Shaping Thin Glass Mirrors using Air Bearings

Shaping Thin Glass Mirrors using Air Bearings Mark L. Schattenburg, Mireille K. Akilian and Ralf K. Heilmann Space Nanotechnology Laboratory Kavli Institute for Astrophysics and Space Research Massachusetts Institute of Technology, Cambridge, MA 02139 International Workshop on Astronomical X-Ray Optics Prague, Czech Republic Dec. 9, 2009

Thin Sheet Glass is Produced in High Volume with High Quality Hard Disk Drives Liquid Crystal Displays

Sheet Glass Manufacturing Processes Slot Draw Process Corning’s Fusion Process

Traditional Slumping Technology • Large thermal asymmetry: • Very long slumping cycles • Edge curl up • CTE mismatch ripples

Problems with Conventional Slumping Technology • Sticking • Dust particles, ripples … • Large thermal asymmetry • Long thermal cycles (days) The traditional approach is not working … We need a new idea! Glass sheet slumped on dusty mandrel

Problem: How to faithfully replicate mandrel surface.Solution: Porous air bearing prevents glass from touching mandrel. Glass surface from manufacturer Glass surface after slumping Flatness ~400 μm P-V (Peak-to-Valley) Flatness <0.4 μm RMS

Air Bearing Slumping Concept Glass sheet is mechanically supported by tungsten wires 100 mm Glass centerline is forced to bearing mid-point

Fluid Dynamics Governing Equations Flow inside porous mandrel Where k: permeability p’: pressure in porous mandrel p: pressure in gap h: air gap height H: porous mandrel thickness Flow in gap h = 15 μm h = 5 μm h = 10 μm

Glass is Pushed to Centerline with 1/h3 Restoring Force Assume: h is constant kx=ky=kz=k Fact:permeability k << h2 h1 > h2 p1 < p2

Slumping onto Flat Surfaces Easiest Plan to Start • OUR THREE-PART PLAN • Crawl: Slump onto flat mandrels: • Low cost mandrels • Simplified metrology • Develop understanding of fluid dymamics • Walk: Slump onto Wolter mandrels (new effort) • Run: Continuous slumpingprocess

Prototype Mandrel Assembly Concept • Tantalum spacers maintain 50 mm glass-mandrel gaps • (temporary solution). • Spacers eliminated in future design. MIT-Kavli Institute

Assembly Steps

Assembly Steps (Continued)

Mandrel Plumbing Layout All experiments to date performed with open-loop pressure control. Closed-loop control has been implemented and is under test.

Slumping Results: Glass Sheet Surface Metrology P-V 2.26 μm; RMS 0.39 μm Frequency RMS 12.22 arcsec RMS 7.53 arcsec Frequency Frequency

Slumping Repeatability: Sheet-to-Sheet RMS 0.35 μm Frequency RMS 8.13 arcsec RMS 3.30 arcsec Frequency Frequency

Deep-UV (l=250 nm) Shack-Hartmann Metrology Tool Repeatability ~ 40 nm P-V

Thin Optic Constraint Double-sided flexures (3) Silicon wafer Reference block Repeatability 55 nm Vertical tilt stage Antenna flexures (4) Horizontal tilt stage

The Next Steps for Air Bearing Slumping … • Implement closed-loop pressure control. • (Completed and under test.) (3) Develop continuous processing of Wolter Optics (2) Eliminate spacers

Summary • Demonstrated a new method for slumping glass using porous air bearings. • Eliminates high-frequency errors due to dust particles 1/h3 restoring force. • Demonstrated sheet-to-sheet repeatability of 3.3 arc sec. • New funding source obtained (2010-2014) Two new Mech. E. Ph.D. students hired Larger slumping facility under development We are grateful to the NASA ROSES program for financial support.

Shaping Thin Glass Mirrors using Air Bearings