Nano-coatingsthe 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 2012 JGW-G1201029 1

• First visit a number of reasons to study nanocoatings

• Then present the status of R&D

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First interest for nanocoatings

• RDS participated to the PXRMS conference Big Sky – Montana

• X-ray mirror coating community

• Report LIGO-G080106-00-R

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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

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• 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 2012 JGW-G1201029 5N.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 SiO2 doped TiO2

• 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?

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Shiuh Chao, et al., "Low loss dielectric mirror with ion beam sputtered TiO2-SiO2 mixed films" Applied Optics. Vol.40, No.13, 2177-2182, May 1, 2001.

Layer thickness vs. Annealing

• Annealing temperature decreases losses

• In co-sputtering large percentages of dopant (SiO2 in Ti2O5)are needed

• Thin layers require less % SiO2 for the same annealing stability

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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 TiO2

• Comparing:• stratified 66%TiO2 with 36%SiO2

• Equivalent refraction index to:• Ta2O5 doped TiO2

• First gain:• If mech. losses in Ta2O5 = TiO2 =>

Gain in dissipation ~ 36%

14 may 2012 GWADW 2012 JGW-G1201029 8Doped Tantala

Silica

Titania

How much gain from layered TiO2

• Further gain: Consider now the measured loss angles:

• Doped Ti2O5 3.66±0.29 10-4

• TiO2 1.2 - 1.4 10-4

• Gain in dissipation = 65%

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Mixture theory: Distribution of “dopant” makes a difference in refraction index

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Higher refraction Index, less material, less loss

Titania Doped Tantala

• Years after Chao introduced SiO2-TiO2 coatings

• LMA discovered that TiO2-Ta2O5 coatings have – less mechanical noise, – better thermal noise performance

• Is it because TiO2-Ta2O5 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 2012 JGW-G1201029 11

Example: hydrogen dissipation

• a metal has P orbitals

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Example: hydrogen dissipation

• Proton resides in electron cloud• = > Double well potential !• Flip-flops between wells• Indifferent equilibrium

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In presence of acoustic wave• horizontal compression:

– Proton jumps down

• Vertical compression– Proton jumps up

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Losses in a glass• Double well potential

• Oscillating stress• Well jumping

• Each jump is a loss

• How to stop it?14 may 2012 GWADW 2012 JGW-G1201029 15

Stress the glass ! !

• Static stress

• Biassed double well

• State lives always in the lower hole

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Acoustic oscillation in double well potential

• No stress Stress• Well jumping No well jumping• Dissipation No dissipation

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Effects of Stress in Si3N4

• High stress• Low loss x 100

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How to add Stress the coating

• Adding TiO2 in Ta2O5 introduce random stress– Stress from different oxidation pattern (random distribution)

• Observed Lower losses

• Alternating thin layers TiO2 to SiO2 introduce ordered stress– Stress from different atomic spacing (ordered)

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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?

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S.Chao, et al., Appl. Optics, 40 (2001) 2177.

• We have seen the reasons to trynanolayered coatings

• Now let’s look at the experimental activity at National Tsing Hua University in Taiwan

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Refurbished ion-beam-sputterer

• Fast cycling Coater for SiO2, TiO2, Ta2O5

• For multi-layers and mixtures

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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 neutralizer14 may 2012 GWADW 2012 JGW-G1201029 23

Nano-layer coating preparations

• Calibrating deposition rate for TiO2 and SiO2

SiO2 first

SiO2 second

0 2 4 6 8 10 12 14 16 18 20 22 24 26 280

102030405060708090

100110120130140150160170180190200

0 5 10 15 20 25 30 350

102030405060708090

100110120130

Time(min)Time(min)

TiO2

Fitting curve

SiO2

Fitting curve

Thic

knes

s (n

m)

Thic

knes

s (n

m)

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/min Deposition rate : 7.62 nm/min

QW :115nm QW :183nm

VB=1000V V A =-200V IB=50mA±2%Ar-Gun : 10-4 mbarAr-PBN : 10-4 mbarO2-Chamber: 5*59*10-4 mbar ±5%

VB=1000V V A =-200V IB=50±2%mAAr-Gun : 10-4 mbarAr-PBN : 1.5*10-4 mbarO2-Chamber: 3.27*10-4 ±5.5% mbar

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Nano-layer coating preparations

• Uniformity distribution for TiO2 and SiO2

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 680

82

84

86

88

90

92

94

96

98

100 SiO2 first

SiO2 second

Position(cm)

1% , 3.16cm

3.6% , 5cm

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

767880828486889092949698

100

TiO2 first

TiO2 second

Position(cm)

4.1% , 5cm

1% , 2.66cm

%

%

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Q experimental setupChamber

Cold cathode ion gauge

Pirani gauge

Gate valve

Valve

Turbo vacuum pump

Scroll pumpConcrete block

Laser

QPD Detector

electrode

Clamp

SampleWindow

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Loss hunting

O-ringTight screws

Frequency(Hz)Frequency(Hz)

τ = 156.91926

φ = 3.75*10-5

τ = 287.14776

φ = 2.05*10-5

• Support losses

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Neutralizing clamp losses• oxy-acetylene welding

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Fused Silica cantileverFused 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

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Neutralizing pump vibrations

Frequency(Hz) Frequency(Hz)

Pump on10-6torr

Pump off10-4torr

Pump on10-6torr

• Added flexible tube sank in lead pellets– Allow continuous pumping

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Frequency(Hz)

Preparing Silicon cantilevers

• For cryogenic measurements

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KOH wet etching

150ml

KOH

DI water

Ultrasonic cleaner

heater

Top view

Si(s) + 2(OH)- + 2H2O → Si(OH)2O2

2- +2H2(g)14 may 2012 GWADW 2012 JGW-G1201029 32

Silicon cantilevers Etch side Protected side

5.5mm

50092 μm

44.35 mm

10 mm

34 mm 0.35mm

500 μm

54.74 °

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Time(min) Ra(nm) error0 0.48 -120 4.296 0.36882240 7.376 1.51907360 8.782 0.34932469 3.539 0.16881

Roughness of cantilever

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Incidentally . . .

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Silicon cliff

• We live here !

• That’s Scary ! ! !

• Is this the reason why cryogenic mirrors do not improve?

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Better cryo coatings?

• What can we do to get better cryo coatings ? ?

• Is getting away from silica a simple answer???

• Should we switch to Al2O3 instead ? ? ?

• More work to do

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