High reflecting dielectric mirror coatings deposited with plasma assisted reactive magnetron sputtering H. Hagedorn*, J. Pistner Bühler Alzenau GmbH, Siemensstr. 88, D-63755 Alzenau, Germany
ABSTRACT Manufacturing all dielectric mirror coating with reflectivity values of more than 99.99 % is still a challenge to achieve. Losses caused either be transmittance, absorption or scattering have to be maintained well below 100ppm. Increasing the layer number for minimizing the transmittance losses usually increases the scattering by the growth of the roughness. High energy processes are required to minimize or avoid this behavior, but which are a challenge for avoiding unwanted contamination and interface absorption due to unwanted sputtering. As high energy process we used for the preparation of high reflecting dielectric mirrors plasma assisted reactive magnetron sputtering with a Helios 800 system. The machine was equipped with 3 cathode position for low and high index materials. We used metallic tantalum and hafnium targets for the preparation of the high index, silicon and silica targets for the low index. Metallic targets were powered with mid frequency, whereas the quartz target was sputtered by RF. As substrate we used either super polished fused silica or standard silicon wafer. The optical properties of the substrates we characterized by CRD, Laser calorimetry and spectrophotometric measurements. All combination allowed us to reach reflectivity values above 99.99%, with total deficit levels as low as 36ppm. Keywords: Plasma assisted reactive magnetron sputtering, RF sputtering, high reflecting mirror, Dielectric coating, Tantala, Hafnia, Silica,
1. INTRODUCTION Plasma assisted reactive magnetron sputtering was introduced into the market in the beginning of 2004. The patented technology allows the precise deposition of metallic or dielectric layers. In turn-table systems it uses up to 3 magnetron positions that are equipped with metallic targets. In combination with a plasmas source the dielectric layers are build-on in a two-step process. A very thin sub-stoichiometric layer is transferred into a low absorption dielectric layer by alternate passing sputter and plasma source. The layer thickness of each individual pass is in the region of 1-2 Ångstrom to keep the absorption as low as possible. Using co-sputtering with more than one sputter source allows mixing of different materials and the thereby the creation of intermediate refractive indices. Mean deposition rates are in the range of 0,5 - 0,7nm/s. Layer thickness determination is done by means of direct optical monitoring or simply by counting the number of passes beneath the cathode. Lateral homogenous thickness uniformity is created by precise masking underneath each sputter position. Complex interference filters can by created with optical thickness uniformity of +-0.1% on limited area on all 12 substrate position. Over the whole life time of the target material the uniformity can be kept better than +-0.5% by means of a tunable magnet array behind the target. The refractive index stability over time is excellent. By means of single layers and spectral ellipsometry we determined index variation at a wavelength of 550nm for Nb2O5 of 1 = 0.16% and SiO2 of 1 =0, 04% over one target life cycle (fig. 1, 2). Plasma assisted reactive magnetron sputtering allows the use of very thin layers to realize advanced optical thin film designs. There lateral uniformity is given by each pass underneath the sputtering source. No averaging about multiple rotations, like in a planetary system, is necessary. There thickness can be controlled precisely by counting the number of revolutions of the turntable, using the actual deposition rate per pass under the sputter source. Transmission electron images of notorious inhomogeneous materials, like HfO2, do not show any structure change in the growth direction from layer thicknesses much thinner than 10nm on, when deposited by co-sputtering with SiO 2. Oxide materials like SIO2, Ta2O5 and Nb2O5 have the same behavior without the use of co-sputtering. The refractive indices of these materials can
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be assumed to be independent from layer thickness. More details about the technique is described various publications [1] [2] [3]. For the investigation of high reflecting dielectric mirror coatings deposited with plasma assisted reactive magnetron sputtering we used a Helios 800 system. In contrast to the standard configuration, we used for deposition of the SiO 2 layers a newly developed rf-sputtering source.
kwh 15,9 35,5 72,5 91 105,3 119 148,5 185 198
n @550nm 2,353 2,352 2,351 2,361 2,3605 2,3585 2,36 2,3555 2,354 2,3562 0,0037 0,16%
Figure 1. Uniformity and index stability over target life time (Nb2O5) Helios 400
kwh 7,6 26,5 35,8 54,6 56,9 136,2 156,8 167 192,7 n
n @550nm 1,482 1,4827 1,4826 1,4826 1,4825 1,4828 1,483 1,4832 1,4845 1,4829 0,0007 0,04%
Figure 2. Uniformity and index stability over target life time (SiO2); Helios 400
2. RF-SPUTTERING SOURCE The rf-sputtering source for the Helios 800 uses one rectangular 320*200mm target and a similar uniformity mask as the mid-frequency powered dual-magnetrons, as can be seen in fig.3. It is powered with frequency of 13.56 MHz up to power levels of 10kW via an auto-tuning matching network. The rf-source can be mounted at any of the 3 sputter position of the Helios 800. Any combinations with dual-magnetron sources are possible. We used either one or two sources in combination with one or two dual magnetrons. As target material we investigated only high purity quartz of 8mm thickness. The achievable rates are depending linear on the total power. With one target deposition rates of 0.12nm/s were possible. Using two targets simultaneously, rates up to 0,25nm/s were achieved (fig. 4). The magnetic field was optimized for high target utilization of more than 40%.
Figure 3. RF-Sputtering source and auto-tuning match box mounted on the top of a Helios 800 sputtering equipment
Helios 800RF, SiO2 RF Rate vs. Leistung, Einzel- oder Paarquellen 0.3
0.25
Rate [nm/s]
0.2
0.15
Rate vs. Leistung
0.1
0.05
0 0
2000
4000
6000
8000
10000
12000
14000
16000
18000
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Leistung [W]
Figure 4. SiO2 sputtered with one or two RF sputtering sources, deposition rate vs. input power;
3. RESULTS
Single Layer We compared the absorption losses of the rf-sputtered SiO2 films with MF-sputtered SiO2 films from the dual magnetron at three laser wavelength of 355nm, 532nm and 1064nm. The rf-films revealed slightly lower losses compared with the MF ones. We further investigated the losses by measuring the transmittance of 420nm single layers on Suprasil300 substrates. Once again the rf-films showed slightly higher transparency. After 12 hour heat treatment at 300°C the traces of the RF coated sample could not be distinguished from an uncoated Suprasil 300 sample down to the short wavelength measuring limit of 185nm of the PerkinElmer Lambda 900 spectrometer.
Deposited
Film
Film
Thickness nm 100 500
SiO2 MF SiO2 RF
Abs 1064nm per 100nm ppm 0,8 0,3
Abs 532nm per 100nm ppm 3,9 2,1
Abs 355nm per 100nm ppm 17 10,2
Process Conditions
MF: 5kW RF: 7kW
Figure 5. Plasma assisted reactive magnetron sputtered SiO2 films: Single layer properties of rf-sputtered compared to mfsputtered.
High Reflecting Mirrors at 1064nm and 633nm To explore the potential for high reflecting dielectric mirror coatings with high laser damage threshold we deposited an L (LH)^18 layer stack for a wavelength of 1064nm on super polished fused silica substrates. For L–Index material we used silica deposited by RF- Sputtering. For the H-Index material we used Hafnia deposited from the MF-Source. The total losses were determined by cavity ring down (CRD) measurements and achieved a value of only 36ppm. From the design we calculated transmission losses of 12ppm of the component. By laser calorimetric measurement we determined the absorption loss and achieved a value of only 6ppm. Therefore the total scattering loss of the component has to be below 20ppm and total reflectivity values of more than 99.99% at a wavelength of 1064 nm are reachable. We determined the LIDT S-on-1 according to ISO 21254-2 at this wavelength as well and achieved remarkable high value as can be seen in fig. 6 and reported in [3] for films with tantala as high index material.
Reflection
99.9964%
Total loss ( CRD)
36ppm
Absorption (laser calorimetric)
6ppm
Transmittance (calculated)
12ppm
Figure 6. Characteristic damage curve and optical properties of a L (LH)^18 layer stack hafnia/silica @1064nm
In combination with Ta2O5 as H-index material we deposited an HL^17H design on super polished zerodur and fused silica substrates for a wavelength of 633nm. CRD measurements for two consecutive runs achieved on all substrates values for the reflectivity of more than 99.99% (fig. 7).
Figure 7. Reflectivity values of a 4 samples coated with a HL^17H layer stack tantala/silica @633nm
CONCLUSIONS The combination of RF- Sputtered SiO2 films from a dielectric quartz target with MF- sputters High index films of hafnia and tantala allows the production of very high reflecting dielectric mirror coatings with reflectivity’s far above 99.99% and very high laser damage thresholds. Total scattering losses of less than 20ppm could be achieved. The normal disadvantage of RF- Sputtering, the low deposition rate could by improved noticeable by the combination with the high rate of the mid-frequency sputtered high index materials, the high power capabilities of the new developed RF- Sources and the combination of two simultaneously used Low index sources. Even taking into account that most Designs contain twice as much low index material, mean deposition rates of 0,3 – 0,4nm/ s are possible, which makes RF sputtering more attractive.
REFERENCES [1] Scherer M., Hagedorn H., Lehnert W., Pistner J.; "Innovative thin film laser components"; Advances in Optical Thin Films II, Proc. Of SPIE Vol. 5963, 2005 [2] Pearce S.J. et al. ; "Structural characteristics and optical properties of plasma assisted reactive magnetron sputtered dielectric thin films for planar waveguiding applications"; Surface Coatings Technology 206 (2012) 4930-4939 [3] Hagedorn H., Lehnert W., Pistner J., Scherer M., Zöller A, "Plasma Assisted Magnetron Sputtering of Demanding Interference Filters", 55th Annual Technical Conference Proceedings, Santa Clara, CA April 28-May3 , 2012
