Cryogenic Si-Si Bond Strength Testing 2nd ET Annual Workshop 14-16 October 2009 Cryogenic Si-Si Bond Strength Testing Nicola Beveridge1, Mariëlle van Veggel1, Jim Hough1, Sheila Rowan1, Ronny Nawrodt1, Stuart Reid1, Bostjan Besentzek1, John Davidson2, Donald Nicholson3 1 Institute for Gravitational Research, University of Glasgow 2 Faculty of Mechanical Engineering, University of Glasgow 3 Faculty of Electrical Engineering, University of Glasgow

Introduction to Silicate Bonding Originally developed for NASA’s Gravity Probe B mission, launched April 2004. (Gwo et al., patent) Construction of the ultra-rigid, ultra-stable optical benches for the LISA Pathfinder mission. GEO600 currently operates with quasi monolithic fused silica suspensions and mirrors. Advanced LIGO upgrades include silica suspensions similar to that used in GEO600 High bond strength and ability to withstand the forces and thermal cycling associated with space launches and conditions J R Smith, G Cagnoli, D R M Crooks - Class. Quantum Grav. 21 (2004) S1091–S1098

Hydroxy-Catalysis Bonding Step 1 of 3 – Hydration The surface of silica is hydrophilic and will attract OH− ions to fill any open bonds from the silica (hydration). During the bonding process, cleaning the bonding surface with cerium oxide makes the surface hydrophillic.

Hydroxy-Catalysis Bonding Step 2 of 3 – Etching Placing a solution with a high concentration of OH− ions on the surface of silica causes etching to take place.

Hydroxy-Catalysis Bonding Step 3 of 3 – Polymerisation When the concentration of Si(OH)4 molecules reaches ~2%, the solution polymerises and becomes “rigid”. pH < 11: the silicate ion hydrolyses to soluble Si(OH)4 and OH− when the pH is below 11 Si(OH)4 is a monomer which likes to form a polymer arrangement Bonding requires a silica bulk to bond to R.K. Iler, 1979, The Chemistry of Silica

Reducing Thermal Noise A fundamental noise source caused by thermally driven fluctuations in the interferometer optics and their suspensions Intent is to reduce thermal noise [Punturo, ET talk at the LSC meeting, Amsterdam 2008]

Reducing Thermal Noise Cryogenic operating temperatures Monolithic Suspension Low thermal noise Low loss in pendulum modes Matched thermal expansion High thermal conductivity Silicon suspension technology (E.T.) GEO600 Advanced LIGO Cryo temp necessitates a change in material – high dissipation peak in silica Mono sus has low loss in pendulum modes in operational frequency band.

3rd Generation Detectors Schematic of a 3rd gen monolithic suspension Use silicate bonding to joint the Si suspensions to the Si mass

Research Aims Strength testing of silicate bonding for use in 3rd generation detectors Strength of silicon-silicon silicate bonds at cryogenic temperature To determine if bonding silicon to silicon is strong enough to bear the weight of the mass. Zwick-Rowell 250 machine

Sample Preparation Silicon pieces oxidised in wet N2 environment In order to bond,

Testing Set-up ASTM C1161-02c four point ¼ point flexural strength test P L Silicon piece size: 5 x 10 x 20 mm (b x d x l) Bonding surface has PV flatness < 60 nm

Previous Research Presented in Erice, October 2009 Average silica-silica bond strength Presented in Erice, October 2009

Previous Research Average strength of silicate bonds between samples of oxidised silicon higher than silicate bonds between pieces of silica Strength of bond is not significantly changed by cooling to cryogenic temperatures No apparent correlation between oxide layer thickness and bond strength 70 Silicon samples oxidised at reduced times 10 samples left with native oxide (<10nm) 40 Pairs bonded – 6 failed prior to testing

Results

Conclusions Combined oxide layer minimum of ~80 nm

Future Work Add control samples Shear and tensile strength tests Alternative oxidation techniques Thermal conductivity Bond loss measurements