LSST detector components and assemblies: Metrology issues

1
LSST detector components and assembliesMetrology
issues

• Peter Z. Takacs
• Brookhaven National Laboratory
• 23 March 2005
• LSST Camera Project Meeting
• SLAC

This research was sponsored (in part) by the U.S.
Department of Energy under Contract No.
DE-AC02-98CH10886.
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Detector Metrology Issues

• Focal plane flatness requirements - from proposal
  draft doc 2/9/05
• Measure over temperature range and at various
  angular attitudes
• What does it take to insure these requirements
  are met?

1 µm 0.000040 2.5 µm 0.0001 1 ?
0.6 µm
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Example of what could happen

• Several microns of bow and twist possible in
  detector surface

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FPA Metrology Error Budget

• Goal - Focal plane planarity 10 µm PV over 64cm
  dia. area
• Maintain planarity at -100C and at 45 tilt
  attitude angle.
• FPA comprised of
• CCD modules
• Raft plates
• Raft assemblies
• Integrating structure
• Adjustable kinematic mounts
• Other items hanging from raft assemblies
• Specified tolerances
• CCD module planarity 5µm PV
• Raft plate planarity and parallelism 1.0µm
• Raft assembly planarity 6.5µm PV
• Integrating structure planarity 1.0µm,
  attitude deflection 1.0µm
• Measurements on individual components and
  subassemblies required to insure final tolerances

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FPA component structures
5 µm
1 µm
1 µm
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CCD module metrology
Require non-contact measurement methods

• PMI Interferometer for surface flatness -
  aperture 60mm
• White-light micro PMI for edge height
  discontinuity
• Gauge block reference for absolute height
• Vendor guarantee on absolute height (?)

PMI ? Phase Measuring Interferometer
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CCD module metrology considerations

• 201 live individual sensor modules
• Planarity 5µm over 40x40 mm2 area
• Datum plane defined by shoulders on 3 pins
• Absolute height (piston) and tilt relative to
  datum is important
• General case is warped CCD surface --gt not flat
• Do we believe vendor???
• Warping and distortion after cryo test cycles?
• Height measurement requires non-contact optical
  method
• Use gauge block as absolute standard
• White-light micro PMI needed for comparison
  measurement
• Conventional PMI required for non-contact 2D
  topography measurement
• Useful for in-situ measurement through dewar
  window during cryotesting

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Raft plate metrology

• Area 120 x 120 mm2
• Measure before any items assembled onto it
• Thickness tolerance 1µm

Unknowns Kinematic mount configuration - impact
on thickness tolerance??? Measure plate with
kinematic mount assembled?
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Raft plate metrology considerations

• Raft construction - 120 x 120 mm2
• Double-sided plates, top surface lapped plano to
  1µm
• Thickness secondary issue. Adjust with kinematic
  mounts?
• Is 1µm PV parallelism tolerance too tight?
• Insure planarity - want no bowing or twisting
• Changes vs. time and temperature -- causes
  deformation
• Measure surface 2D topography
• Easiest method by conventional PMI -- 200mm
  aperture
• Require top surface to be specular reflector --
  lapped smooth
• Full aperture in one measurement - short time
• Contact probe if non-specular surface -- discrete
  points
• Need CMM with lt 1µm accuracy
• Not good for in-situ cryotesting
• Takes time for complete surface measurement

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Raft assembly metrology

• Flat and coplanar 6.5 µm PV
• Absolute height with kinematic mounts attached,
  ZR
• Distortion at cryo temperatures and off-normal
  attitudes
• Surface deformation of modules after assembly -
  torque effects from pins

• ZR

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Raft assembly metrology considerations

• CCD module mounting pins - force and torque
  transmission into surface
• Easy with 200mm PMI
• Kinematic mount static height adjustment
• Need to know absolute height of detector plane
  above IS surface
• Require dummy IS reference surface during
  assembly
• Gauge block standard with micro PMI for absolute
  edge heights
• Edge discontinuity between modules ? height
  ambiguity
• Requires white light PMI
• Full aperture (200mm) PMI for dynamic surface
  distortions
• Configure for viewing through dewar window and at
  45

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FPA Integrating Structure metrology
Nominal max OD 1 meter

• Planarity tolerance 1 µm PV
• Interface to raft assemblies through kinematic
  mounts
• Map topography of interface surfaces
• Measure distortion vs. temperature and attitude
• Testing with unassembled raft plates installed???
• Requires BIG interferometer

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FPA metrology

• Assemble rafts onto IS
• Edge height continuity
• Dynamic distortion vs. attitude and temperature
• Monitor changes over time
• Full aperture measurement requires steerable
  750mm dia aperture PMI

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Major optical instrumentation required

• White-light micro PMI
• Eliminates edge height ambiguity
• Microscope-based system
• ADE Phase Shift MicroXAM w/200mm stage

• 200mm aperture PMI
• Add beam steering for attitude testing
• (Fisba stock item, on order)

• 750mm aperture white-light Fizeau PMI
• with variable optical delay line
• 4D Technology concept with Jim Burge, UofA

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FPA instrumentation issues

• 200mm aperture Twyman-Green PMI interferometer
• Available from Fisba, currently on order, 110K
• Customize for variable attitude, variable
  stand-off distance
• White-light micro PMI
• FOV 3.3x2.5mm2 at 2.5x magnification
• Essential for edge discontinuity problem
• Available from several sources, start at 55K,
  87K as shown
• Large aperture (750mm) white-light Fizeau
  interferometer
• Remote sensing of module problems within FPA
• New concept, licensed by Zeiss to 4D Technology
• Will require significant development work,
  collaboration with Jim Burge, UofA
• Cost TBD
• Surface plates for clean room workstations
• Gauge blocks
• Fixtures for cryotesting and variable attitude
  testing

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