Silicon detectors in 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/cm 3 A mip particle creates about 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  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 devices High precision (< 5  m) 1-D coordinate measurement Large 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 devices True 2-D measurement (20  m 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 devices Just 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

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! L max ~10 34 cm -2 s -1 At  = 4 cm ~ (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 SiO 2 layer and SiO 2 -Si interface Fixed positive oxide charge Accumulation of electrons at the interface Additional interface states are created at the SiO 2 -Si border R. Wunstorf, PhD thesis 1992

Radiation Levels in some LHC experiments total dosefluence 1MeV n eq. [cm -2 ] after 10 years ATLAS Pixels50 Mrad1.5 x ATLAS Strips7.9 Mrad~2 x CMS Pixels~24Mrad~6 x * CMS Strips7.5 Mrad1.6 x ALICE Pixel500 krad~2 x LHCb VELO- 1.3 x /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 SD Standard Geometry Leakage path S D Gate Enclosed Geometry Enclosed gate (S-D leakage) Guard ring (leakage between devices)

Front end technology choices of the different experiments TechnologyChip ALICE Pixel0.25µm CMOSALICE1 ALICE Strips0.25µm CMOSHAL25 ALICE Drift0.25µm CMOSPASCAL ATLAS StripsDMILLABCD ATLAS PixelDMILL->0.25µm CMOSFE-D25 CMS PixelDMILL->0.25µm CMOSPSI CMS Strips0.25µm CMOSAPV25 LHCb VELODMILL/0.25µm CMOSSCTA/Beetle LHCb Tracker0.25µm CMOSBeetle Deep sub-µm means also: speed, low power, low yield, high cost

Radiation Damage in Detectors Surface 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