Sudeep Chatterji GSI, Darmstadt DPG Spring Meeting 4 March 2013

Sudeep Chatterji GSI, DarmstadtDPG Spring Meeting4 March 2013Silicon microstrip detectors for the CBM STS

Batches of Double sided Silicon Strip Detectors (DSSDs) explored till date. Problems with previous batches of DSSDs. Design improvement using TCAD simulation.

Development of an operating scenario for CBM experiment taking into account periods of shutdown/maintenance.

Radiation damage studies Impact of neutron fluence on a) Charge Collection efficiencyb) Capacitive/Resistive noisec) Required operating voltage Comparison of various Isolation techniques to optimize DSSDs performance.

Outline

q=150 (P-side)5.5 cm Pitch=50.7mm CBM01 1024 stripsCBM021.5cmPitch=50 mmCBM036.2 cmq=7.50Double metal interconnections for short corner stripsPitch=58 mm

CBM prototype detectors

Problem with Double metal interconnections (Batch CBM03)Pinholes/Broken capacitorsHuge series resistive noise (ENC a )Remedy (Implemented in Batch CBM03 & CBM05)Used stacked dielectrics like ONO to overcome pinhole problemThickness of metal interconnection increased to 1.0 mm from 0.5 mmTCAD tool SYNOPSYSRAPHAEL used to design kapton cables (See talk by M.Singla)SDEVICE used to design rad-hard sensors

Fluence profile for CBM STSInitial resistivity of silicon = 5.33 K-cmRuntime~2 months (T = -100 C), Shutdown~9 months (T = -100 C), Cold Maintenance~3 weeks (T=+100 C), Warm Maintenance~1 week (T=+210 C)Carrier lifetime estimation: Krambergers model te,h-1 = be,h (t,T).f, t = Annealing time When CCE = 85%, Vbias = VOP VOP ~ (1.5-2.0)*Vfd

YearFluence f(neq cm-2) Neff (cm-3)te(ms)th (ms)Vfd (V)Vop (V)1210132.8010111.141.052870241013-1.541011.57.5272050361013-5.351011.38.35144110481013-8.841011.285.26375150511014-12.11011.228.211100200

Simulated grids for various Isolation Techniques (SYNOPSYS)Metal workfunction = 4.29eVBarrier height (n-type) = 0.7eVBarrier height (p-type) = 0.58eVP-stopFloating implanted P-type dopant of high concentration between n-strips P-sprayFloating spray of P-type dopant of medium/low concentration between n-strips Modulated P-sprayCombination of P-stop and P-spray Schottky contact M-S contact having rectifying properties

p-sprayMeasured/Simulated ENC ComponentsGood match after depletionSchottky slightly better than P-Stop after full depletion

CCE/Rint, Conventional Vs. Schottky Schottky discarded (for UNBIASED schottky contact)

Various combinations for modulated p-spray p-stop => region with higher p-dose in modulated p-sprayOptimized isolation techniqueIn optimized sensor design V bd ~ 63 %C int ~ 25 %

p-spray con. (cm-3)11015(Very low con.)41016(Low con.)81016(Medium con.)121016(High con.)p-stop width (m)Vbd (V)Cint (pF cm-1)Vbd(V)Cint (pF cm-1)Vbd(V)Cint (pF cm-1)Vbd(V)Cint (pF cm-1)n-sidep-siden-sidep-siden-sidep-siden-sidep-side511251.551.764952.461.772202.441.761602.431.771011251.461.764902.461.772202.441.761602.431.771511501.61.764902.441.772202.441.761602.431.77207602.031.764802.511.772102.441.761602.501.77

Isolation TechniqueFluence(neqcm-2)Vbd(V)Cint(pFcm-1)CCE@Vop(%)P-stop2x10131x10149807202.092.0393.1588.87P-spray2x10131x10145134952.562.4493.1789Optimized Modulated P-spray2x10131x1014160011501.581.6093.2289

Summary Various batches of DSSDs with different isolation techniques explored. Problems faced with earlier batches overcome in subsequent batches.ENC components along with their values estimated for present batches.Expected loss of CCE with fluence, higher operating voltage needed limited by breakdown.An optimized design proposed to minimize the ENC and maximize the breakdown limit.Paper S.Chatterji et.al. Exploring various isolation techniques to develop low noise, radiation hard double-sided silicon strip detectors for the CBM Silicon Tracking System Accepted in IEEE Trans. Nucl. Sci. (Manuscript Id: TNS-00656-2012).

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