Nano-coatings the thought and the actions Riccardo DeSalvo, Shiuh Chao, Innocenzo Pinto, Vincenzo Pierro, Vincenzo Galdi, Maria Principe, Huang-Wei Pan, Chen-Shun Ou, Vincent Huang 14 may 2012 GWADW 2012JGW-G

First visit a number of reasons to study nanocoatings Then present the status of R&D 14 may 2012 GWADW 2012JGW-G

First interest for nanocoatings RDS participated to the PXRMS conference Big Sky – Montana X-ray mirror coating community Report LIGO-G R 14 may 2012 GWADW 2012JGW-G

Lessons from x-ray community Extremely thin layers are always glassy More stable ! = > Natural doping due to inter- diffusion may also play a relevant role 14 may 2012 GWADW 2012JGW-G Different atomic radius and oxydation pattern assure glassy structure around the interface between different materials Sub-layer thickness

Lessons from x-ray community Good glass formers remain glassy even for large thicknesses Poor glass formers – first produce crystallites inside the glass Invisible to x-rays – Then crystallites grow into columnar-growth poli-crystalline films Crystallites are bad for scattering Probably bad for mech. losses also (Dopants induce better glass formers) 14 may 2012 GWADW 2012JGW-G N.S. Gluck et al., J. Appl. Phys., 69 (1991) 3037 Ghafor et al. Thin Solid Films, 516 (2008) 982

Lessons from x-ray community Not surprising that Chao first managed to reduce scattering in gyrolaser dielectric mirrors by inventing the SiO 2 doped TiO 2 But the important message is that thinner coatings are more stable ! They will probably have even less scattering (crystallite free) Will they also have less mechanical losses? 14 may 2012 GWADW 2012JGW-G Shiuh Chao, et al., "Low loss dielectric mirror with ion beam sputtered TiO2- SiO2 mixed films" Applied Optics. Vol.40, No.13, , May 1, 2001.

Layer thickness vs. Annealing Annealing temperature decreases losses In co-sputtering large percentages of dopant (SiO 2 in Ti 2 O 5 )are needed Thin layers require less % SiO 2 for the same annealing stability 14 may 2012 GWADW 2012JGW-G W.H. Wang and S. Chao, Optics Lett., 23 (1998) 1417; S. Chao, W.H. Wang, M.-Y. Hsu and L.-C. Wang, J. Opt. Soc. Am. A16 (1999) 1477; S. Chao, W.H. Wang and C.C. Lee, Appl. Opt., 40 (2001) 2177

How much gain from layered TiO 2 Comparing: stratified 66%TiO 2 with 36%SiO 2 Equivalent refraction index to: Ta 2 O 5 doped TiO 2 First gain: If mech. losses in Ta 2 O 5 = TiO 2 => Gain in dissipation ~ 36% 14 may 2012 GWADW 2012JGW-G Doped Tantala Silica Titania

How much gain from layered TiO 2 Further gain: Consider now the measured loss angles: Doped Ti 2 O ± TiO Gain in dissipation = 65% 14 may 2012 GWADW 2012JGW-G

Mixture theory: Distribution of “dopant” makes a difference in refraction index 14 may 2012 GWADW 2012JGW-G Higher refraction Index, less material, less loss

Titania Doped Tantala Years after Chao introduced SiO 2 -TiO 2 coatings LMA discovered that TiO 2 -Ta 2 O 5 coatings have – less mechanical noise, – better thermal noise performance Is it because TiO 2 -Ta 2 O 5 is a more stable glass? Or because of atomic level stress introduced by doping? Or both? Why stress may be important? 14 may 2012 GWADW 2012JGW-G

Example: hydrogen dissipation a metal has P orbitals 14 may 2012 GWADW 2012JGW-G

Example: hydrogen dissipation Proton resides in electron cloud = > Double well potential ! Flip-flops between wells Indifferent equilibrium 14 may 2012 GWADW 2012JGW-G

In presence of acoustic wave horizontal compression: – Proton jumps down Vertical compression – Proton jumps up 14 may 2012 GWADW 2012JGW-G

Losses in a glass Double well potential Oscillating stress Well jumping Each jump is a loss How to stop it? 14 may 2012 GWADW 2012JGW-G

Stress the glass ! ! Static stress Biassed double well State lives always in the lower hole 14 may 2012 GWADW 2012JGW-G

Acoustic oscillation in double well potential No stressStress Well jumpingNo well jumping DissipationNo dissipation 14 may 2012 GWADW 2012JGW-G

Effects of Stress in Si 3 N 4 High stress Low loss x may 2012 GWADW 2012JGW-G

How to add Stress the coating Adding TiO 2 in Ta 2 O 5 introduce random stress – Stress from different oxidation pattern (random distribution) Observed Lower losses Alternating thin layers TiO 2 to SiO 2 introduce ordered stress – Stress from different atomic spacing (ordered) 14 may 2012 GWADW 2012JGW-G

How to add Stress the coating How thick an optimal layer? – 1 interlayer diffusion length thick ? Uniformly graded concentration => uniform stress ? – Will it lead to Lower mechanical losses? 14 may 2012 GWADW 2012JGW-G S.Chao, et al., Appl. Optics, 40 (2001) 2177.

We have seen the reasons to try nanolayered coatings Now let’s look at the experimental activity at National Tsing Hua University in Taiwan 14 may 2012 GWADW 2012JGW-G

Refurbished ion-beam-sputterer Fast cycling Coater for SiO 2, TiO 2, Ta 2 O 5 For multi-layers and mixtures 14 may 2012 GWADW 2012JGW-G

Refurbished ion-beam-sputterer Kaufman-type ion beam sputter system in a class 100 clean compartment within a class 10,000 clean room Previously used to develop low-loss mirror coatings for ring-laser gyroscope Sputter target and rotator Exchangeable twin target holder Kaufman ion gun and neutralizer 14 may 2012 GWADW 2012JGW-G

Nano-layer coating preparations Calibrating deposition rate for TiO 2 and SiO 2 SiO 2 first SiO 2 second Time(min) TiO 2 Fitting curve SiO 2 Fitting curve Thickness (nm) 8.6% 6.7% -1.2% -1.8% 2.7% -5% -10% -0.3% -1.4% 0.6% 0.2% Deposition rate : 3.40 nm/minDeposition rate : 7.62 nm/min QW :115nmQW :183nm V B =1000V V A =-200V I B =50mA±2% Ar-Gun : mbar Ar-PBN : mbar O 2 -Chamber: 5*59*10 -4 mbar ±5% V B =1000V V A =-200V I B =50±2%mA Ar-Gun : mbar Ar-PBN : 1.5*10 -4 mbar O 2 -Chamber: 3.27*10 -4 ±5.5% mbar 14 may 2012 GWADW 2012JGW-G

Nano-layer coating preparations Uniformity distribution for TiO 2 and SiO 2 SiO 2 first SiO 2 second Position(cm) 1%, 3.16cm 3.6%, 5cm TiO 2 first TiO 2 second Position(cm) 4.1%, 5cm 1%, 2.66cm % % 14 may 2012 GWADW 2012JGW-G

Q experimental setup Chamber Cold cathode ion gauge Pirani gauge Gate valve Valve Turbo vacuum pump Scroll pump Concrete block Laser QPD Detector electrode Clam p Sample Window 14 may 2012 GWADW 2012JGW-G

Loss hunting O-ring Tight screws Frequency(Hz) Amplitude τ = φ = 3.75*10 -5 τ = φ = 2.05*10 -5 Support losses 14 may 2012 GWADW 2012JGW-G

Neutralizing clamp losses oxy-acetylene welding 14 may 2012 GWADW 2012JGW-G Fused Silica cantilever Fused Silica bulk 2 mm 110 μ m τ = 1726(s) φ = 3.01E-06 Frequency = 61.3(Hz)

Neutralizing Residual gas effects Frequency = 54 Hz 14 may 2012 GWADW 2012JGW-G

Neutralizing pump vibrations Frequency(Hz) Amplitude Frequency(Hz) Amplitude Pump on torr Pump off torr Pump on torr Added flexible tube sank in lead pellets – Allow continuous pumping 14 may 2012 GWADW 2012JGW-G Frequency(Hz)

Preparing Silicon cantilevers For cryogenic measurements 14 may 2012 GWADW 2012JGW-G

KOH wet etching 150ml KOH DI water Ultrasonic cleaner heater Top view Si (s) + 2(OH) - + 2H 2 O → Si(OH) 2 O H 2(g) 14 may 2012 GWADW 2012JGW-G

Silicon cantilevers Etch sideProtected side 5.5mm μm mm 10 mm 34 mm 0.35mm 500 μm ° 14 may 2012 GWADW 2012JGW-G

Time(min)Ra(nm)error Roughness of cantilever 14 may 2012 GWADW 2012JGW-G

Incidentally may 2012 GWADW 2012JGW-G

Silicon cliff We live here ! That’s Scary ! ! ! Is this the reason why cryogenic mirrors do not improve? 14 may 2012 GWADW 2012JGW-G

Better cryo coatings? What can we do to get better cryo coatings ? ? Is getting away from silica a simple answer??? Should we switch to Al 2 O 3 instead ? ? ? More work to do 14 may 2012 GWADW 2012JGW-G