Comparison of P- and N-TYPE structures for both un-irradiated and irradiated MSSD sensors

1. Comparison of P- and N-TYPE structures for both un-irradiated and irradiated MSSD sensors

2. Simulation of MSSD : Cintvs. Vbias (un-irradiated) N type P type • Simulation is mostly in good agreement with measurements for both P and N-type.

3. Five Trap model • Two shallow acceptors and one • shallow donor in addition to two deep levels • Able to remove accumulation e- • Produce very high E field near n+ • Reproduce experimental observed good • Rint and Cint • With one deep acceptor, it is not possible to create enough E field (similar to measurement) • near n+ strip along with correct current. • We can not use deep acceptors with higher introduction rates as it will change space charge • significantly leading to very high avalanche multiplication & simulated current become very • high compare to measured one. • Moreover, in reality also, shallow levels are created in much more amount compare to deep • trap levels

4. Simulation of MSSD : Cintvs. Vbias(Irradiated) Red- Experimental result (flux-5e14) Blue - Flux=5e14neq, & QF =8e11cm-2 Green – Flux=1e15neq, & QF=1.2e12cm-2 Red- Experimental result (flux-5e14) Blue - Flux=5e14neq, & QF =8e11cm-2 Green – Flux=1e15neq, & QF=1.2e12cm-2 N type P type • Simulation is mostly in good agreement with measurements for both P and N-type • Cint changes slightly with change in combination of bulk damage (flux) + surface damage (QF)for low bias values.

5. Simulation of MSSD : Rint vs. Vbias (un-irradiated) N type P type Structure no-1 QF= Vary P type • N-type: • All 12 structures follow the similar good Rint characteristics for all values of QF. • P-Type: • Good isolation for all 12 structures for low values of QF. • Strip-isolation decreases on increasing the QF. QF= 1x1011cm-2 QF= 3x1011cm-2

6. Simulation of MSSD : Rint vs. Vbias(Irradiated) : P-TYPE • Measurement (Wolfgang) • DC-CAP Simulation (flux = 1x1015 cm-2) P and Y types Different QF • Simulated Rint show trends similar to the Measurements. • Rint decreases on increasing the QF. • Rint is a strong function of the combination of surface damage (QF) and Bulk Damage (flux). Bulk damage compensates for surface damage. • Good isolation even at high flux and high QF. Simulation Different Flux QF = 5x1011cm-2

7. Simulation of MSSD : Rint vs. Vbias (Irradiated) : N-type • Measurement (Wolfgang) • DC-CAP Simulation (flux = 1x1015 cm-2) Different Structures N-Type: Different QF • Isolation remains good for all values of QF. • Simulation shows decrease in Rint for high values of QF at high Bias values. Experimentally different structures show similar behaviour. • Electric field near the curvature of p+ strip is quite high & increases with QF . This high E field can initiate a localized avalanche & can decrease Rint

8. Simulation of MSSD : E. Field (Irradiated): P & N-types Efield along the surface (1.3 um below) Efield along the surface (0.1 um below) P-Type P-Type N-Type N-Type Flux = 1x1015cm-2; QF = 1.2x1012cm-2; Bias = 500 V • Peak electric field is more for N-type as compared to P-type sensor for a given bias. • Micro-discharge possibility is more in N-type sensors.

9. Simulation of MSSD : E. Field (Irradiated): QF variation P-Type N-Type Flux = 1x1015cm-2; QF : Vary ; Bias = 500 V • N-TYPE: • As QF increases = > Peak Efield increases. • Micro-discharge possibility is more in N-type sensors. • P-TYPE: • As QF increases = > Peak Efield decreases.

10. Simulation of MSSD : E. Field (Irradiated): Temp. variation Efield along the surface (0.1 um below) N-Type Flux = 1x1015cm-2; QF = 8x1011cm-2 ; Bias = 500 V • N:TYPE : Peak E field increases with increase in Temperature.

11. Design Rules for P-type

12. Effect of P-stop doping concentration : Rint Pstop-5e17cm-3 Pstop – 5e17cm-3 Pstop-5e16cm-3 Bias = 200V Flux=1e15cm-2 QF = 1.5e12cm-2 Cutline is 0.1µm below SiO2 Rint (ohm) Pstop-5e15cm-3 E field (V/cm) P-stop doping conc. Variation; QF=1.2e12 • Strip pitch : 90 micron (width = 20 micron) • Double Pstops (4µm each, separation - 6µm) • Flux = 1e15cm-3 Pstop – 5e15cm-3 • Increase in Pstop-doping conc. Increases Rint but decreases breakdown voltage. • Higher Pstop doping leads to very high E field at lower biases near Pstop curvature which can lead to sensor breakdown or probably microdischargesalso. • Lower Pstop-doping concentration is preferred.

13. Effect of P-stop doping width : Rint • Strip pitch : 90 micron (width = 20 micron) • Single Pstop (14µm and 28µm ); Pstop doping conc. = 5x1016cm-3 • Flux = 1e15cm-3 • For the values of the Pstop width considered in the simulation, the results of Rint and Efield are almost independent of that.

14. Backups!

15. Effect of QF variation for Pstop = 5e16cm-3 E field (V/cm) • For low values of QF, E field peak is under MO as well as near Pstop also • For higher values of QF, very high E field peak near Pstops, which increases with • increase in QF

16. Back up-One of the Rint measurement (Robert Eber) Measurement Simulation • Simulation indicate toward QF ~ 1.2e11 cm-2 • Good measurements can be used to predict value of QF using simulations!

17. Simulation structures and defects • Three strip structure (dimensions that of no-5 ) having central strip and two neighboring strips • which are shorted together • For Rint simulations, a small 0.2V bias is given to Anode1 electrode and Rint is calculated from • difference in Anode1 and Anode2 currents. • Simulations were carried out for structure having double Pstop isolation structure (The width • of each pstop is 4µm and they are separated by 6µm, pstop doping depth 1.5µm and doping • density 5x1015cm-3 ) and without any isolation structure at all.

18. Simulation of Rint without any bulk damage • - Three different type of Rint curves were observed. • -For low values of QF, good strip isolation was observed. • For intermediate values of QF, strip isolation is very poor for low biases but improves with higher • reverse biases. Electrons from accumulation layer are progressively removed by higher reverse • bias resulting in better Rint. • But for higher values of QF, Rint remain very low even at higher reverse bias. • Further, it can be observed that pstop doping density 5x1015cm-3 is not sufficient (Fig. 2(a) ) to maintain strip isolation with oxide charge density QF= 5x1011cm-2. • Similarly, it can be inferred from figure 2 (b) that without any isolation structure, strip isolation would not be possible, up to 800V, even for QF = 3x1011cm-2. Figure 2 (a) Figure 2 (b) Without any isolation str With HPK Double Pstops 1x1011 3x1011

19. Summary • Bulk damage and surface damage models are used to investigate the • strip isolation, micro - discharge problem and higher leakage current • for strip sensors • p+n- sensors are more prone to micro - discharge problem • Because of very high electric fields in curved regions of strips, Strip • sensors can have more leakage current compare to diodes • Rint measurement curves can also be understood qualitatively by • simulations • Further tuning of simulations is going on

20. Why two more acceptors with higher introduction rates ? – continue… • Ionized Acceptor trap density inside Si sensor • Ionized Shallow levels (green and blue) are much less compare to deep levels (Red color). • Ionized Acceptors just below SiO2/Si • Interface • In some of the region, Ionized shallow • traps (green and blue) are much more • compare to deep one Cutline is 0.1 um below SiO2 Cutline is perpendicular to n+ strip (Through middle)

21. Why micro discharge is quenched! • E field inside irradiated sensors is a strong function of space charge charge. • So, when breakdown happen near curved area of p+ strip, a lot of free e/h carriers are produced which will change the nearby space charge significantly, changing the electric field, thus stopping the further breakdown. • Moreover, in irradiated sensors, because of presence of high density of traps, free path length of a charge carrier will be very low, particularly in middle of sensors where E field is very low. These fact would stop the avalanche from turning global and continuous

22. Measurement of E-field in a irradiated Si strip sensor (n+p-)G. Kramberger et all , 2009, IEEE conference E field profile for a non-irradiated sensors <8000V/cm for reverse bias = 200V • E field profile for a irradiated sensors • Can be as high as 80000V/cm, near the strips for reverse bias = 200V ! • - Formation of high density negative space charge near n+ strips (flux=5e14cm-2) - The negative space charge will act as Pspray and increases with irradiation ! Hence, we never had much problem of strip isolations in hadron irradiation expt!

23. Electron conc. in the interstrip region decreases as flux increases

24. Rintvs. Vbias(Irradiated) : Strip pitch and Implant width Flux=1e15cm-2 QF = 1.2e12cm-2 • No significant dependence of Rint on changing the strip pitch and width.