Charge Integration Analysis

imagesensorex02.in : Charge Integration Analysis

Requires: S-Pisces/Luminous
Minimum Versions: Atlas 5.22.1.R

In this example we examine the time integration of charge in the N well of the imaging device and look at crosstalk caused by charge blooming into an adjacent cell. In this case we simulate two half cells representing adjacent red and green pixels. This implies that we will implement color separation filtering and perform integration of charge during a time domain simulation.

The simulation begins with a description of the device structure and mesh using the MESH , X.M , Y.M , REGION , ELEC , and DOPING statements.

In this example we add a couple of extra oxide regions to represent the color filters for the adjacent pixels. The optical characteristics of these regions will be modified to pass or not pass the source light.

Next, we specify some material models and parameter defaults. Of significance is the specification of the complex index of refraction of aluminum using the REAL.INDEX and IMAG.INDEX parameters of the MATERIAL statement. In this case we set the imaginary index to a very high value. This makes the aluminum blocking regions highly reflective/absorptive to light.

Here we also specify the imaginary index of the red filter. In this case the thickness of the filter was used to calculate an index that will absorb 99% of the incident light. We do not need to specify the index of the green filter since it is assumed to pass all of the incident light.

Next we specify the optical source on the BEAM statement. We specify the light with normal incidence from above with a wavelength of 0.5325 microns. This wavelength corresponds to the band pass of the green filter. We also define the sampling in the FDTD mesh. This sampling should typically be a small fraction of the incident wavelength. The TD.WAVES parameter specifies how many source wavelengths are propogated in this simulation. Finally, we specify a file for capture of the structure as represented in the final FDTD mesh solution. This is useful to examine the interference and diffraction patterns that might not be otherwise discernable in the standard finite-difference structure file.

Next, we specify the lenslets above the two pixels.

We add perfectly matched layers (PMLs) at the top and bottom of the FDTD domain to absorb reflected and transmitted light. This is specified on the PML statements.

After obtaining the initial solution, we define probes using the PROBE statement to allow us to measure the integrated electron concentration as a function of time for green and red pixel wells.

Next we empty the well for the initial conditions. To empty the well, we perform a zero carrier solution with the electron quasi-Fermi level set to a very high potential. This insures that the electron concentration is negligible.

Finally, we quickly ramp the beam up to an 'on' state and continue to integrate the charge over 1 micro-second.

At the end of the simulation we can plot the integrated charge in the two wells to see at what time a) the green well charge saturates and b) when the red well charge begins to increase due to charge blooming out of the green well.

We then plot the electron concentration in the final structure. This shows how the electrons have diffused out over the barrier between the two collection wells.

To load and run this example, select the Load example button in DeckBuild. This will copy the input file and any support files to your current working directory. Select the run button to execute the example.