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DoE Basic Energy Sciences (BES)Neutron & Photon Detector Workshop
August 1-3, 2012
Gaithersburg, Maryland
Detector Electronics
Helmuth Spieler
Detector System Tutorials athttp://www-physics.lbl.gov/~spieler
or simply web-search “spieler detectors”
More detailed discussions inH. Spieler: Semiconductor Detector Systems, Oxford University Press, 2005
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Many new detector systems require combinations of extreme
High event rates
Fast Timing
High position resolution
High energy resolution
Single particle detection
Although individual characteristics have been implemented, the combination ofmultiple performance levels often has not been demonstrated.
Novel detectors often build on a range of different concepts.
They are often viewed as impractical
“only demonstrated readout systems will be accepted”
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Detector Readout Functions
Interaction Rates
Position Sensing
Energy Distribution
Timing
Some experiments require only one function, but many require a combination.
Optimizing – i.e. achieving the optimum electrical performance of all components– is often not essential.
Finding a compromise that achieves the required experiment performance isusually the goal.
This does require understanding the experimental goals and cross-coupledcontributions – often the optimum solution is not the original concept.
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Some Aspects of Function Requirements
1. Interaction RatesSignal-to-noise ratio
If the signal-to-noise ratio is too low, counts of noise pulses will besignificant
Rate capabilityCount rates are limited by
Sensor collection times
Electronic bandwidthData readout rate
2. Position Sensing
Sensor segmentationSegment signal distribution
Timing – interactions at various distances, also within the sensor
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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3. Energy Distribution
Energy resolutionSensor statistical fluctuations
Electronic NoiseDigitization accuracy
4. Timing
Sensor charge collection time distribution– measuring the current pulse may be better than the charge
Electronic rise time – i.e. bandwidthChanges in detector signal amplitude – “time walk”
Transport time fluctuations and jitter
Minimum timing requires very different circuitry than energymeasurements,
but compromises with energy resolution are often practical.Timing variations due to changes in detector signal pulse shape.
Timing shifts can also arise in the detector ...
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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z0= 50 m z0= 200 m
0
10
20
30
40
50
60
0 1 2 3 4
TIME (ns)
CU
RR
ENT
(pA
)
Varying delays in signal amplitude due to x-ray interaction location z0:
Induced signal current depends on the coupling of the charge by its electric fieldto the electrode
With small electrodes the magnitude ofthe induced current increases greatlywhen the charge is close to the electrode.
For x-rays this shifts the pulse arrival
Note: Commonly presented energyconservation theories forinduced charge are generally wrong.
Initially, charge is inducedover many strips.
As the charge approachesthe strips, the signaldistributes over fewer strips.
When the charge is close tothe strips, the signal isconcentrated over few strips
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Detector Systems – Conflicts and Compromises
Fast event rate High-speed electronics
Increased electronic noise
Degraded signal-to-noise ratio
Alternative: Segmentation – strips or pixels
Reduce the event rate per channel
Allows longer shaping time
Reduces electronic noise
Data readout rate can limit the event rate
Record time intervals and on the readout chipassign to events
Readout does not have to synchronize with incident hits
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Position Sensing Segmentation – strips or pixels
Strips yield one-dimensional position sensing
Position resolution is determined by strip-pitch and transverse diffusion.
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Position resolution requires a low input impedance of the preamp:
Amplifiers must have a low inputimpedance to reduce transfer ofcharge through capacitance toneighboring readout channels.
For electrode pitches that are smaller than the bulk thickness, the capacitance is dominatedby the fringing capacitance to the neighboring strips CSS.
Example: 1 – 2 pF/cm for strip pitches of 25 – 100 m on Si.
The backplane capacitance bC is typically 20% of the strip-to-strip capacitance.
C C C C
C C C C C
ss ss ss ss
b b b b bSTRIPDETECTOR
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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The input impedance of the amplifier must be small at the relevant frequencies –determined by the pulse shaper.
Two different shapers with the same 100 ns peaking time:
The CR-RC shaper appears narrower, but reaches to higher frequencies.The frequency range scales inversely with shaping time.Charge-Sensitive preamplifiers are commonly viewed as yielding a large effectiveinput capacitance. In reality, they commonly have an input resistance.
0 4 8 12 16 20FREQUENCY (MHz)
0
0.2
0.4
0.6
0.8
1
MA
GN
ITU
DE
0 4 8 12 16 20FREQUENCY (MHz)
0
0.2
0.4
0.6
0.8
1
MAG
NIT
UD
E
CR-RC SHAPER100 ns PEAKING TIME
CR-4RC SHAPER100 ns PEAKING TIME
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Pulse Response of a Basic Amplifier
A voltage step ( )iv t at the input causes a current step ( )oi t at the output of the transistor.For the output voltage to change, the capacitance OC at the output must first charge up.
The output voltage changes with a time constant L OR C ,where LR is the output load resistance.
The time constant corresponds to the upper cutoff frequency :1
2 uf
log A
log
v
v0
v
UPPER CUTOFF FREQUENCY 2 fu
V0
FREQUENCY DOMAIN TIME DOMAIN
INPUT OUTPUT
A
A = 10 V = V t0 ( )1 exp( / ) R
R
1L
LC
Co o
g Rm L gmi Co
=
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Input Impedance of aCharge-Sensitive AmplifierInput impedance
( 1)1
f fi
Z ZZ A
A A
For no amplifier phase shift, afeedback capacitor will yield a largeinput capacitance.
However, amplifier gain vs. frequency beyond the upper cutoff frequency0A
i
Feedback impedance 1ff
ZC
i
Input Impedance0 0
1 1i
f f
ZC C
i
i
Imaginary component vanishes low frequencies ( f < fu): capacitive input
high frequencies ( f > fu): resistive input
Very many charge-sensitive amplifiers operate in the 90 phase shift regime. Resistive input
v
Q
C
C vi
i
f
d o
ADETECTOR
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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However ... Note that the input impedance varies with frequency.
Example: cutoff frequencies at 10 kHz and 100 MHz, low frequency gain = 103
The relevant frequency range is determined by the frequency passband of thepulse shaper. This is 5 – 15 MHz for a typical 20 ns shaper, so in this examplethe ohmic input is effective at much longer shaping times.
103 104 105 106 107 108 109FREQUENCY (Hz)
0.001
0.01
0.1
1
10
100
1000
OP
EN
LOO
PG
AIN
|Av0
|
0
40
80
120
160
200
PH
AS
E(deg)
103 104 105 106 107 108 109FREQUENCY (Hz)
102
103
104
105
106
INP
UT
IMP
ED
AN
CE
()
-100
-80
-60
-40
-20
0
PH
AS
E(deg)
GAIN
PHASE
IMPEDANCE
PHASE
OPEN LOOP GAIN AND PHASE INPUT IMPEDANCE (Cf = 1 pF)
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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In the resistive regime theinput impedance
0
1i
f
ZC
,
where fC is the feedback capacitanceand 0 is the extrapolated unity gainfrequency in the 90 phase shift regime.
Low-power amplifiers with a gain-bandwidth product much greater than in thisexample are quite practical, so smaller feedback capacitances are also possible.
Time Response of a Charge-Sensitive Amplifier
Input resistance and detector capacitance form RC time constant: i i DRC
0
1i D
f
CC
103 104 105 106 107 108 109FREQUENCY (Hz)
0.001
0.01
0.1
1
10
100
1000
OP
EN
LOO
PG
AIN
|Av0
|
0
40
80
120
160
200
PH
ASE
(deg)GAIN
PHASE0
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Crossed strips provide 2-dimensional position sensing
“Fake hits” at high rates
y
x
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Problem: Ambiguities with multiple simultaneous hits (“ghosting”)
HITGHOST
n hits in acceptance field n x-coordinatesn y-coordinates
n2 combinationsof which 2n n are “ghosts”
Pixels
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Energy DistributionPixels are also advantageous in achieving low electronic noise.
Equivalent Noise Charge: 2 2 2 2 21
n n i S n v d vf f dS
Q i FT e F C F A CT
ST Shaping Timeni Spectral noise current density, e.g.
2 2n biasi eI strip lengthdC Detector capacitance strip length
ne Amplifier spectral noise voltage density2 1n
m
eg
, ,i v vfF F F "Shape Factors" that are determined by pulse shaper
In many applications the noise voltage contribution dominates:
The main noise source is within the transistor, forming an output noise current.
Transferred to the input, the amplifier spectral noise voltage density ne resultsfrom dividing the output noise current by the transistor gain, i.e. thetransconductance mg .
How does transconductance depend on the current (power) of theinput transistor?
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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In analog circuitry the current draw is driven by the requirements of noise andspeed.
Both depend on transconductance CmBE
dIgdV
(BJT) or DmGS
dIgdV
(FET).
FET transconductance is a non-linear function of current (W =100, L= 0.8 m):
Power efficiency depends on transconductance per unit current /m Dg I .
0 2 4 6DRAIN CURRENT ID (mA)
0
2
4
6
8
TRA
NS
CO
ND
UC
TAN
CE
(mS
)
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2DRAIN CURRENT ID (A)
10-8
10-7
10-6
10-5
10-4
10-3
10-2
TRA
NSC
ON
DU
CTA
NC
E(S
)
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Measurements on 0.8 m CMOS process for different channel lengths L
For a given device the x-values are proportional to device current.e.g. for W 100 m, /DI W 10 corresponds to a current of 1 mA.
Traditional detector front-ends were designed to minimize noise, but accepting a3 to 5-fold increase in noise reduces power by orders of magnitude!
10-4 10-3 10-2 10-1 100 101 102 103
ID /W (A/m)
0
5
10
15
20
25
g m/I
D(V
-1)
L= 0.8 m
L= 25.2 m
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101ID (A)
101
102
103
104
Qn
(e)
gm/ID = 24
2320
1612
8 42 1
Scaling of transistor size to optimize power
Procedure:
For a small device selectthe current density for agiven /m Dg I from ploton previous page.
Then increase device widthwhile scaling thedevice current proportionally(maintain current density).
increase transconductance Cdet = 10 pF, CFET = 1 fF/m(reduce noise)
Minimum noise when
FET detC C (capacitive matching)For larger device widths the increase in capacitance overrides the reduction in noise.
This yields minimum noise, but is not most power efficient!For /m Dg I 24, minimum noise of 1400 e at 50 A, butfor /m Dg I 20 a noise level of 1000 e is obtained at 30 A.
Given noise level can be achieved at low and high current.
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Noise Cross-Coupling
In strip and pixel detectors the noise at the input of an amplifier cross-couples to itsneighbors.
C C C C
C C C C C
ss ss ss ss
b b b b bSTRIPDETECTOR
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Cross-Coupling Function in Strip and Pixel DetectorsThe center amplifier’s output noisevoltage nov causes a current noise
ni to flow through its feedbackcapacitance fC and the inter-electrode capacitances into theneighboring amplifiers, adding tothe other amplifiers’ noise.
The backplane capacitance bCattenuates the signal transferredthrough the strip-to-stripcapacitance ssC .The additional noise introduced intothe neighbor channels
1 21
2 1 2 /no
no nob ss
vv vC C
For a backplane capacitance /10b ssC C the amplifier’s noise with contributions from bothneighbors increases by 16%.
In pixel detectors additional paths must be included.This requires realistic data on pixel-pixel capacitances (often needs tests).
v v v
CC
C
C
ii
i
i
i
i
i i
i
CC
no1 no no2
ff
b
f
n2b
n
n
n1
n1
n1 n2
n2
ssss
NEIGHBOR 1 NEIGHBOR 2MEASUREMENTSTRIP
Z Zi1 i2
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Strip Detector Model for Noise Simulations
Noise coupled from neighbor channels.Analyze signal and noise in center channel.
Includes:
a) Noise contributions fromneighbor channels
b) Signal transfer toneighbor channels
c) Noise from distributed strip resistance (+ potential effect of strip inductance)
d) Full SPICE model of preamplifiers
Measured Noise of a Module –Test beam experiment, so realistic environment:
p-strips on n-bulk, BJT input transistor
Simulation Results: 1460 el (150 A)1230 el (300 A)
No digital cross-talk
Noise can be predicted with good accuracy. 50 100 150 200 250 300Current in Input Transistor [A]
1200
1300
1400
1500
1600
Noi
se[rm
sel
]
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Advantages of Segmentation
1. Segmentation reduces detector capacitance
lower noise for given power
2. Segmentation reduces the hit rate per channel
longer shaping time, reduce voltage noise
3. Segmentation reduces the leakage current per channel(smaller detector volume)
reduced shot noise
4. Segmentation allows higher overall event rates
5. Overall power per unit area can remain fairly constant with the increase inthe number of segments, since the power per segment can reduce withsmaller segments.
Segmentation is a key concept in large-scale detector systems.
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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CCD – a traditional pixel detector
Signal charge deposited in a pixel is readout by shifting it through the neighboringpixels until it reaches the end.
It is then transferred to the output amplifier.
Multiple excited pixels are transferredsequentially, so all individual pixel signalsare read out.
Pixels can be very small, but readout isslowed by the sequential succession.
Multiple readout column groups with outputamplifiers together with increased clockrates can speed up the output.
High-rate devices are read out in the MHz regime, which greatly increases the powerdissipation.
However, at high readout rates the power is dominated by the power loss in the readoutclock lines.
This is even a problem at sub-MHz rates:
The VXD3 CCD operating at a pixel transfer rate of 200 kHz and a clock level of10 V had peak currents of 1.3 A, leading to potential cross-talk.
SERIAL OUTPUT REGISTER
PIXELARRAY
OUTPUTAMPLIFIER
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Pixels directly coupled to front-ends offer high rates and low noise
The most flexible is the hybrid pixel device:
The sensor electrodes arepatterned as acheckerboard and amatching two-dimensionalarray of readoutelectronics is connected viaa two-dimensional array ofcontacts, for examplesolder bumps.
Hybrid pixels allow independent optimization of sensor and readout,e.g. allows non-Si sensor.
Drawback: Engineering complexity much greater than for common chips.
READOUTCHIP
SENSORCHIP
BUMPBONDS
READOUTCONTROLCIRCUITRY
WIRE-BOND PADS FORDATA OUTPUT, POWER,AND CONTROL SIGNALS
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Example: ATLAS pixel detector – about 108 channels
The initiating complex pixel design, which has been working reliably.
Each pixel cell includes
Charge-sensitive-amplifier + shaper per pixel
Threshold comparator per pixel
Trim-DAC per pixel for fine adjustment of threshold
Time-over-threshold analog digitization
Test pulse circuitry per pixel (dual range)
Buffer memory to accommodate trigger latency
Circuitry to mask bad pixels
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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ATLAS Pixel Cell
0.25 m CMOS, Qn 170 e40 W per cell; total power for 2880 pixels: 200 mW (incl. peripheral circuitry)
FROMCALIBRATIONDAC
ToT TRIMDAC
THRESHOLDTRIM DAC
40 MHz CLOCK
LEADING + TRAILINGEDGE RAM
COMPARATORCHARGE-SENSINGPREAMPLIFIER
COLUMNBUS
GLOBAL INPUTSAND
CONTROL LOGIC
ToT
VTH
DETECTORPAD
DUAL RANGECALIBRATION
GLOBALDAC
LEVELS
SERIALCONTROL
BUS
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Readout Scheme
Pixels continuously active, but don’t send signals until struck (self-triggered).Time stamp for struck pixels stored immediately in Content Addressable MemoryData stored in pixel until Level 1 trigger received for stored time stamp.
COLUMNBUFFERS
PIXELCELLS
CONTENTADDRESSABLE
MEMORY
CONTENTADDRESSABLE
MEMORY
TRIGGER
DATA OUT
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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ATLAS Pixel Detector Fabrication IC components
Pixel size: 50 m x 400 m
Size is historical:could be 50 m x 200 m
Power per pixel: < 40 W
Each chip: 18 columns x 160 pixels(2880 pixels)
Module size: 16.4 x 60.4 mm2
16 front-end chips per module
46080 pixels per module
Fabricated in 0.25 m CMOS
~ 3.5 106 transistors per chip
Functional to > 100 Mrad
Radiation resistant to higher fluences than strips because low noise provides largeperformance reserves.
Pixel IC Pixel IC
Module Readout IC Support and Test ICs
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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ATLAS Pixel ModuleSensor used as substrate to mount 16readout ICs
Two-dimensional arrays of solder bumpbonds connect ICs to sensor.
SIGNAL
SENSOR
READOUT IC
SOLDER BUMP
400 m
50 m
PIXEL ICsSOLDER BUMPSSENSOR
FLEX HYBRID
READOUTCONTROLLER
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Threshold dispersion must be smaller than noise.Small feature sizes large threshold dispersion correct with trim DAC
Threshold dispersion before and after trimming
0 10000 20000 30000 40000PIXEL NUMBER = ROW + (160 x COLUMN) + (2880 x CHIP)
1000
2000
3000
4000
5000
6000
7000
THR
ESH
OLD
(e)
0 2000 4000 6000THRESHOLD (e)
0 10000 20000 30000 40000PIXEL NUMBER = ROW + (160 x COLUMN) + (2880 x CHIP)
3000
3500
4000
4500
5000
5500
THR
ESH
OLD
(e)
3500 4000 4500THRESHOLD (e)
620 e
60 e
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Noise Distribution
Three groups visible: 1. nominal pixels
2. Extended pixels that bridge columns between ICs(spikes every 2880 pixels)
3. Ganged pixels to bridge rows between ICs
0 10000 20000 30000 40000PIXEL NUMBER = ROW + (160 x COLUMN) + (2880 x CHIP)
100
200
300
400
500
NO
ISE
(e)
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Monolithic Pixels in Standard IC Designs
Example: Electron Microscopy (P. Denes et al.)Electron Detection
Essentially all charge collected from thin regionfor good position resolution
Detector uses the passive region between thecircuit layers and the base.
Pixels are small (so there can be more of them,but they are less intelligent than hybrid pixels)
But ...
Radiation damage (electric field in the detection region is not well controlled)
Diffusion (because collection region is not depleted)
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Multi-Tier Electronics (aka “SOI” or “3D”)
CMOS Circuitry
Isolation Oxide
Sensor Layer
MIT Lincoln Lab
3-Tier Design (FNAL)
3 transistor levels11 metal layers
Accommodate additionalcircuitry for givenpixel size.
Also extensive SOIdevelopment at KEK
(R. Lipton, FNAL)
7 µm
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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● After introduction in high-energy physics (LHC), hybrid pixel devices with complexelectronic readouts are now applied in a variety applications, e.g. high-rate x-raydetection and medical imaging.
● The pixel size is limited by the area required by each electronic readout cell.
● Pixel sizes of 30 – 100 μm are practical today, depending on the complexity of thecircuitry required in each pixel.
● The readout IC requires more area than the pixel array to accommodate the readoutcontrol and driver circuitry and additional bond pads for the external connections.
● Multiple readout ICs are needed to cover more than several cm2, so in mounting multiplereadout ICs on a larger sensor requires that the ICs are designed with small edge areas.
● Implementing this structure monolithically would be a great simplification and some workhas proceeded in this direction.
● Appropriate cooling can deal with power dissipation, but heating of the sensor should belimited.
Electronics associated with each pixel can perform signal acquisition and pulse shapingand also record timing and provide local storage, so readout does not have to synchronizewith hit rates.
However, pixel readout designs are more complex than strip detector orconventional designs.
Requires multiple simulations and independent cross-checks to verify capabilities.
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Radiation DamageFor x-rays and low-energy gammas the main cause of radiation damage ischarge buildup at oxide-Si interfaces.
The trapped positive charge attracts electrons in the adjacent active Si region.
adapted from Boesch et al. IEEE Trans. Nucl. Sci (1986) 1191
In detectors this can lead to leakage between adjacent electrodes and inelectronics it leads to MOSFET bias shifts ( digital circuit failure).
Eox
ELECTRONINJECTIONFROM Si
TRAPS CLEARED BYTUNNELING FROM Si
ELECTRON-HOLERECOMBINATION
GATE SiOXIDE
ELECTRON-HOLEPAIRS FORMEDBY RADIATION
HOLE TRAPS
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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OXIDE THICKNESS (nm)
-/D
OSE
(V/M
rad)
V
10
10
10
10
10
10
10
3
2
1
0
-1
-2
-3
FB
1 10 100
2V d
MOSFET Threshold Shift vs. Oxide Thickness
Saks et al., IEEE Trans Nucl. Sci. NS-31 (1984) 1249
“Deep sub-micron” MOSFETs have appropriate oxide thicknesses, so theirradiation resistance is good. Cross-coupling in the detector may be critical.
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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“Grounding” – A Common Cause of NoiseA popular recipe is the “star” ground, i.e. connecting all “ground” lines to acommon point.
Example: Integrated Circuit
The output current is typically orders of magnitude greater than the input current(due to amplifier gain, load impedance).Combining all ground returns in one bond pad creates a shared impedance (inductance ofbond wire). The voltage drop due to the output current couples to the input.
LOADDETECTOR
V+
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Separating the “ground” connections by current return paths routes currents away from thecommon impedance and constrains the extent of the output loop, which tends to carry thehighest current.
LOAD
V+
V-
DETECTOR
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Summary
Interplay of many interacting contributions must be understood.Requires understanding the physics of the
experiment,detector,
readout,rather then merely following recipes.
Physics requirements must be translated to engineering parameters.
Many details interact, even in conceptually simple designs.For example, analysis in time and frequency domain
Single-channel recipes tend to be incomplete.Overall interactions must be considered.
Simulations may provide the correct answers, but they should be cross-checked. Multiple equivalent simulations often give different results.
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DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012
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Challenges No “silver bullets”!
Systems design is crucial in advanced detectors.
It is essential to understand key aspects and their interactions.
Key front-end issues don’t require detailed electronics knowledge of circuits,but understanding of basic underlying physics is essential.
Broad physics education required.U.S. physics departments commonly do not recognize the scientificaspects of instrumentation R&D.
Many developments are essentially made as technician efforts, so thesimplistic perspective doesn’t accept that novel developments require ascientific approach.
Emphasis on theory and mathematical techniques neglects understandingof physics and how to apply it to undefined multidimensional problems.
Novel detectors often build on a range of different concepts.
General detector R&D can build an efficient base for multiple applications.