Development of Radiation Hard Si Detectors

Transcription

1 Development of Radiation Hard Si Detectors Dr. Ajay K. Srivastava On behalf of Detector Laboratory of the Institute for Experimental Physics University of Hamburg, D-22761, Germany. Ajay K. Srivastava Uni.- Hamburg 1

2 Outline Activities of our group Motivation to develop radiation harder Si detectors CMS Silicon Tracker An overview of Si detectors (why?, types, relevant parameters, working principle) Radiation damage in silicon detectors * Macroscopic Radiation damage in Si detectros * Microscopic Radiation damage in Si Effects of microscopic defect on device performance * Strategies to develop radiation hard Si detectors * Experimental studies of radiation damage. * Macroscopic measurements * Microscopic measurements * Kinetics of defects. Summary of my work done for pixel detector (HPAD) development for European Free-electron-laser XFEL at Hamburg, Germany Ajay K. Srivastava Uni.- Hamburg 2

3 Activity of Our Group ZEUS experiment at HERA collider, DESY, Hamburg, Germany: silicon vertex detector, physics CMS experiment at CERN, Geneva, Switzerland: silicon tracking detector, detector calibration, GRID computing, physics Radiation hard detector research & development Since the early 1960s our group has been investigating silicon detectors for nuclear spectroscopy and high-energy physics experiments. Our research covers device physics, detector systems, radiation damage and sensor optimization Development of pixel detector (HPAD) for European Free-electron-laser XFEL at Hamburg, Germany Ajay K. Srivastava Uni.- Hamburg 3

4 Main Motivations for R & D on Radiation Tolerant Detectors LHC (2008), L = cm -2 s -1 (14 TeV pp collider, 25 ns bunch spacing) 10 years 500 fb -1 LHC upgrade: Φ(r=4cm) ~ cm -2 Super-LHC (?), L = cm -2 s -1 5 years Φ(r=4cm) ~ cm fb -1 Detector for the European Free-Electron-Laser XFEL at Hamburg (start in 2013): photon fluxes up to: /cm Gy [10 9 J/kg] (2oo ns distance between pulses) 5 Φ eq [cm -2 ] SUPER - LHC (5 years, 2500 fb -1 ) Pixel (?) Ministrip (?) ATLAS Pixel total fluence Φ eq ATLAS SCT - barrel (microstrip detectors) r [cm] Macropixel (?) neutrons Φ eq pions Φ eq [M.Moll, simplified, scaled from ATLAS TDR] other charged hadrons Φ eq Ajay K. Srivastava Uni.- Hamburg 4

5 CMS Silicon Tracker CMS Silicon Tracker Finely segmented silicon sensors (strips and pixels) enable charged particles to be tracked and their momenta to be measured. They also reveal the positions at which long-lived unstable particles decay. This part of the detector is the world's largest silicon detector. It has 205 m 2 of silicon sensors (approximately the area of a tennis court) comprising 9.3 million microstrips and 66 million pixels. Ajay K. Srivastava Uni.- Hamburg 5

6 Si Detector Why Si detectors for particle tracking in High-Energy-Physics Experiments? 1. Fast response 2. High position resolution (~10μm) 3. Reliable operation Types: I. Si strip detector (single sided/double sided) II. Si pixel detector III.Si pad detector Strip detector Pixel detector Pad detector Ajay K. Srivastava Uni.- Hamburg 6

7 Working principle of Si strip detector (AC coupled) Relevant parameters Depletion depth and Voltage Reverse current = generation current Capacitance Noise Charge collection Ajay K. Srivastava Uni.- Hamburg 7

8 Macroscopic Radiation Damage in Silicon Detectors Bulk (crystal) damage due to Non Ionizing Energy Loss (NIEL) - displacement damage, crystal defects/microscopic defect I. Change of effective doping concentration N eff (higher depletion voltage V dep ) II. Increase of leakage current (increase of shot noise, thermal runaway) III.Increase of charge carrier trapping (reduced charge collection efficiency (CCE)) Surface damage due to Ionizing Energy Loss (IEL) I. Charge build-up in SiO 2 (shift of flatband voltage V fb, II. Traps of Si-SiO 2 interface breakdown of critical corners) III Surface generation current (increase shot noise) Ajay K. Srivastava Uni.- Hamburg 8

9 Microscopic Radiation Damage in Silicon 50 kev Si ion with fluence 3 x cm -2 Interstitials and vacancies are mobile at room temperature Examples for defect reactions: (E k >25 ev) (E k >5 kev) I- Interstitials C s - substitutional C V- Vacancy C i Interstitials C [TRIM-simulation, G. Davies, RD50 workshop Ljubljana 2008, modified] Ajay K. Srivastava Uni.- Hamburg 9

10 Radiation Induced Defects and Impact on Device Performance Influence of defects on the material and device properties charged defects N eff, V dep e.g. donors in upper half of band gap and acceptors close to midgap trapping (e and h) CCE shallow defects do not contribute at room temperature due to fast detrapping generation leakage current levels close to midgap most effective Ajay K. Srivastava Uni.- Hamburg 10

11 Strategies to Develop Radiation Hard Si detector I. Material/Defect engineering - Understanding of radiation damage Irradiation with different particles and energies Thermal treatment to understand kinetics Macroscopic effects to understand detector performance Microscopic defects and simulation Improved sensor performance with oxygen rich material. Study different materials : DOFZ, Cz, MCz, EPI-Si II. Device Engineering - Simulation and study of prototype detectors n + -in-p 3D detectors Thin detectors Ajay K. Srivastava Uni.- Hamburg 11

12 I. Macroscopic Measurements I-V C-V Dark Current & stability of device Effective doping concentration, N eff / V dep ΔI / V [A/cm 3 ] n-type FZ - 7 to 25 KΩcm n-type FZ - 7 KΩcm n-type FZ - 4 KΩcm n-type FZ - 3 KΩcm p-type EPI - 2 and 4 KΩcm Φ eq [cm -2 ] n-type FZ Ωcm n-type FZ Ωcm n-type FZ Ωcm n-type FZ Ωcm n-type CZ Ωcm p-type EPI Ωcm [M.Moll PhD Thesis] Ajay K. Srivastava Uni.- Hamburg 12

13 I. Macroscopic Measurements TCT Analysis 0.20 Electric field in sensor Full depletion voltage Effective trapping time Charge collection efficiency MCz, Φ eq = 1x10 14 cm Electric field in a damaged -2 silicon sensor (Φ eq. = 1 x10 14 cm -2 ) current [arb. units] V 80 V 60 V 50 V 40 V Time [ns] Ajay K. Srivastava Uni.- Hamburg 13

14 I. Macroscopic Measurements CCE (Charge collection efficiency) signal to noise ratio & detector efficiency Cm source 5.8 MeV α-particles CCE EPI-ST, 72 μm EPI-DO, 72 μm FZ, 100 μm FZ, 50 μm Φ eq [cm -2 ] [K. Koch, diploma thesis 2007, modified] Ajay K. Srivastava Uni.- Hamburg 14

15 II. Microscopic Measurements DLTS Φ eq cm -2 TSC Φ eq cm -2 Obtained electrical properties of defects: Concentrations Activation energies Capture cross sections Ajay K. Srivastava Uni.- Hamburg 15

16 Kinetics of Defects Time developments of defects depends on Temperature Impurities I= I 0 + VΦ eq. α Ajay K. Srivastava Uni.- Hamburg 16

17 Radiation Damage of Si Detectors by X-RaysX Only damage in SiO 2 and Si-SiO 2 interface important Test structure: gated diode with 5 gate rings Circuit for I-V and C-V/G-V measurement of gated diode Ajay K. Srivastava Uni.- Hamburg 17

18 Experimental Results on Oxide Charge Density N ox reaches a maximum at 5MGy and then decreases why? Ajay K. Srivastava Uni.- Hamburg 18

19 C-V V Characteristics of MOS Capacitor Accumulation V fb = Ø ms Q ss /C ox Depletion Inversion C HF, inv C ox - Oxide related capacitance (F) C HF, inv- High frequency inversion capacitance (F) C fb - Flat band capacitance (F) 7/10/2008 Ajay K. Srivastava Uni.- Hamburg 19

20 ISE-TCAD Simulation of CMOS Capacitor Ajay K. Srivastava Uni.- Hamburg 20

21 ISE-TCAD Simulation of CMOS Capacitor Ajay K. Srivastava Uni.- Hamburg 21

22 Simulation Result and Discussion S./E.= 1.1 Ajay K. Srivastava Uni.- Hamburg 22

23 Comparison: Simulation with Data Conclusion: I. Encouraging first result. II. Improve modelling. III. Extend to irradiated sensors. Ajay K. Srivastava Uni.- Hamburg 23

24 Summary 1. High resolution silicon sensors are presently used in all collider experiments 2. Group has unique expertise and equipment for macroscopic and microscopic radiation damage studies 3. Detailed simulation for next generation of sensors started Ajay K. Srivastava Uni.- Hamburg 24