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TCAD simulations of isolation structures for n + -on-p silicon microstrip detectors

TCAD simulations of isolation structures for n + -on-p silicon microstrip detectors Claudio Piemonte piemonte@itc.it. Outline. Why is this work needed? Device Simulations - p-spray - p-spray experience @ ITC-irst - p-stop - combined p-spray/p-stop Conclusion.

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TCAD simulations of isolation structures for n + -on-p silicon microstrip detectors

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  1. TCAD simulations of isolation structures for n+-on-p silicon microstrip detectors Claudio Piemonte piemonte@itc.it

  2. Outline • Why is this work needed? • Device Simulations • - p-spray • - p-spray experience @ ITC-irst • - p-stop • - combined p-spray/p-stop • Conclusion

  3. strip 1 strip 2 oxide + + + + + + + + n+ n+ p- substrate electron layer p+ backplane S1 S1 S2 S2 S1 S2 Why are simulations needed? isolation structure needed to interrupt the inversion layer between the strip 3 techniques available (from n+-on-n technology): p-stop p-spray/p-stop p-spray high-field regions high-field regions • Each technique affects differently VBR and Cint • simulations are needed to evaluate impact

  4. ½ strip ½ strip strip 2 strip 1 oxide n+ n+ p- substrate p+ backplane Simulated structure 2D simulations of a cross section orthogonal to the strips Reference structure: pitch = 80mm n+ width = 30mm Na = 7e11cm-3 W = 300mm Qox=4e11cm-2 Pure geometrical Cint (i.e. Qox=0, no isol. struct) w=40mm -> w/p=0.5 w=30mm -> w/p=0.375 w=20mm -> w/p=0.25

  5. p-spray

  6. strip 1 strip 2 oxide n+ n+ strip 1 strip 2 oxide n+ n+ VJBR no impact ionization model enabled VBR Impact ionization enabled Breakdown (1) • Three p-spray peak concentrations: • Np1=4e16cm-3 • Np2=8e16cm-3 • Np3=12e16cm-3 VJBR= potential difference between p-spray and strip which causes breakdown VBR= bias voltage for which we reach VJBR High Np => High VJBR => high VBR Transition voltage depends on the substrate concentration Slope depends on w/p Np1=Np2=Np3

  7. Np2=8e16cm-3 Np3=12e16cm-3 Breakdown (2) VBR depends on: 1)slope of Vp-sprayvs VBIAS which depends on w/p and Na 2)VJBR level which depends on NP, QOX and field-plate VBR increases for increasing Qox because VJBR level increases no isol.

  8. Interstrip capacitance (1) Np1=4e16cm-3 Cint increases with Np Cint decreases with Qox Np3=12e16cm-3 p-spray completely compensated => no isolation

  9. n+ n+ Interstrip capacitance (2) FPW Effect of field-plate Tox increases Cint by: C=0.5 *eox *(FPW*L) *Tox-1 Additional component decreases because p-spray is progressively depleted by field-plate

  10. p-spray profile along cut-line Net-doping conc. dose=5e12cm-2 dose=3e12cm-2 n+ p-spray p-spray @ irst First n+-on-p detectors with p-spray fabricated in 2004 2 p-spray doses were used (labeled as low-dose and medium-dose) Both doses isolate before irradiation

  11. 6 7 9 10 1 2 4 5 8 3 p-spray @ irst - breakdown Vbias=100V - Low dose p-spray no BD - Medium dose p-spray: 50mm pitch no BD 100mm pitch BD - Exception: med. dose 100mm pitch no BD because of larger p+ med-dose p-spray low-dose p-spray

  12. High-dose p-spray Low-dose p-spray p-spray @ irst – interstrip capacitance - At “low” voltages cap. dominated by field-plate: - larger field plate => higher capacitance - higher dose p-spray => longer decay

  13. p-stop

  14. S1 S2 Breakdown Similar approach to p-spray: 1. determine Vp-spray = f(VBIAS) 2. determine VJBR level high-field regions increasing p-stop width VJBR • p-stop potential is higher • for wide implants • better narrow p-stop from the breakdown viewpoint VJBR

  15. S1 S2 Interstrip capacitance inversion layer acts as an extension of the n+ strip • narrow p-stop implies higher capacitance (opposite trend with respect to breakdown) As Qox increases, interstrip capacitance increases

  16. combined p-spray/p-stop

  17. Concept p-spray: VBR - low before irradiation - improves for increasing QOX Cint - improves with irradiation p-stop: VBR - high before irradiation - decreases for increasing QOX Cint - high and deteriorates for inc. QOX improves with QOX deteriorates with QOX • interesting solution is to combine the previous two using: • medium dose p-spray (to have sufficiently high initial VBR) • 20/30mm wide p-stop (to have low capacitance for high QOX) Technology: - moderated p-spray (isolation structure realized with single implantation step) - combined p-spray/p-stop (two implantation steps)

  18. high-field regions high-field regions high-field regions 3 5 4 1 2 S1 p-stop S2 S1 p-stop S2 S1 p-stop S2 Breakdown For increasing QOX: VBR first increases (typical of p-spray) than decreases (typical of p-stop) as QOX grows

  19. S1 p-stop S2 S1 p-stop S2 S1 p-stop S2 Interstip capacitance 3 4 1 2 Cint first decreases (typical of p-spray) than increases (typical of p-stop) as QOX grows

  20. Conclusion • simulations well reproduce both breakdown and interstrip • capacitance behavior in the p-spray case • Among the simulated isolation structures, the combined • p-spray/p-stop technology seems to be the most effective • (as for n+-on-n detectors)

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