1. Bonding as an assembly technique in upgrades to aLIGO and detectors like ET and KAGRA R. Douglas 1, A. A. van Veggel 1, K. Haughian 1, D. Chen 2, L. Cunningham 1, R. Glaser 3, G. Hammond 1, G. Hofmann 3, J. Hough 1, M. Masso Reid 1, P. Murray 1, R. Nawrodt 3, M. Phelps 1, S. Reid 4, S. Rowan 1, Y. Sakakibara 2, T. Suzuki 5, C. Schwarz 3, K. Yamamoto 2 1 SUPA, Institute for Gravitational Research, University of Glasgow, Glasgow, UK 2 ICRR, University of Tokyo, Tokyo, Japan 3 Friedrich Schiller University, Jena, Germany 4 SUPA, University of the West of Scotland, Paisley, UK 5 High Energy Accelerator Research Organisation, Tokyo, Japan Amaldi Gwangju, June 2015

2. LIGO-G1500838 GEO600, aLIGO, and advanced VIRGO use quasi-monolithic fused silica test mass suspensions with superior thermal noise performance at room temperature Hydroxide catalysis bonding (HCB) is used in all to attach interface pieces to the mirrors, allowing attachment of the fibres (which are welded) We are considering upgrades to detectors at room temperature (A+ and ET- HF) and various cryogenic temperatures (Voyager, ET-LF, KAGRA) Introduction 2

3. LIGO-G1500838 Chemistry 3 This method can create strong, durable bonds. Chemistry of bonding between silica surfaces: Dehydration Polymerisation Hydration and Etching Bonding solution with an excess of free OH - ions being released from into the

4. LIGO-G1500838 Feasibility study: how? 4 Can we bond sapphire and silicon using HCB?  Yes, we can  Silicon surfaces must be oxidised to facilitate reliable bonding  Chemistry is different for sapphire: aluminate chains Are there other jointing techniques that may be suitable?  Indium bonding but only under compression and if suspension is not baked out What is the strength of bonds in conditions consistent with those of cryogenic suspension, assembly and operation?

5. LIGO-G1500838 Bond Strength 5 Two types of strength tests carried out: 4 point bend tester to measure tensile strength and a torsional shear strength tester. Load, F Bond 4 contact points d L Bonded sample Where T is the torque, a is the length and b is the width of the cross-section of the sample Where b is the breadth of the sample l

6. LIGO-G1500838 Temperature 120 100 80 60 40 20 0 Shear strength [MPa] R.T.77 K 6 Cryogenic strength of HCBs between sapphire 40x10x5 mm samples, 10x5 mm M-to-M-plane bond in the centre Results published Douglas et al., CQG, 2014 Room temperature pH 12 Pristine samples M-to-M 120 100 80 60 40 20 0 Shear strength [MPa] Sodium aluminate Sodium hydroxide Potassium hydroxide Sodium silicate Bonding solution Load, F Bon d 4 line contacts d L l Bonde d sampl e Tensile strength [MPa]

7. LIGO-G1500838 Bond strength: sapphire, curing time 7 Load, F Bon d 4 line contacts d L l Bonded sample Curing time for bonds between silica has been found to be ~4 weeks. All C-C samples broke clean across the bond. Strength within error the same for the 3 different curing times. Curing time of sapphire doesn’t appear to be longer than 4 weeks. Investigations are ongoing. Strength for A-A, M-M and C- C within error the same. Bonded sapphire sample 5x5x40 mm. C-C plane. 5x5 mm bonding surface

8. LIGO-G1500838 Bond strength: silicon, thermal cycling 8 Load, F Bon d 4 line contacts d L l Bonde d sampl e Check the effect of thermal cycling on the strength of bonds as suspended silicon masses are likely to go through multiple thermal cycles in their lifetime. 40 bonds tested in set 1, 28 bonds tested in set 2. Thermal cycling done in Jena ~4 K – ~293 K, 2 hours per cycle. Oxidised silicon sample 10x5x20 mm Boron doped P-type ~150 nm dry thermal oxide Observations: Overall average strength doesn’t change Two strength groups (thought to be linked to bond quality).

9. LIGO-G1500838 Bond strength: questions remaining? Silicon What is the minimum oxide layer thickness for ion-beam sputtered coatings? What is causing difference in strength between different sets of bonds: bond quality? How can we best to assess this? Can strength/mechanical loss be improved through heat/vacuum treatment? What is the curing time? Sapphire Influence of thermal expansion differences between different crystal orientations on bond strength when bonding e.g. c-a plane or c-m plane? Influence of polishing process/surface damage on strength. Literature study suggests surface reactivity higher for mechanically polished surfaces as opposed to pristine surfaces Can strength/mechanical loss be improved through heat/vacuum treatment? Bond strength of sapphire not polished or cut on specific crystal plane? 9

10. LIGO-G1500838 Thermal conductivity 10 T1T1 T2T2 Heater used to induce heat flow Heater and temperature sensor used to thermally stabilise the clamp ΔLΔL Clamped silicon sample Thermal conductivity has now been measured of: 1.Silicon-silicon HCB bonded cylinders (  25 mm x 76 mm) down to 30 K (Lorenzini, ET meeting Jena, 2010) 2.Silicon-silicon HCB bonded samples (5x5x40 mm) down to 8 K 3.Sapphire-sapphire HCB bonded samples (Poster C. Schwarz, D. Chen, ET meeting 2014, Lyon). 4.Sapphire-sapphire indium bonded sample Results suggest drop in thermal conductivity is very small.

11. LIGO-G1500838 Summary 11 HCB between silicon and sapphire have been extensively demonstrated to be strong enough for typical gravitational wave detector suspensions Thermal cycling of HCB bonded silicon does not negatively impact strength of bonds but more work is needed to be able to assess bond quality Thermal conductivity measurements of HCB and indium bonds appear to be high enough for suspension attachments. Aim: Understand better the influence of cross-sectional size on thermal conductivity For more information see Marielle van Veggel’s recent GWADW talk (LIGO-G15008) which includes details on loss measurements.

12. LIGO-G1500838 Thank you for your attention 12 Thank you for your attention Any questions?

13. LIGO-G1500838

14. Bond strength: what do we know? Silicon In Glasgow we bond with sodium silicate solution. (Potassium hydroxide is also possible, see Dari et al., CQG, 2010) Using a wet thermal oxide layer, we need 50 nm oxide layer to get reliably strong bonds (Beveridge et al., CQG, 2011) Strengths tested at R.T. and at 77 K Ion beam sputtered oxide layer is an excellent and probably more practical alternative (Beveridge et al., CQG, 2013) Average strength values are 20–40 MPa (depending on type of material, crystal orientation, temperature of test. Strength at 77 K higher than at R.T. Recent thermal cycling tests (3,10 and 20 times down to ~8 K) of - silicon (2 sets tested), suggest that the average stays constant. We are developing techniques to assess the quality of bonds during the bonding procedure to ensure only high quality bonds are accepted. 14

15. LIGO-G1500838 Stress due to thermal expansion differences 15

16. LIGO-G1500838 Bond mechanical loss, overview Silica-silica HCB bond made with sodium silicate solution @ RT Bond loss: 0.11 ± 0.02 (Cunningham, CQG, 2010) Silica-silica HCB bond (same as above) after 3 years and after heat treatment 150  C for 48 hours. Bond loss: 0.06 ± 0.01 (Haughian, thesis, 2012) Silicon-silicon HCB bond made with sodium silicate solution @ RT. Not an ideal bond (offset). Bond loss: 0.27-0.52 (Haughian, thesis, 2012) Sapphire-sapphire HCB made with sodium silicate solution @ RT temperature Bond loss @ R.T. 0.03± 0.01 Bond loss @ 20 K (3 - 7)x10 -4 ± 1x10 -4 Sapphire-sapphire indium bond Bond loss @ 20 K (2 – 3)x10 -3 (Talk K. Yamamoto, ET meeting 2014, Lyon). 16

17. LIGO-G1500838 17 Thermal noise associated with HCBs in aLIGO Hild, S., CQG. 29 (2012) 124006 In aLIGO thermal noise arising from bonds was to be 10% of the total thermal noise budget @ 100 Hz Thermal noise budget @ 100 Hz is 2.5  10 -24 /  Hz  Bond thermal noise budget per test mass 7  10 −22 m/√Hz, (T010075- 01,2009) Calculated: 5.4  10 −22 m/√Hz ) assuming bond loss of 0.11 at 293 K Assumptions for calculations following: bond thermal noise budget < 10% of overall thermal noise budget @ 100 Hz Ear size (where unknown): same ratios and average shear stress (0.17 MPa) as aLIGO.

18. LIGO-G1500838 Thermal noise from bonds for cryogenic detectors 18 Name detectorA+ 2 ET-LF 1 (option 1)ET-LF 1 (option 2) TM materialSilicasiliconsapphire Temperature TM [K]29310 Bond area 1 ear [mm x mm]39x11845x136 Assumed bond thermal noise requirement [m/  Hz] 3  10 -22 2  10 -22 Bond loss [-] *1.1  10 -1 **1.1  10 -1 *7  10 -4 Calculated expected thermal noise (±15%) [m/  Hz] 5.3  10 -22 1.4  10 -22 1.2  10 -23 ** Assumed value (bond loss) silica-silica bond @ R.T* Measured value