Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Developement of Radiation Hard Silicon for Tracking Detektors Student Seminar, 10. April 2006 Frank Hönniger University of Hamburg, Institut für Experimentalphysik

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Outline Motivation Properties of Silicon Detectors Radiation Damage Experimental Methods Experimental Results Conclusion and Outlook

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April q q p p n n n n p p p p p n n n p p n p n p Standard model of particle physics Open questions: is there a universal force? (GUT?) what is the origin of mass (Higgs-boson?) unknown types of matter (dark matter, SUSY) HEP-Experiments towards higher energies electromagnetic strong weakgravitation quarks: d, u, s, c, b, t leptons: (e - e ) (  -  ) (  -  )

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April LHC properties Proton-proton collider Energy: 2 x 7 TeV Luminosity: cm -2 s -1 Bunch crossing: every 25 nsec Rate: 40 MHz pp-collision event rate: 10 9 /sec (23 interactions per bunch crossing) Annual operational period: 10 7 sec Expected total op. period: 10 years Experimental request Detector property Reliable detection of mips S/N ≈ 10 reachable with employing minimum minimum detector thickness material budget High event rate excellent time- (~10 ns) and & high track accuracy position resolution (~10 µm) Complex detector design low voltage operation in normal ambients, (hybrid integration) Intense radiation field Radiation tolerance up to throughout operational MeV eq. n/cm² period of 10 years low dissipation power moderate cooling Silicon pixel and microstrip detectors meet all requirements for LHC How about future developments?

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Radiation requirements for LHC-experiments CERN-RD48 radiation damage for ATLAS inner detector annual hadron fluence (1 MeV n equiv.) LHC: L=1e34 cm -2 s -1 technology for Si-detectors available, however serious radiation damage S-LHC: factor 10: L=1e35 cm -2 s -1 5 years  (R=4cm) ~ 1.6E16 cm -2 no technology for Si-detectors at S-LHC available yet (thinner detectors?) coordinated R&D needed developement of radiation hard and cost-effctive detectors CERN-RD50 luminosity upgrade vertex

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Motivation for thinner detectors higher initial doping concentration (more n-type) to maintain reasonable reverse bias during operation Thickness: 300  m Pixel: oxygen enriched silicon – DOFZ SCT: standard silicon - StFZ LHC: use of high resistivity FZ silicon S-LHC: to prevent type inversion but all show „type inversion“ after 2*10 13 p/cm 2 „type inversion“ shorter charge collection time faster signal response less charge loss through trapping better signals thin detectors reduction of e.g. pixel area higher position resolution Si-detectors have to be thin

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Motivation of radiation tests and annealing studies use pad detectors with simple and cheap structures irradiate them with different particles and fluences simulation of the irradiation of the whole operation time for silicon detectors in short time annealing studies at higher temperatures (60°C, 80°C) acceleration of annealing (2  10 extract particle, fluence and time dependencies of the detector parameters make predictions Radiation damage and annealing (thermal treatment) have a big influence on detector performance

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Use of silicon

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Semiconductor detector - principle high energy- and position resolution fast readout of the signals large S/N-ratio (CCE=100%) compact geometry possible silicon: wide availibility, operation at RT, in ambient atmospheres and under low voltages a solid state ionisation chamber segmented detectors by planar process (Kemmer 1984) microstrip-, pixel-detectors, CCD‘s various application in HEP, space, atomic & material physics, and in medicine p + nn + - junction fully depleted

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Parameters of silicon detectors  + – +  –  –  –  – + + – + + – – – p n   donor/acceptor concentration space charge distribution carrier distribution electric field electric potential depletion region you can assume an abrupt p-n-junction under reverse bias depletion voltage V dep V>V dep capacitance C of depletion region influence on S/N-ratio most important: determines the bias supply 1 2

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Operational parameters of silicon detectors leakage current volume generation current (caused by impurities and defects) + diffusion current + surface generation current important for the S/N-ratio and the power consumption prevention by cooling (ATLAS: -10°C) charge collection efficiency CCE CCE = Q/Q 0 ratio of measured charge to the induced charge non-irradiated detector CCE = 1 3 4

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Radiation Damage in silicon mobile Interstitial I mobile Vacancy V CsCiCsCi Point defects: they can have discrete energy levels in the band gap can have electrical influence on detectors: generation and recombination of e-h-pairs carriers can be trapped compensation of the initial doping primäry point defects are V and I secondary complex defects: VP, VO, V 2 O, CO influence of oxygen: Theory (thus DOFZ-Silicon) more O more VO (elect. not active at RT) less O more V 2 O (elect. active at RT) Cluster: regions of dislocations high energy PKA cascades of shifted atoms can locally change the band structure generation and recombination of e-h-pairs not really understood yet impinging particle impinging particle Cluster classify into bulk damage (and surface damages) dislocation of Si -atoms (PKA = Primäry knocked on atom)

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Experimental findings No influence on the macroscopic parameters Influencing doping and current Influencing only doping Influencing only current Defect states of the defect levels VP-defect  „donor removal“ Effective doping: n t is large for a deep acceptor if  p >>  n Reverse current: Generation rate:

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April NIEL - Theory How can I compare the irradiation effects of different particles and energies? Simulation (M. Huhtinen): Initial distribution of vacancies in (1µm) 3 after particles/cm² 10 MeV protons24 GeV/c protonsneutrons with PKA (>2keV), sort of irradiation defects become independent of particle and energy NIEL scales with radiation damage D(E)  NIEL charged particle: coulomb scattering neutrons: elastic scattering both, at higher energies: nuclear react. one can convert the particle fluence to 1 MeV n equivalent fluence hardness factor   eq =   24 GeV/c protons: 0.62 reactor neutrons: MeV protons: 3.99 Li-ions: 50.7

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Damage induced changes of macroscopic properties Degradation of charge collection efficiency due to increase of charge carrier trapping 1/  eff,e,h =   e,h   Increase of leakage current Introduction of defects/clusters with near to mid-gap levels as generation centers, increase of noise and power consumption, thermal run-away  I/V =    Change effective doping concentration  change of voltage for total depletion V dep  Introduction of defects which are charged in the space charge region, (acceptor creation)  e.g.: V + P = VP (donor removal) „type inversion“

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Silicon detectors – used test structures StFZ - Standard Float-Zone Silicon produced at Wacker Siltronic, processed at CiS, standard oxidation (passivation) 305  m thick, orientation n-doped ([P] = 7e11 cm -3 ), [O] = 6e15 cm -3, [C] = 7.5e15 cm -3,  = 6.4 k  cm DOFZ – Diffusion Oxyge- nated Float-Zone Silicon produced at Wacker Siltronic, processed at CiS, add. oxidation 305  m thick, orientation n-doped ([P] = 7e11 cm -3 ), [O] = 2.34e17 cm -3, [C] = 11.7e15 cm -3,  = 6.4 k  cm CZ - Czochralski Silicon produced at Sumitomo/Sitix, processed at CiS, 300  m thick, orientation n-doped ([P] = 3e12 cm -3 ), [O] = 7.3e17 cm -3, [C] = 4.1e15 cm -3,  = 1.2 k  cm EPI - Epitaxial Silicon 25, 50 and 75 μm 25, 50, 75  m n-doped EPI-layer [P] = 7e13 cm -3 on 320  m CZ-substrate n + doped ([Sb] = 5.3e17 cm -3 ) orientation, produced at ITME/Warsaw, processed at CiS EPI-diode standard diode CZ, FZ-processEpitaxy 25, 50, 75 μm

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April CV/IV- measurements prober to pad prober to guard ring easy determination of macroscopic properties (depletion voltage, leakage current) allows a fast check of detector functionality measuring in dark box bias supply to the bottom guard ring is grounded

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April TCT- measurements (transient current technique) cooling possible with nitrogen measurements of current pulses with oscilloscope induced from drift of free carriers front and back illumination with 670 (3  m) and 1060 nm (across) laser penetration depth proportional to also exposure with  (23  m) or  (across) (get absolute values) investigation of elect. field distribution sign of the space charge trapping probability, separated for e and h depletion voltage CCE

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April DLTS and TSC Trap concentration N t is proportional to the peak height DLTS-spectrum after proton irradiation DLTS Method: p/n junction is hold under reverse bias Electrical or optical pulse fills traps inside the SCR with carriers Traps release carriers by thermal emission Emission is monitored as a capacitance signal TSC-spectrum after neutron irradiation TSC Method: Traps filled at low temperature by electrical or optical injection Diode heated under reverse bias Current during the heating is monitored Trap concentration is proportional to the released charge TSC current [A] T [K]

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April DLTS principle [1] A bias pulse towards a smaller voltage will reduce the SCR [2] The junction capacitance is reduced because positive space charge is partially compensated by trapped electrons in the SCR [3] The process of carrier emission can be followed as a capacitance transient

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April DLTS method The emission time constant can be evaluated:

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April CERN Scenario Experiment-Depletion Voltage 160 V 120 V large improvement for EPI-detectors small change in depletion voltage for EPI up to very high fluences no type inversion for EPI limitation for StFZ, DOFZ and CZ for very high luminosity colliders CERN Scenario Experiment: consecutive irradiation steps in between annealing for 4 after annealing CV/IV-measurements quasi „online“ monitoring annealing corresponds to 20 days at 20 °C closely related to stable damage Why is EPI radiation harder: shifted donor removal, because of higher initial donor concentration radiation induced acceptor creation compensated by radiation induced donors High energy protons max. bias supply

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Material Parameters  Oxygen depth profiles SIMS-measurements after diode processing O diffusion from substrate into epi-layer interstial O i + dimers O 2i [O] 25 µm > [O] 50 µm process simulation yields reliable [O]  Resistivity profiles SR  before diode process, C-V on diodes SR coincides well with C-V method Excellent homogeneity in epi-layers

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Typical Annealing Curves Typical annealing behavior of EPI- devices: V fd development: Inversion only(!) during annealing (  ) (100 80C ≈ 500 RT)  EPI never inverted at RT, even for 10 16

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Parameterization of Annealing Results Annealing components:  Short term annealing  N A ( ,t(T))  Stable damage  N C (  ) N C = N C0 (1-exp(-cΦ eq ) + g C Φ eq g C negative for EPI (effective positive space charge generation!)  Long term (reverse) annealing: Two components:  N Y,1 ( ,t(T)), first order process  N Y,2 ( ,t(T)), second order process N Y1, N Y2 ~ Φ eq, N Y1 +N Y2 similar to FZ Change of effective “doping“ concentration:  N eff = N eff,0 – N eff ( ,t(T)) Standard parameterization:  N eff = N A ( ,t(T)) + N C (  ) + N Y ( ,t(T))

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Stable Damage Component N eff (t 0 ): Value taken at annealing time t 0 at which V fd maximum  No space charge sign inversion after proton and neutron irradiation Introduction of shallow donors overcompensates creation of deep acceptors  Protons: Stronger increase for 25 µm compared to 50 µm  higher [O] and possibly [O 2 ] in 25 µm (see SIMS profiles)  Neutrons: Similar effect but not nearly as pronounced most probably due to less generation of shallow donors and as strong influence of deep acceptors (clusters)

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Shallow Donors, the real issue for EPI -Comparison of 25, 50 and 75 µm Diodes- SIMS profiling: [O](25µm) > [O](50µm) > [O](75µm) Stable Damage: N eff (25µm) > N eff (50µm) > N eff (75µm) TSC Defect Spectroscopy: [BD](25µm) > [BD](50µm) >[BD](75µm) Defect spectroscopy after PS p-irradiation Generation of recently found shallow donors BD (Ec-0.23 eV) strongly related to [O] Possibly caused by O-dimers, outdiffused from Cz with larger diffusion constant dimers monitored by IO 2 complex Strong correlation between [O]-[BD]-g C generation of O (dimer?)-related BD reason for superior radiation tolerance of EPI Si detectors ≈ 105 V (25 µm) ≈ 230 V (50 µm) ≈ 320V (75 µm)

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April DLTS (Deep Level Transient Spectroscopy) Φ (25 μm epi) = 1.2 ·10 12 p/cm 2 25 μm epi: defect at 67K T W = 200 ms, tp= 100 ms U R =-20V, U P =-0.1V 25 μm epi

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Charge Collection Efficiency  CCE degradation linear with fluence if the devices are fully depleted CCE = 1 –    ,   = 2.7  cm2  CCE(10 16 cm -2 ) = 70 %  CCE measured with 244 Cm  -particles (5.8 MeV, R  30 µm) Integration time window 20 ns  CCE measured with 90 Sr electrons (mip’s), shaping time 25 ns  CCE no degradation at low temperatures ! CCE measured after n- and p-irradiation  CCE(Φ p =10 16 cm -2 ) = 2400 e (mp-value) trapping parameters = thos for FZ diodes for small Φ, For large Φ less trapping than expected !

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Charge Collection Efficiency  CCE(Φ p =10 16 cm -2 ) = 2400 e (mp-Wert)  CCE mit 90 Sr Elektronen (mip’s), Integrationtime 25 ns

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Current Generation Result almost identical to FZ silicon: Current related damage rate α = 4.1· Acm -1 (Small deviations in short term annealing)

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April S-LHC CERN scenario experiment  Simulation: reproducing the experimental scenario with damage parameters from analysis  Experimental parameter:  Irradiation: fluence steps  2.2  cm -2 irradiation temperature  25°C  After each irradiation step annealing at 80°C for 50 min, corresponding 265 days at 20°C Excellent agreement between experimental data and simulated results  Simulation + parameters reliable! Stable donor generation at high Φ would lead to larger V fd, but acceptor generation during RT anneal could compensate this. Proposed Benefit: Storage of EPI-detectors during beam off periods at RT (in contrast to required cold storage for FZ) Check by dedicated experiment:

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April S-LHC operational scenario simulation results  RT storage during beam off periods extremely beneficial  Damage during operation at -7°C compensated by 100 d RT annealing  Effect more pronounced for 50 µm: less donor creation, same acceptor component  Depletion voltage for full SLHC period less than 300 V S-LHC: L=10 35 cm -2 s -1 Most inner pixel layer operational period per year: 100 d, -7°C, Φ = 3.48·10 15 cm -2 beam off period per year 265 d, +20°C (lower curves) -7°C (upper curves)

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Summary Thin low resistivity EPI diodes (grown on Cz) are extremely radiation tolerant No type inversion observed up to Φ eq = cm -2 for protons and neutrons Radiation induced stable donor generation related most likely to O-dimers Elevated temperature annealing results verified at 20°C Dedicated CERN scenario experiment shows benefits of RT storage Simulation of real SLHC operational scenario with RT storage demonstrated 50 µm EPI Detectors withstand 5y SLHC with V op ≤ 300V (full depl.) Future plans: p-type epi, thicker n-type epi, thin Cz …..

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April To understand the nature of the universe !Destination

Frank Hönniger, Institute for Experimental Physics Student Seminar, 10th April Experimental conditions at LHC Detector requirements reliable detection of charged particles material budget has to kept in mind, because of big detectors high event rate & high track accuracy complex detector design intense radiation field in the whole operational period of 10 years Radiation damage negative influence on detector parameters Silicon detector can handle with all requirements for LHC LHC properties Proton-proton collider Energy: 2 x 7 TeV Luminosity: cm -2 s -1 Bunch crossing: every 25 nsec Rate: 40 MHz pp-collision event rate: 10 9 /sec (23 interactions per bunch crossing) Annual operational period: 10 7 sec Expected total op. period: 10 years