Claire Juramy, Pierre Antilogus, Andrew Bradshaw, Craig Lage, Stuart Marshall, Andrew Rasmussen, Yousuke Utsumi LPNHE, Paris - SLAC - UC Davis ...
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Claire Juramy, Pierre Antilogus, Andrew Bradshaw, Craig Lage,
Stuart Marshall, Andrew Rasmussen, Yousuke Utsumi
LPNHE, Paris - SLAC - UC Davis
DeMo2021 - 16th June 2021 1
§ LSST focal plane: rafts (9 CCDs): 13x T-e2v CCD250,
8x ITL STA3800C (+ 4x corner rafts = 3 ITL CCDs)
§ T-e2v CCD250 exhibit variety of «tearing» patterns
in flat-field images: pollute corrections of Pixel
Response Non-Uniformity, astrometric distortions
§ First tests with single-sensor test stand:
understanding generation of tearing patterns,
mitigation in unipolar mode and in bipolar mode
§ Modelling based on sensor physical analysis
§ Applied to full focal plane, limitations on voltage
sets
2
§ T-e2v CCD250 baseline operation: unipolar mode, Back-
Substrate Bias at -70V
§ 16 channels, 16 Mpix, 2s readout
§ Segmentation: ‘corner turn’
§ Blooming stop implant
§ Operating in bipolar mode can have beneficial effects:
serial CTE, dark current, full well
§ Concerns for sensor safety: BSS to -50V, Parallel Low clock
voltage PL > - 6V
§ Parallel clock (P4) used as reset switch for second stage of
output amplifier: ties parallel clock voltages to serial readout
§ Readout electronics:
§ Can drive CCDs with bipolar voltages
§ Cannot shift voltages between exposure and readout (ZTF)
§ Cannot set slopes on clock shifts (voltage-driven)
3
amplitude ~ 7 %
Ø Effect from channel stops: isolated darker and lighter columns are paired,
surrounded by excess/deficit respectively
Ø Same mechanism as brighter-fatter, changing charge carriers and location
4
§ Skyline pattern: observed at high deltaP (> 9.5V), builds up from middle of sensor
§ Thin tearing: excess of charges comes before deficit in readout: not an issue with
parallel transfer
5
§ Tearing boundaries: better uniformity inside
§ Outside: ‘divisadero’ appears at segment edges
§ Divisadero also appears in frames with no tearing patterns
§ Deficit of holes in the single channel stop between
segments
~ 3% amplitude, symmetrical across segment edges,
5+ columns wide
6
§ Single sensor: operating in bipolar mode with PL < -6.5V
removes the tearing and the divisadero
§ Raft (9 sensors): one sensor with a strong bright defect
required an added ‘purge’ step to remove the tearing
pattern.
§ All four phases of parallel clocks set to low at the same time
§ Right before each exposure
§ Similar operations in p-CCDs
§ In inversion: holes can circulate
§ ‘Purging’ (even with PL = 0V) = smoothing out hole
distributions in channel stops
§ Test procedure: start with a ‘purge’ at -7V, then observe
emergence of patterns
purged at 0V purged at -7V
S1+S2 high 7
Purge at -7V
Bipolar read Unipolar read Unipolar read (no Unipolar clear(s)
(no clear) clear), high PU and read
Divisadero
No distortions
Read (no/fast clear) Read (no/fast clear) Read (no/fast clear) Unipolar clear(s)
and read
No distortions Divisadero Tearing pattern (large) Tearing pattern (thin)
§ Distortions in one frame depend on the readout mode in the previous frame, and on
whether there is a clear, but not on the readout mode of the current frame
§ Incoming photoelectrons are affected by the hole densities fixed before exposure
8
§ Reverse clocking (1,000 rows) in unipolar mode, with high PU value (>9.5 V): reverse
tearing pattern appears near edges of sensor
§ Pattern matches model that holes have been pushed away from the sensor edges
9
§ “Physical and electrical analysis of LSST sensors”, Lage, C., arXiv:1911.09577
§ Based on SEM micrograph of deprocessed CCD, SIMS analysis
§ Channel stops: ‘dots’ of boron implants (not the case for ITL STA3800C)
(before applying area factor) 10§ Model: “Poisson_CCD: A dedicated
simulator for modeling CCDs”, Lage,
C., Bradshaw, A., Tyson, T.,
arXiv:1911.09038
§ Solves Poisson equations numerically
for potential and free carrier densities
§ Simulates photo-electron propagation
§ Can repeat simulation with different
conditions to simulate CCD charge
transport
§ Boron dots act as hole traps
(depending on parallel clock voltages)
11§ Poisson_CCD: compares PL = -6V and PL = -8V: hole density under clock low phases
§ With PU = + 3.5V, 10,000 e-, 190,600 h+
12§ Poisson_CCD: compares PL = -6V and PL = -8V: hole density during ‘purge’ phase
§ With PU = PL, 0 e-, 370,100 h+
13§ Unipolar mode: tearing, or divisadero only (with fast clear)
§ Bipolar mode: variety of structures depending on working conditions and sensor
14§ Path for holes at segment edges, affected
by level of serial clocks
§ Serial clock states during purge
§ Serial flush during exposure
§ Blooming stop
§ Depletion near sensor edges
§ Parallel clock propagation
15§ Fractional displaced signal (per-row): fds = 4*(s-2+s+2-s-1-s+1)/(s-2+s-1+s1+s2)
(from A. Rasmussen, 2014 JINST 9 C04027)
16§ Parallel clock voltages: deltaP = 9.3 V
§ ‘Purge’ duration: 3 ms
§ No serial flush during exposure
§ Longer parallel transfer times (improved full well)
§ Tried other mitigations: reverse flush (issue with bright columns, amplifier boundaries,
reverse tearing)
1718
§ Combination of divisadero and brighter-fatter
§ Shape and amplitude match
19§ Early idea: holes generated and removed
by parallel clocking
§ Simpler model: in unipolar mode, there is a
number of holes confined to each channel
stop.
§ After a purge at -7V, when shifting back to
unipolar voltages, the holes come
back/never really left, and are frozen into
the channel stops with a uniform-looking
distribution
§ Parallel clocking for clearing / readout can § In the other channel stops, holes
move the holes in the same direction, with accumulate on the serial register side
efficiency depending on exact parameters until the hole density reaches a limit.
§ In the channel stops at the segment edges, § These two effects cumulate to create
the holes arriving at the serial register can the “divisadero” pattern.
get out.
20§ Transition from “rabbit ears” to tearing
patterns: the clocking moves the holes
efficiently enough along the channel
stops to empty them, starting from the
mid-line.
§ Wide tearing pattern: the edge of the
hole density moves in the readout
direction, until it is blocked by the
accumulated holes, and the tearing
pattern stabilizes.
§ Thin tearing pattern caused by clearing
(and not readout): clearing sequence is
more efficient at moving the holes.
§ Non-uniformity along columns:
§ Clocking in bipolar mode: holes can
difference in total hole content or in move between channel stops, smooths
hole capacity between channel stops.
out distribution
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