating of substrates vulnerable to high deposition temperatures. Espe- cially IAD-processes using xenon as working gas reveal ®lms with increased refractive indices and packing densi- ties. Simultaneously, the homogeneity and the mechanical properties of the ®lms could be improved with adapted deposition parameters. For the produced YbF3-®lms no considerable increase of absorption has been observed in the visible spectral range, meanwhile the extinction coef®- cient is increased notably by preferential sputtering effects below a wavelength of 400 nm. Acknowledgements This work was supported by the Bundesministerium fuÃr Bildung, Wissenschaft, Forschung und Technologie (BMBF) under contract number 13 N 6989. References [1] P.J. Martin, R.P. Netter®eld, in: Cuomo, Rossnagel, Kaufman (Eds.), Handbook of Ion Beam Processing Technology, Noyes Publications, 1989, p. 373. [2] P. Martin, J. Mater. Sci. 21 (1986) 1. [3] M.L. Fulton, SPIE 2253 (1994) 374. [4] H.S. Niederwald, et al., Proc. SPIE, 3133(24) (1997) in press. [5] J.D. Targove, et al., SPIE 678 (1986) 115. [6] P.J. Martin, et al., Appl. Opt. 26 (7) (1987) 1235. [7] D.A. Baldwin, et al., Proc. Soc. Vac. Coat. 40th Annu. Tech. Conf., 1997, in press. [8] J. Kolbe, et al., SPIE 1624 (1991) 221. [9] J. Kolbe, H. Schink, SPIE 1782 (1992) 435. [10] J. 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