Download Coating mechanical loss at room and low temperatures

Coating thermal noise
S. Reid, I. Martin, J. Hough, S. Rowan
Univ. of Glasgow
GEO
M.M. Fejer, R. Route
Stanford University
A. Remillieux, J.M. Mackowski, C. Comtet, N. Morgado
L. Pinard, Ch. Michel
LMA, Lyon
G. Harry
LIGO MIT
H. Armandula,
LIGO Caltech
S. Penn,
Hobart and William Smith Colleges
3rd ILIAS-GW Annual General Meeting – Imperial College, 27th October 2006
Temperature dependence of the mechanical dissipation
in single layer tantalum pentoxide coatings on thin
single-crystal silicon substrates
•
Experiments suggest
–
Silicon is of significant interest as a mirror substrate
material for future detectors and its mechanical loss is
predicted to decrease at low temperatures.
–
Ta2O5 is the dominant source of dissipation in current ionbeam-sputtered SiO2/Ta2O5 coatings
Doping the Ta2O5 with TiO2 can reduce the mechanical
dissipation
–
•
•
GEO
Mechanism responsible for the observed mechanical loss in
Ta2O5 as yet not clearly identified
Use of thin substrates means coating loss becomes dominant
– interpretation of results is easier
Sample fabrication
• The silicon samples have been fabricated by wet etching from
silicon wafers at Stanford (Stefan Zappe)
• The coatings were deposited by LMA.
• A pair of silicon cantilevers, 52 mm thick were measured.
– control sample (underwent same oxidation/annealing process
as coated sample)
coated cantilever, 52 mm thick, 57 mm long
• Silicon substrate properties:
– ~ 52 mm thick, P-type Boron doped, resistivity = 10-20 W-cm
• Tantala coating properties:
– 0.5 mm single layer tantala, doped with 14.5 ± 1 % titania (Formula 5)
Measured mechanical loss
• Results – 4th bending mode at ~777 Hz
(b)
-5
mechanical loss 
1x10
-6
1x10
(a)
-7
1x10
(c)
measured loss, coated, ~777Hz
measured loss, uncoated, control
calculated thermoelastic loss
-8
1x10
50
100
150
200
250
300
Temperature (K)
Temperature dependence of (a) measured uncoated loss, (b) measured coated loss
and (c) calculated thermoelastic loss for the second bending mode at ~777Hz.
Measured mechanical loss ctd
• Results – 5th bending mode at ~1280 Hz
(b)
-5
mechanical loss 
1x10
-6
1x10
(a)
-7
1x10
(c)
measured loss, coated, ~1280Hz
measured loss, uncoated, control
calculated thermoelastic loss
-8
1x10
50
100
150
200
250
300
Temperature (K)
Temperature dependence of (a) measured uncoated loss, (b) measured coated
loss and (c) calculated thermoelastic loss for the third bending mode at ~1280Hz.
Coated silicon cantilever – energy
distribution
• Interpretation: calculating the energy stored in the coating layer
• The the energy ratios were
calculated analytically and
evaluated by Finite Element
analysis and found to agree within a
few %.
• The energy ratio used:
Ecoating/Esubstrate= 0.0249
deflected beam with a lossy surface
layer (Heptonstall et al PLA 2006)
Young’s modulus of silicon substrate: 162.4 GPa
Reid et al, PLA 2005
Young’s modulus of tantala coating:
140.0 GPa
K. Srinivasan et al, LIGO-T970176-00-D 2001, “Coating Strain Induced Distortion in LIGO Optics”
http://www.ligo.caltech.edu/docs/T/T970176-00.pdf
Second cantilever
• Calculated coating loss
Coating mechanical loss coating
coating loss, ~777Hz mode
coating loss, ~1280Hz mode
4x10
2x10
-4
-4
0
50
100
150
200
250
300
Temperature (K)
Temperature dependence of the measured loss in a single layer doped tantala
coating applied to a 52mm silicon substrate at ~777 Hz and ~1280 Hz.
Conclusions - coatings
• Measurements of the loss of a single layer of tantala coating have
been made on silicon cantilever substrates
• Results at low temperature are consistent with:
the mechanical loss of tantala increases as temperature
decreases
~1.5x10-4 at 290K → ~4.0x10-4 at 80K
possible dissipation peak at lower temperatures (?)
- compare to Yamamoto et al (Phys. Rev. D 74, 022002, 2006) where a
similar increase in mechanical loss for silica-tantala (undoped) multilayer
coatings is observed at the measured points at 300K and 77K.
• Further work: in process of adapting cryostats to cool to liquid
helium temperature to study coating loss down to 4 K