Silicon strip detectors and their readout electronics Francis Anghinolfi CERN 1F. Anghinolfi CERN/PH/ESE SeminarDecember 1, 2009

Silicon strip detectors and their readout electronics

Francis Anghinolfi

CERN

1F. Anghinolfi CERN/PH/ESE SeminarDecember 1, 2009

OUTLINE

• Silicon strips detectors for tracking

• (Micro)Electronics for silicon strips

• Strips readout examples

December 1, 2009 F. Anghinolfi CERN/PH/ESE Seminar 2

NB : I will not talk about silicon pixel detectors nor silicon drift detectors …..

Silicon Strips Detectors

• Solid State• ~100% efficient for particle detection• Small (best when ….)


• No multiplication Small signals, then need smart electronics to detect the signal (large amplification and noise reduction)

• Leakage (DC) current

PROS

CONS



Solid state

• No gas, no liquid stable material• But as a consequence, subject to radiation degradation

• Benefits from silicon wafer industry environment (access to very pure silicon material, precise alignment machines, clean room technologies etc …)



Efficiency

SIGNAL if there is PARTICLEDetection Efficiency close to 100%

Charge is always generated by a MIP crossing particle



Efficiency

SIGNAL if there is PARTICLEDetection Efficiency close to 100%

Inefficiency may result from :Insensitive areas (edges)Charge sharingTraps or defect



Signal formation in Silicium

• High Energy Particle deliver signal in the depletion region, by electron hole pair creation. Positive and negative charges drift in presence of the applied electric field

++++ - -

- - -

c




• The depletion region is obtained by applying a voltage between top and bottom of the detector

• For large voltage and small doping concentration it is possible to obtain the depletion of the full volume




The diode (p-type over n-type bulk in this figure) is needed to block direct current path.Direct current path could be as high as mA per strips ( a killer for signal detection)

The diode is reverse biased. The dynamic signal from particle track is sensed through the capacitance of the blocking diode




• The signal is formed by electron-hole pair creation along the track path

• for a fully depleted silicon volume 300 um thick the average signal is around 22000 electrons (3.5 fC)

The energy loss for a Minimum Ionizing Particle (MIP) is around 260eV/um for 300um thick silicon. One e-h pair creation energy in silicon is 3.62 eV



Charge sharing

Particle track through Si material

e-e-

e-e-

e-e-

Strip length

Strip pitchCollected Charge quantity depends on the depleted silicon thickness

In case of track angle the charge distributes on adjacent strips

The charges are drifting along the electrical field toward the strips. The electronics channels are connected to the strips (directly or through decoupling capacitors)




• The collection time is usually below 10ns for electrons (drifting to one side) and below 25ns for holes (drifting the other side)

• Good time resolution is also possible (~ few ns range, OK for LHC with 25ns bunch interval)



The Signal is small

For example read the signal on 50 ohms resistor : Typical 3.5fC delivered on 50 ohms in ~10ns results in peak voltage of 17uV

The voltage noise of the 50 ohms resistor in the bandwidth required for signal is ~ 3.5uV rms

The ratio (5), is not enough to provide safe signal detection (additional noise sources, charge sharing etc …)



The sensing geometry (strip size) can be small

• Small is good for tracking detectors : 10-20um position resolution attainable (less for pixels)

• Small geometry is beneficial for the signal detection electronics ….



Small is good for the detection

• Small areas are obtained by the detector segmentation• There is no signal loss (dead areas) even with segmented detectors• Small areas have less leakage current and less capacitance, 2 key items which improve the signal detection (signal over noise ratio SNR)



Example : Segmented detector

1280 strips on ~ 10 cm

4 rows of 2.5cm long strips

2,5cm

This detector is made of 4 rows of 1280 strips of length 2,5cm. The strip pitch (horizontal axis) is 80um. The expected resolution is 23 um in X.

(Development for ATLAS upgrade, KEK, Japan)



What we know about electronics for Silicon Strips

• Has to deal with small signal and leakage current

• Small detector segments have less leakage current and less capacitance, 2 key items which improve the signal detection (signal over noise ratio SNR)

• But more channels to cover a fixed detecting area : It creates a trade-off between power (nbe of channels) versus SNR

Readout Electronics


The electronics circuit is necessary for :

signal amplification (signal multiplication factor)

noise rejection

signal “shaping”

Impedance adaptation

Typical “front-end” elements

Final objective :

Signal detection

Amplitude measurement

Time measurement

Z+

-

Particle Detector

Electronic Circuit

Rp

Readout Electronics


Circuit

Low Z output voltage source circuit can drive any load

Output signal shape adapted to subsequent stage (T/H, ADC, Discriminator)

Signal shaping is used to reduce noise vs. signal

ZoZ+

-

High Z

Low Z

Low Z

T

Voltage source

• Impedance adaptation• Amplitude resolution• Time resolution• Noise cut

Rp

Readout Electronics


x(f) y(f)

x(f)f

y(f)f

h(f)

f

Noise floor(white)

f0

f0

f0Improved Signal/Noise

Ratio

Example of signal filtering : the above figure shows a « standard » filter case, where only noise is filtered out.

Readout Electronics


x(f) y(f)

x(f)

f f

h(f)

f

Noise floor

f0

f0Improved Signal/Noise

Ratio

In particle physics, the input signal, from detector, is often a very fast pulse, similar to a “Dirac” pulse. Therefore, its frequency representation is over a large frequency range.The filter (shaper) provides a limitation in the signal bandwidth and therefore signal shape at the filter output is different from the input signal shape.

The output signal shape, different from the input detector signal shape, is chosen to optimize the Signal-to-Noise Ratio

• Doing this there is a trade-off btw. SNR, power and pile-up

y(f)

Readout Electronics


ff0

ff0

Filter cuts noise. Signal BW is preserved

Filter cuts inside signal BW : modified shape

Readout ElectronicsPreamplifier Shaper

(t) Q/C.(t)

I O

What are the functions of preamplifier and shaper (in ideal world) :

• Preamplifier : is an ideal integrator : it detects an input charge burst Q

(t). The output is a voltage step Q/C.(t). Has large signal gain such that noise of subsequent stage (shaper) is negligeable.

• Shaper : a filter with : characteristics fixed to give a predefined output signal shape, and rejection of noise frequency components which are outside of the signal frequency range.

December 1, 2009 23F. Anghinolfi CERN/PH/ESE Seminar

Readout Electronics

24

5 10 15 20 25 30 35

0.02

0.04

0.06

0.08

0.1

2 4 6 8 10

-0.005

-0.0025

0.0025

0.005

0.0075

0.01

Preamplifier Shaper

(t)

1/s RCs /(1+RCs)5x

I O

T.F. from I to O

= = RC/(1+RCs)5

Output signal of preamplifier + shaper with “ideal” charge at the input

t

1 2 3 4 5

0.2

0.4

0.6

0.8

1

t

0.2 0.5 1 2 5 100.1

0.2

0.5

1

2

5

f

t

RCtettO /

4

4

RC

1)(

0.001 0.0050.01 0.05 0.1 0.5 1

0.0001

0.0002

0.0005

0.001

0.002

0.005

0.01

0.02

f

CR_RC4 shaper

Ideal Integrator

Q/C.(t)

December 1, 2009 F. Anghinolfi CERN/PH/ESE Seminar

Readout Electronics

5 10 15 20

0.01

0.02

0.03

Preamplifier Shaper

CR_RC shaperNon-Ideal Integrator(t)

1/(1+T1s) RCs /(1+RCs)2

I O

x

Non ideal shape, long tail

Integrator baseline

restoration


Readout Electronics

2 4 6 8 10 12 14

0.025

0.05

0.075

0.1

0.125

0.15

0.175

Preamplifier Shaper

(t)

1/(1+T1s) (1+T1s) /(1+RCs)2

Pole-Zero Cancellation

I O

T.F. from I to O x

CR_RC shaperNon-Ideal Integrator

Ideal shape, no tail

Integrator baseline

restoration


Readout ElectronicsPile-up

• The detector pulse is transformed by the front-end circuit to obtain a signal with a finite return to zero time

2 4 6 8 10 12 14

0.025

0.05

0.075

0.1

0.125

0.15

0.175

5 10 15 20 25 30 35

0.02

0.04

0.06

0.08

0.1

CR-RC :Return to baseline > 7*Tp

CR-RC4 :Return to baseline < 3*Tp


1 2 3 4 5 8 7

1 2 3

Readout Electronics

For the requirement of strips readout for the LHC experiments, the shaping (peaking) time should be in the order of 25 ns

It allows discrimination of events as close as 75ns

With the CR-RCn circuits as described above, it is difficult to trade 25ns peaking time with low power circuit


Readout Electronics

Different solutions have been implemented to realize low power and fast (25ns) shaping time for the LHC experiments:

• transimpedance amplifiers

• analog signal processing


Readout ElectronicsTransimpedance Amplifier

Cd

Rf

Detector model

Amplifier

Resistive feedback element

Vout

iin(t)


s.Rf.Cd1

Rf

i

vout

in

The main pole is function of the detector input capacitance

Other poles are coming from the amplifier open-loop transfer function.

The preamplifier does a fraction of the shaping function

But this circuit is critically stable and needs careful design

The feedback element Rf contributes to the noise

A

For ideal Amplifier A

Readout ElectronicsAnalogue Signal Processing

programmable gain

charge preamplifier

shaper

analoguepipeline

1 of the 128 channels

SFSF

unity gain inverter

S/H

APSP

differentialcurrentoutput stage

128:1 MUX

500400300200100

0250200150100500


50ns shaperCharge preamp

Analog Processor, 25ns shaping time

The fast shaping is applied only to selected samples of the signal, therefore there is a gain in power

Analogue sampling and storage

Readout ElectronicsThe Signal-to-Noise Ratio

2vna

Rp

G(f)

Cd2qIshot

4kTRp

Rs 4kTRs


Whatever is the solution choosen to realize the shaping function, the overall Signal-to-Noise depends on the overall electronics circuit transfer function up to the measurement output

The signal is from the detector, the noises are from the active (transistor, diode) and passive (resistors) elements

Diode(for strips : reverse bias diode)

Resistance in parallel

Resistance in serie

Input transistor (under assumption that load does not contribute, and that the input gain stage is high enough to neglect 2nd stages contribution…)


2vna

Rp

G(f)

Cd2qIshot

4kTRp

Rs 4kTRs


In case of strips :

Strip capacitance

Reverse bias diode leakage

Bias resistors

Not present

Input device “channel” noise

Front-end circuit with shaping time constant


• All noise contributions are calculated in terms of noise voltage appearing at the input of the amplifier

• Noise sources are from detector element and from amplifier.

4 noise sources are considered here :

1. Ishot current in diode (leakage current in Si Detector element )

2. Rp noise, (any) resistance in parallel to the input3. Rs noise, (any) resistance in serie with the input4. V2

na equivalent input noise of the input device of the amplifier



22

2

22 42

4

8d

nashotp

CVkTRsqI

R

kT

q

eENC

2vna

Rp

G(f)

Cd2qIshot

4kTRp

Rs 4kTRs


A formulation of the noise in “electrons” is given here for the CR-RC2 transfer function G(f)

The signal-to noise ratio is computed by division of the signal expressed in electrons by the ENC calculated with the formula

is the shaping time (0 to peak) of the CR_RC2 shaping function

V2na is the equivalent

voltage noise source of the amplifier input device

gmkTvna

1

3

242

gm is proportional to the square root of the current in the input device


21

222

3

24.

4. d

p

Cgm

q

kTFs

Rq

kTFpENC

0.340.360.400.450.510.630.92Fp

7654321n

1.271.161.110.990.950.840.92Fs

7654321n


CR-RCn “n” factor dependance

Formulation without diode current, without Rs Small detector segment

Shaping time trade-off

Increase transistor current (power !)

To reduce ENC


C=5pF

C=10pF

C=15pF

Shaping time (ns)

ENC dependence to the shaping time (C=10pF, gm=10mS, R=100Kohms)

optimum

Shaping time (ns)


ENC (el.) ENC (el.)

Readout ElectronicsThe Power vs. other parameters trade-off

Numerical example :

Target 10pF detector, ENC as 500el. =25ns, V= 2.5V P = 0.32mW (strips typical)

Target 0.2pF detector, ENC as 120el. =25ns, V= 2.5V P = 3 W (pixel case)


This (approximate, don’t use it in real life !) equation shows the relationship btw. the front-end power (input branch only !), detector capacitance and noise performance

Readout System for highly segmented Si Strips detectors

APV25 description


8.1 mm

7.1m

m

pipeline

128x192

128

x p

ream

p/sh

aper

AP

SP

+ 1

28:1

MU

X

pipe logicbias gen.

CAL FIF

O

controllogic

The CMS silicon tracker analogue architecture; 128 channels of preamplifier/shaper followed by one analogue memory (pipeline) bank.

The APSP is the final “fast” shaping circuit, acting on selected signal samples

The readout is analogue : reading amplitude allows to measure charge sharing across adjacent strips


ABCD/N description


The ATLAS silicon tracker binary architecture; 128 channels of preamplifier/shaper/comparator with two memory banks, one for trigger latency and one readout buffer

The readout is binary (either above or below a programmable threshold). Low threshold setting is required to not loose signal in case of charge sharing


Beetle FE chip


The LHCb VELO frontend chip tracker has128 channels of preamplifier/shaper/discriminator followed by one analogue memory (pipeline) bank. Either analogue or binary data is stored into the pipeline.

The readout is analogue but both amplitude or binary information can be readout.


Performance and system aspects for the LHC experiments

Parameter Number

Spatial precision 20 um range

Channel count ~10M (CMS, ATLAS)

Channel power 3-5 mW

SNR Above 10 after radiation damage

Linearity a few MIPs

Long term performance Should work for 10 “LHC years”

Radiation tolerance LHC radiation levels ~10Mrad and 2 10^14 eqvlt 1MeV neutrons



PHOTOS


The VELO tracker detector of LHCb:

The electronics (Beetle) are on the PCB circling around the radial strips


PHOTOS


The CMS tracker module :

Two large planes detectors are connected together by bonding to get long strips (20cm).

The electronics (APV25) are on the small PCB at the end of the silicon detector


PHOTOS


View of one fraction of the CMS tracker, showing detecting planes and the associated electronics and cabling


PHOTOS


View of one disk of the CMS tracker end-cap: detectors with strips form a “V” shape



View of one barrel of the ATLAS SCT tracker.


PHOTOS


Details of the electronics FE parts on the ATLAS barrel modules


Silicon strips are candidate for the LHC experiments upgrade

SLHC ATLAS expected Tracker Occupancy (P. Nevski)

• Outer Layers – long strips– 2 Layers R= 81.4, 95.4 cm – Granularity 12 cm x 80 um – Z length 2x190 cm

• Middle Layers – short strips– 3 layers at R=38, 47.3, 57.4

cm– Granularity 3 cm x 80 um– Z length 2x100 cm

Example of Barrel layoutR. Nickerson, Oxford

Section of projected SI Tracker geometry(Nov.08)

Barrel Disks


Service bus

TTC, Data & DCS fibers

PS cable

DCS env. IN

Cooling In

Opto

SC

DCSinterlock

SMC Hybrid

Module #1 Module #2 Module #12

Cooling Out

~1.2m

MC MC MC MC MC MC

ABCN-25 : Silicon Strip Module/Stave Readout Concept (ATLAS)

~ 1.2 meter

Cross section

Carbon Honeycomb Cooling PipeSensor, Hybrid, ASIC

61440 strips on each side

10 c

m


Thank you !

Documents

Silicon strip detectors and their readout electronics Francis Anghinolfi CERN 1F. Anghinolfi CERN/PH/ESE SeminarDecember 1, 2009