EXAMPLE 17: Full.Coupled Diffusion 312 SUPREM-IV.GS – 2D ...

EXAMPLE 9: Stress Dependent Oxidation
EXAMPLE 9 Stress Dependent Oxidation
DESCRIPTION
This example shows the use of the stress-dependent oxidation model. Ex-
perimental LOCOS profiles are generally of two distinct types [1]. When
the nitride mask is more than two to three times thicker than the pad oxide,
the oxide/silicon interface and the oxide gas surface is kinked. For thinner
nitride masks, the shape can be approximately be described by an error-
function. The kinks were not observed in the first generation of oxidation
simulators, because they result from stress effects on the growth coeffi-
cients. Early oxidation programs did not take stress into account and found
essentially identical oxide shapes irrespective of nitride thickness. This ex-
ample shows how the stress-dependent model in SUPREM-IV.GS can pre-
dict such second-order effects. The example is given in the file “sdep.s4”
in the “examples/exam9” directory.
The grid structure and nitride/oxide sandwich is very similar to the fully-
recessed oxide example. The substrate grid is as sparse as possible (two
lines). The lateral grid is a little coarse to compensate for the increased
computation time used in the nonlinear model.
The new statements are as follows:
oxide stress.dep=t
The nonlinear stress-dependent model is turned on. The default is to turn it
off since it is much more expensive to run than the linear model.
The activation volume for plastic flow Vc, for the stress-dependent reac-
tion rate Vr, and for the diffusivity Vd, are taken from the defaults in the
model file. The values used were derived by fitting Kao’s cylinder oxida-
tion data [2].
SUPREM-IV.GS – 2D Process Simulation for Si and GaAs 281