A Stress Analysis of Sputter Coatingon the LSST Primary-Tertiary Mirror

Substrate temperature and strain during sputter deposition of aluminum on cast borosilicate glass in a Gemini Observatory coating chamber

by Jacques Sebag et al.

by Blake Coughenour

A Summary of Results from

Scientific Concepts of LSST

• 6 Band: 300nm-1.1μm • Wide Field: f/1.25 viewing

9.62 deg2 (50x moon area)• Camera: 3200 MP• Data: 30 TB of data nightly

Reflective Optical CoatingsMetals Ave Refl Vis / IR High Absorption Comments

Aluminum 92 % / 98 % 700-950nmSensitive deposition necessary to prevent "Blueing.”

Thin layer of protective Al2O3 forms on surface.

Gold 94 % / 98 % 200-550nm Adhesion issues with glass. Very soft surface. Use Chrome as binderlayer.

Silver 95 % / 98 % UV Issues with tarnishing. Very soft surface.

Common Coating ProcessesAluminum Evaporation• Well Tested, Cheap, Simplest Process• Recoated every 12-18 months• Loss of throughput for Visible Observations

"The main advantage of silver is that it reduces the total thermal emission of the telescope. This in turn increases the sensitivity of the mid-infrared instruments on the telescope and allows us to see warm objects like stellar and planetary nurseries significantly better” - Scott Fisher , Gemini Scientist

Silver Sputtering• Allows for Multi-Layered Coating with Thickness Control • Recoated every 12 months• High throughput for Visible-IR observations• Low Absorption yields Low Thermal Background noise

Evaporation vs. Sputtering

Thin Film Evaporation Thin Film Sputtering

ProsFast deposition; Heats Whole Substrate Surface; Good history.

Better adhesion – lasts longer; Target can supply large amounts of material

ConsFilament limits amount of material; Non-uniform adhesion and thickness

Slow deposition rate; Ballistic atoms heat up substrate surface locally

Use Borosilicate Monolithic Mirrors ULE Monolithic Mirrors and ZerodurFigures: http://ns.kopt.co.jp/English/ca_jou-gi/joutyaku.html

Sputtering Procedure

Monolithic Borosilicate MirrorsThermal Characteristics

Thermal Diffusivity (D) 1.11 x10-7 m2/s

CTE () 2.8 ppm/K

Specific Heat (c) 710 J/kg C

Strength Characteristics

Young’s Modulus 58.6 Gpa (8.5 Msi)

Typical Breaking Strength 13.8 Mpa (2000 psi)

Max Tensile Stress 0.7 Mpa (100 psi)

Max Polished Surface Stress 2.1 Mpa (300 psi)

Temperature Stress

Recommended by SOML: 5°C Maximum

Test SystemTested 28mm Borosilicate Samples

with Thermocouples and Strain Gaugesin Gemini Observatory Coating Chamber

Temperature Measurements

Temperature Extrapolation

Strain Measurements

10 kW 40 kW Tensile Limit Surface Limit

Back Surface 14psi 32psi 100 psi 300 psi

Front Surface 24psi 54psi 100 psi 300 psi

Samples had strain gauges mounted on the back surface to measure strain directly (Thermal + Mechanical)

Summary

• A Cast Borosilicate Mirror can be safely coated with Sputtering Deposition Techniques

• Max Temperature Variation below 5°C• Resulting Stresses below 100psi Tensile Stress

Acknowledgements:Jacques Sebag, John Andrew, Douglas Neill, Michael Warner,

Blain Olbert, and Roger Angel

Questions?