Simulation Of The Electrostatic Field Distribution In A Pixel Device (update)

1
Simulation Of The Electrostatic Field
Distribution In A Pixel Device (update)

• Marina, Artuso
• Chaouki, Boulahouache
• Phys. Dept.
• Syracuse University

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Goals Of The Simulation

• Determination of the electrostatic field
  properties of a pixel device in both p-stop and
  p-spray configuration.
• Assess radiation damage before and after
  irradiation such that we,
• Optimize the pixel structure so that the lateral
  field is minimized(to avoid impact ionization and
  avalanche break down).
• Get a feeling how the surface charge density does
  affect both isolations.
• Determine the efficiency of charge collection and
  dynamical properties influencing charge sharing.

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Outline

• Things to be discussed in this presentation,
• The field distribution for p-stop and p-spray
  pixels before after irradiation.
• The comparison between lateral field
  distributions in both cases.
• The surface charge density dependence of the
  electric field of p-spray isolation technique,
  and a qualitative comparison with results
  obtained from an article titled,
• HERA-B Detectors With P-spray Isolation On The
  N-side done by Kari S. H., Trond W. and Berit S.
  A.

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Brief Description Of The Processors Used
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The Geometry

• A simplified geometry has been adopted.

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Grid size

• A finite element analysis requires a grid size
  input, we have adopted the following

• For the width,
• Maximum width 0.24mm
• Minimum width 0.12mm

• For the height,
• Maximum height 0.24mm
• Minimum height 0.12mm

and this is over a region of 100 mm of width
and 9 mm of depth.
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P-stop And P-spray Pixels

• We will study the P-stop and P-spray pixels where
  the basic structure is the same except that the
  doping distribution differs. We used a depth of
  1.75mm for P-stop pixels(as obtained from SINTEF
  measurement.), and 1mm for P-spray pixels.
• An approximate structure to the graded p-spray
  will be investigated.

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Surface charge density and Substrate

• Before irradiation,
• The substrate is of an n-type with a 2e12/cm-3
  concentration.
• The surface charge density is about 3e11 /cm-2 .
• To model the irradiation effect we have to
  change the following parameters,
• The substrate changes to a p-type with a
  9e12/cm-3 concentration.
• The surface charge density increases to 5e14
  /cm-2 .

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n and p Doping profiles for P-stop structure.

• The n doping profile is,
• Gaussian distribution
• PeakValue 1e20 / cm-3
• ValueAtDepth 2e12 /cm-3
• Depth 4.5 mm
• The p doping profile is,
• Gaussian function distribution
• PeakValue 6e18 / cm-3
• ValueAtDepth 1e11 /cm-3
• Depth 1.75 mm

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p Doping profile for P-spray structure.

• The p doping profile is,
• Error function distribution
• maxValue 6e18 / cm-3
• ValueAtDepth 1e11 /cm-3
• Depth 1 mm

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n density distribution for P-stop and P-spray
structures before irradiation.
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Interpixel P Distribution.
For p-stop structure
For p-spray structure
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Electrostatic field distribution.
For p-stop structure
For p-spray structure
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Vector Presentation Of The Electric Field
For p-stop structure
For p-spray structure
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Lateral field distribution and p-density along
the X-axis.
For p-stop structure
For p-spray structure
Lateral Electric Field
p distribution
Lateral Electric Field
p distribution
pink ? p distribution green ? lateral electric
field
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Lateral field distribution and n-density along
the X-axis.
For p-stop structure
For p-spray structure
Lateral Electric Field
Lateral Electric Field
n distribution
n distribution
red ? lateral electric field green ? n
distribution
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Characteristics of the lateral field (Before
irradiation)

• Before irradiation the maximum lateral field,
• For p-stop structure is 2.277e05 V/cm which is a
  factor of 1.9 higher than the number given in a
  paper done by Rohe et al. paper.
• For p-spray structure is 1.719e06 V/cm which is
  a factor of 4.5 higher than the number given in
  the same paper.

Rohe et al. / Nucl. Instr. And Meth. In Phys.
Res. A 409 (1998) 224-228.
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Interpixel P Distribution.
For p-stop structure
For p-spray structure
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Lateral field distribution and p-density along
the X-axis.
For p-spray structure
For p-stop structure
Lateral Electric Field
Lateral Electric Field
p distribution
p distribution
pink ? p distribution green ? lateral electric
field
red ? lateral electric field green ? p
distribution
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Lateral field distribution and n-density along
the X-axis.
For p-spray structure
For p-stop structure
Lateral Electric Field
Lateral Electric Field
n distribution
n distribution
red ? lateral electric field green ? n
distribution
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Characteristics of the lateral field (after
irradiation)

• After irradiation the maximum lateral field,
• For p-stop structure is 1.263e06 V/cm which is a
  factor of 2.6 higher than the number given in
  Rohe paper.
• For p-spray structure is 1.200e06 V/cm which is
  a factor of 6 higher.

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Oxide Charge Effect

• By increasing the surface charge density one can
  assess the effect of a hard radiation damage on
  both kinds of pixels. The following table will
  include simulations done for different surface
  charge densities,

Surface Charge Density
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Lateral field-oxide charge correlation
Surface Charge density P-stop pixels P-spray pixels
31010/cm-2 1.675105 V/cm 1.873106 V/cm
31011/cm-2 2.310105 V/cm 1.870106 V/cm
31012/cm-2 8.426105 V/cm 1.830106 V/cm
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Simplified model of structuredescribed in
"graded p-spray from SINTEF, Hera-B detectors"

• The n doping profile is,
• Gaussian distribution
• PeakValue 1e18 /cm-3
• diffLength 1.63 mm
• The p doping profile has two pieces,
• 1) Error function distribution
• maxValue 6e16 /cm-3
• diffLength 0.47 mm.
• 2) Gaussian distribution
• PeakValue 6e16 /cm-3
• PeakPosition 1 mm
• diffLength 0.47 mm.
• The Bias voltage is at 200 volts

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P and E-field Distributions for r11010 cm-2
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P and E-field Distributions for r21011 cm-2
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P and E-field Distributions for r11012 cm-2
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Total Electric Field distribution.
The total electric filed distribution at a
distance of 0.47mm away from the surface. The
maximum electric field (for r11010cm-2) is
about 4.4105V/cm, it is a factor of 3 higher
than the maximum field of the real graded p-spray
structure(1.6105V/cm).
Total Electric Field
Black ? for r11010cm-2 red ? for r21011cm-2
Blue ? for r11012cm-2
Lateral Distance (mm)
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Conclusion

• The oxide charge is a key element that affects
  the lateral field in n/n/p pixel structures.
• P-spray detectors need to have enough p doping
  to compensate the inversion layer up to the
  saturation charge in the oxide, thus they start
  with very high lateral fields. P-stop starts with
  lower lateral fields, but their properties
  deteriorate with the increased radiation level.