1 An overview of work in Glasgow relevant to the design study Stuart Reid 1 SUPA, University of Glasgow Glasgow University – 22 July 2010

Materials issues for ET 22 The construction of a 3 rd generation gravitational wave observatory within Europe E.T. (Einstein Telescope). Cryogenic operating temperature (ET-LF) –Silicon proposed as test mass and suspension material + construction/bonding –Identification of best mirror coatings material/technology being investigated Frequency [Hz] 3 rd generation Strain [Hz -1/2 ] 1 st generation 2 nd generation Seismic/suspension /Newtonian Noise Thermal Noise Shot Noise

3 Single layers of coating materials applied to silicon cantilever substrates supplied by Stanford / KNT (Glasgow) Cantilever clamped rigidly and bending modes excited electrostatically. Loss calculated from envelope of exponential decay in amplitude Multilayer mirror coatings coated cantilever in clamp tantala coating electrostatic drive 34mm 500  m 50  m thick Coating applied here

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

5 Doping increases the activation energy Transition between two stable states appears to be hindered by doping Effect of doping on loss of Ta 2 O 5 coatings Activation energy of dissipation process: (40 ± 3) meV for titania doped tantala (29 ± 2) meV for undoped tantala

6 Effect of doping on loss of Ta 2 O 5 coatings 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) Topp, Cahill, Z. Phys. B: Condens. Matter 101 (1996) 235

7 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) 18K 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 the start of pre-crystallisation ordering (but still appears amorphous on electron diffraction measurements) 90K 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 800 C 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.

8 Dependence on deposition method/technique Ta 2 O 5 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. SiO 2 Bulk silica (a) & thermal oxide (b) grown on silicon have a dissipation peak at ~ 35 K, 109nm e-beam SiO 2 (5.5 kHz) (c) shows no peak at 35 K, higher loss >40 k than bulk/thermal Ion beam sputtered silica (d) shows broad loss peak at ~ 23 K and significantly lower loss than similar thickness of e-beam and thermal SiO 2 Suggests details of coating deposition procedure may have significant impact on the temperature dependence of the mechanical loss – further study of great interest!

9 Alternative high-index coating materials HfO 2 Different atomic weight/size (differences in dynamics arising from atomic weight) Loss lower than tantala below 125 K Peak position (50 K) and width shifted compared to tantala (no peak in 100 C data) High optical absorption (60 ppm) measured at Stanford 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

10 Useful for probing atomic structure and chemistry Allows us to characterise atomic structure – Imaging – Diffraction – Spectroscopy Studying the atomic structure of IBS coatings What is causing the mechanical loss on an atomic level?

11 Image of multilayer coating, (bright- silica, dark - tantala) Amorphous diffraction pattern of 300 o C tantala Crystalline diffraction pattern of 800 o C tantala Crystalline diffraction pattern of 800 o C tantala Compare TEM results to mechanical loss measurements Compare TEM results to mechanical loss measurements The 800 The 800 o C sample has high loss peak at K probably due to crystallisation To probe the properties of the amorphous samples we need Reduced Density Functions To probe the properties of the amorphous samples we need Reduced Density Functions Initial interesting results: (from Ta 2 O 5 samples heat-treated at a range of temperatures) Transmission electron microscopy

12 Reduced density functions The reduced density function is a Fourier transform of the information gained from the intensity profile [D. J. H. Cockayne, Annu.Rev.Mater.Res, 37:159-87, (2007)] Intensity profileReduced density functionTantala diffraction pattern Silica and tantala are amorphous materials – They do not have long range order – They do have short range order We can probe this short range order with reduced density functions – RDFs give a statistical representation of where atoms sit with regards to a central atom

13 Reduced density functions RDFs of heat-treated Ta 2 O 5 Three Ta 2 O 5 coatings were measured – Each one was heat-treated at a different temperature (300, 400 & 600 o C) – RDFs show differences in local atomic structure as heat treatment temperature rises – From comparison to the structure of crystalline Ta 2 O 5 we can deduce that the first peak arises from Ta - O bonds and second peak from Ta - Ta bonds – Both first and second peaks become more defined and difference in heights between them decrease as heat treatment temperature rises implying that: – Material is becoming more ordered – There is an increase in Ta - Ta bonding

14 Modelling the atomic structure Atomic model of amorphous Ta 2 O 5 with red coloured oxygen and blue coloured tantalum atoms from molecular dynamics simulations showing a stable Ta 2 O 2 fragment (a) Comparison between refined and experimental RDFs, (b) partial RDFs showing individual nearest neighbour distances within the model Atomic structure measurements from this model provides important information including nearest atomic neighbour distances, co-ordination numbers and observed similarities between crystalline and amorphous phases, such as clusters with increased contribution from a Ta 2 O 2 ring fragment.

15 Diffractive optics and waveguide coatings Proposals have been made that potentially allow a significant reduction in the coating thickness required for high reflectivity in future GW detectors, e.g. diffractive optics and waveguide coatings. Both can be fabricated through micromachining silicon surfaces 300nm 400nm 1um Waveguide coating fabricated in JenaSimple grating pattern with comparable geometry to Jena waveguides, fabricated in Glasgow (KNT)

16 Diffractive optics and waveguide coatings Initial results comparing cantilevers with and without grating structures can be seen below (results for the 5th bending mode at ~ 3kHz) Preliminary measurements suggest that the increased surface area (top surface area increased ~ 3x), resulting from the grating pattern, have an insignificant effect on the overall measured mechanical loss of our cantilevers. Further gratings with reduced period (more lines) are currently being fabricated in Glasgow. T-shaped (waveguide) gratings are ready in Jena for measuring. A collaborative effort to characterise the thermal noise of waveguide coatings is underway.

17 Construction quasi-monolithic silicon suspensions Requirements for 3rd generation GW observatories (ET-LF / low T) Bond thermomechanical properties - Thermal noise (mechanical loss + thickness) - Heat extraction (thermal conductivity + bond area) Bond robustness - Mechanical strength - Temperature cycling effects/failures

18 Hydroxide-catalysis bond thickness SEM imaging of the bond material Si SiO 2 Bond material 40 nm

Si-Si hydroxide catalysis bond strength testing ASTM C c four point ¼ point flexural strength test P - break force L - spacing of bottom supports b,d - width and thickness Samples post room temperature testing

Si-Si hydroxide catalysis bond strength testing

Si – Si Hydroxide Catalysis Bonds - temperature cycling The ability of silicate bonds to withstand repeated temperature cycles must be verified, in addition to withstanding the thermal stresses that may be induced during cooling. Repeated cycles from room temperature to 77K were performed on bonded samples of silicon with no bond failures (in addition to this various samples of different materials including SiO 2 -ZnSe, SiO 2 -Ge, SiO 2 -ULE, SiO 2 ‐Al 2 O 3, all of whom have different coeff. of thermal expansion)

Si – Si Hydroxide Catalysis Bonds - thermal conductivity Florence results Thermal conductivity measurements carried out by Matteo Lorenzini Bonded silicon sample fabricated in Glasgow (1” diameter, mm lengths) Thermal conductivity of bonded sample at low T peak similar to modeled pure silicon with thin ( ~ 700nm) glass-like layer Heat flow BOND T meas Lorenzini et al., ET meeting, 1-3 March 2010, Friedrich-Schiller-Universität Jena.

Si – Si Hydroxide Catalysis Bonds - thermal conductivity ATC/Glasgow thermal conductivity investigations: Preliminary thermal conductance measurements have been carried out between the UK Astronomy Technology Centre (Edinburgh) and Glasgow. Plan to study the dependence of the thermal conductivity as a function of thermal oxide layer thickness. Figure 3: Thermal Conductance vs. Temperature for bulk (reference) and bonded samples with standard deviation errors. The sample follows the form G=aT b where here the values for the bulk and bonded sample a=2.65 ± 0.97 and 0.27 ± 0.01 and b= 1.01 ± 0.19 and 1.29 ± 0.01 respectively, with temperature T.

Thermal Noise associated with bonds Thermal noise Cunningham et al., ET meeting, 1-3 March 2010, Friedrich-Schiller-Universität Jena. At 100 Hz thermal noise associated with 2 bonds is expected to be approximately 5.2 x m/√Hz at 290 K and 1.3 x m/√Hz at 18 K. Further investigations are required to verify Si-Si  bond at low temperatures (above calculations use room temperature SiO 2 -SiO 2  bond = 0.11±0.02)

Summary of bonding work in Glasgow Silicate bonding appears to be a highly promising technique for the construction of cryogenic and ultra-low loss monolithic suspensions Ongoing collaborative research:  Characterisation of mechanical loss at low T  Thermal conductivity measurements below 37 K  Establishing the effect of the type of oxide layer  Technique/chemistry used to achieve bond and how it affect thermomechanical properties