Sudeep Chatterji GSI, Darmstadt DPG Spring Meeting 4 March 2013

1. Silicon microstrip detectors for the CBM STS Sudeep Chatterji GSI, Darmstadt DPG Spring Meeting 4 March 2013

2. Outline • 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 efficiency • b) Capacitive/Resistive noise • c) Required operating voltage • Comparison of various Isolation techniques to optimize DSSDs performance.

3. Double metal interconnections for short corner strips 6.2 cm 5.5 cm q=±7.50 Pitch=50 mm q=150 (P-side) Pitch=58 mm Pitch=50.7mm 1.5cm CBM03 CBM02 CBM01 1024 strips

4. CBM prototype detectors

5. TCAD tool “SYNOPSYS” • “SDEVICE” used to design rad-hard sensors • “RAPHAEL” used to design kapton cables (See talk by M.Singla) • Problem with Double metal interconnections (Batch CBM03) • Pinholes/Broken capacitors • Huge series resistive noise (ENC a ) • Remedy (Implemented in Batch CBM03’ & CBM05) • Used stacked dielectrics like ONO to overcome pinhole problem • Thickness of metal interconnection increased to 1.0 mm from 0.5 mm

6. Fluence profile for CBM STS • Initial resistivity of silicon = 5.33 KΩ-cm • Runtime~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: • Kramberger’s model te,h-1=be,h (t,T).f, t = Annealing time • When CCE = 85%, Vbias = VOP • VOP ~ (1.5-2.0)*Vfd

7. Simulated grids for various Isolation Techniques (SYNOPSYS) • P-stop • Floating implanted P-type dopant of high concentration between n-strips • P-spray • Floating spray of P-type dopant of medium/low concentration between n-strips • Modulated P-spray • Combination of P-stop and P-spray • Schottky contact • M-S contact having rectifying properties Metal workfunction = 4.29eV Barrier height (n-type) = 0.7eV Barrier height (p-type) = 0.58eV

8. Measured/Simulated ENC Components p-spray • Good match after depletion • Schottky slightly better than P-Stop after full depletion

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

10. Optimized isolation technique • Various combinations for modulated p-spray • p-stop => region with higher p-dose in modulated p-spray In optimized sensor design V bd~ 63 % C int ~ 25 %

11. 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). Thanks