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

Output

Reference: “Modeling, Design, and Analysis of MonolithicCharged particle Image Sensors” by Shengdong Li, Univ. of California, IrvineThank Howard Matis for birdging the communications with Shengdong Li

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.

• 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.

p Well

• 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 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.

N Wellp Well

p-epi layer

p+ substrate

p Well

PIXEL Response simulation - Boundary conditions

•When electron hits the p-epi and p-well 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.

PIXEL geometry questions

• What’s the gap between pWell and nWell?

•Are they fully depleted?

•If the gap is zero, what are the depletion thickness?

•What’s the thickness of p-epi layer?

•What’s the substrate thichness?

•What’s the size of the p-well and n-well?

•What’s the shape? Is the cubic shape reasonable approximation?

N Wellp Well p Well

p- epi layer

p+ substrate

Question on interface between pixels

N Wellp Well

p- epi layer

p+ substrate

p Well N Wellp Well

p- epi layer

p+ substrate

p Well

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.