Orthogonal-Transfer Charge-Coupled Devices and Low-Noise Charge-Coupled Devices Readout Circuits*

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Orthogonal-Transfer Charge-Coupled Devices and
Low-Noise Charge-Coupled Devices Readout
Circuits

• Barry E. Burke

The MIT Lincoln Laboratory portion of this work
was performed under a Collaboration Agreement
between MIT Lincoln Laboratory and The University
of Hawaii, Institute for Astronomy (IfA).
Opinions, interpretations, conclusions, and
recommendations are those of the authors, and do
not necessarily represent the view of the United
States Government.
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Outline

• Review of Orthogonal-Transfer Charge-Coupled
  Devices (OTCCD)
• Development of the orthogonal transfer array
  (OTA)
• Low-noise CCD readout circuits
• Summary

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Conventional vs. Orthogonal-Transfer CCDs
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Application Areas
Videoout

• Compensation of platform motion
• Imaging from unstable and/or moving platforms
• TDI (time delay and integrate) with variable scan
  direction
• Compensation of scene motion
• Ground-based astronomy

Output register
Frame store
Imaging area
OTCCD can noiselessly compensate for scene motion
across sensor during image integration
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Application of OTCCDs in Astronomy
Star-cluster imagery (M71)

• Use OTCCD to remove blurring due random motion of
  star images (electronic tip-tilt)

No motion compensation, ??0.73"
With motion compensation, ??0.50 SNR increase
1.7?
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Outline

• Review of Orthogonal-Transfer Charge-Coupled
  Devices (OTCCD)
• Development of the orthogonal transfer array
  (OTA)
• Low-noise CCD readout circuits
• Summary

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Pan-STARRS(Panoramic Survey Telescope and Rapid
Response System)

• Four 1.8-m telescopes viewing same sky sector
• 3 FOV, 24 mv sensitivity
• High-cadence, wide-field surveys
• Detect variable or moving objects
• 1.4-Gpixel CCD focal-plane array on each telescope

Proposed Pan-STARRS telescope configuration
Gigapixel focal-plane array (64 CCDs)
First Pan-STARRS telescope on Haleakala (PS1)
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Orthogonal-Transfer Array
2.38 arc min

• Wide field-of-view (FOV) imaging
• Wavefront tilt decorrelates over FOV gt few arc
  minutes
• Need 2D array of OTCCDs, each independently
  clocked to track local wavefront tilt (rubber
  focal plane)
• OTA is a new CCD architecture
• Requires on-chip switching logic
• More complex layout and processing than
  conventional CCDs

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Orthogonal Transfer Array
OTA 8?8 array of OTCCD cells
OTA cell with I/O control
Four-phase OTCCD pixels

• New device paradigm
• 2D array of independent OTCCDs
• Independent clocking and readout of OTCCDs
• Advantages
• Enables spatially varying tip-tilt correction
• Isolated defective cells tolerable (higher yield)

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OTA Operation

• Subset (4 5) of cells chosen to image guide
  stars
• Map of wavefront tilt constructed from guide-star
  data and applied to science cells
• Four redundant views of every patch of sky used
  to fill gaps due to
• Guide-star cells
• Dead cells
• Cosmic rays
• Dead areas between cells and devices

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Device Fabrication

• Four OTAs on 150-mm wafer (die size 49.5?50.1 mm)
• Four-poly, n-buried-channel process
• Fabricated on 5 000 ?cm float-zone silicon
  wafers
• Back-illuminated devices thinned to 75 µm

150-mm wafer with four OTAs
Photo of pixel array
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Sample Imagery

• First devices were fully functional but with some
  issues (noise, logic glow)
• Device redesign resolved issues with prototype
  devices
• Redesigned devices have been fabricated and most
  of them packaged

Image from back-illuminated OTA 10-µm pixel, 22.6
Mpixels
Image from OTA cell with fixed light spot and CCD
gates clocked
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Substrate Bias

• Substrate bias enables thick, fully depleted
  devices
• High quantum efficiency, 800-1000 nm
• Small charge point-spread function

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Quantum Efficiency

• Back-surface p using ion-implant/laser anneal
• Two-layer anti-reflection coating with reflection
  null at 850 nm for reduced fringing
• Thicker device clearly superior beyond 800 nm

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OTA Focal Planes

• TC3 focal plane assembled from 16 prototype
  devices on-sky tests in February
• GPC1 assembled from lots 2 and 3 (summer 2007)

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Outline

• Review of Orthogonal-Transfer Charge-Coupled
  Devices (OTCCD)
• Development of the orthogonal transfer array
  (OTA)
• Low-noise CCD readout circuits
• Summary

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CCD Output Circuits
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Output Circuit Comparison
Sense-node capacitance is lower (?higher
responsivity) for JFET than MOSFET
Noise spectral voltage is lower for JFET than
MOSFET
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Noise Comparison
2000-fps, 160?160-pixel imager, 20 ports with
JFET output circuits
Best MOSFET noise vs. preliminary JFET noise
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Summary

• OTCCD developed for astronomical applications but
  has a potentially much broader range of uses
• OTA development for Pan-STARRS is new OTCCD
  concept, also with other applications
• Recent work with JFETs shows noise levels better
  than BCMOSFETs and nearing 1 e-