Shaping Thin Glass Mirrors using Air Bearings
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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

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Shaping Thin Glass Mirrors using Air Bearings

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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

Thin Sheet Glass is Produced in High Volume with High Quality

Hard Disk Drives

Liquid Crystal Displays


Sheet glass manufacturing processes

Sheet Glass Manufacturing Processes

Slot Draw Process

Corning’s Fusion Process


Traditional slumping technology

Traditional Slumping Technology

  • Large thermal asymmetry:

  • Very long slumping cycles

  • Edge curl up

  • CTE mismatch ripples


Problems with conventional slumping technology

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


Shaping thin glass mirrors using air bearings

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

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

Fluid Dynamics Governing Equations

Flow inside porous mandrel

Wherek: 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 h 3 restoring force

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

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

    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


    Assembly steps continued

    Assembly Steps (Continued)


    Mandrel plumbing layout

    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

    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

    Slumping Repeatability: Sheet-to-Sheet

    RMS 0.35 μm

    Frequency

    RMS 8.13 arcsec

    RMS 3.30 arcsec

    Frequency

    Frequency


    Shaping thin glass mirrors using air bearings

    Deep-UV (l=250 nm) Shack-Hartmann Metrology Tool

    Repeatability ~ 40 nm P-V


    Shaping thin glass mirrors using air bearings

    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

    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

    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.


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