ATLAS pixel detector and radiation damage€¦ · 11 cm-3] N Y,∞ =g Y Φ eq N C N C0 g C Φ eq N...

T. Ince, MPI Physik Muenchen HUPP Seminer Oturumu, 2013/06/25

ATLAS pixel detector and radiation damage

1

HUPP Seminer Oturumu25 Haziran 2013

Tayfun InceMax-Planck-Institute for Physics

Outline:

Technology

Calibration

Performance

Radiation

Summary


ATLAS experiment

2

protonsprotons


Inner detector

3

beam pipe

2 T magnetic field.

Tracking of charged particles with pT > 0.5 GeV (as low as 0.1 GeV) and |η| < 2.5.

Electron id over |η| < 2.0.

Pixel: n-in-n silicon sensors.

SCT: p-in-n silicon sensors.

TRT: polyimide drift tubes interleaved with polypropylene fibers.


Pixel detector

4

250 µm thick silicon sensor with an active area of 16.4x60.8 mm2.

16 front-ends (FEs) per module bump bonded to the sensor.

Radiation hard up to NIEL 1015 1 MeV neq/cm2 and 500 kGy.

47232 pixels per module. Most with size 50x400 µm2. Special pixels (long and ganged) to cover regions between FEs.

Module controller chip (MCC) to send trigger, commands and configuration data, and to perform event building.

2x80 Mb/s readout in b-layer (innermost), 80 Mb/s in layer 1 and disks, and 40 Mb/s in layer 2.

lead-tinor

indiumbumps


Pixel sensor

5

2008 JINST 3 P07007

(a)

Front-end electronics

-Vbias

0V

0V

n-type bulk

depleted

(b)


-Vbias

0V

0V

p-type bulk

depleted

Figure 15. Comparison of depletion zones in n+-in-n pixel sensors before (a) and after (b) type inversion.Before type inversion the electrical field grows from the backside and reaches the pixel implants (full deple-tion). After type inversion the depletion zone grows from the pixel side and allows operation even if the bulkis not fully depleted.

(a)0 0.5 1 1.5 2 2.5 3 3.5

!eq [1014cm-2]

1

2

3

4

5

6

7

|Neff|[1012cm

-3]

100

200

300

400

Vdep[V](300"m)

neutronsneutrons

neutronsneutrons

pionspions

protonsprotons

pionspions

protonsprotons

oxygen rich FZ

standard FZstandard FZ

(b)1 10 100 1000 10000

annealing time at 60oC [min]

0

2

4

6

8

10

#Neff[1011 cm-3]

NY,$ = gY !eq

NC

NC 0

gC !eqgC !eq

NA = ga !eqNA = ga !eq

Figure 16. (a) Evolution of effective charge densities and full depletion voltage in standard and oxygenatedsilicon during irradiation with various hadrons. In oxygenated silicon the increase after type inversion in-duced by charged particles (pions, protons) is significantly lower. (b) Evolution of the effective dopingconcentration due to annealing and reverse annealing effects. The parameterization of this evolution is theso-called “Hamburg model” and represents an important input to the ATLAS pixel sensors, which will op-erate near the point of minimal depletion voltages. In oxygenated silicon, both NC and NY are reduced[48, 49].

– 34 –

before type inversion after type inversion

2008 JINST 3 P07007

(a)


-Vbias

0V

0V

n-type bulk

depleted

(b)


-Vbias

0V

0V

p-type bulk

depleted

Figure 15. Comparison of depletion zones in n+-in-n pixel sensors before (a) and after (b) type inversion.Before type inversion the electrical field grows from the backside and reaches the pixel implants (full deple-tion). After type inversion the depletion zone grows from the pixel side and allows operation even if the bulkis not fully depleted.

(a)0 0.5 1 1.5 2 2.5 3 3.5

!eq [1014cm-2]

1

2

3

4

5

6

7

|Neff|[1012cm

-3]

100

200

300

400

Vdep[V](300"m)

neutronsneutrons

neutronsneutrons

pionspions

protonsprotons

pionspions

protonsprotons

oxygen rich FZ

standard FZstandard FZ

(b)1 10 100 1000 10000

annealing time at 60oC [min]

0

2

4

6

8

10

#Neff[1011 cm-3]

NY,$ = gY !eq

NC

NC 0

gC !eqgC !eq

NA = ga !eqNA = ga !eq

Figure 16. (a) Evolution of effective charge densities and full depletion voltage in standard and oxygenatedsilicon during irradiation with various hadrons. In oxygenated silicon the increase after type inversion in-duced by charged particles (pions, protons) is significantly lower. (b) Evolution of the effective dopingconcentration due to annealing and reverse annealing effects. The parameterization of this evolution is theso-called “Hamburg model” and represents an important input to the ATLAS pixel sensors, which will op-erate near the point of minimal depletion voltages. In oxygenated silicon, both NC and NY are reduced[48, 49].

– 34 –

increase of depletion voltage is much lower in oxygenated silicon

n-in-n sensor allows operationafter type inversion even if not fully depleted

+-+ -+

++

--

-+ -


Optolink calibration (I)

6

Frequency of optolink calibration is driven by replacement of dead Tx plugins on the off-detector side. Done as often as once per week.

On-detector laser power, and off-detector sampling threshold and phase are tuned to settings that avoid data readout errors. One laser power setting per optoboard (i.e. 6 or 7 modules).

~1 m ~80 mon-detector off-detector


Optolink calibration (II)

7

Optical corruption measured in units of bit flips as a function of data link threshold and phase. Optimal point chosen in error-free region (EFR). Boundaries between EFR and error regions are not equally stable, so optimal operating point not in the centre of EFR.

Data (Up) links of b-layer, layer 1 and disks tuned at 80 Mb/s, while layer 2 at 40 Mb/s.

tuningpoint

error freeregion


Threshold calibration (I)

8

Frequency of module level calibration is driven by periods of long-enough (min 8 hours) downtime of the LHC. e.g. technical stop.

Inject # of test charges at target threshold, vary 7-bit DAC per pixel and count generated hits. Optimal value is at 50% efficiency.

If 7-bit DAC per pixel does not produce sufficient range of thresholds for tuning target, use 5-bit DAC per FE first to shift global threshold per FE.

Injected charge [e]2000 3000 4000 5000 6000 7000

Hit e

fficie

ncy

[%]

0

20

40

60

80

100

50

84

16

Noise

Overdrive

Threshold

In!time Threshold

Figure 3.8: Schema representing the S-curve and the corresponding threshold andnoise. The S-curve corresponding to the in-time threshold is also represented.

the number of Level 1 Accept (LVL1A). Increasing the number of read BCs leadsto a better efficiency for low charges. But as the luminosity increases1, readingseveral LVL1A will lead to overlaps between successive minimum bias events andthe LVL1A should be eventually set to one. In this case low charge hits will belost since they pass the threshold too late. An effective threshold, called the in-time threshold, is represented in figure 3.8. It is defined using the hit efficiencywith a timewalk smaller than a given value. The new S-curve is shifted by a valuecalled the overdrive, corresponding to the difference between the threshold andthe in-time threshold. The S-curve shape stays unchanged and thus the noise isnot affected by this shift. It should be noted that the frontend electronics can par-tially correct for this effect online: this is done by defining a minimal ToT valuebelow which the corresponding hit is duplicated in the previous BC.

ToT calibration The discharge current of the pixel preamplifier, and thus thedischarge speed, is fixed by tuning the feedback current. A ToT scan is neededafterwards to calibrate the ToT response of each pixel with the corresponding col-lected charge. The ToT scan is done by injecting known charges into the pixelpreamplifier and measuring the corresponding ToT. The injection can be donethrough two different capacitors: C-low and C-high. The C-low capacitor pro-vides a more precise measurement up to 100 ke. The C-high capacitor extends the1Increase of the number of pileup events and/or decrease of the time between two successive

bunch crossings.

55

tel-0

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525,

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1 -

24 N

ov 2

009 G. Aad, CPPM-T-2009-003


Threshold calibration (II)

9

Threshold tuned to 3500 e. Dispersion is ~40 e.

Pixel noise is ~170 e for normal and long, and ~300 e for ganged.

0.3% of noisy pixels are masked online to reduce noise rate down to ~10-7 hits/pix/evt. After offline masking, rate <10-9 hits/pix/evt.


ToT calibration (I)

10

Time-over-Threshold (ToT) is the time in which signal is above threshold. In units of bunch crossings (BC), i.e. 25 ns. ToT is proportional to the deposited charge.

Tuning target is 30 BC for a charge of 20k e (most probable for m.i.p.). Resolution ~1 BC.

Inject # of test charges at 20k e, vary 3-bit DAC per pixel to find optimal point resulting closest to target ToT.

In addition, can use two 8-bit DACs per FE to achieve target ToT and optimal tuning.

Time [BC]0 5 10 15 20 25 30 35 40 45 50

Char

ge [k

e]

0

5

10

15

20

25

Discriminator threshold

)1

High charge (Q

1LE 1TE

)2

Low charge (Q

2LE 2TE

1ToT2ToT

Figure 3.7: Discriminator input amplitude as a function of time for a high chargeQ1 (red solid line) and a low charge Q2 (blue dashed line) and their correspondingToT.

Threshold scans The pixel thresholds are first defined by tuning the chip’sGDACs that set an overall threshold for all pixels in one chip. Each individualpixel threshold is then tuned more precisely to achieve the preset value and also tominimise the threshold dispersion at the chip level. This step, which sets the indi-vidual TDACs, is called the threshold tuning scan. The GDACs and TDACs canonly be varied by finite steps, therefore the actual threshold will have a dispersionaround the tuned value.A threshold scan is performed to determine the actual pixel threshold and noise

values. This is done by injecting, in each pixel, an increasing charge and countingthe hit efficiency at each value. The resulting histogram is fitted by an S-curvethat represents the integral of a Gaussian. Figure 3.8 represents an S-curve and thecorresponding threshold and noise values: the threshold is defined as the chargecorresponding to a 50% hit efficiency. The noise corresponds to one standarddeviation: assuming a Gaussian noise distribution, this will correspond to thepoints at a hit efficiency of 16% and 84%.

Timewalk scan The raising time of small charges can be long and the corre-sponding leading edge can occur after several BCs, as shown in figure 3.7. Whena L1 trigger occurs, the pixel data in a time range up to 16 BCs can be read andattached to the same event. At the beginning of data taking, several BCs will beread after each L1 (8 for the data used in this chapter). This number is known as

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G. Aad, CPPM-T-2009-003


ToT calibration (II)

11

Once tuned, inject varying test charges to measure ToT per pixel as a function injected charge.

Parametrize with ToT = p0(p1+Q)/(p2+Q) for each front-end and each pixel type for offline reconstruction.

Improved position resolution is an example advantage of measuring ToT.


Timing and timewalk

12

After timing adjustments for trigger delays, different cable lengths and module-by-module variations, dispersion is 0.17 ns.

Low charge hits may end-up in the next BC due to slow rise above threshold. Compensated by hit doubling based on ToT.


Lorentz angle

13

In a magnetic field, charge carriers drift at an angle w.r.t the normal of electrode surface. Displacement is about 30 µm.

Lorentz angle corresponds to track incident angle resulting in min cluster size. 211.4±0.6 mrad for collisions.

Lorentz angle has a linear dependance to temperature changes with a slope of -0.78±0.18 mrad/K.


Hit to track association and particle-id

14

Efficiency of a track having hits for pixel detector layers crossed is about 99%. Dead modules excluded.

Full efficiency in b-layer due to track selection.

Association inefficiency mostly due to known dead regions (pixels and or FEs) in modules.

Charged particles with m>>me traversing material loses energy mostly via ionization and excitation, described by Bethe-Bloch formula.

Energy loss proportional to collected charge.

p

K

π


Silicon radiation damage

15

Radiation damage on the pixel detector will result in:

an increase of leakage current,an increase (decrease before type inversion) of voltage needed for full depletion of sensors,and a decrease in charge collection efficiency.

Most silicon defects can anneal by diffusion. Highly temperature dependent:

charge trapping decreasesleakage current decreasesdepletion voltage first decreases, then increases (reverse-annealing)

5!Stephen Gibson! Royal Holloway | 27th February 2013! 5!

Silicon Radiation Damage!Radiation damage at the LHC!and silicon detector upgrades!

!   There are three main effects caused by radiation-induced damage to the crystal lattice:!

•  Recombination/generation centres increase the leakage current, affecting power consumption and signal to noise (cooling reduces noise).!

•  Acceptor centres modify the effective doping concentration, Neff , leading to type-inversion, after which the voltage required to fully deplete the sensor increases.!

•  Charge-carrier trapping centres reduce the charge collection efficiency. Reduction of signal degrades hit efficiency and hence track resolution, b-tagging etc.!

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66 1 Basic Principles of a Silicon Detector

+

–

donor

acceptor

EV

Ec

Leakage currentIncrease of

generation currentLevels close to midgap

are most effective

Depletion voltage Trapping, CCECreation of charged defects

upper band: donorlower band: acceptors

N ; Veff FD!

Shockley–Read Hall statistics(standard theory)

Inter–center chargetransfer model

(inside clusters only)enhanced after

diffusion – annealing

Enhanced generation!!

leakage current

space charge

electrons

holes

Shallow levels trap e trap h

lower CCE!

(a) (b) (c) (d)

Fig. 1.51 The different defect level locations and their effects. All relevant defect levels due toradiation are located in the forbidden energy gap. (a) Mid-gap levels are mainly responsible fordark current generation, according to the Shockley–Read–Hall statistics and decreasing the chargecarrier lifetime of the material. (b) Donors in the upper half of the band gap and acceptors in thelower half can contribute to the effective space charge. (c) Deep levels, with trapping times largerthan the detector electronics peaking time, are detrimental. Charge is “lost”, the signal decreasesand the charge collection efficiency is degraded. Defects can trap electrons or holes. (d) The theoryof inter-centre charge transfer model says that combinations of the different defects in so-calleddefect clusters additionally enhance the effects

the strong force result mainly in long-range cluster defects. With enough energy theinitial PKA39 acts further on several additional lattice atoms. In the non-relativisticapproach, the maximum transferred energy ER,max can be calculated in the case ofrecoil for a particle with mass mp and kinetic energy Ep with

ER,max = 4EpmpmSi

(mp +mSi)2 (1.45)

For electrons with energy Ee and mass me, higher kinetic energies are needed forlattice damage. ER,max can be calculated by the approximate relativistic relation

ER,max = 2EeEe +2mec2

mSic2 (1.46)

Table 1.6 shows the average and maximum energy transfer of different particle typeswith the incident energy of 1 MeV.

Also, the energies needed to knock an atom from its original lattice place areclearly dependent on the binding forces and therefore on the material. In the case ofsilicon, the minimum energy needed to displace a single lattice atom (Frenkel pair)is Esingle de f ect ! 25eV, while Ecluster ! 5keV is needed to produce a defect cluster.Table 1.7 lists the minimum kinetic particle energies needed to transfer Esingle de f ector Ecluster, subsequently creating a single point defect or a cluster.

39 PKA: Primary Knock on Atom.


Radiation damage monitoring

16

Leakage current per module also monitored via HV power supply with a precision ~80 nA per half-stave (i.e. 6 or 7 modules).

Increases on all layers are within predictions so far.

Before type inversion, decrease of depletion voltage measured via cross-talk method:

if not fully depleted, cross-talk between pixels is much higher.readout a pixel while injecting high enough charge into neighboring pixels to generate a cross-talk hit only when not fully depleted.

annealingradiation

22!Stephen Gibson! Royal Holloway | 27th February 2013! 22!

2011/2012 fluence at 7 TeV / 8 TeV!Radiation damage at the LHC!and silicon detector upgrades!

!   Prediction for both years is based on 7 TeV FLUKA for 2011 and an estimated 8 TeV fluence for 2012 by an interpolation between 7 TeV and 14 TeV FLUKA simulations. The same scaling factors are applied: L0 (1.15) L1, L2 (1.25).!

!   Agrees well until June 2012, after which L0 data show a slightly shallower slope.!

!   Thought to arise from an underestimate of annealing effects in the model: the time binning in the Dortmund (Hamburg) model needs to be finer to capture warm up time structure (per day to per hour or less).!

!   Adding the latest 2012 data…!


Summary

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The ATLAS pixel detector is calibrated and performing well. Regular checks of performance and re-tuning when necessary.

Threshold tuned to 3500 e with a dispersion of 40 e.

ToT is tuned to 30 BC with a resolution of 1 BC.

Timing dispersion of 0.17 ns between modules.

Radiation effects which are within expectations so far are monitored. LHC delivered ~30 fb-1 corresponding to NIEL ~6x1013 1 MeV neq/cm2. Pixel can stand for O(10) more radiation.

Insertable B-Layer (IBL) will take us even closer (3.3 cm) to the interaction point and restore/improve performance.


Questions?

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ATLAS pixel detector and radiation damage€¦ · 11 cm-3] N Y,∞ =g Y Φ eq N C N C0 g C Φ eq N...

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