Erase example

sonosex03.in : Erase example

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

The example shows:

  • How to create a SONOS device structure
  • How to charge and then erase the device
  • Use of PROBE statement to obtain stored charge density

This example illustrates how the position of the trapped charges in the Silicon Nitride vary with time as the device is charged up and then erased by changing the Gate Bias. This example has a 2 nm thick tunnelling layer, a 5 nm Nitride layer and a 5 nm blocking oxide layer.

The NITRIDECHARGING statement sets up a density of 10^20 /cm^3 of electron traps, having a relatively high capture cross-section of 10^-10 cm^2. A bias of 18 V is applied to the gate and the device is charged up. The state of the structure is saved out to a structure file at certain values of charge time. It is charged for 1.0 seconds, after which SOLVE INIT is called, with the SONOS parameter specified.

SOLVE INIT preserves the stored charge density, but on its own tries to put in the steady state equibrium solution. The combination of a large fixed charge, and weak coupling of the drift-diffusion equations through the wide band-gap semiconductors, can result in erroneous free carrier screening of the fixed charge. The SONOS parameter used with SOLVE INIT avoids this.

After this, negative bias is applied to the gate, ramping for 1 nanosecond and then the device is erased for 1.0 seconds. The structure file is output at preset intervals in order to allow the user to inspect the progress of the erasing.

The integrated nett stored charge in C/um is obtained using the PROBE statement

PROBE NAME=nettcharge SONOS.CHARGE INTEGRATE Y.MIN=-0.0075 Y.MAX=-0.0015 X.MIN=0.1 X.MAX=0.2 and this quantity can be plotted as a function of time. When the device is charged, the largest contribution is from the tunnel current into the Nitride conduction band. For erase, it is the tunnelling direct from the traps to the channel. This is illustrated by doing a second erase, with the only change being the parameter ELEC.DEPTH in the NITRIDECHARGING statement. This is increased from 1.5 eV to 2.0 eV, in order to reduce the direct trap-to-channel tunnel current. It can be seen from the log file that the erase takes longer with deeper traps.

The user can employ the TonyPlot cutline tool to see the vertical profile of the Trapped Nitride Charge through the gate stack at various time points of the charging/erasing.

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.