Transcript PowerPoint Slides - STScI Confluence

Near-Infrared Detector Arrays
M. Robberto
(with several slides grabbed from J. Beletic, K. Hodapp et al.)
Intrinsic materials
The bandgap depends on the temperature
Eg  Eg0   T ( <0)
e.g. for InSb: Eg = 0.24 eV and β=
-2x10-4eV
material
 c(mm)
Eg(eV)
AgCl
0.39
3.20
CdS
0.52
2.40
GaP
0.55
2.24
CdSe
0.69
1.80
CdTe
0.71
1.45
GaAs
1.35
0.92
Si
1.11
1.12
Ge
1.85
0.67
PbS
2.95
0.42
InAs
3.18
0.39
PbTe
5.0
0.25
PbSe
5.40
0.23
InSb
5.40
0.23
Pb1-xSnxTe
<12.4
>0.10
Hg1-xCdxTe
<12.4
>0.10
Extrinsic materials
P-type ◄
NASA CDR 05-08-01
► N-type
material
 c(mm)
Eg(eV)
Ge:Au
8.27
0.15
Ge:Hg
13.8
0.09
Ge:Cd
20.7
0.06
Ge:Cu
30.2
0.041
Ge:Zn
37.6
0.033
Ge:Be
40
0.03
Ge:B
119.2
0.0104
Ge:Ga
120
0.01
Ge:Li
140
0.009
Si:In
8.00
0.165
Si:Mg
14.3
0.087
Si:Ga
17.1
0.0723
Si:Bi
17.6
0.0706
Si:Al
18.1
0.0685
Si:As
23.1
0.0537
Homework 1
• Small fractional changes in x lead to large
fractional changes in the gap energy. How well we
need to control x at room Temperature to have a
2% uncertainty in response at cutoff for
– HgCdTe 1.72micron cutoff at 145K [WFC3]
– HgCdTe 2.5micron cutoff at 77K [ground based]
– HgCdTe 5micron cutoff at 35K [JWST]
– HgCdTe 10micron cutoff at 35K [NEOCAM]
• Among these, that is the most demanding material
to grow?
Cross-section of
HgCdTe detector
p-on-n
P-on-N design
PN junction
Semiconductor
EF = Fermi Level
=> ½ occupancy at high T
PN junction
Doped semiconductors
P-type
N-type
Impurities (doping) move the EF closer to the valence (Ptype) or conduction (N=type) bands.
PN junction
Depletion or Space Charge region
P-N Junction
P-type
E
N-type
When the two materials are brought into electrical contact, the electrons and hole
diffuse. Recombination occurs until the Fermi levels are in equilibrium.
Depletion Region
• Not Neutral: there is an electric field from the N-type
(+ charged) to the P-type (- charged)
• Free (depleted) of mobile carriers: extremely low
conductivity, or high resistivity.
• An insulator between two charge distributions is a
capacitance.
• The development of the electric field eventually stops
the diffusion: “built-in voltage” or “contact potential”
• The electric field facilitates the flow of charges in one
direction and prevents in the other: diode
E
PN junction
Reverse biased P-N Junction
P-type
E+E
b
N-type
Reverse bias: apply voltage with the same polarity of the contact potential
+ Voltage to the N-type
- Voltage to the P-type
makes depletion region wider and increases the resistance of the junction.
(but do not exagerate! => breakdown)
PN junction illuminated
Reverse biased P-N Junction
P-type
N-type
Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a
hole-electron pair. They will eventually recombine. However, if the electron (minority
carrier in the P-type material), reaches the junction before recombination, it will be swept
on the other side. There it becomes a majority carrier. It will be sensed out if the bias is
kept constant, or recombines with a hole and discharges the junction
If the bias is “floating”, the other original hole, a majority carrier in the sea of holes, will
drift until recombination, calling an electron from ground. A current is generated in the
reverse direction with respect to the original one that set the junction.
(Same is true for photogenerated holes in N-type material).
PN junction illuminated
Reverse biased P-N Junction
P-type
N-type
Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a
hole-electron pair. They will eventually recombine. However, if the electron (minority
carrier in the P-type material), reaches the junction before recombination, it will be swept
on the other side. There it becomes a majority carrier. It will be sensed out if the bias is
kept constant, or recombines with a hole and discharges the junction
If the bias is “floating”, the other original hole, a majority carrier in the sea of holes, will
drift until recombination, calling an electron from ground. A current is generated in the
reverse direction with respect to the original one that set the junction.
(Same is true for photogenerated holes in N-type material).
PN junction illuminated
Reverse biased P-N Junction
P-type
N-type
Assume a photon is absorbed BY THE BULK MATERIAL on the P-type side, creating a
hole-electron pair. They will eventually recombine. However, if the electron (minority
carrier in the P-type material), reaches the junction before recombination, it will be swept
on the other side. There it becomes a majority carrier. It will be sensed out if the bias is
kept constant, or recombines with a hole and discharges the junction
If the bias is “floating”, the other original hole, a majority carrier in the sea of holes, will
drift until recombination, calling an electron from ground. A current is generated in the
reverse direction with respect to the original one that set the junction.
(Same is true for photogenerated holes in N-type material).
Back to zero bias and beyond
P-N Junction
P-type
N-type
Eventually the junction is discharged but photons are still absorbed. The diffusion current
pushes back to maintain the built-in bias. Dark and photocurrent therefore work in
different directions. An equilibrium is reached: saturation.
Reset
Photon detection
Do you see the cross-talk/MTF problem?
End of integration
Reading out the generated charges
• “Hybrid CMOS sensors”
• Indium bumps are aligned, squeezed and distorted to
establish electric contact between detector layer and
multiplexer: COLD-WELDING
• The addressing and readout electronics is built on Silicon.
More standard technology (still >107 transistors).
HAWAII-2: Photolithographically Abut 4 CMOS
Reticles to Produce Each 20482 ROIC
Twelve 20482 ROICs per 8” Wafer
Submicron Stepper Options
Canon 16mm x 14 mm
GCA 20mm x 20 mm
ASML 22mm x 27.4 mm
Reticle-Stitching: 2048x2048 ROIC
20482 Readout Provides Low Read Noise for Visible and MWIR
RSC Approach
HAWAII - 2RG
HgCdTe Astronomy Wide Area Infrared Imager
with 2k2 Resolution, Reference pixels and Guide Mode
• HgCdTe detector
– substrate removed to achieve 0.6 µm sensitivity
• Specifically designed multiplexer
– highly flexible reset and readout options
– optimized for low power and low glow operation
– three-side close buttable
• Two-chip imaging system: MUX + ASIC
– convenient operation with small number of clocks/signals
– lower power, less noise
HAWAII-2RG
Block Diagram
I/O Pads & output buffers
serial
interface
clock
buffers
decoders for horizontal start and stop address
fast guide shift register + logic
fast normal shift register + logic
5 MHz column buffers
glow and crosstalk shield
decoders for vertical start and stop address
Slow guide shift register + logic
Slow normal shift register + logic
glow and crosstalk shield
4 rows and columns containing reference pixels
Additional row of reference pixels
for diagnostic purposes
2048 x 2048 pixel array
(2040 x 2040 sensitive pixels)
4 rows and columns containing reference pixels
• All pads located on
one side (top)
• Approx. 110 doubled
I/O pads (probing
and bonding)
• Three-side close
buttable
• 18 µm pixels
• Total dimensions:
39 x 40.5 mm²
Single Output Mode
4 Output Mode
Fast scan direction selectable
Fast scan direction individually
selectable for each subblock
default scan
directions
Single output for all
2048 x 2048 pixels
(guide mode always
uses single output)
Slow scan direction selectable
Slow scan direction selectable
Output Options
default scan directions
Separate output for
each subblock of
512 x 2048 pixels
Output Options (2)
Slow scan direction selectable
32 Output Mode
Separate output for
each subblock of
64 x 2048 pixels
Four different patterns for
fast scan direction selectable
default scan directions
Interleaved readout of full field and guide window
• Switching between full field and guide
window is possible at any time
 any desired interleaved readout
pattern can be realized
• Three examples for interleaved readout:
1. Read guide window after reading part of
the full field row
2. Read guide window after reading one
full field row
3. Read guide window after reading two or
more full field rows
FPA
Full field
Guide window
Reset Schemes
Pixel by pixel reset
Full field
Guide
window
Line by line reset
Global reset
MIRI Detectors: Si:As IBC
• Extrinsic (vs. HgCdTe, intrinsic)
• Blocked Impurity Band (BIB) extrinsic (vs. “Bulk”)
n - type : 1017 cm -3
p - type : 1012 cm -3
READOUT INTEGRATED CIRCUIT (ROIC)
•
1024 × 1024 / 25 μm pixels
•
7 K Operation
•
Source-Follower-per-Detector (SFD) PMOS input circuit
•
Low Noise: 10 – 12 e- rms with Fowler-8
•
Low Read Glow
•
Low Power: < 0.5 mW
•
4 outputs with interleaved columns
•
Reference pixels on all outputs mimic "dark" detectors
•
Reference output averages noise from 8 "dark" reference pixels
•
2.75 second read time at 10 μsec per sample (100 kHz pixel data rate)
Diode Bias Voltage
Reset
Reset
0.5 V
kTC Noise
Reset-Read Sampling
0V
Time
Readout
Reset Noise in Capacitors
Energy stored in a capacitor:
1 Q2
E=
2 C
Noise floor energy:
E_n = ½kT
Noise Charge: E=En
Qn = kTC
Problem:
Calculate the Reset noise for JWST detectors, assuming: C= 50 fF, T=37 K
Open Shutter
Close Shutter
kTC noise
Reset
Readout
CDS Signal
Diode Bias Voltage
Reset
0.5 V
Double Correlated Sampling
0V
Time
Readout
kTC noise
Readout
MCS Signal
Diode Bias Voltage
Reset
Reset
0.5 V
Fowler (multi) Sampling
0V
Time
Readout
kTC noise
MCS Signal
Diode Bias Voltage
Reset
Reset
0.5 V
Up-the-Ramp Sampling
0V
Time