1

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

2

N type P type

Simulation of MSSD : Cint vs. Vbias (un-irradiated)

• Simulation is mostly in good agreement with measurements for both P and N-type.

3

Five Trap modelTrap Energy Level Intro. σe (cm-2) σh (cm-2)

Acceptor 0.525 eV 3.0 1x10-14 1.4x10-14

Acceptor 0.45eV 40 8x10-15 2x10-14

Acceptor 0.40eV 40 8x10-15 2x10-14

Donor 0.50eV 0.6 4x10-14 4x10-14

Donor 0.45eV 20 4x10-14 4x10-14

• 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

N type P type

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

• 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.

Simulation of MSSD : Cint vs. Vbias (Irradiated)

5

Simulation of MSSD : Rint vs. Vbias (un-irradiated)

P type

N type P type

QF= 3x1011cm-2

QF= 1x1011cm-2

Structure no-1QF= Vary

• 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.

6

Measurement (Wolfgang)- DC-CAP

Simulation of MSSD : Rint vs. Vbias (Irradiated) : P-TYPE

• 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 (flux = 1x1015 cm-2)

P and Y types

Simulation

Different FluxQF = 5x1011cm-2

Different QF

7

Simulation of MSSD : Rint vs. Vbias (Irradiated) : N-type

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

Measurement (Wolfgang)- DC-CAP

Simulation (flux = 1x1015 cm-2)

Different Structures

8

Simulation of MSSD : E. Field (Irradiated): P & N-types

Efield along the surface (1.3 um below)

P-Type

N-Type

• 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.

Efield along the surface (0.1 um below)

P-Type

N-Type

Flux = 1x1015cm-2 ; QF = 1.2x1012cm-2; Bias = 500 V

9

Simulation of MSSD : E. Field (Irradiated): QF variation

P-Type N-Type

• P-TYPE: • As QF increases = > Peak Efield

decreases.

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.

10

- N:TYPE : Peak E field increases with increase in Temperature.

Simulation of MSSD : E. Field (Irradiated): Temp. variation

Efield along the surface (0.1 um below)

Flux = 1x1015cm-2 ; QF = 8x1011cm-2 ; Bias = 500 V

N-Type

11

• Design Rules for P-type

12

Pstop-5e15cm-3

Pstop-5e16cm-3

Pstop-5e17cm-3

P-stop doping conc. Variation; QF=1.2e12

Rint

(ohm

)

- Strip pitch : 90 micron (width = 20 micron)-Double Pstops (4µm each, separation - 6µm)- Flux = 1e15cm-3

Effect of P-stop doping concentration : Rint

Pstop – 5e17cm-3

Pstop – 5e15cm-3

Bias = 200VFlux=1e15cm-2

QF = 1.5e12cm-2

Cutline is 0.1µmbelow SiO2

E fie

ld (V

/cm

)

- 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 microdischarges also.

- Lower Pstop-doping concentration is preferred.

Effect of P-stop doping width : Rint

- For the values of the Pstop width considered in the simulation, the results of Rint and Efield are almost independent of that.

- Strip pitch : 90 micron (width = 20 micron)- Single Pstop (14µm and 28µm ); Pstop doping conc. = 5x1016cm-3

- Flux = 1e15cm-3

14

Backups!

15

Effect of QF variation for Pstop = 5e16cm-3

-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

E fie

ld (V

/cm

)

16

Back up-One of the Rint measurement (Robert Eber)

Simulation indicate toward QF ~ 1.2e11 cm-2

Good measurements can be used to predict value of QF using simulations!

Measurement

Simulation

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

With HPK Double Pstops Without any isolation str

3x1011

1x1011

- 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 (b)Figure 2 (a)

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/SiInterface-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 sensorsCan be as high as 80000V/cm, near the strips for reverse bias = 200V !- Formation of high density negative space charge near n+ strips

- 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!

(flux=5e14cm-2)

23

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

Flux=1e15cm-2

QF = 1.2e12cm-2

Rint vs. Vbias (Irradiated) : Strip pitch and Implant width

- No significant dependence of Rint on changing the strip pitch and width.