Download p Well - Purdue Physics

PIXEL Slow Simulation
Xin Li
3/16/2008
CMOS Active Pixel Sensor (APS)
Epitaxy is a kind of interface between a thin film and a substrate. The term
epitaxy (Greek; epi "above" and taxis "in ordered manner") describes an
ordered crystalline growth on a monocrystalline substrate.
Can use the standard Integrate Circuit production process for the production.
Rely on charge diffusion instead of drifting.
Simulation Process
Input from
GEANT3
PIXEL
Geometry
PIXEL response
simulation
Reference:
“Modeling, Design, and Analysis of Monolithic
Charged particle Image Sensors” by Shengdong Li, Univ. of
California, Irvine
Thank Howard Matis for birdging the communications with
Shengdong Li
Output
Input for the Simulator
• Track directions and inject locations at PIXEL
• Number of ionized pairs
– For the charged particles: The number of ionized
electrons/hole pairs. There are two options:
• Energy deposition in the detector volume from Geant.
– StMcTrack->dE().
• H. Matis: for thin layer of material better use the Bichsel
distribution, as
– H. Bichsel, "Straggling in Thin Silicon Detectors," Review
in Modern Physics, vol. 60, pp. 663, 1988
Will use the Bichsel distribution as input since it’s tested with
the experiment.
– For neutral particles including photons (see next
slides)
PIXEL Response Simulation
• For charged track
– the ionized electron/hole pairs originate randomly along the track.
– Number of ionized electrons is E/3.6eV where E is the energy deposition
from the Bichsel distribution.
• For high energy photons and neutral particles
– No good reference to our knowledge.
– possible way to deal with it:
• For high energy photons (E>1MeV) where pair production and
Compton scattering dominant:
– Use Bischsel distribution if we know the pair production vertex
or the Compton scattering origin for high energy photons and
the electron tracks direction (need to check it).
– For low energy gamma-rays below 1MeV where p.e. effect
dominant,
» use the energy deposition from GEANT.
» 1MeV may be below the lower threshold of GEANT tracking.
And may have negligible effect. Need to check it.
– Neglect contribution from other neutral particles since they are
rare.
PIXEL Response simulation - Boundary conditions
p Well
N Well
p Well
p Well
p-epi layer
p+
substrate
when the p-epi electron hit p-epi/p+
substrate, because p+ substracte is more
heavily doped, interface is recognized as a
boundary with total reflection for electrons
in the epitaxial silicon.
when the p+ substrate electron hit p-epi/p+
substrate interface, the interface is totally
transparent.
• When the electron fall into the depletion
region between N-Well and P-Well or the
N-well region, it will be fully collected into
the readout electronics.
•Electrons in the p-well region will be
neglected.
•When electron hits the p-epi and pwell interface, the p-well/p-epi
interface can be recognized as a
boundary with total reflection for
electrons in the epitaxial silicon
because pWell are more heavily doped
and electric field in the depletion
region will reflect the electron away.


• When electrons hit the n-well/p-epi
depletion region, has very little chance to
be reflected but pass through.
Consequently, the n-well/p-epi interface
can be recognized as a boundary with total
absorption.
PIXEL geometry questions
p Well
N Well
p Well
• What’s the gap between
pWell and nWell?
•Are they fully depleted?
p- epi layer
•If the gap is zero, what are
the depletion thickness?
•What’s the thickness of pepi layer?
p+ substrate
•What’s the substrate
thichness?
•What’s the size of the pwell and n-well?
•What’s the shape? Is the
cubic shape reasonable
approximation?
Question on interface between pixels
p Well
N Well
p- epi
layer
p+
substrate
p Well
p Well
N Well
p Well
p- epi
layer
p+
substrate
Will the boundary be total transparent to the
electrons going across the pixel boundaries?
Simulation Diagram
Total integration time is 200us. If there’re still electron diffusion after
this time. We will keep it and add it to the simulation for the next track
hit the same pixel. This might have an impact for pileup. But according
to Shengdong’s thesis, this effect is very small.
Output – PIXEL hits collection
• Output of each event is a collection of hits
– Each hits corresponds to an object of fired pixel
• Each pixel object contains
– PIXEL ID
– Number of electrons
– First 3 or more geant tracks information contribute most to the
signal.
• Injecting location and direction of each track
• Contribution to the fraction of signal from each track.
– Fraction of signal from p-epi layer for each track
– Fraction of signal from p-substrate for each track
– Fraction of signal from p-epi layer for all signals
– Fraction of signal from the p-substrate for all signals
– Fraction of signal from previous 200us cycle.
• Pileup effect?
Summary
• Geometry:
Detailed sizes of different layers, depletion region and
boundaries.
• Input:
Deposition energy, incident angle and inject location
• PIXEL response simulation:
How do ionized electron/hole pairs originate from charged
and neutral particles?
Recombination
Action when crossing boundaries
• Output:
Collection of HITS.