Detector Electronics - Stanford University...DetectorElectronics Helmuth Spieler BES Neutron &...

DoE Basic Energy Sciences (BES)Neutron & Photon Detector Workshop

August 1-3, 2012

Gaithersburg, Maryland

Detector Electronics

Helmuth Spieler

[email protected]

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

DetectorElectronics Helmuth SpielerBES Neutron & Photon Detector Workshop, Aug. 1-3, 2012

2

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”


3

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.


4

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


5

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 ...


6

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


7

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


8

Position Sensing Segmentation – strips or pixels

Strips yield one-dimensional position sensing

Position resolution is determined by strip-pitch and transverse diffusion.


9

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


10

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


11

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

=


12

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


13

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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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


15

Crossed strips provide 2-dimensional position sensing

“Fake hits” at high rates

y

x


16

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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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?


18

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

)


19

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


20

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.


21

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


22

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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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

]


24

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.


25

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


26

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


27

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


28

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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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


30

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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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


32

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)


3000

3500

4000

4500

5000

5500

THR

ESH

OLD

(e)

3500 4000 4500THRESHOLD (e)

620 e

60 e


33

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


100

200

300

400

500

NO

ISE

(e)


34

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)


35

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


36

● 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.


37

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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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.


39

“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+


40

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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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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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.

Detector Electronics - Stanford University...DetectorElectronics Helmuth Spieler BES Neutron &...

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