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					                   Status Report: Radiation Damage for X-rays in Si




    Two-Dimensional Numerical Modelling of p+n-n+
     Strixel Si Sensor: Impact on Cint, Rint and VBD

   (Ajay K. Srivastava, E.Fretwurst, R.Klanner, D.Eckstein)

                        Institute for Experimental Physics,
                         University of Hamburg, Germany




CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg        1
                   Status Report: Radiation Damage for X-rays in Si


Outline

•   Introduction
•   Device Structure and Physical Model
•   Device Simulation Procedure
•   Work Done :

Simulation Result and Discussion

Two-dimensional numerical modelling of p+n-n+ strixel Si sensor:
Impact on Cint, Rint and VBD
   • Unirradiated
      • With metal-overhang
      • Without metal-overhang




CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg        2
                       Status Report: Radiation Damage for X-rays in Si


Introduction

Why Cint important?             In the design of Si microstrip detectors,
                                      Cint      Creation of cross-talk noise
                                                 in the readout of electronics

For good S/N ratio  Low Cint required


The relation of equivalent noise charge (ENC) with total detector capacitance (CD)
given by,

                              ENC2∝24kT(CD+CFET)2/(3gm),

where CD consists of two terms backplane capacitance and interstrip capacitance and
other symbols having their usual meaning.

Why Rint important?
   Characterizing the quality of a Si microstrip detectors,
                                         spatial resolution (high)
           Rint should high for good electrical separation between neighbouring
          strixel strips
    CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg                 3
                                   Status Report: Radiation Damage for X-rays in Si

Physics of Cint and Rint

In this work, 2-D numerical device simulation using ISE T- CAD DESSIS version 2005.10 has been exploited in order to evaluate the mutual
capacitance and Rint between two facing strips


 Through small signal AC-analysis, the admittance matrix of the network shown in I.a and I.b structure can be solved at an arbitrary bias point,
from which mutual capacitance and Rint between the facing strips through conductance can be estimated

                 The backplane capacitance (CIi−sub, CIj−sub) has not been analyzed in this work



 The contribution to interstrip capacitance (Cint) between adjacent strips mainly comes from four components:

              (1) The capacitance between metal of ith and jth strips (CMi−Mj),
              (2) The capacitance between the implanted strips (CIi−Ij),
              (3) The capacitances between a metal and adjacent strip’s implant (CMi−Ij, CMj−Ii)
              (4) The coupling capacitances between a metal strip and implanted strip (CMi−Ii, CMj−Ii).


  Coupling capacitances CMi−Ii and CMj−Ij are usually much larger than the remaining parasitic components, so that we can consider their
impedance to vanish in the high frequency range of interest

            From first order approximation, we can assume that the total interstrip capacitance between two facing strips is given by

                           Cint=CMi−Mj+CIi−Ij+CMi−Ij,

where, CMi−Ij includes the contribution of both the capacitances between Mi−Ij and Mj−Ii. Whereas, Rint is the resistance between two neighbouring
strips


                                                Cint and Rint should measures at VFD.



           CERN, 28 January 2009                        Ajay K. Srivastava Uni.- Hamburg                                                 4
                       Status Report: Radiation Damage for X-rays in Si


 Device simulation

Aim:  Detailed simulation of strixel p+n-n+ Si sensor with and without metal-
overhang for Cint, Rint and VBD

          What to do?            Effect of metal-overhang extension ( gap between adjacent
strip) width on Cint , Rint   and VBD


Device structure and physical models

Software:    2-D Device simulation through ISE T-CAD DESSIS version 2005.10
(Device Simulation for Smart Integrated Systems)

Proposed strixel Si sensor structure for Cint, Rint and VBD simulation:

(for fixed w/p=0.25)

I. p+n-n+ with and without metal-overhang (two geomety taken I.a and I.b)
II. n+p-p+ with p-stop, p-spray and mixed (p-stop+p-spray) with and without
    metal-overhang
    CERN, 28 January 2009           Ajay K. Srivastava Uni.- Hamburg                   5
                                   Status Report: Radiation Damage for X-rays in Si


                                                WMO                   CMi-Mj
           tpass                                                                        Dielectric passivation
                              Al                                                                      Al
            tox                    CMi-Ii           CMi-Ij                     CMj-Ii         CMj-Ij             SiO2
                                                                                                                           Al
                        Xj         p+                                                                    p+
                                                                      CIi-Ij
                               W
            WN
                             CIi-sub                                                                   CIj-sub



                                                                       n

                                                              n+
                                                     Al

                                                             strip pitch (P)
CMi−Mj – metal metal capacitance                                                             CIi−Ij – implant implant capacitance
CMi−Ij, CMj−Ii- capacitances b/w metal and adjacent strip’s implant                          CMi−Ii, CMj−Ij – capacitance b/w metal
strip                                                                                         and implanted strip

I.a:Cross-section of device structure (I) used in the present simulation work.
       CERN, 28 January 2009                      Ajay K. Srivastava Uni.- Hamburg                                               6
                              Status Report: Radiation Damage for X-rays in Si

      tpass                          WMO               CMi-Mj      Dielectric passivation
                     Al                                                                    Al
        tCP               CMi-Ii        CMi-Ij                  CMj-Ii tox    CMj-Ij             SiO2
                                                                                                        Al
               Xj         p+                                                             p+
                                                       CIi-Ij
       WN             W
                    CIi-sub                                                            CIj-sub


                                                        n
                                                  n+
                                         Al

                                                 strip pitch (P)
   I.b:Cross-section of device structure (I) used in the present simulation work.

 In the present work, we have compared both structures for low Cint
Suitable boundary conditions: In ISE T-CAD DESSIS, Dirichlet boundary conditions are applied
at ohmic contact, whereas Neumann (reflecting ) boundary conditions are applied at non-
contacted edges of Si sensor
C-V analysis: We have applied DC bias on p+ implant ( ith and jth strip of sensor) and AC on top of Al
and n+ on ground
I-V analysis: Scanning of Vbias (applied on n+), p+ - ground, top of Al- ground
     CERN, 28 January 2009                                                                                   7
                                           Ajay K. Srivastava Uni.- Hamburg
                        Status Report: Radiation Damage for X-rays in Si


Specification for strixel Si sensor
Specs for Cint and Rint
Cint>0.7 pF/cm
Rint>200 G-ohm
                Elementary cell model for Cint calculation for 2-strip Si sensor
                                                                       CMiMj  Cm
                                                                       CIiIj  Cp


                                                                      




                                                                                      tox  TI


         Rm= 20 ohm/cm
         For Rstr , Rs ; sheet resistance after implant is needed because ρ =Rs X j
     CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg                               8
                               Status Report: Radiation Damage for X-rays in Si

 Physics models used:
                                  •SRH recombination
                                  •Auger recombination
                                  • Doping-dependent mobility
                                  •Impact ionization
                                  •Surface recombination
                                  •Trap model at Si-SiO2 interface
                                       solving Poisson and electron/hole current-
                                      continuity equations
Input physical and geometrical parameters used in simulation
- Thin oxide (tcp): 200 nm
-Thick oxide (tox): 1 micron
- Final passivation layer thickness (tpass) :1 micron with different dielectric passivant
- Thickness of aluminium layer (WAl): 1.2 microns, W(n+):1 micron
- Junction depth (Xj): 1 micron
- Interface oxide charge (Qf): 0.7 e11 (for 1 0 0 crystal)
- Doping concentration for n-type bulk (ND) 10^12 /cm^3
- The p+ implants are 5x 10^19 atoms/cm^3 the peak and 10^15 atoms/cm^3 at junction at 1 micron
- Device depth (WN): 300 micron
- w/p=0.25 (width=12.5 micron and pitch=50 micron)
- AC frequency :10khz and 1 Mhz with 50mV ac voltage
- DC bias voltage of 100V , 500V and 5000V for optimized structure.
- Metal overhang width i.e. WMO= 0, 2.5, 5, 7.5, 10 (i.e. gap between adjacent strip 25, 20, 15, 10, 5 micron)
- Tlattice :300K, initial minority carier life time of charge carrier in Si wafer 0.1 ms, S0 , 2.8 cm/sec
- Donor trap at center of band gap , Nit : 2.1x 10^10 cm^-2, effective capture cross-section σeff (n/p) :5.6 x10^-16 cm ^2

         CERN, 28 January 2009                Ajay K. Srivastava Uni.- Hamburg                                        9
                      Status Report: Radiation Damage for X-rays in Si


Simulation procedure




                                                                         Different
                                                                         parasitic capacitance




                                                                           Cp, Gp

                                                                                   Rint




   CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg                     10
                         Status Report: Radiation Damage for X-rays in Si

Simulation result and discussion                                -Vbias =100 volt, f= 10 khz




I.a
                                                                                          I.b




      CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg                      11
                       Status Report: Radiation Damage for X-rays in Si




 -Vbias =100 volt, f= 10 khz




 When no metal overhang (without final passivation)
  CIiIj dominates (less in I.a and high in I.b @ VFD = 69.5 Volt) and other two
components CMiMj and CMiIj not affected the capacitance but the values different
(equivalent to zero)

      Cint lower in I.a than I.b structure
      I.a structure preferred for low Cint and high Rint (order of Mega ohm)
    CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg              12
                       Status Report: Radiation Damage for X-rays in Si

-Vbias =100 volt, f= 10 khz




     Cint increases with WMO and after WMO =5 micron , Cint decreases
     Cint lower for WMO= 10 micron
      VBD increases with WMO and higher for WMO= 10 micron
     Cint lower and high VBD for WMO = 10 micron obtained

    CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg        13
                                  Status Report: Radiation Damage for X-rays in Si




                                                        WMO( micron)
                   1.00E-15
                              0   CIi-Ij 2          4            6               8   10      12
                   1.00E-16
                                     CMi-Ij
                   1.00E-17                    CMi-Mj
 Capacitance (F)




                   1.00E-18
                                                                        -Vbias =100 volt, f= 10 khz
                   1.00E-19

                   1.00E-20

                   1.00E-21

                   1.00E-22

                   1.00E-23

                   1.00E-24


CERN, 28 January 2009                         Ajay K. Srivastava Uni.- Hamburg                        14
                   Status Report: Radiation Damage for X-rays in Si




                                                       -Vbias =100 volt, f= 10 khz




Rint increases with WMO and after WMO =5 micron , Rint saturates

High Rint (order of G ohm) obtained for WMO=10 micron

     Cint low and high VBD with high Rint obtained for WMO=10
   micron
           In order to understand this behavior , we have plotted surface electric
        field and electrostatic potential distribution in unirradiated strixel Si
        sensor (I.a: unpassivated and final passivated) with and without metal-
        overhang
CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg                       15
                            Status Report: Radiation Damage for X-rays in Si

Effect of final dielectric passivants on Cint for I.a structure (Vbias =100 volt, f= 10 khz)


        Table.1: WMO =10 micron @VFD

        Passivation layer                           C int (F)


        Si3N4                                   2.05 x 10-16 F


        SiO2+Si3N4                              2.02 x 10-16 F

        SiO2                                    1.95 x 10-16 F




                  Cint increases with ɛdiel

                 VBD will be high for nitride passivated structure due to high ɛdiel = 7.5




       CERN, 28 January 2009           Ajay K. Srivastava Uni.- Hamburg                  16
                         Status Report: Radiation Damage for X-rays in Si

VBD: Surface electric field for different metal-
overhang at Vbias=5000 Volt
                       (Unpasivated, ɛair=1)
                                                                            WMO=5micron




                                     WMO=0 micron

                                                                            WMO=7.5micron




                                     WMO=2.5 micron                          WMO=10micron




      CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg               17
                         Status Report: Radiation Damage for X-rays in Si

Electrostatic potential distribution for different metal-overhang
                                                                             WMO=5 micron
at Vbias=5000 Volt

                                WMO=0 micron




                                                                              WMO=7.5 micron




                                  WMO=2.5micron




        CERN, 28 January 2009       Ajay K. Srivastava Uni.- Hamburg        WMO=10micron    18
                         Status Report: Radiation Damage for X-rays in Si

Comparison of surface electric field for two WMO= 7.5 and 10 micron at Vbias=500 Volt




(WMO= 10 micron, without passivation, Vbias=500 Volt)         (WMO= 10 micron, with nitride
                                                              passivation, Vbias=500 Volt )




   (WMO=7.5 micron, with nitride passivation, Vbias =500 Volt)
        CERN, 28 January 2009       Ajay K. Srivastava Uni.- Hamburg                          19
                      Status Report: Radiation Damage for X-rays in Si

                                 Electric field distribution




(WMO=7.5 micron, with nitride                       (WMO=10 micron, with nitride passivation, Vbias
passivation, Vbias =500 volt )                      =500 volt )


  WMO=7.5 micron metal-overhang structure –
 good for strixel because of lower Cint, high VBD and high Rint and lower overlap
 electric field between MIMj than WMO=10 micron

     CERN, 28 January 2009          Ajay K. Srivastava Uni.- Hamburg                           20
                           Status Report: Radiation Damage for X-rays in Si



Cint for optimized nitride passivated structure (WMO=7.5 micron) at three different
frequency @ VFD, Vbias=100 Volt

Cint (pf/cm)                                   f(Hz)

2.05                                          10 khz
2.42                                          500 Khz
2.41                                          1000 khz


  Cint increases with frequency (10 to 500 khz) and then saturates

Comparison with analytical calculation
Ccoupl                           Analytical                     Simulation

                            4.31 x 10-12 pf/cm               6.65 x 10-12 pF/cm

Little difference in Ccoupl observed due to effective oxide thickness of oxide
changed in the presence of interface and oxide charge or may be due to length of
strip (default = 1 micron in simulator)

         CERN, 28 January 2009           Ajay K. Srivastava Uni.- Hamburg         21
                        Status Report: Radiation Damage for X-rays in Si

Summary and next step

 Optimized width of metal- overhang - WMO=7.5 micron

    Good for strixel p+n-n+ Si sensor because of lower Cint, high VBD and high Rint
 Next step for simulation of n+p-p+ structure with p-stop, p-spray and mixed (p-
stop+p-spray) for Cint, Rint and VBD




     CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg               22
                     Status Report: Radiation Damage for X-rays in Si



Spare Slides

WMO=10 micron




Optimized structure:
Nitride passivated
WMO= 7.5 micron,
IV: VBias=500 Volt


  CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg        23
                    Status Report: Radiation Damage for X-rays in Si




Inversion layer




 CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg        24
                   Status Report: Radiation Damage for X-rays in Si




                         2-D T-CAD Simulations of
                            Isolation structures
                        for n+-on-p Strixel Si sensor




CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg        25
                                        Status Report: Radiation Damage for X-rays in Si

                strip 1
                            Geometry -1                  strip 2


                              ++++++++              oxide
                                                                                        Why is isolation structure needed?
             n+                                                 n+
                                                                                              Isolation structure needed to
       p- substrate                     electron layer
                                                                                         interrupt the inversion layer between the
                                                                                         strip
           p+


                            backplane



                              +
3 techniques available (from n -on-n technology):



                  p-spray                                                  p-stop                          p-spray/p-stop

  S1                                       S2              S1                                  S2     S1                          S2

                                                                                                              high-field region
         high-field regions                                        high-field regions                           depend Qox




  Each technique affects differently VBD and Cint
     Simulations are needed to evaluate impact

         CERN, 28 January 2009                               Ajay K. Srivastava Uni.- Hamburg                                     26
                       Status Report: Radiation Damage for X-rays in Si

Requirement:
For p-spray and p-stop simulation for Cint and VBD :
• P-spray concentration (min 3) , Qf
• P-stop width for w/p=0.25
• Boundary condition
• C-V (Cint measurement) and I-V analysis
• Rest all paramaters or best guesss!

                                                                     Al



                                               tox
                      tcp

Geometry -2            n+
                                              p-spary                     n+

                 p



                                                     p

                                                         p+
                                                      Al
                                                      Al

    CERN, 28 January 2009         Ajay K. Srivastava Uni.- Hamburg             27

				
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