Silicate bonding on silicon and silica

1. Silicate bonding on silicon and silica S. Reid, J. Hough, I. Martin, P. Murray, S. Rowan,J. Scott, M.v. Veggel University of Glasgow

2. Introduction: Current applications • Originally developed for NASA’s Gravity Probe B mission, launched April 2004. (Gwo et al., patent) • GEO600 currently operates with quasi-monolithic fused silica suspensions and mirrors. This technology allows improved thermal noise in the suspension systems. • Construction of the ultra-rigid, ultra-stable optical benches for the LISA Pathfinder mission. Picture of a GEO600 sized silica test mass in Glasgow with silica ears jointed using hydroxy-catalysis bonding Silica fibres are welded to the ears in the completion of the lower-stage of the GEO600 mirror suspension.

3. 10-21 10-22 10-23 10-24 h Introduction: Planned applications eg. LIGO AdvLIGO • The upgrades for Advanced LIGO plan to incorporate the GEO600 technology for significantly improved thermal noise performance – and under consideration for Advanced VIRGO (in addition to other improvements, e.g higher power lasers). • Construction of the ultra-rigid, ultra-stable optical benches for LISA. Wire loop arm length: 4 km Quadruple stagesilica ribbons/fibres Design sensitivity curves for the LIGO and AdvLIGO detectors.

4. Introduction: Future applications (silicon) • The construction of a 3rd generation gravitational wave observatory within Europe E.T. (Einstein Telescope) and under consideration for the construction of the DUAL resonant mass detector. M. Punturo et al.

5. Studies on silicate bonding of silica in relationto the Advanced generation of GW detectors • Settling time • investigate details of the underlying chemistry e.g. Through the Arrenhius equation • time available for bond adjustment and alignment • Bond structural properties • Close inspection of bonds through electron microsopy can reveal properties such as: bond thickness, molecular structures in addition to imperfections/inhomogeneities at the microscopy-scale. • Bond mechanical properties • Mechanical strength (studying the factors responsible for strength and reliability) • Mechanical loss in addition to bond thickness will allow the level of mechanical dissipation associated with the bond layer to be calculated – thus allowing precise modelling of the level of thermal noise expected from silicate bonds in future gravitational wave detectors.

6. Studies on silicate bonding of silica in relationto the Advanced generation of GW detectors Settling time experiments Bond structural properties Bond mechanical properties SEM SEM Above plot showing two bonded silica cylinders, studied before and after silicate bonding. Above plot showing settling time as a function of temperature for silica-silica bonds TEM Activation energy:Ea = 0.545 eV permolecule of OH− AFM Experiments suggest that the level of loss associated with silicate bonding may lie:fbond ~ (0.3→1.2)×10-1(across the differentmeasured modes). S. Reid et al.,PLA 363 341-345 (2006) TEM (81±4) nm 7.9 GPa

7. Required studies on silicate bondingof silicon in relation to future detectors • Settling time • Bond structural properties • Bond mechanical properties • Surface preparation (oxidisation techniques) • Thermomechanical properties • Temperature cycling effects/failures In addition to characterising these properties in relation to silicon-silicon bonds, it is also necessary to understand the required surface preparation and the cooling performance available for bonded silicon components.

8. Wealth of literature on oxidation techniques • For example: Deal, B.E., Grove, A.S. General relationship for the thermal oxidation of silicon. Journal of Applied Physics, vol. 36, no. 12, pp. 3770 – 3778, 1965 • Found quantitative relations of the rate of growth of thermal oxide

9. Relevant literature knowledge • Qualitative statements B.E. Deal, A.S. Grove • Wet oxidised surfaces give a less dense silicon oxide then dry oxidised surfaces – possibility of having an effect on bond strength and thermal noise. • Carrier gas for wet oxidation doesn’t make a difference in oxidation speed (nitrogen or oxygen). Thus undissociated H2O is the oxidising agent. • In dry oxidation molecular oxygen is oxidising agent. • Flow speed in wet oxidation doesn’t influence speed of oxidation. • Higher oxidation temperature gives higher density.

10. Visable appearance of thermal oxides on silicon • Color of oxide as a function of thickness • Note, this is also dependent on viewing angle.

11. Oxidation results (oxide colors) in Glasgow • Shown layer thicknesses are expected layer thicknesses • Colours don’t match with corresponding layer thicknesses on graph in previous slide • Colours don’t match between wet and dry oxidation of the same prospected thickness

12. Oxidation results (change in flatness) Spikes on edge of surface Localised dip in surface 100 nm wet 1000C 100 nm dry 1000C 50 nm wet 1000C 100 nm dry 920C 200 nm wet 1000C 50 nm dry 1000C 200 nm dry 1000C

13. Oxidation results (change in flatness)

14. Bond thickness • Comparison of silica-silicon SEM images 40 nm

15. Thermal conductivity of silicate bondsin collaboration with Firenze • The first set of silicate bonded silicon-silicon have been fabricated with varying volumes of 1:6 sodium silicate solution at Glasgow. • Samples sent to Florence for thermal conductivity measurements. • Volumes of bonding solution: 0.4 ml cm-2, 0.2 ml cm-2 and0.1 ml cm-2. (Advanced LIGO specification) • See following talk by Enrico Campagna. diameter = 25.4 mm 12.7 mm

16. Mechanical strength of bonds • Initial tests showed that a pair of silicate bonded 1” silicon disks supported 40Kg for 2 week (~1 MPa). clamped sample wire loop rubber ring 40Kg lightly clamped 40Kg load suspended Si-Si sample under load

17. Test setups for mechanical strengthtesting of silicon-silicon • New strength testing setups have been designed and ready for use.(M. v. Veggel) pure shearstrength test Four-point bending test (peeling test)

18. 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 SiO2-ZnSe, SiO2-Ge, SiO2-ULE, SiO2‐Al2O3, all of whom have different coeff. of thermal expansion)

19. Conclusion • Silicate bonding appears to be a highly promising technique for the construction of cryogenic and ultra-low loss monolithic suspensions • Current estimates suggest that the thermal noise associated with silicate bonding will have a negligible contribution to the overall thermal noise in Advanced LIGO and likewise Advanced VIRGO. • Future work: extend the studies of bond thermal noise