1. Overview of coatings research and recent results at the University of Glasgow M. Abernathy, I. Martin, R. Bassiri, E. Chalkley, R. Nawrodt, M.M. Fejer, A. Gretarsson, G. Harry, D. Heinert, J. Hough, I. MacLaren, S. Penn, S. Reid, R. Route, S. Rowan, C Schwarz, P. Seidel, J. Scott, A.L. Woodcraft University of Glasgow GWADW, Kyoto Japan, May 2010 Document #: LIGO-G1000508-x0

2. 2 Things You Should Already Know: Thermal Noise – Limits Detectors – Important component comes from Ta 2 O 5 Doping Ta 2 O 5 has been shown to reduce thermal noise. Temperature Dependence of mechanical loss can tell us about loss mechanisms

3. 3 Things You should Know by the End of this Talk Pt. 1: Ta 2 O 5 Doping changes the activation energy of Tantala’s loss mechanism. – Does anything else do that? Heat treatment also changes the activation energy Different deposition techniques change activation energies.

4. 4 Things You Should Know by the End of this Talk Pt. 2: Additional Notes Ion Beam Sputtered (IBS) Silica coatings have temperature dependant loss too Hafnia (HfO 2 ) is an interesting material for study

5. 5 55 Thermal Noise 5 Coating thermal noise will limit sensitivity of Next-generation detectors at their most sensitive frequencies Coating thermal noise Evans et al PRD 78 102003 (2008)

6. 6 Mechanical Loss Thermal noise of the coatings is related to the mechanical loss of the coating material: [Harry et al. 2002] Y,Y’ … Young’s modulus of the bulk/coating material  … mechanical loss o the coatings

7. 7 Mechanical Loss and Measurement

8. 8 Mechanical loss spectroscopy 34mm 500  m 50  m thick Coating applied here Single layers of coating materials applied to silicon cantilever substrates supplied by Stanford / KNT Cantilever clamped rigidly and bending modes excited electrostatically. Loss obtained from exponential decay of amplitude coated cantilever in clamp tantala coating electrostatic drive

9. 9 Measuring coating loss Mechanical loss of coating layer calculated from difference in loss of a coated and un-coated cantilever Loss of (a) uncoated silicon cantilever with thermal oxide layer, (b) cantilever coated with 500 nm of TiO 2 -doped Ta 2 O 5 (14.5 % Ti) and (c) the calculated loss of the coating layer (a) (b) (c)

10. 10 Measuring coating loss Mechanical loss of coating layer calculated from difference in loss of a coated and un-coated cantilever Loss of (a) uncoated silicon cantilever with thermal oxide layer, (b) cantilever coated with 500 nm of TiO 2 -doped Ta 2 O 5 (14.5 % Ti) and (c) the calculated loss of the coating layer (a) (b) (c)

11. 11 Measuring coating loss Mechanical loss of coating layer calculated from difference in loss of a coated and un-coated cantilever Loss of (a) uncoated silicon cantilever with thermal oxide layer, (b) cantilever coated with 500 nm of TiO 2 -doped Ta 2 O 5 (14.5 % Ti) and (c) the calculated loss of the coating layer (a) (b) (c)

12. 12 Ta 2 O 5 and Doping G. M. Harry et al 2007 Class. Quantum Grav. 24 405.

13. 13 Effect of doping on loss of Ta 2 O 5 coatings Comparison of dissipation in tantala doped with 14.5 % TiO2 and un-doped tantala for 3rd (left) and 4th (right) bending modes.

14. 14 TiO 2 doping reduces the height and slightly increases the width of the dissipation peak TiO 2 doping reduces the loss of Ta 2 O 5 throughout the temperature range studied, with the exception of the wings of the peak Effect of doping on loss of Ta2O5 coatings

15. 15 Analysis of coating loss peak (TiO 2 doped coating) Debye-like dissipation peaks T peak increases with mode frequency indicating a thermally activated dissipation mechanism  0 = relaxation constant E a = activation energy  … relaxation strength  … relaxation time

16. 16 Analysis of coating loss peak Activation energy of dissipation process: (40 ± 3) meV for titania doped tantala (29 ± 2) meV for undoped tantala Doping increases the activation energy Transition between two stable states appears to be hindered by doping

17. 17 Distribution of parameters Amorphous structure results in a distribution of potential barrier heights g(V) Activation energy calculated from Arrhenius law corresponds to the average barrier height in this distribution The barrier height distribution function g(V) can be calculated from temperature dependent loss data Doping shifts distribution of barrier heights to higher energy, thus reducing loss 1 Gilroy, Phillips, Phil. Mag. B 43 (1981) 735 2 Topp, Cahill, Z. Phys. B: Condens. Matter 101 (1996) 235

18. 18 Distribution of parameters Amorphous structure results in a distribution of potential barrier heights g(V) Activation energy calculated from Arrhenius law corresponds to the average barrier height in this distribution The barrier height distribution function g(V) can be calculated from temperature dependent loss data Doping shifts distribution of barrier heights to higher energy, thus reducing loss 1 Gilroy, Phillips, Phil. Mag. B 43 (1981) 735 2 Topp, Cahill, Z. Phys. B: Condens. Matter 101 (1996) 235

19. 19 Heat Treatment

20. 20 Effect of heat treatment temperature on Ta 2 O 5 loss 35 K peak – Observed in Ta 2 O 5 heat treated at 300, 400 C. Evidence suggests may also be present in Ta 2 O 5 heat treated at 600 C – Activation energy 54 meV – Postulate that this may be analogous to dissipation peak in fused silica, involving thermally activated transitions of oxygen atoms Above: Electron diffraction pattern of Ta 2 O 5 heat treated at 300 C Left: Loss at 1.9 kHz of 0.5  m Ta 2 O 5 coatings annealed at 300, 400, 600 and 800 C. 300C 600 C 800 C 400C (i) 300 C 800 C 600 C 800 C 600 C 800 C 600 C 800 C 400C 600 C 800 C 400C 600 C 800 C 300C 400C 600 C 800 C 300C 400C 600 C 800 C

21. 21 Effect of heat treatment temperature on Ta 2 O 5 loss 18 K peak – Observed in Ta 2 O 5 heat treated at 600 C and 800 C – Dissipation mechanism may be related to structural changes brought on by heat treatment close to re-crystallisation temperature – Perhaps some pre-crystallisation ordering (but still appears amorphous on electron diffraction measurements) Above: Electron diffraction pattern of Ta 2 O 5 heat treated at 600 C Left: Loss at 1.9 kHz of 0.5  m Ta 2 O 5 coatings annealed at 300, 400, 600 and 800 C. 300C 600 C 800 C 400C (i) 600 C 800 C 600 C 800 C 600 C 800 C 600 C 800 C 400C 600 C 800 C 400C 600 C 800 C 300C 400C 600 C 800 C 300C 400C 600 C 800 C 600 C

22. 22 Effect of heat treatment temperature on Ta 2 O 5 loss 90 K peak – Observed in coating heat treated at 800 C – Large, broad loss peak likely to be related to (expected) onset of polycrystalline structure due to high temperature heat treatment – Dissipation mechanism could be e.g. phonon scattering at grain boundaries – more analysis required Above: Electron diffraction pattern of Ta 2 O 5 heat treated at 800 C Left: Loss at 1.9 kHz of 0.5  m Ta 2 O 5 coatings annealed at 300, 400, 600 and 800 C. 300C 600 C 800 C 400C (i) 800 C 600 C 800 C 600 C 800 C 600 C 800 C 400C 600 C 800 C 400C 600 C 800 C 300C 400C 600 C 800 C 300C 400C 600 C 800 C

23. 23 Effect of heat treatment temperature on Ta 2 O 5 loss 90 K peak – Observed in coating heat treated at 800 C – Large, broad loss peak likely to be related to (expected) onset of polycrystalline structure due to high temperature heat treatment – Dissipation mechanism could be e.g. phonon scattering at grain boundaries – more analysis required Above: Electron diffraction pattern of Ta 2 O 5 heat treated at 800 C Left: Loss at 1.9 kHz of 0.5  m Ta 2 O 5 coatings annealed at 300, 400, 600 and 800 C. 300C 600 C 800 C 400C (i) 800 C 600 C 800 C 600 C 800 C 600 C 800 C 400C 600 C 800 C 400C 600 C 800 C 300C 400C 600 C 800 C 300C 400C 600 C 800 C

24. 24 Deposition Effects

25. 25 Silica - comparison of deposition methods Bulk silica (a) & thermal oxide (b) grown on silicon have a dissipation peak at ~ 35 K 1,2 e-beam SiO 2 (5.5 kHz) 1 (c) – 109 nm thick e-beam film shows no peak at 35 K – Higher loss than bulk and thermal oxide above 40 k Ion beam sputtered silica (d) – Broad loss peak at ~ 23 K – Significantly lower loss than similar thickness of e-beam and thermal SiO 2 Deposition method can have a significant effect on loss – studies of coatings deposited by different methods planned 1 White and Pohl, Phys. Rev. Lett. 75 (1995) 4437, 2 Cahill and Van Cleve Rev. Sci. Inst. 60 (1989) 2706. (a) (c) (d) (b)

26. 26 ‘ Low water content ’ Ta 2 O 5 from ATF ATF Ta 2 O 5 heat treated at the same temperatures as CSIRO / LMA Ta 2 O 5 has loss peaks at significantly higher temperatures. Full analysis underway Suggests details of coating deposition procedure may have significant impact on the temperature dependence of the mechanical loss – further study of great interest Left: CSIRO / ATF comparison, heat treated at 600 C post-deposition. Right: CSIRO / ATF comparison, heat treated at 300 C post-deposition.

27. 27 ‘ Low water content ’ Ta 2 O 5 from ATF ATF Ta 2 O 5 heat treated at the same temperatures as CSIRO / LMA Ta 2 O 5 has loss peaks at significantly higher temperatures. Full analysis underway Suggests details of coating deposition procedure may have significant impact on the temperature dependence of the mechanical loss – further study of great interest Left: CSIRO / ATF comparison, heat treated at 600 C post-deposition. Right: CSIRO / ATF comparison, heat treated at 300 C post-deposition.

28. 28 Additional Notes

29. 29 Comparison of SiO 2 and Ta 2 O 5 Ta 2 O 5 has higher loss throughout temperature range, but loss of SiO 2 will also be significant below 100 K Scatter at higher temperatures, possibly due to loss into clamp.

30. 30 Temperature dependence of coating thermal noise at 100 Hz If coating loss was constant with temperature, could gain factor of ~ 4 in coating thermal displacement noise of a mirror at 18 K Measured coating losses imply a gain of a factor of ~ 1.7 in coating TN by cooling to 18 K Optimistic estimate – constant coating loss at all temperatures Using measured coating loss

31. 31 Temperature dependence of coating thermal noise at 100 Hz If coating loss was constant with temperature, could gain factor of ~ 4 in coating thermal displacement noise of a mirror at 18 K Measured coating losses imply a gain of a factor of ~ 1.7 in coating TN by cooling to 18 K Optimistic estimate – constant coating loss at all temperatures Using measured coating loss

32. 32 Alternative high index materials - hafnia Hafnia studied – allow comparison of loss in another high-index oxide Different atom weight / size – Differences in dynamics arising from atom weight expected Loss significantly lower than tantala below 125 K Peak position and width shifted compared to tantala High optical absorption (60 ppm) measured at Stanford Coating loss at ~ 1kHz for HfO 2 and Ta 2 O 5 heat treated at 300ºC. (E. Chalkley) loss peak around 50 K loss peak/plateau around 200 K

33. 33 Effect of heat treatment on loss of hafnia As-deposited (100 C) shows no clear loss peaks Peak at ~ 50 K observed in coatings heat treated at 300 and 400 K Electron diffraction measurements show evidence of both crystalline and amorphous structure in all the hafnia coatings Silica-doped hafnia remains amorphous when annealed up to 500 C, and presence of silica appears to only slightly increase loss at room temperature

34. 34 Effect of heat treatment on loss of hafnia As-deposited (100 C) shows no clear loss peaks Peak at ~ 50 K observed in coatings heat treated at 300 and 400 K Electron diffraction measurements show evidence of both crystalline and amorphous structure in all the hafnia coatings Silica-doped hafnia remains amorphous when annealed up to 500 C, and presence of silica appears to only slightly increase loss at room temperature

35. 35 Things You should Now Know Doping changes the activation energy of Tantala’s loss mechanism. Heat treatment also changes the activation energy Different deposition techniques change activation energies. IBS silica coating has temperature related loss Hafnia (HfO 2 ) is an interesting material for study