Silicon detectorsin nuclear and particle physics

A few general remarks

• Basic information carriers: electrons and holes

• Band gap: 1.2 eV

• Energy to create an (e-h) pair: 3.6 eV (30 eV in gases)

• High density: 2.33 g/cm3

• A mip particle creates about 30000 e-h pairs in 300 m Si

• High mobility - Fast signal collection (10 ns in 300 m Si)

• No charge multiplication - Amplification needed

• Radiation damage problems

• First use of silicon detectors in HEP experiments since 50’s for energy measurements

• Precision position measurements up until 70’s done with emulsions or bubble chambers limited rates and no triggering!

• Traditional gas detectors: limited to 50-100 m point resolution • First silicon usage for precision position measuring (late 70’s):

» secondary vertex tagging (charm) in fixed target experiments

» segmented sensors (strips) with fine pitch» first silicon pixel device used in early 80’s (NA32) charm

experiment– Why wasn’t silicon used earlier?

» Needed micro-lithography technology cost» Small signal size (need low noise amplifiers)» Needed read-out electronics miniaturization (transistors, ICs)

Silicon detectors in HEP experiments

..

First silicon usage in collider experiments- Initially avoided due to excessive material (electronics) in active volume- Advances in electronics miniaturization and low mass composite structures allowed its use- Late 80’s: Mark II (SLC) and in the 90’s all 4 LEP experiments (ALEPH, DELPHI, L3, OPAL)- First pixel detector at collider (SLC) in early 90’s (SLD experiment)- Usage of silicon limited to small region near interaction point (2-3 layers around beam pipe): both silicon and electronics were very expensive

Silicon detectors in HEP experiments

Current usage of silicon detectors- Basically all currently operating HEP collider experiments (FNAL p-pbar collider, HERA, B-factories at Cornell, SLAC and KEK) as well as all those in construction (LHC) use silicon vertex detectors.- Many fixed target experiments and non-HEP experiments (space physics) are using them as well.

Silicon detectors in HEP experiments

Next generation of collider experiments pushing the limits of the technology

- High radiation environment prevents usage of gas detectors near interaction point (r<1m)- New developments in radiation-hard silicon and electronics allow use of silicon strip devices for r>20cm- Silicon pixel devices to be used for r<20cm- Reduced cost of silicon and electronics allowing large area detectors

HEP silicon detector technology has greatly benefited from the revolutionary progress in the microelectronics industry (large area silicon wafer processing, CCDs, CMOS devices, radiation hard processes, high density interconnects...)

Silicon detectors in HEP experiments

Silicon detectors in high energy physics

• Silicon detectors are now widely used in high energy

physics, due to good energy and spatial resolution

• Two different approaches for position determination

• Discrete array of readout elements

• Continuous readout

Position sensitive devices

Strip devicesHigh precision (< 5m) 1-D coordinate measurementLarge active area (up to 10cm x 10cm from 6” wafers)Inexpensive processing (single-sided devices)2nd coordinate possible (double-sided devices)Most widely used silicon detector in HEP

Pixel devicesTrue 2-D measurement (20m pixel size)Small areas but best for high track density environment

Pad devices (“big pixels or wide strips”)Pre-shower and calorimeters (charge measurement)

Drift devicesJust starting to be used

– FNAL p-pbar collider» CDF(strip)» D0 (strip)» BTeV (pixel, strip)

– B-factory colliders» Babar (strip)» Belle (strip)» Cleo-3 (strip)

– HERA ep collider» H1 (strip)» Zeus (strip)

– RHIC heavy ion collider

» STAR (strip, drift)» PHENIX (strip,

pad)» PHOBOS (strip,

pad)» BRAHMS (strip)

– Fixed target» HERA-B (strip)» HERMES (strip)» COMPASS (strip)» others

– Space» AMS (strip)» GLAST (strip)» PAMELA (strip)» AGILE (strip)» NINA (strip)» others

– LHC pp/HI collider» ALICE (strip, drift,

pixel)» LHCb (strip)» ATLAS (strip, pixel)» CMS (strip, pixel, pad)

Silicon detectors in HEP experiments

Silicon detectors in HEP experiments

•At LHC, head-on collisions of protons (7 TeV on 7 TeV) and heavy ions (5.5 ATeV) will produce a lot of particles crossing silicon detectors!

Lmax~1034cm-2 s-1

At = 4 cm ~ 3 1015 (neq) cm-2 in 10 years (>85% charged hadrons)

! RADIATION DAMAGE !

Radiation damage

• Many effects (not fully understood) involved in the radiation

damage of silicon detectors

• Dose = Deposited energy/Mass (1 Gray = 1 Joule/kg = 100 rad)

• However, dose is not enough to understand the problem!

• Effects are dose dependent and particle species dependent!

• Bulk effects and Surface effects

Radiation damage

Surface Damage Bulk Damage

Electronics

Sensitive components are located close to the surface

Detectors

Full bulk is sensitive to passing charged particles

Radiation damage

Radiation Damage in Electronics

Cumulative Effects Single Event Effects (SEE)

Total Ionizing Dose (TID)Ionisation in the SiO2 and SiO2-Si interface creating fixed charges (all devices can be affected)

Displacement Defects(bipolar devices, opto-components)

Permanent (e.g. single event gate rupture SEGR)

Static (e.g. single event upset SEU)

Transient SEEs

Total Ionizing Dose

Ionization due to charged hadrons, , electrons,… in the SiO2 layer and SiO2-Si interface • Fixed positive oxide charge• Accumulation of electrons at the interface• Additional interface states are created at the SiO2-Si border

R. Wunstorf, PhD thesis 1992

Radiation Levels in some LHC experiments

total dose fluence 1MeV n eq. [cm-2] after 10 years

ATLAS Pixels 50 Mrad 1.5 x 1015

ATLAS Strips 7.9 Mrad ~2 x 1014

CMS Pixels ~24Mrad ~6 x 1014 *CMS Strips 7.5 Mrad 1.6 x 1014

ALICE Pixel 500 krad ~2 x 1013

LHCb VELO - 1.3 x 1014/year**

*Set as limit, inner layer reaches this value after ~2 years

**inner part of detector (inhomogeneous irradiation )

A radiation tolerant design is important to ensure the functionality of the read out over the full life-time!

Enclosed geometry to avoid leakage

Gate

S D

Standard Geometry

Leakage path

SD

Gate

Enclosed Geometry

Enclosed gate (S-D leakage)Guard ring (leakage between devices)

Front end technology choices of the different experiments

Technology Chip

ALICE Pixel 0.25µm CMOS ALICE1ALICE Strips 0.25µm CMOS HAL25ALICE Drift 0.25µm CMOS PASCALATLAS Strips DMILL ABCDATLAS Pixel DMILL->0.25µm CMOS FE-D25CMS Pixel DMILL->0.25µm CMOS PSICMS Strips 0.25µm CMOS APV25LHCb VELO DMILL/0.25µm CMOS SCTA/BeetleLHCb Tracker 0.25µm CMOS Beetle

Deep sub-µm means also: speed, low power, low yield, high cost

Radiation Damage in DetectorsSurface Damage

• Creation of positive charges in the oxide and additional interface states.• Electron accumulation layer.

Bulk Damage

Displacement of an Si atom and creation of a vacancy and interstitial

• Point like defects (, electrons)• Cluster Defects (hadrons, ions)

Macroscopic Effects

Bulk Damage

• Increase of leakage current• Increase of depletion voltage• Charge trapping

Surface Damage

• Increase of interstrip capacitance (strips!)• Pin-holes (strips!)

Effects signal, noise, stability (thermal run-away!)

• Annealing effects will not be discussed here.But: Do not neglect these effects, esp. for long term running!All experiments have set up annealing scenarios to simulate the damage after 10 years.

• Silicon detectors still largely in use for future experiments

• Several developments in progress

• Radiation damage is a concern

• New materials welcome

Conclusions