Reaction-Diffusion degradation modelling of a Silicon pMOSFET

mos2ex20.in : Reaction-Diffusion degradation modelling of a Silicon pMOSFET

Requires: S-Pisces
Minimum Versions: Atlas 5.24.1.R

This example is similar to mosex18 but uses a different degradation model. It demonstrates :

  • Using Athena to set up a pMOSFET.
  • Setting up a deck with with the Reaction-Diffusion degradation model enabled.
  • Doing stressing and relaxation biasing transient simulations.
  • Using PROBE statement to get hydrogen densities.
  • Obtaining Id-Vg curves for degraded device structures.

The first section beginning
go athena
is much the same as mos2ex18. The next section starting
go atlas
enables the reaction-diffusion model using the DEVDEG.RD parameter
model temp=300 mos hhi devdeg.rd devdeg.h
and the parameter DEVDEG.H specifies that the traps created will become positively charged. The hot hole model is also enabled. Accompanying the models statement is
degradation rd.sihtot=1.0e12 rd.h.hccoef=1.0e8 rd.kf0=2.0e-4 rd.aevar=0.05 rd.kr0=1.0e-13 rd.invhcoef=0.01 rd.coupled
which sets the density of passivated bonds on the interface using the RD.SIHTOT parameter. The contribution of hot holes to the degradation is included by assigning a value to the RD.H.HCCOEF parameter. The depassivation rate constant RD.KF0 is set to 2.0e-4 /s and the repassivation rate constant to 1e-13 cc /s. The default activation energy of 1.5 eV is used with a variance of 0.05 eV as specified by the RD.AEVAR parameter. The RD.INVHCOEF parameter enables the inversion hole contribution to depassivation rate, and the RD.COUPLED flag enables fully self-consistent coupling between the hydrogen generation rates and the other equations.

The PROBE statements probe the atomic hydrogen and molecular hydrogen compositions at various points near to the Si/SiO2 interface.

The device is stressed to -2.0 V on the gate and 0.1 V on the drain and the simulation run for 100 s. The DEVDEG parameter on the LOG statement means that the total interface charge due to degradation is stored in the log file. The depassivation process creates atomic hydrogen which then diffuses and dimerises to molecular hydrogen. The default values for diffusion and dimerisation have been used, details are given in the manual.

In the next section, starting
go atlas
the same models are set and the device structure degraded to 100 s is read in using the LOAD statement. The gate and drain biases are zeroed on a short ramp and the device is relaxed to 1000 s. The presence of atomic hydrogen which has not diffused away from the interface means that the repassivation rate is greater than the depassivation rate, most notably in the channel under the gate. This is seen in the integrated degradation charge which is captured in the log file and is plotted at the end of the relaxation simulation.

The structure files from this and the previous section are all saved with the DEVDEG parameter on the OUTPUT statement causing the degradation trap density to be output. This is then used in the final section, starting
go atlas
where each of the saved structure files is read in turn during the loop statement, and the Id-Vg curves obtained. These are then plotted and you can see that the relaxed structure has retained a small shift in threshold voltage. This is disproportionate to the overall reduction in interface charge density, and is due to the large repassivation occuring at the interface under the gate, where the device is most sensitive to interface charge. Over longer time periods the free hydrogen will diffuse away and repassivation will cease.

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