monolithic active pixel sensors (maps) · maria elisabetta giglio monolithic active pixel sensors...

Monolithic Active Pixel Sensors(MAPS)

Maria Elisabetta Giglio

February 3, 2017

Maria Elisabetta Giglio Monolithic Active Pixel Sensors (MAPS) February 3, 2017 1 / 39

Table of contents

1 IntroductionPixel sensors: key to solve complex eventsPixel sensors in High Energy Physics (HEP) experiments

2 ALICE Monolithic Pixel SensorThe experimentUpgrade of the Inner Tracking System (ITS)Pixel Chip TechnologyRead OutFocus on ALPIDE architecture

3 pALPIDE prototype - Test Beam Resultsπ− irradiationNeutron irradiation

4 Conclusions


Introduction

Outline




4 Conclusions


Introduction Pixel sensors: key to solve complex events

Silicon sensors

Solid state detectorsHigh resolution for particle trackingPrinciple of operation analogous to gasionization devicesLow ionization energy (3.6eV to create anelectron-hole pair)

- gas detectors 15-40 eV per electron-ion pair- scintillators 400-1000 eV per photon

→ Better energy resolution and high signalHigh density and atomic number

- Large energy loss in a short distance→ Thinner detectors→ Diffusion effect smaller than in gas detectors→ Higher spatial resolution

High carrier mobility → Fast!High intrinsic radiation hardness

Figure: Scheme explaining the working principleof a semiconductor as detector.


Introduction Pixel sensors: key to solve complex events

Silicon Pixel Sensors

2D-matrix → unambiguous hits

High granularity - Small pixel area- Low detector capacitance (≈ 1fF/Pixel)- Large signal-to-noise ratio (e.g. 150:1)

Small pixel volume → low leakage current (≈ 1pA/Pixel)

High cost per surface unit (Large number of readout channels)- Large number of electrical connections- Large power consumption


Introduction Pixel sensors in High Energy Physics (HEP) experiments

Pixel detectors - Key to solve complex events L. Musa - CERN (EMMI, GSI 11June, 2015)

Si Pixel detectors are high granularity detectors in a harsh environment close to theInteraction Point (IP)

Position resolution down to few µmHigh track density region → Unambiguous hit info is necessary!High resolution for determination of primary and secondary vertexHigh interaction rate → fast readoutHigh level of radiation



Pixel sensors - At the heart of the LHC Experiments

L. Musa - CERN (Schloss Waldthausen, Germany, August 2016)



The variety of pixel technology



The CMOS Technology

What is a CMOS?

Complementary Metal Oxide Semiconductorsn-channel MOSFET (NMOS)p-channel MOSFET (PMOS)

MOSFET= Metal Oxide Semiconductor Field Effect Transistor



The CMOS Technology

What is a MOSFET?(Metal Oxide Semiconductor Field Effect Transistor)

Figure: Physical structure of the n-channel enhancement-type transistor.



The CMOS Technology


Figure: The enhancement-type NMOStransistor with positive voltage appliedto the gate.

pn-junctions between substrate and D and S,kept reversed biased

VDS is always positive (3-terminal device byconnecting B and S)

VGS controls the current flow from D to S in thechannel region

The device is symmetrical respect with S and D



The CMOS Technology



VGS = 0

2 back-to-back diodes in series between D and S

No current conduction from D to S when a VDSis applied (very high resistance ∼ 1012Ω)



The CMOS Technology



VGS > 0Free holes are repelled from the channel, acarrier-depletion region is formed

e− from n+ regions accumulate under the gate,an n-region is created

An electric field develop from G and the channel,it controls the amount of charge in the channel(”filed-effect transistor”)

VDS makes the current to flow, forming then-channel



The CMOS Technology



Threshold voltage Vt : VGS such that a sufficientnumber of e− accumulate to form a conductingchannel

Overdrive voltage: VOV = VGS − Vt

→V across the oxide must exceed Vt for achannel to form!



The CMOS Technology

What is a MOSFET?

VGS > Vt enhances the channel (MOSFET conducts)small VDS : MOSFET as a resistance whose value is controlled by VGS



The CMOS Technology

What is a CMOS?

Fast switching characteristics → CPUsno ohmic resistors needed → low powereasy to implement capacitors



Hybrid Pixel Technology

Hybrid Pixel Detector (currently used at LHC)

Sensor based on silicon junction detectorsReadout chip: ASIC - CMOS sub-microntechnology (limited number of producers ∼ 10world-wide)Sensor and front-end electronics in two separateSi chips and connected by bump bonds (complexand costly)Bump bonds limited to pitches of 30− 50µm(input capacitance, power consumption)



Beyond Hybrid Pixel Detectors...

How to design a CMOS particle detector?

Monolithic Active Pixel Sensors (MAPS)Monolithic=single process

Sensing part incorporatedinside the ASIC! (signalprocessing inside the pixel)→ high granularity

typical dimensions20× 20µm2

Motivation to reduce cost,power, material budget,assembly and integrationcomplexity



Monolithic Pixel Sensors in Heavy Ion (HI) experiments

L. Musa - CERN (Schloss Waldthausen, Germany, August 2016)

Industrial development of CMOS imaging sensors and intensive R&D work within theHEP community!

Several HI experiments have selected CMOS pixel sensors for their inner trackers!


ALICE Monolithic Pixel Sensor

Outline




4 Conclusions


ALICE Monolithic Pixel Sensor The experiment

The current ALICE detector L. Musa - CERN (EMMI, GSI 11June, 2015)


ALICE Monolithic Pixel Sensor The experiment

The current ALICE Inner Tracking System (ITS) L. Musa - CERN (EMMI, GSI 11June, 2015)

6 concentric barrels, 3 different technologies2 layers of silicon pixels (SPD)2 layers of silicon drift (SDD)2 layers of silicon strips (SSD)


ALICE Monolithic Pixel Sensor Upgrade of the Inner Tracking System (ITS)

Physics motivation for upgrade L. Musa - CERN (EMMI, GSI 11June, 2015)

Provide a characterization of QGP properties for a better understanding of QCD

measurements of rare probes → High statistics (luminosity) required!→ deal with the challenge of expected Pb-Pb interaction rates of up to 50kHz

new high precision measurements→ Coverage in pT as complete as possible (down to very low momenta)→ Very accurate identification of secondary vertices from decaying charm and beauty→ High standalone tracking efficiency

Improve the resolution and the readout rate capabilities is fundamental!



Upgrade objectives

1. Improve impact parameter resolution by a factor of 3Get closer to IP (position of the first layer): 39mm → 22mmReduce x/X0/layer: ∼ 1.14%→∼ 0.3% (for inner layers)Reduce pixel size: 50µmX425µm → 28µmX28µm

2. Improve tracking efficiency and pT resolution at low pT (Increase granularity)6 layers → 7 layerssilicon drift and strips → pixels

3. Fast readout (study of new readout architectures): 1kHz→ 50kHz



The new ITS Layout L. Musa - CERN (EMMI, GSI 11June, 2015)

7-layer geometry based in CMOS Sensors

r coverage: 23-400mm

pseudorapidity: |η| ≤ 1.22 (90% mostluminous region)

3 Inner Barrel Layers

4 Outer Barrel LayersMaterial/Layer: 0.3%X0 (IB), 1%X0 (OB)



PIXEL Chip - General Requirements L. Musa - CERN (EMMI, GSI 11June, 2015)


ALICE Monolithic Pixel Sensor Pixel Chip Technology

A novel Pixel Chip Technology

CMOS Pixel Sensor using TowerJazz 0.18µm CMOS Imaging Process

High Resistivity (> 1kΩcm) p-type epitaxiallayer (18µm to 30µm) on p-type substrate

Small N-well diode (2µm diameter)→ minimize spread of charge over many pixels→ minimize capacitance (∼ fF ) (decisive for

large S/N at low power)→ large depletion volume

Application of reverse bias voltage(−6V < Vbb < 0V ) to substrate to increasedepletion zone around NWELL collection diode

Deep PWELL shields NWELL of PMOStransistors to allow for full CMOS circuitrywithin active area


ALICE Monolithic Pixel Sensor Read Out

Different Architectures


ALICE Monolithic Pixel Sensor Read Out

Different Architectures

Pixel pitch: 28µmx28µmEvent time resolution: < 2µsPower consumption: 39mW /cm2

Dead Area: 1.1mmx30mm

Pixel pitch: 36µmx64µmEvent time resolution: ∼ 20µsPower consumption: 97mW /cm2

Dead Area: 1.7mmx30mm

ALPIDE and MISTRAL-O have the same dimensions (15mmx30mm), identical physicaland electrical interfaces


ALICE Monolithic Pixel Sensor Focus on ALPIDE architecture

ALPIDE-Principle of Operation L. Musa - CERN (EMMI, GSI 11June, 2015)

ArchitectureIn-pixel amplificationIn-pixel discriminationIn-pixel (multi-) hit bufferIn-matrix sparsification (lessdata to send, shorter time,lower power)



pALPIDE-1 - First full scale prototype

ALPIDE Full Scale prototypedimensions: 30mmx15mmPixel Matrix: 1024 cols x 512 rowsFinal pixel pitch: 28µmx28µmPower consumption: ∼ 30mW /cm2

Interface pads over matrixGlobal shutter: triggered acquisition(200kHz) or continuous (int time< 10µs) Figure: Picture of pALPIDE-1



Projected Performance of new ITS L. Musa - CERN (EMMI, GSI 11June, 2015)

∼ 40µm at pT = 500MeV /c


pALPIDE prototype - Test Beam Results

Outline




4 Conclusions


pALPIDE prototype - Test Beam Results

Experimental setup-CERN PS, September 2015

M. Mager, ”ALPIDE, the Monolithic Active Pixel Sensor for the ALICE ITS upgrade”, Nucl. Instrum. Meth., A 824 (2016) 434-438

Telescope made of 6-7 planes of pALPIDE-1 sensors

3 reference planesDUT (Device Under Test)2/3 reference planes

Figure: Photograph of the pALPIDE-1 indicating its splits.Figure: The pALPIDE-1 test beam telescopeset-up.


pALPIDE prototype - Test Beam Results π− irradiation

Detection efficiency and Noise occupancy

P. Yang et al., ”MAPS development for the ALICE ITS upgrade”, JINST 10 (2015) no.03,C03030

Figure: Noise occupancy and detection efficiency of sectors of 1 to 3, beam test results by using a 6GeV π− source.



Detection efficiency and Noise occupancy


Figure: Detection efficiency as a function of the hit position within the pixel of sector 1, beam test results by using a 6GeV π−

source.



Residual and Cluster size


Figure: Residual and average cluster size of sectors of 1 to 3, beam test results by using a 6GeV π− source.



Residual and Cluster size


Figure: Cluster size as a function of the hit position within the pixel of sector 1, beam test results by using a 6GeV π− source.


pALPIDE prototype - Test Beam Results Neutron irradiation

Detection efficiency and Fake Hit RateLarge operational margin with detection efficiencies well above 99% at fake hit rate significantlybelow 10−5 for several sensors!


Figure: Detection efficiencies and fake hit rates of pALPIDE-1 (50µm thick chips) shown for different dies as well as before andafter neutron irradiation. Data is for sector 2 with a reverse bias voltage of −3V .


pALPIDE prototype - Test Beam Results Neutron irradiation

Cluster size and spatial resolution


Figure: On the left, cluster sizes and spatial resolution of pALPIDE-1 shown for different dies as well as before and afterneutron irradiation. On the right, cluster sizes dependent on impact point within 2x2 pixels (elementary layout cell). Whenparticles hit the sensor at the center, the average size is below 2, rising to 3,5 in cases where the particle impinges the sensor atthe corner of a pixel. Data is for sector 2 with a reverse bias voltage of −3V .


Conclusions

Outline




4 Conclusions


Conclusions

ConclusionsMAPS provides:

Very thin sensors and small pixelsReduced power consumption and integration time by an order of magnitude

Tests results of pALPIDE prototype are very promising!DE above 99% and FHR much better than 10−5

Spatial Resolution ∼ 5µm

MAPS are used and considered for many upgrade projects

Further optimisation is foreseen onThe collection electrode to reduce power and achieve lower charge detection thresholdThe pixel array readout circuit (AERD) to increase readout speed

→ Clearly the way to go in future!


Conclusions

Thank you for your attention!


monolithic active pixel sensors (maps) · maria elisabetta giglio monolithic active pixel sensors...

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