1. Dark Current Experiment Preliminary Design Review
25 September Bernie Rauscher
SPACE
TELESCOPE
SCIENCE
INSTITUTE

2. Goals of the Review
Demonstrate that we know how to measure dark current
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5 July 2019

3. Definition of Dark Current
Dark current is the signal that accumulates linearly in the absence of any light source
Important contributors to dark current include
Charge diffusion,
Thermal generation-recombination (GR) of charges within the semiconductor, &
Small leakage currents
Strictly speaking, dark current does not include effects such as amplifier glow and multiplexer glow.
Nevertheless, these effects have much the same effect as dark current and therefore require good characterization.
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4. The importance of Reference Pixels
Experience by G. Finger, D. Hall, and others has shown that reference pixels are necessary for measuring dark current at ultra-low background levels
Reference pixels shall be used in a manner TBD to suppress correlated noise and drifts to a level well below the rms noise in each frame.
The next two slides illustrate the importance of reference pixels.
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5. Darkcurrent Measurement using open In bump bonds
Triangles: measured integration ramp
Diamonds: dead pixels
Open In bump bonds are used to monitor drifts
Squares: drift corrected integration ramp
darkcurrent at 28.5 K: e/s/pixel
This slide courtesy of Gert Finger, ESO
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6. Darkcurrent Measurement using open In bump bonds
Triangles: measured integration ramp
Open In bump bonds are used to monitor drifts
Squares: drift corrected integration ramp
darkcurrent at 25 K: e/s/pixel
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7. NGST Requirements from NGST Doc. 641
NGST does not have a dark current requirement. Rather, total rms noise including contributions from read noise and dark current in a 1000 seconds exposure should satisfy the aforementioned requirements.
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8. Dark Current Experiment Requirements
It shall be possible to measure the dark current of a detector meeting the NGST Project’s requirements to 10% accuracy.
Assume 1/√2 of total = 2.5 e- in a 1000 s exposure is shot noise on dark current
idark = 3.12510-3 e-/s/pixel
This implies a requirement on light-tightness of the detector head enclosure =3.12510-4 e-/s/pixel
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9. Dark Current Experiment Requirements
There are implied requirements on per sample read noise
The procedure (described later) will be to take samples at ~15 minute intervals for several (~4) hours and to fit a line.
For this experiment, the per-sample rms noise requirement is =4.9 e-
Reference pixel corrections shall be made so that drifts are negligible compared to rms noise
Least Square Line Fitting
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10. Proposed Experiment Procedure
Blank off detector
If detector has been warm or exposed to light in past 24 hours, drain depletion regions by allowing enough time (TBD) for trapped charge to randomly bleed out of traps (24 hours?).
Stabilize detector bias and temperature
Obtain bias/dark MULTIACCUM ramp exposure as follows
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11. Summary of Requirements
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12. Proposed Experiment Variations
The following would be decided early on and frozen
Length of ramp
Number of samples (TBD) at each point on ramp
Select 1 pixel and do not clock (Cannot use reference pixels when doing this)
The following would be done for each detector
Temperature: 3 levels covering NGST range
Bias levels: 2 levels covering NGST requirement =6104 e- and goal =2105 e- for well capacity
Sample spacing along ramp: 2 spacings ~7.5 min and ~15 min to allow separating intrinsic dark current from glow
Combinations: 12 combinations per detector -> Approx. 1 week per SCA.
Dewar is big enough for 4 SCAs, could save time by testing 4 detectors at a time.
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13. Proposed Experiment Duration
Dominant step in terms of schedule is length of ramp
Time estimate: 6 days per SCA (excluding cooldown/warm up)
Extended scope:
Single pixel readout using custom hardware reference
Use of off-chip pre-amp
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14. Proposed Experiment Designs
Standard TFST hardware (dewar, Leach controller, etc.)
Blank off “top” for detector enclosure
Blank off “top” must include a light-tight vent to allow pumping. Alternatively, a slide can be closed once the system is fully under vacuum.
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15. Data Reduction/Analysis Procedure
Data collected in passthrough mode
Reference pixel correction (TBD)
Fit slope on per-pixel basis
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16. Expected Performance (Accuracy)
Requirements have been set to allow ~10% accuracy measuring NGST’s goal total noise
May be limited by light leaks
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17. Schedule
Dark current is a fundamental parameter that directly effects sensitivity.
Should be one of the first “real” experiments done in lab.
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18. Costs (Shopping List)
“Top” for detector head
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19. Risks System may not be light tight Bias may be insufficiently stable
Potential for major reworks in detector head. Can mitigate this risk by using aluminized tape for critical joints if found to be necessary.
Bias may be insufficiently stable
Can mitigate this with simple custom voltage reference if found to be necessary.
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20. Actions
5 July 2019