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Medipix3 Irradiation Studies

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Medipix3 Irradiation Studies. Irradiation Studies of a 130nm Large Area Pixel Chip Richard Plackett – CERN Medipix Group TWEPP 2009, Paris, 22 nd September. Overview. Introducing the Medipix3 Chip Updates from Medipix2 Charge Summation Spectroscopic mode Example Application - PowerPoint PPT Presentation
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Medipix3 Irradiation Studies Irradiation Studies of a 130nm Large Area Pixel Chip Richard Plackett – CERN Medipix Group TWEPP 2009, Paris, 22 nd September
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Page 1: Medipix3 Irradiation Studies

Medipix3 Irradiation Studies

Irradiation Studies of a 130nm Large Area Pixel Chip

Richard Plackett – CERN Medipix GroupTWEPP 2009, Paris, 22nd September

Page 2: Medipix3 Irradiation Studies

-2-

Overview

Introducing the Medipix3 ChipUpdates from Medipix2Charge SummationSpectroscopic modeExample Application

Brief Radiation Damage Overview

Specific Design Issues

Irradiation Measurements up to 460MradThreshold, Noise and Gain ResultsDAC Stability

Irradiation Measurements up to 3MradAdditional Run in the Worst Case

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

The Medipix3 Design team is Xavier Llopart, Rafael Ballabriga & Winnie WongThese Measurements taken and analysed by myself, Xavier Llopart & Rafael BallabrigaThanks to them and all the CERN group for their support

Page 3: Medipix3 Irradiation Studies

-3-

Hybrid Pixel Detectors

sensor

Analogue amplification

Ionizing Particle

e-h+

Digital processing

Chip read-out

Positive or negative

sensor bias

Medipix3 adds communication between the pixel analogue electronics

10001110101

A hit in the sensor deposits charge

The charge passes to the analogue amplifiers

The Medipix 3 charge summing circuit operates

The counter is iterated and read out with the shutter

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 4: Medipix3 Irradiation Studies

-4-

Imaging by Photon Counting• Application of HEP idea to x-ray detection and medical imaging,

the concept behind Medipix chips.

• Photon counting devices provide superior contrast and dynamic range to charge integrating devices.

• Counting only hits that pass threshold rejects noise in the sensor giving clearer images.

• Dynamic range is limited only by front end response and depth of in-pixel counters.

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 5: Medipix3 Irradiation Studies

-5-

Medipix 3 • Medipix3 builds on the success of Medipix2 as a single

photon counting imaging chip• Added Features

– Analogue charge summing to keep all charge information– Spectroscopic mode with 8 threshold levels– Continuous readout mode (no dead time)– Increased counter depth increasing dynamic range– Increased readout speed– Increased radiation hardness from 130nm CMOS process

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 6: Medipix3 Irradiation Studies

-6-

Status• First engineering run (12 wafers of

100 chips) delivered early this year

• Wafer probing complete (from 11 wafers – 437 Class A + 166 ClassB)

• Initial readout system working at low speed

• Initial characterisation underway

• One wafer diced and bonded to PCBs

• First bump bonded assemblies with wafers expected soon

• Initial radiation hardness measurements underway

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 7: Medipix3 Irradiation Studies

-7-

Charge Summing

10001110101

A hit in the sensor deposits charge across four pixels

The analogue comparators assign the charge to the pixel with the most hits

This prevents ‘lost’ charge by partial hits not passing threshold

The threshold is applied to the summed charge and read out when the shutter closes

Medipix3 can be set to sum charge across four pixel clusters to prevent hits being lost due to charge sharingIntroduction

to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 8: Medipix3 Irradiation Studies

-8-

Spectroscopic Mode• By connecting 4 pixels into a larger super pixel, eight threshold

levels are available to us in the digital part of the pixel…

Counter 1

Counter 2

Counter 3

Counter 4

Counter 5

Counter 6

Counter 7

Counter 8

Amplifier response

• Each threshold is adjustable, allowing a wide range of settings.

• This gives us enough flexibility to capture reasonable spectra in many different applications

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 9: Medipix3 Irradiation Studies

-9-

MARS Computed Tomography System

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 10: Medipix3 Irradiation Studies

-10-

Radiation Damage• 130nm is expected to be significantly more radiation hard

than 250nm CMOS because of the much thinner gate oxides• Radiation damage occurs because the regular crystal

structure of a device is disturbed. This causes a number of effects among which are:

– Change of effective doping concentration (esp. in diodes)– Increase leakage current (esp. in detectors)– Charge trapping (esp. in detectors)– Oxide charging (esp. in CMOS)– Single Event Upset (in logic circuits)

• Photons and hadrons cause different types of damage, point and cluster respectively.

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 11: Medipix3 Irradiation Studies

-11-

Analogue Switch ProblemOne of the DAC outputs is particularly sensitive due to a minor design oversightThe current drawn by some minimum sized NMOS transistors when the matrix is irradiated can overload the DAC output stage stopping the pixel front end from functioning under nominal bias conditionsHere it happened at less than 1MRad so we were able to take no proper front end measurements during the 400MRad irradiation

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 12: Medipix3 Irradiation Studies

-12-

Solution• The chip operation can be recovered by modifying the

DAC settings.

• In particular the pre-amp reference voltage was reduced modifying the front-end operating point.

• Next wafer production fix Cas DAC issue by eliminating the (unneeded) leaky NMOS

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 13: Medipix3 Irradiation Studies

-13-

400Mrad Irradiation• Used a calibrated X-ray machine (Seifert RP149)• Beam profile is smaller than the Medipix3 → Two runs:

On the Pixel Matrix 60MradThreshold Variation

Gain Variation

Noise Increase

On the Periphery 400Mrad Check DACs

E-fuses

Logic functionality

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

60 MRad

460 MRad

400 MRad

Page 14: Medipix3 Irradiation Studies

-14-

Performance after 460MRadThreshold Noise

Yiel

d ar

tifac

t

b 190.733061= σ 35.304781= Fit 0.000026=

0 2 4 6 8 10 12 14 16 18 200

200

400

600

800

THL [ke-]

Cou

nts

σ=1.72 ke-

µ=9.3 ke-

b1 8.798851= σ1 1.591887= Fit1 0.000247=

0 20 40 60 80 100 120 140 160 180 2000

2000

4000

6000

8000

10000

noise[e-]

Cou

nts

σ=12.9 e-

µ=71.6 e-

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Threshold can be re-tuned using 5 bit equalisation

Page 15: Medipix3 Irradiation Studies

-15-

0 15 30 45 60 75 90 105 120 135 1500

1000

2000

3000

4000

5000Pixel Non-IrradiatedPixel Irradiated at 460 MRad

THL [DAC step]

Pixe

l cou

nts

Performance after 460MRad

Yiel

d ar

tifac

t

Qin=2ke-

0 100 20040

60

80

100

Row Number

Noi

se [e

-]

Row

[0:2

55]

After 460MRad there is essentially no gain variation observed

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 16: Medipix3 Irradiation Studies

-16-

DAC Measurement

0.5

0.525

0.55

0.575

0.6

0.625

0.65

0.675

0.7

0 10 20 30 40 50 60 70 80 90 100 110

Volta

ge [v

]

Hours (1hour→ 4Mrad)

Band Gap

Accumulated dose of 396 Mrad

0.3

0.325

0.35

0.375

0.4

0.425

0.45

0.475

0.5

0 10 20 30 40 50 60 70 80 90 100 110

Volta

ge [v

]

Hours (1hour→ 4Mrad)

Accumulated dose of 396 Mrad

9mV

NMOS DAC (Preamp)

0.95

0.975

1

1.025

1.05

1.075

1.1

1.125

1.15

0 10 20 30 40 50 60 70 80 90 100 110

Volta

ge [v

]

Hours (1hour→ 4Mrad)

Accumulated dose of 396 Mrad

33mV

PMOS DACIntroduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 17: Medipix3 Irradiation Studies

-17-

3MRad irradiation

3 MRad

Worst effect at 3MRad, After this the effect of further radiation is much less.

Irradiated a small area of the chip up to 3Mrad to try and keep Cas DAC working by only damaging a limited number of pixels

The main effect observed is the shift in the threshold and amplifier response caused by the DAC moving…

The front end leaves its normal operating point at nominal bias conditions at 1500krad, although it remains possible to read out the chip up to 3Mrad. The front end can be recovered by increasing the value of the IKRUM DAC.

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 18: Medipix3 Irradiation Studies

-18-

3MRad Noise and Gain Result

Yiel

d ar

tifac

t

0 100 20040

60

80

100

Row Number

Noi

se [e

-]

0 15 30 45 60 75 90 105 120 135 1500

1000

2000

3000

4000

5000Pixel Non-IrradiatedPixel Irradiated at 3 MRad

THL [DAC step]

Pixe

l cou

nts

Qin=2ke-

Row

[0:2

55]

In this worst case situation the noise is still less than 100e- and the gain variation is still minimal

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 19: Medipix3 Irradiation Studies

-19-

Next Steps

• Repeat 400MRad measurement with bare chips and full sensors, recovering performance at each point to take measurements.

• Begin hadronic measurements rather than x-rays, possibly in collaboration with LHCb upgrade projects.

• Measurements with different sensors eg 3D or diamond

• Next wafer production to fix Cas DAC issue by eliminating the (unneeded) leaky NMOS

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 20: Medipix3 Irradiation Studies

-20-

Conclusions

• Medipix3 has been operated successfully after an exposure to a very large x-ray dose.

• This is very encouraging for readout chips in SLHC trackers and other high radiation environments.

• Confirmation in a full chip that 130nm is intrinsically more radiation tolerant than 250nm.

• You still need to bear radiation damage in mind when designing chips to avoid unexpected problems.

• Thank you for your attention.

Introduction to Medipix3

Radiation Damage

460MRad Irradiation

Design Issues

3MRad Irradiation

Page 21: Medipix3 Irradiation Studies

-21-

Backup Slides

• This page has been left intentionally blank

Page 22: Medipix3 Irradiation Studies

-22-

460MRad Threshold variation

0 100 200170

180

190

200

210

220

Row Number

THL

[DA

C st

eps]

After annealing we see ~20 THL DAC steps variation (<1ke-) between areas non irradiated and areas irradiated at 60, 400 and 460 Mrad.This variation can be corrected by the threshold equalization procedure (5 bits)

Row

[0:2

55]

Page 23: Medipix3 Irradiation Studies

-23-

General behavior after 3 Mrad

0 3 6 9 12 15 18 21 24 27 300

200

400

600

THL [ke-]

Cou

nts

Threshold Noise

b1 4.928843= 1 0.991838= Fit1 3508.489764=

0 20 40 60 80 100 120 140 160 180 2000

5000

10000

15000

noise[e-]

Cou

nts

Yiel

d ar

tifac

t

Page 24: Medipix3 Irradiation Studies

-24-

THL Shift at 3MRad

0 100 200180

200

220

240

260

280

300

Row Number

THL

[DA

C st

eps]

Main contributor is the protection diode. In order to minimize the pixel to pixel threshold variation Ikrum is set to 16nA which indicated that the leakage of this diode at 3Mrad is ~5 to 10 nA. This is the worst radiation operation point.

Row

[0:2

55]

Page 25: Medipix3 Irradiation Studies

-25-

Difference in CAS DAC range• Good chip (F7) shows CAS DAC dynamic ranger larger

than other chips → Less ids transistor leakage in analog switches?

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

0 32 64 96 128 160 192 224 256 288 320 352 384 416 448 480 512

Volta

ge [V

]

DAC Code

8-bit DACs

Cas F7

Ideal

Cas E6


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