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
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• 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 !
• Different atomic radius and
oxydation pattern assure
glassy structure around the
interface between different
materials
Sub-layer thickness
• = > Natural doping due to interdiffusion may also play a relevant role
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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 glassN.S.formers)
Gluck et al., J. Appl. Phys., 69 (1991) 3037
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Ghafor et al. Thin Solid Films, 516 (2008) 982
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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?
Shiuh Chao, et al., "Low loss dielectric mirror with ion beam sputtered TiO2SiO2 mixed films" Applied Optics. Vol.40, No.13, 2177-2182, May 1, 2001.
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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
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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%
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Titania
Silica
Doped Tantala
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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
Higher refraction Index, less material, less loss
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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?
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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?
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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
• Well jumping
• Dissipation
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Stress
No well jumping
No dissipation
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Effects of Stress in Si3N4
• High stress
• Low loss
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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?
S.Chao, et al., Appl. Optics, 40 (2001) 2177.
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• We have seen the reasons to try
nanolayered 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
Exchangeable twin target holder
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
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Sputter target and rotator
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Kaufman ion gun and neutralizer
Nano-layer coating preparations
130
120
110
100
90
80
70
60
50
40
30
20
10
0
TiO2
Fitting curve
VB=1000V
V A =-200V
IB=50mA±2%
Ar-Gun : 10-4 mbar
Ar-PBN : 10-4 mbar
O2-Chamber: 5*59*10-4 mbar ±5%
2.7%
-1.8%
Thickness (nm)
Thickness (nm)
• Calibrating deposition rate for TiO2 and SiO2
-1.2%
Deposition rate : 3.40 nm/min
6.7%
8.6%
0
5
10
15
20
Time(min)
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30
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SiO2
Fitting curve
VB=1000V
V A =-200V
IB=50±2%mA
Ar-Gun : 10-4 mbar
Ar-PBN : 1.5*10-4 mbar
O2-Chamber: 3.27*10-4 ±5.5% mbar
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
0.2%
0.6%
-1.4%
-0.3%
-5%
0
QW :115nm
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SiO2 first
SiO2 second
Deposition rate : 7.62 nm/min
-10%
4
6
8 10 12 14 16 18 20 22 24 26 28
Time(min)
QW :183nm
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Nano-layer coating preparations
• Uniformity distribution for TiO2 and SiO2
100
98
96
94
92
90
88
86
84
82
80
78
76
1% , 3.16cm
TiO2 first
SiO2 first
100
TiO2 second
SiO2 second
98
96
4.1% , 5cm
94
3.6% , 5cm
92
%
%
1% , 2.66cm
90
88
86
84
82
80
-6 -5 -4 -3 -2 -1 0 1
Position(cm)
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3
4
5
6
-6 -5 -4 -3 -2 -1 0 1
Position(cm)
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3
4
5
6
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Q experimental setup
Pirani
gauge
Chamber
electrode
Clam
p
Cold cathode
ion gauge
Window
Laser
QPD
Detector
Sample
Gate valve
Turbo vacuum pump
Concrete
block
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Scroll pump
Valve
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Loss hunting
• Support losses
Tight screws
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τ=
156.91926
φ=
3.75*10-5
Frequency(Hz)
Amplitude
Amplitude
O-ring
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τ=
287.14776
φ = 2.05*10-5
Frequency(Hz)
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Neutralizing clamp losses
• oxy-acetylene welding
110 μ m
2 mm
Fused Silica cantilever
Fused Silica bulk
Frequency = 61.3(Hz)
τ = 1726(s)
φ = 3.01E-06
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Neutralizing Residual gas effects
Frequency = 54 Hz
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Neutralizing pump vibrations
• Added flexible tube sank in lead pellets
Pump on
10-6torr
Frequency(Hz)
Pump off
10-4torr
Amplitude
Amplitude
Amplitude
– Allow continuous pumping
Frequency(Hz)
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Pump on
10-6torr
Frequency(Hz)
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Preparing Silicon cantilevers
• For cryogenic measurements
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KOH wet etching
Top view
KOH
150ml
Si(s) + 2(OH)- + 2H2O
→
Si(OH)2O22- +2H2(g)
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DI water
Ultrasonic
cleaner
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Silicon cantilevers
Etch side
Protected side
54.74 °
34 mm
0.35mm
500 μm
5.5mm
500
92 μm
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44.35
mm
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Roughness of cantilever
Time(min)
0
120
240
360
469
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Ra(nm)
0.48
4.296
7.376
8.782
3.539
error
0.36882
1.51907
0.34932
0.16881
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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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